X-ray or gamma ray systems or devices – Source
Patent
1995-10-30
1997-08-05
Porta, David P.
X-ray or gamma ray systems or devices
Source
378120, H05G 200
Patent
active
056549989
DESCRIPTION:
BRIEF SUMMARY
This application claims benefit of international application PCT/GB94/00928, filed Apr. 29, 1994, published as WO94/26080 Nov. 10, 1994.
FIELD OF THE INVENTION
This invention relates to laser-excited X-ray sources and, in particular, to a laser-plasma X-ray source having a target which is irradiated by the output of a suitable laser, preferably an excimer laser.
BACKGROUND OF RELATED ART
Such X-ray sources have been employed in applications such as X-ray spectroscopy, X-ray lithography, radiobiology and X-ray microscopy, and an X-ray source of this kind has been described in considerable detail in a paper entitled "100 Hz KrF Laser-Plasma X-ray Source" presented at a conference entitled "Excimer Lasers and Applications III" at The Hague, Netherlands in 1991 and published in the Proceedings of SPIE--The International Society for Optical Engineering (SPIE Vol. 1503 Excimer Lasers and Applications III (1991), 391-405). As there described, an X-ray source comprises an excimer laser system which generates UV light pulses, at a repetition rate up to 100 Hz, which are focused on to an X-ray target. The target consists of a tape of suitable material, such as copper, steel or mylar, chosen in dependence on the desired frequency of the X-rays to be generated, and a tape transport mechanism which moves the tape so that each light pulse impinges on a fresh part of the tape, undamaged by previous pulses. Each light pulse, impinging on the tape, generates at the tape surface a highly localized volume of plasma which emits the desired X-rays but also has the effect of attenuating the UV light and shielding the tape from it, and thereby limiting the duration of the period in which X-rays are emitted. The incident UV light also causes debris to be ejected from the tape, and the debris tends to settle on surfaces of the optical system which delivers the UV light pulses to the tape and thus reduce the intensity of the incident light. As described in the above-mentioned paper, attempts to minimize these shielding effects include arranging a flow of helium gas across the part of the tape surface on which the UV light is incident, so as to sweep away and remove the debris, and assist in dissipating the plasma rapidly at the end of a pulse, and also to operate the system with the target not under highly reduced pressure but in a helium atmosphere at or approaching atmospheric pressure, which does not affect the emission of X-rays but has the effect of stopping and removing fast ions emitted by the plasma. In spite of such measures, however, it is found that in use of the known apparatus referred to the X-ray pulse generated by a pulse of UV light lasting 20 or 30 nanoseconds is generally limited to a duration of not more than 5 nanoseconds.
This shielding of the target from the incident UV laser light, in the known X-ray source referred to above, and heat loss from the expanding plasma, represent a severe limitation of the "conversion efficiency" (i.e. the ratio of X-ray energy to laser energy) of the apparatus, and a corresponding limitation on its average X-ray output power. These are factors which greatly affect the suitability of such apparatus for use in, particularly, X-ray lithographic work, for example in microcircuit production, where the highest possible average X-ray powers are required in order to minimize processing times.
It has been proposed (App. Phys. Lett. 55 (25), December 1989 and 71 (1), January 1992) to reduce the shielding effect, and improve the conversion efficiency and the average X-ray output power in such apparatus, by arranging that the laser light is emitted not in individual pulses with a pulse duration measured in tens of nanoseconds but in trains of ten to fifteen substantially shorter pulses, each pulse having a duration of about 0.10-0.15 nanoseconds, with the overall duration of the pulse train being about 20 to 30 ns. This proposal enables the conversion efficiency and average X-ray output power, when comparing a pulse train of 20 or 30 ns duration with a single pulse of equal length, t
REFERENCES:
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patent: 5539764 (1996-07-01), Shields et al.
O'Neill et al., "X-Ray Emission From Plasmas Generated By An XeCl Laser Picosecond Pulse Train", Appl. Phys. Lett. (Dec. 1989) 55 (25) pp. 2603-2604.
Turcu et al., "100 Hz KrF Laser-Plasma X-Ray Source", SPIE vol. 1503 Excimer Lasers and Applications III (1991) pp. 391-405.
Microelectronic Engineering, vol. 21, No. 1/4, 1 Apr. 1993, Amsterdam NL, pp. 95-98 I.C.E. Turcu et al "X-ray Lithography with Efficient Picosecond KrF Laser-Plasma Source at 1 NM Wavelength".
Soviet Journal of Quantum Electronics, vol. 21, No. 3, 1 Mar. 1991, New York U.S. pp. 248-249, S.A. Akhmanov et al. "Generation of Picosecond X-Ray Pulses in a Dense Plasma Created by High-Power Femtosecond Laser Pulses of the 308 NM Wavelength".
IEEE Journal of Quantum Electronics, vol. 25, No. 12, 1 Dec. 1989, New York US, pp. 2417-2422, M.M. Murnane et al "Generation and Application of Ultrafast X-Ray Sources".
Applied Physics Letters., vol. 60, No. 18, 4 May 1992, New York US, pp. 2195-2197 G.A. Kyrala et al "X-Ray Generation by High Irradiance Subpicosecond Lasers".
Journal of Applied Physics, vol. 71, No. 1, 1 Jan. 1992, New York US. pp. 85-93 D. Xenakis et al "Laser-Plasma X-Ray Generation Using an Injection-Mode-Locked Xecl Excimer Laser" cited in the application.
Applied Physics Letters, vol. 63, No. 22, 29 Nov. 1993, New York US, pp. 3046-3048 I.C.E. Turcu et al "Efficient Kev X-Ray Emission From Plasmas".
O'Neill Fergus
Ross Ian Norman
Turcu Ion Christian Edmond
Council for the Central Laboratory of the Research Councils
Porta David P.
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