JPH027511B2 - - 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]

Scanning Electron Microscope
A CRT (Cathode Ray Tube) is used to obtain an enlarged 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 (Cathode Ray Tube). This is what I did.

As an application of this SEM, LSI (Large Scale Integrated Circuit)
It is possible to observe the internal workings of. 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 areas with negative potential and areas with positive potential to observe the potential distribution inside the LSI. It is an effective means for analyzing LSI operation or failure.

Furthermore, in synchronous phenomena such as electrical signals propagating inside an IC or LSI, potential changes occur regularly and repeatedly for each phase. 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, by keeping the electron beam at a desired point on the sample surface and electrically changing the phase difference between the excitation of the sample and the pulsed electron beam, the horizontal axis of the CRT shows this transfer amount, and the vertical axis shows the secondary A strobe SEM has been developed that displays the voltage waveform at a desired point on a CRT screen by inputting each electronic signal amount.

By the way, sample observation can be roughly divided into two observation modes. In other words, there is a waveform mode based on the same concept as mechanically contacting a probe on the sample and observing the voltage waveform at the probe contact point with an oscilloscope, and a waveform mode based on the idea of scanning a beam over the sample surface and observing the secondary electron signal. This is an image mode in which the amount is displayed on a CRT and a potential contrast image on the sample surface at each phase point is observed. In order to functionally perform waveform observation using the SEM, a strobe SEM is usually connected to a computer for processing.

FIG. 1 shows the configuration of a conventional strobe SEM system that combines such a data processing device.
In this system, a secondary electron signal obtained from a sample in a strobe SEM 1 having a pulsed electron beam irradiation unit (not shown) is detected by a secondary electron detector 2 using a photomultiplier tube, and is converted into an analog signal. Output. The analog signal output from the secondary electron detector 2 is integrated by a passive RC integration circuit 3 provided to improve the S/N ratio, and then converted into a digital signal by an analog-to-digital (A/D) converter 4. converted into a signal. This digital signal is taken into the computer processing system 5, where predetermined data processing is executed. This computer processing system 5 controls a SEM controller 6 for synchronizing the electron beam irradiation by the pulsed electron beam irradiation unit in the strobe SEM with specimen excitation, and this SEM controller controls the scan controller 7 to control the electron beam A predetermined sample observation point is scanned by This signal from the scan controller 7 and the signal from the secondary electron detector 2 are sent to the image display (CRT) 8.
An image of the sample will be observed on the CRT8.

In the above apparatus, in order to further improve the S/N ratio, data of the same phase is captured N times and the computer processing system 5 performs averaging processing using software.

To explain the operating principle of such a device, first, the signal is sampled N times at the first phase point, the sample outputs are added, and then the signal is moved to the next phase point, and the same operation is repeated. At this time, the time t during which the beam remains at the observation point is expressed by the following equation.

_t =t _P + [(t _A + t _{S )} N + _t time, _tS is the opening time of the sampling gate, N is the number of times the signal is sampled at a certain phase point, _tX is the time required to shift the phase, and M is the number of phase steps.

FIG. 2 shows a time chart when the computer processing system does not perform software processing (N=1). When the power is turned on, the strobe SEM 1 applies an in-sample operation signal shown in FIG. 2 to a specimen installed therein, for example, a MOS type LSI. At this time, the beam pulse shown in FIG. 2 has a phase step number M
The sample is irradiated repeatedly at a specific cycle (which does not necessarily match the cycle of the sample's internal operation) until it reaches a set value. Secondary electron signals from the sample corresponding to these intra-specimen operation signals and beam pulses are detected by a secondary electron detector 2 and sent out as an analog detector output as shown in FIG. The output signal of the detector 2 is input to a passive RC integrating circuit 3, where it is integrated, and then becomes the integrating circuit output as shown in FIG. This integrating circuit output is converted into a digital signal by an A/D converter 4. At this time,
Since a sampling gate signal as shown in FIG. 2 is applied from the computer processing system 5 to the A/D converter 4, n pieces of digital data are output from the A/D converter 4 only during this sampling gate open time _t is taken into the computer processing system 5, where the average value is processed as an N=1 signal.

[Problems with background technology]

However, when performing this kind of software processing, as can be seen from equation (1) above, N
The electron beam will be irradiated onto the sample for twice as long. Sampling of signals by an integral method using the passive RC integrating circuit 3 is effective in improving the S/N ratio, but it takes a long time to measure, and the electron beam irradiation time on the sample surface increases. In general, the adverse effect of electron beam irradiation on devices depends on the electron beam irradiation dose, that is, the product of irradiation current per unit area and irradiation time, so long electron beam irradiation times are not desirable for devices. Moreover, since observation in the waveform mode described above involves point irradiation, energy is concentrated more than in the case of image mode, which is surface irradiation, and the adverse effects of irradiation are more likely to appear.

Furthermore, the interaction between the sample surface potential and the electron beam is not so much of a problem when the sample is a bipolar device, but in the case of a MOS device, electron beam irradiation causes changes in the critical voltage V _TH and other values. , it becomes difficult to accurately measure potential changes on the sample surface.

Furthermore, a fatal drawback of the passive RC integration circuit 3 is that the integral value is affected by the interval of the sample excitation repetition signal. This is not a big problem when observing a sample where the excitation frequency can be freely selected within a certain range, such as a memory circuit, but it is a problem when observing random logic such as a microprocessor (CPU). Become. In particular, before executing any instruction, the CPU must always execute a decoding routine (fetch cycle) for that instruction. For example, if you want to observe the adder circuit of a CPU that operates at a basic clock frequency of 8MHz, the adder instruction will be executed after 4 states (1 state is 125ns), so the observer will observe the adder circuit every 500ns. become. In general, the number of states varies depending on the type of CPU and instruction type, but
There are a maximum of 18 states. When such an instruction with a large number of states is executed, the DC component of the integral value of the integrating circuit 3 decreases, the S/N ratio of the output of the secondary electron detector 2 decreases, or the computer processing system 5. Inconveniences occur such as it may become impossible to read data.

For this reason, conventional strobe SEMs often use peak hold circuits that use expensive operational amplifiers, which also poses a cost problem.

[Purpose of the invention]

Therefore, the present invention reduces the adverse effects of electron beam irradiation on the sample, allows accurate sample observation without being affected by electron beam irradiation or sample excitation, is inexpensive, and fully demonstrates the performance of a strobe scanning electron microscope. The object of the present invention is to provide a strobe scanning electron microscope device that can perform the following steps.

[Summary of the Invention] The present invention provides a data collection unit having two types of circuits with different characteristics to capture predetermined analog data from the output of a detector that detects secondary electrons emitted from a sample irradiated with an electron beam, and performs observation. The circuit configuration is such that one of the two types of circuits of the data collection section can be freely selected depending on the properties of the sample to be analyzed (excitation speed, influence of electron beam irradiation, etc.).

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

FIG. 3 is a block diagram showing the configuration of a strobe scanning electron microscope apparatus according to the present invention, and is a strobe SEM having a pulsed electron beam irradiation section (not shown) that generates a pulsed electron beam to irradiate a sample.
1 includes a secondary electron detector consisting of a photomultiplier tube for detecting secondary electrons emitted from a sample in an operating state by an internal operation signal of the sample due to electron beam irradiation from a pulsed electron beam irradiation unit; 2 is provided. The output of the secondary electron detector 2 is input to a CRT 8 that displays an image of the sample surface, and is also input to a data acquisition circuit 10 that captures predetermined analog data according to the properties of the sample, converts it into digital data, and sends it out. Ru. The digital data from the data acquisition circuit 10 is taken into the computer processing system 5, and soft processing (averaging processing) is executed for each sampling gate signal (for each phase point). The control signal output by the computer processing system 5 is sent to the SEM controller 6 to synchronize pulsed electron beam irradiation and sample excitation in the strobe SEM 1 and to synchronize the sample excitation with the strobe SEM 1.
Controls a scan controller 7 that scans sample observation points within the X and Y directions.

Details of the data acquisition circuit 10 are shown in FIG. This data acquisition circuit 10 includes a first circuit to which a passive RC integration circuit 11 and an A/D conversion circuit 12 are directly connected, and a diode coupling integration circuit 1.
3 and the A/D conversion circuit 14 are directly connected, and a multiplexer circuit 1 to which these two circuits are connected and which receives the output of the secondary electron detector 2 as an input.
It consists of 5. FIG. 5 shows a detailed configuration of the passive RC integrating circuit 11, which includes a resistor 16 and a capacitor 17, and its output is input to an amplifier (OP AMP) 18.
The amplifier 18 is provided with a variable resistor 19 and a resistor 20 for gain adjustment, and the output is sent to the A/D conversion circuit 12.
has been entered. FIG. 6 shows the detailed configuration of the diode coupling integrator circuit 13.
The output of the secondary electron detector 2 is connected to the input diode 21
consisting of a resistor 22 and a capacitor 23 via
An amplifier (OP AMP) 25 is connected to the output point of this RC integrating circuit, and the source and drain of a MOS field effect transistor (MOS FET) 24 are connected between this point and the reference potential point. Electrodes are inserted. This MOS FET
A reset output terminal of the computer processing system 5 is connected to the gate electrode 24. The amplifier 25 is provided with a gain adjusting variable resistor 19 and a resistor 20.

Data acquisition circuit 10 adopting such a configuration
The operation is as follows. multiplexer 15
When the passive RC integrator circuit is selected by
The output of the electron detector 2 is integrated with a good S/N ratio by the resistor 16 and capacitor 17 in the passive integration circuit 11, and then converted into a digital signal by the A/D conversion circuit 12 and sent to the computer processing system 5. However, since a part of the once integrated signal is discharged to the secondary electron detector 2 side, when the excitation frequency is high, the DC component of the integrated output decreases, and in order to obtain a predetermined integrated output, electron beam irradiation is required. It is necessary to take a long time.

On the other hand, when the diode coupling type RC integrating circuit is selected by the multiplexer 15,
The signal integrated by the resistor 22 and capacitor 23 is held by the input diode 21 without being discharged again to the secondary electron detector 2 side, and is converted into a digital signal by the A/D conversion circuit 14 and sent to the computer. It is sent to the processing system 5. In this case, even if the excitation frequency is high, the DC component of the integrated output does not decrease, and the electron beam irradiation time can be shorter than in the case of a passive RC integration circuit. MOS FET24 installed in the diode coupling integration circuit
is for discharging the charge stored in the capacitor 17 when the beam moves from one phase point to the next phase point, and a reset signal issued from the computer processing system 5 is input to its gate terminal. As a result, the integrating circuit constituted by resistor 22 and capacitor 23 is reset.

In order for such a diode-coupled RC integration circuit to fully demonstrate its performance, it is necessary that no current leak anywhere other than the signal path. For this reason, the MOS FET should be one with low leakage, and the capacitor 23 should be one with good retention characteristics.
As the diode 21, it is appropriate to use, for example, a Shottky diode, which has little reverse leakage and low forward voltage.

Also, in a diode coupling type RC integrator circuit, the S/N is worse than in a normal passive RC integrator circuit, and random noise may be added to the output, but in this case, it can be processed by the computer processing system 5 as white noise. .

The multiplexer circuit selects the optimal integration circuit depending on the characteristics of the observation sample, such as the height of the excitation frequency and the magnitude of the influence from electron beam irradiation. For example, for a sample whose excitation frequency is high or where the adverse effects of electron beam irradiation are significant, the diode-coupled integrating circuit 13, which does not have a high S/N ratio but outputs an integrated output in a short period of time, is selected. For a sample that is low and not easily affected by electron beam irradiation, an ordinary passive RC integrating circuit 11 with a good S/N ratio is selected.

By the way, the problem of poor S/N ratio, which is a drawback of the diode coupling type integrating circuit, can be solved as follows. That is,
As shown in FIG. 7, a low-pass filter 28 for cutting off high-frequency noise components may be provided in the data acquisition circuit. In the embodiment of FIG. 7, the low-pass filter 28 is placed before the multiplexer circuit 15. Figure 8 shows low pass filter 2
8, and is a well-known example consisting of an inductance 29 and a capacitor 30.

If such a low-pass filter 28 is used, high-frequency noise on the input signal can be cut off, and the high-frequency noise of the signal that has passed through the low-pass filter 28 can be reduced.

Note that the low-pass filter 28 is inserted not only in the front stage of the multiplexer circuit 15 but also in the S/
Diode coupling integration circuit 13 with poor N ratio
A multiplexer circuit 15 and a diode coupling integrator circuit 1 are used to improve the S/N ratio only for
It can also be between 3 and 3.

[Effects of the Invention] A data acquisition circuit that allows a multiplexer circuit to arbitrarily select between a passive RC integration circuit and a diode coupling integration circuit as means for extracting a digital signal output from the output of the secondary electron detector as described above. In the strobe scanning electron microscope device, it is possible to select the optimal integration circuit according to the characteristics of the observation sample, such as the height of the excitation frequency and the magnitude of the influence from electron beam irradiation.
Accurate sample observation and minimizing the adverse effects on the sample due to electron beam irradiation can be achieved at a much lower cost than conventional strobe scanning electron microscope equipment that includes a peak hold circuit using an expensive operational amplifier. be.

In addition, in the above-mentioned strobe scanning electron microscope device equipped with a low-pass filter in the data acquisition section, even when using a diode-coupled RC integrating circuit with a poor S/N ratio, high-frequency signals riding on the input signal to be integrated are Since noise can be removed, a decrease in the S/N ratio can be prevented.

[Brief explanation of drawings]

Figure 1 is a block diagram showing the configuration of a conventional strobe SEM system. Figure 2 is a time chart showing the relationship among the intra-sample movement signal, beam pulse, secondary electron detector output, integrating circuit output, and sampling gate signal. 3 is a block diagram showing the configuration of the strobe SEM system according to the present invention, FIG. 4 is a block diagram showing the configuration of the data acquisition circuit in the strobe SEM system according to the present invention, and FIG. 5 is a block diagram showing the configuration of the data acquisition circuit in the strobe SEM system according to the present invention. 6 is a circuit diagram showing details of a diode coupling type integrating circuit in the data acquisition circuit. FIG. 7 is a configuration diagram showing a data acquisition circuit equipped with a low-pass filter. The figure is a circuit diagram showing an example of a low-pass filter. 1... Strobe SEM, 2... Secondary electron detector, 3
...Passive RC integration circuit, 4...A/D conversion circuit, 5
...computer processing system, 6...SEM controller,
7...Scan controller, 8...CRT, 9...Data acquisition circuit, 11...Passive RC integration circuit, 12,
14... A/D conversion circuit, 13... Diode coupling integration circuit, 15... Multiplexer circuit, 1
6, 20, 22, 27...Resistance, 17, 23, 30
...Capacitor, 18,25...Amplifier, 19,26
...variable resistor, 21...diode, 24...MOS
FET, 28...Low pass filter, 29...Inductance.

Claims (1)

[Scope of Claims] 1. A scanning electron microscope having a beam irradiation section that irradiates a sample surface with a pulsed electron beam, a scanning electron microscope control section that synchronizes the pulsed electron beam irradiation of the beam irradiation section and sample excitation; a secondary electron detector for detecting secondary electrons emitted from a sample irradiated with a pulsed electron beam in the scanning electron microscope; and a predetermined analog from the output of the secondary electron detector depending on the properties of the observation sample. a data collection section that takes in data, converts it into digital data, and sends it out; a computer processing system that takes in the digital data from the data collection section, performs predetermined processing, and controls the control section; and a display means for displaying an image of the output of the secondary electron detector, the strobe scanning electron microscope apparatus comprising: a passive type strobe scanning electron microscope that integrates the output of the secondary electron detector;
an RC integrator circuit and a first analog signal converting the analog output from the integrator circuit into a digital signal;
A first data acquisition circuit consisting of a digital conversion circuit, a diode coupling type integrating circuit in which a diode element is inserted between the output of the secondary electron detector and a passive RC integrating circuit, and an analog output from this integrating circuit. a second data collection circuit consisting of a second analog-to-digital conversion circuit that converts the data into a digital signal, and the first data collection circuit or the second data collection circuit is arbitrarily selected depending on the properties of the sample. A strobe scanning electron microscope apparatus, characterized in that it has a data acquisition unit comprising: a multiplexer circuit that derives the output of the secondary electron detector to the selected data acquisition circuit. 2. A scanning electron microscope having a beam irradiation section that irradiates a sample surface with a pulsed electron beam, a scanning electron microscope control section that synchronizes the pulsed electron beam irradiation of the beam irradiation section and sample excitation, and a scanning electron microscope in the scanning electron microscope. A secondary electron detector detects secondary electrons emitted from a sample irradiated with a pulsed electron beam, and digital data is obtained by capturing predetermined analog data from the output of the secondary electron detector according to the properties of the observation data. a data collection section consisting of a plurality of data collection circuits that convert the digital data into digital data and send it out; a computer processing system that takes in digital data from the data collection section and performs predetermined processing and controls the control section; and a computer processing system that controls the control section. A strobe scanning electron microscope apparatus comprising display means for displaying an image of the output of the detector, the passive type integrating the output of the secondary electron detector.
a first data collection circuit consisting of an RC integration circuit and a first analog-to-digital conversion circuit that converts an analog output from the integration circuit into a digital signal; and a connection between the output of the secondary electron detector and the passive RC integration circuit. a second data acquisition circuit consisting of a diode coupling type integrating circuit with a diode element inserted therebetween and a second analog-to-digital conversion circuit that converts the analog output from the integrating circuit into a digital signal; a multiplexer circuit that arbitrarily selects the first data collection circuit or the second data collection circuit according to the selected data collection circuit and derives the output of the secondary electron detector to the selected data collection circuit;
A strobe scanning electron microscope apparatus comprising a data acquisition section including a low-pass filter provided before the multiplexer circuit or between the multiplexer circuit and the diode coupling integration circuit.