SURFACE MICROMACHINING USING PLASMA ETCHING

Surface micromachining can also be realised using a dry-etching process rather than a wet-etching process. Plasma wet-etching of the silicon substrate, with SFe/CVbased and CF4 H2-based gas mixtures, is advantageous because high selectivities for photoresist silicon dioxide and aluminum masks can be achieved. However, when using plasma etching, a large undercut of the mask is generally produced. This is due to the isotropic fluorine atom etching of silicon that is known to be high compared with the vertical etch induced by ion bombardment. In contrast, reactive ion etching (REE) of poly-Si using a chlorine-fluorine gas combination produces virtually no undercut and produces almost vertical etch profiles with photoresist used as the masking material. Thus, rectangular silicon patterns, which are up to 30 urn deep, can be formed using chlorine-fluorine plasmas out of poly-Si films and the Si wafer surface. A deep etch process is essential for microactuators and, therefore, the deep RIE process is an attractive option. Here, we illustrate its use to make two MEMS devices:

SURFACE MICROMACHINING USING PLASMA ETCHING 159 Centre-pin-bearing side-drive micromotor

Gap comb-drive resonant actuator

Worked Example E6.6: Centre-Pin-Bearing Side-Drive Micromotor11

Objective:

The objective is to fabricate a centre-pin, variable-capacitance, and side-drive micro-motor, such as the salient-pole and wobble types.

Process Flow:

The flow process in this case adds to what has already been described in the previous Worked Examples 6.3 and 6.4. The rotor is the main component of the micromotor;

however, we also need to incorporate stator poles to form the micromotor. A variable-capacitance side-drive micromotor clearly requires electrically conducting materials for both the rotor and the stator. Heavy doping of the poly-Si with phosphorus to form n-type poly-Si satisfies this requirement. In addition, the stator poles need to be electrically isolated from the substrate, the rotor, and one another. This electrical isolation is achieved by LPCVD of an insulating silicon nitride layer. Figures 6.12 and 6.13 show the top and the cross-sectional views, respectively, of the salient-pole and wobble micromotors.

The process flow is shown in Figure 6.14 and runs as follows:

1. First, an insulation bilayer that consists of 1 um silicon-rich silicon nitride is deposited by LPCVD over a 1 um thermally grown oxide and it completely covers it. This insulation bilayer is required to survive the release etching and to withstand high voltages during the operation of the micromotor. The etch rate of silicon-rich silicon nitride in HF solution is negligibly small compared with that of the oxide. Also,

Figure 6.12 Top view of the salient pole micromotor

" For details, see Mehregany and Tai (1991).

160 SILICON MICROMACHINING: SURFACE

Shield Bushing Bearing Rotor Stator

Figure 6.13 Cross-sectional view of the salient pole micromotor Stator anchor Shield Bushing mould Isolation

(a)

/,—

^{^ /}

Substrate

Air gap Bushing Rotor Stator

2nd LTO Bearing anchor Bearing clearance

Figure 6.14 Process flow for the centre-bearing side-drive micromotor of Figures 6.12 and 6.13 (Mehregany and Tai 1991)

the bilayer insulation structure proves to be very effective in eliminating electrical breakdown through the substrate and permits operating voltages of up to about 250 V on the stator poles.

2. The second step is to deposit a 0.35 um-thick heavily doped poly-Si and pattern the poly-Si layer to form the shield. This step is followed by the deposition of the first thermal low-temperature oxide (LTO) sacrificial layer, which is subsequently patterned for the bushings and stator anchors as shown in Figure 6.14(a).

3. A 2.5 um-thick poly-Si layer is deposited and heavily doped with phosphorus. Phos-phorus doping is achieved using POC1³ at 950 °C. The poly-Si is patterned by RIE to form the rotor, stator, and air gaps (Figure 6.14(b)). In the RIE process, a 0.5 (am

SURFACE MICROMACHINING USING PLASMA ETCHING 161 thermal oxide is used as an etch mask. The final rotor-stator poly-Si thickness is 2.2 urn because of the thermal oxidation used for the mask formation.

4. A second sacrificial LTO layer is grown; this provides 0.3 |im of LTO coverage on the rotor and stator sidewalls and approximately 0.5 urn of LTO coverage on the top surfaces. The bearing anchor is then defined and etched through the two sacrificial oxide layers down to the electric shield below (Figure 6.14(c)).

5. A 1 urn-thick poly-Si layer is deposited, heavily doped with phosphorus, and then patterned to form the bearing as shown in Figure 6.14(d). At this point, the completed device is immersed in HF solution to dissolve the sacrificial LTOs and release the rotor.

Worked Example E6.7: Gap Comb-Drive Resonant Actuator12

Objective:

Comb-drive actuators are widely used, as their output force is easily controlled by the applied voltage, and the output force required to drive passive structures is extracted more easily than that from rotational actuators. A top view of the resonator to be fabricated is shown in Figure 6.15. The drive force of the actuator is obtained by applying a voltage between the stator and the drive electrodes; this force is inversely proportional to the gap width between the electrodes. Therefore, reducing the gap width between the two electrodes is the most effective means of reducing the high drive voltage greater than 25 V that is normally required.

Process Flow:

It is widely acknowledged that masking precisely controlled submicron gaps from thick poly-Si (e.g. ~4 urn as used in this example) using commonly available lithography and etching systems is not an easy process. The process flow described subsequently

Attached to substrate

Suspended above substrate

Figure 6.15 Top view of a gap comb-drive resonant actuator (Hirano et al. 1992) For details, see Hirano et al. (1992).

162 SILICON MICROMACHINING: SURFACE

(a)

(b)

(c)

Fixed Suspended electrode electrode

Thermally grown silicon dioxide (2.5 Jim) Phosphorus-doped LPCVD polysilicon CVD silicon nitride (250 nm) Nickel (100 nm) Silicon dioxide in oxidation machining

I • I

(d)

Figure 6.16 Process flow to fabricate the gap comb-drive resonant actuator in Figure 6.15

outlines a method of fabricating submicron gaps for comb-drive actuators called oxida-tion machining. The process flow for the actuator's fabricaoxida-tion is depicted in Figure 6.16.

1. A 4 um-thick poly-Si (doped by ion implantation and annealed at 1100°C for 1 hour) is deposited by LPCVD on top of a 2.5 um thermally grown oxide on the substrate. A 250 nm-thick silicon nitride layer is then deposited by LPCVD over poly-Si, which protects the top surface during the thermal oxidation step. Finally, a 100 nm-thick nickel layer is deposited by vacuum evaporation (Figure 6.16(a)).

2. The shape of the actuator is patterned on the nickel, followed by the wet etching of the nickel film. Using the nickel pattern as a mask, Si3N⁴ and poly-Si are RIE-etched in SF6 (Figure 6.16(b)).

3. After the removal of the nickel mask, the wafer is cut into 1 cm² pieces and the poly-Si is thermally oxidised (Figure 6.16(c)).

4. Figure 6.16(d) shows the actuator's cross section after it is released in HF solution.

6.5 COMBINED 1C TECHNOLOGY AND ANISOTROPIC