The scanning micromirror has attracted much attention due to its wide range of applications, such as free space communication, projection display, spatial light modulation, etc. Scanners which have a large range of deflection ability, low voltage actuation, and fast dynamic response are preferred. A large amount of work has been done on developing processes which have the capability of realizing 2Dof micromirrors.
Figure 1 shows the schematic principle of micromirror actuation. An off-axis lateral force is acting on the torsional suspension beam and induces the torsional movement of the micromirror. In the previous approach , multi-layer structures were realized through STS timing etch. Timing etch has a few disadvantages, such as wafer-across nonuniformity and process condition variation. The thickness of the low SCS and upper SCS layer is hard to control. This affects the process yield and makes the design difficult.
The new method involves the alignment bonding of patterned SOI/SOI wafers. Unlike the timing etch process, the thickness of the most critical layer, the upper SCS layer, is predetermined by the device layer of the SOI wafer. This approach has much better wafer-across uniformity and the process yield is much higher. Figure 2 shows the process flow. First two SOI wafers are patterned and etched individually. Then they are prebonded by Ksaligner and annealed at 1200 degrees for 24 hours. STS backside etch and handle wafer etch are performed afterwards. Finally, the bonded wafer is released in concentrated HF for a couple of minutes.
Figure 3 shows the picture of a fabricated 2Dof scanning mirror. The preliminary testing result shows that a 1Dof mirror is deflected 11 degrees optically under an actuation of 53 V, compared with the result of 6 degrees at 56 V from the timing etch process. Finer tuning of the design parameter and more tests are on the way.
Figure 1: Torsional movement of a micromirror induced by off-axis lateral actuation
Figure 2: Process flow of patterned SOI/SOI wafer bonding
Figure 3: Picture of a fabricated 2D scanning mirror (left corner: a SEM picture of the multi-layer structure)