In the fabrication of semiconductor wafers, layers of various conductive and nonconductive polymeric materials are typically applied to a surface of the wafer during various stages of production. Polyimide is a polymer material often used in the production of semiconductor substrates such as silicon wafers. Polyimide is a desirable insulating material for semiconductor wafers because of its outstanding physical properties. Unfortunately, polyimide typically requires a long time to cure when conventional heating techniques are used. A cure cycle of several hours is typical and this often becomes the pacing step in semiconductor fabrication. In addition, there are other problems involved with curing polyimide resin with conventional heat. For example, when polyimide resin is cured in a conventional furnace, the outer surface of the resin typically cures faster than the center portions. This can cause various physical defects, such as the formation of voids, and can result in inferior mechanical properties such as reduced modulus, enhanced swelling, solvent uptake, and coefficient of thermal expansion.
Microwave energy is being investigated in a variety of manufacturing operations, including those involving the curing of polymeric materials. Microwave processing of polymeric materials is believed to be advantageous for a number of reasons. The application of microwave energy decreases the time required to cure some polymers as compared with conventional heating methods. This is because the volumetric deposition of microwave energy is more efficient than conduction from the surface resulting from conventional heating techniques. See, for example, Polymer Curing In A Variable Frequency Microwave Oven, R. J. Lauf et al., Oak Ridge National Laboratory. See also, U.S. Pat. No. 5,296,271 to Swirbel et al., which proposes a method of curing photoreactive polymers by exposing them to microwave energy. Additionally, microwave processing is more economically attractive than conventional heating techniques due to the shorter processing time required to cure the resin.
The application of single frequency microwave energy within a single mode resonant microwave furnace cavity to cure polyimide on a semiconductor wafer is described in U.S. Pat. No. 5,317,081 to Gelorme et al., and U.S. Pat. No. 5,241,040 to Cuomo et al. In each of these patents, the rate of cure is controlled by varying microwave power and by changing the physical characteristics of the cavity (referred to as "tuning"). As the polyimide resin cures, it causes the resonance of the cavity to change, thereby requiring compensation via tuning to maintain the cavity at maximum resonance (i.e., where reflected power is at a minimum). Tuning is accomplished by moving a plate within the microwave furnace up or down and moving a coupling probe in and out of the cavity, accordingly.
Unfortunately, the maximum size of a semiconductor wafer and/or the number of wafers that can be uniformly processed with single frequency microwave energy in a single-node or multi-mode cavity are limited. This is because the size and number of modes (also referred to as "hot spots") within a cavity are limited when single frequency microwave energy is used. If the area of the semiconductor wafer to be processed is larger than a mode, it becomes extremely difficult to obtain uniform and repeatable processing at a single frequency. Therefore, large scale microwave processing can be difficult using a fixed frequency of microwave irradiation in either a single mode or multi-mode cavity. In fact, the above patents acknowledge the difficulty of providing uniform curing of sizable electronic components in these types of cavities.