Method of fabricating polycrystalline silicon resistors in integrated circuit structures using outdiffusion

A process of fabricating a polycrystalline silicon resistor on a semiconductor structure is disclosed. According to the process insulating material is fabricated over selected regions of the semiconductor structure 10, and selected impurity 18 introduced into the insulating material, typically in conjunction with other process operations useful in fabricating semiconductor structures. Undoped regions of polycrystalline silicon 21 are then formed on the surface of the insulating material 15 and the entire structure is treated to cause the dopant in the insulating material 15 to out diffuse into the undoped polycrystalline silicon to thereby create resistors. The treating operation is typically the heat treatment performed in conjunction with other process operations in the fabrication of integrated circuits. In one embodiment the introduction of impurities into the insulating material places the impurities at a depth such that no out diffusion will occur at the selected process parameters except where the field oxide has been selectively thinned using an etching or other known process.