Immersion lithography is a
photolithography resolution enhancement technique for manufacturing integrated circuits (ICs) that replaces the usual air gap between the final lens and the wafer surface with a liquid medium that has a
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
greater than one. The
resolution
Resolution(s) may refer to:
Common meanings
* Resolution (debate), the statement which is debated in policy debate
* Resolution (law), a written motion adopted by a deliberative body
* New Year's resolution, a commitment that an individual mak ...
is increased by a factor equal to the
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
of the liquid. Current immersion lithography tools use highly purified water for this liquid, achieving feature sizes below 45 nanometers.
ASML and
Nikon
(, ; ), also known just as Nikon, is a Japanese multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging products. The companies held by Nikon form the Nikon Group.
Nikon's products include cameras, camera ...
are currently the only manufacturers of immersion lithography systems.
History
The idea for immersion lithography was patented in 1984 by Takanashi et al. It was also proposed by Taiwanese engineer
Burn J. Lin and realized in the 1980s. In 2004,
IBM's director of
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
technology,
Ghavam Shahidi
Ghavam G. Shahidi (born 1959) is an Iranian-American electrical engineer and IBM Fellow. He is the director of Silicon Technology at the IBM Thomas J Watson Research Center. He is best known for his pioneering work in silicon-on-insulator (SOI) ...
, announced that IBM planned to commercialize lithography based on light filtered through water.
Immersion lithography is now being extended to sub-
20nm
NM, nm, and variations may refer to:
Businesses and organizations
* Northwestern Mutual, financial services company in Wisconsin, United States
* Air Madrid (IATA airline designator NM), Spanish airline
* Mount Cook Airline (IATA airline des ...
nodes through the use of
multiple patterning
Multiple patterning (or multi-patterning) is a class of technologies for manufacturing integrated circuits (ICs), developed for photolithography to enhance the feature density. It is expected to be necessary for the 10 nm and 7 nm node se ...
.
Background
The ability to resolve features in
optical lithography is directly related to the
numerical aperture of the imaging equipment, the numerical aperture being the sine of the maximum refraction angle multiplied by the
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
of the medium through which the light travels. The lenses in the highest resolution "dry" photolithography scanners focus light in a cone whose boundary is nearly parallel to the wafer surface. As it is impossible to increase resolution by further refraction, additional resolution is obtained by inserting an immersion medium with a higher index of refraction between the lens and the wafer. The blurriness is reduced by a factor equal to the refractive index of the medium. For example, for water immersion using
ultraviolet light
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
at 193 nm wavelength, the index of refraction is 1.44.
The resolution enhancement from immersion lithography is about 30–40% depending on materials used. However, the depth of focus, or tolerance in wafer topography flatness, is improved compared to the corresponding "dry" tool at the same resolution.
Defects
Defect concerns, e.g., water left behind (watermarks) and loss of resist-water adhesion (air gap or bubbles), have led to considerations of using a topcoat layer directly on top of the
photoresist
A photoresist (also known simply as a resist) is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronic industry.
...
. This topcoat would serve as a barrier for chemical diffusion between the liquid medium and the photoresist. In addition, the interface between the liquid and the topcoat would be optimized for watermark reduction. At the same time, defects from topcoat use should be avoided.
As of 2005, Topcoats had been tuned for use as
antireflection
An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the effic ...
coatings, especially for hyper-NA (NA>1) cases.
By 2008, defect counts on wafers printed by immersion lithography had reached zero level capability.
Polarization impacts
As of 2000,
Polarization effect
In social psychology, group polarization refers to the tendency for a group to make decisions that are more extreme than the initial inclination of its members. These more extreme decisions are towards greater risk if individuals' initial tendenci ...
s due to high angles of interference in the photoresist were considered as features approach 40 nm. Hence, illumination sources generally need to be azimuthally polarized to match the pole illumination for ideal
line-space
In typography, leading ( ) is the space between adjacent lines of type; the exact definition varies.
In hand typesetting, leading is the thin strips of lead (or aluminium) that were inserted between lines of type in the composing stick to incre ...
imaging.
Throughput
![Immersion_tool_throughput](https://upload.wikimedia.org/wikipedia/commons/c/c8/Immersion_tool_throughput.png)
As of 1996, this was achieved through higher stage speeds,
[M. A. van den Brink et al., Proc. SPIE 2726, 734 (1996).]
/ref> which in turn, as of 2013 were allowed by higher power ArF laser pulse sources. Specifically, the throughput is directly proportional to stage speed V, which is related to dose D and rectangular slit width S and slit intensity Iss (which is directly related to pulse power) by V=Iss*S/D. The slit height is the same as the field height. The slit width S, in turn, is limited by the number of pulses to make the dose (n), divided by the frequency of the laser pulses (f), at the maximum scan speed Vmax by S=Vmax*n/f.[ At a fixed frequency f and pulse number n, the slit width will be proportional to the maximum stage speed. Hence, throughput at a given dose is improved by increasing maximum stage speed as well as increasing pulse power.
According to ASML s product information about twinscan-nxt1980di, immersion lithography tools currently boasted the highest throughputs (275 WPH) as targeted for high volume manufacturing.]
Multiple patterning
The resolution limit for a 1.35 NA immersion tool operating at 193 nm wavelength is 36 nm. Going beyond this limit to sub-20nm
NM, nm, and variations may refer to:
Businesses and organizations
* Northwestern Mutual, financial services company in Wisconsin, United States
* Air Madrid (IATA airline designator NM), Spanish airline
* Mount Cook Airline (IATA airline des ...
nodes requires multiple patterning
Multiple patterning (or multi-patterning) is a class of technologies for manufacturing integrated circuits (ICs), developed for photolithography to enhance the feature density. It is expected to be necessary for the 10 nm and 7 nm node se ...
. At the 20nm foundry and memory nodes and beyond, double patterning and triple patterning are already being used with immersion lithography for the densest layers.
See also
*Oil immersion
In light microscopy, oil immersion is a technique used to increase the resolving power of a microscope. This is achieved by immersing both the objective lens and the specimen in a transparent oil of high refractive index, thereby increasing the ...
*Water immersion objective In light microscopy, a water immersion objective is a specially designed objective lens used to increase the resolution of the microscope. This is achieved by immersing both the lens and the specimen in water which has a higher refractive index th ...
References
{{DEFAULTSORT:Immersion Lithography
Lithography (microfabrication)
Taiwanese inventions
ja:液浸