Thin film resistors are generally resistors that are formed on a semiconductor substrate using a thin-film deposition process. An exemplary thin film resistor 10 is illustrated in FIG. 1. As depicted, the thin film resistor 10 is formed on a substrate 12 and is shown having metallic interconnects 14 extending from either side of the thin film resistor 10. The substrate 12 may be formed from a wafer and is used as a foundation on which one or more semiconductor devices, such as transistors and diodes, are formed. The interconnects 14 are used to connect either side of the thin film resistor 10 to other electrical components, such as other resistors, inductors, capacitors, transistors, diodes, and the like in an overall circuit that is formed at least in part on the substrate 12. While the interconnects 14 are shown on either side of the thin film resistor 10, the interconnects 14 may be provided entirely or substantially above and below the thin film resistor 10.
In certain applications, the resistance provided between the interconnects 14 by the thin film resistor 10 is critical to the overall performance of the circuit in which the thin film resistor 10 resides. The circuit may be designed to require a resistor with very tight tolerances, and if the resistance provided by the thin film resistor 10 falls outside of a set tolerance, the circuit may not perform as desired. As such, it is important to form the thin film resistor 10 such that the resistance provided between the interconnects 14, or two other contact points, is highly controllable and repeatable during fabrication of the overall circuit on the substrate 12.
Unfortunately, the material from which thin film resistor 10 is formed is prone to oxidizing, and oxidation occurs before the interconnects 14 are formed during the fabrication process. The oxidation results in an oxide layer 16 forming over the exposed surface of the thin film resistor 10 before the interconnects are formed. The oxide layer 16 effectively raises the interlevel contact resistance between the thin film resistor 10 and the interconnects 14, and as a result, the actual resistance provided between the interconnects 14 by the thin film resistor 10 can be significantly different than the desired resistance. While the oxide layer 16 may be removed using various acid-based cleaning steps, such cleaning steps may unintentionally erode or harm other structures that were previously formed on the substrate.
Further, semiconductor fabrication generally involves numerous deposition, etching, and cleaning iterations as the various layers and devices are formed on the substrate 12. As such, numerous etching and cleaning steps may be required after the thin film resistor 10 is formed. These etching and cleaning steps may erode portions of the thin film resistor 10. Erosion of the thin film resistor 10 also has a significant impact on the resistance provided by the thin film resistor 10 between the interconnects 14.
Accordingly, there is a need for a technique that will substantially protect thin film resistors 10 from the undesirable effects of oxidation during fabrication, such that the thin film resistors 10 can be repeatedly formed to provide resistances within relatively tight tolerances. There is a further need for a technique that will substantially protect thin film resistors 10 from erosion during fabrication.