Nanoporous polymer films for extreme low and interlayer dielectrics

A process for producing a nanoporous polymer film of no greater than 10 micron thickness having low dielectric constant value, including the steps of: (a) providing a polymer in a solution with at least two solvents for the polymer in which a lowest boiling solvent and a highest boiling solvent have a difference in their respective boiling points of approximately 50.degree. C. or greater; (b) forming a film of the polymer in solution with at least the two solvents on a substrate; (c) removing a predominant amount of the lowest boiling solvent; (d) contacting the film with a fluid which is a non-solvent for the polymer, but which is miscible with the at least two solvents to induce phase inversion in the film; (e) forming an average pore size in the film in the range of less than 30 nanometers. The present invention is also nanoporous films made by the above process.

BACKGROUND OF THE INVENTION
 The electronics industry utilizes dielectric materials as insulating layers
 between circuits and components of integrated circuits and associated
 electronic devices. Line dimensions are being reduced in order to increase
 speed and storage capability of microelectronic devices (e.g., computer
 chips). Microchip dimensions have undergone a significant decrease even in
 the past decade such that line widths previously &gt;1 micron are being
 decreased to 0.25 microns, with future plans on the drawing boards of as
 low as 0.07 microns. As the line dimensions decrease, the requirements for
 preventing signal crossover (crosstalk) between the chip components become
 much more severe. These requirements can be summarized by the expression
 RC, where R=resistance of the conductive line and C=capacitance of the
 dielectric layer. Decreasing the dielectric constant will decrease the
 capacitance of the interlayer which will aid in minimizing RC.
 Historically, silica with a dielectric constant of 4.2-4.5 has been
 employed as the interlayer dielectric (ILD). However at line dimensions of
 0.25 microns and less silica is no longer acceptable, and is expected to
 be replaced by polymers as the ILD material of choice. Representative
 polymers which meet the demanding requirements for an ILD material include
 the poly(arylene ethers) with dielectric values in the range of 2.6-2.8,
 such as that described in U. S. Pat. No. 5,658,994. Ultimately materials
 with dielectric constant values of 2.0 or less will be required as the
 line dimensions continue to decrease. The only polymers (nonporous) which
 reach values near or below 2.0 are fluorocarbon polymers. Fluorocarbon
 polymers have other drawbacks such as poor adhesion, potential reaction
 with metals at high temperature, poor rigidity at high temperature, and in
 some cases lower thermal stability than acceptable. In order to achieve
 the desired property characteristics and low dielectric constant values,
 nanoporous polymeric materials may be used.
 Incorporation of porosity to polymeric materials is long known to reduce
 the dielectric constant of polymers (L. Mascia, The Role of Additives in
 Plastics, 1974). Microporous polymers (e.g., cyanate ester resins) have
 been noted by Kiefer et al. (Macromolecules, 29 (1996) 8546) to exhibit
 reduced dielectric constant values versus dense precursors. The resultant
 pore diameters were in the range of 10-20 microns which is orders of
 magnitude too large for line widths &lt;0.25 microns. If polymeric materials
 are going be used as the interlayer in applications which require
 dielectric constant .epsilon.&lt;2.0 along with excellent stability,
 excellent adhesion, compatibility with the integration process (e.g.,
 reactive ion etching, fabrication of thin films), ability to have global
 planarization, low water sorption, non-reactivity to metals, and constant
 dielectric constant up to and exceeding 1 GHz, then nanoporous films will
 be one of the alternatives to be considered.
 The design of nanoporous polymers for these applications was reported by
 Hedrick and coworkers at IBM in several papers. Their procedure involved
 the synthesis of diblock copolymers comprised of a high T.sub.g, thermally
 stable block and a lower T.sub.g, thermally labile block. The resultant
 material phase separates into well-defined micellar domains. Upon exposure
 to temperatures above the decomposition temperature of the thermally
 labile block, but below the T.sub.g and thermal decomposition temperature
 of the high T.sub.g block, thermolysis of the former would occur to
 produce a nanoporous structure. Hedrick et al. (Polymer, 34 (1993) 4717)
 also noted the ability to generate porous polymers with nanometer sized
 pores using the above described technique employing
 poly(phenylquinoxaline) as the high T.sub.g stable block and
 poly(propylene oxide) or poly(methyl methacrylate) as the labile block.
 Charlier et al. (Polymer, 36, No. 5 (1995) 987) used this procedure
 employing poly(propylene oxide) as the labile block to produce nanoporous
 polyimides. Hedrick et al. (Chem. Mater., 10 (1998) 39) described
 polyimide nanofoams using aliphatic polyester based thermally labile
 blocks (e.g., poly(.epsilon.-caprolactone)) in polyimide block copolymers.
 Pore sizes of 6-7 nm were reported. While this procedure yields a
 nanoporous material, the dielectric constants reported are primarily above
 2.0 and there appears to be a practical limit to the amount of porosity
 that can be achieved via this approach. The choice of polymeric materials
 with T.sub.g above 400 .degree. C., desired to be even &gt;450.degree. C.,
 are very limited and generally involve very polar materials and/or
 materials with higher water sorption than desired.
 One method for producing porous polymeric materials is termed phase
 inversion, commonly employed in the production of microporous membranes.
 The phase inversion process to produce porous membranes has been well
 described in the literature, including the book by Kesting and Fritzsche
 (Polymeric Gas Separation Membranes, 1993). The phase inversion process
 can involve a non-solvent induced phase separation or a temperature
 induced phase separation to produce pores in the micron to nanometer
 range. Saunders et al. (ASME, 53 (1994) 243) noted that spin coating on
 silicon wafers followed by non-solvent induced phase separation yielded
 microporous polyimide films with a reduced dielectric constant. A
 polyimide (Ultem 1000.TM.) was spin coated onto a silicon wafer from a
 solution of 1,3-dimethoxybenzene followed by immersion in toluene, a
 non-solvent for the polymer. A specific example indicates film thickness
 of 22 microns with porosity of 68% and a pore size of 1.4 microns. The
 dielectric constant decreased from 3.15 for the dense film to 1.88 for the
 porous film. While this reference denotes a porous film with a reduced
 dielectric constant produced via non-solvent induced phase separation, the
 pore sizes being almost two orders of magnitude too large are considerably
 outside the realm of interest for present or future low dielectric
 constant interlayer materials. Therefore the methodology employed by this
 reference requires major changes in order to be able to achieve the
 desired film properties for an interlayer dielectric in microelectronic
 applications (e.g., computer chips). Young, et al., Desalination 103
 (1995) pp 233-247 studied solvent based phase inversion for porous
 polymers and used combinations of solvents and nonsolvents to evaluate
 pore formation.
 The prior art suffers from several disadvantages for using phase inversion
 for low dielectric films for microelectronics. One of the basic problems
 of the phase separation and spin coating process is that low viscosity
 (e.g., low solids content) solutions are required to yield thin films
 (.about.1.mu.) with uniform wafer coverage. However the phase separation
 process using low viscosity, low solids solutions yields large pore
 structures; utilizing high solids solutions (&gt;20 vol %) leads to
 exorbitantly thick films and inconsistent film thicknesses along the
 substrate. Also, the ability to tune porosity to a desired level with
 essentially constant film thickness is not possible with the current
 procedure due to the low viscosity requirements for the spin-on solution
 and the required non-volatile nature of the solvent, e.g., to prevent
 phase separation during casting. Another characteristic inherent with the
 prior art process when employing a low solids solution (as required for
 spin casting) is that the phase separation process leads to loss of
 adhesion to the desired substrate, often resulting in films detaching from
 the substrate during the non-solvent phase separation process. Poor
 adhesion is not acceptable and poor mechanical properties may lead to
 failure of the resulting interlayer during integration processing. A final
 problem which must be addressed for nanoporous ILDs arises from the
 temperature exposure inherent with typical microelectronic device
 manufacturing, e.g., up to 450.degree. C. For a polymeric porous ILD with
 a T.sub.g below processing conditions, collapse of the porous structure
 will occur as the polymer relaxes to the equilibrium state of the material
 (i.e., dense film). It is known to crosslink polymers with photo
 crosslinkable groups (U.S. Pat. No. 4,717,393) and to do so for polyimide
 films for integrated circuits, Lin, et al. Macromolecules, 1988, 21, pp
 1165-1169. This presents a well-defined hurdle which must be overcome to
 allow porous polymeric materials to be employed as ILDs. The present
 invention addresses all of these difficult requirements outlined above and
 demonstrates via experimental results that these issues can be resolved
 using the techniques and materials of the present invention set forth
 below.
 BRIEF SUMMARY OF THE INVENTION
 The present invention is a process for producing a nanoporous polymer film
 of no greater than 10 micron thickness having low dielectric constant
 value, comprising the steps of:
 (a) providing a polymer in a solution with at least two solvents for the
 polymer in which a lowest boiling solvent and a highest boiling solvent
 have a variation or difference in their respective boiling points of
 approximately 50.degree. C. or greater;
 (b) forming a film of the polymer in solution with at least the two
 solvents on a substrate;
 (c) removing a predominant amount of the lowest boiling solvent;
 (d) contacting the film with a fluid which is a non-solvent for the
 polymer, but which is miscible with the at least two solvents to induce
 phase inversion in the film;
 (e) forming an average pore size in the film in the range of less than 30
 nanometers.
 Preferably, the nanoporous film has a thickness of no greater than 2
 microns.
 Preferably, the substrate is a semiconductor.
 Preferably, the lowest boiling solvent has a boiling point of approximately
 100.degree. C. or less and the highest boiling solvent has a boiling point
 of approximately 150.degree. C. or more.
 Preferably, the polymer is selected from the group consisting of
 poly(arylene ethers), polyimides, poly(phenyl quinoxalines), substituted
 poly(p-phenylenes), poly(benzobisoxazoles), polybenzimidazoles,
 polytriazoles and mixtures thereof.
 Preferably, the lowest boiling solvent is selected from the group
 consisting of tetrahydrofuran, acetone, 1,4-dioxane, 1,3-dioxolane, ethyl
 acetate, methyl ethyl ketone, cyclohexanone, cyclopentanone and mixtures
 thereof.
 Preferably, the highest boiling solvent is selected from the group
 consisting of dimethylformamide, dimethylacetamide, N-methyl pyrrolidone,
 ethylene carbonate, propylene carbonate, glycerol and derivatives,
 naphthalene and substituted versions, acetic acid anyhydride, propionic
 acid and propionic acid anhydride, dimethyl sulfone, benzophenone,
 diphenyl sulfone, sulfolane, phenol, m-cresol, dimethyl sulfoxide,
 diphenyl ether, terphenyl, cyclohexanone, cyclopentanone and mixtures
 thereof.
 Preferably, the fluid which is a non-solvent for the polymer is selected
 from the group consisting of water, methanol, ethanol, isopropanol,
 toluene, hexane, heptane, xylene, cyclohexane, butanol, cyclopentane,
 octane and miscible mixtures of these non-solvents.
 Preferably, the polymer is crosslinked before contacting the film with the
 fluid which is a non-solvent.
 Alternatively, the polymer is crosslinked after contacting the film with
 the fluid which is a non-solvent.
 Preferably, the polymer is crosslinked by photochemical crosslinking.
 Alternatively, the polymer is crosslinked by chemical crosslinking.
 Preferably, the film is formed by spin casting.
 Preferably, the contacting the film with the fluid which is a non-solvent
 is by spin casting, alternatively by immersion in non-solvent, further
 alternatively by non-solvent vapor atmosphere.
 Alternatively, the contacting the film with the fluid which is a
 non-solvent is by immersion.
 Further alternatively, the contacting the film with the fluid which is a
 non-solvent is by vapor atmosphere.
 The present invention is also a low dielectric constant nanoporous film
 having average pore size of less than approximately 30 nanometers and no
 greater than 10 micron thickness made by the process of:
 (a) providing a polymer in a solution with at least two solvents for the
 polymer in which a lowest boiling solvent and a highest boiling solvent
 have a variation or difference in their respective boiling points of
 approximately 50.degree. C. or greater,
 (b) forming a film of the polymer in solution with the at least two
 solvents on a substrate;
 (c) removing a predominant amount of the lowest boiling solvent;
 (d) contacting the film with a fluid which is a non-solvent for the
 polymer, but which is miscible with the at least two solvents to induce
 phase inversion in the film;
 (e) forming an average pore size in the film in the range of less than 30
 nanometers.
 The present invention is also a low dielectric constant nanoporous
 poly(arylene ether) film having average pore size of less than
 approximately 30 nanometers and a film thickness of less than 10 microns.
 Preferably, the film has a film thickness of no greater than approximately
 2 microns.
 Preferably, the film has a porosity of approximately 10 to 60 volume
 percent.
 More preferably, the film has a porosity of approximately 30 to 60 volume
 percent.
 Preferably, the poly(arylene ether) comprises a poly(arylene ether) polymer
 consisting essentially of non-functional repeating units of the structure:
 ##STR1##
 wherein m=0 to 1.0; and n=1.0-m; and Ar.sub.1, Ar.sub.2, Ar.sub.3 and
 Ar.sub.4 are individually divalent arylene radicals selected from the
 group consisting of;
 ##STR2##
 and mixtures thereof, but Ar.sub.1, Ar.sub.2, Ar.sub.3 and Ar.sub.4, other
 than the diradical 9,9-diphenylfluorene, are not isomeric equivalents.
 Preferably, the film is supported on a semiconductor substrate of an
 integrated circuit.
 Alternatively, the film is supported on a microelectronic material of an
 integrated circuit.
 Preferably, the film has a film thickness of no greater than approximately
 2 microns, a pore density of 30 to 60 volume percent and being
 crosslinked.
 Preferably, the film has a T.sub.g of at least approximately 400.degree. C.

EXAMPLE 1
 Solution Preparation (BTDA-DAM)
 A sample of polyimide BTDA-DAM (benzophenone tetracarboxylic
 dianhydride-diamino mesitylene; number average molecular weight
 (Mn).about.1-3.times.10.sup.4 g/mol, poly dispersity index (PDI)=2-3 as
 determined by gel permeation chromatography (GPC) in N-methyl pyrroline
 (NMP) using polystyrene standards), was added to a known amount of
 propylene carbonate (PC, 99.7+%) to produce a 10 wt % solids solution. The
 mixture is heated to .about.100.degree. C. to produce a homogeneous amber
 solution. If left to cool the solution becomes slightly turbid and gels,
 and can not be spun onto a wafer to produce a homogeneous thin film. With
 stirring, a known amount of a 50/50 mixture of dioxane(D;
 99+%)/tetrahydrofuran (THF; 99.99%) was added slowly to the hot solution
 to produce a clear, amber solution of .about.5 wt % solids which did not
 gel upon cooling. All solutions were filtered through a 1.0 micron or
 smaller filter prior to spin casting.
 The solution of BTDA-DAM in PC forms a gel at room temperature down to at
 least 2 wt % solids which was found to be thermoreversible; at lower
 solids contents (.about.2 wt %) the mixture also showed signs of being
 thixotropic. If only D is used to dilute the mixture to .about.5 wt % the
 solution still gels upon cooling, however if only THF is used as a diluent
 the mixture does not gel. PC is a good solvent for BTDA-DAM, while D and
 THF are poor solvents. A good solvent is defined as one which can dissolve
 the polymer to at least 5 wt % (with heating and stirring), while a poor
 solvent is one which only partially dissolves a 5 wt % mixture.
 EXAMPLE 2
 Solution Preparation (PAE-2)
 A sample of poly(aryl ether), PAE-2, (Mn.about.1.times.10.sup.4 g/mol, PDI
 .about.2-3) is dissolved in a mixture of cyclohexanone (C; 99.9%) and THF
 or D. The solutions used were .about.5 wt % solids and .about.40% by
 volume C. All solutions were filtered through a 1.0 micron or smaller
 filter prior to spin casting. All liquids are good solvents for this
 polymer as judged by rapid and complete dissolution of polymer at 5 wt %
 solids with stirring. PAE-2 has the following structure:
 ##STR5##
 EXAMPLE 3
 Spinning and Phase Separation/Inversion Procedure
 Typically at a spinning rate of 500-2000 rpm on a 4" wafer, .about.2 mL of
 solution is introduced over a few seconds. The sample typically remains
 spinning for 5-20 seconds after completely dispensing the solution, and
 may remain on the spinner chuck once spinning has stopped for an
 additional 0-120 seconds to allow the film to partially dry and
 equilibrate to the desired composition. The wafer is then removed and
 quickly immersed in a phase inversion solution (large excess relative to
 solvent) to structure the film and remove the solvents, locking the
 structured morphology in place.
 Phase inversion may also be conducted on the spinner chuck by introduction
 of the phase inversion non-solvent fluid onto the sample while spinning.
 For this invention, the phase inversion solution may be any liquid or
 mixtures of liquids which causes a structuring of the polymer film through
 a phase inversion or separation process, as noted in the introduction. It
 was determined that the solution composition, as well as the phase
 inversion mixture and the spinning and phase inversion procedure are
 important to obtain the required thickness, porosity, pore size, and
 adhesion to the Si surface for the final polymer film.
 For phase inversion of BTDA-DAM samples, wafers were left in the phase
 inversion bath typically for 30-90 seconds to fully remove residual
 solvents. For PAE-2 samples, immersion time was much shorter, requiring
 only 1-10 seconds in the phase inversion bath. Samples were then placed on
 a hotplate at 80.degree. C. for 2 minutes and then 280.degree. C.
 (200.degree. C. only for PAE-2 films) to dry off any residual liquids.
 For PAE-2 at 5 wt % solids content, the solvent composition was required to
 be at least 30-35 vol % C to prevent phase separation on the wafer during
 spinning, which would produce a textured film. Excessive texturing of the
 film was assessed visually and quantified by profilometry (a significant
 degree of dispersity in the thickness profile of the film across the wafer
 surface which continually changes over the length scale of several
 microns).
 EXAMPLE 4
 UV Curing Procedure
 To lock the structured morphology in place, a UV curing stage was
 incorporated prior to the phase separation/inversion procedure. Wafer
 samples were placed inside a cylindrical clear glass chamber 1 cm
 deep.times.15 cm diameter with a quartz lid. The samples would then be
 exposed to a UV source (medium pressure Hg lamp from Ace Glass, 30 cm
 distance) for times ranging from 5-300 seconds. The lamp had an output of
 .about.90 watts in the wavelength range of 365 nm to 220 nm. After
 exposure, the samples were typically immediately immersed in a phase
 inversion bath to structure the film.
 The degree of crosslinking was assessed through high temperature bake.
 Samples of porous BTDA-DAM, dense BTDA-DAM, and porous BTDA-DAM which were
 irradiated with UV were first tested for .epsilon. to get initial values.
 The samples were then placed in an oven at 400.degree. C. for 30 minutes
 (air). The T.sub.g of uncrosslinked BTDA-DAM is &lt;375.degree. C. so baking
 at 400.degree. C. should collapse BTDA-DAM films which were structured
 without crosslinking, as indicated by .epsilon. values after baking.
 Dielectric values for films were determined from thickness and capacitance
 values using a Tencor model P-2 profilometer and a Solartron model SI1260
 frequency analyzer with a MSI Electronics model Hg-401 mercury probe with
 background corrections for capacitance of uncoated wafer and equipment.
 The total error in dielectric values is estimated to be &lt;5%. Porosity
 values were determined from elipsometric measurements on a Sentech model
 SE800 elipsometer using the refractive indecies of dense polymer and air
 to model the experimental data. The results compared well with that
 calculated from dielectric values using the Clausius-Masotti equation. All
 wafers were tested initially for background corrections due to surface and
 bulk impurities.
 Prior art in this area (Saunders et al) indicates that porous polymer films
 of tens to hundreds of microns thickness with pores in the micron size
 range can be easily and reproducible made from single solvent polymer
 solutions, in some cases with homogeneous pore size distribution. To be
 useful as an interlayer dielectric film for future generations of
 microelectronics, the smaller features dictate that film thicknesses and
 pore sizes will need to be 1-2 orders of magnitude smaller than that shown
 by Saunders, et al. Using a solution composed of a single, high boiling
 point solvent will not result in the desired final product.
 Firstly, to obtain films with thicknesses of .about.1 micron requires low
 viscosity solutions, implying low solids content (.about.5 wt %). Thinner
 films will also result in faster phase separation, which suppresses growth
 of nuclei, thus resulting in producing smaller pores. However as Saunders,
 et al. has shown, the composition of the film does not change
 significantly during the spinning process when using a single high boiling
 point solvent, thus the porosity of a thin film produced from a 5 wt %
 solids solution would approach 95%. High solvent compositions also favor
 slower phase separation kinetics and would effectively increase void
 sizes. Although a film with 95% porosity would have an extremely low
 .epsilon. the film would have little mechanical integrity. These
 concentrations would also favor nucleation and growth of polymer particles
 from solution, producing an interconnected porous structure which is not
 desirable.
 To achieve both the desired porosity and pore structure (discontinuous
 voids in a polymer continuous phase produced from the nucleation of
 solvent(s) and non-solvent(s) from the polymer phase) while maintaining
 mechanical strength requires high solids content solutions, which are
 unable to form sufficiently thin films as Saunders has shown. In effect,
 there is no direct way in which a single, high boiling point solvent can
 produce both thin films and high solids content which would result in a
 nanoporous structure.
 Conversely, using only a lower boiling point solvent will inevitably
 produce a dense film with a surface structured on the micron level as a
 result of phase separation during the spinning procedure.
 The following examples indicate how polymer solutions composed of multiple
 solvents of significantly different boiling points (.gtoreq.50.degree. C.
 can produce nanoporous, structured thin films on Si wafers with variable
 porosity, and thus variable .epsilon.. Examples are also given for the use
 of UV irradiation to stabilize the porous structure to temperatures in
 excess of the polymer's original T.sub.g, and of optimized phase inversion
 procedures to produce nanoporous films with good adhesion and electrical
 properties.
 BTDA-DAM
 For solutions of BTDA-DAM, it was determined important that the mixture be
 comprised of a solvent mixture of high boiling point and low boiling point
 solvents. Using a single high boiling point solvent solution of BTDA-DAM
 produces porous structures through phase inversion, however the pore sizes
 are generally at least an order of magnitude too large for the current
 application. For example, polyimides may be phase inverted from NMP only,
 producing pore sizes in the range of 1 micron and films which were
 visually opaque.
 BTDA-DAM cannot be spun from solutions containing only propylene carbonate
 or ethylene carbonate (EC) as these mixtures gel upon cooling below
 .about.50.degree. C., making the production of a uniform thin film not
 possible. As well, EC crystallizes from solution when cooled. While
 spinning at high temperatures may circumvent these problems, this work
 also indicated that PC alone has little or no ability to impart a porous
 structure through phase inversion techniques. From studies of bulk gels of
 BTDA-DAM in PC it was determined that upon immersion of a portion of the
 gel in a non-solvent solution typically used for phase inversion of
 BTDA-DAM, the gel appears to proceed through what is better defined as a
 solvent exchange. The material retains its initial structure size and
 appearance, yet PC appears to be replaced by methanol. This, along with
 the gelling behavior of the mixtures of BTDA-DAM in PC, implies that a
 physically crosslinked structure is obtained. Upon drying this gel
 collapses to dense material.
 The addition of both D and THF was found to be important to obtain thin
 films with small pore structures that otherwise could not be achieved.
 Neither THF nor D are good solvents for BTDA-DAM, as determined by
 solubility tests at .about.5 wt % solids. An optimum range of solution
 compositions for producing small voids (&lt;30 nm) with porosities from
 10-50% was determined to be .about.50 vol % PC and 10-40 vol % D and 10-40
 vol % THF. In this range thin films could be produced with .epsilon. down
 to 1.7 having little or no visible opacity to the film. Samples from
 solutions containing 1:1:1-PC:D:THF yield .epsilon. as low as .about.3 for
 similar spin conditions with pores &lt;30 nm.
 For samples spun from high D content solutions (&gt;50 vol %) only dense films
 were produced. Samples spun from solutions with PC levels &lt;.about.10 vol %
 phase separated on the wafer within 10 seconds of solution dispense,
 producing textured dense films. Thus the range of PC content required to
 allow for thin films to be spun and still produce a porous structure was
 determined to be from .about.10 to .about.75 vol %. Above 75 vol % PC thin
 films could only be spun if there were excess THF relative to D, and all
 produced opaque films indicating larger pores. Below .about.10 vol % PC,
 the polymer was not fully dissolved in the solution and phase separation
 was observed soon after solution was dispensed. These results clearly
 indicate that a porous structure with pore sizes &lt;30 nm can only be
 produced using a mixture of solvents. Note that one criteria for the use
 of PC, D, and THF is that they are all transparent to UV in the range
 required for excitation of the benzophenone moiety of BTDA-DAM, which has
 been determined to be the active species in crosslinking of this polyimide
 (Lin et al).
 Other high boiling point liquid solvents were also tested; these included
 NMP, dimethyl sulfoxide (DMSO), pyridine, and C. Other low boiling point
 liquids considered included 3-methyl-1-butanol, glyme, acetonitrile, ethyl
 acetate, hexafluoroisopropanol, and diethyl ether. From SEM micrographs
 for polyimides BTDA-DAM (7 wt % solids) in 50/50:THF/NMP and 6FDA-DDA (20
 wt % solids) in NMP only, it is clear that both composition (i.e., polymer
 and solvent type) and solids concentration have a strong effect on the
 final film character.
 Films of BTDA-DAM in mixtures of PC+D+THF gel upon evaporation of THF from
 solution when spun onto wafers. Once gelled, diffusion rates within the
 film (and ultimately evaporation rates of low boiling point liquids from
 the film) drop substantially. As a result spinning times and drying times
 could be quite long for BTDA-DAM films (.about.3-5 minutes) prior to phase
 inversion and still produce films with significant porosity. Porous films
 produced from mixtures of PC/D/THF are optically clear and smooth (as
 determined by profilometry). Because of the clarity of the films it is
 expected that the majority of the pores are &lt;30 nm in size, which is
 confirmed by SEM. Solution composition, phase inversion mixture, and
 spinning and phase inversion procedure govern the final film properties.
 The composition of the phase inversion solution also has a big role in the
 final film character. Liquids tested extensively include water (distilled
 and deionized), methanol (MeOH; 99.99%), ethanol (EtOH; anhydrous) and
 mixtures thereof. These were chosen based on solubility of the solvent
 mixture in the phase inversion mixture, small molecular size (to enhance
 diffusion kinetics and thus phase inversion kinetics) and their lack of
 any solvency for the polymer. It was found that MeOH produced the films
 with the least opacity and lowest .epsilon.. Films from EtOH were more
 opaque and less porous as a result of larger pore sizes, while those
 produced through phase inversion in water or water/MeOH resulted in films
 which were opaque, but had poor adhesion to the wafer, e.g., the film
 immediately cracked and peeled off the wafer when immersed in solution,
 and were often dense. Immersion time in the non-solvent solution was also
 determined to be important to obtain a porous structure with good
 adhesion; immersion times were optimized for the experiments performed.
 TABLE 1
 Dielectric and selective refractive index measurements
 for porous BTDA-DAM films:
 dielectric refractive %
 cnst. index porosity pore size
 Sample # (.epsilon.) .+-. 5% (.eta.) .+-. 5% (.epsilon.; .eta.)
 (nm)
 107-18C; 300-12; 300-13 4.14 1.6833 0 na
 (16633-7-3; 16633-41-2;
 166333-41-3)
 107-24C 3.53 -- 11 &lt;20
 (16633-19-3)
 107-18D 2.75 1.464 28,32 &lt;30
 (16633-7-4)
 300-19A 1.93 -- 54 &lt;30
 (16633-65-5)
 dielectric - mercury probe + profilometry
 refractive index - elipsometry
 % porosity - volume percent calculated from Clasius-Mosotti relationship;
 model fit from elipsometry data
 pore size - measured by SEM
 Solution compositions:
 107-18C: Soln 16428-92-1; .about.4 wt % BTDA-DAM (16428-12-1) in 50/25/25
 PC/D/THF
 300-12: Soln 16428-63-1; 5 wt % BTDA-DAM (16428-12-1) in C
 300-13: Soln 16428-63-1; 5 wt % BTDA-DAM (16428-12-1) in C
 107-24C: Soln 16428-92-1; .about.4 wt % BTDA-DAM (16428-12-1) in 50/25/25
 PC/D/THF
 107-18D: Soln 16428-92-1; .about.4 wt % BTDA-DAM (16428-12-1) in 50/25/25
 PC/D/THF
 300-19A: Soln 16633-64-1; .about.5 wt % BTDA-DAM (16633-62-1) in 50/25/25:
 propylene carbonate/diphenyl ether/THF.
 Polymer MW (GPC in NMP):
 BTDA-DAM # 16428-12-1: Mn=1.76.times.10.sup.4 g/mol, Mw=4.70.times.10.sup.4
 g/mol
 BTDA-DAM # 16633-62-1: Mn=1.74.times.10.sup.4 g/mol, Mw=3.62.times.10.sup.4
 g/mol
 Scanning electron microscopy imaging of the above materials showed that
 nanoporosity was achieved with uniformity and small pore structure. The
 dielectric values were reduced with increasing porosity (void content or
 pore density). The following decreasing dielectric values were achieved
 with increasing porosity:
 SEM of dense BTDA-DAM @ e=4.14
 SEM of porous BTDA-DAM @ e=3.53
 SEM of porous BTDA-DAM @ e=2.75
 SEM of porous BTDA-DAM @ e=1.93
 Negative examples:
 Scanning electron microscopy showed pores in the micron size range and a
 lack of uniform nanoporosity in the following cases:
 SEM of polyimide 6FDA-DDA from NMP; 20 wt % solids (LR98112)
 SEM of BTDA-DAM from 50/50 THF/NMP (LR94111)
 EXAMPLE 5
 Crosslinking via UV Irradiation
 Locking the morphology in place via UV crosslinking allows the film to
 retain its structure during high temperature baking. Results shown below
 clearly indicate that baking porous BTDA-DAM samples for 10 minutes at
 400.degree. C. causes a significantly higher reduction in porosity for the
 non-irradiated samples versus the irradiated sample. The results for 30
 minute bake at 400.degree. C. clearly show that while the non-irradiated
 samples have high dielectric values (actually higher than dense unbaked
 material at 4.14) and a dense appearance via SEM, the irradiated samples
 have much lower values of F and still retain some porosity. The increase
 in F for the samples to values above the dense film is likely a reflection
 of oxidation or degradation of the film at this high temperatures. While E
 values for the irradiated samples did increase somewhat from their values
 prior to baking, the fact that they are still lower than the dense film
 indicates that porosity has been retained as a result of UV irradiation,
 presumably through a crosslinking mechanism.
 TABLE 2
 Elipsometry and dielectric data of irradiated and non-irradiated
 BTDA-DAM films - oven baking (400.degree. C., air, 10 mins).
 porosity (pre-bake) porosity (post-bake)
 Sample vol. % Irrdn time (sec) vol. %
 111-18D 39 30 27
 (16759-76-5)
 111-21A 32* 0 9
 (16759-81-5)
 111-21B 31* 0 8
 (16759-81-6)
 *estimated from dielectric data
 Samples discolored after bake, turned slight golden color.
 TABLE 3
 Dielectric constant of irradiated and non-irradiated BTDA-DAM films -
 oven baking (400.degree. C., air, 30 mins).
 Sample .epsilon. (pre-bake) Irrdn time (sec) .epsilon.
 (post-bake)
 300-20A 2.41 15 3.84
 (16633-66-3)
 300-20B 2.19 60 3.47
 (16633-66-4)
 300-23B 2.85 0 5.45
 (16633-68-2)
 300-23D 3.01 0 5.47
 (16633-68-4)
 300-23E 2.06 0 5.39
 (16633-68-5)
 Samples discolored after bake, turned golden color but retained their
 respective levels of opaqueness. From SEMs done prior and post-bake it was
 determined that pore structure had been reduced:
 20A: mostly dense, very little visible porosity, few very small pores.
 20B: appears slightly porous; pores 20 nm-40 nm in size.
 20C: mostly dense.
 23B: film mostly dense
 PAE-2
 For PAE-2 it was also determined that the use of multiple solvents as
 solution components was also required for the application of porous thin
 films with significant porosity (&gt;10 vol %) and suitable pore size (&lt;30
 nm) via a spin coating procedure. As with BTDA-DAM, PAE-2 could be applied
 using only a single high boiling point solvent; however, this inevitably
 resulted in pores &gt;50 nm. The use of THF, D or 1,3-dioxolane as a diluent
 allowed for thin films to be applied which had the desired pore sizes
 (e.g., &lt;30 nm). In these cases, these lower boiling solvents are good
 solvents for the polymer, yet solutions employing these lower boiling
 point liquids alone resulted in phase separation on the wafer within a few
 seconds after complete dispensing of the aliquot of solution. It was
 determined that it was preferred to have at least .about.30-40 vol % C in
 the solution to prevent phase separation on the wafer and to produce the
 required porosity and pore sizes. As an example, a solution of 75 vol % C
 with 25 vol % THF (5 wt % solids) produced a film with 70 vol % porosity;
 samples made from high C content solutions (&gt;75 vol %) typically produced
 very low dielectric values compared to mixed solution systems with pore
 sizes which were significantly larger than &gt;50 nm.
 Samples produced are optically clear as determined visually. Because of the
 clarity of the films it is expected that the majority of the pores are &lt;30
 nm in size; this was confirmed by SEM. The amount of spinning time after
 complete dispensing of solution, and the amount of time the wafer is left
 on the spinner after spinning has ceased will dictate the amount of
 residual solvent and thus the final porosity. For PAE-2 solutions, the
 spin times and drying times were shortened considerably as a result of the
 higher volatility of the high boiling point solvent used here (C,
 bp=155.degree. C.) versus BTDA-DAM samples (PC, bp=240.degree. C.) and the
 fact that BTDA-DAM samples produce a gel on the wafer which impedes
 drying. As a result, spinning times and drying times could be longer for
 BTDA-DAM films without eliminating all porosity. The solutions used to
 produce PAE-2 films dry much more rapidly, with C+D films appearing to dry
 more quickly than C+THF films. As a result, there are important time
 windows for spinning, drying, and phase inverting PAE-2 films which have
 been determined for these solutions. This allowed for good reproducibility
 and homogeneity in the film. The differences in spinning and phase
 inversion procedures for the two polymer systems appears to be caused by
 the differences in volatilities of the solutions and the gelating behavior
 of BTDA-DAM films on the surface of the wafer. Additionally solution
 components with very high boiling points, such as diphenyl ether, have
 been used to increase the time-window for phase inversion of PAE-2
 solutions, while still producing clear films with low dielectric values.
 Other poly(aryl ethers), (such as PAE-2 crosslinked with benzophenone
 functional groups, such as:
 ##STR6##
 and mixtures thereof, where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are
 individually H or alkoxy radical, the alkoxy can have a carbon chain of
 C.sub.1-8, normal or branched) were also used for assessing the procedure
 to produce nanoporous films. At similar compositions to PAE-2 based
 mixtures, solutions of PAE-2 crosslinked with benzophenone functional
 groups in cyclohexanone and D or THF were phase inverted in a similar
 manner to produce clear films with dielectric values ranging down to
 .about.1.9 with thicknesses of .about.1 micron.
 Porous PAE-2 films of .about.0.5 micron thickness and 35% porosity
 (.epsilon.=1.95) were tested for dielectric strength; there was no
 breakdown of the dielectric observed up to 0.8 MV/cm (sample 111-17,
 16759-88). These films also showed excellent adhesion to Si substrates as
 determined by ASTM standard test method for measuring adhesion by tape
 test (ASTM-D3359).
 The composition of the phase inversion solution also has a big role in the
 final film character. Fluids tested extensively include water (distilled
 and deionized), methanol (MeOH; 99.99%), ethanol (EtOH; anhydrous) and
 mixtures thereof. These were chosen based on solubility of the solvent
 mixture in the phase inversion mixture, small molecular size (to enhance
 diffusion kinetics and thus phase inversion kinetics) and their lack of
 any solvency for the polymer. Again it was found that MeOH produced films
 with the least opacity and lowest .epsilon.. Films from EtOH were more
 opaque as a result of larger pore sizes, while those produced through
 phase inversion in water resulted in dense films. Water: MeOH mixtures
 (50/50) resulted in clear porous films with poor adhesion to the wafer.
 Table 4 below shows a comparison of the physical properties of a non-porous
 PAE-2 film with results for porous films with the proper structure
 developed from multiple liquid solutions and for high C content solutions
 (&gt;75 vol %).
 TABLE 4
 dielectric refractive Ave. pore
 const.(.epsilon.) index(.eta.) % porosty size
 Sample # .+-.5% .+-.5% (.epsilon.; .eta.) (nm)
 16259-21-2 2.75 1.6808 0 --
 RC1 C710 257-17 -- 1.530 -; 21 (&lt;30)
 (16759-85-1)
 111-17 1.95 1.4325 34; 34 (&lt;30)
 (16759-74-4)
 111-15 1.98 -- 33 (&lt;30)
 (16759-74-5)
 110-18C 1.67 -- 50 &lt;30
 (16759-37-2)
 112-23A 1.93 -- 35 (&lt;30)
 (16853-22-1
 112-23E 1.91 -- 37 (&lt;30)
 16853-22-5
 111-18A 1.81 -- 42 &gt;100
 (16759-76-2)
 111-18B 1.38 -- 70 &gt;100
 (16759-76-3)
 112-22A 2.35 -- 16 &lt;20
 (16665-32)
 112-220 1.92 -- 36 &lt;30
 16665-32)
 dielectric - mercury probe + profilometry
 refractive index - elipsometry
 % porosity - volume percent calculated from Clasius-Mosotti relationship;
 model fit from elipsometry data.
 pore size - measured by SEM; results in brackets indicates SEM not
 performed, samples visually clear.
 Solution compositions:
 16259-21-2: dense PAE-2 on SI wafer from Schumacher
 16759-74-4: Soln 16759-62-1; .about.5 wt % PAE-2 (16759-13) in .about.40vol
 % C/60 vol % D
 16759-74-5: Soln 16759-61-4; .about.6 wt % PAE-2 (16759-38-2) in
 .about.40vol % C/60 vol % THF
 16759-37-2: Soln 16759-1-1; .about.4.5 wt % PAE-2 (16759-13) in .about.35
 vol % C/65 vol % THF
 16759-76-2, 3: Soln 16759-70-2; .about.5 wt % PAE-2 (16759-13) in 75 vol %
 C/25 vol % THF
 16853-22-1: Soln 16853-21-1; .about.6 wt % PAE-2 (16853-9-1) in 40 vol %
 C/60 vol % 1,3-dioxolane
 16853-22-5: Soln 16853-21-2; .about.6 wt % PAE-2 (16853-9-1) in 6 vol %
 diphenyl ether, 34 vol % C, 60 vol %, 1,3-dioxolane
 16665-32 (112-22A): Soln 16853-14-2; .about.6 wt % PAE-2 with benzophenone
 class crosslink (16853-9-2) in 40 vol % C/60 vol % THF
 16665-32 (112-22D): Soln 16853-14-3; .about.6 wt % PAE-2 with benzophenone
 class crosslink (16853-9-2) in 40 vol % C/60 vol % D
 Polymer MW:
 16759-13: formerly 16668-55-1, Mn=9.8.times.10.sup.3 g/mol, d=2.7
 16759-38-2: formerly 16496-11-1, Mn=9.6.times.10.sup.3 g/mol, d=2.8
 Scanning electron microscopy imaging of the above materials showed that
 nanoporosity was achieved with uniformity and small pore structure. The
 dielectric values were reduced with increasing porosity (void content or
 pore density). The following decreasing dielectric values were achieved
 with increasing porosity:
 SEM of dense PAE-2 @e=2.75
 SEMs of porous PAE-2 @e=1.67
 SEM of porous PAE-2 @e=1.81
 The present invention overcomes the drawbacks of film thickness, dielectric
 constant, porosity, stability and adhesion of the prior art, wherein the
 present invention provides low dielectric constant nanoporous thin films
 with good adhesion for use in dielectric layers for integrated circuits
 and various microelectronic devices. The unique process conditions of the
 present invention allow the achievement of these high performance films,
 which will allow polymeric interlayer dielectrics to be achieved in the
 reduced line dimensions contemplated by the electronic fabrication
 industry. The present process and the resulting low dielectric films are
 critical to the movement to decreased line dimension. Thus the present
 invention solves a significant need in the microelectronics fabrication
 industry as it moves to the next generation devices.
 The present invention has been set forth with regard to several preferred
 embodiments, however, the present invention should not be limited to those
 embodiments, but rather the full scope of the present invention should be
 ascertained from the claims below.