Endpoint detection methods for time division multiplex etch processes

During the fabrication of many MEMs devices it is required to etch a layer of material to completion stopping on the layer below (e.g. Silicon on Insulator (SOI) - clearing a Si layer stopping on an underlying silicon dioxide layer). Allowing the etch process to proceed beyond the time when the first layer has been removed can result in reduced thickness of the underlying stop layer, or feature profile degradation (known as "notching" for SOI applications). One method commonly used to detect plasma process termination times is optical emission spectrometry (OES). OES analyzes the light emitted from a plasma source to draw inferences about the chemical and physical state of the plasma process. In semiconductor processing this technique is commonly used to detect material interfaces during plasma etch processes. While this approach works well for single step processes or process with a limited number of discrete etch steps (such as an etch initiation followed by a main etch) it is difficult to apply OES techniques to plasma processes with rapid and periodic plasma perturbations such as time division multiplex (TDM) plasma etching processes for Si etching. At Unaxis USA, we have developed a proprietary optical emission end point algorithm in conjunction with OES to detect material transitions in TDM processes. This technique requires no synchronization of the algorithm to the TDM process and has been applied to silicon on insulator (SOI) structures. The mechanism and performance of the algorithm will be discussed. The sensitivity of the technique has been evaluated over a range of silicon etch loads. Signal to Noise (SNR) ratios of greater than 15:1 have been achieved for samples with less than 10% exposed silicon.