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• Thick photoresists for DRIE dry etching :
When you want to go through a silicon wafer (ie 500 microns) by dry etching method (DRIE), the resist mask must have a thickness of several microns (typically 6 microns). We use resists with viscosity about 25 times greater than 1µm-thick positive resists. For these steps, we use SPR 220 7.0 or AZ 9260 photoresists.
• Thick photoresists for electroplating : UV LIGA technology
One particular technique to fabricate micro-objects is to combine UV photolithography of a thick resist with the deposition of a metal by electroplating : this is the LIGA UV technology used in FEMTO-ST since 1993 *.
* W. Daniau, S. Ballandras, P. Berçot, D. Hauden « Metallic microdevices fabricated by deep-etch UV lithography », Materials Science and Engineering A160 p. L5-L8 1993.
The goal is to get a relatively thick object, so the polymer mold must be made with a viscous photoresist. For objects up to about 50 microns, we use AZ 9260 resist (viscosity : 520 cSt), and beyond the SU-8 2075 resist (viscosity : 22,000 cSt) and the KMPR 1050 resist (viscosity : 13 000 cSt).
To describe the resist and nickel patterns, we used the term Aspect Ratio, defined by dividing the height of the pattern by its smaller width:
The smallest theoretical width lmin that can be obtained is limited by the Fresnel diffraction:
λ is the wavelength, g is the gap between the mask and the resist and h is the height of the resist.
The maximum theoretical aspect ratio that can be achieved for a resist pattern is thus given by:
Here is the graph of this equation for a perfect contact (g = 0 and λ = 365 nm)
Note that this aspect ratio concept is not reserved only for very thick photoesists and that can also be used for thin resists. But a successful high aspect ratio remains linked to the choice of tool used to structure the resist throughout its thickness.
Achieved results :