Sacrificial Layer Applications

Microporous silicon is suitable for sacrificial layer applications because of its high etch rate ratio to bulk silicon, because it can be formed selectively, and because of the low temperatures required for oxidation. PS can be formed selectively if the substrate shows differently doped areas, as discussed in Section 4.5, or if a masking layer is used. Noble metal films can be used for masking as well as Si02, Si3N4 and SiC. Oxidation conditions are given in Section 7.6, while the etch rates of an etchant selective to PS are given in Fig. 2.5 b. 

For applications where micro PS is transformed into Si02 it can be argued that micro PS is not truly a sacrificial layer. However, the application described next has been included in this section for the sake of completeness. 

Besides the application of micromirror arrays, nickel surface micromachining with a copper sacrificial layer is a technology that can be used for various microfabrication concepts. Only recently, the method has been applied for the construction of capacitive RF switches for antenna impedance matching in multiband mobile phones [26]. 

The sacrificial oxide layer is initially formed on top of the silicon substrate by thermal or plasma processing (Fig. 5.3.1a). If insulation of the functional layer from the silicon substrate is required, the sacrificial layer may be preceded by deposition of an insulating film, for example, silicon nitride. The thickness of the sacrificial oxide layer determines the height of the released cantilever above the silicon substrate. It is typically a few micrometers thick, depending on the specific requirements and application. 

M Navarro, JM Lopez-Villegas, J Samitier, JR Morante, J Bausells, A Merlos. Electrochemical etching of porous silicon sacrificial layers for micromachining applications. J Micromech Microeng 7 131-132, 1997. 

This chapter describes the theory, methodology, and application of a microfabrication process that uses phase-changing sacrificial layers (PCSLs) as intermediates to protect microchannel features during bonding or hydrogel polymerization. We focus on key process details associated with the fabrication of microchips, and the application of PCSL-formed microfluidic devices in CE separations and other electric field-based analysis methods. Finally, we provide a brief overview of potential future trends and applications of PCSL fabrication methods in microfluidics. 

Microfabrication using sacrificial layers is well developed in the field of microelectromechanical systems. Reports include the fabrication of micro- and nanomechanical components, electroos-motic micropumps in silicon and glass substrates, and nano- ormicrochannels with potential applications in biology. Unlike bonding protocols, in which a cover plate is affixed to a patterned substrate to seal microchanneis, sacrificial layer methods can obviate the bonding step, making this approach very attractive. ... 

In this chapter, we have presented phase-changing sacrificial layers as enabling tools for making a variety of microfluidic devices in polymers. Several successful uses of PCSLs in biological analysis are discussed herein, which should motivate the pursuit of new developments and applications of this emerging approach. 

Since then, various methods have been adopted for fabrication of photoresist-based microfluidic devices. The first method shown in Figure 20.9a begins with a spin coating of photoresist onto a substrate and patterning with a photomask." Once the open microchannels are created, a sacrificial material is filled into the space of the microchannel. Subsequently, a second layer of photoresist is spin coated and patterned on top to define the access holes for inlet and outlet. Finally, the sacrificial layer is dissolved to create the closed microchannels. The major disadvantage in this process is the slow dissolution, therefore only short microchannels are applicable. 

Macroporous silicon technology found its principal application in integrated microsystems as sacrificial layer multilayered and suspended structures, such as bridges, membranes, and cantilevers, often require fabrication, almost always by isotropic etching and removing, by alkaline-based... 

Lang W, Steinera P, Richter A, Marusczyk K, Weimamib G, Sandmaier H (1994) Application of porous silicon as a sacrificial layer. Sens Actuators A 43 239-242 Lang W, Steiner P, Sandmaier H (1995) Porous silicon a novel material for mierosystems. Sens Aetuators A 51(l) 31-36... 

Kuhl M, O Halloran GM, Geimissen PTJ, French PJ (1998) Formation of porous silicon using an ammonium fluoride based electrolyte for application as a sacrificial layer. J Micromech Microeng... 

Wei, Y, et al., 2013. Screen printing of a capacitive cantilever-based motion sensor on fabric using a novel sacrificial layer process for smart fabric applications. Meas. Sci. Technol. 24 (7), 75104. 

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