This invention relates to a charged particle beam apparatus comprising means for generating a beam of charged particles, and a lens system including a magnetic lens for focussing the beam.
The beam of charged particles may be used to effect a process at a surface of a target. For example, in the field of semiconductor technology 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.
A conventional electron beam column produces a Gaussian, round beam spot which is scanned across the target to write the desired pattern. Although this exposure technique has the advantage of Providing optimum pattern flexibility it suffers from the drawback of being relatively slow.
Shorter exposure times can be achieved by using an electron beam column capable of producing a larger, square-shaped beam spot of fixed size. In this case the pattern is written by moving the shaped beam in discrete steps across the target.
Recent developments in the technology of electron beam columns have made it possible to shorten exposure times still further by using a rectangular-shaped beam spot whose size and shape can be altered to fit the various areas of the pattern to be exposed. This technique is known as variable-shaped beam imaging. The shaping variations are performed while the beam is stepped from one position to the next.
The lens system of a typical variable-shaped electron beam column basically comprises five magnetic lenses. In this context reference is invited to U.S. Pat. No. 4,243,866 and more particularly to the description with reference to FIG. 6. While the electron-beam column described there comprises various other elements, the basic lens constituents are as follows. Firstly, after the electron source there are two magnetic lenses associated with the beam shaping facility. Then, moving along the length of the column away from the electron source, there is a pair of demagnifying lenses and finally a so-called projection lens. Essentially these lenses are all arranged in series in the sense that, travelling the length of the column, the electron beam experiences the field of each lens in turn.
In the present context a magnetic lens normally comprises an electrically conductive coil which is enclosed within a soft ferromagnetic member forming the pole pieces of the magnet. The coil is disposed in the vicinity of the electron beam path so that the beam of electrons can be focussed by the action of the magnetic field produced by the passage of direct current through the coil. The magnetic field strength produced by the magnet is dependent on the magnitude of the current flowing in the coil so that the focussing effect (i.e. the strength) of the lens can be altered simply by increasing or decreasing the current. It is noted here that magnetic focussing is inevitably accompanied by rotation of the beam and the extent of this rotation is dependent on the strength of the magnetic lens.
However, there are occasions when the focussing of the electron beam column has to be altered within a limited range while the machine is actually operating. This may be necessary, for example, when there is a change in the height of the target on which the electron beam is impinging or when the size of the beam spot (and hence the beam current) is to be altered. In order not to erode the advantage of the fast exposure times which are capable with the variable-shaped electron beam column the refocussing should be completed in a time period similar to that taken to change the spot size, typically tens of nanoseconds. Unfortunately the time taken to change the strength of a magnetic lens by altering the current flow is considerably greater than this because of the inductance of the coil. Therefore, although the variable-shaped electron beam column is capable of fast exposure times, the number of wafers which can be processed per unit time by the machine can be limited by the relatively long times necessary for adequate refocussing.