Patent Publication Number: US-2007103151-A1

Title: Inspection method and device for semiconductor equipment

Description:
FIELD OF THE INVENTION  
      The present invention relates to non-destructive inspection devices and such inspection methods, and more specifically to devices and methods suitable for locating a defective portion of semiconductor devices using a microscope with a magnetic field detector.  
     BACKGROUND OF THE INVENTION  
      As a non-destructive inspection method for a sample like semiconductor wafers, an inspection method that employs a scanning laser SQUID (superconducting quantum interference device) microscope is known. When the laser beam is irradiated onto a defective portion or a portion related to the defect, an electric current flows. The laser-SQUID microscope detects the magnetic field induced by the current with a SQUID fluxmeter and obtains images by scanning the laser or samples as disclosed by Nikawa and Inoue, LSITS2000, pp. 203-208 (2000) [Non-patent Document 1]. When the laser beam is irradiated onto a substrate of a semiconductor sample, an electron-hole pair generated by the laser irradiation causes a current to flow by electric field at p-n junctions and so on. The current is called an OBIC (optical beam induced current) current. Since defects within the sample cause an imbalance in temperature gradients by irradiation heat of the laser beam, a current is caused to flow also by the thermoelectric effect [Refer to Non-patent Document 1, Patent Document 1 etc.].  
      Moreover, a detection method that extracts a signal with the same frequency as the modulation frequency from the detected magnetic field signals is employed in the scanning laser-SQUID microscopy to realize a practical S/N (signal to noise) ratio. Non-patent Document 1 discloses a criterion for selecting a modulation frequency having less environmental noise. To be more specific, a frequency of 8.3 kHz is selected. Non-Patent Document 2 also discloses a development of system with 100 kHz at the maximum to reduce time to obtain images by the scanning laser-SQUID microscopy, which improves the conventional system with 10 kHz at the maximum.  
      [Patent Document 1] 
      Japanese Patent Kokai Publication No. JP-P2002-313859A  
      [Non-Patent Document 1] 
      K. Nikawa and S. Inoue, “Novel non-destructive and non-contact failure analysis and process monitoring technique—Scanning laser-SQUID microscopy-”, LSITS2000, pp. 203-208, 2000  
      [Non-Patent Document 2] 
      K. Nikawa and H. Nakayama, “Scanning laser-SQUID microscope showing sub-micron spatial resolution observed from back-side of a wafer”, LSITS2002, pp. 275-280, 2002  
      [Non-Patent Document 3] 
      T. Sakai and K. Nikawa, “Observation of completed LSI after building-in defect using laser-SQUID microscopy”, LSITS2004, pp. 341-345, 2004  
     SUMMARY OF THE DISCLOSURE  
      Positions such as p-n junction where photocurrent is induced by laser irradiation are located by scanning laser-SQUID microscope. There is a problem with such convectional inspection methods that images of the positions (such as p-n junction where photocurrents are induced by the laser irradiation) obtained by the methods are blurred (i.e., out of focus) except certain special cases and poor in spatial resolution than those by optical microscopes.  
      Therefore, the localizing (or focusing) accuracy of the defective position by the conventional inspection methods has been about 100 micrometers at most as disclosed in the Non-patent Document 3, for example.  
      It is an object of the present invention to provide a completely novel method to overcome the above-mentioned problem. According to the present invention, there is provided a novel inspection method characterized by increasing the modulation frequency higher than those in the conventional methods.  
      For example, images of a higher resolution are obtained by the scanning laser-SQUID microscope based on modulation frequencies higher than 100 kHz which has been conventionally 100 kHz at the maximum.  
      According to a first aspect of the present invention, there is provided an inspection method comprising: generating laser light modulated in its intensity with a modulation signal (i.e., synchronized with a reference signal) and scanning a sample with the modulated laser light; detecting magnetic field induced from the sample using a magnetic field detector; extracting a magnetic field signal of the same frequency component as the modulation frequency from magnetic field signals detected by the magnetic field detector; outputting the extracted signal as an image display signal; wherein the modulation frequency is higher than 100 kHz.  
      An inspection device according to the present invention is characterized by comprising: a detector that extracts a magnetic field signal from the sample of the same frequency component as the modulation frequency higher than 100 kHz during the irradiation of the modulated laser light on the sample; and a display that displays images using the signal extracted by the detector as an image display signal.  
      According to a second aspect of the present invention, there is provided a detector that irradiates laser light modulated with a modulation frequency higher than 100 kHz on a sample, receives magnetic field signals from a magnetic field detector that detects the magnetic field induced from the sample and extracts a signal of the same frequency component as the modulation frequency; and a display that displays images using the signal extracted by the detector as an image display signal.  
      The device according to the present invention comprises: a modulated beam generating unit that generates laser light modulated in its intensity with a modulation signal synchronized with a reference signal; an optical system that irradiates the modulated laser light on a sample; a magnetic field detector; a scanner that scans the modulated laser light relatively to the sample; a extractor that receives magnetic field signals from the magnetic field detector and extracts a signal of the same frequency component as the modulation frequency; and a display that generates and displays a magnetic field from the extracted signal; wherein the modulation frequency is higher than 100 kHz.  
      The meritorious effects of the present invention are summarized as follows.  
      The present invention makes it possible to obtain scan images with higher spatial resolution than those by conventional methods. As described in detail in the following, the reason is thought that the travel length through distribution of minor carriers during one period of the modulated frequency becomes shorter as the frequency becomes higher. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows an embodiment of the present invention.  
       FIG. 2A  shows configuration of a device as an embodiment of the present invention, and  FIG. 2B  shows schematically timing waveforms of magnetic field signals, reference signals and modulation signals. 
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION  
      Embodiments of the present invention are described below in more detail referring to the attached drawings. In the present invention a sample for inspection is scanned by relative movement (or displacement) between the irradiation spot (position) of a laser beam and the sample, and the magnetic field is detected with a highly sensitive magnetic field sensor to obtain a magnetic field image.  FIG. 1  shows an inspection method according to one embodiment of the present invention.  
      According to the present invention, as shown first in  FIG. 1 , when an OBIC current (photocurrent) is generated by irradiation of a laser beam modulated in their intensity with a predetermined frequency onto the back surface of an IC chip  110 , a magnetic field is induced and the magnetic flux is detected with a magnetic fluxmeter (a highly sensitive magnetic field sensor)  120  that is arranged against the main side of the chip, to form a magnetic field detector. Two dimensional images that correspond to the magnetic field distribution can be obtained by scanning the laser beam over the back surface of the IC chip (or moving the sample relative to the fixed laser light) and displaying the intensity (distribution) of the magnetic flux detected by the magnetic fluxmeter  120  in association with the scanning position information of the IC chip  110  on a display device based on gradation display (brightness display), for example.  
      Moreover, output voltage that corresponds to strength of the magnetic field is output by the fluxmeter  120 , and a signal that is output from the magnetic fluxmeter  120  and selected for having only the same frequency as the modulation frequency of the laser with the detector  130  is converted to gradation data of the pixel in association with positions irradiated with the laser beam in a signal processor (or a data processor), followed by displaying on a display device  140 . In this way, image data of the magnetic field distribution (scanning image of the magnetic field distribution) are obtained. The purpose of irradiation of the laser beam onto the back surface of the IC chip is to make the laser beam reach p-n junction  112  near the surface of silicon substrate  111  of the IC chip  110 . That is, when they are irradiated from the top side of the IC chip  110 , the laser beams reflect on the metal wiring layer within the upper layers of silicon substrate of the IC chip and so on and does not reach the p-n junction  112  near the surface of the silicon substrate  111 . Moreover, when wafers and chips are inspected, they are preferably finished up with mirror polishing.  
       FIG. 1  is a schematic illustration of scanned images of a p-n junction  112  near and in the surface of the silicon substrate  111  in association with the modulation frequency on a display device  140 . Merely for explanation,  FIG. 1  shows three display images (a), (b) and (c) that correspond to relatively high, medium and low modulation frequencies, respectively, as an example of the scanning microscope image displayed on the display device  140  for explanation. This does not necessarily mean that these three images are always displayed at the same time. On the contrary, each of these three images is displayed individually.  FIG. 1  shows that spatial resolution becomes higher as the modulation frequency becomes higher. As a magnetic fluxmeter  120 , a SQUID fluxmeter is preferably employed in the present embodiment. However, it is not limited to the SQUID fluxmeter. Any highly sensitive magnetic field sensor can be employed.  
      In the following, the reason why the spatial resolution becomes higher as the modulation frequency becomes higher is analyzed. For simplifying the explanation,  FIG. 1  shows a case where images of one p-n junction  112  are obtained by a scanning laser microscope. The laser is modulated and scans the region that includes p-n junctions. When the laser hits the p-n junction  112 , a photocurrent is generated and flows into an LSI internal wiring  113  through electrodes  114  connected with both ends of the p-n junction. A magnetic field induced by the current is detected with the magnetic fluxmeter  120  and used as a magnetic field signal and displayed on the display device  140 .  
      As described above, not all the magnetic field signals are used to display images but only the components having the same frequency as the laser modulation frequency are extracted with the detector  130  and used as image display signals.  
      The photocurrent flows even if the laser beam doesn&#39;t just hit the p-n junction  112 .The reason is explained by the phenomenon called carrier diffusion. The carrier diffusion occurs when carrier densities are not uniform. When electron-hole pairs are generated by the irradiation of the laser beam, the minority carrier meets this condition.  
      When the laser beam is irradiated onto a silicon substrate (Si), electron-hole pairs are generated. When there is a p-n junction at the irradiation position, the electron and the hole are pulled apart immediately by the internal electric field of a depletion layer existing there to form a current.  
      On the other hand, the generated minority carriers evenly spread out toward the surroundings by diffusion, when the irradiated position is not the p-n junction. Then, the minority carriers recombine to extinguish while spreading. When the minority carriers arrive at a p-n junction before extinction, they form a photocurrent by the internal electric field.  
      As understood by the foregoing explanation, the area (boundary) within which the carriers generated by the laser irradiation can reach within one period of the laser modulation frequency expands (i.e., is large) when the modulation frequency is low.  
      An image that extends to form a circle is obtained, for example, as shown in the display image (c) with a caption “low” within the display device  140  in  FIG. 1 .  
      When the frequency is raised beyond that, area of the extension decreases as shown in the image (b) with a caption “medium”.  
      When the frequency is raised still further, an image of almost only the p-n junction is obtained as shown in the image (a) with a caption “high”.  
      In this way, the spatial resolution is improved by raising the modulation frequency. The experiment on the display image and the consideration are the original findings of the present inventor.  
      That is, it has not been recognized conventionally that the size of the image would change with the frequency. Therefore, as stated above, the selection of the frequency has been done as follows. Namely, a frequency with less environmental noise has been chosen as a laser modulation frequency in Non-patent Document 1, for example, and a certain high frequency has been chosen in Non-patent Document 2 in order to decrease the time to obtain scanning laser SQUID microscope images.  
      The frequency with low environmental noise can be selected irrespective of the magnitude of the frequency. Moreover, it has been unnecessary to develop a system in which a frequency over 100 kHz can be selected, because the frequency of at most 100 kHz is enough to decrease the time to obtain the images.  
      According to the present invention, the spatial resolution of about 100 micrometers in the conventional methods are improved by one digit or more by raising the modulation frequency up to a level of higher than 100 kHz. Hereafter, the present invention is explained along with an embodiment.  
      [Embodiments] 
       FIG. 2A  shows one example of a device structure that embodies the present invention.  FIG. 2B  shows an example of a timing waveform of reference signal  1 , modulation signal  2  and magnetic field signal  4  in  FIG. 2A . According to  FIG. 2A , this inspection device comprises: a modulated beam generating unit  10  that generates light modulated in its intensity by a signal  2  synchronized with a predetermined reference signal  1  and focuses to generate a modulated laser beam  61 ; a sample stage  71 , on which a sample  70  is located, that moves so that a predetermined position of the sample is irradiated by the modulated beam  61 ; a magnetic detection unit  20  that detects a magnetic field (magnetic flux) induced by a current generated during the irradiation of the modulated beam  61  onto the sample  70  and outputs magnetic field signals  4 ; a signal extractor  30  that extracts the magnetic field intensity and the phase difference  6  (see  FIG. 2B ) between the reference signal  1  and the magnetic field signal  4  and outputs them as an intensity signal  5  and a phase difference signal  7 , respectively; a system controller (abbreviated as controller) that controls irradiation of the modulated beam  61  onto the sample  70  and positioning of the sample stage  71  according to the information on the irradiation position, inputs the intensity signal  5  and the phase-difference signal  7  and outputs these signals in association with the information on the irradiation position; and a display  50  that receives at least one of the intensity signal  5  or the phase difference signal  7  and information on the irradiation position, and displays resultant images.  
      The modulated beam generating unit  10  comprises: a pulse generator  11  that generates and outputs the reference signal  1  and the modulation signal  2  (see  FIG. 2B ) synchronized with the reference signal  1 ; a fiber laser and the like with a modulation function specifically comprising; a laser light source  12  that generates light (laser light) modulated in its intensity with the modulation signal  2  output from the pulse generator  11 ; an optical fiber  14  that guides the laser light; and an optical unit  13  that focuses the light guided by the optical fiber  14  and generates a modulated beam  61 .  
      The magnetic field detection unit  20  comprises a SQUID fluxmeter  21  that works as a highly sensitive magnetic field sensor and an electronic circuit (also called as a SQUID electronic circuit)  22  that controls the SQUID fluxmeter  21 , generates a magnetic field signal  4  from the output signal  3  (output voltage) of the SQUID fluxmeter  21  and outputs these signals. For example, a high-temperature superconducting SQUID is employed as the SQUID fluxmeter  21 . A flux-locked loop (FLL) circuit may be employed as the electronic circuit  22 . The SQUID fluxmeter  21  should be so highly sensitive as to sense the magnetic flux changing with as specified high frequency in association with the modulated laser light of the specified high modulation frequency.  
      The signal extractor  30  comprises a two-phase lock-in amplifier (not shown in the drawings) for example although it is not restricted to such amplifier, receives the magnetic field signal  4  from the electronic circuit  22  and detect a signal with the same frequency as the modulation signal  2 . Moreover, the signal extractor  30  may output the intensity of the extracted signal (with the same frequency as the modulation frequency) and the phase difference between the magnetic field signal  4  and the modulation signal  2  as output signals  5  and  7 , respectively.  
      The controller  40  controls the position of the sample stage  71  (or the sample  70 ) according to stage scanning signals, for example, controls optical unit  13  of the modulated beam generating unit  10  in  FIG. 1  if necessary and so on and irradiates the sample  70  while scanning the modulated beam  61  over the sample  70 .  
      The control unit  40  uses signals, detected (extracted) by the signal extractor  30 , that have the same frequency component(s) as the modulation frequency as the image display signal and supplies them to the display unit  50 . The control unit  40  receives the intensity signal  5  and the phase difference signal  7  and controls the display of images with the scanning laser SQUID microscope synchronized with the stage scan, the laser light irradiation position or scanning of the SQUID fluxmeter, although it is not limited to this case. The control unit  40  outputs image display signal  8  as a combination of information on the irradiation position of the modulated beam  61  and the intensity signal  5  and phase difference signal  7  both of which are in association with the information on the irradiation position.  
      The display unit  50  comprises a PC (personal computer)  51  and a display  52 , receives the image display signal  8  from the control unit  40 , and output an intensity image  81  in association with the magnetic field at the laser-beam scanning position and a phase difference image  82  corresponding to the phase difference. Moreover, as shown in  FIG. 2A , it is preferred to show both the intensity image  81  and the phase difference image  82  as the image of the magnetic field distribution, although only one image has been shown in the above explanation of  FIG. 1  to make the explanation easier.  
      Next, an example of operations for the present embodiment shown in  FIGS.2A and 2B  is explained. In the present embodiment, IC chips or Si wafers are used as a sample  70 . Needless to say, compound semiconductor wafers, TFT substrates and the like may be also used. The sample  70  is located on the sample stage  71 , a reference signal  1  and a modulation signal  2  synchronized with the reference signal are generated by the pulse generator  11 , the reference signal  1  is output to the signal extractor  30 , the modulation signal  2  is output to the laser light source  12  composed of fiber laser (of 1,065 nm in wavelength, for example) equipped with a modulation function, intensity-modulated laser light is generated, guided to the optical system unit  13  through the optical fiber  14  and the modulated beam (laser beam) is focused on the sample  70 .  
      When the modulated beam  61  is irradiated onto the first irradiation spot, the SQUID fluxmeter  21  detects a magnetic field from the sample  70  and the electronic circuit  22  outputs a magnetic field signal  4 . The signal extractor  30  receives the magnetic field signal  4  and outputs the intensity signal of the magnetic field signal  4  having the same frequency as the modulation signal  2  and the phase difference signal  7  (of the magnetic field signal from the modulation or reference signal) to the control unit  40 .  
      The control unit  40  outputs the intensity signal  5  and phase difference signal  7  corresponding to the irradiation position of the modulated beam  61  to the PC  51 . The PC  51  memorizes the intensity signal  5  and the phase difference signal  7  in a storage unit (not shown in the drawings) of the PC  51 .  
      The irradiation spots within the desired inspection area are selected sequentially by x-y scan of the sample stage  71  controlled by the stage scan signal combined with the scan of the modulated beam  61  controlled by the laser scan signal if necessary; the desired irradiation spots within the inspection region are sequentially selected and irradiated by the modulated beam  61 ; and the intensity signal  5  and the phase difference signal  7  are stored in the storage of PC  51  as the image display signal  8  in accordance with the information on the position of the irradiation spot.  
      The PC  51  displays images on the display  52  with a gradation (brightness) display in correspondence with the intensity signal  5  or phase difference signal  7  and the laser irradiation points of the sample  70  (Refer to  FIG. 2A ). In this case, either intensity image  81  corresponding to the intensity signal  5  or the phase difference image  82  corresponding to the phase difference signal  7  may be displayed. The magnetic field images may be displayed in real time in association with the scanning spot. Moreover, the image of the intensity signal  5  and phase difference signal  7  stored in the storage corresponding to the scanning position may be displayed offline on the display  52  when the entire scans over one sample is finished or at any time afterwards by memorizing the gradation (brightness) data of the intensity signal  5  and phase difference signal  7  corresponding to scanning positions. Of course, printers, file devices and the like may be used instead of the display.  
      In the present embodiment, employment of a fiber laser device equipped with modulation function as a laser generator  12  makes it possible that the frequency of the modulation signal  2  is set to any values up to, e.g., 1 M Hz at a maximum as far as the presently available facility is concerned. When the S/N (signal to noise) ratio is low because of weak magnetic field signals, the modulation frequency should be selected so as to reduce the noise. This does not necessitate changing the frequency in a large extent. Since the laser light of 1,065 nm in wavelength is employed in the present embodiment, the modulated laser beam irradiated onto the back surface of a Si wafer  70  penetrates the silicon substrate and reaches the p-n junction (region) near the Si wafer surface. Silicon wafers with a mirror-polished back surface are preferred, because the modulated beams  61  irradiated onto the back surface of Si wafers reach around p-n junction (region) efficiently.  
      When a HTS (high temperature superconducting) SQUID fluxmeter is used as a SQUID fluxmeter  21 , it is possible to detect a magnetic flux density B of 1 pT or less. Moreover, the SQUID fluxmeter is equipped with a magnetic shield  65 . It usually detects magnetic flux perpendicularly to the surface of the sample  70 .  
      Since the output magnetic field signal  4  from the electronic circuit  22  usually includes noises, extraction of the same frequency component as the modulation frequency with a two-phase lock-in amplifier (signal extractor  30 ) improves the S/N ratio. The employment of a two-phase lock-in amplifier as the signal extractor  30  makes it possible not only to extract only the same frequency component(s) as the modulation frequency  2  but also to output the phase difference signal  7  and the intensity signal  5  as separate data. When the chip size is 6 mm by 10 mm for example, the image of the magnetic field distribution is captured by focusing the modulated beam  61  into a beam of 10 microns diameter and by scanning the sample stage  71  composed of a ceramic stage in x-y direction(s) (x- or y-direction, or x-y directions).  
      A modulation frequency higher than 100 kHz is chosen for example. Then an intensity image  81  and a phase difference image  82  are obtained.  
      Although the configuration shown in  FIG. 2A  has been explained based on an example where the intensity signal and the phase difference signal showing phase difference between the magnetic field signal and the reference signal are output individually from the magnetic field signal detected with a magnetic field detection unit, it is certainly applicable to the case where only the intensity signal is output. Moreover, although the explanation has been based on the example where a SQUID fluxmeter  21  is employed as a highly sensitive magnetic field sensor, the present invention is not limited to the case of scanning SQUID microscopy with a SQUID fluxmeter. The present invention is applicable to scanning microscopy with any highly sensitive magnetic field sensor.  
      Although it has been explained according to the above embodiment, the present invention is not limited to the construction of the above embodiment. It is, of course, that various modifications and variations by persons skilled in the art fall within the spirit and scope of the present invention.  
      It should be noted that other objects, features and aspects of the present invention will become apparent in the entire disclosure and that modifications may be done without departing the gist and scope of the present invention as disclosed herein and claimed as appended herewith.  
      Also it should be noted that any combination of the disclosed and/or claimed elements, matters and/or items may fall under the modifications aforementioned.