Abstract:
A method for forming through hole vias in a substrate uses a partially exposed seed layer to plate the bottom of a blind trench formed in the front side of a substrate. Thereafter, the plating proceeds substantially uniformly from the bottom of the blind hole to the top. To form the through hole, the rear face of the substrate is ground or etched away to remove material up to and including the dead-end wall of the blind hole.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This U.S. patent application is related to U.S. patent application Ser. No. 11/211,625, U.S. patent application Ser. No. 11/211,622 and U.S. patent application Ser. No. 11/211,623 filed on an even date herewith, each of which is incorporated by reference in its entirety. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     STATEMENT REGARDING MICROFICHE APPENDIX 
     Not applicable. 
     BACKGROUND 
     This invention relates to a process for plating through hole vias having high aspect ratios. 
     Electrical vias allow electrical access to electronic devices or microelectromechanical systems (MEMS) within a package or in a circuit. In order to continually reduce the cost of such packages and circuits, the packing density of devices within the packages and circuits has been continually increased. In order to support the increase in packing density, the pitch between electrical vias for the devices has also continued to shrink. As a consequence, there is a desire to form vias of increasingly large aspect ratio, that is, the vias are tending to become increasingly long and narrow. 
     Long, narrow vias are often created by plating a conductive material into a hole formed in a substrate.  FIG. 1  illustrates a typical prior art process for forming an electrical via by electroplating. A hole  14  is created in a substrate  12  by a directional material removal process such as reactive ion etching (RIE). A seed layer  16  is then deposited conformally over the etched surface, to provide a conductive layer to attract the plating material from the plating bath 
     Another known method for making vias is to use an anisotropic etch to form the holes with sloping sidewalls, and to deposit the conductive material on the sloped walls of the holes. However, this method often results in conductive material having non-uniform thickness, and the heat conduction in the thin deposited layer is relatively poor. The aspect ratio must also remain near 1:2 (width=2× depth), further limiting the density of the vias. 
     SUMMARY 
     However, when using the approach illustrated schematically in  FIG. 1 , the plated material has a tendency to concentrate at the corners  18  of the blind hole  14 . This tendency results from the proportionately larger density of field lines emanating from the corners, and from geometric considerations, that is, the aspect ratio of the via Since the via is deeper than it is wide, the build up of material in the cylinder of the via will close off the cylinder before the plated material reaches the top of the substrate and completely fills the hole. Since the aperture to the via has become closed, the plating bath no longer circulates and the confined bath within the hole is exhausted of its plating species. Plating into the hole will then cease, and a void is formed beneath the point of closure of the via aperture. Since these problems worsen as the via becomes longer and narrower, the approach illustrated in  FIG. 1  becomes increasingly difficult for long, narrow vias. Specialized bath chemistries have been developed that reduce the negative effects cited above, but they can be expensive and are difficult to control. 
     Systems and methods are described here which address the above-mentioned problem, and are particularly applicable to the formation of long, narrow vias by plating. The systems and methods expose only a portion of the seed layer, to effect a “partially” exposed seed layer, to the plating bath, that portion being located at the bottom of the blind hole. Since the seed layer is only exposed in the bottom of the blind hole, the plating material may necessarily deposit first at the bottom of the blind hole. As the plating proceeds, the hole may be filled uniformly from the bottom to the top, and no voids are formed. The vias formed using these methods are solid metal, so that the heat conduction through the vias may be excellent. 
     In one exemplary embodiment, the partially exposed seed layer may be formed by depositing an inhibition layer over the seed layer. The inhibition layer may be formed by sputter-depositing a non-conductive material over the seed layer, with the wafer disposed at an angle with respect to the sputter target. The angle of deposition may cause the rim of the blind hole to shadow the lower portion of the opposing wall, so that the inhibition layer may only be deposited above the range of the shadow. The inhibition layer may inhibit the plating of material from the plating bath wherever it is located. The shadowing effect caused by the rim of the blind hole may cause the inhibition layer to cover only the upper portion of the seed layer, leaving only the bottom portion uncoated and therefore effective as a terminal in the plating bath. 
     Since the inhibition layer may cover the seed layer everywhere but at the bottom of the blind trench, the plating of the trench via material necessarily proceeds from the bottom up. 
     The wafer may then be planarized, which removes the seed layer from the top of the wafer, so that the blind hole vias are electrically isolated from one another. 
     To make the through hole vias, the backside of the wafer may be ground before or after bonding to remove material up to the dead-end wall of the blind hole, leaving the conductive vias extending through the substrate. A silicon-on-insulator (SOI) wafer may also be used, wherein the thicker handle wafer may be selectivity wet or dry etched from the thinner device layer to reveal the vias without the need for grinding. 
     The systems and methods may therefore include etching at least one trench with a dead-end wall in a front side of a substrate, forming a partially exposed seed layer in the trench, and depositing a conductive material within the trench. The backside of the wafer may also be processed to remove material up to the dead-end wall of the trench, to form a through hole via. 
     Systems and methods will be described particularly with attention paid to the formation of the partially exposed seed layer. The remaining process steps needed to form the completed through hole via will also be described. 
     These and other features and advantages are described in, or are apparent from, the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary details are described with reference to the following figures, wherein: 
         FIG. 1  is a cross sectional view showing a prior art approach to the formation of a plated via; 
         FIG. 2  is a cross sectional view of an exemplary substrate after a first step of fabrication; 
         FIG. 3  is a cross sectional view of the exemplary substrate after a second step of fabrication; 
         FIG. 4  is a cross sectional view of the exemplary substrate during a third step of fabrication; 
         FIG. 5  is a cross sectional view of the exemplary substrate after the third step of fabrication; 
         FIG. 6  is a cross sectional view of the exemplary substrate after a fourth step of fabrication; 
         FIG. 7  is a cross sectional view of the exemplary substrate after a fifth step of fabrication; 
         FIG. 8  is a cross sectional view of the exemplary substrate after a sixth step of fabrication; 
         FIG. 9  is a cross sectional view of the composite SOI wafer after assembly with the substrate of  FIG. 8 ; and 
         FIG. 10  illustrates an exemplary MEMS switch using the through hole vias. 
     
    
    
     DETAILED DESCRIPTION 
     The systems and methods described herein may be particularly applicable to microelectromechanical devices, wherein the vias may be required to carry a relatively large amount of current. MEMS devices are often fabricated on a composite silicon-on-insulator wafer, consisting of a relatively thick (about 675 μm) “handle” layer of silicon overcoated with a thin (about 1 μM) layer of silicon dioxide, and covered with a silicon “device” layer. The MEMS device is made by forming moveable features in the device layer by, for example, deep reactive ion etching (DRIE) with the silicon dioxide layer forming a convenient etch stop. The movable feature is then freed by, for example, wet etching the silicon dioxide layer from beneath the moveable feature. The moveable features may then be hermetically encapsulated in a cap or lid wafer, which is bonded or otherwise adhered to the top of the silicon device layer, to protect the moveable features from damage from handling and/or to seal a particular gas in the device as a preferred environment for operation of the MEMS device. 
     Through-hole vias are particularly convenient for MEMS devices, because they may allow electrical access to the encapsulated devices. Without such through holes, electrical access to the MEMS device may have to be gained by electrical leads routed under the capping wafer which is then hermetically sealed. It may be problematic, however, to achieve a hermetic seal over terrain that includes the electrical leads unless more complex and expensive processing steps are employed. This approach also makes radio-frequency applications of the device limited, as electromagnetic coupling will occur from the metallic bondline residing over the normally oriented leads. Alternatively, the electrical access may be achieved with through-wafer vias formed through the handle wafer, using the systems and methods described here. 
     The through hole vias may be constructed by first forming a blind trench in the substrate, and then forming a partially exposed seed layer in the blind trench. It should be understood that although the word “trench” is used, the term should be construed as including any shape of opening, including a circular hole. In addition, the term “partially exposed seed layer” should be understood to mean a seed layer which is only exposed or effective over a particular portion, such as its lower extremity, but nonetheless functions as a terminal for the plating process. A “through hole via” should be construed to mean an electrical conduit which extends completely through a material, for example, through a wafer or substrate. 
     The partially exposed seed layer may then be plated with a conductive material, for example, copper. The substrate may then be planarized using, for example, chemical mechanical polishing. The handle layer may then be ground to remove the dead-end wall of the blind trench, to create the through hole via. Alternatively, the device and insulating layers of a silicon-on-insulator composite wafer may be removed, to reveal the through-hole vias. 
       FIG. 2  is a cross sectional view of an exemplary substrate  100  after a first step in the fabrication of the plated through hole via. The substrate  100  may be, for example, silicon, float zone silicon or any of a number of other common substrate materials, such as glass. The substrate  100  is first coated with photoresist  110  and exposed in regions where the blind trenches, or blind holes  120  are to be formed. The photoresist  110  is exposed and developed, such that areas which have been exposed are dissolved and removed, if using a positive photoresist. If using a negative photoresist, the areas which have not been exposed may be dissolved and removed. The means for forming the blind trenches or holes  120  may be, for example, deep reactive ion etching (DRIE), which is performed on the region of the substrate over which the photoresist has been dissolved and removed. The remaining photoresist  110  is then removed from the substrate  100 . At this point, a thermal oxidation process or other electrically insulating deposition may be performed to further electrically isolate the vias from each other. 
       FIG. 3  is a cross sectional view of the exemplary wafer  100  after a second step of fabrication of the through hole via. In  FIG. 2 , a seed layer  130  may be conformally deposited in the blind trenches  120 . The seed layer  130  may be a two part system, for example, a layer of chrome (Cr) as an adhesion layer and a layer of gold (Au) as a plating and conducting layer, are deposited on the substrate  100 . While a Cr/Au seed layer is described here, it should be understood that the seed layer may be composed of any of a number of other materials, which are effective for adhesion and plating of the conductive material into the blind hole, including titanium (Ti), copper (Cu), and nickel (Ni) The Cr/Au seed layer  130  may be deposited by, for example, chemical vapor deposition (CVD), evaporation or sputtering. An initial adhesion layer of Cr, Ti or other material may be deposited at thicknesses of 50 A up to 500 A, while the conductive plating base layer may be deposited at thicknesses of a few thousand Angstroms up to one micron or more, so long as reasonably low resistance conductive path is made to the bottom of the vias. 
       FIG. 4  is a cross section of the exemplary wafer  100  after a third step of fabrication, which includes deposition of the inhibition layer  140 . The deposition technique may be sputter deposition such as ion beam sputter deposition The inhibition layer  140  is deposited by tilting the substrate  100  with respect to the target  150  at an angle with respect to a line normal to the target  150  surface. Because the substrate  100  is disposed at the angle, the deposited species will be ejected from the target  150  at an angle α with respect to the substrate  100 . Therefore, the walls of the trench may effectively shadow the lower portions of the trenches  120 , so that the sputtered inhibition layer  140  may not be deposited in the lower portion as shown in  FIG. 4 . Instead, the sputtered inhibition layer  140  only coats an upper portion of the trench. 
     The inhibition layer  140  may be any number of materials, particularly insulating materials. For example, any oxide material such as silicon dioxide SiO 2 , alumina Al 2 O 3 , tantalum oxide Ta 2 O 5  or chromium oxide Cr 2 O 3  may be used. In addition, any sputter-deposited polymer may also be used, as long as the sputtered film is insulating and reasonably predictable in terms of its location and thickness. However, any material which inhibits the plating of material from the plating bath may be used for the inhibition layer  140 . Conductive materials can also be deposited and then oxidized in a subsequent step. A conductive layer of chrome Cr, for example, may be deposited and then rendered a dielectric by oxidizing it in, for example, an oxygen plasma, to produce chromium oxide. 
     The blind trench  120  may be coated uniformily by the inhibition layer  140  by rotating the tilted substrate 360 degrees. In various exemplary embodiments, the substrate  100  may be disposed at an angle α of between about 45 and about 90 degrees, and preferably between about 60 and about 80 degrees with respect to the axis normal to the target  150 , and rotated at a rate of 1 revolution per 1 minute of sputter time. It should be understood that these details are exemplary only, and that any of a number of alternative sputtering configurations and conditions may exist which may be capable of forming the inhibition layer  140 . 
     Although the systems and methods described here use a substrate  100  tilted with respect to the sputtering target  150 , it should be appreciated that the same effect may be produced by tilting the substrate target  150  with respect to the substrate  100 , and then rotating the tilted substrate target  150  about an axis normal to the surface of the substrate  100 . In this embodiment, the sputtering target  150  may be disposed at an angle α with respect to a line normal to a surface of the substrate. The means for forming the partially exposed seed layer may therefore be a CVD system for depositing the seed layer or any metal deposition technique, such as metal evaporation, sputtering, etc., and a tilted sputtering system for depositing the inhibition layer. 
       FIG. 5  is a cross sectional view of the exemplary substrate  100  after the third step in the formation of the through hole vias.  FIG. 5  shows the structure of the inhibition layer  140 , before plating of the conductive material into the blind trenches  120 . As shown in  FIG. 5 , the inhibition layer may cover only the upper 125 μm of a 150 μm trench, leaving the lower 25 μm of the seed layer  130  exposed This 25 μm portion may constitute the exposed region  126  of the seed layer  130 . More generally, the inhibition layer  140  may come within about 100 μm or less of the end of the blind trench or blind hole  120 . The width of the trench or diameter of the blind hole  120  may be, for example, about 50 μm wide. 
       FIG. 6  is a cross sectional view of the exemplary substrate  100  at the beginning of the deposition of the conductive species  160  into the blind trenches  120 . The means for depositing the conductive material may be a plating system, including a plating bath and a power supply. The deposition may be performed by immersing the substrate into the plating bath, and coupling the seed layer to one terminal of the power supply. The plating species dissolved in the plating solution then may then be deposited as a layer  160  over the seed layer  130  which is only exposed at the bottom of the trenches  120 . The plating of material  160  then proceeds in an upward fashion, beginning from the bottom of the blind trenches  120 , as indicated by the arrows in  FIG. 6 . The plating therefore proceeds uniformly, without forming the voids characteristic of the prior art techniques. Using the techniques described here, blind trenches may be plated with nearly arbitrarily high aspect ratios. 
     The plated species may be copper, for example, plated by immersing the substrate in a plating solution containing copper sulfate and sulfuric acid. However, it should be understood that this embodiment is exemplary only, and that any other suitably conductive material which can be plated on the substrate, including gold (Au) or nickel (Ni), may be used in place of copper. 
       FIG. 7  is a cross sectional view of the exemplary substrate  100  after completion of the plating step. As shown in  FIG. 7 , the plating proceeds to a point at which the plating material  160  is deposited in and over the blind trenches  120 . Therefore, the plating process results in a non-planar top surface profile, which can be planarized using any known technique, such as chemical mechanical planarization (CMP). The CMP process may stop on the original substrate, such as Si, or on the inhibition layer described above. If the latter approach is used, the inhibition layer may be thick enough to remain after CMP of the Cu. This allows it to be used additionally as a top isolation layer if additional circuitry is later patterned on the wafer surface. 
     Finally, the through hole vias need to be formed from the blind trenches, by removing the dead-end walls of the blind trenches. The through vias may be formed by, for example, grinding or polishing the backside  170  of the substrate  100 , to remove material from the backside to a point  170  at which the blind walls have been removed. For example, grinding may be employed to quickly remove about 100 to about 400 μm of silicon from a 500 μm thick substrate, leaving 100 μm of material as substrate  100 . The grinding can be done either before, but typically after the via substrate  100  is bonded to a device substrate. Accordingly, using the methods described here, through hole vias of diameter less than about 50 μm and depths of at least about 100 μm may be made. More particularly, the aspect ratio of the via, that is, the ratio of the depth of the via to its width, may be at least one-to-one. 
     Alternatively, instead of grinding, the through hole vias may be made using a silicon-on-insulator composite substrate. The blind holes may be etched as described above through the thick handle wafer, and coated with the seed layer and plated as before. However, using the silicon-on-insulator wafer, the device layer and oxide layers may then be removed, to expose the end of a via plated in the handle wafer, to create the through-hole. In yet another embodiment, the through holes may be created in the thinner device layer, and the oxide layer and handle wafer may then be removed. 
       FIG. 8  shows the exemplary substrate  100  upon completion of the fabrication steps for the through hole vias  120 . The through hole vias  120  may be completed by polishing the top surface  180  to a point at which the seed layer  130  and inhibition layer  140  have been removed from the top surface  180 , and the bottom surface  170  has been background to remove material until the blind walls have been removed. At this point, there is no conductive path between the through hole vias, as the plated material  162  within each via  122  is electrically isolated from the plated material  164  within every other via  124  by the inhibition layer  140 . Therefore, the techniques described here may be used to make electrically isolated vias  122 ,  124  within a conducting substrate  100 , as well as conducting vias  122 ,  124  within an insulating substrate  100 . 
     Substrate  100  of  FIG. 8  may be assembled into a silicon-on-insulator wafer  1000  as shown in  FIG. 9 . In  FIG. 9 , the substrate  100  has been overcoated with an insulating layer  200  and bonded to a device wafer  300 , in which the features of a MEMS device will be formed. The insulating layer  200  may be formed as part of the via isolation layer described above. In various exemplary embodiments, the insulating layer  200  is silicon dioxide, and the device layer  300  is silicon. Prior to bonding, the silicon dioxide layer may be patterned with an additional set of thin conductive vias  222  and  224 , which correspond to the through hole vias  122  and  124 , that will connect the through hole vias  122  and  124  to the MEMS device, as will be described further below. The silicon device wafer  300  is then bonded to the insulating layer  200 . The MEMS device is then formed in the device layer. The through hole vias as described above, may thereby provide electrical access to a MEMS device, such as that described next and illustrated in  FIG. 10 . 
       FIG. 10  shows an exemplary finished MEMS device  2000 , sealed in a hermetic package. The MEMS device may be a switch or relay  300  having two portions,  322  and  324  which, when the switch is activated, may touch to close a circuit. Since the details of the MEMS switch  300  are not necessary to the understanding of the systems and methods disclosed here, the MEMS switch  300  is shown only schematically in  FIG. 10 . It should be understood that the MEMS switch shown in  FIG. 10  is exemplary only, and that any other MEMS device may make use of the systems and methods disclosed here, including MEMS sensors, actuators, accelerometers, and other devices. Similarly, the systems and methods disclosed here may be applied to non-MEMS devices. 
     Electrical contact with the through hole vias  122  and  124  may be made by depositing a layer of a conductive material  222  and  224 , into a pair of holes made in insulating layer  200 . After securing the device layer  300  to the insulating layer  200 , the features  322  and  324  of the MEMS switch  300  may be formed in the device layer by, for example, deep reactive ion etching through the device layer to the insulating layer  200 . The features  322  and  324  may be formed in locations corresponding to the locations of the through hole vias  122  and  124  and conductive material regions  222  and  224 . The insulating layer  200  may remain under the outboard portions of MEMS features  322  and  324 , in order to anchor the MEMS features  322  and  324  to the substrate surface  100 . Elsewhere under MEMS features  322  and  324 , the insulating layer  200  has been etched away to release MEMS features  322  and  324 , so that MEMS features  322  and  324  are free to move. A wet etchant such as hydrofluoric acid (HF) may be used to remove the insulating layer  200  under the MEMS features  322  and  324 . 
     MEMS switch  300  is then encapsulated in a cap or lid wafer  500 , which has been relieved in areas to provide clearance for the movement of MEMS switch  300 . The hermetic seal may be made by, for example, forming an alloy seal  400  as taught in greater detail in U.S. patent application Ser. No. 11/211,625 and U.S. patent application Ser. No. 11/211,622 incorporated by reference herein in their entirety. The alloy seal  400  may be an alloy of gold (Au) layers  410  and  430  and indium (In) layer  420 , in the stoichiometry of AuIn 2 . 
     While various details have been described in conjunction with the exemplary implementations outlined above, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that are or may be presently unforeseen, may become apparent upon reviewing the foregoing disclosure. For example, while the disclosure describes an embodiment including a MEMS switch, it should be understood that this embodiment is exemplary only, and that the systems and methods disclosed here may be applied to any number of alternative MEMS or non-MEMS devices. Accordingly, the exemplary implementations set forth above, are intended to be illustrative, not limiting.