Elliptic flow measurements have demonstrated that partonic collectivity -- collective flow of partons -- has been developed in 200 GeV Au+Au collisions at RHIC. To pin down the partonic EOS at RHIC, one must address the status of thermalization in such collisions. Since the masses of heavy-flavor, charm quarks for example, are much larger than the maximum possible excitation of the system created in the collision, heavy-flavor collective motion could be used to indicate the thermalization of light flavors (u-,d-, s- quark hadrons). In order to measure the low pt v2 of charmed hadrons at RHIC, we propose to construct a heavy flavor tracker (HFT) for the STAR experiment at RHIC. In this talk, we will present the design of the detector in-depth and its expected performance as studied in detailed simulations and analytic calculations. Further physics potentials of the detector will also be discussed.