Thermophysical Properties of Gases Used in Semiconductor Processing

J.J. Hurly, K.A. Gillis, and M.R. Moldover

Objective: To provide high-accuracy data for modeling chemical vapor deposition (CVD) and for calibration of mass flow controllers (MFCs) used in semiconductor processing.

Problem: Many process gases are toxic, corrosive, and/or pyrophoric. For such gases, measurements of thermophysical properties are sparse and rarely accurate. But accurate data are required to model the hydrodynamics of the gas streams, i.e., the velocity and temperature profiles in the vicinity of the hot susceptor, and the hydrodynamics that evolve within the streams used in CVD processes. MFCs are used to deliver process gases (e.g., Cl₂, HBr, BCl₃, WF₆) for CVD and for other processes (e.g., plasma etching). Calibrated MFCs are needed to scale processes up from prototype to pilot plant and to production. Although MFCs are used with process gases, they are sold with calibrations for surrogate gases. Because the operation of MFCs depends upon heat transfer, converting the calibration from a surrogate gas to a process gas requires the heat capacity, thermal conductivity, density, and viscosity as functions of temperature and pressure.

Approach: We are using acoustic techniques to measure the thermophysical properties of three classes of gases: (1) binary mixtures of CVD carrier gases with process gases, (2) pure process gases, and (3) surrogate gases. We will develop a comprehensive, reliable database for these gases that provides the heat capacity, thermal conductivity, viscosity, and the pressure-density-temperature relation for the gases and also diffusion coefficients for mixtures of the gases. The diffusion coefficient will be obtained from models for the intermolecular potentials between the carrier and the process gases.

Results and Future Plans: We developed a facility for safely measuring the properties of these hard- to-handle gases. During the past year, we have completed measurements on the seven gases identified by the SEMATECH MFC Working Group as having the highest priority. The figure displays speed-of-sound data for chlorine. The data range from somewhat below the boiling temperature to 200 ° C and from 25 kPa to 1500 kPa or 80% of the vapor pressure. The data were analyzed for the ideal-gas heat capacity and the equations of state with uncertainties of approximately ±0.1%. For all seven gases, effective pair potentials have been derived and these pair potentials have been used to estimate the transport properties of these gases. In the coming year, the speed-of-sound will be measured in an organometallic gas and in other process gases. Acoustic measurements of the transport properties and a database are planned.

Publications:

Hurly, J.J., "Thermophysical Properties of Gaseous CF₄ and C₂F₆ from Speed-of-Sound Measurements," Int. J. Thermophys. 20, 455 (1999).

Hurly, J.J., Defibaugh, D.R., and Moldover, M.R., "The Thermodynamic Properties of Sulfur Hexafluoride," Int. J. Thermophys. (in press).