Raman Thermometry of Si Thermal MEMS and GaN Devices

The reliability of many microscale devices depends on the presence of induced temperature or stress fields which may lead to hard or soft failures during operation.  While much modeling and metrology has focused on the measurement of such fields, many times it is their coupled thermomechanical behavior which is of most concern to the device scientist or engineer.  Examples of such coupled behavior arise in the packaging of devices where coefficient of thermal expansion mismatch can cause failure in fragile electronic die or interconnect structures upon thermal loading.  Other examples where thermomechanical effects become important is in the reliability of polar semiconductors like GaN RF-devices, high brightness LEDs, and thermal MEMS (e.g., thermal actuators, heaters, and micro-power sources).  Due to the microscale critical dimensions in such devices, the direct and detailed measurement of these coupled temperature and stress fields have remained a challenge, but are important for verifying device performance and validating models.

In this work, our group is developing methods using Raman spectroscopy to measure the temperature and stress distribution in Thermal MEMS and GaN devices.   Analysis of the Raman Stokes peak position and its shift has been frequently used to estimate either temperature or stress in microelectronics and MEMS devices.  However if both fields are evolving simultaneously, the Stokes shift represents a convolution of these effects, making it difficult to measure either quantity accurately.   By using the relative independence of the Stokes linewidth to applied stress, it is possible to deconvolve the signal into an estimation of both temperature and stress.  Using these characteristics of the Stokes Raman response, we have mapped the temperature and stress in a polysilicon microheater and heated AFM cantilever, simultaneously.  We have also performed measurements on GaN HFETs and TLM devices.  Work is underway to develop the simultaneous temperature and stress measurement capability for GaN devices. Applications will include thermal and mechanical characterization of GaN based LEDs and RF power electronics during operation.

Polysilicon Microheater Temperature and Stress Measurements

Heated AFM Thermal Measurements

GaN Thermal Measurements

       
Graphs showing the difference in temperature measured on a GaN TLM structure using teh Stokes Peak Position vs Stokes Linewidth. Linewidth data show very good agreement with FEA model of device. Stokes Peak Position temperature measurements under estimate the temperature due to stress effects in the device.