Abstract:
The detection part has: a subtraction module for calculating correction data from data of detection systems when a reference-voltage generation module applies a reference voltage to the detection systems; a data-holding module for holding the correction data; an addition module for making a correction of detection data; a comparison module for comparing the detection data with switching data; and a selector for switching data of the detection systems including data subjected to the correction according to the output of the comparison module.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese application serial no. JP 2007-124082, filed on May 9, 2007, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a contamination-inspecting apparatus for inspecting a semiconductor wafer, and specifically it relates to a technique for detecting a signal. 
         [0004]    2. Description of the Related Art 
         [0005]    A contamination-inspecting apparatus for inspecting a surface of a semiconductor wafer for a contaminant has: an irradiation optical system for irradiating a wafer surface with laser light; a detection optical system for detecting light scattered from the wafer surface; a detection unit for converting the scattered light detected with the detection optical system into electric signals to make a correction; and a display unit for making a judgment on a contaminant to display the result. In the detection unit, a detection circuit is incorporated; usually a signal is output through the detection circuit to make a judgment on a contaminant. Now, with reference to  FIG. 5 , a detection circuit of a semiconductor wafer contamination-inspecting apparatus, which was considered as a related art by the inventor, will be described in outline. The conventional contamination-inspecting apparatus irradiates a wafer with laser light while rotating the wafer, and detects light scattered and reflected by a contaminant on a wafer and the like by use of PMT (Photo Multiplier Tube)  1 . Specifically, the PMT  1  is a device which multiplies incident light by the effect of multiplication of photoelectrons, and then outputs an electric current depending on the intensity of incident light. PMT  1  outputs a contamination-detection signal current based on light scattered and reflected by a contaminant and the like. In the detection part  10 , an I-V conversion module  2  including a resistor  4  and an operational amplifier  5  converts the contamination-detection signal current into a detection voltage signal. Further, in the detection circuit  3 , a logarithmic amplifier  6  logarithmically amplifies the detection voltage signal making an input voltage  8  to the detection circuit  3  so that a minute voltage can be detected even with the minimum resolution of an analog-to-digital conversion circuit  7 , and then the analog-to-digital conversion circuit  7  converts the amplified voltage to a digital value. Subsequently, the resultant digital signal having the digital value is output to a data processing circuit (not shown) as an output code  9 , and then a judgment about a contaminant or the like is made. 
         [0006]    In general, the intensity of light scattered from a contaminant with a minute particle size is said to be proportional to about the sixth power of the diameter of the contaminant. Therefore, in the case where a wafer targeted for inspection is irradiated with light, and scattered light produced by the irradiation is used to detect a contaminant, a detection circuit for detecting a contaminant is required to have a wider dynamic range, which depends on the range of the amplitudes of detected signals. This is because the smaller the contaminant on a wafer is, the minuter the detected signal is. Further, when the number of rotations of a wafer is increased in order to shorten the time for inspecting the wafer, scanning of a surface of the wafer is speeded up, which results in an increase in the rate of change of the detected signal per hour. Therefore, it is required to speed up the operation of the detection part. 
         [0007]    Now, as for an analog-to-digital conversion circuit, there is a basic trade-off relation between its processing speed and dynamic range in general. Therefore, the dynamic range of the analog-to-digital conversion circuit tends to decrease with an increase in the speed, and its minimum resolution has a tendency to increase. 
         [0008]    Hence, a method to widen the dynamic range by means of arranging amplifiers in parallel, which amplify a detection current with different amplification factors, and using, of outputs of the amplifiers, an output of an appropriate range as the technique disclosed by JP-A-8-145899 has been proposed in the art. 
       SUMMARY OF THE INVENTION 
       [0009]    According to the technique as disclosed by JP-A-8-145899, an analog-to-digital conversion circuit is arranged for each of the ranges which the amplifiers are responsible for. Therefore, the dynamic range of the whole detection part can be widened even in the situation where the dynamic range of each analog-to-digital conversion circuit is restricted owing to the speedup. 
         [0010]    However, the technique has a problem such that it is required to adjust the amplification factors of the amplifiers or to select their components thereby to fit the ratios of the amplification factors to design values because the amplifiers perform detections with different ranges, and therefore the difference in detection error at the time of switching between their detection ranges arises in nature, which lowers the accuracy of measurement. 
         [0011]    The invention was made in consideration of the problems as described above, and it provides a method for detection and a semiconductor wafer contamination-inspecting apparatus using the method, which can speed up the operation of the detection circuit and widening of the dynamic range thereof, and which can achieve the higher accuracy by correcting the difference in detection error caused by switching between detection ranges. 
         [0012]    A method for detection according to the invention and a semiconductor wafer contamination-inspecting apparatus using the method are characterized in: in a detection part are disposed amplification modules having different amplification factors and detection systems each having an analog-to-digital conversion circuit and a logarithmic conversion module; and an output code composed of an output of at least one of the detection systems is compared with switching data to switch between the output code and an output code composed of an output of the other detection system and selectively output the output codes. 
         [0013]    The detection part according to the invention has the following features. That is, in the detection part are disposed a reference voltage output module for producing a reference voltage, and a switching means for switching between an input voltage to the detection part and the reference voltage to selectively apply the voltages to the detection systems. Also, in the detection part are disposed a means for applying the reference voltage to the detection systems, a means for computing a difference between output codes output by circuits of the detection systems, and a means for holding the resultant difference as a piece of correction data. Thus, when a numerical computation to derive the correction data is performed on an output code of one of the detection systems on input of voltage of a detected signal to the detection part, output codes output by the detection systems are coincident with each other in the condition where a detection voltage equivalent to the reference voltage is input to the detection part. 
         [0014]    As for the detection method, the detection systems speed up the operation of the detection circuit and widen the dynamic range thereof. Further, the difference in detection error between ranges is corrected at the time of switching the outputs of the detection systems, whereby the higher accuracy of the detection part can be achieved. 
         [0015]    Now, the outline of the preferred forms of the invention will be described below briefly. 
         [0016]    (1) According to the first aspect of the invention is provided a contamination-inspecting apparatus for inspecting a wafer surface for a contaminant including: an irradiation optical system for irradiating the wafer surface with laser light; a detection optical system for detecting light scattered from the wafer surface; and a detection unit for converting the scattered light detected with the detection optical system into an electric signal to make a correction, wherein the contamination-inspecting apparatus, the detection unit has a plurality of detection circuits with different amplification factors, and the detection unit has a means for correcting the electric signal by use of a piece of correction data derived from a difference in amplification factor between the plurality of detection circuits and held in advance. 
         [0017]    (2) According to the second aspect of the invention is provided the contamination-inspecting apparatus as stated in (1), wherein the detection unit has a switching means for switching between a piece of output data of a first detection circuit and a piece of data output by a second detection circuit and then corrected based on the piece of correction data thereby to selectively output the pieces of data, provided that the first and second detection circuits are each one of the plurality of detection circuits. 
         [0018]    (3) According to the third aspect of the invention is provided the contamination-inspecting apparatus as sated in (2), wherein the first detection circuit has a first amplification means, and a first analog-to-digital conversion circuit for sampling an output of the first amplification means, the second detection circuit has a second amplification means, and a second analog-to-digital conversion circuit for sampling an output of the second amplification means, and the piece of correction data is calculated from a piece of output data of the first analog-to-digital conversion circuit and a piece of output data of the second analog-to-digital conversion circuit when a reference voltage is applied to the first and second amplification means. 
         [0019]    (4) According to the fourth aspect of the invention is provided a detection circuit which detects a voltage and outputs a code corresponding to the voltage, including: a reference-voltage generation means for producing and outputting a reference voltage; a switching means for performing switching between an input to the detection circuit and an output of the reference-voltage generation means; a first amplification means for amplifying an output of the switching means; a first analog-to-digital conversion circuit for sampling an output of the first amplification means; a second amplification means for amplifying the output of the switching means; a second analog-to-digital conversion circuit for sampling an output of the second amplification means; a computing means for calculating a piece of correction data from a piece of output data of the first analog-to-digital conversion circuit and a piece of output data of the second analog-to-digital conversion circuit when the reference voltage is applied to the first and second amplification means with the switching means; a correction means for making a correction of a piece of output data of at least one of the first and second analog-to-digital conversion circuits by use of the piece of correction data calculated by the computing means; and a switching means for switching between the piece of output data of the analog-to-digital conversion circuit and a piece of data resulting from the correction of the piece of output data to selectively output the pieces of data. 
         [0020]    These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a diagram showing a configuration of a detection circuit according to the first embodiment of the invention; 
           [0022]      FIGS. 2A and 2B  are each a diagram showing the relation between input voltages and output codes in association with the detection circuit according to the first embodiment; 
           [0023]      FIG. 3  is a diagram showing a configuration of a detection circuit according to the second embodiment of the invention; 
           [0024]      FIG. 4  is a diagram showing a configuration of a detection circuit according to the third embodiment of the invention; and 
           [0025]      FIG. 5  is a diagram showing a configuration of a detection part of a conventional contamination-inspecting apparatus, considered as a related art. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    The embodiments of the invention will be described below in detail with reference to the drawings. In all the drawings, which are referred to in explaining the embodiments, like parts are identified by the same reference character, numeral or label in principle, and iteration of the description thereof is eliminated. 
         [0027]    Of an irradiation optical system, a detection optical system, a detection unit and a display unit included in the contamination-inspecting apparatus, particularly the detection part of the detection unit will be described here taking an example. As to the others, it is needless to say that appropriate use of a publicly known art will suffice. 
       Example 1 
       [0028]    A configuration of the detection part of the semiconductor wafer contamination-inspecting apparatus according to the first embodiment of the invention is shown in  FIG. 1 . The detection part is operable to output a digital value according to a predetermined operation as described below based on a detection voltage signal output by the I-V conversion module  2  of the semiconductor wafer contamination-inspecting apparatus as shown in  FIG. 5 , for instance. The detection part  3  includes: a switch  101  to which an input voltage  8  is applied; a reference-voltage generation module  103  for generating a reference voltage; a switch  102  to which a reference voltage generated by the reference-voltage generation module  103  is applied; a calibration control module  104  which performs switching control of the switches  101  and  102  by means of a switching control signal  105 ; amplification modules  107  and  108  to which a voltage is input through the switches  101  and  102 ; analog-to-digital conversion circuits  111  and  112  to which voltages amplified by the amplification modules  107  and  108  are input; logarithmic conversion modules  113  and  114  which perform logarithmic conversion of codes output by the analog-to-digital conversion circuits  111  and  112 ; a comparison module  121  which compares switching data held by a switching data-holding module  120  with the data (A) output by the logarithmic conversion module  113 ; a subtraction module  115  which executes, based on the data (A) and data (B) output by the logarithmic conversion module  114 , a process of subtracting the data (B) from the data (A), expressed by (A)-(B); a data-holding module  117  which holds data output by the subtraction module  115  according to a holding-control signal  106  from the calibration control module  104 ; an addition module  116  which performs a process of adding the data held by the data-holding module  117  and the data (B) to output data (C); and a selector  122  which switches the data (A) and data (C) according to a switching-control signal  123  from the comparison module  121  to output either the data (A) or data (C) as a output code  9  output by the detection part. 
         [0029]    The amplification modules  107  and  108  have amplification factors set to K 1  and K 2  respectively. Here, when the amplification factors are set so that K 1 &lt;K 2  (Expression 1) holds, the relation between the data (A) and data (B) output by the logarithmic conversion modules  113  and  114  after the processing by the analog-to-digital conversion circuits  111  and  112  becomes as shown in  FIG. 2A , i.e. the data (B) are larger than the data (A) with respect to a common input voltage. In  FIGS. 2A and 2B , the axes of input voltages are presented on the logarithmic scale. The conversion characteristic as shown in  FIG. 2A  has been obtained as a result of the conversion by the logarithmic conversion modules  113  and  114 . Specifically, as to the characteristic curves of the data (A) and (B), the ratio of the amplification factors described above results in a fixed code difference as shown in  FIG. 2A  because of the logarithmic conversions by the logarithmic conversion modules  113  and  114 . As in  FIG. 2A , the data (B) increases with an increase of the input voltage, which shows the characteristic that when an output of the amplification module  108  or an input of the analog-to-digital conversion circuit  112  becomes saturated, the data (B) becomes saturated, and makes a fixed output code. 
         [0030]    Before a detecting operation by the detection part  3 , calibration is performed according to a control signal (not shown) from the outside. In the calibration, the calibration control module  104  performs the control using a switching control signal  105  so that the switches  101  and  102  are turned off and on respectively, and the reference voltage from the reference-voltage generation module  103  is applied to the amplification modules  107  and  108 . In this situation, the code (A)-(B) output by the subtraction module  115  is held as correction data by the data-holding module  117  according to the data holding-control signal  106 . Then, the data (B) is corrected with the correction data, whereby the data (C), which is an output from the addition module  116 , is made equal to the data (A). In other words, the data (C) is made to agree with the data (A) in characteristic. 
         [0031]    Subsequently, to execute the detecting operation by the detection part  3 , a control signal (not shown) from the outside is used to bring the detection part  3  to a condition for the detecting operation. Specifically, the calibration control module  104  performs the control using the switching control signal  105  so that the switches  101  and  102  are turned on and off respectively, and the input voltage  8  to the detection circuit  8  is applied to the amplification modules  107  and  108 . In the detection part  3 , a comparison of the data (A) with data (D), which is switching data, is made constantly. When the data (A) is equal to or larger than the data (D), the data (A) is output as the output code  9  through the selector  122 , whereas when the data (A) is smaller than the data (D), the data (C) is output as the output code  9 . In this situation, the data (A) and data (C) are coincident with each other in characteristic as described above. The output code  9  of the detection part  3  with respect to an input voltage to the detection part  3  exhibits the conversion characteristic as shown in  FIG. 2B . 
         [0032]    As stated above, in performing the detection with different ranges using two or more amplifiers, the detection part according to the first embodiment corrects the ratios between amplification factors of the amplifiers based on correction data detected when a reference voltage is applied. Thus, the detection part can reduce the difference in detection error between the ranges at the time of switching the ranges, thereby to enhance the accuracy of a detection part with a higher accuracy. 
         [0033]    In the above description, the principle of operation of the detection part according to the first embodiment has been described with reference to  FIGS. 1 and 2  briefly. However, the detection part may be arranged so that the reference-voltage generation circuit  103  switches the reference voltage to appropriate levels, whereby correction data is detected at two or more points and e.g. CPU performs numerical computations based on the correction data. It is needless to say that the detection part can cope with this case by an arrangement such that the part constituted by the addition module  116  serves as a memory to store LUT (Look Up Table) and the like, into which the CPU writes data. 
       Example 2 
       [0034]    Next, a detection part of a semiconductor wafer contamination-inspecting apparatus according to the second embodiment of the invention will be described with reference to  FIG. 3 . The second embodiment is mainly characterized in that the detection part  3  includes a logarithmic amplification module  109  and a linear amplification module  110 . Other parts of the detection part  3  are arranged in the same way as in the first embodiment. 
         [0035]    In the first embodiment, when the amplitude of the input voltage  8  to the detection circuit is larger, and thus the output voltage of the amplification module  107  is beyond the input-allowable bounds of the analog-to-digital conversion circuit  111 , the output code of the analog-to-digital conversion circuit  111  is saturated and therefore the input voltage  8  of the detection circuit cannot be detected accurately. 
         [0036]    For that reason, in the detection part according to the second embodiment, a logarithmic amplification module  109  is disposed on the input side of the analog-to-digital conversion circuit  111  as shown in  FIG. 3 , and logarithmic conversion of an input voltage  8  to the detection part is performed. As a result, even when the amplitude of the input voltage  8  to the detection part is larger, it becomes possible to prevent the saturation of the analog-to-digital conversion circuit  111 . 
         [0037]    In this case, the digital logarithmic conversion module  113  as described in the first embodiment is not placed in the stage subsequent to the analog-to-digital conversion circuit  111 , and the data output by the analog-to-digital conversion circuit  111  are handled as detection data (A) directly. Further, the logarithmic conversion module  114  is arranged so as to have the same logarithmic conversion characteristic as that of the logarithmic amplification module  109 . Thus, in regard to the relation between the input voltage  8  to the detection part and the output code  9  in the detection part  3 , the characteristic as shown in  FIG. 2B  can be obtained. 
         [0038]    To correct the variation in the conversion characteristic of the logarithmic conversion module  109 , the detection part  3  may be arranged so that the following are performed: switching the output voltage of reference-voltage generation module  103  to an appropriate level; detecting and holding correction data at levels; and conducting numerical computations of the correction data using e.g. CPU; and correcting the detection data making use of the memory function such as the LUT. 
       Example 3 
       [0039]    Next, a detection part of a semiconductor wafer contamination-inspecting apparatus according to the third embodiment of the invention will be described with reference to  FIG. 4 . The third embodiment is mainly characterized in that the detection part  3  includes: a division module  118  which performs a computation of correction data, expressed by (A)÷(B) from the data (A) output by the analog-to-digital conversion circuit  111  and the data (B) output by the analog-to-digital conversion circuit  112 ; a multiplication module  119  which performs a multiplication of the correction data held in the data-holding module  117  and the data (B) thereby to output data (C); a logarithmic conversion module  124  which performs a logarithmic conversion of the data output by the selector  122  thereby to output an output code. Other parts of the detection part  3  are arranged in the same way as in the first embodiment. 
         [0040]    According to the arrangement of the detection part in association with the third embodiment, the division module  118  calculates the ratio of the data (A) to the data (B), and the multiplication module  119  multiplies the ratio by the data (B). Then, the ratio of amplification factors of the amplification modules  107  and  108  is corrected, provided that the amplification factors have been set to be different from each other. This makes the data (A) and data (C) substantially equivalent to data detected with two amplifiers identical to each other in amplification factor. The resultant pieces of data are output through the logarithmic conversion module  124 . Thus, as to the relation between the input voltage  8  to the detection part and the output code  9 , the characteristic as shown in  FIG. 2B  is achieved. 
         [0041]    While the invention made by the inventor has been described above focusing on the embodiments, the invention is not so limited. It is needless to say that various modifications and changes may be made without departing from the subject matter thereof. For instance, as to any of the above-described embodiments, the case where two detection circuits are included has been shown. However, the invention is not so limited, and three or more detection circuits may be incorporated. 
         [0042]    Now, the effects and advantages achieved by the preferred embodiments of the invention disclosed herein will be described briefly. 
         [0043]    The invention can widen the dynamic range of a semiconductor wafer contamination-inspecting apparatus with a higher accuracy. 
         [0044]    Further, according to the invention, a contamination-inspecting apparatus is arranged so that in a detection circuit thereof, correction data is detected based on a reference voltage thereby to correct the difference in detection error caused by switching between detection ranges. Thus, the speedup of the operation of the detection circuit, widening of the dynamic range, and enhancement of the accuracy are achieved. 
         [0045]    The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.