Abstract:
A tip treatment device for use in an ultrahigh vacuum in situ scanning tunneling microscope (STM). The device provides spin polarization functionality to new or existing variable temperature STM systems. The tip treatment device readily converts a conventional STM to a spin-polarized tip, and thereby converts a standard STM system into a spin-polarized STM system. The tip treatment device also has functions of tip cleaning and tip flashing a STM tip to high temperature (&gt;2000° C.) in an extremely localized fashion. Tip coating functions can also be carried out, providing the tip sharp end with monolayers of coating materials including magnetic films. The device is also fully compatible with ultrahigh vacuum sample transfer setups.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The United States Government has rights in this invention pursuant to Contract No. DE-AC05-00OR22725 between the United States Department of Energy and UT-Battelle, LLC. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to scanning tunneling microscopes (STM), and more particularly to an easily removable STM tip treatment attachment for a scanning tunneling microscope that readily converts the STM to a spin polarization STM (SP-STM). 
     2. Description of the Prior Art 
     The scanning tunneling microscope (STM) has revolutionized the field of surface science, and a new field of nanotechnology has developed in which the STM is used as a primary tool for nanofabrication and characterization of nanoscale materials and structures. In a STM system, an atomically sharp wire tip is positioned by piezoelectric actuators above the surface of an electrically conductive sample. When the tip/sample distance is sufficiently small, typically 5-15 angstroms, the application of a small voltage between the tip and the sample leads to a quantum mechanical tunneling current. This tunneling current decays exponentially with increasing tip/sample distance by about one order of magnitude per angstrom. Accordingly, the current is localized to the apex of the tip where tip and sample are the closest. In one use of a STM, if the tunneling current is maintained constant while scanning the tip above the sample surface, the surface topography can be imaged with atomic resolution. 
     STM users have long wanted to detect the local magnetization of the surface below the apex of a STM tip. This would be done by making the tip sensitive to the spin polarization of the tunneling electrons, thereby imaging the magnetic domain structure of the sample with ultimate resolution down to the atomic scale. In order to realize a spin-polarization STM (SP-STM), the STM tip must be made of a magnetic material that exhibits an intrinsic spin polarization close to the Fermi level. 
     Substantial progress toward spin-polarization STM has been made over the last fifteen years (Ref. 1). However, it is still carried out with difficulty. Only a very few groups have succeeded in adding a spin polarization function to their STM systems at all. For those groups that have succeeded, the main problem continues to be that major modifications to the existing STM system are required, and as a result, the system becomes more limited, almost wholly dedicated to SP-STM alone. This means a consequent lack of versatility and compatibility for other needed tasks in STM systems that have been modified for spin-polarization STM. 
     An ideal wire tip for SP-STM must possess a good signal-to-noise ratio that can only be achieved if the apex atom exhibits a high spin polarization. Since the presence of adsorbents typically reduces the spin polarization, a clean environment and an inert tip material must generally be maintained. The most widely used STM tip material is tungsten wire. However, tungsten wire requires an extremely high annealing temperature (&gt;2000° C.) to clean. In one widely used STM tip carrier design, by Omicron Nanotechnology Corporation, the tungsten wire is mounted in a tip carrier that incorporates organic insulators and gold coatings which cannot stand such a high temperature. 
     In order to successfully flash a tungsten tip, a large temperature gradient (&gt;1800° C.) is impressed across the millimeter-long tungsten wire. Dipolar interaction (Ref 1) between the magnetic tip and the sample due to the stray magnetic field should be as low as possible because it may modify or destroy the sample&#39;s intrinsic domain structure that is the subject of the investigation. To fulfill this requirement, the magnetic coating area must be localized to the sharp tip end (apex) of the tungsten wire. 
     The present invention is an easily removable STM tip treatment attachment for a scanning tunneling microscope that readily converts the STM to a spin polarization STM (SP-STM). The STM tip treatment device can not only provides the spin polarization function, but is capable of providing other functions as well, including flashing an STM tip to high temperature (&gt;2000° C.) in an extremely localized fashion, general tip cleaning, and coating the tip apex with monolayers of coating materials. 
     REFERENCES 
     
         
         1. A. Kubetzka, M. Bode, O. Pietzsch, and R. Wiesendanger, “Spin-Polarized Scanning Tunneling Microscopy with Antiferromagnetic Probe Tips,”  Phys. Rev. Lett . Vol. 88, No. 5, Paper No. 057201, 4 pages, (2002). 
         2. O. Pietzsch, A. Kubetzka, M. Bode, and R. Wiesendanger, “Recent Advances in Spin-Polarized Scanning Tunneling Microscopy,”  Appl. Phys . A Vol. 78, pp. 781-785 (2004). 
       
    
     BRIEF SUMMARY OF THE INVENTION 
     In a preferred embodiment, the invention is a device for treating a scanning tunneling microscope (STM) tip, the STM including an STM tip affixed in the tip fixture of a tip carrier, the tip carrier homed in a tip transfer unit, and the tip transfer unit homed in a reception stage mounted on a manipulator head. In particular, the device comprises: 
     a high voltage spring contact in electrical contact with the tip fixture, the high voltage spring contact affixed to a contact block mounted on the tip transfer unit; 
     a high voltage contact bar mounted on the reception stage, the high voltage contact bar in electrical contact with the contact block; 
     a screening plate having an aperture, the screening plate mounted on the tip transfer unit such that the aperture is positioned directly over the apex of the STM tip; and 
     a ring filament heater mounted on the reception stage with the ring portion of the ring filament heater positioned directly over the screening plate aperture, such that the ring portion of the ring filament heater, the screening plate aperture, and the apex of the STM tip are aligned on a common axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of the STM tip treatment device of the present invention. 
         FIG. 2  is an exploded view of a modified tip transfer unit illustrating some parts of the STM tip treatment device mounted thereon. 
         FIG. 3  shows the mounting of a high voltage contact block in the modified tip transfer unit of  FIG. 2 . 
         FIG. 4  shows other component parts of the STM tip treatment device mounted on an STM tip reception stage. 
         FIG. 5  is a photomicrograph showing the magnetic domain present in the surface of a sample material as revealed by the STM tip treatment device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1-4 , the invention is a tip treatment device  11  for use in an ultrahigh vacuum (UHV) in situ scanning tunneling microscope (STM). The device  11  comprises a modified tip transfer unit  23  ( FIGS. 1-2 ) and a specially designed reception stage  74  that is mounted on a manipulator head  76  ( FIG. 4 ). The reception stage  74  includes a receptor  82  that contains a receiver  83  for accepting a tip transfer assembly  73  in the usual manner ( FIG. 4 ). The tip transfer assembly  73  comprises the modified tip transfer unit  23  with a standard STM tip carrier  12  therewithin ( FIG. 1 ). 
     In  FIG. 1 , a standard tip carrier  12  is shown in its usual placement within a tip transfer unit  23 . The tip carrier  12  normally comprises a base  16 , tip fixture  15 , tungsten wire  13  with its STM tip  14 , electrical contact legs  17  and organic leg insulators  18 . The tip carrier is coated with gold. The contact legs  17  are isolated electrically from the tip carrier  12  by means of the organic insulators  18  that cannot withstand high temperature. Typically, a tip carrier cannot be heated higher than 200° C. without damage to the insulators  18 . 
     The problem has been that users who want to carry out standard STM functions as well as those who want spin polarization functionality must frequently clean the tip (apex)  14  of the tungsten wire  13  by heating it to a very high temperature, sometimes to more than 2000° C. In order to avoid permanent damage to the tip carrier  12  during such heating, the tip carrier base  16  has to be kept at a relatively low temperature. This has always been a difficult and critical undertaking. Many users have tried to anneal the tip  14  for the purpose of removing some absorbent from the tip. Such tip cleaning may require only 600° C., for example. But even for this low a tip temperature, users frequently overheat the tip carrier base  16  and compromise the insulators  18  in the base. When the insulators break down, the tip carrier  12  becomes useless, and a new one has to be obtained. 
       FIG. 1  further shows a base plate  24 , upper plate  25 , and support posts  27 ,  53  of a standard tip transfer unit  23 . The STM tip treatment attachment of this invention modifies the standard tip transfer unit  23  to include a screening plate  37  and a high voltage spring contact  35 . The screening plate  37  is constructed as a solid plate with a small aperture  38  located so as to be close to the wire apex  14  when the tip carrier  12  is placed (homed) in the tip transfer unit  23 . The tip treatment device  11  also includes a ring filament heater  33  mounted in the reception stage  74 . More particularly, referring to  FIG. 4 , the ring filament heater  33  is mounted in the receiver portion  83  of the receptor  82  on the reception stage  74 . The ring heater  33  is mounted in the receiver  83  such that when the tip transfer assembly  73  is properly inserted (homed) in the receiver  83  ( FIG. 4 ), the ring portion of heater  33 , the screening plate aperture  38 , and the wire tip  14  are aligned on a common axis  26  as shown in  FIGS. 1 and 2 . The ring filament heater  33  is useful for electron bombardment heating of the STM tip  14  and for a number of other uses. 
       FIG. 2  shows the high voltage spring contact  35  in more detail. In  FIG. 2 , the high voltage spring contact  35  is a spring metal having two prong portions  65 , with a loop portion  66  connecting the two prong portions. Also in  FIG. 2 , the high voltage spring contact  35  is mounted on and electrically connected to a contact block  60  by a machine screw  67  and washer  68 . A portion  69  of the contact block  60  is wedge-shaped for the purpose of establishing an electrical connection with a high voltage contact bar  78  ( FIG. 4 ) when the tip transfer assembly  73  is moved into the receiver  83  of the reception stage  74 . 
     In  FIG. 2 , the tip carrier  12  is inserted into the opening  45  in the lower plate  24 , under the orientation lock plate  50 . It is then moved along the inward projecting support ledges  46  and guide rails  47  (see arrow  44  in  FIG. 2 ) with the guidance of the magnet  49  to its locked (home) position against magnet  49 , as is known. As this movement takes place, the tip fixture  15  enters the loop portion  66  of the high voltage spring contact  35 , and then slides between the two prongs  65 , establishing an electrical connection between the cylindrical tip fixture  15  and the two prongs  65  when the tip carrier  12  is fully homed against the magnet  49 . The construction is such that when the tip carrier  12  is firmly in place against the magnet  49 , the apex  14  of the tip wire  13  will be aligned on the axis  26 . 
       FIG. 3  shows the mounting of the contact block  60  in the tip transfer unit  23  in further detail. In  FIG. 3 , the lower plate  24 , orientation lock plate  50 , insulator support posts  53 ,  53 ′, and upper plate  25  are shown in their usual placement on the machine screws  52 ,  52 ′. The contact block  60  is held in place by insulators  61 ,  61 ′ and  62 ,  62 ′. The nuts  63 ,  63 ′ and  64 ,  64 ′ secure the assembly.  FIG. 3  also shows the attachment of the screening plate  37  directly beneath the upper plate  25 . 
     Referring to  FIGS. 1 ,  2 , the small aperture  38  in the grounded screening plate  37  is located directly above the STM tip  14 . This allows exposure of only the STM tip  14  to the electrons emitted from the ring filament heater  33 . The grounded screening plate  37  is useful for other functions as well. For example, an evaporation source  31  and evaporation source aperture  32  ( FIG. 1 ) may be installed along the axis  26  above the screening plate  37 . This allows applications such as coating of the STM tip  14  without exposing the tip carrier base  16  to the incoming flux. The screening plate  37  thus makes possible a built-in heat confinement function around the STM tip  14 , and also a localized tip coating function. Additional functions are made possible by the tip treatment device as will be described later. 
       FIG. 4  shows the reception stage  74  that receives the tip transfer assembly  73 . The reception stage  74  is mounted on the manipulator head  76  using machine screws  77 . The reception stage  74  includes a high voltage contact bar  78  in addition to the receptor  82  with its receiver  83 . In further detail, the receptor  82  is built on the reception stage base  75 . The receptor  82  is a stainless steel structure  84  supported on insulating legs  85 . The structure  84  incorporates the receiver  83 . The structure  84  is also designed to hold the ring filament heater  33 , which is rigidly affixed to the structure  84  by means of filament connecting blocks  86 ,  87 , filament connecting leads  88 ,  89 , and filament fixture  90 . 
     Also in  FIG. 4 , the high voltage contact bar  78  is affixed to the base  75  by insulating legs  79 , and is further supported at one end (the contact end  81 ) on the structure  84 , but is electrically insulated from the structure  84 . A high voltage connecting wire  80  attached to the contact bar  78  is used to supply a high voltage to the contact block  60  through the contact end  81  of the contact bar  78 . 
     The manner of applying high voltage to the STM tip  14  during electron bombardment heating will now be described. It will be recalled that when the tip carrier  12  is inserted into the tip transfer unit  23  and homed, the tip fixture  15  will be in electrical contact with the high voltage spring contact  35 . In  FIG. 4 , as the tip transfer assembly  73  is inserted into the receiver  83 , the contact bar  78  on the reception stage  74  makes electrical contact with the wedge portion  69  of the contact block  60 . This design allows application of high voltage only to the STM tip  14  by means of the high voltage connecting wire  80  while keeping the tip transfer unit  23  and the tip carrier base  16  grounded.  FIG. 2  shows that a portion  28  of one side of the upper plate  25  has been cut away to make room for the ring heater filament  33 . 
     Referring to  FIG. 1 , a tungsten wire (0.007-inch diameter) with thoria coating may be used as the ring filament heater  33 . The tungsten filament  33  allows a current flow of 3.5 A. A high voltage of about 1000 V from a voltage source  36  is applied between the filament  33  and the STM tip  14 . The STM tip  14  is positively biased so as to draw electrons from the filament  33  for heating of the tungsten tip. The filament  33  is positioned so the STM tip  14  will locate at the focus center of the filament heater ring  33  when the tip transfer assembly  73  is engaged in the receiver portion  83  of receptor  82 . 
     In operation, a current from a DC power supply  34  is applied to heat the filament heater  33 , and then a voltage from the high voltage source  36  is applied to the tip fixture  15 . The role of the screening plate  37  is critical whenever the tip  14  is cleaned, flashed, coated, or spin-polarized. The screening plate  37  protects the rest of the tip carrier  12  from the heating effects of the filament  33 . Without the screening plate  37 , electrons from the filament  33  can go everywhere; to the tip  14 , to the whole of the tip carrier  12 , and to the contact legs  17 . The screening plate  37  is thus very important because it localizes the heating effect, and prevents the electrons from bombarding anything but the tip  14 . The screening plate  37  also protects these structural parts from being coated with iron or other magnetic materials that are put on the tip  14 . 
     Some of the major functions needed for STM tips are tip preparation, tip spin polarization, tip cleaning, tip flashing, and tip coating. All of these functions can be carried out by employing the STM tip treatment attachment  11  in existing STM systems. 
     Tip preparation. An in situ preparation of magnetic thin film tips (Ref. 2) for SP-STM consists of the following preparation steps: Electrochemically etch a polycrystalline tungsten wire in a saturated solution of NaOH in distilled water. The etching procedure produces tips with a typical apex radius of 20-50 nm. Next, heat the tungsten tip to 2000° C. upon introduction into the UHV chamber. This step has been found to be very important for cleaning the tip and epitaxially coating the tip with magnetic monolayers. Lastly, epitaxially coat the tip with a magnetic film to a thickness of several monolayers. 
     Tip spin polarization. The orientation of the tip magnetization may be manipulated using the following steps: Adjust the magnetization direction by the choice of the coating material and coating thickness in monolayers (ML). For example: Gd (7-9 ML), GdFe (10-15 ML) and Cr (25-45 ML) usually provide a magnetization along the tip axis, i.e., perpendicular to the sample plane at low temperature. On the other hand, a Fe (3-10 ML) coating results in a tip magnetization parallel to the sample plane (Refs 1, 2). The magnetization direction may be switched using an external magnetic field as is known in the field. 
       FIG. 5  is a photomicrograph showing the magnetic domain present in the surface of a sample material as revealed by the STM tip treatment device. The bottom graph shows a topographic image of an iron film deposited on a copper substrate. It was taken with a tungsten tip and a standard STM system without the spin polarization function. The top and middle images show the dependence of magnetic domain structures on temperature. They were taken using an iron-coated magnetic tip and a standard STM system with a spin polarization function. When the temperature is lower than the Curie temperature (Tc) of the iron film (top image), bright and dark contrasts are revealed which correspond to two magnetic domains with opposite spin orientations as shown by the two arrows in the top image. When the temperature is above the Tc, the magnetic domain contrast disappears because the magnetic domain structures no longer exist at high temperatures. 
     Tip cleaning. The following steps may be used for UHV in situ STM tip cleaning: Mount an STM tip carrier in a tip transfer unit. This step can be done in UHV on the STM scanner stage using the coarse motion drive. Next, insert the tip transfer assembly into the receiver with the UHV wobble stick. In this step, the tip transfer assembly must be fully homed so that the STM tip is located on the centerline of the ring portion of the filament heater, and the contact end of the contact bar on the reception stage is engaged with the contact block on the tip transfer unit. A current of less than 3.5 A is then applied to the filament heater using the DC power supply. Lastly, flash the STM tip by applying high voltage (˜1000 V) to the STM tip. The tip temperature is monitored with an infrared pyrometer or by controlling the emission current of the filament. The high temperature tip flash is confined to the apex of the tip wire, and the other end of the wire, which is held in the tip fixture, is kept at much lower temperature. The tip treatment device thus achieves a controlled large temperature gradient across the tip wire, which enables cleaning of the tip sharp end while leaving the other tip carrier components unaffected. The tip treatment attachment can be used for general tip cleaning processes in most if not all STM systems. 
     Tip flashing. Tip flashing, described above, is the process of going to high temperature quickly for a very short period of time. Flashing burns off unwanted materials from the STM tip. 
     Tip coating. The following steps for UHV in situ STM tip coating may be carried out with the tip treatment device: Flash the STM tip at the required temperature. Then align the tip to an evaporation source using an UHV manipulator. Lastly, evaporate the coating material on the STM tip to the controlled thickness. 
     By changing the STM tip material and controlling the tip treatment process, scanning probes with special crystalline facets and special crystallographic orientations can also be prepared using this tip treatment device.