Nanocontact Printing and Etching with Phase Separated Membranes

Aloys Senefelder used in 1796 a porous stone (in Greek, lithos) as a tool for printing by patterning the stone with ink attracting (hydrophobic) and ink repelling (hydrophilic) regions. Lithography for semiconductor mass fabrication and other microsystem and nanotechnology applications has nowadays regained interest in inexpensive microprint-ing methods as an alternative or complement on current high tech optical wafer stepper technology. A need exists therefore in the art for a convenient, inexpensive, and reproducible method of plating or etching a surface according to a predetermined pattern. The method would ideally find use on planar or nonplanar surfaces and would result in patterns having features in the micrometer and submicrometer domain. Additionally, the method would ideally provide for convenient reproduction of existing patterns. Additionally, the need exists for the fabrication of surfaces that can pattern portions (e.g., SAMs) amenable to attachment of biological species, such as antibodies, antigens, proteins, cells, etc., on the (sub)micrometer scale.

The study of self-assembled monolayers (SAMs) is an area of significant scientific research. Such monolayers are typically formed of molecules each having a functional group that selectively attaches to a particular surface, the remainder of each molecule interacting with neighboring molecules in the monolayer to form a relatively ordered array. Such SAMs have been formed on a variety of products including metals, silicon dioxide, and gallium arsenide using relief printing with a molded stamp made from polydimethylsilox-ane (PDMS) [79]. The upper relief part of the stamp provided with a suitable SAM coating is then being contacted

Figure 29. SEM micrograph of a high-porosity microsieve made with the lift-off method. Courtesy of Applied Optics Group, University of Twente.

Figure 27. SEM micrograph of the microsieve membrane showing pores with a diameter of 260 nm in a 100 nm thick silicon nitride layer. Courtesy of Nanotechnology.

Figure 29. SEM micrograph of a high-porosity microsieve made with the lift-off method. Courtesy of Applied Optics Group, University of Twente.

with a product with a high affinity for the SAM species and a conformal SAM pattern is formed on the product (e.g., alkanethiol pattern on a gold coated product).PDMS is a rather elastic and relatively strong material very well suited for reproducible contacting purposes on nonplanar surfaces; however, it lacks a microporous microstructure for enabling functional fluid (ink) transport to the product domains or to enable other functional properties as will be described.

In Figure 30 three basically different printing techniques are represented.

Microporous stamps 21 (Fig. 30) with (alternating) regions with a dense skin layer and adjacent regions with a (porous) layer without the skin layer have easily been made with a phase separation process by locally removing the skin layer by, for example, oxygen plasma etching with the aid of a perforated mask shielding the remaining dense skin layer regions. The stamps are made with a phase separation process with the aid of a mold having patterned regions with sharp protrusions penetrating the microporous layers and

Figure 30. Left: the art of relief printing. The upper relief part 2 of the stamp 1 provided with a suitable ink coating 3 is contacted with a substrate 7 with a high affinity for the ink species and a conformal pattern 4 is formed on the substrate 7. The lower relief part 5 may be made ink repelling with a suitable coating (e.g., PVA, PVP) in order to avoid smearing of the pattern 4 of ink originating from sections 5. The upper relief part of the stamp 1 is provided with a macro- or nano-porous structure to contain ink or to transport ink from an injection point 6 for reproduction or continuous printing of the pattern 4 on the substrate 7. Middle: the art of gravure printing. The engraved part 12 of the stamp 11 provided with a suitable ink coating 13 is then contacted with a substrate 17 with a high affinity for the ink species and a conformal pattern 14 is formed on the substrate 17. The nonengraved part 15 may be made ink repelling with a suitable coating (e.g., PVA, PVP) in order to avoid smearing of the pattern 14 of ink from sections 15. The engraved part of the stamp 11 is provided with a macro- or nanoporous structure (cf. SEM picture below: engraved part is micro-porous, nonengraved part has a dense skin layer) to contain ink or to transport ink from an injection point 16 for reproduction or continous printing of the pattern 14 on the substrate 17. Right: the art of plano-graphic printing (i.e., art lithography). The ink delivering part 22 and the nonink delivering part 25 of the stamp 21 are not determined by a difference in height but are made by the provision of suitable ink repelling and ink attracting coating s. The stamp 22 with a suitable ink coating 23 on part 22 is then contacted with a substrate 27 with a high affinity for the ink species and a conformal pattern 24 is formed on the substrate 27. Part 25 may be made ink repelling with a suitable coating (e.g., PVA, PVP) in order to avoid smearing of the pattern 24 of ink to sections 25. Part 22 is provided with a macro- or nanoporous structure to contain ink or to transport ink from an injection point 26 for reproduction or continuous printing of the pattern 24 on the substrate 27. Also in another embodiment part 25 may be microporous and filled with an ink repelling medium (e.g., water; Senefelder 1796).

patterned regions without such sharp protrusions where a dense skin layer is formed. In case the skin layer is not dense but nanoporous the skin layer can of course first be hermetically sealed without sealing the microporous part of the stamp with, for example, a hydrophilic coating (e.g., aliphatic and cyclic olefin-based polymers, or fluoropolymers or silicon based polymers). The stamps may also be subpatterned through use of photosensitive precursors in the casting solution of the product.

In one embodiment a (coplanar) stamp 11 with alternating nanoporous hydrophilic and dense hydrophilic surface regions is locally filled with an aqueous chromium etch solvent and brought into contact with a substrate having a chromium layer with a thickness of 20 nm. Whereas the dense regions 13 locally protect the chromium layer, in the nanoporous regions 12 an exchange between the chromium layer and the etch solvent may result in a locally dissolved and patterned chromium layer. Instead of chromium, many other materials or combinations of materials are applicable (e.g., aluminum, metal oxides and nitrides, metals, semiconductors, polymeric lacquer layers, etc.). The chromium layer may be replaced by a one phase lacquer layer and the solvent may be replaced by a second phase vulcanizing agent for the one phase lacquer layer. Instead of solvents also reactive gases can be used to etch patterns (e.g., SF6 to etch and pattern silicon products). The microporous stamp can also be used to dab or adsorb locally a liquid or viscous layer that has been casted on a product. Dabbing may be improved by locally compressing the microporous regions during the contact of the stamp with the product. See Figure 31.

Figure 31. Left: Microporous stamp 1 produced with a phase separation process on a suitable mold. At left is a cross-section of a poly-imide microporous microprinting tool with a smooth skin layer as obtained with a phase separation process. Courtesy of Aquamarijn Research/Membrane Technology Group, University of Twente.

Figure 31. Left: Microporous stamp 1 produced with a phase separation process on a suitable mold. At left is a cross-section of a poly-imide microporous microprinting tool with a smooth skin layer as obtained with a phase separation process. Courtesy of Aquamarijn Research/Membrane Technology Group, University of Twente.

In order to facilitate mask alignment of different mask stamping steps and to reduce thermal expansion differences between the stamps and the product, stamp regions parts are provided on a transparent (e.g., Pyrex, borosilicate glass) support material with the same thermal expansion coefficient as the substrate. Preferentially the microporous parts leading to the injection point have a high inner porosity to reduce flow resistance and a relatively small total dead volume in order to reduce the amount of adsorbed species.

In some cases it has proven to be useful to first print an ink pattern with the stamp on an intermediate dense or microporous transfer foil that transfers the ink pattern subsequently to the substrate, especially foils which have a well defined wetting contact angle with the selected ink medium.

The substrate may also first be provided with a suitable adsorbive nonsplattering or nanoporous (sacrificial) coating for more adsorption of the ink (e.g., obtained with a phase separation method). In another embodiment this nanoporous coating is made by deposition of an aluminum layer with a thickness of 200 nm on a silicon wafer and transforming this aluminum layer to a nanoporous (porosity 60-90%) honeycomb structure with thin vertical walls (pore spacings 10-50 nm) by anodic oxidation techniques well known in the art. Silicon and many other materials with different layer thicknesses can be transformed as well with anisotropic etching techniques for similar purpose. After the local deposition of the ink or an etch resistant lacquer in and/or on this layer, and preferentially dissolving the remaining uncovered layer, the substrate is ready for further processing steps.

Of course stamps may also be used for the formation of microstructures or microtransfer molding on planar and nonplanar surfaces of polymeric, ceramic, or metallic articles.

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