WO2010011186A1

WO2010011186A1 - A method of fabricating a cantilever structure and a cantilever structure

Info

Publication number: WO2010011186A1
Application number: PCT/SG2009/000259
Authority: WO
Inventors: Yee Chong Loke; Kuan Song Yeong; Erik Johansson Cox
Original assignee: Agency For Science, Technology And Research
Priority date: 2008-07-25
Filing date: 2009-07-22
Publication date: 2010-01-28
Also published as: WO2010011186A9

Abstract

A method of fabricating a cantilever structure and a cantilever structure is provided. The method comprises the steps of providing a self-aligning marker on a beam of the cantilever structure; and forming a tip of the cantilever structure based on self-aligning a tip material at the self-aligning marker.

Description

A Method Of Fabricating A Cantilever Structure And A Cantilever Structure

FIELD OF INVENTION

The present invention relates broadly to a method of fabricating a cantilever structure and to a cantilever structure.

BACKGROUND

An Atomic Force Microscope (AFM) is a tool which allows imaging of topography of solid surfaces at a relatively high resolution. AFM can also be used for a number of other applications. For example, AFM can also be used to generate force- versus-distance curves. Such curves, also known as force curves, provide information on local material properties such as elasticity, hardness, adhesion and surface charge densities etc. Thus, the use of AFM in force curves generation is increasingly desired in different fields of research such as surface science, material engineering, and biology etc.

Another typical application of the AFM is in the analysis of surface forces. The AFM can be used to provide force measurements or colloidal force measurements on single molecules involving rupturing of single chemical bonds and stretching of polymer chains. Such measurements can identify structures and properties of confined liquids since force measurements typically provide information on the energy of a confined liquid film.

In the AFM, the cantilever tip is an important component as the tip localizes the spatial extent of the AFM interaction with a sample and contributes to high spatial resolutions. Currently, modifications to conventional tips can add versatility and broaden the range of AFM applications in imaging and force spectroscopy. For example, tips can be coated with metals for conductivity measurements or coated with diamond for increased hardness and inertness or can be chemically functionalized for chemical force microscopy etc.

Attaching a colloidal sphere (or "bead") to a cantilever tip has been introduced e.g. in Ducker et. al., Nature (London) 353, 239 (1991). The attaching of a colloidal sphere can add versatility to AFM force spectroscopy. This attachment technique is known as the colloidal probe technique. The technique can assist in controlling probe size and geometry for measurements of different force interactions. Using a modified tip, a higher signal-to-noise ratio in force measurements can be obtained and more accurate information can be derived. Bead-modified tips have typically been used in the measurements of friction, contact electrification studies and casimir force measurements etc.

One way to produce a colloidal probe tip is by manual attachment of a spherical particle, typically commercially available in different materials and different sizes, to a cantilever beam. Such a spherical particle can be silica or glass and is typically about 2 to 20 μm in diameter. Manual attachment techniques include the so-called dual-wire technique, cantilever-moving technique and high temperature sintering technique. The dual-wire technique is discussed in Ducker et. al., Nature (London) 353, 239 (1991); Ducker et. al., Langmuir 8, 1831 (1992); Mak et. al., rev. Sci. Instrument, 77, 046104 (2006) and Schmutz et. al., Rev. Sci. Instrument, 79, 026103 (2008). The cantilever- moving technique is discussed in Raiteri et. al., Colloids Surf., A 136, 191 (1998); Bowen et. al., Colloids Surf., A 157, 117 (1999); Huntington et. al., Microscopy Today 9, 32 (2001) and Y. Gan, Microcscopy Today 13, 48 (2005). The high temperature sintering technique is discussed in E.Bonaccurso, Ph. D thesis, Universitat-Gesamthochschule- Siegan, Siegan, 2001 , P.29 and Bonaccurso et. al., Phys. Rev. Lett. 88, 076103 (2002).

The above manual attachment techniques typically require bead/sphere manipulation using an optical microscope and a three-dimensional micromanipulator to manually place spheres on cantilevers. Manual attachment of spheres is typically time consuming (e.g. typically taking one hour for three to four successful attachments of a spherical tip to a cantilever) and incurs substantial setup time during AFM operations. In addition, manual attachment of spheres is typically carried out with a single cantilever at one time and is not feasible if there are multiple cantilevers to work on. Furthermore, manual attachment of a sphere to a cantilever can give rise to a number of problems. One problem is that the cantilever is susceptible to damage due to mishandling. Another problem is that it is typically difficult to attach the sphere to the cantilever due to small dimensions.

Another problem associated with the manual attachment techniques is incomplete evaporation of solvent from the sphere. Trace amounts of solvent is typically trapped between the sphere and the cantilever silicon substrate surface during the attachment process. This typically makes separation of the sphere from the substrate surface (to move to a desired site) difficult. Further, the difficulty in separation due to solvent is also observed between spheres as the trace amounts of solvent typically causes increased attraction between spheres in a cluster making attachment of a single sphere to the cantilever difficult.

In addition, for the dual-wire technique and cantilever-moving technique using glue as adhesive for attachment, contamination with adhesive is another problem. Contamination may change the chemical identity of the sphere surface and can affect the integrity of force measurement results. Further, the amount of adhesive used can also pose an issue since the difficulty of attaching the sphere to the cantilever typically increases as the amount of adhesive used is decreased. If excessive adhesive is used for attachment, yet another problem is that excess adhesive may flow over the sphere surface. Also, contamination with adhesive may occur if the sphere is picked up over a number of attempts due to rolling and spinning of the sphere on the cantilever substrate surface.

In Schmutz et. al., Rev. Sci. Instrument, 79, 026103 (2008), a method is proposed to use focused ion beam (FIB) technology to etch/mill a hole on an existing cantilever to giue or manually attach a sphere to the cantilever. Although the hole seeks to allow excess glue to flow away, there is still a likelihood that glue contamination may occur. Furthermore, the manual attachment method in Schmutz et. al. cannot overcome the time consuming disadvantage inherent to the manual attachment techniques. Also, the manual attachment method in Schmutz et. al. can only be carried out on one cantilever at a time, ie. not suitable for mass production.

For the high temperature sintering technique, the technique of sintering Borosilicate glass to a cantilever is possible due to the low melting temperature and softening point of the Borosilicate glass. In addition to the problems already identified above for manual attachment techniques, it will be appreciated that the high temperature sintering technique is impractical for silica spheres (more widely used and commercially available) due to the relatively high melting temperature of the silica spheres.

In addition, apart from the discussion above regarding micron size spherical tip cantilevers, techniques have been developed for attaching even smaller objects such as single nanoparticles and nanotubes to tips used for e.g. AFM and scanning near-field optical microscopy (SNOM) applications. For example, tips modified with submicron or nano-size particles have been used extensively in apertureless SNOM. Further, a metal (typically Ag or Au) nanoparticle can act as a "nanoantenna" to achieve surface- enhanced Raman spectroscopy (SERS). Another such application is in evanescent photon spectroscopy. In these applications, a glass fiber tip comprising an attached single metallic particle can achieve a higher signal to noise ratio (as compared to a fully metallised tip) because the attached metallic particle scatters light stronger than the glass fiber due to the lower index of refraction of the glass fiber.

For submicron particles or nanoparticles, it will be appreciated that it is not feasible to accurately position a single particle on a tip using the manual tip attachment approaches discussed above due to the resolution limitations of the optical microscope. In this regard, a number of techniques have been developed to modify a tip with a single nanoparticle. These techniques utilise surface chemistry, optics and photocatalysis. The techniques are described in Sqalli et. al., Appl. Phys. Lett. 76, 2134 (2000); Vakarelski et. al., Langmuir 22, 2931 (2006); Kawata et. al., Appl. Phys. Lett. 82, 1598 (2003); Kalkbrenner et. al., J. Microsc. 202, 72 (2001); Pampaloni et. al., Proc. Natl. cad. Sci.

USA 103, 10248 (2006); Barsegova et. al., Appl. Phys. Lett. 81 , 3461 (2002); Okamoto et. al., J. Microsc. 202, 100 (2001) and Sqalli et. al., J. Apply. Phys. 92, 1078 (2002).

However, the above techniques are directed at tip attachment on a single cantilever. One problem that may arise is that the tip attachment is time consuming and production efficiency may be undesirably low. Furthermore, another problem is that these techniques may be too complicated for production processes.

Thus, in view of the above problems, there exists a need for a method of fabricating a cantilever structure and a cantilever structure that seek to address at least one of the above problems.

SUMMARY

In accordance with an aspect of the present invention, there is provided a method of fabricating a cantilever structure, the method comprising the steps of providing a self-aligning marker on a beam of the cantilever structure; and forming a tip of the cantilever structure based on self-aligning a tip material at the self-aligning marker.

The self-aligning marker may function as a cavity for trapping the tip material.

The forming the tip may comprise depositing a layer of intermediate metal in the self-aligning marker prior to self aligning the tip material.

The layer of intermediate metal may comprise one of a group consisting of aluminium, indium and zinc.

The method may further comprise subjecting the cantilever structure to a heat treatment for sintering the tip material and the layer of intermediate metal, the heat treatment being executed at a softening point temperature of the intermediate metal.

The self aligning the tip material may comprise dispersing the tip material over the cantilever structure. The tip material may comprise one or more particles.

The particles may comprise one of a group consisting of silica, borosilicate glass, polystyrene and tungsten.

The self aligning marker may be provided on an elevated portion of the beam of the cantilever structure.

The self-aligning marker may function as a pillar base for selectively supporting the tip material on the beam.

The tip material may have a substantially lower melting point than the pillar base and the tip material has a low wettability characteristic on the pillar base.

The pillar base may comprise silicon dioxide.

The tip material may comprise a metal.

The metal may be gold or silver.

The method may further comprise subjecting the cantilever structure to a heat treatment for annealing the cantilever structure, the heat treatment being executed at a melting point of the tip material.

The self-aligning marker may function as a seed base for the tip material.

Said forming the tip may comprise forming the self aligning marker protruding from the cantilever beam.

The self-aligning marker may comprise a semiconductor material.

The semiconductor material may be silicon. The method may further comprise subjecting the cantilever structure to a heat treatment for annealing the cantilever structure, the heat treatment being executed in hydrogen ambient at a temperature lower than the melting point of the self aligning marker but sufficient for effecting atomic migration for providing the tip material.

The tip may be spherical in shape.

The beam of the cantilever structure may be formed from a material selected from a group consisting of silicon, silicon-on-insulator and silicon nitride.

In accordance with another aspect of the present invention, there is provided a cantilever structure, the structure comprising a beam; a tip formed on the beam; and a self-aligning marker disposed between the tip and the beam.

The self-aligning marker may function as a cavity for trapping a tip material.

The tip material may comprise one of a group consisting of silica, borosilicate glass, polystyrene and tungsten.

The self aligning marker may be provided on an elevated portion of the beam.

The self-aligning marker may function as a pillar base for selectively supporting a tip material on the beam.

The pillar base may comprise silicon dioxide.

The tip material may comprise a metal.

The metal may be gold or silver.

The self-aligning marker may function as a seed base for a tip material. The self aligning marker may comprise a semiconductor material.

The semiconductor material may be silicon.

The tip may be spherical in shape.

The beam may be formed from a material selected from a group consisting of silicon, silicon-on-insulator and silicon nitride.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Figure 1 is a schematic process flow diagram illustrating a fabrication process in one example embodiment.

Figure 2(a) is a scanning electron microscope (SEM) image of a sample cantilever.

Figure 2(b) is a scanning electron microscope (SEM) image of a sample cantilever.

Figures 3(a) and 3(b) are schematic diagrams for illustrating a methodology for attaching a bead/sphere to a cantilever in an example embodiment.

Figure 4 shows scanning electron microscope (SEM) images of a sample cantilever.

Figure 5 is a schematic diagram illustrating forming a spherical AFM tip in one example embodiment. Figure 6 is a schematic process flow diagram illustrating a fabrication process in one example embodiment.

Figure 7 is a schematic diagram showing four different material combinations in an experiment.

Figures 8(a), 8(b), 8(c) and 8(d) are respective scanning electron microscope (SEM) images of the samples in Figure 7.

Figures 9(a) to 9(c) are schematic diagrams illustrating an experiment for proof of concept of an example embodiment.

Figure 10 is a schematic diagram illustrating surface diffusion or atom migration on a silicon profile.

Figure 11 is a schematic process flow diagram illustrating a fabrication process in one example embodiment.

Figure 12 is a schematic diagram illustrating formation of an integrated tip in an example embodiment.

Figure 13 is a schematic process flow diagram illustrating a fabrication process in an example embodiment.

Figure 14 is a schematic process flow diagram illustrating a fabrication process in an example embodiment.

Figure 15 is a schematic flowchart for illustrating a method of fabricating a cantilever structure in an example embodiment. DETAILED DESCRIPTION

The example embodiments described below can provide a method of fabricating cantilever tips for an Atomic Force Microscope (AFM). The example embodiments utilise a self aligning marker to form a tip based on self-aligning a tip material at the self- aligning marker. This can overcome various problems associated with manual attachment.

In one example embodiment, there is provided a method to fabricate AFM cantilevers having integrated spherical tips. This is carried out using "non-manual" attachment of spheres on cantilevers for AFM. Commercially available silica spheres are used in the example embodiment as the spherical ball for the fabrication of a cantilever with an integrated spherical tip. In the example embodiment, a cantilever tip is etched to form a cavity/hole that functions as a mechanical trap to a silica sphere during a dispersion process. During the dispersion process, a single monolayer of the spheres is spread on the cantilever surface with one of the spheres trapped in the cavity/hole. The sphere is then physically bonded to the cantilever via an intermediate material (e.g pre-coated aluminium (Al) thin film) using a low temperature sintering technique. Excess spheres used in the dispersion process are then washed away by ultrasonification.

The method of the example embodiment can be further scaled up to provide a microfabrication process for making multiple cantilever tips, where spheres can be precisely attached on every tip of the multiple cantilevers during a batch fabrication process.

The method combines integration of a microfabrication process and a technique for attaching spherical beads on each apex of multiple cantilevers patterned in a single 4-inch wafer.

Figure 1 is a schematic process flow diagram illustrating a fabrication process in one example embodiment. In the example embodiment, a 4-inch silicon-on-insulator (SOI) wafer 102 is used. In step (a), a device layer 104 of about 2 to 3 μm thick is provided. Features of a cantilever structure are patterned into a device layer 104. The patterning comprises spin coating a photoresist layer. In step (b), reactive ion etching (RIE) with an inductively coupled plasma (ICP) with a recipe of 25mTorr / 50W(RIE) / 450W (ICP)/ 30 seem of SF₆/ 15sccm of C₄F₈ is carried out for forming a basic rectangular cantilever structure 106 shape (e.g. measuring about 200-300μm long, 50μm wide and 3μm thick).

In step (c), prior to transferring processing to the bottom side of the SOI wafer 102, the device layer is first direct-current (DC) sputtered with a thin film 108 of aluminium (about 50 nm thick)to protect the cantilever structure 106 surface. At the bottom side of the wafer 102, a layer of silicon dioxide (SiO₂) 110 of about 4 μm thick is deposited using plasma enhanced chemical vapour deposition (PECVD) to function as a platform for the formation of an etch mask later.

In step (d), the SiO₂ layer 110 for a backside release mask is patterned with photoresist (e.g. Clariant AZ9260) of about 15 μm thick and thereafter, reactive ion etching (RIE) is carried out with a recipe of 30mtorr/ 150W/ 25sccm of CHF₃ / 25sccm of Ar to form an oxide hard mask 112 for a subsequent deep etching of a cantilever handle at a later stage. The backside photoresist is stripped off with an AZ400 developer to obtain the exposed hard maski 12 which also functions as a protective base for a handle layer to be formed later.

In step (e), processing of the wafer 102 is switched back to the device layer 104 where the earlier deposited aluminium film 108 has been etched away by the AZ400 developer during the backside pattern development process at step (d). A final mask with a cavity/hole 114 is patterned using photoresist 115 at the apex of a beam of the cantilever structure 106. The hole 114 is then reactively ion etched (also removal of the photoresist in the hole 114) with a recipe of 175mtorr/ 125W/ 30sccm of SF₆ to a partial depth (about 1.5 - 2 μm), which is about half the desired thickness of the cantilever structure 106. Thus, the hole 114 is a non-through hole in the example embodiment. In step (f), without stripping away the remaining photoresist, a layer of aluminium film 117 (about 300 - 500nm) is deposited via DC sputtering where the etch hole 114 is partially filled by the aluminium film (see 116).

In step (g), a 'lift-off' operation is then performed in an acetone solution to 'shake- off the unwanted aluminium thin film on the remaining photoresist from the device layer 104 such that only the etch hole 114 contains aluminium (see 116). The wafer 102 is then ready for a backside etching process to release an underside of the cantilever structure 106.

In step (h), a deep anisotropic silicon etching 'Bosch' process, making use of an aggressive SF₆/C₄F₈ based plasma, is carried out to etch through the backside of the SOI wafer 102 (or about 500μm depth) until the intrinsic buried oxide layer 118 is reached. The etching can be first performed in a commercial inductively couple plasma etcher (e.g. from Oxford Instruments) based on the so-called Bosch process where the etching methodology is based on switching between etching (25mtorr/ 5OW/ 450W(ICP)/ 75sccm of SF6/ 8sec) and passivation (30mtorr/ 2OW/ 450W(ICP)/ 50sccm of C₄F₈)/ 6sec). A second stage of the etching is performed using a typical RIE system with a recipe of 175mtorr/ 125W/ 30sccm of SF₆ to continue etching the backside silicon (of numeral 102) at a substantially slower rate than the ICP process and also to ensure that the backside silicon (of numeral 102) has been completely etched through to the intrinsic buried oxide layer 118 for the whole of the wafer.

In step (i), the buried oxide layer 118 is removed to release the cantilever structure 106 using a reagent Buffered Hydrofluoric acid (BHF) (about 14% concentration). The wafer 102 is then flipped back to the device layer 104 for silica spheres/beads dispersion. Prior to this step of performing the dispersion, the silica spheres are cleaned in a toluene solution with one or more times of ultrasonification to eliminate roughness and improve the smoothness of the silica spherical surfaces. The silica powder is then dried in an oven to evaporate the remaining toluene solution before adding isopropylalcohol (IPA) solution into the silica powder. IPA is chosen due to recognised good wettability which enhances the spreading of the silica spheres on the cantilever structure 106 surface during dispersion. After the dispersion, as shown at step (i), the cavity/hole 114 functions as a self- aligning marker that can trap a silica sphere e.g. 120, without manual attachment for forming a tip. See also silica beads e.g. 120,122 dispersed on the cantilever structure 106.

In step (j), the cantilever structure 106 is first subjected to heat or sintering treatment in a furnace and the temperature is raised up to the softening point of aluminium (i.e. about 500⁰C ~ 600⁰C) for about two to three hours. This can establish a strong bond between a trapped silica sphere/bead e.g. 120 in the etch hole 114 and the aluminium 116. At the final stage of the process, the cantilever structure 106 is undergoes ultrasonification in a deionised (Dl) solution for about 15 seconds to wash away spheres e.g. 122 that have not been trapped/bonded by the sintering treatment.

Therefore, it will be appreciated that the cavity/hole 114 thus functions as a self- aligning marker for the silica tip material to form the tip 120 on the beam 124 of the cantilever structure 106.

Figure 2(a) is a scanning electron microscope (SEM) image of a sample cantilever. The image 202 is taken at a processing step corresponding to step (i) of the example embodiment. The image 202 shows a multi layer of dispersed silica spheres e.g. 204,206 on the sample cantilever 208. It can be observed that there is a relatively good concentration of spheres on the cantilever 208.

Figure 2(b) is a scanning electron microscope (SEM) image of a sample cantilever. The image 210 is taken at a processing step corresponding to step Q) of the example embodiment. The image 210 shows a trapped silica sphere 212 in an etch hole 214 and bonded to the sample cantilever 216 after a sintering treatment. It is observed that the remaining untrapped spheres have been "shaken off' after ultrasonification.

Figures 3(a) and 3(b) are schematic diagrams for illustrating a methodology for attaching a bead/sphere to a cantilever in an example embodiment. Beads e.g. 302,304 are dispersed to a cantilever 306 via a dispersion technique. For demonstration, silica beads or spheres (commercially available) are used as a spherical ball for the fabrication of the cantilever 306 having an integrated spherical tip. A batch fabrication process comprises the dispersion and sintering of silica spheres on the tips of batch-fabricated cantilevers. For each cantilever 306, the tip 308 of the cantilever 306 is first etched to form a cavity/hole 310 that functions as a mechanical-trap to a silica sphere e.g. 302 during dispersion. An aluminium film 312 is deposited or coated in the hole 310. The size of the hole 310 is about 25% smaller than the intended size of the trapped silica bead e.g. 302 such that more than about 75% of the silica bead/sphere e.g. 302 surface is exposed. Referring to Figure 3(b), annealing is then carried out to promote adhesion between the trapped silica sphere e.g. 302 and the aluminium film 312 inside the hole 310. Spheres e.g. 304 which are not in the hole 310 can be washed away by ultrasonification in Dl solution.

The inventors recognise that the approach of sintering silica beads/spheres to silicon can be made feasible by using an intermediate metal layer where the melting temperature of the metal layer is lower than that of the silica spheres and silicon.

One candidate tested for the metal layer (compare 116 of Figure 1) is aluminium film, where the melting point (of about 680⁰C) is about half that of silica and silicon. To test the validity of silica adhesion on Al film, a silicon substrate sample (having a sample size 2cm x 2cm) was patterned with a rectangular cantilever feature and a hole of about 4 μm in diameter is patterned at the apex of the cantilever pattern. The silicon sample is then reactively ion etched with a recipe of 175mtorr/ 125W/ 30sccm of SF₆ to a depth of about 3μm followed by deposition of an aluminium thin film of about 400nm thick using DC sputtering. A "lift-off' operation ensures that the aluminium film is selectively deposited on the substrate base (not the cantilever surface) and the etch hole of the silicon substrate sample.

In the test, silica spheres (e.g. from Bangs Laboratories, Inc.) in an IPA solution are then manually dispersed over the sample surface. The solution of IPA which has high wettability on the Si substrate sample can provide well dispersed silica spheres where about 80% of the sample surface is covered by a monolayer of the spheres. The sample is thereafter annealed to a temperature range of about 500⁰C - 600°C or slightly below the melting temperature of aluminium, with a duration of about 3 hours. After the annealing process, the sample undergoes about 15 sec of ultrasonification in a Dl solution "to shake off' those spheres which are not on the aluminium film surface and therefore are not been subjected to the adhesion effect due to sintering.

Figure 4 shows scanning electron microscope (SEM) images of a sample cantilever. The SEM images show dispersed silica spheres e.g. 402,404 sitting on the post annealed aluminium thin film after about 15 seconds of ultrasonification. These spheres e.g. 402,404 are on the substrate base (not the cantilever surface). Only one sphere 406 remains trapped in the cantilever hole 408. The cross section SEM images 410,412 of the silica sphere 406 highlight the formation of a 'necking' joint 414 on the aluminium surface 416 after the annealing process or sintering treatment.

For mass production of AFM cantilever tips with integrated spheres, an integrated fabrication process can be provided that comprises initial microfabrication of each cantilever, followed by dispersion of silica spheres and a sintering treatment . It is recognised that sphere dispersion is carried out at the second stage of the fabrication process so as to prevent potential contamination concerns which may also incur roughness issues on the scanning surfaces of the spheres.

In the above described example embodiments, the intermediate metal material is not limited to aluminium and can be indium or zinc. Further, besides silica, borosilicate glass, polystyrene and tungsten material can be used for forming the tip. In addition, it will be appreciated that spherical particles are preferred as the material for forming the tip due to better adhesion properties arising from the relatively well-defined surfaces of the spherical particles. Furthermore, although only one cavity/hole is described as being formed on each cantilever, it will be appreciated that more than one hole can be formed, e.g. on both sides of the cantilever. In addition, the tip particle can be micrometer, nanometer or sub-nanometer in diameter.

In an alternative embodiment, there is provided a method to fabricate nanospherical metallic cantilever tips for AFM. Figure 5 is a schematic diagram illustrating forming a spherical AFM tip in one example embodiment. Two materials 502,504 are placed on each other as a pillar at the end of an AFM cantilever 506. The materials 502,504 can be metal. The choice of the two materials 502,504 is such that Material 1 502 (or a material at the top of the pillar) has a substantially lower melting point than Material 2 504 (or a material forming the base of the pillar contacting the cantilever) and Material 1 502 possesses low wettability on the surface of Material 2 504. Upon annealing at a temperature close to the melting point of Material 1 502, Material 1 502 melts (or undergoes atomic transformation) and forms a high contact angle liquid droplet 508 on Material 2 504. This results in an embedded-like spherical-ball structure 510 on the cantilever 506 surface with the liquid droplet 508 being sub-micrometer in size.

Figure 6 is a schematic process flow diagram illustrating a fabrication process in one example embodiment. The example embodiment seeks to obtain an integrated metallic tip cantilever of about 50nm to 1 μm in diameter.

In step (a), a 4 inch SOI wafer 602 (having orientation <100> on its top Si layer

604) is first deposited with a PECVD grown SiO₂ on both sides of the wafer 602 (see

606,608), The top layer 606 is grown to a thickness of about 1 μm and functions as an oxide platform for an Au sphere while the bottom side 608 having a thickness of about

4μm functions as a hardmask for backside etching.

In step (b), a nano patterning of a circular hole 605 together with a patterning of a mask alignment marker (not shown) on a polymethyl methacrylate (PMMA) resist 607 is carried out using an e-beam lithography (EBL) process. In other words, the resist 607 is coated on the layer 606 and the hole 605 is etched from the resist 607 (to expose SiO₂ in the hole).

In step (c), a thin film 609 of Au (about 40nm thick) is sputtered to cover the formed or etched nano hole.

In step (d), a "lift-off' process is carried out to retain an Au pattern 610 in the strategic position (ie. at the hole). In step (e), the Au pattern 610 is used as a hard mask to etch the SiO₂ layer/platform 606 using RIE with a recipe of 30mtorr/ 15OW/ 25sccm of CHF₃ / 25sccm of Ar, to form a pillar-like tip 612 for a cantilever structure.

Thus, the pillar-like tip 612 comprises a SiO₂ pillar base 613 and the Au pattern 610. The SiO₂ pillar base 613 functions as a self aligning marker for selectively supporting the Au pattern 610.

In step (f), the shape of a cantilever structure beam 614 is formed from a first mask by patterning using optical lithography based on the earlier alignment marker of the EBL process in step (b). The patterning comprises spin coating a photoresist layer. In other words, the first mask is used for patterning of the shape of the beam and a RIE etch of the Si layer 604 is carried out using an etch recipe: 25mTorr / 50W(RIE) / 450W (ICP)/ 30 seem of SF₆/ 15sccm of C₄F₈.

In step (g), a second mask is used to pattern a Si platform 616 from the Si layer 606 for the Au and SiO₂ nano pillar or tip 612. The patterned platform 616 functions as an elevation for the tip 612 height (see 618) while the rest of the cantilever structure beam 614 is RIE etched to a thickness of about 2 - 5μm based on an etch recipe of 175mtorr/ 125W/ 30sccm of SF₆ . In the example embodiment, the elevation is about 10μm to 12μm.

In step (h), a thin film of SiO₂ 620 of about 50 to 100 nm thick is passivated or PECVD grown on the top layers and functions as a protective film for the tip 614 for a subsequent backside alignment and etching process.

In step (i), the backside release hardmask 608 is patterned followed by RIE etching with a recipe of 30mtorr/ 150W/ 25sccm of CHF₃ / 25sccm of Ar to form an oxide hard mask 621.

At step G), a deep RIE (DRIE) etching process based on an aggressive SF₆/C₄F₈ based plasma (under the so-called Bosch Process) is carried out to etch through the bulk of the backside Si (about 500μm depth) (remainder shown at 622) until the buried oxide layer is reached. A second stage of the etching is performed using a typical RIE system to anisotropic etch away the buried oxide (remainder shown at 624) with the CHF₃/Ar plasma using a recipe comprising 30mtorr/ 150W/ 25sccm of CHF₃ / 25sccm of Ar for releasing the cantilever structure 626. One reason for not using BHF in etching away the buried oxide is because the characteristic isotropic etch of BHF may etch away the SiO₂ platform supporting the Au pattern (compare 612).

At step (k), the passivating oxide layer 620 which protects the Au tip 612 and the cantilever structure 626 is removed by a RIE etch with a similar CHF₃/Ar plasma that was used in step Q). The RIE etch is performed in 9 cycles where each cycle comprises about 10min etching and about 5 min of cooling to prevent the wafer from over heating during the relatively long etching time as overheating may result in the Au platform "peeling off' during the etching process.

At step (I), the final released cantilever structure 626 is then annealed in ambient N₂ at a temperature range of about 700⁰C to 800⁰C at a time duration of about 20min to 30mins to form a spherical Au tip (see 628). The annealing temperature is close to the melting point of Au which is about 750⁰C to 800⁰C.

Therefore, it will be appreciated that the SiO₂ pillar base 613 thus functions as a self aligning marker for Au tip material to form the spherical tip 628 on the beam 614 of the cantilever structure 626.

In the above described example embodiments, the thermal annealing can provide spherical metallic Au or Ag tips having smooth surfaces which can enhance AFM applications in optical studies such as SNOM applications.

Discussions are provided below for understanding the effect of annealing on the materials of interest.

Figure 7 is a schematic diagram showing four different material combinations in an experiment. Material 1 can comprise either Au 702 or Ag 704, and Material 2 can comprise Ti 706 or SiO₂ 708. Four different samples 710,712,714,716 are fabricated from a Si substrate 718 with an area of 1cm². In test samples 1 710 and 2 712, a Material 2 layer (for both samples) comprising SiO₂ 708 thermally grown to a thickness of about 150nm using a low pressure CVD (LPCVD) process is used, while a Material 1 layer comprising a thin film (about 25nm thick) of Ag 704 and Au 702 respectively for the two different samples is used. Test samples 3 714 and 4 716 each comprise a Ti 706 thin film of about 50nm as a Material 2 layer followed by respective Ag 704 and Au 702 layers of about 25nm thick as the Material 1 layer. These samples 710, 712,714,716 are placed in a furnace at about 600⁰C for a period of about 15min before cooling down to ambient temperature.

Figures 8(a), 8(b), 8(c) and 8(d) are scanning electron microscope (SEM) images of the samples in Figure 7 respectively.

The SEM images show interesting observations with regard to formation of Ag and Au nanospheres on the surfaces of SiO₂ or Ti under thermal influence at about 600⁰C. The results show that as Au and Ag possess low wettability on the SiO₂ surface, Au/Ag embedded-like- nanospheres are formed upon annealing. See Figures 8(a) and 8(b) for the Ag nanospheres and Au nanospheres respectively. On the other hand, Au and Ag have high wettability on Ti which prevents the formation of spherical droplets. See Figures 8(c) and 8(d) for Ag and Au results respectively. Thus, SiO₂ is adopted as a preferred Material 2 layer with either Au or Ag as a spherical tip material. Therefore, Material 2 can be a semiconductor material.

Figures 9(a) to 9(c) are schematic diagrams illustrating an experiment for proof of concept of an example embodiment. With reference to Figure 9(a), a process using E- Beam lithography (EBL) and liftoff technique is used to pattern Au having structures e.g. 902,904 with a diameter each of about 300nm where the AU pattern sits on a SiO₂ thin film 906. With reference to Figure 9(b), the Au pattern having structures e.g. 902,904 acts as a hardmask while RIE etch is carried out on the SiO₂ layer/film 906 till the etching reaches the Si substrate 908. With reference to Figure 9(c), the interlayer of each 'nano-pillar' e.g. 910,912 is subjected to annealing at a temperature range of about 700⁰C - 750⁰C for a time duration of about 20min. After annealing, the nano-pillars e.g. 910,912 form spherical tips e.g. 914,916. The SEM images 918,920 show the formation of the nano-Au spheres e.g. 922,924 on the SiO₂ platform which shows the feasibility of integrating this technique into a cantilever fabrication process (e.g. as described with reference to Figure 6).

The above fabrication process uses integration of EBL with a conventional microfabrication process. The purpose of EBL is to pattern "nano" dimension circular holes with diameters ranging from about 100nm to about 1μm, where the holes are then "filled up" by the Au film at a later stage. The inventors recognise that EBL typically does not allow multiple layer alignment which is a common practice in optical lithography and thus, EBL may restrict direct integration with the rest of the fabrication process.

Therefore, EBL patterning is used at the beginning stages of the fabrication process where it is also used for the patterned alignment mark for subsequent process steps (see the described example embodiment described with reference to Figure 6).

In an alternative example embodiment, there is provided a method for fabricating a cantilever with a circular silicon (Si) tip and using hydrogen gas (H₂) annealing to form a spherical Si tip.

In the example embodiment, the integration of a cantilever fabrication process with H₂ annealing is provided using fabrication process steps similar to those described with reference to Figure 6 to first produce a cantilever with a circular Si tip and thereafter, using H₂ annealing to form a spherical tip.

The physical (or atomic) transformation of Si under H₂ annealing can be explained from knowing that surface mobility of Si atoms is enhanced by heated H₂ at temperatures slightly lower than the melting point of Si (about 1414⁰C) but sufficient enough for atomic migration of Si. In other words, the Si surface is terminated with hydrogen atoms and triggers a global profile transformation through surface diffusion and evaporation-condensation which contributes to a number of fundamental surface mass transport mechanisms. The transformation due to thermal annealing is discussed in Lee et. al. in Journal of Microelectromechanical Systems 15(2), 338 (2006). In one example embodiment, for a cantilever tip to have surface transformation without a trade off in the surface mass, the annealing temperature is maintained at less than about 1100⁰C such that surface diffusion is dominant in the profile transformation. In the example embodiment, the annealing temperature is set to about 1050⁰C.

Figure 10 is a schematic diagram illustrating surface diffusion or atomic migration on a silicon profile. Surface atoms e.g. 1002, 1004 tend to leave from convex corners e.g. 1006 and accumulate at concave corners (see 1008). Using this mechanism, initial sharp corners e.g. 1006 can become rounded (see 1010).

In an example embodiment, a fabrication process is provided. The focus is on a Si tip. A metallic tip structure is used to function as an initial hardmask to etch a SiO₂ mask for a Si pillar.

Figure 11 is a schematic process flow diagram illustrating a fabrication process in one example embodiment. The example embodiment seeks to obtain an integrated Si spherical tip for a cantilever.

In step (a), a 4 inch SOI wafer 1102 is first deposited with a PECVD grown SiO₂ on both sides (1104, 1106) of the wafer 1102. The top side 1104 is grown to a thickness of about 1 μm while the bottom side 1106 has a thickness of about 4μm and functions as a hardmask for backside etching.

In step (b), a nano patterning of a circular hole 1105 together with a patterning of a mask alignment marker (not shown) on a PMMA resist 1107 is carried out by an EBL process. In other words, the resist 1107 is coated on the layer 1104 and the hole 1105 is etched from the resist.

In step (c), a thin film 1109 of Au (about 40nm thick) is sputtered to cover the formed or etched nano hole 1105. In step (d), a "lift-off¹ process is carried out to retain the Au pattern 1108 in the strategic position (i.e. at the hole).

In step (e), the Au pattern 1108 is used as a hard mask to etch the SiO₂ layer/platform 1104 using RIE with a recipe of 30mtorr/ 15OW/ 25sccm of CHF₃ / 25sccm of Ar, to form a pillar-like tip structure 1110 for a cantilever structure.

In step (f), the shape of a cantilever structure beam 1112 is formed from a first mask by patterning using optical lithography based on the earlier alignment marker of the EBL process in step (b). In other words, the first mask is used for patterning of the shape of the beam and a RIE etch of the Si layer 1114 is carried out using an etch recipe: 25mTorr / 50W(RIE) / 450W (ICP)/ 30 seem of SFg/ 15sccm of C₄F₈.

In step (g), the Au and SiO₂ tip structure 1110 serves as hardmask to RIE etch the Si layer 1114 based on an etch recipe of 175mtorr/ 125W/ 30sccm of SF₆ to form a Si pillar 1116 (under the structure 1110) with a height of about 10μm - 12μm. This correlates with a cantilever structure 1118 having a thickness of about 3 - 5μm.

In step (h), a thin film of SiO₂ 1120 of about 50 to 100nm thick is passivated or PECVD grown on the top layers, and functions as a protective film for the tip structure 1110 for a subsequent backside alignment and etching process.

In step (i), the backside release hardmask 1106 is patterned followed by RIE with a recipe of 30mtorr/ 15OW/ 25sccm of CHF₃ / 25sccm of Ar to form an oxide hard mask 1122.

At step (j), a deep RIE (DRIE) etching process based on an aggressive SF_S/C₄F₈ based plasma (under the so-called Bosch Process) is carried out to etch through the bulk of the backside Silicon (about 500μm depth) (remainder shown at 1124) until the buried oxide layer 1126 is reached .

At step (k),a second stage of the etching started in step (h) uses BHF to release the cantilever structure 1118 with all oxide (e.g. buried oxide layer 1126, layer 1120 and tip structure 1110 having oxide base from layer 1104) etched away in the process. A silicon portion 1127 is thus formed after the etching process. The silicon portion 1127 is a self aligning marker that functions as a seed base for a tip material.

At step (I), the final released cantilever structure 1118 is then annealed in pure hydrogen at about IOtorr pressure and in a temperature range from about 1050⁰C to about 1100⁰C. A Si spherical tip 1128 of about 50 nm to 1μm in diameter is formed after the annealing. In other words, the heat treatment or annealing is executed in hydrogen ambient at a temperature that is lower than the melting point of the silicon portion 1127 but sufficient for effecting atomic migration of silicon for providing the tip material (compare Si spherical tip 1128).

Therefore, it will be appreciated that the silicon portion 1127 thus functions as a self aligning marker for silicon tip material to form the spherical tip 1128 on the beam 1130 of the cantilever structure 1118.

It will be appreciated that the example embodiment is not limited to the above process flow. In other words, other different techniques may be used to fabricate the cantilever with an elevated tip apex, for example by etching a silicon platform using a mask to obtain a cantilever beam with an elevated tip apex and then annealing the beam to arrive at a spherical Si tip.

Figure 12 is a close-up schematic diagram illustrating steps (i) and (j) of Figure

11. A silicon pillar 1202 is formed integrally on a silicon cantilever beam 1204 (compare step (i) of Figure 11). The cantilever beam 1204 comprising the pillar 1202 is then subjected to annealing in hydrogen at a temperature of about 1100⁰C to obtain a silicon spherical tip 1206 (compare step (j) of Figure 11).

It will be appreciated that the above described example embodiment is not limited as such and other different techniques can be used to form the spherical tip, for example, by attaching a semiconductor layer to a cantilever as a seed base for providing a tip material. In an example embodiment, to obtain higher sensitivity in friction force microscopy and less cantilever damping effects, the height of a tip for a colloidal probe cantilever can be elevated by adding an additional step to the fabrication process as described above with reference to Figure 1. The approach comprises patterning a platform in the cantilever followed by a RIE etch process.

Figure 13 is a schematic process flow diagram illustrating a fabrication process in an example embodiment. At step 1302, a SOI wafer 1304 comprising a silicon layer 1306 having a thickness of about 15μm to 20μm is provided. At step 1308, the Si layer 1306 is patterned using a first mask having cantilever features and a RIE etching is carried out using an etch recipe: 25mTorr / 50W(RIE) / 450W (ICP)/ 30 seem of SF₆/ 15sccm of C₄F₈.. At step 1310, a second mask is used to pattern a silicon platform from the silicon layer 1306. A RIE etching is carried out to obtain a cantilever thickness of about 3 to 5μm based on an etch recipe of 175mtorr/ 125W/ 30sccm of SF₆. See resultant platform 1312. The steps subsequent to step 1310 are substantially similar to the steps (c) to (j) of Figure 1. As can be seen at step 1314, the spherical tip 1316 has an elevated height (see 1318). The elevated height provided in this example embodiment is about 12μm to 17μm. Therefore, in this example embodiment, the self aligning marker 1320 is provided on an elevated portion (see 1318) of the beam/platform 1312 of the cantilever, structure.

In an example embodiment, the cantilever material can be switched from silicon to silicon nitride by adding a pre-processing step on a blank silicon wafer and depositing low stress LPCVD silicon nitride layers on both sides of the silicon wafer. However, the inventors appreciate that the tradeoff with using silicon nitride is that the thickness for silicon nitride is restricted to a maximum thickness of about 2μm due to the limitation of the LPCVD process.

Figure 14 is a schematic process flow diagram illustrating a fabrication process in an example embodiment. At step 1402, there is provided a double-sided 4-inch polished

Si wafer 1404 of about 500μm thick with an orientation <100>. At step 1406, SiO₂ with a thickness of about 1μm is LPCVD grown on both sides (see 1408, 1410) of the wafer

1404. The SiO₂ layers 1408, 1410 are non-porous. At step 1412, silicon nitride with a thickness of about 2μm is LPCVD grown having low stress characteristics on both sides (see 1414, 1416) of the wafer 1404. The steps subsequent to step 1412 are substantially similar to steps (b) to (j) of Figure 1. For example, for backside etching (compare step (f) of Figure 1), at step 1418, SiN₄ and SiO₂ function as a hardmask 1420 to etch through the bulk Si 1422 and stop at the top oxide layer 1424. In Figure 14, the material for the final cantilever 1426 is silicon nitride SiN₄.

Thus, for the described example embodiments, the substrate material is not limited to SOI and can be silicon or silicon nitride.

Figure 15 is a schematic flowchart 1500 for illustrating a method of fabricating a cantilever structure in an example embodiment. At step 1502, a self-aligning marker is provided on a beam of the cantilever structure. At step 1504, a tip of the cantilever structure is formed based on self-aligning a tip material at the self-aligning marker.

The above described example embodiments can provide a process that can significantly reduce operational setup time for attaching a sphere to an AFM cantilever tip. This can improve the efficiency of AFM operations in chemical microscopy/colloid probe force measurements, and can increase the number of end users of these measurement techniques. Further, the process can enable batch production such as producing 500-1000 units of cantilevers with attached spherical tips from a 4-inch wafer. In addition, the described example embodiments can eliminate contamination problems as adhesive glue is not used as bonding material. Furthermore, the described example embodiments can allow fabrication of nanometallic and silicon spherical tips that can integrate with microfabrication processes to batch produce cantilevers for plasmonic AFM studies.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive. It will be appreciated that the recipes provided in the described example embodiments are by way of example only, and it will be appreciated that different process values, parameters etc. may be used in different embodiments instead of the described examples.

Claims

1. A method of fabricating a cantilever structure, the method comprising the steps of: providing a self-aligning marker on a beam of the cantilever structure; and forming a tip of the cantilever structure based on self-aligning a tip material at the self-aligning marker.

2. The method as claimed in claim 1 , wherein the self-aligning marker functions as a cavity for trapping the tip material.

3. The method as claimed in claim 2, wherein the forming the tip comprises depositing a layer of intermediate metal in the self-aligning marker prior to self aligning the tip material.

4. The method as claimed in claim 3, wherein the layer of intermediate metal comprises one of a group consisting of aluminium, indium and zinc.

5. The method as claimed in claim 3 or 4, further comprising subjecting the cantilever structure to a heat treatment for sintering the tip material and the layer of intermediate metal, the heat treatment being executed at a softening point temperature of the intermediate metal.

6. The method as claimed in any one of claims 2 to 5, wherein the self aligning the tip material comprises dispersing the tip material over the cantilever structure.

7. The method as claimed in any one of claims 2 to 6, wherein the tip material comprises one or more particles.

8. The method as claimed in claim 7, wherein the particles comprise one of a group consisting of silica, borosilicate glass, polystyrene and tungsten.

9. The method as claimed in any of claims 2 to 8, wherein the self aligning marker is provided on an elevated portion of the beam of the cantilever structure.

10. The method as claimed in claim 1 , wherein the self-aligning marker functions as a pillar base for selectively supporting the tip material on the beam.

11. The method as claimed in claim 10, wherein the tip material has a substantially lower melting point than the pillar base and the tip material has a low wettability characteristic on the pillar base.

12. The method as claimed in any one of claims 10 to 11 , wherein the pillar base comprises silicon dioxide.

13. The method as claimed in any one of claims 10 to 12, wherein the tip material comprises a metal.

14. The method as claimed in claim 13, wherein the metal is gold or silver.

15. The method as claimed in any one of claims 10 to 14, further comprising subjecting the cantilever structure to a heat treatment for annealing the cantilever structure, the heat treatment being executed at a melting point of the tip material.

16. The method as claimed in claim 1 , wherein the self-aligning marker functions as a seed base for the tip material.

17. The method as claimed in claim 16, wherein said forming the tip comprises forming the self aligning marker protruding from the cantilever beam.

18. The method as claimed in any one of claims 16 to 17, wherein the self-aligning marker comprises a semiconductor material.

19. The method as claimed in claim 18, wherein the semiconductor material is silicon.

20. The method as claimed in any one of claims 16 to 19, further comprising subjecting the cantilever structure to a heat treatment for annealing the cantilever structure, the heat treatment being executed in hydrogen ambient at a temperature lower than the melting point of the self aligning marker but sufficient for effecting atomic migration for providing the tip material.

21. The method as claimed in any one of claims 1 to 20, wherein the tip is spherical in shape.

22. The method as claimed in any one of claims 1 to 21 , wherein the beam of the cantilever structure is formed from a material selected from a group consisting of silicon, silicon-on-insulator and silicon nitride.

23. A cantilever structure, the structure comprising, a beam; a tip formed on the beam; and a self-aligning marker disposed between the tip and the beam.

24. The structure as claimed in claim 23, wherein the self-aligning marker functions as a cavity for trapping a tip material.

25. The structure as claimed in claim 24, wherein the tip material comprise one of a group consisting of silica, borosilicate glass, polystyrene and tungsten.

26. The structure as claimed in claim 24 or 25, wherein the self aligning marker is provided on an elevated portion of the beam.

27. The structure as claimed in claim 23, wherein the self-aligning marker functions as a pillar base for selectively supporting a tip material on the beam.

28. The structure as claimed in claim 27, wherein the pillar base comprises silicon dioxide.

29. The structure as claimed in claims 27 or 28, wherein the tip material comprises a metal.

30. The structure as claimed in claim 29, wherein the metal is gold or silver.

31. The structure as claimed in claim 23, wherein the self-aligning marker functions as a seed base for a tip material.

32. The structure as claimed in claim 31 , wherein the self aligning marker comprises a semiconductor material.

33. The structure as claimed in claim 32, wherein the semiconductor material is silicon.

34. The structure as claimed in any one of claims 23 to 33, wherein the tip is spherical in shape.

35. The structure as claimed in any one of claims 23 to 34, wherein the beam is formed from a material selected from a group consisting of silicon, silicon- on-insulator and silicon nitride.