Photoacoustic Cell System and Method

Field

The present invention relates to a photoacoustic system and method. In particular the invention relates to a system and method which utilises sound energy resultant from light excitation of a unit under test to perform structural characterisation thereof. Background

Failure analysis is the process of collecting and analysing data to determine the cause of a failure within materials, structures, devices and circuits fabricated thereon. Such analysis provides vital information when developing new products and improving existing products. Typically, this type of analysis relies on collecting failed components for subsequent examination of the cause of failure using various methods, such as microscopy and spectroscopy.

Within the semiconductor industry, for example, particle contamination is of major concern. Particle contamination during the manufacturing process of devices, such as wafers can result in faulty devices. Thus it is desirable to minimise the risk of contaminating products with foreign bodies during the manufacturing process. Failure analysis techniques that utilise sound energy are known in the art. One such technique utilizes photoacoustics to perform structural characterisation. Noise from the ambient environment can distort the accuracy of photoacoustic measurements and subjected to low S/N ratio which is undesirable.

In a multi-cell photoacoustic system, multiple-cells are placed adjacent to each other. Due to the small distance between the neighbouring cells, the acoustic wave can travel across to the adjacent cells and detected by the micro-phones in the adjacent cell as PA signal making inaccurate measurements. Numerous photoacoustic systems exist in the art, such as JP2013255707 (Canon); US2010/147051 (Tobias); WPO009/007875 (Philips Electronics); WO2006/072867 (Philips Electronics) and GB 2218198 (Mine Safety Appliances).

Reflections back along the path of the incident light will cause no problems in this context as they will pass through the cell window. However a problem arises where there are reflection paths other than the path of incidence (caused by a non-level sample or sample surface structure). This introduces the possibility that this reflected light will be incident on the cell wall. Traditional materials for photoacoustic cell construction are metals and are therefore highly absorbent. This means that in addition to the absorption in the sample, there is now a secondary absorption on the cell wall. Photoacoustic measurements are initiated by light absorption and as a result a secondary photoacoustic signal source is generated. As this secondary source is produced by the same excitation as the signal of interest is not possible to differentiate between the two through traditional lock- in techniques.

Operation with a transparent cell partly solves this problem as it removes the absorption at the cell wall, however if there is now a direct line of sight between the reflection point on the sample and the microphone, this secondary absorption due to light reflections will occur at this location leading to even worse results due to the proximity of the photoacoustic source to the detection position.

It is therefore an object to provide a photoacoustic system and method to overcome at least one of the above mentioned drawbacks.

Summary

According to the invention there is provided, as set out in the appended claims, a photoacoustic cell system comprising:

at least one light source for optically exciting a unit under test (UUT), at least one volume for capturing acoustic energy emanating from the UUT as result of optical excitation thereof,

at least one microphone for receiving the captured acoustic energy and generating a corresponding electrical signal, and

an opaque element configured to isolate the microphone from unwanted energy reflections.

By using the newly designed Photoacoustic (PA) cell a PA signal enhancement with approximately an improvement factor of three or more is achieved over prior art systems. It will be appreciated that the majority of the cell is made of a transparent material and the opaque element makes up a sub element of the photoacoustic cell. The opaque element provides a dual function of eliminating the possibility of reflected light reaching the acoustic pickup and removing the blocked energy from the system by breaking the communication path with the microphone, ensuring more accurate measurements of the UUT. The cell can comprise of a trap volume produced from a material which is transparent to the wavelength of the exciting optical power. In effect the opaque element provides a trap volume and prevents return of unwanted energy readings into the volume.

In one embodiment the the at least one microphone is configured to be acoustically isolated from any secondary sources.

In one embodiment the opaque element comprises a layer of transparent material and a layer of absorption material.

In one embodiment an intermediate layer is provided between the layer of transparent material and a layer of absorption material. In one embodiment the intermediate layer comprises a compliant layer, for example an airgap layer and the system further comprises at least one microphone in acoustic communication with the intermediate layer. In one embodiment the intermediate layer is configured to function as a heat sink.

In one embodiment the opaque element is acoustically isolated from the at least one microphone.

In one embodiment the at least one volume comprises an excitation channel and the at least one microphone is positioned outside the excitation channel. In one embodiment at least one pressure port adapted to alleviate cross talk from a neighbouring cell.

In one embodiment there is provided an optical pyrometer to monitor surface temperature of the area under test in real-time.

In one embodiment there is provided a piezoelectric / accelerometer feedback system and configured to dynamically level the photoacoustic cell array and maintain co-planarity with a surface of the unit under test. In one embodiment the light source comprises one or more excitation lasers operating at a small range of modulation frequencies around a centre frequency to allow differentiation between signals generated within adjacent cells and hence minimise cross-talk. In one embodiment the light source comprises at least one of: a laser; a

VCSEL; or an optical fibre in optical communication with a laser source.

In one embodiment there is provided an optical system configured to generate a secondary laser spot to heat the unit-under-test and create a thermal gradient within the material, such that on application of a primary probe laser spot, heat will be confined within the volume defined by the applied thermal gradient, thereby increasing sensitivity to defects at depth within the unit-under-test. In one embodiment there is provided self-levelling supports with respect to the unit under test.

In one embodiment there is provided means of accurately registering a microphone/laser module in a modular system.

In one embodiment there is provided means for accurately registering a microphone/laser module. A method of operating a photoacoustic cell system comprising the steps of: optically exciting a unit under test (UUT),

capturing acoustic energy emanating from the UUT as result of optical excitation thereof,

receiving the captured acoustic energy and generating a corresponding electrical signal, and

isolating the microphone from unwanted energy reflections using an opaque element.

There is also provided a computer program comprising program instructions for causing a computer program to carry out the above method which may be embodied on a record medium, carrier signal or read-only memory.

Brief Description of the Drawings

The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which :-

Figure 1 illustrates a prior art photoacoustic inspection device;

Figure 2 to 5 illustrates a number of embodiments of a photoacoustic cell system of the present invention;

Figure 6 illustrates an embodiment of a photoacoustic cell system according to the invention;

Figures 7, 8 and 9 illustrate an opaque element or light block according to a number of embodiments of the invention; Figure 10 to 13 illustrates a number of embodiments of a photoacoustic cell system of the present invention tuned to operate at different frequencies;

Figure 14 shows an array of photoacoustic cells where each cell comprises a number of pressure ports to alleviate cross talk between neighbouring cells;

Figure 15 illustrates a means of repeatedly and accurately aligning a removable unit containing microphones and a light source;

Figure 16 illustrates an alternate excitation methodology enabled by the transparent nature of the cell. Here a second light source projects a hollow cone of light onto the sample. The absorption of this light generates a heat source of similar shape which alters the thermal profile within the sample thereby confining the heat generated by the more traditional central probe laser in the later dimensions and encouraging vertical diffusion;

Figure 17 illustrates an alternate means of excitation. By exciting with a rotating "bar"-like beam the central region receives the majority of the excitation (due to its constant exposure) while the outer regions are heated but to a lesser extent. This alters the thermal profile in the vicinity of the measurement, confining the thermal wave laterally and encouraging vertical diffusion;

Figure 18 illustrates an example of a removable PA cell and a supporting frame. The figure depicts a means of ensuring the cell, once in place does not protrude beyond a predefined limit. This can be important when many cell are mounted side-by-side and these must be matched.

Detailed Description of the Drawings

The application will now be described with reference to some exemplary photoacoustic inspection metrology tools which are provided to assist in an understanding of the present teaching. It will be appreciated that for simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. Referring to the drawings and initially to Figure 1 there is provided a photoacoustic inspection device 100 which utilises sound energy resultant from light excitation of a unit under test (UUT) 102 to perform structural characterisation thereof. The device 100 comprises a test member 103 which defines a trap volume 105 for capturing acoustic energy emanating from the UUT 102, in this case, an integrated circuit chip. An optical excitation input 107 is in optical communication with the UUT 102 for exciting the UUT 102. The UUT 102 is in acoustic communication with the trap volume 105 such that acoustic energy emanating from the UUT 102 as result of optical excitation enters the trap volume 105. In the exemplary arrangement, the optical excitation input 107 is in registration with the trap volume 105. One or more acoustic pickups 109 are provided on the test member 103 and are in acoustic communication with the trap volume 105 for picking up acoustic energy resultant from excitation of the UUT 102. Acoustic pick-ups 109, for example a microphone are transducers configured for converting mechanical vibrations resulting from acoustic energy into electrical energy. While the exemplary teaching will be described with reference to the UUT being semiconductor chips, it will be appreciated that a photoacoustic inspection device 100 in accordance with the present teaching may be used with a variety of different UUTs types and dimensions - and indeed whole or part of individual UUTs. For example, the UUT may be a whole or partial substrate and may be carried on a carrier member if desired. It is not intended to limit the UUT to any particular shape or size. It is envisaged that the UUTS could include for example, electronic products such as printed circuit boards (PCBs), LCDs, transistors, automotive parts, aeroplane parts, lids and labels on product packages, agricultural vegetation (seed corn, fruits, vegetables, or the like), and medical devices such as stents or the like. Such a photoacoustic system and method is described in published PCT patent application number WO2014/006167, assigned to Sonex Metrology Ltd, and incorporated herein fully by reference. In one embodiment of the invention the photoacoustic cell shape, materials and formation impact on the sensitivity of measurements are all factors in providing an improved photoacoustic cell. Figures 2 to 5 illustrates a number of embodiments of a photoacoustic system of the present invention. In one embodiment a photoacoustic system comprises at least one light source for optically exciting a unit under test (UUT) and at least one volume for capturing acoustic energy emanating from the UUT as result of optical excitation thereof. A microphone is positioned for receiving the captured acoustic energy and generating a corresponding electrical signal. At least one opaque element 200 is configured to isolate the microphone from unwanted energy reflections. The opaque element 200 can be selected to be in a number of positions to block unwanted energy reflections being picked up by the at least one microphone.

For the purpose of the present invention, the excitation point on the sample shall be referred to as the primary source and any additional excitation points (e.g. on the cell walls) introduced as a result of reflected light shall be referred to as secondary sources. Figure 2 to 5 shows the four light blocking elements 200 configured either side or two microphones 109. The light blocking elements are configured to only allow the microphones measure energy from the primary source and remove any energy measurements from the secondary sources. The opaque element 200 can be any suitable opaque material, for example metal.

It was found that the opaque elements mitigated the problem of optical reflectivity that arises in photoacoustic analysis of a surface. The photoacoustic analysis of even moderately reflective materials in back-reflection mode introduces the possibility of secondary photoacoustic signals originating on the cell walls. This possibility is increased when dealing with rough/contoured, highly reflective (e.g. metal) surfaces. To compound this issue, it is quite often advantageous to operate with reduced cell sizes allowing better photoacoustic responses at increased frequencies. The confined nature of these cells results in a high probability of cell wall capture of reflected light.

In a preferred embodiment the microphones are configured to be acoustically isolated from any secondary sources.

In one embodiment the photoacoustic system and method, as illustrated in Figure 6, takes account of the following elements:

1 . a transparent excitation channel,

a. this a volume of transparent material(s) through which the excitation accesses the sample. The excitation is provided by a light source, for example a laser;

2. a "light block" or opaque material

a. the opaque material defines the extremity of the excitation channel in the lateral dimension

b. it is not necessary for the light block to extend the full height of the excitation channel;

c. the light block should be acoustically isolated from the at least one microphones;

d. the light block should be of sufficient extent to optically isolate the microphones from sources of reflected light (most notably the primary source site).

3. optionally the at least one microphone should reside or be positioned outside the excitation channel.

The opaque element or light block' according to the invention comprises a layer of transparent material and a layer of absorption material so that the element can act as a dual function of blocking out unwanted acoustic and light signals. Interposed between the two layers an intermediate layer, such as an airgap, can be provided. The function of the intermediate layer is to act as a heat sink and remove unwanted heat that can be generated on the absorbing layer. Referring now to Figures 7, 8 and 9 the light block comprises of at least 2 layers; an absorptive layer surrounded by a transparent layer to the extent that the absorptive material is acoustically isolated form the microphones (as shown). An optional intermediary compliant layer can be added, the function of which is to remove unwanted heat and effectively acts as a heat sink.

In operation, reflected light will pass through the transparent layer and be absorbed in the absorptive layer. The absorption prevents the reflected light from reaching the microphone. At this point, there will be a heating of the absorptive layer and any surrounding materials. The function of the transparent layer is to acoustically isolate this heating from the microphone. The intermediary layer can help dissipate this thermal energy.

If the intermediary layers consists of an appropriate material e.g. air and is sufficiently sealed, it can be monitored by a second set of microphones enabling a second data channel giving information on the surface morphology of reflective samples.

The current embodiments, whether 2 or 3 layer configurations serve to remove the optical energy from the photoacoustic cell volume before absorbing it thereby isolating the concomitant acoustic energy from the microphones. Sample materials for the layers when operating at an optical wavelength of 808 nm are polycarbonate (transparent); Al (absorptive) and air (compliant). In one embodiment the optimisation of the cell shape is provided for measurements at target frequencies.

In one embodiment resonance tuning of the cell can be performed. By setting key dimensions of the cell, each cell can be tuned to operate at specific frequencies, e.g. The first embodiment with dimensions a = 1 1 mm, b = 1 mm, c = 3mm, d = 1 mm (shown in Figure 10) gives a resonance peak at 2 kHz demonstrated in Figurel 1 . The second embodiment with dimensions a = 8mm, b = 0.5mm, c = 1 .5mm, d = 3mm (shown in Figure 12) gives a resonance peak at 7.3kHz demonstrated in Figure 13. Figure 14 shows an array of photoacoustic cells, indicated generally be the reference numeral 300, where each cell 301 comprises a number of pressure ports 302 adapted to alleviate any cross-talk with a neighbouring cell 301 . In effect the pressure port provides cross talk buffering. Inserting a pressure port releases interstitial to the cells limits cross-talk between cells, when positioned in an array configuration.

Figure 15 illustrates a means of repeatedly and accurately aligning a removable unit containing microphones and a light source indicated by the reference numeral 400.

Figure 16 illustrates an alternate excitation methodology enabled by the transparent nature of the cell, indicated by the reference numeral 500. Here a second light source projects a hollow cone of light 501 onto the sample. The absorption of this light generates a heat source of similar shape which alters the thermal profile within the sample thereby confining the heat generated by the more traditional central probe laser in the later dimensions and encouraging vertical diffusion.

Figure 17 illustrates an alternate means of excitation indicated by the reference numeral 600. By exciting with a rotating "bar"-like beam the central region receives the majority of the excitation (due to its constant exposure) while the outer regions are heated but to a lesser extent. This alters the thermal profile in the vicinity of the measurement, confining the thermal wave laterally and encouraging vertical diffusion.

Figure 18 illustrates an example of a removable PA cell and a supporting frame. The figure depicts a means of ensuring the cell, once in place does not protrude beyond a predefined limit. This can be important when many cells are mounted side-by-side and these must be matched.

It will be appreciated that the photoacoustic cell design hereinbefore described provides a flexible metrology tool that can be used in many applications. For example multiple photoacoustic cells can be arranged in an array to examine multiple locations of the unit-under-test simultaneously. The light source in each cell can comprise one or more excitation lasers operating at a small range of modulation frequencies around a centre frequency to allow differentiation between signals generated within adjacent cells and hence minimise cross-talk. Multiple laser sources, either standalone lasers supplying light via optical fibres to groups of cells and/or a vertical cavity surface emitting laser (VCSEL) array with optical elements (e.g. lenses, collimators) fabricated directly onto the emitting surface can be provided. Transparent cell interiors can be provided to reduce heating of the cell by back- reflected laser light.

Opaque/reflective microphone housings can be used to prevent optical absorption and subsequent heat generation.

In an array of photoacoustic cells at least one monitoring cell comprises an optical pyrometer to monitor surface temperature of the area under test in realtime. A piezoelectric /accelerometer feedback system can be provided and configured to dynamically level the photoacoustic cell array and maintain co-planarity with a wafer surface. The optical system can be configured to generate a secondary laser spot to heat the unit-under-test and create a thermal gradient within the material(s). On application of the primary probe laser spot, heat will be confined within the volume defined by the applied thermal gradient, thereby increasing sensitivity to defects at depth within the unit-under-test.

There is also provided a double-walled enclosure, containing a vacuum between the walls that surrounds the photoacoustic cell and provides acoustic isolation from the ambient environment.

At least one acoustic pick-up unit (e.g. MEMs microphone) per cell, positioned within or adjacent to a channel providing partial or complete isolation from back- reflected laser light. An array of photoacoustic cells with the array covering an area roughly equivalent to that being investigated. A plurality of pressure release holes interspersed between the cells. Each cell in the array can comprise its own independent light source. The light source can be an optical fibre in optical communication with a laser source.

- where the light source is a VCSEL.

- where the light source is a VCSEL with incorporated processing optics.

Single cell/array suspended on self- levelling supports e.g. accelerometer driven linear actuators.

A cell array where the light sources are mechanically coupled to one another e.g. a single PCB carrying all light sources (e.g. VCSELs). A cell array where a plurality of light sources can be manufactured on a single substrate for example a VCSEL array. This allows tight control over inter-spot distance, necessary for some semiconductor applications.

In another embodiment there is provided a modular photoacoustic cell comprising: - Light source,

- optics,

- microphone,

- processing electronics,

- ADC.

This can be attachable (detachable) to (from) the cell and is in communication with a controller.

In one embodiment the photoacoustic cell comprises a means of accurately registering a microphone/laser module, e.g. precision machined recesses for either the microphone/light source or alignment struts. In one embodiment there is provided an array of cells additionally incorporating sample alignment facilities, e.g. a camera enabled to recognise fiducials on a semiconductor wafer.

The embodiments in the invention described with reference to the drawings comprise a computer apparatus and/or processes performed in a computer apparatus. However, the invention also extends to computer programs, particularly computer programs stored on or in a carrier adapted to bring the invention into practice. The program may be in the form of source code, object code, or a code intermediate source and object code, such as in partially compiled form or in any other form suitable for use in the implementation of the method according to the invention. The carrier may comprise a storage medium such as ROM, e.g. CD ROM, or magnetic recording medium, e.g. a floppy disk or hard disk. The carrier may be an electrical or optical signal which may be transmitted via an electrical or an optical cable or by radio or other means.

In the specification the terms "comprise, comprises, comprised and comprising" or any variation thereof and the terms include, includes, included and including" or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.