JPH02297855A - Signal processing method for electron beam tester device - Google Patents

Abstract

PURPOSE:To eliminate false detection of the logical signal attending on the deterioration in S/N of the secondary electron signal to be generated from a sample and detect the true logical signal by integrating the secondary electron signal between respective logical steps, and performing a logical judgement, and converting the secondary electron signal to the logical value as a means for converting the detected secondary electron signal to the logical value. CONSTITUTION:A secondary electron generated from a sample is converted to the signal including the information about a potential of a sample by an analizer 3 to be entered to a secondary electron detecting unit. The entered secondary electron signal is amplified by a video amplifier at high speed to be input to an integrating circuit 9. The integrating circuit 9 continuously integrates during one cycle period of a reference clock, and the maximum voltage value during this one cycle period is sample-held to be input to a comparator 10. The comparator 10 compares the threshold voltage Vt preset at random with the integrated voltage Vi, and when Vi is larger than Vt, the logical value 1 is output, and when Vi is smaller than Vt, the logical value 0 is output. False detecting caused by the high frequency noise is thereby eliminated to detect the true logical information, and a logical electron beam tester with high accuracy can be realized.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a charged particle beam device, and particularly to signal processing of an electron beam tester device suitable for evaluating logic LSI devices.

[Conventional technology]

For measuring the internal voltage of LSIs, an electron beam tester (EB tester) using a scanning electron microscope is used instead of a conventional general-purpose tester using a stylus method. In general, a so-called strobe EB tester device that pulses an electron beam is predominant. This strobe method is a method in which a signal is applied to the sample, and in synchronization with the signal, an electron beam pulsed to an arbitrary width is irradiated onto the sample, and the voltage on the sample is measured from the secondary electron signal generated from the sample. It is. By controlling the sample application signal and the phase of the pulsed electron beam, a rapidly changing sample potential can be converted to a slow one for detection. Here, since the pulse width of the electron beam determines the time accuracy of the voltage waveform to be measured,
It is set to 1/1000 and is approximately 108 or less. The strobe method EB tester is effective for samples that operate with relatively short repetition periods (1 μs or less), such as LSIs such as memories, but for samples that operate with long repetition periods (10 μs or more), such as logic LSIs such as gate arrays. When similarly measured with high resolution (electron beam pulse width of 1 ns or less), the duty ratio of the electron beam becomes 1/104 or less, resulting in a decrease in S/H. This requires a long time for measurement, making the strobe method unsuitable. The above problem would be solved if an EB tester could develop a secondary electron detection system that irradiates a direct current electron beam without using a strobe method and responds to a sample voltage operating at high speed. This method is called the real-time method. Since the amount of secondary electron signal increases or decreases in proportion to the magnitude of the sample voltage, logical information of the sample voltage of 5v10v (high/low) can be detected. Conventionally, as this type of secondary electron detector, a detector using a combination of a scintillator and a photomultiplier has been used. Recently, microchannel plates (hereinafter abbreviated as MCP) with good high-speed response are also being adopted. There is a report that realizes a real-time method of about 9 MHz using these detectors [Reference: M. Ostrov et al., “Icy Internal Electron Beam Logic State Analysis; Scanning Electron Microscopy No. 563
~572 pages (M., Ostrow, E., Menzel, e.
tc”IC-INTERNAL ELECTRON B
EAM LOGIC5TATE ANALYSIS:
5CANNING ELECTRON MICRO5
COPY/1982/Seal (Pages563-572)
”] Here, the determination of '1'/'O' (high/low) is made by setting an arbitrary threshold voltage for the secondary electron signal generated from the sample, and determining whether it is higher or lower than that voltage level. Obtaining logical information.

Problem 1 to be Solved by the Invention In the above-mentioned prior art, S of the secondary electron signal mainly depends on the primary electron probe current rp and the frequency band f of the detection circuit, and is given by the following equation. Since the real-time method uses a high-speed amplifier circuit (frequency band is about 100 MHz), the S/N ratio of the secondary electron detection signal deteriorates significantly, making it difficult to detect a faithful logic signal from the sample. An object of the present invention is to detect a true logic signal by eliminating erroneous detection of a logic signal caused by deterioration of the S/N ratio of a secondary electron signal generated from this sample. [Means for solving the problem] The above purpose is to synchronize with the reference clock applied to the sample.
Means for integrating the secondary electron signal during the period, means for generating an arbitrary threshold voltage. This is achieved by comprising means for comparing the integrated voltage with an arbitrary threshold voltage and means for storing the result.

[Effect]

Logic LSI elements generally operate logically in synchronization with one or two reference clocks. Therefore, by integrating the detected secondary electronic signal one cycle at a time in synchronization with this reference clock, it is possible to remove high frequency noise higher than the frequency of the reference clock and detect -period average logical information. By comparing an arbitrary threshold voltage and its integral value, the logical value '1' can be obtained.
/'O''. Through the above-described operation, the logical information of the sample can be detected correctly, and a highly accurate logical tester device can be realized.

【Example】

An embodiment of the present invention will be described below with reference to FIG. A primary electron beam 2 emitted from a field emission (FE) electron gun 1 is narrowly focused by an objective lens 4, and is spot-irradiated onto an arbitrary measurement location of a logic LSI 5, which is a sample. An arbitrary test pattern is applied to the sample 5 from the pattern generator 7. This test pattern includes 1, for example, high and low clocks operating at a clock frequency of 50 MHz.
Thousands of 1'/'O' (5V10V) patterns of ow) are applied continuously, and various patterns are applied from hundreds of external pins of the LSI element. FIG. 2(a) shows the reference clock waveform. The secondary electrons generated from the sample are converted into a signal containing sample potential information by the energy analyzer 3. incident on the secondary electron detector. The incident secondary electron signal is amplified by a high-speed video amplifier (frequency band: 100 MHz) 8 and converted into a voltage signal. FIG. 2(b) shows a detected secondary electronic signal waveform of a certain logical pattern. As is clear from the figure, this secondary electron signal waveform has a very poor S/H due to quantization noise, FE noise, amplifier noise, etc. that depend on the amount of electron beam. Conventionally, this signal was converted to a logical value by comparing it with an arbitrary threshold voltage, making it impossible to accurately detect the logic. Therefore, in this example,
This secondary electron signal is input to an integrating circuit 9. The integrating circuit 9 continuously integrates for one period of the reference clock, and its waveform becomes as shown in FIG. 2(c). The maximum voltage value during this period is sampled and held and input to the comparator 10. The comparator compares an arbitrarily set threshold voltage Vt with the integrated voltage Vi, and
i) If V t, output the logical value '1', and Vi<V
If t, a logical value '0' is output. As a result, Figure 2 (d)
becomes the logical value of These logical values are sequentially stored in the memory 11. As a result, false detections due to high frequency noise can be removed and true logical information can be detected. As mentioned above, the logical information stored in the memory is imported into the computer and simulated in advance.
A comparison is made with the D data and a failure analysis of the logic LSI element is performed.

【Effect of the invention】

According to the present invention, since true logic information can be detected, a highly accurate logic EB tester device can be realized.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an EB tester according to an embodiment of the present invention, and FIG.
The figure is a pulse waveform diagram for explaining signal processing in an embodiment of the present invention.Explanation of symbols 1...electron gun, 2...secondary electron, 3...energy analyzer, 4...objective Lens, 5... sample,
6...Secondary electron detector, 7...Patter generator, 8...Video amplifier, 9...Integrator circuit, 10...
·comparator. Tatsumi / 斤JeKetaRator sum 1 (δ2Kho ho sophistry is l

Claims (1)

[Claims] 1. In an electron beam tester device that narrows down the electron beam, irradiates it onto the sample LSI element, detects the generated secondary electrons, and measures the internal voltage of the sample, a logical value is determined from the detected secondary electron signal. 1. A signal processing method for an electron beam tester apparatus, comprising: integrating a secondary electron signal during each logical step, performing a logical judgment, and converting the signal into a logical value.