Method and apparatus for rapid thermal processing and bonding of materials using rf and microwaves

Abstract

A method and apparatus for rapid and selective heating of materials using variable frequency RF and microwaves. The apparatus uses variable frequency solid state electronics as a microwave power source, a novel microwave heating head to couple microwave energy to the target materials and a match-up network to tune the frequency and impedance match between the microwave source and the load. An electronic and computer measurement and control system is employed to monitor and control the microwave heating process. The method teaches the use of inductive microwave coupling for thin conductive materials such as metal film and impurity doped silicon wafers. The method also teaches the use of capacitive microwave coupling for dielectric material such as glass and ceramics. The method further teaches the use of rapid and selective heating of heterostructure for bonding and sealing of mems and integrated circuits. The method and apparatus can provide ultra-high heating speed along with ultra-high heating temperatures for rapid thermal processing of semiconductors and other materials. It also allows the use of bonding materials with high melting temperature for strong bonding and sealing of mems and IC devices. The apparatus further provides for high interconnection density of integrated circuits as connections are made without the use of solder bumps.

1 . A method for bonding and sealing of two substrates using an apparatus comprising a high frequency microwave power source, a microwave heating head and a measurement and control system, the method comprising the steps of:
 placing a thin layer of material, including high conductive thin film and high complex dielectric loss material, on a surface of each of the substrates, wherein the thin layer of material has a microwave absorption stronger than that of the substrates;   selecting an appropriate heating head, wherein the appropriate heating head is selected based on a type of thin layer material to be placed on the substrates;   generating microwaves within the microwave power source; and   heating the thin layer of material with microwaves, wherein a thickness of the thin layer of material is within several orders of magnitude of the microwave skin depth of the material,   wherein the frequency of the microwave power source is variable, wherein the apparatus is adapted to selectively heat materials, wherein the apparatus does not include a microwave cavity, and wherein the microwave power source can sweep a certain bandwidth within a frequency range from 100 MHz to 20 GHz.   
   
   
       2 . The method of  claim 1 , wherein the step of selecting an appropriate heating head further comprises:
 selecting an inductive coil as the heating head when the type of thin layer material to be placed on the substrate is a high conductive film, including gold film; and   selecting two capacitive plates for the heating head when the type of material to be placed on the substrate is a dielectric material, including glass and ceramics.   
   
   
       3 . The method of  claim 1 , wherein the substrates are selected from the group consisting of semiconductor material and silicon (Si). 
   
   
       4 . The method of  claim 1 , wherein the thin layer of material is selected from the group consisting of conductive material and dielectric material, including gold, glass and ceramic. 
   
   
       5 . The method of  claim 1 , further comprising the step of:
 stacking the two substrates so that the thin layer of material placed on the surface of a first substrate faces the thin layer of material placed on the surface of a second substrate.   
   
   
       6 . The method of  claim 5 , further comprising the step of:
 pressing the two substrates together.   
   
   
       7 . The method of  claim 5 , further comprising the steps of:
 positioning the two stacked substrates at a position relative to the heating head so that a microwave power from the heating head is maximized;   applying microwave power to the stacked substrates to heat the thin layers of material to melting temperature; and   holding together the stacked substrates a period of time sufficient for good bonding and sealing.   
   
   
       8 . The method of  claim 7 , wherein the step of positioning the stacked substrates further comprises:
 placing the stacked substrates such that the material to be heated is at a position where a magnetic field is maximized when the heating head selected is an inductive coil; and,   placing the stacked substrates such that the material to be heated is at a position where an electric field is at a maximum when the heating head selected comprises two capacitive plates.   
   
   
       9 . The method of  claim 1 , wherein the microwave heating head includes a match-up network, and further comprising the step of:
 adjusting the match-up network so that a frequency and an impedance of the microwave power source match, respectively, a resonant frequency and an impedance of the heating head when the thin layers of material on respective surfaces of each of the two substrates are heated.   
   
   
       10 . The method of  claim 1 , wherein a driving frequency of the microwaves is greater than 100 MHz. 
   
   
       11 . The method of  claim 1 , wherein microwave power source is able to generate a continuous waveform, a pulsed waveform, and a modulated waveform. 
   
   
       12 . The method of  claim 1 , wherein the step of generating microwaves further comprises:
 sweeping a frequency range so as to compensate for a resonant frequency shifting of a loaded heating head that is caused by a temperature change in the substrates.