Evaluation of the wafer polishing pad capacity and lifetime in the machining of reliable elevations

The purpose of the wafer polishing process is to ensure wide planarization, uniformity, a precise surface finish and a non-defective surface. Its significance is increasing with the trend toward large-diameter wafers. The polishing pad used in these processes is an important factor that influences the wafer surface finish. Maintenance of the pad capacity plays a key role in the machining of reliable elevations. If the machining time increases, the pad capacity decreases, creating the need for a suitable shift time. A rough pad surface condition can contain microscopic holes that serve as an exhaust pathway for minute chip particles. Such particles become reaction materials on the pad surface.In this study, a dressing effect on the machining capacity, deformation degree and pad lifetime are investigated. A 300 mm silicon wafer was polished and the dressing defect and control condition were compared and analyzed with no change of the other machining conditions. The pad capacity and lifetime were evaluated in terms of the pad deformation degree, pad surface variation and silicon wafer surface finish under various pad deformation conditions. The AE (Acoustic Emission) signal of various pad surface conditions was detected using a AE sensor during the silicon wafer polishing process. The polishing process was monitored and analyzed using the AE raw signal, RMS (Root Mean Square) conversion data and FFT (Fast Fourier Transformation) analyses. The AE signal is a spontaneous elastic wave signal caused by a change in the stress condition of each material. Using a loadcell with the wafer head fixture, variations in the applied pressure signal were detected and compared while varying the condition of the pad surface and final wafer surface finish.The silicon wafer polishing process has sensitive machining characteristics due to the interaction between various complex process factors. These can include the pad, slurry, applied pressure, rotation speed, dressing condition and the machining temperature. Among these important machining factors, the pad capacity and lifetime evaluation as they pertain to the dressing control were assessed. These characteristics were investigated through an analysis of the deformation of the pad, the wafer surface finish, the detected sensor signal and the signal process.

► Dressing effect on the machining capacity and pad lifetime are investigated.
► AE is mainly generated by friction between silicon wafer and pad; wear of wafer, pad and particles in the slurry.
► With periodic dressing control, optimal polishing process is obtained.
► With smooth slurry supply, the increase of pad temperature will be stabilized.
► The original pad surface remains for longer time.