Patent Number: 047016230
Section: summary

This invention relates to a charged particle beam apparatus comprising means for generating a beam of charged particles, means for directing the beam towards a target, an electrically conductive tube disposed such that the beam can travel longitudinally through the bore of the tube, and means disposed outside the tube for generating within the tube a variable magnetic field. The beam of charged particles may be used to effect a lithographic process at a surface of a target. For example, in the manufacture of semiconductor devices it is known to use a charged particle beam apparatus in the form of a so-called electron beam column to direct an electron beam towards a semiconductor wafer coated with an electron sensitive resist and to write a predetermined pattern in the resist by computer control of the beam. After exposure the resist is developed to form a masking layer which can be used subsequently in the processing of the semiconductor wafer. It is usual practice in such apparatus to arrange the beam of charged particles to be at all times surrounded by an electrically conductive tube disposed such that the beam can travel longitudinally through the bore of the tube. The conductive tube is generally grounded allowing charge, which otherwise would accumulate on surfaces around the beam, to leak away. Accumulation of charge is undesirable because it can give rise to uncontrollable beam deflections. In normal operation, part of the conducting tube will be subject to a changing magnetic field. For example, in order that the beam of charged particles can be deflected controllably, magnetic deflection coils are commonly employed outside the tube to generate a variable magnetic field inside the tube, and transversely to the axis of the tube. By changing the current in the deflection coils the magnetic field strength is caused to vary thereby deflecting the charged particle beam. Unfortunately, however, a changing magnetic field gives rise to eddy currents which can flow in the conducting tube in closed loops around the magnetic field lines. In tun the eddy currents give rise to another magnetic field which opposes the deflecting magnetic field thereby reducing the overall field strength. Although this is only a transient effect and the full strength of the deflecting field is realized when the eddy currents eventually die away, it is nevertheless a significant disadvantage because deflection times are consequently increased. Unfortunately, this disadvantage is particularly pronounced in apparatus intended for high speed operation because the effect of eddy currents is greater with high frequency magnetic field changes. Attempts have been made to overcome this problem by increasing the overall resistance of the conductive tube in order to impede the flow of eddy currents. To this end it is known to provide the conductive tube as a relatively thin lining on the internal surface of an insulating tube. This conductive lining is usually provided by evaporation but, however provided, it is very difficult to achieve a uniform coating over the whole internal surface of a relatively long thin insulating tube. In any case the lining may be still sufficiently conductive to support appreciable eddy currents. Moreover, when the apparatus is used, contaminants build up on the conductive lining and this eventually necessitates removal of the insulating tube from the apparatus so that the lining can be cleaned. Clearly this is inconvenient particularly when the insulating tube also forms part of a vacuum envelope, as is usual, in which case a complex sealing arrangement is necessary to ensure that the vacuum enveloped is restored when the insulating tube is replaced. According to the present invention there is provided a charged particle beam apparatus comprising means for generating a beam of charged particles, means for directing the beam towards a target, an electrically conductive tube disposed such that the beam can travel longitudinally through the bore of the tube, and means disposed outside the tube for generating within a tube a variable magnetic field, characterised in that the tube comprises a filamentary conductor formed into a close-wound helix. A conductive tube having this helical construction has the advantage that it readily conducts along a helical path and hence allows charge to leak away - along the helical path. Longitudinally, however, conduction is severely impeded because at most there is only minimal (tangential) physical contact between adjacent turns of the helix. This is highly advantageous because the tube thus formed cannot readily support eddy currents, and in the absence of this transient response, the beam of charged particles can be deflected controllably in a significantly shorter time period. Another important advantage of a helically wound conducting tube is that it is relatively straightforward to form and avoids the difficulty associated with the prior art of providing a uniform conductive coating on the internal surface of an insulating tube. Also, as discussed in greater detail below, with this tube construction a heating current can be passed through the helix to deter the build up of contaminants thereon and thereby prolonging the useful operating life of a given tube before requiring it to be changed or cleaned. In order to virtually eliminate the effect of eddy currents it is preferable that the helix is adapted to provide means between adjacent turns of the helical filamentary conductor whereby current flow in the direction of the tube axis is impeded. For example, such current impedance means may comprise a relatively thin resistive film coating provided on the filamentary conductor. This resistive film may be an oxide formed naturally on the conductor material. Preferably the helix is provided inside an insulating tube which may form part of a vacuum envelope. In this case, it is also preferable that the helix is present as a removable insert in the tube so that the helix can be taken out from the apparatus for cleaning or replacement without disturbing the insulating tube. This has the advantage, compared with the prior art arrangement mentioned above, that a less complex sealing arrangement can be used for the vacuum envelope.