JP2012216845A - Sample inspection device and creation method for absorption current image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sample inspection device and a creation method for an absorption current image which can acquire a clear absorption current image from an absorbed current detected by using a plurality of probes without including difference in amplification rate in input, and can improve measurement efficiency.SOLUTION: A plurality of probes 4 are brought into contact with a sample 2, and while irradiating the sample 2 with an electron beam 1, current that flows through the probes 4 is measured, and signals from at least two probes 4 are input into a differential amplifier 6. Output from the differential amplifier 6 is amplified and an absorption current image 7 is created on the basis of the output from the differential amplifier 6 and scanning information of the electron beam 1. Thereby, The clear absorption current image 7 can be obtained without including difference in amplification rate in input, and measurement efficiency in failure analysis of the semiconductor sample 2 can be improved.

Description

  The present invention relates to a sample inspection apparatus for analyzing a semiconductor sample or the like and a method for creating an absorption current image using the same, for example, to a technique for identifying an electrical failure location in wiring of a semiconductor sample or the like.

  In a semiconductor sample in which a circuit is formed on a semiconductor surface, it is difficult to specify a defective portion as the device is miniaturized, and the failure analysis requires a great deal of time. Therefore, analysis devices such as OBIRCH (Optical Beam Induced Resistance Change) and EB tester are currently used.

  In the semiconductor sample defect analysis, as a defect analysis related to wiring, in recent years, an electron beam is irradiated on the surface of a semiconductor sample, and a current absorbed from the wiring or a secondary signal emitted from the semiconductor sample is analyzed / imaged. Is attracting attention.

  For example, in Patent Document 1, in a semiconductor sample on which a wiring pattern is formed, a probe is brought into contact with both ends or one side of the wiring pattern, an electron beam is scanned over the wiring pattern on the semiconductor sample, and a current flowing through the probe is detected. A technique for identifying a defective portion by measuring / imaging the image is disclosed.

  Patent Document 2 discloses a technique for amplifying signals from a plurality of probes, taking a difference between the signals, scanning the differential amplified signal to display an image, and modulating and scanning an irradiation electron beam, Similarly to the above, a technique for scanning a differential amplification signal and displaying an image is disclosed.

JP 2002-368049 A JP 2003-86913 A

  As described in the above prior art, in the measurement of the current input from the probe, one probe that contacts the sample is connected to the current amplifier and the other is connected to the ground, and the signal from the probe is connected to the current amplifier. When measured, the situation is as follows.

  When the probe is brought into contact with both ends of the wiring, and the electron beam is irradiated / scanned on the semiconductor sample in this state, the current (absorption current) input to the wiring is grounded from the point where the electron beam hits, At the other end, a current (absorbed current) flows toward the current amplifier. At this time, the resistance inherent in the wiring is divided from the point where the electron beam hits to the point where each of the probes contacts. The absorbed current input from the electron beam to the wiring is shunted by the resistance value and input to the ground side or the current amplifier side. When this measurement method is used, if there is an abnormality in the wiring, a difference due to an abnormality in the resistance value can be observed, so that a defective portion can be specified. However, when the resistance value of the wiring is smaller than the input impedance of the current amplifier, an absorption current flows to the ground side rather than the current amplifier. If this difference is large, the difference between the absorption currents flowing through both of them will increase accordingly, and the absorption current will hardly flow to the current amplifier side. For this reason, it is impossible to measure a wiring having a small resistance value, and it is impossible to specify a defective portion.

  Similarly, in the case of measurement using two probes, an amplifier is configured before the input to the difference image amplifier in the conventional configuration, and the current input from the probe is amplified by different amplifiers. It is input to the differential amplifier later. In this case, since the amplified signal is input to the differential amplifier in a state including the difference in amplification factor due to the difference between the amplifiers in the previous stage of the differential amplifier, the current input from each probe is different as a result. In some cases, the difference signal between the signals amplified at the amplification factor is amplified by a differential amplifier, and a value different from the actual current is measured.

  In addition, when the signal on one side of the input signal to the differential amplifier is amplified much larger than the other signal, the output may swing to one of the positive and negative. In order to avoid this state, it is necessary to accurately adjust to the same amplification factor and the same offset for adjusting each amplifier, and the measurement itself becomes complicated.

  Furthermore, since a differential amplifier is used, when the probe is brought into contact with both ends of the wiring, a loop is formed between the input terminals by a connection cable from the probe to the input of the differential amplifier. Since this loop is affected by the magnetic field, if the magnetic field is not shielded, it may be directly superimposed on the signal due to the input current from the probe as noise due to the induced electromotive force due to the magnetic field and the impedance of the wiring.

  An object of the present invention relates to improving a measurement efficiency by acquiring a clear absorption current image from an absorption current detected using a plurality of probes without including a difference in amplification factor between inputs.

  In the present invention, a plurality of probes are brought into contact with a sample, an electron beam is irradiated on the sample, a current flowing through the probe is measured, and signals from at least two probes are input to a differential amplifier. The present invention also relates to amplifying the output from the differential amplifier and creating an absorption current image based on this and the scanning information of the electron beam.

  Further, in the present invention, a plurality of probes are brought into contact with the sample, the current flowing through the probe is measured while irradiating the sample with an electron beam, and a signal depending on the current flowing through the one probe is transmitted to the input side of the amplifier. And a signal depending on the current flowing through the other probe is input to the GND of the amplifier. And it is related with producing an absorption current image based on the output from an amplifier, and the scanning information of an electron beam.

  Further, the present invention separates the probe from the sample, irradiates the sample with an electron beam, creates noise information, contacts the probe with the sample, irradiates the sample with an electron beam, creates absorption current information, The present invention relates to creating an absorption current image based on absorption current information and noise information.

  According to the present invention, a clear absorption current image can be acquired without including a difference in amplification factor between inputs, and the measurement efficiency of the failure analysis of a semiconductor sample can be improved.

1 is a schematic configuration diagram of a sample inspection apparatus to which an embodiment of the present invention is applied. It is the figure which showed the structure of the semiconductor inspection apparatus of this Embodiment containing the structure shown in FIG. An embodiment of a method for reducing the influence of disturbance noise on the absorbed current image 7 will be described.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a schematic configuration diagram of a sample inspection apparatus to which an embodiment of the present invention is applied.

  The primary electron beam 1 is applied to the sample 2. A wiring pattern 3 is provided on the surface of the sample 2, and the probe 4 is brought into contact with both ends or pads of the wiring pattern 3. In this state, the surface of the sample 2 including the wiring pattern 3 is scanned with the primary electron beam 1 from the electron beam source 5. Of the irradiated primary electron beam 1, electrons entering the wiring pattern 3 are detected as current from the probe 4 and input to the differential amplifier 6 for amplification. The differential amplifier 6 generates and outputs a difference signal with respect to the input signal. This difference signal is displayed on the display unit 8 as an absorption current image 7 in synchronization with the scanning of the primary electron beam 1.

  The current detected by the probe 4 entering from the wiring pattern 3 is shunted by the resistance value of the wiring pattern 3 between the point where the primary electron beam 1 hits and the probe 4, and the + input of the differential amplifier 6, Alternatively, it is input to the negative input. For this reason, in the wiring pattern 3, there is a difference between light and dark in the absorption current image 7 according to the change in resistance value while being in contact with the probe 4. In the defective part on the wiring pattern 3, since the resistance value is not uniform, the change in brightness is displayed differently compared with other normal places. For this reason, a state different from the others, that is, a defective portion on the wiring pattern 3 can be easily determined as the absorbed current image 7.

  FIG. 2 is a diagram showing the configuration of the semiconductor inspection apparatus of the present embodiment including the configuration shown in FIG.

  In FIG. 2, the sample inspection apparatus includes an electron beam irradiation optical system that can irradiate an electron beam. That is, the primary electron beam 1 irradiated from the electron beam source 5 is irradiated to the sample 2 via the condenser lenses 9 and 10, the diaphragm 11, the scan deflector 12, the image shift deflector 13, and the objective lens 14. At this time, the primary electron beam 1 scans the surface of the sample 2 by the scan deflector 12 or the like.

  A secondary electron beam 15 is emitted from the sample 2 irradiated with the primary electron beam 1 and detected by the secondary electron beam detector 16.

  A secondary electron beam detector 16 that is a detector capable of detecting secondary electrons generated from the sample is controlled by the SEM control unit 17. The SEM control unit 17 has a video board 18 and a recording unit 19. A signal input from the secondary electron beam detector 16 is converted into a digital signal by the video board 18, and an image is displayed on the display unit 8 in synchronization with the scanning of the primary electron beam 1. Since it is displayed on the display unit 8 in synchronization with the scanned primary electron beam 1, the secondary electron beam 15 is displayed as an SEM image. The signal and the SEM image are recorded in the recording unit 19. The entire sample inspection apparatus is also controlled by the SEM control unit 17.

  The sample 2 is fixed to the sample holder 20 and has a structure having means capable of moving in a three-axis method of X, Y, and Z by a sample stage 21 which is a sample stage on which a sample can be placed. The probe 4 that can come into contact with the sample has a structure that can be moved by the probe stage 22 in the three-axis method of the X axis, the Y axis, and the Z axis, like the sample stage 21.

  The sample stage 21 and the probe stage 22 are moved / controlled in the three axis directions of the X axis, the Y axis, and the Z axis, respectively, and the probe 4 is brought into contact with the surface of the sample 2.

  The probe 4 is brought into contact with one or both ends of the wiring formed on the surface of the sample 2. In this state, the surface of the sample 2 including the wiring pattern 3 is scanned with the primary electron beam 1 from the electron beam source 5. Of the irradiated primary electron beam 1, electrons entering the wiring pattern 3 are detected from the probe 4 as a current. The current flowing through the probe is measured by a measuring instrument. The current detected by the probe 4 entering from the wiring pattern 3 is shunted by the resistance value of the wiring pattern 3 between the point where the primary electron beam 1 hits and the probe 4 and input to the differential amplifier 6. The The differential amplifier 6 to which the signal from the measuring instrument is input generates and outputs a difference signal with respect to the input signal. The difference signal output from the differential amplifier 6 is amplified by the amplifier 23 with an amplification factor necessary for displaying the absorption current image 7 by the absorption current from the probe 4 and synchronized with the scanning of the primary electron beam 1. The absorption current image 7 is displayed on the display unit 8 which is an image device that outputs an absorption current image based on a signal from the differential amplifier and a signal depending on scanning of the electron beam irradiation optical system.

  As described above, the SEM control unit 17 includes the video board 18 and the recording unit 19. The signal input from the probe 4 is converted into a digital signal by the video board 18 and displayed on the display unit 8 in synchronization with the scanning of the primary electron beam 1. Thereby, the distribution of the signal (absorption current signal) obtained from the current (absorption current) input from the probe can be displayed as an image (this is referred to as an absorption current image 7). The signal and the absorption current image 7 are recorded in the recording unit 19.

  For this reason, in the wiring pattern 3, there is a difference between light and dark in the absorption current image 7 according to the change in resistance value while being in contact with the probe 4. In the defective part on the wiring pattern 3, since the resistance value is not uniform, the change in brightness is displayed differently compared with other normal places. For this reason, a state different from the others, that is, a defective portion on the wiring pattern 3 can be easily determined as the absorbed current image 7.

  The SEM control unit 17 has a function of switching the signal input system for image display to the secondary electron beam detector 16 and the differential amplifier 6, and displays the absorption current image 7 by the current from the probe 4. When doing so, the probe 4 is switched to the differential amplifier 6 side.

  The absorption current image 7 is displayed by displaying the input signal for generating the absorption current image 7 on the display unit 8 in synchronization with the scanning of the primary electron beam 1.

  A switching unit 24 is mounted in front of the differential amplifier 6. While the primary electron beam 1 is being irradiated onto the sample 2, the probe 4 may be charged because the primary electron beam 1 is also irradiated onto the probe 4. When charged, the probe 4 is discharged when it approaches the sample 2. The probe 4 has a very small diameter of about several hundreds of nanometers and may be broken by damage due to discharge. Further, many of the samples 2 are vulnerable to static electricity, and there is a possibility of damage due to discharge. That is, the probe 4 and the sample 2 may be damaged by bringing the charged probe 4 close to the sample 2. The switching unit 24 is connected to the ground side until the probe 4 is brought into contact with the sample 2. And after the probe 4 contacts the sample 2, it switches to the differential amplifier 6 side. Thereby, the probe 4 can contact the sample 2 without being charged.

  Further, the switching unit 24 is configured to be able to select either a state connected to the differential amplifier 6 or a state connected to the current amplifier 25.

  When the measurement is performed by the current amplifier 25 with the probe brought into contact with both ends of the wiring of the wiring pattern 3, one of the probes brought into contact with both ends of the wiring pattern 3 is switched by the switching unit 24. The other is connected to the power supply GND of the current amplifier through a resistor. This resistance can be selected and can be switched according to the resistance value of the sample.

  When the probe 4 is brought into contact with both ends of the wiring pattern 3 on the surface of the sample 2, a circuit constituting a loop connecting the inputs of the differential amplifier 6 is generated. As a result, when an external magnetic field is generated in this vicinity, an induced electromotive force is generated in a loop such as a wiring connecting the inputs of the differential amplifier 6. Due to the impedance of this loop, current flows by the induced electromotive force, and these are input from the input of the differential amplifier 6. This is superimposed on the absorption current image 7 as noise. In this semiconductor inspection apparatus, the space between the probe 4 and the input of the differential amplifier 6 is covered with a magnetic field shield 26. Thereby, since the influence which a loop part receives from a magnetic field is reduced significantly and an induced electromotive force is reduced, the superimposed noise is also reduced significantly.

  In this embodiment, a signal flowing through each probe is directly received by a differential amplifier, so that a difference in signal amplification factor between the input systems does not occur, and the difference between the input signals itself is amplified. Since an output with no output bias can be obtained, the image quality is dramatically improved. In addition, the influence of each input signal can be reduced as compared with the conventional case, so that an absorption current image can be observed for a case where the input signal itself is smaller than the conventional one. As a result, an absorption current image can be observed even when the resistance value of the sample to be measured is smaller than the conventional one. In addition, since the first stage amplification is a differential amplifier and the effect on the offset by this amplifier is dominant, there is no need to adjust the offset for each input system. In this case, only the differential amplifier is sufficient, and it is possible to reduce the complexity of the apparatus adjustment in the observation of the absorption current image, and the convenience is greatly improved.

  In addition, one of the probes is grounded through a resistor, the other probe is connected to a current amplifier, and an input signal from the probe is displayed in synchronization with the scanning means to display an image of the probe. The current flowing in from the current flows not only on the ground side but also on the current amplifier side, so the resistance value of the sample that can be measured is widened by selecting the resistance value between the ground and the sample with a resistance value smaller than the conventional one Can be measured.

  The detection system according to the first embodiment is sensitive to disturbance noise because it is necessary to increase the amplification factor. FIG. 3 shows an embodiment for reducing the influence of disturbance noise on the absorbed current image 7. Only the main differences from the first embodiment will be described below.

  Before bringing the probe 4 into contact with the sample 2, the primary electron beam 1 is once irradiated, and an absorption current image from the probe 4 is measured. The signal input from the probe 4 is converted into a digital signal by the video board 18 and recorded by the recording unit 19. This signal is used as a background signal, and an image based on this signal is as shown in the background image 27. A signal whose polarity is inverted by the SEM control unit 17 is generated from the signal data of the background signal once recorded in the recording unit 19 and recorded in the recording unit 19. This signal is used as an inverted background signal, and an image based on this signal is as shown in the inverted background image 28. This signal is composed only of signal components of disturbance noise from the surroundings that do not depend on the sample.

Next, the probe 4 is brought into contact with the sample 2, the absorption current at that time is measured, and the input signal from the probe 4 at that time is similarly converted into a digital signal by the video board 18, and the recording unit 19 Record at. This signal is used as an absorption current signal, and an image obtained by this signal is as shown in the absorption current image 7. This absorption current signal is also superimposed with the noise of the disturbance imaged earlier. The inverted background signal recorded earlier is read from the recording unit 19, added to the absorption current signal, and displayed on the display unit 8 in synchronization with the scanning of the primary electron beam 1 (absorption current image + inversion background image 29). As a result, the background, which is a disturbance component, is canceled out, and the disturbance noise can be greatly reduced.

  In this embodiment, image quality deterioration due to noise can be significantly improved by subtracting the background noise of the absorption current image.

DESCRIPTION OF SYMBOLS 1 Primary electron beam 2 Sample 3 Wiring pattern 4 Probe 5 Electron beam source 6 Differential amplifier 7 Absorption current image 8 Display part 9, 10 Condenser lens 11 Aperture 12 Scan deflector 13 Image shift deflector 14 Objective lens 15 Secondary electron Line 16 Secondary electron beam detector 17 SEM control unit 18 Video board 19 Recording unit 20 Sample holder 21 Sample stage 22 Probe stage 23 Amplifier 24 Switching unit 25 Current amplifier 26 Magnetic field shield 27 Background image 28 Reversed background image 29 Absorption Current image + inverted background image

Claims (4)

A method of creating an absorption current image,
Separate the probe from the sample, irradiate the sample with an electron beam, create noise information based on the signal that depends on the current flowing through the probe and the signal that depends on the scanning of the electron beam irradiation,
The probe is brought into contact with the sample, the sample is irradiated with an electron beam, and absorption current information is created based on a signal that depends on the current flowing through the probe and a signal that depends on the scanning of the electron beam irradiation,
A method of reversing the polarity of the noise information and superimposing it on the absorbed current information to create an absorbed current image. A method of creating an absorption current image according to claim 1,
The method is characterized in that the sample is a semiconductor sample on which a wiring pattern is formed. A sample stage on which a sample can be placed;
An electron beam irradiation optical system capable of emitting an electron beam;
A detector capable of detecting secondary electrons generated from the sample;
A probe capable of contacting the sample,
A storage unit for storing an inverted absorption current signal obtained by inverting the polarity of an absorption current signal obtained when the probe is not in contact with the sample;
A sample inspection apparatus comprising: an imaging device that adds an absorption current signal obtained when the probe is in contact with the sample and an inverted absorption current signal stored in the storage unit. In claim 3,
The sample inspection apparatus, wherein the sample is a semiconductor sample on which a wiring pattern is formed.

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