Double contacts for carbon nanotubes thin film devices

A method of fabricating a semiconductor device is disclosed. A first contact layer of the semiconductor device is fabricated. An electrical connection is formed between a carbon nanotube and the first contact layer by electrically coupling of the carbon nanotube and a second contact layer. The first contact layer and second contact layer may be electrically coupled.

BACKGROUND

The present disclosure relates to carbon nanotube devices and, in particular, to methods of fabricating a carbon nanotube transistor from a thin-film of nanotubes.

Carbon nanotubes (CNTs) are carbon allotropes that form a cylindrical structure. Semiconducting carbon nanotubes (CNTs) conduct exceptionally high currents with respect to their nanoscale diameter (e.g., 1-2 nanometers). Among their many uses, CNTs have been integrated into thin-film transistors (TFTs) which use CNTs to form gates of the transistor. The CNTs in these TFTs are generally not straight linear cylinders but rather have various bends and curves. Thus, when assembling a plurality of CNTs, the ends of the CNTs are generally not neatly aligned. This lack of alignment of the CNT ends introduces problems when attempting to electrically couple the CNTs to the various electrical contacts of the TFT. Consequently, there is generally a high contact resistance at interfaces between CNT and contacts in thin-film devices.

SUMMARY

According to one embodiment, a method of fabricating a semiconductor device includes: fabricating a first contact layer of the semiconductor device; and forming an electrical connection between a carbon nanotube and the first contact layer by electrically coupling of the carbon nanotube and a second contact layer to fabricate the semiconductor device.

According to another embodiment, a method of fabricating a transistor, includes: fabricating a first contact layer of at least one of a source and a drain of the transistor on a substrate; forming a second contact layer of the at least one of the source and the drain; and forming an electrical connection between a carbon nanotube and the first contact layer by electrically coupling the carbon nanotube to the second contact layer.

According to another embodiment, a method of forming a thin-film device includes: fabricating a contact on a thin-film substrate, wherein the contact includes a first contact layer and a second contact layer; and coupling a carbon nanotube to the second contact layer to provide an electrically connection between the first contact layer and the carbon nanotube.

Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure. For a better understanding of the disclosure with the advantages and the features, refer to the description and to the drawings.

DETAILED DESCRIPTION

FIG. 1Ashows an exemplary carbon nanotube (CNT) thin film device100that may be fabricated using the methods disclosed herein. In the illustrative example, the thin film device100is a thin-film transistor, such as a carbon nanotube thin-film transistor. The thin film device100may include a supporting substrate102that is generally an insulating substrate. Exemplary substrates may include silicon dioxide on silicon substrate, for example. In one embodiment, the substrate is a flexible substrate. Contacts104and106are formed on the substrate102. In one embodiment, at least one of the contacts104and106may be formed on a surface of the substrate102. In another embodiment, at least one of the contacts104and106may be formed in a trench formed in the substrate102. For illustrative purposes, contact104is referred to herein as a source and contact106is referred to herein as a drain. However, it is understood that the contacts may be used for purposes other than as a source and a drain. A carbon nanotube layer108includes one or more CNTs that are configured to extend from the source104to the drain106. The CNTs may be electrically coupled to the source104at one end of the CNT and to the drain106at an opposing end. In various embodiments, the carbon nanotubes generally do not extend along a straight line from source104to drain106but rather may include various bends and/or curves. Such curviness allows CNTs to maintain their structural and electrical integrity as the substrate flexes. However, this curviness makes it difficult to bond the CNT to contact surfaces, i.e., the surfaces of the source104and of the drain106.

FIG. 1Bshows an exemplary sectional side view110of the drain106as viewed along the direction line B indicated inFIG. 1A. The sectional side view110shows the ends of various carbon nanotubes of the CNT layer108disposed at the drain106. Surface116, also referred to herein as a first contact layer and a bottom contact layer, represents a coupling surface of the drain106. Due to the curviness of the carbon nanotubes, the ends of the carbon nanotubes may pile up upon each other to form nanotube bilayers112, trilayers114or multi-layers greater than three (not shown). For a bilayer112, a bottom layer of the CNT bilayer112is in contact with the surface116while a top layer is away from the surface116. In a trilayer, only one of the CNT layers (i.e., the bottom CNT layer) is in contact with surface116, while two CNT layers are away from the surface. Therefore, generally only the CNTs at the bottom of the bilayer, trilayer, etc. may form a direct electrical coupling to surface116. It is understood that a sectional side view of the drain may show CNT layering similar to that shown in the sectional side view ofFIG. 1B.

As illustrated inFIG. 1B, even for a simple bilayer112, there are a significant number of CNTs that are not in direct contact with the surface116. The contact resistance increases with the number of CNTs not in direct contact with the contact surface. Increasing the number of CNTs that have an electrical contact to the surface therefore reduces contact resistance. A double-contact geometry provided herein is configured to increase the number of CNTs having an electrical contact to the surface, thereby reducing contact resistance. The exemplary double-contact geometry includes a second contact layer that is coupled to at least some of the CNTs that are not in direct contact with the first contact layer, as shown inFIGS. 2 and 3.

FIG. 2shows a CNT bilayer112sandwiched or enclosed between a first contact layer116and a second contact layer118. The first contact layer116may be a bottom contact layer and the second contact layer118may be a top contact layer. The ends of the CNTs are sandwiched or enclosed between the top contact layer116and the bottom contact layer118, as discussed below. The CNT ends at the bottom layer201of the bilayer are electrically coupled to the bottom contact layer116and the CNT ends at the top layer202of the bilayer are electrically coupled to the top contact layer118.FIG. 3shows a CNT trilayer114sandwiched or enclosed between bottom contact layer116and top contact layer118. The CNT ends at the bottom layer301of the trilayer are electrically coupled to the bottom contact layer116. The CNT ends at the top layer303of the trilayer are electrically coupled to the top contact layer118. The CNTs of middle layer302may or may not form an electrical coupling to either the bottom contact layer116or the top contact layer118. Nonetheless, for a CNT bilayer, trilayer or higher number of CNT layers, the contact design disclosed herein increases the number of CNTs having an electrical coupling to the selected contact. Thus, the contact design of the present disclosure reduces a contact resistance between the CNTs and the contacts. Though not shown inFIGS. 2 and 3, first contact layer116and second contact layer118may be form an electrical contact or be otherwise electrically coupled.

FIGS. 4-8illustrate an exemplary process for fabricating the exemplary thin-film device ofFIG. 1.FIG. 4shows an exemplary substrate102that may be provided at a first step in the fabrication process. In various embodiments, the substrate102may be a silicon-based substrate102. Trenches202and204may be formed in the substrate using various etching techniques, for example.

FIG. 5illustrates a metallic layer502formed on the substrate102for formation of contacts in the substrate102. The metallic layer502may be deposited on the substrate102using various methods, including chemical vapor deposition, etc. The metallic layer502may then be polished in order to form bottom contact layers514and516(seeFIG. 6) in the trenches202and204. In an exemplary embodiment, the exposed surfaces of the bottom contact layers514and516are substantially coplanar with the substrate surface. In an alternate embodiment, the bottom contact layers may rest on a surface of the substrate102(i.e., are not embedded therein), as shown inFIG. 1A, for example.

FIG. 6shows CNTs602placed along the substrate102such that ends of the CNTs602overlap the bottom contact layers514and516formed in the substrate102. The CNTs602extend across a section of the substrate102between the bottom contact layers514and516so that one end of a selected CNT extends to the bottom contact layer514and the other end of the selected CNT extends to the bottom contact layer516. The selected CNT may be configured to form a direct electrical connection with the bottom contact layers514and516or may be positioned away from the bottom contact layers, as illustrated inFIG. 1B, for example.

FIG. 7illustrates formation of contacts having a double-contact geometry according to an exemplary embodiment. Contact104includes bottom contact layer514and top contact layer518. Contact106includes bottom contact layer514and top contact layer520. Top contact layer518is placed on top of the ends of the CNTs and the bottom contact layer514in order to form an electrical coupling between the CNTs and the bottom contact layer514. In particular, the top contact layer518forms an electrical coupling to both bottom contact layer514and to the CNTs that are away from the bottom contact layer514, thus completing an electrical connection between the bottom contact layer514and the CNTs away from the bottom contact layer514. Similarly, top contact layer520is coupled to bottom contact layer516to provide an electrical coupling between the bottom contact layer516and the CNTs away from the bottom contact layer516.

FIG. 8shows a completion stage of the exemplary thin-film device. A gate structure802is formed on the portion of the CNTs between the source104and drain106. Voltages at the gate structure802are used to implement the CNTs as gates of the transistor. Electrodes and/or wires (not shown) may be connected to the source104, drain106and gate802to provide operation of the transistor.

FIG. 9shows a flowchart illustrating a method of fabricating a CNT thin-film field effect transistor according to an exemplary embodiment. In box902, at least one trench is etched into a substrate. In box904, the at least one trench is filled with a bottom contact material. In box906, the bottom contact material is polished to provide a bottom contact layer, wherein the bottom contact layer and the substrate may be substantially coplanar. In box908, carbon nanotubes are places along the substrate to extend between bottom contact layers. In box908, for a selected contact, a top contact layer is deposited on a bottom contact layer to form a double contact geometry that sandwiches the CNTs. In general, a coupling surface of the top contact layer has substantially the dimensions of the coupling surface of the bottom contact layer. The top contact layer forms an electrical coupling to various CNTs that extend away from the bottom contact layer and also forms an electrical coupling to the bottom contact layer, thereby creating an electrical path between the various extended CNTs and the bottom contact layer. In box910, a gate dielectric and conductive layer are formed at the CNT layer to complete a field-effect transistor.

The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the disclosure. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed disclosure.