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TimeDomain CVD, Inc. |
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Is Plasma the Answer?Combining TEOS with ozone, in an attempt to preserve the excellent step coverage of TEOS/oxygen LPCVD at lower temperatures, was successful but as we noted, results in significant problems with film stress, moisture absorption, and stability not observed by most other techniques. Another approach to achieving deposition from TEOS at lower temperatures is to add plasma excitation. Plasma-enhanced deposition of oxides from TEOS (PETEOS) was first reported in the late 60's. Extensive commercial development was performed in the 80's. Modern commercial processes employ parallel-plate showerhead reactors with RF plasma excitation, typically operating at pressures of 1 to 10 Torr, with gas flows of a few slpm including a few 10's to hundreds of sccm of TEOS. Bubblers or liquid injection is employed to deliver the TEOS, as in thermal CVD. Film propertiesWith plasma excitation, high deposition rates are obtained at temperatures of 300-450 C from TEOS / oxygen. In single-wafer reactors rates of 5000-10,000 A/min can be achieved, using pressures around 6-10 Torr and very small electrode gaps (<6 mm). The problems encountered in thermal TEOS/ozone deposition (water absorption, silanol, stress, cracking) are also present in plasma-deposited films, but the availability of the plasma allows improved flexibility in dealing with them. Increasing RF power, increasing pressure while decreasing electrode gap, and increasing the oxygen:TEOS ratio, all help produce films which are "dry" and stable in air. The use of dual-frequency reactors is effective in adjusting stress from tensile to compressive with little effect on other parameters. Conformality is greatly superior to that obtained from plasma or thermal silane/oxygen deposition, but inferior to thermal TEOS. "Vertical" deposition, presumably from ionized species, plays a significant role in the total deposition rate. It has been reported that dual-frequency processes achieve better conformality than single-frequency processes. By alternately depositing oxide with PETEOS process, and subjecting the wafer to a sputter etching step (in a separate chamber) which removed the overhanging corners, excellent gap fill may be obtained but at a significant cost in throughput: this is the "dep-etch" process commercialized by Applied Materials in the late 80's. Due to the use of magnetized plasmas for sputter etch, this process also encountered some problems with plasma damage of underlying transistors. Plasma TEOS films have also been observed to display large fixed charge densities, particularly if deposited under conditions producing compressive stress, after high-temperature annealing. Plasma TEOS films are often used to encapsulate thermal TEOS-ozone oxide or spin-on glass, to protect it from ambient moisture. A PETEOS underlayer has better step coverage ability than plasma-enhanced silane films, so the thermal or SOG oxide has an easier task filling trenches or holes with a PETEOS underlayer. However, PETEOS is a relatively poor moisture barrier, and thus conformality and reliability impact must be balanced.
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