High Index, UV Stable NIL Formulation for AR Waveguides
Grace McClintock, Benjamin Balfanz, Mariah Durrant, Selina Monickam, Peter Guschl,
Serpil Gonen Williams
Pixelligent Technologies
6411 Beckley Street, Baltimore, Maryland 21224
Email: pguschl@pixelligent.com

Abstract
High refractive index waveguides are a key requisite to achieving both a large field of view andhigh efficiency in augmented reality displays. Manufacturing these waveguides requires
nanoimprintable formulations that result in highly transparent structures with high refractive
index. This paper focuses on Pixelligent’s UV-stable, high-index nanoimprint formulation,
PixNIL® SCS1, as tested on a EVG®7300 SmartNIL system by the industry-leading EV Group.
PixNIL®SCS1 produces high quality binary, blazed, and high aspect ratio gratings with
unparalleled nanoparticle distribution in the structures while maintaining <2 nm surface
roughness. Additionally, this formulation enables a large process window with > 24h pot lifestability and results in nanocomposites that are stable under UV exposure at > 405 nm.
Keywords: nanoparticle, nanocrystal, nanoimprint, high refractive index, nanocomposites,augmented reality, field of view, low residual layer thickness, waveguide, surface relief gratings

Introduction
Augmented Reality (AR) displays join virtual images with real-world content to create new
possibilities for human and machine interactions. Such displays rely on diffractive, or reflective
waveguides, combine the virtual content with the ambient images seen through the AR lenses.
For next-generation AR displays, diffractive waveguides with Surface Relief Gratings (SRG) on highrefractive index substrates are now considered an essential enabling technology.
High refractive index resins processable by using Nanoimprint Lithography (NIL) are vital whenmanufacturing AR glasses on a wafer level to achieve low production costs and a high field ofview (FOV). In addition to low haze, high visible light transparency, and high refractive indices (RI),
these resins must deliver nanoimprint lithography (NIL) performance with optimal filling of thewaveguide structures, excellent compatibility with industrial stamp materials, and robust
mechanical properties. During the manufacturing process, these resins should be easy toprocess, possess long shelf-life and pot-life, and produce high-quality waveguides with optimaland consistent structures. Common issues associated with high index materials include poor
control over nanoparticle surface chemistry and size, resulting in poor dispersion and largersized particle aggregates that cause light scattering and haze. Additionally, next generation
waveguides will require increasingly smaller feature sizes, i.e., smaller than 100 nm, that will
become difficult to fill homogeneously with larger and non-uniform nanoparticles.

1Pixelligent | 6411 Beckley Street, Baltimore, MD 21224 | 443.529.8310 | www.pixelligent.comConfidential

High Index, UV Stable NIL Formulation for AR Waveguides
Pixelligent has been developing its proprietary method of synthesizing and capping nanocrystal
metal oxides for well over a decade. This has resulted in the company’s industry-leading
PixClear® Zirconia and Titania nanocomposites that have refractive indices ranging from 1.7 –2.0. Through our proprietary capping chemistry processes, Pixelligent can customize the surfaceof nanocrystals to be compatible with a vast array of resin systems. The recently launchedPixNIL® and PixJet® formulations, for example, are an excellent demonstration of these
combined capabilities and were engineered to be compatible with nanoimprint lithography (NIL)
and inkjet printing manufacturing processes respectively. These products exemplify Pixelligent’s
unique abilities to tune RI, viscosity, and mechanical properties while delivering the highest
transparency, lowest haze, and best reliability. Superior control of capped particle sizes with anarrow size distribution ranging from 5nm to 20 nm particles, provides unparalleled optics andcompatibility with even the smallest required grating structures. Table 1 shows Pixelligent’s
PixNIL® solvent-based formulations with film refractive index values ranging from 1.7 to 2.0.
With the recent introduction of the PixCor™ technology, and the corresponding PixNIL® SCS1formulation, Pixelligent has delivered significant improvements in optical film stability under light
exposure above 405 nm. This UV-stable material, based on growing a ZrO2 shell around aPixClear® TiO2 nanocrystal, provides an RI of 1.86, while maintaining excellent waveguide
imprinting that is equivalent to the lower-RI organic-based materials currently available in themarket.
This paper focuses on Pixelligent’s UV-stable, high-index nanoimprint resin formulation, PixNIL®
SCS1, as tested on the industry-leading EV Group’s EVG®7300 SmartNIL system. The analysis will
highlight the best-in-class optical properties, formulation stability, uniform dispersion ofnanocrystals throughout the nanostructures with the NIL process, and the capacity to achieve
high structural fidelity with both binary and slanted gratings. Additionally, SCS1 showcases a wideprocess window, providing greater flexibility during nanoimprint production.
Table 1. Pixelligent’s PixNIL® formulation products

ProductCore

RI (589 nm)

FT Range (nm)

PixNIL® SZ1 PixClear® ZrO2

1.7

200 – 1200

PixNIL® SCS1

PixCor™

1.86

100 – 500

PixNIL® ST2PixClear® TiO2

1.9

100 – 1200

PixNIL® ST5PixClear® TiO2

2.0

100 – 1200

2

Pixelligent | 6411 Beckley Street, Baltimore, MD 21224 | 443.529.8310 | www.pixelligent.com
Confidential

High Index, UV Stable NIL Formulation for AR Waveguides
Methodology

SmartNIL® Replication with EV GroupFor the wafer-level NIL replication process, the SmartNIL® technology was used. The process
flow consists of two steps: First, manufacturing multiple low-cost polymer working stamps (WS)out of a previously manufactured master and then utilizing them for the actual imprints during
the second step. This intermediate approach avoids wearing out the original master templateand improves the overall production economics, as the achievable number of imprints isincreased tremendously. Defective working stamps can be replaced quickly and at low cost,proving particularly advantageous during high-volume production runs. The NIL process isdepicted in detail in Figure 1.

Figure 1. SmartNIL® replication process flow and a PixNIL® resin imprinted on 200-mm wafer

To ensure defect-free WS fabrication, the master is coated with an anti-sticking layer applied byspin coating. The WS material is then dispensed directly on the master, also by a spin coating

3Pixelligent | 6411 Beckley Street, Baltimore, MD 21224 | 443.529.8310 | www.pixelligent.com
Confidential

High Index, UV Stable NIL Formulation for AR Waveguides
process. Next, the transparent SmartNIL backplane is attached to the coated master. After curing
the WS polymer using a UV LED, it is finally demolded from the master. Upon completing thisprocess, the actual SmartNIL imprinting process begins, where a dedicated material is applied
onto the substrate using the same process used for the WS fabrication, e.g., spin coating. The WS
and the substrate with the dispensed material are then brought into contact. Similar to the WSfabrication, this step is followed by UV curing and demolding, allowing multiple imprints to bemade with the final structures. Reusing the WS for multiple imprints during SmartNIL replication
increases the process efficiency and has been well-proven in high volume manufacturing.
Furthermore, even though the full SmartNIL imprint process described is split into two parts, theentire process can be implemented on the same equipment. This work is performed on theEVG®7300 SmartNIL system using 200 mm or 300 mm wafers.

Results and Discussion
One of the challenges of making successful nanocomposite materials is achieving uniform
dispersions of the nanoparticles in specific resins. Pixelligent’s PixCor™ core-shell nanocrystals
have an average particle size of 20 nm with a narrow particle size distribution and unparalleledcompatibility with solvents and monomers. Formulations with Pixelligent’s nanocrystals offer
long shelf-life stability under a range of storage conditions, long pot life for stability on a
production line, and fast and facile UV curing for high-volume manufacturing.
The scope of this white paper will highlight the following aspects of the PixNIL® SCS1 resin:
1.
2.
3.
4.

Formulation Stability
Optical properties of flat films
Nanoimprinting Structures
UV Reliability

Formulation Stability
Shelf-life is an important feature for NIL resins, as the resin needs to remain relatively unchangedwhen used over many weeks to months. This storage stability is required to match not only
customer storage preferences, but also to endure extreme shipping conditions (4C and 50C) usedduring standard shipping processes. Pot life stability is also important as the resins need toremain stable when utilized within a daily / shift time frame. Table 2 displays these four importantstability criteria in terms of viscosity stability for the PixNIL® SCS1 formulation. Overall, thePixNIL® SCS1 formulation maintains comparable values to the initial viscosity (1.64 ± 0.03 cP)under the shelf-life, storage, and pot life conditions within the standard measurement error. Thisis a significant competitive advantage compared to other products in this market.

4Pixelligent | 6411 Beckley Street, Baltimore, MD 21224 | 443.529.8310 | www.pixelligent.com
Confidential

High Index, UV Stable NIL Formulation for AR Waveguides
Table 2. Viscosity stability of PixNIL® SCS1 under various conditions (longer shelf-life testongoing) (standard error in viscosity measurement = 1.7%)

25 C
Shelf Life (cP)
1.70 ± 0.03
(1 month)

4CStorage (cP)
1.67 ± 0.03
(48 hours)

50C
Storage (cP)
1.66 ± 0.03
(48 hours)

25 C
Pot Life (cP)
1.69 ± 0.03
(24 hours)

Optical Properties of Flat Films
PixNIL® SCS1 was formulated to provide a range of film thicknesses between 200 and 500 nm
based on typical waveguide structure feature sizes while providing best-in-class optics for bothtransmission and haze.
In Figure 2, film thickness is varied with spin speed. The one-step process shows PixNIL® SCS1with film thicknesses ranging from roughly 150 to 330 nm at 4000 and 1000 RPM, respectively.
Using a two-step process (i. e., a slow speed for 45 seconds followed by a faster speed for 15seconds) that allows the liquid to gradually wet the substrate gives better film uniformity over a
one-step process (i. e., one fast speed for 45 s). The two-step process produces about 200 nm
thicker films at those spin speeds.

Figure 2. Spin curves for PixNIL® SCS1

Film refractive index and transparency (usually referred to as %Transmission or %T) are essential
optical properties for waveguide applications. Figure 3 shows both the film refractive index curvein the visible and near IR wavelengths and %Transmission curve for PixNIL® SCS1 formulation.
To avoid optical interference due to RI mismatch between the film and the substrate, %Tmeasurement was measured on 1.9 RI Corning EXG glass and shows greater than 95%
transmission above 400 nm.

5Pixelligent | 6411 Beckley Street, Baltimore, MD 21224 | 443.529.8310 | www.pixelligent.com
Confidential

High Index, UV Stable NIL Formulation for AR Waveguides

Figure 3. Film RI and %T versus wavelength for PixNIL® SCS1 at 200 nm thickness.

Nanoimprinting Structures
Imprintability
As described in the Methodology section, the nanoimprinting process comprises the following
steps: spin coat, soft bake, and stamp/cure. An optional post-bake process can also be applied
for certain applications. Two important factors that determine imprint quality are:
1. Soft bake conditions: Once the desired film thickness has been deposited (Figure 2), thesoftbake is applied. Soft bake removes the solvent to prepare the formulation for
imprinting process. For PixNIL®SCS1, softbake conditions covering temperature rangesof 25 to 80°C and time ranges of one to five minutes were tested. The best imprinting
results at 200-nm film thickness were obtained at 50°C/1 min softbake.
2. Wait time before imprint process: It is essential to know the maximum time a film can sit
prior to imprinting, while still showing perfect resolution. Ideally, manufacturers prefermaterials that survive longer wait times, enabling inexpensive batch-type processing. TheSEM images for PixNIL® SCS1 in Figure 4 show that the two imprints produced within aminute after soft bake (a) versus a 24-hour wait time after soft bake (b) are nearlyidentical.

a.)

b.)

Figure 4. Wait time between Softbake and UV Cure (400-nm binary gratings). PixNIL® SCS1: a.)Initial and b.) 24-h wait.

6

Pixelligent | 6411 Beckley Street, Baltimore, MD 21224 | 443.529.8310 | www.pixelligent.com
Confidential

High Index, UV Stable NIL Formulation for AR Waveguides
Refractive index uniformity within the imprinted structures is crucial for device efficiency in
AR/MR devices. When imprinting resins that contain nanoparticles into grating structures on theorder of 100 nm or less, concerns arise about the ability of the nanoparticles to distribute evenly
within the structures. Utilizing the capping materials that have been optimized to surround thePixCor™ nanoparticles, combined with careful resin component selection, the PixNIL® SCS1exhibits excellent nanocrystal uniformity throughout the structure after imprinting. Figure 5
illustrates this result, showing close packing of the nanoparticles within the imprint structures
including sharp edges, without any indications of phase separation between the nanoparticlesand resin. These observations provide additional evidence of the dispersion quality of thenanoparticles within the resin system after imprinting. As mentioned earlier, the ability toproperly fill the structures and distribute evenly along the edges and corners is a highly attractive
benefit of Pixelligent’s nanocomposite systems. This result is evidenced in Figure 6, a 15x15 um
AFM scan across an imprinted surface prepared for PixNIL® SCS1 demonstrates an impressively
low surface roughness values (Rq) of 1.44 nm.

a.)Figure 5. Images of PixCor™ nanoparticles in PixNIL® SCS1 imprints: a.) binary and b.) slantedfeatures.

Figure 6. AFM images of PixNIL® SCS1 with low surface roughness (Rq = 1.44 nm)

7

Pixelligent | 6411 Beckley Street, Baltimore, MD 21224 | 443.529.8310 | www.pixelligent.com
Confidential

High Index, UV Stable NIL Formulation for AR Waveguides
The versatility of imprinted structures prepared from PixNIL® SCS1 formulation were evaluatedby comparing side-by-side to a reference 1.5 RI imprint nanoparticle-free resin (EVGNIL UV/A2)
as seen in Figure 7. Key highlights are: 1) well-defined, slanted gratings with consistent heights(roughly 337 nm) and clean, sharp edges; 2) high-aspect ratio gratings at 6:1 height-to-width (433nm:70 nm); and, 3) small, blazed gratings with 130-nm heights. It is important to note that thefeatures and trenches do not show any adverse roughness due to the presence of thenanoparticles.

H = 337 nm
W = 170 nm
P = 350 nm
 = 36o

H = 433 nm
W = 70 nm
P = 360 nm

1a.)

1b.)

2a.)

2b.)

3a.)

3b.)

H = 130 nm
P = 400 nm
 = 64o, 27o

Figure 7. Using EVG’s Moldnano master various imprinted structures compared withreference 1.5 RI NIL resin “EVGNIL UV/A2” (1a to 3a) compared to SCS1 (1b to 3b) with slanted(top), high aspect-ratio binary (middle), blazed gratings (bottom)

Demonstration of Low Residual Layer Thickness (RLT)
Residual layer thickness is a critical parameter in cases where there is a mismatch between theRI of the substrate and the resin, resulting in an undesirable effect on image quality. When therefractive indices of the substrate and imprint are better matched, then thin RLTs are lessnecessary. Pixelligent’s PixNIL® formulations are commonly selected as index-matching

8Pixelligent | 6411 Beckley Street, Baltimore, MD 21224 | 443.529.8310 | www.pixelligent.com
Confidential

High Index, UV Stable NIL Formulation for AR Waveguides
materials for specific RI grades of glass and PC wafers. Regardless of the required index, PixNIL®
SCS1 can deliver RLT of < 40 nm. Figure 8a demonstrates imprints with 200 nm heights, 360 nm
widths, and 800 nm pitches, that can achieve RLTs close to 30 nm. RLT values of this magnitude
are approximately twice the size of the PixCor™ nanoparticles themselves, and yet exceptional
imprinting is still achieved. Figure 8b demonstrates the uniform distribution and close packing of
the nanoparticles within the residual layer of these imprints.

a.)

b.)
Figure 8. a.) Binary gratings of PixNIL® SCS1 with 31 nm RLT and b.) Close up TEM of residuallayer with close packed nanocrystal distribution

UV Reliability
Under a 405-nm UV exposure test (25 mW/cm2 for 148 hours), the color, film thickness andrefractive index were measured for the PixCor™ containing PixNIL® SCS1 when compared toPixClear® ZrO2 and TiO2 nanocomposite films to show the UV stability benefit of the PixCor™
nanoparticles. Figure 9 displays how the SCS1 performs in between the ZrO 2 and TiO2 in thesethree ways. The ZrO2 nanocomposite has virtually no color change while the E* of TiO2 is greaterthan 1.0, indicating a noticeable color evolution, while the ZrO2 shell of the PixCor™ nanoparticles
passivates the TiO2 core, reducing the impact of color formation under UV exposure by <85%.
Similarly, the photo-active TiO2 causes significant changes in film thickness and RI, due to photodegradation. Again, the SCS1 film changes much less in these properties by maintaining betterstability under UV light at 405 nm.

9Pixelligent | 6411 Beckley Street, Baltimore, MD 21224 | 443.529.8310 | www.pixelligent.com
Confidential

High Index, UV Stable NIL Formulation for AR Waveguidesa.)

b.)

c.)

Figure 9. 405-nm UV exposure behavior of PixNIL® SCS1 with changes in a.) E*, b.) filmthickness, and c.) refractive indexConclusions
The importance of high refractive index materials for advancements in the Augmented Reality
and Extended Reality markets is essential for delivering both the largest field of view possible aswell as highly efficient devices. These materials must meet stringent NIL requirements, including:the highest optical clarity; high refractive indices that match the glass substrate; optimized fluidity
to allow proper filling of binary and slanted grating structures; all while delivering a fully uniform
dispersion of nanoparticles throughout the structures; and the highest degree of UV and productstability. Pixelligent’s PixNIL® SCS1 has demonstrated the ability to meet all these stringent
requirements with 1.86 RI at 589 nm, >95%T in the visible wavelengths, the ability to print a wide
range of complex nanoimprinted structures, UV stability close to that of ZrO2 performance, androbust shelf-life properties.
Acknowledgements
We would like to acknowledge the excellent work of the EV Group team and thank them for theirvaluable contributions to this white paper. Special thanks to Christine Thanner, Patrick Schuster,Thomas Achleitner and Andrea Obendorfer.

10Pixelligent | 6411 Beckley Street, Baltimore, MD 21224 | 443.529.8310 | www.pixelligent.com
Confidential