JPS6244381B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a cold cathode field emission electron gun which is configured to stop operating when the probe current falls below an effective value. The present invention relates to a scanning electron microscope equipped with a scanning electron microscope.

[Prior Art] In general, in a cold cathode field emission electron gun, the current of the electron beam emitted from the emitter of the electron gun, that is, the total radiation current, decreases with time. The extraction voltage is automatically controlled to keep the value constant. Although this makes it possible to keep the total radiation current constant, it may cause changes in the pattern of the electron-emitting part and changes in the lens characteristics of the anode lens as the extraction voltage changes, causing the electrons to reach the sample. The probe current of the electron beam continues to decrease, and at a certain point the emission operation reaches an unstable region (arc discharge region), leading to damage to the emitter. Since it is not possible to control the probe current at a constant level with such an electron gun, conventional methods have been used to detect the rise in the radiation current when it enters the unstable region mentioned above and stop the operation of the electron gun. There is.

[Problems to be Solved by the Invention] However, in this method, the stop control mechanism is activated for the first time when the emitted current increases due to arc discharge, so the tip of the emitter is often damaged by the discharge. Furthermore, there is a very small current region for a considerable time before reaching the unstable region, and the scanning image resolution in this region is extremely poor. That is, as the probe current decreases, the influence of quantum noise on the scanned image increases, and its resolution is generally inversely proportional to the square root of the probe current. Therefore,
Decreasing the probe current will significantly reduce resolution.

An object of the present invention is to provide a scanning electron microscope equipped with a novel field emission type electron gun that can solve the above-mentioned problems.

[Means for Solving the Problems] Therefore, the present invention provides a cold cathode field emission type electron gun, a circuit for applying a high voltage to each electrode of this electron gun, and a method for focusing and projecting an electron beam onto a sample. an electron lens for scanning, a deflection means for scanning the sample with an electron beam, a detector for detecting information from the sample, a display device to which the output signal of this detector is added as a brightness modulation signal, and a total radiation A device comprising a means for detecting a current and controlling an extraction voltage so that the current value is constant, which effectively controls the probe current of the electron beam emitted from the cold cathode field emission electron gun and projected onto the sample. A detecting means is provided, and when the detected probe current becomes less than a reference value, the high voltage from the high voltage circuit to the electron gun is cut off.

[Example] The drawings show an example of the present invention, and 1 is:
For example, a tungsten emitter. The emitter is used as a cold cathode and is placed on a heating filament for flushing. 2 is a power supply for flushing, and the emitter is connected to the negative side of the DC high voltage generation circuit 3 via this power supply. Reference numeral 4 denotes an extraction electrode placed opposite the emitter. A high positive voltage is applied to the emitter by a DC power supply 5, and a strong electric field of 10 ⁷ V/cm or more is formed at the tip of the emitter. Enables field emission of electrons. 6 is an accelerating electrode, which is kept at ground potential. An electron beam emitted from a cold cathode electron gun consisting of an emitter 1, an extraction electrode 4, and an accelerating electrode 6 is focused by a focusing lens 7 and projected onto a sample 8. Reference numeral 9 denotes a scanning deflection coil, which is driven by a signal from a scanning power source 10, and the electron beam scans the sample 8 two-dimensionally by the operation of this coil. Electrons scattered from the sample by electron beam irradiation are detected by a detector 11, and the output signal is introduced into a cathode ray tube display device 14 as a brightness modulation signal via an amplifier 12 and a signal processing circuit 13. The deflection coil 15 of this cathode ray tube is supplied with a scanning signal from the above-mentioned scanning power source and is driven in synchronization with the deflection coil 9, so that a scanned image of the sample is displayed on the screen of the cathode ray tube 14. R is a total emission current detection resistor, and a voltage corresponding to the total emission current detected by the resistor is supplied to a comparator circuit C. Since the comparison circuit is supplied with a reference voltage from the reference voltage source S, the comparison circuit controls the DC voltage source 5 so that the voltage corresponding to the total emission current becomes equal to the reference voltage. Through this control, the positive high voltage (extraction voltage) applied to the extraction electrode 4 from the DC power source is automatically controlled.

An aperture-shaped electron beam detector 16 is placed below the accelerating electrode 6, and the electron beam at the very periphery of the electron beam projected onto the sample is detected as a probe current signal. This signal is sent to the processing circuit 13 via the amplifier 17, and depending on the change in this signal, the detector 1
Process the signal from 1. That is, for example, the degree of amplification of the signal from the detector 11 is automatically controlled, the ratio of the signals from both detectors is determined, and fluctuations in the sample information due to changes in the probe current are canceled out.
Meanwhile, a signal representing the probe current from amplifier 17 is also sent to comparator 18 and compared with a reference voltage from reference signal source 19. This reference signal source sets the lower limit of the probe current, and the amplifier 17
The comparator sends a signal to the control circuit 20 when the detected probe current signal from the probe falls below the set reference signal. This control circuit receives the signal from the comparator 18, controls the DC high voltage generation circuit 3 and the DC power supply 5, cuts off the voltage applied to each electrode of the electron gun, and stops the operation of the electron gun. . The control circuit 20 also controls the flashing circuit 2 and supplies current for flashing the emitter 1 to the emitter holding filament. As a result, the deteriorated emitter tip is regenerated.

In the above embodiment, the probe current signal input to the comparator 18 is transmitted to the detector 16 for signal processing.
However, a detector may be provided separately from the detector. Alternatively, a signal from the detector 11 that detects information from the sample may be used.

[Effects of the Invention] With the above configuration, when the probe current falls below a certain value, the operation of the electron gun is automatically stopped and the emitter is flashed, so that the discharge in the unstable region is prevented. This prevents the emitter from being irreparably damaged and eliminates the need for use in conditions where the resolution of the scanned image is significantly reduced, which is extremely effective.

[Brief explanation of the drawing]

The drawing is a block diagram showing one embodiment of the present invention. 1: Emitter, 2: Flushing circuit, 3:
DC high voltage generation circuit, 4: Extraction electrode, 5: DC power supply, 6: Accelerating electrode, 7: Focusing lens, 8: Sample, 9: Deflection coil, 10: Scanning power supply, 11: Detector, 13: Processing circuit, 14: Cathode ray tube display device, R: Total radiation current detection resistor, C: Comparison circuit,
S: reference voltage source, 16: detector, 18: comparator,
19: Reference signal source, 20: Control circuit.

Claims (1)

[Claims] 1. A cold cathode field emission electron gun, a circuit for applying a high voltage to each electrode of the electron gun, an electron lens for focusing and projecting an electron beam onto a sample, and a A deflection means for scanning with an electron beam, a detector for detecting information from the sample, a display device to which the output signal of this detector is added as a brightness modulation signal, and a total radiation current that is detected and whose value is constant. and means for controlling the extraction voltage so that 1. A scanning electron microscope equipped with a field emission electron gun, characterized in that the high voltage from the high voltage circuit to the electron gun is cut off when the probe current becomes below a reference value. 2. A cold cathode field emission electron gun, a circuit for applying high voltage to each electrode of this electron gun, an electron lens for focusing and projecting an electron beam onto a sample, and a system for scanning the sample with an electron beam. a detector for detecting information from the sample; a display device to which the output signal of this detector is added as a brightness modulation signal; The device comprises means for virtually detecting a probe current of an electron beam emitted from the cold cathode field emission electron gun and projected onto the sample, and the detected probe current is set to a reference value. A scanning device equipped with a field emission type electron gun, characterized in that the high voltage from the high voltage circuit to the electron gun is cut off and the cold cathode (emitter) of the electron gun is flashed when the following conditions occur: electronic microscope.