Solution-Based Photo-Patterned Gold Film Formation on Silicon Solution-Based Photo-Patterned Gold Film Formation on Silicon Nitride Nitride

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[3] Devices and applications exploiting these beneficial native features can be augmented and improved using designer metal overlayers that fulfill structural roles, serve as electrodes, and provide alternative surface chemistry options, including as a platform for subsequent thiol monolayer self-assembly.The field of nanopore single-molecule sensing offers compelling examples of the prospects of merging SiNx thin films and designer metal layers into devices, and does this within a nanofluidic context where the need for versatile metallizing approaches is clear. 3- 7 4][5][6][7][8] The nanoscale dimensions of the SiNx film and pore can be significant barriers to efforts to incorporate such functional metal films, particularly when the interior of the pore must be metallized.Solution-based metallization routes offer an appealing route with natural compatibility with micro-and nanofluidic devices.In this domain, nanopores represent an extreme fabrication challenge owing to their small size in all directions.In fluidic devices more amenable to conventional micro-and nanofabrication routes, such as fluid cells for transmission electron microscopy on liquids, 1,3 the channel dimensions can be nanoscale in height but millimeters in width.In such platforms, the design of low-overhead patterned metallization strategies should carefully account for the potential challenges of the <100 nm thickness of the free-standing, supporting SiNx channel floor and ceiling.] Solution access, rather than line-of-sight as in physical vapor deposition, dictates where surface plating will occur, so that electroless plating is an appealing choice for fashioning nanofluidic devices where even irregular and concealed surfaces may require metallization.] To fully exploit solution-based metallization as a tool for micro-and nanofabrication, however, requires control not just over the plated film composition, thickness, and grain size, but also over its spatial disposition, which must be at least partly independent of underlying substrate patterning. 15We wanted a patterning approach that did not need mechanical access to target surfaces, both to improve the generality of the approach, and to minimize the risk of damage that can accompany repeated handling of thin films-especially of free-standing thin-films.We sought to develop a gentle, solution-based patterned metallization approach [16][17] capable of plating a range of even structured substrates, including inside existing (nano)fluidic channels. 3,7,[14][15]18 he horizons of single-molecule science have recently been dramatically expanded by the development of simple methods for fabricating nanopores: entirely solution-based processes requiring only uncomplicated instrumentation are removing barriers to the widespread use of nanopore methods. 19To conserve the benefits of simple pore formation methods, our focus also included developing similarly widely-accessible, straightfoward solutionbased approaches to patterned metallization.][22][23] In the conventional approach to substrate patterning by optical lithography, the target surface is coated with photoresist, usually by spin-coating.After baking, the photoresist is irradiated through a mask, developed, and then selectively removed by dissolution or etching to reveal the patterned surface.To simplify the processing, we chose instead to only attach the protective layer where it was desired, by photo-patterning the covalent attachment of an organic monolayer to SiNx, 24 and to investigate its ability to then template the substrate metallization.With the use of substrate immersion and an initially liquid patterning precursor (here, 1-octene), we sought to gain greater tolerance to irregularities-including the presence of engineered structures such as micro-and nanofluidic channels-of the SiNx surface.
For metallization, we initially adopted an electroless plating approach that had been specifically developed for gold-plating SiNx. 7,25  approach is outlined in Scheme 1, and full details of materials, instrumentation, and safety precautions are provided in the Supporting Information (SI).We had previously developed a gold electroless plating approach for SiNx that required a hydrofluoric acid (HF) etching step prior to surface metallization 7,25 .The HF-etched surface offered a natural starting point to incorporate patterned monolayer formation in an effort to guide the spatial extent of the substrate metallization.
An alkane monolayer can be covalently linked to a freshly HF-etched SiNx surface through the photochemically-driven hydrosilylation of a 1-alkene. 24Tremendous care must be exercised in the use of HF, and we detail the precautions-including additional protective equipment and monitored work-in the SI.The UV (254 nm) photoirradiation was through copper transmission electron microscopy (TEM) grid masks, with different bar sizes and spacings (see SI for specifications), that had been placed directly on the wafer (without securing them or preventing liquid access underneath), with both wafer and mask then immersed in the 1-alkene.Plating selectivity depended on rigid adherence to the rinsing steps detailed in the SI, and, as in prior work, we ensured compatibility of the process with free-standing ultrathin SiNx membranes by avoiding ultrasonic cleaning steps. 20heme 1: A SiNx substrate is (a) plasma treated and hydrofluoric-acid etched, then (b) immersed in 1-octene for photopatterning (254 nm) through a TEM grid.The patterned substrate is then (c) immersed in a series of metallizing solutions to yield (d) a patterned gold film.Structural features are not to scale.A detailed description of solution compositions and process flow is provided in the SI.
We proposed to spatially pattern LPCVD SiNx metallization by forming a physical barrier on the surface to control where the metal plating could take place.The first step of patterned plating thus involved the formation of this patterned protective layer.In our prior work to develop an electroless gold plating procedure for SiNx, we found it was essential to first etch the SiNx surface with dilute HF. 7 This same initial etching step forms the starting point for the covalent attachment of 1-alkenes (or 1-alkynes) by photochemical (or thermal) hydrosilylation on silicon-rich SiNx 2,24 to form alkane monolayers that could potentially function as a barriers for electroless plating.
Photoirradiation using a UV lamp (254 nm) proved convenient in transferring the spatial patterning offered by a selection of copper transmission electron microscopy (TEM) grids (Figure 1a) to the SiNx surface.Figure 1b is a photograph of a representative substrate after patterned irradiation through a thin (<2 mm) layer of neat 1-octene held under a quartz plate in a specially constructed holder.This optical micrograph taken during the evaporation of a dichloromethane drop placed on the surface reveals the transfer of the TEM grid pattern to the surface-functionalized substrate.
Dichloromethane proved the most expedient of several different solvents to photograph the surface pattern.Such patterned substrates were then electrolessly gold-plated, using the three-solution-Sn (II)/Ag (I)/Au (I)-process beginning with Sn (II) sensitization that had been proven successful for HF-etched SiNx (see SI for complete details of metallization solutions and process flow). 7,25  pink-tinged, hazy film extending across the substrate in Figure 1c is gold plated onto regions of the SiNx that had been subjected to the protection step of photoexposure while immersed in the 1-octene.While gold replicas of the TEM grid masks can be seen in Figure 1c, it is also apparent that the plating spatial selectivity was quite poor compared to its Pd (II)-initiated counterpart, Pd (II)/Ag (I)/Au (I) (vide infra, and calculation details in SI).The chemical nature of the surface sensitizing species, and its interactions with the surface, have a strong influence on the performance of a particular electroless plating formulation.The Sn (II) sensitizer is tolerant to substrate composition, which is frequently beneficial, but it is clearly-in this instance, at leastdetrimental to patterned metallization. 13,23 igure 1d provides a magnified view, by field emission scanning electron microscopy (FE-SEM), of a Sn (II)/Ag (I)/Au (I)-metallized substrate.][5][6][7]  We abandoned Sn (II)-sensitized electroless plating when efforts to improve the spatial selectivity by using different rinsing steps, for example, proved ineffective.We tested, instead, a palladium-based treatment 27 in place of the Sn (II) sensitization step to give an overall process flow of Pd (II)/Ag (I)/Au (I).The use of this Pd (II) surface treatment solution delivered extremely high pattern fidelity, as seen in Figures 1e and 1f.The rich chemistry of the native SiNx surface, and of the palladium species, complicates the determination of the mechanism, and indeed may allow for multiple mechanisms to be simultaneously operational. 3,13,23,28 Fiure S2 shows the results of several process chemistry variations, all delivering lower metallized pattern quality than seen in Figures 1e and 1f, but nevertheless highlighting a setting rich with opportunity for fundamental study and applications.For example, substrate photopatterning through an air layerlikely through a photochemical oxidation route similar to that seen on silicon 29 -instead of 1-octene (Figure S2) yielded spatial selectivity degraded by smudges of gold across the surface.
The patterned monolayer-templated route offers benefits beyond preserving pattern quality.
Photohydrosilylation offers lower process overhead and better compatibility with fluidic channels than conventional photoresist-based approaches, and a suitable hydrosilylated monolayer confers some resistance to any subsequent HF etching, but can be readily removed if necessary (Figure S3). 2,18,24 Te metal plating selectivity when using 1-octene with Pd (II) surface treatment as the first step was easily reproducible across scores of patterned gold depositions when scrupulous adherence to the rinsing steps was maintained.The results shown in Figures 1e and 1f are thus representative and reproducible.
We focus in this work on characterizing the spatial selectivity and the physical structure of the gold layers resulting from this successful initial Pd (II) surface treatment.We present analyses of gold replicas produced after ~30 minute immersions in the Au (I) bath.This duration provides a balanced perspective of film nascence and degree of spatial selectivity.Examination of gold replicas using digital holographic microscopy (DHM; Figure 1g) allowed us to determine that the gold films were ~23±1.5 nm thick.Higher magnification scanning electron micrographs in Figure 2 upheld the quality of selectivity demonstrated in Figures 1e and f.There was only sparse gold coverage where the photoirradiation had installed the protective layer, between the mask grid lines.
The gold grid lines, themselves, could be resolved into gold features with 28±5 nm mean diameters providing ~83% surface area coverage (across 15 different grids, with a 13% standard deviation) after the 30 minutes of immersion in the gold plating bath at ~3°C.This surface coverage is reflected in measured film (sheet) resistivities of ~10.6±2.1 µ•cm that are unsurprisingly higher than the 2.2 µ•cm for bulk gold. 7Systematic studies of surface sensitizer preparations have clearly established principles to control the plated film grain size and grain size distribution, and these can be explored for patterning, as well. 23The degree of infilling shown here is high in the context of low-process-overhead patterned metallization steps, 30 and particularly when targeting suitability for use with structured surfaces incompatible with more involved conventional patterning, such as in enclosed micro-and nanofluidic channels.To explore the spatial patterning in further detail, we focus on gold replicas of 100 mesh copper grids.The copper bars of these grid masks were 54.4±1.3 m wide (measured by FE-SEM with analysis details in the SI), and they were placed on the SiNx surfaces under 1-octene without securing them or attempting to prevent liquid access underneath.The spatial selectivity, defined in a classical signal-to-noise sense (details in the SI), was ~10.1 for the 1-octene-patterned Pd (II)/Ag (I)/Au (I) route that we focus on here, in contrast to ~2.7 for the 1-octene-patterned, Sn (II)-sensitized route, and ~3.2 for the former solution steps with air-patterning in place of 1-octene.In addition to FE-SEM micrographs, we collected elemental maps from representative gold replicas using energy-dispersive x-ray spectroscopy (EDS; also commonly abbreviated EDX).The maps and electron micrographs in Figure 3a,b are consistent with a thin gold overlayer on SiNx that possesses a high degree of infilling and spatial selectivity.We used FE-SEM and EDS line profiles across the open spaces and grid lines to characterize the gold replica lines and the edge resolution, with procedural details provided in the SI.The mean line width of the gold bars in the FE-SEM images of the gold replicas was 44.8±3.3 µm, measured from more than 300 lines from each of 9 chips.To extract the edge resolution, we fit the Au-channel EDS intensity versus linear position to Boltzmann functions and recovered sub-micrometer (0.92±0.24 µm; 15 EDS line profiles) transition widths from metal-free to metallized segments.We developed a solution-based method to form spatially patterned metal features on silicon-rich SiNx thin films.This approach leverages the benefits of electroless plating and establishes a lowoverhead surface-patterning approach suitable for SiNx thin films.We ensured that spatial selectivity could be achieved without using ultrasonic excitation or other mechanically disruptive manipulations so that the patterning approach would be compatible with free-standing thin SiNx membranes useful in a host of other applications, particularly for micro-and nanofluidics.
Photochemical hydrosilylation linkage of organic monolayers to SiNx is a flexible and appealing route to surface-functionalize SiNx, especially in conjunction with spatial patterning.The templating monolayer may serve as a permanent or removable coating, protecting the underlying SiNx or being removed to expose it after metallization.The ability to readily modify the surface functional groups of these high quality monolayers using standard chemical transformations 2 dramatically widens the prospects of this simple patterned metallization approach.The alreadyexcellent metallization selectivity could conceivably be further improved and prolonged by tuning the monolayer electrostatics and hydrophobicity, for example.1][12]23 More tantalizingly, a base monolayer may be used as a platform for further chemical tuning of the surface, in which demonstrated properties and function 2 can be installed around the patterned gold layer.Thus, we contend that the patterned metallization strategy introduced here is promising and useful not only for delivering a spatially-selective solution-derived metal film, but one primed for further development.
ASSOCIATED CONTENT Supporting Information.Experimental details, method, and sample characterizations.This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 1 :
Figure 1: (a) Photographs of 50 and 100 mesh copper TEM grids on a SiNx-coated silicon chip;

Figure 2 :
Figure 2: (a) FESEM image of a subsection of a 100 mesh pattern on a SiNx chip processed with

Figure 3 :
Figure 3: (a) A composite of an electron image (top) and three EDS maps (descending from