Abstract:
Microfabricated inductors with through-wafer vias and including a first wafer and a second wafer, each wafer having a plurality of metal fillings therein, and a plurality of metal conductors connecting the plurality of metal fillings together to form a spiral. A method for producing an inductor including steps of forming a first plurality of vias in a first substrate, filling the first plurality of vias in the first substrate with a first plurality of metal fillings, forming a first plurality of metal conductors, connecting pairs of the first plurality of metal fillings together using the first plurality of metal conductors to form a spiral, performing the foregoing steps similarly on a second substrate formed with a second plurality of vias filled with a second plurality of metal fillings, and bonding the first substrate with the second substrate.

Description:
CLAIM OF BENEFIT UNDER 35 U.S.C. §§120 and 121 
     This application is a divisional of and claims the benefit and priority of U.S. patent application Ser. No. 12/398,942, entitled “MICROFABRICATED INDUCTORS WITH THROUGH-WAFER VIAS,” filed on Mar. 5, 2009. The entire disclosure of the parent application is incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     The present invention relates to microfabricated inductors with through-wafer vias. 
     2. Related Art 
     RF systems often use inductors. However, conventional inductors with an air center require a large amount of physical space and may also have a relatively low Q factor. This can place a limit or hamper the ability of reducing the size of RF systems. Furthermore, the relatively low Q factor can reduce the performance of RF systems at high frequencies. 
     Thus, there is a need for inductors which occupy a reduced amount of space and possess an improved Q factor. 
     SUMMARY 
     In one embodiment, the present invention is an inductor including a first wafer, a first plurality of metal fillings located within the first wafer, and a first plurality of metal conductors connecting the first plurality of metal fillings together to form a first spiral with a first plurality of windings. 
     In another embodiment, the present invention is an inductor including a first wafer, a first plurality of metal fillings located within the first wafer, a first plurality of metal conductors connecting pairs of the first plurality of metal fillings together to form a first portion of a first spiral. The present invention also includes a second wafer, a second plurality of metal fillings located within the second wafer, and a second plurality of metal conductors connecting pairs of the second plurality of metal fillings together to form a second portion of the first spiral. The first portion of the first spiral and the second portion of the first spiral can be bonded together to form the first spiral. 
     In yet another embodiment, the present invention is a method for producing an inductor including the steps of forming a first plurality of vias in a first substrate, filling the first plurality of vias in the first substrate with a first plurality of metal fillings, forming a first plurality of metal conductors, and connecting pairs of the first plurality of metal fillings together using the first plurality of metal conductors to form a spiral. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, wherein: 
         FIG. 1  is a perspective view of an embodiment of the present invention; 
         FIG. 2  is a top view of an embodiment of the present invention; 
         FIG. 3  is a graph of Q factors; 
         FIG. 4  is a side view of a production of an embodiment of the present invention; 
         FIG. 5  is a side view of a production of an embodiment of the present invention; 
         FIG. 6  is a side view of a production of an embodiment of the present invention; 
         FIG. 7  is a side view of a production of an embodiment of the present invention; 
         FIG. 8  is a side view of a production of an embodiment of the present invention; 
         FIG. 9  is a side view of a production of an embodiment of the present invention; 
         FIG. 10  is a side view of a production of an embodiment of the present invention; 
         FIG. 11  is a side view of a production of an embodiment of the present invention; 
         FIG. 12  is a side view of a production of an embodiment of the present invention; 
         FIG. 13  is a side view of a production of an embodiment of the present invention; 
         FIG. 14  is a side view of a production of an embodiment of the present invention; 
         FIG. 15  is a side view of a production of an embodiment of the present invention; 
         FIG. 16  is a side view of a production of an embodiment of the present invention; 
         FIG. 17  is a side view of a production of an embodiment of the present invention; 
         FIG. 18  is a side view of a production of an embodiment of the present invention; 
         FIG. 19  is a side view of a production of an embodiment of the present invention; 
         FIG. 20  is a side view of a production of an alternate embodiment of the present invention; 
         FIG. 21  is a side view of a production of an alternate embodiment of the present invention; 
         FIG. 22  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 23  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 24  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 25  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 26  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 27  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 28  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 29  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 30  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 31  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 32  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 33  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 34  a side view of a production of an alternate embodiment of the present invention; 
         FIG. 35  is a side view of an alternate embodiment of the present invention; 
         FIG. 36  is a top view of an alternate embodiment of the present invention; and 
         FIG. 37  is a top view of an alternate embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Methods and systems that implement the embodiments of the various features of the present invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the present invention and not to limit the scope of the present invention. Reference in the specification to “one embodiment” or “an embodiment” is intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least an embodiment of the present invention. The appearances of the phrase “in one embodiment” or “an embodiment” in various places in the specification are not necessarily all referring to the same embodiment. Throughout the drawings, reference numbers are re-used to indicate correspondence between referenced elements. 
       FIG. 1  is a perspective view of an embodiment of the present invention while  FIG. 2  is a top view of an embodiment of the present invention depicted in  FIG. 1 . As shown in  FIG. 1  and  FIG. 2 , an inductor  2  is formed within a substrate  4 . Substrate  4  can be formed, for example, from silicon (Si), insulators, a combination of silicon and insulators, and/or any other type of material suitable to form a core for an inductor. Substrate  4  can be a silicon wafer, or a silicon on insulator (SOI) wafer. Substrate  4  can, for example, be between approximately 100 μm to approximately 1000 μm thick. In one embodiment, substrate  4  is approximately 400 μm thick. 
     Inductor  2  includes a plurality of vias  6  which can be filled with metal fillings  8 . The metal fillings  8  can be connected by conductors  10 . Metal fillings  8  and conductors  10  can form windings and/or loops within substrate  4 . The metal fillings  8  can be, for example, copper (Cu), copper alloys, silver (Ag), silver alloys, or any other types of conductive metal. Conductor  10  can also be copper (Cu), copper alloys, silver (Ag), silver alloys, or any other types of conductive metal. In one embodiment, metal fillings  8  and conductor  10  are formed from the same type of conductive metal. In another embodiment, metal fillings  8  and conductor  10  are formed from different types of conductive metal. The conductors  10  can be formed on a surface of the substrate  4 . 
     A current can flow, for example, into metal filling  8   a , from metal filling  8   a  to conductor  10   a , from conductor  10   a  to metal filling  8   b , from metal filling  8   b  to conductor  10   b , from conductor  10   b  to metal filling  8   c , and so on such that the metal current traverses the loops formed by metal fillings  8  and conductors  10  within substrate  4 . 
     Since inductor  2  is formed within substrate  4 , inductor  2  can utilize less space. Inductor  2  can be 10 times smaller in each of three dimensions, thickness, width, and length when compared with a conventional inductor with an air center. Thus, inductor  2  can be approximately 1,000 times smaller than conventional inductors formed with wires, such as conventional inductors with air centers. 
       FIG. 3  is a graph of Q factors. The Q factor for inductors is governed by the equation 
             Q   =       ω   ⁢           ⁢   L     R           
where R is the inductor&#39;s internal electrical resistance and ωL, is the inductor&#39;s capacitive or inductive reactance at resonance.
 
     As seen in  FIG. 3 , although the Q of a silicon-core inductor may not be as high as that of an air-core inductor at high frequencies, the Q of the inductor of the present invention is comparable to the Q of an air-core inductor at frequencies less than approximately 500 MHz. The present invention may be particularly valuable for applications below 500 MHz where the present invention can be used without compromising the Q. As seen in  FIG. 3  at 0.1 GHz, the Q factor of the inductors with air centers can have a Q of approximately 10. In contrast, in one embodiment of the present invention, the Q factor of inductor  2  is 60 at 0.1 GHz as indicated by point  42 . In another embodiment of the present invention, the Q factor of inductor  2  is 180 at 0.1 GHz as indicated by point  44 . 
       FIG. 19  is a side view of an embodiment of the present invention.  FIGS. 4 to 18  are side views of a production of an embodiment of the present invention disclosed in  FIG. 19 . As shown in  FIG. 4 , vias  6  are etched in substrate  4 . Substrate  4  can have a first side and a second side. In  FIG. 4 , the first side is a top side of substrate  4  while the second side is a bottom side of substrate  4 . In  FIG. 5 , substrate  4  is coated with an insulation layer  12 . Insulation layer  12  can be, for example, Parylene, silicon oxide (SiO 2 ), or any other type of suitable insulation material. To coat insulation layer  12  on substrate  4 , substrate  4  can be thermally oxidized or insulation layer  12  can be coated by performing deep reactive ion etching using a time sequenced etch and passivation process, such as the Bosch process, on substrate  4 . 
     In  FIG. 6 , gold layer  14  is evaporated on the second side of substrate  4 . In  FIG. 7 , gold layer  14  is deposited on insulation layer  12  of a dummy wafer  16 . Gold layer  14  can be deposited on insulation layer  12  of dummy wafer  16  through evaporation. Dummy wafer  16  can be formed, for example, from silicon  4 . In  FIG. 8 , dummy wafer  16  is bonded to substrate  4  through gold layer  14  for both substrate  4  and dummy wafer  16 . In  FIG. 9 , a metal  26  is used to fill vias  6 . Metal  26  can be, for example, copper, silver, nickel or any other metal with a relatively high conductivity such as the metal used for metal fillings  8 . Furthermore, metal  26  can be filled in vias  6  by electroplating metal  26  through vias  6 . 
     In  FIG. 10 , photoresist  18  is placed over the first side of substrate  4  encompassing metal  26  and insulation layer  12 . Photoresist  18  provides protection for substrate  4  and the layers of materials between substrate  4  and photoresist  18  such as metal  26  and/or insulation layer  12 . In  FIG. 11 , dummy wafer  16  is removed along with photoresist  18 . The removal of photoresist  18  exposes metal  26  and insulation layer  12  on the first side of substrate  4 . Dummy wafer  16  can be removed by grinding and etching dummy wafer  16 . After dummy wafer  16  is removed, insulation layer  12  which was located on the second side of substrate  4  between dummy wafer  16  and gold layer  14  is also removed through etching. 
     In  FIG. 12 , metal  26  that juts above substrate  4  and insulation layer  12  on the first side of substrate  4  is removed through polishing. In  FIG. 13 , both gold layers  14  are removed. Also, metal  26  that juts above substrate  4  and insulation layer  12  on the second side of substrate  4  is also removed. In  FIG. 14 , seed layer  20  is deposited on both sides of substrate  4 . Seed layer  20  can be, for example, copper, gold (Au), or any other suitable material to form conductors  10 . In one embodiment, seed layer  20  can be between approximately 200 Å to approximately 3000 Å. In another embodiment, seed layer  20  is approximately 1000 Å. 
     In  FIG. 15 , photoresist  22  is layered on both sides of substrate  4 . The layer of photoresist  22  on a first side of substrate  4  is patterned with holes where conductors  10  are formed. In  FIG. 16 , metal  26  is deposited in the holes, created during the patterning of photoresist  22 , on the first side of substrate  4 . Metal  26  can be deposited in the holes created during the patterning of photoresist  22 , for example, through electroplating. In  FIG. 17 , the layer of photoresist  22  that was patterned in  FIG. 15  is overlayed with photoresist covering up the holes. The layer of photoresist  22  on the second side of substrate  4  is now patterned with holes where conductors  10  will be formed. In  FIG. 18 , metal  26  is filled in the holes of the layer of photoresist  22  on the second side of substrate  4 . In  FIG. 19 , the layers of photoresist  22  and seed layer  20  on both sides of substrate  4  are removed to form inductor  2  with substrate  4 , metal fillings  8  and conductors  10 . With the embodiment depicted in  FIG. 19 , inductor  2  can have a Q of approximately 60. 
       FIG. 35  is a side view of an alternate embodiment of the present invention.  FIGS. 20 to 34  are side views of a production of an embodiment of the present invention disclosed in  FIG. 35 . In  FIG. 20 , a first substrate  4  is coated with insulation layer  12 . In  FIG. 35 , the first substrate  4  is an SOI substrate with a first side and a second side. In  FIG. 21 , insulation layer  12  is patterned on one side of substrate  4  including forming holes on insulation layer  12  on the first side of the first substrate  4 . In  FIG. 22 , the patterned insulation layer  12  on the first side is coated with photoresist  22 . A pattern is created on photoresist  22  including holes above locations on the first substrate  4  where vias  6  should be created on the first substrate  4 . 
     In  FIG. 23 , holes are etched in substrate  4  where vias  6  should be formed. In addition, a layer of photoresist  22  is deposited on the second side of the first substrate  4 . The layer of photoresist  22  on the second side of the first substrate  4  is patterned and holes are created in the layer of photoresist  22  corresponding to the location of vias  6 . In  FIG. 24 , holes are etched completely through the first substrate  4  to form vias  6 . In  FIG. 25 , photoresist  22  is removed on both the first side and the second side of the first substrate  4  exposing the patterned insulation layer  12  on both the first side and the second side of substrate  4 . 
     In  FIG. 26 , trenches  24  are formed in the first substrate  4  by etching the first substrate  4  at locations corresponding to the holes of the patterned insulation layer  12 . The trenches  24  can be formed using deep reactive ion etching according to a time sequenced etch and passivation chemistry such as the Bosch process. By forming the trenches through deep reactive ion etching, rather than for example, using molds formed by photoresist, a closer packing of the trenches may be achieved. An aspect ratio of a depth of the trenches  24  to spacing between trenches  24  can be increased, for example, the aspect ratio can be equal to or greater than 2:1, such as 10:1. The larger aspect ratio increases an amount of metal fillings and metal conductors per area thus increasing the winding density, or the amount of windings per area, of the inductor  2 . With a higher winding density, an inductance of the inductor  2  can be improved. The closer packing of the trenches  24  can thus improve the performance of inductor  2 . 
     In  FIG. 27 , insulation layer  24  is placed inside trenches  24 . Insulation layer  24  can be placed, for example, inside trenches  24  through oxidizing the first substrate  4 . In  FIG. 28 , seed layer  20  is sputtered on the first side of the first substrate  4 . In  FIG. 29 , metal  26  is plated on the first side of the first substrate  4 . 
     In  FIG. 30 , dummy wafer  18  is resist bonded onto metal  26  using photoresist  26  on the first side of the first substrate  4 . In  FIG. 31 , metal  26  is used to fill vias  6 . Metal  26  can fill vias  6 , for example, through electroplating. In  FIG. 32 , dummy wafer  18  is removed along with photoresist  22 . In  FIG. 33 , excess metal  26  is removed from both the first side and the second side of the first substrate  4 . Furthermore, both the first side and the second side of the first substrate  4  can be polished. In  FIG. 34 , gold layer  14  is deposited on an end of metal  26  in via  6  on the second side of the first substrate  4 . In  FIG. 35 , the process depicted in  FIGS. 20 to 34  is repeated for a second substrate  4 . The second substrate  4 , which has gone through the process depicted in  FIGS. 20 to 34 , is bonded with the first substrate  4  using a gold thermocompression bond at gold layers  14  to form inductor  2 . Gold layers  14  can be, for example, gold bumps. With the embodiment depicted in  FIG. 35 , simulations have shown that inductor  2  can have a Q as high as 180. Although gold is used, other ductile metals can be used and the first substrate and the second substrate can be bonded with ductile metal bumps. 
     In both the embodiment depicted in  FIG. 35  and the embodiment depicted in  FIG. 19 , metal fillings  8  and/or conductors  10  can be advantageously formed in a rectangular or elliptical shape in cross section. By having metal fillings  8  and/or conductors  10  formed in a rectangular or elliptical structure, they can be packed closer while taking up the same area thus increasing a winding density of the inductor  2  over conventional inductors. Conversely, with the same winding density, that is, the same amount of metal fillings  8  and/or conductors  10  per area, there is more current passing through the metal fillings  8  and/or conductors  10  due to the skin-effect. That is, the skin-effect drives the current to an outside edge of conductive materials. Rectangles and/or ovals beneficially have larger outside edges per unit area which thus reduces an amount of resistance in metal fillings  8  and/or conductors  10  and increases the amount of current passing through metal fillings  8  and/or conductors  10 . 
       FIG. 36  is a top view of an alternate embodiment of the present invention. In  FIG. 36 , metal fillings  30  have a larger width than metal fillings  8  disclosed in  FIG. 1 . In  FIG. 36 , metal fillings  30  have a rectangular shape in cross section, but metal fillings  30  can have any shape such as circular, oval, triangular, etc. By increasing the width of the metal fillings, the resistance of inductor  2  is decreased. Since the Q factor is governed by the equation 
             Q   =       ω   ⁢           ⁢   L     R           
where R is the inductor&#39;s internal electrical resistance and ωL is the inductor&#39;s capacitive or inductive reactance at resonance, any decrease in the amount of resistance results in an overall increase in the Q factor.
 
       FIG. 37  is a top view of an alternate embodiment of the present invention.  FIG. 37  includes metal fillings  34  and metal contacts  38  which are connected in parallel to metal fillings  8  and metal contacts  10 . By doing so, two inductors can be sandwiched into an area for a single inductor. However, since the two inductors are connected in parallel, simulations have shown that while the inductance may be decreased, the resistance will be decreased at a greater amount than the decrease in inductance. For example, the resistance can, for example, be decreased to approximately 50% of its original value. Therefore, the inductance would be decreased by less than 50% of its original value. Since the corresponding decrease in resistance is greater than the corresponding decrease in inductance, the overall Q value would be increased. 
     Furthermore, as shown in  FIG. 37 , metal fillings  32  are connected in parallel with metal fillings  8 , while metal fillings  36  are connected in parallel with metal fillings  34 . Since the metal fillings  32  are connected in parallel, the overall resistance of the metal fillings in inductor  2  is reduced. Since the resistance in inductor  2  is reduced, the Q factor is increased through metal fillings  32  being connected in parallel with metal fillings  8  and metal fillings  36  being connected in parallel with metal fillings  34 .