Abstract:
A method for micromachining a material, including configuring an optical system to provide illumination of an illumination wavelength to a site via a given element of the optical system, the illumination generating returning radiation from the site. The method further includes configuring the optical system to receive the returning radiation via the given element, and to form an image of the site therefrom, calculating an actual position of a location at the site from the image and outputting a signal indicative of the actual position of the location, generating a beam of micromachining radiation having a micromachining wavelength different from the illumination wavelength, positioning the beam to form an aligned beam with respect to the location in response to the signal, and conveying the aligned beam to the location via at least the given element of the optical system so as to perform a micromachining operation at the location.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application 60/816,332, filed 26 Jun., 2006, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to optical alignment, and specifically to optical alignment of a target to be processed in a printed circuit board. 
     BACKGROUND OF THE INVENTION 
     Laser micromachining is employed to form holes, inter alia, in printed circuit boards (PCBs). As elements of the PCBs shrink in size, demands on the location and accuracy of the laser machining increase. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the present invention, a micromachining apparatus micromachines a location at a site, the site typically comprising an object, such as a conductive pad, embedded in an electrical circuit dielectric substrate of a printed circuit board (PCB). The apparatus comprises an optical system which illuminates the site with a radiation source, receives returning radiation from the site in response to the illumination, and conveys a micromachining beam from a beam source to the location. There is at least one common element in the optical system, such as a steerable mirror, which is used for all three functions. The radiation source and the beam source operate at different wavelengths. The beam source is typically a laser. The radiation source is typically a laser diode, although in some embodiments the radiation source may be a light emitting diode (LED). 
     An image sensor images the site using the returning radiation, and a processor calculates an actual position of the location to be micromachined, such as a center of the conductive pad, from the image. The processor generates a signal indicative of the actual position and uses the signal to align the micromachining beam with respect to the location, typically by adjusting the steerable mirror. The processor then operates the beam source to micromachine the location with the aligned beam. The beam may micromachine a hole of substantially any shape at the location. By using at least one common element for the functions of site illumination, site imaging, and beam transfer, the apparatus is able to provide local high intensity illumination to the site, so forming a good image of the site, and thus quickly and accurately align the micromachining beam with the location. 
     Typically, the apparatus is used to micromachine at multiple sites, each site having a different position, in the PCB. For each site, the processor may calculate nominal coordinates of a location to be micromachined, for example by analyzing a computer aided manufacturing (CAM) file of the circuit, and use the nominal coordinates to locate the substrate so that the site is nominally aligned with the beam and the illumination. At each site, the actual position for the beam is determined as described above. For at least some of the multiple sites, the realignment of the beam from site to site is performed by only operating the steerable mirror, thus enhancing the rate of micromachining of the PCB while maintaining the accurate beam alignment for all the sites. 
     In a disclosed embodiment, the image sensor acquires an image of the locality where the beam strikes the site, typically by the processor operating the beam source at a low power below an ablation threshold for the site. From the image of the site and the image of the locality where the beam strikes, the processor determines an offset to be applied to the beam so as to perform the beam alignment described above. 
     In some embodiments the radiation source may generate fluorescent radiation as the returning radiation, and the image sensor forms an image of the site and/or of a calibration target from the fluorescent radiation. The processor may adjust the wavelength and/or the power of the radiation source, typically according to fluorescent characteristics of the site. The adjustment may be made so that radiation from the radiation source penetrates the site and/or a region surrounding the site so that the image of the site from the fluorescent radiation is optimal. Using fluorescent radiation eliminates the problem of speckle if the radiation source is a laser. 
     In an alternative embodiment of the present invention, the radiation from the radiation source is linearly polarized, and the returning radiation is polarizeably analyzed. For a site comprising an embedded conductive object, returning radiation from the object is typically at least partially depolarized due to surface roughness of the object. The image sensor is thus able to form a well-contrasted image of the object in relation to its surroundings, for which the returning radiation is typically not depolarized. 
     In a further alternative embodiment of the present invention, the radiation source comprises a laser producing a coherent beam having a short coherence length, to substantially eliminate the effect of speckle. Alternatively or additionally, the radiation source comprises other speckle reducing and/or removing components, such as a plurality of optic fibers having different optical lengths. 
     In another disclosed embodiment, the radiation source is configured to illuminate the site using structured illumination, such as by forming an annular ring about an object at the site, and the substrate is diffusive. The combination of annular irradiation and a diffusive substrate effectively “backlights” the object. 
     There is therefore provided, according to an embodiment of the present invention, a method for micromachining a material, including:
         configuring an optical system to provide illumination of an illumination wavelength to a site of the material via a given element of the optical system, the illumination generating returning radiation from the site;   configuring the optical system to receive the returning radiation via the given element, and to form an image of the site therefrom;   calculating an actual position of a location at the site from the image and outputting a signal indicative of the actual position of the location;   generating a beam of micromachining radiation having a micromachining wavelength different from the illumination wavelength;   positioning the beam to form an aligned beam with respect to the location in response to the signal; and   conveying the aligned beam to the location via at least the given element of the optical system so as to perform a micromachining operation at the location.       

     Typically, the site includes an object embedded in one or more dielectric substrates, and providing illumination to the site may include providing structured illumination that illuminates only a region surrounding the object. The structured illumination may be formed with a diffractive element. 
     In an embodiment, providing illumination to the site includes selecting the illumination wavelength to be a wavelength at which the site fluoresces, and the returning radiation includes fluorescent radiation generated at the site in response to the provided illumination. The method may include filtering the fluorescent radiation to optimize the image of the site. 
     In an alternative embodiment, providing illumination to the site includes providing polarized illumination to the site, and forming the image of the site includes polarizeably analyzing the returning radiation from the site. 
     In some embodiments the given element includes a steerable mirror. The site may include a plurality of different sub-sites wherein micromachining is to be performed, and positioning the beam may include directing the beam to the plurality of different sub-sites by only steering the mirror. 
     In a further alternative embodiment the given element includes an optical element train which is configured to focus the beam and the illumination to the site. 
     The site may include a site area, and providing illumination to the site may include providing illumination to the site area and to a further area no larger than the site area and contiguous therewith. Typically, forming the image may include forming the image on an image sensor, and the illumination may have an intensity that generates the image on the image sensor in 3 milliseconds or less. Forming the image may include forming the image on an image sensor having an array of pixels, and selecting pixels for analysis of the image from the array in response to the area and the further area. 
     The method may also include determining a nominal position of the location prior to providing the illumination to the site, and providing the illumination in response to the nominal position. 
     In a yet further alternative embodiment, generating the beam of micromachining radiation includes:
         generating a low-power beam at a power for the beam below an ablation threshold for the site;   conveying the low-power beam to the site; and   determining an offset for the beam in response to an image of the low-power beam at the site.       

     Typically, positioning the beam includes positioning the beam in response to the offset, and conveying the positioned beam to the location includes setting the beam to have a power equal to or greater than the ablation threshold. 
     The method may include configuring the illumination wavelength to have a value for which the site is non-absorbing. 
     In an alternative disclosed embodiment the site includes an external surface, and providing illumination to the site includes illuminating the site with imaging radiation normal to the external surface. 
     Providing illumination to the site may include providing coherent imaging radiation at the site, the coherent imaging radiation have a coherence length equal to or less than twice a dimension of the site. 
     In a yet further alternative disclosed embodiment, calculating the actual position includes:
         providing a theoretical relationship in accordance with an expected image of the site;   determining an actual relationship from the image; and   fitting the actual relationship to the theoretical relationship.       

     Forming the image of the site may include adjusting at least one of the illumination wavelength and a power of the illumination so as to vary a depth of penetration of the illumination at the site. 
     In one embodiment the site includes an object embedded in a diffusive layer, and the method includes compensating for a deviation resulting from the image being formed of the object embedded in the diffusive layer. 
     There is further provided, according to an embodiment of the present invention, a method for micromachining a material, including:
         operating a source to provide a beam of radiation to a site of the material including a location at an operational wavelength at which the material fluoresces, at a beam power insufficient for micromachining, so as to generate fluorescent radiation from the site;   forming an image of the site in response to the fluorescent radiation;   positioning the beam in relation to the location in response to the image; and   operating the source to provide the beam of the radiation to the location at the operational wavelength and at a micromachining power sufficient to cause micromachining of the location.       

     Typically, operating the source at the beam power includes providing the beam of radiation to the site via a beam directing optical system, and forming the image includes transferring the fluorescent radiation via at least one element of the beam directing optical system to an image sensor. The method may include filtering the fluorescent radiation to optimize the image of the site. 
     There is further provided, according to an embodiment of the present invention, apparatus for micromachining a material, including:
         a radiation source which is configured to provide illumination of an illumination wavelength to a site of the material via a given element of an optical system, the illumination generating returning radiation from the site;   an image sensor which is configured to receive the returning radiation via the given element, and to form an image of the site therefrom;   a beam source which is configured to generate a beam of micromachining radiation having a micromachining wavelength different from the illumination wavelength; and   a processor which is configured to calculate an actual position of a location at the site from the image and to output a signal indicative of the actual position of the location, to position the beam to form an aligned beam with respect to the location in response to the signal, and to operate the beam source so that the aligned beam is conveyed to the location via at least the given element of the optical system so as to perform a micromachining operation at the location.       

     The apparatus may include a set of filters configured to filter the fluorescent radiation, and the processor may be configured to select one of the set to optimize the image of the site. 
     The illumination may include polarized illumination, and the apparatus may include a polarization element enabling the image sensor to polarizeably analyze the returning radiation from the site. 
     The given element may include a steerable mirror. 
     Alternatively, the given element may include an optical element train which is configured to focus the beam and the illumination to the site. 
     There is further provided, according to an embodiment of the present invention, apparatus for micromachining a material, including:
         a beam source which is configured to provide a beam of radiation to a site of the material including a location at an operational wavelength at which the material fluoresces, at a beam power insufficient for micromachining, so as to generate fluorescent radiation from the location;   an image sensor which is configured to form an image of the site in response to the fluorescent radiation; and   a processor which is configured to position the beam in relation to the location in response to the image, and to operate the beam source to provide the beam of the radiation to the location at the operational wavelength and at a micromachining power sufficient to cause micromachining of the location.       

     The apparatus may include a beam directing optical system, and operating the beam source at the beam power may include providing the beam of radiation to the site via the beam directing optical system, and forming the image may include transferring the fluorescent radiation via at least one element of the beam directing optical system to the image sensor. 
     The apparatus may include a set of filters configured to filter the fluorescent radiation, and the processor may be configured to select one of the set to optimize the image of the site. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings, a brief description of which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a beam alignment apparatus, according to an embodiment of the present invention; 
         FIG. 2  is a graph of the percentage transmission of different types of Ajinomoto Build-up Film (ABF) resin; 
         FIG. 3  is a schematic graph of the normalized fluorescence of different types of ABF resin and FR4 resin; 
         FIG. 4  is a flowchart showing steps performed to operate the beam alignment apparatus, according to an embodiment of the present invention; 
         FIG. 5A  shows a schematic diagram of a surface of an optical sensor, according to an embodiment of the present invention; 
         FIGS. 5B and 5C  show schematic diagrams of images on the sensor of  FIG. 5A , according to an embodiment of the present invention; 
         FIG. 6  is a schematic diagram of a beam alignment apparatus, according to an alternative embodiment of the present invention; 
         FIG. 7  is a schematic diagram of a beam alignment apparatus, according to a further alternative embodiment of the present invention; and 
         FIG. 8  illustrates an imaging illumination configuration provided by sources in the apparatus of  FIG. 1 ,  FIG. 6 , and/or  FIG. 7 , according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference is now made to  FIG. 1 , which is a schematic diagram of a beam alignment apparatus  20 , according to an embodiment of the present invention. Apparatus  20  is used to micromachine a site  43 , which, by way of example, is assumed hereinbelow to be included in a printed circuit board (PCB)  24 . Site  43  typically comprises dielectric substrate material, such as epoxy resin with glass beads and/or fibers, and/or conductive material such as copper pads or traces. Typically, although not necessarily, site  43  comprises conductive material embedded in dielectric substrate material. Apparatus  20  incorporates a beam source  22  which projects a radiation beam  26  via a collimator  27 . Beam  26  is used to micromachine a hole at a location in site  43 . In one embodiment, source  22  comprises an ultra-violet (UV) laser operative at a beam wavelength of approximately 350 nm. The UV laser may be operated as a short pulse laser, the pulses being of the order of femtoseconds in length, using the nonlinear interactions of the short pulses to cause ablation. In an alternative embodiment source  22  comprises a carbon dioxide laser operative at a beam wavelength of approximately 10 μm. However, apparatus  20  may use any suitable radiation source configurable to supply radiant energy that site  43  can absorb, in a form and of a level that may be used for micromachining. Hereinbelow, by way of example, source  22  is assumed to comprise a laser, so that beam  26  is a laser radiation beam. 
     A set  31  of optical components, comprising a beamsplitter  28 , an optic element train  30 , and a mirror  34 , acts as a beam directing system to convey the beam onto the PCB. Typically, mirror  34  is a front surface mirror and beamsplitter  28  is a narrow-band dichroic cube beamsplitter that transmits the beam wavelength and reflects other wavelengths. Optic train  30  and PCB  24  are mounted on respective translation stages  33 ,  45 . Mirror  34  is mounted on a beam steering stage  35 , typically a galvanometer based steering stage, or a two axis fast beam steering stage such as is described in U.S. patent application Ser. No. 11/472,325. U.S. patent application Ser. No. 11/472,325 is assigned to the assignees of the present invention and is incorporated herein by reference. Laser beam  26  is transmitted, via the beamsplitter, to the optic element train which directs and focuses the beam. 
     Apparatus  20  is configured as a “post-scan” system, wherein there are no optical elements between mirror  34  and PCB  24 . In this configuration, the mirror typically has a field of view (FOV) of approximately ±3°. 
     The following description, unless otherwise stated, is drawn to micromachining PCB  24  using one laser beam. However, it will be understood that embodiments of the present invention may use more than one laser beam operating substantially simultaneously. 
     An operator  23  operates apparatus  20  using a workstation  21  which comprises a memory  25  and a processing unit (PU)  32 . PU  32  uses instructions stored in memory  25  to control individual elements of apparatus  20 , such as laser  22  and the translation and beam steering stages. In addition to operating stages  33 ,  35 , and  45 , PU  32  may vary the focus of optic train  30  as a particular hole in site  43  is being micromachined. The hole is micromachined at a selected region  42  on a top surface  36  of PCB  24 . An inset  44  shows site  43 , which includes region  42  and an area surrounding the region, in more detail. 
     In some embodiments of the present invention, an object  46  is located beneath region  42 , the object being embedded in PCB  24  so that there is a layer  38  of the PCB above the object, and a layer  40  below the object. Typically, other embedded objects are in proximity to object  46 , and other layers may be comprised in PCB  24 , but the other embedded objects and layers are not shown in  FIG. 1  for clarity. Object  46  is typically part of an electrical circuit, and layers  38  and  40  act as a substrate on which the electrical circuit is formed. In one embodiment, object  46  is an approximately circular metal pad, having an approximate diameter of 100 μm. Typically, layers  38  and  40  are dielectrics, and are formed from filled epoxy resin. In some disclosed embodiments, layers  38  and  40  are assumed to be formed from one of the varieties of Ajinomoto Build-up Film (ABF), manufactured by Ajinomoto Fine-Techno Co., Inc, NJ, that are known in the art, and which are described below in reference to  FIG. 2  and  FIG. 3 . In one embodiment, layers  38  and  40  are implemented from ABF type GX3, and have a thickness of approximately 35 μm. However, it will be appreciated that layers  38  and  40  may be formed from any material suitable for construction of printed circuit boards. For example, layer  38  may comprise an ABF material and layer  40  may comprise FR4 material. 
     In order that PU  32  may align PCB  24 , the PCB is illuminated by illumination from a radiation source  50 , typically a laser diode, providing imaging radiation at an imaging radiation wavelength. In some embodiments, source  50  comprises a light emitting diode (LED), typically a high brightness LED. If source  50  comprises a laser diode, the source typically includes a speckle removing system, such as a bundle of fiber optics. Alternatively or additionally, the source may be selected to have a short coherence length, as is described in more detail below. Apparatus  20  includes a second dichroic beamsplitter  52 , which is transparent to the beam wavelength and which acts as an approximately 50/50 beamsplitter at the imaging radiation wavelength. In some embodiments of the present invention, as described below, beamsplitter  52  comprises a polarizing beamsplitter. The imaging radiation is conveyed via a focusing lens system  49  through beamsplitter  52 , so as to be generally coaxial with beam  26 . The imaging radiation reflects from mirror  34  so that the imaging radiation at PCB  24  is substantially normal to surface  36 . The imaging radiation arriving at surface  36  is configured to illuminate a relatively small area surrounding and contiguous with region  42 , rather than an extended area of the surface, the area typically being of the order of four times the area of the site being micromachined. For example, for the exemplary 100 μm pad described above, focusing lens system  49  may be configured to provide imaging radiation in a circle having a diameter of the order of approximately 200 μm. 
     By configuring the imaging radiation to illuminate a relatively small area surrounding the location where the micromachining is to be performed, high intensity illuminating radiation may be efficiently provided to the area, so that good quality images of the area may be generated. By directing the imaging radiation via elements of apparatus  20  that are also used to direct micromachining beam  26  to the area being micromachined, as apparatus  20  is realigned to micromachine new areas the high intensity illuminating radiation automatically realigns to the new areas. In addition, as described below, returning radiation that is used for imaging also returns via common elements of apparatus  20  that direct beam  26  and the illuminating radiation, so that as apparatus  20  is realigned to micromachine new areas, the new areas are also automatically imaged. As is explained in more detail below, the above combination of features allows embodiments of the present invention to align beam  26  with its site substantially in real time, thus increasing the overall rate for micromachining PCB  24 . 
     Returning radiation from site  43  is reflected by mirror  34  via beamsplitter  52  to optic train  30 , as shown schematically by arrow  54 , and transfers to beamsplitter  28  from the optic train. Train  30  directs the returning radiation, via beamsplitter  28  and a focusing lens  55 , to an optical sensor  56 , optionally via a filter system  53  which typically comprises a set of selectable filters, including band-pass and long-pass filters. Such a filter system may be utilized if site  43  generates fluorescent radiation, as is described below. For an object, such as object  46 , that is present in site  43 , sensor  56  is configured to provide signals to PU  32  according to the location of the object, and the processing unit uses the signals to align and orient beam  26  correctly with respect to PCB  24  and the object. The operation of sensor  56  is described in more detail with respect to  FIGS. 5A ,  5 B, and  5 C. 
     In some embodiments, source  50  is used to generate fluorescent returning radiation from site  43 , so that, inter alia, images formed from the returning radiation are inherently free of speckle. U.S. patent application Ser. No. 10/793,224, which is assigned to the assignee of the present invention and which is incorporated herein by reference, describes generation of fluorescent images. In such cases, source  50  may advantageously comprise a laser diode operative at approximately 405 nm, and typically there may be no need for a speckle removing system. Furthermore, beamsplitter  52  may advantageously be configured as a dichroic beamsplitter, reflecting radiation from source  50 , and transmitting beam  26  and the fluorescent returning radiation. Advantageously, PU  32  may be configured to adjust the wavelength and/or the power of the imaging radiation generated by source  50 . By adjusting the wavelength and/or power, the effective depth of penetration of the imaging radiation into site  43  may be varied, so that the image generated by the fluorescent radiation may be optimized. If site  43  incorporates an object that does not fluoresce, such as a metal pad, generating an image with fluorescent radiation enhances the contrast of the image. Since, as is explained below, site  43  typically comprises layers having different fluorescent properties, PU  32  and/or operator  23  may choose filters from filter set  53  to optimize the image 
     In some embodiments source  50  is selected to have an operating wavelength or range of wavelengths to which the PCB is substantially transparent, such as the wavelengths given below in reference to  FIG. 2 . In this case, typically for objects  46  that are at least partially specular, the objects may be imaged as bright objects against a relatively dark background. This type of “bright field” imaging may be generated when relatively long source wavelengths, such as are given below in reference to  FIG. 2 , are used in conjunction with materials, such as SH9K ABF resin, GX3 ABF resin, or GX13 ABF resin, that are relatively transparent to these wavelengths. 
     Typically, PU  32  uses translation stage  45  to perform coarse alignment for PCB  24 , and stages  33  and  35  for fine alignment, so that region  42  is at a desired position on surface  36 , and so that beam  26  is in a desired orientation with respect to the surface. However, any other convenient combination of operations of translation stages  33 ,  45 , and beam steering stage  35 , may be used to position and orient beam  26 . 
     In order to micromachine a hole in PCB  24  with beam  26 , the material machined needs to be at least partially effectively absorbing, so that energy of the beam is absorbed. The effective absorption may be caused by absorption of the beam by the PCB resin at the beam wavelength, or by objects incorporated in the resin, for example, glass particles or fibers, or by objects such as object  46  embedded in the PCB. Alternatively or additionally, in the case of short pulse lasers referred to above, the effective absorption to the beam may be caused by nonlinear interactions of the short pulses with the PCB resin or embedded object. In general, since the micromachining operates by ablation of portions of the PCB, the efficiency of micromachining increases as the effective absorption of the beam increases. 
     A number of other factors may influence the ability of apparatus  20  to efficiently micromachine in PCB  24 :
         The required effective absorption of the PCB portions to be micromachined at the beam wavelength may limit effective imaging at the beam wavelength of objects beneath surface  36 , such as object  46 .   Some of the optical elements of apparatus  20  convey both the beam radiation from source  22  and the imaging radiation from source  50 . In addition, the optical elements may convey fluorescent radiation if it is generated. The three radiations have differing wavelengths, and some of the wavelengths may be very different from each other. In such cases optical elements of apparatus  20  may be advantageously selected to comprise reflective elements, refractive elements, or a combination of the two types of elements, and/or other elements such as diffractive elements so as to correctly transfer the differing wavelengths. The selection of elements will be apparent to those having ordinary skill in the art.   There are practical limits to the wavelength that may be selected for the beam, as well as for the wavelength, or range of wavelengths, that may be selected for the imaging radiation and for fluorescent radiation, if it is used.       

     The choice of beam and imaging radiation wavelengths is a function of these and other factors, including optical characteristics of the constituents of PCB  24  and object  46 . As a consequence, in some embodiments of the present invention the beam wavelength and the imaging radiation wavelength are selected to be approximately the same. For these embodiments, the imaging radiation wavelength is separated from the beam wavelength by about 50 nm or less. In other embodiments the two wavelengths are selected to be different, so that the imaging radiation wavelength is separated from the beam wavelength by about 100 nm or more. For the case of fluorescent imaging, the imaging radiation wavelength is chosen so as to generate fluorescence, and there is inherently partial absorption of the imaging radiation by the PCB resin. 
     Apparatus  20  may be used to micromachine multiple holes in PCB  24 , the holes typically being used for micro vias and/or blind vias. The steps involved in micromachining multiple holes are to align beam  26  with region  42 , micromachine the hole through the region, and realign the beam on a new site having a region to be micromachined. This process is repeated iteratively. For the process to be efficient, the alignment and realignment of the beam should be performed as quickly as possible. Alternatively or additionally, multiple sets of apparatus  20  may be configured to micromachine multiple holes substantially simultaneously. In one embodiment of the present invention,  18  sets of apparatus  20  are operated simultaneously on the PCB. 
     In some embodiments of the present invention, apparatus  20  comprises an element  5   1 . The function of element  51  is described below, in reference to  FIG. 8 . 
       FIG. 2  is a schematic graph of the percentage transmission of different types of ABF resin for a resin thickness of 45 μm at different wavelengths. 
     Inspection of the graph shows that at a wavelength of approximately 350 nm, corresponding to that provided by laser  22  if the laser is a UV laser, SH9K ABF resin transmits approximately 20%, whereas GX3 ABF resin is highly absorbing. Thus, if layer  38  is SH9K ABF resin, source  50  may have approximately the same wavelength as laser  22 , and produce returning radiation from object  46 . If layer  38  comprises GX3 ABF resin, then to achieve the same, or more, returning radiation as for SH9K, the source wavelength should be approximately 430 nm or more. In addition to the transmission factor given by the graph of  FIG. 2 , other factors affecting imaging of PCB and object  46  include diffusion of the illuminating radiation, which varies due to the size and density of the glass beads used to fill the epoxy resin comprising layers  38  and  40 . 
     The inventors have found that both types of resin are substantially transparent at near infra-red wavelengths, of the order of 800 nm or more. The inventors have also found that good images of embedded objects such as object  46  are produced if source  50  operates at these wavelengths, irregardless of the diffusion caused by embedded beads in layers  38  and  40 . 
       FIG. 3  is a schematic graph of fluorescence of different types of resin. The graphs, for ABF resins GX3, SH9K and GX13, and for an FR4 material, plot a normalized fluorescence intensity vs. the fluorescent wavelength for each of the resin materials. The graphs were generated for an excitation wavelength of approximately 300 nm, but the inventors have verified that generally similar graphs occur for other excitation wavelengths, including the wavelength of 350 nm for a UV laser exemplified above. Some embodiments of the present invention use the property of fluorescence illustrated by the graphs of  FIG. 3  in operating apparatus  20 . For example, if layer  40  ( FIG. 1 ) comprises FR4 resin, and layer  38  comprises GX3 resin, the two layers may be well distinguished by using a band-pass filter operating at approximately 450 nm, or a long-pass filter having a cut-off at approximately the same wavelength. To observe fluorescence from both layers a shorter wave band-pass or long-pass filter may be used. 
       FIG. 4  is a flowchart  60  showing steps performed in operating apparatus  20 , according to an embodiment of the present invention. 
     Prior to micromachining with apparatus  20 , the apparatus is initially calibrated with respect to PCB  24 . The initial calibration may be by marking a panel such as a specific calibration panel (different from PCB  24 ), imaging the marks with apparatus  20 , and from the imaged marks determining a calibration offset for the apparatus. In some embodiments, a part of PCB  24  may be marked and the marks used for the calibration. 
     Alternatively or additionally, the property of fluorescence illustrated by the graphs of  FIG. 3  may be advantageously used for registration of apparatus  20 , as is described in more detail below. 
     The following description of the steps of flowchart  60  describes a calibration process and a micromachining process. 
     In a first calibration step  62 , operator  23  positions a special calibration panel, or PCB  24  if the PCB is to be used for calibration, on stage  45 . The operator provides apparatus  20  with calibration target coordinates, typically for 2 to 4 targets, as well as shapes corresponding to the targets, in the calibration panel or in PCB  24 . The operator may provide the target coordinates and shapes from a computer aided manufacturing (CAM) file, or they may be input directly by the operator. As is described above, the targets may be configured to be non-destructive or destructive. Alternatively, the calibration panel or PCB  24  may be positioned mechanically, typically using reference pins, corners, or other mechanical reference regions of the panel or PCB. 
     In a second calibration step  64 , the operator operates a registration system of apparatus  20  so as to illuminate and locate the calibration targets. The illumination may be from source  50 , which, as described above, may advantageously have its imaging radiation wavelength selected so that the returning radiation is fluorescent radiation. As is also described above, PU  32  may adjust the wavelength and/or the power of source  50  to optimize the image generated. 
     Alternatively or additionally, if fluorescence of the calibration targets is used, a region including the targets may be illuminated by operating laser  22  at a power below an ablation threshold power for the PCB. In this case the region may be illuminated by operating laser  22  in an “area illumination” mode, typically by defocusing beam  26  with optic train  30 . Alternatively, the area illumination mode may be implemented by scanning mirror  34  and thus the laser beam using beam steering stage  35 . The calibration targets are imaged on sensor  56 , and PU  32  uses the target images formed on the sensor to calibrate apparatus  20 . If fluorescence is used, PU  32  and/or operator  23  may select one of the filters in set  53  to optimize the image produced, typically in the case that layers  38  and  40  comprise different resins such as are described above, and as is exemplified in the description of  FIG. 3 . 
     The following steps assume that PCB  24  has been used for the calibration, and that the PCB is in place in apparatus  20 . Also in the following steps, object  46  is assumed, by way of example, to be an isolated approximately circular pad, and a hole is to be micromachined vertically to surface  36 , through the center of the pad. Those having ordinary skill in the art will be able to adapt the description of the steps of the flowchart, mutatis mutandis, for other types of object  46 , such as a circular pad connected to a rectangular conductor, or to an array of connected circular pads. 
     In a first micromachining step  65 , operator  23  loads a CAM file, corresponding to an electrical circuit implemented in PCB  24 , into memory  25 . 
     In a second micromachining step  66 , PU  32  uses the CAM file to determine a shape, and nominal coordinates of the shape, wherein a hole is to be micromachined. In the following description, it is assumed that a hole is to be micromachined in the center of object  46 , so that the nominal coordinates may be those of object  46 , or of site  43  comprising the object. Alternatively, the nominal coordinates and shape of object  46  may be found from analysis of an image of the electrical circuit, the analysis being performed by operator  23  and/or PU  32 . 
     In a third micromachining step  68 , PU  32  uses a signal corresponding to the nominal coordinates to provide coarse adjustment control signals to the motion stages holding PCB  24 , train  30 , and/or mirror  34 , so that object  46  moves into the field of view of sensor  56 . The positioning may be performed completely automatically by the processing unit. Alternatively, operator  23  may at least partially implement the positioning, typically by providing the nominal coordinates to PU  32 . 
     From step  68 , PU  32  follows one of two possible paths. A first path  69  leads to an object illumination step  74  via beam registration steps  70  and  72 . A second path  71  leads directly to an object illumination step  74 . First path  69  is followed by PU  32  when flowchart  60  is first operated, and periodically afterwards, so that the beam registration performed in steps  70  and  72  is not performed for each object that is micromachined. Rather, the beam registration is performed intermittently every t seconds, where t is a parameter chosen by operator  23 , and is typically approximately of the order of 10. 
     In path  69 , in first beam registration step  70 , laser  22  is operated at low power, below an ablation threshold, so as to impinge on site  43 . The laser beam typically causes fluorescence where it impinges on site  43 , herein assumed to be region  42 , in which case the returning fluorescent radiation is focused at sensor  56 , to form an image at the sensor of region  42 . Alternatively, rather than using the fluorescence of the PCB, an ablative calibration board may have been previously added to site  43 . 
     In path  69 , in second beam registration step  72 , PU  32  records the location of the laser beam on sensor  56 . 
     In an object illumination step  74 , PU  32  switches laser  22  off, and operates source  50  to illuminate object  46 . Alternatively or additionally, in step  74  PU  32  may maintain laser  22  at a low power and/or in the area illumination mode described above. Typically, PU  32  uses generated returning fluorescent radiation from the PCB, in the vicinity of object  46 , to produce the image described in the following step  76 . The fluorescent radiation may be generated from the radiation of laser  22  and/or source  50 . The image may be formed solely from the returning fluorescent radiation, or together with returning radiation at the wavelength of source  50 . Typically, such as for the example described above of layers  38  and  40  comprising different resin types (such as ABF and FR4), in the case of returning fluorescent radiation PU  32  selects a filter from set  53  to optimize the image. 
     In an object record step  76 , PU  32  records an image of the object that is generated at sensor  56 . PU  32  analyzes the signal levels from sensor  56  to determine a signal corresponding to actual coordinates for the center. An example of the analysis is described in reference to  FIGS. 5B and 5C . If path  69  has been followed, the processing unit records and determines an offset between the actual coordinates of the center of the circular pad and the beam position found in step  72 . If path  71  has been followed, the processing unit uses the offset found in the most recent implementation of path  69 . 
     In a motion step  78 , PU  32  uses the offset determined in step  76  to adjust the beam position in relation to the center of object  46 . Typically, the adjustment is made by operating beam steering stage  35  to correctly align mirror  34 . 
     In an operate laser step  80 , PU  32  switches the power of source  22  above the ablation threshold so that the beam ablates layer  38  and object  46 , and thus micromachines a hole at the actual coordinates of the center of object  46 . In some embodiments, during the micromachining, the processing unit may also use optic element train  30  to change the focus of beam  26  as the micromachining proceeds. 
     In a first decision  82 , PU  32  checks if there are further micromachining operations to be performed on PCB  24  at other sites of the PCB. If there are no more operations, flowchart  60  ends. If there are more operations, herein assumed to be machining holes in the center of objects substantially similar to object  46 , flowchart  60  continues to a second decision  84 . 
     In second decision  84 , PU  32  determines if the distance from object  46  to the nominal location of a next object to be machined is greater than a preset distance, typically of the order of 10 mm. If the distance is greater than the preset distance, a counter N is set to 0, and the flowchart returns to step  66 , to machine the next object. 
     If the distance is less than or equal to the preset distance, then in a third decision  86  PU  32  checks if the offset recorded in step  76  is less than a preset value. If the offset is less than the preset value, then in a step  88  PU  32  operates apparatus  20  by performing steps  78  and  80  for N next objects, where N is the counter referred to above, and where N is set to a predetermined value, typically approximately 10. Operator  23  may set the predetermined value of N at the loading of the CAM file in step  65 . 
     While performing step  88 , PU  32  checks after each machining operation if the distance between objects exceeds the preset distance, in which case the flowchart returns, as shown by a broken line  67  in the flowchart, to step  66 . If the preset distance is not exceeded as the N objects are machined, PU  32  completes machining the N objects, increments N, and then returns the flowchart to step  66 . 
     If in decision  86  the offset is greater than or equal to the preset value, then PU  32  decrements N, to a minimum value of 0. In a step  90  PU  32  operates the apparatus by performing steps  78  and  80  for N (the decremented value) next objects. While performing step  90 , PU  32  checks after each machining operation if the distance between objects exceeds the preset distance, in which case the flowchart returns, as shown by a broken line  73  in the flowchart, to step  66 . If the preset distance is not exceeded as the N objects are machined, PU  32  completes machining the N objects and then returns the flowchart to step  66 . 
     Decision step  84  allows operator  23  to configure apparatus  20  so that objects within a preset distance of an object wherein registration steps  66 - 76  have been performed may be machined without performing the registration steps. In other words, the offset determined for a given object is used to position the beam for a group of objects close to the given object. 
     Decision step  86  allows the operator to configure the apparatus so that the size of the offset found in step  76  determines how many objects are in the group referred to above. Thus, if the determined offset is below the preset offset, the value of N, the number of objects in the group, is incremented for the next group of objects to be machined. If the determined offset is greater than the preset offset, the value of N is decremented for the next group of objects to be machined. 
     The operator typically inputs values of the preset distance and preset offset in step  65 . 
     The description above applies to micromachining a circular hole vertical to surface  36 , through the center of a circular pad. Apparatus  20  may also perform other micromachining operations, such as micromachining a hole non-vertically, and/or micromachining a non-circular hole, for example a hole in the shape of a slit, and/or micromachining a hole at a position different from the position corresponding to the actual coordinates determined in flowchart  60 . It will also be understood that the micromachining may be applied to form a hole that completely penetrates the PCB, or a hole that does not completely penetrate the PCB. Those having ordinary skill in the art will be able to adapt the description above for such other micromachining operations, typically by the processing unit implementing, in steps  78  and  80 , further operations of translation stage  33 , translation stage  45 , and/or beam steering stage  35 . 
     Typically, the coarse alignment corresponding to step  68 , if performed automatically, takes approximately 1-3 ms from a previously micromachined hole. The shorter times typically apply if beam steering stage  35  ( FIG. 1 ) is galvanometer based, the longer times typically apply if the stage is a two-axis scanning system. Advantageously, the fine alignment procedure described above in step  78  takes less than approximately 1 ms. The times are achieved because of, inter alia, the high intensity imaging radiation that is directed to each site that is micromachined. 
     The inventors have found that, because of these times, substantially no time is lost in application of flowchart  60  to machining PCBs, compared to prior art systems that do not apply the steps of the flowchart for such machining. Furthermore, steps such as decision steps  84  and  86  may be performed during machining of the PCB. Thus, flowchart  60  may be implemented to operate substantially in real time. By operating at the times stated, deleterious relatively long term effects, such as thermal drift, may be eliminated. Furthermore, by only performing registration steps  70  and  72  intermittently, as described above, overall operation time is reduced without affecting the accuracy of the micromachining. 
       FIG. 5A  shows a schematic diagram of a surface of optical sensor  56  that may be used in apparatus  20 , according to an embodiment of the present invention. Typically, in order to generate alignment signals in the alignment times given above, sensor  56  uses complementary metal oxide semiconductor (CMOS) technology. Alternatively, sensors  56  may comprise one or more CCDs (charge coupled devices), or other suitable sensing devices. 
     A diagram  164  illustrates the surface of sensor  56 . Sensor  56  typically comprises a rectangular array of detector elements  170 . Some examples of suitable image sensors are described hereinbelow. Micron Technology, Inc of Boise, Id., provide an MTM001 CMOS 1.3 Mpixel rectangular array sensor, which the inventors have found is suitable. The number of elements of the sensor that are addressed may be restricted using a programmable area of interest (AOI), allowing the array to be used for short acquisition times of the order of 1-3 ms. Hamamatsu Photonics K.K., of Japan, provide a 256×256 detector element S9132 array which may be operated as two one-dimensional arrays, giving summed outputs described in more detail below. Other arrays which are suitable for use as sensor  56  will be familiar to those having ordinary skill in the art. 
     PU  32  may advantageously use signals from elements  170  to accurately determine a particular position with respect to object  46 .  FIGS. 5B and 5C  show examples of images of object  46 . By way of example, object  46  is assumed to comprise a circular pad, and the center of the circular pad is to be micromachined. In  FIG. 5B , object  46  comprises an isolated approximately circular pad, generating an image  166 . In  FIG. 5C , object  46  comprises an approximately circular pad connected to a rectangular conductor, generating an image  176  comprised of a circular portion  178  connected to a rectangular portion  180 . 
     If sensor  56  comprises a rectangular array of individual pixels such as the Micron array referenced above, then for image  166  PU  32  may reduce the number of pixels to be analyzed to a rectangular set of pixels  168  surrounding image  166 , the reduction of pixel numbers reducing the acquisition time of the image. PU  32  may then fit all the imaged pixels to a circle, typically by using an edge-detection algorithm, to identify the center of image  166  to sub-pixel accuracy. 
     For example, by using 100×100 pixels of the 1.3 Mpixels, the image acquisition time may be improved by a factor of nearly 100 compared with the nominal frame rate of 30 Hz, providing a sub-millisecond acquisition time. Such a short acquisition time requires a high image illumination intensity, as is provided by the directed site illumination from source  50  via mirror  34  ( FIG. 1 ). 
     For image  176 , PU  32  may reduce the number of pixels to be analyzed to a rectangular set of pixels  179  surrounding portion  178 , possibly cutting off some of the pixels of rectangular portion  180 . By using an edge-detection algorithm, PU  32  may then fit imaged pixels forming a non-linear edge to a circle, to identify the center of circular portion  178  to sub-pixel accuracy. Alternatively, PU  32  may use an edge-detection algorithm to fit all the pixels to an expected theoretical edge generated by a circle intersected by two parallel lines on one side of the circle. 
     Typically, pixels selected for analysis by PU  32  do not need to be simple rectangular arrays. For example, the imaged site may comprise a small circular pad attached to a large circular pad, in which case the pixels selected by PU  32  may be configured as a generally irregular set of pixels chosen to just encompass the site. 
     Sensor  56  may comprise an array which may not give an output for each pixel of the array, such as the Hamamatsu array referenced above. In this case PU  32  may apply curve fitting to the summed outputs of the array, to find the centers of images  166  and  178 . 
       FIG. 6  is a schematic diagram of a beam alignment apparatus  320 , according to an alternative embodiment of the present invention. Apart from the differences described below, the operation of apparatus  320  is generally similar to that of apparatus  20  ( FIG. 1 ), and elements indicated by the same reference numerals in apparatus  20  and  320  are generally similar in construction and in operation. 
     Apparatus  320  includes a beamsplitter  326 , and beamsplitter  52  is removed. Beamsplitter  326  is configured to transmit imaging radiation from source  50 , and to reflect radiation returning from site  43  to sensor  56 . If the returning radiation has the same wavelength as that of source  50 , beamsplitter  326  may be a 50/50 beamsplitter. If the returning radiation is fluorescent radiation, beamsplitter  326  may be configured as a dichroic beamsplitter. Alternatively, as described below, beamsplitter  326  may be a polarizing beamsplitter. 
     In apparatus  320 , optical element train  30  is separated into two sets of optics. A first set  324  typically comprises movable optic elements that may be used to change the magnification of the beam from source  22 . A second set  322  typically comprises fixed optical elements. By dividing train  30  into the two sets, the magnification of the beam from source  22  may be adjusted without affecting the illumination and imaging path between beamsplitter  28  and mirror  34 . 
     Elements  323  and  325  in apparatus  320  are described below. 
     If the normal imaging illumination provided in apparatus  320  is generally uniform over site  43 , i.e., if there is little or no structure to the illumination, the resulting image of a specular object  46  is typically a bright image of the object against a dark background image of a region surrounding the object, and the two images have good contrast. 
     Consideration of apparatus  20  and  320  shows that optical elements such as steerable mirror  34  and optic train  30  may convey at least two differing wavelengths, i.e., the beam wavelength of beam  26  and the imaging radiation wavelength of source  50 . If fluorescence is used, then the optical elements may convey three differing wavelengths, i.e., the beam wavelength, the imaging radiation wavelength, and the fluorescent wavelength. Configuring the same elements to convey two or three different wavelengths significantly reduces the number of optic elements that may be needed if separate sets of elements are used for the differing wavelengths. 
       FIG. 7  is a schematic diagram of a beam alignment apparatus  330 , according to a further alternative embodiment of the present invention. Apart from the differences described below, the operation of apparatus  330  is generally similar to that of apparatus  20  ( FIG. 1 ) and apparatus  320  ( FIG. 7 ), and elements indicated by the same reference numerals in apparatus  20 ,  320  and  330  are generally similar in construction and in operation. 
     Apparatus  330  comprises a lens system  336  between mirror  34  and site  43 . Lens system  336  typically comprises a telecentric lens, which allows mirror  34  to have an FOV of approximately ±20°. Adding the lens system configures apparatus  330  as a “pre-scan” system. The larger FOV of the mirror, compared with the post-scan systems described above, allows the mirror to both project beam  26  onto a larger area of PCB  24  and to image the area. 
     Optical sets  324  and  322  are typically respectively reconfigured to a first set  334  comprising movable elements, and a second set  332  comprising fixed elements, set  334  and set  332  being selected to accommodate lens system  336 . 
     The descriptions above for apparatus  20 ,  320 , and  330  have assumed that the imaging illumination is generally normal to surface  36 , and that it is generally unstructured. In some embodiments of the present invention, as described below, the imaging illumination may also be configured so that the illumination has structure, as described below. 
       FIG. 8  illustrates an imaging radiation configuration  344  provided by source  50 , according to an embodiment of the present invention. A cross-section  340  and a top-view  342  of PCB  24  are shown for radiation configuration  344 . In configuration  344  the imaging radiation on surface  36  is structured, for example as a generally annular ring  346  of imaging radiation. The imaging radiation penetrates layers  38  and  40 , and is also partially scattered within the layers because of diffusion within the layers, due, inter alia, to the fill material incorporated in the layers. The combination of penetration and partial scattering effectively “backlights” object  46 , as shown schematically by arrows  348 , so producing a high contrast image at sensor  56 . The high contrast image is generated irregardless of whether object  46  is specular or non-specular. Furthermore, the high contrast image produced by the backlighting effectively compensates for blurring of the image that may be caused by the radiation diffusion within the layers. Without using the backlighting effect, the image blurring may cause deviations in measured position of the image. 
     Radiation configuration  344  may be advantageously provided in apparatus  20  by positioning an element  51  ( FIG. 1 ), typically a stop, between lens  49  and beamsplitter  52 . Although not illustrated in the interests of clarity, configuration  344  may also be provided in apparatus  320  by positioning an appropriate stop between lens  55  and beamsplitter  28 . Other methods for producing an annular ring of radiation in apparatus  20 ,  320 , and  330 , such as by using diffractive elements designed to give structured illumination, will be apparent to those having ordinary skill in the art, and are assumed to be comprised within the scope of the present invention. For example, element  51  may comprise such a diffractive element. Other forms of structured illumination may be provided by source  50 , the illumination typically being structured according to the site being imaged. For example, a rectangle of illumination may be used to illuminate a region around a generally linear trace. All such forms of structured illumination are assumed to be comprised within the scope of the present invention. 
     For configuration  344 , source  50  may be selected to be a laser emitter with very short coherence length, so that there is substantially no speckle. The inventors have found that lasers with a coherence length of the order of 1-2 times a dimension of an object being machined, such as the diameter of a circular pad, are suitable. 
     Referring back to  FIG. 6 , an alternative radiation configuration uses polarized illuminating radiation. As illustrated in  FIG. 6 , a polarizer  323  may be positioned after source  50 , and an analyzer  325  is positioned before sensor  56 . Alternatively, since source  50  typically provides polarized radiation, there may be no need for polarizer  323 . The orientation of polarizer  323 , or of source  50  if its radiation is polarized, and of analyzer  325 , may be controlled by PU  32 . Alternatively, the orientations may be preset to generally fixed values by operator  23 . Reflections from surface  36 , and from intermediate surfaces of PCB  24  such as the interface between layer  38  and layer  40 , have practically the same polarization as the incoming polarized radiation at low incident angles. The returning scattered radiation from layers  38  and  40  is relatively weak and is mainly polarized in the same direction as the incoming polarized radiation. However, if object  46  has even a partially roughened metallic surface, as is typically the case so as to improve adhesion of the object with its embedding resin or resins, the radiation it reflects is substantially depolarized, so having a component at 90° to the incoming polarized radiation. In the alternative configuration described here, PU  32  arranges that polarizer  323  and analyzer  325  have crossed polarizations, or operator  23  presets these orientations, so that the specular reflection from the surfaces and interiors of layers  38  and  40  is absorbed, whereas the depolarized radiation from object  46  is transmitted. The crossed polarizations thus provide a good image of object  46  having high contrast with material surrounding the object. 
     In an alternative implementation for polarizing illuminating radiation, neither polarizer  323  nor analyzer  325  are used. Rather, source  50  is implemented to provide polarizing illumination, and beamsplitter  326  is configured as a polarizing beamsplitter which transmits the polarizing illumination from the source. The polarizing beamsplitter acts to reflect the depolarized radiation, comprising radiation from object  46 , to sensor  56 , so forming a good image of the object, as described above. 
     Referring back to  FIG. 1 , beamsplitter  52  may be configured as a polarizing beamsplitter at the wavelength of source  50 , so that sensor  56  in apparatus  20  forms an image of object  46  substantially similar to that formed in apparatus  320 . 
     The polarizing embodiments described above enable sensor  56  to polarizeably analyze the returning radiation from object  46  and its surroundings. 
     For polarizing embodiments, to reduce speckle source  50  may comprise a laser emitter with a coherence length less than the dimensions of an object being machined. For example, for a circular pad, the coherence length may be significantly smaller than the diameter of the pad. Other methods for reducing speckle, such as are exemplified above, may also be used. 
     The embodiments described above have related to using an optical image of PCB  24  and/or an embedded object  46  to adjust actual positions for micromachining the PCB. However, it will be appreciated that other types of images of the PCB and/or an embedded object may be used by PU  32  to determine the required actual positions. In addition, it will be understood that embodiments of the present invention may be used to image objects embedded in, or on a surface of, materials other than PCBs, such as ceramic or glass. Those having ordinary skill in the art will be able to modify the description above, without undue experimentation, to accommodate changes required by the other types of images. 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.