In situ endpoint detection by pad temperature in chemical-mechanical polishing of copper overlay
As the number of metal levels and the wafer size increase, the global planarity and effective removal of metal overlay across the wafer becomes more crucial. Chemical-mechanical polishing (CMP) has been recognized essential to achieve this goal. Accurate in situ endpoint detection and monitoring method significantly improves the yield and throughput. Previous methods have been proposed, which either require the rearrangement of the machine set-up, or can only be implanted on certain types of machines. In this study, a model for pad temperature capable of predicting the endpoint of CMP in situ is established based on the total consumed kinematic energy between wafer and pad. Limited assumptions of thermal and kinematic conditions are made. The model of temperature rise uses the integral of the relative polishing speed and is verified by on-line measurement. Since the coefficient of friction between the pad and dielectric layer is distinguishably lower than that between the pad and the metal layer, the pad temperature increases milder than polishing the metal layer. In use of the proposed regression method applied to the measured temperature rise, the endpoint of the process can be detected.
Statistical and Experimental Analysis of Correlated Time-Varying Process Variables for Conditions Diagnosis in Chemical–Mechanical Planarization
During chemical-mechanical planarization (CMP) of semiconductor wafers, chemical and mechanical process variables are strongly correlated and jointly affect polishing performance. The correlation among these process variables could potentially be utilized to characterize process conditions for the purpose of diagnosis. However, process variables measured during CMP, such as the temperature distribution and coefficient of friction between wafer and pad, vary with time and present in a functional form. This significantly increases the complexity of analyzing correlation patterns and relating them with process conditions. The focus of this paper is therefore twofold: 1) experimental investigation of the correlation between sensing process variables and the implication of correlation pattern changes on process conditions and 2) statistical analysis of correlation patterns between process variables in functional form. In the designed CMP experiment, we investigated two failure modes during CMP process: pad failure and slurry failure. Slurry failure was generated by reducing the percentage of oxidizer to investigate its effects on polishing performance and heat generation on the pad. Pad failure was due to variation of diamond abrasive sizes in the conditioner. The post-CMP study of nonuniformity and defects such as scratches on the wafer was conducted to characterize process conditions. The experimental and statistical results support the investigation of correlation among process variables for condition diagnosis.
Microstructure of the solder-casting zone in bridges of dental gold alloys.
The composition and structure were studied in the cast and in the solder of bridges which had failed clinically. Both materials were gold alloys but with different contents of Pt, Ag and Zn. The metallographic investigation revealed defects mostly in the solder, situated in the subsurface layer. SEM studied of the fracture surface revealed large porosites and a structure of dense parallel lines--striations--, indicating that the material had failed from fatigue. The materials in the fractured bridges were identified by micro-probe measurements. It was stated that cast material and solder material used together, were of different composition.