Schematic of the nanoimprint lithography process. The thickness contrast is
created in the resist by imprinting (1). The pressure is maintained until the polymer flow fills the cavities of the mold (2). The mold and substrate are cooled below the glass transition temperature and separated (3).
The nanoimprinting is a potential method for submicron scale patterning for various applications, for example, electric, photonic and optical devices. The
patterns are created by mechanical deformation of imprint resist using a patterned
imprinting mold called also a stamp. The bottle-neck for imprint lithography
is availability of the stamps with nanometer-scale features, which are typically
fabricated by electron beam lithography. Therefore, patterning of a large
stamp is time consuming and expensive. Nanoimprint lithography can offer a
low cost and a high through-put method to replicate these imprinting molds.
In this work, stamp replication process was developed and demonstrated for
three different types of imprint molds. Replication relies on sequential patterning
method called step and stamp nanoimprint lithography (SSIL). In this method a
small master mold is used to pattern large areas sequentially. The fabricated
stamps are hard stamps for thermal imprinting, bendable metal stamps for roll
embossing and transparent stamps for UV-imprinting.
Silicon is a material often used for fabrication of hard stamps for thermal imprinting.
Fabrication process of silicon stamps was demonstrated using both the
imprinted resist and lift-off process for pattern transfer into silicon.
Bendable metal stamp for roll-to-roll application was fabricated using sequential
imprinting to fabricate a polymer mold. The polymer mold was used for
fabrication of a nickel copy in subsequent electroplating process. Thus fabricated
metal stamp was used in a roll-to-roll imprinting process to transfer the patterns
onto a CA film successfully.
Polymer stamp for UV-imprinting was fabricated by patterning fluorinated
polymer templates using sequential imprinting and a silicon stamp. The imprinted
polymer stamp was used succesfully for UV-NIL.
In the stamp fabrication process the features of the silicon stamp were replicated
with good fidelity, retaining the original dimensions in all of three stamp
types. The results shows, that the sequential imprinting is as a potential stamp
replication method for various applications.
Basics of nanoimprint lithography
* The imprint parameters
* Residual layer
* Thermoplastic materials
* Imprint molds
* Antiadhesion treatment
Stamp fabrication by imprinting
* Sequential imprinting method
* Imprint tools
– Imprint machines
– Stamp holder
* Evaluation methods
– Optical microscopy
– Scanning electron microscopy (SEM)
– Atomic force microscopy (AFM)
– Replication of silicon stamps
– Bendable nickel stamps
– UV-NIL stamps
Bendable metal stamps were fabricated using imprinting to define a polymer
mold and electroplating to deposit nickel onto the mold. Thus, the fabricated metal stamp was used in a roll-to-roll imprinting process to transfer the patterns
onto a CA film successfully.
The fabrication method of polymer stamps for UV-NIL was demonstrated by
patterning fluorinated polymer templates by thermal SSIL using a silicon stamp.
The features of the silicon stamp were replicated with good fidelity, retaining the
dimensions. The tests on the polymer stamps by UV imprinting demonstrated
good mold-release properties along with satisfactory pattern replication of submicron
features, thus proving the concept useful.
In the field of thermal imprinting, the emphasis has been on the parallel processing
method with large molds. In recent years, the sequential method has attracted
wider interest, and a growing number of research groups have shown
interest in developing sequential imprinting. The benefit of the method is flexibility, which allows patterning of each chip with a different stamp.
The parallel method has attracted a growing number of researchers due to its
simplicity and low cost. In thermal step and repeat imprinting, the bottleneck is
low throughput, compared with the parallel method, but the development of
rapid heating and cooling of the stamp can improve the throughput significantly.
Materials are also being developed all the time, and better suitability for imprint
purposes can be expected in the future. Another challenge in electronic applications
is multilevel capability with an overlay accuracy of tens of nanometers on
wafers larger than 150 mm in diameter. Applications relying on single layer
patterning, such as optical gratings, are not as demanding in overlay accuracy
and can be realized sooner in mass production.
There are many applications in sight in nanoimprint lithography that require
sophisticated stamp fabrication technology for optical, photonic, electrical, and
biological applications. Nanoimprint lithography is suitable for patterning 3D
features used in, for example, optical devices such as sawtooth diffractive grating
elements or anti-reflection surfaces. There are many other applications in the
field of optics, such as photonic crystals, light directional elements for extracting
light from LEDs, and control deviation of light in window glass surfaces for use
in natural lighting for housing. There are different sensors for nano-bio research,
such as interdigitated nanoelectrodes. In electrical application, imprinting can be
used for, for example, fabrication of MOSFETs and organic FETs.