Challenges for Scaling UV NIL to Production Speeds using Roll-to-Roll Manufacturing1Xin Chen, Manogna Jamhapuram, Ron Markum, Stephen Qi, D.A. Lucca and J.K. GoodOklahoma State University
Roll-to-roll (R2R) manufacturing methods have existed for over 200 years. This manufacturing method
remains one of the most productive in existence. Products that cannot be made in discrete form and result inprofit can be made cheaply with better repeatability with R2R methods. R2R methods are used to make andcoat substrates called webs. Coatings can be continuous or discrete by the gravure process and may be used
to print graphics, text or even electronics. What coating processes have in common is that they must be atleast partially cured or dried prior to contact with a roller surface in the R2R machine or coating defects will
result. They must be completely cured or dried prior to winding the web at the end of the R2R machine. The
coated web is wound in rolls, to date this is the only efficient means for storing web lengths on the industrial
scale of kilometers for subsequent processing. While multiple processes can occur on one R2R machine it isunusual. These processes run optimally at unique web velocities and tensions.
Moving UV-NIL from discrete embossing to continuous R2R production has unique challenges. Uncured
resins are coated onto a web. This coated web enters a convergence between an imprint roller and a backup
roller as shown in Fig.1 in schematic, the R2R imprinter is shown in Fig. 2. The backup roller is required toforce the resin to fill the nanoimprint mold which is mounted on the surface of the imprint roller. Then, while theresin/web remains in contact with the imprint roller, the resin must be sufficiently cured with UV light such thatthe cured resin/web can be peeled from the roller with no damage to the imprinted nano features. Although thebackup roller is a necessity for overcoming the fluid dynamics of mold-filling there are additional effects that
may be unobvious to those practiced in UV-NIL processes. Scaling to higher velocities requires the elastohydrodynamics of coating to be considered. Sufficient resin must be provided to fill the imprint mold but excess
resin thickness will result in mold fouling and requires higher UV power intensity to ensure curing sufficiently
prior to peel [1].
In R2R manufacturing there is an entire field dedicated to the contact mechanics of webs entering nips
such as the convergence of the imprint/backup rollers used in UV NIL. What is known is that not only a contactpressure distribution forms that assists the mold filling operation but also tension stresses and shearing forces
are induced that cause contact shear stresses to form between the molded resin and imprint roller while
subject to UV exposure [2]. Such shearing could be disastrous if the result is shearing or distortion of the nanoimprinted features in the resin. In the current research imprint speeds of 20 mpm have been achieved. At thisspeed, the resin coated web has ~1/6 second residence time in contact with the imprint roller while UV curing
prior to peeling. We have found that light curing photo chemistry versus dark curing aspects of UV resins 2 may
be a factor that will limit the speeds of UV-NIL R2R production (Fig. 3). We are finding the shear strength of thelight cured resin may be but a fraction of the dark cured strength. If the shearing stresses due to nip mechanics
exceed the shear strength of the resin, imprint damage is expected. With knowledge of the tensile (Fig. 4) andshearing stresses, especially in the vicinity of the peeling region (Fig. 5), the web/resin tension at the exit of theimprint roller can be used to attenuate slippage and prevent damage. UV resins will continue to dark cure after
peeling from the imprint roller and the mechanical strength of the resin will continue to rise over periods ofminutes to hours. While additional UV curing is possible in unsupported web spans downstream from theimprint roller the imprinted surface will become subject to contact pressure when transiting a roller and more sowhen wound where contact pressures in the MPa range are possible.
Scaling discrete UV-NIL stamping and curing processes to R2R manufacturing processes often requires
detailed analysis of fluid mechanics, contact analysis on the imprint roller, properties of UV curable resins whilecuring and control of pressure on imprinted surfaces downstream of the imprint roller. Note with increased
speeds these analyses become critical and can result in partial mold filling (Fig. 6). Means of characterizing themoving imprinted surface are necessary such that web and gravure roller velocities, nip force and webtensions upstream and downstream of the imprint can be controlled to maintain the quality of an imprinted
surface. While such analyses can be challenging, the potential reward is immense for producing nanoimprinted
surfaces continuously at web velocities of 20 mpm or faster.

12

The research was funded by the National Science Foundation, Grant #5545312.
UV15, Masterbond, 154 Hobart Street, Hackensack, NJ 07601 USA

Peelregion𝜙 = 2.52 in
Imprint
RollerPotential
SlipRegion

365 nmUV cure4 W/cm24rad/s

Idler roller forpeel control
𝜙 = 1.50 in

ϕ = 1.38 in
Rubber coveredbackup roller

𝜙 = 0.79 𝑖𝑛

Imprint Roller Entry

1200

200
1801601401201008060
40
200

1.5 mW/cm2Slip Begins
Imprint Roller Exit

1000

7.5 mW/cm275 mW/cm2150 mW/cm2

800
600
400
20000204060
UV Exposure Time (s)Fig.3 Storage Modulus UV1580

100

Imprint Roller Entry

1Contact shear stress (psi)Fig.2 R2R UV-NIL ImprinterWeb Tensile Stress (psi)Storage Modulus (psi)

Nip load
Web tension
10 pliweb thickness
500 psiResin coating0.0014 in
Web tension
thickness 0.0006 in
500 psi
Fig.1 Schematic of UV-NIL ImprinterNip Exit
0.5

Stuck to mold051015Web path (in)Fig.4 Web Stresses resulting from Nip Contact
2 mpm

5 mpm

10 mpm

20 mpm

Imprint
Slip Begins
Roller
Imprint Abrasion
Exit

0-mu*P+mu*P
Shear stress-0.5-1
135Web Path (in)Fig.5 Web Shear Stresses and Slippage

7Fig. 6 AFM characterization of replicas from trials at imprint
speeds of 2, 5, 10 and 20 mpm

References
1. J. P. Gomez-Constante, P. R. Pagilla & K. R. Rajagopal, “Effect of Process and Transport Parameters on Mold Filling in Roll-toRoll Nanoimprint Lithography,” Fifteenth International Conference on Web Handling, Oklahoma State University, June 2019.
2. Y. Ren and J.K. Good, The Nip Mechanics of Nano-Impression Lithography in Roll-to-Roll Process Machines, FourteenthInternational Conference on Web Handling, Oklahoma State University, June 2017.