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Preparing and Storing BoronPlus Sources

Cleaning: The BoronPlus sources are cleaned of processing contaminants before shipping. If additional cleaning is desired, the procedures outlined in Table II should be followed.

Table II
Cleaning Procedures

• 8 minutes in NH₄ OH/H₂0₂/H₂0(1/1/5) at 80°C or
  8 minutes in a megasonic cleaning system at room temperature
• 2 minutes in DI WATER
• 10 minutes DRY at 90°C

The BoronPlus sources should not come in contact with HF or HCI at any time.

Preparation: The sources should be held at the intended deposition temperature before they are used to be sure all moisture has been removed. Although some processors begin immediately to use the sources after this initial drying period, others prefer to hold the sources at the use temperature for an additional time until a more constant B₂O₃ evolution rate occurs. The recommended minimum aging times can be obtained from Figure 8.

Storage: Since the B₂O₃ is contained within the source and not totally on its surface, the BoronPlus sources exhibit a minimum amount of water absorption. However, since absorption of even small amounts of moisture can cause problems in silicon processing, we recommend they be stored in diffusion boats in a dry environment at an elevated temperature. The best procedure is to hold the sources in the hot zone of the diffusion furnace at 600°C in dry nitrogen.

Typical Doping Procedures with
BoronPlus Sources

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°C are normally obtained with a four-rail quartz boat with a design similar to Figure 9. When depositions are made above 1100°C, 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 900°C and at least 100°C 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.

LTO Cycles,Gas Flow Rates

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 1000°C. Small amounts of oxygen are sometimes blended with the gas during the deposition. The oxygen concentrations are usually less than 1% below 1000°C and could be as high as 5% at deposition temperature above 1100°C, as shown in Figure 10. The sources should not be used in the presence of steam.

LTO Cycles: High concentrations of oxygen can be used during the deposition cycle in the presence of the BoronPlus sources since oxygen has a negligible effect upon the subsequent performance of the sources. The oxygen diffuses through the deposited glassy film and oxidizes the boron-silicon phase the forms on the silicon surface during the deposition. The oxidized phase may then be easily removed with a conventional HF etch. This in-situ LTO step significantly reduces the overall processing time as schematically illustrated in Figure 11.

The in-situ LTO cycle can be used with any of the BoronPlus sources. The predictable results of using the in-situ LTO with the GS-278 BoronPlus sources for a typical p-type emitter diffusion are shown in Figure 12.

Although the in-situ LTO has been successfully used in other p-type emitter, isolation, etch stop (micromachining) and similar types of applications, the conventional low temperature oxidation cycle is usually recommended for high sheet resistivity 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°C 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."

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