Self-Assembled Monolayers of Alkanethiolates on Palladium Are Good Etch Resists

Abstract

Self-assembled monolayers (SAMs) formed from alkanethiols on palladium resist corrosion by solution-phase chemical etchants, regardless of the chain length and wettability of the SAM. This insensitivity to chain length contrasts with SAMs on gold, for which a hydrophobic, well-ordered, crystalline alkane film is required to prevent access of water-soluble etchants to the underlying metal. 1,2 This paper demonstrates that dense SAMs form on contact of hexadecanethiol (C 16 SH) and palladium, and that these SAMs are notably more useful resists against wet-chemical etchants than are those on gold. 3 This system, however, differs from SAMs on gold in that the interfacial layer between the palladium and the organic film contributes the major part of the etch resistance of the system: a number of thiols differing substantially in hydrophobicity and chain length (C 16 SH, C 12 SH, HO(CH 2 ) 2 SH, C 6 H 5 SH, HO 2 -CCH 2 SH) provide comparable levels of contrast as etch resists.SAMs of alkanethiolates on the coinage metals (Au, Ag, Cu) have been used for a range of purposes, 4 including etch resists for microcontact printing (µCP). 5 The use of µCP of SAMs is generally not practical for patterning sub-500-nm features on these metals for two reasons: (i) it leaves defects and surface pits during etching (for gold) and (ii) spontaneous formation of an oxide layer in air complicates the system (for copper and silver). In addition, the coinage metals are incompatible with complementary metal-oxide semiconductor (CMOS) fabrication, and require barrier layers to prevent their contamination of the silicon with deep-level traps for minority carriers. 6 Palladium and its alloys are commonly used in microelectronic elements such as resistors and capacitors. 7 Palladium has several physical properties that suggest it would be a useful substrate for patterning SAMs of alkanethiolates: (i) it is very reactive toward sulfur-containing compounds, (ii) it has a slightly smaller lattice spacing in the (111) plane (2.75 Å) than gold and silver, 8 (iii) it resists oxidation in air below 400°C, and (iv) it forms smaller grains (15-25 nm) by e-beam evaporation than either gold (50-100 nm) or silver (30-50 nm). Here we report that alkanethiols form well-ordered SAMs on palladium, and resist etching with greater selectivity than SAMs on gold. 9 The details of the formation and characterization of the hexadecanethiolate SAM on palladium are described in the Supporting Information. Infrared data indicate that a hexadecanethiolate monolayer forms rapidly on palladium with crystalline molecular packing, and that the order of the SAM is comparable to that of C 16 S SAMs on gold and silver. A high advancing contact angle of water (θ a ∼ 120°) and high hysteresis (θ a -θ r ∼ 20°) suggest that this SAM may be more heterogeneous or disordered at the molecular scale than C 16 Sderived SAMs on either gold or silver. 10 Figure 1 shows structures that we can generate by the combination of µCP and etching on palladium and on gold. 11 Both films were pat...

Soft lithography for micro- and nanoscale patterning

“…Conformal, molecular/atomic-level contact between a PDMS stamp and the surface of a substrate is the key to successful transfer of ink molecules from the stamp to the substrate. µCP is widely used in printing alkanethiols on thin films of Au 10 , Ag 45 , Pd 46 and Pt 47 , and to a smaller extent (and with more difficulties) of alkylsiloxanes on silicon/silicon dioxide or glass 48 . The ink molecules form SAMs on these surfaces during the printing process.…”

Sub-Micrometer Patterning Using Soft Lithography

“…The dimensions of the printed pattern are determined by the size of the raised features on the stamp: they can range from several millimeters to below 100 nm, and are typically reproduced at a 1:1 ratio. Microcontact printing (µCP) [8,11,75] has become a method of choice for patterning self-assembled monolayers (SAMs) of alkanethiols on gold [21,[76][77][78] and on other coinage metals, including silver [79,80], copper [81,82], palladium [83,84], and platinum [85]. This application takes advantage of the high affinity of the sulfur moiety for noble metals, leading to rapid adsorption of molecules under the formation of a covalent and thermodynamically stable metal-sulfur bond.…”

Writing self-assembled monolayers with Cs: Optimization of atomic nanolithography imaging using self-assembled monolayers on gold substrates

“…This resolution issue was addressed by Love et al [25] who reported that Pd substrates yielded better post-etch resolution than their Au counterparts when coated with alkanethiol monolayers for nanolithographic purposes. We have recently reported [26] that a resolution limit of ≃ 20 nm is found when 1-nonanethiol SAMs are adsorbed on sputtered Au substrates.…”

“…There have been numerous reports of substrate-induced effects on the quality of the patterns produced after etching in atomic nanolithography processes [25,27]. However, all such reports have apportioned blame for poor quality on either the roughness of the substrate or on the grain size.…”

“…It can be observed that each domain consists of an ordered arrangement Having characterized the influence of the Au surface on the coverage uniformity by the alkanethiol monolayer, we now turn to the exposure of this monolayer by a cold atomic Cs beam. Many groups report that alkanethiol resist-based nanolithography is not very reproducible [25,52]. This lack of reproducibility is partially due to the quality of the Au substrate.…”