The isotropic etchant attacks the Si surface very strongly and non-uniformly, and the reaction rate is very sensitive to localized temperature changes due to the exothermic nature of the reaction. As a result, this etching process leads to varying, and sometimes excessive, surface roughness--which is what you observe. The acetic acid enables a more controlled reaction and minimizes localized overetching. Hope this helps. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | Shuvo Roy | | Research Assistant, Microelectromechanical Systems Group | | | |Office: Rm. 811 Glennan Bldg. | | Department of Electrical Engineering & Applied Physics | | Case Western Reserve University | | Cleveland, Ohio 44106 | | USA | | | |Telephone: 216-368-3051 Fax: 216-368-2668 | | | |E-mail: roy@mems4.eeap.edu sxr37@po.cwru.edu | | | |WWW: http://mems.cwru.edu/roy | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^