Polymerization monitoring in plasma etching systems.

Abstract

In plasma etching processes, the polymers used to enhance etch anisotropy and selectivity also deposit on various parts of the reaction chamber. This polymerization on reactor surface not only strongly affects the concentration of reactants in the plasma discharge, eventually changing the etching characteristics, but also can produce particulates which lower yield. This thesis explores the development of a direct in-situ polymerization monitoring sensor to minimize the drifts in plasma etching processes. In addition, polymerization dependencies on basic processing parameters and polymerization effects on etching characteristics have been explored for the first time using a direct in-situ sensor. The polymer buildup process is a strong function of parameters such as power, base pressure, and flow rate, and is also dependent on the reactor materials used, temperature, and the hydrogen/oxygen concentrations present. Experiments performed in an Applied Materials 8300 plasma etcher show a significant increase in polymerization with increased pressure and flow rates and a decrease as a function of power. These experiments provide insight into how the chamber state changes under the different processing recipes used for etching specific material layers and also suggest how the chamber seasoning process can best be carried out. The reactor surface, which serves as both a source and a sink for reactive gas species, not only strongly affects the concentration of reactants in the plasma discharge, eventually changing the etching characteristics, but also can produce particulates which lower yield. The etch rate and selectivity variations for specific silicon dioxide and silicon nitride etching recipes have been explored as a function of the polymer thickness on the reactor walls. The etch rates of nitride and polysilicon decrease dramatically with polymer thickness up to a thickness of 60nm, while the oxide etch rate remains virtually constant due to the polymerization-suppressing nature of the oxide etch. A new sensor for monitoring polymer buildup in plasma etching systems was designed, fabricated, and tested as part of this work. The device is mounted flush in the chamber wall and uses an electrothermal oscillator to measure the thermal mass change of a micromachined dielectric window as polymer deposits on it. The variation in the oscillation pulse width (cooling time) is used as the sensor output. The device operates with a typical cooling time of 2.7msec and has a thickness resolution of <1nm. The dielectric windows are 0.5mm on a side and about 1mum thick; the overall die size is 3.5mm x 7.5mm. After initial measurements using a passive sensor, CMOS (complementary metal oxide semiconductor) control circuitry was successfully designed and integrated on-chip to provide temperature compensation and a low-impedance interface with the external world. The double-poly, single-metal CMOS process was modified to add the needed micromachining steps, and photoresist masking was explored for the first time to protect the devices from silicon etchant (tetramethyl ammonium hydroxide : TMAH) attack. A chevron-based support structure is used on the back side of the wafer to hold the devices in wafer form after micromachining to allow testing, high-yield die separation, and non-lithographic post-TMAH processing. The device metallization, package, and O-ring-based placement structure have been designed to ensure compatibility with the plasma etching environment.