Case study: MOSFET instability due to charging of remnant stringers from a disposable polysilicon spacer process

Incompletely etched polysilicon "stringers" can produce a wide range of failure mechanisms in advanced integrated circuits. Parasitic leakage paths are often the direct cause of failures, but stringers can also form unwanted implant masks, or even act as the conducting channels of parasitic thin film transistors. A failure analysis case study was presented in which remnant stringers from a disposable polysilicon LDD spacer process led to large scale product fallout during burn-in. As a first step in the investigation, a series of circuit and transistor level stress experiments were performed. These produced evidence of significant transistor instability as the likely failure mechanism. Next a detailed Cross-section Transmission Electron Microscopy (XTEM) analysis was performed in order to relate the microstructure of the transistors to the observed electrical performance. The XTEM images clearly showed very small (30-70 nm) remnant polysilicon stringers at the edge of the gate poly reoxidation, above the transistor source/drain regions. The stringers were attributed to incomplete removal of the sidewall spacer polysilicon. This was confirmed by XTEM images of devices pulled from the fab before spacer etch. Finally, additional transistor electrical tests were performed to prove that these stringers acted as parasitic floating gates, gradually charging under saturation bias conditions. It was shown that this trapped charge, located just above the drain region, had a profound effect on subsequent device operation.