Hej! It could be possible, that the chamberwalls need to be cleaned. I have no idea why the plasma only is unstable while using mixed gases. Is the gas flow stable? But plasma tends to get unstable when recently hard-to-etch materials (like Ni) were etched. Those materials coat the chamber walls with a conductive film, which causes then RF breakdowns. /Andreas On Sep 19, 2008, at 3:22 AM, yuzhul@eden.rutgers.edu wrote: > Hi, We have a STS ICP, working fine until recently. > > Problem is when using pure gas, such as O2 or CF4 alone, then plasma > is > stable. However if using mixed gas, O2+CF4, then plasma unstable. > Lower > top coil power helps but etch rate would be much slower compare to > before. > > Can someone give a suggestion? Thanks.