Thick porous silicon formation using implanted mask technology

Silicon Porosification: State of the Art

“…For defining the regions where Si is exposed to the electrolyte, different types of etch masks can be used. The peculiarities of using different masks were discussed by Connolly et al, 372 Splinter et al 373 and Hedrich et al 156 Masking layers have to withstand Downloaded by [Stanford University Libraries] at 04: 49 11 October 2012 the aggressive HF electrolyte for durations from ≈30 min up to a 10 hours; the time required for etching n-Si-macro pores to a depth of >400 µm. It is quite convenient to use the structured photoresist directly for obvious reasons.…”

“…The photoresist layer lifts off from the substrate and the result is a lateral underetching. 373 A 15 nm layer of chromium between the silicon wafer and the photoresist improves the adhesion and reduces underetching. This mask can withstand the electrolyte for about 30 minutes with underetch of about 10 µm.…”

“…90,155,156,372,373 It was established that silicon carbide gives good results but it is expensive and has a high intrinsic stress and poor adhesion. Silicon nitride tends to shows better results but is not completely resistant against fluoric solution.…”

“…Silicon nitride can be easily deposited using standard Low-Pressure Chemical Vapor Deposition (LPCVD) or Plasma Enhanced Chemical Vapor Deposition (PECVD) methods, but the thickness of a stoichiometric Si 3 N 4 layer deposited by those methods is limited to 200 nm as a result of residual stress building up. 373 Moreover, stripping this mask without damaging to the porous silicon layer obtained after the etching may be difficult without damaging PSi.…”

Optimized plasma-polymerized fluoropolymer mask for local porous silicon formation

“…In the literature, different kinds of HF resistant materials have been tested and proposed as a masking layer for deep PS formation. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Among them, silicon nitride (Si x N y ), 15,16 bilayer of polycrystalline silicon/silicon dioxide, 18,22 or tri-layer stack of poly-Si/Si x N y /SiO 2 12 are the most widely used masking materials due to the maturities of deposition techniques and the compatibility with current silicon processing. However, the common drawback of all these masking materials is the complicated post-etching mask removal procedures when bare silicon surface is ultimately required.…”