WO2012060782A1

WO2012060782A1 - Hard tip for scanning probe microscopy and method of its production

Info

Publication number: WO2012060782A1
Application number: PCT/SK2011/000022
Authority: WO
Inventors: Ján ŠOLTÝS; Vladmir Cambel; Karol Frohlich; Kristina HUŠEKOVÁ; Alexander ŠΑΤΚΑ; Goran Karapetrov
Original assignee: Elektrotechnický Ústav Sav; Centrum Vedecko-Technických Informacii Sr
Priority date: 2010-11-05
Filing date: 2011-10-26
Publication date: 2012-05-10
Also published as: SK50412010A3

Abstract

The solution relates to hard tip for scanning probe microscopy which is covered by RuO2 layer of the thickness from 0.5 to 40 nm using the atomic layer deposition method. The solution considers also the method of production of hard tips for scanning probe microscopy consist of heating the deposition and evaporation chambers of reactor to temperatures of 290 and 200 °C, respectively. Subsequently organometallic precursor dissolved in pyridine is injected in the reactor vacuum chamber using electromagnetic valve, while precursor and solvent vapours are transported into the reaction chamber using argon as a carrier gas. Subsequently, chemisorption and formation mono molecular layer of the precursor on the substrate take place. This is followed by argon purging step. Subsequently oxygen gas is introduced in the reactor and a chemical reaction takes place that results in RuO2 formation on the tips. Finally, the reactor is purged by argon. This cycle is repeated until desired thickness of RuO2 is obtained (usually between 0.5 up to 40 nm).

Description

Hard tip for scanning probe microscopy and method of its production Technical field

Present disclosure relates to hard tips for scanning probe microscopy (SPM) and methods of its production. In particular, it deals with the methods of covering these tips by thin oxide layers using chemical vapour deposition (CVD) in the regime of layer growth by atomic layer deposition (ALD).. Methods describe preparation of mechanically hardened tips that are electrically conducting.

Background of the art

Scanning probe microscopy (SPM) represents itself a set of experimental methods for characterization of surfaces with high spatial (atomically resolved) resolution. SPM techniques utilize the interaction of a sharp tip (probe) with the surface of the sample explored. SPM is used for imaging of the surface morphology and for spatial imaging of material characteristics - local viscosity, thermal conductivity, elasticity, surface potential, resistance, capacity, or magnetic (electric) field spatial distribution.

Moreover, the sharp tip is used for surface modification - for local oxidation of the samples, for material deposition (atoms, molecules), and for realization of devices at nanoscale (D.M. Eigler, E.K. Schweizer, Nature, 344, 524 (1990) a K. Matsumoto, M. Ishii, K. Segawa, Y. Oka, B. J. Vartanian, J. S. Hariss, Appl. Phys. Lett., 68 (1), 34 (1996)).

The key feature of the SPM techniques that guarantees excellent spatial resolution is the sharp tip. The tip is mostly several micrometers long, located at the end of cantilever. The cantilever serves for sensing interactions between the tip and the sample surface. Its movement follows the surface profile and deflects mostly in the vertical direction (perpendicular to the sample surface) depending on the local surface properties. The movement is usually detected by a laser beam reflected from the backside of the cantilever, although other detection methods are known to those skilled in the art. Tips are mostly fabricated from silicon, and the radius of the tip apex is from 5 to 50 nm.

Cantilever bending is used to detect Van der Waals forces between the tip and the sample in the case of atomic force microscope (AFM, one of the SPM techniques). Commonly bare Si tips are used in this case. Such tips used in scanning experiments become damaged depending on the hardness of scanned material, scanning technique and scanning parameters set. This is the main reason for covering tips by thin hard layers that increase their stiffness and prolong their lifetime. In general, one needs to cover the tips with either conductive or isolating layer.

In addition to scanning of the sample surface to probe its physical characteristics, AFM is used also for lithography, a method in which the surface of the sample is modified either by scratching or by local oxidation induced by the tip. Since lithography techniques are more invasive, the tips are destroyed at much faster rate than in conventional scanning. That is why one has to use very hard tips in this case. Moreover, in the case of local oxidation the tip must be conductive, because the current flows through the tip to the sample that is being oxidized. Up to now, mostly following layers were used to cover the tips: Ti/Pt, Pr/Ir, W₂C, TiN or diamond (nanodiamond) layer (B. Bhushan, M. Palacio, K. J. Kwak, Acta Materialia 56, 4233 (2008)).

There are several other reasons for covering the scanning probe tips by other materials. For example, to sense local magnetic forces in Magnetic force microscopy (MFM). Tips with evaporated or sputtered layer of magnetic material are fabricated (Z. Deng, E. Yenilmez, J. Leu, J. E. Hoffman, E. W. J. Straver, H. Dai, K. A. Moler, Appl. Phys. Lett. 85, 6263 (2004)). For example, to map local electric fields over the sample in Electric force microscopy (EFM, A. S. Hou, F. Ho, D. M. Bloom, Electron. Lett. 28-25, 2302 (1992)), which uses conductive tips fabricated by evaporation of thin metallic layer onto the AFM tips. During the EFM scanning, various voltages are applied between the tip and the sample surface.

The process of coating sharp tips is technologically difficult due to the particular shape of the AFM tip. Silicon sharp tips have usually conic shape with the diameter around tens of nanometers at the very end, with the diameter increasing due to coating to 40 - 60 nm.

Mechanical hardness of the protective coatings varies with the material, with highest durability exhibited by tips covered by nanocrystalline diamond layer. The technology of preparation of regular hard diamond layers with good adhesion onto the tip is difficult. Several CVD techniques are used which differ by the activation of the key gas. The methods used to coat the tips with nanocrystalline diamond include plasma enhanced chemical vapor deposition (PE-CVD), ion beam assisted CVD, UV enhanced CVD, and laser assisted CVD (Ph. Niedermann, W. Hanni, D.Morel, A. Perret, N. Skinner, P.-F. Indermuhle, N.-F. de Rooij, P.-A. Buffat, Appl. Phys. A 66, S31-S34 (1998)).

Common feature of all these continuos CVD processes is presence of excited gas phase in the vicinity of the substrate, formed by the gas activation. Gas phase should contain a particular form of carbon precursors, or particles specific to formation of diamond film and effective selective etchant ( e.g. hydrogen, oxygen, fluorine) serving to remove non-diamond carbon phases formed on the substtate surface or on the growing diamond layer. For the needs of local oxidation induced by AFM, the diamond layer should be conducting, and therefore these films are doped to increase their conductivity.

Mentioned drawbacks of the SPM tips covered by hard conducting films are solved by deposition of thin films of relevant oxide on Si tips using ALD technique. This technique allows for conformal oxide deposition with excellent adhesion and thickness controlled on atomic layer level. (K. Husekova, E. Dobrocka, A. Rosova, J. Soltys, A. Satka, F. Fillot, K. Frohlich, Thin Solid Films 518, 4701 (2010)).

Disclosure of the Invention

The origin of the enhancedhardness of the scanning probe microscopy tip is due to the coating by Ru02 thin film with thickness from 0.5 up to 40 nm by atomic layer deposition.

The method of production of hard tips for scanning probe microscopy consist of heating the deposition and evaporation chambers of reactor to temperatures of 290 and 200 °C, respectively. Subsequently organometallic precursor dissolved in pyridine is injected in the reactor vacuum chamber using electromagnetic valve, while precursor and solvent vapours are transported into the reaction chamber using argon as a carrier gas. Subsequently, chemisorption and formation mono molecular layer of the precursor on the substrate take place. This is followed by argon purging step. Subsequently oxygen gas is introduced in the reactor and a chemical reaction takes place that results in Ru02 formation on the tips. Finally, the reactor is purged by argon. This cycle is repeated until desired thickness of Ru02 is obtained (usually between 0.5 up to 40 nm). Organometallic compound bis(2,2,6,6-tetrametyl-3,5-heptandionato)(l,5- cyklooktadien)rutenium(II) is used as a precursor.

Application of the above mentioned production method of Ru02 material described in this invention will result in higher quality, enhanced hardness and conductivity of SPM tips. We will describe detailed preparation procedure of hard conducting tips for SPM.

Ru02 layers will be grown on SPM tips at low pressure in a hot wall reactor. Organometalic precursor dissolved in pyridine is injected into the vacuum reactor chamber using controlled valve, e.g. electromagnetic micro-valve. Precursor and solvent vapours are transported into the reaction chamber using carrier gas, e.g. argon. Deposition cycle consist of following steps: deposition of the precursor layer, removal of the precursor excess by carrier gas (argon), precursor decomposition caused by oxygen, layer growth and of subsequent removal of reaction by-products by carrier gas. Mentioned procedure is applied cyclically until desired thickness is obtained.

Sharp tips with or without special over layer are used for surface imaging and for the surface modification by scanning probe techniques. Tips without protective overlayer show decreased hardness and therefore lower lifetime, tips with over layer exhibit decreased sharpness and lower homogeneity of the tip coverage.

The methods described in this patent application solve improving the tip hardness, tip lifetime, and also homogeneity of their coverage. According to the patent application, Ru02 layers cover the tips using chemical vapour deposition in the regime of atomic layer deposition growth. Tips prepared using this method show high homogeneity of the Ru02 layer thickness, show enhanced hardness and at the same time high conductivity.

Brief Description of Drawings

Schematic of the ruthenium oxide (Ru02) deposition setup using ALD technique according to the invention is on the figure 1 and it is described as an example of the invention implementation, while by relational mark 1 is labeled precursor solution, 2 is injector, 3a depicts heater of the evaporation part of the reactor, 3b heater of the deposition part of the reactor, 4 is holder with AFM tips, 5 is pressure gauge, 6 is liquid nitrogen trap, 7 is regulating valve, 8 is rotary pump and 9 is mixing device. Figure 2 shows SPM tip on the holder and the detail of the top of the tip after deposition of Ru02 (image obtained by scanning probe microscopy).

Best Mode for Carrying Out the Invention

Example 1

AFM tip with the Ru02 layer.

AFM tip is placed on ceramic holder 4 (Fig. 1) and put into the deposition part of the reactor. As a next step vacuum is produced in the reactor by a rotary pump 8. Constant flow of argon (45 ml/min) through the reactor is set. Argon is employed as a carrier gas in the first step of the ALD process (a. injection of the precursor - deposition of the precursor on the tips) as well as purging gas in the second and fourth steps (purging pauses). Before starting of the ALD process solution of ruthenium precursor for Ru02 layer is prepared - organometal compound bis(2,2,6,6-tetrametyl-3,5-heptandionato)(l,5- cyklooktadien)rutenium(II), pyridine solvent, concentration of the solvent 0.02 M.

Reservoir of the injector 2 is filled by the solution. Reactor is heated up to the temperature 290 C (deposition part) and 200 C (evaporation part), followed by ALD deposition cycle:

a. Precursor injection - coating the tips by precursor - two injections with one second delay, time for the microvent opening is 0.003 s, mass of the injected drop is ~ 5.1 mg. b. Purging cycle - reactor is purged by argon for 30 s

c. Chemical reaction - oxygen, reactive gas, is introduced into the reactor (flow 200 ml/min) for 80 s.

d. Purging cycle - reactor is purged by argon for 50 s.

Deposition cycle is repeated from 1 to 100 times to cover the tips by oxide of the thickness from 0.5 to 40 nm.

As a result, hard SPM tips covered by 12.5 nm thick Ru02 layer were fabricated. Example 2

The device for SPM tip coating is depicted in the Fig. 1. The device consists of following parts: the precursor solution is injected from the vessel 1 using the electromagnetic micro-valve 2 and controlled by computer. After the liquid injection the solution evaporates in the evaporating part, in the mixing part 9 it is mixed with carrier and reactive gases. The layer grows on the sample surface located on the sample holder 4. Both parts of the reactor (evaporating and deposition parts) have separate heating (3a, 3b), so different temperature can be adjusted in each part. The pressure is measured using capacitance pressure sensor 5 in the apparatus. Reactive waste products are captured in a cooler trap 6 filled with liquid nitrogen. Pressure in the apparatus is set on wished value by control valve 7. Whole apparatus is evacuated by vacuum pump 8 which keeps the pressure in the apparatus on the wished value.

Industrial applicability

Hard SPM tips covered with Ru02 layers can be used for standard surface scanning, in contact lithography resulting in extended lifetime, and in local anodic oxidation utilizing the combination of their enhanced hardness and electrical conductivity.

The method of oxide tip coating represents the process of production of tips with Ru02 surface layers.

Claims

1. Hard tip for scanning probe microscopy which is covered by Ru02 layer of the thickness from 0.5 to 40 nm using the atomic layer deposition method.

2. The method of production of hard tips for scanning probe microscopy consist of heating the deposition and evaporation chambers of reactor to temperatures of 290 and 200 °C, respectively. Subsequently organometallic precursor dissolved in pyridine is injected in the reactor vacuum chamber using electromagnetic valve, while precursor and solvent vapours are transported into the reaction chamber using argon as a carrier gas. Subsequently, chemisorption and formation mono molecular layer of the precursor on the substrate take place. This is followed by argon purging step. Subsequently oxygen gas is introduced in the reactor and a chemical reaction takes place that results in Ru02 formation on the tips. Finally, the reactor is purged by argon. This cycle is repeated until desired thickness of Ru02 is obtained usually between 0.5 up to 40 nm.

3. Method of production of claim 2, in which the organometallic precursor based on the compound bis(2,2,6,6-tetrametyl-3,5-heptandionat)(l ,5-cyklooktadien)rutenium(II) is used.