JP3515063B2 - Scanning electron microscope - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for observing a semiconductor device using an electron beam, and more particularly to a method for enabling a fine observation of the surface and the inside or recess of a semiconductor device, and a scanning electron microscope used therefor. .

[0002]

2. Description of the Related Art A scanning electron microscope which scans an electron beam on a sample and detects secondary electrons or backscattered electrons generated from the sample is widely used in various fields of industry and research. Particularly in the semiconductor industry, the degree of integration and miniaturization of patterns in semiconductor devices has advanced, and scanning electron microscopes have been used instead of optical microscopes for the inspection. The main applications of scanning electron microscopes in the semiconductor industry are inspection of semiconductor elements during the development or manufacturing process, such as dimensional measurement of resist patterns, inspection of etching state or presence of residues, and the like.

A scanning electron microscope used for inspecting a semiconductor element has a low energy with an accelerating voltage of 1 kV or less in order to avoid charging of an insulator coated on the semiconductor element. When the insulator is charged, an electric field is generated by the charge, and the irradiated electron beam is bent,
Secondary electrons generated from the sample are pulled back to the sample and do not reach the detector, which causes a problem that a highly accurate scan image cannot be obtained. Therefore, it is extremely important to perform the observation under the condition that the semiconductor element is not charged.

The reason why the insulator can be prevented from being charged by using the low energy electron beam is as follows. FIG. 2 shows how, when an insulator is irradiated with an electron beam, the electron emission ratio δ defined by the following equation changes depending on the energy of the irradiated electron beam. δ = (secondary electron amount + reflected electron amount) / (irradiated electron beam amount) As can be seen from FIG. 2, δ gradually increases as the electron energy decreases, and exceeds 1.0 when the energy becomes 1 kV or less. become. Under the condition that δ is 1.0 or more than 1.0, the balance between the amount of irradiation electron beam entering the sample and the amount of secondary electrons (including the amount of backscattered electrons) from the sample occurs, and the insulator surface is charged. You can observe without doing.

Further, low-energy electrons of 1 kV or less lose energy on the very surface of the sample, and secondary electrons and reflected electrons are generated there. Therefore, the obtained scan image reflects the structure of the surface. Sensitivity to this surface structure is another feature of low energy scanning electron microscopes. That is, the reason why the low-energy scanning electron microscope is utilized in the semiconductor process is that it is possible to observe an insulator and to observe a surface such as an etching residue.

[0006]

As a result of the progress of pattern miniaturization and multi-layering of patterns in the manufacture of semiconductor devices, a low-energy electron beam is used for observing defects inside the semiconductor device and for evaluating alignment of the multilayer structure. It is desired to observe not only the surface state by the above method, but also the inside of the semiconductor element or the concave portion such as the contact hole at the same time.

In order to observe the internal structure and recesses of the semiconductor element, it is necessary to use a high-energy electron beam capable of further penetrating from the surface of the sample, and at the same time, the insulator coated on the surface of the semiconductor element. It is necessary to avoid charging. This is because, as described above, when the semiconductor element is charged, the irradiated electron beam is bent and the secondary electrons generated from the sample cannot be detected, and a satisfactory scan image cannot be obtained. It is an object of the present invention to provide a method for simultaneously observing the surface and the inside of a semiconductor element whose surface is covered with an insulating material without charge and a scanning electron microscope used therefor.

[0008]

A method of observing a semiconductor element according to the present invention is a method of scanning a semiconductor element coated with an insulating thin film with an electron beam and detecting generated electrons or X-rays to obtain a scanned image. In addition, scanning is performed using an electron beam in a low energy region and a high energy region in which the insulator thin film is not charged.

Specifically, a semiconductor element coated with an insulating thin film is placed in a sample chamber of a scanning electron microscope, and when observing the surface, an electron beam in a low energy region that does not charge the insulating thin film is used. When observing the inside or the recess, an electron beam in a high energy region that penetrates the insulating thin film is used. The electron beam in the low energy region has energy such that the amount of electron beam irradiated to the insulator thin film and the total amount of secondary electrons and reflected electrons generated in the insulator thin film by electron beam irradiation are balanced, The electron beam in the region generates an electron-hole pair in the insulator thin film due to the penetration of electrons through the insulator, and energy that neutralizes the charge on the surface of the insulator thin film by conduction of the electron-hole pair. Have.

The scanning electron microscope according to the present invention comprises a sample stage on which a semiconductor element coated with an insulating thin film as a sample is mounted, an electron source, and an electron beam accelerating means for accelerating an electron beam generated from the electron source. An accelerating voltage switching means for switching the accelerating voltage of the electron beam accelerating means, a means for converging the accelerated electron beam on the sample, a means for scanning the electron beam on the sample, and an electron or X generated from the sample.
A detection means for detecting the line, and a scanning image display means for displaying the output of the detection means in synchronization with the scanning of the electron beam,
The acceleration voltage switching means is characterized in that the energy of the electron beam can be switched between a low energy region and a high energy region in which the insulator thin film is not charged.

The low energy region in which the insulator thin film is not charged may be realized by providing a means for applying a negative potential to the sample below the sample and decelerating the electron beam incident on the sample. The signals from the detection means input to the scanning image display means are secondary electron detection signals, backscattered electron detection signals, X
It may be a line detection signal or a combined signal of either of them, and it is particularly preferable to display the scanning image by the low energy electron beam and the scanning image by the high energy electron beam in an overlapping manner.

The electron beam detector can be provided above the final lens of the means for focusing the electron beam on the sample, and between the final lens of the means for focusing the electron beam on the sample and the electron beam detector. A means for applying an electric field and a magnetic field orthogonal to the electric field may be provided so as to attract the secondary electrons generated from the sample to the electron beam detector without deflecting the electron beam with which the sample is irradiated. The sample chamber is preferably one in which a sample with a diameter greater than 4 inches can be inserted.

[0013]

When the film thickness of the insulator coated on the surface is as thin as several μm or less, it is possible to observe the insulator by using a high-energy electron beam without charging the insulator by a principle different from that at low energy. It will be possible. This will be described with reference to FIG. The high-energy irradiation electron beam 4 can easily pass through the insulator 8 on the order of several μm of the semiconductor element 7. At this time, electron-hole pairs 9 are generated in the insulator 8. When the electron-hole pair 9 is generated in the insulator 8, for example, when the positive charge 10 is accumulated on the surface of the insulator 8, the electron-hole pair 9 is generated.
Negative charge of flows to the surface, neutralizing the positive charge 10.
That is, the insulator 8 becomes a conductor by the electron beam irradiation. The non-charged observation according to this principle depends on the thickness of the insulator, but in a normal semiconductor element in which the thickness of the insulator is several μm or less, it can be observed by this principle when the electron beam acceleration voltage exceeds 50 kV. become.

FIG. 4 shows the superiority and inferiority of the observation using the low energy electron beam and the observation using the high energy electron beam. (A) shows an electron energy region in which the semiconductor element can be observed without being charged. As described above, it is possible in the low energy region of 1 kV or less and the high energy region of 50 kV or more. The high-energy region is a condition that is possible only with a semiconductor (insulating film having a thickness of several μm). The intensity of the signal from the surface is shown in (B). The signal on the surface is large at low energy, but small at high energy. On the other hand, as shown in (C), signals from recesses such as internal structures or contact holes can be obtained only at high energy. Further, (D) shows the resolution, and the higher the energy, the higher the resolution can be obtained.

As is apparent from FIGS. 4B and 4C,
A scan image with a low energy electron beam forms an image near the surface, and a scan image with a high energy electron beam forms an image from the inside or a concave portion. Therefore, when the same portion of the sample is scanned with the low-energy electron beam and the high-energy electron beam and the respective scan images are displayed in an overlapping manner, an image in which the inside or the recess is finely displayed at the same time as the surface can be obtained.

Further, when an unobservable internal structure, a concave portion such as a contact hole, or a portion desired to be observed with a higher resolution is encountered during inspection with an electron beam in a low energy region, the same energy can be obtained by switching to high energy. Since the observation is performed with the mirror body, the internal structure and the like at that portion can be observed in a short time. In routine inspection work,
By selecting high and low energy depending on the measurement target location, reliable inspection can be performed. Secondary electrons or backscattered electrons provide information about the shape and size of the sample, while X-rays generated from the sample provide information about the constituent elements. Therefore, scanning X-ray images identify the substances present at each position in the scan image. It can be performed.

[0017]

EXAMPLES The present invention will be described in detail below with reference to examples. [Embodiment 1] FIG. 1 is a schematic view of an embodiment of a scanning electron microscope according to the present invention. The electron source 11 is a thermal field emission type. The electron source 11 is heated by a current from the heating power source 12, and an extraction voltage of several kV is applied to the extraction electrode 13 from the power source 14 to extract the electron beam 15. The suppressor electrode 16 blocks electrons emitted from other than the tip portion of the electron source 11 with a negative voltage supplied from the suppressor power supply 17. The control of the emitted electrons 15 can be performed by the power supply 14.

The electrons 15 having passed through the extraction electrode 13 are adjusted to a desired energy by the first acceleration voltage applied from the power source 18 to the first acceleration electrode 19. For example, the first acceleration voltage is 1 kV, which corresponds to low energy. When observing with low-energy electrons, the second accelerating voltage from the power source 20 is not applied, so the first accelerating electrode 19 becomes the ground potential, and the energy of the electrons irradiating the sample 25 is 1
It becomes kV.

When observing with high-energy electrons,
A second acceleration voltage from the power source 20 is applied. For example, when 199 kV is applied as the second acceleration voltage from the power source 20,
The electron beam 15 is accelerated to 200 kV and 20
Electrons with an energy of 0 kV are irradiated. In this embodiment, the second accelerating voltage is divided by the dividing resistor 21 and applied, and is accelerated stepwise by the three second accelerating electrodes 22 fixed by the insulator 47. Reference numeral 51 denotes a container that covers high voltage portions such as the electron source 11 and the acceleration electrode 22, and is filled with an insulating gas. 50 is a cable for supplying voltage.

First acceleration voltage from power supply 18 and power supply 20
The second accelerating voltage due to is selected so that the electron beam 15 does not charge the sample 25 as described above. The electrons 15 selectively accelerated to low energy or high energy are converged on the sample 25 by the condenser lens 23 and the objective lens 24. An aperture stop 52 determines the aperture angle of the irradiation electron beam. The deflected coil 28 scans the sample with the converged electron beam. The deflection coil 28 is composed of an upper deflection coil 26 and a lower deflection coil 27, and is adjusted so that scanning always passes through the lens center of the objective lens 24. Illustration of the scanning power supply is omitted.

The secondary electrons 29 generated by the electron irradiation are supplied to the suction electrode 30 installed through the objective lens 24 and the power source 4
It is pulled up above the objective lens 24 by the suction voltage applied from 0. When the secondary electron 29 thus pulled up passes through the suction electrode 30, it is decelerated because the surrounding member is at the ground potential. Since the energy of the decelerated secondary electron 29 is several V, it is easily attracted to the scintillator 31 to which 10 kV is applied, and this time it is accelerated to 10 kV to make the scintillator 31 emit light. The light passes through the light guide 33 and is amplified and converted into an electric signal by the photomultiplier tube 32. This electric signal is further amplified, and the scanning image display means, for example, CRT 60.
It becomes the brightness modulation signal of.

The backscattered electrons generated by the electron irradiation are detected by a backscattered electron detector 34 such as a multi-channel plate (MCP), and similarly used as a brightness modulation signal of the CRT 60. The X-ray generated from the sample is detected by the X-ray detector 65, and the detection signal is also used as the brightness modulation signal of the CRT 60. Although each detection signal can be directly input to the CRT 60 in synchronization with the scanning of the electron beam, it is temporarily stored in the image memory 61, and if necessary, subjected to image processing, and then CR.
It can also be displayed at T60.

Photomultiplier tube 32 which is a secondary electron detection signal
Can be added to the output signal of the backscattered electron detector 34 to form a luminance modulation signal of the CRT 60. Also,
The scanning image by the low-energy electron beam and the scanning image by the high-energy electron beam are stored in the image memory, and the image size due to the difference in magnification of both scanning images, the positional deviation, the rotation, etc. are corrected and then superimposed. It can be displayed on the CRT 60. In this case, it is possible to obtain a high-definition image in which the surface of the sample and the inside of the sample or the recesses such as contact holes can be observed simultaneously.

A typical example of the sample 25 used in the present invention is a semiconductor process wafer. In this case,
Wafer 37 housed in pre-evacuated vacuum spare chamber 35
Are sequentially transferred to the stage 38 by opening and closing the gate valve 36. The stage 38 is measurement-controlled by a laser length measuring device or the like and can be moved to a position to be inspected. A tilting mechanism 39 may be provided depending on the inspection purpose. It goes without saying that the device settings such as the current values of the condenser lens 23 and the objective lens 24 are switched according to the switching of the acceleration voltage.

[Embodiment 2] FIG. 5 is a schematic view of another embodiment of the scanning electron microscope according to the present invention. The same functional parts as those in FIG. 1 are designated by the same reference numerals and detailed description thereof will be omitted. The electron beam generation method is the same as that of the above-mentioned embodiment, but the method of low-energy observation is different from that of the above-mentioned embodiment. In this embodiment, the sample 25 is insulated from the sample stage 38 and the tilting mechanism 39 by the insulating base 42. A negative deceleration voltage is applied from the power source 41 to the sample table 55 on which the insulated sample 25 is placed, and the energy of the electron beam 15 is reduced to a desired energy immediately before the sample 25. For example, the first acceleration voltage 18 is 2 kV and the deceleration voltage 41 is 1 kV.
By setting V, the substantial energy incident on the sample 25 is set to 1 kV. According to this method, the electron beam passes through the electron lens at a high speed, so that the aberration is reduced and a resolution higher than that obtained in the embodiment of FIG. 1 can be obtained. In the embodiment, 2 kV is set to 1 kV, but it may be decelerated from a higher voltage.

In this embodiment, the secondary electrons generated in the sample 25 are accelerated by the deceleration voltage 41 of 1 kV applied to the sample 25 and pass through the objective lens 24. The secondary electrons 29 pass through the objective lens 24 and travel further upward. The secondary electrons 29 traveling upward irradiate a plate 43 having an opening for passing the electron beam 15 in the center, and secondary electrons 46 are generated again here. The plate 43 is made of a non-magnetic material such as gold or copper that generates a lot of secondary electrons. The irradiation area of the plate 43 is adjusted to be larger than the opening of the plate 43. This adjustment is performed by the first acceleration power supply 18 or the suction voltage power supply 40.

Since the energy of secondary electrons generated on the plate 43 is several V, it can be easily attracted by an electric field. In the present embodiment, the scintillator 3 is generated by the electric field created by the detection electrodes 44, 44 '.
Bias in the direction of 1. The electrode 44 'is a mesh electrode. Since the electron beam 15 is also deflected by this electric field, the correction coil 45 forms a magnetic field orthogonal to the electric field to cancel the deflection of the irradiation beam. The method of deflecting the secondary electrons to the detector by the electric field can also be used in the embodiment of FIG.

[Embodiment 3] FIG. 6 is a schematic view of another embodiment of the scanning electron microscope according to the present invention. 1 and 5
The same functional parts as those in FIG. In this embodiment, the electron beam 15 is accelerated to a desired energy by one-step acceleration. The switching of the acceleration voltage is performed by adjusting the single acceleration power source 49.

The electrons 15 extracted by the extraction electrode 13 in the electron source 11 are accelerated by the acceleration voltage from the acceleration power source 49 applied to the ground. The electron source 11, the suppressor electrode 16, and the extraction electrode 13 are supported by a high voltage insulator 48. The conducting wire connected to each electrode is connected to an acceleration power source 49 or the like by a cable 50. The acceleration voltage is selected to be 3 kV for low energy and 50 kV for high energy.
By applying a deceleration voltage of −2 kV from the deceleration voltage 41 to the sample 25 when the energy is low, the electron beam energy at the time of incidence of the sample becomes 1 kV. In the case of high energy, as in the case of FIG. 1, the secondary electrons 29 that have passed through the objective lens 24 are deflected toward the scintillator 31 by the electric field created by the detection electrodes 44 and 44 ′ and detected.

When the accelerating voltage is switched by adjusting a single accelerating power source, a relatively high accelerating voltage (3 kV) and a decelerating power source 41 are used together to lower the electron beam energy at the time of sample incidence, as in this embodiment. If the configuration is adopted, the voltage adjustment range of the acceleration power supply becomes small, so that the power supply design becomes easy.

[Embodiment 4] FIG. 7 is a flow chart of in-line inspection using the apparatus of the present invention. In a normal inspection, the inspection location and the inspection method in the wafer are predetermined. For example, consider the case where the scanning electron microscope shown in FIG. 5 is used to measure the dimensions of n1 lines / spaces and n2 contact holes in a wafer for N wafers.

First, low energy is selected as the electron beam energy of the scanning electron microscope (step 71),
The second accelerating power supply 20 is cut off, a low-voltage accelerating voltage of several kilovolts is applied between the electron source 11 and the first accelerating electrode 19 at the ground potential by the low-voltage first accelerating power supply 18, and the deceleration power supply 41 is set. Further, the current value of the electronic lens such as the condenser lens 23 and the objective lens 24 is set to a predetermined value for low energy. Then, the wafer is scanned with the electron beam 15 in the low energy region that does not charge the insulator thin film of the wafer, the stage 38 is driven, and the observation position on the wafer 25 is sequentially moved to a preset point. Dimension measurement of n1 lines / spaces on the wafer. The wafer for which the measurement has been completed opens the gate valve 36 and discharges it into the vacuum preliminary chamber 35.
Next, the next wafer 37 is attached to the sample table 55 on the stage 38.
And perform the same measurement. This operation is repeated until the measurement of all N wafers is completed (step 7).
2).

Subsequently, high energy is selected as the electron beam energy of the scanning electron microscope (step 7).
3), the second acceleration power supply 20 is connected together with the first acceleration power supply 18. The current value of the electronic lens such as the condenser lens 23 and the objective lens 24 is set to a predetermined value for high energy. Then, the wafer is scanned with the electron beam 15 in the high energy region that does not charge the insulator thin film of the wafer, and the stage 38 is driven to sequentially move the observation position on the wafer 25 to a preset point.
Inspection of n2 contact holes is performed on each wafer. This operation is repeated for all N wafers to complete the inspection (step 74). In this inspection, it is possible to observe the same portion with low energy and high energy, and after the observation is completed, image processing such as comparison or superposition of the both is performed.

[0034]

As described above, according to the present invention, by selecting low energy as the energy of the scanning electron beam, it becomes possible to inspect the semiconductor element mainly on the surface, and by selecting high energy, the low energy is selected. It is possible to inspect internal structures and contact holes that cannot be observed with energy. Therefore, the inspection in the process of manufacturing the semiconductor device can be more complete, and the effect of promoting the device development and improving the yield is very large. Further, by combining and superimposing the scan image of the low energy electron beam and the scan image of the high energy electron beam and displaying them, a high-definition image can be obtained from the surface to the deep portion.

[Brief description of drawings]

FIG. 1 is a schematic view of an embodiment of a scanning electron microscope according to the present invention.

FIG. 2 is a diagram illustrating the principle of observing an insulator in a low energy region.

FIG. 3 is a diagram illustrating the principle of observing an insulator in a high energy region.

FIG. 4 is a diagram comparing advantages and disadvantages of a low energy region and a high energy region.

FIG. 5 is a schematic view of another embodiment of the scanning electron microscope according to the present invention.

FIG. 6 is a schematic view of another embodiment of the scanning electron microscope according to the present invention.

FIG. 7 is a diagram illustrating a flow of a semiconductor process inspection using the device of the present invention.

[Explanation of symbols]

7: semiconductor element, 8: insulator thin film, 9: electron-hole pair, 1
0: positive charge, 11: electron source, 12: heating power source, 13: extraction electrode, 15: electron beam, 16: suppressor electrode, 1
7: suppressor power supply, 18: first acceleration power supply, 19: first
Acceleration electrode, 20: second acceleration voltage, 21: division resistance, 2
2: 2nd acceleration electrode, 23: condenser lens, 24: objective lens, 25: sample, 26: upper deflection coil, 27: lower deflection coil, 28: deflection coil, 29: secondary electron, 3
0: suction electrode, 31: scintillator, 32: photomultiplier tube, 33: light guide, 34: backscattered electron detector, 3
5: vacuum reserve chamber, 36: gate valve, 37: wafer, 38: stage, 39: tilting mechanism, 40: suction voltage, 41: deceleration voltage, 42: insulating base, 43: plate,
44, 44 ': Detection electrode, 45: Correction coil, 46: Secondary electron, 47: Insulator, 48: High voltage insulator, 49: Acceleration power supply, 50: Cable, 51: Container, 52: Aperture stop,
55: sample stage, 60: CRT, 61: image memory, 6
5: X-ray detector

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tadashi Otaka 882 Ichige, Oita, Hitachinaka City, Ibaraki Prefecture Hitachi, Ltd. Measuring Instruments Division (56) Reference JP-A-62-234859 (JP, A) JP HEI 5-258703 (JP, A) JP-A 5-266855 (JP, A) JP-A 60-47358 (JP, A) Actual development Shou 51-96494 (JP, U) (58) Fields investigated (Int .Cl. ⁷ , DB name) H01J 37/28 H01J 37/244

Claims (4)

(57) [Claims] 1. An electron source and an electron beam emitted from the electron source are focused on a sample .
The objective lens and the objective lens disposed closer to the electron source than the objective lens.
Based on the irradiation of a sample with an electron beam focused by
In a scanning electron microscope equipped with a secondary electron detector for detecting secondary electrons generated by the method, and a power source for applying a negative potential to the sample, and arranged on the electron source side of the detector , The electron beam has a passage opening for the electron beam and is formed by the negative potential.
Secondary electrons are generated by the collision of the secondary electrons accelerated by the field.
A conductive member that generates a child, and the secondary electron on the sample side of the conductive member, the detector
A scanning electron microscope , which is equipped with a deflector for guiding to . 2. An electron source and an electron beam emitted from the electron source are focused on a sample .
The objective lens and the objective lens disposed closer to the electron source than the objective lens.
Based on the irradiation of a sample with an electron beam focused by
A secondary electron detector for detecting secondary electrons detected by a scanning electron microscope, and a power source for applying a negative potential to the sample, and arranged on the electron source side of the detector , The electron beam has a passage opening for the electron beam and is formed by the negative potential.
Secondary electrons are generated by the collision of the secondary electrons accelerated by the field.
A non-magnetic material for generating a large amount of children, to the sample side of the non-magnetic material, the detection of the secondary electrons
A scanning electron microscope , which is equipped with a deflector for guiding the scanning electron microscope. 3. The two electrodes are provided on the sample side of the conductive member.
The scanning according to claim 1 or 2, further comprising : a deflection electrode for guiding a secondary electron to the detector, and a magnetic pole for forming a magnetic field in a direction orthogonal to an electric field formed by the deflection electrode. Scanning electron microscope. 4. The scanning electron microscope according to claim 3, wherein the magnetic field formed by the magnetic pole acts so as to cancel the deflection of the electron beam due to the electric field.