Surface roughness of the lower electrode plate is a decisive quality parameter in electrowetting liquid lens manufacturing, directly governing optical resolution, contact angle hysteresis, dielectric integrity, and long-term reliability. This article presents a complete technical overview of the lower electrode plate structure, the roughness parameters that must be controlled, the contact and non-contact measurement methods used by accredited third-party laboratories, and the ISO, ASTM, and GB standards that govern the inspection process.
Technical Overview
A liquid lens based on electrowetting-on-dielectric (EWOD) technology adjusts its focal length by applying a voltage between electrodes, which changes the wettability of a hydrophobic surface and thereby reshapes the meniscus between a conductive aqueous liquid and an insulating oil. The lower electrode plate is the foundational component of this stack and is typically built up as follows:
- A glass or quartz substrate providing optical transparency and mechanical support.
- A transparent conductive layer, most commonly indium tin oxide (ITO) or a patterned metal electrode ring.
- An insulating dielectric layer such as silicon dioxide, aluminum oxide, hafnium oxide, parylene, or DLC, deposited by sputtering, ALD, CVD, or vapor deposition.
- A thin hydrophobic fluoropolymer coating that directly interfaces with the conductive liquid.
Because the moving liquid contact line travels across this plate during every focal-length transition, surface roughness has an outsized influence on device behavior. Excessive roughness increases contact angle hysteresis, raises the threshold voltage, slows the response, and can cause contact-line pinning, image jitter, and liquid sticking. Rough or granular films also promote pinholes in the dielectric, leading to dielectric breakdown and electrolysis of the conductive liquid, and they scatter light at the optical interface, degrading modulation transfer function (MTF) and increasing stray light. Typical production specifications demand nanometer-level control: polished glass substrates often require Ra below 1–2 nm, ITO and dielectric layers below 5–10 nm, and finished hydrophobic surfaces Sa below 10–20 nm. Verifying these values requires carefully selected instrumentation, filtering, and reporting in accordance with modern surface texture standards.
Testing Samples
Bare Substrate Samples
Glass or quartz wafers and cut plates are inspected after polishing and cleaning to establish the baseline roughness of the optical surface before any film deposition. Coupons are typically sampled from multiple wafer positions to capture polishing non-uniformity across the batch.
Patterned Electrode Samples
After ITO sputtering, photolithography, and etching, the transparent electrode surface is measured to verify that film growth and patterning have not introduced excessive graininess, hillocks, or step-edge artifacts at trace boundaries.
Dielectric-Coated Samples
Witness coupons coated alongside production lots are used to evaluate the roughness evolution of the insulating layer, since columnar growth or particulate contamination in dielectric deposition is a common source of elevated Rz and latent pinhole defects.
Hydrophobic Surface Samples
The final fluoropolymer coating is tested in its as-deposited state because this is the surface that actually contacts the working liquid. Dewetting, aggregation, or dust inclusion in this soft polymer film can more than double the roughness relative to the underlying dielectric.
Assembled and Aged Plates
Lower electrode plates extracted from finished lens assemblies, or coupons subjected to reliability stress such as thermal cycling, damp heat, or continuous voltage aging, are measured to characterize surface degradation, delamination, and electrochemical attack for failure analysis purposes.
Testing Items
Roughness evaluation of the lower electrode plate covers profile (2D) and areal (3D) parameters, along with related geometric checks:
- Ra and Rq (arithmetic and root-mean-square profile roughness): primary lot-release parameters for each layer of the stack.
- Rz and Rt: maximum height parameters that detect isolated peaks, particles, or scratches which may pierce thin dielectric films even when Ra is compliant.
- Rsk and Rku: skewness and kurtosis values that distinguish porous, valley-dominated films from spiky, defect-dominated surfaces.
- RSm and Rmr(c): mean spacing and material ratio curves used to characterize grain structure and bearing behavior of coated films.
- Sa, Sq, Sz, Ssk, and Sku: areal parameters measured over defined evaluation regions (for example 50 × 50 µm or 250 × 250 µm) that capture directional film growth textures invisible to single profiles.
- Waviness (Wa) and step height of electrode traces and contact vias, confirming that form deviations and interconnect topology remain within optical and assembly tolerances.
- Nanoscale roughness maps by AFM over 1 × 1 µm to 10 × 10 µm regions, resolving fine grain structure of ITO and dielectric layers.
Testing Methods
Stylus Profilometry
Traversing stylus instruments conform to ISO 21920-2 and ISO 21920-3 (which superseded ISO 4287 and ISO 4288), with instrument characteristics per ISO 3274 and calibration per ISO 12179. A diamond stylus with a 2 µm tip radius and measurement force reduced to the minimum practical level (typically ≤ 0.75 mN, and lower for polymer surfaces) is traced along multiple parallel profiles. Evaluation uses four to five sampling lengths with a cut-off (λc) of 0.25 or 0.8 mm selected according to the expected roughness step. Stylus methods are robust for step heights and larger areas, but the soft hydrophobic fluoropolymer may be plastically deformed, so forces must be verified to leave no visible trace.
White Light Interferometry (Coherence Scanning Interferometry)
Interferometric optical profilers provide fast, non-contact areal measurement in accordance with the ISO 25178 framework. Phase-shifting interferometry (PSI) mode resolves sub-nanometer roughness on smooth polished substrates, while vertical scanning interferometry (VSI) mode handles rougher coated surfaces and steps. Caution is required with transparent multilayers: thin-film interference can displace the apparent surface, so measurement wavelength, objective magnification, and, where necessary, model-based film correction must be validated against reference methods.
Laser Scanning Confocal and Chromatic Confocal Microscopy
Confocal techniques, also operating under ISO 25178 measurement standards, offer excellent lateral resolution and reliable performance on gently curved or transparent samples. Height discrimination through the pinhole rejects haze from underlying layers, making confocal microscopy well suited to patterned ITO surfaces and coated coupons where interferometric fringes are difficult to interpret.
Atomic Force Microscopy
AFM delivers the highest vertical and lateral resolution for quantifying grain size and nanoscale texture of ITO and dielectric films. Tapping-mode imaging avoids damaging soft hydrophobic coatings, and results are reported as Sq or Sa over standardized scan areas, providing reference data against which faster production-line optical methods are correlated.
Data Processing and Measurement Conditions
- Filtering follows the ISO 16610 series: an S-filter and L-filter (or Gaussian profile filter per ISO 16610-21) remove short-scale noise and long-scale form, and an F-operator performs levelling and form removal before parameter calculation.
- Laboratory conditions are maintained at 23 ± 2 °C with vibration-isolated granite tables, since thermal drift and building vibration are significant error sources at nanometer levels.
- Samples are cleaned with lint-free wipes and filtered solvents or dry nitrogen to remove particles that would otherwise be reported as false Rz peaks.
- Instruments are verified daily using certified roughness artifacts traceable to national metrology institutes, and test reports issued by accredited third-party laboratories follow ISO/IEC 17025 management requirements.
Applications
Roughness inspection of the lower electrode plate supports every stage of the liquid lens product lifecycle:
- Process development: optimizing glass polishing, ITO sputtering power and temperature, ALD/CVD dielectric deposition, and fluoropolymer dip or spin coating recipes against roughness targets.
- Incoming quality control: lot release of substrates, coated wafers, and finished plates using statistical sampling plans and SPC control charts of Ra, Rz, and Sa.
- Camera module manufacturing: qualifying autofocus lenses for smartphones, machine vision cameras, barcode readers, and biometric identification systems.
- Medical imaging: ensuring optical-grade surface quality in compact endoscopic and surgical visualization lenses.
- Automotive optics: validating durability-critical surfaces for driver monitoring and cabin vision systems subjected to wide temperature ranges.
- Failure analysis: correlating elevated roughness with dielectric breakdown, contact-line sticking, image jitter, and MTF degradation in returned or stress-tested units.
FAQ
Why is lower electrode plate roughness more critical than in conventional optical components?
Because the plate is not merely a passive window: the liquid contact line moves dynamically across its surface under voltage, and the dielectric must withstand sustained electric fields of hundreds of volts per micron. Roughness simultaneously degrades the optical interface, the electro-wetting mechanics, and the dielectric strength, so tolerances are far tighter than for static glass elements.
What roughness values are typically specified?
Typical targets are Ra ≤ 1–2 nm for polished glass substrates, Ra ≤ 5 nm for ITO and dielectric layers, and Sa ≤ 10–20 nm for the finished hydrophobic surface, with Rz limits placed on isolated defects. Exact limits depend on lens aperture, drive voltage, and the reliability class of the end application.
Which method should be chosen for transparent coated samples?
White light interferometry or laser scanning confocal microscopy is preferred for routine areal inspection of transparent stacks, with AFM used as the reference method for nanometer-level film texture. Stylus profilometry remains valuable for step height and large-area scans, provided the low-force settings are validated for soft coatings.
Which standards apply to the measurement and reporting?
Profile measurement follows the ISO 21920 series (replacing ISO 4287, ISO 4288, and ISO 12085), areal measurement follows ISO 25178, stylus instruments follow ISO 3274 and ISO 12179, and filtration follows ISO 16610. In the Chinese market, the corresponding national standards GB/T 3505, GB/T 1031, and GB/T 6062 are commonly referenced, while optical drawing notation may follow ISO 10110-8.
How many points must be measured for a valid judgment?
Since deposition textures are spatially non-uniform, best practice combines multiple areal maps distributed across the wafer or plate (center, mid-radius, and edge zones) with several profile traces per zone. Results are evaluated statistically, and acceptance criteria normally require every measured location to satisfy both the Ra (average texture) and Rz or Sz (isolated defect) limits.