Doping density, not valency, influences catalytic metal-assisted plasma etching of silicon

Metal-assisted plasma etching (MAPE) of silicon (Si) is an etching technique driven by the catalytic activity of metals such as gold in fluorine-based plasma environments. In this work, the role of the Si substrate was investigated by examining the effects of the dopant concentration in both n- and p-type Si and the dopant atom type in n-type Si in SF6/O2 mixed gas plasma. At the highest dopant concentrations, both n- and p-type Si initially exhibit inhibition of the MAPE-enhanced etching. As the etch progresses, MAPE initiates, resulting in catalytic etching of the underlying Si at the metal–Si interface. Interestingly, MAPE-enhanced etching increases with decreasing doping concentrations for both n- and p-type Si substrates, distinct from results for the similar but divergent, metal-assisted chemical etching of silicon in liquid. Our findings show that the metal–Si interface remains essential to MAPE, and surface enrichment of the dopant atoms or other surface chemistries and the size of metal nanoparticles play roles in modulating catalytic activity.

nm Au and 10 nm Cr were patterned by electron beam lithography and metal deposition on ntype phosphorus doped Si substrates of varying levels of doping (resistivities 0.001-0.005,0.08-0.5,1-10, and 10-20 Ω-cm).The circular nanoparticles measured 200 nm in diameter and were patterned in 12x12 arrays.The level of doping decreases from the heaviest doped substrate in the leftmost column (resistivity 0.001-0.005Ω-cm) to the lightest doped substrate in the rightmost column (resistivity 10-20 Ω-cm).Si substrates were etched for 1 min (top row), 2 min (middle row), or 3 min (bottom row) in mixed SF 6 /O 2 plasma at 25% O 2 concentration.
At 1 min, inhibition of the MAPE enhanced etching occurred in the heaviest doped Si substrates (resistivities 0.001-0.005and 0.08-0.5 Ω-cm), which was visualized by the formation of Si pillars instead of etched pits underneath each Au/Cr nanoparticle.No inhibition of MAPE was seen in the lighter doped substrates (resistivities 1-10 and 10-20 Ω-cm) at 1 min etch.This inhibition effect was not seen in the n-type Si substrates at 2 and 3 min as MAPE appeared to proceed as expected.At 2 min, all substrates, regardless of doping level, exhibited etch pits characteristic of locally enhanced etching underneath each nanoparticle of the array.These etch pits became more irregular in border and shape as the etch advanced to 3 min of etching.Images for 1 min etches were taken at 45º tilt to better visualize the etch features.All scale bars represent 2 µm.The nanoparticles measured 200 nm in diameter and were deposited with thin films of 5 nm Au and 10 nm Cr.The level of doping decreases from the heaviest doped substrate in the leftmost column (resistivity 0.001-0.005Ω-cm) to the lightest doped substrate in the rightmost column (resistivity 10-20 Ω-cm).Si substrates were etched for 1 min (top row), 2 min (middle row), or 3 min (bottom row) in mixed SF 6 /O 2 plasma at 25% O 2 concentration.Similar to the n-type Si, early inhibition of MAPE in the heavily doped substrates (resistivities 0.001-0.005and 0.005-1 Ω-cm) was evident from the formation of Si pillar instead of etched pits at 1 min.This inhibition did not occur in the lightly doped substrates (resistivities 1-10 and 10-20 Ω-cm).
By 2 and 3 min, all substrates exhibited MAPE enhanced etching as etch pits formed underneath each nanoparticle.No enhancement of etching occurred outside the boundaries of the original array even as the borders of the etch pits became more irregular as the etch proceeded.Images for 1 min etches were taken at 45º tilt.All scale bars represent 2 µm.

Figure S1 .
Figure S1.MAPE enhanced etching of n-type Si substrates.Circular nanoparticles with 5

Figure S3 .
Figure S3.MAPE etching of undoped Si substrates.Undoped Si substrates were patterned with circular nanoparticles of 5 nm Au/10 nm Cr in 12x12 arrays and etched in SF 6 /O 2 mixed plasma at 25% O 2 concentration.Panels a-b show tilted SEM images of etching at 1 min, panel c shows etching at 2 min, and panel d displays etching at 3 min.All undoped substrates exhibited MAPE-catalyzed etching at the Au-Si interface and no inhibition of MAPE.Scale bars for panels a, c, d represent 2 µm, and the scale bar for panel b represents 500 nm.

Figure S4 .
Figure S4.APT analysis of the 2 min etched structure found in the n-type Si substrate.(a) APT specimen collected from the edge of the etch pits (peak region).(b) APT analysis obtained from the region marked in (a).Au (yellow) was observed at the interface of Cr (pink) and Si (gray).(c) Top-down view showing Au was found as clusters and particles near the surface of the Si.The characteristics of small Au particles are similar to what observed in 1 min etch sample, but they are found further away from the original position of the as prepared nanoparticle.(d) Mass spectrum of the Au + peak; under 650 Au + ions were detected in this sample.