Lecture 3 - ME 601: Manufacturing in Micro- and Nanosystems...

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ME 601: Manufacturing in Micro- and Nanosystems Lecture 3: Advanced Lithography Topics, Immersion Lithography
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Effect of Diffraction Light is diffracted through mask features (a ~ λ ) Narrower features lead to broader spread of intensity Also depends on wavelength . .. 3, 2, 1, , sin = = m a m λ φ
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Effect of Diffraction Diffracted light can be directed to the image plane (resist to be exposed) using a focusing lens Finite diameter of the lens means some light (information) is lost (higher spatial frequency components) Other familiar example: Airy disk pattern Diffraction is usually described in terms of two limiting cases Fresnel diffraction - near field Fraunhofer diffraction - far field
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Effect of Diffraction Idealized exposure (no diffraction or large features) As features get smaller, diffraction becomes more significant Higher spatial frequency (m increasing) light provides sharper definition = “contains more information”
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Effect of Diffraction Madou, UC Irvine
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Resolution Limits Rayleigh criterion: resolution (distance between A and B) separated by one Airy disk interval Practical resolution: How do we improve resolution? NA k R λ 1 =
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Limitations of Optical Lithography Simple answer #1: decrease wavelength: early 1980s – 436 nm, mercury arc lamp (g-line) late 1980s – 365 nm, mercury arc lamp (i-line) early 1990s – 248 nm, excimer (KrF gas) laser (DUV) mid 1990s – 193 nm, excimer (ArF gas) laser (DUV) late 1990s – 157 nm, excimer (fluorine gas) (DUV) Sounds great – so what’s the catch? Mask and lens materials and photoresist must be re-engineered at each wavelength This became increasing harder as wavelength got smaller At 157 nm, absorption of mask materials was a problem: fused silica is OK for 193 nm and above but CaF required for 157 nm; introduces problems with birefringence (ray bifurcation and phase-front distortion)
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Limitations of Optical Lithography Earlier predictions pointed to reduced wavelength as answer, but complexity with materials limited development of 157 nm lithography Industry has jumped to embrace significantly lower wavelength at 13 nm: extreme ultraviolet lithography (EUV) Further advances have continued to push the limits of 193 nm DUV
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Limitations of Optical Lithography
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Depth of Focus Simple answer #2: increase numerical aperture Resolution @ i-line w/ 0.45 NA ÅÆ g-line w/ 0.54 NA Big tradeoff problem: depth of focus (depth of field) Defines distance over which light remains in focus Small DOF = wafers must be extremely flat, limits maximum wafer size and requires perfect stage tilt/leveling Large DOF = optical system is more forgiving to changes in thickness or tilt () 2 2 NA k DOF λ =
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Depth of Focus As NA goes up to reduce minimum feature size, DOF also decreases As wavelength goes down so does DOF! DOF actually more significant
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Lecture 3 - ME 601: Manufacturing in Micro- and Nanosystems...

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