JPH0329867A - Observation of voltage distribution image by electron beam apparatus - Google Patents

Abstract

PURPOSE:To obtain a highly faithful voltage distribution image by respectively irradiating a sample for correcting a lattice-like electrostatic latent image pattern and a sample to be observed with electron beam and comparing the detector outputs of the secondary electrons from both samples and using the comparing result. CONSTITUTION:Electron beam EB is projected on a sample 30 for correcting a lattice-like electrostatic latent image pattern and deflected by a deflector 14 and the entire surface of the sample is subjected to two-dimensional scanning by electron beam and the secondary electron SE generated from the irradiation point with the electron beam is collected by a detector 10 and a peak/bottom non-uniform waveform due to the potential effect of a member is outputted. An A/D converter 16 digitalizes said waveform based on the signal of a controller 28 and the digital data is sampled along with the irradiation point position data to be written in an image memory 20 to be set to correction data. Next, a sample 20 to be observed is subjected to two-dimensional scanning by electron beam in place of the sample 30 and the generated secondary electron is collected by the detector 10 to be written in an image memory 40 in the same way. This digital value is divided by that of the memory 20 while correspondence is taken based on the position data to be written in an image memory 44. By this method, the potential distribution image of the sample 20 can be displayed highly faithfully.

Description

[Detailed Description of the Invention] [Summary of the Invention] Regarding a method for observing potential distribution images of a sample such as an LSI chip using an electron beam device, the detected secondary electron signal is corrected and emitted from the sample surface at the same voltage. The secondary electron signals are made to be as similar as possible,
The aim is to provide a voltage distribution image with high fidelity.The sample surface is scanned with an electron beam, the secondary electrons emitted during the scanning are collected by a detector, and the output of the detector is used to detect the sample. In a method of observing a voltage distribution image on a surface, a correction sample having a grid-like electrostatic latent image pattern is used, the sample is replaced with the observation target sample, and the resulting secondary electron detector is scanned with an electron beam. The output is stored in a memory, and then the surface of the sample to be observed is scanned with an electron beam, the obtained secondary electron detector output is divided by the readout output of the memory, and the result is calculated as a voltage. It is planned to be used for distribution image observation.

[Industrial application field]

The present invention relates to a method of observing a voltage distribution image of a sample such as an LSI chip using an electron beam device.

2. Description of the Related Art Techniques for observing the voltage distribution of a sample using a scanning electron microscope (SEM) have been known.

This technique is used to obtain voltage distribution images of wiring to observe the operating state of LSIs, and to observe electrostatic latent images. The former is, for example, Furukawa, Inagaki: "Non-contact diagnostic technology for ISI", Transactions of the Institute of Electrical Engineers of Japan, Vol. 107. No. 3, 245J
The latter is, for example, Omotani, Tanaka, and Hoshino: "SEM Observation of Surface Charges Created by Corona Ion Irradiation," Proceedings of the Electrostatic Society of Japan '86. 1 7 9 I am familiar with J.

[Prior Art] As a method of obtaining a voltage distribution image on a surface, there is a method in which a metal needle with a minute tip is brought into contact with the surface and the voltage is measured.
If this is done with a miniaturized LSI, there are problems such as it is difficult to bring the metal needle into contact with the internal electrode wiring, there is a risk of damaging the LSI, and it is difficult to measure high-speed signals because of the capacitance.

Observation using an electron beam involves scanning the surface of an LSI chip (sample) in an operating state with an electron beam, guiding the secondary electrons generated at this time to a secondary electron detector, and using the output of the detector to modulate the brightness of a CRT display. This can be done by displaying an image of the potential distribution on the sample surface on the surface.

Secondary electrons are emitted when the sample surface is irradiated with an electron beam, and the amount of secondary electrons detected by the secondary electron detector is influenced by the potential of the sample surface and the electric field distribution formed on the sample surface. receive. Generally, a portion with a low potential has a large number of emitted secondary electrons, and this portion of the voltage distribution image on the display appears bright. Conversely, in a high-potential area, fewer secondary electrons are emitted, and the potential distribution image becomes dark. This brightness indicates the potential distribution on the sample surface. Electron beams are very effective because voltage distributions can be obtained quickly and with high resolution.

[Problem to be solved by the invention]

However, it is generally not easy to obtain a secondary electron signal that faithfully reflects the voltage distribution. As shown in FIG. 4, even if the wiring has the same voltage, the level of the secondary electron signal generally differs. In this figure, 10 is a secondary electron SE detector, 12
is an amplifier, l4 is a deflector for the electron beam EB, 20 is a sample (LSI chip), and ■■...■ is a wiring formed on the sample, which is supplied with the same constant voltage by a power source 22. ing.

Even if the wiring has the same voltage, the secondary electron signal is as shown in Figure 4 (b)
The reason why the levels differ is that the trajectory of secondary electrons is affected by various electric fields, and the amount of detected secondary electrons varies depending on the position of the wiring.

The present invention focuses on this point, and corrects the detected secondary electron signals so that the secondary electron signals emitted from the sample surface of the same potential are as similar as possible, thereby improving the fidelity. The purpose of this is to provide a high voltage distribution image.

[Means to solve the problem]

As shown in FIG. 1, in the present invention, a correction sample 30 having a grid-like electrostatic latent image pattern is prepared.

In FIG. 1(a), the shaded area is, for example, a high potential area, and the shaded area is the opposite low potential area. As shown in FIG. 1(b), such an electrostatic latent image pattern is created by uniformly charging a medium in which a conductor 32 and a semiconductor 34 are laminated, which is used in a copying machine or the like.
It can be easily visualized by optical image projection.

The correction sample with this electrostatic latent image is set on the SEM sample stage, scanned with an electron beam, the generated secondary electrons are collected by a secondary electron detector, and the output of the detector is stored in memory. Remember.

After that, set the sample to be observed on the SEM sample stage,
Scanning is performed with an electron beam, the generated secondary electrons are collected by a secondary electron detector, the output of the detector is divided by the readout output of the memory, normalized, and the result is used for voltage distribution image observation. It should be used for

[Effect]

When a correction sample having an electrostatic latent image as shown in Fig. 1(a) is scanned with an electron beam, for example, when scanning the line A-A in Fig. 1(a), the output of the secondary electron detector is It should look like Figure 1(C), but it actually looks like Figure 1(d).

That is, in addition to the high and low detection outputs due to the high voltage portions (hatched portions) and low voltage portions (white portions) of the sample, the high and low values depending on the positions of these portions are added.

The height and low depending on the position are unique and determined by the structure of the SEM, the mounting position of the secondary electron detector, etc. Therefore, even when the sample to be observed is placed on the sample stage and scanned with an electron beam, the secondary electrons cannot be detected. This is what comes into the device output. Therefore, the secondary electron detector output when the correction sample 30 is scanned with an electron beam is stored in a memory, and the secondary electron detector output obtained when the observation sample is scanned with an electron beam is read out from the memory. By dividing by I can do it.

〔Example〕

FIG. 2 shows a means for obtaining a voltage distribution correction signal, and FIG. 3 shows a means for correcting with the obtained correction signal to obtain a voltage distribution image with high fidelity.

First, in FIG. 2, numeral 30 is a correction sample having the grid-like electrostatic latent image pattern shown in FIG. Semiconductor 3 of sample 30
Selenium (Se) or the like is generally used as the material 4, and uniform charging is normally achieved by corona discharge. A narrowly focused electron beam EB is projected onto the sample 30 and deflected by a deflector 14 to two-dimensionally scan the entire surface of the sample 30 with the electron beam. This scanning data (X, Y deflection data) is generated by a data generation circuit 26, the data generated by the circuit is converted into an analog signal by a D/A converter 24, and then sent to the X, Y deflection data via an amplifier (driver) 22. Add to 14.

When the sample 30 is irradiated with the electron beam EB, secondary electrons SE are generated from the irradiation point. The generated secondary electrons SE are small when the potential of the irradiation point is high, and large when the potential is low, and secondary electrons from areas with high potential have low energy, and those from low areas include those with high energy. This can be solved with a high degree of accuracy by using an energy analyzer (
Enables voltage measurement (with an error of 1% or less, etc.). Detector IO
It collects the ephemeral secondary electrons SE and generates a voltage output. The output of this detector is as shown in FIG. 1(d).

In other words, since it is uniformly charged and the charge is removed to form a striped pattern, the output should originally be as shown in Figure 1 (C), but due to the influence of the potential of various parts on the sample 30, secondary The peak of the waveform of the detection output and the volume may become uneven due to the mounting position of the electronic detector.

The amplifier 12 amplifies this, and the A/D converter 16 converts it to digital. In the A/D conversion, sampling and conversion of each obtained sample (analog) into a digital value are performed, and the sampling is performed according to a signal from the controller 28.

The controller 28 takes in the deflection data output by the data generation circuit 26, and since this corresponds to the coordinates of the irradiation point of the electron beam EB on the sample 30, it is used as the position data shown in FIG. /D converter 16 to sample, and the A/D converted value and the position data are stored in the image memory 2.
Write it to 0 and use it as correction data.

The interpolation processing circuit 18 performs interpolation processing on the output of the A/D converter 16 to form a dotted line in FIG. 1(d) (creating an envelope line).

That is, what is needed as correction data is not a signal representing the grid-like voltage distribution, but a fluctuation component that causes it to swing high/low, and this is the envelope shown in FIG. 1(d).

Note that the sampling at the A/D converter l6 is shown in Figure l (d
) for each peak point of the waveform (although it is the same even if it is performed only for each bottom point), and if the peak value and peak point coordinates are written to the image memory 20 and used as sleeve correction data, interpolation processing is performed. is not necessary. In this case, between each sampling point, use the correction data before/after that point.
For example, correction data read from memory is interpolated and used. The size of the lattice spacing of the lattice potential distribution pattern is determined according to the required correction accuracy. The size of the correction sample 30 is set to match the size of the observation sample, for example, an LSI chip, or to match the scanning range of the electron beam.

Since the correction data is unique to the SEM, it only needs to be collected once. However, the fineness/coarseness of the latent image pattern of the correction sample may be changed in accordance with the fineness/coarseness of the voltage distribution pattern of the observation target sample, and the sample may be re-sampled.

In FIG. 3, the observation sample 20 is placed on the SEM sample stage instead of the correction sample 309 10 and two-dimensionally scanned with an electron beam. The generated secondary electrons are collected by a detector 10,
The signal is amplified by the amplifier l2, A/D converted by the A/D converter 16, and temporarily written into the image memory 40. Again, the digital value of each sample is written along with its position data. Next, this image memory 40 and the image memory 20 storing the correction data are read out, and the digital value of each sample from the image memory 40 is divided by the digital value of each sample from the image memory 20 while making correspondence with the position data. A circuit 42 is a circuit that performs this division. In the figure, this is a hardware image, but it may also be software. Also memory 20.4
0, the position data is a memory address, and if a sample digital value is written in that memory address, correspondence with the above position data can be easily taken.

The division result is written to the image memory 44, and when it is repeatedly read out and displayed on the CRT display 46, the sample 2
A potential distribution image of 0 is displayed with high fidelity with the effects of potentials of various members removed.

The observation sample 20 may be an LSI chip or a sample storing an electrostatic latent image. Samples that store electrostatic latent images include X-ray microscope samples. This sample is also a laminate of a conductor 32 and a semi-circular body 34 as shown in Figure 1(b), and after being uniformly charged, when X-rays are irradiated through the observation object, the structure of the observation object (X-ray transmittance distribution ) is obtained.

When this is scanned by the electron beam EB and displayed on the display 466 as described above, the structure to be observed can be visually observed.

The obtained voltage distribution image can not only be displayed on a display, but also be made into a hard copy, subjected to logical/arithmetic processing, and the like.

〔Effect of the invention〕

As explained above, according to the present invention, the internal wiring of an LSI and the voltage distribution of an electrostatic latent image can be observed with high fidelity using an electron beam.
This is very effective for I diagnosis and surface voltage distribution measurements.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the principle of the present invention, FIGS. 2 and 3 are block diagrams showing embodiments of the present invention, and FIG. 4 is an explanatory diagram of a conventional voltage distribution image acquisition method. In FIG. 1, 30 is a sample for correction, in FIG. 2, EB is an electron beam, SE is a secondary electron, 20 is an image memory for storing correction data, and in FIG. 3, 42 is a dividing circuit.

Claims (1)

[Claims] 1. A method in which a sample surface is scanned with an electron beam, secondary electrons emitted during the scanning are collected by a detector, and a voltage distribution image on the sample surface is observed from the output of the detector. , using a correction sample with a grid-like electrostatic latent image pattern, replacing the sample with the observation target sample and scanning with an electron beam;
The obtained secondary electron detector output is stored in a memory, and then the surface of the sample to be observed is scanned with an electron beam, and the obtained secondary electron detector output is divided by the readout output of the memory. A method for observing voltage distribution images using an electron beam device, characterized in that the results are calculated and the results are used for observing voltage distribution images.