Abstract:
An optical semiconductor wafer inspection system and a method thereof are provided for classifying and inspecting defects such as scratches, voids and particles produced in a flattening process by a polishing or grinding technique used for semiconductor manufacturing. The present invention is an optical semiconductor wafer inspection system and a method thereof characterized by obliquely illuminating a scratch, void or particle produced on the surface of a polished or ground insulating film at substantially the same velocity of light, detecting scattered light at the time of oblique illumination from the surface of an inspection target at different angles and thereby classifying the scratch, void or particle.

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to an optical semiconductor wafer inspection system and a method thereof for classifying and inspecting a defect such as scratch, void or particle as an example of extraneous material produced in a flattening process with a polishing or grinding processing technique used during semiconductor manufacturing. 
     As a conventional technique for classifying and inspecting defect such as scratch and particle on a semiconductor wafer, there is known a technique described in JP-A-2006-201179. That is, the technique described therein combines a high-angle detection optical system and middle-angle detection optical system which condense and receive scattered light generated from locations illuminated by a right-overhead illumination system and oblique illumination system and convert the scattered light to a luminous intensity signal, thereby sets a plurality of detection conditions and classifies defects on an inspection target based on a relationship between luminous intensity signals detected under the respective conditions. 
     Patent Document 1: JP-A-2006-201179 
     BRIEF SUMMARY OF THE INVENTION 
     However, the above described conventional technique necessarily needs to be switched between the right-overhead illumination system and oblique illumination system as for the illumination system as the plurality of detection conditions and does not consider fixing the illumination system to the oblique illumination system and switching between the high-angle detection optical system and middle-angle detection optical system or between the high-angle detection optical system and low-angle detection optical system. 
     Therefore, the conventional technique does not consider carrying out processes by the high-angle detection optical system and low-angle detection optical system in parallel and increasing the inspection speed either. Here, the “middle-angle” and “low-angle” do not mean absolute angles but mean relative positions of angles lower than the position of high-angle. 
     It is an object of the present invention to solve the above described problems and provide an optical inspection system and an optical inspection method capable of speedily classifying defect such as scratch, void or particle which exists on the surface of an inspection target such as a semiconductor wafer. 
     In order to attain the above described object, the present invention provides an optical inspection method for an optical inspection system including a stage on which an inspection target is placed, an oblique illumination system that obliquely illuminates a surface of the inspection target placed on the stage, a high-angle detection optical system that directs light toward the surface at a high angle and detects high-angle scattered light generated from the inspection target with oblique illumination, and a low-angle detection optical system that directs light toward the surface at a low angle and detects low-angle scattered light generated from the inspection target with oblique illumination, wherein luminous intensity detected by the high-angle detection optical system is compared with luminous intensity detected by the low-angle detection optical system to classify a defect which exists on the inspection target. 
     The present invention provides an optical inspection system including a stage on which an inspection target is placed, an oblique illumination system that obliquely illuminates a surface of the inspection target and a detection optical system that detects scattered light generated from the inspection target through illumination by the oblique illumination system, wherein the detection optical system includes a high-angle detection optical system that directs light toward the surface at a high angle and a low-angle detection optical system that directs light toward the surface at a low angle, and a judgment section that compares luminous intensity detected by the high-angle detection optical system with luminous intensity detected by the low-angle detection optical system and classifies a defect on the inspection target. 
     The present invention can speedily inspect defects. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a configuration example of an inspection system according an embodiment of the present invention; 
         FIG. 2  shows examples of defects to be classified according the embodiment of the present invention; 
         FIG. 3  shows examples of a defect classification method according the embodiment of the present invention; 
         FIGS. 4   a  and  4   b  show examples of an illumination optical system and a detection optical system according the embodiment of the present invention; 
         FIG. 5  is an example of a flowchart of classification according the embodiment of the present invention; 
         FIG. 6  is scatter diagram data of a luminous intensity ratio between particles and voids explained in the embodiment of the present invention; 
         FIG. 7  is a relational expression between a reflection factor and an angle of incidence of light explained in the embodiment of the present invention; 
         FIG. 8  is a graph showing the reflection factor and angle of incidence of light explained in the embodiment of the present invention; 
         FIG. 9  shows a state of void scattered light explained in the embodiment of the present invention; and 
         FIG. 10  shows a state of particle scattered light explained in the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of an optical semiconductor wafer inspection system and a method thereof aimed at stable operation of a flattening process used in a semiconductor manufacturing process according to the present invention will be explained with reference now to the attached drawings. 
     As shown in  FIG. 2 , when forming an SiO2 film (processing target)  22  on an Si wafer  21  and applying CMP (Chemical Mechanical Polishing), this embodiment classifies a scratch  23 , void  24  and particle  25  generated on a wafer  10 . 
     However, the Si substrate  21  does not always exist below the SiO2 film  22 , but a wiring layer may exist instead. 
     In the CMP process, polishing is performed to flatten the surface of this SiO2 film  22 . Therefore, the scratch  23  which is a polishing scar is produced on the surface of the SiO2 film  22  as shown in  FIG. 2 . 
     Furthermore, the void  24  corresponds to a bubble (void) existing inside the SiO2 film  22  that emerges when the surface is polished. The particle  25  may be dust generated from within a semiconductor manufacturing system stuck to the surface of the SiO2 film  22 . 
     For a stable operation of the flattening process in this way, it is important to quickly carry out a defect classification process simultaneously with detection of defects, estimate the mechanism of occurrence of defects and take remedial actions. 
     Next,  FIG. 1  shows an example of the optical semiconductor wafer inspection system to implement the embodiment. 
     The optical semiconductor wafer inspection system includes a wafer  10  which is an inspection target placed on a stage  15 , position coordinates of which are measured and traveling in the XY direction of which is controlled, a light source  2  made up of, for example, an Ar laser having a wavelength of 488 nm and an oblique illumination optical system  1  made up of a reflecting mirror  4 . 
     Furthermore, the optical semiconductor wafer inspection system has a high-angle detection optical system  5   a  and a low-angle detection optical system  5   b  made up of condensing lenses  6   a ,  6   b , photoelectric transducers  7   a  and  7   b  made up of a photomultiplier, CCD camera, CCD sensor and TDI sensor or the like respectively and A/D converters  15   a  and  15   b  that convert analog luminous intensity signals outputted from the photoelectric transducers  7   a  and  7   b  to digital luminous intensity signals. 
     The optical semiconductor wafer inspection system further has a stage controller  14  that controls the traveling of the stage  15  based on the position coordinates measured from the stage  15 , judgment sections  17   a  and  17   b  that detect defects in synchronization with the traveling of the stage  15  and calculates luminous intensity signals thereof, and an overall control section  9  that controls the stage controller  14 , further controls the judgment sections  17   a  and  17   b  and receives inspection results obtained from the judgment sections  17   a  and  17   b.    
     Examples of the judgment sections  17   a  and  17   b  include dedicated digital signal circuits that can perform pipeline processing in synchronization with scan clocks of the photoelectric transducers  7   a  and  7   b  that perform scanning in synchronization with the traveling of the stage  15 . 
     Instead of performing the above described synchronization processing as the judgment sections  17   a  and  17   b , there is also a method of storing the outputs of the A/D converters  15   a  and  15   b  in a memory and performing processing asynchronously, for example. In this case, the inspection speed is slower than the synchronous processing. 
     The illumination optical system and detection optical system will be explained by quoting  FIG. 4 . 
       FIG. 4(   a ) is a plan view of the arrangement of the illumination optical system and detection optical system seen from right above. 
     The low-angle detection optical system  5   b  having a low angle is located at 180° with respect to the Y axis (minus direction: counterclockwise direction). The oblique illumination optical system  1  can be changed in three directions of 0°, 45° and 135° with respect to the Y axis. 
     In other words, the positional relationship between the low-angle detection optical system and oblique illumination system is arbitrarily selectable within a range except 0° to 45° within the XY plane on the surface. 
       FIG. 4(   b ) is a front view of the arrangement of the illumination optical system and detection optical system seen from right abeam. 
     The high-angle detection optical system  5   a  is located at 90° with respect to the XY plane. The low-angle detection optical system  5   b  is located at 12° with respect to the XY plane. The oblique illumination optical system  1  can be changed in three directions of 3°, 5° and 20° with respect to the X axis. YAG laser 355 nm can be used for the inclined illumination wavelength. 
     In other words, the oblique illumination system is arbitrarily selectable within a range of angle of elevation of 3° to 20° with respect to the XY plane (on the surface). 
     Furthermore, the low-angle detection optical system is kept to an angle of elevation of approximately 12° with respect to the XY plane (on the surface). 
     Next, the detection procedure will be explained. 
     Oblique illumination light  12  is irradiated onto the CMP plane of an insulating film  22  on the wafer  10  so as to prevent the oblique illumination light  12  from being directly irradiated onto the surfaces of the condensing lenses  6   a  and  6   b  and prevent regularly reflected light of the oblique illumination light  12  from the wafer  10  from being directly irradiated onto the surfaces of the condensing lenses  6   a  and  6   b.    
     While eliminating the regularly reflected light component generated from the insulating film  22 , only the scattered light (low-order diffracted light component) emitted from the scratch  23 , void  24  or particle  25  as the defect on the insulating film  22  is condensed by the condensing lenses  6   a  and  6   b  onto the light-receiving surfaces of the photoelectric transducers  7   a  and  7   b  made up of a CCD, a TDI sensor or the like. While moving the stage  15 , the photoelectric transducers  7   a  and  7   b  such as a CCD and a TDI sensor are made to scan. 
     The outputs of the photoelectric transducers  7   a  and  7   b  are A/D-converted by the A/D converters  16   a  and  16   b  in synchronization with the scanning of the photoelectric transducers  7   a  and  7   b  such as a CCD and a TDI sensor. 
     The outputs of the A/D converters  16   a  and  16   b  are inputted to the judgment sections  17   a  and  17   b  and the judgment sections  17   a  and  17   b  calculate scattered light luminous intensity information and defect position information from the scratch  23 , void  24  or particle  25  as the defect on the insulating film  22  in synchronization with the movement of the stage  15  and the scanning of the photoelectric transducers  7   a  and  7   b  and record the information in the overall control section  9  as the inspection result. 
     As the method of judging a defect, the existence of a defect is judged, for example, when the luminous intensity level of the scattered light from the insulating film  22  is a certain threshold or above. 
     Next, classification after the existence of a defect is judged will be explained. 
     The principle of classification for implementing the above described embodiment according to the present invention will be explained using  FIG. 3 . 
     In the case of the scratch  33  which is a defect on the insulating film  32 , since this is a shallow concave defect, the luminous intensity signal of the high-angle detection optical system  5   a  is smaller than the low-angle detection optical system  5   b  irrespective of the defect size. 
     In the case of the void  34 , since this is a deep concave defect, exposure at the edge of the hole seen from the direction of the low-angle detection optical system  5   b  is small and the luminous intensity signal of the high-angle detection optical system  5   a  is larger than the low-angle detection optical system  5   b  irrespective of the defect size. 
     In the case of the particle  25 , since this is a high convex defect, the luminous intensity signal of the high-angle detection optical system  5   a  is substantially the same as the luminous intensity signal of the low-angle detection optical system  5   b  irrespective of the defect size. 
     Therefore, it is possible to classify the scratch  33 , void  34  and particle  35  according to the following relationship. 
     [Case of Scratch  33 ] 
     Luminous intensity signal of high-angle detection optical system  5   a &lt;luminous intensity signal of low-angle detection optical system  5   b    
     [Case of Void  34 ] 
     Luminous intensity signal of high-angle detection optical system  5   a &gt;luminous intensity signal of low-angle detection optical system  5   b    
     [Case of Particle  35 ] 
     Luminous intensity signal of high-angle detection optical system  5   a ≦luminous intensity signal of low-angle detection optical system  5   b    
     As shown above, when the luminous intensity signal of a defect is small, it is not possible to judge whether the defect is the small-sized particle  35  or large-sized scratch  33  using only one detection optical system, whereas it is possible to classify the defect by observing relative intensities of the luminous intensity signal of the high-angle detection optical system  5   a  and the luminous intensity signal of the low-angle detection optical system  5   b.    
     In the example above, the classification of the scratch  33 , void  34  and particle  35  on a semiconductor wafer has been explained, but it is also possible to classify a scratch or particle produced on, for example, a hard disk in a manufacturing process of the hard disk. 
     The above described classification will be explained by quoting the flowchart shown in  FIG. 5 . 
     The process starts in step  501  and a stage scan starts (step  502 ). Signals condensed by the high-angle detection optical system and low-angle detection optical system are A/D-converted (step  503 ). In step  504 , defects are detected from the A/D-converted signals by the defect judgment section and their luminous intensity levels are calculated. In a comparison between the luminous intensity of high-angle detection and the luminous intensity of low-angle detection calculated (step  505 ), if the luminous intensity of high-angle detection&lt;luminous intensity of low-angle detection, the defect is judged to be a scratch (step  506 ). 
     On the contrary, in the comparison between the luminous intensity of high-angle detection and the luminous intensity of low-angle detection calculated (step  507 ), if the luminous intensity of high-angle detection&gt;luminous intensity of low-angle detection, the defect is judged to be a void (step  508 ). 
     On the other hand, in the comparison between the luminous intensity of high-angle detection and the luminous intensity of low-angle detection calculated (step  509 ), if the luminous intensity of high-angle detection≦luminous intensity of low-angle detection, the defect is judged to be a particle (step  510 ). 
     The above described judgment is repeated until all defects detected during stage scanning are judged. (step  511 ) 
     When repeated stage scanning on the entire area to be inspected on the wafer is completed (step  512 ), the process ends (step  513 ). Since the above described judgment is made in synchronization with stage scanning, the inspection process is speedily carried out. 
     Furthermore, since detection processes of the high-angle detection optical system and the low-angle detection optical system are carried out in parallel, this contributes to speed enhancement of the inspection. 
     The above described classification will be explained in further detail by quoting  FIGS. 6 to 10 . 
       FIG. 6  shows scatter diagram data of a luminous intensity ratio between particles and voids. This is a data example where the particle  35  and void  34  are actually classified. 
     The relative intensities of the luminous intensity signal of the high-angle detection optical system  5   a  and the luminous intensity signal of the low-angle detection optical system  5   b  are defined as the luminous intensity signal of the low-angle detection optical system  5   b ÷luminous intensity signal of the high-angle detection optical system  5   a =luminous intensity ratio. 
     Assuming the threshold is 0.6 in the scatter diagram of the luminous intensity ratio, it is apparent that a defect equal to or greater than the threshold can be classified as the particle  35 , a defect equal to or less than the threshold can be classified as the void  34 . This threshold varies depending on the gain of each photoelectric transducer  7   a ,  7   b , but the ability to classify defects based on a certain threshold is invariable irrespective of the size of the particle or void. 
     This principle will be explained using  FIGS. 7 to 10 . 
       FIG. 7  shows a general relational expression between a reflection factor and an angle of incidence of light on a certain plane. Here, P-wave (Rp) and S-wave (Rs) mean that an electric field vector of light is parallel and perpendicular to the plane of incidence respectively.  FIG. 8  shows a graph of the reflection factor on an interface with n=1.7 between matter and air as an example. This graph is equal to a relationship when the numerical value (Expression 1) in the relational expression of  FIG. 7  is assumed.
 
(nI=1.0, nII=1.7)  [Expression 1]
 
     The shape of the graph generally varies depending on the refractive index of matter or the like, but the reflection factor increases as the angle of incidence of light approximates to parallel to the plane. When there is no polarization of the electric field vector of the incident light, the reflection factor of light becomes Rp+Rs of the graph. Furthermore, in  FIG. 8 , the angle perpendicular to the plane is assumed to be 0° and the angle parallel to the plane is assumed to be 90°. If this is applied to  FIG. 4 , 
     (Scattering direction of high-angle detection optical system  5   a  in 90° direction . . . 0° direction in  FIG. 8 ) 
     (Scattering direction of low-angle detection optical system  5   b  in 12° direction . . . 78° direction in  FIG. 8 ) 
     The reflection factor “Rp+Rs” in  FIG. 8  becomes approximately 0.07 at 0°. Furthermore, the reflection factor becomes 0.35 at 78°. The reflection factor increases approximately five-fold. There is also an approximately five-fold difference between the particle distribution center in  FIG. 6  and void distribution center. 
     This principle will be explained when applied to the state where the scatter diagram data of the luminous intensity ratio in  FIG. 6  is acquired. 
     Like the state of the void scattered light shown in  FIG. 9 , there are not many blocking planes in the scattering direction of the high-angle detection optical system  5   a  in the 90° direction shown in  FIG. 4 . This direction corresponds to the 0° direction shown in  FIG. 8 . 
     On the contrary, there is an interface in the scattering direction of the low-angle detection optical system  5   b  in the 12° direction shown in  FIG. 4 , where there is a situation in which scattered light is significantly reflected, hardly reaching the low-angle detection optical system  5   b . This direction becomes the 78° direction shown in  FIG. 8 . In this way, the luminous intensity ratio shown in  FIG. 6  is assumed to decrease in the case of the void. 
     On the other hand, in the state of the particle scattered light shown in  FIG. 10 , there is nothing that blocks in the scattering detection of the high-angle detection optical system  5   a  in the 900 direction or in the scattering detection of the low-angle detection optical system  5   b  in the 12° direction. This causes the luminous intensity ratio shown in  FIG. 6  to increase in the case of the particle. 
     As described above, it is appreciated that keeping the angle of the low-angle detection optical system  5   b  to approximately 12° or below makes it possible to classify the particle and void. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.