I n s i t u stress monitoring and deposition of zero-stress W for x-ray masks

We have developed and tested a computer-controlled system for monitoring in situ the stress of tungsten sputter deposited onto x-ray mask membranes. This system allows us to achieve zero stress (i.e., <5×107 dyn/cm2) tungsten. The stress is monitored via the resonant frequency of the x-ray mask membrane which changes during deposition due to W stress, mass loading, and temperature rise. Differences in W stress give rise to differences in curvatures of the plot of resonant frequency versus W thickness. This effect can be used to ensure that a film will have stress below 1×108 dyn/cm2 and can also be the basis of an automated closed-loop, in situ stress control system.