Most silicon ingots are manufactured using growers designed for the chip industry, where the cost of the semiconductor wafers derived from that ingot is insignificant compared to the cost of chips. Hence, chip makers do not optimize productivity or the cost of growing silicon ingots.
For solar, a wafer is essentially single, large photovoltaic diode. Consequently, wafer cost is a huge part of the end product. To meet those needs, Solaicx optimized its manufacturing facility to grow only ingots for the solar industry.
"We went back to basics and designed a machine which removed every cost from the ingot growing process that was not related to the solar industry," said Sedgwick. "As a result, our machines manufacture ingots for the solar industry better, faster and cheaper than semiconductor growers can."
The major problem solved by Solaicx was ingot uniformity. Semiconductor ingot growers, when used for making solar cell wafers, have a resistivity that changes from one end of the ingot to the other. As a result, wafers cut from the ingot must be sorted based on their ability to convert sunlight into electricity. Solaicx claims its ingots can be sawed into wafers that uniformly produce solar cells of the highest possible efficiency.
In a traditional IC ingot grower, silicon and its dopants are melted in the bottom crucible. A seed is then lowered into the grower so that it just touches the molten mixture. As the molten material crystallizes on the bottom of the seed, it is slowly pulled upward. After about 12 hours, a pure, single-crystal ingot several meters long and either 6- or 8-inches in diameter is produced. The ingot is then cut into 100- to 300-micron-thick wafers, the major component of a solar cell.
The problem with this method for the solar industry is that as the ingot is grown the silicon-to-dopant ratio changes. This causes the resistivity of the resulting wafers to increase from one end of the ingot to the other.
Solaicx claims it solved the problem by constantly feeding silicon and dopant into the crucible as the ingot grows, thereby maintaining a constant ratio throughout the entire growing process. Hence, wafers cut from its ingots have a more consistent resistivity that boosts solar efficiency.
"The level of impurities ordinarily gets higher as you pull an ingot out of the molten silicon," said Sedgwick. "But with our process we are continuously feeding fresh silicon and dopant into the crucible under computer control to keep their ratio the same throughout the entire run."