WO2006035403A2

WO2006035403A2 - Cleaning device and process for scanning tunneling microscopy (stm) tip

Info

Publication number: WO2006035403A2
Application number: PCT/IB2005/053195
Authority: WO
Inventors: Maurizio Iannilli; Nunzio Motta; Daniele Pecchi; Anna Sgarlata
Original assignee: Universita' Degli Studi Di Roma 'tor Vergata'
Priority date: 2004-09-29
Filing date: 2005-09-28
Publication date: 2006-04-06
Also published as: WO2006035403A3; ITRM20040464A1

Abstract

The present invention is referred to a device for effectively cleaning tips of a scanning tunnelling microscope (STM) probe in ultra high vacuum comprising means for bombarding (11, 20, 21, 22) a tip (1) with an electron beam, to a microscope as above specified including the latter a cleaning process for tips of a scanning tunnelling microscope probe in ultra high vacuum comprising the steps of: transferring a tip (1) on an insulated housing system (6); approaching a conducting filament (20) to said tip; applying a predetermined current to said conducting filament (20); applying a predetermined voltage between said conducting filament (20) and said tip (1) to be cleaned for a predetermined time, whereby said tip (1) is subject to an electron flux.

Description

CLEANING DEVICE AND PROCESS FOR SCANNING TUNNELING

MICROSCOPY (STM) TIP

DESCRIPTION

The present invention is referred to a device for cleaning tips of a Scanning Tunnelling Microscope (STM) suitable for annealing the tip to be cleaned "in situ" by electron beam bombardment, and to the process for using said device. The invention finds application in the improvement of performances of a Scanning Tunnelling Microscope (STM) that works in Ultra High Vacuum (UHV). In several research fields probe microscopes are used, being capable to get observations and measurements at high resolution. In particular, a Scanning Tunnelling Microscope allows to reach resolutions of the order of a fraction of an angstrom. Such resolutions require probes with extremely fine and sharp tips, even being terminated by only one atom. Tips irregularly shaped or covered by impurities cause false or low-resolution images.

When a certain experiment or measurement is started with a specified tip, whose characteristics are well determined, the possibility to regenerate the tip without substituting it represents an essential element to the success of the experiment, as well as an opportunity to save money, considering the high cost of tips.

The above problems are felt especially in Scanning Tunnelling microscopes that operate in Ultra High Vacuum (UHV, 10^"11 mbar); in fact it is necessary to limit the number of opening of the microscopy pre-chamber to insert new tips. Every opening, in fact, worsens the vacuum present in the vessel, essential element to guarantee the cleanliness of the sample surface and, as a consequence, the success of the observation. Several different devices for tip cleaning are knowN, based on a resistive heating in order to cause the desorption of the more external layers, usually oxidized.

These devices reveal a number of drawbacks, and among these the fact that, besides the tip, also the tip-holder, where the tip is inserted, is heated, causing the desorption of a large amount of impurities. Moreover, in many systems, the temperature that can be reached is limited, and often not sufficient to obtain the degree of cleanliness required for the tip.

Other devices for the "cleaning" of the tips are based on bombardment with an ion beam, but also in this case it is extremely difficult to control the effect of the "ion-etching", that sometimes can worsen rather then improve the state of the tip. The technical problem at the root of the present invention is providing a device for cleaning in Ultra High Vacuum the tips of a microscope probe, allowing to overcome the above drawbacks with reference to the known state of the art. Such problem is solved by a device for cleaning tips of the probe of an

STM Microscope, comprising means for bombarding a tip with an electron beam.

The main advantage of a device according to the present invention resides in heating only the tip towards which the electrons are focalised, while all the surrounding system stays unperturbed at ambient temperature.

Another advantage of the device consists in obtaining, at the same time of cleaning, a sharpening of the tips.

According to the same inventive concept, the present invention is referred moreover to a microscope that comprises said device and to a process for carrying out the cleaning of Scanning Tunnelling Microscopy

(STM) tips in Ultra High Vacuum comprising the following steps consisting in:

* transferring a tip on an insulated housing system;

* approaching a conducting filament to the tip;

* applying a predetermined current to the filament; * applying a predetermined potential difference between said wire and said tip to be cleaned, for a predetermined time.

The present invention will be in the following disclosed according to a preferred embodiment, provided with exemplificative and non limiting purpose with reference to the annexed drawings wherein: * Figure 1 shows a perspective view of a tip on which is acting a device according to the present invention;

* Figure 2 shows a perspective view of the tip in Figure 1 in a tip-holder element;

* Figure 3 shows a perspective view of a component of the device according to the invention;

* Figures 4a and 4b show different perspective views, illustrating the mounting of the tip-holder member of Figure 2 on the component of Fig 3;

* Fig 5 shows an elevational section of the device according to the invention; * Fig.6 shows a perspective view of the device according to the invention;

* Fig.7 show a tip object of the process according to the invention; * Figure 8 shows a STM image(50 nm x 50 nm) of a Si(IOO) surface with a misorientation of 8° (miscut angle) on which 5 MoI of Ge have been deposited at 600°C;e (a) and (b) panels are STM images acquired with the same STM tip respectively after and before the application to the tip of the cleaning process , as disclosed in the following;

* Figure 9 shows a STM image (1 μm x 1 μm) of a Si(100) surface measured with the same tip of Figure 8, with a different current; e

* Figure 10 shows STM images of a Si(001 ) surface; in panel (a) (300 nm x 300 nm) ordered holes realized by STM nanolithography are visible; in panel (b) (300nm x 300nm) it is shown the same sample after deposition of 5 MoI of Ge at 600° C and after the tip cleaning; in panel (c) an image (200 nm x 200 nm) is shown, acquired at a later time, where it is possible to appreciate the resolution and hence the good tip quality.

Each of the components till now introduced will be now described with greater detail, referring to the specific embodiments presented herein.

It will be intended that the present invention is suited to a number of embodiments alternative to that described till now.

The embodiment of the device described in the following is referred to the cleaning of a metallic tip 1 , preferably made by tungsten, of a probe generally used in the scanning tunnelling microscopes in Ultra High Vacuum. With reference to figure 1 , the probe comprises a disk shaped support 2 on which three legs 3, 4, 5 are attached and said tip is inserted at the centre of the support 2 and is electrically connected to the first leg 3 by means of a conducting medium not shown. According to the invention, the device comprises a housing system 6 wherein the tip to be cleaned are inserted. The housing system 6 is realised in insulating material and, according to the preferred embodiment the insulating material is a machinable glass ceramic named Macor^®. The shape of the housing system 6 is a parallelepiped, showing on a first side 7 and on a second side 8, respectively a first aperture 9 and a second aperture 10. The second aperture 10 is crossed by an insulated electrical feedthrough 11 connected to a high voltage power supply (not shown), while the purpose of the first entry opening is to house the first leg 3.

The first side has a first groove 12, with square cross-section, joining the corner of the first side 7 with the first aperture 9, and a second groove 13, with a square cross section, parallel to the first groove and with reduced length. The grooves 12, 13, carved out in the thickness of the insulating material, have the role of guides for the legs. Inside the housing system, the electrical feedthrough 11 and the first leg 3 are electrically connected.

According to the proposed embodiment, the probe is inserted in a tip- holder member 14 comprising a bottom plate 15 and a top plate 16, square or rectangular shaped, attached through four bars 17 placed in proximity of the four corners of the plates 15 and 16.

The bottom plate presents notches to insert the legs 3, 4, 5 of the probe, while the top plate 16, at the centre thereof, has a hole 18 that put the tip 1 in communication with the outside. The top plate has moreover a clamping device 19 that enable the transport of the tip-holder member 14 in the housing system 6 of the cleaning device where it is to be attached.

The present invention comprises, moreover, a conducting filament 20 apt to be passed through by a current, connected to a low voltage power supply 21 and attached to a translation device (not shown) through which it is possible to vary the distance between the conducting wire and the tip. According to a preferred embodiment, the translation device is a lamellar bellow.

According to the disclosed embodiment, the housing system for the tip to be cleaned, the electrical feedthrough apt to be electrically connected to the tip, and the conducting apt suited to be passed through by a current constitute the means whose function is to determine an electrical potential between the tip and the conducting filament so as to generate an electron beam which allows to clean and sharpen the tip.

The operation of the above-disclosed device will be apparent from the description of the relative use process that will be developed in the context of a preferred application, and precisely in the context of tip cleaning, preferably a tungsten tip of a Scanning Tunnelling Microscope.

As mentioned in the introduction, a Scanning Tunnelling Microscope allows to reach a very high resolution operating in different environments, e.g. gaseous, liquid and Ultra High Vacuum (UHV), enabling the application in a wide range of research fields like the study of materials for electronics, nanostructures, biotechnology, only to quote a few.

The Scanning Tunnelling Microscope according to the invention comprises an analysis chamber, a manipulator, a wobble stick and a tip cleaning device as described above.

The process of the invention for a Scanning Tunnelling Microscope according to the invention, operating in ultra high vacuum, provides, to be operated, that the analysis chamber is UHV compliant and that the electrical feedthrough as well is suited to UHV.

According to the preferred embodiment for the invention, the housing system 6 is bound to the manipulator and the tip-holder element 14 with the probe is carried and inserted by the wobble stick in the housing system 6.

In a first step of the cleaning process of the tip 1 , the manipulator is positioned so that the filament 20, through a translatory movement obtained preferably through a lamellar bellow, can reach a predefined distance from the tip. Such a predetermined distance in a cleaning process in the preferred context, considering the Microscope operating in Ultra High Vacuum, is less than about 5 mm and preferably between about 1 and 2 mm.

Then, a voltage is applied to the ends of the filament, determined in such a way that a predetermined current flows through it, with the purpose of producing an electron beam suitable to bombard the tip to be cleaned.

Preferably, this current will be in about the range 1 ,0+1 ,3 A, and more preferably about 1 ,2 A.

To create the electron flux, a voltage is applied to the tip 1 by a high voltage power supply 22 connected through the electrical feedthrough 11 while the filament 20 is grounded (fig 6).

Such voltage is preferably in the range 250+600 V, and more preferably in the range 350+400 V.

Under these conditions, such electron flux, suitable to heat the tip causing the desorption of the more external layers, is equal to an emission current preferably in the range 100+700 μA, and more preferably in the range 450+650 μA.

A tip is subjected to the above described cleaning process in an time interval between about 5 and 25 minutes, preferably at least 10 minutes, so that a sensible improvement of the images acquired through the cleaned and sharpened tip is obtained.

It has been possible to demonstrate that such cleaning process not only allows to improve the quality of acquired images on samples different for structural and electronic properties, but also allows to clean tips used for the so-called "STM nanolithography". This last process consists in using the tip of the microscope to create, in a controlled way, ordered matrices of holes or impurities that can represent preferential nucleation sites for nanometric size islands on the sample surface. For every system, the emission current reached by the tip during the treatment (l_em in microamperes) and the cleaning process duration (t in minutes) are reported.

Example 1 : Ge/Si(001) sample The first example concerns the effect of the tip cleaning in the case of a measurement on a sample obtained evaporating 5 MoI of Ge on a Si(IOO) surface with a misorientation of 8° (miscut angle). The (a) and (b) panels in fig 8 present STM images acquired with the same STM tip respectively after and before the application to the tip of the cleaning process described in the patent with an emission current equal to 700 μA and a duration of 9 min.

It should be noticed how, after the cleaning, the increased resolution allows to distinguish the single ordered atoms forming the SiGe islands facets.

Example 2 In figure 9, the STM images are shown of a Si(001) surface measured with the same tip of the preceding example, respectively before and after a 3 minutes cleaning process with l_em=400 μA.

Example 3

Recently, a special technique called Nano-lithography has been developed to create ordered nanostructures on surfaces.

This implies to use the STM microscope tip to generate on the surface an ordered distribution of holes and/or defects with sizes and periodicity of the order of the nanometers.

For achieving this aim, it is possible to apply to the tip a chosen value of bias voltage, or of the tunnelling current, or, as in the case shown in figure

10, it is possible to apply to the tip a "z" pulse. In other words, the tip is mechanically inserted into the surface, in order to create in a controlled fashion an ordered defect matrix at chosen positions. Of course, the same tip used to generate such defect array must allow to visualize the surface at a later stage, so that it is important to have the possibility to clean or regenerate the tip for the measurement in case it gets dirty or it is modified during the Nanolithography process.

In figure 10, an ordered matrix of holes realized by applying to the tip a "z" pulse and then inserting the tip in the sample surface is shown. The defects created in this way on the surface represent preferential nucleation sites for the successive growth of Ge islands (Fig 10b). The tip, once used for nanolithography, can be regenerated with the described cleaning process and becomes again capable to acquire high resolution images as shown in

Figure 10b and 10c.

* * *

A man skilled in the art, with the purpose of satisfying further and specific needs, will be able to apply a number of modifications and variations to the above described cleaning device for tips of a Scanning Tunneling Microscope, all however included in the frame of protection of the present invention and afferent to the same inventive core, all included in the sphere of protection of the claims set forward hereafter.

Claims

1. Tip cleaning device of a probe of a Scanning Tunnelling Microscope (STM) in ultra high vacuum comprising means for bombarding (11 ,20,21 ,22) a tip (1 ) with an electron beam.

2. Device according to the claim 1 , wherein said means for bombarding (11 , 20, 21 , 22) said tip (1 ) with said electron beam comprises:

* a housing system (6) apt to receive said tip (1 ) to be cleaned;

* an electrical feedthrough (11 ) apt to be electrically connected to said tip (1 );

* a conducting wire (20) apt to be passed through by a current; and

* means for establishing a voltage (22) between said tip (1 ) and said conducting filament (20) through said electrical feedthrough (11 ).

3. Device according to the claim 1 or 2 wherein said housing system (6) is made of an insulating material.

4. Device according to the preceding claim, wherein said insulating material is a ceramic material.

5. Device according to the preceding claim, wherein said ceramic material is Macor^®.

6. Device according to the preceding claim, wherein said means for establishing a voltage (22) between said tip (1 ) and said conducting filament comprise a high voltage power supply (22) capable to produce a voltage in the approximate range pf 250÷600 V.

7. Device according to the claim 6, wherein said voltage is in the range 300÷450 V.

8. Device according to anyone of the preceding claims, wherein said tip is electrically connected to said electrical feedthrough through a leg (3) of a support (2) of the tip (1 ).

9. Device according to anyone of the preceding claims, wherein said conducting filament (20) is positioned at a predetermined distance from said tip (1 ).

10. Device according to the claim 9, wherein said predetermined distance is about 1÷2 mm.

11. Device according to anyone of the preceding claims, wherein said conducting filament (20) is passed through by a predetermined current.

12. Device according to the claim 11 , wherein said current is in the approximate range 1 ,0 ÷ 1 ,3 A.

13. Scanning Tunnelling Microscope (STM) comprising:

* an analysis chamber;

* a manipulator; * a wobble stick

* a device for cleaning tips in ultra high vacuum according to anyone of the preceding claims.

14. Scanning Tunnelling Microscope according to claim 13, wherein said analysis chamber is characterised in that it is an Ultra High Vacuum chamber (UHV).

15. Microscope according to anyone of the preceding claims 13 or 14, wherein said electrical feedthrough is an electrical conductor provided by an ultra high vacuum electrical insulating sheath.

16. Microscope according to any of the claims from 13 to 15, where a translating device vary the distance of said filament (20) from said tip (1 ).

17. Microscope according to the claim 16 wherein said translating device is a lamellar bellow.

18. Process for carrying out the cleaning of tips of a Scanning Tunnelling Microscopy probe in UHV comprising the following steps: * transferring a tip (1 ) on an insulating housing system (6);

* approaching a conducting filament (20)to said tip;

* applying a predetermined current to said conducting filament (20);

* applying a predetermined voltage between said conducting filament (20) and said tip (10) to be cleaned for a predetermined time, whereby said tip (1) is subject to an electron flux;

19. Process according to claim 18, wherein said predetermined distance is less than 5 mm.

20. Process according to the claim 19, wherein said predetermined distance is in the range 1÷2 mm.

21. Process according to any of the claims from 18 to 20, wherein a predetermined voltage is applied at the leads of the conducting filament (20)

22. Process according to the claim 21 , wherein this voltage is in the range 2÷3 V.

23. Process according to any of the claims from 18 to 22, wherein said voltage generates a predetermined current.

24. Process according to any of the claims from 18 to 23, wherein said predetermined current applied to said conducting filament (20) is in the range 1 ,0÷1 ,3 A.

25. Process according to any of the claims from 18 up to 24, wherein a voltage in the range 250÷600 V is applied between said tip (1 ) and said conducting filament (20).

26. Process according to the claim 25, wherein such voltage is in the range 300÷450 V.

27. Process according to any of the claims from 18 up to 26, wherein said electron flux is in the range 100÷700 μA.

28. process according to the claim 27, wherein said electron flux is in the range 450-M350 μA.

29. Process according to any of the preceding claims, wherein said electron flux on said tip (1 ) is applied for a predetermined time.

30. Process according to the preceding claim, wherein said predetermined time is in the range 10÷25 minutes.

31. Process according to the preceding claim wherein said predetermined time is at least 10 minutes

32. Process according to any of the preceding claims, wherein said process occurs in Ultra High Vacuum.