[Novellus,Nano-Porous Strengthened ]: Low K porous materials having improved mechanical strength by Parylene AF4 Coating

• [Applicant]: Novellus Systems, Inc.
• [Inventors]: Jeffrey C. Benzing, John Kelly
• [Application No.]: US6528153 B1
• 

ABSTRACT

The present invention relates to porous materials, typically xerogels or aerogels, having a low dielectric constant but relatively poor mechanical strength. The present invention relates to polymeric coatings, preferably parylene, coated on inorganic xerogels or aerogels so as to increase the mechanical strength while not substantially degrading the dielectric properties of the resulting coated material. Silica xerogel conformally coated with Parylene AF-4 is described.

Highlight of patents & Parylene AF4 related

• Porous materials have been used for a variety of applications including thermal insulators, heat storage systems, acoustic damping materials, and electrical insulators. In recent years, there has been particular interest in developing porous materials as insulators for use in semiconductor devices.
• As a material is made more porous, the dielectric constant is decreased, but at the cost of a decrease in mechanical strength. Another disadvantage of silica xerogels is their sensitivity to moisture.
• Deposition and coating of xerogels where very thin coatings are required should be performed under conditions of slower deposition than typically used for coating a substantially flat parylene film onto a substrate. In the experiments conducted, temperatures as high as +20 C. were used to slow the deposition rate to a convenient value.

 BACKGROUND OF THE INVENTION(selected)

1. Porous materials have been used for a variety of applications including thermal insulators, heat storage systems, acoustic damping materials, and electrical insulators. In recent years, there has been particular interest in developing porous materials as insulators for use in semiconductor devices.
2. The dielectric constant of vacuum or dry air is about 1.0. Thus, one way to obtain a material with a low dielectric constant is to use a porous or sparse, low density material in which a significant fraction of the bulk volume consists of space or air.
3. One class of porous materials is foams. Typically, foams are made by producing a structure of the material and blowing air or other gases through the structure to create voids or by liberating gas throughout the material to create cells or pockets. Foams may be useful as thermal insulators and energy absorbers.
4. Another class of porous material that has been extensively investigated is sol-gel derived materials termed xerogels or aerogels. Sol-gel synthesis of oxide materials is typically based upon the hydrolysis and condensation of alkoxides M(OR)n where M is typically a metal atom (Si, Ti, Al, etc.) and R is typically an alkyl group. A common precursor for SiO2 xerogels and aerogels is tetraethoxysilane (“TEOS”), when M=Si, R=C2H5 and n=4.
5. The potential utility of porous silica as a low dielectric constant insulating material has been recognized.
   • As a material is made more porous, the dielectric constant is decreased, but at the cost of a decrease in mechanical strength.
   • Another disadvantage of silica xerogels is their sensitivity to moisture. The inner surface of silica xerogels is very polar due to residual hydroxy or alkoxy groups bonded to silicon, which promotes adsorption of water.

DETAILED DESCRIPTION OF THE INVENTION

• Parylene Deposition
  • The crystalline solid dimer is vaporized to a pressure of about 100 mT (milliTorr) at a temperature of about 100° C.
  • The resulting vapor is allowed to flow into a pyrolysis chamber maintained at a temperature of approximately 650° C. where the dimer cracks into the monomer tetrafluoro-p-xylylene as depicted in Eq. (2).
  • The monomer vapor is then directed into the deposition chamber which is maintained at a pressure of approximately 20-40 mT. The monomer gas condenses onto a cold surface in the deposition chamber where the polymerization reaction occurs to yield the parylene AF-4 film, as depicted in Eq. (2).
  • The substrate temperature is typically maintained in the range 0° to −20° C. The deposition rate increases with decreasing substrate temperature in this temperature range. Deposition rates of 1000 A are easy to obtain under these conditions and deposition rates up to 1 μm per minute can readily be achieved. Deposition and coating of xerogels where very thin coatings are required should be performed under conditions of slower deposition than typically used for coating a substantially flat parylene film onto a substrate. In the experiments conducted, temperatures as high as +20 C. were used to slow the deposition rate to a convenient value.
• Techniques of analytical chemistry were used to confirm the coating of parylene AF-4 onto the silica xerogel
  • Fourier transform infrared spectroscopy (“FTIR”)
  • Rutherford backscattering spectroscopy (“RBS”)
  • secondary ion mass spectroscopy (“SIMS”)
  • scanning electron microscopy (“SEM”), UV spectroscopy
• Historically, the term “aerogel” has been used to indicate a porous structure in which supercritical drying is employed and “xerogel” when dried by solvent extraction

ClaimS:

1. A dielectric material comprising:
2. a) a nanoporous material comprising a skeletal structure of interconnected fibers separated by pores; and,
3. b) a polymer coated on the surfaces of the fibers generally throughout said nanoporous material without filling said pores, thereby increasing the mechanical strength of said nanoporous material.
4. A dielectric material as in claim 1 wherein said nanoporous material is selected from the group consisting of silica, alumina, zirconia and mixtures thereof.
5. A dielectric material as in claim 1 wherein said polymer is conformally coated.
6. A dielectric material as in claim 1 wherein said polymer has a low dielectric constant.
7. A dielectric material as in claim 4 wherein said polymer is a parylene.
8. A dielectric material as in claim 5 wherein said polymer is parylene AF-4.
9. A semiconductor device comprising at least one insulating medium therein wherein said at least one insulating medium comprises a dielectric material according to claim 1.
10. A semiconductor device as in claim 7 wherein said nanoporous material is selected from the group consisting of silica, alumina, zirconia and mixtures thereof.
11. A semiconductor device as in claim 7 wherein said polymer is conformally coated.
12. A semiconductor device as in claim 7 wherein said polymer has a low dielectric constant.
13. A semiconductor device as in claim 10 wherein said polymer is a parylene.
14. A semiconductor device as in claim 11 wherein said polymer is parylene AF-4.
15. A method of increasing the mechanical strength of a dielectric material comprising:
16. a) providing a porous dielectric material, said dielectric material comprising a skeletal structure of interconnected fibers separated by pores; and,
17. b) coating the surfaces of the fibers generally throughout said porous material with a polymer without filling said pores, thereby increasing the mechanical strength of said dielectric material.
18. A method as in claim 13 wherein said porous dielectric material is a nanoporous material.
19. A method as in claim 14 wherein said porous material is selected from the group consisting of silica, alumina, zirconia and mixtures thereof.
20. A method as in claim 14 wherein said polymer is conformally coated.
21. A method as in claim 13 wherein said polymer has a low dielectric constant.
22. A method as in claim 17 wherein said polymer is a parylene.
23. A method as in claim 18 wherein said polymer is parylene AF-4.