Abstract:
A three dimensional integrated circuit and method for making the same. The three dimensional integrated circuit has a first and a second active circuit layers with a first metal layer and a second metal layer, respectively. The metal layers are connected by metal inside a buried via. The fabrication method includes etching a via in the first active circuit layer to expose the first metal layer without penetrating the first metal layer, depositing metal inside the via, the metal inside the via being in contact with the first metal layer, and bonding the second active circuit layer to the first active circuit layer using a metal bond that connects the metal inside the via to the second metal layer of the second active circuit layer.

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. §119  
       [0001]    This application claims the benefit of Provisional Application No. 60/766,526, filed Jan. 25, 2006, herein incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates generally to integrated circuits. More particularly, the invention relates to buried via technology for three dimensional integrated circuits. 
         [0004]    2. Description of Related Art 
         [0005]    Three dimensional integrated circuit (IC) technology provides powerful capability for increased IC functionality. The three dimensional IC technology utilizes a multi-layer of active circuitry stacked up one on top of the other. Each active layer may consist of several metal layers with a thickness of about 1 micron each, forming an electric interconnect network between active devices, such as transistors. 
         [0006]    To fully exploit three dimensional IC technology, high density vertical interconnects with conductive wiring between stacked active circuit layers is required.  FIG. 1  illustrates a prior art vertical interconnect, such as a through via  11 , between metal layers  13 ,  15 ,  17  and  19  for active circuitry. The size of through via  11  should be compatible with feature size of the underlying lateral process technology. 
         [0007]    Most approaches to three-dimensional IC technology rely on through via  11 . However, through vias  11  have several disadvantages. First, through vias  11  create an exclusion zone that interrupts the routing in all metal layers  13 ,  15 ,  17  and  19 , as shown in  FIG. 1 . Through vias  11  penetrate, not only through a wafer  21 , but also through the stacked metal layers  13 ,  15 ,  17  and  19  and interrupt the circuit routing. 
         [0008]    This creates exclusion area constraints that make combination with state of the art 2-dimensional circuit technologies difficult and inefficient. 
         [0009]    Second, routing streets in line with through via  11  are blocked by the through via  11 , as shown in  FIG. 2 . A conductive wiring  23  coated with an interlayer dielectric travels through the via  11 . This wiring  23  blocks the routing of streets  25  and  27  in line with the through via  11 . Hence, the routing streets are blocked in both dimensions, on all metal layers. 
         [0010]    Third, the top level metal routing street in line with the landing pad  33 , for example metal layer  19 , is blocked by the landing pad  33 , as shown in  FIG. 2 . The conductive wiring  23  travels through the via  11 , comes out of via  11 , and goes through at least the top metal layer  19  at the location of the landing pad  33 . 
         [0011]    Consequently, the landing pad  33  blocks the routing in streets  29  and  31  in line with the landing pad  33 . Hence, the routing streets are blocked on the top metal layer  19  at the landing pad  33  as well. Since the top metal layer  19  typically has the lowest electrical resistance of all metal layers in an integrated circuit process, it is used for power routing. Blocking this power routing layer is problematic. 
         [0012]    With an ever increasing demand for improved integrated circuits technology, there remains a need in the art for buried via technology in three dimensional integrated circuits that provides a high density vertical interconnect with minimal exclusion zones while maintaining compatibility with two dimensional processed integrated circuits. 
       SUMMARY OF THE INVENTION 
       [0013]    A three dimensional integrated circuit and method for making the same. In one embodiment, the three dimensional integrated circuit has a first, a second and a third active circuit layer. The first active circuit layer is deposited on a substrate wafer. The second active circuit layer is coupled to the first active circuit layer using conventional hybridization techniques. The second active circuit layer has a buried via and a first metal layer. The first metal layer is embedded in a first dielectric material in the second active circuit layer. The buried via is etched through the first dielectric material to expose the first metal layer. The buried via contains metal in contact with the first metal layer of the second active circuit layer. The third active circuit layer has a second metal layer. The second metal layer is embedded in a second dielectric material in the third active circuit layer. The second dielectric material has an opening that exposes the second metal layer of the third active circuit layer. The opening is aligned above the buried via of the second active circuit layer. The opening contains a metal bond that mechanically couples the third active circuit layer to the second active circuit layer and electrically couples the first metal layer of the second active circuit layer to the second metal layer of the third active circuit layer. 
         [0014]    According to an embodiment of the invention, the fabrication method for the three dimensional integrated circuit includes placing a first active circuit layer on a first substrate and a second active circuit layer on a second substrate, the first active circuit layer having a first metal layer embedded in a first dielectric material of the first active circuit layer, the second active circuit layer having a second metal layer embedded in a second dielectric material of the second active circuit layer, hybridizing the first active circuit layer to a handling wafer, etching the first substrate to uncover the first dielectric material of the first active circuit layer, etching a via through the first dielectric material to uncover the first metal layer in the first active circuit layer, depositing metal inside the via, the metal inside the via being in contact with the first metal layer, etching an opening in the second dielectric material to uncover the second metal layer in the second active circuit layer, aligning the opening in the second active circuit layer with the via of the first active circuit layer, and hybridizing the second active circuit layer to the first active circuit layer using a metal bond that connects the metal inside the via to the second metal layer of the second active circuit layer. 
         [0015]    In another embodiment, the three dimensional integrated circuit has an active circuit layer of non-separated dies on an entire wafer, a first known good die and a second known good die. The active circuit layer is deposited on a substrate wafer. The first known good die has a buried via, a buried oxide layer and a first metal layer. The first metal layer is embedded in a first dielectric material of the first known good die. A via hole is etched through the buried oxide layer, the semiconductor substrate layer and the first dielectric material to expose the first metal layer without penetrating it. The buried via contains metal in contact with the first metal layer. The first known good die is coupled to the active circuit layer in a hybridization step. In this hybridization, electrical connections are made between the first metal layer of the first known good die and the active circuit layer wafer. There is one good die coupled to every non-separated die on the active circuit layer wafer. All known good dies on the active circuit layer wafer are then thinned using the buried oxide layer as an etch stop. The second known good die is then coupled to the first known good die. The second known good die has a second metal layer. The second metal layer is embedded in a second dielectric material of the second known good die. The second dielectric material has an opening that exposes the second metal layer. The opening is aligned above the buried via of the first known good die. The opening contains a metal bond that mechanically couples the second known good die to the first known good die and electrically couples the first metal layer of the first known good die to the second metal layer of the second known good die. 
         [0016]    According to an embodiment of the invention, the fabrication method for the three dimensional integrated circuit includes etching a via hole in the first known good die to expose the first metal layer without penetrating it, depositing metal inside the via hole, the metal inside the via hole being in contact with the first metal layer, and bonding the second known good die to the first known good die using a metal bond that connects the metal inside the via hole to the second metal layer of the second known good die. The metal bond may be indium, gold or solder. The via hole is about 5 μm deep with an aspect ratio less than or equal to 20. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The exact nature of this invention, as well as the objects and advantages thereof, will become readily apparent from consideration of the following specification in conjunction with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein: 
           [0018]      FIG. 1  is a cross sectional view of a prior art three dimensional integrated circuit with a through via. 
           [0019]      FIG. 2  is a top view of the prior art three dimensional integrated circuit of  FIG. 1 , illustrating an interruption of routing streets by a landing pad and the through via. 
           [0020]      FIG. 3  is a cross sectional view of a three dimensional integrated circuit with a buried via at the wafer level, according to an embodiment of the invention. 
           [0021]      FIG. 4-10  are graphic illustrations of the fabrication steps for buried interconnect vias at the wafer level, according to an embodiment of the invention. 
           [0022]      FIG. 11  is a cross sectional view of a three dimensional integrated circuit with a buried via using know good dies on top of an active circuit layer wafer, according to an embodiment of the invention. 
           [0023]      FIGS. 12-21  are graphic illustrations of the fabrication steps for buried interconnect vias at the die level, according to an embodiment of the invention. 
           [0024]      FIG. 22  is a flow chart illustrating the fabrication steps for buried interconnect vias at the wafer level, according to an embodiment of the invention. 
           [0025]      FIG. 23  is a flow chart illustrating the fabrication steps for buried interconnect vias at the die level, according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    An apparatus and method is provided herein for a buried via in three dimensional integrated circuits. The buried via does not interrupt the routing at all metal layers and avoids full penetration of the wafer. The buried via may be used to interconnect between metal layers on adjacent active circuit layers from separate dies or wafers with no or less exclusion area constraints as compared to a through via interconnect. As a result, the apparatus and method provided herein allow improved electrical interconnects of three dimensional integrated circuits with state of the art IC technologies. 
         [0027]      FIG. 3  is a cross sectional view of a three dimensional integrated circuit  35  with a buried via  59 , according to an embodiment of the invention. The three dimensional integrated circuit  35  has three layers: a first active circuit layer  36 , a second active circuit layer  37  and a third active circuit layer  38 . A bond material  56 , such as indium, gold or solder, may be used to couple the first active circuit layer  36  to the second active circuit layer  37  and/or the second active circuit layer  37  to the third active circuit layer  38 . An under fill  57  may be used to enhance mechanical strength between the layers  36 ,  37  or  38 . 
         [0028]    Each active circuit layer  36 ,  37  or  38  may have one or more metal layers  13 ,  15 ,  17  or  19  to create lateral electrical interconnects between circuit components. Preferably, a dielectric layer  39 , such as silicon dioxide, is used as an electrical insulator to surround metal layers  13 ,  15 ,  17  and/or  19 . 
         [0029]    The second active circuit layer  37  also includes a buried oxide layer  51 , a semiconductor substrate layer  40 , and a buried via  59 . The second active circuit layer  37  is processed using a wafer with a buried oxide layer  51  as used, for example, in Silicon On Insulator (SOI) CMOS process technologies. The buried oxide layer  51  serves as an etch stop when removing the underlying wafer material. Preferably, the second active circuit layer  37  has a thickness of about 10 micron. The buried via  59  is processed by etching through the buried oxide layer  51 , the thin substrate layer  40  and the dielectric layer  39 . The buried via  59  connects metal layer  13  of layer  37  to metal layer  19  of layer  38 . The buried oxide layer  51  is preferably about 1 μm thick. The semiconductor substrate layer  40  may be a silicon layer of about 5 μm thickness. 
         [0030]    To manufacture three dimensional integrated circuits  35 , a plurality of layers  36 ,  37  and  38  of active circuitry are stacked up one on top of the other.  FIGS. 4-10  are graphic illustrations of the fabrication steps for buried interconnect vias at the wafer level, according to an embodiment of the invention.  FIG. 22  is a flow chart illustrating the fabrication steps for buried interconnect vias at the wafer level. 
         [0031]    Referring to  FIGS. 4 and 22 , the first active circuit layer  36  may be deposited and/or grown on one side  48  of a first wafer  45  by processes well known in the art ( 101 ). Preferably, the first active circuit layer  36  has one or more metal layers  13 ,  15 ,  17  or  19  for lateral interconnects between active circuitry. Each metal layer  13 ,  15 ,  17  or  19  has a thickness of about 1 μm. A dielectric layer  39 , such as silicon dioxide, may be deposited between metal layers  13 ,  15 ,  17  and/or  19 . The first active circuit layer  36  may have an opening  60  in the dielectric layer  39  to expose metal layer  19 . The first wafer  45  may be about 200 to 400 μm thick, while the first active circuit layer  36  may be about 5 μm thick. 
         [0032]    The second active circuit layer  37  may be deposited and/or grown on one side  50  of a second wafer  49  ( 103 ), as shown in  FIG. 5 . Preferably, the second active circuit layer  37  has one or more metal layers  13 ,  15 ,  17  or  19  for lateral interconnects between active circuitry. Each metal layer  13 ,  15 ,  17  or  19  has a thickness of about 1 μm. Dielectric layer  39  may be deposited between metal layers  13 ,  15 ,  17  and/or  19 . The second active circuit layer  37  may have an opening  60  in the dielectric layer  39  to expose metal layer  19 . The second active circuit layer  37  may also include the substrate layer  40  and the buried oxide layer  51 . The substrate layer  40  may be a silicon layer of about 5 μm thick. The buried oxide layer  51  may be silicon dioxide of about 1 μm thick. The buried oxide layer  51  may be grown on the second wafer  49 . The substrate layer  40  and metal layers  13 ,  15 ,  17  or  19  are then deposited on top of the buried oxide layer  51 . The second wafer  49  may have a thickness of about 200 to 400 μm. 
         [0033]    The third active circuit layer  38  may be deposited and/or grown on one side  50  of a third wafer  52  ( 105 ), as shown in  FIG. 6 . Preferably, the third active circuit layer  38  has one or more metal layers  13 ,  15 ,  17  or  19  for lateral interconnects between active circuitry. Each metal layer  13 ,  15 ,  17  or  19  has a thickness of about 1 μm. Buried oxide layer  51  may be grown on the third wafer  50 . Metal layers  13 ,  15 ,  17  or  19  are then deposited on top of the buried oxide layer  51 . Dielectric layer  39  may be deposited between the metal layers  13 ,  15 ,  17  and/or  19 . The third active circuit layer  38  may have an opening  60  in the dielectric layer  39  to expose metal layer  19  ( 115 ). The third wafer  52  may be about 200 to 400 μm thick, while the third active circuit layer  38  may be about 5 μm thick. 
         [0034]    Metal layers  13 ,  15 ,  17  and/or  19  have a predetermined shape and size. Depending on design characteristics, the metal layers  13 ,  15 ,  17  and/or  19  may be selectively deposited in layers  36 ,  37  and/or  38 . A first stencil mask (not shown) may be used to selectively deposit a metal layer  13 ,  15 ,  17  or  19  using lithography techniques well known in the art. A dielectric  39 , such as silicon dioxide, may be deposited thereafter to embed the metal layer  13 ,  15 ,  17  or  19  within the layer  36 ,  37  or  38 . 
         [0035]    Referring to  FIG. 7 , the second wafer  49  may be inverted to allow the bonding of the second active circuit layer  37  to the first active circuit layer  36  ( 107 ). The bond material  56  may include indium, gold and/or solder. Under fill  57  may be used to enhance the bonding between the second active circuit layer  37  and the first active circuit layer  36 . 
         [0036]    Next, the second wafer  49  is removed in a controlled manner using, for example, a dry etch process, as shown in  FIG. 8 . To provide depth control, the buried oxide layer  51  may be used as an etch stop. When the second wafer  49  is removed, the buried oxide layer  51  becomes exposed ( 109 ). 
         [0037]    In  FIG. 9 , the buried via  59  may be formed, for example, by selectively etching through the buried oxide layer  51 , the substrate layer  40  and the dielectric layer  39  until metal layer  13  is exposed ( 111 ). The buried via  59  may be used to connect between two metal layers of different active circuit layers. The buried via  59  preferably has a depth of about 5 μm and an aspect ratio less than or equal to 20. According to an embodiment of the invention, the buried via  59  is filled with metal ( 113 ), as shown in  FIG. 10 . Since the buried via  59  is relatively shallow because of its low aspect ratio, metal deposition inside the buried via  59  is fairly simple and unrestrained by capillary forces or deposition shadowing effects. The metal inside buried via  59  is in contact with the metal layer  13  of layer  37 . While not shown, dielectric isolation layers may also be applied to electronically isolate the metal of via  59  from the substrate layer  40 . 
         [0038]    To add more layers to the three dimensional integrated circuit  35 , the fabrication steps illustrated in  FIGS. 7-10  may be used. For example, the third wafer  52  may be inverted to allow the bonding of the third active circuit layer  38  to the second active circuit layer  37  ( 117 ). Bond material  56  and under fill  57  may be used to enhance the bonding between the third active circuit layer  38  and the second active circuit layer  37 . The opening  60  in the third active circuit layer  38  may be aligned above the buried via  59  to expose metal layer  19  of the third active circuit layer  38  to the bond material  56 , as shown in  FIG. 3 . Since bond material  56  is coupled to the metal inside the buried via  59 , which is coupled to the metal layer  13  of layer  37 , the buried via  59  connects metal layer  13  of layer  37  to metal layer  19  of layer  38  ( 119 ). Finally, the third wafer  52  may be removed in a controlled manner using, for example, a dry etch process ( 121 ). To provide depth control, the buried oxide layer  51  may be used as an etch stop. Depending on design characteristics, more layers with or without buried vias  59  may be stacked on top of third active circuit layer  37  to construct the three dimensional integrated circuit  35  ( 123 ). 
         [0039]    In one embodiment, the first active circuit layer  36  is a sacrificial layer that provides mechanical support for the thin second active circuit layer  37 . The first active circuit layer  36  may be removed after the second active circuit layer  37  is hybridized to the third active circuit layer  38 . 
         [0040]    Since wafers may be defective, for example, due to manufacturing, the use of wafers in three dimensional integrated circuits provides an accumulation of yield loss. If one wafer, or parts of it, is not functioning, then a second wafer coupled to the first wafer, will also not function. To minimize yield loss, known good dies may be used in three dimensional integrated circuits. The dies are tested separately and known good ones are used. 
         [0041]      FIG. 11  is a cross sectional view of a three dimensional integrated circuit  61  with one or more buried vias  63 , according to an embodiment of the invention. The three dimensional integrated circuit  61  has three layers: a first active circuit layer  65 , a second active circuit layer  67  and a third active circuit layer  69 . The first active circuit layer  65  may be a handling wafer or handling circuit. The second active circuit layer  67  may have a plurality of known good dies  75  with buried via  63 . The third active circuit layer  69  may have a plurality of known good dies  77 . If another layer is stacked above the third active circuit layer  69 , then known good dies  77  may also include buried via  63 . 
         [0042]    Each active circuit layer  65 ,  67  or  69  may have one or more metal layers  13 ,  15 ,  17  or  19  for lateral interconnects between active circuitry with a thickness of about 1 μm. Preferably, a dielectric layer  79 , such as silicon dioxide, is used as an electrical insulator to surround metal layers  13 ,  15 ,  17  and/or  19 . 
         [0043]    The second active circuit layer  67  also includes a buried oxide layer  81 , a semiconductor substrate layer  83 , and a buried via  63 . The second active circuit layer  67  is processed using a wafer with a buried oxide layer  81  as used, for example, in Silicon On Insulator (SOI) CMOS process technologies. The buried oxide layer  81  serves as an etch stop when removing the underlying wafer material. Preferably, the second active circuit layer  67  has a thickness of about  10  micron. The buried via  63  is processed by etching through the buried oxide layer  81 , the substrate layer  83  and the dielectric layer  79 . The buried via  63  connects metal layer  13  of layer  67  to metal layer  19  of layer  69 . The buried oxide layer  81  is preferably about 1 μm thick. The substrate layer  83  may be a silicon layer of about 5 μm thickness. 
         [0044]    A bond material  71 , such as indium, gold or solder, may be used to couple the first active circuit layer  65  to the second active circuit layer  67  and/or the second active circuit layer  67  to the third active circuit layer  69 . An under fill  73  may be used to enhance the bonding between the layers  65 ,  67  or  69 . An etch protection fill  85  may be used to provide mechanical support to known good dies  75  on the second active circuit layer  67 . Similarly, the etch protection fill  85  may be used to provide mechanical support to known good dies  77  on the third active circuit layer  69 . 
         [0045]      FIGS. 12-21  are graphic illustrations of the fabrication steps for buried interconnect vias at the die level, according to an embodiment of the invention.  FIG. 23  is a flow chart illustrating the fabrication steps for buried interconnect vias at the die level. Referring to  FIG. 12 , the first active circuit layer  65  may be deposited and/or grown on one side  87  of a first wafer  89  by processes well known in the art ( 131 ). Preferably, the first active circuit layer  65  has one or more metal layers  13 ,  15 ,  17  or  19  for lateral interconnects between active circuitry. Each metal layer  13 ,  15 ,  17  or  19  has a thickness of about 1 μm. A dielectric layer  79 , such as silicon dioxide, may be deposited between metal layers  13 ,  15 ,  17  and/or  19 . The first active circuit layer  65  may have an opening  99  in the dielectric layer  79  to expose metal layer  19 . The first wafer  89  may be about 200 to 400 μm thick, while the first active circuit layer  65  may be about 5 μm thick. 
         [0046]    The second active circuit layer  67  may be deposited and/or grown on one side  91  of a second wafer  93  ( 133 ), as shown in  FIG. 13 . Preferably, the second active circuit layer  67  has one or more metal layers  13 ,  15 ,  17  or  19  for lateral interconnects between active circuitry. Each metal layer  13 ,  15 ,  17  or  19  has a thickness of about 1 μm. Dielectric layer  79  may be deposited between metal layers  13 ,  15 ,  17  and/or  19 . The second active circuit layer  67  may have an opening  99  in the dielectric layer  79  to expose metal layer  19 . The second active circuit layer  67  may also include the substrate layer  83  and the buried oxide layer  81 . The substrate layer  83  may be a silicon layer of about 5 μm thick. The buried oxide layer  81  may be silicon dioxide of about 1 μm thick. The buried oxide layer  81  may be grown on the second wafer  93 . The substrate layer  83  and metal layers  13 ,  15 ,  17  or  19  are then deposited on top of the buried oxide layer  81 . The second wafer  93  may have a thickness of about 200 to 400 μm. 
         [0047]    The third active circuit layer  69  may be deposited and/or grown on one side  95  of a third wafer  97  ( 135 ), as shown in  FIG. 14 . Preferably, the third active circuit layer  69  has one or more metal layers  13 ,  15 ,  17  or  19  for lateral interconnects between active circuitry. Each metal layer  13 ,  15 ,  17  or  19  has a thickness of about 1 μm. Buried oxide layer  81  may be grown on the third wafer  97 . Metal layers  13 ,  15 ,  17  or  19  are then deposited on top of the buried oxide layer  81 . Dielectric layer  79  may be deposited between the metal layers  13 ,  15 ,  17  and/or  19 . The third active circuit layer  69  may have an opening  99  in the dielectric layer  79  to expose metal layer  19  ( 137 ). The third wafer  97  may be about 200 to 400 μm thick, while the third active circuit layer  69  may be about 5 μm thick. 
         [0048]    Metal layers  13 ,  15 ,  17  and/or  19  have a predetermined shape and size. 
         [0049]    Depending on design characteristics, the metal layers  13 ,  15 ,  17  and/or  19  may be selectively deposited in layers  65 ,  67  and/or  69 . A first stencil mask (not shown) may be used to selectively deposit a metal layer  13 ,  15 ,  17  or  19  using lithography techniques well known in the art. A dielectric  79 , such as silicon dioxide, may be deposited thereafter to embed the metal layer  13 ,  15 ,  17  or  19  within the layer  65 ,  67  or  69 . 
         [0050]    To minimize yield loss for three dimensional integrated circuits  61 , known good dies  75  in the second active circuit layer  67  are used. The dies  75  are tested separately in the second active circuit layer  67  ( 139 ). The second active circuit layer  67  along with the second wafer  93  are then diced, as shown in  FIG. 15 , and the known good dies  75  are selected ( 141 ). 
         [0051]    Referring to  FIG. 16 , the known good dies  75  of second active circuit layer  67  may be inverted to allow the bonding of the second active circuit layer  67  to the first active circuit layer  65  ( 143 ). The bond material  71  may include indium, gold and/or solder. Under fill  73  may be used to enhance the bonding between the second active circuit layer  67  and the first active circuit layer  65  by countering the effect of mismatches in coefficient of thermal expansion between the layers  65  and  67  and the bond material  71 . 
         [0052]    Next, an etch protection fill  85 , such as a photo resist, epoxy, or other curable liquid, may be used to enhance the mechanical stability of known good dies  75  on the first active circuit layer  65  ( 145 ), as shown in  FIG. 17 . The gaps between known good dies  75  are filled in with the etch protection fill  85  to mechanically hold the dies  75  together. 
         [0053]    The second wafer  93  may then be removed in a controlled manner using, for example, a dry etch process ( 147 ), as shown in  FIG. 18 . To provide depth control, the buried oxide layer  81  may be used as an etch stop. When the second wafer  93  is removed, the buried oxide layer  81  and the etch protection fill  85  become exposed. The exposed etch protection fill  85  protruding from second active circuit layer  67  is then removed ( 149 ), as shown in  FIG. 19 . 
         [0054]    In  FIG. 20 , the buried via  63  may be formed, for example, by selectively etching or drilling through the buried oxide layer  81 , the substrate layer  83  and the dielectric layer  79  until metal layer  13  is exposed ( 151 ). The buried via  63  may be used to connect between two metal layers of different active circuit layers. The buried via  63  preferably has a depth of about 5 μm and an aspect ratio less than or equal to 20. According to an embodiment of the invention, the buried via  63  is filled with metal ( 153 ), as shown in  FIG. 21 . Since the buried via  63  is relatively shallow because of its low aspect ratio, metal deposition inside the buried via  63  is fairly simple and unrestrained by capillary forces or deposition shadowing. The metal inside buried via  63  is in contact with the metal layer  13  of layer  67 . While not shown, dielectric isolation layers may also be applied to electrically isolate the metal of via  63  from the substrate layer  83 . 
         [0055]    To add more layers to the three dimensional integrated circuit  61 , the fabrication steps illustrated in  FIGS. 15-19  may be used. For example, the third active circuit layer  69  is tested and known good dies  77  are identified ( 155 ). The third active circuit layer  69  along with the third wafer  97  is diced and the known good dies are selected ( 157 ). The known good dies  77  of the third active circuit layer  69  may be inverted to allow the bonding of the third active circuit layer  69  to the second active circuit layer  67  while exposing the third wafer  97  for dry etching. Bond material  71  and under fill  73  may be used to enhance the bonding between the third active circuit layer  69  and the second active circuit layer  67 . The opening  99  in the third active circuit layer  69  may be aligned above the buried via  63  to expose metal layer  19  of the third active circuit layer  69  to the bond material  71  ( 159 ), as shown in  FIG. 11 . Since bond material  71  is coupled to the metal inside the buried via  63 , which is coupled to the metal layer  13  of layer  67 , the buried via  63  connects metal layer  13  of layer  67  to metal layer  19  of layer  69  ( 161 ). 
         [0056]    Next, an etch protection fill  85 , such as a photo resist, epoxy, or other curable liquid, may be used in the hybridization process of known good dies  77  and the third wafer  97  ( 163 ). The gaps between the known good dies  77  are filled in with the etch protection fill  85  to mechanically hold the dies  77  together. The third wafer  97  may be removed in a controlled manner using, for example, a dry etch process ( 165 ). To provide depth control, the buried oxide layer  81  may be used as an etch stop. Finally, any exposed etch protection under fill  85  protruding from third active circuit layer  69  are removed ( 167 ). Depending on design characteristics, more known good dies with or without buried vias  63  may be stacked on top of third active circuit layer  69  to construct the three dimensional integrated circuit  61  ( 169 ). 
         [0057]    In one embodiment, the first active circuit layer  65  is a sacrificial layer that provides mechanical support for the thin second active circuit layer  67 . The first active circuit layer  65  may be removed after the second active circuit layer  67  is hybridized to the third active circuit layer  69 . 
         [0058]    While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other changes, combinations, omissions, modifications and substitutions, in addition to those set forth in the above paragraphs, are possible. Those skilled in the art will appreciate that various adaptations and modifications of the just described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.