X-ray or gamma ray systems or devices – Source
Reexamination Certificate
2001-03-26
2004-02-10
Patel, Harshad (Department: 2855)
X-ray or gamma ray systems or devices
Source
C378S084000
Reexamination Certificate
active
06690764
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to, inter alia, X-ray sources, more specifically to X-ray sources useful for any of various X-ray apparatus such as X-ray microscopes, X-ray analysis devices, and X-ray microlithography apparatus. Even more specifically, the invention pertains to X-ray sources that produce X-rays from a plasma produced by a target material highly energized by laser pulses or electrical discharge.
BACKGROUND OF THE INVENTION
Laser-plasma X-ray sources (hereinafter abbreviated as “LPX” sources) produce X-rays from a plasma generated by focusing a pulsed laser light on a target material situated inside a vacuum chamber. The laser light pulses convert the target material into the plasma, from which the X-rays are produced. LPX sources are small but nevertheless generate X-rays having an intensity comparable to the intensity of X-rays produced by undulators. Other small X-ray sources include dense plasma focus (DPF) sources that produce X-rays from an electrically produced discharge plasma. DPF sources also produce large quantities of X-rays, and have a higher conversion efficiency of X-rays to input power, and are lower in cost, than LPX sources.
In LPX and DPF sources, the target material and any other material located in or near the plasma are atomized, ionized, or generally fragmented (the products of such fragmentation are termed herein “flying debris”). The particles of flying debris propagate to neighboring components (e.g., X-ray optical elements) to which the debris adheres and on which the debris accumulates. These deposits diminish the performance (e.g., reflectivity or transmissivity) of the components. Also, collisions of particles of the flying debris with neighboring optical components damage the components.
According to one conventional approach to reducing the problem of flying debris in LPX sources, the target material is a gas at room temperature (e.g., nitrogen, carbon dioxide, krypton, and xenon). The gaseous target material is discharged from a nozzle while a pulsed beam of laser light is being irradiated onto the discharge stream of gas. According to another approach, the discharged target material is configured as a gaseous cluster produced by adiabatic expansion. Because they are gaseous, target materials produced in such manners tend not to accumulate on neighboring optical components. However, the plasma itself produces and emits high-velocity atoms, ions, and electrons that collide with the discharge nozzle and with components near the discharge nozzle. These collisions erode the nozzle and the components, producing flying debris that propagates to surrounding regions and accumulates on neighboring optical components. Consequently, an LPX source that produces no flying debris has yet to be realized.
Meanwhile, to decrease the rate at which flying debris is produced in LPX and DPF sources, efforts have been made to fabricate components of these sources (such as nozzles and electrodes) using materials having high melting points and high hardness, such as tungsten or tantalum. Another approach has been to decrease the operating voltage of the source. Unfortunately, neither approach has resulted in zero flying debris.
In addition, flying debris is not emitted uniformily in all directions. Rather, the particles tend to be emitted preferentially according to a certain asymmetric angular distribution. For instance, in LPX sources that utilize a gas-jet nozzle, fewer particles of flying debris propagate in the gas-discharge direction (i.e., along the gas-discharge axis). The quantity of flying debris increases with increases in the angle from the discharge axis.
For X-ray illumination purposes as exploited in X-ray microlithography apparatus, for example, illumination-optical systems have been proposed that utilize fly-eye mirrors. In this regard, reference is made to FIG.
7
(B) depicting a system that receives a collimated beam
702
of X-rays that is reflected successively from two fly-eye mirrors
703
,
704
before being reflected by illumination mirrors
705
-
706
to a reflective reticle
707
. From the reticle
707
, the X-rays are reflected by a projection-mirror array
708
to a substrate
709
. As shown in FIG.
7
(A), a typical fly-eye mirror
700
comprises multiple arc-shaped micro-elements grouped together. Each fly-eye mirror, such as that shown in FIG.
7
(A), facilitates the achievement of a constant X-ray intensity distribution at the reticle
707
. (See Japan Kôkai Patent Document No. Hei 11-312638). If X-rays incident to a fly-eye mirror have an axially symmetric distribution of X-ray intensity around the center axis of the fly-eye mirror, then the beamlets of reflected X-ray light from the various micro-elements of the fly-eye mirror reinforce each other and make uniform the intensity distribution of X-ray light at the reticle. However, if the X-ray beam incident to the fly-eye mirror is asymmetric around the center axis of the fly-eye mirror, then the fly-eye mirror will not adequately compensate for intensity variations of the incident beam. Consequently, the intensity distribution of the X-ray beam reflected from the fly-eye mirror will not be uniform at the reticle.
The angular distribution of X-rays radiated from a gas-nozzle LPX generally is rotationally symmetric around the gas-discharge axis. If a paraboloidal mirror (i.e., a mirror having a reflective surface configured as a paraboloid of revolution) were situated such that its axis of revolution is coincident with the gas-discharge axis, then X-rays reflected by the paraboloidal mirror should be a collimated beam having an intensity distribution nearly symmetrical to the gas-discharge axis. Thus, an X-ray flux suitable for the illumination-optical system described above could be formed. However, the angular distribution, relative to the gas-discharge axis, of emitted flying debris typically is not symmetrical. Rather, the angular distribution of the flying debris depends upon the plasma producing the debris and on the position of the nozzle (in the case of a gas-discharge LPX source) or the electrode (in the case of a discharge-plasma DPF source).
As a result of the phenomena summarized above, operation of an X-ray source for a long period of time is accompanied by a progressively more asymmetric distribution of X-ray intensity produced by the source, due to the axially asymmetric accumulation of flying debris on neighboring optical components. With respect to use of such a source in an X-ray microlithography apparatus, this asymmetric distribution of X-rays results in variations in the axial distribution of X-rays illuminating a reticle, with corresponding inaccuracies in the transfer of a reticle pattern to a substrate.
SUMMARY OF THE INVENTION
In view of the shortcomings of conventional apparatus and methods as summarized above, an object of the invention is to make any deposits of flying debris on an X-ray optical component situated adjacent the X-ray source rotationally symmetric about a propagation axis of the X-rays. Thus, the intensity distribution of the X-ray flux from the source is maintained rotationally symmetric, even in situations in which the X-ray source exhibits an asymmetrical distribution of emissions of flying debris.
Another object is to provide X-ray optical systems, situated adjacent the X-ray source, configured to rotate one or more neighboring optical components about the optical axis (propagation axis) of the X-ray beam. As a result, in the context of X-ray microlithography for example, the intensity distribution of the X-ray beam at the reticle remains uniform about the optical axis. In an X-ray microlithography apparatus, this axial uniformity of the beam allows the reticle pattern to be transferred accurately to the substrate.
To such ends, and according to a first aspect of the invention, X-ray sources are provided that generate X-rays from a plasma produced by directing pulsed laser light onto a target material in a vacuum chamber evacuated to a subatmospheric pressure. An embodiment of such a source includes a device
Klarquist & Sparkman, LLP
Nikon Corporation
Patel Harshad
LandOfFree
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