Patent Application
20130321805
1. Field of the Invention
The invention relates generally to non-contact measurements of thin film layers applied to a generally transparent substrate; and more particularly for assessing at least the relative surface roughness of the thin film by reference to an optical absorption edge of the thin film material.
2. Related Art
Advanced manufacturing processes involving depositing thin films on substrates often depend on the ability to monitor and control a property of a semiconductor material, such as its temperature, surface roughness, thickness and/or optical absorption properties with high precision and repeatability.
As is now well known, a sudden onset of strong absorption occurs when the photon energy exceeds the band gap energy. In “A New Optical Temperature Measurement Technique for Semiconductor Substrates in Molecular Beam Epitaxy,” Weilmeier et al. (Canadian Journal of Physics, 1991, vol. 69, pp. 422-426) describe a technique for measuring the diffuse reflectivity of a relatively thick substrate having a textured back surface, and inferring the temperature of the semiconductor from the band gap characteristics of the reflected light. The technique is based on a simple principle of solid state physics, namely the practically linear dependence of the interband optical absorption (Urbach) edge on temperature.
Briefly, a sudden onset of strong absorption occurs when the photon energy, hv, nears the band gap energy Eg. This is described by an absorption coefficient,
α(hv)=αg exp [(hv−Eg)/E0], (Equation 1)
where αg is the optical absorption coefficient at the band gap energy. The absorption edge is characterized by Eg and another parameter, E0, which is the broadening of the edge resulting from the Fermi-Dirac statistical distribution (broadening ˜kBT at the moderate temperatures of interest here). The key quantity of interest, Eg, is given by the Einstein model in which the phonons are approximated to have a single characteristic energy, kB. The effect of phonon excitations (thermal vibrations) is to reduce the band gap energy according to:
E
g(T)=Eg(0)−SgkBθE/[exp (θE/T)−1] (Equation 2)
where Sg is a temperature independent coupling constant and θE is the Einstein temperature. In the high T case where θE<<T, which is well-obeyed for high modulus materials like Si and GaAs, one can approximate the temperature dependence of the band gap by the equation:
E
g(T)=Eg(0)−SgkBT, (Equation 3)
showing that Eg is expected to decrease linearly with temperature T with a slope determined by Sg kB. This is well obeyed in practice and is the basis for contemporary absorption edge thermometry, also known as band edge thermometry (BET).
As mentioned above, control of the temperature, surface roughness, thickness and/or optical absorption properties of a semiconductor material, be it the substrate itself or a thin film deposited onto the substrate, can be achieved through non-contact, real-time monitoring of diffusely scattered light emanating from the semiconductor material. The BandiT™ system from k-Space Associates, Inc., Dexter Mich., USA (kSA), assignee of the subject invention, has emerged as a premier, state-of-the-art method and apparatus for measuring temperature, among other properties. Diffusely scattered light from the semiconductor material is detected to measure the optical absorption edge characteristics. From the optical absorption edge characteristics the temperature is accurately determined, as well as other properties such as film thickness. The kSA BandiT can be set up to run in both transmission and reflection modes. In transmission mode, a substrate heater (or other source) may be used as the light source. In reflection mode, the light source is mounted in a non-specular geometry. The kSA BandiT is available in several models covering the spectral range of about 380 nm-1700 nm. Typical sample materials measured and monitored include GaAs, Si, SiC, InP, ZnSe, ZnTe, CdTe, SrTiO3, and GaN. The kSA BandiT system is described in detail in U.S. Pat. No. 7,837,383, the entire disclosure of which is incorporated here by reference.
One emerging area in which these types of equipment may be applied is the so-called thin-film solar cell. Thin-film solar cells, also known as thin-film photovoltaic (PV) cells, are devices that are made by depositing one or more thin layers (thin films) of photovoltaic material having semiconductor properties on a generally transparent substrate. The thickness range of these thin films varies from a few nanometers to tens of micrometers depending on application. Many different PV materials are deposited with various deposition methods on a variety of substrates. These PV materials may, for example include: Amorphous silicon (a-Si) and other thin-film silicon (TF-Si), Cadmium Telluride (CdTe), Copper indium gallium diselenide (CIS or CIGS), textured poly-silicon, organic solar cells, etc.
The ability to monitor real-time optical band gap properties (that is, optical absorption edge properties) enables manufactured products such as solar panels to achieve consistently high quality and high performance specifications. Although these thin films do, typically, possess semiconductor properties in the aspect of an optical absorption edge, the extremely small thickness of these thin films creates new challenges for the application of existing BET methods and equipment. This is due in part to the increased difficulty of measuring the light absorption properties when transparent and/or non-semiconductor substrate materials are used, because non-semiconductor substrate materials do not have a measurable optical absorption edge and are typically transparent to all practical wavelengths of light. Furthermore, in the field of thin-film PV panel production, manufacturing throughput is increasing so rapidly that thermometry techniques used in the production processes must be compatible with highly automated assembly line conditions. Still further, these types of absorber layers are often very rough and scatter light more substantially than do smooth surfaces. For some applications, an assessment of the surface roughness of a thin film layer may be useful for quality control and manufacturing considerations.
Some in-line film thickness measurement techniques have been proposed for production line thin film PV processes, such as those described in the March/April 2009 issue of Photovoltaics World, Pages 20-25 (www.pvworld.com), the entire disclosure of which is hereby incorporated by reference. However, these prior techniques have been based on certain analytical methods that do not yield consistent or reliable results. In another example, which for the avoidance of doubt is not admitted prior art to the subject application, US Publication No. 2010/0220316 to Finarov discloses a method for thin film PV quality control in which an illuminated line is projected onto the thin film. A detector samples points along the line to derive a spectral signal which is used to compute certain parameters of the thin film.
There is therefore a need in the art to advance and adapt the BET techniques to account for new materials, high throughput production techniques, and increased demands on quality control which are considered necessary to compete in the future markets, including but not limited to PV panel production and other related fields.
According to one aspect of the invention, a method is provided for assessing at least the surface roughness of a thin film applied to a generally transparent substrate. A generally transparent substrate is provided. A thin film of material is deposited onto the substrate. The film material composition is of a type that exhibits an optical absorption (Urbach) edge, and has an upper exposed surface with a measurable surface roughness. White light is allowed to interact with the film deposited on the substrate to produce diffusely scattered light. The diffusely scattered light emanating from the film is detected with a detector that is spaced apart from the film, and then routed to a spectrometer to produce spectral data in which the detected light is resolved into discrete wavelength components of corresponding light intensity. An optical absorption (Urbach) edge is then identified in the spectral data. From the characteristics of this absorption edge, an assessment of the relative surface roughness of the film can be made.
The invention is distinguished from prior art techniques in its use of the absorption edge as a metric to assess surface roughness. This approach is more robust and reliable than prior art techniques, and has been determined to yield consistently reliable results particularly in the highly automated, large throughput assembly line conditions.
According to another aspect of this invention, an assembly is provided for assessing the relative surface roughness of a thin film applied to a generally transparent substrate. The assembly comprises: a generally planar substrate fabricated from a non-semiconductor material having no measurable optical absorption edge. In particular, the substrate comprises a glass material composition. A thin film of a material is deposited on the substrate. The thin film has a material composition exhibiting an optical absorption edge, and an upper exposed surface with a discernible surface roughness. A light source is disposed on one side of the thin film for projecting white light toward the thin film. As a result, diffusely scattered light emanates from the thin film. A first detector is spaced apart from the thin film on the same side of the thin film as the light source for detecting the diffusely scattered light reflected from the thin film. A second detector is spaced apart from the thin film on the same side of the thin film as the light source for detecting the diffusely scattered light reflected from the thin film. A third detector is spaced apart from the thin film on the opposite side of the thin film from the light source for detecting the diffusely scattered light transmitted through the thin film. At least one spectrometer is operatively connected to the first, second and third detectors for producing spectral data from the respective detections of diffusely scattered light. A conveyor means moves the thin film and substrate as a unit relative to the detector while maintaining a substantially constant normal spacing therebetween.
These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein:
Referring to the figures, wherein like numerals indicate like or corresponding parts throughout the several views, an absorption edge measurement system according to this invention is generally shown at 20. The system 20 is particularly adapted for inline measurement of materials 22 that are moved along a conveyor system 24. Typical materials 22 include the manufacture of PV solar panels on which is applied a thin film absorption layer 26 over a glass (or other suitable) substrate 28. The substrate 28 and thin film 26 layers are shown illustratively in
In the specific example of PV panel manufacture, wherein the material 22 comprises a component of a solar panel assembly, it is typical for such materials 22 to comprise rigid sheet-like materials formed to rectangular dimensions and moved as a unit over a conveyor 24 for purposes of absorption edge measurement and/or real time BET measurement techniques using the system 20 of this invention. However, the general principles of this invention are not limited to PV panels, or applications only of sequentially fed sheet materials, but are also applicable to continuous strip applications, disc-like wafers, as well as other conceivable applications. The system 20 includes a light source 30 which may be comparable, generally or specifically, to that described in detail in the applicant's U.S. Pat. No. 7,837,383. The light source 30 produces a beam of white light 32, and in particular non-polarized, incoherent light 32, directed onto the material 22. As shown in
A first absorption edge detector 36 is located in a non-specularly opposed position, i.e., outside the angle of incidence, from the beam 32 so as to collect scattered/reflected light 34. The absorption edge detector 36 is in this arrangement configured as a “reflection mode” detector 36 constructed generally in accordance with that described in U.S. Pat. No. 7,837,383. One or more spectrometers 58 (
A second thin film measurement detector, generally indicated at 38, is also disposed at a non-specularly opposed position relative to the light source 30 so as to collect scattered/reflected light 34 from the material 22. Both the first 36 and second 38 detectors are disposed on the same side of the thin film 26 as the light source 30, and thus both configured for reflectance mode operation. The thin film measurement detector 38 is manufactured substantially in accordance with that described in the applicant's co-pending international patent application WO 2010/148385, published Dec. 23, 2010, the entire disclosure of which is hereby incorporated by reference and relied upon.
Both the reflection mode absorption edge detector 36 and thin film measurement detector 38 may be fitted with laser alignment devices as described in U.S. Pat. No. 7,837,383, and configured to produce respective laser beams 36′, 38′ useful in connection with setup to align the detectors 36, 38 relative to the point at which the light beam 32 impacts the material 22. The alignment lasers 36′, 38′ are deactivated during the detection modes.
Further, a third transmission mode detector, generally indicated at 40, is positioned below the material 22 so as to receive transmitted light 34′. The transmission mode detector 40 may include an alignment laser 40′ for use during the initial setup phases of the system.
A highly simplified construction for the system 20 is shown in
Turning now to
In operation, the light source 30 emits radiation for both film thickness determination and diffuse reflectance of the film side and thin film 26 absorption edge detection via transmission mode detector 40. Although not shown, a secondary light source may be located on the underside of the material 22 for use in measuring the absorption edge of any films applied to the bottom edge of the substrate 28, as is the case in some applications. If a secondary light source is used, it may be configured to emit visible radiation for absorption edge detection on any bottom-applied films via diffusive reflection. In the case of a supplemental light source, both light sources will preferably be focused at the same position on the material 22 via a focusing lens as taught in U.S. Pat. No. 7,837,383. Lenses are preferably used as well for the detectors 36, 38, 40 to provide optimal results in terms of total counts, S/N ratio and minimizing stray light collection.
Relative film 26 surface roughness determinations can be made in many ways using the absorption edge derived by the system 20. According to one such technique, spectral data collected from the reflectance mode absorption edge detector 36 are used. Referring to
In yet a still further application of the principle that the absorption edge is useful to assess relative surface roughness conditions between discrete materials samples 22 or different locations within the same materials sample 22,
The first and third detectors 36, 40 may be utilized to monitor the temperature of the film 26, whereas the second detector 38 may be utilized primarily to monitor the thickness of the film 26. In some cases, and in particular when monitoring temperature during the deposition process, it may be desirable to account for changing film thickness. The general dependence of the transmission of light through a semiconductor material is provided by Equation 4 below.
I(d)/I(0)=exp(−αd) (Equation 4)
wherein d is the thickness of the film 26, I(d) is the intensity of the diffusely scattered light collected from the film 26 at the film thickness (d), I(0) is the intensity of diffusely scattered light collected from the substrate 28 without the film 26, and α is the absorption coefficient of the material of the film 26 below the band gap energy of the material. The absorption coefficient of the material (α) accounts for the dependence of the optical absorption on the band gap energy of the material, which is temperature-dependent. The absorption coefficient (α) is also referred to as α(hv) in the equation given above: α(hv)=αg exp [(hv−Eg)/E0] (Equation 1).
Equation 1 illustrates that the optical absorption of the film 26 is thickness-dependent and the behavior of the optical absorption is exponential. In applications wherein the substrate 28 has no measurable optical absorption edge wavelength, light 32 diffusely scatters from the surfaces of the thin film 26, the interface between the film 26 and the thick substrate 28, and the surfaces of the substrate 28, like substrates formed of semiconductor materials. For substrates 28 formed of semiconductor materials, the light 32 is affected by the substrate 28, which has a large thickness, so the incremental changes in the thickness have virtually no significant effect on the optical absorption edge. However, when the substrate 28 is formed of a material having no measurable optical absorption edge wavelength, such as a non-semiconductor, the light 32 is essentially not affected by the substrate 28. The substrate 28 in these situations is typically either transparent (e.g. glass or sapphire) or completely reflective (e.g. steel or other metal). Thus, the light 32 is only affected by the semiconductor film 26. Since the film 26 is thin, the incremental increases or changes in the film thickness will have a significant effect on the measured optical absorption edge wavelength of the film 26. An incremental change or increase in the film thickness is typically a 1.0 μm increase or decrease in thickness.
In one exemplary embodiment shown in
The method, apparatus, and system of the present invention can be configured to account for the incremental changes in the thickness of the film 26 by determining the optical absorption edge wavelength of the film 26 as a function of the film thickness, which is then used to determine the temperature of the film 26. The optical absorption edge wavelength and temperature are determined at a time during the manufacturing process when adjustments can be made to the film 26 to correct undesirable temperatures which yield undesirable properties.
The first step includes performing spectra acquisition to correct potential errors due to equipment artifacts, such as a non-uniform response of the detector used and non-uniform output light signals. These errors could prevent raw diffuse reflectance light signals from yielding a measurable optical absorption edge at the correct wavelength position. When performing the spectra acquisition, it can be assumed the errors are steady-state.
The spectra acquisition first includes producing a reference spectrum representing the overall response of the system, i.e. the combination of light source output signature and detector response, which are both wavelength dependent. The reference spectrum is produced by illuminating the substrate 28 with light, without the film 26, for example bare sapphire, and collecting diffusely scattered light in the detector 40. Next, the spectrometer 58 is used to generate the reference spectrum based on the diffusely scattered light collected from interacting light with the substrate 28 alone. The spectra acquisition concludes by normalizing the reference spectrum.
Each time a raw spectrum is produced based on the diffusely scattered light from the film, the method includes normalizing the raw spectrum, and dividing the normalized raw spectrum, by the normalized reference spectrum to produce a resultant spectrum. Dividing the raw spectrum by the reference spectrum is performed on every incoming raw spectrum, and is necessary to determine an accurate film thickness, in addition to enhancing the optical absorption edge signature. The resultant spectrum is normalized and used to determine the optical absorption edge wavelength. The resultant spectrum provides a resolvable optical absorption edge wavelength, which is used to determine the temperature or another property of the film 26.
The spectra acquisition, including creating a normalized reference spectrum, is performed each time a component of the system changes. For example, a view port of the detector 40 can become coated over time, which affects the collected light. The spectral acquisition can be performed one time per run, one time per day, one time per week, or at other time intervals, as needed. Performing the reference spectrum acquisition one time per run will typically provide more accurate results than once per week.
The spectrum of the present method and system, including the reference spectrum, raw spectrum, and the resultant spectrum, are typically produced by resolving the light signals from the substrate 28 into discrete wavelength components of particular light intensity. The spectrum indicates the optical absorption of the film 26 based on the diffusely scattered light from the film 26. The spectrum typically includes a plot of the intensity versus wavelength of the light, as shown in
The resultant spectra are used to determine the optical absorption edge wavelength. As discussed supra, the optical absorption edge wavelength is the abrupt increase in degree of absorption of electromagnetic radiation of a material at a particular wavelength. The optical absorption edge wavelength is dependent on the specific material, the temperature of the material, and the thickness of the material. The optical absorption edge wavelength can be identified from the spectra; it is the wavelength at which the intensity sharply transitions from very low (strongly absorbing) to very high (strongly transmitting). The optical absorption edge wavelength is used to determine the temperature of the substrate 28, as well as to make the relative surface roughness assessments described above.
The method may further include producing a temperature versus wavelength calibration table (temperature calibration table) of the film 26 at a single thickness. The temperature calibration table can also be provided to a user of the method, rather than produced by the user of the method. The temperature calibration table indicates the temperature versus optical absorption edge wavelength at a constant thickness of the film. The temperature calibration table provides subsequent temperature measurements of the film based on the optical absorption edge wavelength obtained from the spectra. However, unlike in the prior art system and method, the present system and method further includes determining the temperature of the film 26 by accounting for the effect of the thickness of the film 26 on the optical absorption edge wavelength, or the dependence of the optical absorption edge wavelength on film thickness, which will be discussed further below.
As stated above, the method and system of the present invention includes determining the optical absorption edge of the film 26, which may optionally be determined as a function of the film 26 thickness if under the circumstances it is relevant that the optical absorption edge wavelength of the film 26 depends on the thickness of the film 26. The film thickness has an especially significant impact on the optical absorption edge of thin films 26, and thus the determination of the temperature of the thin films 26, such as the top layer 64 of the sample of
The thickness of the film 26 can be determined by a variety of methods. In one embodiment of the invention, the thickness of the film 26 is conveniently determined from the spectrum produced by the light diffusely scattered from the film 26 and used to determine the optical absorption edge wavelength, discussed above. The spectrum, often includes oscillations below (to the right of) the optical absorption edge region of the spectrum. The oscillations are a result of thin film interference, which is similar to interference rings sometimes observable on a thin film of oil. A derivative analysis of the wavelength-dependent peaks and valleys of the oscillations is employed to determine the thickness of the film 26. Equation 5 below can be employed to determine the thickness of the film 26,
wherein d is the thickness of the film, λ1 is the wavelength at a first peak of the oscillations and λ2 is the wavelength at a second peak of the oscillations adjacent the first peak, or alternatively λ1 is the wavelength at a first valley of the oscillations and λ2 is the wavelength at a second valley of the oscillations adjacent the first valley, n1 is a predetermined index of refraction dependent on the material of semiconductor at λ1, and n2 is a predetermined index of refraction dependent on the material of semiconductor at λ2. The wavelengths used for λ1 and λ2 can be any two successive peaks or any two successive valleys of the oscillations. The oscillations and value obtained for thickness of the film 26 have a non-linear dependence on all layers 60, 62, 64 of the film 26. The thickness of the film 26 can also be determined using other methods. For example, the thickness can be estimated based on previous measurements of thickness as a function of deposition time or by laser-based reflectivity systems such as the Rate Rat™ product available from k-Space Associates, Inc., Dexter, Mich. USA.
As stated above, the step of determining the optical absorption edge of the film 26 as a function of the film 26 thickness includes accounting for the dependence of the optical absorption of the film 26 on the film thickness. The step of determining the optical absorption edge of the film 26 as a function of the film thickness can also include adjusting a measured optical absorption edge wavelength value of the film 26 obtained from the spectra due to the step of depositing the film 26 of a semiconductor material having a measurable optical absorption edge and a measurable thickness on the substrate 28. The step of determining the optical absorption edge of the film 26 as a function of the film thickness can also include identifying the semiconductor material of the film 26 and adjusting a measured optical absorption edge wavelength value determined from the spectra based on the semiconductor material and the thickness of the film 26 to obtain an adjusted absorption edge wavelength.
The step of determining the optical absorption edge of the film 26 as a function of the film thickness typically includes using a thickness calibration table. Each semiconductor material has a unique thickness calibration table. The thickness calibration table indicates optical absorption edge wavelength versus thickness at a constant temperature of the film.
The thickness calibration table can be acquired by growing a film 26 of the semiconductor material at a constant temperature and measuring the optical absorption edge wavelength at each incremental increase in thickness to produce a spectrum for each thickness. The thickness calibration table can also be prepared by depositing the film 26 on the substrate 28 at a constant temperature and measuring the optical absorption edge wavelength of the film 26 at the constant temperature and a plurality of thicknesses. Preparing the thickness calibration table at a constant temperature also allows a user to determine the dependence of the optical absorption edge wavelength on the thickness.
The spectra acquisition is performed on each spectrum, as described above. Next, from each spectrum a raw optical absorption edge wavelength value is determined for each thickness at the constant temperature. An nth order polynomial fit is performed on the raw optical absorption edge wavelength values to produce the optical absorption edge wavelength versus thickness curve, where n is the order of the polynomial providing the best fit to the data. This nth order polynomial dependence is used to create the thickness calibration table. The thickness calibration table is used as a thickness correction lookup up for subsequent temperature measurements. The thickness calibration table illustrates the dependence of the optical absorption edge wavelength on film thickness. The optical absorption edge wavelength increases as the film thickness increases. The thickness calibration table is produced for each unique semiconductor material, as different materials produce different results. The thickness calibration table can also be provided to a user of the method, rather than produced by the user. However, for each unique material, only one thickness calibration table is needed to determine temperature of the film at various thicknesses and temperatures. The method can include identifying the semiconductor material of the film and providing the thickness calibration table and temperature calibration table for the identified semiconductor material. The temperature of the film at a certain thickness is determined based on the spectrum, the thickness calibration table, and the temperature calibration table.
In alternative constructions, it may be desirable to move the system 20 relative to the material 22. Such relative movements may include relative lateral as well as longitudinal directions, or even curvilinear motions, so as to scan either sequentially or intermittently different surface locations of the material 22. As shown in
Transmission mode detector 40 may incorporate an optical trigger mechanism capable of sensing the presence or absence of material 22 crossing the beam 32. Alternatively, a stand-alone or other type of optical trigger can be used to accomplish a similar purpose. This data can be used for quality control and material 22 tracking purposes. As shown in
The functionality of the three detectors 36, 38, 40 described above can be consolidated into one single detector 136 as shown in
The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention.
This application claims priority to Provisional Patent Application No. 61/362,938 filed Jul. 9, 2010, the entire disclosure of which is hereby incorporated by reference and relied upon.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US11/43507 | 7/11/2011 | WO | 00 | 8/13/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61362938 | Jul 2010 | US |
