JP2683951B2 - Scanning electron microscope for cross-section observation and cross-section observation method using the same - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning electron microscope for observing a cross section and a method for observing a cross section using the same, and particularly to a scanning type for observing a cross section of a semiconductor wafer. The present invention relates to an electron microscope and a cross-section observation method using the electron microscope.

[Prior Art] Conventionally, a scanning electron microscope (hereinafter referred to as SEM) is known as a device for observing a surface or a cross section in defect analysis and process evaluation of a semiconductor integrated circuit. This SEM
In the cross-section observing method using (Scanning Electron Microscope), a method of cutting the wafer so that a desired cross-section of the wafer can be observed has been adopted.

FIG. 7 is a perspective view of a semiconductor wafer required when a conventional cross-sectional observation method by SEM is used. A conventional cross-sectional observation method using SEM will be described with reference to FIG. A plurality of chips are formed on the semiconductor wafer 27. First, the cross-section observation test chip 28 for cross-section observation is selected. After that, the semiconductor wafer 27 is cut so that the observation section 129 of the section observation test chip 28 appears. After the observation cross-section 129 of the cross-section observation test chip 28 was exposed in this manner, the observation cross-section 129 was observed by SEM. As described above, conventionally, the semiconductor wafer 27 is manually cut in order to expose the observation cross section 129 of the cross section observation test chip 28.

[Problems to be Solved by the Invention] As described above, in the method of observing the cross section of the semiconductor wafer using the conventional SEM, the observation cross section 129 of the test chip 28 for observing the cross section is exposed when observing the cross section. Therefore, the semiconductor wafer 27 was manually cut. In the method of cutting the semiconductor wafer 27, the portion corresponding to the observation cross section 129 desired to be observed manually is cut, so that it is very difficult to accurately expose the cross section of a specific portion. Further, this method has a disadvantage that it takes a long time to prepare the sample and observe the cross section. Further, since the semiconductor wafer is cut to observe the cross section, the wafer cannot be subjected to the subsequent wafer process. As a result, there is a problem that a wafer for cross-section observation must be prepared for each wafer process. That is, in the cross-section observation method using the conventional SEM,
Since the wafer is manually cut, it is impossible to accurately expose the cross section of a specific portion, and it takes a long time to observe the cross section. Further, a wafer for cross-section observation is required for each wafer process.

Therefore, as an improvement of the conventional cross-section observation method using SEM, FIB (Focused-Ion-Beam) was added to the SEM barrel.
A cross-section observation method using an electron microscope for cross-section observation in which a lens barrel portion is integrated has been proposed. These are, for example, J.
Appl.Phys.62 (6) 15 September 1987 P2163 ~ P2
168. As a section observation method using the proposed electron microscope for section observation, first, an electron beam and an ion beam are aligned with a reference mark. Then, the processing place is observed with the electron beam, and the processing is performed with the ion beam. Using such an electron microscope for cross-section observation,
There is an advantage that it is not necessary to manually break the semiconductor wafer in order to observe the cross section.

However, in this proposed electron microscope for cross-section observation,
Since SEM observation is performed using secondary electrons generated from a semiconductor wafer, SEM observation cannot be performed simultaneously with FIB processing. That is, in the scanning of the semiconductor wafer by the FIB and the SEM, the secondary electrons are generated from the semiconductor wafer, so even if the cross-section observation is performed by the SEM at the same time when the cross-section of the semiconductor wafer is cut by the FIB, the scanning by the SEM is performed. Only the secondary electrons cannot be detected. Therefore, in this proposed electron microscope for cross-section observation,
When cutting out the observation cross section, it is necessary to repeat the step of cutting out a predetermined amount with the FIB and then observing the cross section with SEM, which makes it difficult to shorten the time for cutting out the observation cross section. Further, since it is not possible to perform observation by SEM at the same time as cutting out the observation section by FIB, there is a problem that it is difficult to obtain an observation section at a specific position more accurately.

The present invention has been made to solve the above problems, and a cross-section observing scanning electron microscope capable of observing a cross section of a specific portion of a semiconductor wafer by cutting out more accurately in a short time and the same. It is an object of the present invention to provide a cross-sectional observation method using.

[Means for Solving the Problem] The invention according to claim 1 is a scanning electron microscope for observing a cross section for irradiating a cross section of an object to be observed with an electron beam and observing the cross sectional state of the object. It includes a beam scanning unit, an electron beam scanning unit, a first electron detecting unit, a second electron detecting unit, and a scanning region observing unit. The ion beam scanning means is for scanning an object to be observed with an ion beam. The electron beam scanning means is installed at a position inclined by a predetermined angle from the ion beam scanning means, and is for scanning the electron beam.
The first electron detecting means is for detecting secondary electrons generated from the object to be observed by scanning the ion beam and the electron beam. The second electron detecting means is for detecting reflected electrons reflected from the object to be observed by scanning the electron beam. The scanning area observation means is the first
And for observing the cross-sectional state of the region scanned by the electron beam scanning means based on the detection result of the second electron detecting means. Here, in the invention described in claim 1, the first electron detecting means and the second electron detecting means are constituted by one electron detector, and the one electron detector switches the bias voltage. Bias voltage switching means is included. 1 by the bias voltage switching means
By switching the bias voltage of one electron detector, two types of electrons, that is, secondary electrons and reflected electrons, are detected.

The invention according to claim 2 is a cross-section observing method using a cross-section observing scanning electron microscope for observing a cross-sectional state of an object to be observed by irradiating a cross-section of the object to be observed with an electron beam. The cross section is cut out by scanning the ion beam at the position where the cross section is observed. The electron beam is scanned at the same time while cutting out the cross section, the backscattered electrons reflected from the object to be observed are detected, and the cutting process of the cross section is simultaneously observed while cutting out the cross section. By cutting the cross section, the cut cross section is scanned with an electron beam, and 2
Secondary electrons are generated, the secondary electrons are detected, and the cross-sectional state of the cross section scanned by the electron beam is observed.

In the cross-section observing method according to a third aspect, in the above-mentioned configuration of the second aspect, the reflected electrons are detected by the first electron detecting means and the secondary electrons are detected by the second electron detecting means.

According to a fourth aspect of the present invention, in the structure of the second aspect, the backscattered electrons are detected by the first electron detecting means, and the secondary electrons are switched by the bias voltage of the first electron detecting means. To detect.

According to a fifth aspect of the present invention, in the structure of the second aspect, the step of observing the cutting process of the cross section includes a step of stopping the scanning of the ion beam when a necessary cross section is obtained. To do.

[Operation] In the scanning electron microscope for observing a cross section according to claim 1, 2
The first electron detecting means for detecting the next electron and the second electron detecting means for detecting the backscattered electron are constituted by one electron detector, and the one electron detector switches the bias voltage. The secondary voltage and the reflected electron are detected by switching the bias voltage of the electron detector by the bias voltage switching means. Therefore, the secondary electron and the reflected electron are easily detected as one electron. It becomes possible to detect with a detector.

In the cross-section observing method using the scanning electron microscope for cross-section observation according to any one of claims 2 to 5, while scanning the ion beam to cut out the cross section, at the same time, scanning the electron beam to detect backscattered electrons and cut out the cross section. Since the process is observed simultaneously while cutting out the group, it becomes possible to observe the process of cutting out the cross section in real time. This allows
It becomes possible to more accurately cut out the cross section to be observed in a short time.

[Embodiment of the Invention] FIG. 1 is a schematic view of a cross-sectional observation scanning electron microscope showing an embodiment of the present invention. Referring to FIG. 1, the scanning electron microscope for observing a cross section includes an SEM lens barrel portion 100, a FIB lens barrel portion 200, and a sample chamber portion 300.

The SEM lens barrel portion 100 is focused by the electron gun 1 for generating an electron beam, the SEM first lens 2 for focusing the electron beam 30 generated by the electron gun 1, and the SEM first lens 2. The SEM blanking coil 3 for turning the electron beam on and off, the SEM deflection coil 5 for scanning the electron beam 30 passing through the SEM blanking coil 3, and the electron beam 30 passing through the SEM deflection coil 5 are again And an SEM second lens 4 for collecting light. Electron gun 1, SEM
An electron gun driving power source 6, a SEM lens power source 7, a SEM blanking power source 8 and a SEM deflection power source 9 are connected to the first lens 2, the SEM second lens 4, the SEM blanking coil 3 and the SEM deflection coil 5, respectively. ing.

The FIB barrel portion 200 includes a liquid metal ion source 10 for generating an ion beam and a FIB first lens 1 for focusing an ion beam 31 generated from the liquid metal ion source 10.
1 and the ion beam focused by the FIB first lens 11
FIB blanking electrode 12 for turning ON / OFF 31 and FIB
The FIB second lens 13 for refocusing the ion beam 31 that has passed through the blanket electrode 12 and the FIB deflection electrode 14 for deflecting the ion beam 31 focused by the FIB second lens 13 are included. Liquid metal ion source 10, FIB first lens
11 and FIB second lens 13, FIB blanking electrode 12, FIB
An ion source driving power source 15, a FIB lens power source 16, a FIB blanking power source 17, and a FIB deflection power source 18 are connected to the deflection electrode 14, respectively.

The sample chamber portion 300 includes a sample table 19 for setting the semiconductor wafer 27 and a sample table driving device for moving the sample table 19.
20, a backscattered electron detector 21 for detecting backscattered electrons of the electron beam 30 reflected by the semiconductor wafer 27, and detection of secondary electrons generated from the semiconductor wafer 27 by scanning the electron beam 30 and the ion beam 31. Secondary electron detection means 23 for The backscattered electron detector 21 and the secondary electron detector 23 are connected to a backscattered electron detector control power supply 22 and a secondary electron detector control power supply 24, respectively. Further, a detection signal amplifier 25 is connected to the backscattered electron detector 21 and the secondary electron detector 23, and an observation CRT 26 is connected to the detection signal amplifier 25.

The observation CRT 26 has a SEM deflection power supply 9 and a FIB deflection power supply 1
8 is connected. Further, the SEM lens barrel portion 100 is attached to the sample table 19 at an incident angle of about 60 degrees so that the cross section of the sample can be observed. The FIB lens-barrel part 200 is the sample table 19
Mounted almost perpendicular to. The acceleration voltage of the SEM lens barrel portion 100 is set to 1 KV or higher. Also,
The backscattered electron detector 21 and the secondary electron detector 23 are controlled by a backscattered electron detector control power supply 22 and a secondary electron detector control power supply 24. That is, backscattered electron detectors 21 and 2
The signal detected by the secondary electron detector 23 is amplified by the detection signal amplifier 25 and displayed on the observation CRT 26 in synchronization with the scanning synchronization signal 35. A bias voltage is applied to the backscattered electron detector 21 in order to prevent detection of secondary electrons generated during scanning of the electron beam 30 and the ion beam 31.

FIG. 2 is a plan view of a semiconductor wafer used in a cross-section observation method using the cross-section observation scanning electron microscope shown in FIG. A plurality of chips are formed on the semiconductor wafer 27, and a cross-section observation test chip 28 for cross-section observation is selected in advance.

3 to 5 are schematic views for explaining a cross-section observation process for performing cross-section observation with the scanning electron microscope for cross-section observation shown in FIG. A cross-section observation method will be described with reference to FIGS. 1 to 5. First, the third
A process of setting an ion beam scanning region for cutting out a cross section by scanning the ion beam 31 will be described with reference to the drawings. The sample table drive device 20 (see FIG. 1) moves the sample table 19 (see FIG. 1) to the vicinity of the cutout region 50 for automatic or manual cross-section observation. The ion beam 31 is scanned near the cutout region 50 to detect the secondary electrons 34 generated from the semiconductor wafer 27, and the image is displayed on the observation CRT 26. Based on the display of the CRT 26 for observation, the position of the ion beam scanning area 29 for cutting is specified by an input device such as a keyboard or a mouse. In this case, as the detector, only the secondary electron detector 23 is operated.

Next, with reference to FIG. 4, a process of cutting out a section to be observed by using the sputter etching function by the scanning of the ion beam 31 will be described. First, the ion beam scanning area
Scan the ion beam at 29 and start the cross-section cutting.
At the same time, the backscattered electron 32 of the cross section cut out by scanning the electron beam 30 is detected by the backscattered electron detector 21. The detection signal of the backscattered electron detector 21 is amplified by the detection signal amplifier and displayed on the observation CRT 26. Thereby, the cutting process by the ion beam 31 can be monitored. In this case, only the backscattered electron detector 21 is operated and the secondary electron detector 23 is not operated. In this way, the cutting process is monitored in real time and the ion beam scanning is immediately stopped when the required cross section is obtained. This makes it possible to more accurately obtain the cross section of the ultrafine region at a specific location as compared with the conventionally proposed improved example. In addition, the observation cross section can be cut out in a shorter time as compared with the conventionally proposed improved example, and the cross section observation process can be speeded up.

Next, with reference to FIG. 5, a process of observing a cross section cut out by the scanning of the ion beam 31 will be described. Only the electron beam 30 is scanned on the cross section cut out by the scanning of the ion beam 31. Secondary electrons 33 generated from the semiconductor wafer 27 by scanning with the electron beam 30 are detected by the secondary electron detector 23. The detection signal of the secondary electron detector 23 is amplified by the detection signal amplifier 25 and displayed on the CRT 26 for observation. By this secondary electron observation, highly accurate cross-sectional observation can be performed. As described above, in this embodiment, an arbitrary cross section can be cut out with extremely high accuracy and immediately observed, so that the cross section of a minute region can be efficiently observed. Further, since it can be applied to the cross-section observation of minute foreign matter mixed in a certain process, it is also effective in controlling the cleanliness of the wafer process.

Further, as the backscattered electron detector 21, the secondary electron detector 23,
A scintillator and a photomultiplier tube are used. In this embodiment, the backscattered electron detector 21 and the secondary electron detector 23 are independently provided, but the same one may be used. FIG. 6 shows a backscattered electron detector and 2 according to another embodiment of the present invention.
It is a schematic diagram of a scanning electron microscope for cross-section observation when a common electron detector is used as a secondary electron detector. Referring to FIG. 6, the secondary electron detector 23 is also used as a backscattered electron detector. That is, when a common one is used as the backscattered electron detector 21 and the secondary electron detector 23, this effect can be obtained by switching the bias voltage with the changeover switch 40. Specifically, secondary electrons are usually tens to hundreds of eV, while reflected electrons are incident electron beams.
Close to 30 energies, for example 10 KeVSEM, ~ 10
If it is KeV, 1KeVSEM, it is ~ 1KeV. Therefore, if the bias voltage of the detector is set to a reverse bias of a little less than 1 KeV, only backscattered electrons can be detected. As a result, it is possible to monitor only the image of the reflected electrons by the electron beam 30. In the present embodiment, the secondary electron detector 23 is attached so that it can detect both the secondary electrons 33 generated by the electron beam 30 and the secondary electrons 34 generated by the ion beam 31. When the relative symmetry becomes a problem depending on the irradiation position of the image brightness, a plurality of secondary electron detectors may be attached at appropriate positions for optimization. Further, in the present embodiment, the same ion beam is used for the ion beam for cutting out the cross section and the ion beam for setting the scanning region, but the present invention is not limited to this, and the ion beam on the sample surface is scanned by the ion beam. If irradiation damage is a problem, try to reduce the irradiation damage by reducing the ion beam current or by taking the secondary electron image into the image memory to minimize the ion beam irradiation dose. Good. Furthermore, in the present embodiment, physical sputtering of sample constituent atoms by direct etching with an ion beam was used, but the present invention is not limited to this, and a reactive gas is introduced through a nozzle, and an assist etching function by a chemical reaction is performed. You may make it cut out a cross section using together. In addition, in the present embodiment, the process evaluation was performed by providing the one-chip cross-section observation test chip 28 in the wafer 27 and monitoring the entire process, but the present invention is not limited to this, and is applicable to semiconductor device defect analysis. You may use.

As described above, in the cross-section observation scanning electron microscope shown in FIG. 1, the FIB lens-barrel portion 200 is integrally attached to the SEM lens-barrel portion 100, and in addition to the secondary electron detector 23, a backscattered electron detector is also provided. Ion beam 31 by installing 21 newly
When the observation cross section is cut out, the electron beam 30 is scanned to detect the reflected electrons, and the cross section can be observed in real time. Therefore, it is possible to more accurately cut out and observe a cross section of a specific portion of an arbitrary chip in the wafer in a short time.

In the cross-section observation method using the cross-section observation scanning electron microscope shown in FIG. 4, the ion beam 31 is scanned to start the cross-section cutting, and at the same time, the electron beam 30 is scanned to reflect the backscattered electrons 30 of the cross-section being cut out. Is detected by the backscattered electron detector 21, the process of cutting out the observation section by the ion beam 31 can be observed in real time. Therefore, when the necessary cross section is obtained, the ion beam scanning can be immediately stopped, and the observation cross section can be processed more accurately.

[Effect of the Invention] According to the scanning electron microscope for observing a cross section according to claim 1, two types of secondary electrons and reflected electrons are obtained by switching the bias voltage of the electron detector by the bias voltage switching means of the electron detector. Since it is configured to detect the above-mentioned electrons, it is possible to easily detect two types of electrons, that is, secondary electrons and reflected electrons, with one electron detector.

According to the cross-section observing method of any one of claims 2 to 5, while the ion beam is scanned to cut out the cross section, at the same time, the electron beam is scanned onto the cross section to detect backscattered electrons and the cross section cutting process is performed. By observing at the same time while cutting out, the cutting process of the cross section can be observed in real time, and as a result, the cross section to be observed can be cut out more accurately and observed in a short time.

[Brief description of the drawings]

FIG. 1 is a schematic view of a scanning electron microscope for observing a cross section showing an embodiment of the present invention, and FIG. 2 is a semiconductor wafer used in a method of observing a cross section by the scanning electron microscope for observing a cross section shown in FIG. 3 is a plan view, FIG. 3 to FIG. 5 are observation process diagrams for explaining a sectional observation method using the scanning electron microscope for sectional observation shown in FIG. 1, and FIG. 6 is another embodiment of the present invention. FIG. 7 is a schematic view of a scanning electron microscope for observing a cross section when a common electron detector is used as a backscattered electron detector and a secondary electron detector according to the example, and FIG. 7 is used for a cross section observing method by a conventional scanning electron microscope. It is a perspective view of the semiconductor wafer used. In the figure, 1 is an electron gun, 5 is an SEM deflection coil, 10 is a liquid metal ion source, 14 is a FIB deflection electrode, 21 is a backscattered electron detector, 23 is a secondary electron detector, 25 is a detection signal amplifier, and 26 is a detection signal amplifier. An observation CRT, 27 is a sample (semiconductor wafer), 30 is an electron beam, 31 is an ion beam, and 40 is a changeover switch. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (5)

(57) [Claims] 1. A scanning electron microscope for observing a cross section of an object to be observed by irradiating the cross section of the object to be observed with an electron beam, wherein the object to be observed is scanned with an ion beam. Ion beam scanning means, electron beam scanning means for scanning the electron beam installed at a position inclined by a predetermined angle from the ion beam scanning means, and the object to be observed by scanning the ion beam and the electron beam. First electron detecting means for detecting secondary electrons generated from the second electron detecting means, second electron detecting means for detecting reflected electrons reflected from the object to be observed by scanning the electron beam, Scanning area observing means for observing the cross-sectional state of the area scanned by the electron beam scanning means based on the detection results of the first and second electron detecting means. The first electron detecting means and the second electron detecting means are configured by one electron detector, and the one electron detector includes bias voltage switching means for switching a bias voltage, and the bias voltage switching is performed. A scanning electron microscope for observing a cross section, wherein two types of electrons, that is, the secondary electrons and the reflected electrons, are detected by switching the bias voltage of the electron detector by means. 2. A cross-section observing method using a cross-section observing scanning electron microscope for observing a cross-sectional state of an object to be observed by irradiating an electron beam onto the cross section of the object to be observed. A step of scanning the cross section by scanning an ion beam to a position where observation is performed, and simultaneously scanning the electron beam on the cross section while cutting the cross section, and detecting backscattered electrons reflected from the object to be observed to detect the cross section of the cross section. Simultaneously observing the cutting process while cutting the cross section, and detecting the secondary electron by causing the electron beam to scan the cross section cut by the step of cutting the cross section to generate secondary electrons from the observed object. And observing a cross-sectional state of the cross section scanned by the electron beam.
Section observation method using a scanning electron microscope for section observation. 3. The reflected electrons are detected by a first electron detecting means, and the secondary electrons are detected by a second electron detecting means.
A cross-section observation method using the scanning electron microscope for cross-section observation according to claim 2. 4. The reflected electrons are detected by a first electron detecting means, and the secondary electrons are detected by switching a bias voltage of the first electron detecting means.
A cross-section observation method using the cross-section observation scanning electron microscope described in. 5. The scanning electron microscope for observing a cross section according to claim 2, wherein the step of observing the cutting process of the cross section includes a step of stopping the scanning of the ion beam when a necessary cross section is obtained. Section observation method used.