Method and device for measuring the thickness of thin films even on rough substrates

Abstract

The present invention relates to a method and device for fast and accurate mapping of the thickness of a thin film (10), particularly on a silicon wafer. The method comprises of irradiating the thin film (10) with excitation radiation of at least two wavelengths, wherein a luminescent image is captured during irradiation. In a preferred embodiment, the silicon wafer can move, for example during transport on a belt in a production line. These procedures can be used for online diagnostics of silicon wafer thicknesses in the production of solar cells. Exemplary embodiments include a method and device for obtaining images of an entire silicon wafer and can provide quick feedback for process control if preferably connected to a computing unit.

Claims

1-13. (canceled)

14. A method of measuring the thickness of a thin film of material exhibiting at least partial excitation radiation absorbance or at least partial luminescence radiation absorbance, wherein the thin film is located on a substrate having luminescent properties, the method comprising: a) irradiating the thin film on the substrate by a first source of excitation radiation; and simultaneously, or at least partially simultaneously; b) detecting and recording of luminescence radiation emitted by the substrate in response to irradiation from the first source of excitation radiation; c) irradiating of the thin film on the substrate by a second source of excitation radiation; and simultaneously, or at least partially simultaneously; d) detecting and recording of luminescence radiation emitted by the substrate in response to irradiation from the second source of excitation radiation; e) comparing the recorded luminescence radiation from step (b) and (d), wherein the detecting steps are performed through an optical filter transmitting radiation with a wavelength greater than 870 nm; and the irradiating steps are performed through optical filters transmitting radiation of a wavelength of less than 750 nm; and wherein the comparing includes the step of calibrating the measured data, noise reduction of measured images and thickness calculation using an algorithm based on the Beer-Lambert law.

15. The method according to claim 14, wherein the thin film of material is a thin film of silicon material which is placed on a crystalline silicon wafer, and detected.

16. The method according to claim 15, wherein the thin film of material is a thin film of amorphous silicon material which is placed on a crystalline silicon wafer, and detected.

17. The method according to claim 15, wherein the thin film of material is a thin film of microcrystalline silicon material which is placed on a crystalline silicon wafer, and detected.

18. The method according to claim 15, wherein the thin film of material is a thin film of polycrystalline silicon material which is placed on a crystalline silicon wafer, and detected.

19. The method according to claim 14, wherein the irradiation from the excitation source is performed by LEDs.

20. The method according to claim 19, wherein the first excitation radiation source emits radiation with a mean wavelength of about 465 nm; and the second excitation radiation source emits radiation with a mean wavelength of about 625 nm.

21. The method according to claim 19, wherein the first excitation radiation source emits an optical radiation using blue LED; and the second excitation radiation source emits an optical radiation using red LED.

22. The method according to claim 14, wherein the detection is performed via a GaAs wafer absorbing excitation scattered radiation and solar-control glass which absorbs the infrared component of blue 465 nm and red 625 nm diodes and detected radiation so as not to detect parasitic signals.

23. The method according to claim 14, wherein the detection is performed via a GaAs wafer absorbing excitation scattered radiation and solar-control glass which absorbs the infrared component of blue 465 nm and red 625 nm diodes or detected radiation so as not to detect parasitic signals.

24. The method according to claim 14, wherein the control unit is displaying the calculated thickness and is communicating to the system depositing the thin film.

25. The method according to claim 14, wherein the intensity of excitation radiation is varying.

26. A method of measuring the thickness of a thin film of material exhibiting at least partial excitation radiation absorbance or at least partial luminescence radiation absorbance, wherein the thin film is located on a substrate having luminescent properties using a device comprising: a source of monochromatic excitation radiation capable of emitting electromagnetic radiation of at least two different wavelengths in succession; at least one detector adjustable to detect electromagnetic radiation emitted from the substrate simultaneously, or at least partially simultaneously, with the emission of excitation radiation; wherein the detector is fitted with a filter transmitting electromagnetic radiation at wavelengths exceeding 870 nm; and the excitation radiation sources are provided with filters transmitting radiation at wavelengths of less than 750 nm; and wherein the device further comprises a computing unit storing data on the electromagnetic radiation intensities from the substrate and processing the data so as to be adapted to determine the thickness of the thin film on the basis of the Beer-Lambert law.

27. The method according to claim 26, wherein the first source of excitation radiation is at least two LEDs emitting radiation with a mean wavelength of about 465 nm; and the second source of excitation radiation emits radiation with a mean wavelength of about 625 nm.

28. The method according to claim 26, wherein the device comprises of an excitation radiation intensity modulator.

29. The method according to claim 26, wherein the device comprises of a control unit communicating with the deposition system applying the individual films in such a way that the control unit is able to affect the deposition conditions according to the desired thickness of the thin film.

30. A method for detecting the thickness of thin films of solar cells comprising the method according to claim 26, wherein the thin film is a thin film of amorphous hydrogen-doped silicon placed on a crystalline silicon wafer.

31. A method according to claim 30, wherein the device further comprises of a sliding band on which crystalline silicon wafers with deposited thin films move.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0056] FIG. 1 illustrates an algorithm of the method for determining the thickness of thin films according to the present invention and its preferred embodiments.

[0057] FIG. 2 is a schematic diagram of the device according to the present invention.

[0058] FIG. 3 is a schematic diagram of the device according to the present invention in preferred embodiments.

[0059] FIG. 4 is a schematic diagram of the device according to the present invention in a preferred embodiment focused on an assembly of excitation radiation source, detector and a set of filters.

[0060] FIG. 5 is a detailed drawing of a portion of the assembly according to FIG. 4.

[0061] FIG. 6 shows a measurement record of a photoluminescence image of amorphous silicon bands excited by blue excitation.

[0062] FIG. 7 shows a measurement record of a photoluminescence image of amorphous silicon bands excited by red excitation.

[0063] FIG. 8 shows the resulting representation of amorphous silicon bands using the present invention.

[0064] FIG. 9 is a direct comparison of the results obtained by the present invention (luminescence) method with the results obtained by Raman spectroscopy.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0065] FIG. 1 shows the steps of the method for determining the thickness of thin film 10. The thin film 10 must be of a material capable of at least partially absorbing the excitation radiation or at least partially absorbing the luminescence radiation of the substrate 11. Examples of such materials are thin-film silicon, carbon films, GaAs films or hybrid organic-inorganic perovskites.

[0066] The thin film 10 is placed on the substrate 11. Example of combinations of the thin film 10, substrate 11 and excitation sources 21 and 22 is a silicon wafer, a thin film of amorphous silicon and an LED, with the emission of about 625 nm and with the emission of about 465 nm.

[0067] In the first step of the method according to the invention, the thin film 10 on the substrate 11 is irradiated by the first source 21 of excitation radiation. In the case of a silicon wafer with an amorphous silicon thin film, the first source 21 is suitably selected as a red LED with the emission of about 625 nm. The substrate 11 absorbs excitation radiation and in response emits luminescence radiation which passes through the thin film 10. Luminescence radiation passing through the thin film 10 is detected simultaneously or at least partially simultaneously. The detected luminescence radiation intensity is stored in the computing unit 41. In the next step, the same thin film 10 is irradiated by the second excitation radiation source 22 and again simultaneously, or at least partially simultaneously, the luminescence radiation of the substrate 11 passing through the thin film 10 is detected, the intensity data being stored in the computing unit 41. The radiation detection always passes through the optical filter 3, which is suitably selected with respect to the excitation radiation or the luminescence radiation passing through the thin film 10. Examples of suitable optical filters 3 are e.g., GaAs wafer, layer of hybrid organic-inorganic perovskites, interference filters with suitable edge. The excitation sources 21 and 22 are also provided with filters 321 and 322 so as to transmit radiation with a wavelength of less than 750 nm. The optical filter 3 transmits radiation with a wavelength greater than 870 nm.

[0068] In the preferred embodiment of the method according to the invention, in particular, the silicon thin film 10 is measured on the silicon wafer 11.

[0069] In another preferred embodiment, the first source 21 of excitation radiation is a blue LED. In another embodiment, e.g., an Xe lamp is used as the radiation excitation source 21. Irradiation is performed through the filter 321, more preferably through a set of filters. Examples of individual filters are edge interference filters transmitting a suitable region of the spectrum or colour filters.

[0070] In another preferred embodiment, the excitation radiation intensity can be varied, thereby changing the density of the excited charge carriers in the substrate 11. Based on the modulation of the intensities of sources 21 and 22, it is possible to determine the quantum efficiency of the thin film and thus to predict the solar cell efficiency.

[0071] In the next step of the method according to the invention, the recorded luminescence radiation intensities are compared. The thickness calculation is based on a computer programme using an algorithm based on the Beer-Lambert law. Specifically, the intensities stored in the computing unit 41 are compared at each location of the thin film 10 or they are substituted in a formula according to the Beer-Lambert law (Equation 1) and the thickness of the thin film 10 is calculated.

[0072] To carry out the method according to the present invention, the device shown in FIGS. 2 and 3 is preferably used. FIG. 2 shows the substrate 11 on which the thin film 10 is applied. The thin film 10 was deposited in the deposition system 42 shown in FIG. 3. The thickness of the thin film 10 is determined using the method and device of the present invention.

[0073] Preferably, the substrate 11 is placed on the band 12 which moves with the substrate 11. The device according to the invention further comprises the detection and excitation system 5, comprising at least two excitation radiation sources 21 and 22 emitting radiation of different wavelengths, preferably blue and red LEDs for detecting the thicknesses of the thin films 10 on the silicon wafer. The system 5 further comprises the detector 31, which is located above the filter 3, which transmits luminescence radiation passing through the thin film 10 of the substrate 11. The system 5 is further connected to the computing unit 41 which monitors and stores the intensities detected by the detector 31 and processes the thin film 10 thickness information. The computing unit 41 can also display the film thickness values in real time, which has the advantage of immediately evaluating the quality of the thin film 10 in real operation.

[0074] In the preferred embodiment of the device according to the present invention, the computing unit 41 is further connected to the deposition system 42 and is able to control the deposition conditions of depositing the thin film 10 on the substrate 11 according to the thin film 10 thickness information from the previous measurement. The band 12 is preferably connected to the deposition system 42 so that the substrate 11 with the thin film 10 is measured immediately after the deposition of the thin film 10.

[0075] The excitation radiation sources 21 and 22 are located above the filters 321 and 322, which filter the excitation radiation, the filter transmitting radiation with a wavelength of less than 750 nm. Filters 321 and 322 therefore transmit radiation which is able to excite the charges in the substrate 11. On the other hand, the excitation radiation sources 21 and 22 can also emit radiation of wavelengths in the infrared spectrum, which is the case of e.g., LED sources. Filters 321 and 322 effectively remove components of unwanted infrared radiation originating from sources 21 and 22 and the filter 3 transmits only infrared radiation originating from the substrate 11.

[0076] FIGS. 4 and 5 show the preferred embodiment of the system 5 above the band 12 and below the detector 31. In one embodiment, the filter 3 may be part of the system 5, as described in the following paragraph. The system 5 is oval in shape and consists of the bottom part 51 and the upper part 52. The bottom part 51 comprises filters 321 and 322 in the annulus, which transmit excitation radiation. The filters 321 and 322, together with the sources 21 and 22, are suitably alternately positioned so that the excitation radiation covers the entire area of the thin film 10. The top part 52 includes the excitation and radiation sources 21 and 22, which are located above the respective filters 321 and 322. In the middle of the system 5 there is an empty space for the passage of luminescence radiation coming from the thin film 10, whereby preferably the top part 52 can be provided with the filter 3. Such an embodiment can be preferred in particular with regard to the simple replacement of the detector 31.

[0077] The preferred embodiment of FIGS. 4 and 5 further allows homogeneous illumination of the thin film by excitation radiation, using a number of sources 21 and 22. Four sources 21 and four sources 22 were used in a particular arrangement according to this exemplary embodiment. Each source 21 and 22 was provided with filter 321 or 322, which was placed in the empty space at the bottom part 51 of the system 5.

[0078] The measured image is first denoised using the Gaussian smoothing. The smoothing parameters depend on the optical system of the embodiment. After creating the thickness map, it is possible to perform noise reduction of the measured data using methods based on the Fourier transform.

[0079] For samples with a low level of excited radiation, the measurement sensitivity and accuracy can be increased by detecting multiple images of a single sample irradiated with a single monochrome light. These images are then averaged or combined using a suitable algorithm. For example, using kappa-sigma clipping. Furthermore, the images are treated as images obtained from the sensor.

Experimental Results

[0080] Article (2) discloses the method for measuring the thickness of thin film profiles using Raman spectroscopy. This method is practically unusable industrially due to its time-consuming nature, but it is precisely calibrated and was therefore used to verify the method accuracy according to the present invention.

[0081] In carrying out the invention, the inventors performed the experiment described below. FIG. 9 shows a comparison of the thickness determined by the method of the present invention and by Raman spectroscopy.

[0082] In implementing the measuring device, the method of measuring the thickness of the thin film 10 was performed on a sample of an amorphous silicon film (aSi:H) deposited on the substrate 11 consisting of a crystalline silicon wafer.

[0083] The amorphous silicon thin film 10 was irradiated with blue light as one possible embodiment of the method. The LEDs were placed in a circular location with a hole in the middle to accommodate the detector 31 provided with the filter 3. The experiment avoided the emission of LEDs in the infrared region of light, which can also occur with these sources 21 and 22. This was achieved by using solar-control glass with light transmittance at wavelengths only up to 750 nm. The detector 31 with lens was placed above the hole in the excitation ring and provided with the filter 3. A silicon CCD camera with optimised sensitivity in the NIR part of the spectrum was used as the detector 31. Filter 3 used was a double-sided polished GaAs wafer.

[0084] The diodes emitted excitation radiation over the entire area of the sample with the measured thin film 10. Appropriate placement of light sources 21 and 22 and diffusers ensured uniform illumination of the sample. The red diode emitted light with a mean wavelength of 625 nm, while the blue diode emitted light with a mean wavelength of 465 nm. The choice of these wavelengths is particularly suitable for absorption by the film 10 made of amorphous silicon.

[0085] LEDs were preferably used due to the speed of measurement in the production or inspection process in production halls. Unlike laser spectroscopy, the entire wafer is irradiated and identified at one time. In addition, it is not necessary to aim the laser beam at a given wafer, therefore saving time. The silicon wafer as a whole, or as part of a larger whole, can move along the production line as part of the production process and stop moving at some point, triggering the said measurement method over a given part of the sample.

[0086] During irradiation of the sample with blue light, photoluminescence caused by excitation by blue light was measured using the optical focal plane array 3 (FIG. 6).

[0087] In the experimental setup, detector 31 was equipped with filter 3 transmitting emitted infrared light (wavelength of radiation is above 870 nm).

[0088] The photoluminescence information was stored in the computing unit 41 connected to the detector 31. Photoluminescence was measured with the optical detector 31 sensitive in the near infrared region. At the same time, it was necessary to filter out parasitic light from the visible area of the radiation using the edge filter 3 with a transmittance above 870 nm.

[0089] The wafer was subsequently irradiated with red light and photoluminescence was detected by the same detector 31 as a function of the excitation radiation of the red light (FIG. 7). The information on the photoluminescent response to red radiation was again stored in the computing unit 41.

[0090] Focal plane array capable of detecting photoluminescence (PL) radiation was used for each image. It is possible to use detectors, such as CCD, CMOS, etc. The PL detector 31 can preferably be positioned perpendicular to the wafer. The geometry can be defined by different detector slots or by the defining silicon wafer region from the detector's point of view. Therefore, excitation conditions are created on the moving wafer and photoluminescence radiation emanating from the wafer is detected at steady state.

[0091] FIG. 8 shows the resulting representation of amorphous silicon bands, the record corresponding to the actual thickness of the thin film 10 deposited on the pyramid texture of the surface of the silicon wafer—the substrate 11.

[0092] FIG. 9 shows a comparison of the two methods. The thickness of the thin film 10 was determined independently in two ways. The first was an accurate but slow process of Raman spectroscopy—the bottom diagram in FIG. 9. In the second case, the thickness of the thin film 10 was determined on the basis of the method according to the present invention—the top diagram in FIG. 9. The change in the thicknesses of the thin film 10 depending on the position, i.e., the presence of bands formed during deposition through the shielding mask, can be clearly seen from the record. The record shows the thickness of the thin film 10, which reaches a height of approximately 45 nm. The record also shows that the method according to the present invention can also be used for the detection of thickness up to 10 nm, since the detected noise, and therefore the measurement error caused, reaches several units of nm at most.

[0093] In certain embodiments, the noise can be further removed or computerised by means of suitable computer programmes which advantageously make it possible to determine the thickness of the thin film 10 in several units of nm.

INDUSTRIAL APPLICABILITY

[0094] The present invention presents the method and device for measuring the thickness of thin films. In one embodiment, the invention can be used during a manufacturing process to monitor the film thickness online and the measurement is further used as a guide to control the quality of the manufacturing process.

[0095] In the preferred embodiment, the efficiency of the conversion of light into electrical energy can be optimised by adjusting the thickness of the thin silicon films deposited on the silicon wafer of future solar cells.