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Chapter 30

Vapor-Depositable Polymers with Low Dielectric Constants 1

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 15, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch030

J. A. Moore , Chi-I Lang , T.-M. Lu , and G.-R. Yang 1

Department of Chemistry, Polymer Science and Engineering Program and Center for Integrated Electronics and Department of Physics, Rensselaer Polytechnic Institute, Troy, NY 12180-3590 2

Chemical vapor deposition (CVD) has been one of the preferred processes for generating thin films of inorganic insulators in the fabrication of microelectronic devices. Currently, this methodology is being studied to generate organic thin films for use as dielectrics. Interaction between synthesis of the potential organic precursors for vapor deposition polymerization and design of the deposition apparatus has become a very active and demanding area in our research effort. Several thin polymer films, such as Parylene-N, Parylene-F (fluorinated), Teflon, Teflon AF (amorphous), poly(naphthalene) (PNT-N), poly(fluorinated naphthalene) (PNT-F) and poly(bis-benzocyclobuteneF ) (fluorinated), which have very low dielectric constants( 10 A/s. Spectroscopic analysis (XPS) indicates the material suffered little change in overall composition upon evaporation and redeposition and there was no change observed in the infrared spectrum between Teflon AF (1600) before and after deposition. In comparison to the conventional one step deposition process, two step deposition has been shown to give an even smoother surface, and more uniform films. It has also been shown that Teflon AF (1600) thin films deposited by the two step deposition process are much smoother than laser-ablated thin films (21). The key difference between the conventional one step deposition and the two step deposition processes is the annealing period at 300°C., which melts the source material completely with no large solid particles remaining during the deposition process. We believe the rough surface we observed from the one step deposition process is caused by the presence large solid particles. The powder-free and completely pyrolyzed Teflon AF (1600) gives better and smoother films. The vapor-deposited Teflon AF (1600) thin films possess a reasonably high breakdown field strength (6 χ 10 V/cm). However, this value is lower than that of spin-coated thin films (1.5 χ 10 V/cm). We believe that the lower breakdown field strengths of vapor deposited thin films may be primarily attributed to the lack of complete polymerization. In the vapor deposition process, Teflon AF (1600) is first pyrolyzed into a vapor, and subsequently repolymerized on the substrate (22). As is well-known, the repolymerization process strongly depends on the deposition conditions and the resulting polymer usually has a shorter chain length than the bulk material. The fact that the vapor deposited Teflon AF (1600) thin films show a lower mechanical strength than spin-coated thin films supports this argument. Although Teflon and Teflon AF (1600) possess an outstanding combination of physical, electrical, and chemical properties, the thermal stability of these polymers is still a major concern for microelectronic application. The reported thermal stability of Teflon AF (1600) is only 360 °C in air. 5

6

IV. Poly(naphthalene) and PoIy(fluorinated naphthalene) Fundamental considerations of the parameters which must be dealt with in designing a thermally stable, low dielectric constant polymer naturally lead one to consider, aromatic rigid rod polymers containing no "lossy" functional groups. A structure such as poly(naphthalene) (Scheme 4) is a likely candidate. The formation of O C bonds between aromatics rings is an important step in many organic syntheses and may be accomplished by chemical, photochemical, or electrochemical means. Poly(naphthalene) is chemically similar to poly(p-

In Microelectronics Technology; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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MICROELECTRONICS TECHNOLOGY

phenylene), which is insoluble, infusible, resistant to oxidation and radiation degradation. The successful synthesis of high molecular weight samples of these rigid-rod macromolecules has been an ongoing problem. Via conventional synthetic methods the poor solubility of these aromatic polymers result- ed in low molecular weight and poor processability, attributes which excluded them from thin film applications. Bergman's study of the thermal cycloaromatization of enediynes led to the suggestion of a benzene 1,4-radical intermediate (23,24). Tour and coworkers theorized that 1,4-naphthalene diradicals generated in solution might couple to eventually form a polymer (25). Even though large excesses of radical terminators were employed in their work, polymerization was still the predominant process . The solubility and processability problems for these types of rigid-rod polymers should be surmountable using chemical vapor deposition from appropriate precursors which can be activated to form intermediates which polymerize readily. The requisite monomers, 1,2-diethynylbenzene or 1,2-diethynyltetrafluorobenzene, are thermodynamically labile, espe- cially when the polymerization is carried out in a sealed tube. They are, however, easily and safely deposited in the vapor deposition apparatus. The polymerization process is outlined in Scheme 4. The vapor deposition polymerization was followed by following the pressure changes accompanying the beginning of the reaction. The vapors of the monomers caused an increase in pressure which returned to the initial base vacuum of the system when the reaction reached completion. Scheme 4

R.-^Y

C5CH

^>^C=CH

350 °C 0.25-0.45 Τοπ-

Ι ,2-diethynyltetrafluorobenzene 1,2-diethynylbenzene η

R = H poly(naphthalene) R = F poly(fluorinated naphthalene)

The general syntheses of these monomers are described in a current publication by Professor Tour (24). Polynaphthalene (PNT-N) and poly (fluorinated naphthalene) (PNT-F) were synthesized by vaporizing 1,2diethynylbenzene or 1,2-diethynyl tetrafluorobenzene in vacuo and condensing the vapor on a hot surface at 350 °C. A schematic of the apparatus used for the



30.

MOORE ET AL.

Vapor-Depositable Polymers with Low Dielectric Constants

preparation of PNT-N and PNT-F in this work is shown in Figure 6. The unit consists, basically, of four sections: a source (1,2-diethynylbenzene) vessel with a needle valve, a vapor introduc- tion channel, a high temperature area with substrate holder, a recycling trap and a pumping system. No catalyst or solvent was involved in this process and unreacted monomer could be recovered from the cold trap. The best film was obtained when oxygen wasrigorouslyexcluded from the system. Film which was deposited in an oxygen rich ambient exhibited lower thermal stability and poor adhesion. These films have low dielectric constants of 2.2 to 2.5. In comparison to PA-F films made from dimer, PNT-N and PNT-F films have higher dissociation temperatures (>570 °C), better thermal stability (>530 °C), and no film cracking until annealed at 600 °C in nitrogen. The presence of an inert vaporous diluent in the pyrolysis process is not preferred because the films deposited with argon as a carrier gas do not adhere well to silicon surfaces. This puzzling result is different from the vapor deposition of PA-F film, in which inert, vaporous diluents such as nitrogen, argon, carbon dioxide, steam and the like can be employed to change the total effective pressure in the system (26). The films, generated by vapor deposition on hot surfaces such as glass or silicon, are not soluble in common laboratory solvents. A difference in reactivity between 1,2-diethynylbenzene and 1,2-diethynyltetrafluorobenzene was observed in the polymerization process. These polymers (PNT-N and PNT-F) have been synthesized by both chemical vapor deposition and solution polymerization and the reaction rate for PNT-F is faster than PNT-N. This observation might be an important factor when high deposition rates are required.

V. Poly(fluorinated benzocyclobutene) The Parylene family has very attractive properties for dielectric use, as was mentioned above. When other types of monomers as possible precursors to be used in this approach were considered, the isomeric o-xylylene structure (Scheme 5) came to mind as a candidate. This pathway involves the thermolysis of benzocyclobutene derivatives to generate a reactive o-dieneoid intermediate which could then undergo a Diels-Alder reaction of two quinodimethane molecules, one as a diene and one as a dienophile, to yield an intermediate spirodimer. The spirodimer would then fragment to give a benzylic diradical species which may undergo intramolecular coupling to give dibenzocycloocta-l,5-diene or oligomerize to provide poly (o-xylylene). If two of these benzocyclobutene units were joined together they should undergo polymerization upon heating to yield an insoluble, crosslinked system. In solution, benzocyclobutenes undergo thermally induced ring-opening to produce o-qui nodi methane (o-xylylene) intermediates which then undergo cycloaddition or dimerization reactions (27). These transformations have been used in solution or melt polymerization reactions where the monomers contain one or more benzocyclobutene groups per molecule (28). Tan and co-worker have reported a thermosetting system based on the Diels-Alder cycloaddition between the terminal benzocyclobutene units and alkyne groups (29).



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30.

MOORE ET AL.

Vapor-Depositable Polymers with Low Dielectric Constan

Polymers prepared from some multifunctional benzocyclobutene monomers exhibit a combination of low dielectric constant and low dissipation factor, slight moisture sensitivity, and good thermal stability, leading to the use of these materials in microelectronic applications (30,31). Under appropriate thermal conditions, the strained four-membered ring of benzocyclobutene undergoes electrocyclicring-opening.The temperature at which such a concerted process occurs depends primarily on the substituents of the alicyclic, rather than the aromatic portion of the molecule. Scheme 5

Benzocyclobutene

o-Quinodimethane

Spirodimer

η Benzylic diradical

Poly(o-xylylene)

To date, the systems reported have used R groups (Scheme 6) which are oligomeric and, therefore, have little or no volatility. If, however, R were to be made small enough that the mass of the monomer was close to that of paracyclophane, it should be possible to vapor deposit polymers derived from such monomers. Scheme 6

Using a fluorinated benzocyclobutene monomer should provide at least one advantage over the already promising properties offluorinatedpoly(p-xylylene). All the good properties such as low dielectric constant and low affinity for water should remain but the thermal stability should be enhanced because of the


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crosslinking which would accompany the generation of these films. It is also possible that the coefficient of thermal expansion will also be reduced. We were gratified to find that poly(fluorinated benzocyclobutene) was able to be synthesized by polymerization of activated species that are generated by UV irradiation of 7,7,8,8'-octafluoro-4,4'-bis (1,2-dihydrobenzocyclobutene) in the vapor state or in solution (Scheme 7). The polymer is obtained as a film and is not soluble in common laboratory solvents. The synthesis of the monomer involves a multi-step synthetic sequence which proceeds in high yield and is described in a current published paper (32). XPS analysis indicates that the film contains fluorine and we are currently trying to prepare enough of this material to characterize its structure by solid state nuclear magnetic resonance.


t

Scheme 7

bis-Benzocyclobutene-Fg

Poly(fluorinated benzocyclobutene) ,

Our original goal was to try to activate 7,7',8,8'-octafluoro-4,4 -bis (1,2dihydrobenzocyclobutene) thermally to undergoring-openingof the cyclobutene ring and subsequently polymerize it to form a crosslinked film on the substrate. The cure chemistry of these systems is primarily based upon the fact that under appropriate thermal conditions, the strained four-membered ring of benzocyclobutene undergoes an electrocyclic ring opening. The temperature at which such a concerted process occurs depends principally on the substituents at the alicyclic, rather than the aromatic position. It is predicted that an electrondonating substituent at C and/or C will favor ring-opening, but electron withdrawing groups at those positions will make thering-openingenergetically more demanding (28a). 7,7 ,8,8 -Octafluoro-4,4'-bis( 1,2-dihydrobenzocyclobutene) has four fluorine atoms on each ring and we expected the reaction temperature to be higher than for an unfluorinated system but that we would gain enhanced thermal stability and low dielectric constant after it polymerized. However, the electronic effect is so overwhelming that this compound did not polymerize or undergo DielsAlder reactions with added dienes or dienophiles at the temperatures or conditions which were useful for benzocyclobutene. However 7,7',8,8'-octafluoro-4,4'-bis( 1,2dihydrobenzocyclobutene) did polymerize when it was irradiated in solution or the vapor state with a UV source. The film collected from the UV polymerization is not soluble in common laboratory solvents. The film is very thin and we were not able to grow thicker films. This result might be caused by absorption of the incident radiation by the film formed on the wall of the quartz reactor, thereby blocking the incoming U V light and preventing the activation of monomer and continuous 7

,

8

,


30. MOORE ET AL.

Vapor-Depostiable Polymers with Low Dielectric Constants

polymerization. A different reactor geometry and the use of intense laser sources may overcome this difficulty.


Conclusions: The main objective of our research is to find new precursors and new techniques to vapor deposit thin films of organic polymer with very low dielectric constants for microelectronic interconnection applications. During the dielectric constant measurements, we found that the measured values were very dependent on the quality of the deposited films. Boron doped silicon wafers (Si(100)) coated with poly (naphthalene) or poly(fluorinated naphthalene) to a thickness of 0.5 m were covered with a contact mask containing numerous circular holes 0.8 mm and 1.6 mm in diameter. The wafers were placed in a Airco Temescal CV-8 vacuum chamber and aluminum was deposited by electron-beam evaporation at 10~6 Torr. In this manner, circular capacitors with two different diameters were fabricated. Capacitance measurements were carried out with the aid of a Hewlett-Packard, model 4280 A 1MHz C meter/C-V plotter. Contact was made to the back side of the wafer and via a surface probe to one of the aluminum contacts. The capacitance was measured using a 1-MHz ac signal. The capacitance of 30 individual capacitors were measured and the results averaged. From this capacitance data, the dielectric constants were calculated and averaged using the equation: ε = dC/Αεο in which ε is the dielectric constant, d is the thickness of the film, C is the capacitance, A is the area and ε is the permittivity in a vacuum. Although the materials we have studied have very low dielectric constant, some of them still fall short of certain requirements, such as thermal stability. Parylenes (PA-N or PA-F) made from different dimers, depending on the type, have dielectric constants ranging from 2.38 to 3.15 and poly(naphthalenes) have dielectric constants lower than 2.5. Teflon AF has a value of 1.9. But VLSI interconnection and packaging applications also require high thermal stability of the films being used. A further consideration is that the diffusion in, and adhesion of metal t* polymer films depends strongly on the thermal stability of the polymer film. For instance, Cu diffusion into PA-N starts at a temperature of 300-350 °C which corresponds, roughly, to the onset of thermal degradation. Adhesion failure between Cu and PA-N also starts at 300 °C. The thickness of PA-N film begins to shrink at 350 °C while annealing in nitrogen(33). The thermostability of these thin films were measured by annealing the film in a nitrogen flow in a tube furnace. The film thickness changed as a function of annealing temperature. The furnace was first stabilized at the annealing temperature and then samples were directly introduced into the hot zone and held at each temperature for 30 minutes. After annealing, samples were removed and cooled to room temperature in air. The decomposition temperature was taken as the temperature where the normalized thickness was reduced by more than 5%. Film thickness was measured with a profilometer (Alpha-step 200) made by the Tencor instrument company.


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In Table I an overview of the properties of those organic thin films which are being studied by chemical vapor deposition in our laboratory is presented. We can see that exciting possibilities for new dielectrics have been uncovered. These polymers, PA-N, PA-F, PNT-N, PNT-F and poly(bis-BCB-Fg), would have been impossible to synthesize or process into thin films by conventional methods because of their limited solubilities. Table I. An Overview of the Properties of Organic Thin Films Film Film Deposition Source Material Toxicity Dielectric Constant Electric Breakdown Dissocia tion

T(°o Thickness vs. Annealing Temp. Structure As Deposited Cracks Film Thickness Adhesion to Si Adhesion toAl

PA-N vapor

PA-F vapor

dimer (solid) yes 2.6010.1

precursor (liquid) hazard 2.2 - 2.3

Teflon AF vapor

PNT-N vapor

PNT-F vapor

precursor (solid) NA 2.3 ±0.1 (best) 2xl0 V/M 5 xl0 V/M 5 x l 0 V / M 3 x l 0 V / M 5 x l 0 V / M (best) 6 430 °C in 530 °C in Ϊ60 Ù in 570 °C in NA air N N N 7

2

polymer (solid) hazard 1.93

precursor (liquid) NA 2.4 ±0.1

7

7

2

7

7

2

no change no change to350°C NA to500°C in N2 in N2

no change NA to530°C in N 2

crystalline

crystalline/ amorphous amorphous

crystalline

crystalline

yes 0.1-10 μΜ

no 0.1-1 μΜ

yes 0.1-10 μΜ

no 0.5-4 μΜ

no 0.5-2 μΜ

poor

good

poor

good

NA

OK

NA

poor

poor

NA

Additionally, this synthetic approach provides a new route for the synthesis of thin films without the use of toxic solvents. However, there remains the challenge of choosing or synthesizing potential source materials for this methodology. The right precursors should have certain characteristics, such as compatible chemistry, ease of evaporation or sublimation in the vacuum system, no volatile fragments released in the process, and, certainly, the final polymeric film should have the desired thermal stability and dielectric properties. Future work will will continue to expand on this approach including the development of new precursors as well as studying the crystallinity of these materials and its influence on the morphology and utility of thesefilmsas dielctric materials.


30. MOORE ET AL.

Vapor-Depositable Polymers with Low Dielectric Constants

Acknowledgement: We thank Prof. Tour for providing details of his synthetic approach prior to publication. Financial support of this effort has been provided, in part, by the IBM Corporation and is gratefully acknowledged.


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24. Lockhart, T. P.; Cornita, P. B.; Bergman, R. G.J.Am. Chem. Soc. 1981, 103, 4082. 25. John, J. Α.; Tour, J. M.J.Am. Chem. Soc. 1994, 116, 5011. 26. Chow, S.-W. (to Union Carbide Corp), U.S. Pat. 3,268,599 (1966) 27. (a) Boekelheide, V.; Ewing, G. Tetrahedron Lett 1978, 44, 4245. (b) Schiess, P.; Heitzmann, M.; Rutschmann, S.; Staheli, R. Tetrahedron Lett. 1978, 46, 4569. (c) Oppolzer, W. Synthesis 1978, 973. 28. (a) Kirchhoff, R. Α.; Carriere, C.J.;Bruza, Κ.J.;Rondan, N. G.; Sammler, R. L.J.Macromol. Sci.-Chem. 1991, A 28(11&12), 1079. (b) Hahn, S. F.; Martin, S. J.; Mckelvy, M. L. Macromolecules 1992, 25, 1539. (c) Kirchhoff, R. Α.; Bruza, K. J. CHEMTECH 1993, 23, 22. 29. (a) Tan T.S.; Arnold F. E. J. Polym. Sci., Polym. Chem. Ed. 1988, 26, 1819. (b) Walker, Κ. Α.; Markoski, L.J.;Moore, J. S. Macromolecules 1993, 26, 3713. 30. Burdeaux, D.; Townsend, P.; Carr,J.J.;Garrou, P. E. J. Electron. Mat. 1990, 19, 1357. 31. Chinoy, P. B.; Tajadod,J.IEEE Trans. CHMT 1993, 16, 714. 32. Moore, J. Α.; Lang, C.-I.; Lu, T.-M.; Young, G.-R. Polym. Mat. Sci. Eng. 1995, 72, 437. 33. Yang, G.-R.; Dabral, S.; You, L.; McDonald,J.F.; Lu T.-M. J. ofElectronic Materials, 1991, 20, 571 RECEIVED September 1, 1995


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