X-ray or gamma ray systems or devices – Source
Reexamination Certificate
2000-08-23
2001-11-27
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Reexamination Certificate
active
06324256
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an extreme ultraviolet light source, and more particularly, to a laser-plasma, extreme ultraviolet light source for a photolithography system that employs a liquid spray as the target material for generating the laser plasma.
2. Discussion of the Related Art
Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process, well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current state of the art for photolithography light sources generate light in the extreme ultraviolet (EUV) or soft x-ray wavelengths (13.4 nm).
Different devices are known in the art to generate EUV radiation. One of the most popular EUV light sources is a laser-plasma, gas condensation source that uses a gas, typically Xenon, as a laser plasma target material. Other gases, such as Krypton, and combinations of gases, are known for the laser target material. The gas is forced through a nozzle, and as the gas expands, it condenses and forms a cloud or jet of extremely small particles known in the art as clusters. The condensation of cluster jet is illuminated by a high-power laser beam, typically from a Nd:YAG laser, that heats the clusters to produce a high temperature plasma which radiates the EUV radiation. U.S. Pat. No. 5,577,092 issued to Kublak discloses an EUV radiation source of this type.
FIG. 1
is a plan view of an EUV radiation source
10
including a nozzle
12
and a laser beam source
14
.
FIG. 2
is a close-up view of the nozzle
12
. A gas
16
flows through a neck portion
18
of the nozzle
12
from a gas source (not shown), and is accelerated through a narrowed throat portion
20
of the nozzle
12
. The accelerated gas
16
then propagates through a flared portion
24
of the nozzle
12
where it expands and cools, and is expelled from the nozzle
12
. As the gas cools and condenses, it turns into a jet spray
26
of clusters
28
.
A laser beam
30
from the source
14
is focused by focusing optics
32
on the clusters
28
. The heat from laser beam
30
generates a plasma
34
that radiates EUV radiation
36
. The nozzle
12
is designed so that it will stand up to the heat and rigors of the plasma generation process. The EUV radiation
36
is collected by collector optics
38
and is directed to the circuit (not shown) being patterned. The collector optics
38
can have any suitable shape for the purposes of collecting the radiation
36
, such as a parabolic shape. In this design, the laser beam
30
propagates through an opening
40
in the collector optics
38
.
The laser-plasma EUV light source discussed above suffers from a number of drawbacks. Particularly, it is difficult to produce a sufficiently large droplet spray or large enough droplets of liquid to achieve the desirable efficiency of conversion of the laser radiation to the EUV radiation. Because the clusters
28
have too small a diameter, and thus not enough mass, the laser beam
30
causes some of the clusters
28
to break-up before they are heated to a sufficient enough temperature to generate the EUV radiation
36
. Typical diameters of the droplets generated by a gas condensation EUV source are less than 0.01 microns and it is exceedingly difficult to produce clusters that are significantly larger than 0.1 microns. However, particle sizes of about one micron in diameter would be more desirable for generating the EUV radiation. Additionally, the large degree of expansion required to maximize the condensation process produces a diffuse cloud or jet of clusters, and is inconsistent with the optical requirement of a small plasma size.
What is needed is a laser-plasma EUV radiation source that is able to generate larger droplets of liquid to enhance the EUV radiation generation. It is therefore an object of the present invention to provide such an EUV radiation source.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, a laser-plasma EUV radiation source is disclosed that generates larger liquid droplets for the plasma target material than previously known in the art. The EUV source forces a liquid, preferably Xenon, through the nozzle, instead of forcing a gas through the nozzle. The geometry of the nozzle and the pressure of the liquid propagating though the nozzle atomizes the liquid to form a dense spray of liquid droplets. Because the droplets are formed from a liquid, they are larger in size, and are more conducive to generating the EUV radiation. A heat exchanger is used to convert gaseous Xenon to the liquid Xenon prior to being forced through the nozzle.
Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 4723262 (1988-02-01), Noda et al.
patent: 5577092 (1996-11-01), Kublak et al.
patent: 6002744 (1999-12-01), Hertz et al.
patent: 6007963 (1999-12-01), Felter et al.
McGregor Roy D.
Orsini Rocco A.
Petach Michael B.
Harness & Dickey & Pierce P.L.C.
Porta David P.
TRW Inc.
Yun Jurie
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