Electron-beam proximity effect model calibration for fabricating scatterometry calibration samples

Abstract

Scatterometry has been proven to be effective in critical dimension (CD) and sidewall angle (SWA) measurements with good precision and accuracy. In order to study the effectiveness of scatterometry measurement of line edge roughness (LER), calibration samples with known LER have to be fabricated precisely. The relationship between ITRS LER specifications and the feature dimension design of the LER calibration samples is discussed. Electron-beam-direct-write lithography (EBDWL) has been widely used in nanoscale fabrication and is a natural selection for fabricating the designed calibration samples. With the increasingly demanding requirement of lithography resolution in ITRS, the corresponding LER feature of calibration samples becomes more and more challenging to fabricate, even for EBDWL. Proximity effects in EBDWL due to electron scattering can cause significant distortion of fabricated patterns from designed layouts. Model-based proximity effect correction (MBPEC) is an enhancement method for EBDWL to precisely define fine resist features. The effectiveness of MBPEC depends on the availability of accurate electron-beam proximity effect models, which are usually described by point spread functions (PSFs). In this work, a PSF in a doubleGaussian function form at a 50 kV accelerating voltage, an effective beam size, and a development threshold energy level of the resist are calibrated with EBDWL exposure tests. Preliminary MBPEC results indicate its effectiveness in calibration sample fabrication.Keywords: scatterometry, programmed LER structures, electron beam direct write lithography, proximity effect, proximity effect correction, model calibration INTRODUCTIONAs the minimum half pitch of integrated circuits is approaching 32 nm and below, urgent needs for near-ideal metrology technology are being called. Typical requirements for metrology include richness of measurement information, measurement speed, non-destructiveness of measurement processes, and capability of measuring process fluctuation 1 . Amongst various techniques, scatterometry is a promising candidate for meeting all the requirements. It can rapidly acquire average properties of periodic features over a relatively large area, and with proper set-ups by connecting to a scanner and a track, sub-nanometer variations can be identified and achieved 2, 3 . Evaluations of the ability of scatterometry to measuring CD, SWA, and overlay have been made 4-7 . Feature sizes down to 20 nm have been evaluated and correlated to critical dimension scanning electron microscopy (CD-SEM) with good results 1, 8 .Line edge roughness (LER) and line width roughness (LWR) are among the factors influencing device performances because they can affect dopant concentration profile and are responsible for gate leakage 9, 10 . Scatterometry has shown good sensitivity to roughness of 7.5 to 10 nm 10, 11 . The performance of scatterometery has been compared to alternative tools such as CD-SEM, atomic force microscopy, dual beam focused ion beam, and high-voltage SEM. It appears th...