Ellipsometry is used to characterise properties of thin films such as thickness, roughness, optical constants, composition, crystallinity, quality and concentration, and may obtain sub-nanometre surface sensitivity.

When light, including polarisation components in both the s- and p-planes, interacts with a substrate, the relative ratio of the polarisation components may change. Ellipsometry determines the change in polarisation by measuring the amplitude ratio, Y, and phase difference, D, of the reflected or transmitted light. The

“ellipsometric ratio (ER)”, p, (related to the changes in polarisation) may then be calculated to be tan( )e ^_iA . This data is then typically fitted to a model that has been constructed to describe the sample in order to extract parameters of interest such as optical constants and layer thickness.

Model analysis for complex materials, such as organic semiconductors, becomes increasingly computationally expensive and time consuming with an increasing numbers of layers. The application of SE to in situ, real-time analysis of thin films, as the layers are being deposited, for example, may be limited to determining the thickness of the sample only, owing to the increased complexity of the resultant spectra due to absorption of the incident radiation by multiple layers of the substrate. This results in manufacturers often relying on thickness monitoring (which provides no information on the optical parameters or layer quality) as the only form of quality control of thin film devices in situ, followed by intensive post- production quality testing. As a result, product defects are not identified until post production, resulting in high material wastage and process inefficiency.

The present invention aims to provide an improved method of processing SE data (e.g. in real-time). When viewed from a first aspect the invention provides a method of processing spectroscopic ellipsometry data to monitor interfacial processes, the method comprising:

(i) capturing spectroscopic ellipsometry data as a function of the wavelength of electromagnetic radiation that is incident upon a sample to be analysed, wherein the spectroscopic ellipsometry data is captured from a portion of the incident electromagnetic radiation that has been reflected or transmitted from the sample; wherein the spectroscopic ellipsometry data captured comprises data representative of a change in an amplitude ratio, Y, and a change in a phase difference D; wherein the change in the amplitude ratio, Y, represents changes in the amplitudes of the polarisation components of the reflected or transmitted portion of electromagnetic radiation relative to the incident electromagnetic radiation; and wherein the change in the phase difference, D, represents the shift in the phase difference between the polarisation components of the reflected or transmitted portion of electromagnetic radiation relative to the incident electromagnetic radiation;

(ii) determining an ellipsometric ratio, p _t , as a function of the wavelength of the incident electromagnetic radiation using data from a first spectroscopic ellipsometry data set captured at a first time;

(iii) determining the ellipsometric ratio, p ₂ , as a function of the wavelength of the incident electromagnetic radiation using data from a second spectroscopic ellipsometry data set captured at a second time;

(iv) determining a differential in the ellipsometric ratio, dr, with respect to time, as a function of the wavelength of the incident electromagnetic radiation, using the determined ellipsometric ratios, p _t and p ₂ ; and

(v) using the determined differential of the ellipsometric ratio to compare the measured data sets to a reference data set.

When viewed from a second aspect the invention provides a spectroscopic ellipsometry data processing system for monitoring interfacial processes, the spectroscopic ellipsometry data processing system comprising: an ellipsometer comprising: an electromagnetic radiation source for generating electromagnetic radiation to be incident upon a sample to be analysed; a detector for detecting a reflected or transmitted portion of the electromagnetic radiation incident upon the sample, wherein the detector is arranged to: capture spectroscopic ellipsometry data as a function of the wavelength of electromagnetic radiation that is incident upon a sample to be analysed, wherein the spectroscopic ellipsometry data is captured from a portion of the incident electromagnetic radiation that has been reflected or transmitted from the sample; wherein the spectroscopic ellipsometry data captured comprises data representative of a change in an amplitude ratio, Y, and a change in a phase difference D; wherein the change in the amplitude ratio, Y, represents changes in the amplitudes of the polarisation components of the reflected or transmitted portion of electromagnetic radiation relative to the incident electromagnetic radiation; and wherein the change in the phase difference, D, represents the shift in the phase difference between the polarisation components of the reflected or transmitted portion of electromagnetic radiation relative to the incident electromagnetic radiation; and processing circuitry for processing the ellipsometry data, wherein the processing circuitry is configured to:

(i) determine an ellipsometric ratio, _x , as a function of the wavelength of the incident electromagnetic radiation using data from a first spectroscopic ellipsometry data set captured at a first time;

(ii) determine the ellipsometric ratio, p ₂ , as a function of the wavelength of the incident electromagnetic radiation using data from a second spectroscopic ellipsometry data set captured at a second time;

(iii) determine a differential of the ellipsometric ratio, dr, with respect to time, as a function of the wavelength of the incident electromagnetic radiation, using the determined ellipsometric ratios, p _t and p ₂ ; and

(iv) use the determined differential of the ellipsometric ratio to compare the measured data sets to a reference data set. The present invention thus provides a method of, and an ellipsometry data processing system for, processing spectroscopic ellipsometry data to monitor interfacial (e.g. surface) processes of a sample being analysed. Such spectroscopic ellipsometry is a surface sensitive technique, and can thus be used to study a whole variety of surface phenomena.

Spectroscopic ellipsometry data is captured by directing electromagnetic radiation from a source of electromagnetic radiation on a sample (on which interfacial processes are being performed) to be analysed, such that the interfacial processes may be monitored. A detector captures ellipsometry data by measuring a change in an amplitude ratio, Y, and a change in a phase difference, D, of a reflected or transmitted portion of electromagnetic radiation. The data is captured as a function of the wavelength of the electromagnetic radiation incident upon a sample to be analysed. (Whether the portion of electromagnetic radiation that is detected by the detector is reflected or transmitted by the sample being analysed, may depend on the geometry of the measurement being taken, e.g. of the sample and/or ellipsometer.)

The change in amplitude ratio, Y, measures the change in the amplitudes of the polarisation components (s- and p- components) of the portion of the electromagnetic radiation reflected or transmitted from the sample relative to the amplitudes of the polarisation components (s- and p- components) of the electromagnetic radiation incident on the sample. The change in amplitude ratio may therefore be expressed as: where E _p/S is the complex p- and s- component of the electromagnetic radiation.

The change in phase difference, D, measures the shift in the phase difference between the polarisation components (s- and p- components) of the portion of the electromagnetic radiation reflected or transmitted from the sample relative to the phase difference between the polarisation components (s- and p- components) of the electromagnetic radiation incident on the sample. For example, the phase difference between the polarisation components of the incident electromagnetic radiation, D _έ , may be represented as: Aj — F^,ί ^— F r,ί where F _5/R is the phase of the s- and p- polarisation components of the electromagnetic radiation respectively. Similarly, the phase difference between the polarisation components of the electromagnetic radiation reflected or transmitted from the sample, A _sam , may be represented as:

Asam ^— F S,sam ~ F P,sam such that the shift in the phase difference, D, may then be represented as:

In some preferred embodiments, the incident electromagnetic radiation is linearly polarised. In embodiments where the incident electromagnetic radiation is linearly polarised, the phase difference between the polarisation components of the incident electromagnetic radiation will be substantially equal to zero, i.e. , D _; = 0.

In such embodiments, the change in phase difference, D, may then be represented by the phase difference of the reflected or transmitted portion of the electromagnetic radiation from the sample, A _sa m· This may then allow the change (shift) in the phase difference (between the reflected or transmitted portion of electromagnetic radiation and the incident electromagnetic radiation) simply to be measured by measuring the phase difference, D _5a)h , between the polarisation components of the electromagnetic radiation reflected or transmitted from the sample (without having to determine the shift from the phase difference of the incident electromagnetic radiation).

Thus preferably the data representative of the change in the phase difference D (between the reflected or transmitted portion of electromagnetic radiation and the incident electromagnetic radiation), comprises data representative of the phase difference, D _5a)h , between the polarisation components of the electromagnetic radiation reflected or transmitted from the sample. Preferably the method comprises (and the detector is configured to) capturing data representative of the phase difference, D _5a)h , between the polarisation components of the electromagnetic radiation reflected or transmitted from the sample. ln some embodiments, e.g. when the incident electromagnetic radiation is linearly polarised, it may be preferable for the amplitudes of the polarisation components (s- and p- components) of the electromagnetic radiation incident on the sample to be arranged to be substantially equal, e.g. (E _p /E _s ) _incident = 1. In such embodiments, the change in amplitude ratio, Y, may then be represented by the amplitude ratio of the polarisation components of the portion of the electromagnetic radiation reflected or transmitted from the sample. This may then allow the change in the amplitude ratio (between the reflected or transmitted portion of electromagnetic radiation and the incident electromagnetic radiation) simply to be measured by measuring the amplitude ratio, tan = E _{P Sam} /E _{S Sam} , of the polarisation components of the electromagnetic radiation reflected or transmitted from the sample (without having to normalise the amplitudes to those of the incident electromagnetic radiation).

Thus preferably the data representative of the change in the amplitude ratio, Y (between the reflected or transmitted portion of electromagnetic radiation and the incident electromagnetic radiation), comprises data representative of the amplitude ratio, Y _5aίh , of the polarisation components of the electromagnetic radiation reflected or transmitted from the sample. Preferably the method comprises (and the detector is configured to) capturing data representative of the amplitude ratio, Y _5aίh , of the polarisation components of the electromagnetic radiation reflected or transmitted from the sample.

Thus preferably the method comprises (and the detector is configured to) measuring (at the first and second times) (e.g. parameters representative of) the changes in amplitude ratio, Y, and phase difference, D, of the reflected or transmitted portion of electromagnetic radiation incident upon the sample to be analysed (relative to the incident electromagnetic radiation), as a function of the wavelength of the incident electromagnetic radiation (and capturing data representative of these measurements). As outlined above, the change in the amplitude ratio, Y, is the ratio of the amplitudes of the polarisation components (s- and p- components) of the portion of the electromagnetic radiation reflected or transmitted from the sample relative to the amplitudes of the polarisation components (s- and p- components) of the electromagnetic radiation incident on the sample. As also outlined above, the change in the phase difference, D, is the shift in the phase difference between the polarisation components (s- and p- components) of the portion of the electromagnetic radiation reflected or transmitted from the sample relative to the phase difference between the polarisation components (s- and p- components) of the electromagnetic radiation incident on the sample.

The method preferably comprises (e.g. as part of the measuring step) (and the detector is configured to) measuring the amplitude ratio, Y, and phase difference, D, of the reflected or transmitted portion of electromagnetic radiation, as a function of the wavelength of the incident electromagnetic radiation (and capturing data representative of these measurements). When the components of the incident electromagnetic radiation have a substantially equal amplitude and/or the incident electromagnetic radiation is linearly polarised, these may be the only measurements that are required to be made to determine the changes in the amplitude ratio, Y, and phase difference, D, e.g. without also measuring (or having knowledge of) the amplitude ratio and phase difference of the polarisation components of the incident electromagnetic radiation. The changes in amplitude ratio and phase difference (Y, D), resulting from the electromagnetic radiation being incident on and reflected (or transmitted) from the sample on which the interfacial process is being formed, are measured by the detector. Thus preferably the changes in amplitude ratio, Y, and phase difference,

D, of a reflected or transmitted portion of electromagnetic radiation incident upon a sample to be analysed, are measured as a function of the wavelength of the incident electromagnetic radiation.

The ellipsometric ratio (ER), p, upon interaction of the incident electromagnetic radiation with the surface is determined, using data from first and second spectroscopic ellipsometry data sets captured at a first and second time respectively, wherein the measured spectroscopic ellipsometry data represents the measured changes in amplitude ratio and phase difference (Y, D)). The ER is representative of the change in polarisation of the incident electromagnetic radiation upon interaction with the surface. The ER may be determined using p = tan Y e ^l& or p = tan e ^iA .

The differential with respect to time in the ER, dr, is determined (e.g. at the first and second times), using the determined ER of the reflected or transmitted electromagnetic radiation at the two different times. By determining the differential with respect to time, it is also determined with respect to the thickness of a layer of a (e.g. thin film) sample that is being deposited.

The Applicant has appreciated that determining the differential in the ER provides an approximation of the optical properties (synonymous with the refractive indices) of the sample being monitored. Comparing the measured data (using the differential in the ER) to a reference data set, such as the optical properties (e.g. of a sample of known quality or desirable parameters such as thickness), thus allows the interfacial process to be monitored over time. It will be appreciated that a non-zero value of dr corresponds to a change in the sample (e.g. a layer being deposited or modified) and thus helps to provide a direct measurement of an interfacial process.

Therefore, through simple (e.g. numerical) differentiation (from the calculation of the differential of the ER (representative of changes in polarisation, 6Y and 6A)), a (substantially) real-time approximation of the optical properties of a sample, such as a thin-film device being manufactured, may be obtained. This may allow the development of a process, e.g. the deposition of multiple thin-film layers or the evolution of a chemical reaction, to be monitored.

Furthermore, this may allow the interfacial process to be monitored as a function of time and in situ, e.g. owing to the simple nature of the calculations that are performed. This contrasts with the computationally expensive model analysis of conventional ellipsometry techniques that may only be possible to be performed post-production, e.g. because of the data needing to be fitted to reference SE data in order to extract layer thickness and optical parameters (such as refractive index).

The present invention may be used in a number of different fields, including, but not limited to the field of surface science and the manufacturing of thin-film devices.

The (substantially) real-time analysis helps to provide in situ detection of inadequate quality during production, thus allowing the termination of the production procedure (saving material wastage) and/or the in situ modification of process parameters, such as temperature or contamination. This may help to remediate any detected issues and/or identify the point in time at which optimal parameters are reached. In turn, this helps to reduce unnecessary material usage and/or inefficient processing time. Thus, in some preferred embodiments of the present invention, the process may provide a feedback loop mechanism for manufacturing processes.

The electromagnetic radiation source of the ellipsometer, for generating electromagnetic radiation to be incident upon a sample to be analysed, may comprise any suitable and desired electromagnetic radiation source. The electromagnetic radiation source may be arranged to output non-polarised radiation, e.g. the electromagnetic radiation source may comprise a thermal bulb (e.g. halogen bulb) or an arc-discharge plasma lamp, a polarised radiation source (e.g. a laser), or a circularly polarised radiation source. The electromagnetic radiation source may, for example, be continuous wave.

In some embodiments the electromagnetic radiation source comprises one or more polarisation manipulating elements, e.g. half-waveplates, compensators and/or polarisers. These may be used to alter the polarisation of the output electromagnetic radiation before it is incident upon the sample, e.g. to arrange the incident electromagnetic radiation to be linearly polarised and/or have equal amplitude polarisation components. In some embodiments the (e.g. one or more polarisation manipulating elements of the) electromagnetic radiation source is arranged to set the s-polarisation and p-polarisation components at any required amplitude ratio, e.g. ranging from 0:1 or 1:0 to 1:1. In some embodiments the s- polarisation and p-polarisation components are set to an amplitude ratio of 1:1, e.g. a 45° polarisation angle relative to the sample.

In some embodiments the electromagnetic radiation source is arranged to output a plurality of wavelengths, e.g. broadband radiation. In some embodiments, the plurality of wavelengths may be output at the same instance in time. Thus the electromagnetic radiation source may comprise a blackbody radiator, e.g. a broadband laser. In some embodiments, the plurality of wavelengths may be output at discrete instances in time, e.g. a wavelength tunable laser. This allows the optical properties of a sample surface to be analysed at multiple different (e.g. a continuous range of) wavelengths, e.g. within a single measurement. This may be useful for complex samples with multiple absorption features across a broad range of wavelengths that may vary differently as a function of the number of (e.g. deposition) layers of the sample being analysed.

Thus preferably the changes in amplitude ratio and phase difference (Y, D) of a reflected or transmitted portion of electromagnetic radiation incident upon a sample to be analysed, as a function of the wavelength of the incident electromagnetic radiation, are measured (and data representative of these measurements is captured) for a plurality of wavelengths of the incident electromagnetic radiation (for each of the first and the second data sets at the first and second times respectively).

Preferably the ER (as a function of the wavelength of the incident electromagnetic radiation) is determined (from the first and second data sets) for a plurality of wavelengths of the incident electromagnetic radiation. Preferably the differential of the ER (as a function of the wavelength of the incident electromagnetic radiation) is determined (from the first and second data sets) for a plurality of wavelengths of the incident electromagnetic radiation.

In some embodiments the electromagnetic radiation source is arranged to output narrowband radiation, e.g. a single wavelength. Thus the electromagnetic radiation source may comprise a monochromatic radiation source, e.g. a narrowband laser. It will be appreciated that narrowband radiation sources, e.g. monochromatic lasers, have a narrower energy distribution with respect to wavelength, and thus may be able to be used on thicker samples without saturation of the absorption signal.

As spectroscopic ellipsometry measurements are (e.g. highly) interferometric in nature, with the radiation reflected or transmitted from the first layer of a sample material interfering with the radiation reflected or transmitted from a second layer of the sample material, and so on, it will be appreciated that the thickness of the sample that can be studied should preferably be less than the coherence length of the radiation source. Thus, highly coherent radiation sources, e.g. lasers, may be more suited to studying thicker samples than non-coherent light sources, e.g. black body radiators. Ellipsometry measurements may thus require a compromise between the bandwidth of a radiation source and the coherence length.

In some embodiments, the electromagnetic radiation source is arranged to output into a super-continuum generation system (e.g. a sapphire) and/or particle accelerator to generate radiation with long coherence lengths and broad spectral ranges. An electromagnetic radiation source with a longer coherence length may help to increase the thickness of a sample that is able to be analysed. For example, samples having thicknesses of up to approximately five times the wavelength of the incident electromagnetic radiation may be able to be analysed, e.g. using an ellipsometer with a white light source. This may be taken as an approximation to the coherence length.

In some embodiments, the ellipsometer comprises an imaging system, e.g. positioned between electromagnetic radiation source and the sample, coupled to the electromagnetic radiation source, for directing the incident radiation beam onto the sample. The imaging system may comprise any suitable and desired components, for example, one or more (e.g. all) of: mirror(s), lens(es) and polariser(s).

The detector of the ellipsometer, for detecting a reflected or transmitted portion of the electromagnetic radiation incident upon the sample, may comprise any suitable and desired detector.

In some embodiments, the detector comprises a single channel device (e.g. a photodiode or photomultiplier). In some embodiments, the detector comprises a multichannel device (e.g. a charge-coupled device (CCD)). Single channel devices may be more sensitive than multi-channel detectors. However, multichannel detectors may be able to measure the data representative of the ellipsometric parameters (e.g. the changes in tan( ) and D) for all wavelengths simultaneously, thus helping to improve data collection efficiency.

In some embodiments, the detector is coupled to a spectrometer. This may allow better precision, sensitivity and spatial resolution of the radiation’s component wavelengths when capturing measurements using the detector (e.g. compared to using a CCD).

In some embodiments, the ellipsometer comprises a polarisation analyser, e.g. positioned between the sample and the detector, arranged to be coupled to the reflected or transmitted portion of the radiation beam from the sample. The polarisation analyser may comprise any suitable and desired components, for example, one or more (e.g. all) of: lens(es) and polariser(s).

When the ellipsometer comprises a polarisation analyser, preferably the polarisation analyser is used to capture data as a function of the analyser angle. This may allow the measurements (e.g. of the intensity of the reflected or transmitted portion of the electromagnetic radiation on the detector) that are captured by the detector to be measured as a function of the analyser angle. In turn, this may allow (e.g. Fourier) analysis to be performed on the measured data, to determine the SE parameters (Y and D).

At a first time, the ellipsometer measures a first spectroscopic ellipsometry data set comprising data representative of a change in amplitude ratio, Y _c , and change in phase difference, A _t , (the ellipsometric parameters) of a reflected or transmitted portion of the electromagnetic radiation incident upon the sample to be analysed, as a function of the wavelength of the incident electromagnetic radiation. At a second, later time, the ellipsometer measures a second spectroscopic ellipsometry data set comprising data representative of the change in amplitude ratio, Y ₂ , and change in phase difference, D ₂ , (the ellipsometric parameters) of a reflected or transmitted portion of electromagnetic radiation incident upon the sample to be analysed, as a function of the wavelength of the incident electromagnetic radiation. The ellipsometric parameters may be measured and/or determined in any suitable and desirable way, for example, being directly output by the detector or determined by processing circuitry (e.g. external to the detector).

The detector may capture the SE data (SE parameters) in any suitable and desired way. In one embodiment the detector measures the intensity of the reflected or transmitted electromagnetic radiation (as a function of the wavelength of the incident electromagnetic radiation). Preferably the detector (or the processing circuitry) is arranged to extract the SE data (the changes in amplitude ratio and phase difference (Y, D)) from the measured intensity. Preferably the detector (or the processing circuitry) extracts the SE data from the intensity measurements using (e.g. Fourier) analysis (e.g. of the intensity of a particular wavelength of the electromagnetic radiation as measured by the detector).

In some embodiments, the method comprises (and the processing circuitry is arranged to) determining a corrected change in phase difference data set, D ₂ ', from the change in phase difference data, D ₂ , of the second data set (the data representative of the change(s) in phase difference from the second measured ellipsometry data set), i.e. correcting the set of phase difference data value(s) in the second set of data. Correcting the change in phase difference value(s) helps to account for the cyclic nature of the phase change data value(s). Preferably the corrected change in phase difference data set is produced by applying a correction function to the measured (i.e. original) change in phase difference data set (thus preferably correcting the change in phase difference data value(s) using the correction function).

In some embodiments, the corrected change in phase difference data set, D ₂ ' is determined by comparing the change in phase difference data, D ₂ , of the second data set with the change in phase difference data, A _t , of the first data set. Thus, preferably the method comprises (and the processing circuitry is arranged to) determining a corrected change in phase difference data set, D ₂ ', from the change in phase difference data, A _t , of the first data set (the data representative of the change(s) in phase difference from the first measured ellipsometry data set). However, first, it may be determined whether it is necessary to correct the change in phase difference data, D ₂ .

Thus, in some embodiments, the method comprises (and the processing circuitry is arranged to) determining whether to correct a value of the change in phase difference data set for the second data set, D ₂ , by calculating the difference between the respective values of the change in phase difference data in the first and second data sets (e.g. for a particular wavelength, e.g. each of the wavelengths of the incident electromagnetic radiation) of the incident electromagnetic radiation. When the magnitude of the difference in the change in phase difference values (e.g. for a particular wavelength of the incident electromagnetic radiation) is greater than 180°, the method comprises (and the processing circuitry is arranged to), applying a correction factor to the phase change value of the second data set (e.g. for each wavelength of the incident electromagnetic radiation for which the difference in the phase change values is greater than 180°).

In some embodiments, the difference of the two measured change in phase difference data values is determined by calculating D ₂ - A (e.g. for the change in phase difference data values in the first and second data sets for each wavelength). When the value of the difference is greater than +180°, the correction factor is applied by subtracting 360° from the change in phase difference value of the second data set, D ₂ (e.g. for a particular wavelength of the incident electromagnetic radiation). When the value of the difference is less than -180°, the correction factor is applied by adding 360° to the change in phase difference value of the second data set, D ₂ (e.g. for a particular wavelength). When the value of the difference is greater than or equal to -180° and less than or equal to +180°, no correction factor is applied.

These correction factors together (e.g. for all the wavelengths of the second data set (for which it has been determined to correct the change in phase difference data values)) thus preferably form the correction function applied to the (e.g. every) change in phase difference values of the second data set. Applying the correction function helps to account for the cyclical nature of the change in phase difference values, i.e. the phase values measured by the detector may range between 0° and 360°, such that after 360°, the values start to increase from 0° again. Alternatively, the phase values measured by the detector may range between -180° and + 180°, such that after +180°, the values start to increase from -180° again.

In embodiments in which a series of data sets are captured, e.g. third, fourth, fifth, etc., data sets, preferably the change in phase difference data values (e.g. for each of the wavelengths of the incident electromagnetic radiation) are corrected (when necessary) for each subsequent data set, preferably using the respective (e.g. corrected) change in phase difference data values of the previously captured data set. For example, the change in phase difference data values of the fourth data set (e.g. for each of the wavelengths of the incident electromagnetic radiation) are corrected (when necessary) using the (e.g. corrected) change in phase difference data values of the third data set. Similarly, were there to have been a data set captured prior to the first data set, the change in phase difference data values of the first data set (e.g. for each of the wavelengths of the incident electromagnetic radiation) may be corrected (when necessary) using the (e.g. corrected) change in phase difference data values of the previously captured data set.

The ER, p, as a function of the wavelength of the incident electromagnetic radiation, determined using the data from the data set representative of Y and D, may be determined in any suitable and desirable way, e.g. using the equation p = tan(W) e ^~lA . In some embodiments, the corrected change in phase value(s), D', is used instead of D, to determine the ER, p.

Once the ER at two different times has been determined, the differential in the ER, e.g. assessed at these two different times is determined. The differential of the ellipsometric ratio, dr, with respect to time, as a function of the wavelength of the incident electromagnetic radiation, using the determined changes in polarisation, p _t and p ₂ , may be determined in any suitable and desirable way, for example, using a (e.g. numerical) differentiation.

In some embodiments, the differential of the ER, dr, is determined by differentiating the ER with respect to time, e.g. using the equation: dr = e ^_iA [((sec( )) ² X dY — i X tan( ) X dD], when ER is determined using p = tan e ^_iA , or: dr = e ^iA [((sec( )) ² X dY + i X tan( ) X dD], when ER is determined using p = tan Y e ^lA , where dY = Y ₂ - Y _c and dD= D ₂ - A _t . This equation makes the assumption that the data sets are measured as a temporally even distribution, e.g. the time between measuring the first and second data set is equal to the time between measuring the second and third. In some embodiments, the differential of the ER is determined using a method of second order accurate central differences, e.g. using the numpy. gradient^, t) function in Python to determine 6Y and/or numpy. gradient^, t) function in Python to determine 6D. In these embodiments, when the data sets are evenly distributed temporally, dY = Y ₂ - Y _c and dD= D ₂ - A is only used to determine dr for the first and last data sets. For all other data sets, the value of dY and dD is determined using the equation:

Sx _j x _i+1 - xt- St 2h where x is Y or4, j is the time point at which the differential is being determined and h is the temporal step size between data sets.

In some embodiments, the dr data set is presented graphically. In some embodiments the values of Sp are plotted as a function of the energy (and thus wavelength) of the incident electromagnetic radiation. In some embodiments the values of Sp are plotted as a function of time, in order to show the development of Sp as the interfacial process progresses (e.g. as a thin film layer is deposited). One or more (e.g. all) of the real part, the imaginary part and the magnitude of the dr values may be used, e.g. for plotting and/or comparing the data set.

In some embodiments, the evolution of dr with respect to time provides an indirect measure of the evolution of the interfacial process being monitored, e.g. in situ. Thus the method of the present invention may be performed over a time scale that is substantially equivalent to the time over which the interfacial process being monitored occurs. For example, the time between the first and second times, for measuring the first and second data sets may correspond to substantially the same time over which the interfacial process being monitored occurs. This helps to provide effective monitoring of the process in “real time” (e.g. between the deposition of layers).

The measured data is then compared to a reference data set, using the determined differential of the ER. The comparison may be performed in any suitable and desired way, e.g. as a function of the wavelength of the incident electromagnetic radiation. In one embodiment the method comprises (and the processing circuitry is configured to) comparing the determined differential of the ER to the reference data set, e.g. as a function of the wavelength of the incident electromagnetic radiation. Thus, the determined differential of the ER may be compared directly to (e.g. a comparable measure in) the reference data set, e.g. rather than calculating a measure for the comparison or fitting the ER data to a model (wherein the model determines the refractive index, n, and the extinction coefficient, k). In one embodiment the comparison uses the real part of the determined differential of the ER. Preferably this is compared to an extinction coefficient, k, determined from an absorption spectrum or through model fitting to the SE data of the reference data set for a sample. In one embodiment the comparison uses the imaginary part of the determined differential of the ER. Preferably this is compared to a refractive index, n, determined from an absorption spectrum or through model fitting to the SE data of the reference data set for a sample. In one embodiment the comparison uses the magnitude of the determined differential of the ER. Preferably this is compared to Vn ² + k ² determined from an absorption spectrum or through model fitting to the SE data of the reference data set for a sample. The comparison may use one, more or all of the real part, the imaginary part and the magnitude of the determined differential of the ER, when comparing with the reference data set.

The reference data set may be obtained from any suitable and desirable source, e.g. a data set obtained from another distinct experiment, a data set from the same sample (that is currently being monitored) at an earlier point in time or a computational model. Preferably the reference data set is obtained from or produced for the same interfacial process as the interfacial process being monitored. Thus, for example, the reference data set may be obtained from or produced for the same deposition process and/or the same material (e.g. thin film layer) as that for the interfacial process being monitored.

The comparison may be performed in any suitable and desirable way, for example, using a numerical difference (e.g. between the measured data and the (e.g. corresponding measure of the) reference data set) and/or visual inspection.

In some embodiments, the method comprises (and the processing circuitry and, e.g., the ellipsometer are configured to) using the comparison to the reference data set to provide feedback to the interfacial process being analysed. Preferably the method comprises (and the processing circuitry and, e.g., the ellipsometer are configured to) using the feedback to control the interfacial process.

For example, when (as part of the comparison) the difference(s) between the determined value(s) of dr and the corresponding value(s) of the reference data set are less than a (e.g. particular, e.g. predefined) threshold (indicating, for example, that the interfacial process has been completed and/or is being performed to a sufficient quality), then the (e.g. particular) interfacial process may be confirmed as being completed and/or being performed to an acceptable standard. This may allow the (e.g. particular) interfacial process to be stopped (e.g. at a particular location of the sample), which may then allow a further interfacial process to be performed on the sample (e.g. a new layer may be deposited) or the interfacial process to be performed at a different location of the sample.

Thus preferably the method comprises (and the processing circuitry is configured to) calculating the difference(s) between the determined value(s) of dr and the corresponding value(s) of the reference data set, e.g. for a plurality of wavelengths of the incident electromagnetic radiation.

When, for example, the differences between the determined values of dr and the corresponding values of the reference data set are greater than a (e.g. particular, e.g. predefined) threshold (indicating, for example, that the interfacial process has not been completed and/or is not being performed to a sufficient quality), then the process may be stopped. This may, for example, allow the interfacial process to be performed again (e.g. on a new sample, with, for example, the previous sample being discarded). Alternatively, when the differences are greater than a threshold, the interfacial process may, for example, be continued. This may, for example, allow an uncompleted process (e.g. layer being deposited) to be completed.

In some embodiments, such “feedback control” may provide in situ identification of anomalies in the interfacial process being performed (e.g. errors in deposition) thus allowing, for example, optimisation of the procedural parameters (e.g. temperature, e.g. pressure) and/or the premature cessation of a process (e.g. to minimise material loss).

In some embodiments, the method comprises (and the processing circuitry is configured to) determining a second differential (or derivative) of the ER, d ² r, with respect to time, as a function of the wavelength of the incident electromagnetic radiation (e.g. for each of a plurality of different wavelengths). For these measurements, d ² r may be determined using the differential of the ER for the respective data). The second differential may be determined in any suitable and desirable way, for example, using (e.g. numerical) differentiation.

In some embodiments, the second differential of the ER, d ² r, is determined by differentiating the (first) differential of the ER, dr, with respect to time, e.g. using the equation: d ² r = e ^iA [((sec( )) ² (6 ² - 2 c ί c dY c dD + 2 c tan( ) * (dY) ² ) — tan( )(i X d ² D + (dD) ² )], when ER is determined using p = tan e ^ίD , or: d ² r = e ^iA [((sec( when ER is determined using p = tan Y e ^lA , where dY = Y ₂ - Y _c and dD= D ₂ - A _t . This equation makes the assumption that the data sets are measured as a temporally even distribution, e.g. the time between measuring the first and second data set is equal to the time between measuring the second and third. It will also be appreciated that the second differential of the ER, d ² r, may be determined directly from the values of Y and D.

In some embodiments, the second differential of the ER is determined using a method of second order accurate central differences, e.g. using the numpy. gradient^, t) function in Python to determine 6Y and/or numpy. gradient^, t) function in Python to determine 6D. In these embodiments, when the data sets are evenly distributed temporally, dY = Y ₂ - Y _c and dD= D ₂ - A (and correspondingly for the second order differentials) is only used to determine d ² r for the first and last data sets. For all other data sets, the value of dY and dD is determined using the equation: where x is Y orA, i is the time point at which the differential is being determined and h is the temporal step size between data sets. A corresponding equation is used for the second order differentials.

In some embodiments, the d ² r data set is presented graphically. In some embodiments the values of d ² r are plotted as a function of the energy (and thus wavelength) of the incident electromagnetic radiation. In some embodiments the values of d ² r are plotted as a function of time, in order to show the development of d ² r as the interfacial process progresses (e.g. as a thin film layer is deposited).

One or more (e.g. all) of the real part, the imaginary part and the magnitude of the d ² r values may be used, e.g. for plotting and/or comparing the data set.

In some embodiments, the evolution of d ² r with respect to time provides additional insight into the interfacial process (e.g. thin film growth) being monitored, e.g. in situ.

Thus, in some embodiments, the second differential of the ER is used to compare the measured data to the reference data set. In one embodiment the method comprises (and the processing circuitry is configured to) comparing the determined second differential of the ER to the reference data set, e.g. as a function of the wavelength of the incident electromagnetic radiation. As with the comparison using the determined first differential of the ER, the real part, the imaginary part and/or the magnitude of the determined second differential of the ER may be used when comparing the measured data to the reference data set.

In some embodiments, the sample is (arranged to be kept) stationary with respect to the ellipsometer, e.g. while the interfacial process is being performed and the sample is being analysed. This helps to allow in situ analysis of the changes in a sample with respect to an interfacial process, e.g. monitoring the deposition of thin film layers. In in situ analysis, preferably substantially the same position on a sample is interrogated by the same ellipsometer at different time points.

In some embodiments, the sample is (arranged to be kept) translated with respect to the ellipsometer (for example, the sample travels along a production line), e.g. while the interfacial process is being performed and the sample is being analysed. This helps to allow in line analysis of a sample. In line analysis may be used, for example, to monitor the production of parts via a sequential process (e.g. roll-to-roll production processes, e.g. products that require different constituent layers and/or different deposition conditions).

In some embodiments, the ellipsometry data processing system comprises a plurality of ellipsometers. Preferably each of the plurality of ellipsometers is arranged as outlined in the first and second aspects of the invention. This helps to allow the sample to be analysed in line, by at least two ellipsometers. Preferably each of the plurality of ellipsometers is arranged to monitor a respective stage (e.g. a respective location) in the in line interfacial process, e.g. the manufacturing process. This enables the SE data sets that are captured (and the measures that are subsequently determined) by an ellipsometer to belong to a particular location. For example, in some embodiments each ellipsometer may be arranged to monitor the deposition of a different material layer. This may be used to compare sample quality and consistency of the process, e.g. in production.

In some embodiments, the frequency at which the ellipsometer measurements are captured (i.e. the frequency corresponding to the time between the first and second times) is greater than 0.1 Hz, e.g. greater than 0.5 Hz, e.g. greater than 1 Hz, e.g. greater than 2 Hz, e.g. greater than 5 Hz, e.g. greater than 10 Hz, e.g. greater than 30 Hz, e.g. greater than 100 Hz. In some embodiments, the frequency at which the ellipsometer measurements are captured (i.e. the frequency corresponding to the time between the first and second times) is less than 0.1 Hz, e.g. less than 0.01 Hz, e.g. less than 0.001 Hz, e.g. less than 0.0005 Hz, e.g. less than 0.0001 Hz. Conventional techniques are limited by the speed of the analysis of the ellipsometry measurements, because this requires complex modelling that scales in complexity (and thus time required) with increasing numbers of layers and/or species present at the sample interface.

As the ellipsometry analysis, according to at least preferred embodiments of the present invention, does not scale in duration with respect to the complexity of the sample surface, e.g. increasing number of layers, embodiments of the present invention may allow in situ monitoring and feedback (e.g. measurements being captured and the analysis being performed) on timescales corresponding to the interfacial process being studied, e.g. on the timescales of layer deposition.

As the ellipsometry analysis, according to at least preferred embodiments of the present invention uses the change in dr with respect to time (and not the absolute value of dr), this may allow samples to be studied that may be inaccessible to other (e.g. analytical) ellipsometric methods, for example samples with more layers and/or samples with complex absorption spectra.

In some embodiments, the frequency of ellipsometer measurements (and thus the difference in time between the first and second times) may (be selected to) correspond to the timescale of the sample process of interest. For example, in interfacial processes comprising thin film deposition, the measurements may be taken every 1 to 2 seconds, corresponding to a change of between 0.01 Angstrom to 100 Angstrom in the thickness of a layer being deposited on the sample being analysed. In some embodiments the increment of the thickness of a layer being deposited on the sample being analysed that may be monitored is on the Angstrom to nanometre scale (e.g. depending on the frequency of measurement). The time scales used for measurement may then be chosen accordingly, e.g. between 0.001 seconds and 10 seconds as appropriate. In some embodiments, the time between taking ellipsometer data set measurements may be on the order of hours to days. This may be suitable or desirable, for example, when the ellipsometer is being used to monitor the stability of a sample, e.g. the process of degradation, or biological processes, e.g. antibody to antigen binding interactions. In some embodiments therefore, the reference data set may comprise the determined differential of the ER that was determined from the same sample at an earlier point in time, e.g. an hour earlier, e.g. a day earlier. In some embodiments, the acquisition time of each ellipsometry measurement (e.g. each measurement at a first or second time) may be any suitable and/or desirable time. For example, the acquisition time for each ellipsometry measurement may be less than 10 seconds, e.g. less than 5 seconds, e.g. less than 1 second, e.g. less than 0.5 seconds, e.g. less than 0.1 seconds. In some embodiments, the acquisition time for each ellipsometry measurement may be greater than 10 seconds, e.g. greater than 30 seconds, e.g. greater than 1 minute, e.g. greater than 2 minutes, e.g. greater than 5 minutes, e.g. greater than 10 minutes. It will be appreciated that smaller acquisition times have the advantage of allowing finer resolution of the data representing the process being studied. In contrast, a greater acquisition time allows an improved signal-to-noise ratio and thus may be advantageous for complex systems comprising weak signals and/or high noise (e.g. biological and/or liquid based systems).

The method and the ellipsometry data processing system may be used to monitor (the surface of) any suitable and desired interfacial process, or lack thereof, e.g. in the case of monitoring the stability (e.g. degradation) of a layer. For example, the method may be used to monitor the deposition of thin film layers, both in real-time (as the layers are being deposited) and post processing (e.g. to determine the degradation rates). However, the method and the ellipsometry data processing system may be used to monitor any interfacial process, for example, in biology or chemistry. The method and the ellipsometry data processing system may be used to monitor processes at any surface substrate boundary in real time, including liquid/liquid, liquid/solid, solid/solid, gas/solid and gas/liquid. ln one embodiment, the method and the ellipsometry data processing system is used to monitor the production of thin film layer devices, e.g. solar cells. In such embodiments, the thin film production may be monitored in situ (e.g. by one ellipsometer) or in line (e.g. by at least two ellipsometers) as the thin film layers are formed. The thin films may be formed by any suitable and desirable method, including, but not limited to deposition, vacuum deposition, sputtering, and/or by physical or chemical evaporation.

In one embodiment, the ellipsometer is used to monitor the change in material properties of thin films after fabrication, e.g. under different environmental conditions. In such embodiments, if the coating is stable, (both the real and imaginary parts of) dr should be approximately 0.

In one embodiment, the ellipsometer is used to monitor biological and/or chemical processes, e.g. protein electrochemistry, biofouling, enzymatic degradation of surface layers, adsorption of proteins onto immobilised antibodies or catalytic kinetics.

The processing circuitry that is used to process the measured ellipsometry data may be integral to the (e.g. detector of the) ellipsometer. For example, the (e.g. detector of the) ellipsometer may comprise the processing circuitry itself. However, in some embodiments, the processing circuitry is remote from the (e.g. detector of the) ellipsometer, e.g. in a separate computer, with the measurements captured by the (e.g. detector of the) ellipsometer being transferred from the (e.g. detector of the) ellipsometer to the processing circuitry, e.g. via a wired or wireless link between the detector and the processing circuitry. Thus preferably the ellipsometer comprises a transmitter for acquiring (e.g. data representative of the) measurements from the detector and transmitting the (e.g. data representative of the) measurements from the ellipsometer to the processing circuitry.

The method of, and data processing system for, analysing the data per se, is considered to be novel and inventive in its own right. Thus when viewed from a further aspect the invention provides a method of processing spectroscopic ellipsometry data to monitor interfacial processes, the method comprising: (i) for spectroscopic ellipsometry data captured as a function of the wavelength of electromagnetic radiation that is incident upon a sample to be analysed, wherein the spectroscopic ellipsometry data is captured from a portion of the incident electromagnetic radiation that has been reflected or transmitted from the sample; wherein the spectroscopic ellipsometry data captured comprises data representative of a change in an amplitude ratio, Y, and a change in a phase difference D; wherein the change in the amplitude ratio, Y, represents changes in the amplitudes of the polarisation components of the reflected or transmitted portion of electromagnetic radiation relative to the incident electromagnetic radiation; and wherein the change in the phase difference, D, represents the shift in the phase difference between the polarisation components of the reflected or transmitted portion of electromagnetic radiation relative to the incident electromagnetic radiation; determining an ellipsometric ratio, p _t , as a function of the wavelength of the incident electromagnetic radiation using data from a first spectroscopic ellipsometry data set captured at a first time; determining the ellipsometric ratio, p ₂ , as a function of the wavelength of the incident electromagnetic radiation using data from a second spectroscopic ellipsometry data set captured at a second time;

(ii) determining a differential of the ellipsometric ratio, dr, with respect to time, as a function of the wavelength of the incident electromagnetic radiation, using the determined ellipsometric ratios, p _t and p ₂ ; and

(iii) using the determined differential of the ellipsometric ratio to compare the measured data sets to a reference data set.

When viewed from a further aspect the invention provides a spectroscopic ellipsometry data processing system for monitoring interfacial processes, the spectroscopic ellipsometry data processing system comprising: processing circuitry for processing ellipsometry data sets, wherein the processing circuitry is configured to: for spectroscopic ellipsometry data captured as a function of the wavelength of electromagnetic radiation that is incident upon a sample to be analysed, wherein the spectroscopic ellipsometry data is captured from a portion of the incident electromagnetic radiation that has been reflected or transmitted from the sample; wherein the spectroscopic ellipsometry data captured comprises data representative of a change in an amplitude ratio, Y, and a change in a phase difference D; wherein the change in the amplitude ratio, Y, represents changes in the amplitudes of the polarisation components of the reflected or transmitted portion of electromagnetic radiation relative to the incident electromagnetic radiation; and wherein the change in the phase difference, D, represents the shift in the phase difference between the polarisation components of the reflected or transmitted portion of electromagnetic radiation relative to the incident electromagnetic radiation; determine an ellipsometric ratio, _x , as a function of the wavelength of the incident electromagnetic radiation using data from a first spectroscopic ellipsometry data set captured at a first time; determine the ellipsometric ratio, p ₂ , as a function of the wavelength of the incident electromagnetic radiation using data from a second spectroscopic ellipsometry data set captured at a second time; determine a differential of the ellipsometric ratio, dr, with respect to time, as a function of the wavelength of the incident electromagnetic radiation, using the determined ellipsometric ratios, p _t and p ₂ ; and use the determined differential of the ellipsometric ratio to compare the measured data sets to a reference data set.

As will be appreciated by those skilled in the art, these aspects may (and preferably do) comprise one or more (e.g. all) of the optional and preferable features outlined herein.

Preferably the method of the present invention is (and preferably the processing circuitry is configured to) repeated for each of a plurality times while the interfacial process is being performed. Thus, preferably at each time, an ellipsometry data set is measured, an ellipsometric ratio in the detected electromagnetic radiation (compared to the incident electromagnetic radiation) is determined, a differential of the ellipsometric ratio (for subsequent times) is determined, with this being used to compare the measured data sets to a reference data set.

Preferably the first time (when the measurements for the (very) first data set are obtained) is at a time before the interfacial process to be monitored commences.

For example, when the interfacial process comprises a deposition process, preferably the measurements for the first data set are taken before any material is deposited on the substrate. After the measurements for the first data set have been taken, preferably the interfacial process commences. Preferably the second time (at which the measurements for the second data set are taken) is after the start of the interfacial process.

The methods in accordance with the present invention may be implemented at least partially using software, e.g. computer programs. It will thus be seen that when viewed from further embodiments the present invention provides computer software specifically adapted to carry out the methods herein described when installed on a data processor, a computer program element comprising computer software code portions for performing the methods herein described when the program element is run on a data processor, and a computer program comprising code adapted to perform all the steps of a method or of the methods herein described when the program is run on a data processing system. Thus the invention extends to a computer readable storage medium storing computer software code which when executing on a data processing system performs the methods described herein.

The present invention also extends to a computer software carrier comprising such software arranged to carry out the steps of the methods of the present invention. Such a computer software carrier could be a physical storage medium such as a ROM chip, CD ROM, RAM, flash memory, or disk, or could be a signal such as an electronic signal over wires, an optical signal or a radio signal such as to a satellite or the like.

It will further be appreciated that not all steps of the methods of the present invention need be carried out by computer software and thus from a further broad embodiment the present invention provides computer software and such software installed on a computer software carrier for carrying out at least one of the steps of the methods set out herein.

The present invention may accordingly suitably be embodied as a computer program product for use with a computer system. Such an implementation may comprise a series of computer readable instructions either fixed on a tangible, non- transitory medium, such as a computer readable storage medium, for example, diskette, CD ROM, ROM, RAM, flash memory, or hard disk. It could also comprise a series of computer readable instructions transmittable to a computer system, via a modem or other interface device, over either a tangible medium, including but not limited to optical or analogue communications lines, or intangibly using wireless techniques, including but not limited to microwave, infrared or other transmission techniques. The series of computer readable instructions embodies all or part of the functionality previously described herein.

Those skilled in the art will appreciate that such computer readable instructions can be written in a number of programming languages for use with many computer architectures or operating systems. Further, such instructions may be stored using any memory technology, present or future, including but not limited to, semiconductor, magnetic, or optical, or transmitted using any communications technology, present or future, including but not limited to optical, infrared, or microwave. It is contemplated that such a computer program product may be distributed as a removable medium with accompanying printed or electronic documentation, for example, shrink wrapped software, pre-loaded with a computer system, for example, on a system ROM or fixed disk, or distributed from a server or electronic bulletin board over a network, for example, the Internet or World Wide Web.

Certain preferred embodiments for the invention will now be described, byway of example only, with reference to the accompanying drawings in which:

Figures 1(A) and 1(B) illustrate a unit cell in a crystal and the dependence of the optical properties on the orientation of molecules in the crystal;

Figures 2(A), 2(B) and 2(C) show a multi-layered sample and associated optical properties and thicknesses derived from model fitting to ellipsometry data; Figure 3 shows an ellipsometry data processing system according to an embodiment of the present invention;

Figure 4 shows a flow diagram of a method of processing ellipsometry data according to an embodiment of the present invention;

Figures 5(A), 5(B) and 5(C) shows ellipsometry data captured with the ellipsometry processing data system shown in Figure 3, using the process shown in Figure 4;

Figures 6(A), 6(B) and 6(C) show respectively the real part, imaginary part and magnitude of the differential of the ellipsometric ratio, calculated from the amplitude ratio and corrected phase difference values shown in Figures 5(A) and 5(C);

Figures 7(A), 7(B) and 7(C) show respectively the real part, imaginary part and magnitude of the second differential of the ellipsometric ratio, calculated from the amplitude ratio and corrected phase difference values shown in Figures 5(A) and 5(C);

Figures 8(A), 8(B) and 8(C) show respectively the real part, imaginary part and magnitude of the differential of the ellipsometric ratio, dr, as shown in Figures 6(A), 6(B) and 6(C);

Figures 8(D), 8(E) and 8(F) show respectively the imaginary part (extinction coefficient, k), real part (refractive index, n)and magnitude (i.e. the magnitude of the complex refractive index, N) of a reference C ₆ o film;

Figures 9(A), 9(B) and 9(C) show respectively the real part, imaginary part and magnitude of the differential of the ellipsometric ratio, dr for a layer of hexapropyl-truxene;

Figures 9(D), 9(E) and 9(F) show respectively the imaginary part, real part and magnitude of a reference refractive index for a hexapropyl-truxene film; and

Figure 10 shows an experimental ellipsometry set up of the present invention for studying the dynamics of biological systems.

Embodiments of the invention will now be described in the context of ellipsometry, which may be used to characterise properties of thin films such as thickness, optical constants, composition, crystallinity, quality and concentration, and may obtain sub nanometre surface sensitivity. The growth of thin films comprising complex materials may be monitored using ellipsometry, as shown in Figure 1. Figure 1(A) shows the possible orientations of Triclinic Pentacene, which has three alignments relative to a substrate on which it is grown, represented by the axes a, b and c in the figure inset.

Figure 1(B) shows ellipsometry data that has been simulated for the crystal of Triclinic Pentacene shown in Figure 1(A). This shows the difference in optical properties (£ = £ _! + ie ₂ ) of these orientations and when interrogating the crystal along each different axis orientation, which can be clearly distinguished from each other owing to the different real 102a, 104a, 106a and imaginary 102b, 104b, 106b optical components that are extracted from the ellipsometry data for b, a and c directions respectively.

It will be appreciated that extracting and modelling the optical properties from the ellipsometry data requires complex processing, which thus takes time to perform. This may prevent the growth of the sample being monitored and analysed in real time.

As the orientation of a molecule is intrinsically linked to its optoelectronic properties, it can be envisaged that monitoring the growth of such a material may allow early identification of the growth of a less desirable orientation and thus may allow the reduction of material waste (e.g. by terminating an interfacial process) and/or the tuning of parameters of the interfacial process to optimise growth.

Figures 2(A), 2(B) and 2(C) show a multi-layered sample and associated ellipsometry data obtained from the sample. Figure 2(A) shows a complex multi layered device 400 that may be manufactured by depositing successive thin film layers onto a glass substrate 401. The multi-layered device 400 comprises, as one of its multiple layers, a a-sexithiophene (a-6T) layer 402.

Figure 2(B) shows the extinction coefficient, k, that is derived from model fitting to spectroscopic ellipsometry data that is captured as the a-6T layer 402 is being deposited. The extinction coefficient is determined as a function of the wavelength of the incident electromagnetic radiation used when capturing the spectroscopic ellipsometry data. Figure 2(B) shows the extinction coefficient determined for a number of different thicknesses of the a-6T layer 402 (e.g. during deposition of the a-6T layer 402).

The extinction coefficient is shown in Figure 2(B) for thicknesses of the a-6T layer 402 of 10 nm 404, 30 nm 406 and 55 nm 408. It can be seen from Figure 2(B) that the shape of the extinction coefficient as a function of the incident wavelength changes as the a-6T layer 402 is being deposited. This helps to allow, for example, the thickness of the layer being monitored to be extracted from the captured and processed ellipsometry data (using the real part, n, of the refractive index).

Again, however, the conventional technique of processing the spectroscopic ellipsometry data to obtain the extinction coefficients shown in Figure 2(B), requires complex processing in order to extract the optical properties of the sample in the absorbing region (representative of the quality of the sample), which thus takes time to perform, particularly when the sample involves multiple complex layers. This may prevent the growth of the sample being monitored and analysed in real time.

Figure 2(C) shows the thickness of the a-6T layer 402 that is determined over time (i.e. as the a-6T layer 402 is being deposited) from the real part, n, corresponding to the determined extinction coefficients (e.g. as shown in Figure 2(B)). Figure 2(C) also shows the thickness of the subsequent layers of the multi-layered device 400 that is determined over time as these layers are deposited sequentially on top of the a-6T layer 402.

Figure 3 shows schematically an ellipsometry data processing system 200 in accordance with an embodiment of the present invention. A sample substrate 206 is positioned with respect to an evaporation source 208 that is used to vaporise a material 210 to be deposited on the substrate 206 to form a thin film layer 212.

The ellipsometry data processing system 200 comprises an ellipsometer comprising an electromagnetic radiation source 202 that generates an incident beam of electromagnetic radiation 204 that is directed towards the sample to be monitored, i.e. to analyse the thin film layer 212 that is being formed. The ellipsometer also comprises a detector 216 that detects the electromagnetic radiation 214 that is reflected from the thin film layer 212. The detector 216 is arranged to capture ellipsometry data from the reflected electromagnetic radiation 214 and transmit the data to a computer 218 (comprising processing circuitry for processing the ellipsometry data) to be analysed.

The ellipsometry data processing system 200 shown in Figure 3 may be used for analysing thin film layers that are deposited in situ with respect to the ellipsometer, i.e. the substrate 206 and the thin film layer 212 deposited remains stationary during this interfacial process relative to the ellipsometer.

It will be appreciated, however, that the ellipsometry data processing system 200 shown in Figure 3 may be used for analysing thin film layers that are deposited in line with respect to the ellipsometer, i.e. the substrate 206 and the thin film layer 212 deposited move relative to the ellipsometer during this interfacial process. In this embodiment, the ellipsometry data processing system 200 may (and preferably does) comprise a plurality of ellipsometers, e.g. each arranged as the ellipsometer shown in Figure 3.

Operation of the ellipsometry data processing system 200 shown in Figure 3 will now be described with reference to Figures 3 and 4. Figure 4 shows a flow diagram 300 of a method of processing ellipsometry data in accordance with an embodiment of the present invention.

Prior to using the ellipsometry data processing system 200 to analyse a thin film layer 212 being deposited on the substrate 206, the ellipsometry data processing system 200 is set up, e.g. as shown in Figure 3, and the substrate 206, on which the sample is to be deposited, is prepared (step 302, Figure 4).

Once the ellipsometry data processing system 200 has been set up, the electromagnetic source 202 is energised to direct a beam of electromagnetic radiation 204 onto the surface of the substrate 206. The reflected component of the electromagnetic radiation 214 is detected by the detector 216 and the ellipsometric parameters (the changes in amplitude ratio, Y, and phase difference, D, as a function of the wavelength of the incident electromagnetic radiation) are determined by Fourier Analysis of the data measured by the detector 216 and output to the computer 218, to provide a first measurement of these parameters (at time t=0) (step 304, Figure 4).

This first measurement is taken as an initial reference point, e.g. with the substrate 206 blank before any material is deposited on the substrate 206. The data captured by the detector 216 and output to the computer 218 (the measured values of Y and D), is used by the processing circuitry of the computer 218 to determine the ellipsometric ratio, p, of the incident electromagnetic radiation, as a result of its interaction and reflection from the substrate 206, using the equation p = tan(W) e ^~lA . The ellipsometric ratio (ER) is determined as a function of the wavelength of the incident electromagnetic radiation.

The determined ER is stored as part of a data stack (e.g. in a memory of the computer). For every subsequent measurement captured by the detector and added to the stack, a correction function is applied relative to the previous measurement (step 306, Figure 4). The correction function will be described in more detail below.

An interfacial process can then be started (step 308, Figure 4), e.g. thin film layer growth using deposition as shown in Figure 3. (It will be appreciated that the method described with reference to Figure 4 may be applied to a number of different interfacial processes, such as deposition or subjecting a (e.g. thin film) surface to range of different environmental conditions, e.g. heating, cooling and/or pressure.)

During the interfacial process being performed, the ellipsometer is used, at each of a plurality of time steps, to measure the ellipsometric parameters (amplitude ratio, Y, and phase difference, D). For a time step at which the ellipsometric parameters have been measured while the interfacial process is being performed (i.e. at any time after the initial measurement at time t=0), a correction function is applied to the measured change in phase difference. The correction function helps to ensure that the values of the change in phase that are to be used in determining the changes in polarisation are handled correctly, owing to their cyclical nature. First, it is determined whether or not it is necessary to correct a measured change in phase difference value. This is done by taking the difference of two measured change in phase difference values (for adjacent points in time), A _t and D ₂ , recorded at the same energy (and thus wavelength), for each of the energies (wavelengths) at which the change in phase difference values are captured. When the magnitude of the difference is greater than 180°, a correction factor is applied to the change in phase difference value of the two adjacent values at the later time, D ₂ . When the magnitude of the difference is less than 180°, no correction factor is applied.

The difference of two measured ellipsometry data values for the change in phase,

D ₂ - D- _L , is calculated. When the value of the difference is greater than +180°, the correction factor is applied by subtracting 360° from D ₂ . When the value of the difference is less than -180°, the correction factor is applied by adding 360° ΐoD ₂ . When the value of the difference is between -180° and +180°, no correction factor is applied.

The correction function (and the correction factors, if necessary) are applied to all of the measured ellipsometry data values for the change in phase, i.e. for all energies of the incident electromagnetic radiation. The measured amplitude ratio, Y, and the corrected change in phase difference A' values (for the values of the energy spectrum used for the incident electromagnetic radiation) are added to the data stack (as a separate stack) for storing.

Using these corrected changes in phase, the processing circuitry of the computer 218 determines the ER, p, for the particular time step, using the measured change in amplitude ratio, Y, and the corrected change in phase difference A'. This is calculated using the equation p = tan(W) e ^~lAl , determined as a function of the wavelength of the incident electromagnetic radiation.

For a time step at which the change in polarisation has been determined while the interfacial process is being performed, the differential of the ER, dr, is calculated relative to the change in polarisation determined at the immediately previous time step, using dr = e ^_iA [(sec( )) ² x 6Y - i x tan( ) x 6D] (step 310, Figure 4) (or alternatively, dr = e ^iA [(sec( )) ² x dY + i x tan( ) x 6D]), using the corrected change in phase difference values where appropriate.

This is done by first converting Y and D into units of radians (if necessary) and then performing (e.g. numerical) differentiation with respect to time of the Y and (corrected) D values, for each time step of the collected data (i.e. the data stored in the data stack). These 6Y and dD (or dD') values are then used to calculate the differential of the ER, for each time step, using dr = e ^_iA [(sec( )) ² x dY - i x tan( ) x dD] (or alternatively dr = e ^iA [(sec( )) ² x dY + i x tan( ) x dD]), for each wavelength of the incident electromagnetic radiation.

Once the differential of the ER has been calculated, the dr values (for the energy spectrum used for the incident electromagnetic radiation) are compared, e.g. quantitatively and/or qualitatively, to a reference data set of dr values, e.g. from a data library (step 312, Figure 4), across the energy spectrum. The reference data set may, for example, be obtained from another distinct prototypical experiment, a data set from the same sample (that is currently being monitored) at an earlier point in time or from a computational model, e.g. as appropriate for the interfacial process being monitored. The reference data set is obtained from or produced for the same interfacial process as the interfacial process being monitored.

If the differences between the measured data set (of the dr values) and the reference data set are, for example, greater than a predefined threshold (indicating that the measured values are outside acceptable limits and/or show growth of unexpected peaks), feedback control to the interfacial process is provided (step 314, Figure 4).

Such feedback may include altering one or more parameters of the interfacial (e.g. deposition) process, such as changing the temperature and/or pressure. The feedback control may comprise identifying contaminants in the sample being subject to the interfacial process, or stopping the process (e.g. to minimise waste).

If the feedback control has not stopped the interfacial process and the process is to be continued, the process continues and ellipsometry data is continued to be collected. Thus the steps of measuring the ellipsometry data, calculating the differential of the ER and comparing the determined differential of the ER with a reference data set (steps 310 and 312, Figure 4) are repeated.

When comparing the determined differential of the ER with a reference data set, if the differences are less than a predefined threshold (indicating that the measured values are within acceptable limits), this indicates that the interfacial process is being performed as expected.

It is then determined whether a set condition (a desired goal) has been met, e.g. whether the desired thickness of a sample being prepared has been reached or whether the interfacial process has been performed for the desired period of time, (step 316, figure 4). If the condition has not been satisfied (e.g. the sample has not yet reached the desired thickness or the interfacial process has not yet been performed for the desired period of time), the interfacial process being performed is continued (step 318, Figure 4).

Thus the steps of measuring the ellipsometry data, calculating the differential of the ER and comparing the determined differential of the ER with a reference data set (steps 310 and 312, Figure 4) are repeated, e.g. until the desired goal has been achieved.

If the condition has been satisfied (e.g. the sample has reached the desired thickness or the interfacial process has been performed for the desired period of time), the interfacial process and the taking of the associated spectroscopic ellipsometry measurements is terminated (step 320, Figure 4).

Data that has been captured and resultant measures that have been determined by the ellipsometry processing data system 200 shown in Figure 3, using the process shown in Figure 4 will now be described with reference to Figures 5-8. Figures 5-8 show these various data and determined measures, from monitoring the deposition of C ₆ o on a quartz substrate at every 1 nm of thickness deposited.

Figures 5(A) and 5(B) show the change in amplitude ratio (Y) and change in phase difference (D) respectively, as measured by the ellipsometry processing data system 200 shown in Figure 3, as a function of the wavelength of the incident electromagnetic radiation. Figure 5(C) shows the corrected change in phase difference, D'.

These measurements are shown as a function of the energy (wavelength) of the incident electromagnetic radiation, for a sample of C ₆ o deposited on a quartz substrate. The measurements shown have been taken at every 1 nm of thickness of the O _d o deposited.

From Figure 5(B), it can be seen that the change in phase difference (D) values measured by the ellipsometer range between 0° to 360° and are cyclic in nature. The step of applying the correction function corrects for the step change with Figure 5(C) showing the corrected phase difference, D', plotted as a function of the energy (wavelength) of the incident electromagnetic radiation. It will be appreciated that the application of the correction function to the phase difference values does not (significantly) alter the ER calculated.

Figures 6(A), 6(B) and 6(C) show respectively the real part, imaginary part and magnitude of the differential of the ER, dr, that have been calculated from the amplitude ratio and corrected change in phase difference values shown in Figures 5(A) and 5(C), shown as a function of the energy (wavelength) of the incident electromagnetic radiation, for every 1 nm of thickness of the C ₆ o deposited.

As the real and the imaginary part of the differential of the ER are equivalent to the extinction coefficient, k, and the real part of the refractive index, n, Figures 6(A),

6(B) and 6(C) show how the optical qualities (and thus representative of the quality) of the thin film being deposited change with respect to the (quality of the) thin film at a previous time step.

For example, starting with a blank substrate such as quartz (or glass or silicon wafer), the ER calculated when the first layer is deposited provides information on the quality of this layer with respect to the substrate. Similarly, the ER for (n+1)th layer provides information on the quality of this layer with respect to the nth layer. When the ER is zero then this indicates that the quality of the (n+1)th layer is same as that of the nth layer. When the ER is non-zero and follows a similar shape or pattern (its form as a function of wavelength) as the thin film is deposited (e.g. as shown in Figures 6(A), 6(B) and 6(C)), this indicates that the film quality of the (n+1)th layer is different from the nth layer (assuming no change in the n layers below upon the deposition of the (n+1)th layer).

The magnitude of the ER may be used as a metric to quantify this difference between layers of a thin film that are deposited. The smaller the value, the more similar is the (n+1)th layer with respect to the nth layer. However, if the ER of (n+1)th is non-zero but the shape is different (e.g. the appearance of a new feature such as a peak), then the quality of the (n+1)th layer is different compared to the nth layer. This may be due to a number of reasons such as orientation changes of the organic semiconductor and contamination by another material.

Figures 7(A), 7(B) and 7(C) show respectively the real part, imaginary part and magnitude of the second differential of the ER, d ² r, that have been calculated, as a function of the energy (wavelength) of the incident electromagnetic radiation, for every 1 nm of thickness of the C ₆ o deposited. The second differential of the ER gives an indication of the rate of change of the first differential of the ER (5p).

Figures 8(A), 8(B) and 8(C) show respectively the real part, imaginary part and magnitude of the differential of the ER, dr, as shown in Figures 6(A), 6(B) and 6(C). Figures 8(D), 8(E) and 8(F) show respectively the imaginary part, real part and magnitude of a reference refractive index (N = n + ik) of the corresponding bulk film, obtained under static conditions using a C ₆ o film having a thickness of 13 nm, and shown as a function of the energy (wavelength) of the incident electromagnetic wavelength. It should be noted that the data presented in Figure 8(A)

(corresponding to Figure 6(A)) has been mirrored in the x-axis for ease of comparison with the calculated extinction coefficient k of 8(D).

The imaginary part of the reference refractive index (as shown in Figure 8(D)) corresponds to the extinction coefficient, k, of the sample; the real part of the reference refractive index (as shown in Figure 8(E)) corresponds to the refractive index, n, of the sample. The determined parts of the differential of the ER (as shown in Figures 8(A), 8(B) and 8(C) may thus be compared with the corresponding parts of the reference refractive index (as shown in Figures 8(D), 8(E) and 8(F)), e.g. as part of the process shown in Figure 4 (in particular step 312).

It will be appreciated from the comparison of the measured and reference data sets that the profile of the different parts of the refractive index in the reference data sets is qualitatively reproduced by dr and thus embodiments of the present invention provide a first approximation of the refractive index (and thus the optical properties) without expensive computational analysis.

Thus, it is possible to obtain a comparison, for example, by deriving the refractive indices (N = n + ik) for a set of different thicknesses (e.g. 1, 4, 7, 10, ... nm) of a thin film being deposited and correlating these values to the calculated differential of the ER at the same thicknesses. Analysis of the differential of the ER thus provides an efficient and fast characterisation, e.g. compared to obtaining the refractive indices (N = n + ik) through model fitting, which is complex and time-consuming, particularly with complex materials and increasing number of layers.

The skilled person will appreciate that the above discussion is by way of example only and not intended to limit the invention to the study of Obo thin films or quartz substrates.

Figure 9 thus presents the same analysis as Figure 8 but for hexapropyl-truxene deposited on a substrate comprising a 5 nm layer of molybdenum oxide on a 23 nm layer of silicon oxide on silicon wafer.

Figures 9(A), 9(B) and 9(C) show respectively the real part, imaginary part and magnitude of the differential of the ER, dr, shown as a function of the energy (wavelength) of the incident electromagnetic radiation, for a range of thicknesses of the deposited layer of hexapropyl-truxene. Figures 9(D), 9(E) and 9(F) show respectively the imaginary part, real part and magnitude of a reference refractive index of the corresponding bulk film, obtained under static conditions using a hexapropyl-truxene film having a thickness of 4 nm, and shown as a function of the energy (wavelength) of the incident electromagnetic wavelength. Figure 10 shows schematically an ellipsometry data processing system 1000, in accordance with an embodiment of the present invention, suitable for studying biological systems, in particular the binding dynamics of antigens and antibodies. A biomolecule (e.g. antigen) 1012 is immobilised on a sample substrate 1006 which is then enclosed in a liquid cell 1010 with quartz windows 1008, 1009. Liquid solution 1014 (e.g. phosphate buffer saline solution) is injected into the liquid cell 1010.

The ellipsometry data processing system 1000 comprises an ellipsometer comprising an electromagnetic radiation source 1002 that generates an incident beam of polarised electromagnetic radiation 1004 that is directed at an angle (e.g. 70° with respect to perpendicular to the substrate) and (e.g. perpendicularly) through the quartz windows 1008, 1009 of the liquid cell 1010 (and the liquid 1014 held therein) towards the immobilised biomolecule 1012 sample to be monitored, i.e. to analyse the binding dynamics of the biomolecule 1012 with species 1018 present in the liquid 1014 held in the liquid cell 1010. The ellipsometer also comprises a detector 1016 that detects the polarised electromagnetic radiation 1005 that is reflected from the immobilised sample layer (e.g. at the same above angle). The detector 1016 is arranged to capture ellipsometry data from the reflected electromagnetic radiation 1005 and transmit the data to a computer (comprising processing circuitry for processing the ellipsometry data) to be analysed.

The ellipsometry data processing system 1000 shown in Figure 10 may thus be used to analyse the interactions arising in biological systems. For example, a plurality of spectroscopic ellipsometry measurements may be acquired at any suitable and/or desirable frequency (e.g. on the order of seconds to minutes, e.g. 5 seconds, e.g. 10 seconds, e.g. 60 seconds, e.g. 120 seconds) during which time a solution containing the biomolecule (e.g. antibody) expected to interact with the immobilised biomolecule (e.g. antigen) may be injected into the liquid cell at periodic intervals (e.g. between ellipsometry measurements) such that the evolution in the spectroscopic profile of the immobilised biomolecule may be analysed to indicate binding or interaction dynamics. The ellipsometry data processing system 1000 shown in Figure 10 may thus be used for analysing biological dynamics in situ with respect to the ellipsometer. Alternatively, when the ellipsometry signal is very weak, it may be preferable to measure the real part, imaginary part and magnitude of the first and second differentials of the ER ( dr and d ² r ) using two ellipsometry measurements with relatively large acquisition times (e.g. 2 minutes) and recorded approximately 60 minutes apart, wherein the first measurement may be taken prior to the second biomolecule (e.g. antibody) being added to the liquid cell and the second measurement taken 60 minutes after the second biomolecule (e.g. antibody) has been added. It will be appreciated that operation of the ellipsometry data processing system 1000 shown in Figure 10 will be functionally the same as that described with reference to ellipsometry data processing system 200 and the description above relating to Figures 3 and 4. It can be seen from the above that, in at least preferred embodiments, the invention provides a method of, and a spectroscopic ellipsometry data processing system for, processing spectroscopic ellipsometry data to monitor interfacial processes of a sample being analysed that is simple to perform and helps to provide a direct measurement of the interfacial process being monitored.