Etching Rate Ratio

Alkaline etchants are anisotropic. The etch rate for the (111) crystal planes of the Si crystal is smaller by about two orders of magnitude than the etch rate of any other crystal plane. The etch rate ratio between other crystal planes like (100) and (110) depends on etchant concentration and temperature, but doesn t usually exceed a factor of two [Sa6]. Addition of oxidizing agents reduces the anisotropy. The etch rate of (100) Si and Si02 in KOH at different temperatures is shown in Fig. 2.2. 

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. 

Figure 9. Silicon to Si02 etch rate ratio for atomic fluorine etching as a function of substrate temperature. Constructed from etch rate expressions...

Table II. Relative etch rate ratios for ester X-substltuted methacrylates, 4CH2-C(CH3)C02X)-.

X. I PE Etch Rate Ratio vs Si02 RIE Etch Rate Ratio vs Si02 lon-millingc Rate Ratio g vs Si02 Gs Gx... 

Electron beam positive (top) and negative (bottom) resist etching rate ratios for three sets of etching conditions. 

Table VI Plasma etch rate ratios for copolymers.

Table VII Commercial photoresist etch rate ratios.

The hydrogen was introduced both as an additive gas (CF, —mixtures) and intramolecularly by using CHFj as the etch gas. This concept has been extended by several groups and impressive SiOj-to-Si etch-rate ratios have been obtained. Ephrath , for example, using CF —mixtures in a reactive-ion etching system has observed ratios of 30 1. An example of this work is shown in Fig. 3.2. Lehmann and Widmer using CHFj gas also in a reactive ion etching mode have obtained ratios of 15 1. We have previously shown that mixtures of CF, and CjF behave similarly to CF —Hj mixtures with respect to the SiOj-to-Si etch-rate ratio. 

The mechanism we believe is responsible for the large SiOj-to-Si etch-rate ratios which have been obtained in fluorine-deficient discharges is based on several experimental observations. First of all, it has been shown that there are several ways in which carbon can be deposited on surfaces exposed to CF, plasmas. One way is to subject the surface to bombardment with CF ions which are the dominant positive ionic species in a CF plasma. The extent to which this can occur is shown by the Auger spectra in Fig. 3.3. Curve (a) is the Auger spectrum of a clean silicon surface and curve (b) is the Auger spectrum of the same surface after bombardment with 500 eV CFj" ions. Note that the silicon peak at 92 eV is no longer visible after the CFj bombardment indicating the presence of at least two or three monolayers of carbon. Another way in which carbon can be deposited on surfaces is by dissociative chemisorption of CFj or other fluorocarbon radicals. 

Figure 3. Plot of etching rate ratios as a function of...

Brominated poly(l-trimethylsilylpropyne) is an example of a substituted polyacetylene that is suitable for bilevel-resist processes (34). Requiring both exposure and postexposure bake (PEB) steps, samples of the polypropyne having a mole fraction of bromine from 0.1 to 0.2 per monomer unit exhibit sensitivities in the order of 25 mj/cm. Submicrometer resolution has been demonstrated, and etching-rate ratios relative to hard-baked photoresist planarizing layers are —1 25. 

The use of organometallic polymers in the bilevel RIE-PCM scheme was first reported by Shaw et al. (185). They used polysiloxane (structure 3.16) as a negative-working electron beam imaging layer. The etch rate ratio... 

FIGURE 7.32. Etch rate ratio of (100)/(111) and (110)/(111) in different etching solutions. Data from Table 7.1. 

Etch rate Ratio of X/(1I1) Etch rate Ratio of X/(111)... 

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