JPH0428284B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an alternating current electrical signal generated on a sample such as a semiconductor wafer by intermittently scanning the surface of the sample with a luminance-modulated light spot. The present invention relates to an improvement in a scanning photon microscope, which is a property measuring device that detects and displays an image of the property distribution of a sample, and particularly relates to improving the quality of images in this type of microscope.

[Background of the invention]

Conventionally, brightness modulation using AC signals has been used as a device for non-contact and non-destructive measurement and inspection of the distribution of electrical properties such as lifetime, diffusion length or surface potential of decimal carriers in semiconductor wafers, and for displaying the results as images. A structure has been proposed in which a semiconductor wafer is scanned with a light spot generated by the laser beam, and the alternating current photovoltage generated at this time is detected via the capacitance of the air gap (Japanese Patent Application Laid-Open No. 155543/1983, etc.). In this case, in order to ensure the positional accuracy of the measurement point and to simplify the apparatus, it is necessary to scan the optical spot on the semiconductor wafer intermittently.

However, as a result of prototyping a characteristic measuring device for this purpose, it was found that there was a problem in that periodic striped image noise was superimposed on the displayed image of the electrical characteristic distribution, resulting in a significant loss of image quality. Some kind of countermeasure is desired.

[Purpose of the invention]

Therefore, an object of the present invention is to provide a scanning photon microscope that can obtain high quality images.

[Summary of the invention]

In order to achieve the above object, the present invention includes: a light spot generating means for generating a light spot whose brightness is modulated at a predetermined frequency; a light spot focusing means for focusing the light spot on the surface of a sample having a photoelectric effect; A light spot intermittent scanning means for intermittently scanning a spot on the surface of a sample; a signal detection means for detecting an alternating current electrical signal generated on the sample by scanning the light spot; and a position of the detected alternating current electrical signal. The present invention is characterized in that the scanning photon microscope is constructed from an image display means for displaying a change in output as an image, and a synchronous operation means for synchronously operating the light spot generation means and the light spot intermittent scanning means.

This characteristic configuration of the present invention makes it possible to remove striped image noise in images displaying characteristic distributions, and as a result, it becomes possible to provide a scanning photon microscope that can obtain high-quality images. Ta.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail based on the drawings.

FIG. 1 shows the basic configuration of a scanning photon microscope that was prototyped prior to the present invention. The configuration and operation of this microscope will be explained using the signal waveforms of each part shown in FIGS. 2a to 2f.

In FIG. 1, a light spot on the screen of a cathode ray tube 1 is imaged onto a sample (semiconductor wafer) 3 by a focusing lens 2. In FIG. This light spot corresponds to the AC signal from the AC oscillator 4 (waveform 2 in Figure 2d).
Based on 1), the brightness is modulated by the brightness modulation circuit 5 and the cathode ray tube 1 to become intermittent light, and as a result, an alternating current photovoltage is induced in the sample 3. This alternating current photovoltage is extracted via a capacitance formed by a transparent electrode 6 and a metal electrode 7 placed above the sample 3 with a gap in between, and a lock-in amplifier 8
The signal is synchronously rectified using the AC signal from the AC oscillator 4 as a reference signal, digitized by the analog-to-digital converter 9, and written into the image memory 10. In addition, the light spot is a cathode ray tube 1
The image is scanned intermittently on the screen using the following method. The clock signal (waveform 18 in FIG. 2a) from the pulse oscillator 11 is counted by the counter 12, and the counting result is converted into an analog quantity by the digital-to-analog converter 13 (waveform 18 in FIG. 2b).
The waveform 19) is, for example, a horizontal deflection signal of the cathode ray tube 1. Then, the carry signal of the counter 12 is counted by the counter 14, and the counting result is converted into an analog quantity by the digital-to-analog converter 15, and is used as, for example, a vertical deflection signal of the cathode ray tube 1. The output signal of the oscillator 11 is delayed by the delay circuit 16 (waveform 20 in FIG. 2c) and is converted into an analog signal.
This is used as a starting signal for the digital converter 9. The image memory 10 writes the conversion result of the analog-to-digital converter 9 using the output signals of the counters 12 and 14 as address signals, and displays the contents on the monitor television 17 as a gray scale image.

In the prototype device described above, the oscillator 4 and the oscillator 11 are independent of each other, so that the scanning signal of the light spot (waveform 19 in FIG. 2b) and the brightness modulation signal of the light spot (waveform 19 in FIG. 2d) The waveform 21) is mutually asynchronous. That is, as is clear from a comparison between waveform 19 and waveform 21, the state of the change in brightness of the light spot during each period of time when the light spot is stationary differs depending on the stationary point of the light spot. On the other hand, sample 3 is a semiconductor wafer, and the alternating current photovoltage follows the temporal brightness change of the light spot with a time delay. the result,
Even if the sample 3 has uniform characteristics, the input signal to the lock-in amplifier 8 is, for example, the waveform 22 in FIG. 2e.
The amplitude of the AC optical voltage differs depending on the scanning position. The output of lock-in amplifier 8 is waveform 2
2 is synchronously detected using waveform 21 as a reference signal, and includes a low frequency fluctuation component as shown in waveform 23 in FIG. 2f. Therefore, the image on the monitor television 17 is loaded with striped image noise corresponding to the above-described low frequency fluctuations, resulting in a significant loss of image quality. Naturally, when the scanning speed of the light spot is changed, the above-mentioned low frequency fluctuation component changes, and therefore the pitch of the striped noise on the image changes.

Therefore, as a result of conducting various experiments in order to remove the striped noise that weighs heavily on the image on the monitor television 17, it was found that the following method can be used.

That is, the lock-in amplifier 8 in FIG.
The frequency Δ of the above-mentioned low frequency fluctuation component included in the output signal (waveform 23 in FIG. 2 f) of the AC oscillator 4 (waveform 21 in FIG. 2 d) is expressed as m,
Clock signal from pulse oscillator 11 (Fig. 2a)
The period T _s of the waveform 18) is given by Δ| _n /[ _n ·T _s ]−1/T _s |. However, here [ _n・T _s ] is
It means the integer value closest to _n・T _s . The condition for removing low frequency fluctuation components and eliminating striped image noise is Δ=0, which holds true when [ _n・T _s ]=, _n・T _s in the above equation. do. In other words, the period _Ts of the clock signal used to scan the light spot is equal to the period (1/1) of the AC signal used to modulate the brightness of the light spot.
_n ), Δ=0, and striped image noise can be removed. That is, by synchronizing the scanning of the light spot with the brightness modulation of the light spot, striped image noise can be removed.

Next, one embodiment of the present invention will be described with reference to FIG. 3 and FIGS. 4 a to 4 e. In this embodiment, the pulse oscillator 11 for optical spot scanning in the configuration of the scanning photon microscope shown in FIG. This is replaced with a combination of frequency dividers 25. That is, in Figure 3,
Output signal of AC oscillator 4 (Fig. 2 d and Fig. 4 a)
The waveform 21) is shaped by the waveform shaper 24 into the waveform 26 shown in FIG. Convert it into a signal with a period that is an integral multiple of the period (waveform 27 in Figure 4 c), and use this as a clock signal to scan the light spot using the same method as the scanning photon microscope described in Figure 1. This makes it possible to precisely synchronize the scanning of the light spot with the brightness modulation of the light spot, and as a result, remove striped image noise.

According to one embodiment, the image noise removal effect is as follows. In Fig. 1, the oscillation frequency of the pulse oscillator 11 for scanning is 1 kHz (time interval for scanning the optical spot is 1 ms), the frequency of the AC oscillator 4 for modulation is 2.01 kHz, and the number of scanning points of the optical spot is set vertically and horizontally. When both were set to 512 points, about 10 stripes with a contrast ratio of 2:1 appeared on one screen in the observed image of the silicon wafer with an oxide film. Furthermore, the appearance of these stripes changes slightly even if the frequency of the AC oscillator 4 is slightly changed (for example, by 1 Hz). These fringes clearly degrade the image quality and further mislead the interpretation of the image observations. On the other hand, in FIG. 3 according to the present invention, even when the oscillation frequency of the AC oscillator 4 was set to 2.0 kHz or 2.01 kHz, the above-mentioned stripes did not appear at all, and a high-quality image could be obtained.

Although the sample has been described using a semiconductor wafer as an example, the sample is not limited to a semiconductor wafer, but may be any other sample as long as it has a photoelectric effect. It goes without saying that the light spot generating means and the light spot intermittent scanning means may be constructed using a semiconductor laser source or a gas laser source in addition to the above-mentioned cathode ray tube.

〔Effect of the invention〕

As described above, according to the present invention, in a scanning photon microscope that displays an image of the characteristic distribution of a sample by intermittently scanning a brightness-modulated light spot, a light beam generated by the interaction between a brightness modulation signal and an intermittent scanning signal is used. It is now possible to remove striped image noise. As a result, it becomes possible to provide a scanning photon microscope that can obtain high-quality images.

[Brief explanation of drawings]

Fig. 1 is a basic configuration diagram of a scanning photon microscope prototyped prior to the present invention, Fig. 2 is an output signal waveform diagram of each component of Fig. 1, and Fig. 3 is an example of an embodiment of the present invention. The basic configuration of the scanning photon microscope shown in FIG. 4 is a diagram of output signal waveforms of each component of the embodiment shown in FIG. 1... Cathode ray tube, 2... Focusing lens, 3... Sample, 4... AC oscillator, 5... Brightness modulation circuit, 6
...Transparent electrode, 7...Metal electrode, 8...Lock-in amplifier, 9...Analog-digital converter,
10... Image memory, 11... Pulse oscillator, 1
2... Counter, 13... Digital-analog converter, 14... Counter, 15... Digital-analog converter, 16... Delay circuit, 17... Monitor television, 18... Optical spot scanning clock Signal waveform, 19... Optical spot scanning signal waveform, 20
...Analog-digital converter start signal waveform,
21... Brightness modulation signal waveform, 22... Lock-in amplifier input signal waveform, 23... Lock-in amplifier output signal waveform, 24... Waveform shaper, 25... Frequency divider, 26... Basic clock signal waveform, 27... …
Divided clock signal waveform.

Claims (1)

[Scope of Claims] 1. A light spot generating means for generating a light spot whose brightness is modulated at a predetermined frequency; a light spot focusing means for focusing the light spot on the surface of a sample having a photoelectric effect; intermittent scanning means for scanning a light spot intermittently on the surface of the sample; signal detection means for detecting an alternating current electrical signal generated on the sample by scanning the light spot; A scanning device comprising: an image display means for displaying a change in positional output as an image; and a synchronous operation means for operating the light spot generating means and the intermittent light spot scanning means in synchronization. Photon microscope. 2. The light spot generating means comprises an AC oscillator, a brightness modulation circuit that generates a brightness modulation signal based on the output signal of the AC oscillator, and a cathode ray tube that receives the generated brightness modulation signal as input. The first term is a scanning photon microscope. 3. The optical spot intermittent scanning means includes an AC oscillator, a waveform shaper that shapes the output signal of the AC oscillator into pulses, a frequency divider that divides the repetition frequency of the pulse, and counts the frequency-divided pulses. a second counter that counts the carry signal from the first counter; and first and second D/A that converts the output signals from the first and second counters into digital signals, respectively. converter and the above first and second D/A
A first device comprising a cathode ray tube whose output signals from the converter are horizontal and vertical deflection signals, respectively.
Sectional scanning photon microscopy. 4 A transparent electrode and a metal electrode arranged so that the signal detection means is capacitively coupled to the sample, and a lock-in amplifier connected to the electrode and using the brightness modulation frequency in the light spot generation means as a reference input. The scanning photon microscope according to item 1, characterized in that it consists of: 5 The image display means includes an A/D converter that converts the output signal from the signal detection means into a digital signal, and an output signal from the A/D converter using the scanning signal from the intermittent scanning means as an address signal. A scanning photon microscope according to item 1, comprising an image memory for storing images, and a monitor television for displaying output signals from the image memory as gray scale images. 6. The scanning photon microscope according to item 1, wherein said synchronous operation means comprises an AC oscillator shared by said optical spot generating means and said optical spot intermittent scanning means.