JPH041458B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a strobe scanning electron microscope apparatus.

[Technical background of the invention and its problems]

Scanning Electron Microscope
Microscope] generally obtains a microscope image of the sample surface by irradiating the sample surface with an electron beam and displaying the response corresponding to potential changes on the sample surface on a CRT.

This SEM can be used to observe the internal operation of LSIs (large scale integrated circuits). This involves observing an LSI with a potential distribution on its surface using a secondary electron image using an SEM, and using the difference in contrast between negative potential areas and positive potential areas to observe the internal potential distribution of the LSI. It has become an effective means of analyzing LSI operation or failure.

Furthermore, in synchronous phenomena such as electrical signals propagating inside an IC or JSI device, potential changes occur regularly and repeatedly each time. Therefore, if a pulsed electron beam is repeatedly applied only to a certain phase, the output signal will correspond to the potential at this phase. Therefore, the beam is stopped at a desired point on the sample surface, the phase difference between the sample excitation and the pulsed electron beam is electrically changed, and this phase shift amount is plotted on the horizontal axis of the CRT.
A strobe SEM has been developed that displays a voltage waveform at a desired point on a CRT screen by inputting the amount of secondary electron signals on the vertical axis.

As explained above, sample observation can be roughly divided into the following two observation modes. In other words, a waveform mode similar to that in which a mechanical probe is placed on a sample and the voltage waveform at the probe contact point is observed with an oscilloscope, and a beam is scanned over the sample surface.
Display the secondary electron signal amount on a CRT and observe the potential contrast image on the sample surface at each phase point.
This is an image mode using SEM. FIG. 1 shows such a conventional strobe SEM system. In this system, a pulsed electron beam irradiation section 1
The secondary electron signal obtained from the sample 3 by the strobe SEM 2 having the SEM 2 is detected by the secondary electron detector 4 using a photomultiplier tube, and is output as an analog signal. The analog signal output from this detector 4 is taken in to a display device 6 via an amplifier 5, where the amount of secondary electron signal is displayed and a potential contrast image 7 on the sample surface at each phase point is observed. Ru.

In addition, in the above device, the sample 3 is
The surface is scanned and the secondary electron image is displayed on the display device 6.
In order to display the image above, a ramp wave shown in FIG. 2 is applied to the X-axis scanning coil 8 and the Y-axis scanning coil 10. This ramp wave is generated by an internal oscillator 21 connected via amplifiers 12 and 14 and a changeover switch 16. On the other hand, when irradiating an arbitrary position on the sample 3 with an electron beam, the changeover switch 16 connects the variable power supply circuits 22 and 24 to apply DC voltage to the X- and Y-axis scanning coils 8 and 10. , determine the point irradiation position. When the position of the beam is moved, the emission position of the secondary electrons is changed, and therefore the bright spot on the display device 6 is also moved accordingly.

Therefore, in order to obtain a voltage waveform at an arbitrary observation point on a sample using a conventional strobe SEM system, first, the surface of the sample 3 is scanned and a secondary electron image is displayed on a display device. Next, by switching to the waveform mode and changing the voltage applied to the scanning coils 8 and 10 to scan the beam position, the position of the observation point is detected from the bright spot 26 on the display device 6. Here strobe SEM
Since the display CRT 6 has a high afterimage property, the image 7 remains even when switching from image display to point irradiation. Therefore, while observing the image 7, move the bright spot 26 and set the beam position to an arbitrary position. I can do it.

However, with conventional strobe SEM systems,
Measurement is extremely complicated because each observation point must be positioned and measured every time. Furthermore, since positioning is performed using afterimages, the positioning accuracy is poor, and if a high magnification display is used to accurately align the position, the amount of irradiation per unit area becomes large, which greatly affects sample movement. Furthermore, since the beam is irradiating the sample surface even during alignment, it has a great influence on the sample, as described above.

[Purpose of the invention]

The present invention was developed in view of the above points, and it is possible to realize accurate positioning of observation points, to specify multiple points at one time, and to shorten measurement time.
The object of the present invention is to provide a strobe scanning electron microscope device in which the specimen is not affected by the irradiation by not irradiating the specimen with an electron beam during the alignment operation.

[Summary of the invention]

The present invention provides a scanning electron microscope having a beam irradiation section that irradiates a sample surface with a pulsed electron beam;
a scanning control unit that synchronizes the pulsed electron beam irradiation of the beam irradiation unit, sample excitation, and a scanning signal; and a scanning control unit that detects electrons or X-rays emitted from the sample irradiated with the pulsed electron beam in the scanning electron microscope. a detector, a storage device for storing the output from the detector, a display device for displaying the stored contents, and a desired address in the storage device specified by the display device is read and the address is sent to the scanning control section. A strobe capable of automatically measuring multiple points from the same screen by being equipped with a means for inputting signals, and eliminating the influence of irradiation by not irradiating the sample with an electron beam while aligning the observation points. This is a scanning electron microscope device.

[Embodiments of the invention]

Hereinafter, the present invention will be described in detail based on examples with reference to the drawings. FIG. 3 shows the configuration of a strobe scanning electron microscope apparatus according to the present invention. In the figure, the same parts as in FIG. 1 are designated by the same reference numerals and will be explained. In Figure 3, strobe SEM2
Inside is a pulsed electron beam irradiation unit 1 that generates a pulsed electron beam to irradiate the sample surface with the beam.
is provided. Due to the electron beam irradiation from this pulsed electron beam irradiation unit 1, 2 is emitted from the sample 3 which is in the operating state according to the intra-specimen operation signal.
Secondary electrons are detected by a secondary electron detector 4. Here, to the X-axis scanning coil 8 and the Y-axis scanning coil 10, a staircase wave obtained by converting a signal generated by a scanning control section 31 by DA converters 32 and 33 is applied. These staircase waves are in a free-running state, and their waveforms are shown in FIG. The scanning coil controlled by this step wave allows the electron beam to scan the sample surface in two directions.
Scan dimensionally. Note that the sample excitation, electron beam irradiation, and scanning signal are synchronized by the CPU 34 to keep the amount of electron beam irradiated to each point of the sample constant.

Further, the signal detected by the secondary electron detector 4 is input to the AD converter 38 via the amplifier 36 and converted into a digital signal. This converted digital signal is stored in the image memory 40 and then displayed on a display device such as a CRT 6. Here, the number of pixels of the image memory 40 matches the number of scanning lines of the display device, and at the same time, the number of steps of the staircase wave is also made to match. For example, if the number of scanning lines of a display device is
If it is 512 x 512, the number of pixels in the image memory is also 512 x 512.
At the same time, the number of steps of each staircase wave is 512.
Make it into a book. Therefore, one pixel corresponds to one step. Further, a cross-shaped cursor 42 is displayed on the CRT 6 at the same time as the sample image, and its position can be arbitrarily varied by a controller 44.

Next, when observing the voltage waveform at any point on the sample 7 obtained on the CRT 6, use the cursor 4 mentioned above.
is moved by the controller 44, and the cursor 42 is placed on the point where the waveform is to be measured. The address in the image memory corresponding to the pixel on which the cursor is placed is read by the address reading device 46 and stored in the address storage device 48. At this time, only one waveform observation point may be measured, but an arbitrary number of addresses of images successively read by the cursor are stored in the address storage device 48. Next, these addresses are scanned by the scanning control unit 31.
and pass through the DA converters 32 and 33,
By applying a voltage to the scanning coils 8 and 10, it is possible to continuously irradiate an arbitrary number of designated positions with the beam.

Next, the main parts of the strobe scanning electron microscope apparatus shown in FIG. 3 according to the present invention will be explained in detail based on FIGS. 5 to 8. In FIGS. 5 and 7, the same reference numerals as in FIGS. 1 and 3 are given to one part for explanation. FIG. 5 shows the image memory 40, display device 6, controller 44, and
4 is a configuration diagram for explaining in detail an address reading device 46 and an address storage device 48. FIG.

In Figure 5, the reading operation of the measurement position is as follows:
By operating the X-axis and Y-axis cursor switches 50 and 51 provided on the controller 44, the cursor 42 displayed on the CRT 6 in a superimposed manner with the secondary electron image 7 is moved to detect the measurement position. ing. The amount of movement by the cursor switches 50, 51 is read by cursor address counters 52, 53 for the X and Y axes. With a cursor switch, pressing the switch once moves the cursor by one pixel. On the other hand, the synchronization signals _HD and _VD for image display are generated by the display synchronization signal generator 54, and are constantly counted and output by the display X-axis counter 56 and Y-axis counter 58, respectively.
Here, as shown in Fig. 6, a pulse of the synchronizing signal V _D is generated every 512 pulses of the synchronizing signal _HD .
The CRT6 screen is scanned sequentially. The outputs of the cursor address counters 52, 53 and the display counters 56, 58 are compared by comparators 60, 62 for X and Y, respectively, and when the two outputs match, the comparators generate an output. The X and Y display counter outputs are converted into scanning signals by the memory address selector 64, and the image memory 40 is sequentially scanned to read out secondary electronic image signals. The contents of the image memory are displayed through the data selector 66.
This data selector 66 is controlled by the outputs of the X and Y comparators 60 and 62.
That is, the cursor address counter 52,5
3 and the outputs of the display counters 56 and 58 match, the output from the image memory becomes blanking, and instead, the cursor display data generated by the cursor data generator 68, such as white, is displayed on the CRT. Data for displaying a cross is output onto the CRT 6 via the DA converter 70.

According to the operations described above, the output of the cursor address counter is superimposed on the secondary electronic image on the CRT, resulting in a white cross (cursor) display. As a result, by aligning the cursor on the CRT with the measurement position, the value can be obtained as the output value of the cursor address counter corresponding to the address on the image memory. These values are sequentially stored in the coordinate storage device 48.

Next, the scan control section of the present invention will be explained in detail based on FIGS. 7 and 8.

Normally, when observing a secondary electron image, the electron beam is transmitted to the XY scanning clock generator 7 as shown in FIG.
The scanning is controlled by an X scan clock and a Y scan clock generated by 0. Here, the X scanning clock and Y scanning clock are shown in FIG. Each scanning clock is synchronized with synchronizing signals _HD and _VD shown in FIG. Each clock input is an X counter 72, a Y counter 72,
Counter 74 counts X, YDA
It is converted into a binary signal for converters 32 and 33.

Next, when the beam is stopped and a single point is irradiated, the scanning clock input to the counter is disabled by the beam scanning inhibit signal from the CPU 34, and the CLK inhibit gates 76 and 78 are inhibited, and the input from the X and Y coordinate storage device 48 is disabled. Depending on the data, the binary signal output corresponding to the measurement position is output to the DA converter 32,3.
3, and the beam position is set by the scanning coil.

In the above embodiment, the observation was performed by detecting an electron beam, but the observation is not limited to this, and it is also possible to perform the observation by detecting an X-ray.

〔Effect of the invention〕

As explained above in accordance with the embodiments, since the work of searching for and aligning the measurement point at any position on the sample can be performed on the image memory, there is no need to irradiate the sample with an electron beam during the alignment work. Since the sample is irradiated only when displaying the image of the sample on the CRT and when irradiating the beam to a positioned location, changes in sample characteristics due to irradiation can be kept to a minimum compared to conventional methods. Therefore, as the ultra-high density of integrated circuits progresses, the characteristics of the specimen become more susceptible to fluctuations due to the reduction in drive voltage and the shrinkage of the design room. It is suitable for use.

Further, since the address reading device is provided with an address storage device for temporarily storing image memory, a large number of measurement points can be specified. As a result, many specified positions can be automatically measured, reducing measurement time and improving operability.

Furthermore, since the resolution of the position setting corresponds to one pixel on the image display, the accuracy of the position setting is improved. Furthermore, since the measurement accuracy is improved by the method of the present application, high magnification observation as in the conventional method can be avoided, and changes in the characteristics of the sample can be kept to a minimum.

[Brief explanation of drawings]

FIG. 1 is a basic configuration diagram of a conventional strobe scanning electron microscope device, and FIG. 2 is a diagram showing scanning signal waveforms applied to the X-axis and Y-axis scanning coils of the device in FIG.
FIG. 3 is a basic configuration diagram of a strobe scanning electron microscope according to an embodiment of the present invention, and FIG. 4 is a diagram showing scanning signal waveforms applied to the X-axis and Y-axis scanning coils of the apparatus in FIG. 3. 5 and 7 are diagrams for explaining in detail the main parts of the strobe scanning electron microscope according to the present invention, and FIGS. 6 and 8 are signal waveforms for explaining FIGS. 5 and 7. It is a diagram. In the figure, 1...pulse beam irradiation section, 2...stroboscopic scanning electron microscope (SEM), 3...sample, 4...2
Secondary electron detector, 5, 12, 14, 36...Amplifier, 6...Display device (CRT), 7...Secondary electron image, 8...X-axis scanning coil, 10...Y-axis scanning coil, 31 ...Scanning control unit, 32, 33...DA
Converter, 34... CPU, 38... AD converter, 40... Image memory, 42... Cursor,
44 controller, 46...address reading device, 48...address storage device.

Claims (1)

[Claims] 1. A scanning electron microscope having a beam irradiation unit that irradiates a sample surface with a pulsed electron beam, a scanning control unit that synchronizes the pulsed electron beam irradiation of the beam irradiation unit, sample excitation, and a scanning signal, and A detector that detects electrons or X-rays emitted from a sample irradiated with a pulsed electron beam in a microscope, a signal converter that converts an analog signal from this detector into a digital signal, and a digital signal from this signal converter. a storage device for storing signals; a display device for displaying the stored contents; an address reading device for reading an address corresponding to desired data in the storage device; and an address storage device for inputting the stored addresses to the scan control section, and the scan control section directs the pulsed electron beam to measurement points on the sample corresponding to the plurality of stored addresses from the address storage device. What is claimed is: 1. A strobe scanning electron microscope apparatus characterized by having a control means for sequentially irradiating and repeating this irradiation.