Manipulating the adhesion of electroless nickel-phosphorus film on silicon wafers by silane compound modification and rapid thermal annealing

Additively Manufactured 3D Micro-bioelectrodes for Enhanced Bioelectrocatalytic Operation

The drive toward miniaturization of enzyme-based bioelectronics established a need for three-dimensional (3D) microstructured electrodes, which are difficult to implement using conventional manufacturing processes. Additive manufacturing coupled with electroless metal plating enables the production of 3D conductive microarchitectures with high surface area for potential applications in such devices. However, interfacial delamination between the metal layer and the polymer structure is a major reliability concern, which leads to device performance degradation and eventually device failure. This work demonstrates a method to produce a highly conductive and robust metal layer on a 3D printed polymer microstructure with strong adhesion by introducing an interfacial adhesion layer. Prior to 3D printing, multifunctional acrylate monomers with alkoxysilane (-Si-(OCH₃)₃) were synthesized via the thiol-Michael addition reaction between pentaerythritol tetraacrylate (PETA) and 3-mercaptopropyltrimethoxysilane (MPTMS) with a 1:1 stoichiometric ratio. Alkoxysilane functionality remains intact during photopolymerization in a projection micro-stereolithography (PμSLA) system and is utilized for the sol-gel reaction with MPTMS during postfunctionalization of the 3D printed microstructure to build an interfacial adhesion layer. This leads to the implementation of abundant thiol functional groups on the surface of the 3D printed microstructure, which can act as a strong binding site for gold during electroless plating to improve interfacial adhesion. The 3D conductive microelectrode prepared by this technique exhibited excellent conductivity of 2.2 × 10⁷ S/m (53% of bulk gold) with strong adhesion between a gold layer and a polymer structure even after harsh sonication and an adhesion tape test. As a proof-of-concept, we examined the 3D gold diamond lattice microelectrode modified with glucose oxidase as a bioanode for a single enzymatic biofuel cell. The lattice-structured enzymatic electrode with high catalytic surface area was able to generate a current density of 2.5 μA/cm² at 0.35 V, which is an about 10 times increase in current output compared to a cube-shaped microelectrode.

Chemical-Structure Evolution Model for the Self-Assembling of Amine-Terminated Monolayers on Nanoporous Carbon-Doped Organosilicate in Tightly Controlled Environments

Amine-terminated self-assembled monolayers are molecular nanolayers, typically formed via wet-chemical solution on specific substrates for precision surface engineering or interface modification. However, homogeneous assembling of a highly ordered monolayer by the facile, wet method is rather tricky because it involves process parameters, such as solvent type, molecular concentration, soaking time and temperature, and humidity level. Here, we select 3-aminopropyltrimethoxysilane (APTMS) as a model molecule of aminosilane for the silanization of nanoporous carbon-doped organosilicate (p-SiOCH) under tightly controlled process environments. Surface mean roughness (R_a) and the water contact angle (θ) of the p-SiOCH layers upon silanization at a 10% humidity-controlled environment behave similarly and follow a three-stage evolution: a leap to a maximum at 15 min for R_a (from 0.227 to 0.411 nm) and θ (from 25 to 86°), followed by a gradual decrease to 0.225 nm and 69^o, finally leveling off at the above values (>60 min). The -NH₃⁺ fraction indicating monolayer disorientation evolves in a similar fashion. The fully grown monolayer is highly oriented yielding an unprecedented low -NH₃⁺ fraction of 0.08 (and 0.92 of upright -NH₂ groups). However, while having a similar thickness of approximately 1.4 ± 0.1 nm, the molecular layers grown at 30% relative humidity exhibit a significantly elevated -NH₃⁺ fraction of 0.42, indicating that controlling the humidity is vital to the fabrication of highly oriented APTMS molecular layers. A bonding-structure evolution model, as distinct from those offered previously, is proposed and discussed.

Solvent-Dependent Adhesion Strength of Electroless Deposited Ni-P Layer on an Amino-Terminated Silane Compound-Modified Si Wafer

Amino-terminated silane compound modification was wet-processed on a silicon wafer using four different solvents to investigate the property of the self-assembled monolayer (SAM) and its influence on the adhesion of electroless deposited nickel-phosphorus (Ni-P) films. Analyzed by various tools including dynamic light scattering, the atomic force microscope, X-ray photoelectron spectroscopy, inductively coupled plasma with mass spectroscopy, a proper link between the processing solvent and SAM quality is established. It is found that at least the chemical compatibility, the polarity, and the acidity of solvents can affect the final morphology of the resultant SAM. Unlike toluene and ethanol that are most frequently chosen in literature, we conclude that isopropyl alcohol (IPA) is a superior solvent for amino-terminated silane compounds. Owing to the good SAM quality formed in IPA, the adhesion of electroless deposited Ni-P films is largely strengthened, even as high as the bulk strength of silicon wafers.

Conformal Electroless Nickel Plating on Silicon Wafers, Convex and Concave Pyramids, and Ultralong Nanowires

Nickel (Ni) plating has garnered great commercial interest, as it provides excellent hardness, corrosion resistance, and electrical conductivity. Though Ni plating on conducting substrates is commonly employed via electrodeposition, plating on semiconductors and insulators often necessitates electroless approaches. Corresponding plating theory for deposition on planar substrates was developed as early as 1946, but for substrates with micro- and nanoscale features, very little is known of the relationships between plating conditions, Ni deposition quality, and substrate morphology. Herein, we describe the general theory and mechanisms of electroless Ni deposition on semiconducting silicon (Si) substrates, detailing plating bath failures and establishing relationships between critical plating bath parameters and the deposited Ni film quality. Through this theory, we develop two different plating recipes: galvanic displacement (GD) and autocatalytic deposition (ACD). Neither recipe requires pretreatment of the Si substrate, and both methods are capable of depositing uniform Ni films on planar Si substrates and convex Si pyramids. In comparison, ACD has better tunability than GD, and it provides a more conformal Ni coating on complex and high-aspect-ratio Si structures, such as inverse fractal Si pyramids and ultralong Si nanowires. Our methodology and theoretical analyses can be leveraged to develop electroless plating processes for other metals and metal alloys and to generally provide direction for the adaptation of electroless deposition to modern applications.