Patent Number: 047016230
Section: description

The electron beam lithography apparatus shown in the FIGURE comprises an electron optical column 1 including an electron gun 2 for generating an electron beam 3 and for directing the beam as a Gaussian round spot beam towards a target 4 such as a semiconductor wafer coated with an electron sensitive resist. The semiconductor wafer 4 is mounted on an X-Y stage 5 in an evacuable workchamber 6 which is connected to a vacuum pump (not shown) via a conduit 7. The X-Y stage 5 is driven from outside the workchamber 6 by a motor 8. Before impinging on the wafer 4, the beam 3 passes through various electron-optical stages of the column in the following order: a first demagnifying magnetic lens 9, an electrostatic beam blanker 10, a second demagnifying lens 11, a pair of magnetic deflection coils 12a, 12b, and a final magnetic lens system comprising an objective lens 13a and separate lens assembly 13b for automatic focus and stigmation. It is noted that all the lenses are shown schematically in the FIGURE. As is well known to those skilled in the art, a magnetic lens comprises an electrically conductive coil enclosed within a soft ferromagnetic member forming the pole pieces of the magnet. The features of the electron beam column described so far are well-known in the art and so no further details need be given here. For more information reference is invited, for example, to the paper by J. P. Beasley and D. G. Squire entitled "An Electron Beam Maskmaker" which appeared in IEEE Transactions on Electron Devices, Vol. ED-22, No. 7, July 1975, at pages 376-384. As shown in the Figure the column also comprises a central vacuum envelope 14 which at its lower end communicates with the workchamber 6. At its upper end the envelope 14 houses the electron gun 2 and is connected to a vacuum pump (not shown) via a conduit 15. Between the electron gun 2 and the workchamber 6 the vacuum envelope comprises two electrically conductive tube parts 16a, 16b made of metal such as stainless steel. The magnetic demagnifying lenses 9 and 10 are disposed outside the vacuum envelope and surround the tube part 16a in a concentric manner. Similarly the magnetic lenses 13a, 13b of the final lens system are disposed outside the vacuum envelope and concentrically surround the tube part 16b. Between the metal tube parts 16a and 16b in the vicinity of the deflection coils 12a, 12b the vacuum envelope is completed by a glass tube 17. In the present case the bore of the glass tube 17 is similar in diameter to that of the metal parts 16a, 16b which diameter may be, for example 10 mm. It is noted, however, that this dimension is not critical and may be different from the bore diameter of the metal tube parts 16a, 16b. The deflection coils 12a, 12b surround the glass tube 17 in a concentric manner and are disposed approximately mid-way along the length thereof, although at least longitudinally the relative position is not critical. Also, the length of the glass tube 17 is not critical but may be, for example, 15 cm. In this example tube 17 projects above the upper deflection coil 12a and below the lower deflection coil 12b. At its upper and lower ends the glass tube is sealed to a wall portion 18 of the column by respective O-rings 19,20. Also O-ring seals 21, 24 are present respectively between the upper side of wall portion 18 and an outwardly extending flange 26a at the lower end of the metal tube part 16a, and between the lower side of wall portion 18 and an outwardly extending flange 26b at the upper end of the metal tube part 16b. Thus an effective vacuum seal is provided between the glass tube 17 and the metal tube parts 16a and 16b of the vacuum envelope 14. Present inside the glass tube 17 is an electrically conductive tube comprising a single wire formed into a close-wound helix 22. In this case the radial dimensions of the helix match approximately the internal diameter of the glass tube 17, but the fit is sufficiently loose to allow the helix 22 to be removed and re-inserted easily. It is noted however, that in an alternative arrangement the diameter of the helix may be significantly smaller than the bore of the glass tube 17. At its lower end the glass tube 17 comprises an inwardly extending flange 17a on which the bottom of the helix 22 rests. In the case where the helix is significantly narrower than the glass tube the flange 17a may also be provided with locating projections for centering the helix. The helix 22 may be made for example of 1/2 mm diameter nichrome wire covered with an oxide film coating. In the case of nichrome a relatively thin layer of oxide is formed naturally on the wire and this natural oxide forms a resistive coating which is desirable for present purposes. The wire is formed into a helix by winding it around a former having appropriate dimensions in such manner that adjacent turns of the helix are contiguous, and the former is subsequently removed. The thin resistive film serves to impede the flow of current in the longitudinal direction of the helix whereas, to all intents and purposes, an electron beam is passing through the bore of the helix is always surrounded by a conductive surface. At each end of the helix a short length of the nichrome wire is left unwound to provide electrical connection 23a and 23b to the metal tube parts 16a and 16b respectively. The unwound length of wire 23a is led out of the upper end of the glass tube 17 and sandwiched between an upwardly directed face of the wall portion 18 and the flange 26a of the tube part 16a, and may be fastened to the wall portion 18, for example with a screw. At the lower end the unwound length of wire 23b is led out of the glass tube 17 and sandwiched between a downwardly directed face of the wall portion 18 and the flange 26b of the tube part 16b, and may be fastened to the wall portion 18, for example with a screw. Thus by connecting the metal tube part 16a to ground potential as shown, the helix 22 and the tube part 16b, being electrically connected, are also grounded so that the electron beam 3 in passing along the length of the column is at all times surrounded by a grounded conductive surface whereby stray charge-which otherwise would accumulate on the internal surfaces of the column-can leak to ground. As discussed previously it will be necessary from time to time to remove the conductive helix for cleaning or replacement because of the gradual build up of contaminants thereon. For this purpose the top and bottom sections of the whole column 1 can be separated at the area of the boundary between the metal tube part 16a and the glass tube 17 as represented by the broken line in the Figure. The helix 22 can then be removed very easily without disturbing the glass tube 17. It is believed that the rate at which contaminants build up on the conductive helix 22 is reduced at elevated temperatures. With this in mind, instead of maintaining the helix at a uniform potential, as described above, a potential difference may be applied across its ends to cause a heating current, e.g. in the order of 1 Amp, to flow therethrough. It is thought that the uniform rate of change of potential over the length of the glass tube 17 and the relatively low potential difference (compared with the accelerating voltage) necessary to generate this current would not adversely affect the operation of the apparatus to any significant extent. Moreover, it is noted that a heating current may be passed through the helix not only when the electron beam is in use, but additionally or alternatively at other times to prolong the useful operating period of the tube before needing to be changed or cleaned. In view of the description so far it will be evident to a person skilled in the art that many modifications may be made within the scope of this invention, and it is noted that the particular configuration of the various elements of the electron beam column described previously is merely exemplary. Thus for example, arrangements are envisaged where the deflection coils are situated in alternative locations in the column. Hence the deflection coils may be disposed on the opposite (downstream) side of the final lens system, that is to say, between the final lens system 13a, 13b and the workchamber 6. Alternatively, the deflection coils may be disposed entirely or partially within the final lens system, and in the latter case the deflection coils may project from either the upstream or downstream side of the final lens system. Also it is noted that use of the helically wound conductive tube is not restricted to the vicinity of deflection coils but may be used elsewhere in the electron beam column where the tube may be subject to a varying magnetic field. Finally, attention is drawn to the fact that while the specific embodiment described above relates to an electron optical column producing a Gaussian round beam with a fixed spot size, the invention is equally applicable to electron optical columns producing a beam spot whose size and shape are both variable. Moreover, the invention also has applications outside electron beam lithography. Thus for example, apparatus in accordance with the invention may take the form of an electron microscope or a machine producing a beam of charged particles other than electrons.