Problem stsetch SNF 2010-07-17 07:46:20: Wafer-to-wafer etch rate is not constant

Etched 8 wafers using Deep recipe. The etch rate got slower as etching more wafers. The etch rate of 1st wafer was 2.28um/min, but it decreased gradually and became 1.75um/min for 8th wafer.
During etching, He process flow was initially 1.60, but it dropped to 1.54~1.55 later.