Reduction of diffraction effect of UV exposure on SU-8 negative thick photoresist by air gap elimination

Abstract

This paper reports a novel way to compensate the air gap between photomask and photoresist for eliminating UV light diffraction on photoresist, which greatly increases the sidewall straightness of high-aspect-ratio resist structures. In this research, SU-8 negative tone photoresist was used for experiments, and glycerol was employed as an index match material for bridging air gap between photomask and photoresist during exposure. Results showed that a high aspect ratio wall structure of 156 lm thick and 25 lm wide had a 45% pattern width error when exposed under 100 lm air gap, while glycerol compensated process accomplished a straight resist wall without appreciable error. This method is simple and cheap to employ, compared to the usage of costly thickphotoresist-film spinner for resist planarization. Numerical simulation on the diffraction effect upon the structure wall has also been conducted. The calculated and experiment wall profiles showed similarity in trend.

SU-8 Photolithography as a Toolbox for Carbon MEMS

“…Such random variation will negatively impact the photolithography process during the exposure step, when the distance between the mask and the resist is supposed to be uniform throughout, by allowing for air gaps between the resist and the mask. These gaps enable light to diffract at the film-mask interface and can lead to the broadening of the structures at the top, a well-known effect commonly known as T-topping [26,29,30]. For example, when exposing thick layers (>200 µm) coated in a single step, it was common to find T-topped features in one area of the wafer, while they are perfectly fine in other ones, even though the exposure and post-baking times were the same.…”

“…The exposure of dense arrays of features greatly benefits from using the hard contact mode, since the resist is pressed against the mask, and air gaps throughout the exposure area are minimized. Alternatively, one may use an index matched material to fill the air gaps; glycerol for example [30]. Yet another way to eliminate air gaps is a multi-step coating process for layers above 100 µm thick.…”

Moving-mask Lithography for 3D Microstructure Molding

“…These results suggest that the effect of diffraction should be considered. According to the literature [21], the proximity gap has a significant effect on the pattern size, especially in the case of ultra thick resist exposure, where calculations predicted that a 100 µm proximity gap would expand the mask pattern 25 µm in the radial direction. In the present study, the thickness of the glass substrate and proximity gap acted as an actual proximity gap of at least 180 µm.…”

Optically clear biomicroviscometer with modular geometry using disposable PDMS chips

“…A mask containing the negative of the design was then aligned over the spin-coated silicon wafer. To improve the contact between the wafer and the mask, a thin layer of glycerol was used between the mask and wafer due to it having a similar refractive index to SU-8 50 [92]. Choosing a fluid with a similar refractive index would ensure that the light would not refract in the gap (or material) between the mask and the wafer, to result in straight-walled channels.…”

“…Due to the non-uniform surface structure of the SU-8 layer, any air gaps can cause the UV rays to create features with "over-hangs", as shown in Figure 73 (left) or in Figure 74 (left to right) [92], [111]. [92].…”