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Bio-MEMS Technologies and Applications - Wang and Soper (Eds) Part 3

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Tham khảo tài liệu 'bio-mems technologies and applications - wang and soper (eds) part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The LIGA Process 53 a b c FIGURE 3.6 a Approximately 1 pm-diameter gold post on intermediate mask top and copy into approximately 6 pm-thick PMMA resist bottom . b 2 pm-thick silicon nitride mask covering a 4 wafer with seven exposure fields. c Be mask with gears and a patterned field of 85 mm 0 top and detail of Au absorber of test structures bottom . 2007 by Taylor Francis Group LLC 54 Bio-MEMS Technologies and Applications a b FIGURE 3.7 a Three millimeter-thick PMMA sheet solvent bonded to a silicon substrate. b Four millimeter-thick SU-8 layer applied to a silicon substrate using the double-dry chip casting method. spin-coated SU-8 layers is possible up to a height of approximately 500 pm thicker layers show a nonuniformity of the solvent contents over the height and consequently nonuniform lithography performance compromising the structure quality 36 . Although hard x-ray photons easily penetrate through matter in particular organic materials like resists photon absorption and energy transfer into the polymer 38 has to be considered when choosing the right exposure conditions. Consequently the amount of absorbed energy in units of J cm3 is a measure for dose instead of the incident power per area commonly used in optical lithography 39 . Furthermore two dose values are commonly used the top dose defining the amount of energy absorbed at the resist surface and the bottom dose indicating the minimum dose required at the resist-substrate interface to fully destroy the resist and make it soluble see Figure 3.8 . In order to properly expose the resist the top to bottom dose ratio should not exceed a value of 5 for PMMA resist. This ratio further decreases for thicker resist layers in order to prevent foaming of the surface layer. A third dose is related to the dose that is absorbed underneath the gold absorber. This dose the so-called critical dose determines the minimum gold absorber thickness needed to sufficiently absorb the synchrotron radiation. Dose .