Low-Temperature Epi Silicon

As device geometries become smaller and smaller, the epi layers also scale down and become thinner. The thinner the epi layer, the more serious the autodoping problem becomes. Very sharp transitions are needed, and cannot be ob tained readily at normal temperatures ( 1000°C). Therefore, there is great interest in finding a low-temperature epi process that produces good quality epi layers. 

A number of researchers have grown epi films at temperatures as low as 800°C.25 However, in all cases, a high temperature (1040 to 1180°C) clean step 

Using these techniques, epi films have been deposited at temperatures as low as 650°C. At these temperatures, there is no autodoping problem. However, whether or not these are useful films for devices is not yet proven. Also, it remains to be seen whether a reactor based on these techniques will be economically viable. 

Another approach to this problem involves heating the wafer at 750 F at very low pressures ( 10 10 Torr prior to deposition.28 This has the effect of removing the native oxide by evaporation of SiO. Depositions were achieved in the temperature range of 750° to 850°C in SiH4 + H2. Since the authors were developing a hot-wall system with many wafers stacked close to each other, the deposition was carried out at 2 mTorr. Deposition rates of 20 to 45 A/min were achieved. As expected, dopant transition widths were very narrow, several hundred angstroms. Again, device studies on such a system have not yet been done. 

Chemical Methods of Film Deposition, in Thin Film Processes, 

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