WO2014084575A1 - Method and device for detecting microdefects - Google Patents

Abstract

The present invention provides a method for detecting microdefects, comprising the steps of: irradiating a pump laser beam of a constant frequency upon a sample; changing periodic reflection intensity by enabling the temperature of defect surface to change due to a photothermal effect of a defect in an area upon which the pump laser beam is irradiated; and measuring a change of the reflection intensity by irradiating a probe beam upon the sample.

Description

Method and apparatus for detecting microdefects

The present invention relates to a technique for detecting fine defects, and more particularly, by irradiating a pump laser beam to a sample to induce a change in the periodic reflection intensity due to the photothermal effect of the defect present in the irradiation area, and to the probe A method and apparatus for detecting minute defects in a sample by irradiating a beam and measuring a change in reflection intensity.

In the manufacture of thin films for active / passive devices such as semiconductor devices, for example transistors, solar cells and semiconductor lasers, cracks, scratches, structural defects and contaminants during the process Various types of micro defects in the thin film may be generated. Since these defects act as a major cause of degrading the performance or shortening the life of the thin film device, it is very important to detect the defects of the thin film or to identify the cause of the defects in the manufacture of semiconductor devices.

For example, in the case of an optical thin film deposited for the purpose of controlling the optical performance (reflection, transmission, absorption, etc.) of an optoelectronic device such as a semiconductor laser, a photodetector, an optical amplifier, microdefects in the thin film may be It can act as a local hot source to increase the ambient temperature, and this change in temperature and the difference in coefficient of thermal expansion in the thin film results in thermal stress, resulting in a laser-induced laser damage threshold. damage threshold (LIDT) can be lowered. As a result, the lowering of the laser damage threshold caused by laser exposure ultimately results in damage and permanent destruction of the thin film, thereby degrading the performance of the optoelectronic device and further shortening the life of the device.

Therefore, in order to prevent damage to the thin film and to secure high quality optical properties, the necessity of detecting the fine defects in the thin film has been continuously demanded.

It looks at the prior art for this. Photothermal microscopy (PTM) [B. Bertussi et al ., "High-resolution photothermal microscope: a sensitive tool for the detection of isolated absorbing defects in optical coatings," Appl. Opt., 45 (7) 2006, US patent, "Photothermal imaging scanning microscopy", registration number: 07075058, registration date: July 11, 2006, is one of the most widely used thin film defect detection techniques. It is a method of estimating a defect location by checking a change in refractive index around a defect due to a stimulus to a degree of deflection / refraction of a probe beam.

However, because the method obtains defect distribution by raster scanning using a sample stage, the data acquisition time is long (more than a few minutes), and the photothermal effect around the defect predominates when the pump beam size is larger than the measurement defect. There was a problem that the accuracy of the detection falls.

In addition, since the light-heat signal is very sensitive to the relative position of the probe beam and the pump beam, it is difficult to precisely adjust the probe beam irradiation angle for each sample measurement.

On the other hand, this problem has been described in the case of a thin film, for example, but in the case of a bulk material, a wafer, etc., there was the same problem even when detecting a fine defect.

Under this background, an object of the present invention is to provide a novel technique using a reflection mode photothermal reflection microscopy technique that measures the position of the defect through the degree of change in reflectance instead of the change in refractive index due to the photothermal effect.

In order to achieve the above object, a first aspect of the invention is a step of irradiating a pump laser beam of a predetermined frequency (f) to the sample; Changing the periodic reflection intensity by changing the defect surface temperature due to the photothermal effect of the defect in the area irradiated with the pump laser beam; And irradiating a probe beam to the sample to measure a change in the reflection intensity.

The sample for detecting the microdefects is not particularly limited, such as a thin film, a thick film, a wafer, a bulk material, and can be various kinds.

Preferably, the method further comprises the step of measuring from the change in reflectance of the sample by phase-locked heat reflection and converting it into a heat distribution. The detector may further include triggering a sample at a multiple of the frequency for temperature-modulating the sample.

On the other hand, the light source for generating the pump laser beam may be configured using a wavelength tunable laser diode and a wavelength selection filter (not shown) to irradiate the beam of various wavelengths.

The pump laser beam is preferably irradiated with a surface light source, but may also be modified to be irradiated in the form of a line light source.

When the pump laser beam is irradiated with a surface light source, since the probe beam imaging plane may be moved into the sample through vertical movement of the sample stage, three-dimensional defect information of the sample may be realized through the stage Z-axis scan.

Another aspect of the invention the sample mounting unit for mounting a sample; A pump light source for irradiating the pump laser beam at a predetermined frequency f on the sample; A probe light source for irradiating visible light onto the sample; A detector for detecting light reflected by the probe light source and reflected from the surface of the sample; And a control unit and an image processing unit at multiple times of a period in which the sample is temperature-modulated by irradiation of the pump laser beam.

Preferably, the apparatus further includes a light splitter, and transmits the beam emitted from the probe light source to the sample and delivers the beam transmitted from the sample to the detector.

According to the invention as described above, since the system implementation is relatively simple and the high speed measurement is possible in a relatively large area compared to the conventional defect detection method, it is possible to provide a faster and more efficient defect inspection environment in the field application It works.

In addition, according to the present invention, by using a two-dimensional array sensor using a signal detector, the probe beam irradiation area is acquired at a time without a separate scan, thereby greatly reducing the imaging time (several seconds or more), and the probe beam and the pump beam. Since high-precision light alignment of the liver is not required, there is an effect that the defect site can be identified more quickly.

1 is a flowchart illustrating a method of detecting microdefects according to an embodiment of the present invention.

2 is a schematic structural diagram of a system for implementing a method for detecting microdefects of the present invention.

3 to 5 are views for explaining a method of irradiating a sample to the pump light source according to the present invention.

6 is a photograph showing an example of detecting impurities in a uniform medium using a method for detecting microdefects according to an embodiment of the present invention.

Figure 7 is a photograph showing the results confirmed through the system for the micro-defects inside the uniform PDMS produced additionally.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, embodiments of the present invention illustrated below may be modified in many different forms, and the scope of the present invention is not limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

1 is a flowchart illustrating a method of detecting microdefects according to an embodiment of the present invention.

Referring to FIG. 1, in the method (S101) in which a pump laser beam of frequency f is irradiated onto a sample, and the temperature around the defect is changed due to the photothermal effect of the defect in the irradiated region, a periodic change in refractive index is achieved. Causing a periodic reflection intensity to be changed (S103), and irradiating a probe beam to the sample to measure the change of the reflection intensity (S105).

More specifically, when the sample is irradiated with a pump beam of frequency f, the defect in the sample absorbs the pump beam and the absorbed light energy is converted into thermal energy and transferred around the defect. This thermal energy increases the temperature around the defect, which results in a change in refractive index (thermal lens effect). Therefore, as a result, the change in the refractive index leads to a change in the light reflection intensity, and the relationship between the relative change in the light reflection intensity and the temperature change can be expressed as follows.

, ΔT 는 각각 광 반사세기의 변화도, background 반사세기, 열반사 보정 계수, 그리고 샘플 표면의 온도변화를 나타내고 있다. 본 발명의 하나의 특징은 결함 위치 판별에 있으므로 상대적인 반사율 변화량 (ΔR/R)을 결함 위치 측정을 위한 파라미터로 사용할 수 있다.Where ΔR, R,

, ΔT represent the change of light reflection intensity, background reflection intensity, heat reflection correction coefficient, and temperature change of sample surface, respectively. One feature of the present invention is in determining the defect position, so that the relative reflectance change amount ΔR / R can be used as a parameter for measuring the defect position.

If this thermo-optic action results in a periodic change in the reflection intensity of the microscopic defects, the probe beam can be used to measure it. Preferably, after the probe beam is evenly incident on the sample through a microscope objective with a long working distance, the sample reflected light can be detected by a CCD camera which is recondensed by the objective and operates at a frequency of 4f.

2 is a schematic structural diagram of a system for implementing a method for detecting microdefects of the present invention.

Referring to FIG. 2, two off-axis beams (pump beam 120 and probe beam 100) and CCD-based thermal reflection microscopy (TRM) are configured. Thermal reflection microscopy is an optical technique for measuring the microscopic distribution of thermal changes in an optoelectronic circuit, which can calculate the actual temperature by measuring the relative reflection intensity change of the sample caused by the temperature change. On the other hand, it is also possible to provide a band pass filter 270 in front of the CCD camera to block the pump beam reflected light by the sample. The reflected light intensity recorded on the CCD arbitrary pixels (x, y) can be expressed as follows.

는 샘플의 반사세기이며,

는 펌프 빔 여기에 의한 샘플의 상대적인 반사 세기 변화를 나타내고, f와

는 각각 반사빔의 변조 주파수와 광열변조에 기인된 위상 지연값을 나타내고 있다. CCD 카메라는 펌프 빔과 동기화 되어 있어 저주파 대역의 신호 복조 기법으로 잘 알려진 호모다인 위상장금검출법(homodyne lock-in detection)을 이용하여 CCD 카메라에 기록된 반사광 세기로부터 상대적인 반사율 변화량만을 추출할 수 있게 된다.here,

Is the reflectivity of the sample,

Represents the change in the relative reflection intensity of the sample due to pump beam excitation,

Denotes the phase retardation values caused by the modulation frequency and the optical thermal modulation of the reflected beam, respectively. Since the CCD camera is synchronized with the pump beam, it is possible to extract only the relative reflectance variation from the reflected light intensity recorded in the CCD camera by using homodyne lock-in detection, which is known as a low frequency signal demodulation technique. .

2, the system of the present invention includes a probe light source 100, a pump light source 120, a detector 300, a controller, and an image processor 400, and further includes an optical splitter 250. The beam emitted from 100 may be transferred to the sample, and the beam transmitted from the sample may be transferred to the detector 300. Meanwhile, various lenses C and L may be disposed at the front end of the light source or the detector to assist in converging light.

The probe light source 100 is a light source that provides light in which light rays having a plurality of wavelengths are mixed in the visible light wavelength region. The type includes a wavelength filter (not shown) that selects only a predetermined wavelength together with a light source capable of obtaining a wide wavelength line width such as a white light, an LED, a solid light source having a broad wavelength line width, or a line width of about 10 nm to 50 nm. LEDs having a specific wavelength band can be used.

The pump light source 120 is for irradiating a sample with a beam of frequency f, and may be irradiated using a laser diode of 808 nm. It is, of course, also possible to use a multi-mode fiber or a bundle of fiber to guide the pump light source 120 to the sample.

In addition, the pump light source 120 may be configured using a wavelength tunable laser diode and a wavelength selection filter (not shown) to irradiate beams of various wavelengths. That is, more effective defect imaging can be realized by selecting a wavelength having good transmittance and light absorption by irradiating beams of various wavelengths and using a filter for wavelength selection.

Meanwhile, since the pump laser beam is irradiated with a surface light source and the probe beam imaging plane may be moved into the sample through vertical movement of the sample stage (focal plane position is moved into the sample), the stage Z-axis scan is performed. Three-dimensional defect information of the sample can be implemented. In the case of a large area sample, this process is followed by a scan of the transverse axis of the sample stage, thereby making it possible to obtain three-dimensional defect information on the entire area of the sample. In the case of a large area, it is possible to implement three-dimensional defect information on the entire area of the sample by stitching by moving the sample stage horizontally by one step.

In addition, the beam irradiated to the surface light source from the pump light source 120 enables to intensively obtain information about a certain depth in the depth direction inside the sample (for example, a thin film), and relatively outside the corresponding depth There is a vulnerable problem. Therefore, it is also possible to scan in the depth direction in such a way that the pump light source 120 adjusts the depth irradiated in the sample. Specifically, by adjusting the distance between the pump light source 120, the condenser lens 295, and the sample 500, a focal plane in which the beam irradiated from the pump light source 120 is mainly intensively irradiated deeply. It is possible to change in the direction. According to another method, by changing the wavelength of the pump light source 120 to change the depth irradiated intensively in the sample it is possible to ensure whether the internal defects in the three-dimensional image. However, changing the wavelength of the pump light source has an effect of changing the image plane according to the wavelength when the lens is used, but the light absorption of the defect is changed according to the wavelength, so it is difficult to obtain the same defect information.

The detector 300 may include a plurality of optical signal detectors including a charged coupled device (CCD), a photo detector, an avalanche photo diode (APD), and a photo multiplier tube (PMT).

The system controller and the image processor 400 generate a signal for synchronizing the pump light source 120 and the detector 300, and are composed of hardware and software for processing the measured image information.

According to this embodiment, the phase lock nib is applied by applying an optical signal that is a sample, and simultaneously illuminating the object with visible light through an optical microscope to detect the distribution of reflected light, for example, by using a CCD camera. The exothermic distribution of the object is measured by measuring by means of law.

More specifically, the sample is temperature-modulated by the pump beam at a particular frequency f, with heating and cooling repeated periodically. Such periodic heating and cooling drive signals cause periodic temperature changes around defects in the sample. In this case, the CCD which is the detector 300 can detect the light reflected from the sample. The detector 300 is triggered by multiple times (eg, 4 times) the frequency that modulates the sample, thereby generating a series of images multiple times (eg, 4 times) within one period of temperature modulation of the sample. It can be secured. The data secured through the CCD is sent to the controller and the image processor 400 to process the data.

3 to 5 are views for explaining a method of irradiating a sample to the pump light source according to the present invention. FIG. 3 illustrates a case where the pump light source is irradiated in an off-axis manner, FIG. 4 illustrates a case where the pump light source is irradiated in a collinear manner, and FIG. 5 illustrates a case where the pump light source is irradiated in an inverted manner.

6 are photographs showing an example of detecting impurities in a uniform medium using a method for detecting microdefects according to an embodiment of the present invention. As an imaging sample, a PDMS (polydimethylsiloxane) mixture containing microparticles was prepared. 6 (a) is an image of microparticles located at a depth of 50 μm from the PDMS surface, FIG. 6 (b) is a light heat reflection image when the pump beam is operated, and FIG. 6 (c) is a light heat reflection image when the pump beam is turned off. Indicates. The image acquisition time was about 50 seconds after 50 averaging processes and the acquired image size was 200 μm (X) × 148 μm (Y).

Referring to FIG. 6, when the pump beam is operated (FIG. 6 (b)), due to the photothermal effect of the polystyrene beads, only the reflectance change of the beads is mapped as shown in FIG. 6 (b), while the pump beam is turned off. The photothermal effect of the beads is stopped and the overall reflectance change is not seen as shown in FIG.

Figure 7 is a photograph showing the result of confirming the micro-defects inside the uniform PDMS produced further through the system. 7 is a result of detecting the micro defects in the PDMS for each depth, (a), (c) is an optical microscope image and a light-heat reflection image corresponding to the optical microscopy image at a depth of 15 ㎛, 20 ㎛ from the PDMS surface is (b), (d )to be. Submicron impurities are more clearly displayed in the expanded rectangular box.

That is, FIGS. 7 (a) and 7 (c) are optical microscopic images at depths of 15 μm and 20 μm from the PDMS surface, respectively, and as shown in the drawing, no defects appear to the naked eye. However, during pump beam operation, locally located submicron impurities (enlarged box markings) were found at each position, as shown in FIGS. 7 (b) and 7 (d).

As described above, the present invention relates to a method and apparatus for detecting submicron micro defects in a sample, by applying a two-dimensional image sensor-based heat reflection microscopy technique, by measuring a relative change in reflectance caused by the photothermal effect of impurities, where Can be imaged.

In the present invention, image acquisition time is tens of times faster than conventional detection systems, and a separate optical alignment process is not required for defect measurement, so that defect inspection can be performed more quickly. In particular, in the multi-layered thin film, the three-dimensional defect inspection can be performed much faster than the conventional inspection equipment of the scan driving method, which can be said to be of great technical value and industrial application.

Although a preferred embodiment of the defect analysis method according to the present invention described above has been described, the present invention is not limited to this, and the present invention is not limited thereto, and various modifications can be made within the scope of the claims and the accompanying drawings. It is possible and this also belongs to the present invention.

Claims (11)

Irradiating a pump laser beam of a constant frequency (f) onto the sample; Changing the periodic reflection intensity by changing the temperature around the defect due to the photothermal effect of the defect in the area irradiated with the pump laser beam; And Irradiating a probe beam on the sample to measure a change in the reflection intensity. The method of claim 1, And measuring the phase lock thermal reflection method from the change in reflectance of the sample and converting the sample into a temperature distribution. The method of claim 1, The light source for generating the pump laser beam is a method for detecting microdefects configured by using a wavelength variable laser diode and a wavelength selection filter to irradiate a beam of various wavelengths. The method of claim 1, The pump laser beam is a method for detecting a micro-defect irradiated in the form of a surface light source. The method of claim 4, wherein When the pump laser beam is irradiated with a surface light source, the probe beam imaging plane may be moved into the sample through vertical movement of the sample stage. Thus, the microscopic defect that secures three-dimensional defect information of the sample through the stage Z-axis scan How to detect. A sample mounting unit for mounting a sample; A pump light source for irradiating the pump laser beam at a predetermined frequency f on the sample; A probe light source for irradiating visible light onto the sample; A detector for detecting light reflected from the sample irradiated by the probe light source; And And a control unit and an image processing unit at a multiple of a frequency at which the sample is temperature-modulated by irradiation of the pump laser beam. The method of claim 6, The control unit and the image processing unit detects a micro-defect from the reflectance change by the phase-locked thermal reflection method and converting it into a thermal distribution. The method of claim 6, The pump light source is a device for detecting a micro-defect configured by using a wavelength variable laser diode and a wavelength selection filter to irradiate beams of various wavelengths. The method of claim 6, The pump laser beam is a device for detecting a micro-defect irradiated in the form of a surface light source. The method of claim 6, The method of irradiating the sample to the pump light source detects fine defects irradiated in an inverted manner that is reflected by a mirror and is incident from the bottom of the sample when the pump light source is irradiated in an off-axis manner or when irradiated in a collinear manner. Device. The method of claim 6, Further comprising a light splitter, And transmitting a beam emitted from the probe light source to a sample, and transmitting a beam transmitted from the sample to a detector.

Images