Advanced Etch-to-Depth Process Control Using Adaptive Interferometric Endpointand Endpoint/Host Parameter Exchange

摘要

Accurate endpoint control solutions for fabrication of trench capacitor structures can be complicated by twornthings: etch rates which vary significantly with increasing etch depth, and etch mask thickness variations that arernpresent wafer-to-wafer and lot-to-lot. This work reports on two phases of effort, and includes relative measures ofrnsuccess of each phase, which have addressed these process control challenges. The first phase is thernimplementation of an reactive/adaptive approach to the etch-to-depth control problem that accommodates I) etchrnrate variation within a wafer run and ii) wafer to wafer mask thickness variations. The second phase includesrnfurther refinement of the adaptive control algorithm with feed-forward information related to the lateral criticalrndimension (CD) of wafer trench capacitor structures. The feed-forward CD information allows the controlrnalgorithm to more accurately "track" the changing ER earlier within the wafer process.rnThe etch target of a recess etch process is ideally the difference in height of the top of the capacitor electrodernbeing etched and the bottom of the mask, it is not the total etch depth from the start of the process. Thus, therncapability to measure and compensate the mask thickness on a run-to-run basis is the key to achieving optimalrnprocess control. The process control algorithm must also account for pre recess, or "dishing" of the capacitorrnstructures from two modes: over-etching in the blanket etch back of the poly trench fill (in recess 2, or R2), andrnCMP selectivity (in recess 3, or R3). In the course of this work, an advanced endpoint system was introduced intornthe production environment and the performance improvements evaluated.rnThe recess process of record (POR) relied upon daily characterization of chamber etch rate, with the ER resultrnthen used to adjust chamber etch times so as to hit the etch target. Etch time adjustments were made to compensaternvariations in incoming CD and mask thickness and were unique to each chamber. In line metrology used forrncompensation included mask thickness measurement by Optiprobe and atomic force microscopy for recess depthrndeterminations. A summary of typical variation of the POR is shown normalized about the mean value in Figure 1.rnThe production variation of the etch depth around the normalized target depth for the POR is 0.08 (1σ).rnAn advanced feature endpoint system was implemented to provide closed loop control of the recess process.rnThis endpoint system (Applied Materials EyeD-IEP) performed in-situ mask thickness measurements prior to the recess etch, and then during the recess etch, the endpoint algorithm adapted to changing etch rate with depth tornarrive at the depth under mask measurement and the endpoint determination. The results across a number ofrnproduction lots for the variation in etch depth is 0.044 (1σ). The results are presented in Figure 2.rnThe second phase of the work added run-to-run process control to the endpoint approach: lateral CD valuesrnwere downloaded from the fab host to the endpoint system. This parameter exchange capability is bidirectional, sornthe host could both send and receive parametric information to the endpoint system on a run-to-run basis. In thisrnwork, the CD values were used to compensate the endpoint algorithm, providing a measure of correction for thernetch rate dependency on CD. The results across a sampling of production lots for the variation in etch depth isrn0.038 (1σ). These results are presented in Figure 3.rnDetails regarding the results and approaches will be presented in the full paper.