Method and Device for the Regulation of a Magnetic Deflection System

The invention relates to a method for regulating a magnetic deflection system for a particle radiation optical device, particularly an electron beam writer, in which the current flowing through the deflection coils is measured and a control signal for the readjustment of the coil current is generated by means of comparison with a command value. The invention further relates to a device for implementing this method.

In order to achieve short write times with an electron beams writer in the exposure of semiconductor wafers, a rapid and high-precision deflection system for the electron beam is required in addition to a high beam current density and a rapid data processing. Given the employment of magnetic deflection systems, the magnetic field is determined by measuring and regulating the coil current in deflection coils. The coil current can be simply measured with sufficient precision in order to achieve the required precision of the magnetic fields from 10.sup.-4 through 10.sup.-5 even given the small magnetic fields of a few 10.sup.-4 Tesla hereby occurring. Rapid changes of magnetic field, however, are influenced by eddy currents which are induced in surrounding, conductive components. Under certain conditions, thus, the magnetic field cannot quickly enough follow the command value. A current regulation with additional differential behaviour is only admissible insofar as no overshoot (or: ringing) occurs beyond the allowed tolerances by so doing.

A control system which is based on a direct measurement of the magnetic field could avoid these problems. In so doing, however, there is difficulty in statically and dynamically measuring the small magnetic fields with the precision required here.

The greatest possible employment of nonconductive materials, ferrite shields and the observance of the greatest possible intervals from conductive surfaces only produces limited success.

The object of the present invention is to expand a method and a corresponding device of the type initially cited in such manner that the magnetic field always quickly and precisely tracks (or: follows) the command value independently of influences due to eddy currents.

This object is inventively achieved by means of the method steps specified in the characterizing part of claim 1.

A combined control is here inventively proposed. In the static case, the control ensues via the measurement of the coil current, whereby the required precision can be observed. Given changes of command value, a second control loop is superimposed: with the assistance of a magnetic field measuring means, a signal proportional to the field rate of change dB/dt is generated and compared to the command value rate of change dx/dt gained by means of differentiation of the command value x. If the magnetic field does not track the command value quickly enough then an additional actuating signal occurs as a function of the error between the rate signals.

It is provided in an advantageous further development of the invention that the additional control signal influences the readjustment of the coil current. However, it is possible with the same advantage that the additional control signal directly influence the magnetic field. A high degree of compensation of delays caused by eddy currents is achieved by means of these simple measures given high precision at the same time. The measures for improving the control behavior initially cited, for example, by means of incorporating a differential component, can also be advantageously applied within the framework of the inventive principle.

Given a device for implementing the inventive method with a known current regulation, the solution of the said object is characterized in that a magnetic measuring device is provided in which the signal proportional to the field change dB/dt is generated; in that the command value is additionally supplied to a differentiating circuit; and in that the outputs of the magnetic field measuring means and the differentiating circuit are connected to a respective input of a differential amplifier whose output is connected to the control means for the coil current. It is just as advantageously possible that the output of the differential amplifier is connected to auxiliary coils in the deflection system. An induction coil with impedance buffer or a Hall generator with differentiating circuit or a magnetoresistor with linearization and differentiating circuit can be employed for the magnetic field measuring means, for example.

In the following, an exemplary embodiment of the invention is described and explained in greater detail on the basis of a FIGURE on the accompanying drawing sheet; and other objects, features and advantages will be apparent from this detailed disclosure and from the appended claims.

The sole FIGURE shows a block diagram of a device for the rapid and precise magnetic field regulation for an electron beam writer.

Greatly simplified, the FIGURE shows a deflection system 1 with a single deflection coil 2. In practice, at least one pair of coils symmetrically arranged with respect to the axis of the electron beam writer, preferably, however, a two stage deflection system with two pairs of coils lying behind one another along the beam path, is employed in electron beam writers for deflecting the electron beam. The deflection system 1 is driven via a current control 3. The command value x is supplied to the current control at its input 4. For example, a via a computer-controlled program. In this exemplary embodiment, a simple current control is indicated in which the current I flowing through the deflection coil 2 generates a voltage drop to ground at a resistor 5. This voltage, like the command value x, is supplied to an amplifier 8 equipped with the two input resistors 6 and 7, being supplied for the purpose of comparison. This regulation of the magnetic field via the current regulation suffices, in the static operating case, in order to observe the required magnetic field with the precision demanded. Given rapid changes of magnetic field, a second control circuit is now superimposed. With the assistance of a measuring means 10, for example, an induction coil with impedance buffer (or impedance converter), a signal b.multidot.dB/dt proportional to the rate of change of magnetic field intensity B is generated. The multiplication factor b depends on the type of measuring means employed and is necessary in order to render a comparison with the differentiated command value dx/dt possible. The command value x is differentiated in a differentiating circuit 11. A signal a.multidot.dx/dt derives. The factor a is a multiplication factor which takes the type of differentiating circuit employed into consideration. The two signals generated in this manner are supplied to a differential amplifier 12. In case the magnetic field does not track (or: follow) the command value x quickly enough, then a further actuating signal arises in the differential amplifier which either influences the current regulating circuit via an element 13 with a resistor 14 or directly influences the magnetic field via an element 15 with an auxiliary coil 16.

Since every differentiating circuit in practice fundamentally exhibits an error dependent on the frequency, it can be advantageous under certain conditions to insert a correction element into the branch between the measuring means 10 and the differential amplifier 12, said correction element in turn eliminating the error as a function of frequency.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts and teachings of the present invention.