Manufacturing Precision Polysilicon Resistors Using Ion Implantation Available to Purchase

Two techniques exist to form high resistance, precision polysilicon resistors. In both cases polysilicon films are degenerative doped with phosphorus to form the gate electrode. In the first method, phosphorus at a dose near 1×10¹⁵at/cm² is implanted in a portion of the second level polysilicon that is masked during the previous implant. Resistors can also be formed in a portion of the first level polysilicon by counter doping with boron. This provides resistors with lower temperature coefficients of resistance and the ability to manufacture precision resistors in a single poly process. As resistors comprise only a small portion of the die (<2%), the wafer is almost entirely free of resist in the first process and entirely covered with resist in the second process. Using two types of high current, batch implanters, resistors were successfully fabricated using the first method. For the boron resistors, consistent uniformity could only be obtained using one type of implanter. In the other type of implanter, variations in resistor values of more than 300ohms were observed which far exceeded the design specification. Variation was dependent on lot size, with small lots showing better uniformity. Variation across the wafer manifested as horizontal bands with the lowest values in the center of the wafer. The cause of the nonuniformity was determined to be photoresist outgassing and ion neutralization. Photoresist breaks down under ion bombardment releasing large quantities of hydrogen gas. The released gas neutralizes incoming positive ions. This results in overdosing, which lowers the resistance most dramatically in the center of the wafer where the quantity of photoresist is greater.