Mu-San Chen

Fabrication and selective surface modification of 3‐dimensionally textured biomedical polymers from etched silicon substrates

A new method is described for producing biomedically relevant polymers with precisely defined micron scale surface texture in the x, y, and z planes. Patterned Si templates were fabricated using photolithography to create a relief pattern in photoresist with lateral dimensions as small as 1 micron. Electroless Ni was selectively deposited in the trenches of the patterned substrate. The Ni served as a resilient mask for transferring the patterns onto the Si substrate to depths of up to 8.5 microns by anisotropic reactive ion etching with a fluorine-based plasma. The 3-dimensional (3-D) textured silicon substrates were used as robust, reusable molds for pattern transfer onto poly (dimethyl siloxane), low density poly (ethylene), poly (L-lactide), and poly (glycolide) by either casting or injection molding. The fidelity of the pattern transfer from the silicon substrates to the polymers was 90 to 95% in all three planes for all polymers for more than 60 transfers from a single wafer, as determined by scanning electron microscopy and atomic force microscopy. Further, the 3-D textured polymers were selectively modified to coat proteins either in the trenches or on the mesas by capillary modification or selective coating techniques. These selectively patterned 3-D polymer substrates may be useful for a variety of biomaterial applications.

Color changes in chitosan and poly(allyl amine) films upon metal binding

The intense coloration of butterflies, snakes, hummingbirds and arthropods is due to reflective interference by stacked thin film layers of alternating high and low index of refraction materials. By controlling film thickness, it is also possible to create a single layer film of similar materials with well-defined color. Cross-linked chitosan and poly(allyl amine) hydrochloride thin films were assembled by dropping a dilute solution onto a spinning silicon substrate. Film quality and reproducibility were investigated, as well as the effects of cross-linking. Control of thickness could be used to determine film color. When dipped into a variety of metal ion solutions, the cross-linked films changed in thickness and color.

Ion beam modification and patterning of organosilane self‐assembled monolayers

The patterning and modification of organosilane self‐assembled monolayers on Si native oxide surfaces by low‐ and high‐energy ion beams were investigated. The nature and extent of low‐energy (50–140 eV) Ar+ ion‐induced modification of a 2‐(trimethoxysilyl) ethyl‐2‐pyridine monolayer was studied by x‐ray photoelectron spectroscopy and by the quality of the electroless Ni patterns obtained. C(1s) and N(1s) core level x‐ray photoelectron spectroscopy indicated that the ion‐induced modification of the monolayer involved loss of the ethylpyridyl chain by sputtering and/or decomposition. The type of modification was independent of the ion energy and fluence, but the extent of modification depended on both parameters. The modification of the pyridine monolayer was monitored by the percent loss in the N(1s) peak area; modification commenced at a fluence of 5×1014 ions/cm2 and was observed for all ion energies studied. However, selective electroless metallization occurred only for monolayers that suffered ≳50% los...

Channel‐Constrained Electroless Metal Deposition on Ligating Self‐Assembled Film Surfaces

Channel-constrained metallization is described as a novel process for fabrication of metal features useful as etchmasks and electrical interconnects in microelectronics applications. The method creates a requisite surface reactivity template through patterned exposure and development of photoresist films to open channels to an underlying ligand self-assembled film. Subsequent electroless metal deposition occurs selectively at exposed ligand sites in the channels, which constrain lateral metal growth detrimental to feature critical dimension (CD) control during plating. A characterization of the individual process steps is presented using a positive tone photoresist system as an example. Determination of the exposure and development conditions that promote clearance of photoresist residues from the channels while maintaining adequate feature CD control is identified as an important issue in successfully performing the process. The process has been successfully demonstrated using optical exposure sources and is compatible with a range of substrates relevant for electronics applications, including Si. The high plasma etching selectivity of a thin Ni metal masking layer was used in the fabrication of high aspect ratio structures (≤5:1) in Si.

Adhesive Electroless Metallization of Fluoropolymeric Substrates

A process for producing patterned metal deposits on fluoropolymeric substrates is described. A metal ion—chelating organosilane is chemisorbed by self-assembly onto a fluoropolymer surface after radio-frequency glow discharge plasma surface hydroxylation. Positional modulation of the surface hydrophobicity is illustrated by wetting. The silane covalently binds an aqueous palladium catalyst and subsequent electroless deposition yields homogeneous or patterned metal deposits that exhibit excellent adhesion to the fluoropolymer.

Size‐Controlled Colloidal Pd(II) Catalysts for Electroless Ni Deposition in Nanolithography Applications

A new Pd(II) electroless metal deposition catalyst dispersion, PD2, prepared by quenching a PdCl 4 2 solution with HCl and excess NaCl following rapid hydrolysis at pH ∼ 7 and ∼0.8 mM NaCl is described. The precursors to the catalytic Pd(0) species are shown to be chloride-rich Pd(II) colloidal particles having negative surface charge by x-ray photoelectron spectroscopy, UV-visible spectroscopy, centrifugation, and chemical tests. The particles bind selectively and covalently at ligand-modified surfaces with complete surface coverage occurring for treatment times ≥2 min. Atomic force microscopy indicates that the average and maximum sizes of the bound particles are 9 ± 3 and 18 nm, respectively. A correspondingly narrow distribution (15 to 33 nm) of Ni particles of average size 21 ± 5 nm is obtained following metallization of catalyzed surfaces. The ability to control Ni particle morphology using PD2 is successfully exploited in the selective metallization of ∼15 nm features patterned by scanning tunneling microscopy. Metallization occurs with minimal distortion of feature geometries and no pattern degradation due to Ni overgrowth or bridging of adjacent features. Catalyst behavior is well described by a model in which domination of particle nucleation events and dispersion medium chemistry during colloid formation determine particle surface binding, stability, size, and dispersity

Characterization of a colloidal Pd(II)-based catalyst dispersion for electroless metal deposition

Abstract An aqueous Pd(II) dispersion, useful as a catalyst for the selective electroless deposition of nickel metal at ligand-bearing surfaces, is prepared by the hydrolysis of PdCl 4 2− at pH 5 in an approximately 0.01 mol dm −3 NaCl solution. The catalyst dispersion is characterized by UV-visible absorption spectroscopy, electroless metallization, ultracentrifugation, and electrophoresis. The dispersion is found to consist of a distribution of anionic and uncharged Pd(II) species ranging in type from monomeric to colloidal. The species responsible for the initiation of electroless metal deposition at the ligand surface are identified as colloidal. Atomic force microscopy indicates that the colloidal catalysts are bound at the surface and range in diameter from approximately 4 to 53 nm with an avarage size of 30 ± 12 nm. The behavior of the catalyst dispersion is consistent with a model in which colloid formation is initiated by polymerization of monomeric precursors generated by successive hydrolytic Cl − loss from PdCl 4 2− , and deprotonation of the corresponding aquo-Pd(II) complex(es).

Deep ultraviolet patterning of monolayer films for high resolution lithography

A new process has been developed for high resolution photolithography that employs chemisorbed monolayer films as the surface imaging layers. Organosilane treated surfaces are exposed to patterned deep UV radiation, either from excimer laser or lamp sources. The photochemical process modifies the surface wettability and reactivity of the film. Organosilane films patterned by deep UV radiation are treated with a Pd/Sn catalyst and then metallized with electroless copper and nickel baths to yield metal films several 100 A thick. The metal is selectively deposited in the unexposed regions of the film to produce a positive tone image. The patterned metal film is then utilized as a plasma hard etch barrier in a reactive ion etch, allowing efficient pattern transfer into the underlying substrate and producing features with linewidths to 0.4 μm. Electrical testing of processed substrates demonstrates compatibility of the process with subsequent device performance, and working transistor test structures have been...

Imaging layers for 50 kV electron beam lithography: Selective displacement of noncovalently bound amine ligands from a siloxane host film

We report the development of an imaging layer technology for 50 kV electron-beam lithography based upon the displacement of noncovalently bound amine ligands from a siloxane host film. The patterned films were used as templates for the selective deposition of an electroless nickel film resulting in a positive tone imaging mechanism. The deposited nickel was sufficiently robust to function as an etch mask for pattern transfer by reactive ion etching. Metallized and etched patterns with linewidths to approximately 40 nm are demonstrated using an exposure dose of 500 μC/cm2.

Formation of Aromatic Siloxane Self-Assembled Monolayers

We describe reproducible protocols for the chemisorption of self-assembled monolayers (SAMs), useful as imaging layers for nanolithography applications, from p-chloromethylphenyltrichlorosilane (CMPS) and 1-(dimethylchlorosilyl)-2-(p,m-chloromethylphenyl)ethane on native oxide Si wafers. Film chemisorption was monitored and characterized using water contact angle, X-ray photoelectron spectroscopy, and ellipsometry measurements. Atomic force microscopy was used to monitor the onset of multilayer deposition for CMPS films, ultimately allowing film macroscopic properties to be correlated with their surface coverage and nanoscale morphologies. Although our results indicate the deposition of moderate coverage, disordered SAMs under our conditions, their quality is sufficient for the fabrication of sub-100-nm-resolution metal features. The significance of our observations on the design of future imaging layers capable of molecular scale resolution in nanolithography applications is briefly discussed.