Chamber Wall Interactions with HBr/Cl2/O2 Plasmas Studied by the “Spinning Wall” Method
Srivastava, Ashutosh K.
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Plasma etching is a widely used method to pattern materials in the fabrication of microelectronic devices. As the minimum feature sizes, or so-called critical dimensions, shrink beyond 14 nm, plasma etching processes need to be ever more tightly controlled. At low pressures in the range of 1-100 mTorr, typical in current plasma processes, the ~cm mean free path of species ranges are comparable to reactor dimensions. Consequently, gas phase reactions (especially three-body processes) become less likely and heterogeneous reactions on chamber walls become increasingly important. The surface layers formed on the reactor walls become a source of production or loss of species. As a result, shifting plasma composition leads to process drifts, leading to changes in etching rates, profiles, selectivity, and yields. Hence, it is of prime importance to understand the interactions of plasmas with the dynamic chamber wall surfaces. HBr plasmas are used to etch Si, as well as GaN, PZT, InP, indium zinc oxide and other materials. In Si etching, HBr plasmas create better anisotropic profiles than Cl2 plasmas, with better selectivity toward SiO2. Selectivity can be further improved by adding oxygen to the plasma. The feed gas composition of HBr/Cl2/O2 plasmas is optimized to best meet the needs of the particular application. Keeping such a complex process stable over time requires tight control over all plasma parameters, including reactor wall conditions. Here, we have studied the interaction of HBr/Cl2/O2 inductively-coupled plasmas with reactor chamber wall deposits, with and without Si etching, using the “spinning wall” technique. The spinning wall is part of the reactor chamber walls, allowing near-real time analysis of the composition of surface layers via Auger electron spectroscopy, and determination of species desorbing off the walls by mass spectrometry. In HBr plasmas with no bias voltage on the Si substrate, and hence no Si etching, HBr is ~30% dissociated and H2 and Br2 form in the plasma. Layers deposited on the reactor chamber contained little if any Br under these conditions. Adding O2 to an HBr plasma leads to formation of Br2 and H2O products that desorb from the spinning wall. H2O has a very long residence time on the surface. With bias voltage applied to the Si substrate in an HBr plasma, SiBr and SiBr3 are prominent mass spectrometer signals, SiBr2 and SiBr4 appear to be the major gas phase components, and a SiOxBry layer deposits on the spinning wall. Adding 20% O2 to HBr stops etching and eliminates Br from the surface layer, indicating that Br on the reactor walls is a result of SiBrx impingement, and not from bromination by impinging Br. With HBr/Cl2 plasmas and no bias on the stage, a SiOxCly layer deposits; no Br is detected. In addition, the mass spectrum of HBr and Cl2 gas mixture without plasma revealed HCl, Br2 and BrCl species. Further experiments revealed that these products were the result of reactions between HBr and Cl2 on the plasma reactor walls. With plasma and bias on the Si substrate, both Br and Cl incorporate in a depositing layer. Adding 20% O2 to a HBr/Cl2 plasma with substrate bias suppresses Br adsorption, but Cl still adsorbs. In 40% O2/HBr/Cl2 plasmas with stage bias, Cl adsorption also ceases.