A Novel Technique to Measure the Glass Transition of Ultra-Thin Polymer Films

Christopher White, Wen-li Wu
Polymers Division
Materials Science and Engineering Laboratory Bldg. 224, Rm. B320 National Institute of Standards and Technology Gaithersburg, MD 20899

This presentation will detail developments made at NIST for the measurement of complex viscoelastic coefficients (h*) and glass transition temperature (Tg) for ultra-thin films based upon the dual torsional quartz crystal resonator.

One of the most important properties in the selection of materials for polymer interlayer dielectrics (ILDs) is Tg. It has been assumed that traditional techniques such as differential scanning calorimetery on large samples would produce an accurate value for the thin film application of these same materials. Recent experiments based on x-ray reflectivity, ellipsometry and positron annihilation spectroscopy, etc. have demonstrated that the physical properties of polymer films thinner than 1,000 nanometers may deviate significantly from the expected bulk values.

Tg has been determined for ultra-thin films by several different techniques as mentioned above. Each of these techniques exploits a measurable change in the properties of a polymer sample. One of the most important changes that occurs at Tg is the change in the viscosity. At Tg, h* can change by eight orders of magnitude. It is this change and the possibility of flow or creep of the ultra-thin polymer film above Tg that can lead to failure of a polymer ILD. Knowledge of h* is also critical in determining the processing conditions for the interlayer dielectrics.

No method, to date, is able to measure the viscosity of a thin film on the order of 1,000 nanometers within a reasonable frequency (<1MHz) or frequency range (2 decades). Bulk rheological instrumentation techniques are quite effective at measuring h* and employs one of two well established limits: gap loading or surface loading. Neither of these techniques are applicable to polymer films of 1,000 nanometers or less. To measure h* of such films we have developed new working equations and instrumentation that complements these developments. These developments will allow us to measure h* of a <1,000 nanometer films with a frequency range of 10kHz-~1MHz.