Design and fabrication problems encountered in this thesis need to be considered more carefully in the future. To ensure the flip-up micro mirror to work, the SU-8 needs to be patterned on silicon. Besides, the precision of the aligner limits the finger gap. Small finger gaps can produce good scanning performance. Furthermore, the misalignment of the comb fingers would affect the maximum scan angle [42]. For example, 10 % error of the finger gap spacing reduces the maximum scan angle to 80
% of the value without misalignment [17]. In order to solve all the problems, a new fabrication process with four masks is proposed. The vertical comb fingers fabricated by the new fabrication process are self-aligned to minimize the finger gap and eliminate the finger misalignment. The detailed fabrication flow is shown in Figure 5-1.
The SOI wafer is first cleaned by the standard RCA clean process. Then a
LPCVD nitride and a PECVD oxide are deposited on the both side and back side of the SOI wafer, respectively. Mask 1 defines the SOI device layer pattern except the fingers as shown in Figure 5-1 (c). Mask 2 defines the through-wafer holes under the push pads and the mechanical connection of the micro scanning mirror. Then a PECVD oxide is deposited on the front side and Mask 3 is the coarse definition of the moving comb fingers as shown in Figure 5-1 (f). Mask 4 defines the SU-8 pattern of both fixed and moving comb fingers as shown in Figure 5-1 (g). The comb fingers are defined in a single mask so the minimum gap of the fingers is limited by the mask and photolithography resolution. Compared to the original finger gap of 7 μm, 3 μm finger gap can be achieved in the new process. Then the front side oxide is etched by RIE as Figure 5-1 (f). Back side ICP and front side ICP are followed as shown in Figure 5-1 (i) and (l). The SU-8 on the moving comb fingers is stacked on the oxide and can be removed in the release step as shown in Figure 5-1 (k). Finally gold is deposited for electrical connection.
In the current design, the mirror was locked by the friction between the interlock and the sidewall of the mechanical connection substrate due to the misalignment of the double side aligner. Therefore the dimensions of the back mechanical connection must be carefully designed.
(a) RCA cleaned SOI wafer
(b) Front side nitride and back side oxide deposition.
(c) Front side nitride patterning (Mask 1).
Figure 5-1. New fabrication flow.
Buried oxide LPCVD nitride PECVD oxide Silicon Buried oxide Silicon
Buried oxide LPCVD nitride PECVD oxide Silicon
FH 6400 photoresist
(d) Back side oxide patterning (Mask 2).
(e) Front side PECVD oxide deposition.
(f) Front side oxide patterning (Mask 3).
Figure 5-1. New fabrication flow (continued).
Buried oxide LPCVD nitride PECVD oxide Silicon
AZ4620 photoresist
Buried oxide LPCVD nitride PECVD oxide Silicon
Buried oxide LPCVD nitride PECVD oxide FH 6400 photoresist Silicon
(g) SU-8 photoresist patterning.
(h) Front side oxide etching.
(i) Back side ICP.
Figure 5-1. New fabrication flow (continued).
LPCVD nitride PECVD oxide Buried oxide
LPCVD nitride SU-8 photoresist Silicon
PECVD oxide Buried oxide
LPCVD nitride SU-8 photoresist Silicon
PECVD oxide Buried oxide
SU-8 photoresist Silicon
(j) Front side ICP.
(k) Release.
(l) Gold deposition.
Figure 5-1. New fabrication flow (continued).
LPCVD nitride SU-8 photoresist Silicon
PECVD oxide Buried oxide
SU-8 photoresist Silicon Buried oxide
SU-8 photoresist Gold Silicon Buried oxide
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