JPH047534B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a focused ion beam device, and more particularly to a focused ion beam device in which an electrostatic lens for focusing an ion beam is improved.

(Prior Art) A focused ion beam device is a device that ionizes atoms, extracts them to form a beam, and irradiates a substance with the ion beam to change the shape and properties of the substance. This type of device has recently come to be used as a maskless ion implantation device for semiconductor microfabrication. FIG. 2 is a diagram showing an example of the electrical configuration of a conventional focused ion beam device. In the figure, 1 is an accelerating voltage generation circuit that generates high voltage for accelerating the ion beam, 2 is an emitter that emits ions, 3 is an extraction electrode (extractor) that extracts ions emitted from the emitter 2,
Reference numeral 4 denotes a power source for applying an extraction voltage that applies a potential to the extraction electrode 3. As the output voltage of the accelerating voltage generation circuit 1, for example, about 200 KV is used. As the voltage generated by the power source 4 for applying the extraction voltage, for example, about 5000V is used.

5 is an acceleration tube that accelerates the ion beam passing through it in multiple stages;
Reference numeral 6 denotes a voltage divider that applies multistage accelerating voltages to the accelerating tube 5. As the voltage divider 6, for example, a high voltage dividing resistor is used. 7 is a condenser lens that focuses the ion beam, and 8 is a mass separator (mass filter) that separates ions with different masses among the passing ions. The mass separator 8 applies a magnetic field and an electric field orthogonal to the magnetic field to the passing ions to remove unnecessary ions. In other words, by utilizing the property that the trajectory of ions passing through a magnetic field is bent in descending order of mass, and by applying an electric field perpendicular to the magnetic field to make the necessary ion beams travel straight, unnecessary ions are removed. It is.

9 is an objective lens, 10 is an ion beam X,
A deflector 11 that scans in the Y2 direction is a sample (target) to which the ion beam is finally irradiated. 12 is a voltage divider to which the output voltage of the accelerating voltage generating circuit 1 is applied. As the voltage divider 12, for example, a voltage dividing resistor is used like the voltage divider 6. The voltage divider 12 is provided with taps A and B as shown in the figure. The divided voltage of tap A is the objective lens 9.
The divided voltage of tap B is applied to the condenser lens 7. When the acceleration voltage is 200KV, the voltage applied to the objective lens 9 is, for example,
The voltage applied to the condenser lens 7 is, for example, about 50 KV. In addition, condenser lens 7
The objective lens 9 serves as an electrostatic lens that focuses the beam using an electric field. The operation of the device configured as described above will be explained as follows.

A high-intensity ion beam generated from the emitter 2 and passed through the opening of the extraction electrode 3 is accelerated by a six-stage acceleration tube 5. The high-speed ion beam that has passed through the multistage accelerator tube 5 is narrowed down to a beam diameter by a condenser lens 7, and then unnecessary ions are removed by a mass separator 8. The ion beam that has passed through the mass separator 8 is narrowed down to a beam diameter again by an objective lens 9, deflected in a predetermined direction by a deflector 10, and then irradiated onto a target 11. As a result, the surface of the target 11 is scanned in a predetermined direction, and ions are implanted or sputtered with an ion beam.

FIG. 3 is a diagram showing the trajectory of the ion beam thus formed until it irradiates the target. The numbers in the figure correspond to the numbers of the components in FIG.

(Problems to be Solved by the Invention) By the way, in conventional focused ion beam devices, electrostatic lenses as described above are used as condenser lenses and objective lenses. In the case of an electrostatic lens, it must be vacuum insulated, which causes the following problems.

Micro discharges occur in the spacer between the electrodes, along the creeping surface of the insulator, etc.

Vacuum minute discharge occurs between the electrodes.

In the conventional example shown in FIG. 2, the condenser lens 7 and objective lens 9 are supplied with 50 KV and 100 KV, respectively, from the voltage divider 12 as described above. For example, in a focused ion beam device with an accelerating voltage of 200 KV, a high resistance of about 1000 MΩ is used as the voltage divider 12. Therefore, when there is a minute discharge as explained in , a voltage drop occurs due to the resistance due to the discharge current of this minute discharge, and the above-mentioned applied voltages of 50 KV and 100 KV fluctuate, respectively.
For example, assume that a minute discharge occurs in the objective lens 9. If the peak value of the discharge current at this time is 1μA, the voltage dividing resistance value that gives 100KV is 1/1000MΩ.
Since it is 500MΩ of 2, the amount of voltage fluctuation is 500MΩ×1μA=500(V). The error for the rated value of 100KV will be 0.5%. As a result, the ion beam on the sample 11 deviates from the optimal focusing conditions and becomes out of focus. A similar problem may also occur with the condenser lens 7.

The present invention has been made in view of these points, and its purpose is to provide a focused ion beam that does not deviate from the optimum focusing conditions of the ion beam even if a minute discharge occurs in the electrostatic lens section. The purpose is to realize the device.

(Principle of operation) In order to solve the above-mentioned problems, it is sufficient to reduce the number of micro-discharges, but the reality is that it is currently impossible to completely eliminate micro-discharges. Therefore, it is necessary to consider countermeasures on the premise that minute discharges will occur. One of the countermeasures is the method described below. That is, even if a micro discharge occurs and the load current increases, the voltage applied to each electrostatic lens may be prevented from changing. In order to satisfy these conditions, it is necessary to use a voltage divider or a high voltage generating power source with small internal resistance.

(Means for Solving the Problems) The present invention, which solves the above problems, accelerates an ion beam emitted from an ion source, focuses it with an electrostatic lens, and then produces a focused ion beam that irradiates a target surface. In the device, as an accelerating voltage generating circuit, a Cottcroft-Walton circuit is used, in which series circuits of series-connected capacitors are arranged in parallel, and rectifier diodes are cross-connected between each stage of capacitors in both series circuits. A series circuit of diodes connected in series is inserted between the stages of the series circuit of the capacitors in the Kotscroft-Walton circuit at the same position so that the cathodes are butted, and the electric potential is supplied to the electrostatic lens through the series circuit of the diodes. A Kotscroft-Walton circuit is used as an accelerating voltage generation circuit from the cathodes of diodes constituting the circuit, and a diode series circuit in which diodes are connected in series so that the cathodes are butted is provided between the same number of stages in the Kotscroft-Walton circuit. The electric potential is supplied to the electrostatic lens directly from the cathode of the diode without using a resistor.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is an electrical circuit diagram showing an embodiment of the present invention realized based on the above-described principle. The circuit shown in the figure shows a specific configuration of a portion corresponding to the accelerating voltage generation circuit 1 of FIG. 2, and the configuration of FIG. 2 is used as is for the other portions. In the figure, 20 is a well-known Kotscroft-Walton circuit constructed by connecting diodes and capacitors in multiple stages. As shown in the figure, the Kotscroft-Walton circuit consists of capacitors C ₁ to Cn (n is an integer, the same applies hereinafter) and C ₁ ′ to
A rectifier diode D ₁ is connected between each stage of Cn′ as shown in the figure.
~Dn, D ₁ ′ ~ Dn′ cross-connected,
Used as a high voltage doubler rectifier circuit.

21 and 22 are high frequency power supplies that generate high frequency high voltages whose phases are different by 180 degrees. The high frequency power supply 22 is connected to one end of the series circuit of capacitors C ₁ to Cn.
The high frequency power supply 21 is connected to one end of the series circuit of capacitors C ₁ ' to Cn', respectively. D _c1 , D _c1 ′ are diodes that take out the high-pressure rectified output from the third stage column of the Kotscroft-Walton circuit 20; D _c2 ,
Similarly, D _c2 ' is a diode Dck that takes out the high-pressure rectified output from the sixth stage column, and Dck' is a diode that takes out the high-pressure rectified output from the k-th stage (top stage). The high voltage taken out from the top stage of the Kotscroft-Walton circuit 20 is applied to a multistage accelerator tube (see 5 in FIG. 2), and is further negatively fed back to the output voltage control circuit 23 via a resistor R. The output voltage control circuit 23 controls the output amplitudes of the high frequency power supplies 21 and 22 so that the output of the Cottcroft-Walton circuit 20 is constant. Cm ₁ ~ Cm ₆ ,
Cm ₁ ′ to Cm ₆ ′ are capacitors C ₁ to C ₆ , C ₁ ′ to
These are capacitors connected in parallel to C ₆ ' to increase the capacity. The operation of the circuit configured as described above will be explained as follows.

Kotscroft-Walton circuit 20 shown in the figure
When a high frequency high voltage is applied from the high frequency power sources 21 and 22, respectively, the Kotscroft-Walton circuit 20 generates a high voltage at the top stage corresponding to the number of stages. Partial voltages of this high voltage are taken out from the third column and the sixth column, respectively, and directly applied to the condenser lens and the objective lens, respectively. These applied voltages are, for example, 50 KV and 100 KV as described above.

Now, let's find the internal resistance R ₆ of the voltage divider circuit taken out from the 6th stage of this Kotscroft-Walton circuit 20. Generally, the number of stages of the Kotscroft-Walton circuit 20 as shown in the figure is N, and the high frequency power supply is 2.
When the frequency of 1 and 22 is f and the capacitance of the capacitor is C, the internal resistance R _N is expressed by the following equation.

R _N =1/(f・C)×{(N ³ /4) −(N ² /2)+(N/2)} (1) N=6, f=20KHz, capacitor C ₁ ~Cn,
If the capacities of C ₁ ′ to Cn ′, Cm ₁ to Cm ₆ , and Cm ₁ ′ to Cm ₆ ′ are all 4000PF, then from the above formula, N=6 and 6
The voltage dividing resistance R ₆ of the stage is calculated as follows: R ₆ =1/(2×10 ⁴ ×8×10 ⁻⁹ )×(6 ³ /4−6 ² /2+6/2) = 244 (KΩ). Therefore, if a minute discharge occurs in the objective lens of the 6th stage, and a discharge current of 1 μA flows at its peak, the amount of fluctuation voltage will be 244KΩ x 1μA = 0.244 (V), and the error with respect to the rated value of 100KV can be ignored. . In a focused ion beam device using a liquid metal field emission ion source, the half-width of the initial velocity distribution of ions generated from the ion source is usually 10 eV.
The voltage fluctuation caused by the discharge current of about 1 μA that occurs in each electrostatic lens is about 0.2 to 0.3 V at most, and is not a problem at all. (not shown) functions as a feedback resistor to feed back the output voltage.
That is, the top output of the Kotscroft-Walton circuit 20 is given to the output voltage control circuit 23. The output voltage control circuit 23 performs negative feedback control to compare the input actual output voltage with a reference voltage and control the output amplitudes of the high frequency power supplies 21 and 22 so that the output voltage becomes equal to the reference voltage.

Capacitors Cm ₁ to Cm ₆ are connected in parallel to both ends of capacitors C ₁ to C 6 and C _{1 ′} to C _{6 ′} , respectively.
Cm ₁ ′ to Cm ₆ ′ are used to increase the value of the capacitance C in equation (1) and to reduce the value of the internal resistance R _N of the voltage dividing circuit as a whole. Capacitors C ₁ to _{C 6} and
If the capacitances of C ₁ ′ to C ₆ ′ are increased in advance, it is not necessary to provide such parallel connection capacitors.

In this way, the high voltage output applied to the multistage accelerator tube is controlled to be constant, but the output voltage of the power supply section to the condenser lens and objective lens is not controlled by negative feedback. However, as mentioned above, the internal resistance of the Kotscroft-Walton circuit is 200KΩ~
It is as low as 300KΩ, and even with a minute discharge of about 10μA, the voltage fluctuation is about 2 to 3V, so in an ion beam device that accelerates and focuses ions with an acceleration distribution of about 10eV, it has almost no effect on the focusing effect. .

(Effects of the Invention) As explained in detail above, in the present invention, as an accelerating voltage generation circuit, a series circuit of series-connected capacitors is arranged in parallel, and a rectifier diode is installed between each stage of the capacitors of both series circuits. Using a cross-connected Kotscroft-Walton circuit, inserting and connecting a series circuit of diodes connected in series so that their cathodes butt up between stages at the same position of the series circuit of the capacitors in the Kotscroft-Walton circuit, Since the electric potential is supplied to the electrostatic lens directly from the cathode of the diode constituting the diode series circuit without going through a resistor, the internal resistance of the voltage divider circuit that feeds the electric potential to the electrostatic lens is reduced. becomes smaller, and even if a minute discharge occurs in the electrostatic lens, voltage fluctuations can be reduced. Therefore, it is possible to realize a focused ion beam device in which the optimum focusing conditions of the ion beam are always maintained.

[Brief explanation of drawings]

FIG. 1 is an electric circuit diagram showing an embodiment of the present invention;
FIG. 2 is a diagram showing an example of the configuration of a conventional device, and FIG. 3 is a diagram showing the trajectory of an ion beam. 1... Accelerating voltage generation circuit, 2... Emitter, 3
...Extraction electrode, 4...Power source, 5...Acceleration tube,
6, 12... Voltage divider, 7... Condenser lens,
8... Mass separator, 9... Objective lens, 10...
Deflector, 11... Sample, 20... Cottcroft-Walton circuit, 21, 22... High frequency power supply,
23...Output voltage control circuit.

Claims (1)

[Claims] 1. In a focused ion beam device that accelerates an ion beam emitted from an ion source, focuses it with an electrostatic lens, and then irradiates the target surface, a series circuit of series-connected capacitors is connected in parallel as an accelerating voltage generation circuit. A Kotscroft-Walton circuit is used in which a rectifier diode is cross-connected between each stage of the capacitors in both series circuits, and a cathode is connected between stages at the same position in the series circuit of the capacitors in the Kotscroft-Walton circuit. A diode series circuit connected in series is inserted and connected such that the two diode series circuits face each other, and the electric potential is supplied to the electrostatic lens directly from the cathode of the diode constituting the diode series circuit without going through a resistor. A focused ion beam device featuring: