Electroless Deposition of NiB on 15 Inch Glass ... - ACS Publications

Langmuir 2003, 19, 5923-5935

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Electroless Deposition of NiB on 15 Inch Glass Substrates for the Fabrication of Transistor Gates for Liquid Crystal Displays Emmanuel Delamarche,*,† Matthias Geissler,† James Vichiconti,‡ William S. Graham,‡ Paul A. Andry,‡ John C. Flake,‡,§ Peter M. Fryer,‡ Ronald W. Nunes,‡ Bruno Michel,† Eugene J. O’Sullivan,| Heinz Schmid,† Heiko Wolf,† and Robert L. Wisnieff‡ Science and Technology Department, IBM Research, Zurich Research Laboratory, 8803 Ru¨ schlikon, Switzerland, and Advanced Display Technology Laboratory and Magnetoelectronics Department, IBM Research, T. J. Watson Research Center, Yorktown Heights, New York 10598 Received January 31, 2003. In Final Form: April 9, 2003 The thin-film transistor (TFT) array of liquid-crystal displays (LCDs) comprises a number of metallic, semiconducting, and insulating layers, which need to be deposited and patterned accurately with very high yields on a (large) glass substrate. We are exploring how to fabricate the gate metal lines of the TFT array in an entirely new and potentially cost-effective waysby depositing the metal layer of the TFT array using electroless deposition (ELD) and by patterning the gates using microcontact printing (µCP). To achieve this goal, we separately explore first the plating conditions to deposit a gate metal on 15 in. glass substrates, and second the printing process to finally combine them later in the work. Here, we review in depth the metallization of the glass by ELD of NiB as gate material, and we demonstrate the patterning of the gate layer using a conventional photoengraving process (PEP, i.e., photolithography and wet etching). We selected NiB because this material can fulfill the conductivity requirements for making an SXGA (1280 pixels × 1020 pixels) display having a 157 pixel per inch resolution. Because ELD requires the presence of a catalyst on the substrate, we derivatized the glass by grafting 3-(2-aminoethylamino)propyltrimethoxysilane (EDA-Si) from an aqueous solution, which serves as linker between the glass and colloidal Pd/Sn particles. We identify the optimum conditions for the derivatization of the glass and to activate it with colloidal Pd/Sn in a uniform manner so as to electroless deposit high-quality NiB layers. We plated uniform NiB films of 120 nm thickness on both faces of 15 in. glass substrates, and we removed the NiB from one face of the substrate using HNO3 dissolved in water. The remaining NiB layer was patterned using a mask of photoresist and an etch bath comprising an aqueous solution of 3-nitrobenzenesulfonic acid (NBSA) and ethylenediamine (EDA) at pH ∼ 9. This etch system minimizes the galvanic coupling between the Pd/Sn particles and the NiB, and it enabled patterning the gates with an accuracy better than 1 µm. Annealing the NiB layer at 400 °C reduces its specific resistivity from 25 to 13 µΩ cm, and the roughness and adhesion of the layer to the glass enable the plasma deposition of silicon nitride (SiNx) and amorphous silicon (a-Si) layers over the patterned array of gates. Building an array of TFTs for a SXGA display using the NiB as the gate layer yielded transistors with transfer and output characteristics similar to those fabricated using a conventional gate material. The work presented here may spur the introduction of novel surface chemistry processes into flat-panel-display factories.

Introduction Liquid-crystal displays (LCDs) have emerged as the most promising type of display in the past two decades.1 These displays can be operated using low voltage; hence, they have a low power consumption. They are compact, can be fabricated in different sizes, and provide good resolution and brightness.2 These displays rely on an electrical field to orient liquid crystals (LCs) to attenuate or block the transmission of light from the back of the display to the outside.3,4 Small areas of the LC layer are * Corresponding author. E-mail: [email protected]. † IBM Research, Zurich Research Laboratory. ‡ Advanced Display Technology Laboratory, IBM Research, T. J. Watson Research Center. § Now at Motorola, Austin, TX. | Magnetoelectronics Department, IBM Research, T. J. Watson Research Center. (1) Castellano, J.; Mentley, D.; Weichert, A. Flat Information Displays. Market & Technology Trends, 7th ed.; Stanford Resources Inc.: San Jose, CA, 1996. (2) Flat Panel Displays and CRTs; Tannes, L. E., Jr., Ed.; Van Nostrand Reinhold Company: New York, 1985.

oriented independently using an array of driving elements; the smallest independent area controlling the transmission of light is the subpixel. Light transmitted by the LCs is colored by passing through one type of color filter. Overall, a pixel consists of an ensemble of adjacent red, green, and blue subpixels, and in most LCDs the driving elements consist of TFTs.5 A typical TFT comprises the gate layer patterned on a glass substrate, the insulator and semiconductor parts of the thin-film transistor (TFT) over and next to the gate layer, and finally the patterned data layer. A subpixel is addressed by driving data voltage onto the subpixel capacitor (formed by the patterned storage capacitor and the LC cell itself) by switching the TFT via its associated gate and data lines. The fabrication of TFT-LCDs is complex because it involves handling large substrates and materials as dissimilar as sputtered (3) Liquid Crystals. Applications and Uses; Bahadur, B., Ed.; World Scientific: Singapore, 1990 (reprinted 1993); Vol. 1. (4) Kim, S. S. Inf. Display 2001, 17, 22-26. (5) Tsukuda, T. TFT/LCD. Liquid Crystals Addressed by Thin-Film Transistors; Japanese Technology Reviews; Vol. 29; OPA Amsterdam B. V.: Amsterdam, The Netherlands, 1996.

10.1021/la0341714 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/14/2003

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metals, plasma-enhanced chemical vapor deposited (PECVD) insulators and semiconductors, photoresists, liquid crystals, spacers, and colored polymers. Defects in any layer are difficult to repair but are easily apparent in the final product.6,7 These difficulties account for the relatively high prices of TFT-LCDs compared with television screens and explain in part why TFT-LCDs tend to be smaller in size than the latter. Overall, we want to introduce as many novel methods as possible in the fabrication of TFT-LCDs. In particular, we aim to pattern the gate level of the TFT array using electroless deposition (ELD)8 and microcontact printing (µCP)9 instead of sputtering of a metal and photolithography, as is done currently. Sputtering a metal and spincoating and exposing a photoresist on substrates that approach (and sometimes exceed) 1 m2 translate into formidable equipment resources and long processing times. For these reasons, fabricating the gate structures in a metallic layer placed on glass using ELD and patterning them by means of µCP is technologically and economically interesting and can represent an inspiring paradigm for chemists who wish to influence fabrication processes and see soft lithography10,11 as a firmly established technology in factories. We have chosen to fabricate arrays having the following characteristics: the arrays are 10.5 in. in diagonal and centered on 15 in. glass plates, have a resolution of 1280 pixels × 1024 pixels (SXGA), with 162 × 54 µm2 subpixels, and a 157 per inch density of pixels. Displays with such a resolution are of high quality and are called “paperlike” displays.12 ELD and µCP are powerful but delicate techniques,8,11 and their application to the fabrication of gate patterns on large glass substrates is far from being trivial. ELD is based on the reduction and deposition of metallic ions from a solution to a surface in the absence of an external electric current source. This deposition requires the immersion of a catalytically activated surface into a plating bath containing complexed metal ions and a reducing agent. ELD for our specific application involves (i) placing a catalyst from solution onto the glass substrate, (ii) devising or employing an electroless chemistry for depositing a metal having the required adhesion and conductivity properties, and (iii) having uniform plating across 15 in. substrates. The glass used here is a very smooth fusion-drawn glass13 having a rms roughness of ∼0.5 nm, as measured using an atomic force microscope (AFM). It cannot be roughened to enhance the adhesion as one does for ELD on polymers14,15 or other substrates16,17 (6) Wright, S. L.; Warren, K. W.; Alt, P. M.; Horton, R. R.; Narayan, C.; Greier, P. F.; Kodate, M. IBM J. Res. Dev. 1998, 42, 445-457. (7) Tsuji, S.; Tsujimoto, K.; Iwama, H. IBM J. Res. Dev. 1998, 42, 509-515. (8) Electroless Plating: Fundamentals and Applications; Mallory, G., Hajdu, J. B., Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990. (9) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 20022004. (10) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550-575. (11) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker, D.; Kind, H.; Renault, J.-P.; Rothuizen, H.; Schmid, H.; Schmidt-Winkel, P.; Stutz, R.; Wolf, H. IBM J. Res. Dev. 2001, 45, 697719. (12) Colgan, E. G.; Alt, P. M.; Wisnieff, R. L.; Fryer, P. M.; Galligan, E. A.; Graham, W. S.; Greier, P. F.; Horton, R. R.; Ifill, H.; Jenkins, L. C.; John, R. A.; Kaufman, R. I.; Kuo, Y.; Lanzetta, A. P.; Latzko, K. F.; Libsch, F. R.; Lien, S.-C. A.; Millman, S. E.; Nywening, R. W.; Polastre, R. L.; Powell, C. G.; Rand, R. A.; Ritsko, J. J.; Rothwell, M. B.; Staples, J. L.; Warren, K. W.; Wilson, J. S.; Wright, S. L. IBM J. Res. Dev. 1998, 42, 427-444. (13) Lapp, J. C.; Moffatt, D. M.; Dumbaugh, W. H.; Bocko, P. L.; Anma, M. Dig. Tech. Pap., Soc. Inf. Display Int. Symp. 1994, 851-853. (14) Rozovskis, G.; Vinkevicius, J.; Jaciauskiene, J. J. Adhesion Sci. Technol. 1996, 10, 399-406.

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because roughening this glass can compromise the entire TFT fabrication process and possibly even the glass transparency. Developing an etch chemistry to pattern the metallic layer, postprocessing the plated metal, and resolving issues with device integration might be additional concerns. µCP is a technique that uses a micropatterned elastomer replicated from a lithographically prepared mold to print a self-assembled monolayer onto a substrate.9,18 The regions of contact define where the monolayer forms, and the printed pattern can be transferred into the substrate by using the printed monolayer as a resist during a wet-etch step. This technique was originally demonstrated by printing alkanethiols onto noble metals such as Au.9 From a chemist’s point of view, reaching the goals pursued here is best done by establishing ELD and µCP independently before combining them. For this reason, we divided the work necessary to achieve this task into three parts, and each of them will be described in separate publications.19-21 In this paper, we mainly focus on the ELD part and provide the conditions for derivatizing and activating the glass to achieve a highquality metal deposit, which is NiB in this case, on a 15 in. substrate. We patterned the plated films using a standard photoengraving process and achieved gate structures matching the requirements for the fabrication of functional TFTs. We termed this part of our work “Plate & PEP”. The next parts described in forthcoming publications rely on the findings presented here and combine ELD and µCP in different manners. One approach, called “Plate, iPlate & Print”,19 uses a mask, which is electroplated atop the electroless-deposited metal and microcontact printed. Alternatively, the electroless-deposited metal on the glass substrate itself is printed and etched directly to fabricate the gate structures, an approach named “Plate & Print”.20 We also attempted to direct the ELD of the gate material on the glass by using a printing step to defining those regions where ELD should occur on the substrate. We called this part of the work “Print & Plate”.21 Choosing a Metal for “Plate & PEP”. The specific resistivity is probably the most important criterion in choosing a material for fabricating the gate pattern of an active-matrix LCD because it determines in part the scan rate, and hence the electrical and front-of-screen performance, of the TFT array, and the metal must be sufficiently conductive to allow good signal propagation along narrow gate lines connected to the TFTs.22 Narrow features in an array of TFTs leave more space for transmitting light through the pixel electrode, which results in displays having a higher brightness (large aperture ratio).12 We selected three types of displays, with increasing resolution, and indicate the thickness of a few metal candidates for fabricating gate patterns for these displays in Table 1. The materials in the first half of the table can be sputtered. (15) Suzuki, M.; Kawamoto, M.; Takahashi, A. Polym. Eng. Sci. 1999, 39, 321-326. (16) Schammler, G.; Springer, J. J. Adhesion Sci. Technol. 1995, 9, 1319-1342. (17) Calata, J. N.; Quan, G.; Tze, L.; Chuang, J. Surf. Interface Anal. 2001, 31, 673-681. (18) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (19) Delamarche, E.; Geissler, M.; Magnuson, R. H.; Schmid, H.; Michel, B. Langmuir, in press. (20) Delamarche, E.; Vichiconti, J.; Hall, S. A.; Geissler, M.; Graham, W.; Michel, B.; Nunes, R. J. Am. Chem. Soc. (submitted). (21) Geissler, M.; Kind, H.; Schmidt-Winkel, P.; Michel, B.; Delamarche, E. Langmuir, in press. (22) Fryer, P. M.; Colgan, E.; Galligan, E.; Graham, W.; Horton, R.; Jenkins, L.; John, R.; Kuo, Y.; Latzko, K.; Libsch, F.; Lien, A.; Nywening, R.; Polastre, R.; Rothwell, M. E.; Wilson, J.; Wisnieff, R.; Wright, S. Mater. Res. Soc. Symp. Proc. 1998, 507, 37-46.

Electroless Deposition of NiB

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Table 1. Required Thickness of the Metallic Gate Layer as a Function of the Metal Used and the Display Characteristics

metal or alloy

specific resistivity (µΩ cm)

14.1 in. XGA thicknessa,b for 0.55 Ω/0 (nm)

10.5 in. SXGA thicknessc for 0.28 Ω/0 (nm)

16.3 in. QSXGA thicknessc for 0.08 Ω/0 (nm)

Ag Cu Au Al alloy sputtered Co Ni MoW sputtered Ag as platedf/annealedg at 350 °C

1.59d 1.71d 2.35d 5 6.24d 6.84d 15 4.7/2.3

29e 32 43 91 114 125 273e 86/42

57 62 84 179 223 245 536 168/81

199 214 294 625 780 855 1875 588/283

NiPh as plated/annealed at 400 °C NiWPj as plated/annealed at 400 °C

65/50 85/35

1182/910 1546/637

2322/1786 3036/1250

8125/6250 10625/4375

NiBk as plated/annealed at 400 °C

25/13

455/237

893/465

3125/1625

CoPi as plated/annealed at 400 °C Cul as plated/annealed at 250 °C

136/54 6.4m/5.2

2473/982 117/95

4858/1929 229/186

17000/6750 800/650

comments

bath unstable, inhomogeneous deposition ∼60 nm on 15 in. glass platei ∼60 nm on 15 in. glass platei >110 nm on 4 in. glass platei >500 on 15 in. glass platei >1 µm on 4 in. glass platei ∼60 nm on 15 in. glass platei ∼300 nm on 15 in. glass platei

a d (nm) ) 10(R b c spec (µΩ cm)/Rsheet required (Ω/0)). For a 14.1 in. XGA display with standard performances. For single-sided driven gate lines, fast switching time, and an aperture ratio of ∼30%. d Values from Metals Handbook, Vol. 1, 8th ed., 1961. Experimental data are indicated by default. e The thickness in bold meets the requirements for fabricating the gate pattern of the displays described here. The requirements can be relaxed by as much as 75%, for example, using adjustments in the design, modifying the aperture ratio of the display, and using double-sided driven gate lines. A thickness in italic can be reduced by 50% using relaxed conductivity requirements. f Ag ELD bath prepared using silver nitrate and formaldehyde at a pH of ∼10; the substrate was sensitized with SnCl2 first to initiate the ELD. g All annealing experiments were done in the presence of a forming gas (90% N and 10% H ). h Enplate Ni-434 E (IMASA AG, Da ¨ llikon, 2 2 Switzerland) prepared as recommended by the supplier, operated at 88 °C and pH 5.0. i Maximum thickness of the electroless-deposited metal layer using EDA-Si-treated glass and Pd/Sn colloids as the catalyst. j Proprietary bath. k NiB Shipley Niposit 468 leading to a low amount of deposited B (0.25%, weight). l Thru Cup PEA bath from Uyemura. m The specific resistance of the deposited Cu converges to 3.6 µΩ cm as the Cu thickness becomes 200 nm.

Some of these materials are hypothetical, whereas others are employed in current fabrication processes, for example, Al alloys and MoW.23 The second series of metals and alloys can be electroless deposited onto a substrate. A thickness in bold indicates a material that fulfills the conductivity requirement for either type of display (integration issues are not considered here) but with a thickness that does not exceed ∼250 nm. 250 nm is the practical limit in the thickness of gate layers to fabricate TFT arrays with high yield.24 The first display is a 14.1 in. display with a XGA (1024 × 768) resolution and has a sheet-resistance requirement for the gate layer of 0.55 Ω/0. Typically, such displays are fabricated using gates made of a sputtered MoW alloy because this alloy is inexpensive and has excellent processing properties.25 Ag, NiB, and Cu are the only electroless-deposited metals that can meet the conductivity requirements of the XGA display. Electroless-deposited Au is not considered here because of its exceedingly high cost. The next display is smaller in size, with a diagonal of 10.5 in., but has many more pixels and hence a higher resolution than the first one. It is difficult to use MoW to fabricate such a display. Ag and Cu meet the conductivity requirements of this type of display, and electroless-deposited NiB could be employed in conjunction with relaxed design rules (thickness in italic). Such displays are currently fabricated using Al alloys. Fabricating larger, higher-resolution displays using an electroless-deposited gate layer can be done only with Cu, in principle. The fabrication of these latter displays is our long-term goal. Outline of the Process. The process flow for “Plate & PEP” of NiB gate lines on large glass substrates is depicted (23) Takatsuji, H.; Colgan, E. G.; Cabral, C.; Harper, J. M. E. IBM J. Res. Dev. 1998, 42, 501-508. (24) Gates thicker than 250 nm can be difficult to cover by the subsequent layers in a well conformal manner, and many published designs for TFTs comprise gates 250 nm thick. See refs 12 and 22, for example. (25) Note that design rules can relax the conductivity requirements for the gate layer by more than 50%.

Figure 1. The process to fabricate an array of TFTs on large glass substrates relies here on the ELD of NiB and PEP. The ELD of NiB requires grafting of EDA-Si to the glass substrate and immobilizing a Pd/Sn catalyst. The catalyst initiates ELD of NiB on both sides of the glass substrate. One NiB layer is removed from one side, and the remaining one is patterned using standard photolithography and wet etching. The patterned NiB layer, which corresponds to the gate layer of an array of TFTs, is annealed and characterized before proceeding with the TFT array build.

in Figure 1. Our strategy starts by derivatizing the glass substrate by grafting 3-(2-aminoethylamino)propyltrimethoxysilane (EDA-Si), a trimethoxysilane bearing functional amino groups. In the simplest case, this step is done by immersion of the glass into a 1% solution of EDA-Si in water. In practice, the derivatization of 15 in. glass substrates was done by spraying the solution of EDASi, followed by subsequent rinsing and drying of the plates in an automated tool. The pretreatment of the substrate is necessary to immobilize the catalyst needed for ELD on

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Table 2. Influence of the Solvent for Grafting EDA-Si on the Electroless-Deposited NiB solventa heptane dichloromethane ethyl acetate 2-propanol acetonitrile methanol ethanol N,N-dimethylformamide acetone water

solution cloudye

becomes becomes cloudyg becomes cloudye no change noticed becomes cloudye no change noticed no change noticed becomes cloudyg no change noticed no change noticed

strikeb

NiBc deposit

max. thicknessd (nm)

irregular, very slow irregular, slow irregular good good very good excellent very good excellent excellent

inhomogeneous, grainy inhomogeneous inhomogeneous some spots not plated patches, very grainy uniform uniform excellent uniformity excellent uniformity excellent uniformity

unknownf unknownf unknownf >500 >1000 >500 >500 >1000 >1000 >1000

a Solvents to graft EDA-Si onto 1 × 3 in.2 glass substrates from a 1% EDA-Si solution at room temperature for 10 min. b Refers to the initiation of ELD of NiB. c Uniformity assessed using an optical microscope, after rinsing and drying the deposited NiB layer. d Maximum thickness refers to the thickness of NiB that could be plated without blistering or loss of adhesion of the NiB in the ELD bath. e Becomes cloudy within a few minutes because of the formation of white precipitates. f The maximum thickness was difficult to determine but was not relevant due to the low quality of the deposited NiB. g Becomes cloudy within a few hours.

the glass. We employed colloidal Pd/Sn particles26-29 dissolved in concentrated HCl (termed HClc) as the catalyst because they are very active for initiating ELD, widely used in technology, commercially available, and relatively stable.30 It is preferable to “accelerate” the Pd/ Sn particles once they are immobilized, and the substrates covered with Pd/Sn can be immersed in a dilute solution of HBF4 to remove some of the Sn and to make the Pd-rich core of the particles more accessible to the reagents of the ELD bath.26,30 ELD of NiB constitutes the next step in our strategy and is performed with careful control of the temperature, pH, and recirculation of the bath.31,32 Both sides of the glass substrates are covered with EDA-Si, Pd, and NiB during these process steps, and NiB is removed from one side by covering one of the NiB layers with resist and wet etching the other layer. The resist is then stripped, and the NiB layer remaining on the glass substrate is patterned using photolithography and wet etching. After removing the patterned photoresist, the NiB layer is annealed and characterized. Building the array of TFTs proceeds with the same steps as when the gate layer consists of a sputtered alloy.12 Results and Discussion Derivatizing a 15 in. Glass Substrate with EDASi. We chose self-assembling silanes on glass to immobilize the Pd/Sn catalyst for several reasons. First, Pd/Sn colloids do not bind to glass spontaneously but require a pretreatment of the glass substrates. Pretreating substrates to bind Pd-based catalysts is standard in most ELD applications.33 One type of treatment is to deposit a thin organic film comprising amines on the substrate to provide numerous interaction sites for catalysts.34,35 Second, a large variety of silanes can be immobilized on glass from (26) van der Putten, A. M. T.; de Bakker, J.-W. G.; Fokkink, L. G. J. J. Electrochem. Soc. 1992, 139, 3475-3480. (27) Matijevic´, E.; Poskanzer, A. M.; Zuman, P. Plat. Surf. Finish. 1975, 62, 958-965. (28) Osaka, T.; Takematsu, H.; Nihei, K. J. Electrochem. Soc. 1980, 127, 1021-1029. (29) Dressick, W. J.; Chen, M.-S.; Brandow, S. L. J. Am. Chem. Soc. 2000, 122, 982-983. (30) Sard, R. J. Electrochem. Soc. 1970, 117, 864-870. (31) Mallory, G. O. Plating 1971, April, 319-327. (32) Evans, W. T.; Schlesinger, M. J. Electrochem. Soc. 1994, 141, 78-82. (33) Electroless Nickel Plating; Riedel, W., Ed.; ASM International: Metals Park, OH, 1991. (34) Chen, M.-S.; Brandow, S. L.; Dulcey, C. S.; Dressick, W. J.; Taylor, G. N.; Bohland, J. F.; Georger, J. H.; Pavelchek, E. K.; Calvert, J. M. J. Electrochem. Soc. 1999, 146, 1421-1430. (35) Wu, S.; Kang, E. T.; Neoh, K. G.; Tan, K. L. Langmuir 2000, 16, 5192-5198.

solution or from the gas phase.36-39 Methods for preparing SAMs can be fast and inexpensive. Third, it is likely that homogeneous, self-assembled layers can be formed on large glass substrates.40 This is important to ensure a reproducible, homogeneous coverage of the derivatized glass by Pd/Sn colloids because the electroless-deposited NiB may reflect variations present in the Pd/Sn layer,41 or indirectly in the silane layer. The NiB layer should be as smooth and regular in thickness as possible to yield a good coverage by the next layers and TFTs with equal performance, respectively.22 Table 1 indicates that the thickness of the NiB layer to fabricate the targeted 10.5 in. display can be as low as ∼120 nm using relaxed design rules. The thickness variation in the active area should not exceed (5% of the average layer thickness if possible, and too rough or too inhomogeneous NiB layers will invariably lead to failures during the fabrication processes of the TFT arrays.42 We use trimethoxyorganosilanes rather than silanes having trichloro- anchoring groups because trichlorosilanes are too reactive with moisture and nucleophiles and should be preferably grafted from the gas phase or from nonpolar (and dry) solvents.37,38,43 Triethoxysilanes tend to be less reactive than trimethoxysilanes,44 but adjusting their rate of hydrolysis using basic or acidic solutions is possible. We derivatized the glass substrates with EDA-Si to immobilize Pd/Sn particles dissolved in an acidic solution via the amine functions of EDA-Si.34,41,45 The solvent used to graft silanes onto an SiO2-type surface can influence the properties of the grafted layer and the cost of the process to a large extent.46,47 Table 2 summarizes the influence of the solvent used to prepare (36) An Introduction to Ultrathin Organic Films; Ulman, A., Ed.; Academic Press: New York, 1991. (37) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92-98. (38) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. (39) Ulman, A. Adv. Mater. 1990, 2, 573-582. (40) Thin Films on Glass; Bach, H., Krause, D., Eds.; SpringerVerlag: Berlin, 1997. (41) Brandow, S. L.; Dressick, W. J.; Marrian, C. R. K.; Chow, G.-M.; Calvert, J. M. J. Electrochem. Soc. 1995, 142, 2233-2243. (42) Unexpected concerns, which arose during this work, were the possibility of contaminating the line of fabrication of prototype displays by losing NiB particles in plasma deposition and photoexposure tools, for example. The high quality of the deposited NiB layers precluded encountering these problems. (43) Jo¨nsson, U.; Olofsson, G.; Malmqvist, M.; Ro¨nnberg, I. Thin Solid Films 1985, 124, 117-123. (44) Sol-Gel Science; Brinker, C. J., Scherer, G. W., Eds.; Academic Press: New York, 1990. (45) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375-1380. (46) Vandenberg, E. T.; Bertilsson, L.; Liedberg, B.; Uvdal, K.; Erlandsson, R.; Elwing, H.; Lundstro¨m, I. J. Colloid Interface Sci. 1991, 147, 103-118.


Figure 2. XPS surveys of glass substrates derivatized with EDA-Si using various solvents, and high-resolution spectra showing how much Pd/Sn catalyst was immobilized on the corresponding modified glass substrates. See text for experimental details.

a solution of EDA-Si on the stability of the solution of silanes, the initiation of the ELD (termed “strike”), and the homogeneity and maximum thickness of the NiB layer obtained. Solvents such as heptane, dichloromethane, ethyl acetate, 2-propanol, and acetonitrile were not considered for grafting EDA-Si because using them resulted in plating nonuniform NiB layers. The remaining solvents in Table 2 lead to a good strike, and the NiB deposit was of the best quality when the solution of EDASi was prepared with dimethylformamide, acetone, or water.48 We observed, however, that rinsing the freshly grafted EDA-Si layer with 2-propanol, acetonitrile, methanol, or ethanol tended to remove a fraction of the silanes from the substrate, resulting in only partial plating across 15 in. glass substrates. We selected water as the solvent to graft EDA-Si onto glass and to rinse the derivatized glass plates because water is inexpensive, nonflammable, and simple to dispose of. The XPS spectra shown in Figure 2 (left column) characterize the chemical composition of glass substrates derivatized with EDA-Si using some of the solvents mentioned above.49 The magnitudes of the N 1s (at 400 eV) and C 1s (extending from 285 to 287 eV) signals corresponding to EDA-Si deposited from heptane and (47) McGovern, M. E.; Kallury, M. R.; Thompson, M. Langmuir 1994, 10, 3607-3613. (48) These findings indicate that the critical density of Pd/Sn bound to the EDA-Si layers was reached in all cases. See also: Okinaka, Y.; Osaka, T. Adv. Electrochem. Sci. Eng. 1994, 3, 55-116.

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acetone are much higher than those when ethanol and water are employed for the glass derivatization. This is consistent with the deposition of “particle-like” aggregates of EDA-Si onto the glass that formed within a few minutes after dispensing EDA-Si in these solvents. The important average thickness of these deposited layers of EDA-Si is evidenced by the strong attenuation of the Si 2s, Si 2p, Ba 3d, and O 1s signals of the glass by the EDA-Si overlayer. In comparison, the EDA-Si layers deposited from water and ethanol are much thinner, whereas the ratio between the measured C 1s and N 1s signals is equivalent for all spectra, which suggests that the composition of the EDASi film on the glass is similar in all four experiments. Ellipsometry measurements done on EDA-Si grafted from water onto the native oxide of a Si wafer revealed a thickness of 0.5 ( 0.1 nm,50 consistent with having a monolayer of EDA-Si grafted, on average.34 Even though the display glass substrate and the Si/SiO2 wafer differ in their surface composition, we can probably infer from this thickness determination that a nearly complete film of EDA-Si was grafted onto glass using water as the solvent. The quality of the plated NiB is the most important criterion to determine the best conditions and chemistry to treat the glass substrates. The amount of Pd/Sn deposited on the grafted glass substrates is examined by measuring the Pd 3d signal using XPS (right column in Figure 2). Interestingly, the quantity of Pd on the surface is similar for all samples but does not correlate with the amount of grafted material. The same amount of Pd/Sn is present on EDA-Si grafted from water as on a glass having several times as much EDA-Si grafted from heptane. This observation suggests that all surfaces examined in Figure 2 can be covered with Pd/Sn up to saturation, regardless of the solvent used. In light of the grainy texture of the electroless-deposited NiB, we suggest that the deposition of EDA-Si particles formed in heptane or acetone onto glass leads to a locally excessive deposition of Pd/Sn particles and indirectly to the plating of NiB grains. The observed differences in the ELD strikes further suggest that the Pd/Sn particles bind to the EDA-Simodified glass with variable strength and that even if the amount of catalyst found on the surface of all samples is the same, some catalyst might be lost from the surface before or during the strike. How is EDA-Si Grafted onto Glass? As it is preferable to use water as the solvent for grafting EDA-Si onto display glass substrates, we will next examine the hydrolysis of EDA-Si in water in more detail and how the conditions for grafting EDA-Si affect the homogeneity of the NiB layer deposited. Trimethoxysilanes hydrolyze in water, form silanols, R-Si(OH)3, and then condensate to form a variety of silane oligomers.44,51,52 We are interested in determining the magnitude of this hydrolysis by water and the time scale on which it occurs,53 and therefore, we (49) In all cases, 1 × 3 in.2 glass substrates were immersed in a 1% solution of EDA-Si for 10 min; the samples were removed from the solutions, rinsed in 100 mL of the same solvent for 10 s, dried with a stream of N2, and stored for at least 10 min at room temperature before proceeding with measurements or deposition of Pd/Sn. (50) These experiments were done using a 1% solution of EDA-Si in water (at room temperature, without stirring) and an adsorption time of 10 min. We did not notice variations in the thickness of the deposited EDA-Si film on the wafer even for longer adsorption times or when solutions of EDA-Si in water were up to a few weeks old. (51) Okumoto, S.; Fujita, N.; Yamabe, S. J. Phys. Chem. A 1998, 102, 3991-3998. (52) Silanes and Other Coupling Agents; Mittal, K. L., Ed.; VPS BV: Utrecht, The Netherlands, 1992. (53) Beari, F.; Brand, M.; Jenkner, P.; Lehnert, R.; Metternich, H. J.; Monkiewicz, J.; Siesler, H. W. J. Organomet. Chem. 2001, 625, 208216.

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Figure 3. DEPT-135 13C NMR spectra used to monitor the hydrolysis of the trimethoxysilane groups of EDA-Si in various solvents. (A) These spectra of EDA-Si in D2O and DMSO-d6 demonstrate that hydrolysis of EDA-Si in D2O is fast, whereas the trimethoxysilanes do not hydrolyze noticeably in dry DMSO-d6. (B) This series of spectra of EDA-Si in DMSO-d6 reveals the rapid hydrolysis and oligomerization of EDA-Si after the addition of water.

performed C13 NMR experiments in deuterated solvents, (Figure 3). First, we use the spectra in Figure 3A to attribute the C signals to EDA-Si. The spectrum obtained within a few minutes after dissolving EDA-Si in D2O shows a series of C signals from the alkyl chain of the silane and a strong peak at ∼50 ppm. This signal already corresponds to the hydrolysis product, CH3-OD, because adding methanol to the sample increases this signal. In contrast, the C signal from nonhydrolyzed methoxysilanes (-SiOCH3) is the only visible signal from the trimethoxy groups when EDA-Si is dissolved in dry, deuterated DMSO. Adding methanol to this solution adds a C signal at a slightly lower frequency, corresponding to C from methanol. These experiments indicate that EDA-Si is quickly hydrolyzed in water (at least faster than the few minutes necessary to acquire the spectrum) but not in dry DMSO. We therefore performed a second series of experiments in DMSO-d6 having 1.5% H2O and EDA-Si. The first spectrum in Figure 3B was acquired within a few minutes after adding H2O. Unlike EDA-Si dissolved in D2O, the hydrolysis of EDA-Si is not immediate but takes at least 1 h to become noticeable: the C signal at 48 ppm is related to methanol, as determined in the earlier set of experiments. The C signals between 48 and 50 ppm correspond to -Si-OCH3 partially hydrolyzed and/or already polymerized. The C signal at 7 ppm (B-type CH2) broadens toward a higher frequency range as a result of this

oligomerization. All of the EDA-Si molecules were hydrolyzed and oligomerized after at least 6 h. The experiments reported in Table 3 evaluate the influence of the concentration of EDA-Si in water for derivatizing the glass substrate as well as that of baking the grafted substrate on the NiB deposit. Irrespective of the grafting and baking conditions, the NiB layers deposited exhibit a consistent specific resistivity of ∼22 µΩ cm, indicating that, with respect to the ELD conditions, the ELD of NiB was comparable for all experiments. The homogeneity of the deposited layers was not constant, however, but could be divided into two categories depending on whether the deposits have large NiB grains or not. The granular appearance of the NiB stems from the inclusion of micrometer-sized grains in the NiB layer deposited. Many of these grains are aligned along the direction of immersion of the substrate in the EDA-Sigrafting bath. Varying the grafting time or rinsing the freshly grafted substrates with water before immobilizing Pd/Sn did not influence the texture of the NiB deposited. Because no large grains were observed when the concentration of EDA-Si was less than or equal to 0.1% for unbaked samples, we hypothesized that too high a concentration of EDA-Si in water leads to an oligomerization of the silanes and deposition of EDA-Si particles, which may bind too much of the Pd/Sn catalyst locally, as was the case when EDA-Si was grafted from heptane, for example. Several publications reporting the grafting of


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Table 3. Influence of the Concentration of EDA-Si and Baking Conditions of the EDA-Si Grafted Film on the ELD of NiB thickness sheet [EDA-Si] bake of the NiB resistance specific homoin water timeb platedc of the NiB resistivity geneous (vol %) (min) (nm) plated (Ω/0) (µΩ cm) NiB?d 5a 5 2 2 1 1 0.5 0.5 0.2 0.2 0.1

0 15 0 15 0 15 0 15 0 15 0

230 210 240 190 180 170 200 165 185 140 160

0.95 1 1 1.12 1.56 1.24 1.2 1.3 1.4 1.65 1.45

22 21 23 23 25 22 20 23 26 24 22

no yes no yes no yes no yes no yes yes

a Clean 15 in. glass substrates were immersed for 3 min in the EDA-Si solution, removed, and rinsed for 30 s with water. Plates were immersed into the solution of Pd/Sn catalyst directly or after they were dried with a stream of N2 and baking under N2. bAt 150 °C under N2 after grafting. c After the grafting step, the substrates were rinsed with water for 30 s, baked or not, immersed in a solution of Pd/Sn colloids 15% in HClc for 30 s, rinsed with water, activated in HBF4 5% in water for 30 s, rinsed with water for 30 s, prewarmed in water at 55 °C for 30 s, and plated using a half-strength Niposit 468 bath at 55 °C for 6-7 min. d Refers to the presence of large NiB grains in the plated layer, as can be observed by eye and under an optical microscope.

trimethoxysilanes or triethoxysilanes onto SiO2-type surfaces suggest that baking at ∼150 °C after the grafting step removes multilayers, oligomers, and weakly coordinated molecules from the surface and helps prepare more homogeneous SAMs.46,54,55 We indeed noticed that baked substrates were free of large NiB grains, regardless of the concentration of EDA-Si used. However, we note that in our experiments drying the samples after the grafting step was crucial for depositing highly uniform NiB layers over the entire 15 in. glass plates. Any sample dried after depositing EDA-Si onto the glass substrates did not require a baking step to deposit large-grain-free NiB. We suggest that drying the substrates after removing them from the EDA-Si solution favors dehydration of the silane layer(s) in contact with the glass and helps to cross-link the ensemble of molecules further with the substrate or between themselves. This may reduce the mobility of the silanes polymerized56 on glass in the solution of catalyst and prevent the binding of excessive amounts of Pd/Sn particles. We were able to use an immersion time of the 15 in. glass substrates into a 1% solution of EDA-Si in water as short as a few seconds while plating uniform NiB layers. The pH of a 1% solution of EDA-Si in water at 22 °C is 10.0. A significant portion of the amines of EDA-Si are protonated at this pH. We think that electrostatic interactions between positively charged amines and the negatively charged glass surface55,57,58 promote a quick covering of the glass with EDA-Si molecules, irrespective of the state of hydrolysis and/or the oligomerization of the silanes. Irreversible attachment of the silanes to the glass may occur during the drying step.57 In support of this (54) Bernard, A.; Bosshard, H. R. Eur. J. Biochem. 1995, 230, 416423. (55) Vrancken, K. C.; Possemiers, K.; van der Voort, P.; Vansant, E. F. Colloids Surf., A: Physicochem. Eng. Aspects 1995, 98, 235-241. (56) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268-7274. (57) Herder, P.; Vågberg, L.; Stenius, P. Colloids Surf. 1988, 34, 117-132. (58) Horr, T. J.; Arora, P. S. Colloids Surf., A: Physicochem. Eng. Aspects 1997, 126, 113-121.

hypothesis, we observed that trimethoxysilanes bearing only one amino group could not be deposited onto glass as fast as EDA-Si whereas trimethoxysilylpropyl-modified polyethyleneimine (PEI) could be deposited within seconds on glass from more dilute solutions in water. Using PEI invariably resulted in the inclusion of large grains in the NiB layer, which suggests again that EDA-Si oligomers are responsible for too strong an uptake of Pd/Sn and the development of NiB grains during the ELD. Activation of the Derivatized Glass Substrates. We will now verify that immobilizing the Pd/Sn catalyst from solution onto the treated glass surface can also be achieved with sufficient control for producing high-quality NiB. Pd compounds are widely used as catalysts for ELD, mainly in the form of Pd(II) complexes59-65 or as Pd/Sn colloids.26-29,66 We found it preferable to use Pd/Sn colloids because they are more stable against oxidation effects than Pd(II) compounds, and the glass can be derivatized with EDA-Si but does not need to be sensitized with SnCl267 in order to immobilize them. Pd/Sn colloids are prepared and kept in acidic solution (usually HCl) to prevent oxidation of the particles: partially reduced Sn is stable below pH 1.4,68 and free Cl- ions help separate the particles. We observed aging of the colloidal suspension when diluting it with water, which resulted in the deposition of Pd/Sn stains on EDA-Si-coated glass substrates. These large particles had two adversary effects even if there were only a few of them per plate: they dramatically lowered the adhesion of the NiB layer to the glass, which inevitably resulted in blistering of the NiB during or after deposition in those regions where the particles were located. The second effect was a strong underetch of the NiB in the vicinity of these particles during the NiB patterning step. Consequently, we diluted the colloidal suspensions only with HClc. The XPS surveys in Figure 4 quantify how much Pd/Sn is immobilized on glass derivatized with EDA-Si depending on the concentration of the colloids in HClc. The Pd 3d and Sn 3d signals at 337 and 490 eV, respectively, reveal that almost no colloids were deposited on the surface for a concentration of the colloids e 0.01%, whereas the amount of catalyst on the surface increases sharply if the concentration of the colloids is g1%. The immersion time of the samples was 30 s in these experiments, and all samples were rinsed subsequently for 30 s using water. These results suggest that a solution of at least 1% Pd/Sn colloids in HClc can saturate the EDA-Si-treated glass surface with catalytic particles. In fact, homogeneous initiation of the ELD can be achieved as soon as a critical density of Pd/Sn colloids on the substrate has been reached for a given ELD bath.41 (59) Moberg, P.; McCarley, R. L. J. Electrochem. Soc. 1997, 144, 151153. (60) Kind, H.; Bittner, A. M.; Cavalleri, O.; Kern, K. J. Phys. Chem. B 1998, 102, 7582-7589. (61) Dressick, W. J.; Dulcey, C. S.; Georger, J. H.; Calabrese, G. S.; Calvert, J. M. J. Electrochem. Soc. 1994, 141, 210-220. (62) Shafeev, G. A.; Bellard, L.; Themlin, J.-M.; Marine, W.; Cros, A. Appl. Surf. Sci. 1995, 86, 387-391. (63) Brandow, S. L.; Chen, M.-S.; Aggarwal, R.; Dulcey, C. S.; Calvert, J. M.; Dressick, W. J. Langmuir 1999, 15, 5429-5432. (64) Chen, Y.; Kang, E. T.; Neoh, K. G.; Huang, W. Langmuir 2001, 17, 7425-7432. (65) de Minjer, C. H.; van der Boom, P. F. J. J. Electrochem. Soc. 1973, 120, 1644-1650. (66) Sun, R. D.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1999, 146, 2117-2122. (67) Dumont, E.; Dugnoille, B.; Petitjean, J. P.; Barigand, M. Thin Solid Films 1997, 301, 149-153. (68) Atlas of Electrochemical Equilibria in Aqueous Solutions; Pourbaix, M., Ed. National Association of Corrosion Engineers: Houston, TX, 1974.

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Figure 5. The ELD of NiB seems to be independent of the concentration of Pd/Sn colloids in HClc used to activate EDASi-grafted glass substrates if the colloid concentration in the acid exceeds ∼0.02%. The ELD of NiB is evaluated using three criteria, namely, the thickness of the deposited NiB layer after 5 min, the specific resistivity of the as-plated NiB, and the roughness of the NiB layer as shown in the AFM images for a Pd/Sn concentration of 0.1% (A) and 50% (B).

Figure 4. XPS surveys of display glass substrates derivatized with EDA-Si and treated with acidic solutions of Pd/Sn colloids of varying concentration. A similar amount of Pd/Sn catalyst is immobilized by EDA-Si on the glass as soon as the concentration of Pd/Sn is at least 1% in HClc.

An “acceleration” step after the deposition of the Pd/Sn and before the ELD step is usually performed when Pd/ Sn colloids are employed.69 This step consists of immersing the substrate coated with Pd/Sn particles into an aqueous solution containing HBF4. The particles used in this work have ∼30 times more Sn than Pd atoms (as revealed by elementary analysis). Some of the oxidized Sn that forms the shell of the colloids is dissolved during this step, as indicated by XPS experiments, which helps make the Pd core better accessible to reactants in the ELD bath. The concentration of the acceleration bath and the time of immersion of the substrate had little effect on the ELD of NiB if the conditions for grafting EDA-Si, depositing Pd/Sn, and plating NiB were already optimized.70 The thickness and specific resistivity of electroless-deposited (69) Kim, J.; Wen, S. H.; Jung, D. Y.; Johnson, R. W. IBM J. Res. Dev. 1984, 28, 697-709.

NiB layers on EDA-Si-treated glass are reported in Figure 5 as a function of the relative concentration of Pd/Sn colloids in HClc. A clear threshold exists for a colloid concentration between 0.02 and 0.03%, above which ∼300 nm of NiB having a specific resistivity of ∼22 µΩ cm are deposited within 5 min using a bath operated at full strength. The AFM images of NiB obtained using relative concentrations of Pd/Sn of 0.1 (Figure 5A) and 50% (Figure 5B) reveal similar morphologies of the NiB surfaces. The rms roughness of a freshly deposited NiB layer is ∼1.5 nm in both cases, and the average diameter of the NiB grains is 60 ( 20 nm. Although this roughness is larger than the 0.5 nm rms roughness of the glass substrate alone, it should not compromise conformal coverage of the NiB pattern by the next layer during plasma deposition. Electroless Deposition of NiB on 15 in. Glass Substrates. The preceding sections on how to treat a glass substrate and render it active have anticipated the conditions used for ELD of NiB. NiB was deposited from a commercial bath using (N-dimethylamine)borane (DMAB) as the reducing agent. It was selected because it yields Ni with only 0.25% B, which is an important requirement to have low-resistivity Ni layers. We operated this bath at half of its recommended strength because a lower plating rate helped maintain a good adhesion between the deposited layer and the glass substrate by (70) Immersion times > 4 min using a 5% solution of HBF4 did compromise the strike of the NiB in the bath, which might occur due to dissolving the Sn shell of some particles entirely and displacing them from the glass surface.


having less intrinsic stress in the deposited layer, and a slower evolution of H2. All substrates were prewarmed to the temperature of the bath in warm water after the acceleration step to ensure a fast and homogeneous strike of the NiB. The ELD bath required no filtration, an indication either that no NiB and catalyst particles were lost from the substrate to the bath or that lost particles were inactivated by inhibitors from the bath. The recirculation of the bath was from bottom to top, and the excess of solution continuously fell over a compartment placed in the bath and was collected in the direction of the heater and pump for recirculation. This ensured a constant uptake of O2 in the bath, which helped stabilize it. A Teflon frame was used to carry and maintain the glass substrates in place in the various tanks. The frames for maintaining the substrates in the ELD bath were used solely for this bath to prevent contaminating the bath with Pd/Sn and shifting the pH of the NiB bath. The glass substrates were plated on both faces for ∼6 min and rinsed thoroughly with water upon removal. The plates were then dried under a stream of N2, characterized, and stored for further processing. Characterization of the Plated NiB and Removal of the NiB on the Back of the Plate. It was possible to plate more than 1 µm of NiB on both sides of the glass before a failure of adhesion of the NiB layer in the plating bath occurred. A 10% thickness variation of the NiB layer was usually observed across the entire 15 in. substrates, as indicated by sheet resistance measurements. The recirculation pattern of the NiB bath and the setup to warm the bath represent two important sources of this thickness variation if they are not optimized. The asdeposited NiB layers were typically smooth but showed a much weaker adhesion to the glass71,72 than sputtered metals and alloys. The as-plated NiB failed the “tape test” when it was thicker than 60 nm, for example. The density of dust particles or Pd/Sn stains on the NiB layer was very low and was in the same range as that for the conventional process using sputtered MoW as the gate material (data not shown). The NiB layer on the back of the plate was removed in HNO3 (25% per volume with water) after spin-coating and baking (at 100 °C) a resist on the NiB on the front to protect it. After dissolving the resist, rinsing with water, and drying the plate, we determined the stress in the plate by measuring the radii of curvature along both axes of the glass substrate.73-75 The mechanical stress of a glass plate covered with a 150 nm thick NiB layer was tensile and on the order of 100 MPa in both directions, which corresponds to a radius of curvature of ∼400 m of the plate. The glass used here had a thermal expansion coefficient of 37.6 × 10-7 °C-1, which is smaller than that of NiB of ∼130 × 10-7 °C-1. The electroless deposition occurred at a temperature of 57 °C, and cooling the NiBcovered glass substrate generates a tensile stress because of the larger contraction of the alloy than that of the glass.74 Although this stress does not account alone for the entire stress in the deposit,76 it should be minimized as much as possible. In our experiments, this was done by operating the bath near its lower limit of (practical) operating temperature. Rinsing the freshly deposited NiB with warm (71) Saubestre, E. B. M. Plating 1965, 52, 982-1000. (72) van der Putten, A. M. T. J. Electrochem. Soc. 1993, 140, 23762377. (73) Kerr, C.; Barker, D.; Walsh, F. Spec. Publ. R. Soc. Chem. 1997, 207, 296-317. (74) Parker, K.; Shah, H. Plating 1971, 58, 230-236. (75) Parker, K. ASTM Spec. Technol. Publ. (Test. Met. Inorg. Coat.) 1987, 947, 111-122. (76) Chen, C.-J.; Lin, K.-L. Thin Solid Films 2000, 370, 106-113.

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water and cooling the plate slowly may also be helpful in reducing the stress further. Patterning the Gate Lines in the NiB Layer. A resist (positive tone) was spin-coated onto the NiB, baked for 1 min at 100 °C, exposed to UV light, and then developed. A 1 min O2 plasma was applied to clean the NiB from any residual resist contaminants. The thin Ni oxide film on the NiB layer was removed by immersing the plate for 30 s in a solution of sulfuric acid in water (pH 1.5). The NiB was then dissolved selectively using the resist as a wetetch mask for an etch system composed of NBSA (7.5 g L-1) and EDA (75 mL L-1) in water (pH adjusted to 9.3 using H2SO4, danger77).78 This etch step is critical because of the galvanic coupling between NiB, Pd/Sn, and species from the bath, which can be reduced on the surface of Pd/Sn.79 The formation of such a galvanic cell accelerates the oxidation and removal of Ni where it is electrically connected to Pd/Sn. Using a solution of HNO3 (25% per volume in water) to etch NiB, for example, induced a strong underetching of the NiB pattern by reducing protons into H2 and using Ni as a source of electrons. The smallest features of the NiB gate lines were underetched by ∼1 µm (up to 10% of their width) when using the NBSA/EDA etch system. It is therefore desirable to employ relatively large alignment marks because these structures are the smallest ones in the pattern and thus the ones most vulnerable to underetching. The etch bath using NBSA and EDA minimized the galvanic coupling by having a more neutral pH than that of the acidic HNO3 etch bath. NBSA oxidizes Ni and tends to remain on the oxidized surface, coordinated to Ni2+. EDA exchanges with NBSA and forms a more soluble complex with Ni via its primary amino groups.80 Glass plates covered with 150 nm thick NiB and the patterned resist were mounted on a Teflon frame and immersed in the center of a 28 L etch bath. Stirring was done by slowly moving the plate across the bath. The time necessary to etch a 150 nm thick NiB layer was ∼4 min, and the average undercut of the pattern was 18.2 MΩ cm-1. EDA-Si (chemical abstract number [1760-24-3]) was from Gelest (Tullytown, PA). Pd/Sn colloids were Cataposit 44 (Shipley Company, Marlborough, MA), and the accelerator was a Fidelity Acid Accelerator #1019 (Captree Chemical Corporation, Amytiville, NY). Derivatization of Glass Substrates with EDA-Si. Glass substrates were alkaline-earth, aluminosilicate-type glass (10K LUX glass 1.1 mm thick, code 1737 from Corning Japan), 15 ×

Electroless Deposition of NiB 15 in.2 in size, and were packaged and used in clean room (class 100) environments. Every glass substrate was inscribed with an identification number on the back, cleaned with water, brushed, and dried using automated substrate cleaners to remove particles from the surface. Some glass plates were cut into 6 × 6 in.2 or 1 × 3 in.2 pieces to develop the grafting and ELD processes on smaller samples first. EDA-Si was grafted onto the 15 in. glass substrates using a SEMI Solvent tool (SEMITOOL, Kalispell, MT) equipped with four 40 L tanks and programmed to spray a solution of EDA-Si in water, rinse, and dry the plates. A cassette with up to six 15 in. glass plates was loaded into this tool; the sequence programmed was to rotate the entire cassette at 30 rpm, to spray EDA-Si 1% in water for 3 min, to rinse the plates with water for 6 min, and to dry the plates by spinning them at 600 rpm and pulsing N2 heated at 150 °C. This tool was also used to wash, rinse, and dry glass plates any time when necessary during a TFT-build sequence. Electroless Deposition of NiB. The setup for the ELD experiments consisted of eight 20 L and two 28 L tanks spaced regularly inside a ∼4 m long custom-made chemical hood (Reynolds Tech, East Syracuse, NY) operating under clean room conditions. Every other tank (i.e. the rinsing tanks) had an overflow system connected to a joint drain to allow the continuous addition of water and to keep the tanks free of contaminants. A water-spray gun and a N2 gun were mounted in the center of the hood. Most of the tanks had a N2 and a water supply line. Two tanks were equipped for recirculation as well as with heaters (supplied by Reynolds Tech) located near the back of the hood inside the maintenance area of the clean room. The tank used for prewarming the plates prior to ELD of NiB was connected to a water heater (Reynolds Tech). Draining the tanks was done using electrical pumps and valves installed on the Pd/Sn, HBF4, and NiB tanks to reclaim or collect these chemicals in separate waste containers. The EDA-Si treated plate was immersed in the Pd/Sn bath for 30 s, removed, and rinsed in the second tank for 30 s (while bubbling some N2 for 10 s to increase the rinsing efficiency), immersed in the third tank for acceleration for 30 s, and finally rinsed in the fourth tank with water for 30 s. The plate was then placed for 30 s in warm water, and from there in the center of the plating bath (Niposit 468, Shipley Company). The bath was prepared by adding 0.56 L of component A, 0.7 L of component M, and 0.35 L of component B to 28 L of water. This composition corresponds to half the strength recommended by the supplier, and helped minimize stress and/or adhesion failure of the NiB layer during deposition on the 15 in. glass substrates. The bath had a pH of 7.2 and plated ∼30 nm of NiB per minute at 57 °C. Plates were removed from the ELD bath after ∼6 min, rinsed in the last tank with water, and dried with a stream of N2. The pH was monitored daily and adjusted to 7.2 using NH4OH. Photoengraving Processes. Spin coating the 15 in. plates covered with NiB was done on a Convac M-600 LCD cleaner/ coater (Fairchild Technologies, Fremont, CA). The photoresist employed was Microposit S 1808 (Shipley Company, Marlborough, MA) and was exposed using a MRS 5001 G line Panel Printer (Azores Corp., Wilmington, MA). The exposed resist was developed using a SSEC Evergreen Model 15 FPD Processor (Solid State Equipment Corporation, Horsham, PA) and a Microposit Developer LDD-26 W from Shipley and removed using a resist stripper from Shipley. The developed resist was briefly ashed using a Plasma-Therm System VII, Series 7000 (PlasmaTherm Inc., St. Petersburg, FL). The tool for the PECVD steps, such as the deposition of the trilayer, was a Plasma-Therm PECVD System. Metals and alloys were sputtered using a Balzers-Pfeiffer VIS 350 (Amherst, NY), and deposition targets were Mo (99.95% purity) and Al (having 2.5% Cu) from Pure Tech (Brewster, NY). Spray etching was done using a SEMITOOL SAT 5180S.

Langmuir, Vol. 19, No. 14, 2003 5935 Instrumentation. The 13C DEPT-135 NMR spectra were recorded on a Bruker Avance 300 MHz spectrometer (Bruker Biospin AG, Fa¨llanden, Switzerland). Baking the NiB-covered plates was done in a programmable Blue M Electric oven, model CC-04-C-E-HP (Blue M Electric, Asheville, NC) equipped with a N2 line. Plates were mounted on a stainless steel frame and positioned vertically in the oven. Experiments on annealing under a forming gas were done with 1 cm2 samples in a Jetfirst Processor oven (J. I. P. ELEC, Crolles, France). Visual inspection of the samples was done optically using a Nikon (Melville, NY) microscope equipped with both a motorized, vibration-free holder capable of receiving 15 in. glass plates and a digital camera. Optical metrology was done using a similar microscope equipped with pattern-recognition software, a programmable motorized stage, and a digital camera. Detection, classification, and counting of the defects were done using an FPI Micro Inspection system (Orbotech Inc., San Jose, CA). SEM images were obtained using a Hitachi S-4000 electron microscope. XPS spectra were acquired on a Sigma Probe VG Scientific spectrophotometer operating at a base pressure of 1.5 × 10-9 mbar and equipped with a monochromatized Al KR source (E ) 1486.6 eV). The X-ray spot was focused down to 400 µm for all experiments. The analyzer had an angle of 37° to the sample, and samples were cut to ∼1 × 2 cm2 and mounted on a multisample holder stage for examination under the same conditions. Spectra were referenced to the O 1s peak from the glass substrates at 532 eV. For all samples, surveys were acquired with a pass energy of 80 eV (0.1 eV steps for 20 ms). The current of the primary beam was set to 10 mA for all spectra and varied within 10%. The charge compensation used a flood gun (∼0.25 µA emission current) at a partial pressure of Ar of ∼5 × 10-8 mbar. Local roughness measurements were performed with an AFM Nanoscope Dimension 3000 (Digital Instruments, Santa Barbara, CA) operated under ambient conditions in tapping mode. The profile and radius of curvature of the 15 in. substrates were obtained using a Tencor P-11 Surface Profilometer (KLA Tencor, Mountain View, CA). The radii of curvature were measured in both directions on the glass plates as received and were measured again after each processing step. The conductivity probing system was a Flat Panel Display Prober model LCD 2424 (TNP Instruments Inc., Carson, CA), and the array tester was custom built.82

Acknowledgment. We are grateful to R. W. Nywening, H. Kind, and L. T. Romankiw for their initial help with electroless deposition, to F. R. Libsch, M. Mastro, and R. J. Polastre for the characterization of the TFT arrays, to M. B. Rothwell, K. F. Latzko, H. Ifill, S. H. Libertini, R. G. John, and D. Lisounenko for their invaluable help in building TFT arrays and assistance in the clean room, and to E. G. Colgan and K. Schleupen for their advice on TFT and display performances. We thank our colleagues A. Bietsch, P. Schmidt-Winkel, J. J. Ritsko, R. Magnuson, T. L. Breen, S. Hall, and R. Stutz for useful discussions. We acknowledge financial support from P. F. Seidler, E. P. Harris, and from the IBM Display Business Unit. LA0341714 (82) Jenkins, L. C.; Polastre, R. J.; Troutman, R. R.; Wisnieff, R. L. J. IBM Res. Dev. 1992, 36, 59-68.

Electroless Deposition of NiB on 15 Inch Glass ... - ACS Publications

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