The use of Fluorinated Hydrocarbons (HFC's) for plasma-enhanced etching of surface layers of SiO2 and Si3N4 is widely used today. Obtaining high aspect ratios and precise dimension control becomes essential when operating at geometries below .25 µ. This is used in several critical semiconductor manufacturing process steps, from via etch to shallow/deep trench isolation.
A relatively large amount of experimental data is now available on this subject. A very good source for this is the AEDEPT database published by DuPont, which is currently available through their web site at www.dupont.com/zyron.
There are also three papers currently available which describe several interesting aspects of the use of HFC's for plasma etching. They are as follows:
- L. Murugesh et al., Semiconductor Fabtech, pp.185, 1998
- R. Lindley et al., Solid State Technology, pp.93, 8/97
- Y. Zhang et al., J. Vac. Sci. Technol., A 14(4) pp. 2127, Jul./Aug. 96
Some of the salient facts from these works are the following:
Low C/F ratio was compared to high C/F ratio for etch rates and selectivity, and it was concluded that chemistries with a high C/F ratio (with small amounts of H2) produce high SiO2-to-Si3N4 selectivity (>28). On the other hand, low C/F ratio chemistries achieve poor SiO2-to-Si3N4 selectivity, but do enable SiO2-to-Si selectivity. These results are rationalized on the basis of polymeric film formation and radical chemistry in the plasma. High C/F compounds used were C2F4 and C3F6, while low C/F compounds were CF4 and CHF3
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Along the same lines, chemistries based on C4F8/CO/CHF3/N2 were found to provide high selectivity to overlayers and underlayers. These chemistries were developed specifically for 0.35 and sub-0.35 µ applications requiring high selectivity. When compared with "standard" CHF3/CF4 based chemistries, the new chemistries provided dramatically increased selectivity with little drop in etch rate. This higher selectivity can provide definite advantages in several processes. For example bilevel etching, where selectivity higher than 60:1 are obtained for TiS2, allows processes to stop on this underlayer and continue to etch Poly or Si on different parts of the surface. This in turn eliminates additional masking and etching steps. Other advantages can be found in processing of high aspect ratios contacts and vias and self-aligning contacts.
It is worth noting that some recent data from DuPont indicates that the purity of C4F8 may influence the selectivity Si-Photoresist, where the impurities in question are chlorinated hydrocarbons.
While not directly related to etching, another very new use for HFC's is the deposition of low-k films to be used for IMD applications. In this case, initial data indicates very promising results for amorphous fluorinated Carbon (a-FC) films, obtained by HDP-CVD processes. The compatibility of these films and processibility are now being studied for processing at or below 0.25 µ, where dielectric constants lower than 2.5 can be obtained.
Scott Specialty Gases offers the most complete line of HFC's and PFC's (Perfluorocarbons), allowing process choices in all ranges of C/F ratios. Materials like C4F8 are available in several grades and containers. The latest development ion in this field is C5F8, offering an even higher C/F ratio than C4F8, and thus potentially higher selectivity at comparable etch rates. Scott also offers exclusive products from the Zyron® line of Fluorinated hydrocarbons manufactured by DuPont.
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