This paper discusses the basic concepts
and current state of development of EUV lithography (EUVL), a relatively new
form of lithography that uses extreme ultraviolet (EUV) radiation with a
wavelength in the range of 10 to 14 nano-meters (nm) to carry out projection
imaging. Currently, and for the last several decades, optical projection
lithography has been the lithographic technique used in the high-volume
manufacture of integrated circuits. It is widely anticipated that improvements
in this technology will allow it to remain the semiconductor industry's
workhorse through the 100 nm generation of devices. However, some time around
the year 2005, so-called Next-Generation Lithographic will be required. EUVL
is one such technology vying to become the successor to optical lithography.
This paper provides an overview of the capabilities of EUVL, and explains how
EUVL might be implemented. The challenges that must be overcome in order for
EUVL to qualify for high-volume manufacture are also discussed.
INTRODUCTION
Microprocessors, also called
computer chips, are made using a process called lithography. Specifically,
deep-ultraviolet lithography is used to make the current breed of microchips
and was most likely used to make the chip that is inside your computer.
Lithography is akin to photography in that it uses
light to transfer images onto a substrate. Silicon is the traditional substrate
used in chip making. To create the integrated circuit design that's on a
microprocessor, light is directed onto a mask. A mask is like a stencil of the
circuit pattern. The light shines through the mask and then through a series of
optical lenses that shrink the image down. This small image is then projected
onto a silicon, or semiconductor, wafer. The wafer is covered with a
light-sensitive, liquid plastic called photo-resist The mask is placed over the
wafer, and when light shines through the mask and hits the silicon wafer, it
hardens the photo-resist that isn't covered by the mask. The photo-resist that is
not exposed to light remains somewhat gooey and is chemically washed away,
leaving only the hardened photo-resist and exposed silicon wafer.
The key to creating more powerful microprocessors is
the size of the light's wavelength. The shorter the wavelength, the more
transistors can be etched onto the silicon wafer. More transistors equal a more
powerful, faster microprocessor.
Deep-ultraviolet lithography uses a wavelength of 240 nano-meters As chip-makers reduce to smaller wavelengths, they will need a new chip making
technology. The problem posed by using deep-ultraviolet lithography is that as
the light's wavelengths get smaller, the light gets absorbed by the glass
lenses that are intended to focus it. The result is that the light doesn't make
it to the silicon, so no circuit pattern is created on the wafer. This is where
EUVL(Extreme Ultraviolet Lithography will take over. In EUVL, glass
lenses will be replaced by mirrors to focus light and thus EUV lithography can
make use of smaller wave lengths. Hence more and more transistors can be packed
into the chip. The result is that using EUV lithography, we can make chips that
are up to 100 times faster than today’s chips with similar increase in storage
capacity.
EXTREME ULTRAVIOLET
LITHOGRAPHY
WHY EUVL?
In
order to keep pace with the demand for the printing of ever smaller features,
lithography tool manufacturers have found it necessary to gradually reduce the
wavelength of the light used for imaging and to design imaging systems with
ever larger numerical apertures. The reasons for these changes can be
understood from the following equations that describe two of the most
fundamental characteristics of an imaging system: its resolution (RES) and
depth of focus (DOF).
