Patent Application
20070186629
During the last few years, the atomic force microscope (AFM) has increasingly been used as a powerful tool for performing biological studies. AFMs have been used to image such objects as molecules, cells, tissues and biomaterials. AFMs have additionally have been used to measure such forces as adhesion forces between individual proteins and interaction forces between polymeric systems and AFM tips functionalized with various molecules. AFM tips functionalized with proteins have been used to investigate the interaction of individual ligand-receptor complexes.
Additionally, a demand exists for single-cell manipulation to enable investigation of where and when molecules exhibit their various functions in controlling the activity of a cell. A functionalized AFM probe can be used to transfer a molecule of interest, such as a nucleic acid, a protein or other chemical compound into a living cell at a defined location and at a known time. The reaction of the cell in response to the molecule of interest can then be monitored in real-time. Such techniques can be applied not only for the investigation of cell activity but also in controlled differentiation or therapy of living cells.
A functionalized AFM probe whose probe tip is functionalized with a molecule of interest can be used to perform manipulations of the type just described.
Deposited on conventional AFM probe tip 12 is a thin metallic film 16. In a typical example, metallic film 16 is composed of a 5 nm-thick layer of chromium (Cr) in contact with the AFM probe tip and a 10 to 50 nm-thick layer of gold (Au) covering the layer of chromium.
Determinations of the interaction force between polymeric and biological systems and conventional AFM probe tips functionalized as just described have been demonstrated. However, the relatively large size of conventional AFM probe tip 12 results in AFM probe 10 having a relatively low spatial resolution. Moreover, since the area of the external surface 14 of conventional AFM probe tip 12 is relatively large and since substantially all the external 14 surface of conventional AFM probe tip 12 is covered with functionalizing molecules 18, multiple interactions occur when conventional AFM probe tip 12 is used to measure an interaction force between functionalizing molecules 18 and an object 20. In
The low aspect ratio and relatively large cross-sectional area of conventional AFM probe tips similar to AFM probe tip 12 renders them unsuitable for use inside living cells. As used in this disclosure, the aspect ratio of an AFM probe tip is the length-to-width ratio of the probe tip. The length of a probe tip is the dimension of the probe tip in a length direction that extends between the base of the probe tip and the tip of the probe tip, and the width of a probe tip is the dimension of the probe tip in a width direction, orthogonal to the length direction.
Living cells are of the order of several micrometers thick: the length of a conventional AFM probe tip is insufficient to allow the probe tip to penetrate deep inside the cell. A conventional AFM probe long enough to penetrate as far into the cell as needed for some investigations would be so wide at its proximal end that it would cause the cell to rupture. Moreover, when a conventional AFM probe tip is used to deliver molecules to a point inside a cell, the width of the AFM probe tip in relation to the dimensions of the internal features of the cell makes the location to which molecules are delivered uncertain. Furthermore, a conventional AFM probe does not have a sharp end, which makes it difficult for the conventional AFM probe tip to penetrate through the cell membrane due to the small local pressure the probe tip can exert. Thus, conventional functionalized AFM probe tips have a limited capability for use in studying intracellular interactions.
Recently, long AFM probe tips with a high aspect ratio and a smaller cross-sectional area have been introduced, and penetration of such AFM probe tips into living cells has been successfully demonstrated. Such high aspect ratio AFM probe tip is fabricated by subjecting a conventional pyramidal AFM probe tip to focused ion beam (FIB) etching. However, functionalized versions of such high aspect ratio AFM probe tips suffer from the above-described problems of low spatial resolution, difficult-to-interpret data, relatively large lateral dimensions and functionalization over a wide area. Moreover, the FIB process used to make such probe tips causes damage to the probe tip material, which makes the electrical properties of the probe tips unpredictable.
In accordance with an embodiment of the invention, a functionalizable nanowire-based AFM probe comprises a cantilever element, a semiconductor nanowire and a catalyst nanoparticle. The cantilever element comprises a crystalline growth surface at one end. The semiconductor nanowire extends substantially orthogonally from the growth surface. The catalyst nanoparticle is located at the distal end of the nanowire, remote from the growth surface. The catalyst nanoparticle comprises a material having a greater tendency to bond with a functionalizing molecule moiety than the semiconductor material of the nanowire. The nanowire and catalyst nanoparticle constitute at least part of the probe tip of the functionalizable AFM probe.
The functionalizable AFM probe is functionalizable in the sense that it is structured to enable the catalyst nanoparticle at the distal end of the AFM probe tip to be selectively functionalized with desired functionalizing molecules by any suitable functionalizing process. A functionalizing molecule is any molecule that provides the AFM probe tip with a desired chemical or physical property and that is capable of bonding with the catalyst nanoparticle in preference to the nanowire. A functionalizing molecule typically comprises a bonding moiety linked directly or indirectly to a moiety that will be called a molecule of interest. Typical molecules of interest include antibodies, antigens, DNA, RNA, oligonucleotides, peptides, proteins, receptors, enzymes, ligands, polymers and small molecules such as biotins. In some cases, the molecule of interest can be a cell, a bacterium or a virus. When the AFM probe tip is functionalized by dipping it in a solution of functionalizing molecules, for example, the greater tendency of the catalyst nanoparticle to bond with a moiety of the functionalizing molecules ensures that the functionalizing molecules bond to the catalyst nanoparticle rather than the nanowire.
In accordance with another embodiment of the invention, a functionalized AFM probe comprises a cantilever element, a semiconductor nanowire, a catalyst nanoparticle and functionalizing molecules. The cantilever element comprises a crystalline growth surface at one end. The semiconductor nanowire extends substantially orthogonally from the growth surface. The catalyst nanoparticle is located at the distal end of the nanowire, remote from the growth surface and has an external surface. The functionalizing molecules are localized to the external surface of the nanoparticle.
In the above embodiments, the dimensions and aspect ratio of the nanowire depend on the intended application of the AFM probe. With current fabrication technology, the nanowire can have a diameter as small as about 5 nm and a length of the order of micrometers. As a result, the probe tip can have a small cross-sectional area and can quite easily have an aspect ratio of the order of 100. The diameter of the catalyst nanoparticle is similar to that of the nanowire. A probe tip having such an aspect ratio and a small cross-sectional area can penetrate a living cell without causing the cell to rupture. The orthogonal orientation of the nanowire with respect to the cantilever element allows the AFM probe to be mounted conventionally in the host atomic force microscope.
The radius of the nanoparticle is substantially smaller than that of the conventional AFM probe tip described above with reference to
A crystalline growth surface such as crystalline growth surface 120 is a defined crystalline plane of the semiconductor material underlying the growth surface. In typical embodiments, the growth surface is the (111) crystalline plane of the underlying semiconductor material. A silicon nanowire grown on a silicon (111) crystalline plane will grow epitaxially, i.e., the crystallographic orientation of growth surface will be imposed on the nanowire, and the nanowire will grow in a direction substantially orthogonal to the growth surface. Typically, a nanowire will grow in a direction within ±10° of the orthogonal direction. Hence, a nanowire grown on a growth surface disposed parallel to the cantilever element will extend substantially orthogonally to the growth surface, and, hence, will additionally extend substantially orthogonally to the cantilever element.
In other embodiments, the growth surface is a (100) crystalline plane or a (110) crystalline plane of the underlying semiconductor material. It is typically more difficult to grow a silicon nanowire with good material quality on a silicon growth surface that is the (100) crystalline plane or the (110) crystalline plane than on a silicon growth surface that is the (111) crystalline plane. However, the (100) crystalline plane and/or the (110) crystalline plane can give better material quality than the (111) crystalline plane in nanowires grown from semiconductor materials other than silicon.
In functionalizable AFM probe 100, cantilever element 110 is composed of a cantilever arm 112 and a frusto-pyramidal probe tip base 114 located at one end of cantilever arm 112. As used in this disclosure, the term frusto-pyramidal encompasses frusto-conical, a cone being a pyramid having a base with an infinite number of sides. A probe tip base that is closer to one end of cantilever arm 112 than to the middle of cantilever arm 112 will be regarded as being at one end of cantilever arm 112.
Probe tip base 114 has crystalline side facets, an exemplary one of which is shown at 116, and, at its distal end, remote from cantilever arm 112, a crystalline end facet 118. In this first embodiment, end facet 118 provides growth surface 120, i.e., nanowire 130 extends from end facet 118. End facet 118 is substantially parallel to cantilever arm 112, i.e., end facet 118 is parallel to cantilever arm 112 typically within ±10°. End facet 118 is typically less than about 0.01 μm2 in area.
In a typical embodiment, a monolithic, a single-crystal semiconductor AFM probe having a frusto-pyramidal single-crystal silicon probe tip is used as cantilever element 110. Such monolithic, single-crystal semiconductor AFM probes are sold by NanoWorld AG of Neuchâtel, Switzerland. In such an AFM probe, the cantilever arm and probe tip are respective portions of a single piece of single-crystal silicon. In embodiments in which cantilever element 110 is electrically conducting, as is typically the case, the single-crystal silicon is doped with a suitable dopant such as arsenic. In other embodiments, cantilever arm 112 and probe tip base 114 are separate components joined together. In such embodiments, the material of cantilever arm 112 need not be a semiconductor.
Nanowire 130 extends substantially orthogonally from growth surface 120 provided by crystalline end facet 118 at the distal end of probe tip base 114, i.e., nanowire 130 extends in a direction typically within ±10° of the direction orthogonal to end facet 118. The material of nanowire 130 is a single-crystal semiconductor material, such as a single-crystal group IV semiconductor, e.g., silicon (Si); a single-crystal group III-V semiconductor, e.g., gallium arsenide (GaAs); or a single-crystal group II-VI semiconductor, such as zinc oxide (ZnO). In embodiments in which nanowire 130 is electrically conducting, the single-crystal semiconductor material of the nanowire is doped with a suitable dopant.
As will be described in more detail below, nanowire 130 is grown from catalyst nanoparticle 170 deposited on growth surface 120. Throughout the growth process, catalyst nanoparticle 170 remains at the distal end of the nanowire, remote from cantilever element 110. Catalyst nanoparticle 170 is a somewhat oblate spheroid. The material of catalyst nanoparticle 170 is an alloy of the semiconductor material of nanowire 130 and a catalyst metal. The material of catalyst nanoparticle 170 has a greater tendency to bond with a functionalizing molecule moiety than the semiconductor material of nanowire 130.
In functionalizable AFM probe 200, a cantilever arm 212 is used as cantilever element 210.
In the example shown, cantilever arm 212 is an elongate piece of single-crystal semiconductor material in which one of the crystalline planes of the semiconductor material coincides with a major external surface 218 of the cantilever arm. In this second embodiment, the crystalline external surface 218 of cantilever arm 212 that coincides with one of the crystalline planes of the semiconductor material of cantilever arm 212 provides growth surface 220, i.e., nanowire 230 extends from external surface 218. In an embodiment, external surface 218 is substantially parallel to the longitudinal axis of cantilever arm 212, i.e., external surface 218 is parallel to the longitudinal axis of cantilever arm 212 typically within ±10°.
In a typical embodiment, a tipless, monolithic, single-crystal semiconductor AFM probe is used as cantilever element 210. Such tipless, monolithic, single-crystal semiconductor AFM probes are sold by NanoWorld AG of Neuchâtel, Switzerland. Such tipless AFM probe is a single piece of single-crystal silicon. In embodiments in which cantilever element is electrically conducting, as is typically the case, the single-crystal silicon is doped with a suitable dopant such as arsenic. In other embodiments (not shown), the material of the cantilever element is not a semiconductor. In such embodiments, the cantilever element has at one end a layer of crystalline semiconductor material on at least part of one of its major external surfaces. The exposed surface of the semiconductor material provides growth surface 220.
Nanowire 230 extends substantially orthogonally from growth surface 220 provided by the crystalline external surface 218 of cantilever arm 212, i.e., nanowire 230 extends in a direction typically within ±10° of the direction orthogonal to external surface 218. The material of nanowire 230 is a single-crystal semiconductor material, such as a single-crystal group IV semiconductor, e.g., silicon (Si); a single-crystal group III-V semiconductor, e.g., gallium arsenide (GaAs); or a single-crystal group II-VI semiconductor, such as zinc oxide (ZnO). In embodiments in which nanowire 230 is electrically conducting, the single-crystal semiconductor material is doped with a suitable dopant.
As will be described in more detail below, nanowire 230 is grown from catalyst nanoparticle 270 deposited on growth surface 220. Throughout the growth process, catalyst nanoparticle 270 remains at the distal end of the nanowire, remote from cantilever element 210. Catalyst nanoparticle 270 is a somewhat oblate spheroid. The material of catalyst nanoparticle 270 is an alloy of the semiconductor material of nanowire 230 and a catalyst metal. The material of catalyst nanoparticle 270 has a greater tendency to bond with a functionalizing molecule moiety than the semiconductor material of nanowire 230.
Functionalized probe tip 340 is composed of nanowire 130, catalyst nanoparticle 170 and functionalizing molecules localized on the surface of the catalyst nanoparticle. The surface 176 of catalyst nanoparticle 170 is covered by functionalizing molecules. An exemplary one of the functionalizing molecules is shown at 190. Reference numeral 190 will be used to refer to the functionalizing molecules collectively. Typically, the functionalizing molecules are organic thiols. Such molecules are polar molecules having at one end a thiol moiety comprising an —SH group. The —SH groups of the functionalizing molecules bond to the surface 176 of catalyst nanoparticle 170 to form a self-assembling monolayer that covers surface 176. The —SH group has a greater tendency to bond with surface 176 than with the semiconductor material of nanowire 130. As a result, functionalizing molecules 190 are localized on the surface 176 of catalyst nanoparticle 170.
A molecule of interest is located at the other end of each functionalizing molecule 190. A molecule of interest is any molecule that gives functionalized probe tip 340 a desired chemical or physical property. Typical molecules of interest include antibodies, antigens, DNA, RNA, oligonucleotides, peptides, proteins, receptors, enzymes, polymers, ligands and small molecules such as biotins. In some cases, the molecule of interest can be a cell, a bacterium or a virus. The molecule of interest can be directly modified by covalently bonding to a thiol moiety. Alternatively, the molecule of interest can be indirectly modified by bonding to a thiolated linker molecule via a non-covalent interaction, for example, an electrostatic interaction, hydrogen binding or a specific interaction such as the molecular recognition between antibody-antigen, streptavid-biotin, donor-receptor, etc. Techniques for thiolating typical molecules of interest to form the functionalizing molecules are known in the art and will therefore not be described here. Alternatively, pre-thiolated molecules of interest may be purchased for use as functionalizing molecules 190.
As noted above and as will be described in more detail below, the functionalized AFM probe may be used to measure an interaction force or another parameter resulting from an interaction of the molecules of interest with another object 20. Additionally or alternatively, the functionalized AFM probe may be used to deposit the molecules of interest at a defined location, for example, at a specific location inside a living cell.
Organic selenolates, which are analogous to respective organic thiols but have a selenium atom instead of a sulfur atom, are also capable of forming self-assembling monolayers and may be used as the functionalizing molecules instead of such organic thiols. Organic selenolates have a greater adsorptivity with respect to gold than corresponding organic thiols. Organic selenolates are described further by Huang et al., Selenolates as Alternatives to Thiolates for Self-Assembled Monolayers: a SERS Study, 14 L
Functionalizable AFM probe 100 may be functionalized by its manufacturer to provide the functionalized AFM probe. In this case, the manufacturer of functionalized AFM probe 100 sells the functionalized AFM probe. In an alternative, functionalizable AFM probe 100 may be functionalized by its user to provide the functionalized AFM probe. In this case, the manufacturer of functionalizable AFM probe 100 sells functionalizable AFM probe 100. In another alternative, functionalizable AFM probe 100 may be functionalized by a third party to provide the functionalized AFM probe. In this case, the manufacturer of functionalizable AFM probe 100 sells functionalizable AFM probe 100 and the third party sells the functionalized AFM probe.
An example of a method in accordance with an embodiment of the invention for making functionalizable AFM probes in accordance with the invention will be described next with reference to
A cantilever element having a crystalline growth surface at one of its ends is provided.
As noted above, a conventional monolithic single-crystal AFM probe with a frusto-pyramidal probe tip is typically used as cantilever element 110. Also as noted above, a conventional tipless monolithic single-crystal semiconductor AFM probe is typically used as cantilever element 210. Such conventional AFM probes can be commercially supplied mounted in the wafer of single-crystal silicon in which they are defined. Thus, cantilever elements including cantilever element 110 or cantilever element 210 can be commercially supplied mounted in the wafer of single-crystal silicon (not shown) in which they are defined. This wafer will be referred to as a probe wafer. The cantilever elements are joined to the probe wafer by narrow beams that extend from each cantilever element to the remainder of the probe wafer. Many functionalizable AFM probes similar to AFM probe 100 or AFM probe 200 are fabricated at a time by subjecting the probe wafer in which cantilever elements are defined to the processing described below with reference to
In the example shown, cantilever arm 112 and probe tip base 114 are respective portions of a single piece of single-crystal silicon. The end facet 118 of probe tip base 114 is substantially parallel to cantilever arm 112, as defined above, and is typically the (111) crystalline plane of the silicon of the probe tip base. A group IV or group III-V semiconductor nanowire grown on the (111) crystalline plane of silicon will grow epitaxially, i.e., the crystallographic orientation of the semiconductor material at the end facet of the probe tip base imposes a specific crystallographic orientation on the nanowire, and the nanowire will grow in a direction substantially orthogonal to the crystalline plane. Hence, nanowire 130 that later will be grown on the growth surface 120 of cantilever element 110 will extend substantially orthogonally from the growth surface, and, hence, will additional extend substantially orthogonally to cantilever arm 112. Probe tip base 114 may alternatively be a suitably-shaped piece of single-crystal silicon mounted on cantilever arm 112. In such an embodiment, the end facet 118 of probe tip base 114 that provides growth surface 120 is the (111) crystalline plane of the silicon of the probe tip base so that the growth direction of nanowire 130 is defined as described above. In other embodiments, end facet 118 is a (100) or a (110) crystalline plane, although, as noted above, it is more difficult to grow silicon nanowires with good material quality on such crystalline planes than on the (111) crystalline plane. In the fabrication of AFM probe 200, cantilever element 210 is composed of cantilever arm 212, which is a piece of single-crystal silicon. At least part of the external surface 218 of cantilever arm 212 provides crystalline growth surface 220.
In specific examples, cantilever arm 112 and probe tip base 114 are the cantilever arm and probe tip, respectively, of a monolithic single-crystal AFM probe having a frusto-pyramidal single-crystal silicon probe tip sold by NanoWorld AG of Neuchâtel, Switzerland, and cantilever arm 212 is a tipless monolithic single-crystal AFM probe sold by NanoWorld AG.
The probe wafer in which the cantilever elements including cantilever element 110 are supplied typically has apertures extending between its major surfaces. The apertures make the probe wafer incompatible with the vacuum chucks used in some of the processes described below. At least to remedy this incompatibility, the probe wafer is temporarily mounted on the major surface 152 of a handle wafer 150 with growth surface 120 facing away from major surface 152, as shown in
In an embodiment, handle wafer 150 is a wafer of single-crystal silicon and the probe wafer is temporarily attached to the handle wafer using clips (not shown). Alternative handle wafer materials include ceramics, sapphire and other suitable materials. In another embodiment, cantilever elements similar to cantilever element 110 are supplied temporarily mounted on a handle wafer.
The cantilever element is covered with sacrificial material leaving at least part of the growth surface exposed. In the example shown, a sacrificial layer 160 of sacrificial material is deposited with a nominal thickness greater than the distance from handle wafer surface 152 to growth surface 120 so that the sacrificial layer initially covers cantilever element 110, including growth surface 120, as shown in
In an embodiment, the sacrificial material constituting sacrificial layer 160 was photoresist. The photoresist was deposited on the probe wafer by spin coating to cover cantilever element 110, including cantilever arm 112 and probe tip base 114, as shown in
As a further alternative, a layer of a material that covers underlying elements conformally may be deposited with a thickness sufficient to cover growth surface 120 to provide sacrificial layer 160. An example of a conformally-covering material is silicon dioxide (SiO2) deposited by chemical vapor deposition (CVD).
The portion of the sacrificial material constituting sacrificial layer 160 that is removed to form window 164 was defined by electron beam lithography. Other lithographic techniques such photolithography and nanoimprint lithography are known in the art and may alternatively be used. The size of window 164 determines the size of the catalyst metal (174 in
In an embodiment in which the material of sacrificial layer 160 was silicon dioxide, the portion removed to form window 164 was removed by subjecting the probe wafer to a wet etch process using dilute hydrofluoric acid (HF) as etchant. Alternatively, in such embodiment in which the material of sacrificial layer 160 was silicon dioxide, the portion removed to form window 164 was removed by subjecting the sacrificial material to chemical mechanical polishing (CMP) to expose growth surface 120. CMP should not be used to remove the portion of sacrificial layer 160 to form window 164 in the fabrication of functionalizable AFM probe 200.
The processing described above with reference to
In an embodiment, the layer of native silicon dioxide was removed from growth surface 120 by subjecting the probe wafer to a wet etch process using dilute hydrofluoric acid (HF) as the etchant. In another embodiment, the layer of native silicon dioxide was removed by subjecting the probe wafer to a dry etch process using HF vapor as the etchant.
Catalyst metal suitable for catalyzing a vapor-liquid-solid nanostructure growth process is then deposited on the growth surface.
The material of catalyst metal layer 172 is a metal capable of catalytically decomposing a gaseous precursor to release a respective constituent element of the semiconductor material of nanowire 130 (
In an embodiment, catalyst metal layer 172 is deposited using electron beam evaporation. Catalyst metal layer may alternatively be deposited by a conventional electroplating process or an electroless plating process.
In another embodiment, galvanic displacement is used to deposit catalyst metal selectively on growth surface 120. In an example in which the catalyst metal is gold, an electrical connection is made to probe tip base 114 via cantilever arm 112 and handle wafer 150, and the probe wafer is placed in a solution of gold potassium cyanide (AuK(CN)2) or another suitable electrolyte. A suitable anode is also placed in the electrolyte and a current is passed through the electrolyte between the anode and the probe wafer. The silicon of growth surface 120 acts as a reducing agent and the catalyst metal is selectively deposited on the growth surface through a redox mechanism.
The sacrificial material is then removed. Removing the sacrificial material leaves the catalyst metal deposited on the growth surface.
In an embodiment, sacrificial layer 160 of photoresist is removed by a lift-off process in which the probe wafer is immersed in acetone ((CH3)2CO). Sacrificial layer 160 of silicon dioxide may be removed by subjecting the probe wafer to a wet etch process in which dilute hydrofluoric acid (HF) is used as etchant.
A semiconductor nanowire is then grown extending from the growth surface using the catalyst metal remaining on the growth surface as catalyst.
In an embodiment, handle wafer 150 is placed on the susceptor 180 of a chemical vapor deposition (CVD) reactor (not shown) and the susceptor and, hence, the handle wafer and the probe wafer, are heated to a deposition temperature near the eutectic point of an alloy between catalyst metal 174 and the semiconductor material from which nanowire 130 will be grown. In an embodiment in which catalyst metal 174 was gold and the semiconductor material from which nanowire 130 is grown was silicon, the susceptor was heated to a growth temperature of about 450° C.
A growth pressure is established inside the CVD reactor and a gaseous precursor mixture is passed over the probe wafer. In
Further details of the growth of nanowire 130 will now be described with reference to an example in which the semiconductor material of nanowire 130 has a single constituent element, namely, silicon. The description below can readily be applied to the growth of a nanowire whose semiconductor material is a compound semiconductor. The precursor and adatoms of the dopant will not be mentioned in the following description.
Molecules of the precursor in gaseous precursor mixture 182 that contact catalyst metal 174 (
Catalyst nanoparticle 170 is capable of catalytically decomposing the precursors in precursor mixture 182. Consequently, additional adatoms of the constituent element(s) are deposited on the surface 176 of catalyst nanoparticle 170 and increase the fraction of the constituent element in the alloy until the alloy becomes saturated with the constituent element. Then, further adatoms of the constituent element cause a corresponding number of atoms of the constituent element to be released from catalyst nanoparticle 170 at its surface adjacent growth surface 120. The released atoms grow epitaxially on the growth surface to form a solid nanowire 130 that extends orthogonally from the growth surface.
Further deposition of adatoms of the constituent element on molten catalyst nanoparticle 170 cause the release of additional atoms of the constituent element from the molten catalyst nanoparticle and an increase in the length of nanowire 130, as shown in
Nanowire 130 has a lateral surface 132 that, during the growth of the nanowire, is also exposed to gaseous precursor mixture 182. Some of the molecules of the precursor in gaseous precursor mixture 182 contact lateral surface 132 and decompose non-catalytically to deposit respective adatoms of the constituent element on lateral surface 132. Such adatoms accumulate on lateral surface 132. The rate of lengthways growth of nanowire 130 is substantially constant, so the time that an annular segment of lateral surface 132 is exposed to gaseous precursor mixture 182 is inversely proportional to the distance of the annular segment from growth surface 120. Consequently, adatoms accumulated on lateral surface 132 cause the cross-sectional area of nanowire 130 to increase towards growth surface 120. As a result, nanowire 130 has a tapered shape, rather than the non-tapered shape shown. If the taper is not severe, such a tapered shape is acceptable, and may be desirable, in some applications.
In applications in which the non-tapered shape of nanowire 130 shown in
In an embodiment, gaseous etchant 184 was a halogenated hydrocarbon, such as halogenated methane. In one example, the halogenated methane was carbon tetrabromide (CBr4). In another example, the halogenated methane was carbon tetrachloride (CCl4). Not all the hydrogen atoms of the halogenated hydrocarbon need be substituted. Moreover, ones of the hydrogen atoms may be replaced by different halogens. In another embodiment, gaseous etchant 184 was a hydrogen halide (HX), where X=fluorine (F), chlorine (Cl), bromine (Br) or iodine (I).
In another embodiment, gaseous etchant 184 is provided by using a halogen-containing precursor as the precursor for at least one of the constituent elements. The halogen-containing precursor forms part of gaseous precursor mixture 182 passed over the probe wafer. The halogen-containing precursor is catalytically decomposed at the surface 176 of nanoparticle 170. Adatoms of the constituent element are deposited on surface 176 and the halogen is released into the gaseous precursor mixture. The carrier gas carries the halogen released from the halogen-containing precursor to the lateral surface 132 of nanowire 130 as gaseous etchant 184. Additional halogen may be released by non-catalytic decomposition of the halogen-containing precursor at the lateral surface. At lateral surface 132, the halogen combines with adatoms newly-deposited on the lateral surface to form a volatile compound and the carrier gas carries the volatile compound away from the lateral surface.
The probe wafer in which the functionalizable AFM probes including AFM probe 100 have been fabricated is then detached from handle wafer 150. In an embodiment, the probe wafer including AFM probe 100 is detached from handle wafer 150 by removing the clips (not shown) holding the wafers together.
In the above description, the probe wafer is detached from handle wafer 150 after nanowire 130 has been grown. Alternatively, the probe wafer can be detached from the handle wafer after sacrificial layer 160 has been removed but before nanowire 130 is grown. In this case, the probe wafer is placed directly on the susceptor 180 (
In embodiments of the above method in which the probe wafer lacks apertures extending between its major surfaces, the handle wafer referred to above is unnecessary.
In embodiments of the above method in which the cantilever elements do not constitute part of a probe wafer, such cantilever elements may be mounted on the handle wafer (
In an embodiment, the sacrificial material of sacrificial layer 260 was polymethylmethacrylate (PMMA). The PMMA sacrificial material was deposited by spin coating to cover handle wafer 150 and cantilever element 110, including cantilever arm 112 and probe tip base 114. The viscosity of the sacrificial material and the spin speed were set to obtain a nominal layer thickness about 100 nm less than the distance from handle wafer surface 152 to growth surface 120 so that the deposition process leaves growth surface 120 exposed. Sacrificial layer 260 of PMMA is later removed in the process described above with reference to
Photoresist deposited by spin coating may be used instead of PMMA as the sacrificial material of sacrificial layer 260. Other materials that are compatible with the subsequently-performed processing and that can be applied in a manner that produces a planar surface 262 are known in the art and may alternatively be used as the sacrificial material of sacrificial layer 260.
The catalyst metal constituting nanoparticles 272 and 274 is one capable of catalytically decomposing a gaseous precursor to release a respective constituent element of the semiconductor material of which nanowire 130 (
The size of nanoparticle 274 determines the diameter of nanowire 130 (
In an embodiment, a solution containing colloidal nanoparticles of catalyst metal is spun onto the surface 262 of sacrificial layer 260. Handle wafer 150 is then gently heated to evaporate the liquid component of the colloidal solution. This leaves nanoparticle 274 located on growth surface 120 and nanoparticles 272 distributed over the surface 262 of sacrificial layer 260. In another embodiment, an aqueous solution of colloidal nanoparticles of catalyst metal is mixed with methanol (CH3OH) and the resulting mixture is dropped onto the surface 262 of sacrificial layer 260. The mixture rapidly spreads over surface 262 and growth surface 120. The handle wafer is then gently heated to evaporate the liquid component of the dilute colloidal solution. This leaves nanoparticle 274 located on growth surface 120 and nanoparticles 272 distributed over surface 262 of sacrificial layer 260.
The processes described above with reference to
The processes described above with reference to
The thiol moieties of the functionalizing molecules in solution 194 that contact catalyst nanoparticle 170 form a self-assembling monolayer that coats the external surface of the nanoparticle. Corresponding activity with respect to the functionalizing molecules that contact nanowire 130 occurs minimally, if at all.
The resulting functionalized AFM probe 300 is withdrawn from solution 194, as shown in
Functionalized AFM probe 300 or a functionalized embodiment of AFM probe 200 is used by mounting it as the AFM probe of a host atomic force microscope (not shown) and using the atomic force microscope to perform measurements and manipulations that need a functionalized probe tip.
In an example of the use of functionalized AFM probe 300, the atomic force microscope is operated to place functionalized probe tip 340 adjacent a test subject whose activity is to be measured by measuring an interaction force between the test subject and the functionalizing molecules 190 coating functionalized probe tip 340. A stimulus is provided to the test subject and the atomic force microscope is used to measure the interaction force applied to the functionalized AFM probe by interaction between the functionalizing molecules coating functionalized probe tip 340 and the test subject. Alternatively, the AFM may be used to measure another parameter detected by functionalized AFM probe 300.
In an example of the use of functionalized AFM probe 300 to deliver its functionalizing molecules to a location of interest inside a living cell (not shown), the host AFM is operated to move functionalized AFM probe in the x- and y-directions to align functionalized probe tip 340 with the location of interest. The AFM is then further operated to move functionalized AFM probe 300 in the z-direction to bring functionalized probe tip 340 to the location of interest inside the cell. This causes functionalized probe tip 340 to penetrate the membrane of the cell. The small diameter and high aspect ratio of functionalized probe tip 340 allows functionalized probe tip 340 to penetrate the membrane without causing the membrane to rupture. Once functionalized probe tip 340 is at the location of interest inside the cell, a small voltage is applied to functionalized AFM probe 300. The voltage breaks the bonds between functionalizing molecules 190 and catalyst nanoparticle 170 and releases the functionalizing molecules into the cell at the location of interest.
The reaction of the cell to the functionalizing molecules released at the location of interest is then monitored. The monitoring may involve the AFM measuring the force applied by the cell to probe tip 340, now defunctionalized, as a result of the reaction. In another example, monitoring involves the AFM detecting a change in a mechanical vibration property, such as amplitude or frequency, of the cell. Other monitoring techniques may be used instead or in addition.
This disclosure describes the invention in detail using illustrative embodiments. However, the invention defined by the appended claims is not limited to the precise embodiments described.
This application is also related to U.S. patent application Ser. No. ______ of Sungsoo Yi et al. entitled High Aspect Ratio AFM Probe and Method of Making (Agilent Docket No. 10060079-1) and to U.S. patent application Ser. No. ______ of Bo Curry et al. entitled Insertable Nanoscale FET Probe (Agilent Docket No. 10060080-1), both filed on the filing date of this application. The above applications are all assigned to the assignee of this application and the disclosures of the above applications are incorporated into this application by reference.
