JPH07153410A - Charged particle beam device - Google Patents

Abstract

PURPOSE:To always focus charged particle beams to fix a surface charge at a minimum state by changing a voltage of an extraction electrode when an irradiated area is changed, and interlocking with this, controlling a coil current of an objective lens. CONSTITUTION:Charged particle beams 102 are radiated from a charged particle gun and pass inside of an objective lens 105 and an extraction electrode 106 directly before irradiating an IC 110 to be measured. In order to focus the charged particle beams 102 in interlocking with variation of an irradiated area to be swept, a magnetic field corresponding to each speed of the charged particle beams 102, namely, an optimum refractive index is imparted by the objective lens 105. A voltage of an extraction electrode control voltage source 40 is read by a current control part 50, and a current of the objective lens 105 suitable for this voltage is determined and the current value thereof is indicated to an objective lens current source 60. Consequently, a clear and high quality picture can be obtained. Since it is excellent in reproducibility, a measuring effect is remarkable.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charged particle beam apparatus used when designing and developing a semiconductor integrated circuit (hereinafter referred to as "IC"), and more particularly to a device under test I to be measured.
The present invention relates to a technique for preventing surface charging of C.

[0002]

2. Description of the Related Art In general, in order to analyze the inside of a prototype IC, a defective part is specified by operating a charged particle beam device and referring to a voltage waveform flowing through each part of a prototype device.

FIG. 3 shows an example of a lens barrel 100 of a conventional EB tester. The interior of the lens barrel 100 is a vacuum chamber, which is evacuated by a turbo molecular pump or an ion getter pump, and the degree of vacuum is on the order of 10 ⁻⁴ Pa to 10 ⁻⁵ Pa. The high-energy pulsed charged particle beam 102 emitted from the charged particle gun 101 is decelerated by the electrostatic lens 103 and the electromagnetic lens 104 without losing the brightness. Then, the objective lens 105 focuses on the IC 110 to be measured with a spot having a diameter of 0.1 μm. Further, the charged particle beam 102 is deflected by scanning within a range of 2 mm × 2 mm, and sweeps and irradiates the measured IC 110.

The IC 110 to be measured emits secondary electrons by sweeping irradiation of the pulsed charged particle beam 102.
The amount of secondary electrons depends on the magnitude of the electric potential at the irradiated point. The secondary electrons are efficiently captured by the secondary electron extraction electrode 106 and detected by the secondary electron detector 107. The secondary electron detector 107 once converts the secondary electrons into a light signal, and thereafter, a potential contrast image (SEM image:
Scanning Electron Microscope, SIM image: Scanning I
on Microscope) to convert to an electrical signal for display. The brightness of the potential contrast image changes according to the amount of secondary electron flow.

To observe the waveform, the voltage is sampled by a pulse beam created in synchronization with the signal of the IC 110 to be measured, as in the sampling oscilloscope. It is measured as a waveform by changing the generation timing of this pulse beam on the phase of the waveform to be measured.

FIG. 4 shows an enlarged view of the IC to be measured 110, the secondary electron extraction electrode 106, and the objective lens 105. The IC 110 to be measured is made of SiO 2 to protect the electrode 111.
It is covered with an insulating film such as ₂ (silicon oxide).
When the insulating film 112 is irradiated with the charged particle beam 102, secondary electrons 108 are emitted. When the emitted charges of the secondary electrons 108 are equal to the charges of the incident charged particle beam 102 and cancel each other out, the surface of the insulator 112 is not charged, but the acceleration voltage of the charged particle gun 101 is about 1 KV, and the insulating film 112 has SiO ₂ or In the case of SiN or the like, since the secondary electron emission rate is 1 or more, the surface of the insulating film 112 is positively charged. When the charged IC to be measured 110 is observed and measured, the emission of secondary electrons fluctuates, which causes many difficulties not only in quantitative characteristic tests but also in qualitative tests. The amount of charge on the surface of the insulating film 112 depends on the amount of incident charged particle beam, accelerating voltage, irradiation area,
Irradiation scanning speed changes intricately depending on secondary electron extraction electric field strength, type of insulator, and the like.

In the charged particle beam device, this surface charge 11
Irradiated area (equivalent to magnification) to minimize the charging of 3
And the secondary electron extraction electric field strength, and other specifications were kept constant. That is, when the magnification is changed, the voltage of the secondary electron extraction electrode 106 is automatically changed to set the secondary electron extraction electric field strength to the optimum value. However, when the voltage of the secondary electron extraction electrode 106 is changed, the focal point of the charged particle beam 102 is deviated as shown in b and c as shown in FIG.

[0008]

SUMMARY OF THE INVENTION The present invention provides a charged particle beam device in which the focus of the charged particle beam is focused on the surface of the IC to be measured when the magnification is changed and the voltage of the extraction electrode is changed accordingly. The purpose is to provide.

Here, the potential distribution on the front surface of the IC to be measured when the charged particle beam is irradiated on the IC to be measured will be considered once again. Initially, the secondary electron extraction voltage Ve of the extraction electrode
Is positive and the insulating film potential Vs of the IC to be measured is zero,
Only the accelerating electric field for secondary electrons exists on the front surface of the IC surface to be measured. When irradiation of the charged particle beam is performed here,
For the above reason, the irradiated area on the insulating film is positively charged. As the amount of charge increases, the electric field in front of the irradiation area reverses from positive to negative. However, further away from the illuminated area by the front surface, the electric field becomes positive again. That is,
A potential saddle point occurs here. The position and potential of this potential saddle point strongly depend on the irradiation region, its charge amount, and the magnitude of the extraction electric field strength.

In FIG. 5, the electrode width is 2a and the potential Vs is 5
It is an example of a potential distribution diagram in the case of V (volt) and 0 V except for electrodes. Consider this potential distribution map. The surface charging makes the potential Vs of the irradiation region positive, and the potentials on the Z and X planes when the extraction electric field strength is E [V / mm] can be expressed by the following equation. [Japan Society for the Promotion of Science Application of charged particle beam to industry 132nd Committee: Electron Beam Testing Handbook Electron Beam Research Volume 7: May 1987] Φ (z, x) = E * z + Vs / π * [ tan ^-1 {(ax) / z} + tan
^-1 {(a + x) / z}] where the position coordinates of the potential saddle point are (0, Zm) and the potential is Φ
If m, then Zm = {(2aVs / πE) −a ² } ^1/2 Φm = E * Zm + (2Vs / π) * tan ⁻¹ (a / Zm), which is shown in the graph of FIG. . Φm and Vs
Becomes a potential barrier and acts to suppress secondary electrons.

The following is obvious from FIG. When the secondary electron extraction electric field strength is constant, the height of the potential barrier becomes smaller as the irradiation area becomes larger. That is, the lower the magnification, the smaller the height of the potential barrier. As the height of the potential barrier is smaller, the secondary electrons emitted from the irradiation region are more likely to be taken into the extraction electric field, so that the amount of charge on the surface of the irradiation region further increases. That is, when the secondary electron extraction electric field strength is constant, the height of the potential barrier depends on the irradiation area, and thus the surface charge amount increases as the irradiation area increases (lower magnification).

In order to avoid the disadvantage that the surface charge increases as the magnification becomes lower, the secondary electron extraction electric field may be made smaller as the magnification becomes lower automatically in association with the change in magnification. As a result, it is possible to prevent the amount of charge from increasing as the irradiation area increases while keeping the height of the potential barrier at the same level. However, at a high magnification higher than a specific magnification, the extraction electric field strength is set to a constant value in order to avoid problems (change in matching between CAD layout and SEM image, shift of probe points, etc.) that occur when the extraction electric field strength is changed. It is realistic to keep.

Along with this change in the extraction electric field strength, the lens action due to the electric field changes, and a new problem arises in that the focus of the charged particle beam cannot be determined. This is a problem because the amount of charge on the surface of the irradiation region changes depending on the degree of focus. The present invention provides a charged particle beam device that focuses even if the voltage of the extraction electrode is changed.

[0014]

In order to achieve the above object, the present invention automatically changes the voltage of the extraction electrode in response to the change of the magnification (corresponding to the irradiation area) and changes the extraction electric field strength. At the same time, the coil current of the objective lens is controlled so that the charged particle beam is focused in conjunction with the voltage of the extraction electrode.

According to the structure of the present invention, the relationship between the voltage of the secondary electron extraction electrode and the coil current of the objective lens on which the charged particle beam is focused is clarified, and linked with the voltage of the secondary electron extraction electrode. Since the coil current of the objective lens is controlled,
The charged particle beam is always in focus, so the surface charge is always constant in the minimum state and the measurement error is almost eliminated.

[0016]

FIG. 1 shows an embodiment of the present invention. Portions corresponding to those in FIG. 4 are designated by the same reference numerals. The present invention includes, in addition to the conventional charged particle beam device, an objective lens current source 60 for the objective lens 105, a current control unit 50 for controlling the objective lens current source 60, and the extraction electrode 10 for the current control unit 50.
The extraction electrode control voltage source 40 for instructing the changing voltage of No. 6 is used.

As shown in FIG. 1, the charged particle beam is emitted from the charged particle gun and passes through the objective lens 105 and the extraction electrode 106 immediately before irradiating the IC 110 to be measured.

As shown in FIG. 2A, the secondary electron extraction electrode 106 is interlocked with the irradiation area in order to minimize the charging of the surface of the IC 110 to be measured even if the irradiation area is changed, as described above. Then, the voltage is changed to optimize the extraction electric field strength. Changing the voltage of the secondary electron extraction electrode 106 changes the accelerating voltage of the charged particle beam 102. Therefore, the velocity of the charged particle beam changes, and the focus shifts.

Therefore, it is preferable that the objective lens 105 gives a magnetic field corresponding to each velocity of the charged particle beam 102, that is, an optimum refractive index so that the charged particle beam 102 always focuses. That is, as shown in FIG. 2B, the coil current of the objective lens 105 is controlled in accordance with the change in the voltage of the secondary electron extraction electrode 106 so that the objective lens 105 is always focused.

In the present invention, the current control unit 50 reads the voltage of the extraction electrode control voltage source 40, determines the current of the objective lens 105 suitable for the voltage, and instructs the current value to the objective lens current source 60. The objective lens variable current source 60 is a device configured to flow a specified current to the objective lens 105. Here, since the optimum current value differs depending on the structure of the device, it needs to be determined depending on the model. That is, the number of turns of the coil of the objective lens 105, the secondary electron extraction electrode 106
And the distance from the surface of the IC 110 to be measured and the type of the insulating film 112. Also, since it varies depending on the pulse width of the charged particle beam 102, the sweep irradiation speed, the acceleration voltage, etc.,
What can be fixed should be a fixed value, and the variable number should be small. Therefore, the optimum current value may be formed into a table based on a variable number and controlled according to the table, or the value is linear within the section as shown in FIG. It may be converted into a control and controlled by calculation.

[0021]

As described above, in the charged particle beam apparatus according to the present invention, the surface charge 11 of the IC to be measured 11 is measured.
3 is minimized, and the charged particle beam 102 is always in focus, so that a clear and high-quality screen can be obtained and the reproducibility is excellent, so that the measurement effect is great.

[Brief description of drawings]

FIG. 1 is a diagram of an embodiment of the present invention.

FIG. 2 is an explanatory view of the embodiment shown in FIG.

FIG. 3 is a diagram illustrating an example of a conventional main technique.

FIG. 4 is an enlarged view of a part of the conventional example shown in FIG.

FIG. 5 is a potential distribution diagram.

FIG. 6 is a relationship diagram between a potential barrier and an irradiation region.

[Explanation of symbols]

 40 Extraction Electrode Control Voltage Source 50 Current Control Unit 60 Objective Lens Current Source 100 Lens Barrel 101 Charged Particle Gun 102 Charged Particle Beam 103 Electrostatic Lens 104 Electromagnetic Lens 105 Objective Lens 106 Secondary Electron Extraction Electrode 107 Secondary Electron Detector 108 secondary electron 110 IC to be measured 111 electrode 112 insulating film 113 surface charge

Claims (1)

[Claims] 1. A charged particle beam device for sweeping and irradiating a surface of a semiconductor integrated circuit with a charged particle beam, and detecting secondary electrons generated at each irradiation point to analyze the semiconductor integrated circuit. The extraction electrode control voltage source (4) for changing the voltage of the secondary electron extraction electrode (106) in conjunction with
0), a current control unit (50) for controlling the coil current of the objective lens (105) in association with a change in the voltage of the extraction electrode control voltage source (40), and an objective lens according to instructions from the current control unit (50). (105)
An objective lens current source (60) for changing the coil current of
A charged particle beam device comprising: