WO2010106533A2

WO2010106533A2 - A method and apparatus for accurate determination of parameters of a stack of thin films

Info

Publication number: WO2010106533A2
Application number: PCT/IL2010/000174
Authority: WO
Inventors: Moshe Finarov
Original assignee: Brightview Systems Ltd.
Priority date: 2009-03-14
Filing date: 2010-03-03
Publication date: 2010-09-23
Also published as: WO2010106533A3; WO2010106533A4

Abstract

A method for accurate determination of optical parameters of a stack of thin films with at least one of the films being an opaque film. The method includes measurement of the reflected or transmitted through the stack illumination and mutual correlation of the determined thin film parameters.

Description

A METHOD AND APPARATUS FOR ACCURATE DETERMINATION OF PARAMETERS OF A STACK OF THIN FILMS

TECHNOLOGY FIELD

[001 ] The method and apparatus are in the field of optical measurement technologies and in particular of optical technologies for accurate measurement of thin film parameters.

BACKGROUND

[002] Thin films and in particular, thin film photovoltaic panels are multi-layer structures, also termed stacks, produced by sequential deposition of a plurality of transparent and partially opaque thin films and patterning processes on a substrate. Typically, each of the films may have different parameters such as the film thickness (d), the film refractive index (n), film extinction coefficient (k), surface roughness (R_x), and others.

[003] Thin films manufacturing processes require stringent control of different parameters of each of the thin films, forming the stack. On-line or off-line optical measurement methods are usually employed to determine and control the parameters of each of the thin films. The methods are based on measurement of the intensity and wavelength of the reflected or transmitted by the stack illumination. These methods are accurate and effective in dealing with transparent thin films.

[004] Some of the thin films are partially or fully opaque in the spectral range of interest (e.g. in visible light) and characterized by relatively large optical radiation absorption or dissipation (e.g. by light scattering on the rough film surface). The opaque layers, usually made of semiconductor materials, are typically located between one or more transparent thin films. The large loss of intensity of the transmitted and/or reflected optical radiation reduces substantially the accuracy of the determination the parameters of the thin layers located behind the absorbing layer when considering the direction of the illumination or optical radiation propagation.

[005] It is however important for the deposition process control to determine optical and other properties of each of the thin films forming the stack after completing all the stack deposition or processing steps. Unfortunately, in cases when measurements are performed on the entire stack, the absorbing film/s or layers affect the reflected or transmitted radiation such that accurate determination of the properties of each separate layer forming the stack is almost impossible.

[006] There is a need in a method of measurement of the thin films forming the stack parameters a method of measurement free of above mentioned drawbacks.

BRIEF SUMMARY

[007] The method and apparatus present a system for accurate determination of parameters of a stack of thin films deposited on a transparent substrate. The stack may include an absorbing, opaque or semitransparent layer. Among the parameters that may be determined by the present method are the thin film thickness (d), thin film refractive index (n), thin film extinction coefficient (k), film surface roughness, gap energy, crystallinity, phase composition, chemical composition (stoicheometry), and others. The substrate may be a glass or a plastic substrate, which are transparent in the useful spectral range.

[008] The system measures reflected optical radiation directed from both sides of the stack thus allowing sharing parameters measured at each direction. The method allows improving the measurement accuracy by alleviating the adverse effects introduced by the absorbing optical radiation by opaque or/and dissipating layer inside the stack. GLOSSARY

[009] The term "Opaque film" or "Semiopaque film," "Semitransparent film", or "Absorbing film" in the context of the present disclosure means a film the transparency of which in the same spectral range is substantially lower of the other transparent films forming a stack.

[0010] The term "Optical radiation" or "Illumination" in the context of the present disclosure, have the same meaning.

BRIEF DESCRIPTION OF THE DRAWINGS

[001 1 ] In order to understand the apparatus and the method and to see how it may be carried out in practice, a number of exemplary preferred embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0012] FIG. 1 is a schematic illustration of a typical thin film stack. [0013] FIG. 2 is a schematic illustration of a typical prior art apparatus for measurement of thin film parameters. [0014] FIG 3 is a schematic illustration of an exemplary embodiment of the present apparatus for measurement of thin film parameters. [0015] Figure 4 is a schematic illustration of matching the actually measured spectra data of radiation reflected (or transmitted) from a thin film to a theoretical thin film spectrum. [0016] FIG. 5 is a flow chart illustrating the process of the determination of the parameters of a typical thin film stack according to the present method.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0017] FIG. 1 is a schematic illustration of a typical thin film stack. Such stacks are produced for different applications employing thin films and in particular in - A - photovoltaic panels. Panel 100 includes a transparent film 104 e.g. TCO

(Transparent Conductive Oxide) deposited upon a transparent substrate 108 and an opaque or partially opaque film 1 12. In photovoltaic panels the absorbing thin film 1 12 would typically be a semiconductor material like silicon (Si), cadmium telluride (CdTe), copper indium gallium di-selenide (SIGS) or the like. Another transparent (TCO or opaque metal) film 1 16 covers the absorbing film 1 12. This typical photovoltaic thin film structure is used in solar cells production. The absorbing film 1 12 contains p-n junctions and films 104 and 1 16 serve as conductive contacts to these junctions.

[0018] FIG 2 illustrates a typical prior art apparatus for measurement of the thin film parameters. Apparatus 200 includes a support (not shown) for stack 100, a polychromatic illumination device 208 operative to provide a polychromatic optical radiation 212, a detector 216 operative to detect the spectrum (i.e. intensity and wavelength) of reflected from the stack 100 polychromatic optical radiation 220, and a processor 224 operative to receive the detected intensity and wavelength of the reflected radiation and calculate the parameters of the stack 100. The measurements of the reflected radiation intensity and wavelength provide sufficient data for accurate determination or calculation of the transparent thin film parameters. Black arrowheads mark the incident optical radiation, white arrowheads 220 mark reflected by each stack layer optical radiation, and arrows 228 mark the scattered optical radiation. It should be noted that reflection from the external substrate surface could be mostly eliminated by focusing the illumination on the thin film stack and thus neglected in calculation of the stack parameters.

[0019] Stack 100 includes at least one semitransparent thin film 1 12 made of a semiconductor material possessing a photovoltaic effect. The semitransparent (or opaque or absorbing) thin film 1 12 may absorb or dissipate a significant portion of the incident optical radiation employed in the measurement process. Because of this, the portion of light reflected from layer 104 is weak and sensitivity of the spectrum reflected from the whole stack 100 to parameters of this film is not sufficient for accurate thin film parameters determination . [0020J FlG 3 is a schematic illustration of the present apparatus for thin film measurement parameters. Apparatus 300 includes a support (not shown) for the thin films stack 100, a first polychromatic illumination device 304, which is operative to illuminate by a illuminating beam 308 a first area 310 on the stack 100 from a first side or most outer stack side 312, and a detector 316. The area illuminated by the illumination device may be a point, a line, or a two dimensional area. Detector 316 would typically be a spectrometer operative to detect the intensity and spectral composition of illumination reflected by each of the thin films 104, 1 12, 1 16, and substrate 108 interfaces. Black arrowheads 320 mark the incident optical radiation, white arrowheads 324 mark optical radiation reflected by each thin layer of the stack 100 interface, and arrows 328 mark the scattered optical radiation.

[0021 ] The support enables to illuminate the stack from the first or outer side 332 and a second or proximate to substrate 108 side 336. A second polychromatic illumination device 304' identical to the device 304 is located on the opposite side of the stack 100. The second illumination device 304' is operative to illuminate by a illumination beam 338 an area 340 which is almost the same area or area adjacent to area 310, although it is located on the side 336. A detector 316' identical to detector 316 is operative to detect reflected by the stack of the thin films illumination 324'. Illumination devices located at the first side 312 and at the second side 336, which illuminate respective areas 310 and 340. may operate concurrently or sequentially. The illuminating beams 308 and 338 are shown shifted for explanation purposes only. Generally, they are coaxial beams. Tagged numbers mark similar located on the second side 336 of the stack 100 elements of the apparatus 300.

[0022] Apparatus 300 further includes a processor 346 operative to synchronize operation of the illumination sources 304, detectors 316, receive and process the intensity and wavelength of the reflected illumination provided by the first 316 and second 3 16' detectors, and determine the optical parameters of the thin films forming the stack. The optical parameters of the stack may be at least one of a group consisting of the thin film thickness (d), thin film refractive index (n), thin film extinction coefficient (k), surface roughness, energy gap, crystallinity, phase composition, and film stoichiometry. Apparatus 300 may further include a mechanism providing a relative movement between the stack of thin films 100 or the support of the substrate and the illumination sources 304, 304'and detectors 316. 316' enabling determination of optical parameters in practically every point of the stack. Processor 346 is operative to synchronize the operation of the illumination sources, the detectors and the support movement.

[0023] Apparatus 300 may be employed for accurate determination of thin film optical parameters of stacks including semitransparent or absorbing thin films. This may be done by matching the actually measured from each stack side spectra data (See FIG 4) of radiation or illumination reflected (or transmitted) from a thin film to a theoretical thin film spectrum. Numeral 400 refers to the actually measured spectrum reflected by the thin film 108 and numerals 404, 408 and 412 relate to the theoretical spectra calculated for different sets of thicknesses of the same thin film. Based on some merit function criteria of measuring the spectral differences, of spectrum 412 could be selected as the one most closely matching spectrum 400. Theoretical spectrum 412 was calculated for a predetermined set of expected thin film parameters. As such, the thin film parameters included in the calculation of the theoretical spectrum are at least one of the film thickness (d), film refractive index (n), film extinction coefficient (k), surface roughness, energy gap. crystallinity. phase composition, and film stoichiometry, which are the closest to the parameters of the measured spectrum. These parameters become the parameters characterizing the measured thin film.

[0024] Figure 4 indicates that the spectrum has peaks and valleys. Generally, the film thickness determination could be supported by measurements at the peaks only. However, since other optical parameters such as dispersion of the refractive index (n) and the extinction coefficient (k) should be determined, it is necessary to perform the measurements through the whole spectrum or at least at such sections of the spectrum where the sensitivity to selected variables is the highest one. Such sensitivity analyses may be carried out at the setup stage. [0025] FIG. 5 is a flow chart of the process of accurate determination of the parameters of a typical thin film stack according to the present method. Initially a thin films stack 100 parameters of which have to be determined is provided and mounted on the support of apparatus 300. Concurrently, an optical model enabling to characterize interaction of light with the measured stack of thin films by a set of optical and geometrical parameters of each thin films of said stack is provided (Block 500). Illumination device 304 (see FlG 3) illuminates from the first side 312 an area 310 of the stack 100 by a polychromatic illumination 308 (Block 504). Detector 316 detects illumination 324 reflected by the stack 100 and communicates the detected amplitude and corresponding wavelength to processor 346. The processor processes the received data and determines parameters of the thin films measured from the first side 312. The thin films parameters may be at least one of a group consisting of the thin film thickness (d), thin film refractive index (n), thin film extinction coefficient (k), film surface roughness, gap energy, crystallinty, phase composition, and stoichiometry (Block 508).

[0026] Because film 1 12 is a film made of semiconductor material possessing a photovoltaic effect and it is partially transparent , the film 1 12 and film 104 parameters cannot be reliably and accurate determined, unless sufficient reflected illumination intensity exists. This condition does not exist when the illumination illuminates the absorbing thin film 1 12 attenuates the illumination from one side only.

[0027] The parameters of the thin film 104 may be accurately determined by illuminating by polychromatic illumination the corresponding to area 310 located on the second (or proximate to the substrate 108) surface 336 of the stack 100 area 340 (Block 512). Since substrate 108 is a transparent substrate, it does not obstruct access of optical radiation to the second side 336 of the stack 100. Detector 316' detects reflected by the stack illumination 324' and communicates the detected intensity and wavelength values to processor 346 that determines (Block 516) the parameters of the thin films measured from the second side. Illumination from the the first side 312 and from the second side 336 may illuminate respective areas 310 and 340 concurrently or sequentially, provided they illuminate corresponding areas.

[0028] Processor 346 shares the parameters of the stack measured from the first and second sides to determine highly sensitive parameters of the stack for each side measurement and fit the determined parameters into the updated optical model (Block 520). Fore example, parameters of the film 1 16 will be taken from the top side measurement while parameters of film 104 will be taken from the bottom side measurement.

[0029] The updated optical model of the stack 100 may now be used for recalculation of the parameters of the stack (Block 524) accounting or correlating with measurements made from each side. Since the updated optical model is more accurate, the determined parameters of the stack 100 will be also more accurate.

[0030] The illuminated areas may be at least one of a group of areas consisting of a point, a line, or an area of predetermined size. They should be selected such as to correspond to almost identical, although located on different sides areas 310 and 340. Typically, the same incident illuminating beam 308 and 338 would exist with respect to a line passing through these area and perpendicular to the stack of thin films.

[0031 ] The method and apparatus of measurement of thin films forming the stack from both sides of the stack enable accurate determination of the parameters of thin films forming a stack that includes at least one absorbing thin film. The method and apparatus may be employed for on-line or off-line thin film and in particular photovoltaic thin films quality control.

[0032] While the exemplary embodiment of the method of and apparatus for accurate measurement and determination of optical and geometric parameters of thin films forming a stack have been illustrated and described, it will be appreciated that various changes can be made therein without affecting the spirit and scope of the method. The scope of the method, therefore, is defined by reference to the following claims:

Claims

What I claim is:

1. A method for accurate determination of parameters of a stack of thin films, said method comprising:

- providing an optical model enabling to characterize interaction of optical radiation with the measured stack of thin films by a set of optical and geometrical parameters of each of the thin films of the stack;

- illuminating an area of the stack of thin films by a polychromatic illumination from a first side of the stack, detecting a spectrum of illumination reflected by the stack , and determining parameters of the thin films measured from the first side;

- illuminating an area of the stack of thin films by a polychromatic illumination from a second side, detecting a spectrum of illumination reflected by the stack, and determining parameters of the thin films measured from the second side;

- fitting the determined parameters to the said optical model and updating the model; and

- recalculating all the unknown parameters of the stack using said updated optical model and the measured spectra reflected by the first and second side of the stack.

2. A method for accurate determination of parameters of a stack of thin films, said method comprising:

- illuminating an area of the stack of thin films by a polychromatic illumination from a first side of the stack, detecting reflected by the stack illumination, and determining parameters of the thin films measured from the first side;

- illuminating an area of the stack of thin films by a polychromatic illumination from a second side, detecting reflected by the stack illumination, and determining parameters of the thin films measured from the second side.

3. The method according to claim 2, wherein the stack of thin films includes at least one transparent thin film and at least one absorbing thin film.

4. The method according to claim 3, wherein the stack of thin films is illuminated by the polychromatic illumination from the first side and from the second side.

5. The method according to claim 3, wherein the absorbing thin film is a photovoltaic semiconductor material film.

6. The method according to claim 2, wherein the illumination sources illuminate similar areas on the stack of thin films located on opposite sides of the stack of thin films.

7. The method according to claim 2, wherein the illuminated areas are is at least one of a group of areas consisting of a point, a line, or an area of predetermined size.

8. The method according to claim 2, wherein the thin films parameters are at least one of a group consisting of the thin film thickness (d), thin film refractive index (n), thin film extinction coefficient (k), film surface roughness, energy gap, crystallinity, phase composition, and stoichiometry.

9. A method for accurate determination of optical parameters of each of thin films forming a stack, said method comprising:

- illuminating by a polychromatic illumination a similar area located on a second side of the stack opposite the first area, detecting reflected by the stack illumination, and determining parameters of the thin films measured from the second side; and

- sharing parameters measured at each direction by the two measurements and recalculating parameters of at least one thin film measured from at least one side.

10. The method according to claim 9, wherein detecting the reflected illumination includes detection of at least one of a group consisting of the illumination intensity, illumination wavelength and phase.

1 1. An apparatus for accurate measurement of a stack of thin films parameters, said apparatus comprising:

- a support for a stack of thin films the support enabling to illuminate the stack from a first side and a second side;

- a polychromatic illumination device configured to illuminate the stack of thin films from the first side and a detector configured to detect reflected by the stack of the thin films illumination; - a polychromatic illumination device configured to illuminate a similar area located on a second side of the stack opposite the first area and a detector configured to detect reflected by the stack of the thin films illumination; and a processor configured to receive the value and wavelength of the reflected or transmitted illumination provided by the first and second illumination devices, shares the data, and determine optical parameters of the stack.

12. The apparatus according to claim 1 1 , wherein the processor determines the optical parameters of the stack of thin films from the first and second side and shares the data to recalculate the thin film parameters determined from at least one of the stack sides.

13. The apparatus according to claim 1 1 , wherein the thin films parameters are at least one of a group consisting of the thin film thickness (d), thin film refractive index (n), thin film extinction coefficient (k), surface roughness, energy gap, Crystallinity , phase composition, and film stoichiometry.

14. The apparatus according to claim 1 1 wherein the polychromatic illumination device illuminates an area on the first and on the second sides of the stack being one of a group consisting of a point, a line, or an area.

15. An apparatus for determination of thin films stack parameters, said apparatus comprising: one or more polychromatic illumination sources operative to illuminate corresponding areas located on opposite sides of a thin films stack; one or more illumination detectors operative to detect the illumination reflected by the corresponding areas of the thin films stack; a processor operative to synchronize operation of the illumination sources and the detectors, receive and process the detected illumination and derive optical parameters of the thin films forming the stack.

16. The apparatus according to claim 15, wherein the thin films stack is supported by a support enabling the polychromatic illumination sources to illuminate corresponding areas located on opposite sides of a thin films stack.

17. The apparatus according to claim 16, wherein the support is a mechanism providing a relative movement between the thin films stack and the illumination sources and detectors.

18. The apparatus according to claim 16, wherein the processor is further operative to synchronize the operation of the illumination sources, the detectors and the support movement.

19. The apparatus according to claim 15, wherein the illumination detector is a spectrometer operative to determine the spectral composition of reflected by the corresponding sides of the thin films stack illumination.

20. A method for stack of thin films production process control with accurate determination of optical parameters of each of thin films forming a stack, said method comprising:

- providing a stack of thin films deposited on a transparent substrate;

- illuminating by a polychromatic illumination through the substrate a similar area located on a second side of the stack opposite the first area, detecting reflected by the stack illumination, and determining parameters of the thin films measured from the second side;

- sharing the data of the two measurements and recalculating parameters of at least one thin film measured from at least one side; and wherein the determined parameters of the at least one thin film are used in the thin film production process control.

21. A method for accurate determination of optical parameters of a stack of thin films with at least one of the films being an opaque film, said method comprising:

- illuminating an area of the stack of thin films by a polychromatic illumination from a second side, detecting reflected by the stack illumination, and determining parameters of the thin films measured from the second side;

- sharing parameters measured at each direction between the two measurements and performing mutual correlation of the thin film parameters determined from the both sides.

22. A method for accurate determination of optical parameters of a stack of thin films with at least one of the films being an opaque film, said method comprising:

- - sharing parameters measured at each direction between the two measurements and performing mutual recalculation of the thin film parameters determined; and wherein the mutual recalculation of the determined thin films parameters compensates for the opaque thin film influence .

23. An apparatus for determination of optical parameters of a stack of thin films with at least one of the films being an opaque film, said apparatus comprising:

- one or more polychromatic illumination sources operative to illuminate corresponding areas located on opposite sides of a thin films stack;

- one or more illumination detectors operative to detect the illumination reflected by the corresponding areas of the thin films stack;

- a processor operative to synchronize operation of the illumination sources and the detectors, receive and process the detected illumination, and derive optical parameters of the thin films forming the stack; and wherein the derived thin film parameters are mutually recalculated to compensate for the opaque layer influence .

24. A method for use in quality control of optical parameters of a photovoltaic panel production, said method comprising:

- providing a photovoltaic panel comprising a stack of thin films deposited on a transparent substrate with at least one of the films being an opaque film;

- illuminating an area of the stack of thin films by a polychromatic illumination from the most outer thin film of the stack, detecting reflected by the stack illumination, and determining a set of parameters of the thin films measured from the most outer thin film; - illuminating an area of the stack of thin films by a polychromatic illumination from a side deposited on the transparent substrate, detecting reflected by the stack illumination, and determining parameters of the thin films measured from the transparent substrate side;

- sharing parameters measured at each direction between the two measurements and performing mutual recalculation of the thin film parameters determined; and wherein the mutual recalculation of the determined parameters to compensates for the opaque layer influence.

25. An apparatus for use in the process quality control of multilayer photovoltaic thin film panels production with at least one of the films being an opaque film, said apparatus comprising:

- one or more polychromatic illumination sources configured to illuminate corresponding areas located on opposite sides of a thin films stack;

- one or more illumination detectors configured to detect the illumination reflected by the corresponding areas of the thin films stack;

- a processor configured to synchronize operation of the illumination sources and the detectors, receive and process the detected illumination, and derive optical parameters of the thin films forming the stack; and wherein the optical parameters derived from each of the sides are shared in recalculating parameters of at least one thin film measured from at least one side and compensating for the opaque layer influence .

26. A method for accurate determination of parameters of a stack of thin films, said method comprising:

- providing a stack of thin films having a first and a second side and an optical model enabling to characterize interaction of optical radiation with the stack of thin films by a set of optical and geometrical parameters of each of the thin films of the stack;

- illuminating from a first direction an area on the first side of the stack of thin films by polychromatic illumination and measuring the illumination reflected from the stack and determining parameters of the thin films proximate to the first side; - illuminating from a second direction an area on the second side of the stack of thin films by polychromatic illumination, measuring the reflected from the stack illumination and determining parameters of the thin films proximate to the second side;

- fitting the determined parameters to the optical model and updating the optical model;

- employing the updated optical model and at least one of the measured reflected illumination to recalculate all the earlier determined parameters of the stack.