Sacrificial oxide etch

Typically, the vapor phase of hydrofluoric acid is used instead of HF liquid. HF vapor is known to strip native oxides during cleaning of silicon wafers [21] and was suggested for sacrificial oxide etching [22]. However, even using the vapor phase can be critical in terms of stiction, because a byproduct of the chemical reaction is water ... 

For automotive applications, the various functions require several special material properties. Here, we concentrate on properties of thin films rather than on their production processes. A wide range of publications deal with sensor-specific processes, for example, silicon reactive ion etching (RIE) using the Bosch trench process [2] and sacrificial oxide etching [3, 4]. The details of standard deposition and structuring processes are described in numerous books on semiconductor technology (e.g., [5, 6]), and they are not discussed in depth here. 

The vibrating ring/disk structure as well as the drive mechanism consists of 1 l- rm-thick poly-Si, which has been structured by deep RIE and released from the sacrificial oxide layer underneath by HE vapor phase etching. For the deposition of the thick poly-Si, a modified epitaxy deposition process (EPI poly) has been used [24]. However, as can be seen in Fig. 14.6, the deposition process leads to a rough poly-Si surface with Ra 100nm. For the removal of underlying topography, the surface has to be planarized by CMP in order to... 

FIGURE 14.7 The roughness of the EPI-poly layer can be planarized by using CMP. After removal of the sacrificial oxide layer by vapor phase etching, the sensor structures are released. 

Sacrificial oxide. The sacrificial oxide layer is deposited by low-pressure chemical vapor deposition (LPCVD) or plasma-enhanced chemical vapor deposition (PECVD). The layer is made of either undoped glass or phosphorous-silicon glass (PSG)-doped resulting in a final thickness of 1.5-2.5 pm. Low defect density, good etch rate control, uniformity and stress control have been accomplished for updoped oxide with a Novellus Concept One tool. 

Anchor. The next step is to open contact holes through the sacrificial oxide to the ground-plane polysilicon layer and/or the n+ runners. These contacts become the electrical and mechanical interconnections or anchors for the mechanical polysihcon structure (Fig. 5.2.6). A wet-dry etch combination provides a... 

Microstructure stabilization. Before complete removal of the sacrificial oxide, small cavities are etched around and under the mechanical polysilicon and down to the ground-plane polysilicon below the sacrificial oxide layer. These cavities are then backfilled with photoresist, resulting in pillars that support the polysilicon. In a subsequent masking operation, strips of photoresist are placed across the micromachined elements. This results in a web of photoresist material that holds the polysilicon elements in place after complete removal of the sacrificial oxide layer. Subsequent etching of the sacrificial oxide with a buffered... 

One approach suitable for high-volume production is to stabilize the movable beam structure with pedestals of photoresist [19]. Rectangular openings are created in the sacrificial oxide layer and are isotropically underetched to create circular holes. These holes are filled with photoresist that is patterned to allow the etching liquid to enter the entire area under the movable beam structure. Subsequently the pedestals are removed in an oxygen plasma. Temperature control during this step is essential to avoid capillary forces due to low viscosity in the resist. 

Vapor HF etching for sacrificial oxide removal in surface micro machining, Elec-trochem. Soc. Fall Meeting, Miami Beach, FL USA, Abstr. No. 671, p. 1056, 1994. 

In automotive sensors, aluminum bonding pads are the obvious choice they can be used for bonding both aluminum and gold wires, and the bonding quality is sufficiently good for the lifetime of the device. However, for surface-microma-chined structures, special care has to be taken regarding the bonding pads to prevent corrosion when the sacrificial oxide is etched. 

Etching of the sacrificial oxide by vapour-phase HF etch to release comb and ring/disk structures... 

Figure 18.12(a) shows the roughness of the epi-poly layer after deposition and Figure 18.12(b) shows a detail of the interdigital capacitors of the drive structure after poly-Si CMP, DRIB and vapour-phase HF release etch of the sacrificial oxide layer. Only the smooth surface after CMP allows the precision etch of the capacitor spaces <1 pm. 

Figure 1.10 Stiction that can occur during the sacrificial release etch, (a) Before sacrificial etching, the sacrificial oxide is below the mechanical layer, (b) After the chip is removed from the etch bath it begins to dry and the remaining fluid forms a bridge between the substrate and the mechanical layer, (c) Capillary forces from the meniscus of the fluid exert a downward force on the cantilever and cause it to come into contact with the substrate, (d) The surface forces, such as Van der Waals attraction, that dominate at the microscale cause the cantilever to become stuck to the substrate. (Reprinted with permission from lOP Publishing Ltd.) [15].

Structural elements, polysilicon for resistors and wires, and nitride for electrical insulation. The nickel sidewalls can be further coated with a thin gold layer to improve the electrical conductivity for improved electrical contact in relays. Structural elements can be released by either using sacrificial oxide or undercutting with trenches formed in a bulk KOH etch. An example of a microrelay formed in the MetalMUMPS process is shown in Figure 1.15. A cross section showing all of the layers used to fabricate the relay is shown in Figure 1.16. 

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. 

Next, the sacrificial layer is patterned and holes are etched into the oxide using established lithography and etching processes. These holes will be filled and thus act as anchor points on the left end of the two cantilevers formed later (Fig. 5.3.1 e). In the next step, the functional polysilicon layer is deposited (Fig. 5.3.1b). The thickness of this layer determines the mechanical properties of the movable beam. The thicker it is, the stiffer the beam will be in the z axis, which is desirable for structures intended to move only in the xy direction. But its thickness is limited by the capabilities of the deposition process used. The functional layer is next patterned and etched (Fig. 5.3.1c). Depending on the thickness of the polysilicon layer, specific trench etch processes (as described later on) may be required, especially when this layer is rather thick. Finally, the sacrificial layer is removed (Fig. 5.3.1 d). This is typically done with wet or vapor phase etches to dissolve the silicon dioxide and leave parts of the functional structures free-standing and movable. When using wet etching, special care has to be taken to prevent Stic-... 

Etch-stop at dielectric interfaces is another common occurrence in surface micromachining, when the etch terminates on the sacrificial or isolation oxide beneath the active silicon layer. Notching effects appear in high-aspect ratio (narrow)... 

Next, the sacrificial layer is deposited as a TEOS (Tetraethylorthosilane) oxide about 1.5 pm thick and is coated with photoresist, patterned and etched to form contact openings to the buried polysilicon layer (Fig. 5.3.11b). 

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