**3.2 Silicon-on-insulator processing**

A wide variety of microsystems devices such as microactuators, optical switches, accelerometers, and nanopositioners are fabricated with deep reactive ion etching (DRIE) using SOI (silicon on insulator) materials due to the high aspect ratios that can be achieved (Herrera et al., 2008). DRIE silicon etching is commonly referred to as Bosch etching and was patented by Lärmer and Schlip (1992). A thorough review of DRIE high aspect ratio silicon etching is presented by Wu et al. (2010). In SOI MEMS fabrication, the initial wafer has three layers: a single crystal silicon substrate wafer, a thin thermally grown silicon dioxide layer referred to as the buried oxide, and a mechanically thinned single crystal silicon layer called the device layer. A DRIE process enables high-aspect ratio, deep etching of features in silicon wafers using repeated cycles of conformal polymer deposition, ion sputtering, and chemical etching of the silicon. DRIE can be performed on both the device and substrate layers in order to pattern thermal microactuators from the device layer and remove the substrate underneath the microactuators (Milanović, 2004) to reduce heat loss and required power during operation (Skinner et al., 2008). Typically a metal layer is deposited on top of the device layer to improve electrical connections when the parts are packaged.

Example SOI thermal microactuator designs are pictured in Fig. 6 (Phinney et al., 2011). SOI thermal microactuators were fabricated from a wafer with: a 550 m thick substrate, a 2 m buried oxide layer, and a 125 m thick device layer. Three bent-beam thermal microactuators were fabricated with four actuator legs having lengths from the anchor to the shuttle of 5500 m or 7000 m and leg widths of 50, 65, or 85 m. During packaging, wires were bonded to the 0.7 m aluminum layer that is deposited on top of the bond pad. Figure 6 shows a packaged die with the three thermal microactuators and bond wires visible.

Fig. 6. Picture of SOI thermal microactuators. Two wires bonded to each bond pad are visible in the image. The square bond pads are 900 m x 900 m. The connections to the package are outside of the image.

Thermal Microactuators 421

represents an edge bias which is subtracted from each side of thermal actuator leg nominal width. If a bias is not specified, the nominal width, 4.0 m, is used in the model calculations. As the current is increased, the displacement versus current data exhibits an inflection point and roll-off in the curve. This is attributed to the maximum temperature in the thermal actuator legs becoming hot enough, above 550°C, that the polysilicon is softened or even melts (Baker et al., 2004). The thermal actuator legs have been observed to glow red under

**Displacement Comparison**

0 5 10 15 20 25 **Applied Current (mA)**

Figure 8 shows the total electrical resistance for the actuators versus applied current for the P123 thermal microactuator. The resistance curve exhibits an inflection point, followed by a

Figure 9 shows the force for the P123F thermal actuator versus displacement when actuated at a constant current and voltage, 15 mA and 6.1V (Baker et al., 2004). For this test, the thermal actuator was held at a constant applied current and allowed to displace to its maximum unloaded position, which corresponds to the point on the graph where the curve intersects with the X-axis. Then using a probe tip, the force gauge was pulled away from the actuator, stretching the folded-beam spring in series between the probe tip and the actuator and applying a force to the thermal actuator center shuttle. The spring elongation was used to calculate the applied force and was recorded along with the actuator displacement. As the actuator is pulled back, the force increased to a maximum of 205 N at ~6.75 m. When pulled beyond this, the force begins to decrease due to buckling of the actuator legs. It is important to understand that this force curve represents the available output force of this single actuator design at this single applied power level. To fully characterize the force

Fig. 7. Displacement versus current for the P123 thermal actuator

maximum, and then a decrease in resistance as the current is increased.

these conditions.

0

**4.2 Resistance** 

**4.3 Force** 

5

10

15

**Displacement (m)**

20

25

Data Model (0.1 bias) Model (0.15 bias)
