Study on a Two-Dimensional Scanning Micro-Mirror and Its Application ...-2

In order to realize the decoupling measurement for two deflection angles and obtain the large piezoresistive coefficients for the high measurement sensitivities, an n-type silicon substrate in (110) wafer is selected for the flexible beam. Two p-type silicon piezoresistors RT1 and RT2 are oriented along ±45 degrees off y-axis and a p-type silicon piezoresistor RB is oriented along y-axis in <110> crystal orientation. The directions and crystal orientations of piezoresistors and the connection of two Wheatstone bridges are shown in Figure 3 and Figure 4. With the design of piezoresistors, the deflection angles of micro-mirror in the two directions can be measured by the two Wheatstone bridges and indicated by the output voltages VT and VB, respectively [12].

Figure 3.

Directions and crystal orientations of piezoresistors.

Figure 4.

Piezoresistors connection of two Wheatstone bridges.

2.3. Fabrication

The fabrication process flows are shown in Figure 5. The two-dimensional scanning micro-mirror is fabricated using a bulk silicon process, starting with an n-type silicon substrate of 300 μm thickness (Figure 5(a) and (b)). Boron doping produces p-type piezoresistors on the surface (Figure 5(c)). In order to obtain the desired resistivity r0 = 1.1 × 10−2 Ω cm, the dimensions of the piezoresistors are set to 100 μm × 10 μm with 0.5 μm depth and the boron ion implantation density is 8.0 × 1018 ions/cm2 at the temperature of 1,100 °C. After depositing a layer of silicon dioxide again on the top of the Boron-doped region and metallization (Figure 5(d)), the sputter and lift-off process was adopted and golden thin film lines with a width of 10 μm are connected and laid on the flexible beam (Figure 5(e)). The micrographs of the flexible beam and piezoresistors are shown in Figure 6. Finally, the micro-mirror structure is released by inductive coupled plasma (ICP) dry etching (Figure 5(f)). In addition, the piezoelectric actuator is fabricated by precision machining and connected to the excited part by the epoxy resins with high strength and adhesion.

Figure 5.

Fabrication process flows. (a) n-type Si substrate, (b) deposit SiO2, (c) diffuse p-type piezoresistors, (d)metallization, (e) sputter and lift-off, (f) ICP dry etching.

Figure 6.

Micrographs of flexible beam and piezoresistors. (a) the flexible beam, (b) piezoresistors RT1 and RT2, (c)piezoresistor RB.

The micro-mirror is packaged in a stainless steel case with the size of 28 mm × 20 mm × 18 mm as shown in Figure 7. The top of the package is open for the reflector and it can be closed with a translucent optical glass. The actuation and detection signal are applied and recovered through access points on the package side.

Figure 7.

Package of two-dimensional scanning micro-mirror.