Phosphorus-doped oxide films

Table III. Quantitative Analysis of Phosphorus-Doped Oxide Films by EDS...

The immediate concern about the presence of phosphine in doped oxide films was safety in handling these materials. The permissible exposure limit (PEL) set by OSHA is 0.3 ppm, and the concentration immediately dangerous to life and health (IDLH) is 200 ppm. All precautions must be taken when etching plasma PSG of BPSG wafers. A basket of twenty 3 inch wafers, 5000 angstroms thick, containing 5 wt. % phosphorus could release as much as 2500 ppm of phosphine or 12.5 times the IDLH. 

To Identify the foul smelling gas, a closed system was built in which the film was dissolved. The liberated gas was driven to a reaction chamber and oxidized. If the gas were phosphine, it would be converted to phosphate ion and as such could be analyzed. This test was positive for every plasma and plasma enhanced phosphorus doped oxide that was analyzed. Deductively it was concluded that the gas... 

It is also well known (2) that not all oxoacids of phosphorus react with molybdenum ion to form a positive blue complex at the same rate. Time studies were done on plasma PSG wafers. Figure 1 shows distinct differences between the phosphate ion and doped oxide film from a wafer when analyzed with and without oxidation. 

If desired, plasma oxide films can be doped much as the plasma nitride film we discussed earlier. In fact, doping with boron and phosphorus has been carried out as an alternative to the standard atmospheric-pressure thermal CVD process for BPSG.11 12 The latter process has the drawbacks of high defect density and poor thickness uniformity, so it was hoped that plasma BPSG would be an improvement. However, there are differences in the films in terms of H2 and N2 content, and their effect on reflow temperature, intrinsic stress and passivation effectiveness had to be examined. 

As an example of the use of this technique, a silicon wafer lightly doped with phosphorus is doped with additional phosphorus by ion implantation (dose of 3.5 x 10ncm"2). A thermal oxide film of 857 A thickness was initially grown on the wafer. The variation of dopant concentration with depth from the oxide-silicon interface is shown in Figure 16. The rise in dopant close... 

In past years, the analysis of doped dielectric MOS films has always led to the conclusion that the dopant materials existed in their fully oxidized state. Thus, phosphorus existed as phosphorus pent-oxide and as such could easily be quantitatively measured in aqueous solutions as the phosphate ion. Although many suspected the presence of phosphorus trioxlde, there was no evidence of its existence. 

The determination of specific phosphorus compounds in thin films is important. Only through wet chemical analysis was it possible to first discover the presence and then to accurately measure the quantities of P2Os, P203, and phosphine found in plasma, plasma-enhanced, LPO-LTO (low-pressure oxide-low-temperature oxide), and CVD (chemical vapor deposition) processes (3). Methods such as X-ray or FTIR spectroscopy would have seen all phosphorus atoms and would have characterized them as totally useful phosphorus. In plasma and plasma-enhanced CVD films, phosphine is totally useless in doping processes. 

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