NIL process requirements for optical coatings
Nanoimprint lithography replicates nanostructures by pressing a patterned mold into a resist material, curing it (typically UV), and releasing the mold. For optical applications — waveguide coupling gratings, metalens structures, diffractive optical elements — the imprinted material must maintain precise refractive index after cure, with feature fidelity at the sub-100 nm scale.
The critical material requirements are: controlled and precise RI, low viscosity for mold filling, dimensional stability during and after cure, low cure shrinkage, high mold release quality, and optical transparency. Meeting all of these simultaneously — especially at elevated RI values — is the formulation challenge that limits the supply of NIL-grade optical materials.
Why solvent-free (100% solids) matters for NIL
Solvent-containing resist materials create problems in NIL processing:
- Volume loss — solvent evaporation reduces film thickness by the solvent fraction, making it difficult to control final feature height
- Residual layer variation — uneven solvent evaporation across the imprint area causes thickness non-uniformity
- Void formation — trapped solvent can create bubbles during UV cure, producing optical defects
- Process speed — solvent removal adds a drying step before imprinting, reducing throughput
Kriya's 100% solids formulations eliminate all solvent-related artifacts. What is deposited is what cures — the imprinted feature height equals the dispensed height, with only cure shrinkage (minimised through formulation) as a dimensional variable.
Available RI range and viscosity grades
Kriya's NIL-compatible 100% solids UV-curable coatings are available across RI 1.34 to 1.65. Within this range, custom RI values are formulated to specification. Multiple viscosity grades are available to match different NIL process requirements:
- Low viscosity (below 50 mPa.s) — for capillary-driven mold filling and thin residual layers
- Medium viscosity (50 to 500 mPa.s) — for dispensing and controlled film thickness
- High viscosity (above 500 mPa.s) — for screen printing and thick-layer applications
Cure conditions and shrinkage
UV cure is performed using standard mercury or LED UV sources. Cure conditions are optimised for each formulation grade:
- UV dose: compatible with standard NIL tool UV sources
- Cure shrinkage: minimised through formulation; specific values provided per grade
- Post-cure optional: thermal post-cure available for enhanced cross-link density and hardness
- Oxygen sensitivity: formulated for low oxygen inhibition — no nitrogen purge required for most grades
Applications
Kriya's NIL-compatible coatings serve two primary application categories:
- Waveguide coupling gratings — diffractive structures that couple light into and out of AR/VR waveguides. The RI of the grating material directly determines coupling efficiency and angular bandwidth.
- Metalens replication — nanostructured flat optics that replace conventional lenses. Ultra-high RI at the nanostructure level is critical for metalens performance. See metalenses and photonics.
R2R NIL integration
The transformation of waveguide and metalens manufacturing from wafer-based batch processing to roll-to-roll (R2R) nanoimprint is the key to cost reduction — estimated at approximately 10 times lower cost per unit at scale. Kriya's 100% solids formulations are specifically designed for R2R NIL compatibility:
- Continuous dispensing — no solvent drying step between stations
- Fast UV cure — compatible with R2R line speeds
- Consistent rheology — stable viscosity during continuous processing
- Good mold release — reducing mold cleaning frequency and improving mold lifetime