Abstract:
The presence of particles on the wafer holder of semiconductor exposure equipment is detected by analyzing any up-and-down movement of the stage during the alignment, for exposure, of a wafer mounted on the wafer holder. The analysis of the up-and-down movement of the stage is conducted based on signals from a motor which moves the stage in the vertical (Z-axis) direction, or based on signals from a sensor indicative of the distance that the stage deviates vertically from a reference position. A feedback mechanism produces information representative of the up-and-down movement of the stage, and a computer processes the signals to produce data indicative of whether particles are present beneath the wafer on the stage. The feedback mechanism can be either a feedback loop from the control of the motor which moves the stage in the direction of the Z axis, or from a sensor which senses the position of the surface of the wafer at a fixed location where specific portions of the wafer are brought for alignment in preparation of their exposure. The computer includes an input device for inputting the signals as analog signals, a processor which processes the analog signals to generate data indicative of whether particles are present on the wafer holder, and an output device for outputting the data as digital signals.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor exposure equipment which irradiates a wafer with light passed through a design pattern in order to expose a photoresist on the wafer to the design pattern. More particularly, the present invention relates to a method of and apparatus for detecting the presence of particles on a wafer holder of the semiconductor equipment which supports the wafer, and to a method of determining whether the presence of such particles is significant enough to constitute a defect. 
     2. Description of the Related Art 
     Generally, when manufacturing semiconductor devices, photomasking is used to transfer a semiconductor design pattern onto a wafer. This is achieved by first depositing photoresist on a wafer. Then the wafer/resist combination is inserted into a mask aligner, which contains the optics, ultraviolet (UV) light source, and the circuit layer image contained on a mask or reticle, which is to be transferred to the resist film. The mask or reticle pattern image can be transferred to the wafer on a scale of one-to-one, or at a reduced scale using projection optics to focus and reduce the image. 
     The exposure equipment (mask aligner) also contains a wafer holder for mounting the wafer thereon. The wafer holder itself is disposed on a stage, with the stage being driven by a motor or other driving device. After the wafer is mounted on the wafer holder, the reticle and wafer are aligned by driving the stage. Then, the design pattern is transferred onto the photoresist of the wafer using a light source which irradiates the reticle, and a reduction projection lens through which the light is passed to the wafer. 
     It is common for registration tolerances in high density chips to be on the order of ±2000 Å. The total exposure energy is the combination of light intensity and exposure time, which must be tightly controlled to assure proper image reproduction. 
     However, this exposure process is often carried out while particles inadvertently remain on the wafer holder beneath the wafer, which can not be detected by conventional methods. Because the exposure process is carried out repeatedly, an accumulation of particles on the stage often results in some malfunction of the exposure equipment. Accordingly, the particles may also cause a mass of defects in the final semiconductor devices or, at the every least, may adversely affect the production yield. 
     SUMMARY OF THE INVENTION 
     An object of the invention, therefore, is to substantially overcome the problems, limitations and disadvantages in the prior art by providing a method of and apparatus for detecting the presence of particles beneath a wafer on the wafer holder of semiconductor exposure equipment. 
     To achieve this and other objects, the present invention provides a method of detecting the presence of particles on the wafer holder of semiconductor exposure equipment characterized by a step of analyzing any up-and-down movement of the stage supporting the wafer holder during the wafer alignment step(s) to determine whether particles are present on the wafer holder beneath the wafer. 
     The analysis of the up-and-down movement of the stage is conducted based on signals from a motor which moves the stage in the vertical (Z-axis) direction, or based on signals from a sensor indicative of the distance that the stage deviates vertically from a reference position. Preferably, the signals are voltage signals generated at the time a shutter of the exposure equipment has opened. 
     The present invention also provides a method of determining a defect caused by the presence of particles on the wafer holder during the exposure of a wafer in the manufacture of a semiconductor device. In this case, as respective portions of the wafer are aligned for exposure, each portion is checked for the presence of particles therebelow according to the method outlined above. The number of respective portions of the wafer which are determined to have a particle(s) therebelow is tracked. When this number reaches a predetermined value, it is judged that the presence of the particles is so serious as to constitute a fatal defect. 
     The present invention also provides an apparatus for detecting the presence of particles on the wafer holder of semiconductor exposure equipment, the apparatus including feedback means for producing signals representative of the up-and-down movement of the stage, and processing means for processing the signals in a manner that produces data indicative of whether particles are present on the wafer holder beneath the wafer. 
     The feedback means can be either a feedback loop (of a type well known to those of ordinary skill in the art) from the control of the motor which moves the stage in the direction of the Z axis, or a sensor which senses the position of the surface of the wafer at a fixed location where specific portions of the wafer are brought for alignment in preparation of their exposure. The processing means includes an input device for inputting the signals as analog signals, a computer processor which processes the analog signals to generate data indicative of whether particles are present on the stage, and an output device for outputting the data as digital signals. 
     The feedback means may be provided for each of several sets of exposure equipment. In this case, the input device of the processing means is connected to each feedback means. That is, the processing means may be connected to at least two sets of exposure equipment so that the processor can simultaneously monitor the exposure of several wafers at a time for the presence of potentially harmful particles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram of exposure equipment of a semiconductor device fabrication system; 
     FIG. 2 is a diagram of a wafer map (representation of a wafer) showing the path along which respective portions of the wafer are irradiated by the exposure equipment of FIG. 1; 
     FIG. 3 is a schematic diagram of one embodiment of the apparatus for detecting the presence of particles on the wafer holder of semiconductor exposure equipment according to the present invention; 
     FIG. 4 is a side view of a wafer showing a particle thereon beneath the wafer, and the presence of which is to be detected according to the present invention; 
     FIG. 5 is a graphical representation of the waveforms of signals produced according to the present invention; 
     FIG. 6 is a flow chart of one embodiment of the method of detecting the presence of particles on the wafer holder of the semiconductor exposure equipment according to the present invention; 
     FIG. 7 is a flow chart of a portion of a method of determining whether particles on the wafer holder of semiconductor exposure equipment would create a defect according to the present invention; 
     FIG. 8 is a diagram of a wafer map on which the output of the detection process according to the present invention is displayed; 
     FIG.  9 ( a ) is a schematic diagram of a feedback device showing its operation at the time a specific portion of a wafer having no particle(s) therebelow is aligned for exposure, and at which time the wafer holder is at a position coinciding with a reference position; 
     FIG.  9 ( b ) is a schematic diagram of the feedback device showing its operation at the time where another portion of the wafer having a particle therebelow is misaligned for exposure, and at which time the wafer holder coincides with the reference position; and 
     FIG.  9 ( c ) is a schematic diagram of the feedback device showing its operation at the time the portion of the wafer referred to with respect to FIG.  9 ( b ) is aligned for exposure, but at which time the wafer holder deviates from the reference position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First, a preferred embodiment of the present invention will be generally described with respect to the accompanying drawings. 
     Referring to FIG. 1, the exposure equipment of a semiconductor fabrication system comprises a wafer holder  12 , on which a wafer  10  is mounted, and a stage  14  supporting the wafer holder  12 . The stage  14 , and hence the wafer holder  12  thereon, are movable along X, Y and Z axes (as oriented with reference to FIG. 1) by any suitable motor or driving device  14   a . A light source  16  irradiates the wafer  10  with light through a reticle  18  having the design pattern formed thereon in order to transfer a semiconductor design pattern onto the wafer  10 . A reduction projection lens  20  projects the design pattern of the reticle  18  onto the wafer  10 . 
     FIG. 3 shows a processing means  30  for determining, based on the up-and-down (Z axis) movement of the wafer holder  12 , whether a specific location on the wafer holder  12  deviates from a reference (ideal) location. The processing means  30  comprises an input unit  32  (analog converter) for inputting a signal representative of the deviation as an analog signal, a microprocessor  33  to which the analog signal is input, and an output unit  34  (digital converter) for outputting data from the microprocessor  33  as a digital signal. A display monitor  35  is also connected to the microprocessor  33 . 
     In addition, an error display unit  36  displays a numeral indicative of an error derived from the processing means  30 , and an alarm unit  38  issues an alarm when a given magnitude of the error is derived and displayed. The error display unit  36  is connected to the output unit  34 , and the alarm unit is connected to the error display unit  36 . 
     The input unit  32  of the processing means  30  of the present invention is connected to the semiconductor exposure equipment  40  through a feedback device represented by reference numeral  37  in FIG.  3 . The processing means  30  can be connected to at least two sets of the exposure equipment  40  via its input unit  32 . In the illustrated embodiment, the processing means  30  is connected to six different sets of exposure equipment  40 . 
     FIG. 4 shows the wafer  10  mounted to the wafer holder  12  on which a particle(s) P has remained. The deviation (t 1 ) at a specific portion of the wafer holder  12  where a particle(s) P remains is greater than that (t) at a portion of the wafer holder  12  where no particles remain, the deviation referring to the distance along the Z axis between a surface of the wafer at the portion of the wafer holder in question and some fixed reference position on the wafer holder. 
     FIG. 6 shows the method of detecting the presence of particles P on the wafer holder  12 . Generally, the method comprises the steps of mounting the wafer  10  on the wafer holder  12 ; aligning the wafer  10  and the reticle  18  in the directions of the X, Y and Z axes; based on the up-and-down movement of the wafer holder  12  in the direction of the Z axis during the alignment step, determining locations on the wafer holder  12  where the wafer holder  12  deviates from a reference position at the time a portion of the wafer above that location on the stage was aligned; counting the number of locations where such a deviation occurred; summing these values; and displaying these locations where the deviations occur. 
     The results of the counting step are displayed as a numeral or integer on the error display unit  36 , whereas the locations corresponding to the wafer are displayed on a wafer map  39  which can, as shown in FIG. 3, be displayed on the monitor  35  of the processing means  30 . As shown in the wafer map  39  of FIG. 8, each time a portion of the wafer  10  is determined to have a particle therebelow, a corresponding portion of the wafer map is marked with a numeral, e.g., with the integer “1” at the location where the first deviation is detected, with a integer “2” at the next location where a deviation is detected, and so forth. 
     Referring generally now to FIG. 7, there is shown a subroutine of a method of detecting whether the presence of particles on the wafer holder  12  would give rise to a defect, in which errors in Z-axis alignment of respective portions of the wafer  10  and the reticle  18  are counted. (Again, refer to the wafer map of FIG. 2 showing the sequence in which the respective portions of the wafer and reticle are aligned, and to FIG. 8 showing a display of the wafer map.) An error signal is generated and summed as errors are detected. Once the number of such errors exceeds a predetermined value the error signal is displayed and the alarm unit  38  is activated. Finally, the particular set of exposure equipment associated with the excessive errors is interlocked with the processing means  30  through the output unit  34 , for identification and corrective action. 
     According to the present invention, the predetermined number of alignment errors required to cause an alarm signal to be generated is at least two, and in the preferred embodiment, is greater than three. 
     Now, the embodiment of the method and apparatus for detecting the presence of particles on the wafer holder of wafer exposure equipment according to the present invention will be described in more detail. The following discussion is based on a wafer being placed on a wafer holder, with the wafer holder being supported by a stage. However, one of ordinary skill in the art would understand that the same apparatus and method of particle detection of the present invention is equally applicable where the wafer is placed directly on stage (i.e., if the wafer holder is eliminated) such that particles are detected on the stage underneath the wafer. 
     Referring to FIG. 6, the wafer  10  is mounted on the wafer holder  12  in a start step S 1 . Then one portion of the wafer  10  (corresponding to one of the square sections of the wafer map  39  ) is aligned in step S 2  with a corresponding portion of the reticle  18  in preparation for the exposure of such one portion. In the alignment step S 2 , the stage  14  drives the wafer holder  12  in the X, Y and Z axis directions, if necessary, by the motor or driving device  14   a.    
     If there was no alignment error, namely no movement of the stage and wafer holder  12  once it was brought to a position for exposing a select portion of the wafer, the select portion of the wafer  10  is exposed in step S 3 , after which the shutter for the exposure equipment is closed in step S 3   a . Then, as shown by the wafer map  39  in FIG. 2, the process proceeds to the next portion of the wafer that is to be exposed. Incidentally, concerning the X-Y alignment of the wafer holder  12 , as should be evident from FIG. 2, once Y-axis alignment is completed for a particular row, it is not necessary to carry out another Y-axis alignment for the next several portions of the wafer. 
     Returning to FIG. 6, if only an X-axis and/or Y-axis alignment (i.e., no Z-axis alignment) of a portion of the wafer  10  was required, a check mark is placed on the wafer map  39  (see FIG. 8) in step S 4  at a location corresponding to that portion of the wafer  10 . 
     If a deviation occurs in the direction of the Z axis, namely if abnormal up-anddown movement of the wafer holder  12  during the alignment step S 2  reveals a Z-axis alignment error, it is determined that a particle(s) is present at that location where the alignment error occurred. In step S 5 , the error at that location is counted. If the total error count (step S 51 ) is less than three for example, a numeral or integer (i.e., “1” or “2”) representing the total count is marked (step S 7 ) on the wafer map  39  at a location corresponding to that portion of the wafer as shown in FIG.  8 . If the total error count is greater than or equal to three in step S 51 , a black mark B is placed at the location where the alignment error occurred. 
     An alignment error of the wafer in the direction of the Z-axis is determined with the aid of a feedback loop  37  from the motor or driving device  14   a  of the semiconductor device exposure equipment, or from the feedback provided to the processing means  30  by a photosensor  25 . 
     FIGS. 9 a ,  9   b  and  9   c  illustrate an exemplary operation of a photosensor  25 . FIG. 9 a  schematically represents the case of (t) where the wafer holder  12  is in its reference position for ideal Z-axis alignment. Light emitted from a photoemitter  25   a  of a known type is reflected from the surface of the wafer  10  to a photoreceiver  25   b  at a certain angle (θ 1 ) and reaches the photoreceiver  25   b  with a given intensity I 1 . 
     FIG. 9 b  schematically represents the case of (t 1 ) where the wafer holder  12  is in the same reference position and therefore, a Z-axis misalignment is occurring. When a deviation of the wafer surface such as that shown at (t 1 ) occurs, the intensity of the light  12  (dashed lines) received by the photoreceiver  25   b  differs, due to the different angle of reflection (θ 2 ) of the light (solid line), from that received when the wafer is at an exposure position (t) where no deviation occurs (θ 1 ). In other words, the intensity  12  of the light received by the photoreceiver  25   b  is reduced since the light is scattered somewhat, depicted as the different solid and dashed lines, due to the uneven surface at the position of the particle underneath the wafer  10 . 
     FIG. 9 c  shows that the wafer holder  12  has been moved in the Z-axis direction by a distance ΔZ, based on information generated by the photosensor  25 , to a position where the intensity I 3  of the light received by the photoreceiver  25   b  at an angle (θ 3 ) is equal to the intensity I 1  of the light received by the photoreceiver  25   b  at angle (θ 1 ) as shown in FIG. 9 a.    
     Therefore, not only is the photosensor  25  used for the alignment of the wafer  10  in the Z-axis direction, but for generating a signal representative of the need to have moved the wafer holder  12  up or down in the Z-axis direction relative to a reference position such as the one depicted in FIG.  9 ( a ). Alternately, the signal representative of the need to move the wafer holder  12  up or down in the Z-axis direction relative to a reference position can be generated by the motor that is moving the stage. 
     This alignment signal, indicative of the location where a Z-axis deviation has occurred, is input to the processing means  30  as an analog signal by the input unit  32 . That is, referring now to FIG. 5, the microprocessor  33  of the processing means  30  analyzes the voltage of the input analog signal, and, for example, if the voltage is in excess of 200 mV, the misalignment is judged to be abnormal, indicating that a particle(s) is present under that portion of the wafer being exposed. In FIG. 5, the signal whose voltage is analyzed begins being generated by the input unit  32  at the time a shutter of the exposure equipment opens. 
     As shown in FIG. 7, once the abnormal deviation or Z alignment error signal is processed, the output unit  34  counts or sums the number of abnormal deviations (S 5 ) which is equal to the number of portions of the wafer that experience Z-axis misalignment, and displays this number on the error display unit  36  (S 70 ) if more than a predetermined number of errors are detected (S 51 ). For example, if two, or preferably three abnormalities are determined in connection with deviations of portions of a single wafer  10 , the processing means  30  activates the alarm unit  38  (S 80 ) through the output unit  34 , and interlocks the exposure equipment  40  (S 90 ) that was processing the wafer responsible for the alarm. The processing means also places a mark of a specific color, such as black, on the wafer map (see Step S 6  in FIG. 6) at a location corresponding to the portion of the wafer where the abnormal deviation triggered the alarm. Note that the error display unit  36  could be designed to incrementally show the number of errors numerically, or just show when the predetermined number of errors has been detected. 
     As shown in step S 8  of FIG. 6, if an alignment error was determined to be present, but the processing means was unable to correlate the error with a misalignment in the X, Y or Z axis direction, the processing means executes an analysis of the error. 
     The present invention thus provides numerous advantages. Particles on the surface of the wafer holder underneath the wafer are detected by the vertical movement of the stage supporting the wafer holder and wafer. This prevents irregularities in the exposure process and accordingly improves device yields. Also, by displaying the location of the deviation on a wafer map, the exact location of the particles can be identified. The operator is also alerted of if the presence of such particles is significant enough to constitute a defect. 
     Although the present invention has been described in detail, it should be noted that various changes, substitutions and modifications can be made thereto without departing from the spirit and scope of the invention as defined by the appended claims.