**2. Need for dry etching**

A serious limitation of suspended MEMS structures is that they tend to deflect through stress gradient or surface tension induced by trapped liquids during the final rinsing and drying step. Problems like stiction and bridge collapse are associated with producing a free standing structure. The stiction is described as a process of bonding the top and bottom

Plasma Based Dry Release of MEMS Devices 271

difficult to estimate because of the variation in deposition process or the density of

The liquid bridging occurs due to the surface tension of the trapped capillary liquids. The drying of this trapped liquid is difficult due to the presence of concentrated soluble impurities. These trapped impurities increase surface tension while decreasing the vapor pressure. A third possible adhesion cause can occur if suspended membrane is placed in contact with the lower contact surface due to some external force. This adhesion can occur due to deliberate placement of collapsing forces or can be due to shock effect (Mastrangelo,

The removal of sacrificial layer to achieve a suspended microstructure is the final step in the surface micromachining process. This process mostly requires a wet etching for removal of sacrificial layer. In some cases the removal is also done using plasma etch when sacrificial layer is other than a metal layer like polyimide or photo-resist. After the wet etching the microstructure is rinsed using DI water to remove the residues left during the etching. When the microstructure is pulled out of DI water a strong capillary force develops. A meniscus forms at the interface under the microstructure when the microstructure is pulled out of water. The curved interface creates a pressure called Laplace pressure which is given

*L l*

a definite angle of contact between the liquid and the substrate as shown in figure 1.

*P*

*a b*

*r r*

The liquid surface tension is denoted by γ*l* and two radii of curvature of liquid surfaces are given as *ra* (parallel to surface normal of the substrate) and *rb* (in the plane of the substrate). In most cases, the liquid droplet on the surface of the substrate will not wet it. It will present

(a) (b)

In equilibrium condition, the contact angle between liquid and solid is determined by the balance between the surface tension of the three interfaces. The contact angle *θ* at the junction of three interfaces is defined as the angle formed between solid-air, liquid-air and liquid-solid interfacial tensions in equilibrium. The contact is given by the Young's equation

> 

Fig. 1. Contact angle at solid liquid interface of (a) non-spreading (b) spreading liquid

 *SA SL LA* 

 

1 1 (1)

*cos* 0 < *θ* < *π* (2)

deposited material. In any case, the adhesion strength tends to be significant.

2000).

**3.2 Stiction due to capillary forces** 

by (Israelachvili, 1991)

(Israelachvili, 1991) as

electrodes together by a microscopic surface due to the planner nature of the electrodes. Stiction of MEMS is a common concern. When a sacrificial layer is removed and rinsed in deionized water, the surface tension of rinse water pulls the delicate micro structure to the substrate as the wafer dries. Risk of stiction is caused by the capillary forces originating from the dehydration of meniscuses, van der Waals force or the electrostatic force formed between the suspended beam structures and the substrate following the wet etching (Madou, 2002). These forces keep the structure firmly attached with the substrate. Stiction remains a reliability issue due to contact with adjacent surfaces after release.

Stiction is an inevitable problem we deal with for achieving the working RF MEMS devices. With increase in cantilever length, its flexibility perpendicular to the substrate increases which also increases the susceptibility to stiction. When the structure gets attached with the substrate due to stiction, the mechanical force required to dislodge it from the surface is large enough resulting in damage to MEMS structure (Modou, 2002). The surface morphology has a strong influence on stiction and is a serious problem particularly in metal to metal contact switches (Varadan et al., 2003).

In order to achieve a released structure, contact between the structural elements and the substrate should be avoided during processing. Etching can be done by physical damage, chemical damage or combination of both. Release of these suspended beam structures can be done either through wet etching or dry etching. Etching in a plasma environment has several advantages as compared to wet etching. In the wet etching, this may become impossible or very difficult due to large surface tension forces. Moreover, if a MEMS structure is left too long in the etchant, the structure can be over etched and damaged (Harsh et al., 1999). Plasmas are easier to start and stop than simple immersion wet etching (Campbell, 1996). Also sensitivity of plasma etch is less prone to small changes in the temperature of the wafer. Above mentioned factors make plasma etching more repeatable than wet etching.

Different techniques over a period of time have been used to avoid stiction. Method of creating stand-off bumps on the underside of a polysilicon plate was introduced (Abe et al., 1995) which added meniscus shaping microstructures to the perimeter of the microstructure for reducing the chance of stiction. To avoid stiction critical point drying technique using CO2 dryer is used (Chan et al., 2007) to release the structures.
