bk-1994-0537.ch028

Chapter 28

Base-Catalyzed Photosensitive Polyimide D. R. McKean, G. M. Wallraff, W. Volksen, N. P. Hacker, M. I. Sanchez, and J. W. Labadie

Downloaded by FUDAN UNIV on December 16, 2016 | http://pubs.acs.org Publication Date: November 23, 1993 | doi: 10.1021/bk-1994-0537.ch028

IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099

A new scheme for photosensitive polyimide is described. This imaging scheme is based on the amine-catalyzed conversion of soluble polyamic esters to relatively insoluble, partially imidized polymers. The amine is generated photochemically from o-nitrobenzyl carbamate precursors which were made photochemically active at long wavelength either by using carbamate derivatives containing electron donating substituents or by using triplet photosensitizers. Imidization percentages ranging from 20-80% were obtained following irradiation and postexposure bake without any imidization in unexposedfilm.The imidization percentage of exposed and baked film was dependent on a number of factors including the polymer structure, the nature of the ester group, and the conditions of the postexposure bake. The differential solubility between polyamic esters and partially imidized polymer was greatest for polymers with relativelyrigidpolymer backbones and this resulted in significantly improved imaging properties (lithographic contrast of 3.0). A final cure of the patterned film results in fully imidized polymer.

Polyimides are widely used as dielectric materials because of a combination of desirable properties including good thermomechanical properties, the solution processability of precursor polymers (polyamic acids or esters), good planarizing properties, low dissipation factor, and a reasonably low dielectric constant \e = 2.7-3.5) (i). The fabrication of electronic devices containing metal lines and vias embedded in polyimide is made considerably easier when the polyimide films can be made photosensitive and thus directly imagable. Multilayer processing of nonphotosensitive polyimide films by either dry (2) or wet (3) etch pattern transfer has been demonstrated but these imaging schemes increase the number of process steps and thus decrease manufacturing throughput compared with photosensitive polyimide (PSPI) (4). Patterning of nonphotosensitive polyimide films can also be done by laser ablation (5). Most of the commercially available PSPI systems use either precursor polymers with pendant methacrylate functionality (Siemens technology) (6) or soluble, preimidized polymers such as Ciba-Geigy Probimide 412 (7). Both of these approaches to PSPI function lithographically in a negative mode by producing crosslinked polymers which are insoluble in a developer solvent. The photolithographic contrast is low for both systems and is typically about 1.0. Low 0097-6156/94/0537-0417$06.00/0 © 1994 American Chemical Society Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


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contrast limits the resolution that can be achieved during the imaging process which imposes a physical limitation on feature size reduction. Siemens-type PSPI has a high percentage of shrinkage on final cure (> 50%) requiring imaging of films with thicknesses greater than twice the desired final thickness. This places additional demands on the exposure step for high aspect ratio imaging and requires a larger depth of focus. Patterned image distortion frequently results from the final cure process due to the extensive shrinkage as well as higher stress. This paper describes the development of a new approach to PSPI which is based chemically on the propensity for poly(amic alkyl esters) to imidize rapidly in the presence of catalytic quantities of amines (8). By incorporating amine photogenerators along with poly(amic alkyl esters), it has been possible to develop schemes for imaging polygamic alkyl esters). A key aspect of this scheme is the effective photogeneration of amines, for which we utilized compounds of the type recently reported by Frechet and coworkers (9). These compounds include 2-nitrobenzyl carbamates and 3,5-dimethoxy-a,a-dimethylbenzyl carbamates which have been applied to the imaging of other polymer films including epoxy resins (10). Polymer films containing polyamic ester and amine photogenerator are irradiated to produce an amine, which subsequently catalyzes partial imidization in the exposed portion of the film (Figure 1). The unexposed film containing only polyamic ester and amine photogenerator is more soluble in developer solvents than the partially imidized film and can be selectively removed. Subsequent to development, the negatively imaged polymer film is cured to polyimide at high temperature. Because of the catalytic nature of the imidization step and the greatly decreased solubility of partially imidized polymer, base-catalyzed photosensitive polyimide is a chemically amplified imaging process (11). Chemically amplified imaging schemes are generally high sensitivity, high contrast processes which are advantageous for high resolution imaging. Another potential advantage of this scheme is that the imaging and curing chemistry are identical. EXPERIMENTAL Materials l-(2-Nitrophenyl)ethyl N-cyclohexylcarbamate (1) and l-(2-Nitro-4,5-dimethoxyphenyDethyl N-cyclohexylcarbamate (2) were prepared by reaction of the corresponding substituted benzyl alcohols with sodium hydride and cyclohexylisocyanate. l-(4,5-Dimethoxy-2-nitrophenyl)ethanol was prepared in two steps by nitration of 3,4-dimethoxyacetophenone followed by reduction with sodium borohydride. Polyamic esters 3-5 were prepared by the standard literature procedure (12). Diglyme and N-methylpyrrolidone (NMP) were purchased from Aldrich and used as received. Coumarin 6 was purchased from Kodak. Measurements 13

*H and C N M R analyses were performed on an IBM Instruments AF250 spectrometer. Infrared spectra were done on an IBM FTIR Model 44. Ultravio-

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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let absorption spectra were recorded on a Hewlett-Packard Model 8450A UV/Visible Spectrometer. Exposure doses were measured with an Optical Associates Exposure Monitor Model 355. Film thickness measurements were done with a Tencor Alpha Step 200.


Lithographic Evaluations Solutions were prepared by dissolving 1.0 g of polyamic ester and 0.05-0.2 g of amine photogenerator in 6.5 mL of NMP. Films were prepared on silicon substrates by spin-coating filtered solutions followed by baking to remove excess solvent. Exposures were performed through a mask using broadband irradiation. The films were then heated on a hot plate at temperatures ranging from 80 to 150 °C for 10 min. Development was done by immersion in solvent mixtures containing NMP diluted with an appropriate polymer nonsolvent such as diglyme. RESULTS AND DISCUSSION Short Wavelength Imaging The initial imaging studies were conducted on poly(amic alkyl ester) 4 containing amine photogenerator 1 (Figure 2). The use of a poly(amic alkyl ester) with a glycolate ester group facilitated the base-catalyzed imidization reaction at lower temperatures. Carbamate 1 absorbs most strongly at deep-uv (230-290 nm) wavelengths and so the imaging experiments were conducted using filtered light in this spectral region. However, because of the high absorbance of poly(amic alkyl esters) at deep-uv wavelengths (Figure 3), deep-uv base-catalyzed PSPI was limited to imaging of films with a thickness less than 0.3 /tm. Following exposure to broad band deep-uv irradiation, the films were baked at 100 °C for 10 minutes and gave negative images upon development with NMP. This experiment established the feasibility of the base-catalyzed PSPI scheme. Long Wavelength Imaging For implementation of PSPI in electronic device fabrication, thick film (> 2/im) imaging is required. For the base-catalyzed PSPI scheme, the photogeneration of amines at long wavelength (y > 350 nm) is necessary for thick film imaging. Several methods for long wavelength amine photogeneration were explored including triplet photosensitizers and red-shifted amine photogenerators, as well as the use of more transparent polymer backbones. Polymer Backbones with Higher Transparency Polymer backbones with spacer groups between aromatic rings improve the transparency of these materials in the 350-400 nm range. The poly (amic alkyl ester) derived from oxydiphthallic anhydride (ODPA) and O D A (3) which contains oxygen spacer groups between aromatic rings was used for thick film imaging along with 20% of amine photogenerator 1. Polymer 3 is nearly completely transparent at wavelengths longer than 360 nm and thus imaging can


420



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"

CH CH " S

2

~If

H

o

Figure 2. Structures of Amine Photogenerators and Poly(amic alkyl esters) Used for Imaging Studies.



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be carried out using 365 nm light most likely due to the presence of a weak carbamate 1 absorption band which extends out to 365 nm. The imaging of films containing 1 and polymer 3 was best done with broadband irradiation. The use of the activated ester functionality (trifluoroethyl) allowed the use of a lower postexposure bake temperature - 120 °C. The images were developed using a 1:1 solution of cyclohexanone and ethanol. Films with up to three microns of film thickness could be imaged using polyamic ester 3; however, the solvent resistance of the exposed and postbaked films was limited. Extensive exposed film thinning (50%) was observed during development as well as swelling and solvent-induced stress cracking. The photoresist contrast (Figure 4), which was measured from the slope of a semilog plot of film thickness versus exposure time, was only 1.1 for this experiment. The extensive film thinning and low contrast is likely due to the use of the more flexible polymer backbone which diminishes the solubility differential between polyamic esters and partially imidized polyamic esters. Thus, while the introduction of oxygen spacer groups produces a polymer with greater transparency which facilitates long wavelength imaging, the solvent resistance of the partially imidized polymer is also decreased as a result of this structural change. Triplet Sensitization of Amine Photogeneration Thick films of more rigid polyamic esters were also imaged by using combinations of amine photogenerator 1 and a long wavelength absorbing triplet photosensitizers. The coumarin 6 (73), which has an absorption band extending out to 400 nm and a triplet energy of 57 Kcal/mol, was used as the triplet sensitizer.

—O

6 Films containing 13% carbamate 1, 15% coumarin 6, and 72% polyamic ester 4 were imaged using filtered 404 nm light. Using the same process conditions as the deep-uv imaging experiment, negative images with greater than one micron thickness could be obtained. However, the imaging experiment required large doses (2.5 J/cm ) at 404 nm which is probably due to the inefficiency of triplet sensitization. 2

Long Wavelength Sensitive Amine Photogenerators The imaging of thicker films of poly(amic esters) could also be carried out using a long wavelength absorbing amine photogenerator. The o-nitrobenzyl carbamate derivatives when substituted with electron donating substituents have



422


Wavelength (nm)

Figure 3. Ultraviolet Absorption Spectra for 3.5 micrometer thick film of P M D A / O D A based poly(amic ethyl ester).

Exposure Time (sec)

Figure 4. Contrast curve for imaging of poly(amic ester) 3 containing amine photogenerator 1.



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strong absorption bands at longer wavelength. The dimethoxy carbamate 2, prepared in three steps from commercially available 3,4-dimethoxyacetophenone, absorbs out to 400 nm and could be used for imaging experiments on highly absorbing poly(amic ester) backbones. Polymer films were prepared from an NMP solution containing the poly(amic ethyl glycolate ester) 4 derived from P M D A / O D A along with 20% of carbamate 2. The films were exposed to broadband irradiation and then baked at 120 °C to affect partial imidization. Following development with 9:1 diglyme/NMP, negative images were obtained for film thicknesses up to 6 ^m; however, 20% film thinning occurred in the exposed regions of the film and a significant amount of swelling and cracking was observed. The percent imidization for these films was studied by integration of the imide carbonyl infrared absorption band, which indicated that at 100 °C approximately 80% imidization occurred in the exposed portion of the film, while no imidization took place in the unexposed film (Figure 5). Despite the large percentage of imidization, the solvent resistance of the partially imidized meta substituted poly (amic ester) 4 was not sufficient to prevent swelling and cracking during the development step. Better imaging results for base-catalyzed PSPI were obtained by using the para substituted P M D A / O D A derived poly (amic ethyl ester) 5 which has both a more rigid backbone structure as well as a lower molecular weight ester group. The use of a smaller ester group was beneficial in producing less shrinkage after final cure. Imaging experiments were conducted on films prepared from solutions of 5% 2 and 95% polyamic ester 5. With this low amine photogenerator percentage the dose requirement was about 500 mJ/cm for imaging of 4 fim films. The use of the unactivated ethyl ester required a higher temperature for the post exposure thermolysis step (150 °C). To define the optimum postexposure bake for poly(amic ester) 5, films were heated to various temperatures and the imidization was monitored by infrared spectroscopy. No appreciable imidization occurred until a temperature of 170 °C was reached. Based on this analysis, a postexposure bake temperature of 150 °C was used for the imaging experiments. When 1% of cyclohexylamine was added to poly(amic ester) 5 and the resulting film heated to 150 °C for 10 minutes, infrared analysis indicated that 25% imidization had occurred which was in agreement with the imidization percentage observed when PSPI films were exposed and postbaked. Despite the low imidization percentage, the development of exposed and thermolyzed film with a solvent mixture containing 10% NMP and 90% diglyme produced negative images with minimal film thinning (10%) and no observable swelling or cracking (Figure 6). As a further consequence of high solubility differentiation, the photoresist contrast exceeded 3.0, which is an exceptionally high value for photosensitive polyimide and provides a method for overcoming many of the problems which are encountered during the lithographic exposure step (Figure 7). The higher contrast as well as better solvent resistance of the partially imidized structures is probably due to the more rigid nature of the para-substituted poly(amic alkyl ester) backbone. Furthermore, because the smaller ethyl ester group was used along with a low percentage of photoamine generator (5%), the shrinkage after final cure (37%) was less than shrinkage from Siemens-type PSPI (> 50%). 2


424


1.0


Exposed Film Unexposed Film

0.0

•A, 0

10

20

30

40

Time (minutes)

Figure 5. Relative imidization of exposed and unexposed films containing amine photogenerator 2 in poly(amic alkyl ester) 4.

Figure 6. Optical micrograph of images obtained from base-catalyzed PSPI using amine photogenerator 2 in poly(amic ethyl ester) 5. Smallest resolvable features are 5.0 micron lines and 5.0 micron spaces which are shown in the 5th row of features on the left.



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Exposure Time (sec)

Figure 7. Contrast curve for imaging of films containing amine photogenerator 2 in poly(amic ethyl ester) 5. Lithographic Performance vs. Structure of Amine Photogenerators and Polyamic Esters During the course of these studies, several important structural features were found to be important for the base-catalyzed imaging of poly(amic alkyl esters). Both amine photogenerator and poly(amic alkyl ester) structure are significant for optimal lithographic performance. Polymer structural features which are significant for imaging include the reactive ester group, polymer backbone flexibility and isomeric constitution. Three classes of amine photogenerator were examined for this application: cobalt-ammine complexes (14), benzyl carbamates (9, 15), and o-nitrobenzyl carbamates (9, 16). The most effective class of amine photogenerator for this application has been the o-nitrobenzyl carbamates 1-2. Both the ester group and the polymer backbone structure of the poly(amic alkyl esters) (3-5) were important structural features for optimal performance of the base-catalyzed PSPI imaging. Studies of the imidization rate of poly(amic alkyl esters) (8) revealed that certain ester groups showed a higher propensity for amine-catalyzed imidization. This class of activated esters includes esters derived from ethyl glycolate and 2,2,2-trifluoroethanol and are known to undergo more facile thermal imidization relative to conventional alkyl esters (12b, 17). Imaging of poly(amic esters) with activated ester groups for base-catalyzed photosensitive polyimide allowed the use of a lower postexposure bake temperature to achieve partial imidization of films. However, due to relatively rapid imidization rates, the shelf life of solutions prepared from poly(amic esters) with activated ester groups was limited.


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The polymer backbone structure was also important for the imaging of poly(amic esters). Poly(amic alkyl esters) derived from pyromellitic dianhydride (PMDA) and oxydianiline (ODA) are only transparent at wavelengths greater than 400 nm but the transparency at shorter wavelength can be improved by use of alkyl or heteroatomic spacer groups in the polymer backbone. The polymer backbone structure also affects polymer solubility and film development. Poly (amic alkyl esters) with more rigid backbones become insoluble at a lower percentage of imidization. Thus, although the more flexible polymer backbone may afford greater amine photogeneration, and hence, imidization, better images were often obtained with less transparent rigid backbones due to the effect on solubility and swelling properties of the partially imidized film. CONCLUSIONS A new scheme for photosensitive polyimide has been developed which involves the use of amine photogenerators along with poly(amic alkyl esters). Amines are generated on exposure of the films which are used to catalyze the partial imidization of the polymer. Negative images result from the lowered solubility of the partially imidized polymer. There are several advantages of base-catalyzed imaging of poly(amic esters) over other PSPI schemes. The ability to image simple ethyl or methyl esters of poly(amic esters) is an advantage in terms of cost of the polymer and also gives a lower percentage of shrinkage relative to Siemens-PSPI. The imaging and cure chemistry for base-catalyzed PSPI are the same and the process is a chemically amplified scheme which generally give higher contrast. The contrast for imaging of para-PMDA/ODA poly(amic ester) was greater than three. ACKNOWLEDGEMENTS The authors would like to thank Professor Jean Frechet and Dr. James Cameron of Cornell University for providing samples of the amine photogenerators and valuable discussions regarding their use and Phil Brock of IBM for preparation of amine photogenerators. The authors also thank Dr. Grant Willson of IBM for his strong encouragement and valuable technical discussion regarding this work. REFERENCES 1. Endo, A.; Yoda T. J. Electrochem. Soc. 1985, 132, 155; Jensen, R. J.; Vora H . IEEE Trans. Components, Hybrids, Manf. Technol. 1984, CHMT-7, 384; Wilson A. M . Thin Solid Films 1981, 83, 145; Rothman, L. B. J. Electrochem. Soc. 1980, 127, 2216. 2. Paraszczak, J.; Cataldo, J.; Galligan, E.; Graham, W.; McGouey, R.; Nunes, S.; Serino, R.; Shih, D. Y.; Babich, E.; Deutsch, A.; Kopcsay, G.; Goldblatt, R.; Hofer, D.; Labadie, J.; Hedrick, J.; Narayan, C.; Saenger, K.; Shaw, J.; Ranieri, V.; Ritsko, J.; Rothman, L.; Volksen, W.; Wilczynski, J.; Witman, D.; Yeh, H . Proc. Electronic Components and Technology Conference 1991,



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362. Lichtenberger, A. W.; Lea, D. M.; Li, C.; Lloyd, F. L.; Feldman, M . J.; Mattauch, R. J.; Pan, S. K.; Kerr, A . R. IEEE Trans. Magn. 1991, 27, 3168. 3. Harada, Y.; Matsumoto, F.; Nakakado, T. J. Electrochem. Soc. 1983, 130, 129. 4. Rickerl, P. G.; Stephanie, J. G.; Slota P. IEEE Trans. Components, Hybrids, Manf. Technol. 1987, CHMT-12, 690; Endo, A.; Takada, M.; Adachi, K.; Takasago, H.; Yada, T.; Onishi Y. J. Electrochem. Soc. 1987, 134, 2522. 5. Srinivasan, R. J. Appl. Phys. 1992, 72, 1651. 6. Ahne, H.; Domke, W. D.; Rubner, R.; Schreyer M . In Polymers for High Technology Electronics and Photonics; Bowden, M . J.; Turner, S. R., Eds.; ACS Symposium Series 346; American Chemical Society: Washington DC, 1987, p 457. 7. Pfeifer, J.; Rohde O. Proc. 2nd Int. Conf. on Polyimides 1985, 130. 8. Volksen, W.; Pascal, T.; Labadie, J.; Sanchez, M . Proc. ACS Polym. Mat. Sci. Engr. 1992, 235. 9. Cameron, J. F.; Fréchet J. M . J. J. Org. Chem. 1990, 55, 5919; Cameron, J. F.; Fréchet J. M . J. J. Photochem. Photobiol. A : Chem. 1991, 59, 105; Cameron, J. F.; Fréchet J. M . J. J. Am. Chem. Soc. 1991, 113, 4303; Beecher, J. E.; Cameron, J. F.; Fréchet J. M . J. Proc. ACS Polym. Mat. Sci. Engr. 1991, 71. 10. Fréchet J. M . J. Pure Appl. Chem. 1992, in print; Matuszczar, S.; Cameron, J. F.; Fréchet, J. M . J.; Willson, C. G. J. Mater. Chem. 1991, 1, 1045; Winkle, M . R.; Graziano, K. A. J. Photopolym. Sci. Technol. 1990, 3, 419. 11. Ito, H.; Willson, C. G. In Polymers in Electronics; Davidson, T., Ed.; American Chemical Society: Washington, DC, 1984; 11. 12. a. Nishizaki, S.; Moriwaki, T. J. Chem. Soc. Japan 1967, 71, 1559; b. Volksen, W.; Yoon, D. Y.; Hedrick, J. L.; Hofer, D. MRS Symposium Proc. 1991, 23. 13. Specht, D. P.; Martic, P. A.; Farid, S. Tetrahedron 1982, 38, 1203. 14. Kutal, C.; Willson C. G. J. Electrochem. Soc. 1987, 134, 2280. 15. Birr, C.; Lochinger, W.; Stahnke, G.; Lang P. Liebigs Ann. Chem. 1972, 763, 162. 16. DeMayo P. Adv. Org. Chem. 1960, 2, 367; Patchornick, A.; Amit, B.; Woodward, R. B. J. Am. Chem. Soc. 1970, 92, 6333. 17. Volksen, W.; Yoon, D. Y.; Hedrick, J. L . IEEE Trans-CHMT 1992, 15, 107. Received December 30, 1992


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