Sacrificial layer technique

Sensors for measurements of physical parameters such as pressure, rotation or acceleration are commonly based on elongation or vibration of membranes, cantilevers or other proof masses. The electrochemical processes used to achieve these micromechanical structures are commonly etch-stop techniques, as discussed in Section 4.5, or sacrificial layer techniques, discussed in Section 10.7. 

A combined fabrication technology made possible through a sacrificial layer technique was developed to obtain partly or totally movable microstmctures together with fixed structures on a single substrate. Many practitioners consider this ability to be the hall mark of the LIGA process. 

The sacrificial layer techniques can be difficult to apply as they require a delicate choice of materials and etching methods that might not be compatible with the CPs. Also, the method introduces a step and thus a weak spot in the actuator (see Fig. 4A4). Therefore, a new method was developed that utilizes the poor adhesion between two materials, e.g., between Au and Si, the so-called differential adhesion method (Smela et al. 1995). First, an adhesive frame is patterned on the substrate that surrounds the actuator, except for an anchoring point (Fig. 4D). In the case of Si and... 

Polymer ashing is another process related method. This process is fairly complex and involves patterning of a polymer layer during the release. First, structures are partially released by a timed etch. Next, a polymer film is deposited onto the partially released structures. This film is patterned into support posts that hold the structure in position as the remainder of the sacrificial layer is etched away. Because the polymer support structures hold the devices in place, there is no concern for special drying techniques. Finally, the polymer supports are burned away, typically by ashing in an oxygen plasma.This leaves behind fully released and free-standing microstructures. 

Bulk micromachining relies on several etching techniques and creates projections of planar photolithographic masks in 3 dimensions. Surface micromachining relies on sacrificial layer and wafer bonding techniques. It creates true 3D structures as a stack of 2D patterned layers. Hence, it is more correct to refer to both micromachining techniques as two and a half dimensional (2% D). 

In our study, a three-layered Al/Cu/Ti film was employed as the seeding layer for electroless Cu deposition process. These metal films were deposited using the electron-beam evaporation technique and the substrates employed were thermally oxidized <100> silicon wafers. Ti is employed as the first layer, to serve as a barrier/adhesion promotion layer since Ti adheres well to most dielectric substrates and can prevent Cu diffusion into Si02. The second layer, Cu is the best homogenous catalyst for electroless Cu deposition. The last layer, A1 is a sacrificial layer to prevent Cu oxidation before immersing into the electroless deposition solution. 

Besides the layer-by-layer technique, which can be applied with or without the use of sacrificial cores [165,166] and usually requires polyelectrolytes, the miniemulsion technique is a highly suitable and versatile method for the formation of capsule formation with sizes down to 100 nm. Even the formation of inorganic capsules (e.g., [167]) by the miniemulsion polymerization is possible. For the formation of polymeric nanocapsules, three general approaches (see Figs. 16, 17, and 23) can be distinguished ... 

FIGURE 20.9 Photolithography-based microfabrication techniques (a) sacrificial layer, (b) lamination, and (c) multiwave exposure. 

Given the birth of MEMS from the IC industry, the dominant material used in the early devices was silicon. The use of silicon as a substrate and structural material, and the use of polysilicon as a thin film structural material, has continued to the present day for several reasons. The microfabrication techniques for silicon are highly developed and flexible, the microfabrication equipment has been designed for silicon, the properties of silicon are very well known and can be tightly controlled, silicon has excellent mechanical properties, and silicon has an insulating native oxide that can be used as a sacrificial layer. Other thin film materials that are commonly used in MEMS include silicon nitride, metals, and conventional polymers, such as polyimide. 

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