MACHINE FOR CONTROLLED DEPOSITION OF A THIN-FILM MULTILAYER

TECHNICAL FIELD

The present invention concerns a machine for controlled deposition of a thin-film multilayer.

BACKGROUND ART

The treatment of optical components by deposition of a thin- film multilayer obviously requires control of the thickness of the film deposited, on which the optical behaviour of the component as a whole will depend.

One of the conventional methods used provides for timed control, and is based on the deposition rate over a set time for calculation of the thickness of the film deposited. This method suffers from excessive margins of inaccuracy due to the variability of the deposition rate and, furthermore, does not permit verification of the optical response of the film.

Another method is control by means of piezoelectric quartz microbalance . This method has the drawback of depending on the position of the measuring head in the deposition chamber, and of measuring only the physical thickness of the film deposited, ignoring the optical response.

Lastly, the optical monitoring method by means of monochromatic photometer has the advantage, with respect to the methods previously illustrated, of monitoring the real optical response of the film during its growth, but has the limit of monitoring according to one single wavelength and not according to the utilisation band of the multilayer to be obtained. A further limitation of this method consists in the difficult identification of the exact moment when the signal monitored reaches the required value and, therefore, when the

deposition must be interrupted.

DISCLOSURE OF INVENTION

The aim of the present invention is to produce a machine for the deposition of thin-film multilayers with technical characteristics such as to overcome the drawbacks of the known art.

The subject of the present invention is a machine for the controlled deposition of a thin-film multilayer, comprising a deposition chamber and means for deposition of thin films on a substrate housed inside said deposition chamber, said machine being characterised in -that it comprises a light source suitable for generating an optical signal; means for detecting an optical response of a thin-film multilayer; optical conduction means suitable for conveying an optical signal from said light source to said thin-film multilayer and from said thin-film multilayer to said detection means; and a computer suitable for controlling at least said deposition means and said detection means.

BRIEF DESCRIPTION OF THE DRAWINGS

The following example is intended as a non-limiting illustration in order to aid understanding of the invention with the help of the figures of the accompanying drawing, in which: figure 1 is a schematic form of the machine of the present invention; figure 2 is a flow chart which schematises the operating principle of the method of the machine of the present invention; and figures 3a and 3b are two diagrams illustrating a comparison between measured optical response and theoretical optical response during two distinct moments of deposition of the

film.

BEST MODE FOR CARRYING OUT THE INVENTION

In figure 1 the machine for deposition of a thin-film multilayer of the present invention is indicated overall by 1.

The machine 1 comprises a deposition chamber 2, means for deposition illustrated schematically by 3 , a substrate 4 on which a thin-film multilayer 5 is deposited, a pumping unit illustrated schematically by 6 for creating, a vacuum inside the deposition chamber 2, a lamp 7 for generating the optical signal to be monitored, a fibre optic spectrometer 8 with CCD detector for instantaneous measurement of the optical response over a wide spectral range, and a computer 9 for control of the various components of the machine 1 as a whole.

To convey the optical signals, the machine 1 comprises a plurality of optical fibres 10, which cross the deposition chamber 2 to convey the optical signal from the lamp 7 to the multilayer 5, and from the latter to the spectrometer.

The optical signals are conveyed to and from the multilayer 5 by means of a fibre optic probe 11. In this specific case, the fibre optic probe 11, by means of one single measuring head, physically illuminates the multilayer 5 with the signal coming from the lamp 7 and collects the reflected optical response in order to transfer the signal to the spectrometer 8.

As illustrated in the flow chart in figure 2, for each layer deposited, the automated method provided by the machine 1 and implemented via software performs the following operations: - (OPl) gives the command for deposition of a given layer,-

(0P2) calculates the theoretical optical response of the given layer corresponding to the design thickness;

(OP3) acquires at set intervals the measured optical

response ;

(0P4) compares the theoretical optical response with the measured optical response;

(OP5) if the measured optical response is equal to the theoretical one, taking account of the pre-defined tolerance, it gives the command for interruption of the deposition process ;

- (0P6) if the measured optical response is different from the theoretical one, it estimates the remaining deposition required to reach the design thickness and gives the command for deposition of another layer.

For calculation of the theoretical optical response, the refractive index value n and extinction coefficient value k of the materials, therefore deriving from the characterisation of said materials deposited in the same machine, are used. Furthermore, the theoretical optical response of each n-th layer is calculated also considering the n-1 layers previously deposited starting from the substrate.

The optical response subject of the comparison between measured and theoretical is the specular reflectance, at an almost normal angle, of the multilayer growing on its substrate. The optical response is monitored in the utilisation wavelength range of the multilayer being deposited.

In said regard it should be specified that the optical response subject of the comparison can be different from the reflectance, for example the transmittance.

Before performing the comparison, the measured optical response is mathematically processed for subtraction of the noise and for a uniform comparison with the simulated theoretical response.

The comparison between theoretical response and measured response is performed by subtraction, in absolute value, between the areas subtended by the respective curves in the wavelength range concerned.

The parameter calculated, indicative of the comparison, is the deviation between the areas S:

When the deviation S equals a pre-defined minimum value, the system interrupts the process of deposition of the n-th layer, interpreting said condition as indicating that the design thickness and optical response have been reached.

If, on the other hand, the deviation S is superior to the pre- defined minimum value, the system estimates the remaining process duration in order to reach the design thickness and then resumes deposition until the next acquisition. The estimate is made possible by linear interpolation of the value of the deviation S corresponding to two successive acquisitions alternating with a period of deposition.

The parameter that indicates the remaining deposition required to reach the design thickness can be the deposition time or the number of times the substrate passes in front of the material source.

Figures 3a and 3b show the steps of comparison between theoretical and measured optical response, corresponding to two acquisitions during the deposition of a layer. In particular, in figure 3a the layer growing (the 15th of a structure with 36 layers) has not yet reached the design thickness, while in figure 3b this condition has been reached and the system has given the command for interruption of

deposition of the layer.

As is evident from the above description, the machine of the present invention permits _. controlled depositions of thin-film multilayers. In this specific case, the machine of the present invention guarantees accurate control of the thicknesses and the optical response of the films during their deposition, permitting the production of particularly critical multilayers, such as narrow band filters, clean-cut dichroic filters and metal-dielectric structure filters.

Furthermore, the automation introduced facilitates the deposition of structures with many layers, without the need for control and constant intervention by the operator, guarantees greater process repeatability, with minimum variations between the deposition batches, and optimises the production cycles, reducing the reject components because optical performance is controlled directly during deposition and not after the end of the process .

Lastly, a further advantage of the machine of the present invention lies in the fact that the components are readily available on the market and reasonably priced.

As will be obvious to a person skilled in the art, the particular deposition technique used by the machine

(sputtering, evaporation by electronic gun etc.) is not limiting for the present invention, as the specific characteristics of the machine can be applied to different deposition techniques.