Processing Parameters

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Insertion, Ramping and Withdrawal Procedures

The benefits of using BoronPlus and PhosPlus planar dopant sources in the production of semiconductor devices are well known. To obtain the best results, however, consideration must be given to a number of key processing parameters. One of these parameters is the thermal response of the sources when heated to and cooled from the deposition temperature. This bulletin suggests techniques for insertion, ramping and withdrawal of the dopant sources. Using these suggestions, process engineers should be able to achieve the following:

  • Reduce or eliminate source warpage
  • Improve sheet resistivity uniformity
  • Increase source lifetime
  • Reduce thermal gradients across the silicon wafer and across the dopant source
  • Increase product yields

Recommended Processing Parameters

Many factors can affect the results of a deposition cycle. Of particular importance is the furnace temperature profile. A very steep temperature gradient normally occurs between the front of the flat zone and the mouth of the diffusion tube. If the carrier is rapidly pushed or pulled through this portion of the diffusion tube, a large temperature gradient will form across the radius of the sources. If this gradient is too high, the stresses that develop can warp or break either the silicon wafer or the dopant source. The gradient can also introduce slip planes in the silicon wafer which usually result in decreased product yields.

To minimize these temperature gradients and to improve the performance of the BoronPlus and PhosPlus sources, the insertion, ramping and withdrawal parameters outlined in Table I are suggested. These parameters should be considered as starting points in the development of the optimum deposition cycle for a given process.

Comments on Processing Parameters

Insertion and Withdrawal Temperatures: The insertion and withdrawal temperatures given in Table I will minimize warpage and are the maximum temperatures that should be used. Lower temperatures are certainly acceptable, however, since they will not harm the sources and they could have other beneficial effects on the silicon (less damage) and final device properties (higher minority carrier lifetime). Generally, the difference between the deposition temperature and the insertion/withdrawal temperature should be greater than 100°C.

Insertion and Withdrawal Rates

The insertion and withdrawal rates given in Table I are the maximum rates that should be used when the temperature gradient in the diffusion tube is about 50°C/inch. If the insertion rate is too fast, breakage of the sources could occur. If the withdrawal rate is too fast, the sources could warp. The withdrawal rate is probably too fast if the center of the source is observed to be red when the boat is exiting from the diffusion tube.

Pulling the boat into an ampule (elephant) is also recommended. The ampule decreases temperature gradients across the sources and the silicon wafers when the boat is out of the furnace, and it minimizes the amount of room air, which often contains various concentrations of particulates, from flowing between the sources and the silicon during cooling.

Many of the modern furnaces exhibit temperature gradients which are much steeper than 50°C/inch. Consequently, slower push/pull rates through the temperature gradient may be necessary to prevent warpage of the sources and to minimize silicon damage. Since these furnaces are usually controlled by microprocessors, the diffusion engineer could consider a program which has a rapid push to the beginning of the temperature gradient, a slow push through the gradient, and then a rapid push into the hot zone.

Stabilization Time

The boatload of sources should be allowed to come to thermal equilibrium at the insertion temperature after reaching the hot zone. This will help to insure that both ends of the boat will reach the deposition temperature at the same time after the furnace ramp has been completed. The result will be an improved doping uniformity across the boat.

The minimum recommended stabilization times given in Table I increase with increasing source diameter because of the greater mass of material which must be heated. The diffusion engineer may find that even longer times are necessary if a large number of sources are used and/or if the furnace recovery time is slow.

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