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Chapter 27

Application of Polyisoimide as a Polyimide Precursor to Polymer Adhesives and Photosensitive Polymers Amane Mochizuki and Mitsuru Ueda

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Department of Materials Science and Engineering, Faculty of Engineering, Yamagata University, Yonezawa 992, Japan Recent developments and application of polyisoimide (PII) to high temperature adhesive and photosensitive polymers are described. First, new high temperature adhesives based on PII have been developed. PIIs showed stronger adhesions to copper foils because of favorable anchoring between PII and copper foil due to a good flow of PII. Next, a positive working photosensitive polyimide precursor based on PII and 2,3,4-tris[1-oxo-2-diazonaphthoquinone-4-sulfonyloxy] benzophenone (5) as a photoreactive compound has been developed. The photosensitive polyimide-precursor containing 20 wt% of 5 showed a sensitivity of 250 mJ/cm and a contrast of 2.4 with 435 nm-light when it was postbaked at 150 °C for 10 min followed by developing with 5 % aqueous tetramethylammonium hydroxide solution at 45 °C. Further, a new amine photo-generator{[(4,5-dimethoxy-2-nitrobenzyl)oxy] carbonyl}-2,6-dimethyl piperidine (6) was prepared from 2,6-dimethyl piperidine and 4,5-dimethoxy-2-nitrobenzyl-p-nitrophenylcarbonate. The PII containing 10 wt% of 6 functioned as photosensitive resist having a sensitivity of 900 mJ/cm and a contrast of 3.4 with 365 nm-light when it was postbaked at 150 °C for 5 min followed by development with cyclohexanone at 45 °C. 2

2

High-performance plastics are currently receiving considerable attention for their potential uses in aerospace, automotive, electronic and related industries. In particular, polyimides (Pis) have been widely used as insulation materials for microelectronic devices because of their excellent properties, such as thermal and chemical stability, and low dielectric constants (1). However, aromatic Pis are usually intractable because they are insoluble and high melting temperatures. Therefore, Polyamic acids (PAAs) are prepared first, processed into shaped objects and then converted to Pis by thermal cyclization. On the other hand, polyisoimides (PIIs) are the processable isomeric form of the corresponding Pis and a commercial acetylene-terminated isoimide oligomer (IP-600) is an example of a high temperature thermo-setting resin (2). In a recent paper (3), we reported the preparation and properties of polyisoimide (PII) as a Pi-precursor, and found that PII has a lower glass transition temperature (Tg) than that of the corresponding PI, and is easily converted to PI without elimination of volatile compounds. 0097-6156/95/0614-0413$12.00/0 © 1995 American Chemical Society

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This paper describes the recent developments and application of PIIs to high temperature adhesive and photosensitive polymers. Experimental

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Materials. N-Methyl-2-pyrrolidone (NMP), triethylamine (TEA), and pyridine (Py) were purified by distillation. 4,4'-[Hexafluoroisopropylidenebis(pphenyleneoxy)]dianiline (BAPF) (2a) and 4,4'-[isopropylidenebis(pphenyleneoxy)]dianiline (BAPP) (2b) were purified by recrystallization from cyclohexane and 2-propanol, respectively. 4,4'-Hexafluoroisopropylidenebis(phthalic anhydride) (6FDA) (la) was obtained from American Hoechst Co. Oxydiphthalic anhydride (ODPA) (lb) was obtained from Occidental Chem Corp. Rolled copper foil (35 μτη thickness ) was used as an adherent Other reagents and solvents were obtained commercially and used as received. Polymer Synthesis. A typical example is as follows. Polyisoimide (3a) from l a and 2a. A solution of 2a (2.22 g, 5.0 mmol) in NMP (43.2 ml), was cooled with an ice-water bath. While stirring, l a (2.59 g, 5.0 mmol) was added to this solution. The resulting mixture was stirred at room temperature for 4 h. The viscous solution was diluted with NMP (48.2 ml) and TEA (1.4 ml, 10.0 mmol) was added dropwise with continued stirring. The reaction mixture was then cooled with an ice-water bath, and trifluoroacetic anhydride (1.54 ml, 11.0 mmol) was added dropwise with stirring. The mixture was stirred at room temperature for 4 h and then poured into 2-propanol (1000 ml) to precipitate the polymers. The precipitated polymer wasfilteredoff and dried in vacuo at 40 °C. The yield was 4.54 g (98 %). The inherent viscosity of the polymer in DMAc was 0.40 dl/g at a concentration of 0.5 g/dl at 30 C . IR (KBr), ν (cnr ) 1810, (C=0), 920 (C-O) Analysis: Calculated for C46H22N2O6F12H2O :C.,58.48; H, 2.56; N, 2.96. Found: C., 58.56; H, 2.78; N , 2.90. e

1

Evaluation of Adhesion Strength. A typical procedure is as follows. F i l m Casting. A solution of polymer 4a was made by dissolving the polymer in chloroform to afford a 15 wt% solution. Films cast on glass plates were prebaked at 80 °C for 60 min and dried in vacuo at 150 °C for 12 h. The thickness of polymer film was 25μπι. Hot Press. Test press No. 1541 (Gonno Hydraulic Press Manufacturing Co.) was used for the adhesion between a polymer film and a copper foil. The polymer film was placed between 50 mm χ 50 mm pieces of copper foil. The copper foil was washed with acetone prior to use. The assembly was placed in the hot press and melt processed at 200-260 °C under 14.7 Mpa. After holding 15 min, the platens were cooled under pressure to room temperature. At least five samples were tested and the average values are reported. The peel strength was measured by Tensilon UTM-III-500 (Toyo Baldwin) at a peelrateof 50 mm/min atroomtemperature (0°-peel method) ( 19). Photosensitivity. 3- 5 μηι- thick PIIfilmson a silicone wafer were exposed to 365 nm or 435 nmradiationusing afilteredsuper high pressure mercury lamp. Exposed films were postbaked at 150 °C for 5 min, developed in 5 % aqueous tetramethylammonium hydroxide (TMAH) solution or cyclohexanone at 45 °C., and subsequendy rinsed with 2-propanol. The characteristic sensitivity curve was obtained by plotting a normalized film thickness against logarithmic exposure energy.

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

27. MOCHIZUKI & UEDA

Polyisoimide as Polyimide Precursor

415

Measurement. The infrared spectra were recorded on a Hitachi 1-5020 FT-IR spectrophotometer. Viscosity measurements were carried out by using an Ostwald viscometer at 30 °C. Thermal analyses were performed on a Seiko SSS 5000-TG/DTA 200 instrument at a heating rate of 10 °C/min for TG and a Seiko SSS 5000-DSC220 at a heating rate of 10 °C/min for differential scanning calorimetry (DSC) under nitrogen. Molecular weights were determined by a gel permeation chromatography (GPC) with polystyrene calibration using a JASCO HPLC system equipped with a Shodex KD-80M column at 40 °C in DMF. The film thickness was measured using a Dektak 3030 system (Veeco Instruments Inc.). Dynamic mechanical analysis (DMA) was performed with a DVB-V4 FT Rheospectra (Rheology Co., LTD) in tensile mode at a frequency of 100 Hz and at a heating rate of 2 °C min* with film specimens 5 mm mode and 25 μτα thick. 1

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Result and Discussion Polymer Synthesis. We prepared the polyisoimides (PIIsX3) and polyimides (Pis) (4) by chemical cyclization methods to investigate their properties as melt processable adhesives and photosensitive polyimide precursors. Although many dehydrating agents had been proposed for the preparation of isoimides from amic acids (4), it had not been clear which reagents would be the best for PII synthesis. In a previous investigation (3), we found that trifluoroacetic anhydride (TFAA)-triethylamine (TEA) was the best dehydrating agent for the formation of isoimide. Thus, ring-opening polyadditions of tetracarboxylic dianhydrides and diamines were carried out in NMP for 4h at room temperature equation 1, yielding poly(amic acids), which were subsequently converted to PIIs and Pis by using TFAA-TEA and acetic anhydride-pyridine (Py), respectively. Table I indicates that polymers 3 and 4 were produced in excellent yields with inherent viscosities of up to 0.75 dl/g . Polymers 3 and 4 were characterized by infrared spectroscopy and elemental analysis.

kCF CO) 0-TEA 3

2

I °vY

-LAr -N Ο J η Polyisoimide(3)

Polyamic acid — Η

2

(1)

(CH CO) 0-Py 3

2

Polyimide(4)

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

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Table I. Synthesis of Polyisoimide» (3) and Polyimides (4) " Ar,

Ar

la

polymer

Yield(%)

2a

3a

96

0.40

215

la

2a

4a

99

0.45

250

la

2b

3b

94

0.75

190

la

2b

4b

95

0.71

240

la

2c

3c

96

0.40

203

lb

2c

3d

98

0.36

190

2

e

r>m( 300 °C). On the other hand, a polyisoimide (PII) has a lower glass transition temperature (Tg) than that of corresponding PI and can be converted easily to PI without elimination of volatile compounds. Therefore, PII is of considerable interest as a candidate for high temperature adhesives. Adhesion Properties of Polymer 3 and 4. Films of polymer 3 a and 4a were prepared by casting cyclohexanone solutions onto glass plates which were then heated on a hot plate at 60 °C., and subsequently dried at 150 °C for 12h in vacuo. Dynamic mechanical analysis (DMA) was performed in tensile mode at a frequency of 100 Hz with film specimens 5 mm and 25 μπι thick. Figure 1 shows the dynamic storage modulus of polymer 3a and 4a. The storage modulus for polymer 3a declined sharply around 200 °C., which is agreed with the glass transition temperature of polymer 3a dertermined by DSC. Polymer 4a maintained mechanical integrity at this

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

27. MOCHIZUKI & UEDA

temperature and the storage modulus did not drop until 250 °C. These results indicate that a large difference in the flow behavior between polymers 3a and 4a can be expected above 200 °C The adhesion test was evaluated by the peel strength of copper foil at 180 degrees from the polymer film as shown in Figure 2. The heat compression was conducted employing the process as illustrated in Figure 3. The assembly was placed in the hot press and compressed at 200-260 °C for 15 min under 14.7 Mpa. Subsequently, the platens were cooled to a temperature below 50 °C and the assembly was removed from the heat press tool. The resulting peel strengths of PIIs (3a-b) and Pis (4a-b) films on copper foils are shown in Table II. PII (3a) exhibited peel strengths of780 gem and 400 gem at the press temperatures of 260 °C and 230 °C., respectively. On the other hand, the peel strengths of PI (4a) film on copper foils were very weak even at a press temperature of 260 °C. In addition, PII (3a) film wasflexibleand free of voids after compression at 260 °C., presumably due to the lower Tg. The isomerization to the corresponding PI was confirmed by IR spectroscopy. 1

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Polyisoimide as Polyimide Precursor

-1

Table Π. Peel strength of polymer films on copper fou *

a

Run

Polymer

Press Temp.(°C)

1

3a

260

780

2

3a

230

415

3

3a

200

b

4

4a

260

77

5

4a

230

b

6

4a

200

b

7

3b

250

590

8

3b

200

180

9

4b

250

110

10

4b

200

b

bonded at 14.7 M P a ,

b

Peel Strength (g/cm)

no adhesion

The greater adhesive nature of PII can be elucidated by considering the lower Tg and the drastic changes in elasticity above it's Tg compared to that of the corresponding PL In addition, the advantage of using the PII films as a high temperature adhesive has also been shown with the conversion to die corresponding Pis during the heat compression process without the generation of volatile compounds. Application of PII to the Photosensitive Polyimide (PSPI) System using Diazonaphthoquinone (NQD) (9). Positive resists based on novolak resins with o-diazonaphthoquinone (NQD) are standard materials used in semiconductor manufacturing, where NQD acts as a dissolution inhibitor for aqueous base development of the novolac resin. Several groups (10-11) have reported the resists consisting of polyamic acids and NQD. However, dissolution rates of polyamic acids

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

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100

200

300

400

Temperature CC) Figure 1. Dynamic mechanical analysis of polymer films: ( • ) polymer 4a; (O) Polymer 5a.

Mat surface -

" Copper Foil Polymer Film - Copper Foil

Figure 2. Measurement of peel strength.

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

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Polyisoimide as Polyimide Precursor

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are essentially too high to get a sufficient dissolution contrast, and as a result the dissolution rates of polyamic acids have to be reduced by prebaking, or post exposure bake(PEB). Herein we describe a new photosensitive polyimide (PSPI) system consisting of PII and NQD as the polymer matrix and a photosensitive compound, respectively. We have chosen PII (3c) and 23,4-tris[l-oxo-2-diazonaphthoquinone-4-sulfonyloxy] benzophenone (5) as the candidate polymer matrix and photoreactive compound, respectively. In order to investigate dissolution behavior of the exposed (500 mJ/cm ) and unexposed area, the effect of the loading of 5 on the dissolution rate in a developer after post exposure bake (PEB) at 150 °C for 10 min was studied, and the results are shown in Figure 4. Development was performed at 45 °C using 5 % TMAH solution as a developer. The dissolution rate of the exposed area increased clearly with an increase in 5, and the exposed film dissolved faster than the unexposed film at all concentrations of 5. Furthermore, dissolution rates of unexposed films is not effected by 5. This result indicates that S acts as a dissolution promoter in ΡΠ (3c) film, and that 20 wt% of 5 is necessary to achieve good dissolution contrast. The most widely used positive resists are generally two-component materials consisting of an alkaline soluble matrix resin that is rendered insoluble in aqueous alkaline solutions through addition of hydrophobic radiation-sensitive materials. In this case, although the PII matrix is insoluble in aqueous alkaline solutions, the HI is susceptible to hydrolysis to give alkaline soluble polyamic acid. Hydrolysis of PII by aqueous alkaline may occur in the exposed area due to the diffusion of the developer into the PII film. Presumably, this diffusion is facilitated by the presence of hydrophilic moieties (e.g., carboxylic acid, sulfonic acid or hydroxyl) in the exposed regions. Therefore, the accelerated dissolution behavior in the exposed regions can be attributed to the polarity change of PII, as a result of PII hydrolysis in the development process and the photochemical reaction of compound S.

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2

Ο

Ο

D

Ο

so

2

Ο

I After a preliminary optimization study involving compound 5 loading, postbaking temperature, and developing temperature, we prepared a photosensitive polyimideprecursor system consisting of PII and 20 wt% of 5. The sensitivity curve for a 5 μια thick PII film shown in Figure 5 was consistent with the dissolution behavior studied above, indicating that the sensitivity (D°) and contrast (γ °) were 300 mJ/cm and 4.5, 2

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

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MICROELECTRONICS TECHNOLOGY

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place in heat press tool

Jj

I

01

15

L 30

Time (min) Figure 3. Typical hot press cycle.

10° I 0

1

1 ι 1 ι I 10 20 30 Compound £ contents (%)

ι

I 40

Figure 4. Relationship between Compound 5 Contents and Dissolution rate.

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

27. MOCHIZUKI & UEDA

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Polyisoimide as Polyimide Precursor

with 365 nm-light, respectively after PEB treatment at 150 °C for 10 min. Furthermore, with 436 nm-light the values of D° and γ° were 250 mJ/cm and 2.4. Figure 6 illustrates the scanning electron micrograph of the positive image projection printed in PII by postbaking at 150 °C for 10 min after the fdm exposed to 400 mJ/cm . This resist is capable of resolving a 2.5 μτη feature when a 5 /im thick films is used. 2

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2

Application of PII to the PSPI System using Amine Photo-generator (12). It is well known that aromatic nitro compounds containing benzylic hydrogens at the ortho to the nitro group are light sensitive (13). These photosensitive protecting groups are of potential importance in many areas of synthetic chemistry and formulation of resist materials (14-15). Recently, we found that the isomerization reaction of isoimide to imide was accelerated by a catalytic amount of base (2). This finding led us to synthesize a new amine photo-generator, {[(4,5-dimethoxy-2-nitrobenzyl)oxy]car^ piperidine (6). 2, 6-dimethylpiperidine (DMP) was chosen as the hindered photogenerated base in order to avoid the nucleophilic addition of amine to isoimide. It is known that isoimide tend to react with nucleophiles such as amines or alcohols ( 16-17). The new amine photo-generator 6 was prepared by the reactions shown in equation 3. The reduction of 4,5-dimethoxy-2-nitrobenzaldehyde gave 6-nitroveratryl alcohol. This compound was converted to 43-dimethoxy-2-nitrobenzyl-p-nitrophenylcarbonate by the treatment with p-nitrophenyl chloroformate in the presence of triethylamine. The reaction of 4^-dimethoxy-2-nitrobenzyl-p-nitrophenylcarbonate with DMP in the presence of 1-hydroxybenztriazole (HOBt) (18) yielded the new amine photo-generator 6. Recrystallization from a mixture of benzene and n-hexane gave pale yellow needles. The structure of amine photo-generator (6) was confirmed to be the corresponding carbamate by C-NMR, infrared spectroscopy, and elemental analysis. The IR spectra exhibited characteristic absorptions at 1700, 1525, and 1320 c m due to carbamate carbonyl, nitro asymmetric, and nitro symmetric stretching vibrations, respectively. The base catalyzed isomerization of PII to the corresponding PI was carried out by irradiating PII (3d) films containing 10 wt % of 6 (Figure 7). PII (3d) is significantly converted to the corresponding PI (4d) upon exposure to 1000 mJ/cm of 365 nm irradiation followed by post-exposure bake (PEB) at 150 °C., as evidenced by strong absorptions due to imide C=0 at 1780 cm* and imide C-0 at 1380 cm* . On the other hand, the unexposed polymer (3d) film containing 10 wt% of 6 was quite stable under the thermal treatment at 150 °C for 5 min. 13

1

2

1

1

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MICROELECTRONICS TECHNOLOGY

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422

Figure 6. Scanning electron micrograph of pattern from PII containing 5. (After development with 5% TMAH solution)

After preliminary optimization studies involving the loading of 6, postbaking temperature, developer, and developing temperature, we formulated a photosensitive polyimide- precursor system consisting of PII and 10 wt% of 6. The PII films (ca. 3 μτη thick) containing 10 wt % of 6 were exposed to 365 nm UV irradiation, postbaked at 150 °C for 5 min and developed with cyclohexanone at 45 °C. The sensitivity curve for a 3-/*m-thick PII film shown in Figure 8 is consistent with the isomerization study, indicated that the sensitivity (D - ) and contrast (V °- ) were 900 mJ/cm and 3.4, respectively. Figure 9 shows the scanning electron micrograph of a negative image projection-printed in PII by postbaking at 150 °C for 5 min after exposure to 1000 mJ/cm . 0

5

2

2

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

5

27. MOCHIZUKI & UEDA

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?

Polyisoimide as Polyimide Precursor

423

801-

PEB temperature (°C) Figure 7. Thermal isomerization of PII in the presence of 10 wt % of 6.

1.0 gg

i



M 0.8

1 J

0.6

i 1 g

0

1 £

4

0.2

\

0 10

2

/ L



-

L\ # # # / 1

-

1 1 10 Exposure Dose (mJ/cm ) 11 1,13

10

4

2

Figure 8. Exposure characteristic curves for the system of PII and 10wt%of 6.

Conclusions PIIs were successfully prepared from the corresponding polyamic acids using trifluoroacetic anhydride-TEA system as a dehydrating agent PII exhibited better flow properties than the corresponding PI and also functioned as a suitable high-temperature adhesive for bonding copper foil. The PII containing 20 wt% of 5 was found to be a positive-type PSPI-precursor, in which S acts as the dissolution controller. Furthermore, the new amine photo-generator 6 was effective for the isomerization of PII to PI. PII containing 10 wt% of 6 functioned as a negative type-PSPl with good contrast due to a large difference in the solubility between PII and PI.

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

MICROELECTRONICS TECHNOLOGY

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424

Figure 9. Scanning electron micrograph of pattern from PII containing 6. (After development with cyclohexanone)

Literature Cited 1. Wood, A.S.; Modern plastics international., 1989,June, 26 2.

Landis, A. L.; Naselow, A. B."Polyimides: Synthesis, Characterization, and Aplications", vol.1, 39, K. L.Mittal, Ed, Plenum Publishing Corp., New York (1982), p 39

3. 4. 5. 6.

Mochizuki, Α.; Teranishi T.; Ueda, M . Polym.J., 1994, 26 (3), 315. R.J.Cotter, C.K.Sauers, andJ.M.Whelan,J.Org.Chem.,1961, 26, 10 A. Mochizuki, T. Teranishi, and M . Ueda, Polymer, 1994, 35 (18), 4022 . St. Clair, A. K., and St. Clair,T. L. Sci. Adv. Mater. Process Eng. Ser. 1981, 26, 165 Hergenrother, P. M., Wakelyn, Ν. T. and Havens, S.J.J.Polym.Sci., Polym.Chem.Ed., 1987, 25, 1093 Pratt, J. R., Blackwell, D. Α., and St. Clair,T. L. Polym. Eng. Sci., 1989, 29, 63 A. Mochizuki, T. Teranishi, K. Matsushita, and M . Ueda, Polymer, in press Moss, M.G., Cuzmar, R.M. and Brewer, T. SPIE Advances in Resist Technology and Processing VI, 1989, 1086, 396 Hayase, S., Takano, K., Mikogami, Y. and Nakano, Y. J. Electrochem. Soc., 1991, 138, 3625 A. Mochizuki, T. Teranishi, and M. Ueda, Macromolecules, 1995, 28, 365(1995). Ciamician, G.; Silber, P. Ber., 1901, 34, 2040 Cameron, J. F.; Fréchet, J. M .J.J.Am.Chem.Soc.,1991, 113, 4303 Mckean, D. R.; Briffaud, T.; Volksen, W.; Hacker, N. P.; Labadie, J. W.Polymer preprints, 1994, 35(1), 387 Hedaya, E.; Hinman, R. L. Theodoropulos, S. J. Org. Chem., 1966, 31, 1311 Fan,Y. L.; Pollart, D. F. J. Org. Chem., 1968, 33, 4372 Konig, W.; Geiger, R.Chem.Ber., 1973, 106, 3625 Wake, W. C. Polymer, 1978, 19, 291

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

RECEIVED July 7,

1995

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