Abstract:
The invention is a method of fabricating electrically passive components or optical elements on top or underneath of an integrated circuit by using a porous substrate that is locally filled with electrically conducting, light emitting, insulating or optically diffracting materials. The invention is directed to a method of fabricating electrically passive components like inductors, capacitors, interconnects and resistors or optical elements like light emitters, waveguides, optical switches of filters on top or underneath of an integrated circuit by using porous material layer that is locally filled with electrically conducting, light emitting, insulating or optically diffracting materials. In the illustrated embodiment the fabrication of voluminous, solenoid-type inductive elements in a porous insulating material by standard back- and front-side-lithography and contacting these two layers by electroplating micro-vias through the pores is described. By using a very dense interconnect spacing, an inter-pore capacitor structure is obtained between the metalized pores and the pore walls utilized as insulators.

Description:
RELATED APPLICATIONS  
       [0001]    This application is related to and claims priority from U.S. Provisional Patent Application, serial No. 60/162,570, entitled Structures In Porous Substrates, filed Oct. 29, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates to the field of microelectronics and in particular to the fabrication of microcircuits on porous substrates with high resolution and the structures fabricated thereby.  
           [0004]    2. Description of the Prior Art  
           [0005]    Interest in porous materials has mainly been because of its altered material properties in comparison to compact matter. Examples are nano- or aerogel thermal insulators or in micro devices using porous silicon membranes. Porosity may also lead to lowered dielectric constants of matter or changes in electrical resistivity. If the pores are well ordered like in anodized alumina, they could be additionally used in functional structures where the pores serve as line connections between various levels of a three dimensional structure.  
           [0006]    Micromachining bears some problems in the case of large scale precision machining of porous matter, because the widely used technique of resist lithography does not work on porous materials. The resist is sucked into the inner pores where dissolution is later hindered or difficult to achieve. Alternative strategies which have been used to surmount this problem are methods which deposit the porous material through a hard mask, such as in metal-oxide gas sensors, or which depose the porous material on top of an already patterned resist prepared for lift off.  
           [0007]    For those layers, which have to be made porous after depositing a bulk material, this starting layer may be structured before processing. Etching the starting material or partially covering it with an anodizing-resistant layer may do this. However, these techniques do not provide parallel pores at the edges of the porous layer, as the processes are isotropic. As a rule of thumb the width of edge disordering will be equal to the starting material thickness.  
           [0008]    Therefore, if ordering of pores will be essentially required, the present state of the art of structuring in porous material leads to large minimum feature sizes since the isotropic porous edge region cannot be used. A new method of patterning porous material is needed which maintains pore ordering and which makes feature sizes down to the inter-pore distance possible. From this, new applications of porous layers will arise.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    The invention is a method of fabricating electrically passive components or optical elements on top or underneath of an integrated circuit by using a porous substrate that is locally filled with electrically conducting, light emitting, insulating or optically diffracting materials. The invention is directed to a method of fabricating electrically passive components like inductors, capacitors, interconnects and resistors or optical elements like light emitters, waveguides, optical switches of filters on top or underneath of an integrated circuit by using porous material layer that is locally filled with electrically conducting, light emitting, insulating or optically diffracting materials. In the illustrated embodiment the fabrication of voluminous, solenoid-type inductive elements in a porous insulating material by standard back- and front-side-lithography and contacting these two layers by electroplating micro-vias through the pores is described. By using a very dense interconnect spacing, an inter-pore capacitor structure is obtained between the metalized pores and the pore walls utilized as insulators.  
           [0010]    The invention is a method of fabricating an intermediate structure in a porous substrate in which microdevices are fabricated and which porous substrate has a frontside with a plurality of open pores and an opposing backside wherein the plurality of pores are closed. The method comprises the steps of disposing a patterned photolithographed mask on the backside. Selected portions of the backside are removed to leave a portion of the porous substrate intact to define open pore portions and to leave a portion of the porous substrate with the backside removed to define a plurality of disconnected segments. The disconnected segments in turn define pore walls for a subplurality of doubly-opened pores which are open on each opposing end of the pore. By this means an intermediate structure is formed in which microdevices may be fabricated.  
           [0011]    The method further comprises the step of selectively removing the plurality of disconnected segments to form a plurality of disconnected open pore substrate portions as an intermediate structure in which microdevices may be formed.  
           [0012]    The method further comprises electroplating the backside to fill the doubly-opened pores with a filler material to define a filled pore portion and to create a starting electroplated layer on the backside.  
           [0013]    The method further comprises further electroplating the backside to form a final electroplated layer of material thereon acting as a backing layer.  
           [0014]    The method further comprises disposing an insulating layer on the backside prior to further electroplating the backside to form a final electroplated layer in order to reduce thickness of the final electroplated layer on the backside.  
           [0015]    The method further comprises selectively removing open pore substrate portions to leave a filled pore portion.  
           [0016]    The method further comprises removing the final electroplated layer on the backside to leave a free standing filled pore portion.  
           [0017]    The method further comprises utilizing the free standing filled pore portion as an interconnect layer in a flip-chip hybrid circuit.  
           [0018]    The method further comprises utilizing the free standing filled pore portion as a two-dimensional Zebra-connector between two surfaces bearing contact bumps which in turn are attached to connectors formed on both sides of the Zebra-connector.  
           [0019]    The method further comprises filling in the disconnected segments with a filler material, selectively filling in the open pore portions with a conductive material and forming an array of two parallel rows of connectors through the porous substrate, and disposing metallizations on the frontside and backside of the porous substrate coupling alternate ones of the connectors in each of the two parallel rows of connectors to form a conductive coil in the porous substrate.  
           [0020]    In one embodiment the step of filling in the disconnected segments with a filler material comprises filling in the disconnected segments with a magnetically permeable material.  
           [0021]    The step of disposing metallizations on the frontside and backside of the porous substrate comprises disposing diagonal metallizations on the frontside and backside of the porous substrate to connect opposing connectors in the two rows.  
           [0022]    In another embodiment the step of disposing metallizations on the frontside and backside of the porous substrate comprises disposing double metallizations which are insulated from each other so that two electrically separate coils are formed in the porous substrate.  
           [0023]    The step of forming an array of two parallel rows of connectors through the porous substrate further comprises forming four parallel rows of connectors so that two concentric coils are formed therefrom.  
           [0024]    The method further comprises forming a conductive contact on the frontside and backside of the porous substrate and coupling each of the conductive contacts with one end of the coil.  
           [0025]    The method further comprises selectively removing the open pore substrate portions to leave the plurality of disconnected segments. A first conductive layer is disposed on a first end of the disconnected segments. A second conductive layer is disposed on the pore walls, on the first conductive layer exposed in the pore, and on a second end of the disconnected segments opposing the first end. An insulating layer is disposed on the second conductive layer. A third conductive layer is disposed on the insulating layer to fill the pores and to form a conductive backside layer so that a capacitor is formed.  
           [0026]    The method further comprises electroplating a starting layer on the first end of the disconnected segments.  
           [0027]    The step of disposing a first conductive layer on a first end of the disconnected segments comprises electroplating a conductive partial layer partially extending into the pores. disposing a seed layer within the pores and on the second end of the disconnected segments and on the conductive partial layer within the pores, and electroplating a conductive final layer on the seed layer. The conductive partial layer, seed layer and conductive final layer comprise the second conductive layer.  
           [0028]    The step of disposing an insulating layer on the second conductive layer comprises partially converting the conductive final layer into a first insulating layer and disposing a second insulating layer thereon.  
           [0029]    The step of disposing a third conductive layer on the insulating layer comprises disposing a second seed layer on the insulating layer and electroplating an electrode layer thereon to fill the pores and to provide the backing.  
           [0030]    The invention is also defined as the intermediate structures and apparatus formed by the above methods. Although the methods have for the sake of grammatical ease been described in many cases as steps, it is to be expressly understood that the invention is not to be limited by the construction of means or steps based on 35 USC 112. The invention can be better visualized by turning to the following drawings wherein like elements are referenced by like numerals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1 is a side cross-sectional view in enlarged scale of a partially process porous substrate according to the invention.  
         [0032]    [0032]FIG. 2 is a side cross-sectional view of the porous substrate of FIG. 1 being subjected to additional process steps.  
         [0033]    [0033]FIG. 3 is a side cross-sectional view of the porous substrate of FIGS. 1 and 2 after being completed as an intermediate structure for use in micromaching applications.  
         [0034]    [0034]FIG. 4 is a side cross-sectional view of the porous substrate of FIG. 1 being processed to form a different intermediate structure.  
         [0035]    [0035]FIG. 5 is a side cross-sectional view of the porous substrate of FIG. 4 after additional process steps.  
         [0036]    [0036]FIG. 6 is a side cross-sectional view of the porous substrate of FIG. 4 after being completed as an intermediate structure for use in micromaching applications.  
         [0037]    [0037]FIG. 7 is a side cross-sectional view of the intermediate structure of FIG. 6 being applied to a flip-chip hybrid.  
         [0038]    [0038]FIG. 8 is a side cross-sectional view of the intermediate structure derived from that of FIG. 1 being used as a micro-inductor.  
         [0039]    [0039]FIG. 9 is a top plan view of the micro-inductor of FIG. 8.  
         [0040]    [0040]FIG. 10 is a side elevational view of the micro-inductor of FIGS. 8 and 9 after additional packaging steps.  
         [0041]    [0041]FIG. 11 is a side cross-sectional view of a microcapacitor derived from the open pore region of the intermediate structure of FIG. 1.  
         [0042]    The invention and its various embodiments can now be understood in the context of the illustrated embodiments described below. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]    The problem of resist being sucked into the pores is solved by the invention by uncovering a closed pore bottom layer and using photolithography for creation of the pore openings. This increases the structural resolution up to the pore ordering constant. Various applications arise from this fabrication method, ranging from the fabrication of ion masks or through contacts in for example the fabrication of micro-inductors, high volume capacitors and electroactive photonic crystal devices.  
         [0044]    The invention makes use of the fact that at the end of the conventional pore formation process pore endings  10  form a closed surface  12  as shown in FIG. 1. Pores  18  are formed into a substrate  14  thereby defining pore openings  20  on one surface and an opposing connected structure or pore ending  10  adjacent a back surface  12 . Back side surface  12  of the porous substrate  14  is subjected to conventional patterned lithography on the reverse side, which comprises surface  12 . Since resist  16  cannot seep or enter into pores  18 , high spatial resolution can be achieved in resist  16 . The exposed areas of pore endings  10  may now be opened by conventional etching to remove pore endings  10  and open an open pore region  22 . Thus two different types of pore structures are created, having closed or open pore endings. This forms a first structure which may be further processed to entirely remove open pore regions  22 , or by filling them.  
         [0045]    Open pore regions  22  may be entirely removed by a chemically assisted dry etch process illustrated in FIG. 2. Etchant gas, symbolically denoted by arrows  24 , is provided to the open pore side of the first structure of FIG. 1 while a plasma or ion beam, symbolically denoted by arrows  26 , is directed onto closed pore surface  12  after resist  16  has been removed. Reactive gas and ion beam or plasma  26  will interact on the exposed open pore region  22  in substrate opening  28 . As a rule, the flux of reactive gas must be chosen to be low, as it will be completely consumed within open pore region  22  in order to avoid damage of the closed pore domes  30 . The resulting structure is shown in FIG. 3 and will then have a complete and high aspect ratio opening  28 .  
         [0046]    In the cases where open pore region  22  is not removed, but refilled, electrochemical methods are suited for filling region  22  and opening  28  in FIG. 2. As shown in FIG. 4 after deposition of a starting metal  32 , electrochemical deposition of a filling material  34  may be provided, which fills open pore region  22 . In order to avoid the growth of a thick backside metallization, the deposition of starting layer or a layer  32  laid down by a first period of electroplating leads to almost closing of opened pore region  22 . This closure or near closure allows deposition of another, insulating backside-layer  36 . Collimated sputtering may be useful for pore closing as well. If the final electroplated layer (not separately shown from layer  32 ), which is added onto layer  32  within open pore region  22 , exceeds the thickness of starting or first period electroplated layer  32 , backside layer  36  may be removed by isotropic etching. If different materials are used for starting and electroplating, then their etch ratio will determine the minimum electroplating thickness needed. After this treatment the previously open pore regions  22  will be filled whereas the closed pore regions  30  be hollow, altogether forming the third basic structure as shown in FIG. 4.  
         [0047]    A fourth structure shown in FIG. 5 is fabricated from the structure of FIG. 4 by isotropically etching of closed pore regions  30  leaving the filled region  38  and insulating layer  36  almost remnant and removing the hollow closed-pores regions  30  as well as the starting layer  32 . Alternatively, for instance, if filling material  34  cannot withstand the latter etching, the whole front side of the structure of FIG. 4, including all open pores, is covered with an etch resistant material  40  up to an extent which yields a closed surface on top of filling material  34 , but incomplete surface coverage inside hollow pores  18 . Again an isotropic etch removes closed pore regions  30 , and a double side opened and filled substrate segment  38  is obtained as shown in FIG. 6.  
         [0048]    Based on these structures some novel applications can be made. For example, the structures described above may serve as highly stable mechanical ion etching masks where the structure of FIG. 3 provides pore-sized nano-patterned fields. The structures of FIG. 3 and FIG. 5 provide a high aspect ratio mechanical mask with pore-size resolution. The structure of FIG. 5 also provides a filled mask that has high mechanical stability.  
         [0049]    The structure of FIG. 6 allows various through-contacts to be realized. Within the whole porous layer, comprised of insulating material, conducting areas reaching from one to the other side of the porous layer are realized. The structure of FIG. 6 may serve, for instance, as interconnect layer  38  in a flip-chip hybrid, or in general, as a two-dimensional Zebra-connector between two surfaces bearing contact bumps  42  as shown in FIG. 7, which are attached to connectors formed on both sides  44 . With parallel, well-ordered pores defined in substrate  14 , the maximum line density of the inter-pore distance can be realized. For less well ordered pore substrates, a line density of twice the maximum inter-pore distance can easily be achieved. Further corrections may be made according to pore domain tilting.  
         [0050]    Another application are micro-inductors which are illustrated in FIGS. 8 and 9, e.g. of type 0201 or smaller for surface mount purposes, having long solenoid properties. Pores  18  are used to provide a plurality of parallel rows of pore interconnects  52  between the front and backside of substrate  14  on which metal stripes  46  and  48  are deposited, which stripes  46  and  48  connect two rows of micro-inductors. A plurality of such structures as depicted in FIGS. 8 and 9 are simultaneously fabricated in substrate  14 . By inclining metal stripes  46  and  48  on at least one of the sides of substrate  14  which carries the two adjacent parallel rows of pore interconnects  52 , a large volume coil is formed having a longitudinal axis lying in the plane of substrate  14 . As stated a plurality of such coils are formed in substrate  14  at one time, although FIGS. 8 and 9 show an illustration of only one such coil.  
         [0051]    The fabrication steps are directed to providing interconnects and backside lithography and etching of the openings for the parallel pore interconnects  52 . After selectively filling interconnecting pores  18  with conductive filling material  34  by electroplating, the remaining open pores  18  at the front side of substrate  14  are filled with insulating material  50 . Planarization at the front side of substrate  14  can ensure that front side lithography and stripe metallization  48  contacts pore interconnects  52 , which is an array of exposed conductively filled pores  18 . On the backside of substrate  14  stripes  46  can be structured by removing starting or electroplating layer  32  and lithography followed by metallization or, by using the starting/electroplating layer  32  and etching stripes  46  from starting/electroplating layer  32 .  
         [0052]    The electrical contacts of individual coil inductors are prepared during the same steps by electroplating two rectangular fields  54  at the ends of the parallel rows of pore interconnects  52 , thereby connecting to one end of the coil. After deposition of an insulating layer (not shown) over the bare coil windings comprised of stripes  46  and  48  on both the back and front side of substrate  14  as shown in FIG. 10, multiple replications of the coils of which only one is depicted in FIGS. 8 and 9, are ready for separation. A wafer saw is used for this purpose.  
         [0053]    Simple backend processing can be achieved by designing the inductors with an insulating space between the parallel pore rows, which space is larger than the saw line  58 . By this means the pore windings remain insulated from each other after sawing. End contacts  54  of two adjacent coils may be designed advantageously by lying a bit closer than the orthogonal saw lines  58  and  60  will be. This results in exposed bare metallization of end contacts  54  being made accessible after separation. After these steps the coils may undergo normal end processing like bulk solder tinning  62  of electrical contacts  54  and bulk glazing  56 .  
         [0054]    Insulative filling material  50  may be chosen advantageous by using a low dielectric, k, material or even partially leaving air within the core of the coil, i.e. in those pores  18  between the rows of pore interconnects  52 . This leads to improved high frequency properties of the coils, as the dielectric losses will be reduced when compared to using the homogeneous bulk equivalent to the geometric coil structure. On the other hand, low frequency, highly inductive solenoids can be obtained by using a filling material  50  with high magnetic permeability.  
         [0055]    Also, miniaturized transformers can be made by fabricating two coils around the same core by using double layers for stripes  46  and  48  which are insulated from each other, and corresponding double parallel rows of interconnects  52 , either interlaced or concentric with each other, which are disposed on each side or around a common permeable core.  
         [0056]    In addition to inductors capacitors with a high capacitance per mounting surface can be fabricated according to the invention. As shown in FIG. 11 one starts with the structure of an open pore region  22  from a substrate  14  on which a starting layer  65  is disposed in such a way to define a channel for the through contacts  67 . The through contacts may be improved by partially electroplating a conductive layer  64  into pores  18 . In the next step a seed layer  66  used for a subsequent electroplating step is isotropically deposited into pores  18  by means other than electroplating as well as onto the pore openings  20 . A metal layer  68 , such as titanium is electroplated and partially transformed into barium titanate by a hydrothermal treatment in a Ba(OH) 2  solution. After this the remaining, thinner titanium layer which has not been converted to barium titanate will be completely covered with a BaTiO 3  layer  70  which serves as a high-k insulator between the capacitor plates. Alternatively other metals may be oxidized inside pores  18  by anodization or annealing in an oxygen containing atmosphere. After deposition of another seed layer  72  a metal electrode  74  is electroplated into pore  18  filling the rest of pore  18  and the top of the once open pore region or structure  22 . This finishes the capacitor&#39;s fabrication which is comprised of an interdigitated pair of metal plates  22 ,  65 ,  64 ,  66 ,  72 , and  74  separated by a thin insulating layer  70 .  
         [0057]    Another application of the structure of FIG. 5 is as an active photonic crystal device which can be made by making the freestanding portions  34  of open pore region  22  from a material having an index of refraction different to that of the porous material which comprised substrate  14 . See for example, the copending application entitled, “A Method Of Electroplating Of High Aspect Ratio Metal Structures Into Semiconductors And Structures Made From The Same,” serial no. (Q016) ______, filed ______, assigned to the same assignee as the present application, which is incorporated herein by reference. Incorporation of an electroluminescent or birefringent material according to the method of FIG. 6 and contacting both sides of the photonic crystal device with metallizations results in an active electrooptical device.  
         [0058]    Thus, the invention can now be understood to be a method of structuring porous layers having closed pore endings by using its backside for lithography and etch to:  
         [0059]    1. Open the pores and entirely and selectively remove pore endings to fabricate freestanding well-ordered parallel structures as shown in FIGS.  1 - 10 ;  
         [0060]    2. Fill the opened pores by electroplating into the pores and/or using a backside insulation layer;  
         [0061]    3. Fill the opened pores and removing the hollow pores by isotropic etching by partial frontside deposition of an etch resistant layer to cover the filled pores but leaves the hollow pores partially uncovered, and by subsequent frontside etching;  
         [0062]    4. Fabricate ion etch masks using the method of the invention;  
         [0063]    5. Fabricate through contacts in a porous layer;  
         [0064]    6. Interconnect layers for Flip-Chip hybrids;  
         [0065]    7. Fabricate micro-inductors using porous substrates by frontside filling of the hollow (non interconnecting, plated) pores, planarizing, filling with low-k dielectric or filling with a material of high magnetic permeability, creating electroplated pores adjacent to the coil windings in preparation of front side contacts, disposing an insulating layer on top of the winding sections, using a saw line design to leave the winding side walls insulating, using a saw line design for cutting into the front side walls for bulk solder plating, and/or fabricating transformers using more than one parallel row of interconnects in porous substrates;  
         [0066]    8. Fabricate capacitors using porous substrates by disposing bottom contacts fabricated through pore endings, electroless plating into the inner pores, hydrothermally oxidizing a metal layer in the pores, oxidizing a metal layer inside the pore using anodization, thermally oxidizing a metal layer inside the pore, and/or electroplating a metal layer into the pore to fill it.  
         [0067]    9. Fabricate structured photonic crystal devices by filling the pores with electroluminescent/birefringent material/using plating, and/or etch mask layers as contacts (self-aligned contact)  
         [0068]    Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations.  
         [0069]    The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself.  
         [0070]    The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination.  
         [0071]    Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements.  
         [0072]    The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.