Abstract:
An electron beam heating device with the temperature up to 2200 K is provided for heating a sample and a tip for a scanning tunneling microscope (STM). The electron beam heating device includes a base stage for mating respectively with an electron beam sample heating carrier and an electron beam tip heating carrier, both carriers include a filament. The integration of the filament into the transferable electron beam sample heating carrier and electron beam tip heating carrier enables filament exchange without venting the vacuum system. A fixed distance between the sample and the filament enables reproducible sample temperature control and the filament is mounted at a back of the sample, allowing optical access for temperature measurement, and allowing sample preparation processes without changing positions of the sample or the filament. Once the tip is loaded, a fixed relative position between the tip and the filament enables reproducible control of heating. A tip holder includes an electrically isolated post connected to the tip, enabling a separate electrical potential to be applied to the tip.

Description:
[0001]     This application claims the benefit of U.S. Provisional Application No. 60/705,123, filed on Aug. 3, 2005. 
     
    
     CONTRACTUAL ORIGIN OF THE INVENTION  
       [0002]     The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG- 38  between the United States Government and Argonne National Laboratory. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention relates to scanning tunneling microscopes, and more particularly to an electron beam tip and sample heating device for a scanning tunneling microscope (STM), such as an Omicron variable temperature scanning tunneling microscope (STM).  
       DESCRIPTION OF THE RELATED ART  
       [0004]     Scanning tunneling microscopy (STM) is an important tool for surface science characterization. STM has developed into a powerful surface analysis technique because it can provide three dimensional, real-space images of the surfaces at high spatial resolution. Providing a clean and flat sample and a sharp STM tip, atomic resolution can be achieved.  
         [0005]     By varying the tip-sample bias, STM can also probe the local electronic structures of the surface. This is often called scanning tunneling spectroscopy (STS).  
         [0006]     Scanning tunneling microscopes (STMs) can work in many different conditions, from ambient atmosphere to Ultra-high vacuum (UHV). UHV-STM is of particular interest for basic science research as the sample can be prepared and investigated in an environmentally clean manner. Recently, STM has also been successfully demonstrated to probe the magnetization orientation of the sample surface via tip magnetization modulation and/or spin polarized STS techniques.  
         [0007]     STM tips are typically made from W (tungsten), Au (gold), or Ptlr (platinum iridium) wires. Among them, W tips are most commonly used. STM W tips are usually prepared outside the UHV chamber with an electro-chemical etching method. The W tips are covered by a thin native oxide layer before being introduced into the microscope. The oxide layer needs to be removed in order to obtain stable tunneling conditions. Recently development of spin-polarized STS technique can provide magnetic imaging with high resolution, but requires heating the tip above 2200 K to remove the oxide layer.  
         [0008]     Omicron Nanotechnology, one of the manufacturers for a scanning tunneling microscope (STM), currently provides three techniques for heating the sample. One of these techniques is direct heating, by passing a current through the sample. The direct heating technique requires the sample to have a certain minimum resistance (2Ω). The direct heating technique can only reach a maximum temperature of 1500 K. Another of these techniques is radiation heating by a pyrolytic boron nitride (PBN) heating element, which is located behind the sample. However, this radiation heating method can only reach a temperature of 750 K. A third technique is electron beam heating, which can be applied to a low resistance sample and can reach high temperature. The electron beam heating design, while able to heat both the tip and the sample to high temperatures, places the filament in front of the sample surface where it blocks the optical access of a pyrometer for temperature measurement. Temperature measurement, however, is very important to annealing, since insufficient heating results in a rough surface and overheating can kill the sample. Further, the design requires tedious alignment between the filament and sample, which makes it difficult to achieve reproducible heating conditions.  
         [0009]     Principal aspects of the present invention are to provide an electron beam tip and sample heating device for a scanning tunneling microscope (STM), such as, an Omicron variable temperature scanning tunneling microscope (STM).  
         [0010]     Other important aspects of the present invention are to provide such electron beam tip and sample heating device substantially without negative effect and that overcome some of the disadvantages of prior art arrangements.  
         [0011]     Other important aspects of the present invention are to provide such electron beam tip and sample-heating device that utilizes a single electron beam heating stage that allows for heating of both the sample and the tip higher than 2300 K.  
       SUMMARY OF THE INVENTION  
       [0012]     In brief, an electron beam heating device is provided for heating a sample and a tip for a scanning tunneling microscope (STM). The electron beam heating device includes an electron beam sample heating carrier and an electron beam tip heating carrier. Both the electron beam sample heating carrier and the electron beam tip heating carrier include a filament. The filament of both the electron beam sample heating carrier and the electron beam tip heating carrier is respectively mounted between a pair of contact bars that are electrically isolated from the sample carrier and the tip carrier. A base stage is arranged for mating engagement with the electron beam sample heating carrier and the electron beam tip heating carrier. The base stage couples electrical current and high voltage to the electron beam sample heating carrier and the electron beam tip heating carrier.  
         [0013]     In accordance with features of the invention, the filament of the electron beam sample heating carrier is mounted at a back of the sample, allowing optical access for temperature measurement, and allowing sample preparation processes without changing positions of the sample or the filament. A fixed distance is provided between the sample and the filament, enabling reproducible sample temperature control.  
         [0014]     In accordance with features of the invention, two filaments or a double wire filament surrounds the tip on electron beam tip heating carrier. A relative position between the tip and the filaments is fixed once the tip is loaded, enabling reproducible tip temperature control. A tip holder includes an electrically isolated post connected to the tip, enabling a separate electrical potential to be applied to the tip.  
         [0015]     In accordance with features of the invention, the electron beam heating device can be mounted on a linear bellows with electrical feed-throughs, and can be easily installed into the transfer path of a vacuum load-lock without further modification of the main sample preparation chamber. The electron beam heating device can be used for heating of the sample and the STM tip higher than 2300 K.  
         [0016]     In accordance with features of the invention, the filaments are integrated into the transferable electron beam sample heating carrier and electron beam tip carrier, enabling replacement of filaments without venting the vacuum system. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred embodiments of the invention illustrated in the drawings, wherein:  
         [0018]      FIG. 1  is a side view illustrating an exemplary electron beam sample heating carrier in accordance with the preferred embodiment;  
         [0019]      FIG. 2  is a side view illustrating an exemplary electron beam tip heating carrier in accordance with the preferred embodiment;  
         [0020]      FIG. 3A  is a plan view illustrating a bottom plate of the exemplary electron beam tip heating carrier of  FIG. 2  in accordance with the preferred embodiment;  
         [0021]      FIG. 3B  is a side view of the bottom plate taken along the line A-A in  FIG. 3A  in accordance with the preferred embodiment;  
         [0022]      FIG. 4A  is a plan view illustrating a contact bar of the exemplary electron beam tip heating design of  FIG. 2  in accordance with the preferred embodiment;  
         [0023]      FIG. 4B  is a side view of the contact bar taken along the line B-B in  FIG. 4A  in accordance with the preferred embodiment;  
         [0024]      FIG. 5A  is a front view illustrating an exemplary base stage design receiving the electron beam sample heating carrier of  FIG. 1  and the electron beam tip heating carrier of  FIG. 2  in accordance with the preferred embodiment;  
         [0025]      FIG. 5B  is a side view illustrating the exemplary base stage design of  FIG. 5A  in accordance with the preferred embodiment;  
         [0026]      FIG. 5C  illustrates the exemplary base stage design of  FIG. 5A  mounted on a linear motion bellows with electrical feedthroughs in accordance with the preferred embodiment;  
         [0027]      FIGS. 6A, 6B ,  6 C, and  6 D are SEM images illustrating the exemplary tip before and after flash cleaning; and  
         [0028]      FIG. 7A  illustrates an exemplary topographic image; and  
         [0029]      FIG. 7B  illustrates a spectroscopy or magnetic image for the exemplary topographic image of  FIG. 7A  obtained by the use of the exemplary base stage design of  FIG. 5A  receiving the electron beam sample heating carrier of  FIG. 1  and the electron beam tip heating carrier of  FIG. 2  in accordance with the preferred embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]     In accordance with features of the preferred embodiments, an electron beam tip and sample heating device is provided for a scanning tunneling microscope (STM), such as, an Omicron variable temperature scanning tunneling microscope (STM). The electron beam tip and sample heating device includes a single electron beam heating base stage which is arranged for mating with an electron beam sample heating carrier of the preferred embodiment and the electron beam tip heating carrier of the preferred embodiment. The electron beam sample heating carrier and an electron beam tip heating carrier are enhanced arrangements used instead of conventional sample and tip carriers provided by Omicron Nanotechnology.  
         [0031]     Having reference now to the drawings, in  FIG. 1  there is shown an exemplary electron beam heating sample carrier generally designated by the reference character  100  in accordance with the preferred embodiment. The electron beam heating sample carrier  100  includes a bottom plate  102 , such as a molybdenum (Mo) bottom carrier plate. A plurality (four) of threaded rods  104  are fixed to the bottom plate  102  with a respective nut  106 , such as 4 Mo nuts.  
         [0032]     A respective ceramic bushing  108  on top of each of the bottom Mo nuts  106  is used to fix a pair of contact bars  110  with electrical isolation from a sample  112  and other parts of the sample carrier  100 . An S-shape filament  114  is mounted between the two contact bars  110 .  
         [0033]     In addition, a U-shape electron reflector  120  is connected to one of the contact bars  110  to reflect the electrons from heating the molybdenum (Mo) bottom carrier plate  102 . On top of the ceramic bushings  108 , a corresponding one of a plurality of threaded spacers  122  is used for fixing the ceramic bushings  108  and support the sample  112  in the desired position. The sample  112 , typically 4×6×1 mm 3 , is clamped between a sample support spring  124  and a top ceramic plate  126  which is fixed with the top Mo nuts  128 .  
         [0034]     In the electron beam heating sample carrier  100 , the sample  112  is electrically connected to the Mo bottom plate  102 . Molybdenum advantageously is used to from for the bottom plate  102  and nuts  106 , 128  due to its high temperature, oxidation resistant, and machining properties.  
         [0035]     Importantly, none of the contact bars  110  are electrically connected the bolts  104  or nuts  106 . This allows building a potential difference between a sample  112  and the filament  114  required for electron beam heating.  
         [0036]     During STM measurements, a cooling block clamp (not shown) is used to ground the Mo bottom plate  102 , and the electrically connected sample  112  is grounded, enabling the STM measurements. The electron beam heating sample carrier  100  requires parallel mounting between the ceramic plate  126  and the Mo bottom plate  102 .  
         [0037]     As the filament  114  is mounted at the back of the sample  112 , the electron beam heating sample carrier  100  allows easy optical access for optical temperature measurement; and simultaneous sputtering and annealing. In addition, the filament  114  is integral to the sample carrier with a fixed distance between the sample  112  and filament  114 . This enables reproducible control of the sample temperature and easy replacement of the filament  114  without breaking vacuum.  
         [0038]     The electron beam heating sample carrier  100  in accordance with the preferred embodiment has been successfully used to clean and anneal a Ru(0001) crystal and an Fe(001) whisker.  
         [0039]     Referring now to  FIG. 2 , there is shown an exemplary electron beam tip heating carrier generally designated by the reference character  200  in accordance with the preferred embodiment. The electron beam tip heating carrier  200  includes a bottom plate  202  and a top plate  204  that are fixed together with a plurality (four) of bolts  206 . The bottom plate  202  also is illustrated and described with respect to  FIGS. 3A , and  3 B.  
         [0040]     A pair of orientation locks or horizontal bars  210  and a magnet indicated by reference character  216  in  FIG. 3A  are used to orientate a STM tip holder generally designated by the reference character  220 . The STM tip holder  220  includes a plurality (three) of vertically extending posts  222 A,  222 B,  222 C and a horizontally extending member  224 .  
         [0041]     The central post  222 B is connected with a STM tip  226  and is thus in the same position relative to the orientation locks  210  and tip carrier  200 . A tip end  227  of the SMT tip  226  is positioned above the orientation locks  210 . This allows the positioning of a spring contact  518  for this post  222 B on a base stage  500  at a different potential from the rest of the tip holder  220  and carrier  200 . The base stage  500  is illustrated and described with respect to  FIGS. 5A, 5B , and  5 C.  
         [0042]     A plurality (four) of ceramic bushings  232  are used to electrically isolate a pair of contact bars  234  from the rest of the tip carrier  200 . The two contact bars  234  are illustrated and described with respect to  FIGS. 4A , and  4 B. A double wire filament  236  is used between the two contact bars  234  to provide thermo-emitted electrons for tip heating. The double wire filament  236  surrounds the tip  226 . A plurality of respective spacers  238 ,  240 ,  242  are provided on bolts  206  respectively between the base plate  202  and the orientation locks  210 , between the orientation locks  210  and the ceramic bushing  232 , and between the ceramic busing  232  and the top plate  204 .  
         [0043]     In the electron beam tip heating carrier  200 , the relative position between the tip  226  and the filament  236  is fixed once the tip is loaded. Therefore, reproducible conditions are achieved. The filament  236  is integrated into the electron beam tip heating carrier  200 , enabling easy replacement of the filament without breaking the vacuum. The electron beam tip heating carrier  200  has been successfully used to clean W tips and obtained magnetic imaging on MnFe(001) system with an Fe coated tip.  
         [0044]      FIGS. 3A and 3B  are respective plan and side views of the bottom plate  202  of the exemplary electron beam tip heating carrier  200  in accordance with the preferred embodiment. The bottom plate  202  conforms in shape and dimensions to a conventional base plate of a conventional tip carrier manufactured by Omicron Nanotechnology. The bottom plate  202  includes a plurality of openings  250  for receiving the bolts  206  and a predetermined shaped opening  252  for receiving the STM tip holder  222 . An outwardly extending tab  254  with a central opening  256  is provided for engagement with a gripper or a wobble-stick (not shown).  
         [0045]      FIGS. 4A and 4B  are respective plan and side views of the contact bars  234  of the exemplary electron beam tip heating carrier  200  in accordance with the preferred embodiment. Each contact bar  234  includes a pair of openings  260  for receiving the bolts  206  and a tapered recessed portion  262  generally centrally disposed between the openings  260 . Each contact bar  234  includes an outside flange or ledge portion  264 , as shown in  FIG. 4B .  
         [0046]     Referring to  FIGS. 5A, 5B , and  5 C there is shown an exemplary base stage generally designated by the reference character  500  in accordance with the preferred embodiment. The base stage  500  is arranged for receiving and for coupling electrical current and high voltage to the electron beam sample heating carrier  100  and the electron beam tip heating carrier  200  in accordance with the preferred embodiment.  
         [0047]     The base stage  500  is made of a UHV compatible and machinable insulating ceramic, named Macor (by Corning Inc). The base stage  500  is a rectangular bar of Macor with a plurality of (three) slots  501 ,  502 ,  504 , and a plurality of sets of thru-holes  506 ,  508 , 510 ,  514 . A first large square slot  501  mates with the bottom plate of the sample or tip carrier  100  or  200 , another smaller square slot  502  provides clearance of the three posts  222 A,  222 B,  222 C of the tip holder  220 , and one long slot  504  holds a W wire spring  518  for HV contacting to the back plate of the sample carrier  200  or the post  222 B connected to the tip  226  on the tip holder  220 .  
         [0048]     The two outer pairs of small holes  506 ,  508  are contact brush mounting openings used to mount the contact brushes (not shown) and each of the contact brushes are electrically connected to the electrical feed thru where the filament current is applied. The two inner pairs of small holes  510 ,  512  are clamp mounting openings used to mount clamps for securing the bottom plate  102 ,  202  of the sample carrier  100  and the tip carrier  200 .  
         [0049]     When the sample and tip carriers  100 ,  200  are inserted into the base stage  500 , the bottom plate  102 ,  202  is fixed by the clamps and the contact bars  110 ,  234  in the carriersl 00 ,  200  are engaged with the respective contact brushes. In such assembled condition, the current can pass through the filament  114 ,  236  and thermo-emitted electrons can be generated. The single small hole  516  is used to mount a W spring wire contact  518  in slot  504 .  
         [0050]     When the tip carrier  200  is inserted to the stage  500 , only the post  222 B of the tip holder  220  that is connected with the tip  226  can touch the W spring  518  where the high voltage is applied. Therefore, electrons can be focused onto the tip end  227  to flash the tip  226 .  
         [0051]     When the sample carrier  100  is inserted to the stage  500 , the base plate  102  can touch the W spring  518  where the high voltage is applied. Therefore, electrons can be focused onto the back of the sample  104  to heat the sample.  
         [0052]     As shown in  FIG. 5C , the bottom two big holes  514  are used to mount the stage  500  to an elongated plate  522  and a 2.75 inch flange  524  with the electrical feedthroughs. The flange  524  is mounted on a linear motion bellows  530 , for example, as shown in  FIG. 5C . The whole setup is mounted to a flange perpendicular to the transfer path of a vacuum load-lock (not shown) without further modification of the main sample preparation chamber (not shown)  
         [0053]     Referring to  FIGS. 6A, 6B ,  6 C, and  6 D, there are shown SEM images illustrating the exemplary tip before and after flash cleaning. Scanning electron microscopy images of a fresh prepared tip are shown in  FIGS. 6A, 6C  before flash cleaning and of the tip after flash cleaning are shown in  FIGS. 6B , and  6 D.  
         [0054]      FIG. 6A  and its magnified view in  FIG. 6C  presents the fresh tip imaged with scanning electron microscopy (SEM). The tip  226  is prepared using electro chemical etching with 5% NaOH acid. After etching, the tip  226  is rinsed with de-ionized water for 10 min before introducing into the SEM for imaging with 10 kV electrons. The images of  FIGS. 6A, 6C  show a pointed tip  226  with a diameter of about 50 nm. After SEM imaging, the tip  226  was transferred into the UHV system via a load-lock system and flashed using the electron beam heating tip carrier  200 .  FIGS. 6B , and  6 D presents the same tip  226  shown in  FIGS. 6A, 6C  after flashing. Macroscopically, there is no significant change in tip shape. At higher magnification in  FIG. 6D , one can see that the tip  226  becomes blunt after flashing. The estimated diameter of the tip end  227  is about 300 nm suggesting that parts of the tip end were melted.  
         [0055]     We choose Mn/Fe(001) system to demonstrate the magnetic imaging capability as the system has shown layer by layer antiferromagnetism with both spin polarized (SP)-STM and SP-STS. The experiments are performed in a UHV chamber with the base pressure of 4×10 −11  mBar. The system is equipped with an Omicron variable temperature STM, low energy electron diffraction, Auger electron spectroscopy and a Magneto-optical Kerr effect apparatus. After flash cleaning the tip  226 , the tip is coated with 6-10 ML Fe at room temperature. In-plane magnetization is expected with this thickness range of coating.  
         [0056]     Before being introduced into the STM, the coated tip is further annealed at about 670 K for 1 min and cooled down to room temperature in a magnetic field of 1700 Oe and in the direction of the Fe whisker long axis. The Fe whisker with the dimension of 4×1×0.5 mm 3  is initially annealed in H 2  with the pressure of 5×10 −4  mBar for 5-7 days to deplete the C, O, S and N impurity as described previously. After pumping the H 2  away from the system, the Fe whisker is subsequently hot sputtered with Ar +  ions at ˜870 K. The ion energy of 800 V and the angle of incidence of 60° are used for the sputtering. After sputtering, the whisker is further annealed at ˜1020 K for 3 min far below the bcc-fcc phase transition temperature of 1220 K. Mn with the purity of 99.99% is deposited by electron beam evaporator at the substrate temperature of ˜370 K. After the deposition, the sample is transferred into the STM chamber for the measurement. STM and STS are performed at room temperature. A lock-in technique with typical modulation frequency of 9.2 kHz and amplitude of 30-50 mV is used for the STS measurements.  
         [0057]     We find that Mn grows in a Stranski-Krastanov mode on Fe(001) at 370 K, in good agreement with previous findings. The first 3 monolayer (ML) Mn grows in a layer-by-layer fashion. Above 3 ML, 3 dimensional growth appears, resulting in Mn layers with several different heights.  
         [0058]      FIG. 7A  presents an exemplary topographic image of ˜7.4 ML equivalents of Mn on Fe(001) that was obtained with bias voltage of −0.4 V and feedback setting of 1 nA. In this image, we can see 5 different layers of Mn are exposed. The different Mn layers are indicated by the corresponding thickness in ML.  
         [0059]      FIG. 7B  shows the spectroscopy (magnetic) image simultaneously obtained with topographic image shown in  FIG. 7A . It shows alternating dark and bright contrast corresponding to the odd/even Mn layer. The contrast changes its sign at +0.25 V. We note that no contrast is observed when the W tips without Fe coating are used. This evidences that Mn on Fe(001) has the layer-by-layer antiferromagnetism in good agreement with previous findings.  
         [0060]     While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.