Etch depth, poly

In the selection of etch mask for deep glass etching, thick SU-8 is a choice, but SU-8 cannot be used in a HF bath (48%) because SU-8 does not adhere well to Si02 [123]. However, with a polycrystalline amorphous Si seed layer SU-8 adheres very well. For instance, with a 1.5-pm-thick polished poly silicon, a 50-pm-thick SU-8 can be deposited as the etch mask, leading to a maximum etch depth of 320 pm. Usually photoresist (2 pm thick) is only useful for shallow etch, less than 50 pm [123]. 

Polished or unpolished polysilicon (by low-pressure CVD at 620°C) is another etch mask option. Utilizing 2.5-pm-thick unpolished polysilicon, a maximum etch depth of 160 pm was reached using a HF/H20/HN03 (6 40 100) solution. Further etching causes large pits (1.5-2.2 mm dia.) to form on the glass. With polished poly silicon (1.5 pm thick), etch depth up to 250 pm can be achieved. When amorphous Si is used as the etch mask, a maximum etch depth of 170 pm can be reached [123]. 

The oxidizing and etching effects that help in particle removal may lead to a rougher surface on the substrate such as poly-Si [37,38]. Therefore, a balance must be maintained between the need for greater etching depth to remove... 

Fig. 3 Etch depth vs. log fluence for laser ablation of poly(methyl methacrylate) at 193 nm wavelength...

Typical plots of the etch depth/pulse as a function of fluences are shown in Figure 9.5 for poly(methyl methacrylate) and in Figure 9.6 for poly(ethylene terephthalate) [2031, 2032]. 

Fig. 9.6. Etch depth versus fluence in poly(ethylene terephthalate) with 193 nm laser radiation (O) irradiation in vacuum and ( ) irradiation in air [2032]. (Reproduced with permission from [2032] published by John Wiley Sons, Inc., 1984.)...

The rapid mixer is composed of layers that are fabricated separately and then assembled together. The main four channel device, represented by the cartoon of Fig. 12.2, is etched through a 1-mm-thick silicon wafer using an anisotropic Bosch process RIE (Unaxis 770, Unaxis). The depth of this etch requires a thick mask. We use a 7 pm layer of PECVD silicon dioxide (GCI PECVD Group Sciences Incorporated, San Jose, CA). This mixer is sandwiched between two 100 pm thick poly(dimethylsiloxane) (PDMS) layers (Duffy et al, 1998), which contain channels in a T configuration. 

A number of effects lead to loss of profile depth resolution. The effect of crystallite orientation on sputtering rate is shown in Fig. 4.20, in which 10 keV Kr+ ions at 50° incidence were used to sputter poly crystalline iron [76]. This ion etching may be useful to bring out grain structure but leads to loss of depth resolution... 

Combining the lithographic and etch mask functions into a single polymer can be a major challenge, especially for deep-UV lithography. The latitude in resist design is limited, because at least 10 and preferably 15 wt % of the polymer structure must be reserved for silicon. A few materials, like silicon-substituted poly(methyl methacrylates) (6) and polysilanes (7, 8), have been used as positive two-layer resists for deep-UV lithography, but these materials suffer from either poor to moderate sensitivities to deep-UV radiation or an excessive absorption in the UV that limits exposure depth in the resist layer. 

A silicon wafer that has one surface oxidized to a controlled depth is coated (on the oxide surface) with a photoresist, such as poly(vinyl cinnamate), to produce a thin and uniform coating several micrometers thick when dry. Exposure to UV light through a mask insolubilizes part of the polymer. The uncross-linked polymer is washed off solvents. The bare substrate parts that thus reappear are etched through the oxide layer down to the silicon layer by a fluoride solution in water or by a plasma that contains reactive ions. 

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