imprint lithography by Douglas J . Resnicka*, S . V . Sreenivasanb, and C . Grant Willsonc

The escalating cost of next generation lithography
(NGL) is driven in part by the need for complex phase- sources and optics . The cost for a single NGL tool

been demonstrated that

could soon exceed $50 million, a prohibitive amount
I

for many companies . As a result, several research groups are looking at alternative, low-cost methods

tolithography can produce sub-100 nm

features. Such improvements come with an everic tools . As

for printing sub-100 nm features . Many of these methods are limited in their ability to do precise overlay . In 1999, Willson and Sreenivasan developed step and flash imprint lithography (S-FILTM) . The use of a quartz template opens up the potential for optical alignment of the wafer and template . This paper reviews several key aspects of the S-FIL process, including template, tool, ultraviolet (UV)curable monomer, and pattern transfer . Two applications are also presented : contact holes and surface acoustic wave (SAW) filters .

aMotorola Labs, 2100 East Elliot Road, Tempe,
AZ 85284, USA
E-mail d oug.resnick@motorola .co m

bMolecular Imprints, Inc ., 1807-C West Braker Ln .,
Austin, TX 78758, USA

cDepartment of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712, USA

34

materialsladay

February 2005

The development of light sources and optics are primarily responsible for the rise in the cost of an NGL tool . 157 nm lithography, for example, requires the use of CaF 2 as a tens material . In the case of extreme-UV lithography, no source has yet been identified with sufficient output that will meet the industry's throughput requirements . Clearly, a technology that can reduce tool cost by an order of magnitude will have a significant effect on the economics of the fabrication process 1 .
Imprint lithography is essentially a nanomolding process in which the topography of a template defines the patterns created on a substrate . Investigations by several researchers in the sub-50 nm regime indicate that imprint lithography resolution is only limited by the resolution of the template fabrication process 2-4 . This approach possesses important advantages over photolithography and other NGL techniques since it does not require expensive projection optics or advanced illumination sources .

ISSN: 1369 7021 0 Elsevier Ltd 2005

REVIEW

Devices that require several lithography steps and precise

EE IlR .]2E

features in a glass substrate . Although sub-100 nm

overlay need an imprinting process capable of addressing

geometries were demonstrated, critical dimension (CD) losses

registration issues . A derivative of imprint lithography, S-FIL,

during etching of the thick Cr layer make the fabrication

solves this problem by using a transparent fused silica

scheme impractical for 1x templates . It is not unusual, for

template, facilitating the viewing of alignment marks on the

example, to see etch biases as high as 100 nm 8.

template and wafer simultaneously 5 '6 . In addition, the

More recently, two methods have been employed to

imprint process is performed at low pressure and room

fabricate templates 9 ' 10 . The first method uses a much thinner

temperature, minimizing magnification and distortion errors .

(15 nm) layer of Cr as a hard mask . Thinner layers still

In the S-FIL process, a low-viscosity photocurable

suppress charging during electron-beam exposure of the

monomer (known as the etch barrier) is dispensed onto the

template and have the advantage that CD losses during the

surface of the substrate . The transparent template is

pattern transfer through the Cr are minimized . Because the

brought into contact with the monomer, causing it to spread

etch selectivity of glass to Cr is better than 18 :1 in a fluorine-

across the surface