Abstract:
A method and system for providing different images representing plural depths of field of an electronic device. The vision system has a beamsplitter for receiving an image of the device illuminated by the at least one light source, the beamsplitter providing one of the plurality of images of the device based in a wavelength of the light source; an aperture having a plurality of effective diameters based on the wavelength of light from the at least one light source, the aperture determining a depth of field of the image of the device; and an optical element for receiving the image of the device, the optical element magnifying the image by a predetermined magnification factor to produce a magnified image having the determined depth of field.

Description:
This application is a Continuation-in-Part of application Ser. No. 10/336,458 filed on Jan. 3, 2003, now U.S. Pat. No. 6,760,161 which is a Continuation of application Ser. No. 09/961,742 filed on Sep. 24, 2001 and issued as U.S. Pat. No. 6,529,333 on Mar. 3, 2003. 

   FIELD OF THE INVENTION 
   This invention relates generally to machine vision systems for semiconductor chip bonding/attaching devices. More specifically, the present invention relates to a multi-wavelength aperture providing different depths of field of an observed object based on a wavelength of light and a system and method using such a multi-wavelength aperture. 
   BACKGROUND OF THE INVENTION 
   Semiconductor devices, such as integrated circuit chips, are electrically connected to leads on a lead frame by a process known as wire bonding. The wire bonding operation involves placing and connecting a wire to electrically connect a pad residing on a die (semiconductor chip) to a lead in a lead frame. Once all the pads and leads on the chip and lead frame have been wire bonded, it can be packaged, often in ceramic or plastic, to form an integrated circuit device. In a typical application, a die or chip may have hundreds or thousands of pads and leads that need to be connected. 
   There are many types of wire bonding equipment. Some use thermal bonding, some use ultra-sonic bonding and some use a combination of both. Prior to bonding, vision systems or image processing systems (systems that capture images, digitize them and use a computer to perform image analysis) are used on wire bonding machines to align devices and guide the machine for correct bonding placement. 
   Machine vision systems are generally used to inspect the device before, during or after various steps in the fabrication process. During such process steps, it may be necessary to obtain multiple views of the device under different magnification levels to determine whether the device meets predetermined quality standards. One measurement may require a large field of view to include as many fiducials as possible, while a second measurement may require a high resolution to image fine details. Further, these various measurements may need to narrow or expand the depth of field of the observed object in order to view certain details. 
   In conventional systems, such multiple magnifications are handled by having a separate camera for each desired magnification level. Such a conventional device is shown in FIG.  1 . In  FIG. 1 , imaging device  100  includes objective lens  104 , aperture  106 , beam splitter  108 , mirror  110 , relay lenses  112 ,  114 , and cameras  116 ,  118 . In operation an image of device  102  is transmitted through object lens  104  as transmitted image  120  and in turn through aperture  106  as image  122 . Image  122  is incident on beam splitter  108 , which in turn divides the light from image  122  into first divided light rays  124  and second divided light rays  126 . Divided light rays  126  are then redirected by mirror  110  as divided light  128 . 
   Relay lenses  112  and  114  are selected so as to provide the desired magnification of divided light  124  and  128 , respectively, resulting in magnified images  130  and  132 , which are incident on cameras  116  and  118 , respectively. This system has drawbacks, however, in that it requires a separate camera for each level of magnification desired, and also require that multiple apertures be provided to handle different depths of field, thereby resulting in greater complexity and increasing size and cost. 
   A second conventional system is shown in  FIGS. 2A and 2B . In  FIGS. 2A and 2B , a shutter  218  is used in combination with a second beam splitter  222  to receive two magnifications of device  202  with a single camera  216 . As shown in  FIG. 2A , first beamsplitter  208  separates light rays  224  into light rays  226 ,  228 , each being of about equal illumination, that is each of light rays  226 ,  228  is about half the illumination of light rays  224 . When shutter  218  is in a first position, light rays  226  are prevented from reaching relay lens  214 . On the other hand, light rays  228  are magnified by relay lens  212  to become magnified light rays  230 . In turn, magnified light rays  230  are incident on second beamsplitter  222 , a portion (about 50%) of which is transmitted to camera  216  as light rays  236 . The remaining portion of magnified light rays  230 , however, is deflected by second beamsplitter  222  as lost light rays  234 . As a result, only about 25% of the light used to illuminate device  202  is actually received at camera  216 . In addition, the inclusion of shutter  218  increases the complexity and cost of this system. 
   Alternatively, and as shown in  FIG. 2B , when shutter is in a second position, light rays  228  are prevented from reaching relay lens  212 , while light rays  226  are directed through relay lens  214  by mirrors  210 ,  220  as magnified light rays  232 . Similar to  FIG. 2A , a portion  236  of magnified light rays  232  are received by camera  216  while remaining light rays  234  are lost. As is evident, a large portion of the illumination available for imaging is sacrificed due to the losses associated with first beam splitter  208  and second splitter  222 . The light from a single channel hits the second splitter and is split into a reflected portion  234  and transmitted portion  236 . Only one of these will be directed to camera  216  while the other is lost. This approach can also have reliability issues with respect to the moving shutter mechanism. 
   SUMMARY OF THE INVENTION 
   In view of the shortcomings of the prior art, the present invention is directed to an aperture having different effective diameters based on a wavelength of light passing therethrough to provide one of multiple depths of field of the device being viewed. 
   The present invention is a vision system for use with at least one light source and providing a plurality of images representing plural depths of field of a device. The system comprises a beamsplitter for receiving an image of the device illuminated by the at least one light source, the beamsplitter providing one of the plurality of images of the device based in a wavelength of the light source; an aperture having a plurality of effective diameters based on the wavelength of light from the at least one light source, the aperture determining a depth of field of the image of the device; and an optical element for receiving the image of the device, the optical element magnifying the image by a predetermined magnification factor to produce a magnified image having the determined depth of field. 
   According to another aspect of the invention, the aperture is a dichroic aperture. 
   According to a further aspect of the invention, the optical detector is a camera. 
   According to still another aspect of the invention, the light has a wavelength in the visible spectrum. 
   According to yet another aspect of the present invention, the beamsplitters are dichroic splitters. 
   According to a further aspect of the invention, the aperture comprises a first region having a first reactive property to a first wavelength of light from the at least one light source; and a second region adjacent the first region and having a second reactive property to a further wavelength of light from the at least one light source, such that the first reactive property provides a first depth of field of the object and the second reactive property provides a second depth of field of the object. 
   According to still a further aspect of the invention, the first reactive property results in a first effective diameter of the aperture and the second reactive property results in a second effective diameter of the aperture. 
   According to yet a further aspect of the invention, the aperture comprises a region having a plurality of reactive properties based on a wavelength of light from the light source; and a further region adjacent the first region and absent a reactive property to any wavelength of light from the light source, such that the plurality of reactive properties provide a respective plurality of a depth of field of the object based on the wavelength of light from the light source. 
   These and other aspects of the invention are set forth below with reference to the drawings and the description of exemplary embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following Figures: 
       FIG. 1  is schematic representation of a vision system according to the prior art; 
       FIGS. 2A and 2B  are schematic representations of another vision system according to the prior art; 
       FIGS. 3A and 3B  are schematic representations of a vision system according to a first exemplary embodiment of the present invention; 
       FIG. 4  is a schematic representation of a vision system according to a second exemplary embodiment of the present invention; 
       FIGS. 5A-5C  are views of a dichroic aperture according to an exemplary embodiment of the present invention; 
       FIG. 5D  is a plan view of a dichroic aperture according to another exemplary embodiment of the present invention; 
       FIG. 6  is a schematic representation of a vision system according to an exemplary embodiment of the present invention utilizing an exemplary dichroic aperture; and 
       FIGS. 7A-7B  are a schematic representations of vision systems according to another exemplary embodiment of the present invention utilizing an exemplary dichroic aperture. 
   

   DETAILED DESCRIPTION 
   Referring to  FIGS. 3A and 3B , an exemplary embodiment of the present invention is shown. In  FIG. 3A , device  302  is illuminated by a light source (not shown) having a predetermined wavelength. In a preferred embodiment, this wavelength is within either the visible spectrum of light or ultraviolet spectrum of light. Light rays  330 , representing an image of device  302 , emerges from lens  304  and aperture  306 . Light rays  330  are incident on dichroic splitter  308 , which in turn reflects a substantial portion of light rays  330  as reflected light rays  332 , based on properties of splitter  308  which are dependent upon the wavelength of light illuminating device  302 . As dichroic splitters are not 100% efficient, a small portion of light rays  330  will pass through dichroic splitter  308  as light rays  334 . Light rays  332  are then reflected by mirror  310 , such as a planar mirror, as light rays  336  so as to allow them to be magnified by optical relay  314 . In an exemplary embodiment, optical relay  314  is a doublet type lens assembly having a predetermined magnification factor. Based on this magnification factor, light rays  336  are magnified and emerge from optical relay  314  as magnified light rays  338 . As is understood by those of skill in the art, magnified light rays  338  represent an enlarged image of device  302 . 
   Magnified light rays  338  are again redirected by mirror  320  as magnified light rays  342  to be incident on a surface of dichroic splitter  322 . In addition, light rays  334 , having been magnified by a predetermined magnification factor by optical relay  312 , are incident on an opposite surface of dichroic splitter  322  from that of magnified light rays  342 . In an exemplary embodiment, the magnification factors of optical relays  312  and  314  are different from one another. Dichroic splitter  322  has properties, based on the wavelength of light illuminating device  302 , such that the undesired image rays  340  do not pass through splitter  322 , but rather are reflected away as discarded light  344 . In this way multiple images are not provided to optical detector  316 . On the other hand, dichroic splitter  322  has properties, based on the wavelength of light illuminating device  302 , allowing magnified light rays  342  to be directed toward optical detector  316  as image rays  346 . As a result, optical detector  316  “sees” only a single magnified image of device  302 . In a preferred embodiment of the present invention optical detector  316  may be a camera, such as a CCD or CMOS camera, or a position sensitive detector (PSD). 
   Referring now to  FIG. 3B , device  302  is illuminated by a light source (not shown) having a predetermined wavelength different from the wavelength of light that illuminated device  302  as described above with respect to FIG.  3 A. In a preferred embodiment, this wavelength is within the visible spectrum of light. In  FIG. 3B , light rays  350 , representing another image of device  302 , emerges from lens  304  and aperture  306 . Light rays  350  are incident on dichroic splitter  308 , which in turn passes a substantial portion of light rays  350  as light rays  352 , based on properties of splitter  308  which depend upon the wavelength of light illuminating device  302 . Once again, as dichroic splitters as not 100% efficient, a small portion of light rays  350  will be reflected by dichroic splitter  308  as reflected light rays  354 . These light rays will in turn be redirected by mirror  310  as light rays  356 , which will in turn be magnified by optical relay  314  as magnified light rays  358 , which are then redirected toward dichroic splitter  322  by mirror  320  as reflected light  360 . 
   Light rays  352  that emerge from dichroic splitter  308 , pass through and are magnified by optical relay  312  to become magnified light rays  362 . As a result, magnified light rays  362  are incident on dichroic splitter  322 . As discussed above with respect to  FIG. 3A , dichroic splitter  322  has properties, based on the wavelength of light illuminating device  302 , such that undesired light rays  360  pass through splitter  322 , and thus are directed away from optical detector  316  as discarded light  364 . On the other hand, dichroic splitter  322  has properties, based on the wavelength of light illuminating device  302 , allowing magnified light rays  362  to pass through splitter  322  as image rays  366 . It is image rays  366  which are now “seen” by optical detector  316 . In this way multiple images are not provided to optical detector  316  and different magnifications of device  302  may be provided merely by changing the wavelength of light that illuminates device  302 . 
     FIG. 4  illustrates a second exemplary embodiment of the present invention in which more that two light sources are used to illuminate device  302  and provide more than two different magnifications of device  302 . In  FIG. 4 , device  302  is illuminated by one of light sources  406 ,  416 ,  428 , each having a different wavelength. In a preferred embodiment, these wavelengths are within either the visible spectrum of light or ultraviolet spectrum of light. Illumination emitted by each of light sources is directed toward device  302  though a series of dichroic splitters  404 ,  418 ,  420 , and  430 . In the exemplary embodiment, only one light source is used to illuminate device  302  depending on the magnification desired. In the example illustrated in  FIG. 4 , light source  406  is used to provide magnification of device  302  through lens  412 , light source  416  is used to provide magnification of device  302  through lens  424 , and light source  428  is used to provide magnification of device  302  through lens  434 . The magnification factor of each of lenses  412 ,  424 ,  434  is selected as desired. In a preferred embodiment of the present invention the magnification factor of lenses  412 ,  424 ,  434  is 2×, 6×, and 8×, respectively. 
   To illustrate how the second exemplary embodiment functions, a specific example is now discussed. If for example, it is desired to magnify an image of device  302  by a specific magnification factor achieved through lens  434 , light source  428  is activated and the remaining light sources  406 ,  416  are deactivated. Light rays  444  pass through dichroic splitters  430 ,  420  and  418  and are reflected by dichroic splitter  404  based on the wavelength of the light rays. These light rays are then re-directed by mirror  402  to illuminate device  302 . In turn, light rays  440 , representing an image of device  302 , emerges from lens  304 , are reflected by mirror  402  as reflected light rays  442  and directed toward dichroic splitter  404 . As mentioned above, the wavelength of the light rays  446  are such that they are reflected by splitter  404  and pass through splitters  418 ,  420 . The bottom surface of splitter  430  has different properties than that of the top surface of splitter  430 . As a result, light ray  446  are reflected by splitter  430  rather than passing through it. These reflected rays  448  pass through aperture  432  and are in turn magnified by lens  434 . Light rays  450 , representing the magnified image of a portion of device  302  are next redirected by mirror  436  as reflected light rays  452 , which in turn, based on the wavelength of the light rays, pass through dichroic splitters  426  and  414 , and are received by detector  316 , such as a CCD or CMOS camera, or a position sensitive detector (PSD). As such, detector  316  received a magnified image of device  302  based on the wavelength of the light used to illuminate the device. Similarly, the path of light used to illuminate device  302  and its reflected image is based on the wavelength of light sources  406  and  416 . 
   Referring now to  FIGS. 5A-5C , an exemplary dichroic aperture  500  has various regions  502 ,  504  and  506 . As shown in  FIG. 5A , in aperture  500 , region  502  represents a portion of the aperture where no light can penetrate, region  504  has a diameter d 1  and represents a portion where light having a first wavelength λ 1  can penetrate, and region  506  has a diameter d 2  smaller than d 1  and represents a portion where light having a second wavelength λ 2  can penetrate. With respect to region  506 , light having the first wavelength will also pass through this region. As is known to those skilled in the optical arts, the diameter of an optical aperture affects the depth of field (DOF) and Modulation Transfer Function (MTF) (or optical resolution) of the object being observed. Therefore, as a result of illuminating the object to be observed by light having different wavelengths (in this example λ 1  or λ 2 ), the DOF and MTF may be controlled. For example, and as shown in  FIGS. 5B and 5C , if light having wavelength λ 1  is used, aperture  500  has diameter d 1  resulting in a short DOF  510  and a greater MTF. On the other hand, if light having a wavelength λ 2  is used, aperture  500  has a diameter d 2  resulting in a greater DOF  512  and lower MTF. Although not shown in  FIG. 5C , the portion of light having wavelength λ 2  that does not pass through aperture  500  is reflected. 
   Dichroic aperture  500  may be formed using well-known thin film coating and masking techniques, for example. Although the exemplary dichroic aperture  500  is illustrated with two regions ( 504 ,  506 ), the invention is not so limited. As shown in  FIG. 5D , for example, it is contemplated that any number of regions may  510   a ,  510   b , . . .  510   n  be provided, each tuned to a different wavelength of light, to provide a variety of Depths of Field, as desired. 
   Referring now to  FIG. 6 , an exemplary embodiment of a vision system  600  using dichroic aperture  500  is illustrated. In  FIG. 6 , device  302  is illuminated by light source  602  having light rays  604  of a predetermined wavelength and/or light sources  406  or  428  also having a wavelength equal to that of light source  602 . Light source  602  may be capable of providing illumination in one or more discrete wavelengths as desired. Further light source  602  may be combined with either light source  406  or  428  to provide both oblique and perpendicular illumination to device  302 . Those of skill in the art understand that, although it is desirable for the wavelength of light source  406  or  428  to be equal to that of light source  602 , due to manufacturing tolerances the wavelengths may vary slightly. Similar to the embodiment described above, illumination for light sources  406 ,  428  are incident on device  302  via dichroic splitters  404 ,  408 . 
   Light rays  330 , representing an image of device  302 , emerge from lens  304 , such as an achromatic or chromatic lens as desired. Light rays  330  are incident on dichroic splitters  404 ,  408 , which in turn reflect a portion of light rays  330  as reflected light rays (not shown), based on properties of splitter  308  which are dependant upon the wavelength of light source  602 . The remaining light is incident on dichroic aperture  500 . Based on the wavelength of the light, dichroic aperture  500  adjusts its effective diameter as discussed above and passes the light onto relay lens  412 , such as an achromatic lens having a predetermined magnification factor, either positive or negative. This resultant image is incident on optical detector  316 . Because of the reaction of dichroic aperture to the wavelength of light from light sources  602 ,  406 ,  428  on device  302 , the depth of field may be either narrow  608  or deep  610 . 
   In another exemplary embodiment, light source  602  may have a variable wavelength to adjust the DOF of the object being observed, as desired. 
   Although the exemplary embodiment illustrates three light sources  602 ,  406 ,  428 , the invention is not so limited. It is also possible to add additional light sources similar to those of  406 ,  428  with appropriate dichroic splitters as desired. Of course, as the number of available wavelengths increase, the number of active areas in dichroic aperture  500  should also increase by a like number. 
     FIGS. 7A-7B  illustrate other exemplary embodiments of the present invention in which dichroic aperture  500  is incorporated into the embodiment described above with respect to FIG.  4 . In an effort to provide a more concise representation, however, this exemplary embodiment addresses only two magnification paths, rather that the three magnification paths of FIG.  4 . The invention is not so limited and it is contemplated that the invention may be used with any number of light sources (including variable wavelength light sources) and magnification paths, as desired. 
   As shown in  FIG. 7A , device  302 , disposed on substrate  301  for example, is illuminated by one of light sources  406 ,  428 , each having a different wavelength. In a preferred embodiment, these wavelengths are within either the visible spectrum of light or ultraviolet spectrum of light. Illumination emitted by each of light sources is directed toward device  302  though a series of dichroic splitters  404 ,  408 , and  430  and dichroic aperture  500 . Light for the one active light source  406 ,  428  changes the effective diameter of dichroic aperture  500 , thereby adjusting the DOF of observed device  302 . 
   In the exemplary embodiment of  FIG. 7 , only one light source at a time is used to illuminate device  302  depending on the desired magnification and DOF. For example, light source  406  is used to provide magnification of device  302  through lens  412  at a first DOF, and light source  428  is used to provide magnification of device  302  through lens  434  at a second DOF. The magnification factor of each of lenses  412 ,  434  is selected as desired, as is the DOF. In a non-limiting exemplary embodiment of the present invention, the magnification factor of lenses  412 ,  434  is 2×, and 8×, respectively. Furthermore, filters  706 ,  710  may be added to respective magnification paths as desired to eliminate cross coupling between the wavelengths of light by removing any remaining undesired wavelengths of light that may have passed through dichroic splitters  404 ,  406 , and  430 . Additionally, and as shown in  FIG. 7B , achromatic apertures  708 ,  712  may also be added to eliminate stray light that may be present in light rays  702 ,  704  respectively. 
   As can be appreciated by one of skill in the art, this approach may be modified and expanded to use more than two light sources and magnification paths as desired. 
   Although the invention has been described with reference to exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the true spirit and scope of the present invention.