I Situ Spectroscopic Ellipsometry Studies of Silicon Film Crystallinity and Interface Structure Deposited by DC Reactive Magnetron Sputtering.
This work reports a systematic series of in situ spectroscopic ellipsometry (SE) studies on silicon film growth by reactive magnetron sputtering of a Si target in (Ar + H_2) at low temperatures (<=600^circ C). The complete crystalline film growth zones have been identified for the first time. muc-Si:H films (fine grained polycrystal silicon hydrogen alloy) are deposited at temperatures from 150 to 300^circC with hydrogen partial pressure above 4 mTorr. In this growth region, SE studies show clearly that energetic reflected hydrogen (fast hydrogen) implants into the subsurface region (~45A deep), and leads to the network reconstruction which forms muc-Si:H. This is the first direct experimental data substantiating the "chemical annealing" hypothesis which appeared in the amorphous silicon literature in 1987. In addition, epitaxial growth on Si (100) is observed being extended by hydrogen injection in the plasma at substrate temperature of 230^ circC; the modification of a silicon wafer in the near surface region is also observed, presumably due to fast hydrogen. There is no macroscopic etching observed in our RMS system even with pure hydrogen plasma. SE studies show that etching by thermal atomic hydrogen generated by a hot tungsten filament has a selectivity of ~4 for muc-Si:H over a-Si:H and generates a very rough surface. The "selective etching", i.e., H preferentially etching out a-Si:H phase, is not the dominant mechanism for muc -Si:H deposition by RMS. In fact, muc -Si:H films deposited by RMS are always less rough compared to those by plasma enhanced chemical vapor deposition. The polycrystalline silicon (px-Si) films with mean grain diameter >=400 A and (110) preferred crystal orientation are deposited at 470 ^circC or above, and hydrogen injection interrupts px-Si growth in this temperature region. SE studies show that the px-Si formation is limited by crystallite nucleation. For direct deposition on glass substrates at 470^circC, the initial ~0.3 μm is amorphous silicon, and the growth becomes fully polycrystalline by ~0.6 mum. However, if a ~100 A muc -Si:H layer is deposited first on glass, px-Si can grow on this seeded substrate with no detectable amorphous interfacial layer at the same 470^circC. Since magnetron sputtering is a large-area, high throughput technique, the latter approach appears very attractive for the emerging px-Si thin film transistor technology for flat panel displays. A general analytical formalism which permits the analysis of SE data for thin films on transparent substrates has been developed in this work, which enhances SE as a technique for in situ thin film characterization under conditions typical of actual opto-electronic device-making. The interfaces between silicon films and transparent conductive oxide (TCO) substrates have been studied. The reduction reaction of SnO_2 exposed to a H or Si-containing deposition flux has been observed and quantified. By contrast, no reduction of ZnO is found during silicon film growth. However, silicon films need 100A to coalesce on ZnO compared to ~15A on glass substrate.
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- Engineering: Materials Science; Engineering: Electronics and Electrical; Physics: Condensed Matter