Method of and apparatus for attaining focusing following variation in magnification in electron microscope

Claim 2

In an electron microscope comprising a three-stage image forming electron lens system consisting of an objective lens, an intermediate lens and a projection lens arranged in this order in the direction of an electron beam axis, and excitation current sources each connected to each of said electron lenses for supplying excitation currents thereto and for stabilizing said excitation currents, each of said excitation current sources including means for adjusting said excitation currents; a focusing apparatus comprising means for mechanically coupling said excitation current adjusting means to simultaneously control said excitation currents, so that when the magnification of said electron microscope is varied by varying the excitation current of said intermediate lens while keeping the excitation current, of said projection lens constant, the excitation current of said objective lens is set at the value compensating for the variation of said excitation current of said intermediate lens to provide proper focusing.

Claim 3

In an electron microscope comprising a three-stage image forming electron lens system consisting of an objective lens, an intermediate lens and a projection lens arranged in this order in the direction of an electron beam axis, and excitation current sources each connected to each of said electron lenses for supplying excitation currents thereto and for stabilizing said excitation currents, each of said excitation current sources including means for adjusting said excitation currents; a focusing apparatus comprising means for mechanically coupling said excitation current adjusting means to simultaneously control said excitation currents, so that when the magnification of said electron microscope is varied by varying the excitation current of said intermediate lens while keeping the excitation current, of said projection lens constant, the excitation current of said objective lens is set at the value compensating for the variation of said excitation current of said intermediate lens to provide proper focusing.

Claim 4

In an electron microscope comprising a three-stage image forming electron lens system consisting of an objective lens, an intermediate lens and a projection lens arranged in this order in the direction of an electron beam axis, and excitation current sources each connected to each of said electron lenses for supplying excitation currents thereto and for stabilizing said excitation currents, each of said excitation current sources including means for adjusting said excitation currents; a focusing apparatus comprising means for mechanically coupling said excitation current adjusting means to simultaneously control said excitation currents, so that when the magnification of said electron microscope is varied by varying the excitation current of said intermediate lens while keeping the excitation current, of said projection lens constant, the excitation current of said objective lens is set at the value compensating for the variation of said excitation current of said intermediate lens to provide proper focusing.

Claim 5

In an electron microscope having a three-stage image producing electron lens system consisting of an objective lens, an intermediate lens and a projective lens which are disposed in the above order in the direction of the electron beam axis, an apparatus for attaining the focusing following a variation in the magnification comprising an excitation power supply connected to each of said electron lenses for supplying excitation current to said electron lenses and stabilizing the excitation current, each said excitation power supply including an excitation current control means for controlling the excitation current flowing through the lens coil, a detecting resistor for detecting the excitation current flowing through said lens coil, means for varying the resistance of said resistor, a source of reference voltage for supplying a reference voltage for comparison between it and the voltage detected by said detecting resistor, and an amplifier for detecting and amplifying the difference between said detected voltage and the voltage of said reference voltage source, said amplifier having its first input terminal connected to said detecting resistor, its second input terminal connected to said reference voltage source and its output terminal connected to said excitation current control means, and means mechanically coupling said resistance varying means for said detecting resistors in said excitation power supplies for the simultaneous control of the resistances of said detecting resistors so as to set the excitation current for said objective lens at a value corresponding to a variation of the value of excitation current for said intermediate lens when the value of excitation current for said intermediate lens is varied while maintaining the excitation current for said projective lens at a constant value.

Claim 6

In an electron microscope having a three-stage image producing electron lens system consisting of an objective lens, an intermediate lens and a projective lens which are disposed in the above order in the direction of the electron beam axis, an apparatus for attaining the focusing following a variation in the magnification comprising an excitation power supply connected to each of said electron lenses for supplying excitation current to said electron lenses and stabilizing the excitation current, each said excitation power supply including an excitation current control means for controlling the excitation current flowing through the lens coil, a detecting resistor for detecting the excitation current flowing through said lens coil, means for varying the resistance of said resistor, a source of reference voltage for supplying a reference voltage for comparison between it and the voltage detected by said detecting resistor, and an amplifier for detecting and amplifying the difference between said detected voltage and the voltage of said reference voltage source, said amplifier having its first input terminal connected to said detecting resistor, its second input terminal connected to said reference voltage source and its output terminal connected to said excitation current control means, and means mechanically coupling said resistance varying means for said detecting resistors in said excitation power supplies for the simultaneous control of the resistances of said detecting resistors so as to set the excitation current for said objective lens at a value corresponding to a variation of the value of excitation current for said intermediate lens when the value of excitation current for said intermediate lens is varied while maintaining the excitation current for said projective lens at a constant value.

Claim 9

In an electron microscope comprising a three stage image forming electron lens system including an objective lens, an intermediate lens, and a projection lens arranged sequentially in the direction of the electron beam axis, and excitation sources connected to each respective electron lens for supplying excitation currents thereto and for stabilizing said excitation currents, each of said excitation sources including means for adjusting said excitation currents, an improved focusing apparatus comprising: control means, coupled to each of said current sources and said lenses, for simultaneously maintaining the excitation current of said projection lens constant, for varying the excitation current of said intermediate lens, and for adjusting the excitation current of said objective lens to a predetermined value to provide proper focusing in response to said variation of the excitation current of said intermediate lens, wherein each of said excitation sources comprises a power supply, a switching transistor connected between said power supply and a respective electron deflection element associated with each lens and a control circuit responsive to said control means for appropriately energizing said switching transistor to control the flow of current to said deflection element, and wherein said deflection element comprises a lens coil and wherein said control circuit comprises a voltage dividing circuit, connected to said lens coil, and a differential amplifier having one input connected to a respective source of reference potential and another input adjustably connected to said voltage dividing circuit, the output of said differential amplifier being connected to control the operation of said switching transistor.

Claim 10

In an electron microscope comprising a three stage image forming electron lens system including an objective lens, an intermediate lens, and a projection lens arranged sequentially in the direction of the electron beam axis, and excitation sources connected to each respective electron lens for supplying excitation currents thereto and for stabilizing said excitation currents, each of said excitation sources including means for adjusting said excitation currents, an improved focusing apparatus comprising: control means, coupled to each of said current sources and said lenses, for simultaneously maintaining the excitation current of said projection lens constant, for varying the excitation current of said intermediate lens, and for adjusting the excitation current of said objective lens to a predetermined value to provide proper focusing in response to said variation of the excitation current of said intermediate lens, wherein each of said excitation sources comprises a power supply, a switching transistor connected between said power supply and a respective electron deflection element associated with each lens and a control circuit responsive to said control means for appropriately energizing said switching transistor to control the flow of current to said deflection element, and wherein said deflection element comprises a lens coil and wherein said control circuit comprises a voltage dividing circuit, connected to said lens coil, and a differential amplifier having one input connected to a respective source of reference potential and another input adjustably connected to said voltage dividing circuit, the output of said differential amplifier being connected to control the operation of said switching transistor.

Claim 11

An improved focusing apparatus according to claim 4, wherein said control means further includes a mechanically ganged switching arrangement having respective switches connected between each voltage divider and the corresponding input to each of said differential amplifiers.

Claim 14

A method of varying the magnification of an electron microscope according to claim 6, in which said one electron lens of the step (b) is said first projection lens.