Several techniques exist for estimating the potential lifetime of a planar diffusion source. One method is to periodically dope a silicon wafer with a source held in a diffusion furnace and observe how the resulting sheet resistivity varies with time. Figure 3 shows the sheet resistivity obtained on silicon wafers doped for 30 minutes with GS 139 BoronPlus sources that were held at 975oC and 1025oC. Little change is observed during the 700-800 hours of test time. Similar data are shown for GS245 sources.
A second method of estimating the source's lifetime is to measure the amount of weight loss at a use temperature as the B2O3 evolves. When the source no longer loses weight, the evolution of B2O3 has ended. A weight loss curve plotted in Figure 4 for the GS139 BoronPlus source also indicates that hundreds of hours of use are available at 975oC.
The typical weight loss curve in Figure 4 shows that the B2O3 evolution rate from the BoronPlus sources gradually decreases with continued use time. In fact, the linear relationship obtained when the data are plotted versus the square root of use time (Figure 5) indicates that the B2O3 is actually evolving through a diffusion-controlled process. This means that as B2O3 evolves from the surface of the source, a B2O3 concentration gradient will develop from its interior. This gradient causes additional B2O3 to diffuse to the surface replenishing the supply of B2O3 needed for continued evolution. This "reservoir" of B2O3 contained within the diffusion source is sufficient for many hours of use. Holding the B2O3 within the source instead of on its surface is also directly responsible for decreased moisture absorption from the room air between runs.
The actual lifetime of BoronPlus sources used in typical plant production environments depends upon many factors such as temperature of use, care in handling, the device being manufactured, the sensitivity of the process to the gradual decreasing evolution rate, etc. Typical use times of up to 500 hours near 1000oC and 100 to 150 hours near 1100oC have been reported.
BoronPlus® is a registered trademark of Techneglas Inc.
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