Photomask

Publications & Papers

Effect of Cleaning on EUV Masks

Source: SPIE Advanced Lithography 2013 – Extreme Ultraviolet (EUV) Lithography IV
Author: Jeffery W. Roberts (n&k technology)
Abstract: A major difference between euv lithography and its conventional optical predecessors is the lack of a usable pellicle. No material has been found that is both transmissive at the euv lithography wavelength of 13.5nm and that has sufficient structural integrity to serve as a pellicle. Therefore, during exposure to euv light the mask is unprotected and accretes particles. This necessitates repeated cleanings of the mask, which raises concerns about changes in the mask induced by the cleaning process. It is only through metrology that these concerns can be addressed. This paper describes an optical characterization method to determine the effect of cleaning an euv mask.

 


Phase Shift Mask Etch Process Development Utilizing a Scatterometry-based Metrology Tool

Source: Photomask Conference 2008, Japan
Authors: Jason Plumhoff (Oerlikon), Alexander Gray (n&k Technology)
Abstract: Phase, along with defect levels and CD, must be closely monitored on 45nm technology node masks. The final phase shift of a mask is highly dependent on the ability of the etch tool to stop at precisely the correct depth. Developing etch processes and endpoint recipes for successful phase shift processing depends on rapid and accurate measurement of etch depth. In many mask shops, these measurements are made by either direct phase measurement tools or atomic force microscopes (AFM). These tools have relatively low throughput. In the case of the direct phase measurement tool, the large measurement spot size precludes the measurement of the small features most interesting to mask makers. A need exists for a relatively fast measurement tool that can be applied to features <1μm in size.

As part of Oerlikon USA’s continuous etch process improvement efforts, the etch depth measurement capabilities of a scatterometry based metrology tool were explored. Phase shift masks (one EAPSM, one AAPSM) were created to act as standards for our experiments. Regions of each mask were etched to various depths using an Oerlikon Mask Etcher system, and then measured with both a commercial AFM and an n&k Technology 1700-RT scatterometry tool. Using this data, recipes capable of measuring quartz trench features, partially-etched MoSi trench features, and bulk MoSi films were developed on the n&k 1700-RT. Phase uniformity data taken from actual etch experiments will be provided, as well as data showing the repeatability of each system, and a comparison of the relative measurement times.

 


Measuring Contact Hole Rounding Uniformity Using Optical Scatterometry

Source: European Mask and Lithography Conference, 2008
Authors: Jan Richter (Advanced Mask Technology Center), John C. Lam, Alexander Gray, Stanley Chen (n&k Technology)
Abstract: Uniformity distribution of the corner rounding radius of curvature is investigated using reflectance and transmittance based optical scatterometry. Arrays of square contact holes are measured at multiple locations on an ACI photomask using a broadband spectrophotometer capable of collecting polarized reflectance (Rs and Rp) and polarized transmittance (Ts and Tp) spectra in 190 ­1000 nm wavelength range in one-nanometer intervals. The measured spectra are analyzed using two-dimensional Rigorous Coupled-Wave Analysis algorithm (2D RCWA) in conjunction with the Forouhi-Bloomer dispersion relations for n and k. As a result of the analysis, the values of contact hole width and the radius of curvature associated with the corner rounding are determined at every measurement location. The measurements are presented as uniformity distribution maps and correlation plots, comparing the results with the values obtained using a conventional CD-SEM.

 


Measurements of Corner Rounding in 2D Contact Holes on Phase-Shift Masks Using Broadband Reflectance and Transmittance Spectra in Conjunction with RCWA

Source: SPIE Photomask technology, September 2007, California, USA
Authors: Alexander Gray, John C. Lam and Stanley Chen of n&k Technology and Jan Richter of Advanced Mask Technology Center, Germany
Abstract: For the first time, Rigorous Coupled-Wave Analysis (RCWA) is used for the analysis of both polarized broadband reflectance and transmittance spectra with the purpose of measuring the degree of corner rounding in 2D contact holes. The use of transmittance spectra proves to be advantageous for the characterization of the shape of the contact holes. In contrast with the conventional reflectance-only techniques, transmittance measurements prove to be more sensitive to the angstrom-level variations in the shape of the contact hole. Therefore, the new technique is capable of accurately determining the degree of rounding of the contact hole corners and characterizing a variety of shapes – from perfectly round to perfectly square. Additionally, the high intensity of the transmitted spectra improves the signal-to-noise ratio and guarantees better repeatability of the results.

For the current study, 2D arrays of square contact holes with 800 nm pitch are measured on an After Clean Inspection (ACI) phase-shift mask, using a spectrophotometer-based instrument capable of collecting four continuous spectra during one measurement – two polarized reflectance spectra (Rs and Rp) and two polarized transmittance spectra (Ts and Tp). The measured spectra are analyzed using the Forouhi-Bloomer dispersion equations, in conjunction with RCWA. The method provides accurate and repeatable results for the degree of corner rounding of the square contact holes. In addition, the method provides trench depth, critical dimensions, film thickness, and optical properties (n and k spectra from 190 – 1000 nm) of phase-shift photomasks. The results of the measurements are represented as high-resolution uniformity maps obtained for all the parameters mentioned above. The results show excellent correlation with conventional CD metrology techniques.

 


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

Source: SPIE Photomask technology, September 2007, California, USA
Authors: Alexander Gray and John C. Lam of n&k Technology
Abstract: Polarized broadband transmittance (T) plus reflectance (R) measurements, combined with the Rigorous Coupled-Wave Analysis (RCWA) and the Forouhi-Bloomer dispersion equations for n and k, were used to measure 1D grating on AEI and ASI Phase-Shift Masks.

1D array of various pitches were measured on an AEI and ASI phase-shift mask, using a spectrophotometer-based instrument, capable of collecting four continuous spectra during one measurement –Rs, Rp and Ts, Tp. The global CD results of AEI and ASI phase-Shift masks show an excellent correlation with conventional CD-SEM results. Novel Transmittance-based RCWA analysis technique was applied to the measurements of critical dimensions on ACI photomask. Finally, the new Transmittance-based technique was applied to the CD measurements on pellicalized ASI photomask.

 


Polarized Transmittance-Reflectance Scatterometry Measurements of 2D Trench Dimensions on Phase-Shift Masks

Source: SPIE Conference, May 2007
Authors: John C. Lam, Alexander Gray, Rafael Howell and Stanley Chen of n&k Technology
Abstract: Polarized broadband transmittance (T) plus reflectance (R) measurements, combined with the Rigorous Coupled-Wave Analysis (RCWA) and the Forouhi-Bloomer dispersion equations for n and k were used to measure 2D trench dimensions.

2D arrays of square and circular contact holes of various pitches were measured on an After-Clean-Inspection (ACI) phase-shift mask, using a spectrophotometer-based instrument, capable of collecting four continuous spectra during one measurement –Rs , Rp and Ts, Tp. The measured spectra were analyzed using the Forouhi-Bloomer dispersion equations, in conjunction with RCWA algorithm, applied simultaneously to R and T polarized spectra. The method provided accurate and repeatable results for contact hole depths, critical dimensions film thicknesses and n and k spectra. High-resolution uniformity maps were obtained for all the parameters mentioned above.

 


Innovative Application of the RCWA Method for the Ultra-Sensitive Transmittance-Based CD Measurements on Phase-Shift Masks

Source: European Mask and Lithography Conference, January, 2007, Grenoble, France
Authors: Alexander Gray, John C. Lam and Stanley Chen of n&k Technology
Abstract: For the first time Rigorous Coupled Wave Analysis (RCWA) algorithm is applied to the analysis of the transmittance spectra for the determination of trench depths, critical dimensions, profiles, film thicknesses, and optical properties (n and k spectra from 190 – 1000 nm) of phase-shift photomasks.

A spectrophotometer-based instrument (n&k R-T Scatterometer) was used, capable of collecting four continuous spectra during one measurement – two polarized reflectance spectra (Rs and Rp) and two polarized transmittance spectra (Ts and Tp). The light source of the spectrophotometer was equipped with a rotating polarizer, facilitating TE and TM polarizations of the measurement beam. The analysis was performed using Forouhi-Bloomer dispersion equations, in conjunctions with RCWA algorithm, applied simultaneously to reflectance and transmittance spectra. The method provided accurate and repeatable results of above stated parameters, for all materials present in the structure.

Inclusion of the transmittance measurements proved to be advantageous, because there is a greater sensitivity of the T spectra to the sub-nanometer structural and/or material variations, which are difficult to detect with R-only measurements. Furthermore, the intensity of T is much higher than the intensity of R, resulting in a much improved signal-to-noise ratio, since intensity is proportional to number of photons reaching the detector, which in turn is proportional to the signal. Thus, the higher the intensity, the higher the signal-to noise, and the better the repeatability and reproducibility of the results. A linearity study based on this novel reflectance-transmittance scatterometry method, demonstrated excellent correlation with the target values and the conventional CD-SEM measurements and improved repeatability compared to the traditional reflectance-only measurements. The advantages of the 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.

 


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

Source: SPIE Microlithography Conference, 2006
Authors: John C. Lam and Alexander Gray of n&k Technology
Abstract: This article describes the use of an n&k Analyzer to non-destructively measure 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.

 


Optical Measurements of Critical Dimensions at Several Stages of Mask Fabrication

Source: SPIE 31st International Symposium on Microlithography, February 19-24th, 2006
Authors: John Lam and Alexander Grey of n&k Technology
Abstract: Critical dimension (CD) metrology is an essential part of the mask manufacturing process. We present a metrology solution based on broadband reflectometry, with analysis by Forouhi-Bloomer Dispersion equations, in conjunction with RCWA. The results were compared to conventional CD-SEM measurements collected at the same locations. A linearity study was conducted on 760nm and 1120-nm pitch grating arrays with systematically increasing CD width. The results demonstrate excellent correlation with CD-SEM.

 


Broadband Spectrophotometry for Phase-shift-mask Metrology

Source: Semiconductor Fabtech – 21st Edition, 2004
Authors: Phillip Walsh, George Li and A. Rahim Forouhi of n&k Technology
Abstract: A method based on broadband spectrophotometry in conjunction with Forouhi-Bloomer dispersion equiations and hybrid-rigorous coupled wave analysis (RCWA) for monitoring film thickness, film optical properties, trench parameters and phase shift in phase-shift photomasks.

 


Characterization of Ultra-Thick Photoresists

Source: Advanced Packaging (2003)
Author: J.C. Lam (n&k)
Abstract: Discusses advantages of broadband spectrophotometry over conventional optical methods such as ellipsometry/reflectometry when characterizing ultra-thick photoresist (50-100+ nm). The broadband spectrophotometric method holds advantages in advanced algorithm, throughput, R&R, and its good correlation with contact measurement tools.

 


Integrated Phase Angle Measurements for Advanced Mask Etch Process Control

Source: Proceedings of the SPIE – 23rd Annual BACUS Symposium on Photomask Technology and Management September 30 – September 9 2003, Monterey, California, USA
Authors: Turgut Sahin, Corey Collard, Alfred W. Mak, Melissa J. Buie, Philip Walsh, George Li

 


Process Monitoring of Etched Fused Silica (Quartz) Phase Shift Reticles

Source: Proceedings of the SPIE – 22nd Annual BACUS Symposium on Photomask Technology and Management September 30 – October 4, 2002, Monterey, California, USA
Authors: Cynthia B. Brooks, Melisa J. Buie, Nabila L. Waheed, Patrick M. Martin, Phillip Walsh, Glenn Evans

 


Modeling of Optical Constants of Materials Comprising Photolithographic  Masks in the VUV

Source: BACUS Proceedings (1999)
Authors: D. Harrison, J. Lam, G.G. Li, A.R. Forouhi (n&k); G. Dao (Intel)
Abstract: Vacuum ultra-violet lithography is the future technology for 0.10 mm or less IC production. To accommodate the demands of such small geometries, an entirely new class of advanced lithographic tools, along with the appropriate metrology tools, need to be developed and implemented.

From a lithography and metrology point of view, the industry is in the infancy stage. Working with
Giang Dao of Intel, n&k Technology has been able to extend the analytical capabilities of the present UV-Visible n&k Analyzers to provide values of n, k, R, and T at the VUV wavelength of 157 nm for Intel.

Basically, today’s n&k Analyzer, regardless of the model – whether the n&k 3000 or n&k 1200 can be used to establish the foundation for n and k values at 157 nm.

 


Optical Characterization of Attenuated Phase Shifters

Source: SPIE Proceedings (1997)
Authors: A. Callegari, K. Babich
Abstract: Utilizing the n&k Analyzer to fully characterize diamond like carbon (DLC) used for phase shift mask application, it was found that the results obtained agreed well with the results from a DUV Dual Beam Interferometer. The DUV Dual Beam Interferometer is usually used to directly measure phase shift. It is, however, a very expensive, slow, and difficult-to-use method, whereas the n&k Analyzer is less expensive, fast, and easy to use. Furthermore, the n&k Method provides considerably more information about the film properties than an interferometer (such as thickness, n and k spectra, R and T spectra, etc.).

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