Abstract:
A nanoprobe includes a substrate having a layer, which forms a projected portion. A plurality of conductive lines is adhered to the projected portion and the lines extend beyond an end of the projected portion by a distance to form contact points, wherein the lines are connected to material of the projected portion to provide stiffness and the contact points provide flexibility during use.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to probes, and more particularly to a multipoint nanoprobe and method for manufacturing thereof.  
         [0003]     2. Description of the Related Art  
         [0004]     Measuring the resistance of metal films and semiconductor wafers is typically performed using multipoint probes. These probes are employed for making electrical measurements. With increased interest in the local conductivity of films, there is increased demand for smaller probe dimensions. For example, there is a great deal of interest in measuring low resistance area product (RA) tunnel junctions using Current-in-plane Tunneling, in particular, for applications such as read heads for disk drives. To be useful with current technologies, the RA needs to be roughly about 1 Ohm-micron 2 .  
         [0005]     Current in-Plane Tunneling would be ideally suited for research in this highly competitive area; however, the present generation of microscopic four point probes is not well suited to measuring such a low RA stack. Specifically, data is needed at probe spacings of less than 1 micron, whereas the smallest standard microprobe in use today has a probe spacing of about 1.5 microns. There are several difficulties in making such a nanoprobe.  
         [0006]     For example, when the individual probes of a multiprobe structure are brought closer together, they necessarily need to be narrower. This decreases the spring constant, which must be maintained at a constant value in order to ensure reproducible contact to a sample being measured. One solution may include making the probes thicker, but this cannot be continued much beyond an aspect ratio of 1:1 in thickness to width before the probes become susceptible to twisting, or etching the probes becomes difficult. Making the probes shorter to increase the spring constant is also not feasible since this decreases the amount of compliance. That is, one needs to be able to overdrive the probes a few tenths of a micron, at least, in order to make sure that all probes are in good contact. Therefore, a minimum length is perhaps roughly 5 microns, though 10 microns would be safer.  
         [0007]     Therefore, a need exists for a multipoint probe, which maintains its elastic properties (e.g., spring constant), is relatively easy to manufacture and provides dimensional sizes, which are capable of measuring even the smallest features on a device or wafer.  
       SUMMARY OF THE INVENTION  
       [0008]     A nanoprobe includes a substrate having a layer, which forms a projected portion. A plurality of conductive lines is adhered to the projected portion and the lines extend beyond an end of the projected portion by a distance to form contact points, wherein the lines are connected to material of the projected portion to provide stiffness and the contact points provide flexibility during use.  
         [0009]     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]     The invention will be described in detail in the following description of preferred embodiments with reference to the following figures wherein:  
         [0011]      FIG. 1  is a side view of a nanoprobe in accordance with an exemplary embodiment of the present invention;  
         [0012]      FIG. 2  is a top view of the nanoprobe shown in  FIG. 1 ;  
         [0013]      FIG. 3  is side view of a substrate having a dielectric layer, seed layer and a conductive layer formed thereon;  
         [0014]      FIG. 4  is a side view showing the substrate of  FIG. 3  with a photoresist deposited on the conductive layer for patterning the conductive layer;  
         [0015]      FIG. 5  is a top view of the substrate of  FIG. 4  after patterning the conductive layer and removing the photoresist;  
         [0016]      FIG. 6  is a side view showing the substrate of  FIG. 5  with a photoresist deposited on top of the dielectric and conductive layers for patterning the dielectric layer;  
         [0017]      FIG. 7  is a top view of the substrate of  FIG. 6  after exposing and developing the photoresist;  
         [0018]      FIG. 8  is a top view of the substrate of  FIG. 7  after etching the dielectric layer and removing the photoresist;  
         [0019]      FIG. 9  is a side view of the substrate of  FIG. 8  having a processed photoresist formed on the substrate opposite the side having the conductive layer deposited for selectively etching the substrate to expose a portion of the dielectric layer;  
         [0020]      FIG. 10  is a side view of the substrate of  FIG. 9  after the dielectric layer has been exposed by etching;  
         [0021]      FIG. 11  is a side view of the substrate of  FIG. 10  after removing the photoresist; and  
         [0022]      FIG. 12  depicts a nanoprobe during an electrical test of a wafer. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]     In accordance with the present disclosure, a nanoprobe and a method of manufacture are described. The nanoprobe of the present disclosure provides ample compliance and sufficient stiffness to be capable of permitting reproducible contact with a surface to be measured. In addition, these features of the probe are provided with smaller probe dimensions with less risk of twisting or yielding of the probe fingers.  
         [0024]     The present disclosure employs a hand or base that encapsulates the probes or fingers, which extend therefrom. The fingers can be made very short to maintain their stiffness, while the base provides compliance. The base may be considered a wide cantilever.  
         [0025]     A method for making such a nanoprobe includes depositing a metal layer. Then, the metal is patterned and etched to form the fingers. A second lithography step is used to define the base and expose the fingers.  
         [0026]     Referring now in detail to the figures in which like numerals represent the same or similar elements and initially to  FIGS. 1 and 2 , a multipoint nanoprobe  10  is shown in accordance with one embodiment of the present disclosure. Nanoprobe  10  includes four points or fingers  12 . Each finger  12  is employed to make contact with a wafer surface, a semiconductor circuit or chip or any other electrical or electronic component to be measured. Although nanoprobe or probe  10  includes four points the present disclosure is also applicable to other numbers of points or fingers. For example, in one embodiment  12  fingers are employed, in another embodiment only two fingers or even one finger are employed.  
         [0027]     The conductive surface of fingers  12  is formed from a conductive layer  15  patterned on a surface of a substrate  14 . Substrate  14  may include monocrystalline silicon or other material suitable for growing a dielectric layer  17  thereon. If substrate includes silicon, dielectric layer  17  may include, for example, a silicon oxide or a silicon nitride.  
         [0028]     After patterning conductive layer  15  to form fingers  12 , dielectric layer  17  is selectively removed to expose the tips of fingers  12 . This forms a base area or hand  16 . The thickness of hand  16  is adjusted to provide the right balance of stiffness and compliance in probe  10 . In addition, an extended portion  19  of fingers  12  may be adjusted. The longer the fingers  12  are the more compliance and less stiff the device is. Hand  16  may be modeled as a cantilever beam and calculations may be performed to determine appropriate dimensions to achieve desired results.  
         [0029]     In one embodiment, a thickness t of portion  16  is between about 100 nm to about 1000 nm. The corresponding thickness of conductive layer  15  may be between about 1% and about 10% of the thickness t. In a particularly useful embodiment, fingers are 300 nm by 300 nm in cross-section and extend past layer  16  by about 0.5 microns. A length L may be about 10 microns.  
         [0030]     Conductive layer  15 , which is used as the conducting surface of fingers  12 , may include a metal layer of between about 100 angstroms and 1000 angstroms. The metal layer preferably includes a noble metal or a metal that forms a conductive oxide. For example, metal layer  15  may include one or more of Ag, Au, Pt, Ir, Ru, Pd, or alloys thereof. In particularly useful embodiments, a Pt—Ir alloy is employed.  
         [0031]     Contact pads  18  may be formed concurrently with the patterning of conductive layer  15  to form fingers  12 . In other embodiments, pads  18  may be employed to connect to other circuits or systems, or be employed to add chips, devices or other components on substrate  14 , which would provide convenience or functional advantages for employing probe  10 . For example, a 12 point probe may employ a multiplexer (not shown) mounted on substrate  14  or layer  17  to permit selective activation of, e.g., four fingers  12  at a time for making measurements.  
         [0032]     Point-to-point pitch between fingers  12  may be about 600 nm. Other pitches may also be employed. It is advantageous to provide a probe having a smallest possible pitch, but retaining, elastic properties (e.g., spring constant) and compliance. Of course, the minimum pitch of fingers  12  relies on a smallest width possible for fingers  12 , which can still provide the desired properties of probe  10 .  
         [0033]     In useful embodiments, the fingers  12  may extend beyond the hand  16  by a length of about 0.5 microns. The spring constants may be between about 0.01 and about 100 N/m. Two criteria, which are of note, include reproducible positioning of the probe contacts and compliance. Another consideration includes the fatigue limit, measured in the number of successive engages the probe can withstand.  
         [0034]     Referring to  FIG. 3 , a method for fabricating a multipoint nanoprobe will now be described. Beginning with substrate  14 , a dielectric layer(s) is/are deposited by any known process, such as for example physical vapor deposition (PVD) chemical vapor deposition (CVD), or thermal growth. If substrate  14  includes crystal silicon, dielectric layer  17 , preferably includes silicon nitride or silicon oxide. After deposition of layer  17 , an etch step or polish step may be employed to prepare the surface of layer  17 .  
         [0035]     A seed layer  21  is deposited on the surface of layer  17 . Seed layer  21  is deposited to provide good adherance for conductive materials, which will be deposited in subsequent steps. In one embodiment, seed layer  21  includes Ti, Cr or a combination thereof. In particularly useful embodiments, a seed layer  21  of Ti or Cr is deposited at about 100 angstroms thick on the surface of layer  17 . Seed layer  21  may be deposited using, for example, a sputtering technique, or other PVD or CVD process.  
         [0036]     In a same processing chamber and by similar methods, conductive layer  15  is deposited on seed layer  21  taking advantage of seed layer  21  to form strong adhesion between conductive layer  15  and dielectric layer  17 . Conductive layer  15  may include a metal, an alloy or conductive oxides of metals as described above. In one embodiment, layer  15  includes a Pt—Ir alloy deposited on seed layer  21  at a thickness of between 100 angstroms to about 1000 angstroms. Other dimensions are also contemplated. Optionally, layer  15  may itself be used as a seed layer for a later electroless deposition step, as described below. For example, layer  15  may be a TaN layer if it is planned to deposit Ru by electroless plating later.  
         [0037]     As described above, layer  17  is deposited having a sufficient thickness to bolster the stiffness of the probe without losing compliance needed to preload the probe against the surface to be measured. Knowing the material properties and the dimensions of layers  17  and  15 , a calculation of cantilever spring constants for thin film materials can be determined to meet device specifications. These calculations may include assumptions about the elasticity of the materials and may employ superposition theory to calculate the desired quantities, such as spring constant, maximum deflection, cycle fatigue limit, etc.  
         [0038]     The dimensions, e.g., thickness of layers will be determined based on these determinations and any dimension lost to processing steps such as etching, etc. This is also applicable to the widths of fingers  12  which will be formed in the following steps based on lithography techniques.  
         [0039]     Referring to  FIG. 4 , a resist (photoresist or ebeam resist)  20  is spun onto the surface of layer  15 . Resist  20  is then processed in accordance with known lithography techniques to pattern layer  15 . Resist  20  is opened up on selected surfaces of layer  15  and used as a mask to protect portions of layer  15  remaining unexposed by the resist. Layer  15  is then etched by, for example, an anisotropic etching process to leave the illustrative pattern shown in  FIG. 5 .  FIG. 5  shows a top view of substrate  14  with layer  17  thereon left exposed where conductive layers  15  and  21  have been etched away.  
         [0040]     The illustrative structure shown in  FIG. 5  includes pads  18 , which may be used to make electrical connections to instrumentation (not shown) or to other devices or components, which may be formed on the probe or mounted thereon. As noted above,  FIG. 5  shows a four-point probe; however, the present disclosure is applicable to any number of probes.  
         [0041]     Referring to  FIGS. 6 and 7 , a photoresist (or ebeam resist)  32  is spun onto the surface of layers  15  and  17 . The resist  32  is lithographically processed to expose a portion of dielectric layer  17  and conductive layer  15  to form fingers  12  as will be described. This resist step defines the length of the fingers  12  which may be roughly 0.5 microns, for example. The resist  32  masks off a portion of dielectric layer  17 . Part of this masked area will form the base area or hand  16 . In addition, the conductive layer  15  of the fingers  12  (see  FIG. 7 ) masks off a portion of dielectric layer  17 . This masked area will form the fingers  12 . Dielectric layer  17  is then etched to remove it. Then, the resist  32  is removed.  FIG. 8  shows a top view with substrate  14  exposed where dielectric layer  17  has been etched away in regions  34 .  
         [0042]     At this point an optional step of encapsulating the front side of the substrate may be performed to protect the conductive layer  15  from the following step, which etches the substrate from the backside. A blanket layer of photoresist or some other material such as silicon nitride or silicon oxide can be deposited, completely covering layer  15 , the fingers  12 , the exposed portion of the substrate  34  and the dielectric layer  17 . If layer  17  is composed of silicon oxide, the blanket protection layer could be composed of silicon nitride or vice versa.  
         [0043]     Referring to  FIG. 9 , a photoresist  22  is spun onto an opposite side of substrate  14 . Photoresist  22  is lithographically processed to expose a portion of substrate  14 . This process is employed to release the base or hand  16  and fingers  12  (see  FIG. 2 ) by removing substrate  14  from below this area as shown in  FIG. 10 . Substrate  14  is selectively etched with respect to dielectric layer  17  such that layer  17  remains after material of substrate  14  has been removed from selected areas. Resist  22  is then removed as shown in  FIG. 11 .  
         [0044]     At this point, if the optional blanket protective layer was deposited, it is selectively etched with respect to layers  15  and  17 , so that all of the blanket protective layer is removed and none of layers  15  or  17  are removed.  
         [0045]     An optional electroless plating step may be performed to deposit additional metal onto layer  15 . This deposited material may include a metal, an alloy or conductive oxides of metals as described above, and may be of the same or different composition as layer  15 .  
         [0046]     Referring to  FIG. 12 , nanoprobe  10  may be employed in a plurality of applications. One preferred embodiment, employs nanoprobe  10  for taking electrical measurements from a surface, for example, a wafer surface  38 .  
         [0047]     In one application, nanoprobe  10  may be employed for characterizing tunnel junction film stacks. For such junctions, a magnetic field generator is used to generate a magnetic field, and a multipoint probe  10  having four or more probes, where the smallest spacing between any two of the multiple probes used during a resistance measurement has a spacing of, say 1.5 micron or less, and a resistance measuring module  28  coupled to the multi-point probe and adapted to measure resistance. The magnetic field is generated to place a semiconductor wafer  38  having a tunnel junction film stack into one of a plurality of magnetizations for the tunnel junction film stack. A resistance measurement by the resistance module  28  at these magnetizations at least partially characterizes the tunnel junction film stack. Additionally, various probes and contact pad  18  configurations may be provided.  
         [0048]     Advantageously, a multi-point probe  10  is described that permits many different voltage measurements to be taken very quickly. When using the multi-point probe  10 , a multiplexer  36  may be used to couple probes to the resistance-measuring module  28 . Probe spacings are generally selected to be within a predetermined distance from a length scale, which is related to the RA product of a tunnel junction film stack being measured.  
         [0049]     Module  28  and multiplexer  36  or other components, circuits or chips may be mounted or formed on probe  10 , or may be coupled to the probe by electrical connections  30 .  
         [0050]     Having described preferred embodiments of a multipoint nanoprobe and method for fabrication (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.