JP2003297272A - Electron beam apparatus and method of manufacturing device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make shot noise small as in the case of an electron tube and to obtain stable high brightness by using a thermionic emission cathode. <P>SOLUTION: The electron beam apparatus consists of an electron gun 1a with the thermionic emission cathode 1, a condenser lens 12, an NA opening 13 by which an orbit of a primary electron beam is restricted, an objective lens 19, and detectors (23, 24, 25, and 26) which detect secondary electron beams emitted from a sample 20 to which negative voltage has been impressed. The electron gun 1a is operated on space-charge limiting conditions, and the opening 13 is arranged on the sample 20 side of the condenser lens 12. Or the electron gun 1a is operated on conditions in which the brightness depends on an electron gun current greatly, and the feedback control of voltage of a Wehnelt electrode 4 of the electron gun is carried out so that the electron gun current may become an approximately constant value. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention irradiates a sample such as a wafer having a pattern with a minimum line width of 0.1 μm or less with a primary electron beam and detects a secondary electron beam emitted from the sample, The present invention relates to an electron beam apparatus that evaluates a sample based on a secondary electron image obtained in this way, and a device manufacturing method using the electron beam apparatus.

[0002]

2. Description of the Related Art In order to detect defects in a sample such as a semiconductor wafer or a mask, a probe of a primary electron beam which is narrowed down is used to scan the sample to detect secondary electrons generated from the sample by a detector. An electron beam apparatus that detects defects with high resolution and high throughput is known. In such an electron beam apparatus, an electron gun is generally used as a primary electron beam source.

An FE, TFE or Schottky cathode is generally used for the electron gun of the electron beam apparatus for the above-mentioned application, but it is preferable to reduce noise and obtain high brightness.

On the other hand, in electron tube technology, it is known that shot noise is reduced by using the cathode in the space charge limited region. Further, in the electron gun having a L _a B ₆ cathode, to obtain a high luminance, a method for increasing the cathode current density is known.

[0005]

However, in the FE, TFE, and Schottky emission type electron guns described above, the shot time is large, and in order to obtain a signal with a good S / N ratio, the pixel time is increased with a large beam current. I needed to make it longer.

Further, in an electron gun having a thermionic emission cathode, it is necessary to raise the cathode temperature and increase the cathode current density in order to increase the brightness. In this case,
Since the brightness is operated under the condition that it largely depends on the electron gun current, the brightness is likely to be unstable.

The present invention has been made in view of the above facts, and an object of the present invention is to provide means for reducing shot noise as in the case of an electron tube and stably obtaining high brightness with a thermionic emission cathode. To do.

Still another object of the present invention is to provide a device manufacturing method which improves the inspection accuracy and throughput by inspecting a semiconductor device which is in the middle of manufacturing or a finished product by using the electron beam apparatus. And

[0009]

In the present invention, the electron gun is operated under the space charge limiting condition as one means for reducing shot noise. First, the reason why the shot noise is reduced under the space charge limiting condition and the method of maintaining the electron gun under the space charge limiting condition will be described.

When the state of the electron gun is determined by the cathode temperature, that is, when the electron gun is operating in the temperature limited region, the shot noise i _n emitted by the electron gun is represented by the following equation ("Communications Society of Japan" Engineering Handbook "P.471 (1957)
Year))).

I _n ² = 2eIpB _r (1) In the equation (1), i _n ² is the root mean square value of the noise current, e is the electron charge, I _p is the anode direct current, and B _f is It is the frequency band of the signal amplifier.

On the other hand, when the electron gun is in the space charge limited region, i _n ² = Γ ² 2e · Ip · B _r (2) In the equation (2), Γ ² is a reduction coefficient and a value smaller than 1.

When the electron gun operates under the space charge limiting condition in this manner, a potential valley called a virtual cathode is formed near the cathode. Only thermal electrons having energy exceeding the potential valley (barrier for electrons) become the electron gun current. If a large amount of current flows for some reason, this valley becomes deep and it becomes difficult for current to flow. That is, negative feedback will be applied. This is the reason why shot noise is reduced.

When the cathode temperature is sufficiently high, Γ ² is
The minimum is about 0.018, and the noise current is reduced to 13% of that in the temperature limited region. Assuming that the S / N ratio in this case is secondary electrons≈primary electrons, S / N = I _p / {Γ (2e · I _p · B _f ) ^1/2 } = 1 / Γ · { I _p / (2e · B _f )} ^1/2 = n ^1/2 / (Γ · 2 ^1/2 ) (3) When Γ = 0.13, the following S / N ratio is obtained from the equation (3).

S / N = 7.5 (n / 2) ^1/2 (4) (n: number of secondary electrons / pixel) That is, the electron gun operating in the space charge limited region is an electron in the temperature limited region. This is equivalent to detecting the number of secondary electrons per pixel by 55 times (= 1 / Γ ² = 1 / 0.13 ² ) more than that of a gun (TFE). Since the latter has a luminance that is about two orders of magnitude higher than the former, assuming the same beam diameter, the latter can obtain a beam current that is two orders of magnitude larger than the former, but the S / N ratio is 1/55 in comparison with the former. In other words, the electron gun in the space charge limited region is
The measurement time is 100 / 55≈1.8 times, but the dose is 1/55.

Whether the electron gun is operating in the space charge limited region can be checked by the method described below with reference to FIG. FIG. 3A shows the relationship between the electron gun current and the cathode heating current. In the figure, area P
Is a region where the electron gun current hardly increases even if the cathode heating current is increased, and this region P is the space charge limited region.

Further, FIG. 3B shows the relationship between the electron gun current and the anode voltage. In the figure, area Q
Is a region where the electron gun current increases rapidly when the anode voltage is increased, and this region Q is also a space charge limited region.

From the above, the electron gun current is measured by increasing the cathode heating current of the electron gun, and the region P where the electron gun current is saturated or the anode voltage of the electron gun is increased to increase the electron It is possible to determine that the electron gun is operating in the space charge limited area if the gun current is measured and the area Q is where the electron gun current is changing rapidly. Therefore, conditions for operating the electron gun in the space charge limited region can be set.

Therefore, the first aspect of the present invention is to provide an electron gun having a thermionic emission cathode, an optical system for imaging a primary electron beam, and one or more for limiting the trajectory of the primary electron beam. An electron beam apparatus equipped with a detector for detecting a secondary electron beam, the electron gun being operated under a space charge limited condition, and the opening disposed closest to the electron gun. At least one lens is provided between the electron gun and the electron gun.

In the first aspect, in addition to the operation of the electron gun under the space charge limited condition, at least one lens is provided between the electron gun and the opening arranged closest to the electron gun. It was provided with steps. Therefore, the electron reflected from the aperture back to the electron gun is bent by the lens,
The amount returning to the electron gun is extremely small, and the virtual cathode formed in the vicinity of the cathode is less disturbed, so that the increase of shot noise can be prevented.

The second aspect of the present invention is to emit electrons from an electron gun having a thermionic emission cathode, a condenser lens, an aperture for limiting the trajectory of a primary electron beam, an objective lens, and a sample to which a negative voltage is applied. An electron beam apparatus comprising a detector for detecting a secondary electron beam, wherein an electron gun is operated under a space charge limiting condition, and the opening is arranged on the sample side of the condenser lens. And

Also in the second mode, in addition to the operation of the electron gun under the space charge limiting condition, the electrons reflected from the aperture to return to the electron gun are bent by the condenser lens, so that the electron gun returns to the electron gun. The quantity is very small,
Since the virtual cathode formed near the cathode is less disturbed, it is possible to prevent an increase in shot noise. In addition, since the sample is applied with a negative voltage, the secondary electrons generated from the sample are strongly accelerated and focused in the optical axis direction, and as a result, most of the secondary electrons are absorbed in the optical axis of the objective lens. Pass through. Thus, 100 secondary electrons
%, A signal with a good S / N ratio is obtained by the detector.

A third aspect of the present invention is an electron gun having a thermionic emission cathode and a control electrode, a condenser lens, and 1.
An aperture that limits the trajectory of the next electron beam, an objective lens, and
An electron beam apparatus comprising a sample to which a negative voltage is applied, wherein the electron gun is operated under the condition that its brightness largely depends on the electron gun current, and the electron gun current is set to a substantially constant value. The voltage of the control electrode of the electron gun is feedback controlled.

In the third mode, the voltage of the control electrode is feedback-controlled even in a high luminance state (region Q in FIG. 3B) that varies greatly depending on the electron gun current, so that the voltage is stable. High brightness can be obtained. If a negative voltage is applied to the control electrode, it becomes more difficult for the electrons emitted from the cathode to return to the electron gun side, and it is possible to prevent an increase in shot noise.

[0025] Preferably, the thermionic emission cathode is characterized in that its tip has a shape of a part of a spherical surface. As a result, higher brightness can be obtained. If a surface with a minimum axial potential called a virtual cathode is formed near the cathode, this surface serves as a barrier for electron emission, negative feedback is applied, and shot noise is reduced.

The electron beam apparatus of the present invention is an insulating material (eg, glass, insulating spacer, etc.) that supports each electrode of the electron gun.
Preferably, the insulating material is shielded with a metal or a metal film so as not to be directly viewed from the electron beam passage. As a result, even if the surface of the insulator is charged, its potential is prevented from leaking to the cathode and the beam passage, so that the potential of the virtual cathode is not disturbed, and thus shot noise can be prevented from increasing. .

The device manufacturing method of the present invention is characterized by evaluating the sample as a wafer during the wafer process or after the completion of at least one wafer process by using the electron beam apparatus of each of the above aspects.

Other advantages and effects of the present invention will be further clarified by the following description.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings. (First Embodiment: Electron Beam Device) FIG. 1 shows a schematic configuration of an electron beam device according to a first embodiment of the present invention.

The electron gun 1a of the electron beam apparatus shown in FIG. 1 has an L _a B ₆ single crystal cathode 1 having a spherical tip with a radius of curvature of 30 μmR, _a graphite heater 2, a support metal 3, a Wehnelt electrode 4, an anode 11, and a cathode spacer 6. , Wehnelt spacer, shield plates 8, 9, 10 and
It consists of an electron gun chamber.

The electron beam 14 emitted from the electron gun 1a is
It diverges without forming a crossover, is focused by the condenser lens 12, and is narrowed down by the NA aperture 13.
Form a crossover. Further, the electron beam 14 forms a second crossover image on the sample 20 with the objective lens 19.

Further, the electron beam 14 narrowed down is supplied to the sample 20.
A deflector 16 is provided for scanning above. The deflector 16 is set such that electrostatic deflection and electromagnetic deflection are in opposite directions. Further, an E × B equipped with an electrostatic deflector 17 and an electromagnetic deflector 18 is provided after the deflector 16.
A separator is provided. The electron beam 21 deflected by the deflector 16 is deflected by the electrostatic deflector 17 to the right with a strength of 1, and is deflected by the electromagnetic deflector 18 to the left with a strength of 2, and the objective lens 19 is just moved. Is deflected so as to pass through the center of the lens, and is deflected on the sample 20 in the −x direction as shown by the scanning line 27. For high speed scanning, deflector 1
6 and 17 provide a long field of view in the y direction (scan line 27)
Is biased to.

Since −4 kV is applied to the sample 20, the secondary electrons emitted from the area of the scanning line 27 are strongly accelerated in the z-axis direction and are focused, so that a narrow beam is formed. Thus, the secondary electrons emitted in all directions pass almost off the axis of the objective lens 19, are bent to the right by the focusing action of the objective lens, and are deflected by the deflector group 1
It is incident on 7 and 18. At this time, since the electrostatic deflector 17 and the electromagnetic deflector 18 also deflect the secondary electron beam to the right, the secondary electron orbit becomes 22, enters the MCP (micro channel plate) 23, and is multiplied there, and the Faraday cup is generated. Absorbed at 24. The current flowing through the Faraday cup 24 is converted into a voltage by the resistor 25, amplified by the amplifier 26, converted into a digital signal, and then taken into a calculator (not shown). The computer forms a secondary electron image from the digital signal and evaluates the presence or absence of defects in the sample 20 based on the image.

A virtual cathode 5 is formed near the cathode 1 of the electron gun 1a. A surface having a minimum axial potential is formed on the virtual cathode 5, which is a condition for reducing shot noise. The electron beam emitted from this virtual cathode is the Wehnelt 4
Since a negative bias is applied to the anode, it does not enter the Wehnelt, and since the hole of the anode 11 has a diameter sufficiently larger than the beam diameter, it does not enter the anode. Thus, the reflected electrons do not return to the virtual cathode and disturb its potential, thus not increasing shot noise.

Further, the electrons that have entered the NA aperture 13 emit reflected electrons toward the cathode side. However, since there is the lens 12 between the NA aperture 13 and the electron gun 1a, the reflected electrons are orbited. As shown in, the amount of bending back by the lens 12 and consequently back to the cathode 1 is negligible. Thus, the reflected electrons return to the virtual cathode and disturb the potential thereof almost completely, so that shot noise is not increased.

Further, even if the surfaces of the insulating spacer 6 and the insulating glass 7 are charged, a shield for shielding the potential between the cathode and the beam passage so that the potential does not affect the cathode and the beam passage. Since 8, 9, 10 are provided, the potential of the virtual cathode 5 is not disturbed, and therefore shot noise is not increased.

FIG. 2 shows a single crystal L _a B _{6 having a} (100) orientation.
The brightness of the electron beam emitted from the electron gun is shown when the electron gun current is changed by the Wehnelt bias when the cathode is operated at 1800 ° K and the simulation is performed. As shown in FIG. 2, the luminance exceeds 1 × 10 ⁸ A / cm ² Sr in the electron gun current range of 775 μA to 935 μA.

Conventionally, the brightness of the L _a B ₆ electron gun has a limit value of Langmuir, and at a normal cathode temperature and an acceleration voltage of 100 kV or less, 1 × 10 ⁷ A / cm ²
It has been considered that it will never exceed Sr. However, the above-mentioned limit value is significant within a range where a so-called optical model holds, in which the product of the crossover diameter and the beam emission angle is constant. Therefore, when the optical model does not hold and the model is close to the laminar flow model, the inventor interprets that the brightness can be increased by focusing the beam without an upper limit.

In FIG. 2, when it is desired to use a brightness of 1 × 10 ⁸ A / cm ² Sr, it can be used with two electron gun currents of I _e = 775 μA and 935 μA. The inclination is slightly smaller on the high electron gun current side. Still, since the inclination is steep, if the electron gun current changes even a little, the brightness changes, and thus the beam current is likely to change. Therefore, as shown in FIG. 1, the cathode current is measured at 28 as the voltage of the resistor 30 and is fed back to the Wehnelt power supply 29 so that the electron gun current is controlled to be constant.

(Second Embodiment: Semiconductor Device Manufacturing Method) In this embodiment, the electron beam apparatus shown in the above embodiment is applied to wafer evaluation in a semiconductor device manufacturing process.

An example of the device manufacturing process will be described with reference to the flowchart of FIG. This manufacturing process example includes the following main processes. Wafer manufacturing process for manufacturing wafer 20 (or preparation process for preparing wafer) (step 100) Mask manufacturing process for manufacturing mask used for exposure (or mask preparing process for preparing mask) (step 1)
01) Wafer processing step for performing necessary processing on the wafer (step 102) Chip assembling step for cutting out the chips formed on the wafer one by one and making them operable (step 1)
03) Chip inspection step of inspecting assembled chip (step 104) Each step further comprises some sub-steps.

Among these main processes, the main process that has a decisive influence on the performance of the semiconductor device is the wafer processing process. In this step, the designed circuit patterns are sequentially stacked on the wafer to form a large number of chips that operate as memories and MPUs. This wafer processing step includes the following steps. Thin film forming process (using CVD, sputtering, etc.) to form a dielectric thin film to be an insulating layer, a wiring part, or a metal thin film to form an electrode part. Oxidation process to oxidize a formed thin film layer or wafer substrate. Lithography process that forms a resist pattern using a mask (reticle) to selectively process a wafer substrate Etching process that processes a thin film layer or substrate according to a resist pattern (for example, using a dry etching technique) Ion / impurity implantation Diffusion Step Resist Stripping Step Inspection Step for Inspecting Processed Wafer In addition, the wafer processing step is repeated by the required number of layers to manufacture a semiconductor device that operates as designed.

The lithographic process which is the core of the wafer processing process is shown in the flow chart of FIG. This lithography step includes the following steps. A resist coating step (step 2) of coating a resist on the wafer on which the circuit pattern is formed in the previous step.
00) Exposure step of exposing the resist (step 201) Development step of developing the exposed resist to obtain a resist pattern (step 202) Annealing step for stabilizing the developed pattern (step 203) Semiconductors above Well-known processes are applied to the device manufacturing process, the wafer processing process, and the lithography process.

In the above wafer inspection process, when the evaluation apparatus according to each of the above embodiments of the present invention is used, even a semiconductor device having a fine pattern can be evaluated with high throughput and high accuracy. It is possible to improve the yield and prevent the shipment of defective products.

Although the above-mentioned embodiments have been described above, the present invention is not limited to the above-mentioned examples, and can be suitably modified within the scope of the present invention. For example, although a semiconductor wafer is given as an example of the sample 20, the invention is not limited to this.
An arbitrary sample, such as a mask, on which a pattern or the like in which a defect can be detected by an electron beam is formed can be an evaluation target.

[0046]

As described in detail above, according to the electron beam apparatus of the present invention, the following excellent effects can be obtained. 1. The electron gun is operated under space charge limiting conditions,
At least one lens is provided between the electron gun and the aperture closest to the electron gun, or the aperture is arranged on the sample side of the condenser lens, so that backscattering occurs at the aperture. The generated reflected electrons are less likely to return to the virtual cathode, and shot noise can be significantly reduced. 2. Since the electron gun is operated under the condition that its brightness largely depends on the electron gun current, and the voltage of the control electrode of the electron gun is feedback-controlled so that the electron gun current has a substantially constant value, a high brightness is obtained. Can be used stably. 3. Since the tip of the thermionic emission cathode has a part of a spherical surface, the LaB6 cathode can be used with higher brightness. 4. Since the insulating material that supports each electrode of the electron gun is shielded by the metal or the metal film so that it cannot be directly seen from the electron beam passage, the virtual cathode may be disturbed by the influence of the charge on the surface of the insulator. Thus, a signal having a small shot noise and a high S / N ratio can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an electron beam apparatus according to an embodiment of the present invention.

FIG. 2 shows a single crystal L _a B ₆ cathode with _a (100) orientation of 18
It is a graph which shows the brightness | luminance of the electron beam when changing the electron gun current at the time of operating at 00 degree K with a Wehnelt bias, and simulating.

FIG. 3 is a diagram for explaining a region of a space charge limiting condition of an electron beam device.

FIG. 4 is a flowchart showing a semiconductor device manufacturing process.

5 is a flowchart showing a lithography process of the semiconductor device manufacturing process of FIG.

[Explanation of symbols]

1a electron gun 1 L _a B ₆ cathode 2 of monocrystalline graphite heater 3 Bracket 4 Wehnelt electrode 5 virtual cathode 6 cathode spacer 7 Wehnelt spacer 8,9,10 shield plate 11 anode electrode 12 condenser lens 13 NA aperture 14 primary Electron beam 15 Trajectory of reflected electron 16 Electrostatic deflector 17 Electrostatic deflector 18 Electromagnetic deflector 19 Objective lens 20 Specimen 21 Deflected electron beam 22 Secondary electron trajectory 23 MCP (micro channel plate) 24 Faraday cup 25 Resistor 26 Amplifier 27 Scan line area 28 Voltage measurement circuit 29 Wehnelt power supply 30 Resistance

Continued front page    (72) Inventor Toru Satake             11-1 Haneda Asahi-cho, Ota-ku, Tokyo Co., Ltd.             Inside the EBARA CORPORATION F-term (reference) 4M106 AA01 BA02 CA39 DB05                 5C030 BB01 BB02 BB05 BB11 BB12                 5C033 UU02 UU08

Claims (6)

[Claims] 1. An electron gun having a thermionic emission cathode, 1.
An electron beam apparatus comprising an optical system for forming an image of a secondary electron beam, one or more apertures for limiting a trajectory of the primary electron beam, and a detector for detecting a secondary electron beam. The electron gun is operated under a space charge limiting condition, and at least one lens is provided between the electron gun and the opening arranged closest to the electron gun. Line device. 2. An electron gun having a thermionic emission cathode, a condenser lens, an aperture for limiting the trajectory of a primary electron beam, an objective lens, and a secondary electron beam emitted from a sample to which a negative voltage is applied. An electron beam apparatus comprising a detector for operating the electron gun under space charge limiting conditions, wherein the opening is arranged on the sample side of the condenser lens. . 3. An electron gun for evaluating a sample to which a negative voltage is applied, comprising an electron gun having a thermionic emission cathode and a control electrode, a condenser lens, an aperture for limiting the trajectory of a primary electron beam, and an objective lens. In the line device, the electron gun is operated under the condition that the brightness thereof largely depends on the electron gun current, and the voltage of the control electrode of the electron gun is feedback-controlled so that the electron gun current has a substantially constant value. An electron beam device characterized in that 4. The electron beam apparatus according to claim 3, wherein the thermionic emission cathode has a tip portion in the shape of a part of a spherical surface. 5. An insulating material for supporting each electrode of the electron gun is further provided, and the insulating material is shielded by a metal or a metal film so as not to be directly seen from the electron beam passage. The electron beam apparatus according to claim 1. 6. The electron beam apparatus according to claim 1, wherein the sample is evaluated as a wafer during a wafer process or after completion of at least one wafer process. , Device manufacturing method.