Method of fabricating multiple nanowires of uniform length from a single catalytic nanoparticle

[0001] 1. Field

[0002] Embodiments relate generally to nanowires and more particularly, to a method of fabricating nanowires.

[0003] 2. Background

[0004] Nanowires may be used in fabrication of various devices, such as transistors, displays, sensors or other devices. Typically, nanowires are formed using catalytic particles, such as gold particles. One conventional technique for forming nanowires involves providing catalytic particles on a surface of a substrate as described in block 110 of FIG. 1. Then, the catalytic particles are exposed to a vapor precursor, such as silane, under conditions suited to catalyze growth of silicon nanowires, as described in block 120 of FIG. 1. The nanowires 204 grown using conventional techniques may be inadequate for certain applications due to difficulties associated with producing nanowires of uniform lengths. FIG. 2 illustrates nanowires 204 grown using conventional techniques in which the lengths of the nanowires vary considerably even though they were grown under the same conditions.

[0005] Another disadvantage is that conventional techniques have low nanowire growth yield. That is, conventional techniques for fabricating nanowires are typically unable to produce more than one nanowire 204 per each catalytic particle 202 used in the fabrication process, as shown in FIG. 2. Yet another disadvantage is that the catalytic particles typically remain attached to the nanowires even after the completion of the nanowire fabrication process. The catalytic particles attached to the nanowires may not be desirable in fabrication of various devices.

[0006] The invention is illustrated by way of example and not by way of limitation in the Figures of the accompanying drawings in which like references indicate similar elements. It should be noted that the references to “an” or “one” embodiment of this disclosure are not necessarily to the same embodiment, and such references mean at least one.

[0007] FIG. 1 is a flowchart diagram illustrating a conventional process for forming nanowires from precursor nucleation particles.

[0008] FIG. 2 is a schematic diagram illustrating nanowires grown using a conventional process.

[0009] FIG. 3 is a flowchart diagram illustrating a process of growing multiple nanowire segments from a single catalytic particle according to one embodiment.

[0010] FIGS. 4A and 4B are schematic diagrams illustrating dissolving of sacrificial segments according to one embodiment.

[0011] FIG. 5 is a flowchart diagram illustrating a process of removing catalytic particles from a solution containing nanowire segments according to one embodiment.

[0012] In the following description, for purposes of explanation, specific details are set forth to provide a thorough understanding of embodiments of the invention. However, it is understood that embodiments may be practiced without these specific details. In other instances, well-known structures and techniques have not been shown in detail to avoid obscuring the understanding of this description.

[0013] FIG. 3 shows a process of fabricating multiple nanowire segments of uniform length from a single catalytic particle according to one embodiment. The terms “catalytic particles”, “nanoparticles” and “precursor nucleation particles” are used interchangeably herein to refer to any nanoscale particles, such as gold particles, that may be used to catalyze growth of nanowires. The terms “nanowires” and “nanowire segments” are used interchangeably herein to refer to any elongated nanowire structure grown from catalytic particles, such as silicon, germanium, tungsten or molybdenum nanowires. In one embodiment, the diameter of the nanowires is less than 50 nanometers (nm). In another embodiment, the diameter of the nanowires is less than 10 nm.

[0014] The growing process starts with nanoparticles dispersed on a surface of a substrate, as shown in block 310. Any suitable substrate may be used, such as silicon, glass or other substrate that is compatible with process temperatures and chemistries. In one embodiment, gold particles are used as catalyst for growing nanowire segments. However, the catalytic particles are not limited to gold. Other suitable catalyst material may be used, including, but not limited to, molybdenum, zinc, silver, copper, cadmium, iron, nickel and cobalt.

[0015] In general, the size of the catalytic particles may determine the diameter of the nanowires that grow from them. Accordingly, in one embodiment, nanoparticles of approximately mono-dispersed size are used to grow the nanowire segments having approximately the same diameter.

[0016] In one embodiment, multiple nanowire segments are grown from each individual nanoparticle. Multiple nanowire segments produced by each individual nanoparticle may include alternating segments of nanowires made of a first material (e.g., a sacrificial material) and nanowires made of a second material (e.g., non-sacrificial material). In accordance with one embodiment, the first material and the second material have different solubility characteristics. One advantage of using materials of different solubility is that following the growth of the multiple nanowire segments, sacrificial segments may be selectively dissolved such that non-dissolving nanowires are automatically separated into individual segments.

[0017] As noted above, one of the problems associated with conventional techniques for growing nanowires is difficulty associated with producing nanowires of uniform lengths. This is because the start of initial nanowire formation will vary from one nanoparticle to another. As a result, nanowire length, which is typically determined by conditions, such as time and temperature, can vary from one nanowire to another. As shown in illustrated example of FIG. 4A, the lengths of the first nanowire segments grown from nanoparticles will vary. However, the length of subsequent nanowire segments can be better controlled since the nanoparticles have already started their nanowire formation. Accordingly, in one embodiment, a sacrificial nanowire segment is initially grown from each nanowire before the desired nanowire segments are subsequently grown.

[0018] Referring back to FIG. 3, in block 320, a first precursor such as germane (GeH₄) or other vapor precursor is used to initiate growth of a sacrificial nanowire segment (e.g., germanium nanowire segment) from the nanoparticles. Once all or most of the nanoparticles have started their nanowire formation, the nanoparticles are exposed to a second precursor such as silane (SiH₄) or other vapor precursor to catalyze growth of a silicon nanowire segment between the germanium nanowire segment and the catalytic particle, in block 330. In one embodiment, the silicon nanowire segments are grown by maintaining a pressure of less than 100 mTorr and a temperature of about 440° C. while flowing between 10 to 80 sccm of Silane (SiH₄) (10% in He) for about 5 to 10 minutes. The fabrication conditions for the first precursor and the second precursor may be different. Accordingly, the temperature and/or pressure may need to be modified between the two growth processes. Because the silicon nanowire segments are grown after the nanoparticles have already started their nanowire formation, the silicon nanowire segments of uniform length may be produced by controlling the duration of the silane exposure.

[0019] Any suitable process may be used to provide an adequate condition for nanowire growth, including thermal evaporation process, catalytic chemical vapor deposition process, vapor-solid process and laser assisted catalytic growth process.

[0020] Then, in block 340, the process of exposing the nanoparticles to the first precursor (block 320) and the second precursor (block 330) are repeated until a desired number of segments are grown from each nanoparticle. Alternating between the use of germane and silane will result in alternating segments of germanium and silicon nanowire segments as shown in FIG. 4A. The germanium and silicon nanowire segments have different solubility characteristics. Different nanowire segments having different solubility characteristics may be advantageous because certain nanowire segments made be selectively dissolved without negatively effecting the other nanowire segments. Specifically, in one embodiment, nanowire segments made of germanium may be selectively dissolved without dissolving nanowire segments made of silicon.

[0021] Once the formation of nanostructures having a desired number of nanowire segments is completed, the germanium nanowire segments are selectively dissolved in a solution, in block 350. For example, a water solution may be used to selectively dissolve the sacrificial segments. The water solution may contain oxidizing agents (e.g., peroxide) to facilitate the selective dissolving process. Any suitable solution may be used if desired. The selectively dissolving process causes silicon nanowire segments disposed between the germanium segments to separate into individual segments with uniform length. Producing nanowire segments with uniform length may be advantageous in various applications. This may be achieved by controlling the growth conditions during the application of the second precursor (e.g., silane) such that the silicon nanowire segments will have relatively similar length. The length of the silicon segments may be function of exposure time, temperature and pressure.

[0022] It should be noted that during the selective dissolving process, the silicon nanowire segments will automatically be separated from their catalytic particles since the germanium nanowire segment attached between the catalytic particle and the first silicon nanowire segment will be dissolved during this process. Following the selective dissolving process, the catalytic particles are removed from a solution containing the silicon nanowire segments in block 360. The process of removing catalytic particles will be described in more detail with reference to FIG. 5.

[0023] FIG. 4A and FIG. 4B show nanowire segments before and after the selective dissolving process, respectively. The nanostructures 400-1 through 400-N are shown FIG. 4A as each having nine individual segments 41 through 49. These are merely illustrative examples of the nanostructures that may be fabricated. In general, the nanostructures 400 may be formed of any number of nanowire segments. The nanowire segments 41 through 49 preferably alternate between sacrificial material and non-sacrificial material.

[0024] Once a desired number of nanowire segments have been grown from each nanoparticle 402, the nanostructures 400 are immersed within a solution 500 to selectively dissolve sacrificial segments 41, 43, 45, 47, 49, as shown in FIG. 4B. Since the sacrificial segments 41, 43, 45, 47, 49 and silicon nanowire segments 42, 44, 46, 48 have different solubility characteristics, the sacrificial segments can be selectively dissolved without adversely effecting the silicon nanowire segments. In one embodiment, a solution 450 containing water is used to selectively dissolve the sacrificial nanowire segments 41, 43, 45, 47, 49, which are made of germanium.

[0025] As noted above, the first segments 41-1 through 41-N grow in variable length because there is little control over synchronizing the initial timing of nanowire formation from one nanoparticle to another. In the illustrated embodiment shown in FIG. 4A, the first segment 41 of each nanostructure is made of sacrificial material. Accordingly, the silicon nanowire segments 42, 44, 46, 48 which are formed after the initial nanowire formation will grow at about the same growth rate. Additionally, as shown in FIG. 4A, the last segment 49-1 through 49-N of each nanostructure is made of sacrificial material (e.g., germanium). Accordingly, none of the silicon nanowire segments 48-1 through 48-N will be attached to the catalytic nanoparticles 402 after the selective dissolving process.

[0026] Catalytic particles, such as gold particles, may not be suitable in fabrication of semiconductor devices (e.g., transistor devices) since the particles may cause high junction leakage and low breakdown voltages. In one embodiment, a method is described for removing catalytic particles from a solution containing nanowire segments. Following the separation of nanowire segments from nanoparticles, the catalytic particles, such as the gold particles, are removed by mixing the solution containing nanowire segments and catalytic particles with another solution containing compounds to bond to the catalytic particles.

[0027] FIG. 5 shows a process of removing catalytic nanoparticles from a solution containing nanowires according to one embodiment. The nanoparticle removing process begins at block 510, in which a first solution containing nanowires and catalytic nanoparticles is provided. In one embodiment, the nanowire-containing solution contains water and nanoparticles, such as gold particles. A concentration of the catalytic particles is reduced from the first solution by using a second solution containing compounds that are capable of bonding to the nanoparticles. Accordingly, in block 520, a second solution is added to the first solution. For example, a hydrocarbon solution containing long chained thioalkyl compounds may be used as the second solution.

[0028] In block 530, the first and second solutions are mixed such that the nanoparticles residing in the first solution generally bond to thioalkyl compounds contained in the second solution. Representatively, the mixing of the two solutions may cause micelle formation making the gold particles soluble in the hydrocarbon solution. Following the mixing process, the first and second solutions are allowed to separate in block 540. This may be accomplished by maintaining the mixed solution for a period of time such that the first and second solutions are separated into an upper layer containing the hydrocarbon solution (i.e., second solution) and a lower layer containing the water solution (i.e., first solution). Specifically, the gold particles will bond with the long chained thioalkyl compounds during the mixing process and will be brought to the upper layer containing hydrocarbon solution, instead of being in the lower layer solution with the nanowire segments.

[0029] Once the solution has been separated into upper and lower layers, the upper layer solution containing the nanoparticles is removed, in block 550, for example, by decanting the upper layer solution. This results in the first solution containing a reduced nanoparticle concentration than prior to the nanoparticle removing process. The nanoparticle removing process (blocks 510-550) may be repeated a number of times in order to reduce the nanoparticle concentration to a desired level.

[0030] While several embodiments have been described, those skilled in the art will recognize that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.