Specific Process Knowledge/Lithography/MiR

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Resist description

AZ MiR 701 is a positive UV photoresist. It is considered to have high selectivity for dry etching.

Spin coating

Spin curves for AZ MiR 701 (29 cps) using a 30 s spin-off, and a 60 s @ 90°C proximity softbake

Typical spin parameters:

• Spin off: 30-60 s
• Soft bake: 60 s @ 90°C in proximity

Exposure

Dehydration of resist film:
During exposure in the maskless aligners, MiR can be sensitive to dehydration by the air flow applied by the pneumatic auto-focus mechanism. This may lead to insufficient development, especially for small substrates. This problem can be alleviated by switching to the overview camera for a period of 1 minute before starting the exposure, or simply by parking the writehead away from the exposure location for 1 minute before starting the exposure.

The effect can also be reduced by increasing the soft bake.

Post-exposure bake

Typical PEB parameters:

• PEB temperature: 110°C
• PEB time: 60 s

During exposure, interaction between the incoming light and the light reflected by the substrate can cause a standing wave to form in the resist (especially true for single line/wavelength exposure). This leads to a wavy sidewall after development. The standing wave pattern can be removed by introducing a post-exposure bake before development, which allows the activated PAC to diffuse into unactivated regions, thus smoothing the sidewall. The recommended PEB for MiR is 60 s at 110°C (for a 1-2µm film). Thicker coatings may require longer bake, and substrate thickness and material, e.g glass, may also affect the required baking time.

Since the post-exposure bake is at an elevated temperature compared to the soft bake, it will cause the resist film to become 5-10% thinner (probably due to continued evaporation of solvent). A 1.5µm thick MiR resist film will be approximately 1.4µm after PEB.

Development

Development speed:

• Puddle development in 2.38% TMAH (AZ 726 MIF): ~2 µm/min