An electrostatic chuck used in a deposition or etching process is comprised of a ceramic material with an embedded electrode, on which a wafer is placed in the reactor chamber. The gas flow paths on the backside of the wafer can be adjusted to help control the wafer temperature. The dimensions of these flow pathways can be as small several microns, and at typical backside gas operating pressures (1-10 Torr), the flow can be anywhere between continuum and free molecular flow. Though tools are available to model these flows (Direct Simulation Monte Carlo and Navier-Stokes with slip boundary conditions), these flow conditions, typical in wafer processes, bring up some significant computational challenges. Also, the 3D nature of the real chuck flow with all its complex geometry compounds the numerical difficulties. In this paper, the focus will on a 2D approximation to the flow on the backside of the wafer, which is essentially flow between two flat, parallel circular disks. This geometry is quite similar to the 3D geometry in the electrostatic chuck, but without the complex set of gas grooves. Computations will be performed using Navier-Stokes computational fluid dynamics (CFD) with and without slip boundary conditions, Direct Simulation Monte Carlo (DSMC), and an analytical solution to the problem. These results will be compared to experimental data for helium gas flow through that same geometry. Comparisons show an excellent agreement between all the computations and the experiments in the continuum and near-continuum transition flow regime. The DSMC results and analytical solution match well with the data throughout all the flow regimes. However, the CFD with slip shows a capability in this 2D geometry to capture the trends seen in the data and DSMC out to much higher Knudsen numbers than would be typically expected. These CFD with slip results can be used as a design tool to conservatively estimate backside gas pressure distributions and reliably design gas distribution channels.