Step and Repeat Imprint tools perform the following functions;
- Conform and Separate
Research S&R tools and process have been developed by Shrinivasan and Willson at U.
Texas, Hiroshima at AIST and KIMM (Kim 2004).
MII has developed a series of commercial step and repeat tools based on the U. Texas
technology for UV drop dispense called Step and Flash Imprint Lithography (SFIL).
EVG is developing a S&R imprint tool that will be delivered to AMO in early 2006,
Suss have shown a thermal and UV S&R tool in product brochures (Suss 2005), based on
work with a team at VTT (Ahopelta 2005)
Omron described on in house thermal S&R tool that would imprint on 1 x 1 meter display
glass. They claimed 1000 imprints between cleans and a highly proprietary separation
strategy. (Okuno 2005)
The S&R imprint system, mimic optical S&R systems that have dominated high resolution
lithography for 20 years. The wafer stage is a high performance vacuum preloaded air
bearing stage that must be stable to vertical loads at the edge of the wafer.
In order to maximize the number of devices on a wafer, the imprint fields must be placed as
close as possible. In UV imprint this means the either the exposure light must be masked
to only the field being imprinted, or drop dispense must be used before each field.
In drop dispense, only the exact amount of liquid needed to fill the mold at the desired layer
thickness is dispensed, which results in savings in the imprint material used. The field is
defined by a 15 um high raised mesa on the mold. The mesa edge acts a capillary flow
barrier that MII claims constrains the liquid to the field. Street widths between fields down to
50 um have been reported (Truscott 2003).
The biggest concern with thermal S&R is how close imprint fields can be placed before the
heating of one field affects the pattern of the neighboring field.
Alternative forms of imprint that involve reduced imprint area include roller imprint and wave
imprint. While roller imprint is the largest volume of imprint capacity for security holograms,
relatively few studies have been published except by the Princeton team (Tan 1998).
Phillips Research have reported a “wave imprinter” that uses a flexible mold that imprints a
strip and then the strip is scanned over the substrate (Schnieder 2004).
CONFORM AND SEPARATE
The MII tool uses a flexure to ensure that the field levels to the wafer, with the center of
rotation at the front surface of the mold so that as the mold gimbals to be coplanar with the
wafer, there is no side ways displacement (McMakin 2003).
The S&R tool developed by Hiroshima imprints on spin on UV cured materials. The latest
version uses a open frame stage to move the wafer with a fixed soft pad underneath the
imprint field so the wafer conforms to the mold (Hiroshima 2005). The AIST tools use an
atmosphere of fluorocarbon to minimize trapped air (Ref). Previous publications describe
an active imprint head control system (Hiroshima 2004).
UV setting S&R tools have been designed to meet the needs for device fabrication, fine
overlay and low defect density.
The focus of the MII system design appears to be overlay performance. The imprint fluid is a
mixture of monomers with viscosity less than 2 cps. The low viscosity liquid requires only
1/20 th of an atmosphere that minimizes mechanical distortion. The low viscosity also
allows the wafer to be moved relative to the mold when in lubricated contact – “in liquid
align” (Choi 2001, Schumaker 2004 and Resnick 2005-2). The material is UV cured at room
temperature which minimizes thermal distortion, particularly in the latest tools (I250) when a
temperature controlled mini-environment is used. The I250 also includes an anisotropic
magnification correction scheme based on squeezing the sides of the mold, which allows
fine overlay over the entire imprint field (Schumaker 2005).
The systems use a field to field alignment with moiré patterns as targets.
When low viscosity imprint liquid is used it must be dispensed as a series of drops at each
field just before imprint using a “ink jet pump”. The early systems deliver single drops, the
I250 uses a multijet head for reduced drop size and quicker dispense. Resnick 2005-2).
The tools use an inert atmosphere to minimize trapped air (Schumaker 2005). and oxygen
poisoning during crosslinking.
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Step and Repeat tools