Here is an explanation behind the Stensborg Rolling Nanoimprint Optical Engine and the technical innovations that make our patented Optical Engine well-suited for researchers and mass manufacturers of nanoimprinted structures.
Image credit: Stensborg A/S
Roll-to-roll (R2R) and Roll-to-Plate (R2P) Nanoimprint Lithography (NIL) are replication methods whereby precise micro and nano-structures can be manufactured in high volume and low cost. The technology is applied in manufacturing electronics, medical devices, optical components, and many other applications.
The structures are formed by pressing a stamp (a template or mould) into a liquid resin and then curing (hardening) the resin.
Mass production’s processing time per unit must be as low as possible. For this reason, the curing method is preferably exposure to Ultra-Violet (UV) light as this method enables curing within milliseconds. The amount of stamp pressure and rolling speed are equally important process parameters to control and obtain high-yield production.
The part of a Rolling Nanoimprint Lithography (RNIL) machine that performs UV exposure is often called the nanoimprint optical engine. A well-designed optical engine is critical to obtaining a high-yield RNIL mass-production process. Stensborg has more than 20 years of experience in RNIL, and we have used that experience to build our patented, high-performing, flexible and scalable Nanoimprint Optical Engine. We use this Optical Engine inside all our machines, from the Desktop R2P NanoImprinter for prototyping to large custom-built machines for mass production.
This technical note provides insight into our Optical Engine’s technical details and the rationale behind the design.
Figure 1:Illustration of the RNIL process with the optical engine (not to scale).
In Roll-to-Roll manufacturing, the final products are created on a long, continuously rolling, flexible web, as illustrated in Figure 1. The process for Roll-to-Plate is almost identical, except that the imprints are created on rigid individual substrates (plates). For simplicity, we will describe the R2R process only.
This contrasts the traditional Plate-to-Plate (P2P) process whereby each product is manufactured individually by pressing a stamp into resin on a rigid substrate (wafer or plate).
Depending on the final product, a light curable resin is applied to the web across a selected area. A UV-transparent imprint roller with flexible stamps rotates with a surface speed matching the web speed. The optical engine is located inside the transparent imprint roller. When the uncured resin reaches the imprint roller, the stamp is pressed into the resin, and later the resin is cured by exposure to UV light. After leaving the transparent roller, the imprints may get a post-curing treatment.
Objectives and challenges
It is essential to understand the objectives and possible challenges of a typical RNIL high-volume manufacturing process to evaluate the performance of a specific optical engine.
Each machine should output as many products as possible. RNIL processes such as R2R and R2P offer nanoimprinting with very high rolling speeds, which can provide high throughput.
While the throughput should be high, the performance in terms of replication quality cannot be compromised. The fine structures must thus be imprinted error-free and reproducibly for 10,000s of products. By controlling the UV-light intensity, the stamp pressure, and the rolling speed, it is possible to obtain error-free imprints that demould easily from the stamp without leading to trapped air bubbles and defects. Illuminating every resin section with sufficient UV energy during curing is important to obtain consistent product performance.
A final consideration is the process’s power consumption, meaning the UV-light source naturally must be as power-efficient as possible.
Overview of optical engine
Based on the experience gathered over 20 years in the nanoimprint industry, Stensborg has developed a Nanoimprint Optical Engine that addresses the issues listed in the previous section.
The key function of the Stensborg Optical Engine is to provide the right amount of UV-light intensity in the nip. The main feature of the nip technology is that pressure is applied along a narrow line, as shown in Figure 2, rather than over a larger two-dimensional area in the conventional Plate-to-Plate (P2P) NIL process.
Figure 2: Pressure and illumination along a line in the nip design.
The smaller contact area of the nip design requires less imprinting force than wafer moulding / P2P nanoimprinting. The nip is ideally suited for high-speed, continuous RNIL processes, and the throughput can be very high, with up to 50 meters per minute in R2R production.
A further benefit of the nip solution is that the imprinting process can be precisely controlled, leading to better performance and reproducibility. The rolling speed and UV-light intensity can be reduced for imprinting very detailed structures to obtain even higher precision.
The Stensborg Optical Engine design is modular and scalable from small to large imprinting width. Along the width of the rolling substrate, the Optical Engine design can be scaled up to meters. The same basic Optical Engine is used inside our Desktop R2P NanoImprinter for prototyping and for any machines custom-built by our engineers for high-volume production.
Figure 1 provides a schematic side view of the Stensborg Optical Engine, which includes UV light sources and optics to focus the light from the LEDs to a line, as shown in Figure 2. In the following sections, we will describe the various parts in more detail.
UV light source
As mentioned in the introduction, UV light is the preferred method for curing the resin in high-speed RNIL. It takes a certain amount of energy (E) to cure the resin fully. The energy applied to the resin from the UV light source depends on the intensity of the light (I) and the time (Δt) the resin is exposed to.
E = I*Δt
The Stensborg design ensures that the resin is cured while the template is pressed into the resin. This is highly beneficial because it reduces the effect of shrinkage.
When the UV-curing takes place when the template is pressed into the resin, the intensity must be high. In the Stensborg Optical Engine, high intensity is obtained by focusing the UV light in the nip.
Today, UV-light sources for curing resin are LEDs. Compared to legacy gas-discharge lamps, LEDs are more power efficient and have a much longer lifetime. The Stensborg Optical Engine is powerful, and the lifetime of the LEDs is longer than 10,000 hours.
Figure 3: Transmission of PVC, PET and PC in the range from 200 to 500 nm.
The usable wavelengths for most UV-curing resins are in the UVA range from 315 nm to 400 nm. Stensborg has chosen to use LEDs with a centre wavelength of 395 nm as these tend to be the most efficient and lowest-cost option. Our resins are optimised for 395 nm as well.
Most polymers start to absorb UV light at wavelength below ~300 – 400 nm, as illustrated in Figure 3 for Polyethylene terephthalate (PET), Polyvinyl Chloride (PVC) and Polycarbonate (PC). Therefore, a further benefit of using as long a wavelength as possible is that it enables a more extensive range of usable materials for substrates, templates, and other optical elements.
A row of UV-LEDs is used to form the line illumination, as shown in Figure 4. The emitting surface of each UV-LED is squares of around 1.5 mm by 1.5 mm. This enables an efficient light coupling from the LEDs to a narrow straight line.
The LED module holds several LEDs, and the length of the illumination line can be extended by adding more LED modules next to each other.
Figure 4: Light source module with line of UV-LEDs.
Optics
The purpose of the optics is to create the focused line image of the row of LEDs at the nip. The image should be as narrow as possible to create a high-intensity beam. Furthermore, the optics should ensure that as much as possible of the light emitted from the LEDs is coupled to the nip where the resin is cured.
Because of the line geometry, we use cylindrical lenses to couple the light and separately consider the dimensions transverse to and parallel to the illumination line. Parallel to the illumination line, the optics act like a flat window, so there is no focusing effect. In the transverse dimension, the optics must collect the light from the LED and focus it on a line with a width of less than 3 mm.
The optical engine must fit inside the transparent imprint roller (see Figure 1), which limits the lens widths and focal lengths.
Most lens materials (glasses or polymers) absorb undesirable UV light. For this reason, the material used by Stensborg for the lenses and the imprint roller is a special kind of UV-transparent acrylic.
Figure 5: Normal lens and Fresnel lens.
The lenses used inside the Stensborg Optical Engine are Fresnel lenses which are much thinner than conventional lenses, as illustrated in Figure 5. By using Fresnel lenses, the weight and size of the Optical Engine is significantly reduced. Furthermore, the Fresnel lenses result in lower UV absorption than standard lenses.
Try it yourself
Figure 6: Stensborg Desktop R2R NanoImprinter.
If you need to develop a prototype process for UV-NIL, the Stensborg Desktop R2R NanoImprinter shown in Figure 6 is the ideal tool.
The Stensborg Desktop R2R NanoImprinter is built on our patented Optical Engine, as described in this technical note. The tool enables you to adjust pressure and light dose and develop the right UV-NIL rolling process for your specific structures and material.
Scaling from prototyping to mass production is easier because the same modular optical engine is used inside the prototype tool and our mass-production machines.
