J.Q. Liu, Y. C. Tai, J. Lee, K. C. Pong, Y. Zohar and C. M. Ho, "In Situ Monitoring and Universal Modelling of Sacrificial PSG Etching Using Hydrofluoric Acid," Proceedings, IEEE Micro Electro Mechanical Systems Workshop (MEMS '93), Fort Lauderdale, FL, pp. 71-76, Feb. 7-10 (1993).