JPS6340016B2 - - Google Patents

Description

Detailed Description of the Invention The present invention changes the heating power for flashing according to the surface condition of the emitter, thereby suppressing fluctuations in the amount of current of the electron beam emitted from the emitter and widening the angular distribution of electron emission. The present invention relates to a field emission type electron gun which aims to extend the life of an emitter while preventing such problems.

Field emission electron guns have 100 to 1000 times higher brightness than thermionic emission electron guns, so they have recently been used as electron guns for high-performance electron beam devices, such as scanning electron microscopes, transmission electron microscopes, and exposure equipment. It is being used. A field emission type electron gun is composed of an electron emitting emitter, an extraction electrode, an anode, and the like. The degree of vacuum is about 10 ^-5 to 10 ^-6 Torr for conventional thermionic emission electron guns, but
Requires ultra-high vacuum of 10 ^-10 Torr.

FIG. 1 is a diagram showing one configuration of a field emission type electron gun. In the figure, 1 is an emitter for electron emission, 2 is an extraction electrode, and an accelerating voltage is applied to the emitter 1, and an extraction voltage is applied to the extraction electrode 2 (not shown). 3 is an anode, 4 is a scanning coil for scanning the sample with an electron beam, and 5 is an electromagnetic lens that functions as an objective lens. Further, 6 is a sample, and 7 is an exhaust pump for maintaining an ultra-high vacuum inside the electron gun.

In a field emission type electron gun having such a configuration, when a voltage is applied to the extraction electrode 2 and electrons are emitted from the emitter 1, residual gas may be adsorbed on the surface of the emitter 1, or when the electrons are emitted, The residual gas is ionized, and the residual gas ions collide with the surface of the emitter 1 and deform the surface (ion bombardment), resulting in fluctuations in the emission current.
In order to suppress and stabilize such fluctuations in the emission current, some devices detect fluctuations in the emission current and use negative feedback to control the voltage applied to the extraction electrode 2 so that the emission current remains constant. It's under control. At the same time, fluctuations in the high frequency components included in the emitted current are detected, and if fluctuations exceed a certain value, the extraction power source and acceleration power source are turned off.
It is configured to perform flashing (here, the high frequency component is detected by
This is because fluctuations in low frequency components are removed by the negative feedback circuit for making the emission current constant).

Flushing is a process in which heating power is applied to the emitter 1 to remove gas adsorbed matter adhering to the surface of its tip, thereby stabilizing electron emission and increasing the efficiency of the probe current relative to the total emission current by broadening the angular distribution of electron emission. refers to correcting the deterioration of The above flushing was performed when the fluctuation of the emission current exceeded the allowable value, but there are also other methods that operate a timer to turn off the acceleration power source and extraction power source after a certain period of use (for example, several hours). There is a flushing process performed. As mentioned above, flushing suppresses fluctuations in the amount of emitted current due to gas adsorption and ion bombardment on the emitter surface, and also prevents the angular distribution of electron emission from widening to maintain a constant probe current efficiency. However, it also has the disadvantage that increasing the number of times will make the tip of the emitter 1 thicker.

If the tip of the emitter 1 becomes thicker, electrons will no longer be emitted unless the extraction voltage is further increased. Therefore, if the emitter 1 becomes too thick, it is assumed that the emitter 1 has reached the end of its life and is replaced. When replacing emitter 1, set the ultra-high vacuum chamber to atmospheric pressure and replace emitter 1.
After replacing, it is necessary to perform a bakeout process to evacuate to ultra-high vacuum again. Further, the electrodes and the like must be conditioned to prevent discharge, and their replacement requires a great deal of effort.

The present invention has been made in view of these points. Under normal usage conditions, flashing is performed at low power at regular intervals to prevent fluctuations in electron emission current due to intense gas adsorption and ion bombardment on the emitter surface. If the allowable value is exceeded, flashing is performed at high power to suppress fluctuations in the emission current, prevent the angular distribution of electron emission from widening, prevent a decrease in probe current efficiency, and reduce the length of the emitter. This is a field emission type electron gun with a long service life.

Hereinafter, the present invention will be explained in detail with reference to the drawings. FIG. 2 is a configuration diagram showing an embodiment of the present invention. Components that are the same as those in FIG. 1 are designated with the same reference numerals. In the figure, 11 is a flashing power supply that provides heating power to emitter 1.
The heating power can be changed by SW ₁ and SW ₂ . Here, high power is supplied when the contact of switch SW ₁ is closed, and low power is supplied when the contact of SW ₂ is closed. The power of the flushing power supply 11 is insulated and supplied to the emitter side via the transformer T. Reference numeral 12 denotes an extraction power source that provides an extraction potential to the extraction electrode 2, and 13 an acceleration power source that provides an acceleration potential to the emitter 1. Reference numeral 14 denotes a power supply control circuit that performs on/off control of the extraction power supply 12 and the acceleration power supply 13, and the control circuit 14 is a switch SW ₃ or SW _{4 .}
When either contact of the power source 12,1 is closed, the power source 12,1
It works to turn off 3.

U ₁ is an amplifier that extracts the emission current emitted from emitter 1 as a voltage change from resistor R ₁ , and U ₂
is a comparator that compares the output of the amplifier _U1 and the reference voltage Es. This reference voltage Es is given as an allowable value for fluctuations in the electron emission current, and can be set to any value by changing the voltage division ratio of the variable resistor _R2 . Vc is the supply voltage, Q ₁ is the comparator U ₂
This is a transistor that receives the output of A relay RL ₁ is connected as a collector load of this transistor Q ₁ . When transistor _Q1 turns on,
Excitation current flows through relay RL ₁ and the contacts close. The switches SW ₁ and SW ₃ described above use the contacts of the relay RL ₁ . Q ₂ is a transistor driven by timer 15, and a relay is used as its collector load.
RL ₂ is connected. When the transistor Q ₂ is turned on, the excitation current flows through the relay RL ₂ and the contacts close. The aforementioned switches SW ₂ and SW ₄ are connected to the relay.
RL ₂ contacts are used.

The operation of the device configured in this manner will be described next.

Under normal usage conditions, switch SW ₃ ,
Since all SW ₄ are off, the control circuit 14
sends a control signal to the extraction power source 12 and acceleration power source 13 to turn on both power sources. This results in
An accelerating voltage is applied to the emitter 1, an extraction voltage is applied to the extraction electrode 2, and electrons are emitted from the emitter 1. Unless gas adsorption, ion bombardment, etc. occur on the emitter surface, there will be no major fluctuations in the emission current. Therefore amplifier U ₁
Since the output of will never exceed the reference voltage Es,
Transistor _Q1 remains off. That is,
Switches SW ₁ and SW ₂ of the flushing power supply 11 are both off.

In this state, the timer 15 outputs an on signal after a certain period of time has elapsed. The on signal causes the transistor to
Q ₂ turns on. When transistor Q ₂ is turned on, an excitation current flows through relay RL ₂ and the switch is turned on.
SW ₂ and SW ₄ are both turned on. The control circuit 14 is
Detects that the contact of the switch SW ₄ is closed and sends a control signal to the extraction power source 12 and the acceleration power source 13,
Turn off both power supplies. Thereafter, the flushing power supply 11 detects that the switch SW ₂ is closed, and supplies low power to the emitter 1 to perform flushing. In normal usage conditions, the amount of gas adsorbed substances adhering to the surface of the emitter 1 is small, so that the gas adsorbed substances etc. on the emitter surface can be removed by low-power flushing. It is sufficient that the number of times of flashing is performed every time an ON signal is outputted from the timer 15 at a certain fixed time interval.

Next, a case will be described in which the effects of gas adsorption and ion bombardment on the emitter surface become severe. In this case, as described above, the electron emission current fluctuates greatly. The variation of the emission current is determined by the amplifier U ₁
and sent to comparator _U2 . Comparator U ₂ is
This variation is compared with the allowable value Es. The amount of variation is
When the tolerance value Es is exceeded, the output of the comparator U ₂ turns on the transistor Q ₁ , an exciting current flows through the coil of the relay RL ₁ , and its contacts close. That is, switches SW ₁ and SW ₃ are turned on. The control circuit 14 detects that the contact of the switch SW ₃ is closed, and sends a control signal to the extraction power source 12 and the acceleration power source 13 to turn off both power sources. Thereafter, the flushing power supply 11 detects that the contact of the switch SW ₁ is closed, and supplies high power to the emitter 1 to perform flushing. If the fluctuation of the electron emission current exceeds the allowable value, the amount of gas adsorbed on the emitter surface has increased or the influence of ion bombardment has become significant, so high power flushing must be performed.

In this way, under normal usage conditions, low-power flashing is performed at regular intervals, and when fluctuations in the electron emission current exceed the allowable value, high-power flashing is performed to correct the condition of the emitter surface. Since the heating power can be supplied at the minimum necessary optimum value accordingly, it is possible to prevent the tip of the emitter from becoming thick. Therefore, the life of the emitter can be extended.

Note that as a method of supplying heating power, a method of varying the power application time can be considered. For example, if the application time for low power heating is T ₁ , the heating time for high power heating is 10T ₁ .
This is a way to make it 10 times longer. Alternatively, a method may be considered in which the application time is constant and the applied power supply voltage is varied. For example, for low power heating,
A method of applying power of voltage V ₁ for only T ₁ and, in the case of high-power heating, applying power of voltage 2V ₁ for only T ₁ is also considered. Alternatively, it is also possible to consider a case where both the application time and the voltage are varied and controlled.

As explained in detail above, in the present invention, under normal usage conditions, flashing is performed at low power at regular intervals, and gas adsorption and ion bombardment on the emitter surface become intense, causing fluctuations in the electron emission current to exceed the allowable value. According to the present invention, the device is configured to flash with high power when the voltage is exceeded.
It is possible to suppress fluctuations in electron emission current and further suppress widening of the angular distribution of electron emission, thereby preventing a decrease in probe current efficiency and prolonging the life of the emitter.

[Brief explanation of drawings]

FIG. 1 is a diagram showing one configuration of a field emission type electron gun. FIG. 2 is a configuration diagram showing an embodiment of the present invention. 1... Emitter, 2... Extraction electrode, 3...
Anode, 4... Scanning coil, 5... Electromagnetic lens, 6
...sample, 7...exhaust pump, 11...flushing power supply, 12...drawing power supply, 13...acceleration power supply, 14...control circuit, 15...timer, SW ₁
~SW ₄ ...Switch, RL ₁ , RL ₂ ...Relay, U ₁
...Amplifier, _U2 ...Comparator, _Q1 , _Q2 ...Transistor, _R1 ...Resistor, _R2 ...Variable resistor, T...
Trance.

Claims (1)

[Scope of Claims] 1. In a field emission type electron gun consisting of an electron emitting emitter, an extraction electrode, an anode, etc., a flashing power source for flushing the emitter, and a flashing power source for flushing the emitter, and a flashing power source that is connected to the flashing power source at certain fixed time intervals. The present invention includes means for performing low-power flushing, and means for detecting fluctuations in the emission current emitted from the emitter and causing the flushing power source to perform high-power flushing when the amount of fluctuation exceeds a permissible value. Features a field emission type electron gun. 2. The field emission type electron gun according to claim 1, wherein the flushing mode is such that the application time of the flushing power source is varied. 3. The field emission type electron gun according to claim 1, wherein the flushing is performed by changing the applied voltage of the flushing power source.