Photoacid Generating Ligands for Development of Positive-Tone

ARTICLE pubs.acs.org/Langmuir

Photoacid Generating Ligands for Development of Positive-Tone Directly Photopatternable Metal Complexes Christopher E. J. Cordonier,* Akimasa Nakamura, Kazuhiko Shimada, and Akira Fujishima Technology Research and Development Department, Central Japan Railway Company, 1545-33 Ohyama, Komaki City, Aichi, 485-0801 Japan

bS Supporting Information ABSTRACT: Photoacid generating ligands, 4-(2-nitrobenzyloxycarbonyl)catechol and 4-(6-nitroveratryloxycarbonyl)catechol, and indium tin and titanium complexes thereof, were synthesized. These metal complexes perform as positive-tone, directly photopatternable indium tin oxide (ITO) or titanium oxide film precursors. After exposure, acid-bearing selectively soluble complexes could be removed to give patterned films upon developing in aqueous base, which were transformable to the corresponding pattern-preserving metal oxide film. Micropatterning of ITO and titanium oxide films was accomplished with the photoreactivity of the 2-nitrobenzyloxycarbonyl (NBOC) and 6-nitroveratryloxycarbonyl (NVOC) moiety bearing ligands.

’ INTRODUCTION Patterning metal oxide films, typically by photolithography followed by etching of a sputtered film, comprises a large area of activity in the microelectronics industry.1 Some examples include indium tin oxide (ITO) as transparent electrodes in optical, display, and lighting devices and in touch panels,2 ferroelectric3 and chalcogenide4 films in nonvolatile memory, dielectric films in capacitors,5 and titanium oxide in photocatalytic6 or solar energy harvesting devices.7 Direct patterning of solution processed metal oxide precursor films, transformable to the pattern-retaining metal oxide film by a variety of methods, for example, thermal decomposition,8 photolysis9 or decomposition with atmospheric pressure plasma,10 provides potential to dramatically economize and reduce environmental impact in microelectronic fabrication by eliminating the necessity of a high vacuum environment for film deposition and eliminating etching and resist stripping processes.1 Solution deposition additionally allows for good control of atomic ratios in multimetal compositions and more readily adapted to allow coating of very large or irregularly shaped substrates. Direct patterning compositions of metal compounds combined with nitrobenzaldehyde or nitrobenzyl alcohol derivatives or diazonaphthoquinone sulfonate esters (DNQs) among other photoacid generating compounds have been reported,11 where similar to conventional DNQ/novolac photoresists,1 contrast in solubility is induced upon irradiation. Reactions involving a photoinduced sol-gel type mechanism for 2-ethylhexanoate, β-diketone, and R-hydroxyketone complexes have also been reported,12 although requirement of short wavelength irradiation r 2011 American Chemical Society

in high doses and environmental control make application of these precursors difficult. Among a vast array of possible photocleavable and photoacid or photobase generating compounds, nitrobenzyl derivatives have received much attention. Providing a versatile means for photopatterning, 2-nitrobenzyloxycarbonyl (NBOC) and 6-nitroveratryloxycarbonyl (NVOC) derivatives13 appear in numerous applications, recently, for biochip fabrication14 and for carbon nanotubes in organic semiconductor fabrication15 and nanoparticles,16 for example, where NBOC-derived SAMs on the surface of a substrate are selectively photofunctionalized. Here, NBOC-derived indium tin and titanium complexes perform as a new type of efficient positive-tone, directly photopatternable ITO and titanium oxide precursors, respectively. Detailed reviews of photolabile protecting groups have summarized the main types of compounds and include reaction mechanisms, photochemistry, substituent effects, diversity, methods, and applications.17 In many systems, 2-nitrobenzyl esters are cleaved via a photoinduced elimination reaction to yield the respective acid as illustrated in Scheme 1.17a Differences in nature between the ester and acid induce the contrast exploited in photopatterning or photofunctionalizing applications,14-17 in this case, solubility in aqueous base. Although not the most photosensitive, the nitrobenzyl group was chosen based on its stability, inert nature toward metal ions, Received: October 23, 2010 Revised: January 6, 2011 Published: February 11, 2011 3157

dx.doi.org/10.1021/la104259f | Langmuir 2011, 27, 3157–3165

Langmuir Scheme 1. Photocleavage Reaction of NBOC Derivatives17a

ARTICLE

Scheme 2. Anticipated Photocleavage of NBOC-Derived Complexes 2 and 3

Scheme 3. Photopatterning with NBOC-Derived Complexes

versatility, lipophilic nature rendering it sparingly soluble in aqueous solution, and the facility of ligand and complex synthesis. More recently developed R-methyl-6-nitropiperonyloxycarbonyl18 (MeNPOC) and 2-(2-nitrophenyl)propyloxycarbonyl19 (NPPOC) photolabile groups show higher efficiency than that for the original NBOC and NVOC, but the presence of methyl or methylene at the R-position of the oxycarbonyl group leaves them vulnerable to β elimination, a factor we wish to rule out in preparation and reaction of the ligands and the respective metal complexes at this conjuncture. The nitrobenzyl benzoate system was also attractive in consideration of residual decomposition materials and its ability to form a film with high metal concentration, as a ligand should be of minimal size and free of components that may be difficult to remove, particularly other metals such as sodium. Pertaining to metal oxide precursors, 1,2-dihydroxybenzene (catechol) derivatives display strong affinity toward metal ions and thus form stable metal complexes resistant to hydrolysis20 as well as bonds to metal oxide surfaces.21 As a result, catechol complexes can be expected to exhibit good adhesion and film formation on glass or ceramic substrates when applied by solution deposition. In the present work, we designed metal ion chelating ligands with a photosolvolytic function as a means to develop a new metal complex series that can be directly photopatterned. As depicted in Scheme 2, photoacid-generating catechol ligands bearing a carboxylic acid moiety caged by a photolabile ester that is nearly insoluble in aqueous base but should become soluble upon release of the acid induced by irradiation, 2-nitrobenzyl 3,4dihydroxybenzoate (1a) and 6-nitroveratryl 3,4-dihydroxybenzoate (1b) were synthesized and reacted with metal alkoxide or carboxylate in order to form the respective chelate complexes. Photopatterning is then to be accomplished by a typical photolithography method depicted in Scheme 3 (and Figure S8, Supporting Information), where the irradiated portion of the nitrobenzyl ester complex film is transformed to the carboxylic acid complex that can be selectively removed by dissolving in aqueous base developer. The remaining unexposed or unreacted metal containing ester could then be transformed to the corresponding metal oxide by thermal decomposition. To confirm the photoactivity of the nitrobenzyl group in these compounds, benzyl

3,4-dihydroxybenzoate (1c) and complexes thereof were also synthesized for use as experimental controls. Photodecomposition to the respective 3,4-dihydroxybenzic acid complex was investigated by NMR and Fourier transform infrared (FT-IR) analysis of the complex films upon irradiation.

’ EXPERIMENTAL SECTION Preparation of 6-Nitroveratryl Bromide. Under an N2 atmosphere at room temperature (RT) with external cooling, we added powder 6-nitroveratryl alcohol (23.0 g, 108 mmol) to a stirred solution of PBr3 (30.0 g, 111 mmol) in CH2Cl2 (200 mL) in a 500 mL three-neck flask equipped with a reflux condenser and magnetic stirrer. Stirring was continued at RT for 1 h then at reflux for 2 h. The reaction mixture was then poured into H2O (200 mL). This mixture was neutralized with a 2N aqueous KOH solution (200 mL). The organic phase was then separated, and the aqueous phase was rinsed twice with CH2Cl2 (50 mL). The solution was then dried over MgSO4 (15 g) and the solvent was removed to yield 6-nitroveratryl bromide (28.3 g, 103 mmol, 95%) as slightly yellow needles, mp 114-120 °C and 130-131 °C (dec.). 1H NMR (400 MHz, DMSO-d6) 3.96 (s, 3H), 4.00 (s, 3H), 4.87 (s, 2H), 6.94 (s, 1H), 7.67 (s, 1H). 13C NMR (100 MHz, CDCl3) 30.1 (t), 56.4 (q), 56.5 (q), 108.5 (d), 113.6 (d), 127.4(s), 140.2 (s), 148.9 (s), 153.2 (s). UV-vis (MeCN) λmax 251 nm (log ε 4.21), 346 nm (log ε 3.73). IR (KBr) 3470, 1527, 1329, 1279, 1239, 1065 cm-1. MS (EI) m/z 275, 277 m/z (M)þ. HRMS (EI) m/z found 274.9765, 276.9755, calcd for C9H10NO4Br (M) 274.9793, 276.9773. Preparation of 4-(2-Nitrobenzyloxycarbonyl)catechol (1a). 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU; 15.2 g, 100 mmol) and 2-nitrobenzyl chloride (17.2 g, 100 mmol) were added to a solution of 3,4-dihydroxybenzoic acid (18.5 g, 120 mmol) in N,N-dimethylacetamide (DMA; 200 mL) and then agitated until the solution became homogeneous. The solution was stirred at RT for 3 days before removing DMA at 100 °C, 96 kPa for 1 h. 2-Propanol (270 mL) and H2O (30 mL) were added. The precipitate was filtered off through a membrane filter, and rinsed with 2-propanol (2  50 mL). H2O (200 mL) and 3.6N H2SO4 (100 mL) were added to the filtered solution. The precipitate was isolated by filtration through a membrane filter then dried at 80 °C, 96 kPa for 1 h to 3158

dx.doi.org/10.1021/la104259f |Langmuir 2011, 27, 3157–3165

Langmuir yield 1a (24.1 g, 83.3 mmol, 83%) as a yellow powder, mp 203-211 °C. 1H NMR (400 MHz, DMSO-d6) 5.59 (s, 2H), 6.84 (d, J = 8.0 Hz, 1H), 7.36 (dd, J = 8.0, 2.0 Hz, 1H), 7.39 (d, J = 2.0 Hz, 1H), 7.64 (t, J = 8.0 Hz, 1H), 7.74 (d, J = 8.0 Hz, 1H), 7.81 (t, J = 8.0 Hz, 1H), 8.13 (d, J = 8.0 Hz, 1H), 9.40 (br-s, 1H), 9.90 (br-s, 1H). 13C NMR (100 MHz, DMSO-d6) 62.6 (t), 115.5 (d), 116.3 (d), 119.8 (s), 122.1 (d), 124.9 (d), 129.35 (d), 129.44 (d), 131.7 (s), 134.2 (d), 145.2 (s), 147.6 (s), 150.9 (s), 165.2 (s). UV-vis (MeCN) λmax 259 nm (log ε 4.22), 292 nm (log ε 3.91). IR (KBr) 3495, 3359, 1697, 1610, 1525, 1336, 1304, 1227, 1126, 1007, 763, 728, 640 cm-1. MS (EI) m/z 289 m/z (M)þ. HRMS (EI) m/z found 289.0580, calcd for C14H11NO6 (M) 289.0586.

ARTICLE

Table 1. Photosolvolysis of 2 etched film energy @ 310 nm

exposure

(mJ/cm2)

time, te (s)

2a (nm)

2b (nm)

2c (nm) 35

0

0

5.1

3.1

150

10

21

24

300

20

65

67

450

30

85

111

Preparation of 4-(2-Nitroveratryloxycarbonyl)catechol (1b). The same procedure described for 1a was used except 6-nitro-

600

40

126

202

750

50

175

250

veratryl bromide (27.6 g, 100 mmol) was used instead of nitrobenzyl chloride, which gave 1b (28.4 g, 81.3 mmol, 81%) as a yellow powder, mp 171.5-173.5 °C. 1H NMR (400 MHz, DMSO-d6) 3.88 (s, 3H), 3.90 (s, 3H), 5.53 (s, 2H), 6.83 (d, J = 8.0 Hz, 1H), 7.28 (s, 1H), 7.34 (dd, J = 8.0, 2.0 Hz, 1H), 7.37 (d, J = 2.0 Hz, 1H), 7.71 (s, 1H), 9.60 (brs, 2H). 13C NMR (100 MHz, DMSO-d6) 56.1 (q), 56.2 (q), 63.0 (t), 108.3 (d), 111.9 (d), 115.5 (d), 116.3 (d), 120.0 (s), 122.0 (d), 126.2 (s), 140.0 (s), 145.2 (s), 148.1 (s), 150.8 (s), 153.1 (s), 165.2 (s). UVvis (MeCN) λmax 250 nm (log ε 4.16), 294 nm (log ε 3.92), 344 nm (log ε 3.72). IR (KBr) 3400, 1698, 1603, 1521, 1278, 1219, 1065 cm-1. MS (EI) m/z 349 m/z (M)þ. HRMS (EI) m/z found 349.0784, calcd for C16H15NO8 (M) 349.0798. Preparation of 4-Benzyloxycarbonylcatechol (1c). 3,4-Dihydroxybenzoic acid (30.0 g, 195 mmol) and DBU (22.3 g, 147 mmol) were dissolved in DMA (300 mL). We then ddded benzyl bromide (25.0 g, 147 mmol) and agitated the solution until it became homogeneous. We let the solution stand at RT for 3 days. We then removed DMA at 90 °C, 96 kPa for 1 h. We extracted the product with 1N H2SO4 (400 mL), then twice with H2O (200 mL), then recrystallized it from boiling H2O. The isolated product was then dried at 80 °C, 96 kPa for 1 h to yield 1c (30.9 g, 126.6 mmol, 87%) as colorless needles, mp 146147 °C. 1H NMR (400 MHz, DMSO-d6) 5.26 (s, 2H), 6.82 (d, J = 8.0 Hz, 1H), 7.34-7.45 (m, 7H), 9.60 (br-s, 2H). 13C NMR (100 MHz, DMSO-d6) 65.6 (t), 115.4 (d), 116.3 (d), 120.4 (s), 121.9 (d), 127.9 (d), 128.0 (d), 128.5 (d), 136.5 (s), 145.1 (s), 150.6 (s), 165.5 (s). UVvis (MeCN) λmax 258 nm (log ε 4.05), 293 nm (log ε 3.74). IR (KBr) 3500, 3349, 1689, 1609, 1445, 1300, 1228, 1103, 766 cm-1. MS (EI) m/z 244 m/z (M)þ. HRMS (EI) m/z found 244.0737, calcd for C14H11NO6 (M) 244.0736.

900 1050

60 70

246 261

315 457

1200

80

297

447

1350

90

336

506

1500

100

364a

544a

1800

120

383a

512a

4500

300

375a

505a

40

600

a

444a

13

Preparation of 4-(2-Nitrobenzyloxycarbonyl)catechol Indium Tin Acetate Complex (2a). We dissolved 1a (5.26 g, 18.2 mmol), indium(III) acetate (5.00 g, 17.1 mmol), and tin(II) acetate (0.250 g, 1.06 mmol) in 1-methylpyrolidone (NMP; 40 mL). We then heated the mixture under an inert atmosphere at 130 °C for 1 h before removing volatile components at 120 °C, and then dried it at 130 °C, 96 kPa for 1 h to yield 2a (8.80 g, 17.0 mmol, 94%) as a brown solid. 1H NMR (400 MHz, DMSO-d6) 1.90 (br-s, 3H, OAc), 5.54 (br-s, 2H), 6.58 (br-s, 1H), 7.14 (br-s, 2H), 7.60 (br-s, 1H), 7.75 (br-s, 1H), 7.80 (br-s, 1H), 8.10 (br-d, J = 8.0 Hz, 1H). 13C NMR (100 MHz, DMSO-d6) 22.0 (q, OAc), 61.7 (t), 113.7 (d), 113.9 (d), 119.6 (s), 124.8 (d), 128.9 (d), 129.0 (d), 132.8 (s), 134.1 (d), 142.6 (s), 147.2 (s), 154.0 (s), 161.5 (brs), 166.2 (s), 177.8 (br-s, OAc). UV-vis (MeCN þ 0.05% Et3N) λmax 322 nm (log ε 4.05). IR (KBr) 3466, 1704, 1647, 1588, 1528, 1500, 1432, 1281, 1209, 1125, 1097, 813, 768, 731, 668 cm-1. MS (MALDI Neg.) m/z found 980.1, calcd for C42H31N3O18In 980.1. TGA found 25%, calcd (FW 517.27) 27%. Approximate formula estimated by 1H NMR, assuming no volatilization of metal or 1a, was (1a - 2H)-InSnOAc:1/2NMP [517.27 g/molInSn].

Preparation of 4-(2-Nitroveratryloxycarbonyl)catechol Indium Tin Acetate Complex (2b). The same procedure described for 2a was used except 1b (6.35 g, 18.2 mmol) was used instead of 1a,

9000 a

383

36

10

Indicates complete removal of the film.

which gave 2b (10.64 g, 17.5 mmol, 94%) as a brown solid. 1H NMR (400 MHz, DMSO-d6) 1.90 (br-s, 3H, OAc), 3.84 (s, 6H), 5.48 (s, 2H), 6.46 (br-s, 1H), 7.14 (br-s, 2H), 7.27 (s, 1H), 7.70 (s, 1H). 13C NMR (100 MHz, DMSO-d6) 24.0 (q, OAc), 56.1 (q), 62.1 (t), 108.3 (d), 110.8 (d), 112.5 (d), 113.7 (d), 115.5 (d), 119.5 (s), 127.4 (s), 139.6 (s), 147.8 (s), 153.2 (s), 158.7 (s), 161.0 (s), 166.2 (s), 179.4 (s, OAc). IR (KBr) 3450, 1647, 1584, 1524, 1334, 1281, 1220, 1126, 1094, 1068, 800 cm-1. UV-vis (MeCN þ 0.05% Et3N) λmax 326 nm (log ε 4.17). MS (MALDI Neg.) m/z found 1160.0, calcd for C48H43N3O24In 1160.1. TGA found 24%, calcd (FW 620.49) 22%. Approximate formula estimated by 1H NMR assuming no volatilization of metal or 1b: (1b-2Hþ)InSn-OAc:1NMP [620.49 g/molInSn].

Preparation of 4-Benzyloxycarbonylcatechol Indium Tin Acetate Complex (2c). The same procedure described for 2a was used except 1c (4.42 g, 18.1 mmol) was used instead of 1a, which gave 2c (9.00 g, 17.5 mmol, 96%) as a brown solid. 1H NMR (400 MHz, DMSOd6) 1.90 (br-s, 3H, OAc), 5.22 (br-s, 2H), 6.53 (br-s, 1H), 7.14 (br-s, 2H), 7.29-7.44 (m, 5H). 13C NMR (100 MHz, DMSO-d6) 22.0 (q, OAc), 64.6 (t), 113.7 (d), 114.9 (d), 119.3 (d), 120.5 (s), 127.3 (d), 127.7 (d), 128.4 (d), 137.2 (d), 151.0 (s), 161.0 (br-s), 166.6 (s), 177.8 (br-s, OAc). UV-vis (MeCN þ 0.05% Et3N) λmax 286 nm (log ε 3.86), 321 nm (log ε 4.05). IR (KBr) 3448, 1704, 1643, 1590, 1498, 1429, 1281, 1215, 1122, 1116, 814, 768 cm-1. MS (MALDI Neg.) m/z found 845.5, calcd for C42H34O12In 845.1. TGA found 25%, calcd (FW 515.41) 27%. Approximate formula estimated by 1H NMR, assuming no volatilization of metal or 1c, was (1c - 2Hþ)-InSn-OAc:1NMP [515.41 g/molInSn].

Preparation of 4-(2-Nitrobenzyloxycarbonyl)catechol Titanium Complex (3a). We dissolved 1a (5.79 g, 20.0 mmol) and titanium(IV) isopropoxide (2.84 g, 10.0 mmol) in DMA (20 mL). We then heated the solution under an inert atmosphere at 100 °C for 1 h before removing volatile components at 100 °C, and then dried it at 120 °C, 96 kPa for 1 h to yield 3a: 7.64 g, 9.59 mmol (96%) as a red/ brown solid. 1H NMR (400 MHz, DMSO-d6) 1.95 (s, 3H, DMA), 2.78 (s, 3H, DMA), 2.93 (s, 3H, DMA), 5.57 (s, 2H), 6.30 (d, J = 8.0 Hz, 1H), 6.78 (d, J = 2.0 Hz, 1H), 7.33 (dd, J = 8.0, 2.0 Hz, 1H), 7.64 (t, J = 8.0 Hz, 1H), 7.74 (d, J = 8.0 Hz, 1H), 7.81 (t, J = 8.0 Hz, 1H), 8.11 (d, J = 8.0 Hz, 1H). 13C NMR (100 MHz, DMSO-d6) 21.4 (q, DMA), 34.4 (q, DMA), 37.4 (q, DMA), 62.3 (t), 110.7 (d), 110.8 (d), 119.1 (s), 122.7 (d), 124.8 (d), 129.2 (d), 131.3 (s), 134.1 (d), 147.5 (s), 158.5 (s), 164.7 (s), 165.8 (s) 169.5 (s, DMA). UV-vis (MeCN) λmax 259 nm (log ε 4.55). IR 3159

dx.doi.org/10.1021/la104259f |Langmuir 2011, 27, 3157–3165

Langmuir

ARTICLE

Scheme 4. Composition of Indium Tin Complexes 2a-c

Scheme 5. Composition of Titanium Complexes 3a-c

(KBr) 3462, 1711, 1614, 1526, 1278, 1202, 1114, 1092, 826, 767, 733, 667 cm-1. MS (MALDI Neg.) m/z found 908.6, calcd for C42H27N3O18Ti 909.1. TGA found 12%, calcd (FW 796.59) 10%. Approximate formula estimated by 1H NMR assuming no volatilization of metal or 1a: (1a 2Hþ)2-Ti:2DMA [796.59 g/molTi].

Preparation of 4-(2-Nitroveratryloxycarbonyl)catechol Titanium Complex (3b). The same procedure described for 3a was used except 1b (6.98 g, 20.0 mmol) was used instead of 1a, which gave 3b (8.66 g, 9.42 mmol, 94%) as a red/brown solid. 1H NMR (400 MHz, DMSO-d6) 1.95 (s, 3H, DMA), 2.78 (s, 3H, DMA), 2.94 (s, 3H, DMA), 3.876 (s, 3H), 3.883 (s, 3H), 5.51 (s, 2H), 6.27 (d, J = 8.4 Hz, 1H), 6.73 (d, J = 2.0 Hz, 1H), 7.27 (s, 1H), 7.29 (dd, J = 8.4, 2.0 Hz, 1H), 7.70 (s, 1H). 13C NMR (100 MHz, DMSO-d6) 21.4 (q, DMA), 34.5 (q, DMA), 37.4 (q, DMA), 56.1 (q), 56.2 (q), 62.7 (t), 108.3 (d), 110.7 (d), 110.8 (d), 111.7 (d), 119.3 (d), 122.6 (s), 126.4 (s), 140.0 (s), 148.0 (s), 153.0 (s), 158.4 (s), 164.6 (s), 165.8 (s), 169.6 (s, DMA). UV-vis (MeCN þ 0.05% Et3N) λmax 295 nm (log ε 4.47), 344 nm (log ε 4.36). IR (KBr) 3456, 1711, 1618, 1584, 1524, 1220, 1067, 828, 797, 767, 662 cm-1. MS (MALDI Neg.) m/z found 1089.9, calcd for C48H40N3O24Ti 1090.1. TGA found 11%, calcd (FW 916.65) 9%. Approximate formula estimated by 1H NMR assuming no volatilization of metal or 1b: (1b 2Hþ)2-Ti:2DMA [916.65 g/molTi].

Preparation of 4-Benzyloxycarbonylcatechol Titanium Complex (3c). The same procedure described for 3a was used except 1c (4.88 g, 20.0 mmol) was used instead of 1a, which gave 3c (6.90 g, 9.77 mmol, 98%) as a red/brown solid. 1H NMR (400 MHz, DMSO-d6) 1.95 (s, 3H, DMA), 2.78 (s, 3H, DMA), 2.93 (s, 3H, DMA), 5.25 (s, 2H), 6.27 (d, J = 8.0 Hz, 1H), 6.77 (d, J = 2.0 Hz, 1H), 7.30-7.44 (m, 6H). 13C NMR (100 MHz, DMSO-d6) 21.4 (q, DMA), 34.5 (q, DMA), 37.4 (q, DMA), 65.3 (t), 110.6 (d), 110.8 (d), 119.7 (d), 122.6 (s), 127.7 (d), 127.9 (d), 128.5 (d), 136.8 (s), 158.4 (s), 164.4 (s), 166.1 (s), 169.6 (s, DMA). UV-vis (MeCN) λmax 259 nm (log ε 4.37), 286 nm (log ε 4.35). IR (KBr) 3449, 1709, 1610, 1280, 1281, 1202, 1114, 1089, 827, 767, 669 cm-1. MS (MALDI Neg.) m/z found 773.9, calcd for C42H30O12Ti

774.1. TGA found 16%, calcd (FW 706.55) 11%. Approximate formula estimated by 1H NMR assuming no volatilization of metal or 1c: (1c 2Hþ)2-Ti:2DMA [706.55 g/molTi]. Determination of Photoetching Rate. Indium tin complexes 2a-c (2.0 mmol) were dissolved in 4:1:1 v/v ethyl lactate/γ-butyrolactone/DMA to give a final volume of 5.0 mL, and titanium complexes 3a-c (1.25 mmol) were dissolved in 5.0 mL of 3:1 v/v ethyl lactate/ DMA. All solutions were filtered through a PTFE filter prior to use. The respective solutions (0.5 mL) were then spin-coated onto quartz plates (50  50  2t mm) at 1000 rpm then dried at 100 °C for 1 h on a hot plate. The resulting film was then exposed to parallel beam irradiation from an Hg-Xe lamp (Ushio 250W superhigh pressure mercury lamp USH-250BY through a 75 mm collimator), either with (∼0.02 mW at 310 nm, ∼0.6 mW at 365 nm) or without (∼15 mW at 310 nm, ∼67 mW at 365 nm) a 390 nm sharp cutoff filter (Sigma SCF-50S-39 L, 50%T at 390 nm) for time te before developing in aqueous 0.25 wt % tetramethylammonium hydroxide (TMAH) solution for 30 s. Film thickness of the film before etching was measured by surface profile between the base plate and the film. Amount of film etched, summarized in Tables 1 and 2, was determined by the difference in the UV-vis absorption spectra before and after etching. The absorption areas from 250-320 nm for 2a and 2c, 260-360 nm for 2b, 290-300 nm for 3a, 300-320 nm for 3b or 295-305 nm for 3c were used for the analysis. NMR Analysis of Photoproducts. Complex 2a (2.5 mmol) and 2-methoxyethoxyacetic acid (2.5 mmol) were dissolved in 4:1:1 v/v ethyl lactate/γ-butyrolactone/DMA to give a final volume of 5.0 mL. Four samples were prepared by spin-coating the solution (0.5 mL) onto quartz plates (50  50  2t mm) at 1000 rpm then dried at 100 °C for 1 h on a hot plate. Each film was exposed to irradiation from an Hg-Xe lamp for te = 600 s. Two samples were dissolved in 0.25 wt % solution of TMAH in D2O and the remaining two in DMSO-d6 for which the 1H NMR spectrum of both extracts were recorded. (see Supporting Information) FT-IR Analysis of Photoreaction. The same solutions prepared for the determination of the photoetching rate of 2a and 2c 3160

dx.doi.org/10.1021/la104259f |Langmuir 2011, 27, 3157–3165

Langmuir

ARTICLE

Figure 1. UV-vis absorption of films of 2a (top), 2b (middle), and 2c (bottom) after exposure (left) and after developing (right). (0.2 mL) were spin-coated onto CaF2 discs (20j x 2t mm) at 1000 rpm then dried at 110 °C for 10 min on a hot plate. Two samples were prepared per complex, and each film was exposed to irradiation from an Hg-Xe lamp for te = 600 s. FT-IR spectra were recorded at 60 180, 300, and 600 s intervals. After exposure, the discs were developed in aqueous 0.25 wt % TMAH solution for 30 s, and FT-IR spectra were recorded. Patterning Procedure. Complex films were prepared as above. The resulting film was then exposed to irradiation from an Hg-Xe lamp through a photomask for te = 120 s for 2a,b or te = 600 s for 3a,b, before developing in aqueous 0.25 wt % TMAH solution for 30 s. The patterned films were then baked at 500 °C in air for 10 min, and the resulting ITO films were baked for an additional 10 min at 500 °C in a N2 þ 4% H2 atmosphere. For ITO and titanium oxide samples, the surface profile and laser microscope images was recorded before and after baking, then SEM images and XRD spectrum were recorded. Elemental analysis of all oxide films was performed by XPS. Sheet resistance of ITO samples was measured by the four point method.

’ RESULTS AND DISCUSSION Esters 1a-c were prepared from 3,4-dihydroxybenzoic acid and the corresponding halide using a modified procedure.22 Metal complexes were prepared by heating and removing volatile components from a solution of ester 1 and the respective metal carboxylate or alkoxide as shown in Scheme 4 for indium tin and Scheme 5 for titanium. Substitution of the volatile metal precursor ligands by 1 was confirmed by appropriate upfield shift of benzene ring protons and downfield shift of benzene ring C-O carbon from the free ligand in the NMR spectra accompanied by appropriate reduction of acetate by approximately two-thirds or removal of isopropoxide. Expected broadening of the ligand NMR signals due to dissociation-reformation of In-O bonds was observed for indium tin complexes 2a-c.23 On the basis of 1H NMR spectra of the compositions, the molecular ratio of 1/acetate/ NMP was 1:1:0.5-1.0, suggesting a five or six coordinate species. For titanium complexes 3a-c, expected ligand-metal charge 3161

dx.doi.org/10.1021/la104259f |Langmuir 2011, 27, 3157–3165

Langmuir

ARTICLE

Figure 2. FT-IR of 2a and 2c films during exposure and after developing.

transfer (LMCT) absorption band extending to 500 nm was observed.20 One molecular equivalent of DMA per ligand 1 was observed in the 1H NMR spectra of titanium complexes, suggesting a neutral six-coordinate complex.24 Indium, tin, and titanium complexes are found in 4-6 coordinate mono or multinuclear complexes depending on many variables. As crystallization could not be achieved, there is no evidence to prove the structure of these catecholate complexes, thus Schemes 4 and 5 merely depict molecular ratios. Metal content was also confirmed by the mass reduction ratio in conversion of the complex to metal oxide by TGA, where all residue masses were (2% (except þ3% for 3c) from the estimated values. Assuming conversion to pure oxides, these results suggest negligible volatilization of metal in the pyrolytic conversion process, although inconsistencies in the miniscule samples and impurities remaining in the oxide should be considered. Despite 1:1 and 2:1 molar ratios of ligand to metal atoms in the preparation of 2 and 3 respectively, a peak assignable to a three ligand to one metal atom fragment in the MALDI mass spectrum was observed from all six complexes. Coating solutions were prepared by dissolving 2 or 3 in solvents commonly used in industrial photolithography, which were spin-coated onto quartz substrates. Photosolvolysis experiments were performed by exposing samples to irradiation for time te then developing in aqueous TMAH to remove the reacted portion of the film. The amount of film remaining was determined from the UV-vis absorption, for which results are summarized in Tables 1 and 2 and in the Supporting Information (Figures S9 and S10). The sacrifice of quantum yield for absorbance profile extension to longer wavelengths in NVOC derivatives allows use in the presence of UV-sensitive components.17 Contrary to the slower etching rate expected for NVOC, as shown in Table 1, photosolvolysis of the indium tin complex 2b was etched faster than 2a, although the molar etching rates were similar. The 39 nm-thick ITO film resulting from the 397 nm film of 2a and the 40 nmthick ITO film from the 486 nm film of 2b suggest that both films contain approximately the same atomic amount of material (see Figures S9 and S10). The difference in film thickness of 2a and 2b results from the volume of the methoxy moieties present in NVOC. Titanium oxide films 26 and 30 nm thick were obtained from the respective 324 and 400 nm thick films of 3a and 3b, in a similar trend. As shown in Table 2, titanium complexes behaved as expected where the 3a film was etched faster than the 3b under full exposure from the Hg-Xe lamp, in accord with higher quantum yield reported for NBOC derivatives compared to NVOC derivatives.25 As can be predicted in view of chemical structure, both esters yield similar absorption spectra in free form

Table 2. Photosolvolysis of 3 etched film energy @ 310 nm (mJ/cm2)

exposure time, te (s)

3a (nm)

3b (nm)

3c (nm)

0 900

0 60

0 144

0 84

0 7.5

2700

180

286

222

8.0

4500

300

309

300

9.3

9000

600

328a

392a

19

567

900

7.9

1134

1800

37

65

1701

2700

67

100

2268 2835

3600 4500

128 168

285 400a

3402

5400

240

with filter

@ 365 nm

a

24

Indicates complete removal of the film.

or when in complex, except that the absorption profile of NVOC complexes was extended to 400 nm from 320 nm for titanium and 360 nm for indium tin of the nonsubstituted NBOC complexes. With the sharp cutoff filter in place, the influence of the extended absorbance profile was observed, for which photosolvolysis of 3b occurred slowly, yet more rapidly than 3a. Reduced sensitivity of titanium complexes was most likely due to internal shading by strong absorption of the LMCT band. As apparent in the UV-vis spectra of the exposed NBOC complexes, no significant reduction of the absorption band around 310 nm upon exposure (photobleaching) occurs, thus the etching rates are expected to diminish exponentially, as observed in titanium complex films while only the approximately linear region was considered for indium tin complex films. In contrast, photobleaching occurs to an extent in NVOC complexes where reduction of the absorption band spanning 300-400 nm was observed (see Figure 1 for indium tin films and Figure S3 and S5 for titanium films). The slower photoetching, photobleaching NVOC complexes may prove superior when greater aspect ratios are desirable. To investigate direct patterning of metal complex films by photolithography, samples were irradiated through a custom Lline photomask for the required te to completely remove the film, determined by the photosolvolysis experiment (Tables 1 and 2), 3162

dx.doi.org/10.1021/la104259f |Langmuir 2011, 27, 3157–3165

Langmuir

ARTICLE

Figure 3. Laser microscopic images of ITO from annealing films of 2a (A,B) and 2b (C,D) at 500 °C or TiO2 from films of 3a (E,F) and 3b (G,H) photopatterned using a photomask with positive (A,C,E,G) and negative (B,D,F,H) image L/S 2-100 μm L-lines. The images show the 100x magnified L/S 2-40 μm L-lines with inset images of the 1000x magnified L/S 2 μm L-lines.

then developed in TMAH. Indium tin complexes 2a and 2b resulted in pattern preserved ITO films with sheet resistance of 785 and 499 Ω/0, respectively, and similarly films of titanium complex 3 resulted in pattern preserved TiO2 films after treating at 500 °C. Inspection of micropatterns by laser microscopy (Figure 3) and surface profiling (Figure 4, S15 and S16) showed sharp definition and pattern preservation in transformation to metal oxide by thermal decomposition. Figure 4 shows the profile of the 2a,b films and resulting ITO films L/S 100 μm lines as a representative example. Dimensional comparison between the L/S 100 μm lines of the complex films and their corresponding metal oxide films are summarized in Table 3. Over 10 times contraction was observed in the film perpendicular to the substrate surface with negligible expansion or contraction measured in the parallel plane. Considering that the perpendicular contraction line width measurements were made considerably closer to the base in the oxide samples, expansion in the parallel plane in the titanium samples should be considered a result of a slight taper and disregarded. Metal oxide films were confirmed by XRD and XPS spectra (see Figures S17 and S18 and Table S3 in the Supporting Information), and morphology was observed by SEM images shown in Figure 5. Resultant XRD patterns appropriate for crystalline ITO and anatase TiO2 were observed, although broad due to small particle size. As observed in XPS spectra, all film surfaces were coated by carbon that decreased with etching depth. Carbon in the samples may be the reason for high resistance of the ITO compared to commercially available samples. Lower resistance in the ITO film from 2b may be a result of less residual carbon in the film. In ITO samples from 2a and 2b, both indium and tin content in respective ratios of 10.8 or 9.9 In/Sn was confirmed, which was tin-rich compared to the initial 16.1 In/Sn composition. The indium-to-tin ratio may have changed due to more prominent indium vaporization during synthesis or the baking process, although commercially available ITO-coated glass was found to similarly contain 10.3 In/Sn as opposed to the expected 18.0 In/Sn. SEM images (Figure 5) show the films are composed of 10-20 nm particles with little void area. Adhesion of ITO and TiO2 films were confirmed by a qualitative test where transparent adhesive tape was applied to the oxide films then peeled off. No metal oxide removal from the substrate could be detected upon examination of the substrates and tape from all four samples indicating adequate adhesion. In addition, the ITO

Figure 4. Profile of the photopatterned film of 2a shows the L/S 100 μm L-line before (top) and photopatterned ITO film after (bottom) annealing at 500 °C.

and TiO2 films were found to be harder than 9H according to the JIS-K5400 standardized Pencil Scratch Test. NMR analysis of the components extracted from the irradiated film exhibited signals that could be assigned to the appropriate 3,4-dihydroxybenzic acid complex along with unrecognizable material and appropriate decrease of benzyl proton signal intensity. Signals arising from nitrosobenzaldehyde were not observed, although decomposition of this labile compound would not be unusual. Signals arising from the solvent in the coating solution were negligible showing adequate drying and approximately equivalent ratios of integral regions of 3,4-dihydroxybenzic acid signals and nonvolatile 2-methoxyethoxyacetic 3163

dx.doi.org/10.1021/la104259f |Langmuir 2011, 27, 3157–3165

Langmuir

ARTICLE

Table 3. Dimensional Analysis by Surface Profiling of the L/S 100 μm Lines of the Patterned Films of 2 and 3 Prior to Treatment at 500 °C and the Resulting Patterned ITO and TiO2 Films, Respectivelya before bake (film of 2 or 3)

after bake (ITO or TiO2 film)

complex height (nm) line width (μm) height (nm)

a

line width (μm)

2a

421

103

35

102

2b

518

103

39

100

3a

331

90

24

102

3b

408

95

29

100

Line widths were measured at half height.

atomic level should be realizable, or at least to the optical limits of the photolithography process. With more refined exposure techniques, nanoscale patterning may be possible. Nitrobenzyl compounds were considerably less photosensitive than conventional DNQ/novolac photoresists for which photoreaction typically requires only a few hundred millijoules per square centimeter or less.1,27 This difference in photosensitivity also arises as a result of the photobleaching effect in DNQ that was not observed to a significant extent in the photoreaction here. Although aspect ratios may be limited and exposure times extended when compared to conventional photoacid generators such as in DNQ-based photoresists,27 high photoetching depth control may prove valuable in applications such as 3D patterning or texture formation. In future technical publications we intend to disclose techniques that allow the nitrobenzyl derived complexes to operate with approximately 10 times the photosensitivity reported here and techniques to attain ITO with sheet resistances of less than 30 Ω/0. These ligands may also present interesting surface photofunctionalization applications. Further research on the photosolvolytic reaction, other applicable metals, alternative photoreactive derivatives, and new applications is under investigation.

’ ASSOCIATED CONTENT

bS

Figure 5. SEM images of ITO from annealing patterned films of 2a,b or TiO2 from patterned films of 3a,b at 500 °C.

acid that was included in the solution accounts for most of the complex material and provides evidence that the photouncaging reaction is nearly quantitative in the case of 2a. These results suggest the photoinduced acidity and solubility of these compounds to be due to the generation of the carboxylic acid by the anticipated photocleavage reaction as in Scheme 2. Despite almost unaltered UV-vis absorption profile, changes in absorption profile or lack thereof in the FT-IR spectra of films of 2a and 2c with irradiation present strong evidence that the photoreaction occurring is indeed the nitrobenzyl system because the only difference between 2a and 2c is the nitro moiety. As made clear in Figure 2, distinct change upon irradiation of the 2a film occurs after which the film becomes soluble in TMAH where the film was removed after developing. In contrast, the 2c film IR absorption profile and solubility was unaffected by irradiation or treatment in TMAH.

’ CONCLUSIONS Direct patterning of ITO and TiO2 films was accomplished using a new o-nitrobenzyl functionalized complex. In the complexes of 1, the generated photoacid species is bonded directly to the material to be patterned, and thus problems arising from acid diffusion may be minimized allowing for potentially higher resolution compared to current chemically amplified photolithographic processes.26 In principle, as the contrast-functional moiety is covalently bonded to the metal complex, resolution down to the

Supporting Information. NMR signal assignments, light source output characteristics, XPS data, TGA curves of the complexes, UV-vis spectra attained in the photosolvolysis experiments of the titanium samples and graphs summarizing photosolvolysis and photoetching characteristics, illustration of the photopatterning process, NMR spectra of the irradiated film extracts, microscope images of the patterned complex films prior to annealing, surface profile of the titanium samples, XRD spectra, and images of the photomask used in patterning experiments are available. This information is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel. þ81-568-47-5390. Fax þ81-568-47-5365. E-mail: chris. [email protected]; [email protected].

’ ACKNOWLEDGMENT The authors wish to express gratitude to Professor Donald A. Tryk of the University of Yamanashi for advice and critical revision of the manuscript. ’ REFERENCES (1) (a) Thompson, L. F.; Willson, C. G.; Bowden, M. J. Introduction to Microlithography, 2nd ed.; Oxford University Press: New York, 1994. (b) Willson, C. G.; Dammel, R. A.; Reiser, A. Proc. SPIE 1997, 3051, 28– 41.(c) Deforest, W. S. Photoresist: Materials and Processes; McGraw-Hill: New York, 1975. (d) Fluga, F.; Nicolau, D. V. Microelectron. Eng. 2009, 86, 783–786. (2) (a) Ohta, H.; Orita, M.; Hirano, M.; Tanji, H.; Kawazoe, H.; Hosono, H. Appl. Phys. Lett. 2000, 76, 2740–2742. (b) Chopra, K. L.; Major, S.; Pandya, D. K. Thin Solid Films 1983, 102, 1–46. (3) Chen, S.-Y.; Mo, T.-C.; Lin, S.-H. Ferroelectrics 2001, 263, 247–252. (4) Milliron, D. L.; Raoux, S.; Shelby, R. M.; Jordan-Sweet, J. Nat. Mater. 2007, 6, 352–356. (5) Park, Y. K.; Ahn, Y. S.; Lee, K. H.; Cho, C. H.; Chung, T. Y.; Kim, K. J. Semicond. Technol. Sci. 2003, 3, 76–82. 3164

dx.doi.org/10.1021/la104259f |Langmuir 2011, 27, 3157–3165

Langmuir

ARTICLE

(6) (a) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (b) Fujishima, A.; Rao, T. N.; Tyrk, D. A. J. Photochem. Photobiol. C: Rev. 2000, 1, 1–21. (7) (a) Kay, A.; Gratzel, M. Sol. Energy Mater. Sol. Cells 1996, 44, 99– 117. (b) Arango, A. C.; Johnson, L. R.; Carter, S. A.; Horhold, H. H. Adv. Mater. 2000, 12, 1689–1692. (8) (a) Gallagher, D.; Scanlan, F.; Houriet, R.; Mathieu, H. J.; Ring, T. A. J. Mater. Res. 1993, 8, 3135–3144. (b) Schwartz, R. W. Chem. Mater. 1997, 9, 2325–2340. (9) Bauerle, D. Laser Processing and Chemistry, 2nd ed.; Springer Verlag: Berlin, 1996. (10) Li, Y.; Li, C.; He, D.; Li, J. J. Phys. D: Appl. Phys. 2009, 42, 105303. (11) (a) Bae, S.-W.; Park, H.-H.; Kim, T.-S. Sens. Actuators, A: Phys. 2006, 125, 548–552. (b) Hong, C.-S.; Park, H.-H.; Wang, S.-J.; Moon, J.; Hill, R. H. Appl. Surf. Sci. 2006, 252, 7739–7742.(c) Takahashi, M.; Ogata, T.; Cordonier, C. E. J.; Nakamura, A.; Shichi, T.; Uematsu, T. International Patent Application WO2009139421, 2009. (12) (a) Zhang, X.; Hill, R. H. J. Photopolym. Sci. Technol. 2006, 19, 477–486. (b) Bravo-Vasquez, J. P.; Hill, R. H. Polym. Mater. Sci. Eng. 1999, 81, 16–17. (13) Patchornik, A.; Amit, B.; Woodward, R. B. J. Am. Chem. Soc. 1970, 92, 6333–6335. (14) (a) Fodor, S. P. A.; Read, J. L.; Pirrung, M. C.; Styrer, L.; Lu, A. T.; Solas, D. Science 1991, 251, 767–773. (b) Beier, M.; Hoheisel, J. D. Nucleic Acids Res. 2000, 28, e11. (15) Bardecker, J. A.; Afzali, A.; Tulevski, G. S.; Graham, T.; Hannon, J. B.; Jen, A. K.-Y. J. Am. Chem. Soc. 2008, 130, 7226–7227. (16) (a) Ahmad, S. A. A.; Wong, L. S.; ul-Haq, E.; Hobbs, J. K.; Leggett, G. J.; Micklefield, J. J. Am. Chem. Soc. 2009, 131, 1513–1522. (b) Campo, A.; Boos, D.; Spiess, H. W.; Jonas, U. Angew. Chem., Int. Ed. Engl. 2005, 44, 4707–4712. (c) Vossmeyer, T.; DeIonno, E.; Heath, J. R. Angew. Chem., Int. Ed. Engl. 1997, 36, 1080–1083. (17) (a) Bochet, C. G. J. Chem. Soc., Perkin Trans. 1 2002, 125–142. (b) Pelliccioli, A. P.; Wirz, J. Photochem. Photobiol. Sci. 2002, 1, 441–458. (18) McGall, G. H.; Barone, A. D.; Digglemann, M.; Fodor, S. P. A.; Gentalen, E.; Ngo, N. J. Am. Chem. Soc. 1997, 119, 5081–5090. (19) Buler, S.; Lagoja, I.; Giegrich, H.; Stengele, K.-P.; Pfleiderer, W. Helv. Chim. Acta 2004, 87, 620–659. (20) Creutz, C.; Chou, M. H. Inorg. Chem. 2008, 47, 3509–3514. (21) Li, S.-C.; Diebold, U. Proc. SPIE 2009, 7396, 1–7. (22) Ono, N.; Yamada, T.; Saito, T.; Tanaka, K.; Kaji, A. Bull. Chem. Soc. Jpn. 1978, 51, 2401–2404. (23) Annan, T. A.; Tuck, D. G. Can. J. Chem. 1988, 66, 2935–2940. (24) Gigant, K.; Rammal, A.; Henry, M. J. Am. Chem. Soc. 2001, 123, 11632–11637. (25) Cameron, J. F.; Frechet., J. M. J. J. Am. Chem. Soc. 1991, 113, 4303–4313. (26) Ito, T.; Terao, A.; Inao, Y.; Yamaguchi, T.; Mizutani, N. Proc. SPIE 2007, 6519, 65190J. (27) (a) Nakayama, T.; Ueda, M. J. Mater. Chem. 1999, 9, 697–702. (b) Dammel, R. A. Diazonaphthoquinone-Based Resists, SPIE tutorial text TT11; SPIE Press: Bellingham, WA, 1991. (c) Suess, O. Liebigs Ann. Chem. 1944, 556, 65–66.

3165

dx.doi.org/10.1021/la104259f |Langmuir 2011, 27, 3157–3165