JPH0315295B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a field emission gun for emitting electrons or cations, in particular for accurately measuring and controlling the current in the part of the beam that is actually used and for compensating for beam noise. The present invention relates to a circuit arrangement for forming a correction signal which is supplied to a control electrode for a control electrode.

The electron or ion beam current from a field emission gun fluctuates as a result of noise caused by dirt and corrosion of the sharp field emission source in the high vacuum chamber used for field emission. For this type of gun to be accurate, this flickering noise must be eliminated or significantly reduced in the portion of the beam used. This inherent noise can be reduced to some extent by cleaning the cathode material and improving operating conditions. Additionally, if a suitable beam current can be measured and the secondary effects associated with beam defocusing and shear can be avoided, this flicker noise can be reduced or suppressed by using feedback techniques. .

Stabilization of the field emission current through the use of feedback techniques requires accurate measurement of the beam current. In most field emission gun devices, only a small portion of the electron beam is actually used, and this small portion is emitted through a small opening in the center of an accelerating electrode located near the cathode. A sample at an anodic potential is reached in the beam path. If the emitted beam then has a uniform and constant electron density, measurements of the beam current can be carried out by sampling the current on this extraction electrode, which generates an electric field at the cathode. . Unfortunately, however, due to an effect known as split noise, the noise in the overall emitted current will be different from the noise in the beam passing through the aperture of the accelerating anode, thus making it difficult to achieve feedback noise compensation. Current measurements must be made with the beam impinging on a real sample exposed to an electric field.

Several known devices, taking into account the problem of distributed noise through the beam, use annular collectors capable of making current measurements. The collector in these devices is provided with a small central aperture for passing the central portion of the emitted beam and sampling the beam current therefrom. Another known device, as described for example in US Pat. No. 3,925,706, actually measures the total beam current used at the anode and supplies the measured value to a control electrode near the anode. used to form a feedback signal. Although this known method provides a measurement of the beam current received at the anode, it is not always possible to use the measured current at the same time and therefore no suitable control of the beam at the cathode can be carried out.
Additionally, current measurements made at or near the reference anode potential result in an error signal that must be converted to a very high negative voltage before being fed to the power supply associated with the cathode, thus increasing the error signal. May occur additively. However, large errors occur due to the following reasons. That is, the current in the emitted beam is typically only a fraction of the current flowing in the high voltage loop. The remaining current then consists of leakage, corona, the current generated by the unshielded floating power supply which radiates energy to adjacent components, as well as the pick-up portion that occurs when transmitting the power from the reference potential to the power supply. All devices of this type are prone to measurement errors between the anode and the control electrode in the vicinity of the cathode. These errors are eliminated in the circuit described below.

Briefly, the circuit of the present invention includes an emission electrode,
All of its floating power supply and current sensing and control circuits are surrounded by a conductive shield at a local reference potential and all of the enclosed circuits are connected to this local reference potential. Thus controlling the emission flow from the emission electrode. The first shield is insulated from and surrounded by the second shield. The second shield is connected directly to a high voltage terminal of a high voltage source which is itself connected to ground potential. The beam current flowing to ground is used to control the emission flow by modulating the voltage at the Wehnelt electrode (negative feedback).
has been converted. The error signal obtained by comparing the voltage-converted current with a reference level is applied to a Wehnelt or control electrode in the vicinity of the beam current emitting electrode and the chip, thereby adjusting the beam current in order to maintain a constant beam current. Modulate the size of the emission. 2 surrounding various circuits that are thus susceptible to pick-up and leakage.
By using a heavy shield, the actual emitted beam current is easily distinguished from spurious currents within the device.

Next, the present invention will be explained in detail based on illustrated embodiments.

A conventional field emission type emission chip 10 is shown in the block circuit diagram of FIG. This field emission type emission chip is electrically connected directly to a local reference potential and is 10 ^-9 mmHg
It is sealed in a chamber (not shown) maintained at a high vacuum on the order of . The tip of the chip 10 is arranged so that its axis passes through a small aperture in the vacuum chamber, so that the electron or ion beam emitted from the emission chip 10 passes through the aperture and onto the grounded electrode 12. Reach the placed sample.

A high voltage source 14 is provided between the ground electrode 12 and a circuit connected to the field emission type emission chip 10 for supplying an accelerating voltage to the emitted beam. If the high voltage source 14 is connected to supply a high negative voltage to the electrodes 12, the device is used to generate positive ions from the chip 10. On the other hand, if the high voltage source 14 were connected to provide a high positive voltage to the electrode 12 as shown, the chip 10 would be removed as desired in today's high density, high speed scanning electron beam equipment. emits an electron beam.

Accurate beam current measurements are made at the potential of the cathode field emission chip 10. The reason for this is, as mentioned at the beginning, that when current is measured with a grounded anode sample electrode, there is an inaccuracy when converting the measured value into a current adjustment signal near the cathode voltage, which is several thousand volts minus the ground potential. This is because it causes sadness. Furthermore, the current sensing circuit must be shielded from external influences to avoid measurement errors caused by corona, leakage currents, etc. Thus, as shown in FIG. 1, the shield 16 completely surrounds the field emission type emission chip 10. This shield 16 is connected to a connection point 17 of the circuit which is grounded to a local reference potential. Additionally, this shield 16 is connected to a second or outer concentric shield 18
insulated from and surrounded by shield 18
is electrically connected directly to the negative pole of the high voltage source 14. Although the outer shield 18 is at a very high potential compared to earth potential, the potential of the local earth connection 17 and therefore also the inner shield 16
, it is at a virtual reference potential. The inner concentric shield additionally provides electrical isolation between the detection circuit and the floating high voltage source.

Beam current detection is performed by detecting the emitted beam current reaching the ground electrode 12 at cathode potential. A conventional current to voltage converter having an operational amplifier 20 with a feedback resistor 22 is provided to detect the beam current. The inverting input side of amplifier 20 is connected to outer shield 18 and feedback resistor 22 . The non-inverting input side of the amplifier 20 is connected to the local reference potential point 17 and to the inner shield 16. The output voltage of the amplifier 20, which corresponds to the value of the product of the beam current and the feedback resistor 22, is connected to the inverting input of a conventional operational amplifier comparator 24. The non-inverting input side of this comparator is connected to a potentiometer 25. The potentiometer connects the local reference potential and the adjustable beam current reference signal to a comparator 24.
connected between the positive reference power supply and the power supply. The error output voltage of comparator 24 is applied to a Wehnelt or control electrode 26 located in close proximity to field emission emission chip 10 to control the current emitted therefrom.

In the embodiment shown in FIG. 1, a first anode 28 having a central opening is used. A part of the emitted beam passes through the aperture and reaches the grounded anode 12. The first anode 28 has
A positive voltage is applied to accelerate the emitted beam. This anode derives its voltage from one of a plurality of local power supplies 30. This power supply device also includes a current-voltage conversion amplifier 20 and a comparator 24.
and a current reference potentiometer 25 as well. As can be seen from the figure, local power supply device 3
0 and amplifiers 20 and 24 are also placed within the inner shield, so that the wetting current from the floating high voltage source or operational amplifiers 20, 24 is
Current sensing between inner shield 16 and outer shield 18 is not involved. That is, since the current detection is performed using the local reference potential of the cathode chip 10, there is an advantage that various wetting currents that interfere with the detection of the true beam current arriving at the ground electrode 12, as described above, can be eliminated. .

FIG. 2 shows that compared to the embodiment shown in FIG.
A field emission bipolar gun is shown in the block diagram, differing only in that the first anode 28 of FIG. 1 is omitted. In the bipolar gun of the embodiment shown in FIG. 2, the accelerating voltage range must be narrowed so that the field emission accelerated beam is
It is used when only one high voltage supply 32 needs to be used to generate from 4 to the grounded electrode 36. In the embodiment illustrated in FIG. 2, current measurements are made in the same circuit and in the same manner as used in the circuit of FIG. Local power supply device 3
0 is again used to provide the necessary power to the current-to-voltage conversion amplifier 40 and comparator 42.
It is connected within the inner shield 38 to a local reference potential.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a first embodiment of the present invention of a field emission gun housed in a double shield, a power supply device connected to it, and a current detection circuit, and Fig. 2 shows a field emission diode. have a gun,
2 is a block diagram of an alternative embodiment to the circuit of FIG. 1; FIG. 10, 34... Field emission type emission chip, 12, 36... Sample electrode, 14, 32... High voltage source, 16, 18, 38... Shield, 20,
40... Current voltage converter, 24, 42... Comparator, 17... Local reference potential connection point, 25...
Potentiometer, 26... Control electrode, 28...
First anode, 30...Local power supply device.

Claims (1)

[Claims] 1. A beam current emitted from a gun that measures a beam current and is connected to a local reference potential by controlling a control voltage of a control electrode placed near an emission chip. a beam adjustment device for adjusting the beam, the gun having a sample electrode connected to ground potential, a first terminal connected to the sample electrode at ground potential, and a second terminal connected to a virtual reference; In a current stabilized field emission gun, the beam conditioning device includes a high voltage source connected to an electrical potential, the beam conditioning device comprising a high voltage source, which is floating substantially at a local reference potential of the emission chip. a current detection circuit for detecting the current flowing through the voltage source, the current detection circuit being connected to the high voltage source and the emission chip and for controlling the emission chip current; the beam conditioning device generates an output voltage corresponding to the current impinging on the earth reference potential, which is converted to a voltage used for 1. A current stabilized field emission gun, comprising a comparator circuit for comparing the voltage and supplying an error output voltage signal to the control electrode disposed in the vicinity of the emission chip. 2. The emission chip, the control electrode in the vicinity of the emission chip, the current detection circuit, the comparison circuit, and the local power supply device connected thereto are connected to a first local reference potential of the emission chip. A current stabilized field emission gun as claimed in claim 1 surrounded by an electrostatic shield. 3. The first electrostatic shield is insulated and surrounded by a second electrostatic shield connected to the second terminal of the high voltage source, and the current sensing circuit is configured to measure the beam current. 3. A current stabilized field emission gun as claimed in claim 2, wherein the gun is connected to first and second electrostatic shields. 4. A high voltage source is connected to the second shield so that a positive voltage is supplied to the grounded sample electrode, thereby causing the emission chip to generate a positive ion beam. A current-stabilized field emission gun according to scope 3. 5. A high voltage source is connected to the second shield in such a way that a negative voltage is supplied to the grounded sample electrode, thereby causing the emission chip to generate an electron beam. A current-stabilized field emission gun according to scope 3. 6. The field emission gun further includes a first accelerating anode disposed in the electron beam path, the anode having an aperture for passing a portion of the electron beam to the sample electrode. and the anode is connected to be supplied with a positive voltage from a power source disposed within the first electrostatic shield such that the anode is at a positive potential compared to a local reference potential of the emission chip. A current stabilized field emission gun according to claim 5.