Abstract:
A device and method for optimally detecting the surface conditions of different types of wafers are disclosed. The device includes a light generating unit for impinging light on a wafer to generate a reflected light from the wafer, a combining unit including a plurality of filters having different light cut-off ratios for reducing the amount of the reflected light to generate a reduced amount of the reflected light depending on the reflection rate of the wafer, and a detection unit for processing the appropriately reduced amount of the reflected light to detect the surface condition of the wafer. A different filter or a different combination of the filters are selected depending on the reflection rate of the wafer being processed in order to appropriately reduce the amount of reflected light to be processed by the detection unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a device and method for optimally detecting a surface condition of different types of wafers by automatically selecting filters to appropriately cut off a certain amount of reflected ray from impinging on an imaging sensor to process wafers with different surface properties. 
     2. Discussion of Related Art 
     FIG. 1 shows elements of a conventional device for detecting the surface condition of a wafer. Referring to FIG. 1, the conventional device comprises an Xe-lamp  1  for emitting a constant emissive ray, a filtering lens  3  for filtering the emissive ray from the Xe-lamp  1  within a band of 400˜700 nm, a reticle  5  for adjusting the focus of the emissive ray transmitted through the filtering lens  3 , a half mirror  7  for directing the filtered ray to a beam splitter  17  and to a mirror  8 , the beam splitter  17  for splitting the ray reflected from the half mirror  7  to each PCD (Position Charge Device) sensor  13  and  15 , the PCD sensors  13  and  15  determining the optimal points of automatic focus for the ray, amplifiers  19  and  21  for respectively amplifying signals output from the PCD sensors  13  and  15 , an image processing device  23  for comparing the signals output from the amplifiers  19  and  21  to each other to determine the best optimal point of automatic focus for the wafer  27 , a piezo driving device  25  for controlling a piezo member  29  to provide the best optimal focus point for the wafer  27  according to a signal output from the image processing device  23 , and the piezo driving device  25  for controlling the piezo member  29 . The piezo member  29  is placed on a side of the stage on which the wafer  27  is placed for three-dimensionally controlling the position of the stage. 
     The conventional device further comprises the mirror  8 , an objective lens  9  for magnifying a ray reflected from a wafer  27  and directing it to the mirror  8 , and a CCD (Charge Coupled Device) sensor  11  for representing surface condition of the wafer  27  based on the reflected ray output from the objective lens  9 . The output of the CCD sensor  11  is processed by the image processing device  23  to determine any abnormality on the surface of the wafer  27 . 
     An operation of the conventional device of FIG. 1 is as follows. 
     Referring to FIG. 1, an emissive ray is projected from the Xe-lamp  1  in the direction of the stage where the wafer  27  is placed. The emissive ray is filtered by the lens  3  to a wave band of 400˜700 nm suitable for detecting particles. The filtered ray is reflected by the half mirror  7 . The beam splitter  17  splits the reflected ray into two beams which are respectively directed to two PCD sensors  13  and  15 . The amplifiers  19  and  21  respectively amplify the outputs of the PCD sensors  13  and  15 . 
     The image processing device  23  determines the best optimal point of automatic focus for the wafer  27  by finding the intersection position of electrical signals output from the two PCD sensors  13  and  15 , and generates a control signal to the piezo driving device  25 . The piezo driving device  25  controls the piezo member  29  based on the control signal so that the Xe-lamp  1  projects light on the wafer  27  at the optimal point of automatic focus. That is, in accordance with a driving signal output from the piezo driving device  25 , the piezo member  29  displaced at one side of the stage having the wafer  29  thereon is moved to achieve the optimal point of automatic focus for the light impinging on the wafer  27 . 
     At the same time, the ray emitted from the Xe-lamp  1  and reflected from the wafer  27  passes through the objective lens  9  and is reflected by the mirror  8 . Then this ray impinges on the CCD sensor  11 , and is visualized subsequently to identify the pattern of the surface of the wafer  27 . The CCD sensor  11  generates digital signals representing any abnormality (e.g., particles, dents, etc.) on the surface of the wafer  27 . The principle of the CCD sensor  11  is to detect any displacement of the wafer surface by reading and analyzing light-receiving spots (photo-diode) of the reflected ray. The image processing device  23  processes the output of the CCD sensor  11  to detect an abnormality on the surface of the wafer  27 . 
     In the conventional device of FIG. 1, the emissive ray from the Xe-lamp  1  is reflected at the surface of the wafer  27 , and then transferred to the CCD sensor  11  so that the wafer surface can be examined. However, since the reflection rate at the surface of each wafer differs from each other, wafer abnormality will be detected only if the reflection rate of the wafer is high. Otherwise, surface abnormality of the wafer can not be detected effectively since the processing of a weak reflective ray by the CCD sensor can result in inaccurate detection results. Therefore, conventional detection devices and methods are not reliable when detecting the surface condition of different types of wafers. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a device and method for detecting particles on a wafer that substantially obviate one or more of the problems due to the limitations and disadvantages of the related art. 
     Therefore, it is an object of the present invention to provide a device and method for effectively detecting the surface condition of different types of wafers based on the reflection rates of the wafers. 
     Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purposes of the present invention, as embodied and broadly described, a device for detecting surface conditions of wafers, comprising a light generating unit for impinging light on a wafer to generate a reflected light; a combining unit, including a plurality of filters having different light cut-off ratios, for receiving the reflected light and outputting a varied amount of the reflected light; and a detection unit for receiving the varied amount of the reflected light from the combining unit and processing the received light to detect a surface condition of the wafer. 
     Furthermore, the present invention is directed to a method for detecting surface conditions of wafers, comprising the steps of impinging light on a wafer to generate a reflected light; generating a varied amount of the reflected light selectively using a plurality of filters having different light cut-off ratios; and processing the varied amount of the reflected light to detect a surface condition of the wafer. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the inventing and together with the description serve to explain the principles of the invention. 
     In the drawings: 
     FIG. 1 shows a conventional device for detecting abnormal particles on a wafer; 
     FIG. 2 shows a device for detecting the surface condition of wafers according to the present invention; and 
     FIG. 3 shows an exemplary block diagram of a filter combining unit of the device shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, example of which are illustrated in the accompanying drawings. 
     FIG. 2 shows a device  200  for detecting a surface condition of different wafers according to the present invention. The device  200  includes certain same elements as the conventional device of FIG. 1, as indicated by the same reference numerals. In addition, the device  200  of the present invention includes a filter combining unit  100 . 
     A light source, e.g., Xe-lamp  1 , emits a constant emissive ray towards a wafer  27 . A filtering lens  3  filters the emissive ray to a predetermined wave band, e.g., 400˜700 nm, and a reticle  5  focuses the filtered ray. A half mirror  7  projects the focused ray from the reticle  5  towards the wafer  27  and towards a beam splitter  17 . The ray from the half mirror  7  is split into two beams by the beam splitter  17 . The split rays pass through PCD sensors  13  and  15  and amplifiers  19  and  21  to an image processing device  23 . The image processing device  23  determines the optimal point of automatic focus for the wafer  27  by determining the position of an intersection point of electrical signals from the two PCD sensors  13  and  15 . Based on this determination signal output from the device  23 , a piezo driving device  25  controls a piezo member  29  to obtain the optimal point of automatic focus for the wafer  27 . 
     On the other hand, the ray from the Xe-amp  1  passing through the half mirror  7  impinges on and is reflected from the wafer  27 . This reflected ray passes through the objective lens  9  and the reflection mirror  8 , and is transmitted to the filter combining unit  100 . The objective lens  9  magnifies the reflected ray onto the CCD sensor  11 . The CCD sensor  11  converts the reflected ray (light) from the filter combining unit  100  into an electrical signal, and checks the surface of the wafer  27  to emit an output signal. The image processing device  23  processes the output signal of the CCD sensor  11  to display an image of the wafer surface on a monitor  31 . The monitor  31  may be a part of the image processing device  23  or a separate unit from the image processing device  23 . 
     The CCD sensor  11  includes a plurality of photo-arrays arranged in a matrix, and these photo-arrays corresponding to different portions of the wafer  27  respond to the reflected ray. The CCD sensor  11  measures a surface displacement or condition of the wafer  27  by reading the address of the photo-arrays that responded to the reflected ray, and provides video signals corresponding to this reading. The image processing device  23  processes the signals output from the CCD sensor  11 , displays the video signals and converts them to digital signals. The output of the CCD sensor  11  may be visually displayed on the monitor  31  or other display units. The image processing device  23  can also control the driving device  25  and the piezo member  29  to move the position of the stage or the wafer  27  according to the surface condition of the wafer  27  detected by the CCD sensor  11 , as needed. 
     The filter combining unit  100  selects one or more filters to control the amount of the reflective ray to be impinged on the CCD sensor  11 . The CCD sensor  11  measures any displacement on the surface of the wafer  27  and the objective lens  9  magnifies the ray reflected from the wafer  27 . 
     FIG. 3 shows a block diagram of the filter combining unit  100  for selectively combining filters according to the present invention. Referring to FIG. 3, the filter combining unit  100  includes a reflection rate detecting part  101  for detecting the reflection rate or reflectivity of each wafer  27  placed on the stage, and a filter controlling part  103  for selecting one or more of filters  111   a  to  111   d  according to the detected reflection rate of the wafer  27 . The filters  111   a  to  111   d  have a light cut-off ratio or attenuation factor of 2, 5, 10, and 15%, respectively, but other light cut-off ratios may be used. 
     The filter controlling part  103  is connected to a solenoid valve driving part  105  for controlling the on/off-state of each solenoid valve  107   a  to  107   d  according to the signals output from the filter controlling part  103 . In this example, air is supplied to air cylinders  109   a  to  109   d  through the solenoid valves  107   a  to  107   d , each air cylinder  109   a  to  109   d  having a corresponding filter  111   a  to  111   d  attached or mounted thereto. By selectively combining the filters  111   a  to  111   d  according to the selective operation of the air cylinders  109   a  to  109   d  (i.e., by moving the filters in the path of the reflected ray), the amount of reflected ray impinging on the CCD sensor  11  is regulated at, e.g., 2 to 32%. 
     If the range of the light cut-off ratio according to the selective combination of filters  111   a  to  111   d  is adequate in controlling the amount of the reflected ray impinging on the CCD sensor  11  based on the scope of the reflection rate of the wafer in use, the combined cut-off ratio of the filters does not need to be greater than 32%. In the alternative, filters having other cut-off ratios can be selected and used depending on the desired performance need and reflection rate of wafers. For example, a filter having a cut-off ratio of 15% may be used for the wiring process of peripheral circuits using aluminum, a filter having a 10% cut-off ratio may be used for a bare wafer, and a filter having a cut-off ratio under 5% may be used for LOCOS process, and first gate, and second gate formation processes. 
     As mentioned above, the filter combining unit  100  includes the solenoid valves  107   a  to  107   d , and the filters  111   a  to  111   d  respectively operated by the air cylinders  109   a  to  109   d , and the solenoid valve operating part  105  for controlling or reducing the amount of the reflected ray impinging on the wafer  27 . The air cylinders  109   a  to  109   d  are operated by opening/closing each solenoid valve  107   a  to  107   d  according to control signals output from the solenoid valve operating part  105 . Although the solenoid valves and air cylinders are used to selectively actuate the filters, other types of mechanisms may be used to selectively place the filters  111   a  to  111   d  in the path of the reflected ray to reduce or increase the amount of reflected ray impinging on the CCD sensor  11 . 
     When a new wafer is placed on the stage supported by the piezo member  29 , and about 90% of the total emissive ray is desired from the Xe-lamp  1  to examine the surface abnormality or surface condition of the wafer  27 , the rest (10%) of the total emissive ray of the Xe-lamp  1  is cut off before it arrives at the wafer  27  by using the filter  111   c  having the light cut-off ratio of 10%. If other cut-off amounts are desired, appropriate filters  111   a  to  111   d  are selectively used or combined with each other to obtain the desired reduction amount. 
     The proper amount of the ray that will result in the optimal wafer detection depends on a layer formed on the wafer. If the reflection rate of the layer formed on the wafer is high, the CCD sensor  11  can detect the abnormality of the wafer surface more accurately. 
     In accordance with the reflection rate of the layer formed on the wafer  27 , a sufficient amount of ray is supplied to the wafer  27  based on the filter selection of the filter combining unit  100 . Otherwise, as in the conventional device of FIG. 1, the process of detecting a surface condition of different types of wafers becomes ineffective and less reliable because the image detected by the CCD sensor  11  is not formed with clarity due to the inappropriate amount of reflected ray impinging on the CCD sensor  11 . 
     The strength of the reflected ray from the wafer  27  is analyzed at the reflection rate detecting part  101  of the combining unit  100  wherein the reflected ray from the wafer  27  passes through the CCD sensor  11  and to the image processing device  23 . If the analyzed amount of the ray is larger or smaller than a reference amount, the filter controlling part  103  generates signals of appropriate cut-off ratios to the solenoid valve driving part  105  to control the valves  107   a - 107   d  and the cylinders  109   a - 109   d . Each air cylinder  109   a  to  109   d  opens or closes the corresponding solenoid valve  107   a  to  107   d  under control of the solenoid valve driving part  105 , whereby the filters  111   a  to  111   d  are reciprocally moved by the shift of each air cylinder  109   a  to  109   d.    
     More specifically, if the reflection rate of the wafer  27  is determined to be higher than that of a clean wafer, a filter having a cut-off ratio equal to or higher than, e.g., 15%, is used to reduce the amount of the reflected ray impinging on the CCD sensor  11 . Otherwise, a filter having a cut-off ratio under, e.g., 5%, is used. The image processing device  23  can also analyze the signals output from the CCD sensor  11  to determine whether the appropriate filters were selected. 
     Hence, the amount of the reflected ray impinging on the CCD sensor  11  is regulated properly by the selective use or combination use of the filters  111   a  to  111   d  according to the reflection rate of the layer formed on the wafer  27 . This provides a proper amount of reflected ray from the wafer  27  to be processed by the CCD sensor  11  to detect abnormality on the surface of the wafer  27 . 
     As mentioned above, a device and method for detecting a surface condition of wafers according to the present invention improve the performance of the detecting process by controlling the amount of reflected ray impinging on the CCD sensor  11  using a proper combination of different filters having different light cut-off ratios based on the reflection rate of the wafer. Furthermore, the selective use of the filters  111   a - 111   d  may be applicable for the light path from the lamp  1  to the wafer  27 . Moreover, the present invention provides efficient filters to cut off the reflected ray by automatically combining certain filters suitable for the reflection rate of the wafers. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the device and method for detecting particles on wafers according to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.