Abstract:
A monolithic probe having an integral fine probe point, pressure spring, conductive line, and connector for contacting semiconductor devices to be tested and a method of construction of said probe is described. Integration of a serpentine spring into the probe body reduces breakage and improves contact reliability. Standard, coaxial, triaxial, and Kelvin probes are described. The methods of construction described utilize standard semiconductor processes. The probes may be fabricated to very small dimensions.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an apparatus for contacting semiconductor devices and circuits to be tested and more specifically, it relates to an improved probe having an integral fine probe tip, pressure spring, conductive line, and connector for contacting said semiconductor devices and a method of constructing the improved probe.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the course of fabricating semiconductor devices and circuits it becomes necessary to electrically probe the devices and circuits to ascertain proper functioning and for analysis of parameters and determination of failure mechanisms. To accomplish this a finely pointed probe or group of finely pointed probes is brought into contact with the device, circuit wiring, or pads connected to the device or circuit.  
           [0003]    A typical probe in wide use is formed by sharpening the end of a fine tungsten wire to a pointed tip. This wire is then mounted in a spring loaded manipulator. As semiconductor devices become smaller and circuits denser it becomes difficult make electrical contact with the device, circuit wiring, or pads for two reasons. Firstly, the probe tips may be too or blunt to make contact only to the intended the device, circuit wiring, or pads connected to the device or circuit and the device, circuit wiring, or pads. Secondly, the probe tips or wires from which they are fabricated are so thin as to bend when contact is attempted and slide off the intended contact point when sufficient pressure is placed on the probe tip to make low resistance electrical contact to the device, circuit wiring, or pads.  
           [0004]    The present invention solves the foregoing problems by providing a finely pointed probe tip small enough to contact only the device, circuit wiring, or pads that combines both stiffness and means to prevent bending when pressure is applied.  
         SUMMARY OF THE INVENTION  
         [0005]    The probe tip of the present invention has a body comprising a finely formed tip tapering to a point, a spring comprising horizontal and vertical members in the form of a serpentine and a connector for hookup to a tester. Normal semiconductor processes are used to fabricate the probe assembly, therefore the probes may be fabricated with tip dimension of a few microns and points in the sub-micron regime.  
           [0006]    The monolithic body is formed from a materials such as polysilicon that may flex many times without breaking. Since this material may not be highly conductive, provision is made for a conductive line, typically a metal, running from the tip along the spring to the connector. Metal silicide may be applied to the tip to improve adhesion of the wiring to the monolithic body. The monolithic body is formed by either filling a trench in an oxide layer with a material such as polysilicon and after forming the metal lines and silicide, the body released by dissolving the oxide. Therefore it is an object of the present invention to provide monolithic micro probes having an integral fine probe points, pressure springs, conductive lines, and connectors for contacting semiconductor devices to be tested and a method of fabrication of such probes.  
           [0007]    Probe tips fabricated by the method of the present invention may also be fabricated having single or double shielding layers effectively providing for coaxial and triaxial wiring up to the probe point. Several probe bodies may be formed at the same time, attached to each other in a tree. Instead of a conductive line, a first conductive layer may be deposited over the entire tree followed by alternating layers of insulator and further conductive layers, affording the capability of coaxial and triaxial protection to the signal in the main body or main body/conductive line. The tip would be selectively dip etched to remove the overlaying layers to expose the first conductive layer. Similar etching operations would be performed at the connector end. Therefore it is further object of the present invention to provide a micro probe having conductive shielding surrounding a central conductor surrounding an integral probe point, pressure spring, and connector.  
           [0008]    After a coaxial version of the probe is fabricated, a Kelvin type probe may be fabricated by plating a conductor over the tip, electrically connecting the inner and outer conductors together at the very tip of the probe, while still maintaining its sharpness. Therefore it is still further object of the present invention to provide a micro Kelvin type probe having conductive shielding surrounding a central conductor that surrounds an integral probe point, pressure spring, and connector, wherein the inner conductor and outer shielding are electrically connected together at the probe tip. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]    The invention as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0010]    [0010]FIG. 1A is cross-section view of the preferred embodiment of the micro probe shown in FIG. 1A;  
         [0011]    [0011]FIG. 1B is an side view of the preferred embodiment of the micro probe according to the present invention;  
         [0012]    [0012]FIG. 1C is side view of an alternative probe tip of the preferred embodiment of the micro probe shown in FIG. 1A;  
         [0013]    [0013]FIG. 2 is a top view illustrating the formation of multiple monolithic probe bodies attached to a tree;  
         [0014]    [0014]FIG. 3 is a side view of a micro probe according to the present invention, illustrating the relative scale of a portion of the probe;  
         [0015]    [0015]FIGS. 4A through 4F are partial cross-sectional views through section AA of FIG. 1 of a first method of fabrication of the micro probe according to the preferred embodiment of the present invention;  
         [0016]    [0016]FIG. 4G is a side view of the probe tip of the micro probe according the preferred embodiment of the present invention;  
         [0017]    [0017]FIG. 4H is a end view of the probe tip of the micro probe shown in FIG. 4G;  
         [0018]    [0018]FIG. 4I is an top view of the probe tip of the micro probe shown in FIG. 4G;  
         [0019]    [0019]FIGS. 5A through 5F are partial cross-sectional views through section AA of FIG. 1 of a second method of fabrication of the micro probe according to the preferred embodiment of the present invention;  
         [0020]    [0020]FIGS. 6A through 6G are partial cross-sectional views through section AA of FIG. 1 of a method of fabrication of the micro probe according to another embodiment of the present invention;  
         [0021]    [0021]FIG. 6H is a top view of the probe tip portion of the micro probe shown in FIGS. 6A through 6G;  
         [0022]    [0022]FIG. 6I is a side view of the probe tip of the micro probe shown in FIG. 6H;  
         [0023]    [0023]FIG. 6J is an end view of the probe tip of the micro probe shown in FIG. 6H;  
         [0024]    [0024]FIG. 7A is a partial cross sectional side view of the probe tip of a coaxial embodiment of the micro probe according to the present invention;  
         [0025]    [0025]FIGS. 7B through 7F are end views of the tip of the micro probe through section BB of  
         [0026]    [0026]FIG. 7A illustrating fabrication of a coaxial micro probe tip according to the present invention;  
         [0027]    [0027]FIG. 7G is a top view showing connecting vias for electrical connection of the micro probe according to the coaxial embodiment to test equipment;  
         [0028]    [0028]FIG. 8A is a partial cross sectional side view of the probe tip of a triaxial embodiment of the micro probe according to the present invention;  
         [0029]    [0029]FIGS. 8B through 8G are end views of the tip of the micro probe through section CC of FIG. 8A illustrating fabrication of a triaxial micro probe tip according to the present invention;  
         [0030]    [0030]FIG. 9A is a partial cross sectional side view of the probe tip of a Kelvin type probe embodiment of the micro probe according to the present invention;  
         [0031]    [0031]FIGS. 9B through 9H are end views of the tip of the micro probe through section DD of FIG. 9A illustrating fabrication of a triaxial microprobe according to the present invention; and  
         [0032]    [0032]FIG. 9I is a top view showing connecting vias for electrical connection of the micro probe according to the triaxial embodiment to test equipment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]    Attention is directed to FIG. 1A which illustrates the present invention. The micro probe comprises micro probe body  10  with a front surface  12 , having a connector portion  20 , a spring portion  30 , and a tip portion  40 , which have been formed monolithically from undoped or doped polysilicon as a preferred material. Polysilicon has been chosen because of its ability to bend with low probability of stress cracking and the ability to form metal silicides, however metal or metal alloys of Al, Cu, Ti, Ta, W, or Au could be used, Formed on connector portion  20  is conductive pad  22 . Between connector  20  and conductive pad  22  is optional pad silicide layer 24 , formed primarily in micro probe body  10 . Spring portion  30  of micro probe body  10  comprises multiple horizontal sections 32  and multiple vertical sections  34  alternately joined to one another to form a serpentine. Athough three horizontal sections are shown, it should be understood that more or may be used as long as the resilting structure has the appropriate strength and resilience. Formed along horizontal sections  32  and vertical sections  34  is conductor  36 . External fillets  38  have been formed at the outside comers where horizontal section  32  and vertical sections  34  meet and internal fillets  39  have been formed at the inside corners where horizontal section  32  and vertical sections  34  meet. This reduces the likelihood of stress cracking at the corners of the serpentine shaped spring portion  30 . Tip portion  40  comprises sidewalls  42  tapering to tip point  44 . Tip gusset  46  has been formed to strengthen the attachment of tip portion  40  to spring portion  30 . Conductor  36  runs onto tip portion  40  terminating in tip conductor  39 . Pad  22 , conductor  36 , and tip conductor  39  may be formed to be one continuous conductor and may be formed of a metal, such as, Al, Cu, Ti, Ta, Ag, Au, Pt, W, TiN, or TaN. Between tip conductor  39  and tip portion  40  is optional tip silicide layer  48 , formed primarily in micro probe body  10 . Pad silicide layer  24  and tip silicide layer  48  which may be contracted of PtSi or CoSi. FIG. 1B shows the relative positions of pad  22 , conductor  36 , and tip conductor  39 . The pad silicide layer  24  and tip silicide layer  48  formed on front surface  12  of micro probe body  10  relative to rear surface  14  and edge surface  16  of micro probe body  10 . The tip point  44  extends from front surface  12  to rear surface  14  on edge surface  16  but silicide layer  48  does not, in this embodiment. FIG. 1C shows tip portion  40  when the tip silicide  48  is not used and the conductor  39 A is desired to extend to the edge surface  16  to become tip point  44 . From FIGS. IA through IC, it should be clear that tip point  44  of the tip portion  40  of this embodiment is “V” shaped when viewed from the top, tip point  44  is in reality a wedge rather than a true point and that conductor  36  and connector silicide layer  24  and tip silicide layer  48  have been formed on the same and only one side of micro probe body  10 . Note also that tip portion  40  extends past spring portion  30  so that spring portion  30  will not block the view of tip portion  40  during alignment to the device to be tested.  
         [0034]    [0034]FIG. 2 illustrates the formation of multiple monolithic probe bodies  10  attached to tree  50  by sprue elements  52  attached to runner  54 . Sprue elements  52  are attached to pad portion  20  of micro probe body  10 . This arrangement allows separation of the micro probes from the substrate. In some of the fabrication methods to be described, the individual probes are completed in tree form and need only be broken off. In other cases the intact tree  50  is subjected to further processing before the individual probes are complete and then broken off.  
         [0035]    [0035]FIG. 3 illustrates the relative scale between the connector  20 , pressure spring  30 , and probe tip  40 . The thickness of tip portion  40  is a function of the size of the device to be probed and could range from 0.5 micron or less to 2 microns or more. The ratio of tip portion  40  height to depth ranges from 5:1 to 100:1 as does the ratio of spring portion  30  height to depth, which would be adjusted to change the degree of elasticity and strength. It is possible to fabricate connector portion  20  thicker than spring portion  30  and tip portion  40 . In fact all three can be different thicknesses.  
         [0036]    Turning to methods of fabricating the preferred embodiment. FIGS. 4A through 4F are partial cross-sectional views through section AA of FIG. 1 showing a first method of fabrication of the micro probe according to the preferred embodiment of the present invention. In FIG. 4A silicon substrate  60  having SiO  2  layer  62  thickener than the desired width of the micro probe has been provided. In FIG. 4B trench  64  has been etched within oxide layer  62 , by patterning a layer of resist and reactive ion etching (RIE) followed by stripping the resist. The pattern used is constructed in the form of a tree  50  illustrated in FIG. 2. Note by forming this pattern in steps, the depth of trench  64  could be made a first depth in the portion of the pattern corresponding to connector portion  20 , a second depth in the portion of the pattern corresponding to spring portion  30 , and a third depth in the portion of the pattern corresponding to tip portion  40 . In FIG. 4C trench  64  has been filled with polysilicon, by chemical vapor deposition (CVD) of polysilicon followed by a chemical mechanical polish (CMP) to make the polysilicon and oxide surfaces coplanar, thus forming micro probe body  10 . In FIG. 4D connector portion silicide  24  and tip silicide layer  48  have been formed in micro probe body  10  by selectively etching the polysilicon and depositing a silicide forming metal such as Pt or Co, followed by an anneal step. The position of the silicide may be controlled by selective removal of metal from areas over polysilicon where silicide is not desired prior to anneal. In FIG. 4E pad  22  and conductor  36  have been formed by evaporation and subetch or reactive ion etch. In FIG. 4F finished micro probe  10  has been released by etching away oxide layer  62  with HF or HF/NH4F aqueous based etchants. FIGS. 4G through 41 are side, end and top views of tip portion  40 , which show that the point of the tip of this embodiment is a “V” shaped structure with tip point  44  being a line rather than a point, the conductor  36  and connector silicide layer  22  and tip silicide layer  48  have been formed on the side of micro probe body  10 .  
         [0037]    Attention is mow directed to FIGS. 5A through 5F are partial cross-sectional views through section AA of FIG. 1 which show a second method of fabricating the micro probe. In FIG. 5A silicon substrate  60  having a SiO2 layer  62  has been provided. A polysilicon layer  66  is formed on top of SiO2 layer  62  by chemical vapor deposition as shown in FIG. 5B. The thickness of polysilicon layer  66  corresponds to the finished depth of micro probe body  10 . In FIG. 5C polysilicon layer  66  has been etched in the form of a tree  50  as shown in FIG. 2., by patterning a layer of resist and reactive ion etching polysilicon layer  66 , but not oxide layer  62 , followed by stripping the resist. The connector portion silicide  24  and tip silicide layer  48  are formed into micro probe body  10  by deposition of a silicide forming metal such as Pt or Co, followed by an anneal step as shown in FIG. 5D. The position of the silicide may be controlled by selective removal of metal from areas over polysilicon where silicide is not desired prior to anneal. Note that there is silicide formation some depth in from tip point  44  as the polysilicon is exposed in this method. A blanket conductive layer is formed over the structure as shown in FIG. 5E. A pad  22  and conductor  36  are formed by evaporation and subetch or reactive ion etch as shown in FIG. 5F. The finished probe may be released by etching away oxide layer  62  with HF or HF/NH4F aqueous based etchants.  
         [0038]    Another method of fabrication of the micro probe is shown in FIGS. 6A through 6G which are partial cross-sectional views through section AA of FIG. 1. A silicon substrate  60  having SiO2 layer  62  is used as the starting material as shown in FIG. 6A. A trench  64  is etched into the oxide layer  62 , by patterning a layer of resist and reactive ion etching oxide down to the silicon substrate  60 , followed by stripping the resist as illustrated in FIG. 6B. The pattern used is shaped in the form of a tree  50  illustrated in FIG. 2. The trench  65  having sloping sidewalls  67  is etched in the silicon substrate  60 . For this method it is critical that the silicon substrate  60  have a crystal orientation of &lt;100&gt; and is etched with an an-isotropic etch. Suitable etchants include: a heated (65° C.) saturated aqueous solution of tetramethyl ammonium hydroxide, a heated saturated solution of potassium hydroxide in 80% isopropanol, a heated 30-40 wt % aqueous potassium hydroxide, or a refluxing ethylenediamine/pyrocatechol/water mixture. These mixtures etch along the &lt;111&gt; crystal plane much slower than along any other plane. The sidewalls of trenches etched in &lt;100&gt; silicon substrates will lie on the &lt; 111 &gt; crystal plane. Note by first etching the portion of the pattern corresponding to connector portion  20  in oxide layer  62  down to silicon and etching the silicon substrate ro a first pre-determined depth, followed by etching the portion of the pattern corresponding to spring portion  30  in oxide layer  62  down to silicon and etching the silicon substrate to a second pre-determined depth, followed by etching the portion of the pattern corresponding to tip portion  40  in oxide layer  62  down to silicon and etching the silicon substrate to a third pre-determined depth, three different depths of probe body in each of the three portions would be obtained. An oxide layer  68  is formed over all exposed silicon by either thermal oxidation or by deposition of silicon oxide. The trench  65  is then filled with polysilicon, by chemical vapor deposition of polysilicon followed by a chemical mechanical polish to make the polysilicon and oxide surfaces coplanar, thus forming micro probe body  10  as shown in FIG. 6D. The connector portion silicide  24  and tip silicide layer  48  are then formed in micro probe body  10  by deposition of a silicide forming metal such as Pt or Si, followed by an anneal step. The position of the silicide may be controlled by selective removal of metal from areas over polysilicon where silicide is not desired prior to anneal. The pad  22  and conductor  36  are formed by evaporation and subetch or reactive ion etch. The probe may be released by etching away oxide layer  62  with HF or HF/NH4F aqueous based etchants. FIGS. 6H through 61 show the tip portion  40  where it is clearly shown that the point of the tip of this embodiment has the shape of a three sided pyramid with tip point  44  being a true point and that conductor  36  and connector silicide layer  22  and tip silicide layer  48  have been formed on the same side of micro probe body  10 .  
         [0039]    [0039]FIGS. 7A through 7G show the steps used in fabricating a coaxial of the micro probe. FIG. 7A shows the tip region of a completed coaxial version of the micro probe. Consider that the process steps described above and illustrated in FIGS. 4A through 4C (optionally  4 D), or illustrated in FIGS. SA through  5 C (optionally  5 D) and, or illustrated in FIGS. 6A through 6E (optionally  6 F) have been completed. Consider that the entire probe body  10  will be coated with a first conductive layer  70 , followed by an insulator  72 , and a second conductive layer  74 . The first conductive layer  70  becomes the center conductor of the coaxial system to replace the pad  22  and the conductor  36  shown in FIG.  1 . The second conductive layer  74  becomes the outer or shield conductor of the coaxial system as will now be described in conduction with FIGS. 7B through 7F. which show side views of the tip  40  through section BB of FIG. 7A. The first step is to create the tip  40  shown in FIG. 7B by one of the processes indicated above, next a first conductive layer  70  is deposited over tip  40 . Suitable materials for the first conductive layer  70  include Al, Cu, Ti, Ta, Ag, Au, Pt, TiN, TaN, W. A first insulating layer  72  is then deposited over the conductive layer  70 . Suitable materials for first insulating layer  72  include SiO2 or Si3N4 formed by CVD or low pressure CVD or plasma assisted CVD processes. Next a second conductive layer  74  is deposited over the first insulating layer as shown in FIG. 7E. Suitable materials for the second conductive layer  74  include Al, Cu, Ti, Ta, Ag, Au, Pt, TiN, TaN, W. A portion of the second conductive layer  74  and first insulating layer  72  are removed from the vicinity of tip point  44  by dip etching or plasma ion etching as shown in FIG. 7F. Finally a via  73  is formed in the first insulating layer  72  and via  75  is formed in conductive layer  74  to provide connection to pad  22  on connector portion  20  of micro probe body  10  for hookup to test equipment. It is desirable that the first conductive layer  70  not be removed when the second conductive layer  74  and the first insulating layer  72  are removed, so compatible materials and etchants must be selected. For example, the first conductive layer  70  could be Au or TaN, the first insulating layer  72  could be SiO2 , and the second conductive layer  74  could be Al. The Al would be etched with a H3PO4/HNO3 acid mixture, and the SiO2 with HF or HF/NH4F aqueous based etchants. Other etchant/conductor combinations include NaHClO for W and H2O2/NH4OH for Cu.  
         [0040]    Steps in fabricating a Kelvin type probewill now be described in conduction with FIGS. 8A through 8G wherein FIG. 8A illustrates a completed Kelvin probe in the region of the spring portion  30  and tip portion  40 . It should be understood that the entire probe body  10  will be coated with a first conductive layer  70 , followed by an insulator  72 , and second conductive layer  74 . The first conductive layer  70  is intended to replace pad  22  and conductor  36  which becomes the center conductor of the Kelvin/coaxial system, and the second conductive layer  74  becomes the outer or shielding conductor of the Kelvin/coaxial system. Tip conductive layer  76  forms the Kelvin tip of the probe. Consider that the process steps described above and illustrated in FIGS. 4A through 4C (optionally  4 D), or illustrated in FIGS.  5 A through SC (optionally  5 D), or illustrated in FIGS. 6A through 6E (optionally  6 F) have been completed. FIGS. 8B through 8F are side views of tip  40  through section CC of FIG. 8A illustrating steps in making a Kelvin/coaxial micro probe. The first step is to create the tip  40  shown in FIG. 8B by one of the processes indicated above, next a first conductive layer  70  is deposited over tip  40 . Suitable materials for the first conductive layer  70  include Al, Cu, Ti, Ta, Ag, Au, Pt, TiN, TaN, W. A first insulating layer  72  is deposited over conductive layer  70 . Suitable materials for first insulating layer  72  include SiO2 or Si3N4 formed by CVD or low pressure CVD or plasma assisted CVD processes. A second conductive layer  74  is deposited over the first insulating layer  72 . Suitable materials for the second conductive layer  74  include Al, Cu, Ti, Ta, Ag, Au, Pt, TiN, TaN, W. A portion of the second conductive layer  74  and the first insulating layer  72  is removed by dip etching or plasma ion etching from the tip portion  40  in the vicinity of tip point  44  as shown in FIG. 8E. It is desirable that the first conductive layer  70  not be removed when the second conductive layer  74  and the first insulating layer  72  are removed, so compatible materials and etchants must be selected. It is preferred that the first conductive layer  70  not be removed when the second conductive layer  74  and the first insulating layer  72  are removed, so compatible materials and etchants must be selected. For example, the first conductive layer  70  could be Au or TaN, the first insulating layer  72  could be SiO2 , and the second conductive layer  74  could be Al. The Al would be etched with a H3PO4/HNO3 acid mixture, and the SiO2 with HF or HF/NH4F aqueous based etchants. Other etchant/conductor combinations include NaHCIO for W and H 2 O 2 /NH 4 OH for Cu. The tip point  44  is now be been plated with copper to form tip conductor  76  which connects the first conductive layer  70  to the second conductive layer  74  as shown in FIG. 8G. Other materials such as Al, Ti, Ta, Ag, Au, Pt, TiN, W can be used formed by deposition and etch.  
         [0041]    A triaxial version of a micro may be fabricated using the present invention. The steps which would be used to make a triaxaill probe are illustrated in FIGS. 9A through 91. FIG. 98A illustrates a region including the end of a spring portion  30  and a tip portion  40  of a completed triaxial probe. It is understood that the entire probe body will be coated with a first conductive layer  70 , followed by the insulator  72 , the second conductive layer  74 , the second insulating layer  78 , and the third conductive layer  80 . First conductive layer  70  is intended to replace pad  22  and conductor  36  which becomes the center conductor of this triaxial system, and the second conductive layer  74  becomes the middle conductor of the triaxial system. Finally the third conductive layer  80  becomes the outer or shield conductor of the triaxial system. Consider that the process steps described above and illustrated in FIGS. 4A through 4C (optionally  4 D) and described above, or illustrated in FIGS. 5A through 5C (optionally  5 D), or illustrated in FIGS. 6A through 6E (optionally  6 F) have been completed. FIGS. 9B through 9F are side views of tip  40  through section DD of FIG. 9A illustrating steps in making a triaxial micro probe system. The first step is to create tip  40  shown in FIG. 9B by one of the processes indicated above, a first conductive layer  70  is deposited over tip  40 . Suitable materials for the first conductive layer  70  include Al, Cu, Ti, Ta, Ag, Au, Pt, TiN, TaN, W. A first insulating layer  72  is deposited over the first conductive layer  70 . Suitable materials for first insulating layer  72  include SiO2 or Si3N4 formed by CVD or low pressure CVD or plasma assisted CVD processes. A second conductive layer  74  is deposited over the first insulating layer  72  which becomes the outer shield of the triaxial system. Suitable materials for second conductive layer  74  include Al, Cu, Ti, Ta, Ag, Au, Pt, TiN, TaN, W. A second insulating layer  78  is deposited over the second conductive layer  74 . Suitable materials for the second insulating layer  78  include SiO2 or Si3N4 formed by CVD or low pressure CVD or plasma assisted CVD processes. FIG. 9G illustrates the tip after the third conductive layer  80  has been deposited on the second insulating layer  78 . Suitable materials for third conductive layer  80  include Al, Cu, Ti, Ta, Ag, Au, Pt, TiN, TaN, W. A portion of the second and third conductive layers  74  and  80  respectively, and first and second insulating layers  72  and  78  respectively, are removed in the vicinity of tip point  44  by dip etching or plasma ion etching as shown in FIG. 9H. A first via  73  is etched in the first insulating layer  72  to expose the first conducting layer  70  and a second via  75  is etched in the second conducting layer  74  to expose first via  73  and to step back the second conductive layer from first via  73  as shown in FIG. 91. A third via  79  is etched in the second insulating layer  78  exposing first via  70  and second via  72  and a forth via  81  is etched in the third conducting layer  80  exposing first via  70 , second via  73 , and third via  79  and to step back the third conducting layer from third via  79 . It is desirable that the first conductive layer  70  not be removed when second conductive layer  74  and first insulating layer  72  are removed from probe tip  44 , so compatible materials and etchants must be selected. It is also desirable that first conductive layer  70  not be removed when the second and third conductive layers  74  and  80  and first and second insulating layers  72  and  78  are removed, so compatible materials and etchants must be selected. For example, first conductive layer  70  could be Au or TaN, first and insulating layer  72  and  78  could be SiO2 , and second and third conductive layers  74  and could be Al. The Al would be etched with a H3PO4/HNO3 acid mixture, and the SiO2 with HF or HF/NH4F aqueous based etchants. Other etchant/conductor combinations include NaHCIO for W and H 2 O 2 /NH 4 OH for Cu.  
         [0042]    The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.