Sacrificial layer

Fig. 24.4. Galvanised steel is protected by a sacrificial layer of zinc.

Peel ply is a sacrificial layer of dry or resin-impregnated fabric that is placed on the bond surface during initial part fabrication. The peel ply is cured along... 

The originality of our approach lies in the elaboration of a sacrificial layer and of a sfrucfural layer made wifh low-femperafure processes, suifable for infegrafion of fhe optical archifecfure on fop of fhe driving circuif realized in fhe subsfrafe (Zamkofsian ef ah, 2002b). 

A piston acfuafor sfrucfure efched wifh a sfrucfural layer on fop of fhe sacrificial layer (500 mum square piston area) is presenfed in Fig. 7. Before efching of fhe sacrificial layer, fhe fop surface is complefely flaf. 

Figure 7. Piston actuator structure etched with a structural layer on top of the sacrificial layer (500 pm square piston area) general view and close-up.

The critical operation is the etching of the sacrificial layer. With a proper wet etching process, the remaining structural layer shows a high quality surface (Fig. 8). The 10 pm thick sacrificial layer has been etched and the layer stays with a perfect plane shape. Stiffness of the structural layer is also visible in Fig. 8b where the substrate have been cleaved near a structure. Attachment points are 500 pm away and there is no bending of the structure. 

Figure 8. Structural layer after sacrificial layer etching (a) on the substrate (b) above the edge of a cleaved sample. Structure is 500 wide and attachment points 500 /um away.

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. 

In the manufacture of micromechanical devices electrochemistry is commonly used to realize etch stop structures or to form porous layers. The first of these is discussed in Section 4.5. In the latter case, the use of PS as a preserved layer or as a sacrificial layer can be distinguished. In the first case PS is an integral part of the ready device, as discussed in Sections 10.4 to 10.6, while in the latter case the PS serves as a sacrificial layer and is removed during the manufacturing process. 

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. 

Fig. 10.22 SEM micrograph of the 500 nm thick polysilicon bridge (straight line, center) that is the heart of the hotwire anemometer. The bridge crosses a 80 pm deep groove produced by a micro PS sacrificial layer. After [St6],...

In order to circumvent these shortcomings, a fabrication process based on macro PS as a sacrificial layer has been proposed [Le30]. The process sequence is shown in Fig. 10.25. First etch pits in the desired pore pattern are formed on the n-type silicon wafer surface by photolithography and subsequent alkaline etching. Then deep macropores are formed by electrochemical etching according to the... 

A 5.5 (xm photoresist layer was patterned as the sacrificial layer, followed by the deposition of a second 4.5 p,m parylene layer. The parylene/photoresist/ parylene sandwich structure formed the electrospray nozzle and channel when the photoresist was subsequently dissolved. A 1500 A sputtered aluminum layer was used as a mask for parylene etching to define the shape of the nozzle. Aluminum was removed by a wet etching process. After SU-8 developing, wafers were left inside the SU-8 developer for 2 days to release the photoresist. A serpentine channel (250 pan x 500 pm x 15 mm) extending from the junction of pump channels to the edge of the chip was patterned in the SU-8 layer. Platinum/titanium lines spaced 200 pm apart were patterned under the channel after the electrode deposition step. 

Parylene C plastic microstructures were formed by an additive process. A sacrificial layer of photoresist was used to define the channel regions. The structures were supported on a PC substrate [139,231] or Si substrate [231,691]. 

To reduce expense, efforts are made to exploit integrated thin film technologies. For example, arrays have been produced via thin film deposition of the pyroelectric onto a sacrificial layer, e.g. a suitable metal or polysilicon, which is then selectively etched away. Thermal isolation of the pyroelectric element is achieved through engineering a gap between it and the ROIC silicon wafer. Yias in the supporting layer permit electrical connections to be made between the detector and the wafer via solder bonds. Imaging arrays have been produced in this way incorporating sputtered PST and sol-gel formed PZT films. 

FIGURE 14.10 Schematic cut-through of the micromirror device. Three CMP steps have to be performed (1) oxide planarization of CMOS passivation, (2) Cu damascene of vias, and (3) CMP of thick Cu sacrificial layer until the Ni posts are exposed. 

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