Optical Measurements of Critical Dimensions at Several Stages of the Mask Fabrication Process

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1 Optical Measurements of Critical Dimensions at Several Stages of the Mask Fabrication Process John C. Lam, Alexander Gray n&k Technology, Inc., Santa Clara, CA ABSTRACT Critical dimension (CD) metrology is an essential part of the mask manufacturing process. We present a metrology solution based on broadband reflectometry, covering a wavelength range from 190 to 1000 nm, in one nanometer intervals. The analysis is performed using Forouhi-Bloomer dispersion equations, in conjunctions with Rigorous Coupled Wave Analysis (RCWA). The method provides accurate and repeatable results for critical dimensions, thickness, and optical properties (n and k spectra from nm) for all materials present in the structure. In terms of throughput (several seconds per point) and suitability for integration, the method has many advantages over conventional metrology techniques. Measurements were performed on two masks, at two different stages of the mask manufacturing process After Etch Inspection (AEI) and After Strip Inspection (ASI). CD uniformity distribution maps at 121 points on the mask were obtained for 800 nm pitch grating arrays. The results were compared to conventional CD-SEM measurements collected at the same locations. A linearity study was conducted on 760 and 1120 nm pitch grating arrays with systematically increasing CD width. The results demonstrate excellent correlation with CD-SEM. Keywords: Optical metrology, critical dimension, broadband reflectometry, Forouhi-Bloomer dispersion equations, RCWA, CD uniformity, CD linearity 1. INTRODUCTION Manufacturing of an attenuated phase shift mask is a multiple-step process. At various stages of this manufacturing process, a photomask can consist of one or several thin film layers and trench structures. The typical films involved in the process are molybdenum silicide (MoSi), which is used as a phase shifting material; chromium and chromium oxide alloy, used as a hard mask; and photoresist, used for the patterning of the mask. The fabrication of a production-worthy phase shift mask requires, among other things, excellent uniformity of critical dimensions (trench width and depth) and optical properties of the phase shift material (MoSi). Traditionally, CD-SEM has been the instrument of choice for the measurement of width; atomic force profilometer (AFP) or conventional profilometer for the measurement of depth; and interferometer for the measurement of phase shift and transmittance of the phase shift material. Each one of these techniques has its drawbacks and limitations. CD-SEM, for instance, can only determine trench width at certain stages of the manufacturing process. Due to severe charging, the measurement becomes impossible for photoresist-on-chrome structures and quartz-only trench structures. AFP measurements are highly localized, and limited by the feature size. Interferometer measurements are only possible for patterned masks without chromium/chromium oxide, and require special test features. Furthermore, all of these techniques suffer a common drawback low throughput. We present a metrology method, based on broadband reflectometry, in conjunction with Forouhi-Bloomer dispersion equations and RCWA for measuring thickness and optical properties of thin films, as well as critical dimensions of trench structures. In terms of throughput (several seconds per point) and suitability for integration, the method has many advantages over conventional metrology techniques. Metrology, Inspection, and Process Control for Microlithography XX, edited by Chas N. Archie, Proc. of SPIE Vol. 6152, 61523C, (2006) X/06/$15 doi: / Proc. of SPIE Vol C-1

2 2. MEASUREMENT METHOD The instrument used for the measurement is a broadband reflectometer, used to collect continuous reflectance and transmittance spectra in the deep-uv through the near-ir wavelength range ( nm) in 1 nm intervals. The light source in the spectrophotometer is equipped with a rotating polarizer, which facilitates TE and TM polarization for the measurement beam. Hence, both reflectance and transmittance spectra can be obtained in two polarization modes (R s and R p, T s and T p ). In general, the presented method can be used with only one spectrophotometer (reflectance only) and with or without a polarizer. Addition of a transmittance spectrophotometer (only useful for samples on transparent substrates) and a polarizer facilitates extra raw data, which helps to constrain the calculation model and provide more stable and accurate results. Measured raw data (reflectance and transmittance) is analyzed using the Forouhi-Bloomer dispersion equations, in conjunction with RCWA, to extract the values of n and k, film thickness, and trench dimensions. The analysis model generates calculated reflectance and transmittance spectra using the nominal parameters, and then optimizes the values, using nonlinear regression analysis in order to obtain the best match between the measured and calculated spectra. Each parameter in the model can be either varied or fixed at a known value. Generally, it is not practical to vary the optical properties of the films present in the grating structure. In most cases n and k of the films can be obtained by collecting the measurements on the blanket areas of the sample. The determined optical properties then can be used in the model for the trench structure, reducing the number of variables in the model. In the case of the chromium, photoresist, and the phase shift materials (MoSi), the n and k spectra were pre-measured from the uniform area and then fixed during the CD measurement. 3. APPLICATION TO CRITICAL DIMENSIONS In the current study, the method described above was used to examine photomasks at two stages of mask manufacturing process: After Etch Inspection (AEI) and After Strip Inspection (ASI). Typical AEI mask consists of two thin film layers MoSi at the bottom and oxidized chromium at the top deposited on a fused silica substrate, with a pattern etched through both layer and into the substrate. The parameter of interest at this stage of the process is the width of trench etched in chromium. Typical ASI mask consists of one layer of phase shift material (MoSi) exposed as the result of chromium layer being stripped off the mask, with a pattern etched through the layer and into the substrate. The parameter of interest at this stage of the process is the width of trench etched in MoSi. After Etch Inspection (AEI) After Strip Inspection (ASI) CrOx Cr MoSi MoSi Quartz Quartz Measured Parameters: n and k spectra ( nm) for quartz, MoSi, Cr and CrO x. Thickness of MoSi, Cr and CrO x. Trench width. Measured Parameters: n and k spectra ( nm) for quartz and MoSi. Thickness of MoSi. Trench width. Proc. of SPIE Vol C-2

3 4. GLOBAL CD UNIFORMITY STUDY Both reticles were measured at 121 locations across the mask (11 by 11 map) in order to examine uniformity across the sample. Both TE and TM polarized light were used to collect the spectra of reflectance between 190 and 1000 nm at near-normal incidence. Obtained TE and TM reflectance spectra were then analyzed simultaneously using RCWA method in conjunction with Forouhi-Bloomer dispersion equations. The two measured features were µm 2 grating arrays of 800 nm pitch. Grating arrays 1 and 2 shared the same pitch (800 nm) but varied in orientation by 90 degrees. Hence, the capability of the tool to measure grating structures with the plane of incidence perpendicular and parallel to the orientation of the grating was examined. Grating Array 1 (Horizontal) Grating Array 2 (Vertical) Pitch = 800 nm Line/Space = 1:1 Pitch = 800 nm Line/Space = 1:1 The analysis of the results produced maps of thicknesses of all layers and trench width. In addition to that, spectra of index of refraction (n) and extinction coefficient (k) in nm wavelength range were obtained for all materials in the structure. For ASI mask, close examination revealed that the optical properties of MoSi (n and k) exhibit variation across the mask. In fact, good correlation with CD-SEM for trench width was only possible to obtain when this variation was taken into account in the calculation model. The resultant uniformity maps were compared to the uniformity distributions obtained with a CD-SEM on the same samples. The comparison charts for 800 nm pitch horizontal grating arrays are presented in Figures 1 through 4. Uniformity distributions for the vertical gratings exhibit comparable similarity. A 1 to 15 nm offset between the values of line width obtained using n&k Analyzer and CD-SEM is typical for structures of various pitch and configuration. In general, some kind of offset ( bias ) is expected when two metrology techniques are compared to each other. These pitch and material dependent biases have been observed and reported in the literature. Proc. of SPIE Vol C-3

4 5. CD LINEARITY STUDY For the purpose of the linearity study, measurements were collected at 88 locations on each mask. Measurements were taken at the center and at the edge of the mask. In order to examine the capabilities of the method to measure gratings of various pitches and orientations, at each location, a set of measurements was taken on gratings with horizontal orientation and vertical orientation. For each orientation, features of 760 nm pitch and 1120 nm pitch were measured. For Pitch = 760 nm, lines with nominal width of 310, 312, 314, 316, 318, 320, 322, 324, 326, 328 and 330 nm were measured. For Pitch = 1120 nm, lines with nominal width of 390, 392, 394, 396, 398, 400, 402, 404, 406, 408 and 410 nm were measured. The following diagram illustrates the measured pitches and nominal line widths. Figures 5 through 8 show the correlation between the measurements collected using n&k Analyzer and CD-SEM. Features Measured for CD Linearity Study (Total, 88 per mask) Pitch = 760 nm Center of Edge of 310 nm 312 nm 314 nm 316 nm 318 nm 320 nm 322 nm 324 nm 326 nm 328 nm 330 nm Pitch = 1120 nm Center of Edge of 390 nm 392 nm 394 nm 396 nm 398 nm 400 nm 402 nm 404 nm 406 nm 408 nm 410 nm Proc. of SPIE Vol C-4

5 6. SUMMARY Line gratings of various pitches, line widths and orientations were measured using an n&k Analyzer a broadband reflectometer, capable of measuring TE and TM polarized reflectance and transmittance spectra between 190 and 1000 nm. The obtained spectra were analyzed using Rigorous Coupled Wave Analysis method, in conjunction with the Forouhi-Bloomer dispersion equations for n and k. Optical properties (n and k spectra from 190 to 1000 nm) of quartz, molybdenum silicide, chromium and chromium oxide, as well as thicknesses of all layers and critical dimensions of trench structures were obtained as the result of the analysis. The results were compared with the measurements taken on the same samples using conventional CD-SEM. Two comparison studies were conducted global CD uniformity and CD linearity. The CD linearity study demonstrated excellent correlation between the values of grating line width obtained using the n&k Analyzer and a CD-SEM for the grating structures of two pitches (760 nm and 1120 nm) and two orientations (perpendicular and parallel to the plane of incidence). The global CD uniformity study revealed that the n&k Analyzer can be used to produce CD uniformity maps which demonstrated excellent correlation between the results obtained using a conventional CD-SEM. The advantages of the optical method are high throughput, non-destructive nature of the measurements and capability to measure a wider variety of structures pertinent to the photomask manufacturing process. Acknowledgements We would like to thank Johnson Hung and W.C. Wang of the Taiwan Semiconductor Manufacturing Company (TSMC) for providing the samples and CD-SEM measurement results. We would also like to thank George Li of n&k Technology for supervising the measurements and analysis of the samples. Proc. of SPIE Vol C-5

Global CD Uniformity Measurement Results and CD-SEM Comparison n&k Analyzer ASI Mask CD (Pitch = 800 nm)

0 nm 3σ Standard Dev.: 3.5 nm Figure 1: CD uniformity measured using n&k Analyzer. Maximum Width: 406.

n&k Analyzer AEI Mask CD (Pitch = 800 nm) CD-SEM : rex: : : : : : : -6!"" c9.e459 0-4 -2- -2- eob.

8 nm 407.8 nm 3.7 nm Figure 3: CD uniformity measured using n&k Analyzer.

6 Global CD Uniformity Measurement Results and CD-SEM Comparison n&k Analyzer ASI Mask CD (Pitch = 800 nm) CD-SEM t H b a -è - (cm) Maximum Width: nm Minimum Width: nm nm 3σ Standard Dev.: 3.5 nm Figure 1: CD uniformity measured using n&k Analyzer. Maximum Width: nm Minimum Width: nm nm 3σ Standard Dev.: 3.5 nm Figure 2: CD uniformity measured using CD-SEM. n&k Analyzer AEI Mask CD (Pitch = 800 nm) CD-SEM : rex: : : : : : : -6!"" c9.e eob.ozc SI I I I I I I I -S Maximum Width: Minimum Width: 3σ Standard Dev.: (cm) nm nm nm 3.7 nm Figure 3: CD uniformity measured using n&k Analyzer {cnl} Maximum Width: nm Minimum Width: nm nm 3σ Standard Dev.: 4.5 nm Figure 4: CD uniformity measured using CD-SEM. Proc. of SPIE Vol C-6

7 CD Linearity Measurement Results and CD-SEM Comparison AEI Mask (Pitch = 760 nm) ASI Line Width (Pitch = 760 nm, Center) ASI Line Width (Pitch = 760 nm, Edge) y = x 4.8 R 2 = y = x 4.8 R 2 = n&k Analyzer (nm) n&k Analyzer (nm) CD-SEM (nm) CD-SEM (nm) Figures 5 and 6: Correlation between measurements using CD-SEM and n&k Analyzer on ASI mask. AEI Mask (Pitch = 1120 nm) ASI Line Width (Pitch = 1120nm, Center) ASI Line Width (Pitch = 1120 nm, Edge) OCD Result (nm) y = x R 2 = OCD Result (nm) y = 0.997x R 2 = SEM Result (nm) SEM Result (nm) Figures 7 and 8: Correlation between measurements using CD-SEM and n&k Analyzer on AEI mask. Proc. of SPIE Vol C-7

Fast Non-destructive Optical Measurements of Critical Dimension Uniformity and Linearity on AEI and ASI Phase-shift Masks

Fast Non-destructive Optical Measurements of Critical Dimension Uniformity and Linearity on AI and ASI Phase-shift Masks Alexander Gray University of California at Davis, CA John C. Lam n&k Technology,