Simple Process

The simplest of surface-micromachining processes involves just one poly-Si layer and one oxide layer. This process is a one-mask process and is illustrated in Figure 6.1 in which it is designed to form a poly-Si cantilever anchored to a Si substrate by means of an oxide layer. The oxide sacrificial layer is deposited first (Figure 6.1 (a)). The poly-Si structural layer is then deposited on top of the oxide. Next, the poly-Si layer is patterned, forming both the cantilever beam and the anchor region (Figure 6.1(b)). Following the poly-Si patterning step, the cantilever beam is released by laterally etching the oxide in an HF solution. The oxide etch needs to be timed so that the anchor region is not etched away (Figure 6.1(c)).

To implement successfully the process described in the preceding paragraph, the release etch must be very carefully controlled. If the release etch is continued for too long a period, the anchor region will be completely cut, resulting in device failure. However, to avoid such a failure, the process may be extended to a two-mask process in which the poly-Si cantilever is directly anchored to the substrate. This two-mask process is shown in Figure 6.2. In this process, the deposited oxide (Figure 6.2(a)) is patterned for an anchor opening by the first mask (Figure 6.2(b)). This is followed by a conformal deposition of poly-Si and subsequent patterning of the poly-Si cantilever beam using the second mask (Figure 6.2(c)). The cantilever is then released by a lateral oxide etch in HF solution (Figure 6.2(d)). Because the anchor region in this case is made out of poly-Si, the oxide release etch poses no threat of device failure.

Modifications of these simple one-mask and two-mask processes are the addition of bushings (or dimples) and/or an insulating layer between the cantilever and the substrate.

The process with an added bushing is shown in Figure 6.3. An additional mask is needed to pattern the bushing mould in either the one-mask or in the two-mask process (Figure 6.3(b)). Because of the difference in depth between the bushing mould and the

2 See Appendix G for tabulated properties of silicon.

SACRIFICIAL LAYER TECHNOLOGY 147

Si substrate

(c)

Si substrate

(d) Silicon dioxide

Poly-Si

Figure 6.1 Process flow for a polysilicon cantilever anchored to a silicon substrate by means of an oxide layer

///////////////

Si substrate

Anchor definition

y/A 1 1/////////,

Si substrate (a)

Si substrate

(b) Poly-Si

Silicon dioxide

Si substrate (c) (d)

Figure 6.2 Process flow for a polysilicon cantilever anchored directly to a silicon substrate

148 SILICON MICROMACHINING: SURFACE

m mrm

Si substrate (a)

Si substrate (c)

Si substrate (b)

Si substrate (d)

Poly-Si Silicon dioxide

Figure 6.3 Process flow for a polysilicon cantilever with a dimple anchored directly to a silicon substrate

anchor hole, it is not possible to pattern both openings simultaneously using the same mask. The preferable mask sequence for the patterning of the two openings is first the patterning of the bushing mould followed by the anchor region definition since the latter opening is the deepest of the openings. The rest of the process proceeds as described in the one-mask and two-mask processes.

The process in which an insulating layer is incorporated between the cantilever and substrate is illustrated in Worked Example 6.1. The insulating layer that works very well with the poly-Si-oxide combination is silicon nitride (see also Section 6.3). This process and three other worked examples are now presented in turn:

• Freestanding polysilicon beam

• Linear motion microactuator

• Rotor on a centre-pin bearing

• Rotor on a flange bearing

Worked Example E6.1A: Freestanding Poly-Si Beam Objective (A):

The objective is to fabricate a poly-Si freestanding beam that rests on the surface of a silicon wafer but is raised above it by a silicon nitride-insulating step.

Process Flow (A):

A layer of silicon nitride is first deposited by LPCVD on the surface of a silicon wafer (Figure 6.4(a)). The thickness of the nitride film corresponds to the height of the step on which the freestanding beam base is to rest. This nitride film also acts as a protective layer for the silicon substrate. A layer of sacrificial SiO2 is then deposited by chemical vapour deposition (CVD) on top of the nitride layer (Figure 6.4(b)) and patterned as shown in Figure 6.4(c). This patterned oxide island is of a thickness that is equal to the height above the nitride-layer surface of the freestanding beam. Poly-Si is then deposited by LPCVD on the patterned oxide as shown in Figure 6.4(d). When the sacrificial SiO2

island is laterally etched, the freestanding beam shown in Figure 6.4(e) is finally created.

SACRIFICIAL LAYER TECHNOLOGY 149

(a)

(b)

(c)

Poly-Si Silicon nitride

Silicon dioxide (Silicon) substrate

Figure 6.4 Process flow for a freestanding polysilicon cantilever beam anchored to a silicon substrate via an insulating nitride layer

Worked Example E6.1B: Freestanding Poly-Si Beam3

Objective (B):

The objective is to fabricate a poly-Si freestanding beam that rests on the surface of a silicon wafer and is on the same level as a nitride layer that has been deposited on top of the silicon wafer.

Process Flow (B):

This second procedure for beam fabrication is based on the localised oxide isolation of silicon (LOCOS) process in which windows are opened in silicon nitride on the silicon substrate and, subsequently, thermal SiO2 is grown in the openings (Figure 6.5(a)). Using this patterned SiO² as a sacrificial layer allows the construction of planar poly-Si beams.

For details see Linder et al. (1992).

150 SILICON MICROMACHINING: SURFACE

Silicon substrate Silicon substrate

EH CVD SiO2 Poly-Si

(b)

| LPCVD Si3N4 I I Thermal SiO2

Figure 6.5 Process flow for freestanding polysilicon beams using (a) CVD and (b) thermal silicon dioxide as the sacrificial layer

HV (w.r.t. ground) electrodes (tungsten)

Piezoelectric thin film (ZnO, PZT, etc)

(a)

Polysilicon

(ground electrode) Output connection point/area

Individual bar expansion/contraction

(b) Net actuator displacement

Figure 6.6 Linear motion microactuator (a) perspective view and (b) expansion/contraction and net displacement (Robbins et al. 1991)

151