X-ray or gamma ray systems or devices – Source
Reexamination Certificate
2000-10-06
2003-01-14
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Source
C378S121000
Reexamination Certificate
active
06507641
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to microlithography (projection-transfer of a pattern, defined on a reticle, onto a suitable substrate) and related technologies. Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. More specifically, the invention pertains to apparatus and methods (such as microlithography apparatus and methods) performed using an X-ray beam as an energy beam. Even more specifically, the invention pertains to X-ray sources that generate a beam of “soft” X-ray radiation for use in an X-ray microlithography apparatus, an X-ray microscope, an X-ray analysis device, or the like, and to microelectronic-device manufacturing methods utilizing such X-ray exposure technology.
BACKGROUND OF THE INVENTION
X-ray sources that utilize a discharge plasma are small, produce a large X-ray flux, convert input electrical energy into X-rays at a higher efficiency than X-ray sources that utilize a laser-generated plasma, and are inexpensive. Hence, there has been substantial activity directed to the development of X-ray microscopes and X-ray microlithography apparatus that include a discharge-plasma X-ray source.
An exemplary discharge-plasma X-ray source is termed a “Dense Plasma Focus” (DPF) source manufactured by Cymer, Inc., San Diego, Calif. Information concerning this source is available on the Internet home page of Cymer, Inc. (http://www.cymer.com/) in the paper by Partlo et al., “EUV (13.5 nm) Light Generation Using a Dense Plasma Focus Device,” presented at the SPIE 24th Annual International Symposium on Microlithography, 1999, incorporated herein by reference. See also, U.S. Pat. No. 5,763,930, incorporated herein by reference.
A schematic diagram of a DPF source is shown in FIG.
10
. The DPF is enclosed in a vacuum chamber (not shown). An electric charge from a DC high-voltage power supply
700
is stored in a capacitor C
0
. After the capacitor C
0
reaches full charge, the charge is transferred to a capacitor C
1
by closing a switch
701
(comprising a thyristor, IGBT (Insulated Gate Bipolar Transistor), or the like). Charging of the capacitor C
1
raises the voltage of the capacitor C
1
, and the voltage is applied between a concentrically arranged (coaxial) anode electrode
703
and cathode electrode
702
. As the applied voltage nears a peak voltage, a hollow-cathode preionization source (not shown) initiates avalanche breakdown of molecules of a gas situated between the anode electrode
703
and cathode electrode
702
. This causes an electrical discharge to begin between the electrodes
702
,
703
.
At the onset of discharge, the electrical current flowing from the capacitor C
1
is momentarily held off by a saturable reactor SR until a uniform plasma sheath has formed at the base of the set of electrodes
702
,
703
. That is, at the surface of an insulator
704
disposed between the anode
703
and cathode
702
, a uniform plasma sheath is generated by a surface discharge occurring at the insulator
704
. As the capacitor C
1
continues to dump electrical current across the electrodes
702
,
703
, the plasma sheath moves toward the tip of the anode electrode
703
. The resulting magnetic forces generated at the tip of the anode electrode
703
compress the plasma in this region toward the axis. Also, as the plasma reaches the tip of the anode electrode
703
, materials situated at the anode tip (e.g., gas molecules, the electrode material, or a target material) are energized sufficiently to enter the plasma. The plasma, compressed within the small region bounded by the strong magnetic field, undergoes further heating, causing soft X-ray (SXR) radiation to propagate therefrom. (SXR radiation also is termed herein “extreme ultraviolet” or “EUV” radiation.)
The saturable reactor SR undergoes charge saturation as the plasma sheath moves between the electrodes
702
,
703
, or at least when the plasma sheath reaches the tip of the anode electrode
703
. Saturation causes a large current to flow between the anode
703
and the cathode
702
, resulting in further heating and compression of the plasma at the tip of the anode electrode
703
.
In the X-ray source summarized above, the target material conventionally is a gas present in an atmosphere in the vicinity of the electrodes
702
,
703
. The target material also can be the material constituting the anode electrode, or a substance on the surface of the anode electrode.
With conventional technology, problems arise whenever a gas (present in the atmosphere in the vicinity of the electrodes) is used as the target material. Because SXR radiation usually is highly absorbed by matter, the pressure of the gas generally cannot be increased. At the same time, the pressure of the gas inside the vacuum chamber generally needs to be about 10 Torr or lower. (Whenever a gas is used as the target material, the gas pressure normally is kept low to reduce the density of the target material.) Unfortunately, with such a scheme, the intensity of the generated SXR radiation also is very weak. For example, the publications noted above describe experiments performed using 0.1 -Torr Xe and 0.2-Torr Ar as target-material gases. However, almost no difference is observed in the SXR spectra generated from these target materials because substantial SXR radiation also is generated by the electrode material (e.g., tungsten). Hence, a gas is not optimal for use as a target material.
Whenever a solid electrode is used to supply the target material, the electrode tends to melt and be eroded away by the large electrical current applied to the electrode from the discharge. This results in rapid substantial changes of electrode shape, rendering long-term stable operation impossible. A melted or eroded electrode requires frequent interruption in use to perform electrode replacement, resulting in decreased operating efficiency. In addition, as the electrode is melted or eroded, material released from the electrode tends to migrate to and deposit on neighboring optical components and other structures. Deposits of such materials tend to reduce the optical performance (e.g., reflectivity or transmittance) of the affected optical components.
If the target material is situated on the anode electrode
703
, then a distinctive spectrum of SXR radiation (i.e., a spectrum distinctive of the target material) can be generated. However, the target material conventionally must be applied to the electrode (and periodically replenished) by a discrete operation including shut-down of the apparatus, which renders continuous long-term operation impossible. An exemplary target material, as discussed in the Partlo et al. reference cited above, is lithium (Li); another exemplary target material, as discussed in the JP '195 patent document cited above is lithium hydroxide (LiH). Unfortunately, both substances are highly reactive and dangerous, and are difficult to handle. Also, both of these substances react violently with water. Whenever atmospheric air is introduced into the vacuum chamber containing these target materials, a substantial risk is created of residual Li or LiH in the vacuum chamber reacting explosively with water vapor in the air.
The shortcomings of conventional X-ray-generation devices as summarized above extend also to X-ray microlithographic apparatus that comprise such X-ray-generation devices as a source of SXR radiation used for pattern transfer. i.e., the apparatus frequently must be shut down temporarily for electrode servicing. As a result, microelectronic-device fabrication using such apparatus tends to exhibit disappointingly low throughput.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide X-ray-generation devices that can generate SXR radiation in a manner characterized by long-term stability both when using an electrode as the target material and when using a substance other than the electrode as the target material. Another object is to provid
Kondo Hiroyuki
Murakami Katsuhiko
Shiraishi Masayuki
Sugisaki Katsumi
Kim Robert H.
Klarquist & Sparkman, LLP
Nikon Corporation
Song Hoon K.
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