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Effect of Resist Thickness. Resists usually do not have uniform thickness on the wafer Edge bead: The build-up of resist along the circumference of the wafer - There are edge bead removal systems Step coverage. Centrifugal Force. Effect of Resist Thickness. - PowerPoint PPT Presentation
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Effect of Resist Thickness
Resists usually do not have uniform thickness on the wafer– Edge bead: The build-up of resist along the
circumference of the wafer- There are edge bead removal systems
– Step coverage
Centrifugal ForceCentrifugal Force
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Effect of Resist Thickness
The resist can be underexposed where it is thicker and overexposed where it is thinner– This can lead to linewidth variations
Light intensity varies with depth below the surface due to absorption
where is the optical absorption coefficient Thus, the resist near the surface is exposed first
– We have good fortune. There is a process called bleaching in which the exposed material becomes almost transparent
i.e., decreases after exposure to light- Therefore, more light goes to deeper layers
)exp()( 0 xIxI
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C. A. Mack, “Absorption and exposure in positive photoresist”, Appl. Opt. C. A. Mack, “Absorption and exposure in positive photoresist”, Appl. Opt. 2727(23), Dec. 1, 1988, (23), Dec. 1, 1988, pp. 4913-4919.pp. 4913-4919.
exposedexposed = B and = B and unexposedunexposed = A+B = A+B
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Photoresist Absorption
If the photoresist becomes transparent, and if the underlying surface is reflective, reflected light from the wafer will expose the photoresist in areas we do not want it to.
However, this leads to the possibility of standing waves (due to interference), with resultant waviness of the developed resist
We can solve this by putting an antireflective coating on the surface before spinning the photoresist increases process complexity
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Standing Waves due to Reflections
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Standing Waves Due to Reflections
http://www.lithoguru.com/scientist/lithobasics.htmlhttp://www.lithoguru.com/scientist/lithobasics.html
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(a) (b) (c)
Diffusion during a post-exposure bake (PEB) is often used to reduce standing waves.
Photoresist profile simulations as a function of the PEB diffusion length: (a) 20nm, (b) 40nm, and (c) 60nm.
http://www.lithoguru.com/scientist/lithobasics.htmlhttp://www.lithoguru.com/scientist/lithobasics.html
Removal of Standing Wave Pattern
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Mask Engineering
There are two ways to improve the quality of the image transferred to the photoresist– Optical Proximity Correction (OPC)– Phase Shift Masks (PSM)
We note that the lenses in projections systems are both finite and circular
Most features on the mask are square We lose the high frequency components of the
pattern We thus lose information about the
“squareness” of the corners
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Mask Engineering
The effects are quite predictable We can correct them by adjusting feature
dimensions and shapes in the masks
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Mask Engineering
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Phase Shift Masks
In a projection system, the amplitudes of the diffracted light at the wafer add– Closely spaced lines interact; the intensity at the
wafer is smeared If we put a material of proper index of refraction on part
of the mask, we can retard some of the light and change its phase by 180 degrees– Properly done, the amplitudes interfere
The thickness of the PS layer is
n is the index of refraction of the phase shift material
12
nd
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Phase Shift Masks (PSM)
Intensity Intensity pattern is pattern is barely barely sufficient sufficient to resolve to resolve the two the two patterns.patterns.