Patent Publication Number: US-2007099397-A1

Title: Microfeature dies with porous regions, and associated methods and systems

Description:
TECHNICAL FIELD  
      The present invention relates generally to microfeature dies with porous regions, and associated methods and systems, including dies with conductive structures formed from porous media, and methods for singulating dies having porous media.  
     BACKGROUND  
      Packaged microelectronic assemblies, such as memory chips and microprocessor chips, typically include a microelectronic die mounted to a substrate and encased in a plastic protective covering. The die includes functional features, such as memory cells, processor circuits and interconnecting circuitry. The die also typically includes bond pads electrically coupled to the functional features. The bond pads are coupled to pins or other types of terminals that extend outside the protective covering for connecting the die to buses, circuits, and/or other microelectronic assemblies. Alternatively, bare microelectronic dies can be connected to other microelectronic assemblies.  
      Multiple microelectronic dies are typically formed simultaneously in a single microelectronic workpiece or wafer, and are then singulated or diced to separate the dies from each other and from the remainder of the workpiece.  FIG. 1  illustrates a workpiece  10  having multiple die portions  11  that are singulated or diced in accordance with the prior art. Each die portion  11  can include microelectronic features  12 , and a scribe area  14  positioned between neighboring die portions  11 . Sacrificial test circuitry  13  is typically positioned in the scribe areas  14  and can extend between the microelectronic features  12 . The test circuitry  13  includes conductive material  17  formed in and/or on the workpiece  10  during formation of the microelectronic features  12 , and is used for diagnostic purposes to determine the quality of the fabrication processes completed on the workpiece  10 .  
      During singulation, a rotating dicing blade  30  is moved downwardly into contact with the workpiece  10  at the scribe area  14  to cut through the workpiece  10  and separate the neighboring die portions  11  from each other. One drawback associated with this method is that the dicing blade  30  can place high lateral stresses on the die portions  11 . In particular, the conductive material  17  can adhere to the dicing blade  30  as the dicing blade  30  cuts through the test circuitry  13 , effectively widening the dicing blade  30  and increasing the lateral stresses the dicing blade  30  places on the die portions  11 . These lateral stresses can cause cracks  16  to form in the scribe area  14 . The cracks  16  can propagate into the die portions  11 , where they can damage the microelectronic features  12 .  
      One approach to reducing the lateral stresses placed on the workpiece  10  is to provide one or more relief grooves  15  in the scribe area  14 . However, the relief grooves  15  may not provide sufficient stress relief to prevent the formation of the cracks  16 . Another approach is to increase the width W 1  of the scribe area  14  so that any cracks  16  that form in the scribe area  14  do not extend into the microelectronic features  12 . A drawback with this approach is that it can significantly reduce the amount of costly workpiece material available for forming the die portions  11 .  
      Another problem associated with existing dies relates to the conductive structures in the dies. Each singulated die typically includes conductive lines and conductive vias that connect microelectronic features  12  within the die. Conductive lines are generally formed in layers of the die oriented generally parallel to the major faces of the die. Conductive vias typically connect conductive lines located in different layers of the die and are therefore oriented generally normal to the major faces of the die. In a typical process, a first set of conductive lines is formed by etching away conductive material in a selected plane of the die. An insulating layer is then disposed over the lines, and conductive vias are formed in the insulating layer. The vias are typically formed by etching holes through the insulating layer, cleaning the holes, coating the holes with a dielectric material, and then filling the holes with a conductive material. Once the vias are formed, an additional plane of conductive material is disposed on the insulating layer, and is selectively etched to form a second set of conductive lines. The second conductive lines are electrically connected to one end of the vias, and the first conductive lines are connected to the other end of the vias. This technique is also used to connect internal vias to external die bond pads. Accordingly, the lines and vias can connect electrical structures spaced laterally apart from each other and/or positioned on different planes of the die.  
      One drawback associated with the foregoing technique for forming vias in the die is that the technique can be time consuming. Another drawback is that the holes in the vias can provide sites from which cracks can propagate through the die. These cracks can damage other structures (e.g., the microelectronic features  12 ) within the die. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a partially schematic, side elevational view of a workpiece being diced in accordance with a prior art method.  
       FIG. 2  is a partially schematic, cross-sectional view of a microfeature workpiece having a porous region located between die portions, in accordance with an embodiment of the invention.  
       FIG. 3  is a partially schematic, cross-sectional view of a microfeature workpiece having a porous region sized in accordance with another embodiment of the invention.  
       FIG. 4  is a partially schematic, side elevation view of a microfeature workpiece having a porous region positioned below test circuitry in accordance with another embodiment of the invention.  
       FIGS. 5A and 58  illustrate a method for implanting ions during formation of a porous region between die portions of a microfeature workpiece in accordance with an embodiment of the invention.  
       FIGS. 6A-6F  illustrate methods for forming porous material in accordance with further embodiments of the invention.  
       FIG. 7  is a partially schematic isometric illustration of a singulated die having porous edge surfaces in accordance with an embodiment of the invention.  
       FIGS. 8A-8D  illustrate a process for forming porous regions in a microfeature workpiece in accordance with an embodiment of the invention.  
       FIGS. 9A-9C  illustrate a process for disposing a conductive material in porous regions of a microfeature workpiece, in accordance with an embodiment of the invention.  
       FIGS. 9D-9E  illustrate microfeature dies having conductive paths that include porous regions filled with conductive material, in accordance with another embodiment of the invention.  
       FIGS. 10A-10F  illustrate a process for forming a conductive path in a microfeature workpiece accordance with another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
      The present invention relates generally to microfeature dies with porous regions, and associated methods and systems. A method for separating a microfeature die in accordance with one aspect of the invention includes forming a porous region between a die and a remainder portion of a microfeature workpiece. The method can further include separating the die from the remainder portion by removing at least a portion of the porous region. For example, at least a portion of the porous region can be removed by making a cut at the porous region, e.g., with a rotating saw blade. In other aspects of the invention, material can be removed using an etching process, or by directing a liquid jet or other ablative stream at the porous region. The microfeature workpiece can have a first surface and a second surface facing away from the first surface, with the porous region extending from the first surface to the second surface, or only part of the distance between the first and second surfaces.  
      In further particular aspects of the invention, the porous region can be formed by applying an electrical current to the microfeature workpiece in the presence of an electrolyte. For example, the porous region can be formed by positioning a first electrode proximate to and spaced apart from the first surface of the workpiece, disposing an electrolyte between the first electrode and the first surface, and connecting a second electrode directly to the second surface of the workpiece. The method can further include moving at least one of the first electrode and the microfeature workpiece relative to the other while passing a current between the first and second electrodes via the workpiece and the electrolyte.  
      Aspects of the invention are also directed to a microfeature die. In one aspect of the invention, the microfeature die can include a microfeature workpiece material having a first surface, a second surface facing generally away from the first surface, and an edge surface between the first and the second surfaces. At least part of the edge surface can be porous. The die can further include at least one microelectronic element carried by the microfeature workpiece material.  
      In further particular aspects of the invention, the edge surface can include a semiconductor material (e.g., silicon), and the porous part of the edge surface can include a porous semiconductor material. At least one microelectronic element can be positioned a first distance from the first surface of the microfeature workpiece material, and the porous part of the edge can extend a second distance from the first surface of the microfeature workpiece, with the second distance being at least as great as the first distance. In still further particular aspects of the invention, the at least one microelectronic element can include at least a portion of a memory circuit, for example, a dynamic random access memory circuit.  
      Other aspects of the invention are directed toward a method for forming a conductive path in a microfeature workpiece. The method can include forming a porous region in the microfeature workpiece, with the porous region being elongated along an axis. The method can further include disposing a conductive material in pores of the porous region, with the conductive material forming a conductive path between a first point along the axis and a second point along the axis. In yet further aspects of the invention, the conductive material can be insulated from adjacent portions of the microfeature workpiece, for example, by oxidizing surfaces of pores in the porous region. The conductive path can link portions of the workpiece, for example, bond pads located at one or more surfaces of the workpiece.  
      A microfeature system in accordance with another aspect of the invention includes a microfeature workpiece that has a substrate material with a porous region elongated along an axis, and a conductive material disposed in pores of the porous region to form a conductive path aligned along the axis. The porous region can include a plurality of interconnected pores having interconnected porous surfaces, and the conductive path can include interconnected conductive path segments positioned in the pores. The microfeature workpiece can include a first surface and a second surface facing generally away from the first surface, and the porous region can include a first part extending generally parallel to the first surface and offset from the first surface, and second and third parts that extend generally transverse to the first surface. The first part can be connected to and extend between the second and third parts.  
      Still further aspects of the invention are directed to methods for processing a microfeature workpiece, and can include disposing ions at a target region of a microfeature workpiece, and disposing an electrolytic liquid in fluid communication with the microfeature workpiece. The method can further include positioning a first electrode in fluid communication with the microfeature workpiece via the electrolytic liquid, and positioning a second electrode in electrical communication with the microfeature workpiece. The method can still further include removing material from the target region to form pores by moving at least one of the first electrode and the microfeature workpiece relative to the other while passing an electrical current along an electrical path that includes the first and second electrodes, the microfeature workpiece, and the electrolytic liquid.  
      In other embodiments, the second electrode can also be positioned in fluid communication with the workpiece via the electrolytic liquid, and removing material from the target region need not include providing relative movement between the microfeature workpiece and one or more of the electrodes. The resulting porous regions formed in accordance with either of the foregoing methods can be used to aid in separating adjacent die portions, or can be filled or partially filled with conductive material to form a conductive path.  
      As used herein, the terms “microfeature workpiece” and “workpiece” refer to substrates on and/or in which microelectronic devices are integrally formed. Typical microelectronic devices include microelectronic circuits or components, thin-film recording heads, data storage elements, microfluidic devices and other products. Micromachines and micromechanical devices are included within this definition because they are manufactured using much of the same technology that is used in the fabrication of integrated circuits. The substrates can be semiconductive pieces (e.g., doped silicon wafers or gallium arsenide wafers), nonconductive pieces (e.g., various ceramic substrates) or conductive pieces. In some cases, the workpieces are generally round, and in other cases the workpieces have other shapes, including rectilinear shapes. Several embodiments of systems and methods for separating microfeature workpiece dies, forming porous regions in microfeature workpieces, and forming conductive structures in microfeature workpieces are described below. A person skilled in the relevant art will understand, however, that the invention may have additional embodiments, and that the invention may be practiced without several of the details of the embodiments described below with reference to  FIGS. 2-10F .  
      The following description describes porous regions and associated singulation techniques, generally with reference to  FIGS. 2-4 , and describes methods for forming the porous regions, generally with reference to  FIGS. 5A-6F .  FIG. 7  describes dies singulated using porous regions, and  FIGS. 8A-10E  describe workpieces having conductive structures formed using porous regions.  
       FIG. 2  illustrates a microfeature workpiece  210  having die portions  211  (shown as a first die portion  211   a  and a second die portion  211   b ) positioned on opposite sides of a scribe area  214 . The microfeature workpiece  210  can also include a porous region  260  that mitigates and/or eliminates the drawbacks associated with singulating or dicing the die portions described above with reference to  FIG. 1 . Much of the following description associated with  FIGS. 2-6F  focuses on removing the first die portion  211   a  from a remainder  220  of the microfeature workpiece  210  that includes the second die portion  211   b . The same process can be used to separate the second die portion  211   b  and other die portions from the microfeature workpiece  210 .  
      The microfeature workpiece  210  can have a first surface  218 , a second surface  219  facing away from the first surface  218 , and microelectronic features or elements  212  located at the die portions  211 , between the first and second surfaces  218 ,  219 . Sacrificial test circuitry  213  can be located in the scribe area  214  between the die portions  211 , and can include conductive material  217  (e.g., conductive lines, vias, and/or circuit elements). The test circuitry  213  can be used to perform diagnostic tests on the microfeature workpiece  210  before the die portions  211  are singulated. In a particular embodiment, the scribe area  214  between the die portions  211  is elongated to form a scribe line extending transverse to the plane of  FIG. 2 , and includes a recess  222  bounded by a recessed surface  223  that optionally includes grooves  215 . The porous region  260  can extend from the recessed surface  223 , adjacent to the test circuitry  213  to a backgrind plane  224 . Accordingly, the porous region  260  in this embodiment can be formed so as to have little or no impact on the efficacy of the test circuitry  213 . In another embodiment, the porous region  260  can be formed in the microfeature workpiece  210  only after the test circuitry  213  is no longer needed, for example, when forming the porous region  260  would adversely affect pre-existing test circuitry  213 . In a particular aspect of either embodiment, the die portions  211  (and/or the corresponding microelectronic features  212 ) can extend to a depth D 1  from the first surface  218 , and the porous region  260  can extend to a depth D 2  (greater than D 1 ) from the first surface  218 . As will be described below, this arrangement can reduce the likelihood for damaging the die portions  211  during singulation.  
      Prior to singulation, material is removed from the second surface  219  of the microfeature workpiece  210  until the backgrind plane  224  is exposed (e.g., using an existing backgrind process). During singulation, a rotating dicing blade  230  can be brought into contact with the microfeature workpiece  210  at the porous region  260 . At least one of the microfeature workpiece  210  and the rotating dicing blade  230  is moved relative to the other until the dicing blade  230  penetrates through the microfeature workpiece  210  to, or at least proximate to, the backgrind plane  224 . At this point, the first die portion  211   a  can be separated from the remainder  220  of the microfeature workpiece  210 . This process can be repeated for other die portions (e.g., the second die portion  211   b ) of the microfeature workpiece  210  until all the die portions  211  are singulated.  
      One feature of an arrangement described above with reference to  FIG. 2  is that the porous region  260  can be less dense than non-porous constituents of the microfeature workpiece  210 , and can extend to a depth D 2  beneath the first surface  218  that is greater than the depth D 1  to which the microelectronic features  212  extend. Because the porous region  260  has a reduced density, it can more readily absorb stresses placed upon it by the dicing blade  230 . For example, if conductive material  217  from the test circuitry  213  adheres to the dicing blade  230  during singulation (effectively widening the kerf of the dicing blade  230 ), the porous region  260  can collapse or otherwise fail in a lateral direction (indicated by arrows A). Accordingly, the porous region  260  can absorb the lateral stresses introduced by the dicing blade  230  and can reduce or eliminate cracks that might otherwise propagate into the microelectronic features  212 .  
      A further advantage of an arrangement described above with reference to  FIG. 2  is that the dicing blade  230  can cut through the reduced density porous region  260  more quickly than it can cut through a conventional, non-porous scribe area. Accordingly, the time required to singulate the die portions  211  can be reduced, thereby increasing the rate at which a manufacturer can produce finished microelectronic products.  
      Another feature of an arrangement described above with reference to  FIG. 2  is that a width W 2  of the scribe area  214  and the porous region  260  can be less than the width W 1  of an existing scribe area  14  ( FIG. 1 ). The width W 2  (which can be on the order of 100 microns or less) may be reduced because the porous region  214  more effectively absorbs lateral stresses introduced by the dicing blade  230  than does the existing, non-porous scribe area  14 . An advantage of this feature is that it can increase the area of the microfeature workpiece  210  available for forming die portions  211 , thereby increasing the yield associated with each workpiece  210 .  
       FIGS. 3 and 4  illustrate microfeature workpieces having porous regions arranged differently than the porous region described above with reference to  FIG. 2 , and also illustrate different techniques for removing material from the porous regions. For purposes of illustration, the techniques for removing material from the porous regions are described in the context of particular porous region arrangements. It will be understood by those of ordinary skill in the relevant art that some or all of the techniques described herein with reference to  FIGS. 2-4  for removing material from porous regions can be used with some or all of the porous regions described and shown herein.  
      Referring now to  FIG. 3 , the microfeature workpiece  210  can include die portions  211   a ,  211   b  spaced apart by a scribe area  314  that is not recessed from the first surface  218 . In a further aspect of this embodiment, the microfeature workpiece  210  can include a porous region  360  that extends entirely through the microfeature workpiece  210  from the first surface  218  to the second surface  219 . Because the porous region  360  extends entirely through the microfeature workpiece  210 , the microfeature workpiece  210  need not be thinned by removing material from the second surface  219  up to a backgrind plane  224  ( FIG. 2 ). Instead, material can be removed from the porous region  360  in a single operation to singulate the first die portion  21  la from the remainder  220  of the microfeature workpiece  210 .  
      In a particular aspect of an embodiment shown in  FIG. 3 , material can be removed from the porous region  360  with a jet  332  directed toward the microfeature workpiece  210  through a high pressure nozzle  331 . The high pressure nozzle  331  can be scanned over the surface of the microfeature workpiece  210  (and/or the microfeature workpiece  210  can be scanned relative to the high pressure nozzle  331 ) to remove material from the porous region  360  and singulate the first die portion  211   a  from the remainder  220  of the microfeature workpiece  210 . The jet  332  can include one or more liquids (e.g., water and/or an etchant), one or more solids (e.g., sand, dry ice particles, or another particulate), and/or other ablative agents. In another embodiment, the first die portion  211   a  can be singulated with a laser. As described above with reference to  FIG. 2 , the porous region  360  can absorb lateral stresses introduced by the jet  332  or other dicing agent, and can accordingly reduce and/or eliminate the formation of cracks in or near the microelectronic features  212 . The porous region  360  can also have a reduced width when compared with conventional scribe areas, allowing the die portions  211   a ,  211   b  to be positioned more closely together.  
       FIG. 4  illustrates a microfeature workpiece  210  having a porous region  460  positioned below the test circuitry  213 . Accordingly, the porous region  460  can be formed in the microfeature workpiece  210  prior to forming the test circuitry  213 . This arrangement can be used when forming the porous region  460  may have an adverse effect on pre-existing test circuitry. In particular, it may be desirable in some cases to form the porous region  460  using a high-temperature process, which may damage pre-existing test circuitry, as well as pre-existing microelectronic features. By forming the porous region  460  prior to forming the test circuitry  213  and the microelectronic features  212 , the high temperature process may be conducted without damaging these structures. Alternatively, when it is desirable to form the porous region after forming the test circuitry  213  and/or the microelectronic features  212  (as described above with reference to  FIGS. 2 and 3 ), the porous region can be formed using an equally effective (though perhaps slower) low-temperature process.  
      In another aspect of an embodiment shown in  FIG. 4 , material can be removed from the porous region  460  using an etchant  433 . The etchant  433  can be disposed on the microfeature workpiece  210 , optionally with appropriate masking  434  over the die portions  211   a ,  211   b , to remove material from the scribe area  214 , including the porous region  460 . If it is too time consuming to remove the non-porous material  425  located in the scribe area  214  over the porous region  460 , another technique (e.g., the dicing blade  230  or the water jet  332  described above with reference to  FIGS. 2 and 3 , respectively) can be used to singulate the die portions  211 . Conversely, the etchant  433  shown in  FIG. 4  can be used to remove material from the porous regions  221  and  321  ( FIGS. 2 and 3 , respectively), which have exposed porous surfaces that are directly accessible to the etchant  433 . In any of these embodiments, the etchant  433  can include hydrofluoric acid or another substance that includes fluorine and/or a fluorine compound. In still further embodiments, the etchant  433  can include other constituents in addition to or in lieu of fluorine and/or fluorine compounds.  
       FIGS. 5A and 5B  illustrate initial steps for forming a porous region in a microfeature workpiece  210 . Referring first to  FIG. 5A , the portions of the microfeature workpiece  210  that are to remain non-porous (e.g., the microelectronic features  212 ), can be protected with a mask  534 . An ion beam  540  can then be directed toward the microfeature workpiece  210  to implant ions  541  in the microfeature workpiece  210  (e.g., at the scribe area  214 ). As shown in  FIG. 5B , the ions  541  can be implanted generally directly beneath an opening  535  in the mask  534 . The depth to which the ions  541  are implanted can be controlled by controlling the strength of the ion beam  540  and/or the length of time during which the microfeature workpiece  210  is exposed to the ion beam  540 , in a manner consistent with techniques generally known to those of ordinary skill in the relevant art. Once the ions  541  are implanted, microfeature workpiece  210  can be annealed by applying heat H to the workpiece  210  and causing the implanted ions  541  to substitute for atoms (e.g., silicon atoms) in the existing lattice structure of the microfeature workpiece  210 . The implanted ions  541  can include boron, arsenic, phosphorous or other elements. The ion concentration and temperature at which the workpiece  210  is annealed can determine or at least influence the porosity of the resulting porous region, as can additional factors described below with reference to  FIGS. 6A-6F .  
       FIGS. 6A-6F  illustrate techniques for electrolytically forming the porous regions in the microfeature workpiece  210 . Beginning with  FIG. 6A , the microfeature workpiece  210  having implanted ions can be positioned in an electrolytic cell or chamber  655  of an electrolytic system  650   a . The electrolytic cell  655  can be filled with an electrolyte  654  (e.g., alcohol and hydrofluoric acid) and can include a first electrode  651   a  spaced apart from a second electrode  652   a , with the microfeature workpiece  210  positioned between the two electrodes  651   a ,  652   a . The electrodes  651   a ,  652   a  can be coupled to a potential source  653  (e.g., a source of varying current, including an AC power source). When the potential source  653  drives the first electrode  651   a  and the second electrode  652   a  to different electrical potentials, the resultant current flowing through the electrolyte  654  and the microfeature workpieces  210  removes material from the microfeature workpieces  210  at the sites of the implanted ions, causing the implanted region to become porous. The removed material can include both the implanted ions and (optionally) at least some surrounding workpiece material. The sizes of the pores, and therefore the overall porosity of the porous regions can be controlled by controlling factors that include the current level, current density, time of exposure and/or electrical potential of the electrical signal passing through the electrolyte  654 . The porosity can also be controlled by the type of ion implanted in the microfeature workpieces  210 , the composition of the electrolyte  654 , and/or other factors that are commonly selected for electrolytic processing.  
       FIG. 6B  illustrates an electrolytic system  650   b  configured in accordance with another embodiment of the invention in which a first electrode  651   b  is spaced apart from a microfeature workpieces  210 , and a second electrode  652   b  has a fixed position in the electrolytic cell  655 . Alternatively, the second electrode  652   b  can be replaced with a plurality of second electrodes  652   b  (shown in phantom lines) to form a “bed-of-nails” arrangement. In either embodiment, the first electrode  651   b  can scan in two orthogonal directions, one of which is indicated by arrow B and the other of which is transverse to the plane of  FIG. 6B . Accordingly, the current provided by the potential source  653  can be precisely directed to specific portions of the microfeature workpieces  210 , if desired. This arrangement can also provide for locally high current densities over portions of, or the entirety of, the microfeature workpieces  210 , without providing a high current density throughout the entire electrolytic cell  655 .  
       FIG. 6C  illustrates an electrolytic system  650   c  having a first electrode  651   c  positioned proximate to the first surface  218  of the microfeature workpiece  210 , and a second electrode  652   c  attached directly to the second surface  219  of the microfeature workpiece  210 . Accordingly, the second surface  219  can have a conductive layer that provides a current path between the second electrode  652   c  and the interior portion of the microfeature workpiece  210 . The microfeature workpiece  210  can be supported on a rotating chuck  657 . Both electrodes  651   c ,  652   c  can be coupled to opposing poles of the potential source  653 , and the electrolyte  654  can be supplied to the first surface  218  via an electrolytic fluid supply line  656 . While the microfeature workpiece  210  is rotated (as indicated by arrow C), the first electrode  651   c  can be moved radially relative to the first surface  218  (as indicated by arrow D) to scan the first electrode  651   c  over the first surface  218 . Accordingly, the two electrodes  651   c ,  652   c  can electrolytically form porous regions in the microfeature workpiece  210 . In one aspect of this embodiment, this arrangement can be used with a microfeature workpiece  210  having implanted regions or other conductive structures that extend entirely through the microfeature workpiece  210  from the first surface  218  to the second surface  219 . Accordingly, the first electrode  651   c  can provide direct electrical communication with these structures by direct electrical contact with the second surface  219 .  
       FIG. 6D  illustrates an electrolytic system  650 d having a first electrode  651   d  and a second electrode  652   d , both of which are spaced apart from the microfeature workpiece  210 . In one aspect of this embodiment, both the first electrode  651   d  and the second electrode  652   d  can be positioned proximate to the first surface  218  of the microfeature workpieces  210  and scanned (as indicated by arrow D) while the microfeature workpiece rotates (as indicated by arrow C). This arrangement can be used with microfeature workpieces  210  having conductive structures and/or ion implanted regions that do not extend entirely through the workpiece to the second surface  219 .  
       FIG. 6E  illustrates an electrolytic system  650   e  having a first electrode  651   e  positioned proximate to and spaced apart from the first surface  218  of the microfeature workpiece  210 , and a second electrode  652   e  positioned proximate to and spaced apart from the second surface  219 . The first and second electrodes  651   e ,  652   e  can be scanned together as indicated by arrow D while the microfeature workpiece  210  rotates, as indicated by arrow C. This arrangement can be employed with microfeature workpieces  210  having conductive structures and/or ion implanted regions that extend from or proximate to the first surface  218  to or proximate to the second surface  219 .  
       FIG. 6F  illustrates still another electrolytic system  650   f  having a “bed of nails” arrangement for which electrodes can be selectively powered or unpowered, and can be selectively coupled to one pole or the other of the potential source  653 . The system  650   f  can include a plurality of first electrodes  651   f , a plurality of second electrodes  652   f , and a controller  658  (e.g., a multiplexer) coupled between the electrodes  651   f ,  652   f  and the potential source  653 . An operator (or an automated program) can direct the controller  658  to couple members of selected pairs of electrodes  651   f ,  652   f  to opposite poles of the potential source  653  for selectively forming porous regions in the workpiece  210 . Accordingly, the array of electrodes  651   f ,  652   f  can extend transverse to the plane of  FIG. 6F  to cover some or all of the workpiece  210 . This arrangement can allow the electrolytic system  650   f  to handle workpieces  210  having a wide variety of patterns of porous regions. The electrodes  651   f ,  652   f  can be positioned adjacent to the first surface  218  of the workpiece  210 , and/or the second surface  219 .  
      Once the pores are formed in the microfeature workpieces  210  (using, for example, any of the methods described with reference to  FIGS. 6A-6F ), the microfeature workpieces  210  can be cleaned and dried. The surfaces of the porous regions within the microfeature workpieces  210  can optionally be oxidized, using a high temperature or low temperature oxidation process in which the microfeature workpieces  210  are exposed to oxygen, water and/or other oxidizing agents. One advantage of oxidizing the surfaces of the pores is that it can make some dicing processes (e.g., those that employ a wet etch) more effective because the etchant can be chosen to selectively remove oxides. Even when an etching process is not used for dicing, the oxide can form an amorphous layer that resists crack propagation. After the pores have been oxidized, the dies adjacent to the porous regions can be singulated, using any of the processes described above with reference to  FIGS. 2-4 . Once the dies have been singulated in accordance with any of the foregoing methods, they can be encapsulated and/or otherwise packaged and provided with external contacts (e.g., pins and/or bond pads) in accordance with conventional methods.  
       FIG. 7  illustrates a singulated die  711  having a first surface  718  and a second surface  719  facing opposite from the first surface  718 . The die  711  further includes edge surfaces  726  positioned between the first surface  718  and the second surface  719 . The edge surfaces  726  can include residual porous material  727 , which remains after the singulation processes described above are completed. In one aspect of this embodiment, the residual porous material  727  can extend over the entire distance between the first surface  718  and the second surface  719 , for example, when the porous region extends from the first surface  218  to the second surface  219  of the corresponding microfeature workpiece  210  (e.g., as shown in  FIG. 3 ), and/or when any non-porous regions adjacent to the porous regions are removed (e.g., by backgrinding, as shown in  FIG. 2 ). In other embodiments, the residual porous material  727  can extend over only part of the distance between the first surface of  718  and the second surface  719  of the die  711  (e.g., as shown in  FIG. 4 ). The die  711  can be positioned on a support member  729  and encapsulated with an encapsulant  728 , which is partially cut away for purposes of illustration. The die  711  can include a memory device (e.g., a dynamic random access memory (DRAM) device) having multiple memory circuits. In other embodiments, the die  711  can include other circuit structures performing other functions.  
      Whether or not the die  711  is singulated by removing porous material, the die can include conductive structures that are formed with porous material.  FIGS. 8A-8D  illustrate a process for forming porous regions in a microfeature workpiece, and  FIGS. 9A-9C  illustrate a process for forming conductive pathways in the porous regions, in accordance with embodiments of the invention. These techniques can be used in lieu of the existing etching and filling techniques, and can be less time consuming to implement than existing techniques. The resulting structures can be more robust than existing structures, which can in turn improve the reliability of the devices formed from the microfeature workpieces.  
       FIG. 8A  illustrates a portion of a microfeature workpiece  810  having a first surface  818  and a second surface  819  facing away from the first surface  818 . A mask  834  can be positioned adjacent to the first surface  818 , and can include mask apertures  835  located at selected regions of the microfeature workpiece  810  at which conductive structures are to be positioned. The masked microfeature workpiece  810  is then exposed to an ion beam  840  to implant ions within the workpiece  810 , between the first surface  818  and the second surface  819 .  
       FIG. 8B  illustrates the microfeature workpiece  810  after it has been exposed to the ion beam  840  for a selected period of time. Implanted ions  841  are now located beneath the first surface  818  of the workpiece  810 , and are aligned with the mask apertures  835  along corresponding axes E. In a particular embodiment, the implanted ions  841  can extend entirely through the microfeature workpiece  810  from the first surface  818  to the second surface  819 . In other embodiments (e.g., as described in greater detail below with reference  FIGS. 10A-10F ), the implanted ions can extend through less than the entire thickness of the microfeature workpiece  810 . In any of these embodiments, the implanted ions  841  can be inserted directly into the lattice structure of the microfeature workpiece  810 , as described above.  
       FIG. 8C  illustrates a process for forming pores from the implanted ions  841 . In one aspect of this process, an electrical charge is applied to the ions  841  via an electrolytic system  850  that is generally similar to the system  650   e  described above with reference to  FIG. 6E . The electrolytic system  850  can include a potential source  853  coupled to a first electrode  851  (located proximate to and spaced apart from the first surface  818  of the microfeature workpiece  810 ) and a second electrode  852  (located proximate to and spaced apart from the second surface  819  of the microfeature workpiece  810 ). An electrolyte  854  provides fluid and electrical communication between the microfeature workpiece  810  and the electrodes  851 ,  852 . As the electrodes  851 ,  852  are scanned over the microfeature workpiece  810 , the electrical current provided by the potential source  853  removes the ions located between the first and second surfaces  818 ,  819  and optionally removes at least some adjacent workpiece material (e.g., silicon) as well.  
      The foregoing electrolytic process forms porous regions  860 , shown in  FIG. 8D . Each of the porous regions  860  can include a plurality of pores  861  having pore walls  862 . The pores  861  are interconnected with each other so that each porous region  860  has a fluid path aligned with a corresponding one of the axes E.  
      In other embodiments, the porous regions  860  can be formed using techniques other than those described above with references to  FIGS. 8A-8D , for example, those described above with reference to  FIGS. 6A-6D  and  6 F. In any of these embodiments, it may be desirable to electrically isolate the interiors of the porous regions  860  from the rest of the microfeature workpiece  810 . One technique for performing this function is shown in  FIG. 9A . The microfeature workpiece  810  is exposed to a oxidizing agent  942 , which penetrates the pores  861 . The oxidizing agent  942  oxidizes the surfaces of the pore walls  862 , forming an oxide layer  963 , as shown in  FIG. 9B . Accordingly, each of the pores  861  can have a surface formed from part of the oxide layer  963 , effectively isolating the interior of each porous region  860  from adjacent non-porous regions  925  of the microfeature workpiece  810 .  
      In other embodiments, other techniques can be used to electrically isolate the porous regions  860  from the adjacent non-porous regions  925 . For example, a jacket or section of cladding can be inserted around the porous regions  860  to provide such isolation. In another embodiment, a liquid dielectric material can be introduced into the pores  861 . This method may be suitable when the pores  861  are relatively large, so that the dielectric material coats the walls of the pores  861  rather than entirely filling the pores  861 . In any of these embodiments, the interior regions of the pores  861  can be filled with a conductive material  973  to provide a conductive path  970  through each porous region  860 . For example, as shown in  FIG. 9B , the conductive material  973  can include a liquid that is positioned adjacent to the first surface  818  of the microfeature workpiece  810 . The liquid conductive material  973  can wick into the pores  861  of the porous regions  860 , under the influence of capillary forces to infiltrate most or all of the pores  861  as indicated by arrows F. In particular embodiments, the conductive material  973  is selected to include at least one of silver, copper, tungsten, aluminum, tin and lead.  
       FIG. 9C  illustrates the microfeature workpiece  810  after the conductive material  973  has been introduced into the porous regions  860 . The conductive material  973  forms corresponding conductive paths  970  at each of the porous regions  960 , with each conductive path  970  aligned along ones of the axes E. In the example shown in  FIG. 9C , the conductive paths  970  can include conductive vias  971  extending from the first surface  818  of the microfeature workpiece  810  to the second surface  819 . Each conductive path  970  can include a multitude of interconnected conductive path segments formed in the corresponding interconnected network of pores  861 . Each conductive via  971  as a whole can be electrically isolated from the rest of the microfeature workpiece  810  by the oxide layer  963  at the outer periphery of the corresponding porous region  860 . The diameter or width W 3  of each conductive via  971  can be selected based on factors that include the electrical current load expected to be carried by the vias  971 . For example, vias  971  carrying a relatively low amount of current can have a width W 3  of one micron or less.  
      In other embodiments, other techniques can be used to dispose the conductive material  973  in the porous regions  860 . For example, the conductive material  973  can be introduced into the pores  861  while in a gaseous state, e.g., using a vapor deposition process. In one embodiment, the pores  861  are exposed to phosphene gas, or a mixture containing phosphene gas (e.g., 95% nitrogen, 5% phosphene). The microfeature workpiece  810  is then elevated in temperature (e.g., to about 800° C.), causing the phosphene to adsorb to the surfaces of the pores  861 .  
       FIG. 9D  illustrates portions of two microfeature dies  911  (shown as a first die  911   a  and a second die  911   b ) having conductive features formed in accordance with an embodiment of the invention, and joined to provide electrical communication between the two dies  911 . The microfeature dies  911  can include memory chips in one embodiment, and can include other devices (e.g., processor chips) in other embodiments. The first die  911   a  can include first conductive vias  971   a  extending between the first surface  918  and the second surface  919  of the first die  911   a . The first conductive vias  971   a  can be coupled to conductive elements  980 , e.g., first upper bond pads  981   a  at the first surface  918 , and first lower bond pads  984   a  at the second surface  919 . Accordingly, the first bond pads  981   a ,  984   a  can be attached to corresponding end regions  974  of the first conductive vias  971   a . The second die  911   b  can include corresponding second conductive vias  971   b  electrically connected to corresponding second upper bond pads  981   b  and second lower bond pads  984   b.    
      The first die  911   a  can be joined to the second die  911   b  with solder or another conductive material connected between the first lower bond pads  984   a  of the first die  911   a  and the second upper bond pads  981   b  of the second die  911   b . For example, as shown in  FIG. 9D , a plurality of reflowed solder balls  982  can provide electrical and physical connections between the first die  911   a  and the second die  91 l b . In this manner, the dies  911   a ,  911   b  can be stacked one above the other and can communicate electrically with one another as a result of the electrical connections between the first vias  971   a  and the second vias  971   b . Each stack of dies can include two dies  911  (as shown in  FIG. 9D ) or more dies in other embodiments. In any of these embodiments, an advantage of this arrangement is that the stacked dies  911  can occupy less surface area of a substrate printed circuit board  929  or other support member upon which they are positioned.  
       FIG. 9E  illustrates a portion of another microfeature die  911   c  having conductive vias  971   c  extending between upper bond pads  981   c  and lower bond pads  984   c . A redistribution layer  983  can be connected between the upper bond pads  981   c  and outer layer bond pads  985 . Solder balls  982  can provide for electrical contact with external devices located proximate to the second surface  918  of the die  911   c , and the outer layer bond pads  985  can provide electrical contact with external devices located proximate to the first surface  918 . In other embodiments, the dies  911   a - 911   c  described above can have other arrangements.  
       FIGS. 10A-10F  illustrate a process for forming conductive structures that are buried in a microfeature workpiece  1010  in accordance with an embodiment of the invention. Referring first to  FIG. 10A , a first mask  1034   a  is positioned adjacent to a first surface  1018  of the workpiece  1010 . The first mask  1034   a  includes a first mask aperture  1035   a  aligned with a target location for a buried conductive structure. An ion beam  1040  is directed toward the microfeature workpiece  1010  to implant ions in the workpiece  1010 . The strength of the ion beam  1040  and the duration for which the workpiece  1010  is exposed to the ion beam  1040  can determine the maximum depth to which ions are implanted, and the extent to which the ions extend upwardly from the maximum depth toward the first surface  1018  of the workpiece.  
       FIG. 10B  illustrates the microfeature workpiece  1010  after first ions  1041   a  are implanted at a first implantation site  1043   a . In one aspect of this embodiment, the first implantation site  1043   a  can be elongated along an axis E 1  oriented generally parallel to the first surface  1018 , and in other embodiments, the first implantation site  1043   a  can have other orientations. In one embodiment, the first implantation site  1043   a  alone determines the extent of a corresponding conductive portion (described below with reference to  FIG. 10E ) and accordingly, the entire conductive portion can be buried beneath the first surface  1018 . The conductive portion can couple two or more buried conductive elements  1012  (e.g., operable microelectronic structures, shown schematically in  FIG. 10B ), that are already formed in the workpiece  210  at points along the axis E 1 .  
      In another embodiment, the conductive portion can couple other conductive portions and/or other conductive elements that are formed in a subsequent process. For example, as shown in  FIG. 10C , a second mask  1034   b  can be positioned proximate to the first surface  1018  of the microfeature workpiece  1010 , and can include second mask apertures  1035   b . In one aspect of this embodiment, the second mask apertures  1035   b  can be aligned with end regions of the first implantation site  1043   a . Accordingly, when the ion beam  1040  is directed toward the microfeature workpiece  1010  with the second mask  1034   b  in place, the first implantation site  1043   a  can be protected while second implanted ions  1041   b  are implanted at a second implantation site  1043   b  extending from the first surface  1018  to the first implantation site  1043   a . Third implanted ions  1041   c  can be simultaneously implanted at a third implantation site  1043   c  located toward the opposite end of the first implantation site  1043   a.    
       FIG. 10D  illustrates a process for activating the implanted ions  1041  using an electrolytic system  1050  that is generally similar to the system  650   d  described above with reference to  FIG. 6D . Accordingly, the electrolytic system  1050  can include a first electrode  1051  and a second electrode  1052  that are both positioned proximate to the first surface  1018 . The first and second electrodes  1051 ,  1052  can be moved relative to the microfeature workpiece  1010  (as indicated by arrow D) while providing electrical current to the microfeature workpiece  1010  via an electrolyte  1054 .  
      Referring now to  FIG. 10E , the electrical current can activate the implanted ions and form a porous region  1060  that includes a first porous portion  1064   a  at the first implantation site  1043   a , a second porous portion  1064   b  at the second implantation site  1043   b , and a third porous portion  1064   c  at the third implantation site  1043   c , the porous region  1060  can be electrically isolated from the surrounding, nonporous regions  1025  of the microfeature workpiece  1010  by forming an oxide layer in a manner generally similar to that described above with reference to  FIG. 9B .  
      As shown in  FIG. 10F , a conductive material  1053  can then be disposed in the porous region  1060  to form a conductive path  1070 . The conductive path  1070  can include a first conductive portion  1075   a  (e.g., a line), a second conductive portion  1075   b  (e.g., a via), and a third conductive portion  1075   c  (e.g., another via). Bond pads  1081   a ,  1081   b  can be attached to end regions  1074   a ,  1074   b  of the conductive portions  1075   c ,  1075   b , respectively. Accordingly, the bond pads  1074   a ,  1074   b  can provide for electrical communication between devices external to the workpiece  1010  and electrical features, devices and/or elements internal to the workpiece  1010 .  
      One feature of embodiments of the conductive paths described above with reference to  FIGS. 8A-10F  is that they can be formed in porous regions of an otherwise generally nonporous substrate material (e.g., crystal silicon). This is unlike existing conductive structures formed in microfeature workpieces, which typically include a hole formed in the substrate, generally without regard for the crystal structure of the substrate. One advantage of at least some of the embodiments described above is that the walls of the pores in the porous region can provide more structural support than is provided by a conventional hole. Accordingly, the microfeature workpiece can be less likely to fail than would be an existing workpiece having relatively large holes extending through portions of the workpiece to accommodate conductive vias or other structures. Another advantage of the foregoing features is that porous regions that are integrated with the existing lattice structure of the microfeature workpiece can be less likely to disrupt the existing crystal structure of the workpiece, and can accordingly be less likely to cause the workpiece to fail, structurally and/or electrically.  
      From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. For example, aspects described in the context of particular embodiments can be combined or eliminated in other embodiments. Accordingly, the invention is not limited except as by the appended claims.