Comparing techniques for fabricating polysilicon contacted emitter bipolar transistors

Four methods for fabricating polysilicon-contacted BJT's have been investigated. In the first method polysilicon was deposited using low-pressure chemical vapor deposition (LPCVD) at 620°C. In the remaining three methods a-Si was first deposited and then recrystallized to form polysilicon. In the second method a-Si was deposited using LPCVD at 580°C. The third method used plasma-enhanced chemical vapor deposition (PECVD) to deposit a-Si-H. The fourth method involved a plasma etch with argon or hydrogen prior to deposition of a-Si:H using PECVD. The results indicated that using the PECVD method for depositing a-Si-H without any prior plasma-etch step and recrystallizing it to form polysilicon resulted in the highest current gain (β) enhancement of 3.5 and also allowed the reduction of the polysilicon anneal temperature down to 800°C or 900°C from 1000°C. The compactness in the spread of the peak β values for the devices fabricated using this technique also reflects its ability to reproducible fabrication of polysilicon contacted shallow emitter BJT's.