Characterising the Surface Micromachining Process

From the definition of the surface micromachining process, one can note that deposition of materials layers is used to create the desired structure of a typical device. Fig. 6.3 illustrates the surface micromachining process.

(d) Pliotomasking Fig. 6.3 Surface micromachining process

6.4.1 Isolation Layer

A silicon substrate is used as the ground plane. The first step in the surface micromachining process is the deposition of a thin isolation layer. This layer is deposited with a dielectric material such as silicon oxide (SiO2) followed by a thin layer of silicon nitrite. The latter can act as an etch stop for many etchants. The isolation layer itself is configured to make a metal contact with the base substrate. A pattern is transferred to the isolation layer using isotropic etchant such as buffered HF (BHF). The ratio of NH4F and concentrated HF is 5:1. This solution can effectively etch the SiO2 layer. A typical etch rate is 100 nm/min. The etching may be monitored by changes of the color pattern or by supervision of the hydrophobic and/or hydrophilic behavior of the etched layer (Hermansson et al. 1991). The resist opening is the same size as the oxide thickness. After isotropic etching, it is emerges into a piranha etch bath. A strong oxidiser is then grown over the cut region. Later it is removed using diluted BHF. This is a known as cleaning. Drying follows cleaning. Once both cleaning and drying is completed, the substrate is ready for further deposition. A dry etch using CF4H2 is applied to cut the oxide. The process prevents the undercutting of the resist. But it has a long processing time. Etching of the isolation layer is possible by using a dry etch process followed by BHF (Tang 1990).

6.4.2 Sacrificial Layer

A phosphosilicate glass (PSG) layer can preferably be used as the sacrificial layer. Adding phosphorous and SiO2 onto PSG can enhance the etch rate in HF (Monk et al. 1992; Tenney and Ghezzo 1973). The important benefit is that SiO2 can behave as a solid-state diffusion dopant source making the polysilicon layer electrically conductive. The deposited phosphosilicate makes a nonuniform etch rate in HF. It is normally carried out in a furnace at 900°C to 1000°C for about two hours in a rich oxygen environment (Madou 1997). PSG undergoes a viscous flow at a certain temperature. This increases the smoothening of the edge and improves the etching condition.

6.4.3 Structural Material

The next step is the deposition of a thin structural material layer over the isolation layer. A chemical vapor deposition technique is mostly preferred, but sometimes sputtering is well suited. In the latter case, it is easy to introduce a tapered edge (Madou 1997). The commonly used structural material in microfabrication is polysilicon (poly-Si). It is essentially deposited at low pressure. The applied pressure is about 100 Pa at 500-700°C. A normal deposition rate of 200 A/min can be achieved in an environment of temperature 500°C, pressure 200 Pa with a silane flow rate of 125 sccm. A layer of 1-p.m film will take about one hour and 30 minutes. The silane is usually diluted with 70 to 80% nitrogen and deposited at temperatures from 500 to 700°C for fine-grain and from 600-800°C for coarse-grain applications. Additionally, annealing in nitrogen will reduce stress formation due to thermal mismatch of expansion coefficients. Depending on the nature of the application (e.g., sensor or actuator) other materials such as silicon nitride, silicon oxynitride, polyimide, diamond, SiC, GaAs, tungsten, a-SiH, Ni, W, and Al are also used as structural materials in surface micromachining. Silicon nitride and silicon oxide can be deposited by CVD methods but they exhibit high residual stress, which hampers their use as mechanical components (Chang et al. 1991). However, a mixed silicon oxynitride can produce substantially low or stress-free components. Other problems include oxidation at 500°C for non-passivated films.

Moreover, they do not provide good ohmic contact, and therefore are unreliable for electric contacts (Obermeier 1995).

6.4.4 Selective Etching

Surface micromachining process sequences are presented in Fig. 6.4. We like to consider silicon as the ground plane substrate because of the reasons mentioned in chapter 5.

Fig. 6.4. Surface micromachining process sequence

Selective etching is employed to create movable mechanical parts in the microstructure. The structure is then freed from the spacer or sacrificial layer. Consider the design of a poly-Si based microstructure utilising RIE (Reactive Ion Etching). After patterning the poly-Si by RIE in SF6 plasma, it has to be immersed in an HF solution to remove the underlying sacrificial layer, which makes it possible to release the structure from the substrate. A typical sacrificial layer of phosphosilicate glass, between 10 to 2000 pm long and 0.5 to 5 pm thick, is etched in concentrated buffered HF. The etch rate has to be fast in order to avoid an undesirable attack on the structural element and the insulation layer. The etch rate of PSG can be increased monotonically with concentration (Monk et al. 1994). High concentration phosphorous-doped polysilicon is prone to attack by HF. Silicon nitride deposited by LPCVD etches slower in HF than in oxide films. Kinetic and diffusion reactions are noticeable in short and long channels, respectively. The reaction therefore, shifts from kinetic- to diffusion-controlled when the channel is longer. Diffusion is observed over 300 pm of the channel in concentrated HF, affecting a large structural area (Fan et al. 1988; Mehregany et al. 1988). Rinsing and drying follow etching. Extended rinsing allows native oxide to form on the surface of the polysilicon structure. Such a layer may be desirable and can be formed more easily by a short dip in 30% H2O2. Surface micromachining processes, therefore, depend on the properties of the materials. Important material properties are discussed in more detail.

A simple surface micromachined cantilever beams is illustrated in Fig. 6.5(g) and (h). The polysilicon has been deposited and patterned using the RIE technique, followed by wet etching of the oxide layer under the beams in order to free them from the substrate (Fig. 6.5(h)). It shows that surface micromachining is a powerful technique for producing complicated 3D microstructures. Some of the other structures are tweesers, gear trains and micromotors.

Silicon substrate Isolation layer Material layer Photoresist layer Poly-Si -

Substrate

Sacrificial layer

(h) Free standing Poly-Si

Fig. 6.5. Surface micromachining (a) Isolation layer (sacrificial or spacer layer) (b) Material layer (c) Photoresist layer (d) Photomasking (e) Removal of excess photoresist (f) Selective etching of spacer layer (g) An example of micromachining a cantilever structure (h) After micromachining (Free standing cantilever)

Substrate

Sacrificial layer

(h) Free standing Poly-Si

Fig. 6.5. Surface micromachining (a) Isolation layer (sacrificial or spacer layer) (b) Material layer (c) Photoresist layer (d) Photomasking (e) Removal of excess photoresist (f) Selective etching of spacer layer (g) An example of micromachining a cantilever structure (h) After micromachining (Free standing cantilever)

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