5 to 5 kg of silicon essentially uniformly over the entire bed particle surface this operation is intended to deposit from 1 5 to 2 0 microns of surface layer depending on the exact particle size distribution during the last 10 minutes of the second deposition period product is withdrawn from the reactor to bring bed back to the level at the start of the cycle the amount withdrawn each cycle is estimated by the operator to be the sum of the weight of seed particles added plus the calculated amounts of first and second silicon depositions once product is withdrawn silane and hydrogen flows are reset to the first deposition conditions seed particles are added to the reactor at the start of each cycle to maximize growth on seed particles over the duration of the run product particle size is controlled by the operator by the number of seed particles added each cycle no attempt is made to change temperature from first to second deposition period because of the slow response time of this high heat capacity system however as feed gases are changed bed temperature does increase slowly about 5 u00b0 c during the second deposition period and return slowly during the first table ii gives the average temperature over the entire cycle table ii shows that surface dust was reduced to the range 0 03 0 08 wt percent practicing this invention for the same productivity and temperature range dust would.