PhosPlus (R) Brochure
Page 4

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Cleaning, Preparing and Storing PhosPlus Sources

Cleaning: As part of the routine cleaning process, the PhosPlus sources are acid etched to remove any foreign matter and to expose a pristine surface. As a result, no additional cleaning is necessary before putting the sources into the diffusion furnace. If additional cleaning is desired, the procedure in Table II is the only one recommended.

Table II
Cleaning Procedures

15 seconds dilute acid at room temperature:

TP-250 4:1 HNO3
TP-470 10-1 HF

2 minutes rinse in fresh DI water
1 minute rinse in fresh DI water
Hold in a clean hood until dry
Store in nitrogen

This procedure is only recommended for initial cleaning. If the sources become contaminated after use, contact your area technical representative for assistance

Preparation: Before using the PhosPlus sources in production for the first time, they should be held at the intended deposition temperature for a period of time. This will ensure that all moisture has been vaporized, and it enables the sources to achieve a constant P2O5 evolution rate. The aging period may last from a few hours for high temperature processes to as long as 24 hours for low temperature processes. Figure 14 gives the recommended minimum aging times for the two PhosPlus sources.

Storage: Since the phosphorus is present within the sources in the form of a complex crystal and not as the extremely hygroscopic P2O5 material, the PhosPlus sources exhibit a minimum amount of water absorption. However, the absorption of even small amounts of moisture can cause various problems in silicon processing. It is therefore recommended that the sources be stored in the diffusion boats in nitrogen at an elevated temperature when the time between runs exceeds about 45 minutes.

If the sources were accidentally left out in a room for a long period of time, however, they can be quickly prepared for the next run by merely inserting them into the diffusion tube at the insertion temperature for about 15 minutes. When they are withdrawn from the tube, the boat is ready for loading with production silicon.

Typical Doping Procedures with PhosPlus Sources

Boats: Although diffusion boats of various designs have been successfully used with the PhosPlus sources, the best results are normally obtained with a four-rail quartz boat having a design as shown in Figure 15. When depositions are made above 1100°C, silicon carbide or polysilicon boats 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 0.010" per inch of diameter.

Insertion and Removal: A furnace ramping technique should be utilized for all deposition cycles. This procedure involves slowly inserting the boatload of wafers into the diffusion tube at a temperature below about 800°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 temperature, at which time the boat is withdrawn. The insertion and withdrawal rates should not be more than 4 in./min. for 100 mm sources. Because of the greater mass of material involved, slower insertion and withdrawal rates should be used with the larger diameter sources.

Ambient Gases: Either of the two PhosPlus sources can be used with the conventional gases of nitrogen or argon without detrimentally affecting their doping performance. Although oxygen does not adversely affect the sources, too high of an oxygen concentration should be avoided because of the potential for masking off the silicon surface to be doped. Steam should also be avoided in the presence of the PhosPlus sources at the deposition temperature. Steam causes the P2O5 to rapidly evolve from the sources resulting in shorter lifetimes of the sources. When using the TP-470 sources above 1000°C, small quantities of oxygen may be blended with the nitrogen or argon. The oxygen concentration in the carrier gas is usually less than 1% below 1050°C and could be as high as 5% at temperatures above 1100°C.

When the TP-250 sources are used near 900°C, the oxygen concentration should be maintained below about 1%. This concentration may be increased up to about 5% when depositions are made at higher temperatures. No oxygen should be used below about 875°C, especially when polysilicon layers are being doped.

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 backstreaming down the diffusion tube, flow rates ranging from as low as 2.0 l/min. to as high as 15 l/min. have been successfully used in a 135 mm diffusion tube. Satisfactory results are most often obtained with a flow rate of 3-7 l/min. for this tube size.

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