UltraViolet Nanoimprint lithography (UV-NIL) is a technology whereby micro and nanometer scale patterns are produced with high resolution and throughput at a low cost. The nanoscale patterns are made by pressing a template into a liquid imprint resin and subsequently curing the resin by exposure to UV light. This application note provides basic guidelines for identifying which resin characteristics are important for a specific application.
Image credit: Stensborg A/S
Delimitations of key terms
Before diving into the choice of resins, defining some key terms is beneficial, as shown in Figure 1. An imprint process aims to create a number of identical replicas of a given master. However, the master typically has been produced using a complex, time-consuming procedure, so the master must be carefully preserved.
The overall nanoimprinting process generally involves a two-step approach;
1) creating a template (sometimes called a stamp or a mould) from the master, and
2) using the template to imprint the replicas.
Figure 1: Definition of terms used for NIL.
Choice of replica resin
The final product of the NIL process will be structures imprinted in a resin on a carrier substrate. The main driver for the choice of resin and substrate is naturally the specific usage of the product. For instance, should the product be rigid or flexible, transparent or opaque and in which environment should the product be used? For an optical application, a key design parameter is also the refractive index. Furthermore, if the product is used in an application with regulatory requirements, this also impacts the choice of resin.
It goes beyond the scope of this application note to provide a complete overview of potential resins and substrates for all possible applications. However, at Stensborg, we have extensive experience in supporting our clients with the selection of resins, and customers are very welcome to contact us to discuss specific applications.
- Curing method
UV-curable resins are often preferred for high-volume, low-cost manufacturing using roll-to-roll and roll-to-plate processes. The rolling process is generally much faster than alternative curing methods. One type of UV-curable resins is acrylate-based resins.
Figure 2: Example of insufficient filling with low viscosity resin.
The feature sizes in the product determine the optimum resin viscosity. The basic requirement is that the resin fills the template cavities without trapping air. Therefore, smaller structures require low viscosity, and larger structures require high viscosity resins. As a rule of thumb, use resins with high viscosity for structures larger than 1 micron and low viscosity resins for structures smaller than 1 micron.
On the other hand, a low viscosity resin will result in a thinner layer than a high viscosity resin; therefore, deep cavities typically require higher-viscosity resins, as illustrated in Figure 2.
The substrate is part of the final product, so the product’s usage also determines the substrate’s desired characteristics. The substrate can be rigid or flexible, transparent or non-transparent. The primary constraint for the replica resin is that it must adhere well to the substrate being worked with. The substrate can be anything from paper to polymers like PVC, PET or PC.
Summary – replica resin
Figure 3 summarises the considerations for choosing replica resin. To start a resin project and prove that the structures can be nano-imprinted, Stensborg suggests two general-purpose resins – the X29 and X166b.
Both resins are UV-curable acrylates that will adhere to common substrate materials like PET and PVC. The difference is that X166b has low viscosity (about 25 mPa*s), and X29 has a medium viscosity (about 300 mPa*s).
Figure 3: Considerations for choice of replica resin.
Choice of template resin
The first consideration is how many copies are required from the same template, as the template material must be durable enough to make the required number of replicas. For prototyping and proof of concept, it is unlikely to need more than 10 – 50 copies, but 1000 – 10,000 units could be made from the same template for mass production. At Stensborg, we have experienced cases where our acrylate-based, UV-cured templates are used for over 50,000 replications. However, the durability of the template for a specific application depends on several factors, as described below.
The template material must not chemically react with the replica resin. Furthermore, the template resin should release easily from the cured replica. For this reason, the template and replica materials should generally not be the same.
Since the suggested curing method for the replicas uses UV light, it is also convenient to use UV-curing of the template. A cured template resin and substrate (if present) must be both transparent for UV light at the curing wavelength.
If the template is nano-imprinted on a flexible substrate, the resin template must adhere well to the substrate. The adhesion should be strong enough to withstand the required number of replications without the template resin detaching from the substrate. Some substrates might be primed with an adhesion promoter if the template resin does not adhere well to the base substrate.
Like the case with the choice of replica resin, the feature sizes determine the viscosity of the template resin. Small structures use low viscosity resins, and larger systems use medium viscosity resins.
Finally, consider how the template resin interacts with the master. Typical materials for the master are metals, polymers and SiO2/quartz. The key requirements for the template resin are that it does not chemically react or destroy the master and releases easily from the master after curing.
Summary – template resin
Figure 4: Considerations for choice of template resin.
Stensborg has developed two template resins – the DM56 and the DM57 – which have been proven to work very well for many applications. Both resins are UV-curable acrylates that will adhere to common substrate materials like PET, PVC and PC.
The difference is that DM56 has low viscosity, and DM57 has medium viscosity. Also, templates made from the Stensborg resins are very durable.
A commonly used template material is PDMS which is a UV-transparent elastomer. PDMS, however, has several issues. First of all, the lifetime of the mould is often limited to a few tens of imprints because the adhesion between the stamp and replica resin increases quickly with the number of imprints. Secondly, using PDMS as template material can prevent metal or organic layers from adhering to the replica because the replica surface properties are changed by using PDMS.
Suggested development process
Knowing where to begin can be challenging when developing a new roll-to-plate or roll-to-roll nano-imprinting process. For this reason, Stensborg has created the Desktop R2P NanoImprinter and a range of UV-curable resins for creating both templates and replicas. This device and the range of resins are a good starting point for the development process that may save time and effort. Click to learn more about the Desktop R2P NanoImprinter.
The resins are summarised in the table below, and more details, including data sheets, can be found here: https://stensborg.com//consumables/
|Features||Viscosity||Resins for Template||Resins for Replica|
The resins adhere to most substrates like PC, PET and PVC. The DM57 resin can even be used to create another template in DM56. This is useful in case the original master has a negative polarisation of the final replica requiring a first positive polarisation and second negative polarisation template to be made. The resin formulations have been made to ensure curing wavelength of 395 nm, corresponding to the Stensborg optical engine. We recommend using the Stensborg resin with Stensborg nano-imprinters; however, this is optional.
The development process towards a full-scale volume production typically involves three steps, as outlined in Figure 5 and described in the following sections. The underlying purpose of the suggested development process is to de-risk the project such that development cost and timeline can be well-controlled and within budget.
At each step, the development process involves tuning at least the following three key parameters:
The Stensborg Desktop R2P NanoImprinter allows control of all three parameters.
Step 1: Proof of concept
In the first step, we suggest using the Stensborg Desktop R2P NanoImprinter and Stensborg resins for template and replica to develop a rolling process for specific features. By using the proven consumables from Stensborg, many of the potential problems mentioned in the previous sections are eliminated. In this way, the focus is on adjusting the parameters of the imprinting process, like speed and force to obtain acceptable structure quality and template-resin release parameters.
For a proof-of-concept whereby the structures are imprinted with a rolling method, and the Stensborg resin (either X29, X166b, or others) works for the prototypes, step 2 can be ignored.
Step 2: Tune the process parameters
The resin chemistry may need fine-tuning if the replica material needs specific characteristics. In most cases, the Desktop R2P NanoImprinter and the templates made in step 1 can be used, but exchange the Stensborg replica resin with a different formulation. Stensborg can also help to develop a new resin formulation if needed.
As in step 1, if proof of the structures can be manufactured in a specific resin using a rolling imprint technique, step 3 can be ignored.
Step 3: Volume production
Once light intensity has been optimised, along with speed, force and chemistry during the previous development work, the essential process parameters for scaling to volume production are known. Therefore, the following scale-up to a dedicated volume production with the nano-imprint process should focus on final application specifications and proper choice of mastering technology and template production and material that satisfy the expected production volumes that might run from the thousands to the billions of replications.
It is important to note that the described development steps are only for the nanoimprinted resin replicas. In most cases, additional post-processing steps, such as adding metal layers and active components, will be needed.
Figure 5: Development process steps from proof of concept to volume production.