Abstract:
One of the limitations to current usage of scanning thermal microscopes arises when one needs to obtain a thermal map of an electrically biased specimen. Current practice is for the conductive parts of the specimen to be passivated to prevent excessive current leakage between the tip and the conductive sample. The present invention eliminates the need for this by coating the probe&#39;s microtip with a layer of insulation that is also a good thermal conductor. Examples of both thermocouple and thermistor based probes are given along with processes for their manufacture.

Description:
FIELD OF THE INVENTION 
     The invention relates to the general field of atomic force microscopy with particular reference to performing scanning thermal microscopy on specimens that are conducting electricity. 
     BACKGROUND OF THE INVENTION 
     Scanning Thermal Microscopy (SThM) is a technique that uses the specimen&#39;s thermal conductivity as a contrast mechanism in imaging microscopic features. The temperature sensing probe in Atomic Force Microscopy (AFM) can be used for semiconductor material and device study such as locating hot spots created by short circuit defects in the sub micron regime. 
     Commercial SThMs use a miniature thermal resistor positioned at the end of a cantilever. If a small current is passed through the resistor, and the resistance is measured as the tip is scanned over the surface, a local temperature map of the specimen is produced based on the resistance changes. If, on the other hand, a large current is passed and the resistor temperature rises significantly above that of the specimen, the probe detects local changes in the local thermal conductivity of the sample. In the latter mode of operation, the thermal conductivity of the specimen, as presented at the surface, is an aggregate of any thermal conductivity variations down into the specimen. Changes in composition below the specimen surface will therefore produce a feature in the thermal map. 
     One of the limitations to current usage of the above probe is that if one needs to obtain the thermal map of an electrically biased specimen, the conductive parts of the specimen must be passivated to prevent excessive current leakage between the tip and the conductive sample. This is illustrated in FIG. 1 where microtip  12 , coated with sensing layer  13 , is seen to be located near the end of cantilever beam  11 . If a current is being passed through  13  and/or through specimen layer  15 , leakage, shown schematically as arrows  14 , will occur between probe and specimen. The prior art solution to this has been to coat the specimen with a layer of insulation. In many cases, however, this may be difficult and/or undesirable to do. 
     Additionally, commercial resistive probes are often prone to short circuiting between the conductors should any conductive contaminants end up between the conductive leads that connect to the tips. This is illustrated in FIG. 2 where cantilever beam  11  is seen extending out from one end of insulating substrate  21 . After fabrication of the sensing tip (as in FIG.1, for example), the substrate was coated with a conductive material which was then formed into connecting leads  22  and  23  by laser machining trench  24  through the metal down to the substrate level. Since said trench is only about 25 microns wide, a particle of conductive material, such as  25 , can, if it bridges trench  24 , short circuit the leads  22  and  23 . 
     The present invention enables SThM to be performed directly on electrically biased conductive samples. In addition, it minimizes the probability of its electrical conductors being shorted to each other by conductive contaminants. 
     A routine search of the prior art was performed and the following references of interest were found: Luo et al. in J. Vac. Sci. Technol. B 15(2) 349-359 discuss manufacturing techniques, some of which were used when the first embodiment of the present invention was first reduced to practice. U.S. Pat. No. 5,969,238 (Fischer) shows a thermoelectric probe tip process. U.S. Pat. No. 5,581,083 (Majumdar et al.) shows a sensor and tip for a scanning thermal microscopy, and U.S. Pat. No 5,811,802 (Gamble) shows a scanning microscope. Pylekki et al. (U.S. Pat. No. 5,441,343) show a thermal sensing scanning probe microscope using resistive sensors while Aslam et al. (U.S. Pat. No 5,488,350) show diamond film structures in different patterns for conducting, generating, and/or absorbing thermal energy. 
     SUMMARY OF THE INVENTION 
     It has been an object of the present invention to provide a scanning thermal microscope probe that may be used to scan a specimen through which an electric current is passing. 
     Another object of the invention has been that said probe be of the thermocouple type. 
     A further object of the invention has been that said probe be of the resistance thermometer type. 
     A still further object has been to provide processes for manufacturing of said novel probe types. 
     These objects have been achieved by coating the tips of both the above probe types with a layer of insulation that is also a good thermal conductor. This allows the probes to be used to scan specimens in which an electric current is passing without the presence of a leakage current between probe and specimen. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the cantilever beam and microtip of a scanning thermal microscope of the prior art. 
     FIG. 2 is a plan view of FIG.  1 . 
     FIG. 3 a  is a plan view of a first embodiment of the present invention. 
     FIG. 3 b  is a closeup view of the beam end of FIG. 3 a.    
     FIG. 3 c  is a cross-section through the microtip seen in FIG. 3 b.    
     FIG. 4 a  is a closeup of the beam portion of a second embodiment of the present invention showing a thermistor element. 
     FIG. 4 b  is a cross-section through the microtip seen in FIG. 4 a    
     FIG. 5 is low scale view of FIG. 4 a,  showing the substrate and leads 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention solves the problems that were noted in the earlier section by depositing a dielectric layer over the SThM tip to prevent current leakage between the tip and the electrically biased sample. The dielectric is made of materials that possess high thermal conductivity so as not to degrade the thermal sensitivity of the probe. The thickness of this layer is sufficient to effectively prevent direct short to the electrically biased samples as well as to minimize degradation to the probe&#39;s thermal sensitivity. 
     We will describe two processes for manufacturing probes of this type. In the course of these descriptions, the structures of the two probes will also become apparent. 
     1 st  Process Embodiment 
     Referring now to FIG. 3 a,  the first process embodiment begins with the provision of a planar substrate  21  from which there extends cantilever beam  11 . Microtip  39  is then formed near the far end of the beam on its lower surface (see closeup in FIG. 3 b ). Said microtip extends downwards and away from the lower surface of  11 . 
     We refer now to both FIGS. 3 a  and  3   c.  Next, a metal layer is first deposited onto substrate  21  followed by a layer of an inert metal. Suitable materials for the metal include titanium or aluminum while suitable materials for the inert metal include platinum and gold. The two layers are then patterned and etched to form connecting leads  32  and  33 . The inert metal serves to protect the metal against oxidation. Then, a buffer layer of the metal is deposited onto substrate  21  for good adhesion between the substrate  21  and metal layer  34  which is deposited onto both substrate  21  as well as cantilever beam  11 , including microtip  39 . On substrate  21 , the buffer layer and metal layer  34  are patterned so as to connect metal layer  34  to lead  33 . Layer  34  has a thickness between about 400 and 800 Angstroms. 
     The next step is deposition of insulating layer  36  over metal layer  34 , including the microtip, and then patterning layer  36  to enable it to insulate layer  34  from layer  35  while at the same time not covering connecting lead  32 . Insulation layer  36  has a thickness between about 800 and 1,200 Angstroms. Suitable materials include silicon oxide and silicon nitride. This is followed by the deposition of metal layer  35 , also over both substrate and microtip, over insulating layer  36  and then patterning layer  35  so as to connect lead  32 . Metal layer  35  has a thickness between about 400 and 800 Angstroms. As can be seen in FIG. 3 c,  layers  34  and  35  are separated from one another by layer  36  everywhere except at point  55  which is the thermocouple junction. The thermocouple junction can be formed by first bringing the probe tip into force-controlled contact with a metal-coated silicon substrate. With the scanner stationary, a voltage pulse is applied between the metal layer on the substrate (positive) and the metal film on the tip (negative). The high electric field in the immediate vicinity of the probe causes local metal evaporation, opening a hole only at the very end of the tip. Layer  36  at the tip will be exposed and then patterned and etched to expose underlying metal layer  34 . With the masking layer still in place, the probe tip is capped with metal layer  35  to form the thermocouple junction only at the probe tip. Suitable materials for layers  34  and  35  include combinations of gold and nickel, gold and platinum and platinum and nickel. 
     There now follows a key feature of the present invention. Insulating layer  38  is deposited on nickel layer  35 . It is important that, in addition to being electrically insulating, layer  38  also be a good thermal conductor. We have found that a thermal conductance greater than about 50 W/m.K should be adequate for proper operation of the present invention. Examples of suitable materials for layer  38  include diamond-like carbon, aluminum nitride, and silicon carbide. This enables the thermocouple junction to operate correctly even when in contact with a current carrying surface. Layer  38  has a thickness between about 200 and 500 Angstroms. A sufficient amount of layer  38  is removed from the surface of substrate  21  to expose leads  32  and  33  so that contact can be made to them. 
     2 nd  Process Embodiment 
     Referring now to FIG. 5, the second process embodiment begins with the provision of a planar substrate  21  from which there extends cantilever beam  11 . Microtip  39  is then formed near the far end of the beam on its lower surface. Said microtip extends downwards and away from the lower surface of  11 . 
     We refer now to both FIGS. 4 a  and  4   b  while continuing our reference to FIG.  5 . An aluminum layer is deposited onto substrate  21  and cantilever beam  11  and then patterned in such a way that microtip  39  is not covered while the aluminum layer is given the form of two, non-touching, halves  41  each of which includes a contact electrode  42 , said electrodes being located on opposing sides at the base of the microtip. 
     Using a focused ion beam, a layer of a thermistor material is then deposited in the shape of ribbon  43  that connects the contact electrodes  42 . The thermistor material should have a temperature coefficient of resistance greater than about 3,900 ppm per ° C. Our preferred material for the thermistor has been platinum but other materials such as copper, aluminum, or tungsten could also have been used. 
     There now follows a key feature of the present invention. Insulating layer  38  is deposited over the entire structure except for a section of the substrate which is left uncovered so as to allow contact to leads  41 . It is important that, in addition to being electrically insulating, layer  38  also be a good thermal conductor. We have found a thermal conductance greater than about 50 W/m.K to be adequate for proper operation of the present invention. Examples of suitable materials for layer  38  include diamond-like carbon, aluminum nitride, and silicon carbide. This enables the thermistor to operate correctly even when in contact with a current carrying surface. Layer  38  has a thickness between about 200 and 500 Angstroms. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.