WO2009153116A2

WO2009153116A2 - Diamond nano-tip and method for production thereof

Info

Publication number: WO2009153116A2
Application number: PCT/EP2009/056100
Authority: WO
Inventors: Alexander Obraztsov; Peter Kopylov
Original assignee: Eesti Nanotehnoloogiate Arenduskeskuse As
Priority date: 2008-05-29
Filing date: 2009-05-19
Publication date: 2009-12-23
Also published as: WO2009153116A3; EE200800038A

Abstract

Diamond nano-tip, usable as a probe in scanning probe microscopy, as an indenter, as sensor head or X-ray detector, with apex sizes of a few nanometers having basal face close to square and coincident with (100) crystallographic plane of the diamond. Diamond nano-tips are represented by pyramid- shaped crystals having basal face close to square and coincident with (100) crystallographic plane of the diamond. The single-crystal diamond pyramid-shaped nano-tips are produced by the chemical vapor deposition process providing (100) textured columnar growth of diamond crystallites embedded into ballas-like small size randomly oriented diamond crystallites (5). The method comprises the steps of inducing a diamond nuclei (1) on a substrate (14), growing the {100} facet (3) of diamond crystallite, growing the {111} facet (4) of diamond crystallite and development of secondary nucleation sites developing into crystallites (5).

Description

Diamond nano-tip and method for production thereof

Technical Field

[1] This invention relates to a novel type of single crystal diamond tips with apex sizes of a few nanometers and to a method for manufacturing of said diamond nano-tips. In particular, though not exclusively, the present invention relates to single crystal diamond tip usable as a probe in scanning probe microscopy, as a nanosized indenter, as nano-sized sensor head and X-ray detector.

Background Art

[2] Diamond is known as material with exceptional characteristics including record hardness and thermal conductivity, outstanding chemical inertness, optical transparency in a wide range of ultraviolet to far infrared, high carrier's mobility etc. [Hugh O. Person, Handbook of Carbon, Graphite, Diamond and Fullerenes. Properties, Processing and Applications. Noyes Publications; NJ (USA) 1993]. These properties are very promising for application of diamond in various fields. Many of these applications require creation tip-shaped diamond crystallites. For example, diamond tips are very attractive for scanning probe microscopy [E. Oesterschulze et al. Appl. Phys. Lett. 1997, V.70, P. 435] and for indenter probe [W.T. Marston. US Patent No 5,046,357] because of its hardness; for X-ray detection [C. Manfredotti et al., Diamond and Related Materials, 1998, v. 7, p. 523]; for temperature sensing [A. Majumdar, Ann. Rev. Material Science, 1999, v.29, p. 505]. To be suitable for high resolution measurements the diamond tips must have apex size as small as possible and preferably of nanometer range.

[3] The diamond tips may be produced by polishing of natural or synthetic (obtained with High Pressure High Temperature (HPHT) method) single crystals [B. Mesa, S. Magonov, J. of Physics: Conf. Ser. 2007, v. 61, p. 770]. However the tips produced by this method are quite expensive and due to individual processing of each tip it is very difficult to reach its exact reproducibility.

[4] Much higher reproducibility and lower manufacturing cost can be achieved with use of chemical vapor deposition (CVD) methods providing thin diamond film on a substrate surface. Using CVD it's possible to produce diamond pyramidal tips by a molding technique employing the fabrication of pyramidal pits in silicon by anisotropic etching and subsequent their filling with polycrystalline diamond film material [E. Oesterschulze et al. Appl. Phys. Lett. 1997, V.70, P. 435]. Main disadvantages of this method are low aspect ratio of the diamond pyramidal tips and large size of the tip apexes. Similar techniques of conformable coating of diamond film on silicon substrate with holes created by using focused ion beam (FIB) milling [Z. Wang et al., Microelectronic Engineering, 2005, v. 78-79, p. 353] provides improvement of the aspect ratio. But the polycrystalline diamond filling the small holes contains a number of structural defects and non-diamond carbon contaminations impairing material properties. Alternatively the CVD may be used to produce diamond film covering substrates with tip-like configuration [D. Alvarez et al., Microelectronic Engineering, 2004, v. 13-1 A, p. 910]. However sizes of the tips obtained with this method exceeds a few hundreds of nanometers which are too large for many potential applications. Diamond tip sharpening may be achieved using reactive ion etching (RIE) [S.-T. Lee et al., Patent US 6,902,716 B2, June 7, 2005; H. Uesuka et al., Diamond and Related Materials, 2008 (in press) DOI 10.1016/j.diamond.2007.12.071], by plasma etching in hydrogen diluted with methane [Q. Wang et al., Diamond and Related Materials, 2006, v. 15, p. 866] or by plasma etching in air atmosphere [E. -S. Baik et al., Diamond and Related Materials, 1999, v. 8, p. 2169]. The disadvantage of these etching techniques is creation of damaged layer on the outer surface of the tip and thus degradation of the most attractive properties of diamond. Alternatively diamond tip- like nanocones may be obtained with selective CVD growth with process conditions providing graphite etching [Q. Yang et al., Diamond and Related Materials 2005, v. 14, p. 1683]. However similar to that in previous examples the crystal ordering of the diamond material is very much worse in comparison with single crystal one.

[5] The present invention is based on the fact that in the CVD process diamond growth starts from the formation of discrete three-dimensional nuclei. Usually the nuclei with different crystallographic orientations are randomly distributed on the substrate and have size of nanometer range. The diamond crystallites can grow with a columnar and textured structure if there is a conspicuous growth direction. Textured diamond films comprising of crystallites with the same (or similar) crystallographic orientations can be obtained by taking advantage of the growth competition between differently oriented diamond grains [C. Wild et al., Diamond and Related Materials, 1994, v. 3, p. 373]. In particular it is possible to provide deposition conditions allowing (100) textured diamond growth. The growth rate for the (100) facet is the lowest in comparison with other facets in diamond. This is a reason for generation of twins and secondary nucleation on the lateral surface of diamond crystal growing with well developed { 100} facet. As a result of these processes it is possible to obtain CVD diamond film containing { 100} faceted diamond incorporated into ballas consisting of small size grained crystallites [G. Knuyt et al. Diamond and Related Materials, 1998, v. 7, p. 1095].

Summary of the Invention

[6] It is an object of the present invention to provide new type of single crystal diamond tips with high aspect ratios and with apex size of a few nanometers.

[7] It is another object of the invention to provide a method of producing said single crystal diamond tips.

[8] These and other objects, together with the advantages thereof over known materials and processes, are achieved by the present invention, as hereinafter described and claimed.

[9] According to the invention in one of its aspects, the diamond single crystal tips are represented by pyramid- shaped crystals having basal face close to square and coincident with { 100} crystallographic plane of diamond while other four faces connected together producing apical angle of the pyramid in range of 5 to 10 degrees. The pyramid height depending on size of the square basal face is in range of few to hundreds micrometers. And the curvature radius on vertex of the diamond pyramid is in range of 1 to 10 nanometers.

[10] According to further aspects of the invention, the single-crystal diamond pyramid- shaped tips are produced by the process having the following steps:

[11] - initially diamond film material is produced on a substrate by CVD technique with process parameters providing (100) textured columnar growth of diamond crystallites embedded into ballas-like small size randomly oriented diamond crystallites; the CVD process parameters also provide required size of the square basal face;

[12] - after the CVD growth the diamond film on substrate is heated up to temperature in range of 600 to 900 ° C in oxygen contaminated environment during time period which is enough to remove small size ballas-like diamond crystallites; the remaining part are single crystal diamond crystallites of pyramidal shape oriented by their vertex to substrate surface or lying along the surface in the case if there are no supporting materials in the film after its oxidation;

[13] - separation of the single crystal diamond pyramid- shaped tip for consequent usage.

Description Of Drawings

[14] Fig. IA is a SEM image of a pyramidal shaped single crystal diamond nano-tip according to the present invention

[15] Fig. IB is a schematically draw of a pyramidal shaped single crystal diamond nano-tip according to the present invention

[16] Fig. 2 is a HRTEM image showing apical end of the pyramidal shaped single crystal diamond nano-tip with atomic resolution

[17] Fig. 3A shows transmission electron diffraction pattern obtained for a single diamond nano-tip in obtained in accordance with the present invention

[18] Fig. 3B shows transmission electron microscopy (TEM) image obtained for a single diamond nano-tip in obtained in accordance with the present invention by a method allowing observation of Kikuchi lines

[19] Fig. 4A illustrates schematically diamond nuclei 1 on substrate surface 2

[20] Fig. 4B illustrates schematically a diamond crystallite grown from nuclei 1 on substrate surface 2 with developed facets having { 100} 3 and { 111 } or { 110} 4 crystallographic orientation

[21] Fig. 4C illustrates schematically formation of areas 5 with increased density of defects in between well developed facets having { 100} 3 and { lll } or { 110} 4 crys- tallographic orientation

[22] Fig. 4D illustrates schematically formation of agglomerate of diamond single crystal core having { 100} 3 crystallographic orientation and nanometrically sized diamond crystallites 6 resulted of secondary nucleation

[23] Fig. 5A shows SEM image of surface of CVD diamond film obtained in accordance with embodiments of the invention; arrows show { 100} facets on the diamond film surface

[24] Fig. 5B shows SEM image of cross section CVD diamond film obtained in accordance with embodiments of the invention; arrows show { 100} facets on the diamond film surface

[25] Fig. 6A shows Raman spectrum measured for as grown CVD diamond film obtained in accordance with embodiments of the invention

[26] Fig. 6B shows Raman spectrum measured for CVD diamond film after selective oxidation of nano- sized crystallites in accordance with embodiments of the invention

[27] Fig. 7 shows dependence of weight of sample of CVD diamond film on temperature during oxidation in air atmosphere

[28] Fig. 8A shows SEM image of surface of CVD diamond film oxidized in accordance with embodiments of the invention

[29] Fig. 8B shows SEM image of cross section of CVD diamond film oxidized in accordance with embodiments of the invention

[30] Fig. 9 is a schematic view of apparatus for use in carrying fabrication diamond films by CVD method according to the embodiments of this invention

[31] Fig. 10 shows SEM image of cantilever 16 with pyramidal single crystal diamond nano-tip 17 obtained accordingly to the embodiments of this invention and fixed by using epoxy 18.

Detailed description of the Invention

[32] The diamond single crystal tips according to this invention have shape of a pyramid with square basal face coincident with { 100} crystallographic plane. The typical image obtained with scanning electron microscopy (SEM) for the pyramid- shaped diamond tip is shown in Fig. IA. A schematic image of the diamond tip is shown in Fig. IB. The edges of the pyramid back side (non-visible through the pyramid body) are shown in Fig. IB by dotes lines. In the particular case of the tip shown in Fig. IA the basal face of the pyramid has square shape with size of approximately 1 ' 1 to 2 ' 2 m m² and the pyramid height is about 5 to 15 m m and apical angle is approximately 6 degrees. These dimensions can be varied in the wide ranges of sub-micrometers to tens of micrometers for edges of the square basal face and of few micrometers to hundreds micrometers for the pyramid height. But ratio of the length of the basal face edge to the pyramid height is approximately the same. This provides approximately same apical angles for the pyramid- shaped tips in range of 5 to 10 degrees. [33] High resolution TEM (HRTEM) image shown in Fig. 2A illustrates the atomic arrangement on the apex of the pyramidal shaped diamond tip. According to the HRTEM image the curvature radius on the vertex end is less than 2 nm. This radius may be varied in range of 1 to 10 nm for different tips.

[34] The electron diffraction pattern for the tip shown in Fig. 3A reveals { 111 } and

{200} facets of equivalent symmetry that illustrates single crystal structure of the tip. The electron diffraction pattern suggest that the basal face of the pyramidal shaped tip coincides with { 100} crystallographic plane.

[35] The single crystal structure of the diamond pyramid- shaped tip is confirmed also by observation of Kikuchi diffraction lines in the electron diffraction mode of the TEM as it is illustrated in Fig. 3B.

[36] At least in its preferred forms, the present invention provides a method for manufacturing the single crystal diamond tips via multiple steps, including:

[37] - growth of (100) textured polycrystalline diamond film by CVD providing process parameters allowing formation of micrometer- sized crystallites surrounded by nanometer- sized ballas-like diamond crystallites;

[38] - oxidation of the nanometer- sized diamond crystallites by heating the CVD film in oxygen contaminated gas environment;

[39] - separation of the diamond tips from the oxidized CVD film for consequent usage.

[40] In preferred embodiments of this invention substrates used for diamond film growth by CVD process are made of electrically conductive materials sustainable to action of high temperatures (up to 1200 ° C) in gas environment containing activated radicals of ionized hydrogen, carbon and hydrocarbon precursors. The substrate surface may be treated to increase number of diamond nucleation centers. To provide growth of diamond the substrate is exposed to action of activated gas mixture containing carbon precursors in the CVD reactor. The deposition parameters, such as gas composition and pressure, gas flow and activation power, substrate temperature, process duration, are chosen to provide selective growth of { 100} facets on the nucleus. The possibility for such selection is based on the known fact that within a single crystallite zones formed by growth from { 100} facets are free of planar defects while zones grown from { 111 } and { 110} facets contain large numbers of planar defects. In metastable conditions of the CVD process morphology of the growing crystallites is determined by concurrence of carbon atoms condensation and etching. The etching rate increases with the increasing of the number of defects. Thus the providing conditions for (100) textured film growth the { 111 } and { 110} facets will be automatically subjected to intense etching and consequent secondary nucleation of diamond on the etched facets. As a result of this process the columnar growth of pyramid- shaped crystallites with { 100} well developed face is occurred in the process as it illustrated by a set of images in Fig. 4A, Fig. 4B, Fig. 4C, and Fig. 4D: image Fig.4A shows a diamond nuclei 1 on substrate surface 2 forming at the deposition process start; image Fig.4B shows an intermediate stage of the diamond growth with { 100} facet formed 3; image Fig. 4C shows increased in size diamond crystallite with highly defected region in contacting zone of the { 100} 3 and { 111 } 4 facets and with secondary nucleated crystallites 5; image Fig.4D shows schematically cross section of the finally obtained pyramid- shaped single crystal diamond surrounded by the nano- sized diamond crystallites 6 developed from secondary nucleation sites. Taking into account that these secondary nucleated crystallites start to grow with time delay after the crystal, developed from initial nuclei, their sizes are much smaller than that for the core crystal. At the same time growth process for each of the secondary crystallites is similar to that for initial one and thus after some time their lateral surface became suitable for sub-secondary nucleation. After multiple recurrence of the described process a dendrite structure with gradually reduced sizes of the crystallites agglomerating together is formed. While these nanosized diamond crystallites produce main part of volume of the CVD film body, the core diamond crystal is developed into pyramidal shaped tip of micrometer size.

[41] Fig. 5A shows SEM image in front view and Fig. 5B shows SEM image obtained for a cross sectional fragment of the CVD film grown in the conditions preferable to the present invention. The { 100} facets and nanometrically sized crystallites are indicated in the Fig. 5A and Fig.5B by arrows. The presence of nanosized diamond crystallites in the grown CVD diamond films is confirmed by Raman spectroscopy (RS) analysis illustrated by Fig. 6A. The Raman spectrum shown in the Fig. 6A is measured for the CVD diamond film obtained at process parameters preferred accordingly to the present invention. This spectrum contains lines corresponding to mi- crocrystalline diamond phase (at about 1330 cm ¹), graphite phase (two lines at about 1350 cm⁴ and at about 1550 cm ¹) and nanocrystalline diamond phase (two lines at about 1140 cm⁴ and at about 1470 cm ¹). The nanocrystalline diamond phase is represented by the diamond crystallites with typical sizes of about 2 nm.

[42] In preferred embodiments of this invention the CVD diamond film grown as described above is then oxidized by heating in oxygen contaminated environment. Large size diamond single crystals are known to start to oxidized at temperatures above 800-900 ° C. But polycrystalline CVD diamond films containing small size crystallites became oxidized at temperatures of about 600 ° C. Fig. 7 shows dependence of weight loss due to oxidation for the CVD diamond film having structure and composition described above and shown in Fig. 5A and Fig. 5B. Accordingly to the dependence shown in Fig. 7 there are two temperature regions corresponding to oxidation of two different fractions of the CVD diamond film. One of the regions spans of approximately 600 ⁰ C tO about 700 ° C and the second one is from approximately 700 ⁰ C tO about 900 ° C. According to Raman spectroscopy exposure of the CVD diamond film to the air atmosphere at 700 ° C during 10-20 min leads to complete removal of the nanocrystalline diamond phase. Fig. 6B shows Raman spectrum measured for the CVD diamond films after the thermal oxidation. The Raman spectra shown in Fig. 6A and Fig. 6B were obtained for the same sample before (Fig. 6A) and after (Fig. 6B) oxidation by heating in air atmosphere. There is also significant reducing of intensities of the lines corresponding to graphite phase in the CVD film after its oxidation. Fig. 8A shows SEM image for front view and Fig. 8B shows SEM image for cross section of the CVD film after the oxidation. It is clearly seen from the images that only core parts of the diamond crystallites agglomerates are presented in the CVD film treated by the thermal oxidation in air.

[43] It should be understood that mentioned above values of parameters characterizing

CVD fabrication and oxidation of the diamond films are indicated for illustration. In the preferred embodiments of the present invention these parameters are chosen to reach certain properties of the diamond tips and process parameters used for the tips manufacturing.

[44] EXAMPLE 1

[45] A textured (100) diamond film shown in Fig. 5A and Fig. 5B, which are the SEM images of plain-view of the film surface and of the film cross section, was prepared by CVD in hydrogen-methane gas mixture activated by direct current (DC) gas discharge. The CVD process parameters were adjusted to produce the diamond film with characteristics corresponding to that presented in the SEM images (Fig. 5A and Fig. 5B) and RS (Fig. 6A). Fig. 9 shows schematically CVD apparatus arrangement used for the diamond films production. The CVD process is carried out in a reactor chamber 7 containing two electrodes 8 and 9 and equipped with a system 10 for filling the reactor with mixture of hydrogen and methane in controlled proportion with controlled flow rate. The air in the reactor chamber is pumped out by vacuum pumping system 11 equipped with system allowing to control the gas mixture pressure during deposition process. The electrodes 8 and 9 are electrically connected with a DC power supply 12 providing voltage and current suitable for ignition and maintaining a DC gas discharge in the gas mixture. One of the electrodes is cathode 8 and another is anode 9. By applying suitable voltage between cathode 8 and anode 9 gas discharge plasma 13 is generated above the anode 9. A substrate 14 for the diamond film deposition is situated on the anode 9. The substrate 14 heats up because of contact with plasma 13. The temperature is controlled by a system 15.

[46] In preferred embodiments of this invention the gas mixture composition is: hydrogen/methane ratio is 95/5; gas mixture pressure is of 9 to 10 kPa; gas flow is of 200 to 300 seem; substrate 14 temperature is of 920 to 970 ° C; DC discharge current density is of 0.5 to 1 A/cm²; cathode-to-anode distance is of 40 to 60 mm; deposition process duration is of 100 to 150 minutes. The process parameters are adjusted by controlling film properties using SEM and RS measurements to reach characteristics of the film material similar to that presented in Fig. 5A, Fig. 5B, Fig. 6A.

[47] The as grown diamond film is heated in air environment up to 680 ° C and exposed at the above mentioned temperature during 30 min. The temperature value may be in range of 650 ⁰ C tO 700 ° C depending on requirements to the diamond tip apex radii and surface roughness of the pyramidal shaped diamond crystal. The optimal temperature and duration may be adjusted also by using SEM and RS measurements to reach characteristics of the treated film material to that presented in Fig. 6B, Fig. 8A, Fig. 8B.

[48] After disclosed procedures the diamond nano-tips may be easily separated from the substrate and are ready to use with purposed mentioned in the Field of Invention section.

[49] EXAMPLE 2

[50] A single crystal pyramidal shaped diamond nano-tip was prepared by the disclosed

CVD method and thermal oxidation. The diamond nano-tip was fixed then on a standard scanning probe cantilever by using an epoxy. Fig. 10 shows SEM image of the cantilever 16 with diamond nano-tip 17 fixed by epoxy 18.

Claims

[1] 1. A diamond tip usable as a probe in scanning probe microscopy, an indenter for mechanical and electrical devices, as a sensors and detectors, with high aspect ratio and small apex size tips, made by thermal oxidation applied to diamond film produced by chemical vapor deposition, which is characterized in that the diamond tip is monocrystalline pyramidal shaped which basal face is preferably square and coincident with { 100} crystallographic plane (3) and tip is sharp- pointed.

2. A diamond tip according to claim 1, which is characterized in that the tip basal face is with dimensions from 0,1x0,1 to 99x99 μm² , height is from 3 to 199 μm, apical angle is in range of 5 to 10 degrees and the apex size of the tip is in range of 1 to 10 nanometers.

3. A diamond tip according to claim 2, which is characterized in that the basal face dimensions of tip are preferably of 1x1 to 2x2 μm², height is about 5 to 15 μm and apical angle is about 6 degrees.

4. A method of producing diamond tip, wherein diamond is deposited on surface of substrate (14) located onto anode (9), while this substrate (14) is faced to the cathode (8) and both these electrodes are in the reactor chamber (7) of the device used for manufacturing diamond tip; the reactor chamber (7) is preliminary pumped out and then is filled with the gas mixture of hydrogen and methane, which is activated with direct current gas discharge initiated between cathode (8) and anode (9), by the system (10); air in the reactor chamber (7) is pumped out by vacuum pumping and hydrogen-methane gas mixture is filled by system (11) allowing control the gas mixture pressure during deposition process; the gas discharge plasma (13) is generated above the anode (9) and heats the substrate (14), which is characterized in that the electrically conductive materials sustainable to action of high temperatures in gas environment containing activated radicals of ionized hydrogen, carbon and hydrocarbon precursors in substrate (14), is used, on which monocrystalline pyramid- shaped core crystals, with tips directed towards the substrate (14) surface (2), surrounded with diamond crystallites (6) made of film material, which heights are oriented perpendicularly to the substrate (14) surface (2) with deviation from this direction of ±5 degrees, comprising diamond film is grown, which comprises next steps:

(a) diamond crystallite nuclei (1) is induced on the surface (2) of substrate (14) on the anode (9) in the reactor chamber (7) for making diamond film,

(b) the { 100} facet (3) of diamond crystallite is grown,

(c) the { 111 } facet (4) of diamond crystallite is grown, by which the defected region in contacting zone of the { 100} (3) and { 111 } facets (4) is increased and secondary nucleation sites are developed and crystallites (5) in them by which the film material is formed on the substrate (14), (d) film material is oxidized by heating in oxygen contaminated gas environment,

(e) diamond tips core tips are released from the film material by selective oxidation of its part consisting of smaller crystallites and non-diamond carbon.

5. A method according to claim 4, which is characterized in that monocrystalline pyramidal shaped diamond crystals are oriented by their apexes to the surface of substrate and with their basal plates having { 100} crystal- lographic orientation coincident with the diamond film surface.

6. A method according to claim 4, which is characterized in that monocrystalline pyramidal shaped diamond tip is separated from other components comprising the diamond film.

7. A method according to any of claim 4 or 6, which is characterized in that the parameters of chemical vapor deposition are the most preferably chosen following: gas mixture is hydrogen/methane in ratio 95:5, gas mixture pressure is of 9 to 10 kPa, gas flow is of 200 to 300 cmVmin, substrate (14) temperature is of 920 to 970⁰C, DC discharge current density is of 0,5 to 1,0 A/cm², cathode (8)-to-anode (9) distance is of 40 to 60 mm, deposition process duration is of 100 to 150 minutes.

8. A method according to any of claim 4 to 7, which is characterized in that the diamond film is obtained in polycrystalline form with (100) texture.

9. A method according to any of claim 4 to 8, which is characterized in that the diamond film is oxidized in the gas environment comprising oxygen.

10. A method according to any of claim 4 to 9, which is characterized in that the diamond film is heated in the temperature range from 600 ⁰C to 900 ⁰C.

11. A method according to claim 10, which is characterized in that the diamond film is preferably heated in the temperature range from 650 ⁰C to 700 ⁰C.

12. A method according to claim 11, which is characterized in that the diamond film is heated in the air atmosphere most preferably until to the temperature of 680 ⁰C.

13. A method according to claim 12, which is characterized in that the diamond film is held in corresponding temperature 30 minutes.