Photolithographic Electroforming & Micro-Mesh Vector Prepress Guide

Mastering atom-by-atom additive electroforming, photoresist stencil prepress, and high-precision aperture mesh fabrication.

1. Micro-Electroforming & UV-LIGA Principles

Photolithographic Electroforming (UV-LIGA) is an additive micro-fabrication technology that produces high-aspect-ratio, stress-free metallic microstructures with sub-micron edge definition and zero mechanical tooling wear. Unlike chemical etching or laser cutting—which are limited by undercutting and thermal recast zones—electroforming builds components atom-by-atom via electrolytic deposition onto a photo-patterned cathode mandrel.

The manufacturing process follows four critical prepress phases:

1. Mandrel Preparation: A dimensionally stable substrate (e.g., mirror-polished stainless steel 316L, titanium, or chromium-coated glass) is passivated with a thin native oxide film to provide sufficient electrical conductivity for plating while permitting post-process mechanical peeling without tearing delicate micro-foils.
2. Photolithography: Thick negative photoresist (e.g., SU-8 or dry film) is spin-coated, exposed through a 1-bit high-contrast vector phototool mask, and developed to create vertical dielectric sidewalls with aspect ratios exceeding $5:1$.
3. Precision Electrodeposition: The mandrel is immersed in a high-purity electrolytic bath (nickel sulfamate $ ext{Ni(SO}_3 ext{NH}_2)_2$, copper fluoborate, or hard gold). Low internal stress additives (saccharin, naphthalene trisulfonic acid) maintain zero curl ($\sigma_{ ext{int}} < \pm 10 ext{ MPa}$).
4. Demolding & Part Separation: The electroformed metal sheet is mechanically peeled from the mandrel, and the photoresist stencil is stripped in organic solvents (NMP / PGMEA).

2. Faraday's Law & Overplating "Mushrooming" Geometry

The theoretical deposit thickness $T$ across cathode surface area $A$ is governed by Faraday's constant ($F = 96,485 ext{ C/mol}$):

T = (J * t * M) / (n * F * rho) * eta_eff

When plating thickness exceeds the photoresist height ($T > H_{ ext{resist}}$), metal begins growing horizontally across the resist top surface. This lateral "mushrooming" occurs at an empirical rate $\Delta w pprox 1.6 \cdot (T - H_{ ext{resist}})$, systematically reducing the final through-hole aperture diameter.

3. 1-Bit Vector Mask & CAM Prepress Guidelines

• Optical Proximity Correction (OPC): UV diffraction causes corner rounding in micro-apertures. Add microscopic serif corner extensions ($+1.5 ext{ µm}$ ear tabs on square apertures) to maintain razor-sharp corners.
• Current Density Thieving Borders: High electrical field concentration at cathode edges causes extreme edge buildup ("dog-boning"). Surround active part arrays with a continuous $5 ext{ mm}$ wide sacrificial thief frame spaced $3 ext{ mm}$ from functional meshes.
• Aperture Funnel Taper for Medical Sieves: By intentionally tuning overplating mushrooming, manufacturers create smooth, bell-mouthed entry nozzles for pharmaceutical inhalation drug delivery (droplet MMAD $2.5 - 4.5 ext{ µm}$).