In-Situ Stress Control During Sputter Deposition

Sputter-deposited tungsten thin films exhibit high intrinsic stress. This stress can result in both in-plane and out-of-plane distortion when the films are deposited on thin membrane structures such as x-ray masks. To minimize these distortions, intrinsic stresses in these absorber films have to be low and reproducible. Several groups have recently reported that by precisely controlling the sputter deposition conditions, W films with low stresses can be produced. However, the reproducibility is limited. We have built a novel acoustic resonance system, in which one electrode, mounted behind the mask membrane, monitors its position and simultaneously provides an electrostatic drive to keep it vibrating at its resonant frequency. For typical membranes and deposition conditions, vibrational modes in the 1–10 kHz range are observed. During tungsten deposition, sputtering pressure is varied in response to changes in the membrane resonant frequency, so that the film stress is minimized. We have made a systematic study of the microstructure and stress of W thin films using a variety of characterization techniques. We have shown the feasibility of depositing low-stress (<10 MPa) W films by in-situ stress monitoring and control of sputtering pressure. By using a proper combination of substrate heating and sputter power density (thermal engineering), the reproducibility of in-situ stress control is greatly improved. The present experimental results of in-situ stress control during W sputter deposition are very promising for the successful utilization of low stress (<10 MPa) W films as absorbers for x-ray masks.