In-Plasma Photo-Assisted Etching of Silicon in a High-Density Chlorine Discharge



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Plasma etching is indispensable in manufacturing of microelectronic devices. It involves the removal of material from selected regions of a substrate in a reactive plasma most often created by a radio-frequency (RF) powered gas discharge. The plasma serves as a source of neutral and ionic reactants, which combine with the atoms in the material to form volatile products. The anisotropic etching profile is achieved through the synergy between neutral species (radicals) and energetic ion bombardment, which is critical for precise pattern transfer, especially for features with lateral dimensions of <10 nm. Low ion energies (10s eV) are required to further advance this capability to etch material with atomic resolution, low damage, and ultra-high selectivity. However, at low ion energies, photo-assisted etching (PAE) has been shown to be very important. In-plasma PAE is an alternative etching pathway catalyzed by vacuum ultraviolet (VUV) photons generated in the plasma, which could cause substantial complications for processes due to its comparable etching rate with ion-assisted etching (IAE), when ion energy is low, and its impact on profile evolution. In this thesis, etching of p-Si in 60 mTorr 10%Cl2/90%Ar Faraday-shielded inductively coupled, high density plasma was investigated under both ion-assisted etching (IAE) and photo-assisted etching (PAE) conditions. Real-time etching rates and after-etching Si surface chemical compositions were obtained by laser interferometry and vacuum-transfer X-ray photoelectron spectroscopy (XPS), respectively. Precisely controlled ion energy distributions (IED) were obtained by applying pulsed negative DC bias on the conductive sample stage. Above a ~ 36 eV threshold at a total flow rate of 250 sccm, the IAE rate increased with the square root of the ion energy. The corresponding PAE rate below the threshold energy was ~ 400 nm/min. At a relativity low flow rate (total flow rate of 50 sccm), the IAE threshold energy was ~ 25 eV and the PAE rate was ~ 300 nm/min. In contrast to DC bias, etching under RF bias did not exhibit a threshold ion energy because of the wide IED. XPS spectra revealed that the surface layer under PAE conditions had a significantly lower chlorine content, composed of only SiCl. Under IAE conditions, however, silicon dangling bonds (Si●), SiCl2, and SiCl3 were found on the surface, in addition to SiCl, with a relative abundance of SiCl>SiCl2>SiCl3. The absence of higher chlorides and Si● under PAE conditions suggested that VUV photons and above threshold-energy ions interacted with the surface very differently. By varying the duty cycle of the pulsed DC bias, it was found that the IAE rate scaled with the energetic ion dose, but only for low duty cycles. For higher duty cycles, the apparent IAE yield fell off with an increasing Cl coverage on the surface, as duty cycle went up, which pointed to a negative synergy (anti-synergism) between PAE and IAE as the explanation. This anti-synergism was further supported by the observed decrease of the total etching rate with increasing period of the pulsed DC bias. A plausible mechanism was that increasing the pulsing period caused more near-surface damage, creating more recombination centers that led to higher loss rate of electron-hole pairs through recombination, thereby reducing the PAE rate.



PAE, Anti-synergism


Portions of this document appear in: Hirsch, Emilia W., Linfeng Du, Demetre J. Economou, and Vincent M. Donnelly. "Evidence for anti-synergism between ion-assisted etching and in-plasma photoassisted etching of silicon in a high-density chlorine plasma." Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 38, no. 2 (2020): 023009.