In-situ Metrology for End Point Detection during Chemical Mechanical Polishing of Shallow Trench Isolation Structure
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In-situ Metrology for End Point Detection during Chemical Mechanical Polishing of Shallow Trench Isolation Structure Parshuram B. Zantye a, b , S. Mudhivarthi a, b and Ashok Kumar a, b, and David Evansc a Department of Mechanical Engineering, b Nanomaterials and Nanomaunfacturing Research Center, University of South Florida, Tampa, FL 33620 c Sharp Laboratories of America, Camas, WA ABSTRACT Efficient end point detection (EPD) helps prevent numerous Chemical Mechanical Polishing (CMP) process defects such as dishing, erosion, excessive over-polish etc. During the CMP of Shallow Trench Isolation (STI) structures, the process should be effectively stopped at the buried Si3N4 layer to prevent any/all of the aforementioned process defects. In this research, novel in-situ metrology technique that used the real time tracking of Coefficient of Friction (COF) data during polishing was employed for EPD during STI-CMP. The experiments were performed on the CETR CP-4 CMP Tester using 1”X 1” sample coupons having a 2 µm pitch STI pattern. The COF signal during polishing is a characteristic signature of the given process parameters and conditions. The in-situ EPD was based on the principle that the COF values are strongly dependent upon the surface of the materials that are being polished. Extended polishing to polish the underlying layer after the process end point was reached, can give an estimate of the polishing slurry selectivity with respect to the buried layer. The ex-situ surface characterization of coupons was performed at different points of the process using Atomic Force Microscopy (AFM). The repeatability and reliability of this technique was evaluated after carrying out multiple tests at similar process conditions. The demonstrated methodology can also be implemented for characterization of polishing pads and slurries besides STI-CMP process optimization. INTRODUCTION
Needed Step Height Reduction of field Oxide
Si3N4 Layer Potential Active Device Area
Trench Oxide
Fig. 1 SEM cross section of STI structure The local oxidation of silicon is a very well understood and manufacturable process which has been used down to 0.25 µm device feature size1. However, the “bird’s beak” structure
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formed after LOCOS device isolation imposes a severe restriction on the device pitch. The increasing implementation of Chemical Mechanical Polishing (CMP)2 in semiconductor manufacturing coupled with the limitations of the LOCOS technique has led to the acceptance of Shallow Trench Isolation (STI) as the technique of choice for device isolation3, 4. In STI structures, the field oxide is embedded in the Si wafer in order to clearly separate the device active areas. This allows smaller device pitches and higher packing density. The process of the fabrication of STI structures (besides minor possible process variations) is relatively well characterized and is already discussed in literature5. Some of the teething challenges in STI process are: 1) effective trench oxide filling, 2) thermal stability and wet etch resilienc
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