Method for lower thermal budget multiple cures in semiconductor packaging
A method for forming a multilayer structure comprises the steps of: depositing a first polymerizable layer on a substrate; applying microwave energy to the polymerizable layer while monitoring at least one property of the layer; and, ending the application of microwave energy when the monitored property indicates that the polymerizable layer has reached a desired degree of cure. The property monitored may be optical, e.g., Raman spectrum, or electrical, e.g., dielectric loss. This process control strategy lowers the overall thermal budget, and is especially suitable for curing polymer films on silicon. The method may be used repetitively to cure multiple layers of polymeric material when a thicker film is needed.
1. A method for forming a multilayer structure, comprising
depositing a first uncured polymerizable layer on a substrate;
applying microwave energy to said first uncured polymerizable layer while monitoring in situ at least one property of said first uncured polymerizable layer;
ending said application of microwave energy when said monitored property indicates that said first uncured polymerizable layer is cured to a desired degree of cure to make a first cured layer;
depositing a second uncured polymerizable layer on the first cured layer;
applying microwave energy to said second uncured polymerizable layer while monitoring in situ at least one property of said second uncured polymerizable layer; and
ending said application of microwave energy when said monitored property indicates that said second uncured polymerizable layer is cured to a desired degree of cure to make a second cured layer,
wherein said first uncured polymerizable layer comprises a material having a first polarizability and said second uncured polymerizable layer comprises a material having a second polarizability that is higher than said first polarizability.
2. The method of claim 1 wherein said substrate comprises a silicon wafer.
3. The method of claim 1 wherein said first uncured polymerizable layer and said second uncured polymerizable layer comprise a material selected from the group consisting of, polyimides, benzocyclobutenes and polybenzoxazoles.
4. The method of claim 1 wherein said microwave energy is applied by a VFM source in which the frequency of said microwave energy may be swept over a selected range about a selected center frequency.
5. The method of claim 1 wherein said monitored property is selected from the group consisting of dielectric loss, UV-visible absorbance, fluorescence, near-IR absorbance, mid-IR absorbance and Raman spectrum.
6. The method of claim 5 wherein said dielectric loss is measured indirectly by measuring changes in microwave power demand needed to maintain a selected heating profile.
7. The method of claim 1 further comprising:
depositing a third uncured polymerizable layer on said second cured layer;
applying microwave energy to said third uncured polymerizable layer while monitoring at least one property of said third uncured polymerizable layer; and
ending said application of microwave energy when said monitored property indicates that said third uncured polymerizable layer has reached a desired degree of cure.
8. The method of claim 7 wherein said first and second and third uncured polymerizable layers comprise resins belonging to the same chemical family, and said family is selected from the group consisting of epoxies, polyimides, benzocyclobutenes and polybenzoxazoles.
9. The method of claim 8 wherein said third uncured polymerizable layer has a higher dielectric polarizability than said second uncured polymerizable layer and said second uncured polymerizable layer has a higher dielectric polarizability than said first uncured polymerizable layer.
10. The method of claim 8 wherein said first and second uncured polymerizable layers have similar properties after curing.
11. The method of claim 1 , wherein the first uncured polymerizable layer is cured with microwave energy at a first temperature and the second uncured polymerizable layer is cured with microwave energy at a second temperature that is lower than the first temperature.
12. The method of claim 1 , wherein the first uncured polymerizable layer is cured with microwave energy at 200° C. and the second uncured polymerizable layer is cured with microwave energy at 185° C.
13. The method of claim 7 , wherein said third uncured polymerizable layer is cured with microwave energy at a temperature lower than said second uncured polymerizable layer.