Patent Publication Number: US-7897435-B2

Title: Re-assembly process for MEMS structures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/374,761, filed Mar. 14, 2006, which is a divisional of U.S. patent application Ser. No. 10/119,963, filed Apr. 10, 2002 (now U.S. Pat. No. 7,010,854). 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a method of fabricating an array of microstructures. 
     An example of microstructures is a spring contact. An array of spring contacts may be used as probes in probe card assemblies (among other uses). Examples of spring contacts are disclosed in U.S. application Ser. No. 08/802,054 filed Feb. 18, 1997, and examples of probe card assemblies are disclosed in U.S. Pat. No. 5,974,662, both of which are incorporated by reference herein. 
     Fabricating a structure comprising an array of microstructures, such as spring contacts, can be difficult. For example, if all of the microstructures are fabricated on a single substrate that is to be their final support substrate in the overall structure being made, a defect in one microstructure may cause the entire array to be discarded. On the other hand, if the microstructures are not fabricated on their final support substrate, it may be difficult to align all of the microstructures with respect to one another. 
     SUMMARY OF THE INVENTION 
     The invention is set forth in the claims below, and the following is not in any way to limit, define or otherwise establish the scope of legal protection. In general terms, the present invention relates to a method of fabricating an array of microstructures by aligning and assembling smaller elements into a single structure. 
     Further objects, embodiments, forms, benefits, aspects, features and advantages of the present invention may be obtained from the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating an initial step in a process showing an exemplary embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of a further step in the exemplary process of  FIG. 1 . 
         FIG. 3  is a cross-sectional view of a further step in the exemplary process of  FIG. 1 . 
         FIG. 4  is a cross-sectional view of a further step in the exemplary process of  FIG. 1 . 
         FIG. 5  is a cross-sectional view of a further step in the exemplary process of  FIG. 1 . 
         FIG. 6  is a cross-sectional view of a further step in the exemplary process of  FIG. 1 . 
         FIG. 7  is an enlarged cross-sectional view of a portion of  FIG. 6 . 
         FIG. 8  is a cross-sectional view of a further step in the exemplary process of  FIG. 1 . 
         FIG. 9  is a cross-sectional view of a further step in the exemplary process of  FIG. 1 . 
         FIG. 10  is a partially cross-sectional view of a further step in exemplary process of  FIG. 1 . 
         FIG. 11  is a partially cross-sectional view of a further step in the exemplary process of  FIG. 1 . 
         FIG. 12  is a cross-sectional view of an assembled structure according to the present invention. 
         FIG. 13  is a partially cross-sectional view of a probe card according to the present invention. 
         FIG. 14A  is a cross-sectional view illustrating an initial step in a process showing an alternate embodiment of the present invention. 
         FIG. 14B  is a cross-sectional view of a further step in the process of  FIG. 14A . 
         FIG. 14C  is a cross-sectional view of a further step in the process of  FIG. 14A . 
         FIG. 15A  is a cross-sectional view illustrating an initial step in a process showing another embodiment of the present invention. 
         FIG. 15B  is a cross-sectional view of a further step in the process of  FIG. 15A . 
         FIG. 15C  is a cross-sectional view of a further step in the process of  FIG. 15A . 
         FIG. 15D  is a cross-sectional view of a further step in the process of  FIG. 15A . 
         FIG. 16A  is a cross-sectional view illustrating an initial step in a process showing another embodiment of the present invention. 
         FIG. 16B  is a cross-sectional view of a further step in the process of  FIG. 16A   
         FIG. 16C  is a cross-sectional view of a further step in the process of  FIG. 16A . 
         FIG. 17  is a cross-sectional view of another example of an assembled structure according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and alterations and modifications in the illustrated device and method and further applications of the principles of the invention as illustrated therein, are herein contemplated as would normally occur to one skilled in the art to which the invention relates. 
       FIGS. 1-3  illustrate a technique for fabricating alignment structures on substrate, which may be a silicon wafer. The alignment structures are shown as pits, but other types of alignment structures such as groves or holes are also contemplated.  FIGS. 1-3  also illustrate a technique for fabricating alignment structures on a master substrate, which may also be a silicon wafer. A master substrate will be discussed in greater detail later in this disclosure. 
     As seen in  FIG. 1 , a substrate  100  having a top surface (as viewed) is used as a substrate for constructing the desired microstructures. On the top surface of the substrate  100  is an oxide layer  110 . A layer of a masking material  120  (e.g., photoresist) is deposited atop the oxide layer  110 . The masking layer  120  is processed in any suitable manner to have a plurality (four shown, although more or fewer are also contemplated) of pits  130 A-D extending through the masking material to the underlying oxide layer  110 . The substrate  100  is then prepared for removal of the oxide layer  110 . 
       FIG. 2  shows the substrate  100  after removal of the oxide layer  110 . This may be accomplished such as by etching the surface of the substrate  100  with hydrofluoric acid (HF) or by other suitable means. The masking layer  120  shields the oxide layer  110  from the etching process creating etched guide pits  200  A-D which pass through the masking layer  120  and the oxide layer  110  to the surface of the substrate  100 . The substrate  100  is now prepared for creation of guide pits  300 A-D in the surface of the substrate  100 . 
       FIG. 3  shows a substrate  100  having a plurality (four shown, although more or fewer are also contemplated) guide pits  300 A-D on its upper surface (as shown). Prior to creation of the guide pits  300 A-D, the masking layer  120  is removed from the oxide layer  110  as seen in  FIG. 2  by any suitable means. This leaves a substrate  100  having an oxide layer  110  with a plurality of holes  200 A-D passing through the oxide layer. Once the masking layer  120  is removed from the substrate  100  of  FIG. 2 , the substrate  100  may be etched to create guide pits  300 A-D in the surface of the substrate  100 . This may be accomplished by etching the pits  300 A-D with potassium hydroxide (KOH) or other suitable means. Indeed, various etching techniques are known in the field, and an etching technique may be selected to achieve a particular etch. For example, a deep reactive ion etch may be selected to achieve pits with approximately vertical sidewalls. Once the guide pits  300 A-D have been etched into the surface of the substrate  100 , the oxide layer  110  may optionally be removed leaving the substrate  100  as seen in  FIG. 3 . Removal of the oxide layer  110  may be accomplished by stripping with HF or other suitable means. 
     Once guide pits  300 A-D have been etched into the surface of the substrate  100 , a layer or layers of metal film  400  may be deposited onto the surface of the substrate  100 . Although only a single metal layer  400  is shown in  FIG. 4 , this for the sake of clarity and is in no way intended to limit the scope of the present invention. The metal layer  400  may be deposited in any suitable method such as by sputtering or plating. The exact composition, thickness, and number of metal layers deposited will vary according to the ultimate structure desired. Typical examples of metals deposited include aluminum, copper, nickel, and titanium. Other metals and non-metal layers may also be deposited on the surface of the substrate  100  at this point in the process. Once any desired metal layers  400  have been deposited, a layer of a suitable masking material  410  is deposited over the metal layer  400  (if any) on the surface of the substrate  100 . 
       FIG. 5  shows a substrate  100  having a plurality (ten shown, although more or fewer are also contemplated) of features  500 A-J developed on its surface through the masking layer  410 . Such features  500 A-J may be processed by any suitable method. For example, masking layer  410  may be made of a photosensitive material, and features  500 A-J formed by exposing portions of the masking layer  410  to light. The exact number, size, shape, and alignment of these features  500 A-J is determined by the type of microstructure desired. The number and arrangement of features  500 A-J shown in  FIG. 5  and the corresponding features  600  A-J shown in  FIGS. 6-12  are for illustrative purposes only and in no way are intended to limit the scope of the present invention. 
       FIG. 6  shows the desired features  600 A-J constructed in the features  500 A-J on the surface of the substrate  100 .  FIG. 7  is an enlargement of a portion of  FIG. 6  showing greater detail of the microstructure features  600 A-J. The microstructure features  600 C-D shown in  FIG. 7  consist of three layers  700 ,  710 ,  720 . These layers  700 ,  710 ,  720  may be comprised of suitable metals or nonmetals depending upon the desired microstructure features  600 A-J to be constructed. The present invention also contemplates the fabrication of microstructure features  600 A-J having greater or fewer layers than those shown in  FIG. 7 . These steps show building up from the substrate  100 . The substrate  100  may be a semiconductor and circuit elements and wiring may be formed on the substrate  100  as in conventional semiconductor fabrication. It is also possible that devices could be built down into the substrate  100 , including transistors or other components, prior to or after the generation of the alignment structures  300 . 
       FIG. 8  shows a substrate having a plurality of microstructure features  600 A-J (ten shown, although more or fewer are also contemplated) and a plurality of guide pits  300 A-D (four shown, although more or fewer are also contemplated) after removal of the masking layer  410  and metal film layer  400  of  FIG. 6 . Removal of the masking layer  410  may be accomplished by any suitable means such as stripping. Optionally, the metal layer  400  may then be removed by a suitable method such as etching. The exact means used to remove the masking layer  410  and the metal layer  400  will vary according to their composition and the composition of the microstructure features  600 A-J constructed on the surface of the substrate  100 . 
     For illustrative purposes,  FIG. 8  also shows a defective microstructure feature  600 I. Defective features may occur during the fabrication of microstructure features for a variety of reasons that vary according to the type of structures being fabricated and their composition. Indeed, although the defective microstructure illustrated in  FIG. 8  has a visible physical defect, the defect may other than physical or visible. As just one example, the microstructure is an electrical connection element, the element may be defective because it was determined that its electrical properties fail to meet a predetermined specification after electrically testing the microstructure. For the purposes of this disclosure, it is only important to note that the defective microstructure feature  600 I is undesirable and must be removed before fabrication of the final substrate. 
     Removal of defective microstructure feature  600 I is accomplished by first separating the substrate  100  into individual dice elements  900 A-B as seen in  FIG. 9 . Separation of the substrate  100  may be accomplished by sawing or any other suitable method. Once the substrate  100  has been separated, dice  900 B containing defective microstructure features (such as  600 I) may be discarded and replaced with non-defective dice. These replacement dice may be taken from another part of the substrate or manufactured separately as desired. 
     Once defective dice  900 B have been replaced, assembly of the final substrate may begin.  FIG. 10  shows a partial cross sectional view of a partially reassembled substrate. A master substrate  1000  is formed by the process previously described in the discussion fewer are also contemplated) corresponding to the plurality of guide pits  300 A-D (four shown, although more or fewer are also contemplated) on the individual dice  900 A, C. Alignment spheres  1010 A-D, which are known in the industry, are placed in the guide pits  1020 A-D of the master substrate  1000 . The spheres may be made of metal, ceramic, or any other suitable material. The individual dice elements  900 A, C are placed such that the guide pits  300  A-B, E-F engage the alignment sphere  1010 A-D of the corresponding guide pit  1020 A-D on the master substrate  1000 . This ensures that the individual microstructure features  600 A-E, K-P of each die  900 A, C properly aligned relative to the microstructure features on the other dice in the X-axis (horizontal as shown), Y-axis (into the page as shown) and Z-axis (vertical as shown) directions. Of course, the more uniform the alignment spheres are with respect to each other, the more precise the alignment of the dice the resulting array of microstructures. Alignment spheres of varying degrees of uniformity are known in the field and may be selected to meet the particular needs at hand. 
     As seen in  FIG. 11 , a backing substrate  1100  is fixed to the individual dice  900 A, C elements. The substrate  1100  may be made of silicon, alumina, or any other suitable material. The backing substrate  1100  is fixed to the individual dice elements  900 A, C using glue or another suitable means such as epoxy (not shown). The glue may then be cured (if necessary) and the assembled substrate  1200  as shown in  FIG. 12  removed from the alignment spheres  1010 A-D. The resulting assembled substrate  1200  contains a plurality (ten shown, although more or fewer are also contemplated) microstructure features  600 A-E, K-P aligned with respect to one another in the X-axis, Y-axis and Z-axis directions. Because the alignment of the microstructure features  600 A-E, K-P is dependent only upon the positioning of the guide pits on the individual die elements  900  A, C and the master substrate  1000  and the alignment spheres  1010 A-D, variations in the backing substrate  1100  or the glue layer do not affect the final positioning of the microstructure features  600 A-E, K-P. 
     Although the present invention was described as using guide pits and alignment spheres, the present invention also contemplates the use of alternate alignment means. For example, straight sided guide holes may be formed in the surface of the substrate using a deep reactive ion etch process. In addition to guide spheres, the present invention also contemplates the use of other suitable alignment devices such as pins or cylinders and correspondingly shaped guide groves or pits on the surface of the substrate. 
     The assembled substrate shown in  FIG. 12  is for illustrative purposes only. Another example of an assembled substrate according to the present invention includes a probe card suitable for testing semiconductor devices as is known in the industry.  FIG. 13  depicts an example of such a probe card  1300  consisting of two individual die elements  1370 A-B fixed to a backing substrate  1360 . Although only two die elements  1370 A-B are depicted in  FIG. 13 , it is understood that the present invention also contemplates the fabrication of probe cards consisting of more or fewer die elements. The backing substrate  1360  as shown could consist of a space transformer such as the one described in U.S. Pat. No. 5,974,662 which is incorporated herein by reference. Alternatively, a backing substrate could be included between the individual die elements and such a space transformer. 
     The microstructure features formed on such a probe card  1300  are probe elements  1320  such as microspring probes previously described and suitable for engagement with contact points or bond pads on a semiconductor substrate or device. Such a probe card  1300  would also incorporate terminals  1340  for engagement with a probe head on the face of the backing substrate  1360  opposite the probe elements  1320 . Wiring interconnects  1350  passing through the backing substrate  1360  would connect each probe element  1320  with a corresponding terminal  1340 . 
     It should be noted that, although the exemplary microstructures  600 A-P illustrated in  FIGS. 6-12  are generally shaped as posts, the microstructures may be fashioned in many different shapes. This is accomplished by patterning openings  500 A-J to form a negative (or mold) of the desire shape of the microstructure. 
       FIGS. 14A-D  illustrate formation of microstructures that are exemplary spring contact structures. These are nonlimiting examples of spring contact structures that may be formed on substrate  100  in place of post-shaped structures  600 A-P. 
     As shown in  FIG. 14A , a plurality of masking layers  1460  patterned to form a reverse molded shape for a spring contact structure are applied to substrate  100 . An opening is left in the masking layers  1460  for terminal  1440 . As should be apparent, these masking layers replace masking layers  410  in  FIGS. 4-7 . Rather than apply a plurality of masking layers as shown  FIGS. 14A-C , a single masking layer could be applied and then stamped, pressed, or otherwise molded to have the desired shape. Thereafter, one or more layers of material are formed on the pattern masking layers  1460  to form a contact spring structure  1450  as shown in  FIG. 14B . This may be similar to the step illustrated in  FIGS. 6 and 7 . The masking layers  1460  are then removed, leaving a spring contact  1450  attached to terminal  1440  on substrate  100 . This may be similar to the step illustrated in  FIG. 8 . 
       FIGS. 15A-D  illustrate another example of spring contact. In this example, distinct post  1552 , beam  1554 , and tip  1556 ,  1558  portions of the contact structure  1550  are separately created. As shown in  FIG. 15A , the post  1552  is created by forming a first masking layer  1562  on substrate  100  with an opening over terminal  1540 . The post portion  1552  is then formed by filling the opening with a material suitable for the post as shown in  FIG. 15A . Thereafter a second masking layer  1564  is formed over the first masking layer  1562 , defining an opening that includes the post  1552  and defines the beam  1554 . The beam  1554  is then created by filling the opening with a material suitable for beam as shown in  FIG. 15B . The process is then repeated with third and fourth masking layers  1566 ,  1568  defining the tip  1556 ,  1558  as shown in  FIG. 15C . The foregoing steps would replace the step of applying masking layer  410  in  FIGS. 4 and 5 . Thereafter, the masking layers  1562 ,  1564 ,  1566 ,  1568  are removed, leaving interconnect structure  1550  attached to the terminal  1540 . This step is similar to removing masking layer  410  illustrated in  FIG. 8 . 
     The array of structures produced by the disclosed process need not be the final desired structure. Rather, the disclosed process may be utilized to create intermediate arrays for use in an assembly process. For example,  FIGS. 16A-C  shows the use of an array created according to the process described in  FIGS. 1-12  to create the final desired structure. In this example,  FIG. 16A  shows an array  1600  of tip structures  1630 A-J which may be created using the process described in  FIGS. 1-12 . The array  1600  comprises a plurality of individual die elements  1620  A-B (two shown, although more or fewer are also contemplated) fixed to a backing substrate  1610 . Attached to the surface of each die  1620  A-B are a plurality of tip structures  1630  A-J (ten shown, although more or fewer are also contemplated). The tip structures  1630  A-J are preferably secured to the surface of the dice  1620  A-B by a layer of a suitable release material  1640  such as alumina. The use of other release materials is also contemplated. 
       FIG. 16B  shows the array  1600  of  FIG. 16A  aligned with a second array of structures  1605 , which may be probe elements forming a portion of a probe card suitable for testing semiconductor devices as is known in the industry. A plurality of probe elements  1660  (ten shown, although more or fewer are also contemplated) are attached to the probe bases  1670 . Wiring interconnects  1680  passing through the substrate  1695  provide electrical connection between the probe elements  1660  and the corresponding terminal  1690  located on the face of the substrate  1695  opposite the probe element  1660 . After being brought into contact with a corresponding probe element  1660 , the individual tip structures  1630 A-J may be fixed to a probe element  1660 . The exact nature of the method of fixation will, of course, vary according to the composition and nature of the individual tip structures and probe elements and may include such methods as soldering, brazing, or any other suitable method. The probe elements  1660  may be any type of probe element known in the field for testing semiconductor devices. Examples of such probes include needle probes, Cobra® brand probes, and resilient spring probes. Examples of spring probes can be found in the following: U.S. Pat. No. 5,476,211; U.S. Pat. No. 5,917,707; U.S. Pat. No. 6,336,269; U.S. Pat. No. 6,268,015; U.S. patent application Ser. No. 09/710,539, filed Nov. 9, 2000; and U.S. patent application Ser. No. 09/746,716, filed Dec. 22, 2000, all of which are incorporated herein by reference. Alternatively, the probes  1660  may themselves have been formed using the procedures as described above with respect to  FIGS. 1-12 . For example, the probes may be probes  1320  shown in  FIG. 13 . 
     Once the tip structures  1630 A-J are fixed to the probe elements  1660 , the release layer  1640  securing the tip structures  1630 A-J to the backing substrate  1610  may be removed using a process suitable to the particular release layer  1640  being used. Once the release layer  1640  is dissolved, the backing substrate  1610  and the die elements  1620 A,  1620 B may be removed yielding an array  1608  of probe elements  1660  with attached tip structures  1630 A-J, as shown in  FIG. 16C . The structure  16 C illustrated in  FIG. 16C  may then form a portion of a probe card assembly. For example, the structure illustrated in  FIG. 16C  may be used as the space transformer illustrated in element 506 of FIG. 5 of U.S. Pat. No. 5,974,662, which is incorporated herein by reference. 
     It should be apparent that the tip structures  1630  illustrated in  FIGS. 16A-16C  are not limited to simple square shapes but may include more complicated shapes and structures. For example, U.S. application Ser. No. 08/819,464, filed Mar. 17, 1997 (now abandoned) (which corresponds to published PCT application WO 97/43653) and U.S. application Ser. No. 09/189,761, filed Nov. 10, 1998 (which corresponds to published PCT application WO 00/28625, (both of the foregoing US applications are incorporated herein by reference) disclose examples of tip structures  1630  or other partial structures that may be formed and attached to probes or other partial structures using the principles described herein. 
       FIG. 17  illustrates that it is not necessary to remove the master substrate when practicing the present invention.  FIG. 17  also illustrates an array  1700  in which the structures formed using the process described in  FIGS. 1-12  are not probe elements or electrical contacts, but rather light emitting diodes (LEDs). Individual die elements  1720 A-B (two shown, although more or fewer are also, contemplated) having a plurality of LEDs (20 shown, although more or fewer are also contemplated) attached to or fabricated within their surface are formed and aligned using the process described in  FIGS. 1-9 . During the assembly process the dice  1720 A-B are aligned by placing alignment spheres  1730  in the guide pits  1740 A-D (four shown, although more or fewer are also contemplated) of the master substrate  1710  and the corresponding die pits  1750 A-D (four shown, although more or fewer are also contemplated) of the dice  1720 A-B. This process is similar to that previously described in  FIG. 10 . In this example, however, once the dice  1720 A-B are aligned, the alignment spheres  1730  are fixed to the master substrate  1710  and individual dice  1720 A-B. By selecting a transparent or semi-transparent substrate  1710 , this process may be utilized to produce, for example, a display screen where the LEDs  1760  are visible through the master substrate  1710 . Wiring to the LEDs may be through the master substrate  1710 , the dice  1720 A-B, or both as desired. The alignment spheres  1730  may provide electrical contact between the master substrate  1710  and the dice  1720 A-B. 
       FIG. 17  shows one example of a structure created by using the process described in  FIGS. 1-12  wherein the master substrate becomes a part of the desired array. In another example, the LED shown in  FIG. 17  may be formed on the surface of the master substrate while electrical contract structures corresponding to the LEDs may be formed on the surface of the dice. In an alternate example, the LEDs may be replaced by radio frequency emitting devices to form a phased array radar. One or more individual die, as shown in  FIG. 17 , can be replaced as a die wears out in use or otherwise fails in use. For example, if a LED on one of the dice failed at any time, the die with the bad LED could be replaced with a new die. 
     It should be apparent that the post-shaped microstructures illustrated in  FIGS. 6-12  and the spring contact microstructures illustrated in  FIGS. 14A-15D  are but three examples of types of microstructures from which an array can be fabricated using the techniques of the present invention. Other nonlimiting examples include spring structures such as those shown in the aforementioned U.S. application Ser. No. 08/802,054 filed Feb. 18, 1997 as well as U.S. application Ser. No. 09/364,855 filed Jul. 30, 1999 and U.S. Pat. No. 6,268,015, all of which are incorporated herein by reference. Non-limiting examples of structures used to probe a semiconductor wafer during testing of the wafer which may be fabricated using the techniques of the present invention include needle probes and Cobra® brand probes. 
     Of course, the microstructures are not limited to contacts. Other nonlimiting examples of microstructures that can be fabricated using the techniques of the present invention include arrays of micro-mirrors, arrays of micro-antennae, photosensitive regions, display pixels, and phosphor dots. Indeed, this invention is applicable to forming arrays of any type of microstructure, including without limitation any type of Microelectromechanical Systems (MEMS), and also active elements, transistors, diodes, or other circuitry. 
     Of course, the physical or mechanical properties of the microstructures may be manipulated by including particular additives in the materials used to form the interconnection elements and/or by heat-treating. Nonlimiting examples are described in U.S. Pat. No. 6,150,168, which is incorporated herein by reference. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. The articles “a”, “an”, “said” and “the” are not limited to a singular element, and include one or more such element.