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Abstract
The yield of a large-area ultra-thin display panel depends on the realization of designed thickness of multilayer films of all pixels. Measuring the thicknesses of multilayer films of a single pixel is crucial to the accurate manufacture. However, the thinnest layer is reaching the sub-20nm level, and different layers feature remarkable divergence in thickness with similar optical constants. This turns to a key obstruction to the thickness characterization by optical spectroscopy. Based on the tiny differences in absorptivity, a fast method for measuring the film thickness in a single pixel is proposed which combines the layer number reducing model and micro-area differential reflectance spectroscopy. The lower layers can be considered as semi-infinite in the corresponding spectral range whose thickness is infinite in the fitting algorithm. Hence, the thickness of the upper layer is fitted in a simplified layer structure. For demonstration, a multilayer silicon microstructure in a single pixel, p-Si/a-Si/n-Si (10nm/950nm/50nm) on complex substrate, is measured. The light spot diameter is about 60 microns with measuring-time in 2 seconds. The measurement deviation is 3% compared by a commercial ellipsometer. To conclude, the proposed method realizes the layer number reduction for fitting multilayer thickness with large thickness difference and similar optical constants, which provides a powerful approach for multilayer microstructure characterizations.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Si based multilayer films is the basic structure of semiconductor technology which has been successfully applied in the display panel and other fields [1–6]. For manufacturing larger and thinner displays, it is necessary to reduce the thicknesses of the function panels and ensure the effectiveness of large-area manufacturing process. For instance, in the TFT-LCD panel used to control light polarization, the thinnest layer of the multilayer structure in single pixel is reaching sub-20nm in thickness, which is much thinner than other layers. However, the optical constants of the layers are similar due to the silicon doping. Therefore, measuring the thickness of such complex multilayer films by the non-destructive optical methods becomes a challenge. In addition, micro-area measurement and time-consuming for single pixel are important in practical application.
The spectroscopy methods are based on the fitting of optical signal curve of the multilayer sample, such as spectroscopic ellipsometer and reflectometer [7–9]. Ellipsometry is well known in thin film measurement [10–14]. However, due to the oblique incidence of light beam, ellipsometry has natural disadvantage in measuring small samples. Hence, the effective medium approximation (EMA) model method [11], optical lens and “knife edge” technology [13] have been proposed to realize the micro-area measurement. To some degree, the improvement of microscopic capability scarifies the film thickness resolution, measurement efficiency and spectral measurement ability. Reflectance spectroscopy is reliable for measuring the thickness of thin films in micro-area [15–18]. The shortness is the thickness resolution and multilayer identification due to the spectrum quality, fixed incident angle and non-polarization. To this end, differential reflectance spectroscopy (DRS) [19–23] and multi angle optical spectroscopy were developed, as well the usage of polarization cameras [24,25]. From the perspective of reported samples, the periodically stacked multilayers feature that the adjacent layer thicknesses are similar with different optical constants [12,24,25], which is opposite of the multilayer microstructure in the display panel studied in this work.
Among these measurement methods, DRS originated from the detection and analysis requirements of atomic-thick layers, with the resolution of sub-atomic and molecular layer thickness. The simple and easily extended optical structure guarantees the optical signal in high stability and quality [26]. Obviously, to enhance the multilayer measuring ability of DRS makes sense for the silicon multilayer microstructure of the display panel. From the instrumentation, the strategy is to increase the measurement parameters by additional optical information. From the sample aspect, the optical properties of layers are used to select wavelength range for reducing the layer parameters. For instance, the stepwise fitting method for transparent thin films is used in spectroscopy. Firstly, the wavelength range with zero absorbance is selected to fit the film thickness. Then, the fixed thickness is used to fit the optical constants globally in full wavelength range. That is, the absorbance of layer is neglected for fitting thickness at the first step. On the contrary, the layer is opaque and can be considered as semi-infinite at some wavelength range due to the absorption characteristics. It means the thickness of this layer is ignored in this wavelength range and the layers underneath are neglected, too. Hence, the layer number is reduced reasonably. This helps a lot for measuring the few-nanometer layer which has several layers much thicker underneath. In this way, the lower layers are, one by one, considered as semi-infinite in corresponding wavelength range to accurately fit the thickness of upper layers. Hence, the parameter dimension of the multilayer is reduced actually.
In this paper, a dual-path micro-area differential reflectance spectroscopy (M-DRS) based on parameter dimension reduction is proposed for measuring the thickness of doped-silicon multilayer microstructures in the first time. The wavelength range is selected for fitting the upper layer thickness while the layer underneath is considered as semi-infinite according to the absorption characteristics. The lower layers are not fitted which means the reduction of the layer number. By this procedure, the thicknesses of all the layers are fitted out theoretically using the corresponding wavelength ranges one by one. The spectroscopy quality is guaranteed by the M-DRS with a sub-nanometer sensitivity of thickness. For demonstration, a multilayer silicon microstructure, p-Si/a-Si/n-Si/Mo/Al/Mo/glass, is measured the first time. The nominal thickness of the silicon layer is 10nm, 950nm and 50nm, respectively. The light spot size is about 60 microns in diameter with 2 seconds measuring time. The measured thickness is in accordance with the nominal value and testified by a commercial ellipsometer. As a conclusion, we propose a micro-area measurement scheme for multilayer thickness with large thickness difference, similar optical constants and complex layer stacking, which provides a powerful approach for multilayer microstructure characterizations.
2. Instrumentation and measurement principle
The proposed M-DRS is shown in Fig. 1, which is updated from our homemade setup. The spot diameter is about 60 microns, and the wavelength range is 450nm-850nm. The standard deviation of DR signal is less than 0.005 after correction. The reflectance spectroscopy of the sample and reference are collected sequentially by controlling the optical shutters in 2 seconds. The light spot size is realized using a fiber with 50 microns in diameter by matching the focal lengths of the objective lens and the collimator lens. The DR signal errors due to the drift of light source and photodetector are depressed by the short time between measuring the sample and the reference. The separated reference is selectable to meet the sample reflectance flexibly. The symmetrical design reduces the influence of the differences of optical components on DR signals. The light source is HPX2000, Ocean Optics. Two fiber collimators, 74-UV Ocean Optics, are used for the collimation of incident beam and the convergence of reflected beam, respectively. The beam splitter is CCM1-BS013/M, Thorlabs. The objective lens is 20X Nikon CFI 60 TU Plan Epi ELWD, Edmund. The Nikon 200mm tube lens is supplied by Edmund. The camera, Panda 4.2 PCO, is used to check the light spot size on the sample. The spectrometer is QE65pro, Ocean Optics. The optical motorized shutter is self-designed program-controlled chopper.
The DR signal is calculated by (1), which is in the same mathematical forms as the existing DR method. RSam and RRef are the reflectance of the sample and the reference, respectively. Here, the sample is a silicon multilayer modelled by optical multilayer structure, and the reflectance calculation is referred to the Fresnel’s equations [27]. The reference is a flat mirror with known reflectance.
3. Sample
The cross-section of the silicon multilayer of a commercial TFT-LCD panel is shown in Fig. 2(a). From top to bottom are: the three doped-silicon layers, p-Si/a-Si/n-Si; three metal layers, Mo/Al/Mo; and the glass substrate. Obviously, the thicknesses of the Si layers differ a lot, especially the top layer, p-Si, being about 100 times thinner than the a-Si layer. Meanwhile, due to the similarity of the optical constants of the three doped Si layer, it is a challenge to accurately measure the thickness of each layer by optical method. The Mo layer is about 200nm which is semi-finite in the spectral range 400nm-900nm.
Fig. 2. (a) The nominal structure of the silicon multilayer on a metal/glass substrate. (b) The thickness corresponding to the 99.9% absorption of every single silicon layer, and the corresponding extinction coefficients are shown by the inserted curves.
However, for the silicon layers to be measured, the light transmittance trends to be 0 with the increase of thickness. That means, the layer can be considered as semi-infinite for some wavelength range with proper thickness. Fig. 2(b) shows the thickness corresponding to the layer absorption of 99.9% at every wavelength, and the inserted shows the corresponding extinction coefficient curve of every silicon layers. For instance, the 1000nm-thick a-Si can be treated as semi-infinite in the wavelength range 400nm-542nm. The layer model is simplified as p-Si/a-Si for reflectance, being not disturbed by the thickness of a-Si and the lower layers. The thickness of p-Si is accurately fitted in this spectral range and then used in the thickness fitting procedure for other layers. We would mention that, in Fig. 2(b), the layer thickness corresponds to the unidirectional propagation of the beam. The actual reflectance is calculated based on the multilayer optical model. Using this strategy, the layers, one by one, can be regarded as semi-infinite in selected spectral range according to the absorbance. Hence, the dimension of fitting parameters, i.e., the number of layer thickness, is reduced by the segmented spectrum fitting. The layer absorption, Abs, is calculated by (2) referring to Beer’s law [11]. I is the light intensity when the original light I0 travels a distance d, and k is the extinction coefficient of the medium. The extinction coefficients of p-Si, a-Si and n-Si are measured by commercial ellipsometer (RC-2, J. A. Woollam) in the layer structure p-Si/glass, a-Si/glass and n-Si/glass, respectively.
4. Simulation and analysis
The reflectance of the silicon multilayer is calculated since normally the light beam pass through one-layer multi-time and the interface should be taken into account. Fig. 3(a) and (b) show the reflectance when the a-Si and n-Si have ±20% change in thickness. Fig. 3(a) demonstrates the reflectance due to the a-Si, while p-Si and n-Si are kept 10nm and 50nm thick, respectively. Obviously, the reflectance stays unchanged in the spectral range 400nm-540nm while the thickness of a-Si changes from 750nm to 1150nm. To be more specific, shown in Fig. 3(c), the standard deviation (SD) of reflectance of single wavelength shorter than 557nm is less than 0.0015 which is caused by a-Si thickness variation, and is out of the signal sensitivity of our M-DRS setup. Hence, the spectral range 400nm-557nm is used for fitting the thickness of p-Si, the first layer, since the a-Si can be treated as semi-infinite. In this case, the layer structure is simplified to p-Si layer on a-Si substrate instead of the actual structure, p-Si/a-Si/n-Si/Mo.
Fig. 3. Reflectance of multilayer: (a) the thickness of a-Si is from 750 nm to 1150 nm while the p-Si and n-Si is 10 nm and 50 nm in thick, respectively; (b) the thickness of n-Si is from 40 nm to 60 nm while the p-Si and a-Si is 10 nm and 950 nm in thick, respectively. (c) standard deviations of reflectance of every wavelength corresponding to the thickness ranges of (a) and (b); the inserted curves show the zoom-in curves at the wavelength range of 520 nm to 580 nm.
Due to the absorption by n-Si, the third layer, the spectral range of quasi-constant reflectivity is extended. Fig. 3(b) is the reflectance due to the thickness change of n-Si, while p-Si and a-Si are kept 10nm and 950nm thick, respectively. The wavelength is extended to 567nm where the SD is less than 0.0015 according to n-Si change, shown in Fig. 3(c). Theoretically, 557nm-567nm is the spectral range where n-Si is considered as semi-infinite for the multilayer. It can be used for fitting the thickness of a-Si, the second layer, since p-Si has been fitted already. Then, the thickness of n-Si, the third layer, is fitted using the spectral range 567nm-900nm.
However, the range 557nm-567nm is too narrow for thickness fitting which requires ultra-high resolution in wavelength and reflectivity. From the other aspect, in the long wavelength range the reflectance changes differently according to the thickness variation of a-Si and n-Si, shown in Fig. 3(a) and (b). And the SD differs a lot accordingly as shown in Fig. 3(c) which predicts an effective fitting for a-Si and n-Si. Here, the optical constants of the three doped silicon layers are measured by the commercial ellipsometer. The corresponding layer structures for optical constants measurement are p-Si/glass, a-Si/glass and n-Si/glass, respectively, with centimeter scale. The refractive index of Mo is from [28].
Fig. 4 shows the DR spectra using (1) when the reference is a flat mirror. Obviously, in the spectral range 400-550nm the DR signal is due to the thickness of p-Si while a-Si and n-Si are thickness ineffective. A 5nm thickness change of p-Si can be easily detected considering the noise level of our M-DRS setup. For fitting the thickness of p-Si, the layer structure is simplified as p-Si/a-Si in this wavelength range. This means a layer reduction from the actual structure, p-Si/a-Si/n-Si/Mo. Hence, the spectral range 400-550nm can be used for the thickness fitting of p-Si layer. For a-Si and n-Si, the evolution of DR signals is related to the nominal thicknesses, respectively. The thicknesses of a-Si and n-Si are fitted using the DR signals in spectral range 550nm-900nm makes sense.
Fig. 4. Differential reflectance spectra of multi-layers with variable p-Si thickness (a), a-Si thickness (b) and n-Si thickness (c).
5. Results and discussion
The sample is a TFT-LCD panel from the production line. The thicknesses of the three doped silicon layers are measured by M-DRS and testified by ellipsomter and atomic force microscopy. Fig. 5(a) shows the top view of the display panel using a 20X microscope. The light spot of M-DRS, being about 60 microns in diameter, is in the frame of the pixel. Fig. 5(b) is the scanning electron microscopy of the pixel section. The thickness of single silicon layer is indistinguishable and only the total thickness can be recognized as about 1 micron. The adjacent Mo layer is about 200 nm thick. By effective medium approximation (EMA), the roughness of the interface between air and p-Si is described as an EMA layer. And the dielectric constant is calculated by Bruggeman formula with porosity 50%. Considering the thickness of the adjacent Mo layer being about 200nm, the actual stacking order is EMA layer/p-Si/a-Si/n-Si/Mo in the spectral range 400nm-900nm. Levenberg-Marquardt method is exploited for optical spectroscopy fitting [23].
Fig. 5. (a) The top view using a 20X microscope. (b) The scanning electron microscopy of the section. The measured and fitted DR spectra for three single pixels located in three different regions, (c) area-1, (d) area-2 and (e) area-3.
Fig. 5(c)-(e) show the measured and fitted DR spectra of three single pixels located at three different regions, which are the panel center, the edge and the corner being distanced further than 10cm between each other. Based on the strategy of parameter dimension reducing by segmented spectrum fitting, the EMA layer/p-Si are fitted firstly in the spectral range 450nm-550nm, where a-Si is considered as semi-infinite. Secondly, a-Si and n-Si are fitted using the 550nm-850nm spectroscopy, known the thickness of EMA layer/p-Si. Here, the light intensity out of the range 450nm-850nm is much noisier so that the DR signal gets more noise superposition. Hence, the ranges of 450nm-550nm and 550nm-850nm are used actually.
The thicknesses measured by M-DRS are summarized in Table 1, which are in good accordance with the nominal value. And the thickness divergences of different panel region are clearly recognized which depend on the limits of the manufacturing process. A 64-time continuous M-DRS measurement shows that the standard deviation of thickness is about 0.3nm, which means a sub-nanometer sensitivity of layer thickness. The fitted thickness of EMA layer is 0 while the Rq value is about 1nm measured by a commercial atomic force microscopy, Dimension Icon Bruker. The reason is the dielectric constant of EMA layer is between that of p-Si and air, being calculated using 50% porosity. The thickness change of EMA layer causes a tiny shift of DRS in 450nm-550nm spectral range when it is only several nanometers in thick. This DRS evolution is similar with that of the p-Si, so that the thickness of EMA layer is superimposed on p-Si.
Table 1. Thickness of multilayer microstructure measured by M-DRS.
To verify the measurement accuracy of M-DRS, the same silicon multilayer is measured by a commercial ellipsometer, RC-2 J. A. Woollam. The flat region without pixels of the display panel is measured, due to the measuring spot of the ellipsometer is much larger than the pixels. The layer stacking is EMA layer/p-Si/a-Si/n-Si/glass. The measured areas by the two methods are overlapped and the spot size of M-DRS is much smaller. Obviously, as shown in Table 2, the results measured by M-DRS shows great consistency with the commercial ellipsometer, while ellipsometry is more powerful for roughness identifications. Thanks to the separated reference, the stability of M-DRS is guaranteed by the robustness to the spectrum shift due to the light source and detector. Benefitting from the symmetrical optical path, the DR signal errors caused by the light ray bending from the objective lens are drastically cancelled out. Combined with the ability of measuring micro-area, M-DRS provide applicability in characterizing the micro-patterned multilayer with ultra-thin layer on top and complicated stacking order.
Table 2. Comparison of M-DRS and ellipsometry on measuring silicon multilayer.
6. Conclusion
A M-DRS based on parameter dimension reduction is proposed for measuring the thickness of doped-silicon multilayer microstructures. The wavelength range is selected for fitting the upper Nano-layer thickness. In this wavelength range, the layer underneath is thick enough to be considered as semi-infinite and not fitted due to the absorption characteristics. Hence, the layers are accordingly simplified which means the fitting parameters reduction of the layer parameters in the corresponding wavelength range. By this procedure, the thicknesses of all the layers are fitted out theoretically using the corresponding wavelength ranges one by one. The spectroscopy quality is guaranteed by the M-DRS with sub-nanometer sensitivity. For demonstration, single pixel of TFT-LCD panel, stacking layer as p-Si/a-Si/n-Si/Mo/Al/Mo/glass, is measured in the first time. The nominal thicknesses of the silicon layers are 10nm, 950nm and 50nm, respectively. The light spot size is about 60 microns in diameter with 2 seconds measuring time. The measured thickness is in accordance with the nominal value and testified by a commercial ellipsometer.
As a conclusion, we propose a micro-area measurement scheme for multilayer thickness with large thickness difference, similar optical constants and complex layer stacking, which provides a powerful approach for multilayer microstructure characterizations in the fields of manufacturing, such as display panel, photovoltaic panel, LED array, etc.
Funding
National Key Research and Development Program of China (2019YFB2005601, 2017YFF0107003); National Natural Science Foundation of China (61927808).
Disclosures
The authors declare no conflicts of interest.
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