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US 4,902,940 AGranted
Current control circuit
AssigneeU.S. Philips Corporation
InventorBrian K. Herbert
Life of this patent
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AssigneeU.S. Philips Corporation
InventorBrian K. Herbert
Estimating the term…
A current control circuit is provided for a stigmator lens of an electron or ion beam machine. The stigmator lens consists of two magnetic quadrupole lenses, each of which has two pairs of coils. Current monitoring resistors R10,R11,R12 and R13 are connected in series with the individual coils L1,L2,L3 and L4 of the quadrupole and a resistor R3 is connected to monitor the sum of currents in the pairs of coils. Voltage signals from the resistors are used to close feedback loops which drive currents into the individual coils so that the sum of currents in a pair of coils is held constant at a value set by an adjustable astigmatism control and so that the ratio of the currents in a pair of coils is held constant at a value set by an adjustable beam centering control. Coil currents are stabilized in spite of coil resistance changes due to Joule heating by the currents. Computer control of astigmatism and centering is facilitated since only signal voltages are required as inputs to the circuit.
A current control circuit for a magnetic quadrupole lens comprising two pairs of coils, characterised in that each coil is connected in series with a respective current monitoring resistor, in that the current monitoring resistors of the first pair of coils are connected together and to one end of a total current monitoring resistor and the current monitoring resistors of the second pair of coils are connected together and to the other end of the total current monitoring resistor to form a current distribution network, in that a first d.c. generator is connected to drive current through one coil and current monitoring resistor of the first pair of coils and thence into the current distribution network, one coil and current monitoring resistor of the second pair of coils forming a current return path, in that first means are provided for controlling the first d.c. generator so that the voltage across the total current monitoring resistor is equal to a predetermined proportion of an adjustable input astigmatism control voltage, in that a second d.c. generator is connected to drive current through the other coil and current monitoring resistor of the first pair of coils and thence into the current distribution network, in that second means are provided for controlling the second d.c. generator so that the difference in voltage across the current monitoring resistors of the first pair of coils is equal to a first adjustable proportion of the adjustable input astigmatism control voltage, in that a third d.c. generator is connected to drive current through the other coil and current monitoring resistor of the second pair of coils and thence into the current distribution network, and in that third means are provided for controlling the third d.c. generator so that the difference in voltage across the current monitoring resistors of the second pair of coils is a second adjustable proportion of the adjustable input control voltage.
Dependencies are inferred from the claim text.