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, Inc., Santa Clara, CA ABSTRACT 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-SM has been the instrument of choice for the measurement of width; AFP (Atomic Force Profilometer) or conventional profilometer for the measurement of depth; and Interferometer for the measurement of phase shift and transmittance of the phase shift material. We present an innovative optical 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 190 1000 nm) for all materials present in the structure. In the current study, the method described above was used to examine photomasks at two stages of mask manufacturing process: After tch Inspection (AI) and After Strip Inspection (ASI). The results were compared with the measurements taken on the same samples using conventional CD-SM. 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 this new optical reflectometry approach and a CD-SM for the grating structures of two pitches (760 nm and 1120 nm). The global CD uniformity study revealed that this presented reflectometry method can be used to produce CD uniformity maps which demonstrate excellent correlation with the results obtained using a conventional CD-SM. 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. Keywords: Optical metrology, critical dimensions, broadband reflectometry, Forouhi-Bloomer dispersion equations, RCWA, CD linearity. I. INTRODUCTION In order to insure quality of the product, photomask manufacturers of today are seeking new ways to reliably measure critical dimensions on their masks. Conventional techniques, such as CD-SM and AFM are being replaced by faster, non-destructive optical instruments, and new metrological methods are being developed and refined continuously. Measurements of critical dimensions is important on every stage of the mask manufacturing process starting with the measurement of the photoresist lines on chrome and ending with the measurement of the final pellicalized reticle. 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). Characterization of the CD and phase shift uniformity distribution on a photomask requires a multiple-point measurement across the photomask surface. This type of measurement may take hours on a CD-SM. Moreover, at some stages of the process, this measurement is altogether impossible on a CD-SM due to the severe charging of the sample. Metrology method we present here is capable of measuring critical dimensions and phase shift at every stage of the mask manufacturing process. The technique is based on the simultaneous measurement of broadband reflectance and Photomask Technology 2006, edited by Patrick M. Martin, Robert J. Naber, Proc. of SPI Vol. 6349, 63491O, (2006) 0277-786X/06/$15 doi: 10.1117/12.686150 Proc. of SPI Vol. 6349 63491O-1
transmittance spectra, covering a wavelength range from 190 to 1000 nm, in one nanometer intervals. The analyses of both types of spectra are performed simultaneously, using the Forouhi-Bloomer dispersion equations, in conjunctions with the Rigorous Coupled Wave Analysis (RCWA). The method provides accurate and repeatable results for critical dimensions, thickness, and optical properties (n and k spectra from 190 1000 nm) for all materials present in the structure. II. MASURMNT MTHOD In the current study, all the measurements were performed using a spectrophotometer-based instrument (n&k R-T Scatterometer), capable of collecting continuous broadband reflectance and transmittance spectra in the range between 190 and 1000 nm in one nanometer intervals. The light source was equipped with a rotating polarizer, which facilitates T and TM polarizations of the incident light. As a result, four spectra (R s, R p, T s and T p ) can be obtained during one measurement of the same location. The measurement spot diameter is 35 microns. For this particular study, transmittance spectra were used strictly for the characterization of the optical properties of the materials used in the film structure. The data was analyzed using the Forouhi-Bloomer dispersion relations in conjunction with the Rigorous Coupled Waves Analysis (RCWA). The two polarized reflectance spectra collected at each site were analyzed using a nonlinear regression algorithm in order to obtain critical dimensions, thickness, and optical properties (n and k spectra from 190 1000 nm) for all materials present in the structure. Generally, the method described above can be used to simultaneously analyze any combination of polarized and/or unpolarized reflectance and transmittance spectra. Analysis of several spectra at the same time is generally a lengthier process; however, it facilitates more stable and unambiguous results. The following figure depicts a typical fit between four measured spectra and four calculated spectra, generated by the analysis software. Rs & Rp / Ts & Tp (%) 100 80 60 40 20 Rs-exp Rs-cal Rp-exp Rp-cal Ts-exp Ts-cal Tp-exp Tp-cal 0 200 300 400 500 600 700 800 900 1000 Wavelength (nm) Figure 1: Typical fit between four measured spectra (R s -exp, R p -exp, T s -exp, T p -exp) and their calculated counterparts (R s -cal, R p -cal, T s -cal, T p -cal). The method can also be used to determine a detailed profile of a periodic grating structure. The following figure illustrates an example of such analysis applied to the characterization of the profile of the photoresist lines on chrome (ADI structure). The cross section of the same grating structure obtained using SM is displayed on the right. Proc. of SPI Vol. 6349 63491O-2
Jwnw Figure 2: Typical ADI trench profile obtained using the technique described above. III. MASURMNT OF AI AND ASI PHAS SHIFT MASKS The method described above was applied to the measurement of CD uniformity and linearity on the After tch Inspection (AI) and After Strip Inspection (ASI) phase shift photomasks. The samples were generously provided by the Taiwan Semiconductor Manufacturing Company. The following grating structures were measured: AFT Mask Cross Section I I Cr I AST Mask Cross Section Quaitz Quaitz Figure 3: Schematic drawing of the cross section of measured AI and ASI trench structures. For the global CD uniformity study, each sample was measured at 121 locations across the mask. At each location an 800 nm pitch grating array was measured with the purpose of obtaining trench depth, line width and thickness of all films present in the structure. The following figure depicts global CD uniformity comparison between n&k R-T Scatterometer and CD-SM for the AI photomask. Proc. of SPI Vol. 6349 63491O-3
n&k R-T Scatterometer AI Mask CD (Pitch 800 nm) CD-SM " + 1-4 + 1- + + + 1-4 + + 1-4 62O24H 64 (cm) Maximum Width: 411.4 nfl' Maximum Width: 411.6_nfl' Minimum Width: 404.8 nfl Minimum_Width: 403.9_mn Mean Width: 407.8 mn Mean_Width: 407.3_nm 3o Standard Dev.: 3.7 nm 3o Standard Dev.: 4.5 nfl' Figure 4: Global AI CD uniformity comparison between n&k R-T Scatterometer and CD-SM. ((n) The following figure depicts global CD uniformity comparison between n&k R-T Scatterometer and CD-SM for the ASI photomask. Figure 5: Global ASI CD uniformity comparison between n&k R-T Scatterometer and CD-SM. For the purpose of the linearity study, 760 and 1120 nm pitch gratings were measured at the center and at the edge of each mask. For 760 nm pitch gratings, at each site 11 grating arrays with nominal line widths of 310, 312, 314, 316, Proc. of SPI Vol. 6349 63491O-4
318, 320, 322, 324, 326, 328 and 330 nanometers were measured. For 1120 nm pitch gratings, at each site 11 grating arrays with nominal line widths of 390, 392, 394, 396, 398, 400, 402, 404, 406, 408 and 410 nanometers were measured. The results were compared to the measurements obtained using a conventional CD-SM. The following plots illustrate typical correlation between the n&k R-T Scatterometer and the CD-SM. AI Mask CD Linearity (Pitch 1120 mn) (Center) AI Mask CD Linearity (Pitch 1120 mn) (dge) 430.0 I I.5. S c 1.0031x 1- C.997x + 12.97 12.97 425.0 0.9892 R10.9936.' V. 5 5 r 410.0 420.0 5, 2 2 405.0 415.0 5' 410.0 /5 0-395.0 I I I I I 405.0 I I I I I 390.0 385.0 390.0 395.0 400.0 405.0 410.0 390.0 395.0 400.0 405.0 410.0 415.0 420.0 CD-SM Result (nni) CD-SM Result (nni) r Figure 6: Correlation between measurements using CD-SM and n&k R-T Scatterometer on AI mask. 330.0 a 5 325.0 5 5 320.0 315.0 (F) 335.0 310.0 305.0 300.0 ASI Mask CD Linearily (Pitch 760 nm) (Center) V. y 1.0099x + 48 0.9728........, 4 /..4 305.0 310.0 315.0 320.0 CD-SM Result (nni) 5 0 335.0 (F) 355.0 350.0 a 5, 345.0 340.0 330.0 ASI Mask CD Linearily (Pitch 760 nm) (dge).,, 0.9908x +4.8 R20.9716 '.4 325.0 325.0 330.0 328.0 325.0 330.0 335.0 340.0 345.0 350.0 CD-SM Result (nni) '4 p 4 Figure 7: Correlation between measurements using CD-SM and n&k R-T Scatterometer on ASI mask. V. APPLICATION TO TH MASURMNT OF PLLICALIZD PHOTOMASKS Modern-day photomasks are usually pellicalized at the end of the mask manufacturing process. A pellicle is a thin protective layer, mounted over the surface of the mask with help of an aluminium frame. The function of the pellicle is to protect the mask surface from contamination. The particles which are stuck to the pellicle have no effect on the wafer image, since they appear significantly out of focus. A prospect of measuring critical dimensions on a pellicalized mask is very tempting, since that way the exposure of the actual mask to particles is minimized. It is, however, very challenging, since the measurement has to be done through the pellicle. Obviously, any type of conventional measurement (AFM, CD-SM, etc.) would be impossible only an optical measurement could be accomplished through the pellicle without damaging the membrane. Proc. of SPI Vol. 6349 63491O-5
Several attempts have been made to use scatterometry-base techniques to measure pellicalized masks, however the intensity of the light reflected off the mask surface and through the membrane is too low for a robust measurement. Therefore, a transmittance-based technique has to be used for an effective CD measurement. Typical polarized transmittance spectra (T s and T p ) obtained on a grating array measured through a pellicle are presented in the figure below. 100 80 a 60 ob I 40 20 0 200 300 400 500 600 700 800 900 1000 Wavelength (nm) Figure 8: Typical polarized transmittance spectra (T s and T p ) obtained on a grating array measured through a pellicle. The most important question raised by the manufacturers who want to be able to use the optical technique for the measurement of pellicalized mask is: How does the pellicle affect the measurement, and how is the measurement obtained through a pellicle compare to the conventional measurement without a pellicle? The figures below illustrate the comparison between the results obtained on the same ASI mask with and without a pellicle. The measurement was performed at 25 locations on the mask, and at each location 5 gratings of various pitches and critical dimensions were measured. ASI Measurement through Pellicle ASI Measurement without Pellicle 0.370 0.370 Line Width Line CD (in (micron) microns) 0.350 0.330 0.310 0.290 0.270 Line Width Line CD (in (micron) microns) 0.350 0.330 0.310 0.290 0.270 P190 P200 P210 P220 P250 0.250 0 5 10 15 20 25 Site Index Number Site Index Number 0.250 0 5 10 15 20 25 Site Site Index Index Number Figure 9: Comparison between the results obtained on the same ASI mask with and without a pellicle. Features P190, P200, P210, P220 and P250 correspond to gratings with pitches 760, 800, 840, 880 and 1000 nm. It is evident that with an appropriate analysis recipe adjustment, the measurements through the pellicle match the results obtained without the pellicle. VI. SUMMARY A fast non-destructive optical metrology method was applied to the measurement of AI and ASI phase shift photomasks. Grating structures of various pitched and critical dimensions were measured using a spectrophotometerbased instrument (n&k R-T Scatterometer) and analyzed using the Forouhi-Bloomer dispersion relations, in conjunction Proc. of SPI Vol. 6349 63491O-6
with RCWA, to extract the values of n and k, film thickness, and trench dimensions. The obtained results demonstrate excellent correlation with CD-SM measurements of the same samples. The same method was used to measure critical dimensions of a pellicalized ASI photomask. The obtained results demonstrate excellent correlation with the results obtained on the same mask with the pellicle removed. The described metrology solution has the advantages of high throughput and non-destructive nature over conventional metrology techniques, such as AFM or CD-SM. The instrument has the capability of measuring a wide variety of structures pertinent to the photomask manufacturing process and can be easily integrated with other tools. 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-SM 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 SPI Vol. 6349 63491O-7