JPH02268254A - Apparatus for inspecting fluorescence characteristic - Google Patents

Abstract

PURPOSE:To make it possible to measure the fluorescence intensity and the life of a sample or the correlation of both at the same time at a high speed by moving the measuring position of the sample, and analyzing and processing the data of the time intensity waveforms at a plurality of measuring points. CONSTITUTION:A pulse light source 12 is used and a sample 10 is excited. The data of the time intensity waveforms of photoluminescence generated in synchronization with the pulse light source 12 from a sample 10 are detected at time resolution of picoseconds - microseconds. Then, the measuring position of the sample is moved with an X-Y stage 16. The data of the time intensity waveform at each measuring point are analyzed and processed in a signal processing device 22. Thus, the space intensity distribution and the life of the photoluminescence or correlation distributions of both are obtained. Therefore, the fluorescence intensity and the life of the sample 10 or the correlation of both can be measured at a high speed. The quality of a GaAs wafer and the like can be accurately evaluated. Furthermore, local defects and the like can be readily inspected.

Description

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

The present invention relates to an apparatus for testing the fluorescent properties of a sample, and in particular, a device for testing the fluorescent properties of a sample, particularly for light emitting diodes (LEDs), laser diodes (LDs), 'I field effect transistors (FETs), photodiodes (PDs), optoelectronic integrated devices, etc. element (OEIC
), obtain spatial intensity distribution images and lifetime distribution images of photoluminescence generated from a sample, which are suitable for use in evaluating the quality of semiconductor wafers such as GaAs wafers used in integrated devices (ICs), etc. The present invention relates to a fluorescent property testing device that can perform the following steps.

[Conventional technology]

LED, LD, FET, PD, OE rc, IC
The quality of the GaAs wafers used to manufacture these products is the biggest issue for these manufacturers, and the current situation is far from reliable. In general, when a semiconductor crystal is irradiated with a light beam with energy greater than the forbidden band width to excite electrons from the valence band, fluorescence is observed as the excited electrons lose energy through recombination, and this light emission occurs. is called 7 otoluminescence. The lifetime of the fluorescence in this photoluminescence is also determined by the quality of the crystal, surface treatment, surface distortion, flaws, etc. Therefore, as the processing process progresses from polishing to etching, the number of recombination centers on the surface decreases and the lifetime of the fluorescence may become longer. This corresponds to looking at the surface recombination rate. In general, the fluorescence lifespan changes due to the effects of crystal quality, crystal defects, surface conditions, surface treatment, etc., and wafers with good conditions, that is, good quality, are While the fluorescence lifespan is longer as shown by the solid line A in the figure, lower quality wafers have a shorter fluorescence lifespan as shown by the solid line B in FIG. Therefore, when evaluating the quality of GaAs wafers, it is important to measure their fluorescence lifetime. In addition, the quality of QaAs wafers is determined not only by the fluorescence lifetime but also by
Fluorescence efficiency (single-child efficiency, absolute value of fluorescence, ie, fluorescence intensity) is also important. Normally, as shown in Figure 17, there is a correlation between fluorescent efficiency and lifetime, but there may be cases where this is not the case.
It is also important to measure the fluorescence intensity.

[Problem to be achieved by the invention]

However, conventional equipment for examining the quality of crystals using photoluminescence irradiates the sample with continuous light (DC light) of wavelength λ1, and generates a wavelength of λ2〉λ1).
The sample was evaluated based on the intensity distribution of photoluminescence. A technique has also been proposed in which a sample is irradiated with pulsed light from a mode-locked pulsed laser or a semiconductor laser, and the fluorescence lifetime of the generated photoluminescence is measured using a sampling streak camera. However, since measurement results can only be obtained from a single measurement point, there is a problem in that it is difficult to detect local defects. The present invention has been made to solve the above-mentioned conventional problems, and it is possible to obtain a spatial distribution image of the fluorescence intensity and fluorescence lifetime of photoluminescence generated from a sample in the picosecond to microsecond range, or the correlation between the two. An object of the present invention is to provide a fluorescent property inspection device that can be obtained at high speed.

[Means to achieve the task]

The present invention provides a fluorescence property testing apparatus, as shown in FIG. , a moving means (XY stage 16 in the figure) for moving the measurement position of the sample 10, and a condensing optical system 18 that extracts photoluminescence generated from the sample 10 and guides it to a detector (20). , the pulsed light ai
12, a high-speed photodetector 20 for obtaining information on the time-intensity waveform of the photoluminescence, and analyzing and processing information on the time-intensity waveform at each measurement point detected while moving the sample measurement position. The above object has been achieved by including a signal processing device 22 that determines the spatial intensity distribution and lifetime distribution of photoluminescence or the correlation distribution thereof. Further, the high-speed photodetector 20 is configured as a streak camera device having a temporal resolution of several picoseconds to several tens of picoseconds. Further, the high-speed photodetector 20 is configured as a time-correlated photon counting device having a time resolution of several tens of picoseconds. Further, the high-speed photodetector 20 is a combination of a high-speed photodiode and a waveform memory, and has a time resolution of several hundred picoseconds. Moreover, the means for moving the measurement position mechanically moves the sample. Moreover, the means for moving the measurement position scans a light beam from a pulsed light source. Further, the measurement position moving means mechanically moves the sample and scans the light beam from the pulsed light source. Further, a spectroscopic means is provided immediately before the high-speed photodetector, so that the spatial intensity distribution and lifetime distribution for each wavelength or the correlation distribution between the two can be determined. Further, the condensing optical system is a confocal optical system, and an aperture is provided on the input imaging plane of the high-speed photodetector or spectroscopy means to provide resolution in the depth direction of the sample. [Operations and Effects 1] In the fluorescent property testing device according to the present invention, the pulsed light source 12
is used to excite the sample 10, and in synchronization with the pulsed light source 12, the high-speed photodetector 20 detects information regarding the time-intensity waveform of photoluminescence generated in the sample with a time resolution of picoseconds to microseconds. , the information regarding the time-intensity waveform at each measurement point detected while moving the sample measurement position is analyzed and processed to determine the spatial intensity distribution and lifetime distribution of photoluminescence or the correlation distribution between the two. . Therefore, the fluorescence intensity and lifetime of a sample, or the correlation between the two, can be simultaneously measured at high speed, making it possible to accurately evaluate the quality of GaAs wafers and the like. Furthermore, since a spatial distribution image of the fluorescence intensity and lifetime or the correlation between the two can be obtained, even a single local defect can be easily inspected. Furthermore, when the high-speed photodetector 20 is a streak camera device, the time-intensity waveform can be easily obtained with a time resolution in the picosecond region. Further, when the high-speed photodetector 20 is a time-correlated photon counting device, the time-intensity waveform is at the level of a single photon,
It can be detected with high sensitivity, high temporal resolution (several tens of picoseconds), and wide dynamic range. If the high-speed photodetector 20 is a combination of a high-speed photodiode and a waveform memory, the temporal light intensity waveform can be detected with a temporal resolution of several hundred picoseconds. Furthermore, when the means for moving the measurement position is one that mechanically moves the sample, there is no need to scan the optical system;
Measurements can be made under the most advantageous conditions for the optical system. Furthermore, when the means for moving the measurement position is one that scans a light beam from a pulsed light source, there is no need to move the sample, and it is possible to scan minute parts at high speed. In addition, when the measurement position moving means is one that mechanically moves the sample and scans a light beam from a pulsed light source, for example, each of them is scanned in a one-dimensional direction orthogonal to each other. This enables scanning in two-dimensional directions. Furthermore, it is possible to roughly move a minute part by scanning a light beam and mechanically scanning it. Furthermore, if a spectroscopic means is provided immediately before the high-speed photodetector 20, it becomes possible to obtain the spatial intensity distribution and lifetime distribution for each wavelength of photoluminescence, and in particular, the fluorescence lifetime differs depending on the wavelength. Suitable for inspecting samples. In addition, when the condensing optical system is a confocal optical system and an aperture is provided on the input imaging plane of the high-speed photodetector 20 or the spectroscopic means, only information on the focused plane can be obtained, so the sample depth can be adjusted. Resolution can also be obtained in the horizontal direction. [Example] Hereinafter, with reference to the drawings, an example of the fluorescent property inspection apparatus according to the present invention applied to a semiconductor wafer evaluation apparatus will be described in detail. The first embodiment of the present invention, as shown in FIG.
a pulsed light source 12 for exciting a sample 10 such as a semiconductor wafer; an irradiation optical system 14 for irradiating the sample 10 with light from the pulsed light source 12;
By moving the position of the sample 10 in a two-dimensional direction, the X-Y stage 16 for moving the measurement position (pulsed light irradiation position) of the sample 10 in the two-dimensional direction and the photoluminescence generated from the sample 10 are extracted. A condensing optical system 18 including a filter 18B and a lens 18C for extracting the wavelength component of the photoluminescence of the beam splitter 18A1 and guiding it to the detector; Detection was performed while moving the sample measurement position by moving the high-speed photodetector 20 and the X-Y stage 16 for obtaining information regarding the time-intensity waveform of photoluminescence (hereinafter referred to as fluorescence waveform). For example, each measurement point in a grid (see Figure 2)
Analyzing and processing information regarding the fluorescence waveform in
A signal processing device 22 that obtains the spatial intensity distribution and lifetime distribution of photoluminescence, and a display device that displays the spatial intensity distribution image and lifetime distribution image obtained by the signal processing device 22, or their correlation distribution image. It consists of 24. The pulsed light source 12 has a wavelength of 600 to 68, for example.
A laser diode (LD) that can stably oscillate pulsed light with a pulse width of about 3 Q psec at about 0r++n.
can be used. Furthermore, when measuring relatively slow fluorescence of several nanoseconds or less, a light emitting diode (LED) can be used. The method for obtaining the trigger signal from the pulsed light source 12 is as follows:
For example, the pulsed light from the pulsed light source 12 can be branched, and one of the lights can be converted into an electrical signal by a high-speed photodiode such as an avalanche photodiode (APD) to be used as a trigger signal for the high-speed photodetector 20. As the high-speed photodetector 20, for example, a streak camera can be used. The streak camera is a photodetector that has an extremely high time resolution of several picoseconds, and is suitable for evaluating samples such as GaAS wafers that have fluorescence lifetime components as short as several tens of picoseconds. As shown in the basic configuration of FIG. 3, this streak camera converts incident light into electrons by applying it to a photocathode 36 of a streak tube 34 through an input optical system consisting of, for example, a slit plate 30 and a lens 32. , deflection electrode 3
By sweeping photoelectrons at high speed as they pass through the fluorescent surface 42, the time-varying incident light intensity is measured as a change in brightness at a position on the fluorescent surface 42. In the figure, 40 is a microchannel plate (MCP') for collecting photoelectrons just before the fluorescent surface 42. The image that appears on the output fluorescent surface 42 in this way is called a streak image, and after capturing this image with a television camera 25 using a vidicon, COD, etc., as shown in FIG. By quantifying the brightness distribution along the time axis direction, it is possible to know the change in the intensity of the measured light over time. Specifically, for example, the output of the television camera 25 is A/D converted by an analog/digital (A/D>converter 26 and stored in the - time storage device 27, and then the signal processing device 2
Output to 2. Note that a one-dimensional photodiode array may be used instead of the television camera 25. When the streak camera is used, a fluorescence waveform as shown in Fig. 5 is obtained for each measurement point, and the fluorescence intensity and lifespan obtained for each measurement point are calculated corresponding to the measurement point. By plotting these two-dimensionally, spatial intensity distribution images and lifetime distribution images can be obtained. Furthermore, as shown in FIG. 6, the high-speed photodetector 20 is constructed by providing a slit plate 39 between the deflection electrode 38 and the fluorescent surface 42 to spatially limit the streak image in the streak camera. A sampling type streak camera that electronically samples streak images and later synthesizes them as necessary can also be used. In this case, for example, as shown in FIG. 7, the intensity of the sampled bright spot is detected by a photomultiplier tube 28 or a photodiode, and after being amplified by an amplifier 29 as necessary,
The signal is outputted to the signal processing device 22 via the A/D converter 26 and the -time storage device 27. Furthermore, a time-correlated photon counting device can also be used as the high-speed photodetector 20. This time-correlated photon counting device
As shown in the basic configuration of FIG. 8, time measurement is started by a photomultiplier tube (PMT) 50 that detects photoluminescence at the level of one photon generated from the sample 10 and a trigger signal input from the pulsed light source 12. , said PMT5
A time-to-amplitude converter (TA) outputs a voltage pulse with a height proportional to the time difference between the two pulse signals by stopping the time counting when a photon is detected at zero.
C) 52, and a pulse height analyzer (PHA) 54 that quantizes and stores the height of the output pulse of the TAC 52. The amount of light incident on the PMT 50 is adjusted to a level such that at most one photoelectron is detected per pulsed light irradiation, for example, by adjusting the pulsed light intensity or by providing a filter in front of the PMT50. Since the PHA 54 stores information on how many pulses of different heights have arrived, the contents stored in the PHA 54 after the light source has emitted thousands of times will be as shown in FIG. The horizontal axis in Figure 9 is proportional to the time difference between when the light source emits light and when one fluorescent photon is detected, and the vertical axis is the probability that a photon will be detected at that time, that is, the fluorescence at that time. Since it is proportional to the intensity, FIG. 9 directly represents the fluorescence waveform. When this time-correlated photon counting device is used, each photon, which is the lowest unit of light, is detected one by one, so highly sensitive detection is possible. Furthermore, the time resolution is also excellent. Furthermore,
The dynamic range is also wide. The intensity distribution of photoluminescence can be obtained by integrating the value 1 (t) of the waveform within a predetermined time, but when acquiring only this intensity distribution at high speed or with good S/N, the output of PMT50 It is also possible to count the pulse signals using a separately provided high-speed pulse counter 56 and obtain an intensity distribution image using the signal processing circuit 22, as shown in FIG. 10, for example. At this time, since time information is not required, the pulse light source 12 may be lit with direct current (DC), and the counting rate can be increased to the maximum counting rate of the PMT, amplifier, or pulse counter. According to this, for example, prior to measuring the fluorescence lifetime distribution, an intensity distribution image can be obtained in a short time, and the measurement range of the fluorescence lifetime distribution can be determined from the image. That is, in the general time correlation counting method, the counting rate of the system is limited mainly by the maximum counting rate of TAC (usually several 100 kcps), but in the photon counting method of the embodiment shown in FIG. Since the measurement time can be reduced significantly, the measurement time can be significantly reduced. Further, as the high-speed photodetector 2o, a combination of a high-speed photodiode such as an avalanche photodiode (APD) and a waveform memory can be used. Analysis and processing of information regarding the fluorescence waveform in the signal processing device 22 is performed as follows. That is, based on the fluorescence waveform as shown in FIG. 5 above,
First, the fluorescence intensity! The fluorescence lifetime τ, which is the time until (t) becomes 1/e (approximately 37%), is determined. Specifically, when there is one type of fluorescence life τ, the waveform in which the vertical axis of the fluorescence waveform is expressed logarithmically is as shown in FIG. 11, so for this waveform, the following equation By applying the equation shown in , the nearest coefficient term A and the life τ are determined by, for example, the method of least squares. 1 (t) −A, exp(−t/r) −
(1) Also, there are multiple fluorescent lifetimes (τ1, τ2, τ3,...
) In some cases, the logarithmic waveform of the fluorescence waveform is
As shown in Figure 2, by applying the following equation, each fluorescent life τ
1, τ2, τ3, . . . I (t)−A+ exp(t/τ1)+A 2
・eXp(-t/τ2) +A3・exa(-t/τ3) +... ・・・・・・・・・(2) On the other hand, the total intensity of the fluorescence is the total area of the waveform ( (shown with diagonal lines in Figures 11 and 12), the value of the waveform is 1.
(t) can be calculated over the entire range, or by calculation from the constant A+ obtained when determining the fluorescence lifetime and the fluorescence lifetime τ1 using the following equation. fl(t)=Atτ1+A2τ2+A3τ3+...
(3) The fluorescence lifetime and fluorescence intensity thus determined are displayed on the display device 24 as a spatial distribution image. An example of a display image of the fluorescent life is shown in FIG. 13. In FIG. 13, the distribution of the fluorescence lifetime τ is represented by, for example, shading. Note that when visualizing the photoluminescence intensity and lifetime, in addition to displaying in black and white density, it is also possible to display a color image with different colors, or to perform three-dimensional display. Also, if the fluorescence lifetime also has a second component τ2 and a third component τ3, for example, only the linear component τ1 is mapped in green, and the linear and quadratic components τ1 and τ2 are mapped in red. The linear, quadratic, and cubic equation components τ1, τ2, and τ3 can be mapped in yellow. Also, by making the linear equation component τ1 red, the quadratic equation component τ2 green, and the cubic equation component τ3 blue, and sequentially overlapping them, the linear equation component τ1 becomes red and the quadratic equation component τ1 becomes red, and the quadratic equation component τ1 becomes red. It is also possible to display τ2 in yellow and the cubic equation component τ3 in white. Furthermore, when displaying an image, it is possible to perform appropriate smoothing processing to make it easier to see. Further, the signal processing device 22 performs calculations such as calculating a correlation between different intensity distribution images and lifetime distribution images, for example, calculating a ratio.
It is also possible to display the calculation results (ie, the correlation distribution image). Next, a second embodiment of the present invention will be described in detail with reference to FIG. This second embodiment includes a pulsed light source 12, an irradiation optical system 14, an XY stage 16, and the like as in the first embodiment.
Condensing optical system 18, high-speed photodetector 20, and signal processing device @
22 and a display device 24, the semiconductor wafer evaluation apparatus further includes a spectrometer 60 for dispersing photoluminescence generated from the sample 10 immediately before the high-speed photodetector 20. By detecting the fluorescence waveform for each wavelength, a spatial intensity distribution image and a lifetime distribution image for each wavelength can be obtained. According to this second embodiment, wavelength information can also be measured at the same time. For example, even if the fluorescence lifespan differs depending on the wavelength, as shown in FIG. 15, the fluorescence lifespan τ1 for each wavelength
, τ2 can be determined accurately. In this second embodiment, if a two-dimensional streak camera is used as the high-speed photodetector 20, a fluorescence waveform containing wavelength information can be obtained immediately, so wavelength scanning in the spectrometer 60 is not necessary. In each of the above embodiments, the X-Y stage 1 is used as a means for moving the measurement position of the sample 10.
6 was used, but the moving means for moving the sample measurement position is not limited to this. For example, when the sample 10 is flowing on a belt conveyor or the like, the pulsed light source 12 can be used as means for moving in a one-dimensional direction perpendicular to the flow direction of the belt conveyor. Furthermore, instead of using mechanical scanning means, a configuration may be adopted in which pulsed light is electro-optically deflected and scanned, or this pulsed light scanning and sample movement may be combined. Furthermore, by providing an aperture 62 on the input imaging plane of the high-speed photodetector 20 or the spectrometer 60 to form a confocal system, as shown by broken lines in FIGS.
It is possible to provide resolution not only in the dimensional direction) but also in the depth direction of the sample. Further, as in the third embodiment shown in FIG.
Separately, an epi-illumination light source 80 is provided, for example, a lens 80.
The sample 10 is irradiated through the mirror 82, and this reflected light is sent to the mirror 82.
A and a lens 82C to guide the TV camera 84, the TV camera 84 captures a reflected image, and the A/D converter 86
After D conversion, it is input to the signal processing device @ 22, stored in an image memory (not shown), and displayed on the display device 24 to confirm the position of the pulsed light on the sample 10 and the measurement area. You can also monitor the status of Further, it is also possible to display this reflection image, a photoluminescence lifetime distribution image, an intensity distribution image, etc. in an overlapping manner. Further, the epi-illumination light source 80 is provided with a wavelength filter 80B matching the absorption wavelength of the sample 10, and the entire surface of the sample 10 is irradiated, and the photoluminescence image generated at that time is transmitted to the TV camera through a predetermined wavelength filter 82B. It is also possible to take an image using 84 and obtain a two-dimensional photoluminescence intensity distribution. Furthermore, a light source 90 for transmitted illumination is provided on the back side of the sample 1o,
Lenses 90A, 90C and wavelength filter 90B are provided,
It is also possible to obtain an image of the transmitted light of the sample using the TV camera 84 or the high-speed photodetector 20 and compare it with the photoluminescence lifetime, intensity distribution image, etc. At this time, the spectroscopic means 60 provided in front of the high-speed photodetector 20
The second harmonic (SHG) due to the nonlinearity of the sample 10 is
) component and scan the sample 10 to obtain a nonlinear optical characteristic image. Furthermore, in each of the above embodiments, the present invention is applied to a semiconductor wafer evaluation device for inspecting semiconductor wafers for defects, but the scope of application of the present invention is not limited to this, and is applicable to dielectrics, fluorescent materials, etc. It is clear that the present invention is equally applicable to devices for testing other fluorescent properties such as light surfaces, drugs, paper, biological tests, etc.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a first embodiment of a fluorescent property testing device according to the present invention; FIG. 2 is a plan view showing an example of measurement points on a sample surface in the first embodiment; FIG. 3 is a sectional view showing the basic configuration of a streak camera, which is an example of a high-speed photodetector used in the present invention. FIG. 4 is a signal processing system when a streak camera is used as a high-speed photodetector. 5 is a diagram showing an example of excitation light and fluorescence waveforms, and FIG. 6 is a sampling type streak camera which is another example of the high-speed photodetector used in the present invention. 7 is a block diagram showing an example of a signal processing system when a sampling streak camera is used as a high-speed photodetector, and FIG. 8 is a sectional view showing the basic configuration of the system used in the present invention. A block diagram for explaining the basic principle of a time-correlated photon counting device, which is another example of a high-speed photodetector, and FIG. 9 is a diagram showing an example of a fluorescence waveform obtained by the time-correlated photon counting device. , FIG. 10 is a block diagram showing a modified example of the time-correlated photon counting device, FIGS. 11 and 12 are diagrams for explaining the method of determining the fluorescence lifetime, and FIG. 13 is a diagram showing the fluorescence lifetime. FIG. 14 is a block diagram showing the configuration of the second embodiment of the present invention; FIG. 15 is a plan view showing a display example of the spatial distribution of lifetime; FIG. FIG. 16 is a block diagram showing the configuration of the third embodiment of the present invention. FIG. 17 is a diagram showing an example of the relationship between semiconductor wafer quality and fluorescence waveform. It is a line diagram. DESCRIPTION OF SYMBOLS 10... Sample, 12... Pulse light source, 14... Irradiation optical system, 16... X-Y stage, 18... Condensing optical system, 18B... Filter, 20... High speed Photodetector, 22... Signal processing device, 60... Spectrometer, 62... Aperture. Fig. f Fig. 2 Fig. Death Fig. 6 Fig./2 〃 9 Fig. Liquid height a: Clear f4 Fig. 4 Fig. 13 Fig. 14 Fig. 15 Fig. 17 Fig.

Claims (9)

[Claims] (1) A pulsed light source for exciting the sample, an irradiation optical system for irradiating the sample with light from the pulsed light source, a moving means for moving the measurement position of the sample, and a means for extracting photoluminescence generated from the sample. a high-speed photodetector for detecting information regarding the time-intensity waveform of the photoluminescence in synchronization with the pulsed light source; A signal processing device that analyzes and processes information regarding the time-intensity waveform at a measurement point to obtain a spatial intensity distribution and a lifetime distribution of photoluminescence, or a correlation distribution between the two. Device. (2) The fluorescent property testing device according to claim 1, wherein the high-speed photodetector is a streak camera device. (3) The fluorescence property testing device according to claim 1, wherein the high-speed photodetector is a time-correlated photon counting device. (4) A fluorescence characteristic testing device according to claim 1, wherein the high-speed photodetector is a combination of a high-speed photodiode and a waveform memory. (5) In claim 1, the means for moving the measurement position:
A fluorescent property testing device characterized by mechanically moving a sample. (6) In claim 1, the means for moving the measurement position:
1. A fluorescent property testing device, characterized in that it scans a light beam from a pulsed light source. (7) In claim 1, the means for moving the measurement position:
1. A fluorescence property testing device that mechanically moves a sample and scans a light beam from a pulsed light source. (8) A fluorescence characteristic testing device according to claim 1, characterized in that a spectroscopic means is provided immediately before the high-speed photodetector, and a spatial intensity distribution and a lifetime distribution for each wavelength are determined. (9) In claim 1 or 8, the condensing optical system is a confocal optical system, and an aperture is provided on the input imaging plane of the high-speed photodetector or spectroscopy means, and the resolution in the sample depth direction is A fluorescent property testing device comprising: