Abstract:
When the intensity of scattering light from a defect on a sample becomes very low according to the diameter of the defect, the dark noise from a sensor device itself accounts which a large proportion of the detected signal outputted from the sensor and thus it is difficult to detect minute defects. Furthermore, since a laser light source is pulsed into oscillation, pulse components from the laser light source are superimposed on the detected signal outputted from the sensor, and therefore it is difficult to detect defects with high accuracy. The present invention is a defect inspection device having irradiation means which producing pulsed operation and irradiating a surface of a sample with a laser beam, detection means which detecting scattering light generated at the surface of the sample in response to the irradiation provided by the irradiation means, and a processing portion which generating a delay signal based on the laser beam emitted by the irradiation means and processing the scattering light detected by the detection means using the delay signal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a defect inspection method and defect inspection device. 
       BACKGROUND ART 
       [0002]    In order to maintain and/or improve product yield on a manufacturing line for semiconductor substrates, thin film substrates, or the like, it is necessary to inspect defects existing on the surfaces of semiconductor substrates, thin film substrates, or the like. 
         [0003]    To detect minute defects on sample surfaces, a method of detecting defects having dimensions of tens of nm to several μm or more is available, for example. The method consists of irradiating a wafer surface with a focused laser beam and gathering and detecting light scattering from defects. 
         [0004]    JP-A-2008-20374 (Patent Literature 1) is available as a background art technique of the present technical field. Disclosed in this publication is “a defect inspection device or tool comprising a light source means emitting a laser in pulsed operation, an irradiation optical system means for controlling the state of polarization of the laser emitted from the light source means and directing the laser at a sample, a detection means for detecting light reflected and scattered from the sample, and a signal processing means for processing a detected signal detected by the detection means and detecting defects on the sample” (refer to the claims). 
         [0005]    Defects referred to herein include particles (foreign matter) adhering to wafers, crystal originated particle (COP) defects, and scratches caused by polishing. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         Patent Literature 1: JP-A-2008-20374 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    In the defect inspection device or tool disclosed in Patent Literature 1, the light source means for emitting laser in pulsed oscillation and the irradiation optical system means for controlling the state of polarization of the laser emitted from the light source means and directing the laser at a sample are disclosed. Where the intensity of light scattered from a defect on a sample is quite small depending on the diameter of the defect, for example, dark noises of the sensor device itself account for a large proportion of the detected signal outputted from the sensor. This makes it difficult to detect microscopic defects. Furthermore, the laser light source is generating light in pulsed operation and it follows that pulse components of the laser light source are also superimposed on the detected signal outputted from the sensor. This makes it difficult to detect defects at high accuracy. 
         [0008]    Accordingly, the present invention offers a defect inspection method and inspection device for reducing the effects of dark noise of a sensor device and of pulsed oscillation of a laser light source. 
       Solution to Problem 
       [0009]    To solve the foregoing problem, configurations set forth in the claims are adopted, for example. 
         [0010]    The present application includes plural means that solve the foregoing problem. One example is a defect inspection device having irradiation means for providing pulsed operation and irradiating a surface of a sample with a laser beam, detection means for detecting scattering light generated at the surface of the sample by the irradiation provided by the irradiation means, and processing portion for generating a delay signal based on the laser beam directed by the irradiation means and processing the scattering light detected by the detection means by the use of the delay signal. 
       Advantageous Effects of Invention 
       [0011]    According to the present invention, microscopic defects can be detected by reducing the effects of dark noise of a sensor device and of pulsed operation of a laser light source. 
         [0012]    Problems, configurations, and advantageous effects other than the foregoing will become apparent from the description of the following embodiments. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is an example of a block diagram of a defect inspection device according to a first embodiment. 
           [0014]      FIG. 2  is an example of operation of the defect inspection device for detection. 
           [0015]      FIG. 3  is a block diagram of a clock detection circuit and an example of operation. 
           [0016]      FIG. 4  is an example of flowchart for finding an optimum set value for a delay control portion. 
           [0017]      FIG. 5  is an example of block diagram of a data processing portion. 
           [0018]      FIG. 6  is an example of monitor view showing the results of detection of defects. 
           [0019]      FIG. 7  is an example of block diagram of a defect inspection device according to a second embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0020]    Embodiments are hereinafter described with reference to the drawings. 
       Embodiment 1 
       [0021]    In the present embodiment, an example of defect inspection device for reducing the effects of dark noise of a sensor device and of pulsed oscillation of a laser light source is described. 
         [0022]      FIG. 1  is an example of block diagram of a defect inspection device of the present embodiment. 
         [0023]    A defect inspection device  100  is configured having a laser light source  2 , a reflective plate  3 , lenses  4 ,  5 , a sensor  6 , an IV converter circuit  7 , an A/D converter circuit  8 , a data processing portion  9 , a CPU  10 , a map output portion  11 , a stage control portion  12 , a rotary stage  13 , a translational stage  14 , a clock detection portion  20 , and a delay control portion  24 . 
         [0024]    The stage portion constitutes the rotary stage  13  and the translational stage  14  on which a sample such as a semiconductor wafer  1  is mounted. 
         [0025]    An irradiation optical system is configured having the laser light source  2  emitting a laser beam (laser light) in pulsed oscillation, the reflective plate  3  for reflecting the laser beam emitted from the laser light source  2  in a direction towards the wafer  1 , and the lens  4  for gathering and focusing the laser beam (laser output)  51  reflected by the reflective plate  3 . 
         [0026]    A detection optical system is configured having the lens  5  for gathering and focusing light scattered at the surface of the wafer  1  irradiated by the irradiation optical system and the sensor  6  for detecting the scattering light gathered and focused by the lens  5  and for providing a sensor output  52 . 
         [0027]    A processing portion is configured having the clock detection portion  20  for detecting the laser beam emitted from the laser light source  2  and generating a clock signal synchronized with the laser light source  2 , the delay control portion  24  for finding an optimum set value of sampling timing based on the clock signal generated by the clock detection portion  20 , the IV converter circuit  7  for subjecting the sensor output  52  to IV conversion and providing an output, the A/D converter circuit  8  for sampling the output from the IV converter circuit  7  based on the delay signal from the delay control portion  24  and providing an ADC output  53 , the data processing portion  9  for data processing the PC output  53  and extracting defects, the CPU  10  for sending the results of the data processing performed by the data processing portion  9 , the map output portion  11  for outputting maps indicative of the results of the data processing delivered from the CPU  10 , and the stage control portion  12  for controlling the motion of the stage portion based on the results of the data processing delivered from the CPU  10 . 
         [0028]      FIG. 2  is an example of operation for detecting the laser output  51 , sensor output  52 , and ADC output  53  in the defect inspection device  100 . 
         [0029]    The laser beam directed at the surface of the wafer  1  via the lens  4  of the irradiation optical system is referred to as the laser output  51 . The signal delivered from the sensor  6  of the detection optical system is referred to as the sensor output  52 . The signal converted and delivered by the A/D converter circuit  8  is referred to as the ADC output  53 . 
         [0030]    At this time, the laser output  51  is in pulsed oscillation. The sensor output signal  52  from the sensor  6  due to the scattering light from defects is also a pulsed signal. Accordingly, in the sensor output signal  52 , the signal is effective only at the instant when laser light is delivered from the laser light source  2 . The signal is ineffective during the period in which no laser light is delivered from the laser light source  2 , and dark noise is generated from the sensor  6  itself. 
         [0031]    During inspection, the whole surface of the wafer  1  is irradiated by moving the rotary stage  13  and translational stage  14  on which the wafer  1  is mounted. That is, under control from the CPU  10 , the stage control portion  12  rotates the wafer  1  through the rotary stage  13  and linearly moves the wafer  1  through the translational stage  14 . Consequently, the laser light incident on the wafer  1  draws a helical trajectory over the whole surface of the wafer  1 . Thus, the whole surface of the wafer  1  can be inspected. 
         [0032]    When there exist defects on the wafer  1 , scattering light is generated at the surface of the wafer  1  by being irradiated with the laser light  51 . The scattering light is detected with the sensor  6  via the lens  5 . The detected signal (sensor output  52 ) delivered from the sensor  6  is sampled by the A/D converter circuit  8  via the IV converter circuit  7 . 
         [0033]      FIG. 3  is an example of block diagram of the clock detection portion  20  and operation. The clock detection portion  20  has a sensor  21  for detecting the laser beam emitted from the laser light source  2 , an IV converter circuit  22  for subjecting the laser beam detected by the sensor  21  to IV conversion, and a clock regeneration circuit  23  (including a comparator circuit  26 , a frequency division circuit  27 , and a multiplier circuit  28 ) for generating a clock signal synchronized with the laser light source  2  based on the signal converted by the IV converter circuit  22 . In the clock detection portion  20 , a clock signal synchronized with the laser light source  2  is generated based on the laser light transmitted through the reflective plate  3  after exiting from the laser light source  2 . 
         [0034]    The clock signal generated by the clock detection portion  20  is adjusted in delay via the delay adjusting portion (delay control portion)  24 . A signal delivered by the IV converter circuit  7  based on this is sampled by the A/D converter circuit  8  to thereby obtain the ADC output  53 . 
         [0035]    The incidence of the laser light generates a signal via the sensor  21  and the IV converter circuit  22 . The signal is compared with a comparison voltage  25  by the comparator circuit  26  and becomes a signal on pulses indicated by a comparison circuit output  61 . Then, the signal is frequency divided by the frequency division circuit  27  into a frequency division circuit output  62  that is half in frequency of the laser oscillation. Furthermore, a clock signal with a double frequency of the frequency division output  62  is produced via the multiplier circuit  28 . As a result, reproduced clock  63  becomes a clock signal that has the same frequency as the comparator circuit output  61 , i.e., laser oscillation, and a duty ratio of about 50%. Using the reproduced clock  63 , the delay adjusting portion  24 , A/D converter circuit  8 , and data processing portion  9  in later stages are operated. 
         [0036]    For the sake of illustration, the frequency division circuit  27  is set to one half, and the multiplier circuit  28  is set to twice. Obviously, the invention is not restricted to this ratio if equivalent effects are obtained by a desired operation of the A/D converter circuit  8  based on the laser oscillation. The frequency division circuit  27  and multiplier circuit  28  included in the clock generating circuit  23  may be integrated into a PLL circuit and it is used. In addition, a clock signal having a duty ratio close to about 50% can be generated using a delay means. 
         [0037]      FIG. 4  is an example of flowchart for finding an optimum set value of the sampling timing in the delay control portion  24 . 
         [0038]    In the defect inspection device  100 , the detected signal is effective only at the instant when laser light is delivered from the laser light source  2  and, therefore, when the detected signal is sampled by the A/D converter circuit  8  based on the reproduced clock, it is necessary to optimally set the sampling timing. 
         [0039]    The optimal set value of the sampling timing in the delay control portion  24  is carried out prior to a defect inspection as a calibration operation of the defect inspection device  100  such as irradiation of the wafer  1  with a laser beam or detection of scattering light. 
         [0040]    In the flow of execution, a variable N is first set to 0 (step  101 ). The set delay value of the sampling timing in the delay control portion is set according to the variable N (step  102 ). Then, M data points are sampled by the D converter circuit  8  (step  103 ), and the average value of the sampled data is calculated (step  104 ). The result of calculation of the average value is stored in a memory such that the variable N is made to correspond to an address (step  105 ). The variable N is increased (step  106 ). Where the variable N is equal to or less than a preset final value (step  107 ), the steps  102  to  106  are repeated. Where the variable N exceeds the final value, the results of calculations of the average value stored in the memory are compared and N giving a maximum average value is identified (step  108 ). Since the sensor  6  output at the moment when laser light is produced is greater than the dark noise of the sensor  6  itself, it follows that the set delay value of the reproduced clock corresponds to N giving the maximum average value. 
         [0041]      FIG. 5  is an example of block diagram of the data processing portion  9  in the defect inspection device  100 . 
         [0042]    The data processing portion  9  is configured having a peak detection circuit  33 , a counter circuit  32  operated by a position detection clock  31 , and a hold circuit  34 . 
         [0043]    The position detecting clock  31  is a clock signal oscillating in synchronism with the operation of the rotary stage  13  for rotating the wafer  1  and of the translational stage  14  for translating the wafer  1 , as well as a signal associated with the position of a beam incident on the wafer  1 . The signal is generated inside the defect detection device  100 , e.g., by the stage control portion  12  (not shown). 
         [0044]    The peak detection circuit  33  detects the peak value of sampled data, based on the output data from the A/D converter circuit  8  and the optimal set value data of the sampling timing determined in the delay control portion  24 , and outputs defect detection information  36  based on it. At the same time, the hold circuit  34  maintains the output from the counter circuit  32  and outputs positional information  37 , based on the results of transmission from the peak detection circuit  33 , on the signal from the counter circuit  32 , and on the optimal set value data about the sampling timing determined in the delay control portion  24 . Also, the output data from the A/D converter circuit  8  becomes defect diameter information  35 . The CPU  10  in the later stage outputs defect detection results as a monitor view via the map output portion  12 , based on the aforementioned defect detection information  36 , defect diameter information  35 , and positional information  37 . 
         [0045]      FIG. 6  is an example of monitor view indicating the results of defect detection. 
         [0046]    This indicates a position where a defect is detected on reference coordinates on the surface of the wafer  1  defined in terms of r and θ. As described in  FIG. 5 , the defect diameter information  35 , the defect detection information  36 , and the positional information  37  are obtained and so defect diameters and positions are displayed as defect information about extracted defects on the monitor view of  FIG. 6 . 
       Embodiment 2 
       [0047]    In the present embodiment, an example of defect inspection device which not only reduces the effects of dark noise of a sensor device and pulsed oscillation of a laser light source but also achieves lower cost by simplifying the instrumental configuration is described. 
         [0048]      FIG. 7  is an example of block diagram of a defect inspection device of the present embodiment. To avoid complicating the explanation, a description of constituent elements indicated by the same reference numerals as in Embodiment 1 is omitted. 
         [0049]    A defect inspection device  100  shown in  FIG. 7  is characterized in that it has a reference clock generating circuit  40 . The laser light source  2  pulse oscillates based on the clock signal delivered from the reference clock generating circuit  40 . A detection operation is performed in the A/D converter circuit  8  and data processing portion  9  via the delay adjustment circuit  34 . 
         [0050]    In comparing the defect inspection device associated with the present embodiment with the defect inspection device  100  shown in  FIG. 1 , both laser oscillation and detection operation can be achieved with a common reference clock without using the clock detection portion  20 . The effects of the dark noise of the sensor device and the pulsed oscillation of the laser light source are reduced. In addition, a stable detection operation is achieved without depending on the laser output intensity. Also, lower cost can be achieved while suppressing the constituent elements of the instrument. 
         [0051]    It is to be understood that the present invention is not restricted to the above embodiments but rather embraces various modifications. For instance, the above embodiments have been described in detail such that the present invention is explained in an easily understandable manner. The invention is not limited to those having all the configurations described. Some of the configurations of some embodiment may be replaced by configurations of other embodiments. In addition, configurations of other embodiments may be added to configurations of one embodiment. Further, with respect to some configurations of each embodiment, addition, erasure, and replacement of other configurations may be made. 
         [0052]    Furthermore, the above-described configurations, functions, processing portions, processing means, and so on may be realized in hardware by designing some or all of them, for example, using an integrated circuit. Additionally, the above-described configurations, functions, and so on may be realized by software such that a CPU interprets programs that achieve respective functions. 
         [0053]    While the aspects of the present invention have been described thus far using its embodiments, the defect inspection device can sample a sensor output in synchronism with pulsed emission of a laser light source at optimum timing by applying the present invention. Dark noise detection of a sensor device contained in the sensor output signal during non-emission can be removed. Since the sampling is done at the same frequency as the pulsed emission of the laser light source, an inexpensive A/D converter of high-bit resolution can be applied without using an expensive high-speed A/D converter with low bit resolution. The detection accuracy of the defect inspection device can be enhanced. Also, lower cost can be accomplished. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1 : wafer;  2 : laser light source;  3 : reflective plate;  4 ,  5 : lenses;  6 : sensor;  7 : IV converter circuit;  8 : converter circuit;  9 : data processing portion;  10 : CPU;  11 : map output portion;  12 : stage control portion;  13 : rotary stage;  14 : translational stage;  20 : clock detection portion;  21 : sensor;  22 : IV converter circuit;  23 : clock regeneration circuit;  24 : delay control portion;  25 : comparison voltage;  26 : comparator circuit;  27 : frequency division circuit;  28 : multiplier circuit;  31 : position detection clock;  32 : counter circuit;  33 : peak detection circuit;  34 : hold circuit;  35 : defect diameter information;  36 : defect detection information;  37 : positional information;  40 : reference clock generating circuit;  100 : defect inspection device