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Electroless metal discharge layers for electron beam lithography
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13.SAL-601ER7, diluted 1:4 by volume with Thinner Type A (both Shipley Co.), was spin coated (4000 rpm, 30 s) onto a Cr on glass mask (Cr thickness∼100 nm), baked (75 °C, 30 min), (Ref. 14) exposed (6 mW/cm2, 10 min, Suss MJB3 deep UV (220–230 nm) Hg lamp contact aligner), thermally cross linked (115 °C, 2 min, vacuum hotplate), and plasma etched (2 min, 10 mTorr 100 W rf power) to form surface OH groups. The ∼0.2 μm thick film (Tencor Alpha-Step 500 Surface Profilometer) functioned as an effective planarizer for the Cr features.
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16.Aqueous EDA (1% wt) in 1 mM acetic acid was puddled onto a substrate for 2 min, spin coated (3000 rpm, 15 s) under a stream of rinse water, air dried on the spincoater (3000 rpm, 15 s), and baked (120 °C, 4 min) to complete the chemisorption (Ref. 15).
17.S. L. Brandow, M.-S. Chen, T. Wang, C. S. Dulcey, J. M. Calvert, J. F. Bohland, G. S. Calabrese, and W. J. Dressick, J. Electrochem. Soc. 144, 3425 (1997).
18.W. J. Dressick, M.-S. Chen, S. L. Brandow, K. W. Rhee, L. M. Shirey, and F. K. Perkins, Appl. Phys. Lett. 78, 676 (2001).
19.Cuposit® 328 bath was chosen due to its commercial availability (Shipley Co.) and its ability to plate Cu under ambient conditions. The bath was prepared immediately prior to use by adding Cuposit® 328A, 328Q, and 328C bath component solutions to water in a 12.5:12.5:2.5:72.5 volume ratio, respectively.
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21.S. L. Brandow, W. J. Dressick, C. R. K. Marrian, G.-M. Chow, and J. M. Calvert, J. Electrochem. Soc. 142, 2233 (1995).
22.Pd/Sn catalyst was freshly prepared from Cataposit® 44 concentrate and Cataprep® 404 salt (both from Shipley Co.) by dissolving 10 g of Cataprep ® 404 salt in 45 mL water in a 50 mL volumetric flask, adding 1.5 mL of Cataposit® 44 concentrate by pipet, and diluting to the mark with water. The catalyst dispersion obtained could be used for at least three days, provided that an Ar gas blanket was replenished in the tightly sealed container after each use. The Pd/Sn catalyst, passed through a 0.2 μm Teflon® filter, was applied to the substrate to be metallized for 5 min and excess catalyst was carefully removed using a Pasteur pipet. The substrate was gently rinsed three times with 0.1 M HCl (aq) solution, treated 90 s using a 10% strength Accelerator 19® (Shipley Co.) solution, carefully rinsed once with water, and immersed in the Cuposit® 328 EL Cu bath to initiate plating. After metallization was complete, the sample was carefully rinsed with water and dried in a stream of filtered gas (liquid boiloff).
23.Ability to fabricate a sufficiently homogeneous, thin, transparent Cu film using the Pd/Sn catalyst was critically dependent on the presence of EDA. The Pd/Sn catalyst did not adsorb well directly to fused silica or Cr, leading to patchy, irreproducible Cu deposition. Although the Pd/Sn catalyst did bind planarizer directly, complete substrate coverage by Cu required the deposition of thick (i.e., >100 nm), essentially opaque Cu films unsuitable for use as discharge layers.
24.Electrical resistivity decreased as expected with increased Cu thickness. For example, Cu thickness (resistivity) values of and were obtained for 60 and 90 s plates, respectively. However, decreases in transparency and film adhesion were observed for Cu plated >90 s. Because air oxidation of the thin Cu films increased electrical resistivity, films not immediately used in e-beam patterning experiments were stored in a glovebox under atmosphere Samples stored in this manner maintained their original electrical resistivity for at least three days.
25.UVII resist (Shipley Co.) was used as received from the manufacturer. The ∼500 nm films were prepared by spin coating (∼2000 rpm, 30 s) the resist solution and baked (130 °C, 60 s) on a vacuum hotplate prior to use. For UVII resist of this thickness, Monte Carlo calculations predicted and experimental results using our e-beam writer verified [vide infra, Figs. 2(b) and 3(b)] that beam induced currents were sufficient to discharge the resist in the presence of our grounded EL Cu layers.
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