Effect of Resist Thickness

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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


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


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


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


Standing Waves due to Reflections


Standing Waves Due to Reflections

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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.

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Removal of Standing Wave Pattern


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


Mask Engineering

The effects are quite predictable We can correct them by adjusting feature

dimensions and shapes in the masks


Mask Engineering


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


Phase Shift Masks (PSM)

Intensity Intensity pattern is pattern is barely barely sufficient sufficient to resolve to resolve the two the two patterns.patterns.

Effect of Resist Thickness

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