RoC Tolerancing and Surface Quality Unlike traditional macroscopic lenses, microlens arrays—especially those fabricated via grayscale lithography or precision replication—enable high-fidelity control over arbitrary profiles, whether spherical, aspherical, or freeform. The challenge is not the shape, but the precision and uniformity of that shape. RMS Surface Error (to Best-Fit Shape) - Typically expressed in µm RMS, and sometimes peak-to-valley (P-V) values. - Quantifies how closely the fabricated surface matches the intended lens profile. - Typical spec range: - <0.4 µm RMS for <100um sag optics - <0.05 µm RMS achievable in smaller sag optics Intra-Array Uniformity (Within-Array) - Describes variation in lens profiles across a single array on one part. - Often spec’d as max RMS or RoC error from median lens. - Typical spec: ±1–2% variation in RoC surface error within the array. Part-to-Part Uniformity (Inter-Array) - Describes variation between parts in a batch, especially critical in volume production. - Controlled via: - Stamper tooling stability - Process control - Typical spec: <2% variation in average RoC or sag height across parts. Design Considerations - As lens sag increases, tighter RoC tolerances become harder to maintain due to steeper profiles. - Measurement techniques (e.g., interferometry, confocal profilometry) must be matched to the curvature range to ensure accurate metrology. - Uniformity specs should include profile fidelity (RMS to design) and repeatability (lens-to-lens, part-to-part RoC variation). - Due to the high repeatability replica to replica, best practice is to optimize system design around final replica lens RoC once master tool is in place. Common adjustments include slight alterations to substrate thickness, replica layer thickness, or refractive indices. Notes (Informational Only — Not Specs) - Lens profile shape (spherical, aspherical, freeform) is a design choice, not a spec constraint—modern MLA fabrication allows all with minimal cost impact. - Numerical Aperture (NA) is derived from lens shape and refractive index, useful for optical modeling but not commonly specified as a standalone fabrication tolerance.
