JP3132796B2 - Method for observing semiconductor device and scanning electron microscope used therefor - 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 inside or a concave portion of the semiconductor device, and a scanning electron microscope used for the method. .

[0002]

2. Description of the Related Art A scanning electron microscope which scans a sample with an electron beam and detects secondary electrons or reflected electrons generated therefrom is widely used in various industries and research fields. In particular, in the semiconductor industry, the integration and miniaturization of patterns in semiconductor devices have been advanced, and scanning electron microscopes have been used instead of optical microscopes for inspection. The main uses of the scanning electron microscope in the semiconductor industry are inspection of semiconductor elements during a development or manufacturing process, such as measurement of the size of a resist pattern, inspection of an etching state or presence or absence of a residue, and the like.

A scanning electron microscope used for inspection of a semiconductor device has a low energy of an acceleration voltage of 1 kV or less in order to avoid charging of an insulator coated on the semiconductor device. When the insulator is charged, an electric field is generated by the charge, and the irradiated electron beam is bent,
Problems such as secondary electrons generated from the sample being drawn back to the sample and not reaching the detector occur, and a high-precision scanning image cannot be obtained. Therefore, it is extremely important to perform observation under conditions where the semiconductor element is not charged.

The reason why the insulator can be prevented from being charged by using a 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 = (amount of secondary electrons + amount of reflected electrons) / (amount of irradiated electron beam) As can be seen from FIG. 2, δ gradually increases as the energy of electrons decreases, and exceeds 1.0 when the energy becomes 1 kV or less. become. Under conditions where δ is 1.0 or exceeds 1.0, the balance between the amount of irradiated electron beams entering the sample and the amount of secondary electrons (including the amount of reflected electrons) exiting the sample occurs, and the surface of the insulator is charged. You can observe without doing.

[0005] Electrons having a low energy of 1 kV or less lose energy at the very surface of the sample, and generate secondary electrons and reflected electrons there. Therefore, the obtained scanned image reflects the surface structure. Sensitivity to this surface structure is another feature of low energy scanning electron microscopes. That is, the reason why a low-energy scanning electron microscope is used in the semiconductor process is that it is possible to observe an insulator and to observe the surface sensitively, such as an etching residue.

[0006]

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

In order to observe the internal structure and the concave portion of the semiconductor device, it is necessary to use a high energy electron beam which can enter the inside of the sample from the surface of the sample. It is necessary to avoid charging. As described above, when the semiconductor element is charged, the irradiated electron beam is bent or secondary electrons generated from the sample cannot be detected, so that a satisfactory scanned image cannot be obtained. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for simultaneously and precisely observing the surface and the inside of a semiconductor element whose surface is covered with an insulator without charge and a scanning electron microscope used for the method.

[0008]

According to the method of observing a semiconductor device according to the present invention, a semiconductor device coated with an insulating thin film is scanned with an electron beam to detect 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 penetrating the insulating thin film is used. The electron beam in the low energy region has energy such that the amount of the electron beam irradiated on the insulating thin film and the total amount of secondary electrons and reflected electrons generated in the insulating thin film by the electron beam irradiation are balanced. The electron beam in the region generates an electron-hole pair in the insulator thin film by passing electrons through the insulator, and generates energy such that conduction on the surface of the insulator thin film is neutralized by conduction of the electron-hole pair. Have.

A 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. Acceleration voltage switching means for switching the acceleration voltage of the electron beam acceleration means; means for converging the accelerated electron beam on the sample; means for scanning the electron beam on the sample; and electrons or X generated from the sample.
Detecting means for detecting the line, including a scanning image display means for displaying the output of the detecting means in synchronization with the scanning of the electron beam,
The acceleration voltage switching means can switch the energy of the electron beam between a low energy region and a high energy region where 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. Signals from the detection means input to the scanning image display means are a secondary electron detection signal, a reflected electron detection signal, and X
It can be a line detection signal or a composite signal of any of them, and it is particularly preferable to display a scan image with a low energy electron beam and a scan image with a high energy electron beam in a superimposed manner.

The electron beam detector may be provided above the last lens of the means for focusing the electron beam on the sample, and between the last lens of the means for focusing the electron beam on the sample and the electron beam detector. Means for applying an electric field and a magnetic field orthogonal to the electric field may be provided so as to attract secondary electrons generated from the sample to the electron beam detector without deflecting the electron beam applied to the sample. The sample chamber is preferably capable of inserting a sample having a diameter greater than 4 inches.

[0013]

When the thickness of the insulator coated on the surface is as thin as several μm or less, it is possible to observe the insulator using a high-energy electron beam without charging the insulator according to a principle different from that at the time of low energy. 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 of the semiconductor element 7 on the order of several μm. At this time, electron-hole pairs 9 are generated in the insulator 8. When the electron-hole pairs 9 are generated in the insulator 8, for example, when the positive charges 10 accumulate on the surface of the insulator 8, the electron-hole pairs 9 are formed.
Negative charge flows toward the surface and neutralizes the positive charge 10.
That is, the insulator 8 becomes a conductor by the irradiation of the electron beam. The non-charge observation based on this principle depends on the thickness of the insulator. However, in a normal semiconductor device having a thickness of the insulator of several μm or less, it can be observed by the principle when the acceleration voltage of the electron beam exceeds 50 kV. become.

FIG. 4 shows a comparison between observation using a low energy electron beam and observation using a high energy electron beam. (A) shows an electron energy region where the semiconductor element can be observed without charging. As described above, it is possible in a low energy region of 1 kV or less and a high energy region of 50 kV or more. The high energy region is a condition that can be achieved only with a semiconductor (an insulating film having a thickness of several μm). (B) shows the intensity of the signal from the surface. The signal at the surface is large at low energy, but small at high energy. On the other hand, as shown in (C), a signal from a concave portion such as an internal structure or a contact hole can be obtained only with high energy. (D) indicates the resolution, and the higher the energy, the higher the resolution.

As apparent from FIGS. 4B and 4C,
A scanning image with a low energy electron beam forms an image near the surface, and a scanning image with a high energy electron beam forms an image from inside or from a recess. Therefore, when the same portion of the sample is scanned with the low energy electron beam and the high energy electron beam, and each scanned image is displayed in a superimposed manner, it is possible to obtain an image in which the inside or the concave portion is displayed finely simultaneously with the surface.

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

[0017]

The present invention will be described below in detail 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 of a thermal field emission type. The electron source 11 is heated by a current from a heating power supply 12, and an extraction voltage of several kV is applied to the extraction electrode 13 from a power supply 14 to extract an electron beam 15. The suppressor electrode 16 blocks electrons emitted from portions 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 passing through the extraction electrode 13 are adjusted to a desired energy by a first acceleration voltage applied from a power source 18 to a 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 acceleration voltage from the power supply 20 is not applied, so the first acceleration electrode 19 is at the ground potential, and the energy of the electrons irradiating the sample 25 is 1
kV.

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

First acceleration voltage by power supply 18 and power supply 20
Are selected such 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 a sample 25 by a condenser lens 23 and an objective lens 24. An aperture stop 52 determines the opening angle of the irradiation electron beam. The converged electron beam is scanned over the sample by the deflection coil 28. The deflection coil 28 includes an upper deflection coil 26 and a lower deflection coil 27, and is adjusted so as to always scan through the center of the objective lens 24. The scanning power supply is not shown.

The secondary electrons 29 generated by the electron irradiation are supplied to a power supply 4 by a suction electrode 30 installed through the objective lens 24.
It is pulled up above the objective lens 24 by the suction voltage applied from zero. The secondary electrons 29 that have been pulled up are decelerated when passing through the suction electrode 30 because the surrounding members are at the ground potential. Since the energy of the decelerated secondary electrons 29 is several volts, the secondary electrons 29 are easily attracted to the scintillator 31 to which 10 kV is applied, and then accelerated to 10 kV, causing the scintillator 31 to emit light. The light passes through a light guide 33 and is amplified and converted into an electric signal by a photomultiplier tube 32. This electric signal is further amplified and scanned image display means, for example, a CRT 60
Of the luminance modulation signal.

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

Photomultiplier tube 32 which is a secondary electron detection signal
And the output signal of the backscattered electron detector 34 can be added to obtain a luminance modulation signal of the CRT 60. Also,
The scan image by the low energy electron beam and the scan image by the high energy electron beam are stored in the image memory, and the image size, position shift, rotation, etc. due to the difference in magnification between the two scan images are corrected and then superimposed. It can be displayed on the CRT 60. In this case, a high-definition image can be obtained in which the inside of the sample or a concave portion such as a contact hole can be observed simultaneously with the surface of the sample.

A typical example of the sample 25 used in the present invention is a semiconductor process wafer. In this case,
Wafer 37 stored in pre-evacuated vacuum pre-chamber 35
Are sequentially conveyed to the stage 38 by opening and closing the gate valve 36. The stage 38 is measured and controlled by a laser length measuring device or the like, and can move to a position to be inspected. An inclination mechanism 39 may be provided depending on the purpose of inspection. It is needless to say 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 portions as those in FIG. 1 are denoted by the same reference numerals, and detailed description is omitted. The method of generating the electron beam is the same as that of the above embodiment, but the method of low energy observation is different from that of the above embodiment. In this embodiment, the sample 25 is insulated from the sample stage 38 and the tilt mechanism 39 by the insulating table 42. A negative potential deceleration voltage is applied from the power supply 41 to the sample stage 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, since the electron beam passes through the electron lens at a high speed, aberration is reduced, and a higher resolution than that obtained in the embodiment of FIG. 1 can be obtained. Although 2 kV is set to 1 kV in the embodiment, the speed may be reduced 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 that have traveled upward irradiate a plate 43 having an opening through which the electron beam 15 passes in the center, where secondary electrons 46 are generated again. The plate 43 is made of a non-magnetic material such as gold or copper that generates a large amount 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 the secondary electrons generated in the plate 43 is several volts, it can be easily attracted by an electric field. In this embodiment, the scintillator 3 is driven by an electric field generated by the detection electrodes 44 and 44 '.
1 direction. Electrode 44 'is a mesh electrode. Since the electron beam 15 is also deflected by this electric field, a magnetic field orthogonal to the electric field is formed by the correction coil 45 to cancel the deflection of the irradiation beam. The method of deflecting secondary electrons to a detector by an 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 portions as those described above are denoted by the same reference numerals, and detailed description is omitted. In this embodiment, the electron beam 15 is accelerated to a desired energy by one-stage acceleration. Switching of the acceleration voltage is performed by adjusting a single acceleration power supply 49.

The electrons 15 extracted by the extraction electrode 13 in the electron source 11 are accelerated by an acceleration voltage from an acceleration power supply 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 conductor connected to each electrode is connected to an acceleration power supply 49 or the like by a cable 50. The acceleration voltage is selected to be 3 kV at low energy and 50 kV at high energy.
By applying a deceleration voltage of −2 kV from the deceleration voltage 41 to the sample 25 at low energy, the electron beam energy at the time of incidence of the sample becomes 1 kV. In the case of high energy, the secondary electrons 29 passing through the objective lens 24 are detected by being deflected in the direction of the scintillator 31 by an electric field generated by the detection electrodes 44 and 44 ', as in FIG.

When the acceleration voltage is switched by adjusting a single acceleration power supply, the electron beam energy at the time of sample incidence is reduced by using a relatively high acceleration voltage (3 kV) and the deceleration power supply 41 together as in this embodiment. When the configuration is adopted, the voltage adjustment width of the accelerating power supply becomes small, so that the power supply design becomes easy.

[Embodiment 4] FIG. 7 is a flowchart of an in-line inspection using the apparatus of the present invention. In a normal inspection, an inspection location and an inspection method in a wafer are determined in advance. For example, consider a case in which the dimensions of n1 lines / spaces and n2 contact holes in a wafer are measured for N wafers using the scanning electron microscope shown in FIG.

First, low energy is selected as the electron beam energy of the scanning electron microscope (step 71).
The second accelerating power supply 20 is shut off, a low-voltage first accelerating power supply 18 applies a low-voltage acceleration voltage of several kilovolts between the electron source 11 and the first accelerating electrode 19 at the ground potential, and sets a deceleration power supply 41. Further, the current values of the electron lenses such as the condenser lens 23 and the objective lens 24 are set to predetermined values for low energy. Then, the wafer is scanned with the electron beam 15 in a low energy region in which the insulating thin film of the wafer is not charged, and the stage 38 is driven to sequentially move the observation position on the wafer 25 to a preset point, and one wafer is scanned. The dimension measurement of n1 lines / spaces is performed on the wafer. The wafer whose measurement has been completed opens the gate valve 36 and discharges the wafer into the pre-vacuum chamber 35.
Next, the next wafer 37 is placed on the sample stage 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 values of the electronic lenses such as the condenser lens 23 and the objective lens 24 are set to predetermined values for high energy. Then, the wafer is scanned with the electron beam 15 in a high energy region where the insulator thin film of the wafer is not charged, 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 the wafers. This operation is repeated for all N wafers, and the inspection is completed (step 74). In this inspection, the same portion can be observed with low energy and high energy, and after the observation is completed, image processing such as comparison or superposition of the two is performed.

[0034]

As described above, according to the present invention, it is possible to inspect a semiconductor element mainly on the surface by selecting low energy as the energy of the scanning electron beam, and to reduce the energy by selecting high energy. Internal structures and contact holes that cannot be observed with energy can be inspected. Therefore, the inspection in the process of manufacturing a semiconductor device can be completed more completely, and the effect of promoting the device development and improving the yield is very large. In addition, by synthesizing a scan image with a low-energy electron beam and a scan image with a high-energy electron beam and displaying the combined image, a high-definition image can be obtained from the surface to the deep part.

[Brief description of the drawings]

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

FIG. 2 illustrates the principle of observation of an insulator in a low energy region.

FIG. 3 illustrates the principle of observation of an insulator in a high energy region.

FIG. 4 is a diagram comparing the superiority and the inferiority 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 apparatus of the present invention.

[Explanation of symbols]

7: semiconductor element, 8: insulating thin film, 9: electron-hole pair, 1
0: positive charge, 11: electron source, 12: heating power supply, 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: second 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, 33: light guide, 34: backscattered electron detector, 3
5: vacuum preparatory chamber, 36: gate valve, 37: wafer, 38: stage, 39: tilt mechanism, 40: suction voltage, 41: deceleration voltage, 42: insulating table, 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 Odaka 882 Ma, Katsuta-shi, Ibaraki Pref. Measuring Instruments Division, Hitachi, Ltd. (56) References JP-A-5-290786 (JP, A) JP-A-4-149944 (JP, A) JP-A-6-243814 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01J 37/252-37/295 H01J 37/22

Claims (11)

(57) [Claims] 1. A method for observing a semiconductor device by scanning a semiconductor device coated with an insulating thin film with an electron beam, detecting generated electrons or X-rays, and obtaining a scanned image, the method comprising: A method for observing a semiconductor element, wherein scanning is performed using an electron beam in a low energy region and a high energy region in which the surface is not charged. 2. A semiconductor device coated with an insulating thin film is placed in a sample chamber of a scanning electron microscope, and electrons or X-rays generated when scanning with an electron beam are detected to obtain a scanned image, thereby obtaining a scanned image. A method for observing a surface and an inside or a recess of a device, the method for observing the inside or the recess using an electron beam in a low energy region that does not charge the insulating thin film when observing the surface. A method for observing a semiconductor device, comprising using an electron beam in a high energy region that does not charge the insulator thin film. 3. An electron beam in the low energy region so that an amount of the electron beam irradiated on the insulating thin film and a total amount of secondary electrons and reflected electrons generated in the insulating thin film by the irradiation of the electron beam are balanced. The electron beam in the high-energy region generates electron-hole pairs in the insulator thin film, and the conduction of the electron-hole pairs neutralizes the charge on the surface of the insulator thin film. The method for observing a semiconductor device according to claim 1, wherein the method has energy. 4. The semiconductor device according to claim 1, wherein a scan image using the electron beam in the low energy region and a scan image using the electron beam in the high energy region are superimposed and displayed. Observation method. 5. A sample stage on which a semiconductor element coated with an insulating thin film as a sample is mounted, an electron source, electron beam acceleration means for accelerating an electron beam generated from the electron source, and said electron beam acceleration. Means for switching acceleration voltage of the means, means for converging the accelerated electron beam on the sample, means for scanning the electron beam on the sample, and detection means for detecting electrons or X-rays generated from the sample And scanning image display means for displaying the output of the detection means in synchronization with the scanning of the electron beam, wherein the acceleration voltage switching means reduces the energy of the electron beam to a low energy area where the insulator thin film is not charged and a high energy area. A scanning electron microscope for observing a semiconductor element coated with an insulating thin film, which is switchable between regions. 6. A low-energy region in which the insulator thin film is not charged by decelerating an electron beam incident on the sample by providing means for applying a negative potential to the sample below the sample. Item 6. A scanning electron microscope according to item 5. 7. A signal input to the scanning image display means from the detection means is a secondary electron detection signal, a reflected electron detection signal, an X-ray detection signal, or a composite signal of any of them. A scanning electron microscope according to claim 5. 8. An electron beam detector is provided above a final lens of the means for converging the electron beam on a sample, and secondary electrons generated from the sample and passed through the lens are detected. The scanning electron microscope according to claim 5, 6, or 7. 9. A secondary electron generated from a sample without being deflected by an electron beam applied to the sample between the final lens of the means for converging the electron beam onto the sample and an electron beam detector. The scanning electron microscope according to any one of claims 5 to 8, further comprising means for applying an electric field and a magnetic field orthogonal to the electric field so as to attract the detector. 10. The apparatus according to claim 1, further comprising means for storing the scan image, wherein a scan image by an electron beam in a low energy area and a scan image by an electron beam in a high energy area are displayed on the scan image display means in a superimposed manner. Claims 5 to 9
The scanning electron microscope according to any one of the above items. 11. The apparatus according to claim 5, further comprising a sample chamber into which a sample having a diameter exceeding 4 inches can be inserted.
The scanning electron microscope according to any one of the above items.