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Publication Date: 2014-Nov-17
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Cleaning for silicon thin-film deposition chamber  

Undesired silicon compounds left in the chamber can be removed by vaporous anhydrous hydrogen fluoride (AHF) in combination with inert gases such as argon (Ar) additive in remote plasma process

Remote plasma generator (RPS) is capable of decomposing NF3 to generate F radicals with high dissociation efficiency (>96%).  Even RPS manufactured by MKS (Astron-i, 6kw, maximum flow rate: 3slm) is originally designed for NF3 etching application, AHF (Supplier: TCL, Purity: 99.9%) was used as main reactant to etch silicon instead of NF3, but it did not contribute to etching.  Ar is often used as assisting gas in the plasma etching process.  For this reason AHF was mixed with post remote-plasma excited Ar to etch silicon.  In contrast to the statement in the literature (Y.Nakazawa, Selective etching of silicon oxide with remote plasma excited anhydrous hydrogen fluoride, Japan Journal of Applied Physics, 37, 536, 1998), it did not etch silicon.  However silicon was successfully etched when plasma-excited AHF+Ar admixture was used in RPS at the condition (chamber temperature: 200oC, working pressure: 5Torr).  Ar/AHF should set at range between 0.4 and 0.6 to boost up etching rate.  Ideally 0.5 is preferable.  Etching rate declined when Ar/AHF sets at 0.5 above.  It may result from the effect of dilution by means of considerable Ar.  In addition uniform etching was achieved by AHF+Ar.  It is assumed that exited Ar makes AHF decompose easily in the plasma region even at low plasma power.  In FIG 1, Ar/AHF was varied to find the optimum ratio on increasing of etching rate of silicon.  Plasma power of RPS is not changeable manually as it is automatically optimized by RPS itself.  Thus intentional control to apply high power is impossible.  In fact HF binding energy (567kJ/mol) is much higher than that of NF3 (243kJ/mol).  Hence further high plasma power is needed to break perfectly the bonding between H-F to generate numerous F radicals.  Current etching rate of silicon is highly expected to be higher than RPS if in-situ plasma applicable high plasma power is used.  AHF+Ar mixture can also be supplied from cylinder.  Otherwise AHF and Ar are dynamically mixed in the blender in advance and eventually it is supplied into the chamber.  AHF was tested in thermal etching process at elevated temperature (up to 200oC) but it did not work.  Further high temperature may be needed to generate active species. 

Figure 1 Etching rate of silicon as a function of Ar/AHF ratio in remote plasma process

Silicon plasma etching with AHF+Ar mixture enables to alleviate cost of ownership compared with currently used NF3 remote plasma cleaning.  It also achieves zero emission of global warming potential in the production.  An apparatus to detoxicate NF3 left in exhaust also is not needed.  

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