X-ray or gamma ray systems or devices – Source
Reexamination Certificate
1999-09-14
2001-09-04
Porta, David P. (Department: 2876)
X-ray or gamma ray systems or devices
Source
C378S143000
Reexamination Certificate
active
06285743
ABSTRACT:
This application claims the benefit of Japanese Application No. 10-259060, filed in Japan on Sep. 14, 1998, Japanese Application No. 10-265888, filed in Japan on Sep. 21, 1998, and Japanese Application No. 10-265889, filed in Japan on Sep. 21, 1998, all of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to soft X-ray generation, and more particularly, to a method and apparatus for generating soft X-rays for semiconductor manufacturing.
2. Discussion of the Related Art
In a conventional process, a pattern from a mask is reduced, projected and transferred onto a silicon wafer using visible or ultraviolet light during the manufacturing of semiconductor integrated circuits. The silicon wafer is coated with a photoresist compound that is sensitive to selected wavelengths of light. As technological advances necessitate the use of increasingly more complex and finer patterns, the diffraction limit of light sources is quickly being approached. Thus, the use of soft X-rays with wavelengths shorter than the wavelengths of ultraviolet light, i.e., wavelengths of about 13 nm or about 11 nm, is becoming necessary to achieve required pattern resolution.
A laser plasma X-ray source (LPX) may be used to produce soft X-rays with a wavelength of about 13 nm or about 11 nm. In the LPX, electrons are stripped from the atoms of a target material by an intense electric field when a pulsed light with an irradiation intensity exceeding about 10
10
W/cm
2
is focused and directed onto the target material. The target material is converted into a plasma, and soft X-rays radiate from the plasma. The soft X-rays have an extremely high brightness. The LPX apparatus is very compact when compared to other light sources capable of producing soft X-rays (for example, a synchrotron facility). Thus, use of the LPX to generate soft X-rays for semiconductor wafer exposure is preferred. LPX's may also be used for X-ray microscopes, as well as other applications.
The generation of debris is a serious problem in the LPX. When a solid target material, such as metal, is irradiated with laser light, ions and atoms which form the plasma are displaced by the rapid expansion of the plasma. Portions of the target material located in the vicinity of the plasma are scattered into the surrounding space. Optical elements, e.g., multi-layer film mirrors, thin-film filters, etc., are frequently installed in the vicinity of the plasma in order to shape the beams of generated soft X-rays. Thus, debris are deposited on the surfaces of the optical elements. This results in a deterioration of their optical performance (for example, reflectivity or transmissivity, etc.). Plasma is typically generated over a long period of time (often many months) at a high frequency (e.g., 1 kHz or greater) during soft X-ray exposure of semiconductor wafers. Accordingly, a large quantity of debris is generated over time. Thus, a reduction in the generation of debris is of great interest.
Kublak et al., in U.S. Pat. No. 5,577,092, show an LPX which uses a gaseous target material to solve the problem of debris generation. In their approach, a gas is directed through a nozzle at a supersonic velocity into a vacuum vessel. Clusters of gas molecules are formed through the resulting adiabatic expansion. Plasma is generated by irradiating these clusters of gas molecules with laser light. Since the target material is a gas, there is no deposition of the target material on the surfaces of the soft X-ray optical elements, even if the gas diffuses into the surrounding areas after plasma formation. Accordingly, there is no drop in the optical performance of these elements. Thus, it is reported that the useful life of the mirrors in such a soft X-ray optical system may reach 10
9
shots.
Richardson et al, in U.S. Pat. No. 5,577,091, show an LPX which uses fine water droplets as a target material. In this method, the fine water droplets are directed into a vacuum vessel. Plasma is formed by irradiating the fine water droplets with laser light. The fine water droplets evaporate at the time of plasma formation. Fine water droplets that are not irradiated and are scattered by the plasma (and adhere to the optical elements) are easily removed. Thus, the degradation of performance as a result of debris generation and scattering is reduced.
However, the prior art soft X-ray sources have the following problems. First, the X-ray spatial intensity distribution is frequently non-uniform and asymmetrical, resulting in uneven exposure of a semiconductor wafer. This often results from the plasma region being nonspherical and asymmetrical. Second, structural components needed to support the nozzle and other elements within the vacuum vessel generate shadows, which further reduce the quality of the X-ray intensity distribution.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a method and apparatus for soft x-ray generation that substantially obviates the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method and apparatus for improved soft X-ray generation.
Another object of the present invention is to provide a method and apparatus for improved soft X-ray generation using a gas, a liquid or a solid target material.
Another object of the present invention is to provide a method and apparatus for improved soft X-ray generation using a solid or liquid target material to generate axially symmetrical parallel soft X-rays beams.
Another object of the present invention is to provide a method and apparatus for obtaining a pattern of uniform resolution from an projection system formed of multiple reflective elements.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in one aspect of the present invention there is provided a soft X-ray source including a mirror having a focal point, the mirror being parabolic or ellipsoidal, a nozzle delivering a target material to a plasma generation point substantially coincident with the focal point, and a laser source delivering a pulsed laser beam to the plasma generation point, wherein a plasma is generated at the plasma generation point upon irradiation by the pulsed laser beam, the plasma emitting soft X-rays that are formed into X-ray beams by the mirror, wherein the X-ray beams are parallel if reflected from the parabolic mirror and converging if reflected from the ellipsoidal mirror, and wherein one of an axis of the nozzle and an axis of the pulsed laser beam is coaxial with an axis of the mirror.
In another aspect of the present invention there is provided a method of generating soft X-rays comprising the steps of ejecting a target material from a nozzle towards a plasma generation piont, irradiating the target material with a pulsed laser beam; and forming soft X-ray beams by reflecting soft X-rays emitted by the plasma from a mirror, the mirror being parabolic or ellipsoidal, the soft X-ray beams being parallel or converging, wherein the pulsed laser beam is directed towards substantially a focal point of the mirror, the focal point being substantially coincident with the plasma generation point, and wherein one of an axis of the nozzle and an axis of the pulsed laser beam is coaxial with an axis of the mirror.
In another aspect of the present invention there is provided a soft X-ray exposure apparatus, a soft X-ray source emitting an axially symmetrical beam of soft X-rays; and an optical system comprising a plurality of reflective elements, the optical system including at least
Kandaka Noriaki
Kondo Hiroyuki
Morgan & Lewis & Bockius, LLP
Nikon Corporation
Porta David P.
LandOfFree
Method and apparatus for soft X-ray generation does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for soft X-ray generation, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for soft X-ray generation will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2502759