microlithtexred - Image characteristics Microlithography...

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Dean P. Neikirk © 2001, last update February 8, 2001 1 Dept. of ECE, Univ. of Texas at Austin Microlithography Geometry Trends Master Patterns: Mask technology Pattern Transfer: Mask Aligner technology Wafer Transfer Media: Photo resist technology imaging system (low pass filter) mask exposing radiation made insoluble made soluble film to be patterned substrate (with topography!) mask blank: transparent, mechanically rigid masking layer: opaque, patternable develop etch NEGATIVE POSITIVE photoresist Dean P. Neikirk © 2001, last update February 8, 2001 2 Dept. of ECE, Univ. of Texas at Austin Minimum feature sizes (DRAMS) trend lines for feature size n n n n n n n n n n n n n n n n u u u u u u u u u u u 0.1 1 10 10 100 1000 1970 1975 1980 1985 1990 1995 2000 2005 Minimum feature (microns) Oxide thickness (Angstroms) year (dram: intro or production) 1K 4K 16K 64K 256K 1M 4M 64M 256M 16M 1G 16G Dean P. Neikirk © 2001, last update February 8, 2001 3 Dept. of ECE, Univ. of Texas at Austin Overlay errors between two patterns goal: align two “identical” patterns one on top of the other σ λ level 1 level 2 λ pure registration error σ distortion error overlay error: sum of all errors really a statistical quantity rule of thumb: total overlay error not more than 1/3 to 1/5 of minimum feature size what can go wrong?? Dean P. Neikirk © 2001, last update February 8, 2001 4 Dept. of ECE, Univ. of Texas at Austin Image characteristics contrast intensity based: scalar quantity “incoherent” imaging electric field based: magnitude AND phase interference effects should be included in “coherent” imaging system spatial variations in image measure of how “fast” image varies line pairs per unit distance is “digital” analogy test pattern made up of periodic clear/opaque bars with sharp edges frequency domain analogy: spatial frequency test pattern is sinusoidal variation in optical transparency Dean P. Neikirk © 2001, last update February 8, 2001 5 Dept. of ECE, Univ. of Texas at Austin Modulation transfer function (MTF) mask intensity position transfer function log (spatial frequency) spectrum of “square wave” MTF of imaging system position Dean P. Neikirk © 2001, last update February 8, 2001 6 Dept. of ECE, Univ. of Texas at Austin Resolution in imaging systems diffraction limits passband of system minimum geometry k λ/ NA k ~ 0.5 to 1, typically ~0.8 λ exposure wavelength NA: numerical aperature (typically NA = 0.5) related to quality and “size” (entrance/exit pupil) of imaging system main difficulties need high NA, low aberrations, short wavelength but: depth of focus ~ λ / 2(NA) 2 restricted set of transparent materials for λ = 350nm very difficult to get large field size and high NA θ θ = sin n NA
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Dean P. Neikirk © 2001, last update February 8, 2001 7 Dept. of ECE, Univ. of Texas at Austin Basic imaging techniques contact gap mask photoresist optical imaging system proximity imaging Dean P. Neikirk © 2001, last update February 8, 2001 8 Dept. of ECE, Univ. of Texas at Austin
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microlithtexred - Image characteristics Microlithography...

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