Boats: Although diffusion boats of various designs have been successfully used with the BoronPlus sources, the best results for depositions made at temperatures below about 1100 oC are normally obtained with a four-rail quartz boat with a design similar to Figure 9. When depositions are made above 1100oC, silicon carbide or polysilicon boats, having a 1/3-round design, are often preferred because of their increased resistance to deformation. Boats made of any of these materials fit on standard paddles and cantilever systems and can be used in automatic transfer systems. The spacing between the silicon surface and the source surface should be constant and should be between 0.060" and 0.100". The slots for the sources should be about 0.010" wider than their thickness. The sources should fit loosely in the boat, allowing room for expansion of at least 0.020" per inch of diameter.
Insertion and Removal: We recommend that a furnace ramping technique be utilized for all deposition cycles. This procedure involved slowly inserting the boatload of wafers into the diffusion tube at a temperature below about 900oC and at least 100 oC less than the deposition temperature. After the furnace and boat have reached thermal equilibrium, the furnace is ramped to the deposition temperature. At the end of the deposition time, the furnace is cooled back to the insertion temperatures at which time the boat is withdrawn. The insertion and withdrawal rates should not be more than 4 in./min. for 100mm sources. Because of the greater mass of material involved, slower insertion and withdrawal rates should be used with the larger diameter sources.
Ambient Gases: The BoronPlus sources can be used with the conventional gases of nitrogen and argon without detrimentally affecting their performance. Although nitrogen is the most common gas, some users prefer to use argon, especially at temperatures above 1000oC. Small amounts of oxygen are sometimes blended with the gas during the deposition. The oxygen concentrations are usually less than 1% below 1000 oC and could be as high as 5% at deposition temperature above 1100oC, as shown in Figure 10. The sources should not be used in the presence of steam.
Use of Moisture: The B2O3 evolution rate from the GS126 sources can be significantly increased when controlled amounts of moisture are present in the nitrogen carrier gas. Figure 11 shows the dependence of deposited glass film thickness as the moisture level is increased from 3ppm (dry nitrogen) to about 100 ppm at 900 oC. These low levels of moisture can be easily and accurately obtained when hydrogen and oxygen are blended into the carrier gas using a low flow range mass flow controller system.
The technique has produced source/drain diffusions of a CMOS IC which were indistinguishable from those made with ion implantation. The deposited glass can also be driven in at a higher temperature to obtain other lower sheet resistivities, which permit one set of sources to be used for many different product lines.
LTO Cycles: High concentrations of oxygen can be used during the deposition cycle since this gas has a negligible effect upon the subsequent performance of the BoronPlus sources. For example, one may desire to use 100% oxygen at the end of a deposition cycle to oxidize the boron-silicon phase that forms under deposited glass for subsequent removal in HF. Figure 12 shows that the oxygen removes about 200 angstroms of this phase at 1150oC when used for 5 minutes at the end of the deposition time. Additional oxidation of this phase occurs if oxygen is used during the cooling to the withdrawal temperature. Although this technique has been successfully used in etch stop (micromachining) and in isolation diffusions, the conventional low temperature oxidation (LTO) cycle is usually recommended for base and source/drain diffusions because of better sheet resistivity control. A typical LTO cycle is to hold the silicon wafer in steam for about 20-30 min. at 800-850oC after removing the deposited glass in 10:1 HF. The sources should not be present during any LTO cycle involving steam.
Gas Flow Rates: The gas flow rate utilized during the deposition depends primarily upon the diffusion equipment such as tube size and end cap design. Although the flow rate must be high enough to prevent room air from backsteaming down the diffusion tube, flow rates ranging from as low as 2.0 I/min. to as high as 15 I/min. have been successfully used in a 135mm diffusion tube. Satisfactory results are most often obtained with a flow rate of 3-7 I/min. for this tube size.
"Information contained herein is derived from in-house testing and outside sources and is believed to be reliable and accurate. TECHNEGLAS, Inc., however, makes no warranties, expressed or otherwise, as to the suitability of the product or process or its fitness for any particular application."
BoronPlus® is a registered trademark of Techneglas Inc.
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