MS Thesis Presentation by
Mark R. Abel
Tuesday, July 12, 2005
(Dr. Samuel Graham, Chair)
"Thermal Metrology of Polysilicon MEMS using Raman Spectroscopy"
The development of microscale and nanoscale devices has outpaced the development of metrology tools necessary for their assessment. In the area of thermal MEMS technology, accurate measurements across a broad range of temperatures with high spatial resolution are not trivial. Thermal MEMS devices are important in microtechnology as they are used in actuation, chemical sensing, nanolithography, biological reactions and power generation. In order to properly design and verify the reliability and performance of thermal MEMS devices, the temperature distribution under device operating conditions must be experimentally determined. Of the technologies which are candidates for thermal metrology, Raman spectroscopy has the ability to provide absolute temperature measurements with spatial scales below 1 m m which is sufficient for most MEMS devices.
In this work, a detailed study of Raman spectroscopy as an optical thermal metrology tool was performed. It is shown that a simple calibration of the Stokes shift with temperature yields a linear calibration for measurements up to 1000 ° C in polycrystalline Si. These linear coefficients were determined for polysilicon processed under various conditions (575-620ºC, B and P doping) to assess the effects of microstructural variations on Raman shift. The Stokes peak was also shown to shift linearly with an applied pure bending stress. Thus in order to evaluate temperature without the influence of thermally induced stresses which may evolve in some MEMS, the ratio of the Stokes to anti-Stokes signal intensities and the linewidth of the Stokes spectral band were calibrated for doped Polysilicon over the same temperature range.
Using the calibration data, Raman spectroscopy was implemented for the evaluation of temperature of thermal MEMS with sub-micron spatial resolution. Heated AFM cantilevers and micro-beam heaters were chosen due to their wide range of applications. Different thermal and mechanical boundary conditions were considered by studying both the beams and cantilevers, resulting in varying levels of thermal stress. By using the three calibrations in a complementary fashion for device measurement, the validity of Raman thermometry was explored. Device temperatures of up to 650ºC with uncertainties less than ±8ºC were realized. Effects of thermally induced stress were taken into account and analyzed simultaneously.
Parameters which affect the Raman thermometry technique were reported and elucidated. This includes aspects such as laser heating, spatial and spectral resolution, light collection efficiency, measurement uncertainty, and instrumental drift. This in-depth analysis helped to establish the accuracy of Raman thermometry for MEMS