The Chemical And Physical Mechanisms Determining The Morphology Of Wet-Chemically Etched Si(100)
Because harsh cleaning processes roughen silicon wafers, etchants that can produce smooth Si(100) surfaces have been a long-standing goal of the microelectronics industry. In this work, scanning tunneling microscopy was used to investigate the morphologies of wet-chemically-etched Si(100) surfaces, whereas the chemical composition was studied using infrared absorption spectroscopy coupled with a polarization deconvolution technique. Using this combined approach, aqueous ammonium fluoride etching of Si(100) is shown to produce near-atomically flat surfaces consisting of alternating rows of silicon dihydrides. A new assignment of the vibrational modes accounts for the presence of both strained and unstrained adsorbates on the etched surface. This smooth morphology was easily disrupted by the accumulation of hydrogen bubbles on the surface during etching. Roughening mechanisms created raised circular pillars and microfaceted etch pits if bubbles were not removed via one of several bubble-reduction techniques. In addition, density functional theory was used to predict the vibrational modes for a variety of hydrogen-terminated silicon surfaces. The calculated mode energies highlight the sensitivity of vibrational frequencies to interadsorbate strain, though in general, the calculated frequencies were not accurate enough to predict vibrational frequencies without substantial experimental confirmation. Lastly, the pH-dependence of buffered hydrofluoric acid (BHF) etching of Si(100) was examined. Below the critical pH of 7.8, etched surfaces became progressively rougher and covered with hillocks, whereas surfaces etched in solutions above this pH exhibit no change from the flat missing-row structure. Prominent stretching modes for the BHFetched H/Si(100) surfaces were also assigned based on trends observed in the pHmodified Si(100) spectra and previously well-characterized morphologies.
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