Synthesis of Polysulfides Containing the Triazeno Group and Their

Chem. Mater. 1997, 9, 485-494

485

Synthesis of Polysulfides Containing the Triazeno Group and Their Application as Photoresists in Excimer Laser Polymer Ablation O. Nuyken* and U. Dahn Lehrstuhl fu¨ r Makromolekulare Stoffe, Institut fu¨ r Technische Chemie, Technische Universita¨ t Mu¨ nchen, Lichtenbergstrasse 4, D-85747 Garching, Germany

W. Ehrfeld, V. Hessel, K. Hesch, J. Landsiedel, and J. Diebel IMM Institut fu¨ r Mikrotechnik Mainz GmbH, Carl-Zeiss-Strasse 18-20, D-55129 Mainz-Hechtsheim, Germany Received June 17, 1996. Revised Manuscript Received October 16, 1996X

Two bifunctional unsaturated monomers containing the photosensitive triazene group were synthesized and reacted with dithiols. A radical polyaddition afforded polysulfides which carry the photosensitive chromophores in their backbone. The molar masses reach up to 10 400 g/mol and the glass transitions range between -2 and 39 °C. With respect to their application as dry photoresists for excimer laser ablation, thermo- and photolytical decomposition of monomers and polymers were investigated. TGA measurements revealed that the polymers were stable up to 255 °C, slightly below those of the monomers (270 °C). For photolysis in solution observed by UV/vis spectroscopy, the triazene absorption vanished on a time scale of seconds. Pulsed laser patterning was performed at 193 nm and with PMMA as a reference system. The sensitivity of the resists to irradiation was higher for PMMA and also the structural precision seemed to be slightly better for the standard, both probably owing to the applied wavelength. In contrast, the amount of redeposited material and the roughness of the polymer surface could be improved by using the triazene-containing resists.

Introduction Recent developments in microlithography and photoresist technique1,2 prove that increasing demands in view of materials properties must and can be met. Structure dimensions, resolution, and sensitivity to the irradiation process are the dominating aspects that qualify the performance of photoresist systems. Promising candidates to fulfill the given prerequisites are those that can be structurized by dry development structuring techniques such as excimer laser lithography.3 In this technique spin-coated, compressed or casted films of the respective materials (with a thickness of up to 100-200 µm) are treated by pulsed excimer laser irradiation, varying pulse number, fluence, and local dimensions. Especially in the fast growing field of microtechnology, there is now an increasing application potential of polymer microstructures,4 that will be needed in large quantities, e.g., for micro and integrated optical components or for microfluidic devices. Excimer laser irradiation is a powerful tool for the generation of such microstructures because of the high precision and resolution achievable (submicrometer with holographic techniques5). However, the long process time required Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Masuhara, H.; Fukumura, H. Polym. News 1992, 17, 5. (2) Lazare, S.; Guan,W.; Drilhole, D.; Bolle, M.; Lopez, J. J. Photopolym. Sci. Technol. 1995, 8, 495. (3) Srinivasan, R.; Braren, B. Chem. Rev. 1989, 89, 1303. (4) Arnold, J.; Dasbach, U.; Ehrfeld, W.; Hesch, K.; Lo¨we, H. Appl. Surf. Sci. 1995, 86, 251. (5) Gower, M. C.; Rumsby, P. T.; Thomas, D. T. Proc. SPIE. 1992, 133, 1835. X

S0897-4756(96)00341-9 CCC: $14.00

for excimer laser micromachining is a serious drawback of this technique and results in relatively high costs. These drawbacks can be overcome by the use of replication techniques that were developed by a variant of the so-called LIGA process,6,7 namely, Laser-LIGA,4 which is a combination of excimer laser micromachining, microelectroforming, and micromoulding processes for low-cost replication. For this purpose of excimer laser microstructuring, several polymers affording positive resists by such a dry processing technique were synthesized and investigated in our group earlier.8 The concept is to build up a polymer backbone by interfacial polycondensation that incorporates the photosensitive 1-aryl-3,3-dialkyltriazeno unit9,10 at regular distances. This group has been proved to be a versatile low molecular dopant and sensitizer for laser ablation of poly(methyl methacrylate) (PMMA).11 With release of N2 a well-defined fragmentation of the polymer chain is induced upon the irradiation. The described resists exhibit good behavior when excimer laser photoablation is performed, with high resolutions and no debris being deposited on the surface around. (6) Ehrfeld, W.; Mu¨nchmeyer, D. Nucl. Instrum. Methods A 1991, 303, 523. (7) Ehrfeld, W.; Lehr, H. Radiat. Phys. Chem., in press. (8) Stebani, J.; Nuyken, O.; Lippert, T.; Wokaun, A. Makromol. Chem. Rapid Commun. 1993, 206, 97. (9) Baeyer, A.v.; Jaeger, C. Ber. Dtsch. Chem. Ges. 1875, 8, 148. (10) Lippert, T.; Stebani, J.; Nuyken, O.; Wokaun, A. J. Photochem. Photobiol. A: Chem. 1994, 78, 139. (11) Bolle, M., Luther, K., Troe, J., Ihlemann, J., Gerhardt, H. Appl. Surf. Sci. 1990, 46, 279.

© 1997 American Chemical Society

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Chem. Mater., Vol. 9, No. 2, 1997

We now modified that first synthetic approach in which the polymer backbone and the photosensitive units were formed at once by interfacial condensation in an electrophilic N-coupling reaction.12 The complementary route is to synthesize bifunctional triazenecontaining monomers which then are reacted in a stoichiometric ratio with different comonomers. Thus, the triazene monomer can be varied as well as the comonomer and the type of polycondensation reactions. To take advantage of the dependence between structure and macroscopic properties, a wide variety of chain topologies may be established. The resulting photopolymers can be adapted to the requirements (the polymer’s photo- and thermostability, and glass temperatures as well as film-forming and adhesion properties) by choosing from the pool of structures provided by this synthetic approach. Experimental Part Materials and Methods. All chemicals were applied as purchased from Aldrich or Fluka. Toluene was treated several times with H2SO4 before drying over K/Na alloy. For flash column chromatography Merck silica gel 60 (particle size 0.040-0.063 mm) was used. NMR spectra (1H and 13C) were recorded on a Bruker ARX 300 spectrometer (300 and 75.5 MHz) and computed with a Bruker Aspect Station using the corresponding software. The chemical shifts are given downfield to tetramethylsilane as internal standard. UV/vis spectroscopy was performed on a Varian Cary 3 spectrophotometer in quarz vials using tetrahydrofurane or chloroform as solvents. Photolysis experiments were made using a Ushio UXM200H HgXe high-pressure lamp with 0.1 W output. The gel permeation chromatography was done on a Waters 510 system fitted with Waters Ultrastyragel package with pores of 103, 104, and 105 Å,using a UV254 and RI detector and polystyrene standards. DSC measurements were made on a Perkin-Elmer DSC 7 differential scanning calorimeter (20 K/min) with nitrogen as purge gas and processed with a TAC7/DX controller. The STA experiments were performed on a Polymer Laboratories STA1500 under synthetic air with 10 K/min heating rate. Synthesis. (A) Bis(triazene) monomers (3a, 3b). 3-Aminostyrene (1, 10 g, 84 mmol) 1 is dissolved in 165 mL of 10% hydrochloric acid. Sodium nitrite (5.8 g, 84 mmol) in 40 mL water is added slowly at 0 °C and the mixture is kept stirring for 20 min. The resulting solution of the aryldiazonium salt 2 is transferred into a cooled and light-protected addition funnel. It is slowly dropped into a mixture of 3.61 g (41.9 mmol) of piperazine and 30 g (0.283 mol) of sodium bicarbonate in 470 mL water at -5 to 0 °C where the pH is kept above pH 7. After the addition, the mixture is stirred for another 30 min while 7 g of NaCl is added, and the temperature is allowed to rise to 20 °C. The brown precipitate is filtered, washed with water and dried at 50 °C in vacuo. Flash column chromatography is performed with CH2Cl2 as eluent and the resulting product is refined by recrystallization from acetonitrile. A dark orange, crystalline solid (3a, 7.1 g, 49%) is collected and dried in vacuo. 1,2-Ethanebis[1,1′-(3-ethenyl-phenyl)-3,3′-dimethylene]triaz(1)ene (3a). 1H NMR (CDCl3) δ (ppm) 4.01 (d, 8H, -N(CH2)2), 5.25-5.83 (2 × m, 4H, CH2dC), 6.7-6.79 (m, 2H, dCH-Ph), 7.25-7.78 (m, 8H, aromatic). 13C NMR (CDCl3) δ (ppm) 46.7 (piperazino C), 136.8 and 114.2 (CH2d), 118.7, 120.2, 124.5, 129.1, 138.4, 150.3 (aromatic C). Anal. Calcd for (C20H24N6): C, 68.94; H, 6.89; N, 24.12. Found: C, 68.96; H, 6.94; N, 24.20. UV/vis (in CHCl3) λmax1 ) 259 nm; λmax2 ) 305 nm. Tm ) 79 °C. 1,2-Ethanebis[1,1′-(3-ethenylphenyl)-3,3′-methyl]triaz(1)ene (3b). 1H NMR (CDCl3) δ (ppm) 3.28 (s, 6H, (12) Elks, J.; Hey, D. H. J. Chem. Soc. 1943, 441.

Nuyken et al. -N(CH3)-), 4.07 (s, 4H, -N(Me)-CH2), 5.21-5.25 and 5.73-5.79 (2 × m, 4H, CH2d); 6.66-6.76 (m, 2H, dCH-Ph), 7.17-7.43 (m, 8H, aromatic). 13C NMR (CDCl3) δ (ppm) 37 (br, -N(Me)-CH2-), 55 (broad, -N(CH3)-); 136.9 113.9 (CH2dCH-); 118.6, 120.0, 123.6, 128.9, 138.3, 150.9 (aromatic C). Anal. Calcd for (C20H22N6)n: C, 69.34; H, 6.35; N, 24.25. Found: C, 69.12; H, 6.15; N, 24.62. UV/vis (in CHCl3) λmax1 ) 259 nm; λmax2 ) 297 nm. Tm ) 69 °C. (B) Polyaddition. 1,6-Dimercaptohexane (278.9 mg, 1.94 mmol) and the bistriazene 3a (669.8 mg, 1.94 mmol) are dissolved in 4 mL of thiophene-free toluene which has been thoroughly dried and degassed. 2,2′-Azoisobutyronitrile (AIBN, 25 mg) is added to the solution which then is slowly warmed to 55-60 °C. After 15 h, the color of the solution has changed from orange to bright yellow. The reaction is completed within another 9 h, and when cooled to room temperature, the mixture is diluted with 4 mL of chloroform. Precipitation in a 1:1 mixture of methanol and diethyl ether and reprecipitation in methanol affords 650 mg (69%) of a very adhesive, bright yellow polymer (4a). Polymer 4a: 1H NMR (CDCl3) δ (ppm) 1.59 and 1.39 (s, br, 8H, aliphatic -CH2-), 2.53 (t, 4H, -S-CH2-); 2.79 and 2.91 (t, 8H, Ph-C2H4-), 3.98 (s, 8H, -N(-CH2-)2), 7.05-7.37 (m, 8H, aromatic). 13C NMR (CDCl3) δ (ppm) 28.5, 29.5, 32.3 (aliphatic -CH2-), 33.8, 36.4 (-S-CH2-CH2-Ph), 46.6 (piperazino C); 118.9, 120.9, 126.8, 129.0, 141.6, 150.1 (aromatic). Anal. Calcd for (C26H36N6S2)n: C, 62.89; H,7.25; N, 16.93. Found: C, 62.77; H, 7.33; N, 16.90. UV/vis (in CHCl3) λmax ) 312 nm. Polymer 4b. 1H NMR (CDCl3) δ (ppm) 1.38 and 1.56 (s, br, 8H, aliphatic, -CH2-), 2.52 (t, 4H, -S-CH2-), 2.76 and 2.85 (2 × t, 8H, Ph-C2H4-), 3.43 (d, br, 6H, -N(CH3)-), 4.04 (s, 4H, -N(Me)CH2-), 6.90-7.29 (m, 8H, aromatic). 13C NMR (CDCl3) δ (ppm) 28.5-32.2 (aliphatic -CH2-), 33.6, 36.5 (SC2H4-Ph), 118.5,120.9, 125.8, 128.9, 141.4, 150.8 (aromatic). Anal. Calcd for (C26H38N6S2)n: C, 62.64; H, 7.62; N, 16.85. Found: 62.94; H, 7.56; N, 16.84. UV/vis(in CHCl3) λmax) 312 nm. Polymer 5a. 1H NMR (CDCl3) δ (ppm) 2.94 and 3.18 (t, 8H, -S-CH2-CH2-Ph), 3.96 (s, 8H, -N-(CH2)), 7.02-7.44 (m, 12 H, aromatic). 13C NMR (CDCl3) δ (ppm) 34.9, 35.6 (-SCH2-CH2-Ph), 46.6 (piperazino C), 119.1, 120.9, 126.5, 126.8, 128.3, 129.2, 129.3, 137.4, 140.9, 150.2 (aromatic). Anal. Calcd for (C26H28N6S2)n: C, 63.94; H, 5.73; N,17.21. Found: C, 63.95; H, 5.80; N, 17.09. UV/vis (in CHCl3) λmax1 ) 260 nm, λmax2 ) 314 nm. Polymer 5b. 1H NMR (CDCl3) δ (ppm) 2.90, 3.17 (2 × t, 8H, -S-CH 2-CH2-Ph), 3.24 (s, 6H, -N(CH3)-), 4.02 (d, 4H, -N(Me)-CH2-), 6.94-7.42 (m, 12H, aromatic). 13C NMR (CDCl3) δ (ppm) 34.8, 35.6 (-S-CH 2-CH2-Ph), 118.6, 120.8, 125.8, 126.3, 128.3, 129.3, 129.4, 137.5, 140.6, 150.9. Anal. Calcd for (C26H30N6S2)n: C, 63.67; H, 6.12; N, 17.13. Found: C, 63.53; H, 6.00, N, 15.71. UV/vis (in CHCl3) λmax) 259 nm. Sample Preparation. A powder of 0.5 g of polymer 5a was converted to a thin film by thermal compression on a 100 mm titanium substrate of 10 mm thickness which was lapped, cleaned, and oxidized according to standard procedures in X-ray lithography.13 The temperature was raised to 100 °C in a time interval of 5 min without any increase of pressure. For 1 min a pressure of 25 bar was applied at a temperature of 100 °C, then the sample was cooled down to 30 °C within 3 min without changing the pressure. The pressure of 25 bar was maintained at this temperature for 1 min. Prior attempts with maximal temperatures of 55 and 65 °C gave discontinuous, turbid thin films. The thickness of the 100 °C/25 bar film was determined to be 220 µm in the middle of the substrate and 173-201 µm at the outer edges. The poly(methyl methacrylate) sample was prepared by a casting process from solution. However, since films of pure, noncrosslinked PMMA cannot be used for the formation of films with sufficient thickness due to the presence of crazes, cross-linked PMMA had to be prepared. The raw material was prepared by polymerization of monomeric MMA (63.5 wt %) (13) Mohr, J.; Ehrfeld, W.; Mu¨chmeyer, D, KFK-report 4414, 1988.

Photoresists in Excimer Laser Polymer Ablation

Chem. Mater., Vol. 9, No. 2, 1997 487 Scheme 1

Scheme 2

in the presence of the cross-linker triethyleneglycol dimethacrylate (2 wt %), the polymer PMMA (27 wt %, MW ∼30 000 g/mol), the initiator benzoyl peroxide (1 wt %), the accelerator p-toluidine (2 wt %) and the adhesion promoter trimethoxysilylpropyl methacrylate (4.5 wt %). Excimer Laser Machining. Excimer laser machining was performed by means of a commercial micromachining workstation (Exitech Series 7008) with beam homogenization and computer control of laser firing, beam attenuation, imaged aperture, and workpiece position.4 For the aperture imaging, a reflective Schwarzschild objective (demagnification ×15) was used. The laser employed was a Lambda Physik LPX 100 i operating at 193 nm. Atomic Force Microscopy (AFM) Measurements. AFM measurements were performed by the use of a SFM-BD2 instrument (Park Scientific Instruments). The spectra were measured with a silicon tip in contact mode at a scan rate of 0.5 Hz. The images are built up of 256 lines, with 256 points forming one line. The size of the images was chosen to be 5 × 5, 2 × 2, or 0.5 × 0.5 µm2, because the detected structural changes occurred in this range. In addition, the surface roughness was characterized by peak-to-valley (PV) and rootmean-square (rms) values from AFM line scans. The PV value is the difference between the maximum and minimum height of the surface topography; the rms value is defined as follows:

rms )

x

1

N

∑(x - x)

N - 1 i)1

2

i

where xi ) height measured at data point i and x ) average height value. Profilometer Measurements and Scanning Electron Microscopy (SEM) Characterization. The etch depth d was measured by profilometer measurements using a Tencor a-step 200 instrument with a scan length of 400 mm. The scanning electron microscopy (SEM) characterization of the structurized samples was carried out with a JEOL JSM 6400 S instrument.

Results and Discussion Synthesis and Characterization. An intriguing issue of the photopolymers presented herein is that beside the triazene units they contain sulfide bonds within the polymer backbone as depicted in the general structure in Scheme 1. The triazeno monomers we chose were R,ω-diolefinic species containing two triazine groups whereas the comonomer is a R,ω-dimercaptan. In a radical polyaddition reaction, which proceeds as a step-growth type of reaction as described by Marvel et al.,14 polysulfide backbones of different topologies are formed. Monomer Synthesis. The difunctional unsaturated bis(triazene) monomers (3a, 3b) were prepared in the classical pathway described by Elks and Hey12 by coupling aromatic diazonium salts with secondary amines at low temperatures. To obtain symmetrical monomers bearing two triazene groups, bifunctional amines were reacted with an aryldiazonium salt in a 1:2 ratio. The coupling was carried out at 0 °C in an aqueous solution of the diamine at pH = 9 as sketched in Scheme 2. To investigate the effects of small changes in monomer architecture the bifunctional amines used for coupling are structurally related to each other: (N,N ′dimethylamino)ethane and piperazinesthe related cyclic compound with only one additional C-C bond. The diazonium salt 2, already procured with the double bond, results from 3-aminostyrene (1) which is readily available from dehydration of 3-(1-hydroxyethyl)aniline.15 (14) Marvel, C. S.; Caesar, P. D. J. Am. Chem. Soc. 1951, 73, 1097. (15) Nuyken, O.; Grunow, A.; Pampus, G. Kautsch. Gummi Kunstst. 1989, 42(4), 284.

488


Nuyken et al. Scheme 3

Figure 1. DSC trace of the monomer 3b (10 K min-1) showing melting point, thermal polymerization, the decomposition process of the triazeno group, and a final endothermic polymer chain degradation.

The resulting monomers 3a and 3b were characterized by 1H and 13C NMR and UV/vis spectroscopy. The UV spectra (THF) consist of two main absorptions. A broad band with two superimposed maxima penetrating each other is found at 270-350 nm. It has been assigned formerly to the π-π* transition of the triazine chromophore and can also be seen in the polymer spectra. At 260 nm an even more intense absorption peak can be detected, probably corresponding to the arylvinyl group. This signal totally vanishes in the UV absorption of the polymers as can be anticipated when a polyaddition mechanism is assumed. To ensure that the monomer will be able to withstand the conditions of the polyreaction a solution of 3a was kept at 80 °C for 10 h and observed with UV spectroscopy within regular intervals. No significant decomposition was detected under these conditions, probably for two reasons: the vinyl group as the aromatic substituent (a) provides higher π-conjugation reaching up to the triazene unit and (b) intrinsically stabilizes the triazene group due to its slightly electron-withdrawing effect.10 Thermoanalytical Measurements. Simultaneous thermoanalysis (STA) investigations of the monomers allows to observe their decomposition dependent on temperature by observing the weight loss (thermogravimetry, TGA) and the heat flow (differential scanning calorimetry (DSC)). The TGA traces exhibit two main decomposition processes at about 270 and 390 °C, respectively. Simultaneous DSC measurements reveal strong exothermic behavior for the first and endothermic behavior for the second step. Thus we assign the exothermal loss of N2 to the weight loss occurring at 270 °C, whereas the evaporation of resulting fragments causes the endothermic process at higher temperatures. For monomer 3b the DSC trace is shown in Figure 1. An exothermic peak without any simultaneous weight loss appears at about 140 °C, between Tm and the first decomposition step. An incomplete thermal polymerization is assumed when estimating the polymerization enthalpy ∆Hpol per styryl unit (32.5 kJ/mol) and comparing it with the polymerization heat of styrene (66.5 kJ/mol).16 Polymer Synthesis. As already stated the polymerforming step-growth is a radical addition of the S-H

group to the CdC double bond in an anti-Markovnikov orientation.17 This has been unambiguously established by former investigations18 and is confirmed by 1H, 13C, and 13C dept135 NMR-spectroscopies. As an example the reaction between 1,6-dimercaptohexane and monomer 3a is sketched in Scheme 3. The radical mechanism is started by either a radical initiator or, in certain cases, by photochemical activation.19 The last way must be excluded right away since photodecomposition of the triazene groups will occur simultaneously or even faster. Initiation by adding a radical starter was tried with two different systems: 2,2′-Azoisobutyronitrile (AIBN) which decomposes fairly slowly even at 60 °C and dimyristoylperoxodicarbonate (DMPD) which already shows considerable decomposition20 at 30 °C. With DMPD as initiator and toluene as solvent it was not possible to obtain polyaddition at (16) Brandrup, J.; Immergut, E. H. Polymer Handbook, II.300, 3rd ed.; Wiley and Sons: New York, 1989. (17) Ashworth, F.; Burkhardt, G. N. J. Chem. Soc. 1928, 1971. (18) Nuyken, O.; Reuschel, G. Makromol. Chem., Makromol. Symp. 1989, 26, 313. Nuyken, O.; Siebzehnru¨bl, F. Polym. Bull. 1988, 19, 371. (19) Nuyken, O.; Vo¨lkel, T. Makromol. Chem. Rapid Commun. 1990, 11, 365. (20) Krieg, B.; Meyer, H. Kunststoffe 1985, 75 (10), 751.


Chem. Mater., Vol. 9, No. 2, 1997 489

Table 1. Summary of the Polymers 4a, 5a, 4b, and 5b Showing Results from GPC, DSC, and TGA Measurements compound

triazeno monomer

dimercaptane

Mw [g/mol]

Mw/Mn

Tg [°C]

Tdec [°C]

4a 5a

3a 3a

piperazine piperazine

HS-C6H12-SH

10430 7410

8.55a 2.82

18 39

213 241

4b 5b

3b 3b

N,N ′-dimethylaminoethane N,N ′-dimethylaminoethane

HS-C6H12-SH

3450 5260

2.05 2.33

-2 26

255 245

amine

HS

HS a

SH

SH

Compound was not completely soluble in the eluent (CHCl3).

60 °C with any appreciable yield. When AIBN was used instead of DMPD under the same reaction conditions, macromolecules were formed with yields and molar masses comparable to those in literature.21 Another point worth mentioning is the competition between thermal decay of AIBN and thermolysis of the triazene groups22 under the conditions needed for this kind of polyaddition. Since at higher temperatures the decomposition of certain particularly substituted triazenes occurs at a considerable rate, great care must be taken when chosing the reaction conditions. Fortunately, when reacting for 50 h at 55-60 °C, the decay of AIBN is sufficient to initiate the chain growth without the triazeno units being affected to a perceptible extent. Two quite contrasting species, the aromatic 1,3dimercaptobenzene and the aliphatic 1,6-dimercaptohexane, were used as dimercapto-monomers. The resulting polymers are yellowish oils or solids soluble in common solvents. UV/vis spectroscopy of the resulting polymers reveals the typical absorption band of the triazeno group (λmax ) 312 nm) mentioned above, confirming that the photosensitive group has been preserved during the polyaddition. In addition, compounds 5a and 5b exhibit an intense absorption at about 250-260 nm which can be assigned to the aromatic thioether unit.23 Properties of the Polymers. The polymers were characterized spectroscopically and by GPC, DSC and TGA. The results are summarized in Table 1. Despite the varying chain topologies the glass transitions Tg range in a fairly narrow temperature interval, between -2 and 39 °C. Tg is influenced by both triazeno and mercapto monomers: As expected, the more flexible species effects depression of Tg. Consequently the lowest glass transition is observed for 4b, and the highest one for 5a, as depicted in Table 1. However, note that when CH3-NH-CH2CH2-NHCH3 is replaced by piperazine, by just adding one more C-C bond, Tg raises by 13 and 20 °C, as realized for the pairs 4a, 4b and 5a, 5b. Investigations on the thermal stability, carried out by TGA, show decomposition above 200 °C for all polymers and a weight loss of 60-70% within the next 150 K. As an example, the TGA curve of 4b is shown in Figure 2. Above the onset of the decomposition slope two decomposition steps can be roughly distinguished, where the first exhibits a sudden weight loss, the second (21) Vo¨lkel, T. Ph.D. Thesis, Universita¨t Bayreuth, 1990. (22) Koningsberger, C.; Salomon, G. J. Polym. Sci. 1946, 1, 200. Lau, A. N. K.; Vo, L. P. Macromolecules 1992, 25, 7294. (23) Lang, L. Absorption Spectra in the Ultraviolet and Visible Region; Akademiai Kiado: Budapest, 1966; Vol. II, p 29.

Figure 2. TGA curve of the polymer 4b (synthetic air, heating rate: 10 K min-1) with two distinct decomposition steps above 255 °C.

a shallower one. By analyzing the extent of the first step (32%) a good correspondence with the theoretical percentage was found, supposing that the extrusion of the more volatile fragment CH3

CH3 N

N

N

CH2

CH2

N

N

N

occurs first. This result is also detected for the other triazeno-polysulfides and is in good agreement with former investigations.24 It was also important to know more about the thermostability of such polymers in solution. Therefore a DMF solution of 4a was kept at 80 °C for several hours. No decay of the absorbance could by visualized by UV/vis spectroscopy. Photolytic decomposition of the polymers was performed by irradiating a CHCl3 solution and its detection by UV/vis spectroscopy. A complete disappearance of the triazene band (at 312 nm) is observed after 48 s irradiation time as shown for 4b (see Figure 3). In contrast the intensity of the second absorption, caused by the thioether unit, is not affected by the irradiation in a similar amount. The course of the fragmentation of the backbone during polymer photolysis can be visualized by investigations on the molar masses as well. GPC measurements of the photolysis products (insoluble parts, probably due to cross-linking reactions had to be removed) exhibit a decrease of both molecular weight and poly(24) Stebani, J.; Nuyken, O.; Lippert, T.; Wokaun, A. Makromol. Chem. Rapid Commun. 1993, 14, 365.

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Nuyken et al.

Figure 3. Photolytic decomposition of the polymer 4b (HgXe high-pressure lamp, 100 mW at a distance of 0.95 m) in chloroform observed by UV spectroscopy; the total irradiation time is noted beside and was not continued after the main absorption had leveled out.

Figure 5. Matrix of 35 equally shaped square holes (100 µm × 100 µm) structured in a 170 µm thin film of the polymer 5a. The real matrix is slightly different from that schematically drawn, since three holes of the last row were actually located outside.

Figure 4. Correlation of the molecular weight Mn and the polydispersity Mw/Mn of polymer 5b to the time of irradiation (HgXe high-pressure lamp, 100 mW), observed by size exclusion chromatography (eluent, THF; calibration standards, polystyrene).

dispersity. The decay of the observed parameters and their change as a function of time are presented in Figure 4. It is worth noting that despite the overall decrease of both parameters, within the first 8 s of irradiation an increase of the molar mass from 5200 up to 9000 g/mol is observed. This surprising behavior can be ascribed to either photo-cross-linking or rather reactions of the polymer end groups which in any combination are able to link by photochemical assistance. Excimer Laser Ablation Experiments The investigation of the ablation properties was exemplarily performed with the polymer 5a as an example using a pulsed ArF* excimer laser at 193 nm. The other polymers were expected to behave similarly but to be processable less conveniently due to their lower glass transitions. Therefore, a matrix of 35 equally shaped square holes (100 µm × 100 µm) was structured in a 170 µm thin film of this polymer (see Figure 5). Each square was exposed to the excimer laser radiation with a variable number of 10 Hz pulses and fluence

at a constant pulse repetition rate of 10 Hz. The number of pulses was set to 1, 5, 20, 100, and 500, the energy densities were 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, and 2.0 J cm-2. The following features of the exposed or unexposed polymeric material were analyzed: (i) sensitivity; (ii) structural precision; (iii) amount of redeposited material; (iv) surface roughness of structured material. The sensitivity was monitored by correlating the depth of ablation to the number of pulses as well as to the energy density. The depth of ablation was determined by measuring the profile of the holes with an R-stepper. The amount of the redeposited material is discussed only qualitatively by examination of the light microscopy and SEM images, because the differences between the two samples were rather large and the quantification of a nonhomogeneously distributed material was difficult. The surface roughness was characterized by determining the root-mean-square (rms) and peak-to-valley (PV) values from AFM area scans. A poly(methyl methacrylate) sample (PMMA) was exposed to the excimer laser radiation under identical conditions and is taken as a reference in the following discussion. Sensitivity. The correlation of the ablation depth to the energy density yields for both polymers (polymer 5a and PMMA) a low-pulse energy density regime with a relatively steep increase and a high-pulse energy density regime reaching a plateau. In addition, a linear dependence of the ablation depth on the number of pulses is found (see Figure 6). Similar relationships


Chem. Mater., Vol. 9, No. 2, 1997 491 Table 2. Correlation of Etch Depth d of the Square Holes Made in Polymer 5a and PMMA to the Process Parameters Number of Pulses n, Energy Density F, and Fluence no. of pulses n 1

5

20

100

Figure 6. Correlation of the ablation depth to the fluence (for 20 pulses) and the number of pulses (for a fluence of 0.2 J cm-2) for the polymers 5a and PMMA.

between these three parameters are known for several other polymers.1,25 It can be seen clearly that under identical conditions PMMA is always a more sensitive material than polymer 5a (see also Table 2). Thus, no increase in sensitivity was observed, which can be a result of a high bulk absorption coefficient of polymer 5a. However, it has to be mentioned that the wavelength of the excimer laser source (193 nm) does not ideally match the absorption characteristics of the material exposed. The strongest peak in the UV absorption spectra of the polymer is found at significantly higher wavelengths from 260 to 370 nm (maximum: 312 nm).26 In addition to photoinduced mechanisms, the decomposition of the polymer main chain (at the triazene linkages) may also occur via a thermal mechanism.1,26 A comparison of all pairs with different number of pulses and energy density is given in Table 2 for polymer 5a and PMMA. In Figure 6, the dependences for either a constant number of pulses or a constant energy density were discussed. Dependences by simultaneous variation of both parameters can be discussed by introducing another parameter that describes the overall energy input per area. This is the product of the number of pulses times the fluence. Some pairs in Table 2 have equal (25) Srinivasan, R.; Braren, B. Chem. Rev. 1989, 89, 1303. (26) Nevertheless, another absorption below 200 nm is detectable in certain cases. For this purpose, a water-soluble model compound with a structure similar to that of 3a (with “-4-SO3Na” instead of “3vinyl-” as aromatic substitutent) was investigated. The absorption maxima are found at λ ) 191, 223, and 325 nm. The absorption coefficients at the wavelengths important for excimer laser patterning are 1 (308 nm) ) 30 124 L mol-1, 2 (248 nm) ) 7864 L mol-1 and 3 (193 nm) ) 36 147 L mol-1. This must not be the same characteristics as for the polymeric systems. In the future a comparison of the quantum yields of photolysis at 308 and 193 nm with the ablation results (e.g., ablation rate) will help to understand which wavelenghts is the most efficient patterning tool.

500

fluence Fa [J cm-2]

total energy input ∑F [J cm-2]

etch depth d [µm] polymer 5a

etch depth d [µm] of PMMA

0.2 0.4 0.6 0.8 1.0 1.5 2.0 0.2 0.4 0.6 0.8 1.0 1.5 2.0 0.2 0.4 0.6 0.8 1.0 1.5 2.0 0.2 0.4 0.6 0.8 1.0 1.5 2.0 0.2 0.4 0.6 0.8 1.0 1.5 2.0

0.2 0.4 0.6 0.8 1.0 1.5 2.0 1.0 2.0 3.0 4.0 5.0 7.5 10.0 4.0 8.0 12.0 16.0 20.0 30.0 40.0 20.0 40.0 60.0 80.0 100.0 150.0 200.0 100.0 100.0 300.0 400.0 500.0 750.0 1000.0

0.14 0.21 0.18 0.17 0.24 0.27 0.23 0.55 0.72 1.19 0.89 0.96 1.07 1.09 2.39 3.11 3.29 3.35 3.57 3.68 3.99 11.81 15.05 15.89 15.89 16.37 16.40 17.06 32.50 87.22 93.54 93.33 98.04 46.48 110.50

b 0.38 1.04 0.95 0.91 1.92 2.37 1.13 2.27 2.68 2.22 2.36 2.42 2.37 3.60 9.35 10.50 10.16 11.03 10.31 11.19 35.31 51.00 56.00 59.33 63.84 65.45 66.44 b b b b b b b

a Single pulse. b Depth could not be determined anymore by an R-stepper due to instrumental limitations.

total energy inputs, but different numbers of pulses and energy density. It can be seen that the etch depth d still varies for these parameter sets.27 This is basically a result of the saturation behavior of d found for high energy densities which is a common feature for excimer ablation of polymers. The parameter d for a given total energy input is comparable to values which are known for other triazene polymers, although the wavelength of the excimer laser applied was significantly different (λ ) 308 nm, here λ ) 193 nm). This supports the existence of a thermal mechanism, besides photoinduced mechanism. Structural Precision. The SEM images of four selected square holes are shown in Figure 7a for polymer 5a and in Figure 7b for PMMA. These holes were selected because the corresponding values of the number of pulses/energy densities were either low/low, high/low, low/high, or high/high. The square hole 1 (Figure 7a, high number of pulses, low-energy density) has a well-defined structural shape. The side walls (as well as the bottom) appear to be smooth, and the total depth of the structure is defined accurately. The square hole 2 (high number of pulses, high energy density) is significantly distorted at its edges and shows deviations from a perfect square. This (27) Lippert, T.; Stebani, J.; Ihlemann, J.; Nuyken, O.; Wokaun, A. J. Phys. Chem. 1993, 97, 12296.

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Figure 7. SEM images of four selected square holes of polymer 5a (a, nos. 1-4) and PMMA (b, nos. 5-8). These holes were selected, because the corresponding values of the number of pulses/energy densities were either low/low, high/low, low/high, or high/high. Note that the magnifications and also the hole diameters in this figure are not the same.


Figure 8. SEM image of the square holes made in polymer 5a (a, top) and PMMA (Figure 8b) which were exposed to 5 pulses of 0.8 J cm-2. Details of the top, sidewall, and bottom are visible.

observation seems to correspond to an effect called hole widening28 which occurs for increasing fluences. It has been ascribed to interactions between the plume of ejected material and the wall of the hole. Square holes 3 and 4 (low number of pulses, high or low energy density) have regular shape and, in the limits of SEM imaging, no difference in surface roughness can be detected. Although only a qualitative analysis by comparison of similar SEM images has been performed, the structural precision of the square holes made in PMMA (Figure 7b) were shown to be higher. Deviations from a perfect square shape such as round-shaped corners and curved or slightly elevated edges were generally not found. In contrast Figure 7b shows the ablation of PMMA, with fairly defined structures even for the most extreme treatment (square hole 6). However, for the holes of very high total energy input morphological changes around the edges were detected by light microscopy investigations which are similar to those observed for hole 2 in Figure 7a. They are most likely caused by a thermal melting process (accompanied by thermal decomposition) with subsequent reorganization of the polymer chains after cooling. In addition, it has to be mentioned that some defects may be due to inhomogeneities within the sample of (28) Ihlemann, J.; Schmitt, H.; Wolff-Rottke, B. Adv. Mater. Opt. Electron. 1993, 2, 87.

Chem. Mater., Vol. 9, No. 2, 1997 493

Figure 9. (a) 2.0 × 2.0 µm2 AFM image of the bottom in one hole (5 pulses, 0.8 J cm-2) made in polymer 5a. (b) 2.0 × 2.0 µm2 AFM image of the bottom in one hole (5 pulses, 0.8 J cm-2) made in PMMA.

polymer 5a and can be minimized by an improved sample preparation procedure (especially the thermal compression step). Instead, sample preparation for the PMMA material was performed according to a standard procedure which has been developed for an application in X-ray lithography. Amount of Redeposited Material. Structuring of polymeric samples via excimer laser is achieved by photoinduced and thermal decomposition leading to an ejection of fragments. A certain amount is not removed completely but redeposited back to the surface. A striking feature of all square holes made in polymer 5a is the nearly complete absence of any redeposited material, the so-called “debris” (see Figure 7a). Even the presence of some roughened surface areas around the edges, as found for the square hole 2 (high number of pulses, high energy density), seems to be related only to the direct interaction with the radiation, namely, partial melting or photoinduced decomposition, and not to redeposition. The absence of redeposition is a consequence of the regular cleavage at the triazene units in the polymer main chain. Photodecomposition leads to a release of gaseous fragments of quite defined low molar mass. These gaseous products, in particular nitrogen, are acting as a driving gas at least in the initial stages of the ablation, thereby removing other polymer fragments from the surface.

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Table 3. Rms and PV Values for the Top and Bottom Areas of One Square Hole (5 pulses, 0.8 J cm-2) Made in Polymer 5a and PMMA rms [nm] PV [nm] scan location area (µm2) polymer 5a PMMA polymer 5a PMMA top bottom

0.5 × 0.5 2.0 × 2.0 0.5 × 0.5 2.0 × 2.0

2.0 4.2 0.9 2.2

1.2 2.7 1.8 28

19 43 7 21

18 66 14 221

In contrast, the edges of the holes made in PMMA are covered by a high number of redeposited polymer grains. Details of the sidewalls for both polymer 5a and PMMA are given in Figure 8a,b. This was found not only for samples of high total energy input but also for samples of relatively low total energy input. Surface Roughness of Structured Material. (A) Surface Roughness at the Bottom. The surface roughness was characterized in the bottom of the holes as well as on the top. In the bottom of some square holes made in polymer 5a irregular features like scratches are found (see Figure 8a). Most of them are due to mechanical deformations which were caused by the profilometric measurement with the R-stepper. Another origin of irregularitites are inhomogeneities which were generated during the preparation of the sample. Their contribution to the overall surface roughness is not directly connected to the interaction of excimer laser radiation with the triazene polymer. Thus, selected areas with absence of these defects were imaged by AFM. A 2.0 × 2.0 µm2 image of the bottom in one hole (five pulses, fluence ) 0.8 J cm-2) made in polymer 5a revealed a smooth surface (see Figure 9a and Table 3). The difference between rms and PV values measured for scan areas of different size (2.0 × 2.0 or 0.5 × 0.5 µm2) is related to a wavy structure of the bottom and/ or to the above-discussed defects. The corresponding hole made in PMMA was also imaged by SEM (see Figure 8b) and AFM (see Figure 9b). Here, the surface is characterized by the presence of relatively large redeposited grains as well as a wavy bottom as visible in Figure 8b. The rms and PV values of the PMMA sample are generally higher compared to those of the polymer 5a sample, even when smaller areas with absence of redeposited grains were measured (0.5 × 0.5 µm2 scans, see Table 1). By extension of the

scan area a higher number of grains is analyzed, thereby increasing the surface roughness. (B) Surface Roughness at the Top. The rms and PV values were also measured for the top areas of the holes which have not been exposed to the excimer laser. These values of polymer 5a were higher than the correponding values of the exposed areas. This decrease in surface roughness may be explained by a thermal annealing effect during exposure to the excimer laser radiation. Similar to the bottom, the top of the PMMA sample is also characterized by redeposited grains on a smooth surface. Since the overall number of grains is smaller compared to that of the bottom, lower values of surface roughness are found at the top. Conclusions We have synthesized new photolabile monomers with incorporated triazene-units. Radical polyaddition stepgrowth yielded photopolymers with sulfide-bridges in the backbone which are appropriate systems for excimer laser lithography. Molecular weights range between 3000 and 10 000 g/mol and glass transitions are found at -2 to 39 °C. TGA and UV spectroscopic investigations revealed appreciable thermostability up to 250 °C for condensed and at least 80 °C for dissolved state. Considerable photolytic sensitivity is detected when irradiating and observing the polymers with UV spectroscopy in defined intervals. The first experiments with the excimer laser, indeed, demonstrated that these triazene polymers can be used for the fabrication of microstructures with sufficient precision by development-free excimer laser ablation. As can be anticipated for laser ablation at 193 nm, the sensitivity is lower than found for the standard material PMMA which is often used for excimer laser structuring at that wavelength. The structures show the absence of any amount of redeposited material and yield very smooth surfaces. Therefore, the triazene polymer seems to be suitable for applications where these two features are strongly demanding, e.g., in microoptics and for smooth packaging microstructures. CM9603415

Synthesis of Polysulfides Containing the Triazeno Group and Their

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