Article pubs.acs.org/Langmuir
Metallic Film Formation Using Direct Micropatterning with Photoreactive 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 S Supporting Information *
ABSTRACT: Palladium, cobalt, and nickel in complex with photoacid-generating ligands, 4-(2-nitrobenzyloxycarbonyl)catechol and 4-(6-nitroveratryloxycarbonyl)catechol, were prepared in solution. Films formed from the metal complex solutions perform as positive-tone, directly photopatternable palladium, cobalt, nickel oxide, or composite 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/metal oxide film. The photodynamics of photoinduced solubility and direct micropatterning of palladium, cobalt, nickel, and palladium/nickel oxide composite films were investigated. Employing palladium as the initiator for autocatalytic chemical plating, selective direct copper plating on palladium film on polyethylene naphthalate and palladium/ nickel oxide composite film on glass was accomplished.
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INTRODUCTION Ongoing development of technology, particularly pertaining to electronics-related fields, continually demands miniaturization, increased density, more complex functionality, and improved reliability. At the same time, fabrication cost and the environmental impact of employed processes must be driven down. With regard to circuit formation for semiconductors and packaging, some trends in fabrication that reflect this situation are, for example, development of through silicon vias and copper post connection1 for stacked chip devices2 and threedimensional large-scale integration,3 transition of subtractive aluminum to damascene4,5 at the wafer level, and development of build-up construction, multilayer substrates, and subtractive to semiadditive methods for patterning at the substrate level. In order to implement frontier construction techniques, development of direct patterning and fully additive metallization technology provides a powerful tool that can satisfy future demands and prerequisites. At the substrate level, circuitry is often electroplated and patterned using a subtractive or semiadditive method where the conductor pattern is transferred via photoresist, typically with a line and space (L/S) width of 5 μm or more. With further miniaturization of L/S to 2 μm or less an aspect ratio of 1:1 will only require 2 μm of plated film, in which case electroless plating becomes a feasible metallization method. Simultaneously, electroless plating circumvents the challenge of forming electrically isolated structures in fully additive construction. Alternately, were circuit formation to be accomplished by electroless plating, a direct-patterning technique, not relying on etching or resist stripping, should prove most efficient. For these reasons, fully © 2012 American Chemical Society
additive direct-pattern metallization and electroless platting are complementary. In contrast, while under cut limits resolution in subtractive and semiadditive methods, overgrowth is apprehensive in fully additive methods. However, anisotropic deposition techniques have been developed to curb shortcomings due to overgrowth.6 Another potential application for fine pattern metallization is in optical electronic devices as less than 2 μm thick wiring is almost invisible to the naked eye and therefore appropriate for replacing electrodes formed of transparent conducting oxide, like indium tin oxide (ITO). o-Nitrobenzyl ester photocleavage has been manipulated to achieve patterning or selective functionalization.7 Solubility alteration via photocleavage has been used to pattern polythiophene by inducing aggregation8 or to pattern metal complexes by adjusting acidity. In a previous account, direct patterning of ITO and titanium dioxide films on glass using respective indium−tin or titanium chelate complexes of 2nitrobenzyloxycarbonyl (NBOC) and 6-nitroveratryloxycarbonyl (NVOC) derivatives 1a and 1b, respectively (Scheme 1), has been accomplished in a photolithographic process.9 The patterning mechanism has been shown to occur due to photogeneration of a benzoic acid bearing compound10 such that the ultraviolet (UV) irradiation exposed film becomes soluble in aqueous base (Scheme 1) to give a positive tone pattern relative to a photomask. More recently, patterning of NBOC and NVOC complexes of cobalt and palladium was Received: June 15, 2012 Revised: August 12, 2012 Published: August 14, 2012 13542
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Scheme 1. Direct Photopatterning Procedure of Metal/Metal Oxide Using the Dual Functionality of NBOC Catechol Compounds, Application to Electroless Plating, and Ligands Applied in Photopatterning of Metal Complex Films
Figure 1. Photosolvolysis of nitrobenzyl 3,4-dihydroxybenzoate palladium, cobalt, and nickel complexes relative to the respective palladium, cobalt, or nickel oxide.
accomplished using the same photoreaction and employing a similar photolithographic method. With use of these complexes, the respective patterned metal films were attained from an in surface plane dimensionally invariant thermal transformation of the photopatterned metal complex films. Furthermore, palladium-containing films performed as an initiating catalyst for autocatalytic chemically reduced copper deposition (electroless copper plating). The overall process is outlined in Scheme 1. In this manner, metallic structures could be constructed in a fully additive fashion using direct patterning. In this study cobalt, palladium, and copper-plated palladium patterned films were formed on glass and polyethylene naphthalate (PEN) film substrates. Additionally, a similar strategy as reported for indium tin to increase the photoefficiency of the palladium solution by replacing the insoluble photoreactive ester in part with more soluble ethyl 3,4-dihydroxybenzoate (1c) as a ligand10 was implemented, for which the patterning chemistry was investigated. Other direct patterning techniques for metallization have been reported. For example, site-selective plating has been accomplished on microcontact printed palladium colloid,11−13 palladium composite photoresist,14 and palladium adsorbed into selectively photomodified resin.6
In contrast, the strategy reported here involves a photopatterned cobalt or palladium film formed by thermal transformation from a photoreactive cobalt or palladium compound. In addition to the process, compatibility with substrates having appropriate physical, electrical, and chemical characteristics is equally important. Attractive substrates for electronic devices include inexpensive polyethylene terephthalate (PET), polyethylene terenaphthalate (PEN), cyclo-olefin polymer (COP) film, and glass.15−17 Although patterned cobalt and palladium films could be formed on glass, adhesion was poor and films blistered and peeled during electroless copper plating. To overcome the adhesion problems inherent to plating on glass, the following strategy was also investigated: Pattern a film from a mixed palladium and nickel photoreactive NBOC solution and then anneal the film to give a palladium-containing nickel oxide film fused onto the glass substrate. Selective plating on patterned palladium on PEN was found adhesive enough to endure plating. Whether on glass or resin, the substrate−metal film interface is smooth, as substrates were not roughened like in conventional methods. Thus, this technique may also prove 13543
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Table 1. Photosolvolysis Parameters of Nitrobenzyl 3,4-Dihydroxybenzoate Metal Complex Films ligand
metal
1a 1b 1c 1a + 1c 1b + 1c 1a 1b 1a
Pd Pd Pd Pd Pd Co Co Ni
metal film removal rate (nm/s)
patterned complex film (nm)
patterned metal film (nm)
te for pattern (s)
remaining film when te = 0 s (%)
monitored absorbance range (nm)
0.0155 0.0132
499 736
27 32
2400 2400
0.0105 0.0405 0.0528 0.0524 0.1018
482 502 305 362 222
30 38 37 35 12
3000 1200 1200 600 100
100 98.5 32.7 99.5 99.7 93.0 97.8 97.2
300−320 300−320 300−320 320−340 320−340 310−320 280−300 250−320
Figure 2. Laser microscope images of the patterned 1b + 1c palladium films (A and B), 1b cobalt films (C and D), and resultant palladium films (E and F) and cobalt films (G and H) photopatterned using a photomask with L/S 2−100 μm L lines (A, C, E, and G) and a L/S = 1.5/10 μm mesh pattern (B, D, F, and H). 1.25 mmol) in 9:9:2 ethyl lactate/acetone/water to give a final volume of 5.0 mL. Cobalt solutions were prepared by dissolving ligand, 1a or 1b (1.25 mmol), and cobalt(II) acetate tetrahydrate (311 mg, 1.25 mmol) in 2-methoxyethanol to give a final volume of 5.0 mL. Nickel solution was prepared by dissolving ligand, 1a (1.25 mmol), and nickel(II) acetylacetonate (321 mg, 1.25 mmol) in 9:9:2 ethyl lactate/ γ-butyrolactone/N,N′-dimethylacetamide to give a final volume of 5.0 mL. All solutions were filtered through a PTFE membrane filter prior to use. In palladium solutions where both 1a + c and 1b + c were included, a 1:1 molar ratio was used to give a combined ligand amount of 1.25 mmol. The mixed palladium and nickel solution was prepared by combining 1 mL of the 1a palladium solution with 4 mL of the 1a nickel solution. Determination of Photoetching Rate. Respective solutions (0.5 mL) were spin coated onto quartz plates (50 × 50 × 1t mm) at 1000 rpm and then dried at 100−120 °C for 1 h on a hot plate. Resulting films were then exposed to irradiation from the Hg−Xe lamp for time te before developing in aqueous 0.25 wt % tetramethylammonium hydroxide (TMAH) solution for 30 s. The thickness of each of the unexposed films was measured from the surface profile between the base plate and the film surface. Listed values are the average of three measurements. The amount of film etched (summarized in Figure 1) was determined by the difference in the UV−vis absorption spectra (Supporting Information, Figures S2−S9) before and after etching, monitoring the area specified in Table 1. Estimation of the etching rates in Table 1 was done by linear approximation of the film removal data in terms of resulting metal or metal oxide. Patterning Procedure. Metal complex films were prepared as above but on Tempax glass (50 × 50 × 0.7t mm) or PEN film (50 × 50 mm). Resulting films were then exposed to irradiation from the Hg−Xe lamp through a photomask for the te specified in Table 1
useful for high-frequency applications because transmission loss due to the skin effect may be minimized.
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EXPERIMENTAL SECTION
2-Nitrobenzyl 3,4-dihydroxybenzoate (1a) and 6-nitroveratryl 3,4dihydroxybenzoate (1b) were prepared as previously reported.9 Ethyl 3,4-dihydroxybenzoate (1c) and other reagents were used as purchased. UV−vis spectroscopy was performed on a Hitachi U3310 spectrometer. Fused quartz glass and Tempax glass were cleaned by reactive (oxygen) ion etching with a Samco RIE-10R prior to coating using a Mikasa 1H-D7 spin coater. PEN film (50 μm Teonex Q65F) was coated as received from Teijin DuPont Films Japan Ltd. The light source used for the photoreactions was parallel beam irradiation from an Ushio 250W super-high-pressure mercury−xenon lamp USH-250BY through a 75 mm collimator: 0.626 mW at 245 nm, 15 mW at 310 nm, and 67 mW at 365 nm (Supporting Information, Figure S1). Microscopy was performed with a Keyence VK-9710 laser scanning microscope. Film thickness and profile were measured on a Veeco Dektak 150 stylus profiler. Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 operated at an acceleration voltage of 0.8 kV. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2100 operated at an acceleration voltage of 200 kV. X-ray diffraction (XRD) patterns were measured on a Bruker D8 Discover system by detector scan (ω = 0.2°) using monochromated Cu Kα radiation. Sheet resistance of metal films was measured on a NPS Σ5 with a KS-TC-40-EP-VR 4-point probe by the 4-point method. Preparation of Photoreactive Solutions. Palladium solutions were prepared by dissolving ligand, 1a−c (1.25 mmol), 2methoxyethoxyacetic acid (168 mg, 1.25 mmol), 2-aminoethane thiol (34.7 mg, 0.45 mmol), and palladium(II) acetate (281 mg, 13544
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Figure 3. Laser microscope images of electroless copper (∼85 nm thickness) on the patterned NiO−Pd film from 1a palladium−nickel solution photopatterned using a photomask with positive (A and B) and negative (C and D) image L/S 2−100 μm L lines. Images show the 100× magnified L/S 2−40 μm L lines (A and C) and 1000× magnified L/S 2 and 4 μm L lines (B and D).
Figure 4. TEM images and element mapping of the cross-section of copper on patterned palladium composite nickel oxide film on glass. patterned palladium film formed on PEN film, the 1b + 1c palladium solution prepared above was diluted with an equal volume of 1:1 ethyl lactate/acetone before coating the substrate and the exposure time was set at te = 600 s. The overall procedure is illustrated in Scheme 1. Thermal Treatment. Palladium films prepared were heated to 170 °C on a hot plate for 1 h. Cobalt films prepared were heated to 300 °C
before developing in aqueous 0.25 wt % TMAH solution for 30 s. The surface profile and laser microscope images of the patterned samples were recorded (Figure 2 and Supporting Information, Figures S10− S13). A film formed from a 4:1 mixture of the 1a nickel solution and 1a palladium solution on Tempax glass was patterned under the same conditions except with an exposure time of te = 300 s. For the 13545
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on a hot plate for 1 h. Nickel and nickel−palladium films were heated to 500 °C at a rate of 10 °C/min and then baked at 500 °C in air for 1 h in an electric furnace. Laser microscope images (Figure 2 and Supporting Information, Figures S10−S13), surface profiles (Supporting Information, Figure S17), SEM images (Supporting Information, Figures S19 and S20), and XRD spectra (Supporting Information, Figure S21) of the resulting metal or metal oxide films were recorded. Film thicknesses determined from the average of three surface profile measurements are listed in Table 1. Plating Procedure. Initially, palladium-containing films were submersed in 10% sulfuric acid for 2 min at 45 °C and then in an aqueous solution of sodium hypophosphite monohydrate (30 g/L) for 2 min at 45 °C to reduce ionic palladium remaining from the complex or from over oxidation. Films were then plated by submersion in 1 L of an aqueous solution containing copper(II) sulfate pentahydrate (10 g, 20 mmol), disodium N,N,N′,N′-ethylenediaminetetraacetate dihydrate (30 g, 40 mmol), 37% formalin (10 mL), sodium hydroxide (10 g), polyethylene glycol 200 (100 ppm), and 2,2′-bipyridyl (10 ppm) and adjusted to a pH of 12 with sodium hydroxide for 5 min at 40 °C with rapid aeration. Finally, plated samples were dried at 150 °C for 30 min. Laser microscope images (Figure 3 and Supporting Information, Figure S24), surface profiles, and XRD spectrum (Supporting Information, Figure S22) of the deposited copper films were recorded. The cross-section of a copper on palladium composite nickel oxide film on glass sample that was treated in the electroless copper solution for 2 min was observed by TEM (Figure 4 and Supporting Information, Figure S23).
film remained in this range; therefore, in order to completely remove the film, the full 18 J/cm2 was necessary for patterning. The photoremoval rate for the 1a cobalt films was approximated from data excluding this region. As the 1b cobalt films could be completely removed with 9 J/cm2, the 1b solution was used as a representative patterning example. Patterned complex films of palladium, cobalt, and nickel were thermally transformed to the respective metal/metal oxide with dimensional preservation in the surface plane, within the resolution limits of the laser microscope (Figure 2 and Supporting Information, Figures S10−S13) and stylus profiler. Rough pattern preservation was also observed in the profile of the 100 μm lines (Supporting Information, Figure S17). SEM images of the palladium (Supporting Information, Figure S19) and cobalt (Supporting Information, Figure S20) films showed a grain size of approximately 20 nm for all samples. Diffraction patterns for the 110, 200, and 220 palladium phases and 111, 200, 220, 311, and 222 nickel oxide phases are apparent in the XRD spectra (Supporting Information, Figure S21) of the annealed palladium and nickel complex films, respectively. Probably due to a large amount of amorphous material, mixed metal and oxide material, and film thinness, XRD spectra of cobalt films were not attainable. With careful control of the thermal treatment, metal oxide formation could be minimized. Even under the best conditions, the electrical resistance of the palladium and cobalt films formed was on the kΩ to MΩ order and inconsistent. Next, application of patterned palladium as a catalyst for electroless plating was investigated. For this investigation a typical EDTA/formalin electroless copper plating solution was employed, although there are a number of electroless plating solutions, for example, hypophosphite reduced nickel or copper−nickel, that could be applied here.18,19 In usual plating processes, catalytic palladium is adsorbed onto a pretreated substrate and then exposed to a plating solution. Pattern formation to create circuits or other functional structures is then accomplished using photolithography and etching in a subtractive or semiadditive method. Utilizing the photopatterned palladium films, patterned films were directly exposed to a plating solution to give electroconductive metal films selectively in regions on the substrate where palladium exists. However, adhesion of palladium and cobalt to the glass substrates was weak. Palladium films could be wiped off the glass substrates with tissue and upon prolonged exposure to the electroless EDTA copper plating solution metal stripped away from the glass before complete coverage could be attained. Nickel oxide, on the other hand, can fuse to and diffuse into glass surfaces (Supporting Information, Figure S23), which resulted in stronger adhesion to the glass substrate. In order to attain plated films on glass, the palladium and nickel solutions were combined. Thermal treatment of the patterned films from the mixed solution complex films resulted in 40 nm thick films of palladium fixed in a nickel oxide matrix with stronger adhesion to the glass substrate. Nickel oxide could be slightly dissolved in 10% sulfuric acid to expose the palladium, which displayed sufficient catalytic activity in the electroless plating solution. Exposure of the patterned palladium composite nickel oxide films to the electroless plating solution resulted in uniform and bright copper deposition selectively on the patterned film. Copper coverage occurred over all areas: from large areas to fine 2 μm thick L line and 2 μm2 bump areas of the patterned films. In this examination an immersion time of ∼5 min at 40 °C resulted in ∼90 nm thick copper plating. As
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RESULTS AND DISCUSSION Similar to the indium tin and titanium complexes of 1a or 1b, cobalt, nickel, and palladium complex films also exhibited photosolvolytic character and likewise patterning was also accomplished. In accord with the previous finding, photosensitivity of the 1b + 1c palladium film increased compared to the 1b palladium film (Figure 1). Despite the solubility of the 1c palladium film, the 1a + 1c palladium film was less photosensitive than the 1a palladium film. As apparent in the overlaid absorption spectra of 1a, 1a + 1c, and 1c palladium films and 1b, 1b + 1c, and 1c palladium films (Supporting Information, Figure S18), there is strong overlap in absorption of the 1a and 1c palladium compositions in the 280−350 nm region. This is the incident wavelength range in which the NBOC photoreaction is understood to occur. Strong overlap in this range resulted in decreased total quantum efficiency for the NBOC photoreaction due to reduced incident light on the 1a compound from competitive absorption by the nonphotoreactive 1c compound. When the photoreactive component was 1b, the excitation wavelength range is extended to 400 nm and therefore the detrimental effect of competitive absorption by the 1c compound is reduced. A complementary finding has been made for photoreaction of 1a and 1b titanium films, where when films were irradiated through a 390 nm sharp cutoff filter the photoinduced solubility of 1a titanium films was more strongly inhibited than 1b titanium films.9 Palladium complexes have relatively strong absorbance in the UVA and UVB ranges followed by the cobalt and then nickel complexes. It is very probable that the difference in the rate of photoinduced solubility between the metals investigated so far is largely influenced by the degree of overlap of the NBOC/ NVOC excitation range and the ligand to metal charge transfer (LMCT) absorption band arising from chelation. The 1a cobalt films exhibited an unusual jump in photoinduced solubility with 1.2−2.1 J/cm2 (at λ = 310 nm) of irradiation (Figure 1) that is yet unclear. This phenomenon was consistently reproduced, although approximately 1% of the 13546
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plating thickness was kept at a minimum, pattern dimensions were not noticeably altered as apparent in the laser microscope images in Figure 3. The XRD spectrum, appropriate for the copper deposit, is shown in Supporting Information, Figure S22. The TEM cross-sectional image in Figure 4 of a sample plated for 2 min shows 21 nm of copper on a 41 nm of palladium composite nickel oxide. The nanostructure was not clear but appears to be composed of ∼10 nm particles. Comparison to the TEM observation of the palladium composite nickel oxide film prior to plating (Supporting Information, Figure S23) shows that the intermediate layer is actually composed of ∼30 nm NiO−Pd and ∼10 nm nickel diffused into glass. Prior to plating the film appears to be composed of spaced 20−30 nm particles, suggesting that interparticle space was filled during plating. Nickel diffusion into glass at the palladium composite nickel oxide−glass interface and penetration of the plated copper into the nickel oxide layer at the copper−nickel oxide interface may be the mechanism responsible for increased adhesion. In contrast to the palladium films, the nickel oxide, palladium composite nickel oxide, and copper-plated composite films were not easily scratched and no material was removed in a Scotch tape test. In the case of the palladium−nickel oxide composite, the pyrolyzed film was significantly thicker than that of the nickel oxide film. This may be a result of increased solution viscosity due to the influence of palladium. Viscosity increase in addition to the competitive palladium complex absorption explain the necessity for extended te of 600 s to completely remove the film compared to that of te = 100 s for the 1a nickel complex. Both the Cu/NiO−Pd and NiO−Pd/glass interfaces appear to be very smooth, which is beneficial for use in high-frequency signal conducting circuits or antennas. However, the surface roughness should be increased to 10−30 nm in order to further improve adhesion. Although adhesion of palladium films to glass was insufficient, adhesion to PEN film after thermal treatment at 170 °C was found to be significantly stronger. Mesh patterned palladium films from 1b + 1c palladium solution on PEN were similarly plated for which laser microscope images are shown in Supporting Information Figure S24. The electrical resistance of the copper-plated films formed on glass was in the range of 0.5−4 Ω/□ depending on copper thickness (under 100 nm) and 1−10 Ω/□ for the copper mesh pattern on PEN. The adhesion mechanism of metal to the PEN substrate has not been investigated, but it may result from adsorption of the organic component of the metal complex into the softened PEN film upon thermal transformation at 170 °C.
on the nanometer scale should be possible using the 1a and 1b complexes. Substrates of potential value for next-generation electronic devices, PEN, and glass are yet to be applied in main-stream electronics because they are considered difficult to plate or to plate a conductive film with sufficient adhesion onto. Pretreatment with the photopatternable palladium compounds provided a technique for selective plating of these materials. While reasonable film adhesion of plated copper on PEN was attained for the palladium complexes by heating to near the melting point of PEN, adhesion on glass required an alternative innovation. By devising a technique of firing a palladiuminclusive nickel oxide film onto glass, sufficient adhesion was achieved and at the same time reducing the consumed amount of palladium and enhancing the photoefficiency. In this technique of transforming metal complexes to metal film, conductive material is directly deposited on a substrate and can therefore conduct electricity through the interface or apply a potential directly to the substrate surface, which is favorable under some circumstances, like in photovoltaics and thin film transistor devices. Metal structures were formed on the unaltered substrate surfaces; therefore, metal−substrate interfaces were approximately as smooth as the initial substrate. Because of the smooth interfaces, this technique may prove valuable for fabrication of high-frequency devices.
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ASSOCIATED CONTENT
S Supporting Information *
Emission spectrum of the Hg−Xe light source used for patterning, UV−vis spectra attained in photosolvolysis experiments of the metal complex films, laser microscope images of patterned Pd, Co, NiO−Pd, and plated Pd or NiO−Pd films, images of the photomask used in patterning experiments, surface profiles, SEM images of Pd and Co films, XRD patterns from Pd, NiO, and plated Cu films, and TEM observation and EDX elemental mapping figures of the NiO−Pd composite film cross-section. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +81-568-47-5390. Fax: +81-568-47-5365. E-mail:
[email protected];
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors express gratitude to Kiyokawa Mekki Kougyou Co. Ltd. for acquisition of TEM data and elemental analysis. This work was supported in part by a MEXT-funded program.
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CONCLUSIONS Photoinduced solubility of the palladium, cobalt, and nickel solutions with photoacid-releasing chelating compounds 1a and 1b was investigated. Similar to previously reported indium tin and titanium photocomplexes, palladium, cobalt, and nickel films were photopatterned. Of the metals reported here, the intensity of the LMCT adsorption also appeared to have the greatest influence on the photoreaction rate. The stability of the planar pattern dimensions in the transformation from complex to metal or metal oxide and less than 40 nm in film thinness enable high resolution and tight pitch of plated metal structures. The nitrobenzyl ester photocleavage reaction does not generate mobile radicals or other components that affect the solubility of surrounding film material; therefore, patterning
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