This invention relates to a plasma X-ray source producing soft X-rays by forming a high temperature and high density plasma by means of pulse discharge in a discharge tube using coaxial electrodes, and in particular to a plasma X-ray source which is suitable for a source of an X-ray aligner for manufacturing submicron integrated circuits.
The plasma focus is well known as a representative example of discharge tubes having coaxial electrodes. Research has been conducted heretofore on the plasma focus as sources generating neutrons, for which gas such as heavy hydrogen filled in a discharge tube having coaxially disposed cylindrical electrodes is turned into plasma by applying a pulse voltage from a capacitor to the electrodes and the plasma is accelerated in the space defined between the electrodes so as to be focussed in the neighborhood of the extremity of one of the electrodes so that a high temperature and high density plasma is formed. However, since strong soft X-rays are also emitted from a high temperature and high density plasma produced by the plasma focus, recently attention is paid thereto also as an X-ray source.
Since the construction and the working mode of the plasma focus are simple and the brightness of the source is high, the plasma focus has a possibility to be an excellent X-ray source, but it has a problem that the position of the spot emitting soft X-rays moves from shot to shot. Although it is expected that the spot of the plasma focus is formed on the axis of the coaxial electrodes, in practice it is deviated often from the axis, and even if it is formed on the axis, its position varies thereon. This aspect is described e.g. in W. H. Bostick, V. Nardi and W. Prior: "X-ray fine structure of dense plasma in a coaxial accelerator", J. Plasma Physics, Vol. 8, pt 1, pp. 7-20 (1972).
For a reason why such variations are produced, heretofore, instability of plasma itself has been known, but other reasons therefor have not been clarified.