Porous materials

Details Porous materials Porous materials

2000-08-08

2002-05-21

Cooney, Jr., John M. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S063000, C521S077000, C521S134000, C521S184000, C521S185000, C521S189000, C438S622000, C438S623000, C438S624000, C438S763000, C438S780000, C438S781000

Reexamination Certificate

active

06391932

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to porous materials. In particular, this invention relates to the preparation and use of porous films containing polyimide materials and having a low dielectric constant.
As electronic devices become smaller, there is a continuing desire in the electronics industry to increase the circuit density in electronic components, e.g., integrated circuits, circuit boards, multichip modules, chip test devices, and the like without degrading electrical performance, e.g., crosstalk or capacitive coupling, and also to increase the speed of signal propagation in these components. One method of accomplishing these goals is to reduce the dielectric constant of the interlayer, or intermetal, insulating material used in the components. A method for reducing the dielectric constant of such interlayer, or intermetal, insulating material is to incorporate within the insulating film very small, uniformly dispersed pores or voids.
Porous dielectric matrix materials are well known in the art. One known process of making a porous dielectric involves co-polymerizing a thermally labile monomer with a dielectric monomer to form a block copolymer, followed by heating to decompose the thermally labile monomer unit. See, for example, U.S. Pat. No. 5,776,990. In this approach, the amount of the thermally labile monomer unit is limited to amounts less than about 30% by volume. If more than about 30% by volume of the thermally labile monomer is used, the resulting dielectric material has cylindrical or lamellar domains, instead of pores or voids, which lead to interconnected or collapsed structures upon removal, i.e., heating to degrade the thermally labile monomer unit. See, for example, Carter et. al., Polyimide Nano foams from Phase-Separated Block Copolymers,
Electrochemical Society Proceedings,
volume 97-8, pages 32-43 (1997). Thus, the block copolymer approach provides only a limited reduction in the dielectric constant of the matrix material.
Another known process for preparing porous dielectric materials disperses thermally removable particles in a dielectric precursor, polymerizing the dielectric precursor without substantially removing the particles, followed by heating to substantially remove the particles, and, if needed, completing the curing of the dielectric material. See, for example, U.S. Pat. No. 5,700,844. In the '844 patent, uniform pore sizes of 0.5 to 20 microns are achieved. However, this methodology is unsuitable for such electronic devices as integrated circuits where feature sizes are expected to go below 0.25 microns.
Copending U.S. patent application Ser. No. 09/460,326 (Allen et al.), discloses porogen particles that are substantially compatibilized with B-staged dielectric matrix materials. However, this patent application does not broadly teach how to prepare porous dielectric layers containing polyimide materials.
Polyimides are well known dielectric materials. For example, U.S. Pat. No. 5,969,088 (Ezzell et al.) discloses a dielectric material for use in an electronic device including a polyimide having a plurality of pendant fluorene groups and having a dielectric constant of less than 2.75. A wide variety of such polyimide materials are disclosed in this patent. Neither porous polyimide dielectric materials, nor methods of preparing porous polyimide dielectric materials are disclosed in this patent.
Carter et al., Polyimide Nanofoams for Low Dielectric Applications,
Mat. Res. Soc. Symp. Proc.,
vol. 381, pp 79-91, 1995 and Hedrick et al., High Tg Polyimide Nanofoams Derived from Pyromellitic Dianhydride and 1,1-Bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane,
J. Polymer Sci.,
vol. 34, pp 2867-2877, 1996, disclose polyimide nanofoams prepared by first forming an A-B-A block (or triblock) copolymer with polyimide endcapping units and then casting a film from a solution of the copolymer. Upon heating, the thermally unstable block copolymer component, either a poly(propylene oxide) or a poly(methyl methacrylate), undergoes thermolysis leaving pores in the film, the size and shape of which are dictated by the initial copolymer morphology. In such porous films, the volume fraction of voids was considerably less than the volume fraction of propylene oxide in the copolymer, thus indicating that at least partial collapse of the pores had occurred. Such partial pore collapse makes uniform distribution of pores in the film difficult to achieve. Also difficult to achieve from this approach is a high volume fraction of pores in the dielectric film. The extension of this block copolymer approach as a route to rigid and semi-rigid polyimide nanofoams was not successful.
U.S. Pat. No. 6,093,636 (Carter et al.) discloses a method for forming an integrated circuit containing a porous high temperature thermoset, such as a polyimide. Such porous thermosets are prepared by using as pore forming material highly branched aliphatic esters that have functional groups that are further functionalized with appropriate reactive groups such that the functionalized aliphatic esters are incorporated into, i.e. copolymerized with, the vitrifying polymer matrix. Such incorporation of the pore forming material into the matrix restricts the mobility of the pore forming material, i.e. incorporation prevents phase separation of the pore forming material from the matrix. By restricting such mobility, the size of the phase-separated domains is also restricted.
Other methods of preparing porous dielectric materials are known, but suffer from broad distributions of pore sizes, too large pore size, such as greater than 20 microns, or technologies that are too expensive for commercial use, such as liquid extractions under supercritical conditions.
There is thus a need for improved porous polyimide dielectric matrix materials with substantially smaller pore sizes and a greater percent by volume of pores for use in electronic components, and in particular, as an interlayer, or intermetal, dielectric material for use in the fabrication of integrated circuits. There is also a need for porous polyimide dielectric materials where the volume fraction of pores in the film is equivalent to the volume fraction of pore forming material.
SUMMARY OF THE INVENTION
It has now been surprisingly found that certain polymeric particles (or porogens) incorporated into polyimide dielectric matrix provide porous films having a suitable dielectric constant and sufficiently small pore size for use as insulating material in electronic devices such as integrated circuits and printed wiring boards. Such polymeric particles provide polyimide dielectric matrix material having a greater percentage of pores by volume and more uniformly dispersed pores than are available from known approaches to polyimide nanofoams.
In a first aspect, the present invention is directed to a method of preparing porous polyimide dielectric materials including the steps of: a) dispersing removable polymeric porogen in B-staged polyimide dielectric matrix material; b) forming a film of the B-staged polyimide dielectric matrix material; c) curing the B-staged polyimide dielectric matrix material to form a polyimide dielectric matrix material; and d) subjecting the polyimide dielectric matrix material to conditions which at least partially remove the porogen to form a porous polyimide dielectric material without substantially degrading the polyimide dielectric material; wherein the porogen is substantially compatible with the B-staged polyimide dielectric matrix material and wherein the porogen includes as polymerized units at least one monomer capable of hydrogen-bonding with the B-staged polyimide dielectric matrix material.
In a second aspect, the present invention is directed to porous polyimide dielectric materials prepared by the method described above.
In a third aspect, the present invention is directed to a method of preparing an integrated circuit including the steps of: a) depositing on a substrate a layer of a composition including B-staged polyimide dielectric matrix material having poly

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