**3. Fabrication**

Thermal microactuators are created using various microfabrication techniques including surface micromachining and silicon on insulator (SOI) processing which will be reviewed. Particular designs for surface micromachined thermal microactuators are presented in detail as characterization data for these designs are reported in Section 4.

#### **3.1 Surface micromachining**

Surface micromachining involves the sequential growth or deposition of thin films, patterning of features, and etching of the films to create multilayer structures and devices. Surface micromachining results in devices with in-plane dimensions from a few microns to millimeters and thicknesses of microns to 10 microns so they have low aspect ratios, i.e., thickness divided by length or width. Typical surface micromachining processes use polycrystalline silicon (polysilicon) for the structural layers and silicon dioxide for the sacrificial layers.

The surface micromachined thermal microactuators for which characterization data will be reported were fabricated using the SUMMiT VTM (Sandia Ultra-planar Multilevel MEMS Technology) process (Sniegowski and de Boer, 2000; SUMMiT V, 2008). The SUMMiT V process uses four structural polysilicon layers with a fifth layer as a ground plane. These layers are separated by sacrificial oxide layers that are etched away during the final release step. The two topmost structural layers, Poly3 and Poly4, are nominally 2.25 m in thickness, while the bottom two, Poly1 and Poly2, are nominally 1.0 m and 1.5 m in thickness, respectively. The ground plane, Poly0, is 300 nm in thickness and lies above an 800 nm layer of silicon nitride and a 630 nm layer of silicon dioxide. The sacrificial oxide layers between the structural layers are each around 2.0 μm thick (Sniegowski and de Boer, 2000; SUMMiT V, 2008).

Figure 3 pictures schematics of an electrically heated bent-beam thermal microactuator with two legs and the cross-sectional area of an actuator leg with the width and thickness dimensions labeled. The SUMMiT V processing constraints on the sacrificial oxide cut between two polysilicon layers result in an I-beam shape for the thermal actuator legs (SUMMiT V, 2008). In this chapter, mechanical, electrical, and thermal characterization results are presented for bent-beam thermal microactuators with two actuator legs (Phinney et al., 2009). The thermal microactuator designs have the actuator legs fabricated from three laminated structural polysilicon layers: Poly1, Poly2, and Poly3 (Figure 4). This actuator design is referred to as the P123 actuator throughout this chapter. The second thermal actuator design is the same thermal actuator as the first design with a force gauge attached to the actuator shuttle (Figure 5) and is referred to as the P123F actuator. The force gauge consists of a linear bi-fold spring attached to the shuttle of the actuator using the Poly3 layer. Table 1 summarizes the geometries of the thermal microactuators with nominal

Thermal Microactuators 419

w2 [m] t1 [m] t2 [m] t3 [m] Force Gauge

w1 [m]

P123 2.0 300 3.5 4.0 2.0 2.5 2.0 2.25 No P123F 2.0 300 3.5 4.0 2.0 2.5 2.0 2.25 Yes

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

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

Actuator Gap to

visible.

package are outside of the image.

Substrate [m]

**3.2 Silicon-on-insulator processing** 

Length [m]

Table 1. P123 and P123F thermal microactuator geometries

Offset [m]

dimensions specified according to the SUMMiT V Design Manual (SUMMiT V, 2008). The shuttle that connects beams at the center is 10 m wide, 100 m long and its thickness is the sum of t1, t2, and t3.

Fig. 3. Schematics of a) bent-beam thermal microactuator with two legs and b) actuator leg cross section showing dimensions that are specified in Table 1

Fig. 4. P123 thermal microactuator schematic

Fig. 5. P123F microactuator with an attached force gauge

dimensions specified according to the SUMMiT V Design Manual (SUMMiT V, 2008). The shuttle that connects beams at the center is 10 m wide, 100 m long and its thickness is the

a) b)

cross section showing dimensions that are specified in Table 1

Fig. 4. P123 thermal microactuator schematic

Fig. 5. P123F microactuator with an attached force gauge

Fig. 3. Schematics of a) bent-beam thermal microactuator with two legs and b) actuator leg

sum of t1, t2, and t3.


Table 1. P123 and P123F thermal microactuator geometries
