Polycrystalline silicon etching

Figure 1. Device for measuring friction coefficient between etched polycrystalline silicon sidewall surfaces. The entire structure is shown in (a), a close-up of the contact area in (b), and schematic of the contact geometry in (c).

C.I. Silicon and Poly silicon. The isotropic etching of silicon (Si) and polycrystalline silicon (Poly-Si) by atomic fluorine (F) is probably the most completely understood of all etch processes, particularly for the cases in which F atoms are produced in discharges of F2 (36) and CF4/O2 (42). Fluorine atoms etch (100) Si at a rate (A/min) given by (36) ... 

Plasma etching is widely used in semiconductor device manufacturing to etch patterns in thin layers of polycrystalline silicon often used for metal oxide semiconductor (MOS) device gates and interconnects (see Plasma technology). 

Whether or not a chemical process step has been successful is difficult to measure, since there are few on-line measurable electrical properties. For example, film thickness and grain structure of polycrystalline silicon can be measured after a deposition step. However, their effect on device performance might not show up until subsequent doping or patterning steps fail. Similarly, it is possible to measure etch rates on-line by laser interferometry, but the etch profiles must be checked by electron microscopy. Unexpected mask undercutting or undiscovered etch residues can result in subsequent contact and device lifetime problems. 

Z. Shi, Observation of K2SiFe crystal growth during Secco etching of polycrystalline silicon, J. Elec-... 

Laser ablation was used [22] to produce SiNWs 20 nm in diameter with a polycrystalline silicon core in a thin silicon oxide sheath with 1/4-1/3 of the nominal diameter and 1/3 of the weight of the SiNW. The oxide layer (which makes the SiNWs surfaces inert) was removed by a 5% H F dip for 5 min resulting in smooth, stable, H-terminated SiNW surfaces [77]. The etched SiNWs were immersed into solutions of silver nitrate and copper sulfate of different concentrations. Silver and copper ions were reduced to metallic aggregates deposited onto the surface of SiNWs. The TEM image of the sample treated with a 10 M silver nitrate solution (Figure 10.28) shows dark, round silver particles 5-50 nm in diameter. The HF-etched SiNWs treated with 1.0 x 10 M copper sulfate show much smaller (a few nm) particles (Figure 10.29) identified by EELS as Cu particles. 

Figure 10-6 Example of a microfluidic valve constructed in silicon I200x 1200fxm) with an etched aperture to form the inlet for gas or liquid. The movable valve plate is supported by four polycrystalline-silicon arms,-400 jim long and 200 p,m wide, attached to the silicon substrate. (From Bien DCS, Mitchell SjN, Gamble HS. Fabrication and characterization of a micrornachined passive valve. J Micromech Microeng 2003 13 557-62.)...

Fig. 21 Etch rale of polycrystalline silicon under the influence of XeF2 gas and 450-eV Ar ion impact and under the combined influence (after Cobum, 1994).

A variety of mammalian cells have been successfully cultured onto porous silicon surfaces. The first publications on this topic by Bayliss et al. demonstrated that attachment of Chinese hamster ovary (CHO) cells proceeded on porous silicon surfaces to a similar extent as on bulk silicon (Bayliss et al. 1997a, b). This was also confirmed with the neuronal cell line B50 (Bayliss et al. 2000). Cell viability in these studies was determined using two colorimetric assays, the MTT based on enzymatic reduction of a tetrazolium salt to a purple formazan and the neutral red uptake assay. B50 and CHO cells were cultured on bulk silicon, porous silicon, glass, and polycrystalline silicon. Both viability assays suggested that the neuronal cells showed preference for porous silicon above the other surfaces, while CHO cells showed the lowest viability on the porous silicon surface (Bayliss et al. 1999, 2000). The surfaces of the porous silicon used in these early studies were not modified post-etching, and it was not until a study utilized porous silicon surfaces with an oxide layer for cell culture that surface chemistry was found to play a crucial factor (Chin et al. 2001). Rat... 

Figure 3.13 illustrates how a polycrystalline silicon cantilever beam is formed onto a Si substrate. We first deposit a layer of SiN onto the Si substrate as an insulator. Next, a layer of SiOj is deposited and patterned onto the substrate via LPCVD, followed by photolithography and masked etching of Si02 to expose the underlying SiN at a designated anchor area. Polycrystalline... 

Note that the Si02 layer underneath serves as a place holder for the ultimate air gap between the polycrystalline silicon cantilever and the substrate. Now we can release the cantilever by etching away the sacrificial Si02. This is done in a wet etching step utilizing hydrofluoric acid (HF). 

Quartz wafers, 4 inch in diameter, were used in the experiments In the first step they were covered with polycrystalline silicon (poly-Si) using Low Pressure Chemical Deposition, LPCVD, technique In the photolithography step that followed the wafers were patterned in photoresist with a designed mask in a Canon 1 I mask aligner The pattern was developed and dry etched in CHF3/O2 plasma through the poIy-Si layer The... 

Samples of Si(lOO) or blanket polycrystalline silicon on Si(lOO) were examined in a scanning probe microscope (Park Scientific Instruments Autoprobe LS) using an etched silicon probe. Probes were untreated, and had a nominal radius of curvature of 10 nm. Imaging forces measured by the deflection of the cantilever (Ultralever B with normal spring constant of 0.24 N/m) were between 30 nN compressive and 5 nN tensile. 

We have recently observed that photoluminescence (PL) from p-type porous silicon (p-PS) surfaces, generated on single crystal or polycrystalline silicon substrates via an oxidative HF etching process,(5,4) is selectively quenched by SO2. An analytical relationship has been observed between SO2 concentration and the luminescence intensity providing the possibility of a new sensing methodology. 

In the selection of etch mask for deep glass etching, thick SU-8 is a choice, but SU-8 cannot be used in a HF bath (48%) because SU-8 does not adhere well to Si02 [123]. However, with a polycrystalline amorphous Si seed layer SU-8 adheres very well. For instance, with a 1.5-pm-thick polished poly silicon, a 50-pm-thick SU-8 can be deposited as the etch mask, leading to a maximum etch depth of 320 pm. Usually photoresist (2 pm thick) is only useful for shallow etch, less than 50 pm [123]. 

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