EV GROUP® | Technologies

Manufacturing of Metalenses
using Nanoimprint Lithography

01/2023
EV Group - DI Erich Thallner Strasse 1 - 4782 St. Florian am Inn - Austria - Contact@EVGroup.com
Toppan Photomasks - 131 Old Settlers Blvd. - Round Rock - TX 78664 - USA - photomask@toppan.co.jp

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Manufacturing of Metalenses
using Nanoimprint Lithography
Introduction

Key Aspects

Metalenses Markets and Applications■

One of the most exciting technologies on the horizon of photonics applications is theuse and integration of metalenses and metastructures. As it provides a fundamental
change in how to design and implement optical components it can create significant
impact basically for any optical device like sensors, emitters, cameras, displays and
AR/VR glasses. Metalenses comprise sophisticated nanostructures which enablefocusing or dispersing of a light beam similar to a refractive lens. However, they havetremendous advantages as compared to conventional refractive lenses. Outstandingis the fact that they can be very thin and thus are also often referred as flat lenses.
This makes them an ideal candidate for the integration into modules as this allows toreduce the size and complexity of lens stacks significantly. On top of this, it is possible
to create best performing lenses which are e.g. aberration free, have highest numerical

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Proven industrial process chain for
metalens mastering and subsequent
replication
Process combination allow large
scaling factor up to HVMHigh pattern fidelity for different
kind of shapes, e.g. triangles, circular
structures, diamonds, donut- andrectangular type in arbitrary
orientations
Realized dimensions down to 65 nm
Aspect ratios of 1:4

apertures or can focus polarization independently. All these advantages can be created due to flexibility in design and thus, metalenses
are seen as next gen. optics. Overall metalenses are considered to replace conventional micro optics and optical elements for many
applications but some of the added functions will also be complimentary to conventional micro optical stacks and modules.
Crucial to obtain these highly sophisticated metalenses or metastructures in general is a precise and high-resolution lithography
technique. Typical designs consist of pillars which can differ in diameter, pitch and shape. They are manufactured in design specificoptical materials. To manufacture best performing designs, highest pattern fidelity physical resolution requirements down to 30 nm areforeseen. Not many lithographic techniques are capable to meet these requirements, but NIL. in combination with high quality e-beammastering is the only one that enables high volume manufacturing without significantly limiting the metastructure design possibilities.
Consequently, a matching combination of a high-quality mastering technique and a subsequent effective and reliable replication
process are needed to unlock the full potential of metastructures and to realize high volume wafer-level fabrication techniques.
This is where NIL technology from EVG combined with the high-quality mastering from Toppan comes into the game. In this work, weshow why and how this collaboration and joint effort enables metalenses and overall photonics manufacturing for the future.
Motivation
Right now, we are reaching a point, where conventional lithography
technologies are reaching their limits when it comes to future
photonic applications like metalenses and, in particular, the creationof the required small and arbitrarily shaped patterns. Here, NIL is a
viable alternative for this market due to the high-resolution capability
for complex structures. The technology can replicate sophisticatedstructures very efficiently, on large areas, with less design constraints,
and with a very streamlined process flow, for both prototyping andhigh-volume manufacturing.
Together with Toppan, a leading supplier of photomasks to thesemiconductor market, the collaboration aims to establish NIL as anindustry standard production process for photonics manufacturing.
This joint work offers the required process capabilities to manufacturehigh-quality metalenses and metastructures and it shows thatthe necessary infrastructure exists and can be deployed on anindustrialised scale to bring mastering and subsequent NIL to HVM
quality and volumes.

01/2023
EV Group - DI Erich Thallner Strasse 1 - 4782 St. Florian am Inn - Austria - Contact@EVGroup.com
Toppan Photomasks - 131 Old Settlers Blvd. - Round Rock - TX 78664 - USA - photomask@toppan.co.jp

Figure 1: 25 Imprints of Metalens Arrays for HVM

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NIL process chain: From single master to HVM
It is key for the high-volume manufacturing of high quality metalenses to have a thorough understanding of all required manufacturing
steps and to preserve the quality over the whole process chain. The main necessary manufacturing steps are described below, in thefollowing order:
1.

E-Beam mastering on a single die 6-inch square quartz template – Provided by Toppan

2.

S&R mastering to full 200 mm or 300 mm wafer-scale – Provided by EVG

3.

NIL replication to high volume – Provided by EVG

Mastering
Toppan’s POR (Process of Record) for NIL master manufacturing has been developedusing basic ingredients of photomask manufacturing. By this it benefits from decades ofexperiences and superior capabilities using tools and processes originally developed for
manufacturing of photomasks for the semiconductor industry. Here a brief overview on
the main steps of the manufacturing flow is given (see also figure 2). It begins with thecustomer transferring the design data for the optical product. The data is converted into
a format used by the electron beam pattern generators to expose the pattern onto theblank material.
For the manufacturing of NIL masters for metalenses, a 6-inch square Quartz formfactor isused. But It should be mentioned as well that there are all necessary tools and processes
available to offer the same on 200 mm round formfactor. The quartz plate (or blank) is
coated with a Chrome (Cr) film which acts as a hardmask.
An electron sensitive resist is coated on the chrome and by electron exposure, the areas
which get exposed change their chemical properties such that they can be washed awayin the later development step. This chemical change is enhanced by a baking step after
the exposure.
In the areas in which the resist is removed the Chrome is etched away (removed) andFigure 2: Schematic flow of NILmaster manufacturingthus, the exposed pattern gets transferred into the Cr-hardmask. The patterned hardmask
is used to further transfer the pattern into the Quartz. By a selective etch process only theareas with open chrome are etched to the required depth. The selectivity of this etch process ensures that the Chrome is not changed
i.e. all patterns are etched into the quartz to a defined depth without any change of the size or shape. Once the quartz is etched tothe defined depth the Cr-hardmask is removed. The critical parameters for the master are controlled using SEM metrology for criticaldimensions and by AFM for the depth.
The following data were gathered on a metalens demo master that was jointly designed and manufactured by Toppan and EVG. The
generic metalens designs were realized to illustrate the capabilities – the results are shown on the next pages
The first results shown are the critical dimension (CD) uniformities of dots of various sizes. One can clearly see the same excellent CD
control across the master for all design sizes from large (CD=300 nm) to small (CD=70 nm) pillars.

Figure 3: Uniformity of Critical Dimension of pattern across NIL master

The most critical parameter of a master is the quartz etch depth. The metalens demo master was etched down to 200 nm.
An optimal uniformity over the entire area of the master was achieved.

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The resolution quality of the meta-atoms of various types was assessed using
CD-SEM images. These were taken after Quartz etch prior to the hardmask
removal, in order to get the best contrast. The hardmask removal will neither
change the resolution capability nor the shape of the pattern, i.e. these images
are representative for final Quartz pillars. Some examples of these images areshown below.
A design kit providing a list of shapes and minimum sizes of meta atoms which
can be manufactured on a master is available.
Additionally, the pattern integrity and defectivity level should be considered
to ensure correct functionality of imprint masters. The majority of metalensdesigns, due to their complexity, cannot be assessed by full pattern inspection
as is standard for photomasks. However, the use (or deployment) of the wellestablished manufacturing environment for photomasks is beneficial. By theexcellent cleanliness level and established control schemes for all processes,
the NIL master is manufactured with guaranteed low defect level.

Figure 4: Uniformity of Quartz etch depth across NIL masterTo summarize, Toppan’s excellent capabilities and experiences of decades of photomask manufacturing serve optimally the needs of
NIL master manufacturing, ensuring delivery of 1st class masters to customers.
S&R Mastering
For increasing production volumes of metalenses, time and cost-efficient scaling is key. As the manufacturing time of high-quality
single die masters is directly linked to the size of the patterned area, meaning the costs scale with the patterned area, it is considered toscale the single die master from Toppan by step and repeat (S&R) which allows high volume manufacturing on wafers very efficiently.
To do so, the 6-inch square Quartz master is replicated multiple times on a 200 mm or 300 mm wafer by the EVG®770 step and repeat(S&R) NIL system to create a full area wafer scale master. The system can dispense resist, align the structures, imprint and demold
in a fully automated procedure. This is required to create multiple elements with a high quality and fidelity as close as possible to theoriginal master. This significantly reduces the production time to achieve full wafer-scale master (S&R master) which are then used forthe following NIL replication – see figure 5.

S&R process

Nanoimprint Lithography | From Master Template To Multiple Imprints
Single DIE Master

EVG®770 S&R System

Working Stamp
NIL imprint process

200 µmS&R Master

EVG®7300

Multiple Imprints

Figure 5: Schematic of S&R and NIL replication process flow
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NIL Replication
For the subsequent wafer-level NIL replication process itself, the SmartNIL® technology is used. The process flow consists of twosteps: First manufacturing multiple low-cost polymer working stamps (WS) out of the previous manufactured S&R master and utilizingthem for the second step, the actual imprints. This intermediate approach avoids wearing out the original template and improves theoverall production economics because the achievable number of imprints is increased tremendously. Defective working stamps canbe replaced quickly and at low cost, which can be particularly advantageous during high-volume production runs.

01/2023
EV Group - DI Erich Thallner Strasse 1 - 4782 St. Florian am Inn - Austria - Contact@EVGroup.com
Toppan Photomasks - 131 Old Settlers Blvd. - Round Rock - TX 78664 - USA - photomask@toppan.co.jp

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The NIL process in detail is depicted in the figure 6 below: To ensure defect free WS fabrication, the S&R master is coated with ananti-sticking layer applied by spin coating. Next, the WS material is dispensed directly on the master, also by a spin coating process.
Afterwards, the transparent SmartNIL backplane is attached on the coated master. The WS polymer is then cured using an UV LED,and finally demolded from the master. Second, the actual SmartNIL imprinting process is performed. To do so, a dedicated material
is applied by e.g. spin coating on the substrate using the same process used for the WS fabrication. The WS and the substrate withthe dispensed material are brought in contact. Similar to the WS fabrication, this step is followed by UV curing and demolding, so thatmultiple imprints are made with the final structures (metalenses). The reuse of the WS for multiple imprints for the SmartNIL replication
is increasing the process efficiency and has been proven already for high volume manufacturing.

10 µm

200 µmFigure 6: SmartNIL® replication process flow in details

Even though the full SmartNIL imprint process described above is split into two parts - manufacturing the working stamp andsubsequently imprinting the wafer - the entire process can be implemented on the same equipment. This work is performed on theEVG®7300 SmartNIL system using 200 mm or 300 mm wafers.
Currently, this metalens manufacturing process is typically followed by an etch process to achieve best optical quality. This process
is also referred to as “sacrificial imprint”, because the NIL imprint is used as etch mask (pattern transfer). Most common materialis amorphous silicon (for infrared applications) and TiO2 for visible light due to their beneficial material properties. However, allsemiconductor and dielectric materials are within scope of metalens and metasurface applications.
Whereas in R&D, permanent (or direct) imprint is gaining momentum, mostly in combination with high refractive index resists, because
this process has the advantage, that one can enable complex structures without additional costs because no etching process isrequired.
100 µmProcess Results
Not only is the cost efficiency and HVM manufacturability of the process described above crucial, but it also preserves the patternfidelity and quality of the metalens structures throughout the whole process. Thus, the replicated structures are evaluated in moredetail in terms of height and dimensions, overall stability and repeatability.
Pattern fidelity from master to final print
To demonstrate the stability of the patterns within the whole process, the critical dimension (CD) / height of the original quartz masteris compared to the height of the structures on the fully populated S&R master and on the final imprints. This is investigated for differentrealized patterns, to see the impact of the different shapes. Within the S&R process step, die 1 and die 6 are measured as representativeexamples.
Process chain

Round pillar height (AFM) average

Donut shape pillar height (AFM) average
200 µmToppan Master Template

196 nm

194 nm

S&R fully populated wafer - Die 1

190 nm

189 nm

S&R fully populated wafer - Die 6

192 nm

188 nmFinal imprint

184 nm

180 nmThe numbers show that the overall die variation is less than 5 % (within metrology error). The height loss from the initial master tothe final imprint is explained by material shrinkage. This factor is already considered when creating the master design, ending up withhighly reproducible dies and replications on wafer-level. Additionally, no missing structures (e.g. pillars) are observable - showcasing
overall an excellent pattern fidelity.
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www.photomask.co.jp/englishRepeatability - Imprint series
Next to the pattern fidelity from the original master to the final imprint, special focus is given to the repeatability over the final imprint(series) itself. To demonstrate this, 25 NIL replications using the EVG UV-A resin were executed. To prove the imprint-to-imprint quality,
the height of the patterns of all 25 wafers has been measured and plotted in Figure 7.

EVG UV-A Resin Imprint - Pattern Fidelity
190

Structure Height [nm]

188
186
184
182
180
178
176
174
172
170

1510152025Imprint Count
Round Pillar

Donut Shape PillarFigure 7: Structure height over 25 imprints (imprint series)

Despite a minor variation of a few nanometers, no systematic degradation of structure height is observed – underlining again theexcellent repeatability, stability and quality along the whole process chain.

Images from design through S&R master to multiple imprint wafers
To demonstrate the results, SEM images are taken: from initial design, from the master and from the imprint.
DesignMasterImprint

1 µm

1 µm

1 µm

01/2023
EV Group - DI Erich Thallner Strasse 1 - 4782 St. Florian am Inn - Austria - Contact@EVGroup.comToppan Photomasks - 131 Old Settlers Blvd. - Round Rock - TX 78664 - USA - photomask@toppan.co.jp

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DesignMasterImprint

1 µm

1 µm

1 µm

1 µm

1 µm01/2023
EV Group - DI Erich Thallner Strasse 1 - 4782 St. Florian am Inn - Austria - Contact@EVGroup.com
Toppan Photomasks - 131 Old Settlers Blvd. - Round Rock - TX 78664 - USA - photomask@toppan.co.jp

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How to access these capabilities
The figure 8 below shows, how to access these capabilities and how Toppan and EVG give the opportunity to implement newinnovations and how to ramp up new ideas using the described technologies – resulting in customer tailored solutions.

Customer Request / IdeaSolutionOptics
Designer

Equipment-,
Process- and IPTransfer to Customer
Design Kit
Design Support

Individual
DesignMaster

NIL Process
Development

Prototyping andDesign Optimization

NILPhotonics®
Competence Center
S&R Mastering

Small Volume Pilot
Line Production

Process Optimization,
Scaling to HVM

Figure 8: Schematic of “How to access these capabilities?”

The innovator has access to the Process Design Kit (PDK) from Toppan to design a new product. The design is then transferred toToppan, using an established data path. An automated data handling process is then deployed to the lithography system to generate
the production grade master. The master is manufactured using semiconductor standards to ensure optimally controlled repeatableprocess. These processes are well controlled, proven to be stable, suitable, and with low defect level, complying to semiconductor
industry standards.
The master will then be used at the EVG NILPhotonics® Competence Center to commence the process optimization of the productfor high volume manufacturing (HVM). The NILPhotonics Competence Center is a 1.300 m² cleanroom area at EVG headquarter
where EVG helps customers to ramp up their ideas by providing NIL process development, S&R mastering service, as well processoptimization and prototyping up to and including ‘pilot line’ production phase. After that, the process and dedicated NIL equipment aretransferred to the customers. This ensures the highest level of IP protection for every aspect of development. The competence center
covers everything that is needed for NIL: equipment, dedicated metrology, materials and access to technical expertise incorporating
the complete NIL supply chain, including Toppan masters. This open access innovation incubator helps customers to shorten
development cycles and time to market for innovative photonic devices and applications.

Summary and Conclusion
Together Toppan and EVG have shown that it is possible to set up a high-volume manufacturing line using precision masters andsubsequent nanoimprint lithography for S&R mastering, as well as for wafer-level production. All process steps from a single die
master to the final imprints are well controlled and provide high pattern fidelity. This enables to scale the manufacturing of complex
devices such as metalenses to mass manufacturing.
This collaboration and the associated synergy of both technologies intend to further fuel the adoption of NIL as a mainstream HVM
technology for the photonics industry, especially to showcase the readiness of equipment and processes to serve the demands tomanufacture sophisticated metalenses and other complex optical structures.

01/2023
EV Group - DI Erich Thallner Strasse 1 - 4782 St. Florian am Inn - Austria - Contact@EVGroup.com
Toppan Photomasks - 131 Old Settlers Blvd. - Round Rock - TX 78664 - USA - photomask@toppan.co.jp

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www.EVGroup.com
www.photomask.co.jp/english