Photoelectrochemical etching of isolated, high aspect ratio micorstructures in n-type silicon (100)

Abstract

Three-dimensional integration techniques have become increasingly popular to meet the ever rising demand of high capacity and reduced package size in microelectronics devices. Through Silicon Vias (TSVs) offer an efficient method to achieve 3D packaging with shorter interconnection length and higher interconnect density relative to conventional wire bonding. Wet electrochemical etching is a simple technique which may be used to create deep structures in silicon and is relatively low cost compared with Reactive Ion Etching or Laser drilling. Historically, a primary challenge is passivating TSV (macropore, microstructure) sidewalls against etching at sidewall thickness greater than twice the depletion region width. Lehmann et al created macropores in n-type silicon (40 Ω-cm) with sidewall thickness ~ six times depletion region width, however the wall surface smoothness differed from the macropores passivated by the depletion region. In this research, an attempt was made to create isolated (sidewall thickness = ∞ times the depletion region width) microstructures in patterned n-type silicon (100). For the first time, high aspect ratio (~5:1) deep microstructures with non-porous sidewalls at isolated pitches (>100 µm) are demonstrated using frontside illumination with photoelectrochemical etching. Further, the microstructure aspect ratio is observed to increase with etch duration. While literature on backside illumination illustrates porous sidewalls at isolated pitches, results from this study show frontside illumination can be used to create non-porous microstructures at large pitches. The microstructure etch rate is a function of light intensity and supporting electrolyte composition whereas the microstructure sidewall etching is demonstrated to be a function of applied anodic bias. Anodic bias controls the depletion region width which governs the dominance of drift and diffusion currents. Isolated microstructures are obtained at a low anodic bias where silicon dissolution is controlled by the diffusion current. The microstructure surface smoothness is affected by incident light wavelength; sidewall roughness is minimized by conducting the dissolution reaction with photons having shallower absorption depth in silicon. The work shows photoelectrochemical etching of isolated, anisotropic, high aspect ratio microstructures is possible using frontside illumination with low wavelength light at low anodic bias.