Method of forming catalyst nanoparticles for nanowire growth and other applications
Methods for forming a predetermined pattern of catalytic regions having nanoscale dimensions are provided for use in the growth of nanowires. The methods include one or more nanoimprinting steps to produce arrays of catalytic nanoislands or nanoscale regions of catalytic material circumscribed by noncatalytic material.
This application is a divisional of prior application Ser. No. 10/281,678, filed on Oct. 28, 2002.
BACKGROUNDGrowth of silicon nanowires offers the possibility of forming arrays with a large surface-to-volume ratio. These arrays can be used for chemical or environmental sensing, for electrical transduction, or for electron emission.
Bulk synthesis of semiconductor nanowires has been traditionally achieved using several variations of transition metal catalyzed techniques such as vapor-liquid-solid (VLS) synthesis. See, e.g., Kamins et al., J. Appl. Phys. 89:1008-1018 (2001) and U.S. Pat. No. 6,248,674. In standard vapor-liquid-solid (VLS) synthesis techniques used for producing silicon nanowires, each wire grows from a single particle of gold, cobalt, nickel or other metal. A vapor-phase silicon-containing species transported to the catalyst inside a high-temperature furnace condenses on the surface of the molten catalyst, where it crystallizes to form silicon nanowires.
Silicon nanowires produced by the standard VLS process are composed of a single crystal. In the standard process, the size of the catalytic particle controls the diameter of the nanowire grown from it. Thus, in order to obtain a uniform nanowire diameter distribution, monodispersed catalyst particles need to be created on a solid substrate. However, creation of nanometer size catalyst droplets is a non-trivial task. The nanoparticles can be formed by deposition techniques, such as chemical vapor deposition or physical vapor deposition. Although they can be registered to previously formed patterns, creating these pattern requires additional processing, usually involving costly lithography. In addition, conventional lithography processes cannot readily form nanoparticles of the desired small dimensions Thus, there is a need for improved methods of forming evenly spaced catalytic particles having dimensions in the nanometer range.
SUMMARY OF THE INVENTIONThe present invention is directed to nanoimprinting or soft lithography methods for creating arrays of catalyst nanoparticles useful for forming nanoscale wires for device applications. The methods of the present invention are capable of forming smaller catalyst islands, move rapidly and less expensively than is possible with conventional lithography or even with electron-beam lithography.
In one embodiment, the method of forming an array of catalytic nanoparticles includes the steps of (1) providing a mold with nanoscale protrusions forming a desired pattern; (2) coating the protrusions with catalytic material; and (3) transferring the desired pattern of catalytic material to a substrate by contacting the substrate with the catalytic material.
In another embodiment, nanoscale regions of catalyst are localized within depressions of a non-catalytic layer by a method, which includes the steps of (1) depositing a layer of masking material on an underlying layer; (2) providing a mold with nanoscale protrusions forming a desired pattern; (3) pressing the protrusions of the mold into the masking material so that depressions are formed in the masking layer in the desired pattern; (4) exposing the underlying layer in the depressions; and (5) localizing catalytic material in the depressions. In a preferred embodiment, the underlying substrate itself is the source of the catalytic material localized in the depressions. Alternatively, the catalytic material is selectively deposited in the depressions.
Another embodiment is a method of forming nanoscale regions of exposed catalyst, comprising the steps of: (1) obtaining a substrate; (2) providing a catalytic layer on the substrate; (3) forming a non-catalytic layer over the catalytic layer; (4) depositing a layer of masking material on the non-catalytic layer; (5) providing a mold with nanoscale protrusions forming a desired pattern; (6) pressing the protrusions of the mold into the masking material so that depressions are formed in the masking layer in the desired pattern; (7) exposing regions of the non-catalytic layer in the depressions; (8) etching the exposed non-catalytic regions to expose regions of the catalytic layer; and (9) removing the masking material.
In a preferred embodiment a mold formed from parallel layers of nanoscale thickness, are used for the imprinting process. The mold can be made by: (1) providing a plurality of alternating layers of a first material and a second material forming a stack of parallel layers, wherein the first material is dissimilar from the second material, each layer having a nanoscale thickness; (2) cleaving and/or polishing the stack normal to its parallel layers, thereby creating an edge wherein each layer of the first and second materials is exposed; and (3) creating a mold having a pattern of alternating recessed and protruding lines by etching the edge of the stack in an etchant that attacks the first material at a different rate than the second material, thereby creating said pattern on the edge of the stack. The mold can then be used to create linear patterns of catalyst in further steps, which include: (1) providing a catalytic layer overlying a substrate and coating the catalytic layer with a masking material layer; and (2) forming a first set of nanoimprinted lines in the masking material layer, by pressing the protruding lines of the mold into the masking material layer exposing strips of the catalytic Iayer; and (3) etching the exposed strips of the catalytic layer to form lines of catalyst having a nanoscale width. Preferably, the method further comprises the steps of: (1) rotating the mold; and (2) applying the rotated mold to the masking material, thereby creating a second set of lines in the masking material, which intersect the first set of lines; and (3) etching the catalytic material that is not protected by the masking material. If the two sets of lines created by sequential application of the mold are orthogonal, a rectangular array of squares is created. Alternatively, the two sets of lines can be oriented at a non-perpendicular angle, thereby creating a skewed array of parallelograms.
Yet another embodiment of the present invention is a method for exposing nanoscale regions of catalytic material surrounded by a noncatalytic layer. Starting materials include a multilayered composite comprised of a substrate, a layer of catalytic material covering the substrate, and a masking layer formed over the catalytic layer. A first set of lines is imprinted in the masking layer using a mold having a patterned edge of alternating recessed and protruding nanoscale strips. The mold is then rotated and reapplied to form polygons of masking material in a regular array. The catalytic material that is not protected by the masking material is then covered with a non-catalytic material and the masking material is removed to expose nanoscale regions of the catalytic material circumscribed by non-catalytic material. The exposed regions of catalytic material can then be used as catalysts for nanowire growth.
These and other features, aspects, and advantages of the present invention will become better understood with respect to the following description, appended claims, and accompanying drawings where:
In accordance with the present invention, methods are provided for creating arrays of catalytic material useful for forming nanowires for device applications. For controlled application of the nanowires, they should be regularly spaced or positioned on the substrate in a predetermined pattern. Growth of each nanowire generally proceeds from a catalyst nanoparticle on the substrate surface. As shown here, when the catalyzing nanoparticles do not have to be registered to underlying structure, they can be formed by “soft lithography,” or nanoimprinting, which involves forming impressions of a mold, having nanoscale features, onto a layer of underlying material.
The mold 10 can be patterned with protruding features 20, such pillars, stripes, rectangular solids, or other three-dimensional designs. Protruding features having a minimum lateral size of 25 nm can be obtained using electron beam lithography, reactive ion etching (RIE) and other appropriate technology. Preferably, protruding features of the mold will have a lateral dimension of 5 nm to 20 nm. A mold 10 having the desired pattern of protruding nanoscale features at resolution levels much less than that of the-state-of-the-art e-beam lithography can be made according to methods described in further detail below, or as disclosed in the related application of Chen et. al. [HP Docket No, 100110197-1] (incorporated herein by reference). The typical depth of a protruding feature is from 5 nm to 500 nm, depending on the desired lateral dimension and the depth of the desired impression to be made.
In general, the mold 10 should be made of a relatively hard material capable of retaining its shape and integrity under the pressure applied during a nanoimprinting process. Accordingly, the mold can be made of materials such as metals, dielectrics, semiconductors, ceramics, or their combination.
As shown in
In general, the catalytic material 30 includes a catalyst capable of catalyzing the growth of nanowires. Accordingly, the catalytic material can include metals used to generate silicon nanowires, such as titanium, gold, zinc, silver, copper, platinum, palladium, nickel, and manganese. Alternatively, the catalytic material can include a catalyst capable of catalyzing the growth of carbon nanotubes or metal nanowires.
In this version of the present invention, the substrate 40 can be any material having a noncatalytic surface 50 capable of accepting the catalytic nanoparticles transferred from the mold, e.g., a silicon, silicon dioxide, silicon nitride or alumina substrate.
As shown in
In one version of the invention, the exposed regions 140 of underlying material 110 in the depression 120 can be the catalyst. In an alternative version, the exposed underlying material 110 can be a non-catalytic material on which the catalyst can be deposited selectively without any deposition on the surrounding region. Such selective deposition can be accomplished by, for example, chemical vapor deposition or liquid-phase deposition.
As shown in
In any case, an array of discrete nanoscale regions of catalytic material are formed at the locations determined by the pattern on the mold, and nanowires are then grown by catalytic decomposition of a silicon-containing gas such as silane (SiH4) or dichlorosilane (SiH2Cl2).
As shown in
The two sets of intersecting lines can be orthogonal, creating a rectangular array of square masking elements 470, as in
As shown in
The previously described versions of the present invention have many advantages. In particular, the methods of the present invention are capable of forming smaller catalyst islands than are possible with conventional lithography or even with electron-beam lithography. The present methods can also form catalyst islands more rapidly and less expensively than electron-beam lithography because nanoimprinting is a parallel process (forming many patterns at the same time), rather than a serial process (forming patterns sequentially) like electron-beam lithography.
Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. For example, the technique described above, of rotating a nanoscale mold having a pattern of recessed strips, can also be used to form an intermediate mold having a regular array of polygonal protrusions. The entire pattern of small nanoislands can then be formed in or on an underlying material in one impression. Alternatively, pattern definition and lift-off techniques can be used, in which a nanoscale pattern is formed in an underlying material and later removed along with any material deposited on top of the pattern. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.
Claims
1-23. (canceled)
24. A device comprising:
- i) a mold with nanoscale protrusions forming a desired pattern; and
- ii) catalytic material coating the protrusions
25. A device comprising: said masking material having a desired pattern of nanoscale depressions exposing the underlying layer.
- i) a layer of masking material; and
- ii) an underlying layer
26. The device of claim 25, wherein the underlying layer contains catalytic material.
27. The device of claim 25, further comprising catalytic material deposited within the nanoscale depressions.
28. A device comprising:
- i) a substrate:
- ii) a catalytic layer on the substrate;
- iii) a non-catalytic layer over the catalytic layer; and
- iv) a layer of masking material overlying the noncatalytic layer, said masking material having a desired pattern of nanoscale depressions exposing the non-catalytic layer.
29. The device of claim 28, wherein the nanoscale depressions also expose nanoscale regions of the catalytic layer.
30. A device comprising:
- i) a substrate; and
- ii) a regular array of catalytic nanoislands on the substrate.
31. A device comprising:
- i) a substrate;
- ii) a catalytic layer on the substrate; and
- iii) a non-catalytic layer over the catalytic layer, said non-catalytic layer having a regular pattern of nanoscale openings exposing the catalytic layer.
Type: Application
Filed: Apr 23, 2008
Publication Date: Dec 4, 2008
Inventors: Theodore I. Kamins (Palo Alto, CA), Philip J. Kuekes (Menlo Park, CA), Yong Chen (Redwood City, CA)
Application Number: 12/150,063
International Classification: H01L 23/48 (20060101);