A three-dimensional MOS transistor formation technique with crystallographic orientation-dependent TMAH etchant

Abstract

Along with continuous scaling of MOS devices on LSI’s since early 1970s, it is widely recognized that three-dimensional (3-D) transistor structures will increasingly be important to extend further scaling in future. In response to the requirement, tetra methyl ammonium hydroxide (TMAH) etchant has been utilized to obtain 3-D MOS transistors named corrugated-channel transistor, CCT with vertical channels. The transistor channel is formed on steep vertical walls of almost exact {1 1 1} Miller index planes of single crystal {1 1 0} silicon substrate. Obtained performance of the CCT with 900 nm high and 54 nm wide beams is satisfactory resulting in almost five times more drivability as compared with planar transistor with the same planar area. Any adverse effect due to possible damages on etched surface has not been detected with respect to inversion carrier transport behavior. In this study, fundamental etching characteristics of TMAH are evaluated and it is clarified that the etch rate strongly depends on small deviation from the exact [1 1 2] direction of mask edge even if orientation selectivity exceeds 100 in appropriate conditions.