Molds are a critical component for imprinting. They are particularly challenging because the
feature size and location on the mold must be the same as the final imprint, a “1 x mask”.

Advanced optical lithography tools use reduction lenses and “4 x masks” that should be 4
times easier, however the latest masks have sub resolution optical proximity effect
correction features that can be as small as ¼ the feature size. Furthermore, phase masks
are more sensitive to small line width changes, an effect measured by the “Mask Error
Factor” (MEF).  MEF can be as large as  4 making requiring “1 X” line width control.  

FORM FACTOR
The patterns are created in resist on either wafers or mask blanks. Some systems accept
the blanks, others require machining to be compatible with the holding mechanism.

MATERIALS
Thermal imprint uses either patterned silicon, or quartz wafers as the mold, or patterned
Nickel films mounted on a carrier often with a rubber backing layer. Patterned silicon is a
readily available in most fabs.

Nickel molds are used in an injection molding machines to replicate CD’s. They are made
by electroless Ni plating from either resist or a master pattern.  Nickel films can be formed
into rolls as a mold for roller imprinting. Nickel replication has the advantage that there are
no cumulative replication errors.

Mechanical properties of mask materials are compared below.

UV imprint requires either a transparent mold or substrate. The most common transparent
mold material is fused quartz, in the form of mask blanks or wafers. Fused quartz is
available in very high quality precoated blanks, and has a hard surface that can be cleaned
in strong oxidizing agents such as H2SO4 / H2O2 or “Pirhana”. Industry standard mask
blanks are the only choice if you want molds made through commercial mask shops.  UV
molds have been made by directly patterning organo – silicon resist materials.

A number of materials have also been used make transparent molded replicas of a master
including; spun on organo-silicons, PDMS, and  organic polymers (Transfer Devices). The
replication process has been used to make molds from existing devices, and to produce
transparent versions of molds that can be fabricated in Si more easily than fused silica.
PDMS molded replicas have been used as low cost working plates by Heptagon.

Solvent cast PDMS has been used very widely in transfer imprinting because of its very low
modulus.

PATTERN CREATION

Electron beam PG
- The recipie for success in creating high resolution molds is to use thin
chrome and thin resist blanks on the highest throughput tool that will just resolve the target
pattern. Keep the pattern as regular as possible to minimize proximity effects. Use test
writes and etch cycles to ensure line widths hit the correct target.

Resolution versus patternable area (throughput) is the performance trade off for pattern
generators.
The resolution is determined by spot size, beam voltage, and resist. The throughput is
gated by either the data rate of the tool, or the energy (beam current x time) required to
expose the resist.

Commercial mask writers are designed to write 4 x optical reticles with sub-resolution
assist features. The nominal minimum reticle feature size is around 100 nm. They meet all
their specifications operating at maximum data rate on a commercial resist. These tools
are supplied by Jeol and NuFlare. The writing style is   “vector shaped beam”, meaning that
rectangular shapes of varying length to width are exposed in single shots, and that the
beam moves by the shortest path between shots. This minimizes the number of shots and
maximizes throughput. The throughput is simply determined by the shot count, the number
of overwrites (4 for highest performance) and data rate (250 MHz). Overwrites are used to
average out pattern positioning errors.  A typical 6” reticle for an IC with 16 G transistors will
take around 6 hours to write with 4 overwrites. Effectively the field will be fully covered with
150-200 nm shots. The programs to fracture the data are as important as the mask writers.
Throughput scales as the square of the minimum feature size.

For highest resolution molds, direct write electron beam machines are required. These can
be raster scan Gaussian beam tools, or vector beam tools, the highest resolution use
higher beam voltage to reduce forward scattering. The highest performance Gaussian  
tools are supplied by Lieca with a beam voltage  of 100 KV, and a data rate of 25 MHz
which  is 10 x slower than the mask writers. In practice the data rate is slowed down to
expose high resolution resists. It is just practical to write a fully populated 50 nm line and
space pattern over 25 x 25 mm in high resolution resists.

Modified SEM’s have been used to write very small areas with < 20 nm features.

Using the thinnest possible resist helps to increase resolution, MII has reported using 50
nm thick resist.

Special low contrast  processes can be used to create grey scale patterns (Little, JPL, RAL)

In all cases, the resist pattern must be transferred in to the mask. A silicon wafer or SiO
2
mask blank can be etched using the resist. The industry standard is to etch a layer of
chrome and then use the chrome as a hard mask for the SiO
2 etch. The chrome is
removed at the end of the process which increases process options. Optical lithography
requires anti reflecting chrome (CrO
2) that is hard to etch, bright chrome (pure Cr)
increases process resolution.

Using the thinnest possible bright chrome helps to improve resolution.  The chrome does
not need to be transparent  in imprint applications.  

The most challenging process control element on election beam patterning is obtaining the
correct line width.

•        The long write times means that the develop times must be completely stable. When
in doubt, a short test write at the target CD before starting a long write is good practice.
•        Electron proximity effects caused by electrons backscattered from the substrate
expose neighboring features up to 4- 6 um away depending on beam voltage, and
electrons scattered inside the tool creating the equivalent of flare in a optical system.
Device patterns require correction to every feature. Regular patterns are much simpler, and
do not require correction except over the 6 um edges of the pattern. Corrections can be
applied by adjusting features size and / or exposure dose. Many systems come with built in
correction  algorithms. These break down at the highest resolution, and additional dose
correction based on a set of rules can help (Manakli 2005). Commercial software correction
programs are available (PDF solutions).
•        There is an bias from the etch processes that must be offset in he targeting of the
resist line width.  
•        Limit each pattern to one critical dimension where possible. Test patterns are often
designed with a large range of line widths. These sever proximity effect problems. It is a
much better strategy to write individually targeted  small molds for each critical dimension.

Laser beam PG
Feature sizes > 250 nm can be written on a scanning laser pattern generator at much lower
cost. All commercial mask shops have these tools.

Interferometry
Many regular patterns can be created by interferometry, it is the technique of choice for fine
resolution gratings down to 75 nm. It has been reported that 50 nm gratings have been
made by immersion  deep UV interferometry.

PATTERN REPLICATION
Making copies of molds has several uses; making a mold from an existing device, making
a mold indirectly from an easier to fabricate intermediate, making low cost copies from a
very expensive master. This technique also supports sub masters and working plates with
the minimum accumulation of errors.

Ni copies from  silicon or resist
Electroless Nickel plating to provides a accurate replica of a surface. First a release layer is
deposited and then a plate base. A thick Ni layer is then plated up. The Ni layer can be
peeled away and mounted to a flat or drum support. For better control, the Ni is  bonded to a
support and the master etched or dissolved away, destroying the master. If the Ni is plated
up directly on the resist pattern, there is no etch bias or distortion, ideal for replication of
grey scale patterns.

Polymer copies from masters or submasters
A number of other materials have also been used make molded replicas of a master
including; spun on organo-silicons, PDMS, and  organic polymers (Transfer Devices.). All
these materials probably have shorter process life than fused silica because of poorer
mechanical properties.    

Imprint S&R
An S&R imprint tool can be used to create a large area master from a smaller area mold.


PATTERN INSPECTION AND REPAIR
For integrated circuit applications, the mold must be perfect which requires both 100%
inspection and repair of a 1 X mold. A team at Motorola, KLA and MII (Nordquist 2004,
Resnick 2005) have published data showing the use electron beam inspection of  1 X
molds, and  the feasibility of electron beam repair of 50m nm features.

MOLD RELEASE

There are many different release strategies, relatively few systematic comparisons have
been reported. These strategies are mostly designed to make “non stick” very low energy
mold surface such as; deposited floro- layers, treatment with self assembly reactive
fluorocarbon-chloro silanes, deposited diamond life carbon, making the mold from a floro-
polymer. Additives that migrate to the surface of  imprint materials have been tried in
combination with mold treatments by AMO.

Reviewed in Guo 2004

Release
Treatments        
Liquid and vapor
EVG report Uses PFOCS as release agent on mold (Schaefer 2002). Also Otto 2004

Si on Ni so to react with chlorosilanes (Maeng 2005)
Flourinated materials
Inorganic (Choi 2005, Kawaguchi 2005, Kubenz 2005)
DLC (Lee 2005)
Surfactants


MOLD CLEANING

MOLD LIFE
Molds / Stamps / Templates