Fluorine-Deficient Discharges and Etch Rate Selectivity

2 Fluorine-Deficient Discharges and Etch Rate Selectivity 

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. 

A second observation bearing on the selective etching mechanism is shown in Fig. 3.4. In this figure the rate of accumulation of carbon caused by CFj bombardment is plotted as a function of ion dose for Si and oxidized Si surfaces. It can be 

Etch rates of SIO and Si as a function of molecular percentage of hydrogen added to CF in a reactive ion etching system (data from Ref. ) 

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