MEMS_04 guiren photolithography_resolution_01_1

MEMS_04 guiren photolithography_resolution_01_1 - Chapter 1...

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Chapter 1 Resolution in photolithography
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Diffraction Huygen’s principle: each point of a wavefront acts as a new source of waves Diffraction: bending of light and other waves into the region which would be shadow if rectilinear propagation prevailed Airy disc: radius of the disc is the distance from the intensity peak to the 1 st 0 intensity Airy disc bending of light Fraunhofer diffraction Huygen’s principle
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Diffraction Abbe (or Rayleigh) diffraction limit: θ c = 1.22 λ/D D: aperture of the lens
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Resolution in shadow printing Including contact and proximity arrangement of mask and wafer Factors affecting resolution: Diffraction limit at the edge of opaque feature in the mask Alignment of wafer to mask Non-uniformities in wafer flatness Debris between wafer and mask
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Resolution in shadow printing Equ. (1.12) based on diffraction limit b min: half grating period and the minimum feature size transferable s: gap between mask and resist surface λ: wavelength of the exposing radiation z: resist thickness Fig 1.12 ) 2 ( ) 2 / 3 ( b = R min z s  
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Resolution in shadow printing Contact printing equ. (1.12) -> (1.13) when s = 0 Can reach 1 μm resolution However, this diffraction limited resolution can seldom be achieved due to other factors Wafer flatness Mask alignment The mask can be damaged Proximity printing equ. (1.12) -> (1.14) when s >> z Reduce damage Resolution will be reduced depending on s ~ 2-3 μm resolution is achievable 2 ) 2 / 3 ( b = R min z
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Resolution in shadow printing Proximity printing Can we use nanoscale mask to replicate? Very difficult! Reason: From Bethe’s theory, the transmission efficiency η B of light through small aperture can be expressed by k is the wave number of incident light and r is the radius of the aperture and λ is the wavelength of incident light. η
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MEMS_04 guiren photolithography_resolution_01_1 - Chapter 1...

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