CHARGED PARTICLE SOURCE

Abstract: A charged particle source for emitting a positive ion or negative ion respectively electron by applying a positive or negative potential to a tip electrode (1) covered with a liquid substance (2) is disclosed in which mechanical vibration is applied to the tip electrode (1) so that a favorable standing wave (3, 4; 3′, 4′) is formed in the liquid substance (2), to vary the shape of a charged-­ particle emitting portion of the liquid substance (2) periodically, thereby changing the intensity of an emitted, charged-particle beam periodically, and thus a pulsed beam having a repetition rate up to the GHz band can be obtained without increasing the energy dispersion of the beam.

Description: The present invention relates to focused ion/electron beam technology, and more particularly to a charged particle source capable of emitting a high repetition-rate pulsed beam up to the GHz band stably, without causing variations in energy of emitted, charged particles. A pulsed, focused beam has not yet been used, but can be produced by making use of a prior art. As is evident from JP-B-52-35839 (published on September 12, 1977), an emission current can be varied in such a manner that a control electrode is disposed in the neighborhood of a tip electrode and a voltage applied to the control electrode is varied. In more detail, the above publication discloses that the emission current can be stabilized by feeding a monitor current signal back to the voltage applied to the control electrode. Accordingly, it is readily thought to produce a pulsed beam by applying an A.C. voltage (for example, a high frequency voltage) to the control electrode. In this case, however, an A.C. electric field (that is, a high frequency electric field) which is generated on the basis of the high frequency voltage applied to the control electrode, is superposed on an acceleration electric field. When ions, which are larger in mass and hence lower in the traveling speed than electrons, are generated and accelerated, the electric field intensity of an acceleration region varies while the ions travel through the acceleration region. Accordingly, the kinetic energy of an accelerated ion depends upon the phase of high frequency voltage at the time when the ion is generated. This causes the energy dispersion of an ion beam. This energy dispersion increases as the repetition rate of the pulsed beam is larger. Furthermore, in a case where a pulsed beam having a repetition rate in the GHz band is generated, it is necessary to use the microwave circuit technology, and it is difficult to apply such technology to a conventional source for emitting a focused, charged-particle beam. Further, JP-A-56-112058 (=EP-A-0 037 455) discloses a high intensity ion source in which a tip electrode is covered with a liquid metal and the liquid metal is subjected to an electric field for emission of ions. This document explains that the shape of the liquid covering the electrode tip varies in response to the accelerating voltage applied between the tip electrode and an acceleration electrode. A similar device is disclosed in EP-A-0 202 685 where the accelerating voltage is varied to produce a pulsed ion beam. The first part of claim 1 has been drafted in view of this prior art document. JP-A-60-249234 discloses a liquid ion source where the electrode tip is mechanically oscillated by a piezo-electric body in order to maintain the electrode tip always in a wet condition, thereby stabilizing the emission of the ion beam. It is an object of the present invention to provide a charged-particle source which can emit a pulsed charged-particle beam having a repetition rate up to the GHz band, without increasing the energy dispersion of the charged-particle beam. This object is met by the invention characterized in claim 1. According to the invention, a liquid substance (such as liquid Galium or some kinds of liquid alloys) for covering a tip electrode is applied with mechanical vibration to make a standing wave in the liquid substance, thereby varying the shape of a charged-particle emitting portion, periodically, and thus the electric field intensity at the emitting portion is varied periodically, which makes possible the emission of a pulsed, charged-particle beam. As for the above-mentioned liquid substance, use may be made of a metal such as Ga, Au, Hg, Al or Bi or an electrically conductive material other than the metal. In more detail, as shown in Figs. 2A and 2B, the shape of an end portion of a liquid substance 2 for covering a tip electrode 1 varies periodically in such a manner that the liquid substance 2 is put in a state 3 or 3′ and another state 4 or 4′ alternately. Fig. 2A shows a case where the liquid substance vibrates at a high frequency, and Fig. 2B shows a case where the liquid substance vibrates at a low frequency. In other words, the radius r of curvature of an end portion of the liquid substance 2 varies periodically, and thus the electric field intensity E at the end portion also varies periodically. According to an experimental formula given by Müller (Advances in Electronics and Electron Physics, Vol. XIII, 1960, pages 83 to 95), the electric field intensity E is expressed as follows: E = V 5r (1) where V indicates a difference in electric potential between the tip electrode and an extraction electrode. As is evident from the above equation (1), the electric field intensity E increases as the radius r of curvature is smaller. As shown in Fig. 3, an ion or electron current increases greatly with the increasing electric field intensity E, when the electric field intensity E exceeds a threshold intensity E₀. When the tip electrode 1 is at a positive potential with respect to the extraction electrode, a positive ion can be emitted from the liquid substance. When the tip electrode 1 is at a negative potential with respect to the extraction electrode, an electron or negative ion can be emitted from the liquid substance. The liquid substance 2 can emit a pulsed ion (or electron) beam by setting the potential difference between the tip electrode and the extraction electrode so that the electric field intensity E at a time the liquid substance 2 is put in the state 4 is smaller than the threshold intensity E₀ and the electric field intensity at a time the liquid substance is put in the state 3 is greater than the threshold intensity E₀. When a supersonic vibrator is used for applying mechanical vibration to the liquid substance, a pulsed beam having a repetition rate of 1 kHz to 10 GHz can be emitted from the liquid substance. Fig. 1 is a schematic diagram showing an embodiment of a charged particle source according to the present invention. Figs. 2A and 2B are schematic diagrams for explaining the operation principle of the present invention. Fig. 3 is a graph showing a relationship between the electric field intensity E and an emission current I of a conventional charged particle source which is provided with a tip electrode. Fig. 4 is a schematic diagram showing an unfavorable standing wave which is made in a liquid substance. Fig. 5 is a schematic diagram showing another embodiment of a charged particle source according to the present invention. Now, explanation will be made of an embodiment of a charged particle source according to the present invention, with reference to Fig. 1. Referring to Fig. 1, a tip electrode 1 covered with a liquid substance 2 is vibrated by a mechanical vibrator 8 which utilizes electrostriction or magnetostriction. These are mounted on a flange 7. The vibrator 8 is driven by a voltage from a power supply 9, which is insulated from ground by an insulation transformer 10. Further, the tip electrode 1 is applied with an ion acceleration voltage from an acceleration power supply 11, and an extraction electrode 6 is applied with, for example, a ground potential. An auxiliary electrode 5 is applied with a bias voltage from a power supply 12. The liquid substance 2 which covers the surface of the tip electrode 1, is applied with an electrostatic force due to not only a voltage applied between the tip electrode 1 and the extraction electrode 6 but also a voltage applied between the tip electrode 1 and the auxiliary electrode 5. Thus, the liquid substance 2 has the form of a circular cone. When the vibrator 8 is driven in this state, a wave is generated in the liquid substance 2 by the mechanical vibration of the tip electrode 1, and a standing wave as shown in Fig. 2A or 2B is formed. The wavelength and shape of the standing wave depend upon not only a vibration frequency but also the surface tension and density of the liquid substance 2. In other words, the liquid substance 2 is not always put in the vibrational state shown in Fig. 2A or 2B, but may be in the state represented by 3" or 4" with the result that a node may be formed at an end portion of the liquid substance 2 as shown in Fig. 4. In this case, it is necessary to change the vibration frequency so that a loop is formed in an end portion of the liquid substance 2, and hence the power supply 9 has an adjusting function of changing the vibration frequency. Thus, a standing wave can be generated so that an end of the liquid substance 2 acts as the loop of the standing wave. Further, a voltage appearing across a resistor 13 for emission current measurement is smoothed, and then negatively fed back to a driving voltage for the generator 8, to control the intensity of vibration, thereby stabilizing an emission current. Alternately, a signal indicative of a current flowing into the extraction electrode 6, or an output signal from a current sensor which is disposed downstream from the extraction electrode 6, may be used in place of the voltage appearing across the resistor 13. Fig. 5 shows another embodiment of a charged particle source according to the present invention. Referring to Fig. 5, an X-deflector 14 and a Y-deflector 15 are disposed under the extraction electrode 6, to deflect a charged particle beam emitted from the liquid substance 2. The deflectors 14 and 15 are operated by signals from a deflection circuit 16. When the signals for operating the deflectors 14 and 15 are synchronized with a signal for driving the vibrator 8, a specimen surface 17 is irradiated periodically with the charged particle beam in each of X- and Y-directions, as indicated by a pattern on the specimen surface 17. Examples of the specimen the surface 17 of which is radiated are semiconductor substrate having chips on which identical patterns are to be drawn, substrates with electron beam resist layer thereon, etc. In the embodiments of Figs. 1 and 5, a positive ion is emitted from the liquid substance 2. However, when the polarity of the acceleration power supply 11 is reversed, an electron or a negative ion can be emitted from the liquid substance 2. According to the above-described embodiments of the present invention, the following advantages are expected. (1) A pulsed, focused beam having a repetition rate in the GHz band which cannot be produced by a prior art, can be obtained without increasing the energy dispersion of the beam. In some application fields, the pulsed, focused beam can be used as a D.C. beam. (2) A pulsed, charged-particle beam can be extracted from the liquid substance by a weaker electric field, as compared with a case where the beam is extracted without vibrating the tip electrode. Accordingly, the vibrational state of the liquid substance is stable, and thus the pulsed beam is emitted stably. (3) The energy dispersion of the pulsed beam is smaller, as compared with a case where an A.C. voltage is superposed on the D.C. acceleration voltage, or an A.C. voltage is applied to the auxiliary electrode.

Claims: A charged-particle beam source, comprising: a tip electrode (1) covered with a liquid substance (2), means (5, 6, 11, 12) for applying a voltage to said tip electrode (1) to generate an electric field, and means (8, 9) for periodically varying the intensity of the electric field between values above and below a threshold value for particle emission to produce a pulsed charged-particle beam, characterized in that said varying means includes means (8, 9) to vary the radius of curvature of the shape taken by a portion of said liquid substance (2) at an end portion of said tip electrode (1) such that the electric field intensity at said liquid substance portion varies above and below said threshold value. The device of claim 1, wherein said varying means (8, 9) includes means for generating a mechanical standing wave (3, 4; 3', 4') in said liquid substance (2). The device of claim 2, wherein said varying means includes a mechanical vibrator (8) utilizing electrostriction and/or magnetostriction. The device of claim 3, wherein said vibrator (8) is connected to an adjustable power supply (9) for adjusting the vibration frequency. The device of claim 3 or 4, wherein an emission current from said tip electrode (1) and/or a monitor current for said emission current is negatively fed back to the power supply (9) of said vibrator (8). The device of any one of claims 2 to 5, including means (14, 15) for deflecting the charged-particle beam in synchronism with said mechanical vibration, the frequency of the beam deflection being equal to the frequency of said mechanical vibration or to a frequency obtained by dividing said mechanical vibration frequency by an integer larger than one.

Inventors: SAKUDO, NORIYUKI, TAMURA, HIFUMI
Applicants: HITACHI, LTD.
Application year: 1987
Main classification: H01J27
IPC codes: H01J27, H01J37, H01J3