回收切割矽泥製成氮化矽坩堝用於多晶矽重複生長之研究

Abstract

隨著太陽光電產業快速發展，2018年全年新增併網量超過100 GW，其中矽晶電池就超過90 %。矽晶鑄碇的過程中，鑄碇過的石英坩堝與矽晶圓切割損失矽泥是太陽能產業兩大廢棄物。近年來，在晶錠切割製程上幾乎完全被鑽石線切割製程取代，使得切削的矽泥損失減少至40 %，然而仍是大量的耗損。因此循環經濟、循環材料及永續發展概念的建立可以處理上述製程上的廢棄物問題，透過使用回收矽粉製成氮化矽坩堝，達到純度比目前商用的石英坩堝更為高，進而提高矽錠的品質以及太陽電池的效率，以及減少廢棄坩堝的產生。 本研究使用鑽石線切割損失矽泥做為製作坩堝的起始原料，接著經過中型酸洗過濾設備去除金屬與硼磷後，利用注漿成型的方法做出坩堝生坯，放慢升溫速率在4~7 °C/h間及維持爐內約0.5 atm 5%H2+N2足夠低的負壓以減少氣氛爐漏氣進行氮化，在最佳化氮化時的燒製熱場後，可以不必額外添加催化劑得到氮化程度超過90%、收縮率小於5%的RBSN (Reaction-Bonded Silicon Nitride)坩堝，後續至少可重複鑄錠4次，晶錠的阻值可以控制在商用範圍1~2 ohm-cm內，由晶錠中央縱切面lifetime mapping的結果，其品質優於石英坩堝，驗證由純化回收矽泥製成的坩堝可以使用於商業鑄錠，除達成循環材料的目的外，也因回收矽的價格低廉，製作成坩堝的成本僅需考慮酸和水電，比起傳統石英坩堝更具競爭力，為長久以來難以處理的切割矽廢料找到一條出路。

As the rapid development in PV industry, more than 100 GW module, which consists of more than 90% silicon-based solar cell, was reached in 2018. Nevertheless, two major wastes, that is, kerf-loss silicon and broken quartz crucibles for casting, are produced. Nowadays, procedure for wafer slicing is almost replaced by diamond wire slicing. Even through, 40 wt.% of weight loss is unavoidable. We try to imply the concept of circular economy and circular material by proposing and presenting procedures for solving problems mention above via converting recycling kerf-loss silicon into silicon nitride crucibles, which can not only lower linear shrinkage than silicon nitride sintering but also become purer after every crystal growth. In this research, we recycle kerf-loss silicon and purify it with acid to remove metals, boron and phosphorus. First, slip casting is applied for green crucible manufacturing. Second, the green crucibles are nitrided with slow temperature rising rate (4~7 °C/h) and about 0.5 atm 5%H2+N2 to prevent the leakage of the furnace. As the result, the degree of nitridation can be over 90% without any additives as catalyst, and also the linear shrinkage of RBSN (Reaction-Bonded Silicon Nitride) crucibles are lower than 5% after optimizing the hot zone of nitridation. Finally, we have proved that the RBSN crucible made from slurry waste can be reused at least 4 times. The resistivity of the ingot cast from RBSN crucible can be in 1~2 ohm-cm which is commonly used in commercial production. Moreover, the minority carrier lifetime of the ingot from RBSN crucible is also better than the one from quartz crucible. Besides the advantage of recycling, the cost of making crucible only needs to consider acid, water and utility. To be concluded, this research has provided a new way out for kerf-loss silicon waste in PV industry.