Electrochemical etching of isolated structures in p-type silicon

The role of the space charge region (SCR) was demonstrated in isolated structures for the first time. Electrochemical etching was conducted in differently pitched structures in p-type silicon. A design of experiment was also performed to evaluate the effect of electrolyte and applied potential. Finally chemical etching was demonstrated as a possible post-etch clean to porous structures. The results showed that the morphology of etched structures in pre-patterned silicon is strongly influenced by the SCR even in isolated structures. Using lightly-doped p-type Si (1,000-10,000 Ω-cm), deep macropores were obtained in wide-pitched patterns. However, the diameter of the pores grew larger from its initial opening size. As the pitch increased, sidewall etching became more severe and the ratio of the final pore wall thickness to the pitch decreased, which was expected from SCR effect. The results agreed with Lehmann’s theory which state that if the pore wall thickness is greater than twice the SCR width, sidewall etching takes place. The etched structure can either be porous or electropolished depending on prevailing local etching condition. The results from design of experiment (DOE) meanwhile showed that deep structures are more favored at higher HF concentration and potential. Five out of 18 samples showed 40% better anisotropy than KOH while 4 samples showed etch rates better than 1 μm/min. The mean etch rate in tight pitch structures were only 18.42% higher than in isolated structures. Lastly, chemical etching in 1:3:2 HF:HNO3:CH3COOH at 5-10 oC showed success in polishing of porous structures with a high etch rate of up to 5 μm/min. In light with the SCR effects, a design specification for macropore can be tailored by carefully adjusting the doping level of silicon while providing optimum etching condition