Abstract:
A method of fabricating a plurality of micro probes comprising the steps of defining the shapes of a plurality of probes as a mask, applying a photoresist to a surface of a metal foil, overlaying the mask on the metal foil, exposing the photoresist to light passed through the mask, developing the photoresist, removing a portion of the photoresist to expose a portion of the metal foil, applying an etcher to the surface of the metal foil to remove the exposed portion to produce a plurality of probes, and chemically polishing and plating the plurality of probes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This patent relates to and claims priority to U.S. Provisional Patent Application No. 60/323,651, filed on Sep. 20, 2001. That Provisional Patent Application is incorporated by reference in its entirety herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    (1) Field of the Invention  
           [0003]    This invention relates to a method for the manufacture of miniature micro probes or electrical contacts for use in testing semiconductor chips. More particularly, this invention relates to a method of photo-defining a plurality of probes on metallic flat stock coated with a chemical resist emulsion and then removing unwanted metal to produce a plurality of micro probes.  
           [0004]    (2) Description of the Related Art  
           [0005]    It is known in the art of testing probe cards for electrical continuity to perform such tests using probes made by mechanically forming a straight piece of fine wire into a desired shape so as to provide the necessary size and spring force. FIGS.  1 - 3  show a conventional “Cobra™” probe test head produced by Wentworth Laboratories, Inc. of Brookfield, Conn. Such probe heads consist of an array of probes  64  held between opposing first (upper)  42  and second (lower)  44  dies. Each probe has opposing upper and lower ends. The upper and lower dies  42 ,  44  contain patterns of holes corresponding to spacing on an integrated circuit contact pad spacing designated herein as lower die hole pattern and upper die hole pattern. The upper end of each of the probes is retained by the upper die hole pattern, and the lower end of each of the probes passes through the lower die hole pattern and extends beyond the lower die  44  to terminate in a probe tip. With reference to FIG. 13, there is illustrated the additional inclusion of mounting film  1301 . Mounting film  1301  is typically formed from a suitable polymeric dielectric such as mylar and holds the etched probes  81  in place. For Cobra™ style probes, the lower die hole pattern is offset from that in the upper die  42 , and the offset is formed into the probe such that the probe acts like a spring. Returning to FIGS.  1 - 3 , when the test head is brought into contact with a wafer to be tested, the upper end of the probe remains predominately stationary, while the lower end compresses into the body of the test head. This compliance allows for variations in probe length, head planarity, and wafer topography. The probe is typically formed by swaging or stamping a straight wire to produce the desired probe shape and thickness. This swaging process flattens and widens the center, curved portion of the probe in order to achieve a desired force per mil of probe deflection.  
           [0006]    The lower and upper ends of the swaged area also prevent the probe from extending too far through the dies. In a conventional probe manufacturing process, the probes are formed from a straight piece of wire, typically of beryllium-copper alloy. Custom tooling is used for each probe size and design. The tooling stamps and forms the center portion of the wire to achieve the desired shape and thickness, thereby generating a desired spring rate.  
           [0007]    With reference to FIG. 9 there is illustrated cross sectional renderings of a wire used in the prior art to produce probes. Cross section  90  illustrates the generally circular form of the pre-stamped wire. Cross section  91  illustrates the generally elliptical shape of a stamped and tooled wire. The cross sectional areas of both cross section  90  and cross section  91  are substantially the same. With reference to cross section  91 , the stamped wire forming the probe has a width  95  of approximately 7 mil (one mil equals 0.001 inch) and a height  97  of approximately 1.8 mils. When assembled in a probe head configuration it is preferable to maintain at least a 1 mil separation between the plurality of probes used in the probe head. As a result of width  95  being approximately 7 mils and requiring a 1 mil separation, conventional probes arranged in a probe head are typically spaced one probe every 8 mils. The wire is then cut to length, and the desired probe tip geometry is ground on the end of the probe. The tolerance on the overall length of the finished probes is +/−0.002″. Because this is too large a variance between probes for proper testing, the probes are assembled into a probe head and the entire array of probes is lapped to achieve a more uniform probe length.  
           [0008]    With reference to FIG. 8, there is illustrated a standard probe  83  known to the art and the etched probe  81  of the present invention. With reference to FIG. 5, there is illustrated the basic components of probes. Probe base  5001  is a relatively short and straight expanse connected to probe shaft  5003 . Probe shaft  5003  is a gently curving expanse of the probe  81 , 83  that terminates in the probe end  5005 . In operation, it is probe end  5005  that comes in contact with the circuit to be tested.  
           [0009]    Conventional stamping processes used to form probes often result in residual stresses in the probes which may cause reduced fatigue life. Because these residual stresses can change over time, changes in probe stiffness may arise. In addition, changes in the requirements for probes require retooling. Such retooling contributes to a high cost for probes manufactured in such a fashion and require a substantial lead time before such probes are available. It is also the case that mechanically fashioned probes are more difficult to redesign as their construction is closely tied to the mechanical means by which they are created.  
           [0010]    There therefore exists a need for a method of manufacturing such probes that avoids the problems which arise from mechanical formation. There is further a need for such a method substantially amenable to producing probes of different designs absent a protracted retooling process.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    One aspect of the instant invention is drawn to a method of fabricating a plurality of micro probes comprising the steps of defining the shapes of a plurality of probes as one or more masks, applying a photoresist to first and second opposing sides of a metal foil, overlaying one each of the masks on opposing first and second sides of the metal foil, exposing the photoresist to light passed through each of the masks, developing the photoresist, removing a portion of the photoresist to expose a portion of the metal foil, and applying an etcher to the surface of the metal foil to remove the exposed portion to produce a plurality of probes.  
           [0012]    Another aspect of the invention is drawn to a micro probe manufactured according to the aforementioned method wherein the micro probe comprises a probe base having a generally uniform thickness bounded by a plurality of edges and extending for a substantially straight length in a plane, a probe shaft connected to the probe base the probe shaft of the generally uniform thickness, bounded by a plurality of edges, and extending along a curved expanse within the plane, a probe end connected to the probe shaft the probe end of the generally uniform thickness, bounded by a plurality of edges, and extending for a substantially straight distance within the plane the straight distance being approximately parallel to the straight length, and a scallop running substantially around a periphery comprised of the edges of the probe base, the probe shaft, and the probe end.  
           [0013]    Yet another aspect of the invention is drawn to a probe test head comprising a first die comprised of first and second opposing planar surfaces the first die further comprising a pattern of first die holes extending through the first die in a direction perpendicular to both of the first and second planar surfaces, a second die comprised of third and forth opposing planar surfaces the second die further comprising a pattern of second die holes corresponding to the pattern of first die holes the second die holes extending through the second die in the direction wherein the third planar surface is arranged in planar contact with the second planar surface such that the second die holes are offset from the first die holes in a substantially uniform direction, and a plurality of probes one each of the probes extending through one of the first die holes and one of the second die holes the probes having a surface finish commensurate with having been formed by etching. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a perspective illustration of a probe test head known in the art.  
         [0015]    [0015]FIG. 2 is a perspective illustration of a cross section of a probe test head known in the art.  
         [0016]    [0016]FIG. 3 is a cross section of a portion of a probe test head known in the art.  
         [0017]    [0017]FIG. 4 is a front view of a probe of the present invention.  
         [0018]    [0018]FIG. 5 is a side view of a probe of the present invention.  
         [0019]    [0019]FIG. 6 is an isometric view of a probe of the present invention.  
         [0020]    [0020]FIG. 7 is a photograph of a mask of the present invention.  
         [0021]    [0021]FIG. 8 is a photograph of a standard probe known in the art and an etched probe of the present invention.  
         [0022]    [0022]FIG. 9 is a cross sectional illustration of a probe known in the art both before and after machining.  
         [0023]    [0023]FIG. 10 is a cross sectional diagram of a probe of the present invention after etching.  
         [0024]    [0024]FIG. 11 is a perspective illustration of the tip of a probe of the present invention.  
         [0025]    [0025]FIG. 12 is a perspective illustration of the configuration of the masks, the photoresist, and the flat stock of the present invention prior to etching.  
         [0026]    [0026]FIG. 13 is a cross section of a portion of a probe test head of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0027]    The present invention is drawn to a method of manufacturing probes in a way that provides improved uniformity while lowering the manufacturing cost of the probes. The probes are manufactured through a photo-etching process rather than a mechanical stamping process prevalent in the art. In this process of the present invention, the probes are etched from thin metal flat stock, typically of Beryllium-Copper alloy. With reference to FIG. 8, there is illustrated an etched probe  81  of the present invention. The desired shape of the probe is specified as an image in graphic form, and the image is used to make a glass mask having a repeating pattern of the desired probe profile. FIG. 7 illustrates a sample of such a mask  73 . Mask  73  is comprised of a plurality of probe shapes  72  and dark spaces  71 . The probe shapes  72  define the areas corresponding to the etched probes of the present invention and are constructed so as to allow light to pass substantially unimpeded through probe shapes  72 . Dark spaces  71  extend predominantly between probe shapes  72  and serve to substantially differentiate one probe shape  72  from each other probe shape  72  on mask  73 .  
         [0028]    With reference to FIG. 12, there is illustrated the probe configuration  1205  employed to produce the etched probes of the present invention. Flat stock  1201  is a predominantly planar sheet of thin metal having opposing planar surfaces. Flat stock  1201  has a width corresponding to the desired width of the finished probe. A preferred width of the flat stock  1201  is approximately 3 mil.  
         [0029]    A photoresist  1001  is then applied to both opposing planar surfaces of flat stock  1201 . Two identical masks  73  are then fastened to opposing sides of flat stock  1201  with one side of each mask  73  in contact with the photoresist  1001  covering a single side of flat stock  1201 . The two masks  73  are aligned such that any one feature in either mask  73  corresponding to an identical feature in the other mask  73  is in exact alignment across an axis perpendicular to the expanse of the planar surfaces of flat stock  1201 . Light is then applied to each mask  73  effective to expose the photoresist  1001  disposed between each mask  73  and flat stock  1201 . Both masks  73  are then separated from probe configuration  1205 . After exposure of the photoresist  1001  to light, the photoresist  1001  is developed and rinsed. As a result of rinsing, exposed photoresist  1001  corresponding to a probe shape  72  on mask  73  remains bonded to flat stock  1201 , while unexposed portions of photoresist  1001  corresponding to a dark space  71  is rinsed off of and out of contact with flat stock  1201 .  
         [0030]    An etcher is then applied at substantially the same time to both surfaces of flat stock  1201 . The etcher begins to dissolve flat stock  1201  in a direction extending from the outer surfaces of flat stock  1201  along an axis perpendicular to the planar expanse of flat stock  1201  and directed into flat stock  1201  from each opposing planar surface. One attribute of applying etcher to a photoresist affixed to a metal substrate in order to dissolve the metal substrate is the presence of under cutting. As used herein, “undercutting” refers to the tendency of an etcher applied to dissolve metal to deviate from an etched path extending perpendicular to the surface to which the etcher was applied. Specifically, the etcher tends to extend outward as it travels into the metal.  
         [0031]    With reference to FIG. 10, there is illustrated the effect on undercutting on the etched probes of the present invention. FIG. 10 is a cross sectional view of the etched probes of the present invention after applying the etcher. As can be seen, the etcher has effectively removed the metal comprising flat stock  1201  from the area bordered by undercut  1005  and etch limit  1007 . As is illustrated, undercut  1005  extends from an exterior surface of flat stock  1201  towards the interior of flat stock  1201 . Note that undercut  1005  deviates slightly from perpendicular axis  1009  running perpendicular to the surfaces of flat stock  1201 . Etch limit  1007  is the boundary designating the extent to which the etcher removes flat stock  1201  up until the etcher is neutralized or otherwise rendered incapable of further etching. Because the etcher etches at a substantially constant rate and follows a path along undercut  1005  deviating from perpendicular axis  1009 , the resultant etch limit  1007  forms a gently curving boundary. By controlling the amount of time that the etcher is exposed to flat stock  1201 , it is possible to produce the cross sectional geometry of each probe as illustrated in FIG. 10.  
         [0032]    The resultant superposition of two opposing etch limits  1007  results in the presence of sharp protrusions or scallops  1003  extending around the perimeter of each etched probe. Note that the distance from scallop base  1013  to scallop tip  1015  forms the scallop dimension  1011 . With reference to FIG. 11, there is illustrated a perspective view of a probe end  5005 . As can be seen, scallop  1003  extends around the edge  1107  of the etched probe  81  including probe tip  1101 . Outer probe tip  1105  is located on opposing sides the flat stock  1201  comprising etched probe  81  at the furthest extreme of probe end  5005 . Probe tip  1101  can be seen to extend beyond outer probe tip  1105  as a result of the scallop  1003  extending around the terminus of probe end  5005 . The resulting extension of probe tip  1101  beyond outer probe tip  1105  allows for better contact with electrical circuits when etched probe  81  is in use.  
         [0033]    Removing the unexposed metal results in an array of probes attached at their top end. The array of probes is then chemically polished and plated. The probes are then removed from the flat stock  1201  and readied for assembly into a probe head. The tops of the probes forming the assembly are lapped while the tips are held referenced to a flat surface to bring the probes to the same length.  
         [0034]    With reference to FIGS.  4 - 6 , there is illustrated the shape of a probe of the present invention. With reference to FIG. 8, as has been described, the etched probes  81  of the present invention are manufactured to a desired configuration absent mechanical stamping or other processes which typically result in residual stresses present in the probes  81 . As used herein, “residual stresses” refers to stresses that remain as the result of plastic deformation. Conventional probes tend to contain residual stresses resulting from the mechanical stamping and machining employed to create a desired probe cross-section. These residual stresses serve to limit the functionality of conventional probes in at least two primary ways. First, residual stresses cause conventional probes to exhibit non-uniform resistive forces in response to a series of constant deflections administered to the probe over a period of time. As a result, conventional probes used regularly over a period of time tend to suffer from degradations in their ability to supply constant resistive forces to uniform deflections administered over a period of time. Second, conventional probes comprised of residual stresses are more likely to break in response to a deflection. In contrast, the etched probes  81  of the present invention are created from an etching process which does not require mechanical stamping or machining to achieve desired cross sectional characteristics. As a result, the etched probes  81  do not contain any residual stresses induced as a result of machining or stamping.  
         [0035]    As used herein, “yield strength” refers to the property of a probe to deflect, or yield, in a predominantly linear direction when a force is applied while retaining the ability to return to its original, non-deflected state absent the application of a force. The greater the yield strength of a probe, the greater the linear deflection that may be exerted upon the probe prior to the probe reaching its yield point, whereupon the probe will not return to its original shape. Applicants anticipate that the etched probes of the present invention exhibits increased yield strength compared to probes formed from mechanical processing. Specifically, Applicants anticipate that the etched probes may be deflected a linear distance approximately 20% greater than that distance through which a conventional probe may be deflected before reaching the yield point.  
         [0036]    In addition, it is anticipated that the etched probes of the present invention will possess improved spring force uniformity over probes formed in the conventional manner. As used herein, “spring force” refers to the opposing resistive force generated in a probe which is deflected through a distance. Specifically, it is anticipated that the maximum difference in the spring forces amongst all of the etched probes in a probe test head will be approximately 20% less than the maximum difference in the spring forces amongst all of the conventional probes in a similar probe test head apparatus.  
         [0037]    With reference to FIG. 10, etched probe  81  has a depth  1017  and a width  1019 . Depth  1017  is typically approximately 3 mils while width  1019  is typically approximately 1 mil. Because the etched probes  81  are considerably narrower than conventional probes  83 , when assembled in a probe head the etched probes  81  may be assembled spaced approximately every 4 mils while conventional probes  83  are typically spaced approximately every 8 mils. Because the center-center distance between the etched probes of the present invention assembled in a probe head can be as small as 4 mils, as opposed to the approximately 8 mils required of conventional probes, the etched probes may be used for testing smaller integrated circuits wherein the distance between contacts on the integrated circuit wafer is as small as approximately 4 mils.  
         [0038]    In addition, because a plurality of etched probes  81  is fashioned from a single flat stock  1201 , each etched probe  81  is substantially similar in its physical characteristics to each and every other etched probe  81  fashioned from the same flat stock  1201 .  
       EXAMPLE 1  
       [0039]    The following example details parameters preferable to practicing the present invention. Preferably, there is practiced a plurality of steps including material preparation, photo masking, etching, chemical polishing, plating, and a process of individualizing the probes thus formed. As used herein, “DI” is a descriptor meaning de-ionized. In addition, as used herein, “UX DI” refers to ultrasonically agitated de-ionized water.  
         [0040]    To prepare the material out of which the probes were to be formed, BeCu  17200  Flat stock was cut into squares with side lengths approximating four inches. The flat stock was then cleaned with Citra-solv (by Citra-Solv, LLC of Danbury, Conn.)/DI H2O 20ML/IL (UX 15 Min.). The surface of the flat stock was then air blown dry and the resulting package was then heat hardened in a vacuum for approximately two hours at 600° F.  
         [0041]    Next, the prepared material was photo masked. To accompish the photo masking, the material was again cleaned with Clean Citra-Solv/DI H2O 20ML/IL (UX 15 Min.). Next the material was provided a dip coat with a withdraw rate of 13.3 Sec./1 in. (Shipley SP2029-1) Thinned to 35 Zon/Sec. at 21° C. The material was then dried for approximately 30 minutes at 90° C. and allowed to cool at room temperature under conditions of greater than fifty percent relative humidity. Next, the prepared surface of the material was exposed to approximately 100 milijules 365 nanometer wavelength UV light. The surface exposed to the light was then developed for approximately 1 min 30 sec (Shipley 303 developer, by Shipley Inc. of Newton Mass., at 85° F.). Lastly, the prepared surface was rinsed in cascading DI water for 15 minutes then air blown dry and stored.  
         [0042]    Next, etching was performed using a Marseco Mod.# CES-24, by Marseco Inc. of Huntington Beach, Calif. Hi-speed circuit etching was then performed using Phibro-Tech High Speed Circuit etching solution with the following parameter settings:  
         [0043]    Temperature setting 128 deg. F. (act 127 deg. F.)  
         [0044]    Pump speed (Pump #1-45%) (Pump #2-73%)  
         [0045]    Conveyor (11%)  
         [0046]    Oscillation (Normal)  
         [0047]    A foil test piece was then mounted to the carrier and run through the etcher. The critical dimensions of the resultant parts created from the foil test piece were then measured and adjustments made if necessary. After adjustments were made, the remaining foils were run through the etcher at 30 sec. intervals.  
         [0048]    Next a chemical polish/bright dip was applied to the probes formed from etching. The probes were submerged in PNA Etch in a 2L beaker at 145-150° F. while stirring. The solution was comprised as follows:  
                                                       Phosphoric Acid   760 ML of a 98% solution           Nitric Acid   40 ML of a 69-70% solution           Acetic Acid   1200 ML of a 60% solution                      
 
         [0049]    First, the etch rate was established using a test piece of material. Next, the probe material was etched to remove 0.0001″ Next the material was rinsed in hot DI, in UX DI for approximately 15 minutes and a DI cascade for approximately 2 minutes. Lastly, the probes are oven dried at 100° C. until dry.  
         [0050]    Next, the probes were plated using a Pallamerse Immersion Palladium 5% solution, by Technic Inc. of Cranston Rhode Island, and a Pd activator 25% solution manufactured by Technic Inc. and a Vertrel solvent by Dupont Fluoroproducts of Wilmington, Del. The probes were then weighed and their weights recorded. The probes were then washed in the Vertrel solvent for approximately two minutes. Next, the probes were rinsed in DI H 2 O for one minute and in a 10% sulfuric acid solution for two minutes followed by another two minute rinse in DI H 2 O. The probes were then immersed for 30 seconds in the Technic Pd activator and once again rinsed in DI H 2 O for 30 seconds. The probes were then immersed for 45 minutes in Technic immersion Palladium while stirring slowly, rinsed with running DI H 2 O and dried. The probes were then re-weighed and their weights recorded.  
         [0051]    Lastly, the probes were individualized. A sample of the probes, preferably five or six probes, is tested to measure the grams of resistive force generated within each of the probes when deflected from one to eight millimeters in one millimeter increments. The results on one such test group of probes is illustrated in Table 1. The results of the test were used to assess the uniformity of the probes created from any one initial flat stock as well as conformity to desired properties. The probes were then put in a vile and labeled with tip and shank dimension.  
                                                                                                     TABLE 1                           Dim. 1 st     Force 1 st                                         Sample   Touch   Touch   1 mil   2 mil   3 mil   4 mil   5 mil   6 mil   7 mil   8 mil                                1   0   .0050   4.80   9.80   12.95   15.63   17.86   20.10   21.41   21.72       2   0   .0053   4.50   8.80   12.23   15.21   17.80   19.81   21.60   18.02       3   0   .0051   4.80   9.90   13.60   17.00   19.70   21.30   22.31   23.31       4   0   .0056   4.91   9.60   13.92   17.70   20.30   22.80   24.80   25.41       5   0   .0045   5.80   11.00   14.90   17.30   19.60   21.72   22.22   22.50       6   0   .0053   4.82   8.66   12.23   14.92   17.30   19.50   21.26   22.15                  
 
         [0052]    There is therefore provided herein a process for mass producing miniature micro probes or electrical contacts for use in the testing of semiconductor chips having the following advantages over the conventional probe manufacturing process. First the method of the present invention provides improved uniformity and dimensional accuracy between the probes. The glass mask determines the geometry of the probes, eliminating mechanical variances between the probes. As a result, the stiffness of the probes are more uniform, allowing for a balanced contact force across the array.  
         [0053]    In addition, there are no stresses induced in the probes during fabrication, resulting in improved probe strength and endurance. The conventional stamping process results in residual stresses, causing reduced fatigue life. The stresses can change over time, causing changes in probe stiffness.  
         [0054]    The present invention provides for lower cost and lead-time in manufacturing. Many probes are manufactured simultaneously, and the tip geometry can be made via the etching process rather than as a follow-on process step. The polishing and plating processes are also done simultaneously.  
         [0055]    The probe design of the present invention can be easily modified. The spring rate can be controlled by varying the artwork used to create the glass mask, and by the thickness of the flat metal stock selected. New designs can be made by simply creating a new mask. There is no need for expensive and time consuming re-tooling.  
         [0056]    Lastly, the etched probes produced by the method for the present invention do not require a swage to achieve the required stiffness. As a result, the probes can be placed closer together, allowing for a denser array.