New phenomena observed in electrochemical micromachining of silicon

Abstract

This paper reports on electrochemical micromachining experiments, including a new passivation technique and several phenomena observed in I-V characteristics of Si in KOH, which have not been published in the literature so far. An energy-band diagram representation of the solid-state physics is utilized as a qualitative tool to understand some of these effects. A series of experiments has been carried out to find the optimum bias voltages for etching the p substrate and for passivation of the n regions, including lightly doped deep n diffusions and heavily doped shallow n + regions. A new passivation effect has been observed during these experiments. This new effect is that in the presence of an exposed p-n junction at the Si/KOH interface, the p-type material does not etch when both sides of the junction are held at the open-circuit potential (OCP), although one would normally expect that both sides of the p-n junction would be etched away. With the aid of the energy-band diagram representation, this fact is explained in terms of n-type inversion layer formation at the surface. Other phenomena observed in the I-V characteristics are associated with the illumination. The first concerns (100) n-type material, which exhibits a negative shift in the I-V characteristics for bias voltages around the passivation potential, PP. This is due to the photoinduced holes, which accelerate the oxide growth, thereby reducing the PP. In p-type Si, illumination causes an anodic shift in the I-V characteristic for positive currents. This is due to the photoinduced electrons, which decrease the band bending by filling the available states of the deeply depleted p-type surface. At voltages less than the OCP, the I-V characteristic shows a current runaway when the sample is illuminated.