Carbon nanotube diodes and electrostatic discharge circuits and methods

The present invention relates to the field of electronic devices; more specifically, it relates to carbon nanotube (CNT) diodes, electrostatic discharge (ESD) circuits using CNT diodes and the method of fabricating the CNT diodes and ESD circuits.

The search for ever faster, lower power consuming and thus smaller electronic devices is leading to structures having geometries of width and length smaller than current semiconductor image forming technology can generate. Of particular interest are structures formed from carbon-nanotubes such as quantum dots, CNT wires and CNT field effect transistors (CNTFETs). Circuits constructed using CNTFETs and CNT wires will require ESD protection and structures and methods for providing such protection are virtually unknown. Accordingly, there exists a need in the art to for compatible CNT diodes and ESD circuits for protecting CNT based electronic devices.

A first aspect of the present invention is a diode, comprising: a p-type single wall carbon nanotube; an n-type single wall carbon nanotube, the p-type single wall carbon nanotube in physical and electrical contact with the n-type single wall carbon nanotube; and a first metal pad in physical and electrical contact with the p-type single wall carbon nanotube and a second metal pad in physical and electrical contact with the n-type single wall carbon nanotube.

A second aspect of the present invention is a diode, comprising: a multiplicity of p-type single wall carbon nanotubes; a multiplicity of n-type single wall carbon nanotubes, each of the p-type single wall carbon nanotubes in physical and electrical contact with one or two of the n-type single wall carbon nanotubes and each of the n-type single wall carbon nanotubes in physical and electrical contact with one or two of the p-type single wall carbon nanotubes; and a first metal pad in physical and electrical contact with every p-type single wall carbon nanotube of the multiplicity of p-type single wall carbon nanotubes and a second metal pad in physical and electrical contact with every n-type single wall carbon nanotube of the multiplicity of p-type single wall carbon nanotubes.

A third aspect of the present invention is a diode array, comprising: a multiplicity of p-type single wall carbon nanotubes spaced apart, extending in a first lengthwise direction and parallel to each other in a first layer; a multiplicity of n-type single wall carbon nanotubes spaced apart, extending in a second lengthwise direction and parallel to each other in a second layer, the first lengthwise direction about perpendicular to the second lengthwise direction; first metal pads in physical and electrical contact with respective p-type single wall carbon nanotubes of the multiplicity of p-type single wall carbon nanotubes and second metal pad in physical and electrical contact with respective n-type single wall carbon nanotubes of the multiplicity of p-type single wall carbon nanotubes; and wherein each p-type single wall carbon nano-tube of the multiplicity of p-type single wall carbon nano-tubes crosses each n-type single wall carbon nano-tube of the multiplicity of n-type single wall carbon nano-tubes at included angles of greater than zero but no greater than 90 degrees, the p-type single wall carbon nanotubes in physical and electrical contact with the n-type single wall carbon nanotubes where the p-type single wall carbon nanotubes cross the n-type single wall carbon nanotubes.

A fourth aspect of the present invention is an electrostatic discharge circuit, comprising: a first diode according to the diode of the first aspect and a second diode according to diode of the first aspect; a cathode of a the first diode connected to a high voltage terminal of a power source and an anode of the second diode connected to a low voltage terminal of the power source; an input pad connected to the anode of the first diode, to a cathode of the second diode and to circuits to be protected.

A fifth aspect of the present invention is an electrostatic discharge circuit, comprising: a first diode according to the diode of the second aspect and a second diode according to the diode of the second aspect; a cathode of a the first diode connected to a high voltage terminal of a power source and an anode of the second diode connected to a low voltage terminal of the power source; an input pad connected to the anode of the first diode, a cathode of the second diode and to circuits to be protected.

The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:

FIG. 1A is a top view and FIG. 1B is a section view through line 1 B- 1 B of FIG. 1A of a first CNT diode is according to the present invention;

FIG. 2A is a top view and FIG. 2B is a section view through line 2 B- 2 B of FIG. 2A of a second CNT diode is according to the present invention;

FIG. 3 is a top view of a third CNT diode is according to the present invention;

FIG. 4 is a top view of a fourth CNT diode is according to the present invention;

FIG. 5 is a top view of a CNT diode array according to the present invention;

FIG. 6A is a top plan view and FIG. 6B is a section view through line 6 B- 6 B of FIG. 6A of a fifth CNT diode is according to the present invention;

FIG. 6C is a cross-sectional view of an alternative arrangement of CNTs illustrated in FIGS. 6A and 6B ;

FIG. 7 is a first CNT diode ESD circuit according to the present invention; and

FIG. 8 is a second CNT diode ESD circuit according to the present invention.

Carbon nanotubes are closed-cage molecules composed of sp 2 -hybridized carbon atoms arranged in hexagons and pentagons. Carbon nanotubes come in two types, single wall nanotubes, which are hollow tube like structures or and multi-wall nanotubes. Multi-wall carbon nanotubes resemble sets of concentric cylinders. The present invention utilizes single-wall carbon nanotubes (SWNT). For the purposes of the present invention, the term carbon nanotube (CNT) denotes a carbon SWNT.

Electrostatic discharge is the sudden and momentary electric current that flows between two objects at different electrical potentials. In electronic devices are particularly sensitive to ESD events occurring on the input and output pins. ESD protection circuits are connected to input and output pins to direct ESD current to, for example, ground rather than allowing the current to propagate to the circuits connected to the pins.

FIG. 1A is a top view and FIG. 1B is a section view through line 1 B- 1 B of FIG. 1A of a first CNT diode is according to the present invention. In FIGS. 1A and 1B , a CNT diode 100 A includes a first CNT 105 extending in a first lengthwise direction from a catalytic nano-particle 110 and electrically contacted by a metal contact 115 at an end opposite the end in contact with the catalytic nano-particle. CNT diode 100 A further includes a second CNT 120 extending in a second lengthwise direction from a catalytic nano-particle 125 and electrically contacted by a metal contact 130 at an end opposite the end in contact with the catalytic nano-particle. In FIGS. 1A and 1B , CNTs 105 and 120 lie in two different but parallel planes that are both parallel to a plane defined by a top surface of an insulating layer 135 .

Through not shown in FIGS. 1A and 1B , contact pad 115 may be formed on the same end of CNT 105 as catalytic nano-particle 110 . Though not shown in FIGS. 1A and 1B , contact pad 130 may be formed on the same end of CNT 120 as catalytic nano-particle 125 (see FIGS. 2A and 2B ). Through not shown in FIGS. 1A and 1B contact pad 115 may be formed in a middle region of CNT 105 between its opposite ends. Through not shown in FIGS. 1A and 1B contact pad 10 may be formed in a middle region of CNT 130 between its opposite ends. Thus, there are three possible positions for each contact and nine possible combinations of contact positions for both contacts together.

CNTs 105 and 120 cross at an included angle α and electrically contact each other where they cross. CNT diode 100 A is formed on insulating layer 135 , which in turn is formed on a substrate 140 . Catalytic nano-particles 110 and 125 and metal contacts 115 and 130 are in contact with insulating layer 135 . Catalytic nano-particles 110 and 125 are, in one example, iron/cobalt (Fe/Co) or iron/cobalt/nickel (Fe/Co/Ni) when CNTs 105 and 120 are formed by chemical vapor deposition (CVD) from carbon containing gases, e.g. a carbon monoxide (CO) and hydrogen (H 2 ) mixture.

CNT 105 has a length L 1 and a diameter D 1 and CNT 120 has a length L 2 and a diameter D 2 . L 1 and L 2 are independently between about tens of nm and about a few cm. D 1 and D 2 are independently between about 0.5 nm and about 5 nm. Contact 115 is spaced a distance S 1 from CNT 120 and contact 130 is spaced a distance S 2 from CNT 120 . S 1 and S 2 are independently between about 0.5 nm and about a few cm.

In one example, CNT 105 is a p-type CNT and CNT 120 an n-type CNT. Alternatively, CNT 120 is a p-type CNT and CNT 105 an n-type CNT. CNTs are naturally p-type and there are several methods for making CNTs n-type or more strongly p-type. Contact with palladium (Pd) makes a CNT p-type (majority carriers are holes). Pd has a high work function and physical and electrical contact of a CNT with Pd creates a Schottky barrier that is higher for electrons than for holes. Contact with aluminum (Al) makes a CNT n-type (majority carriers are electrons). Al has a low work function and physical and electrical contact of a CNT with Al creates a Schottky barrier that is higher for holes than for electrons. When CNT 105 is to be n-type and CNT 120 is to be p-type, contact 115 may be formed from Pd and contact 130 from Al. When CNT 120 is to be p-type and CNT 105 is to be n-type, contact 130 may be formed from Pd and contact 115 from Al. It is advantageous, when the doping of CNTs 105 and 120 are by selection of the metal used in contacts 115 and 130 , that distances S 1 and S 2 be as small as possible.

Another method to dope CNTs is by forming a charge transfer complex coating on the CNTs. In one example, CNT may be made n-type by chemically coating it with amine rich polymer, for example polyethylene amine. In one example, the amine rich polymer layer is a monolayer. In one example, CNT made be made more p-type by treating the CNT by forming a charge-transfer complex (that charges the CNT positive) on the surface of the CNT. One method of forming a charge transfer complex on a CNT is by immersion of the CNT in a solution of triethyoxnium hexachloroantimoate (C 2 H 5 ) 3 O + SbCl 6 − followed by rinsing appropriate solvents. When CNTs 115 are doped chemically, contacts 115 and 130 may be formed from metals such as titanium (Ti), tantalum (Ta), copper (Cu) or tungsten (W), though Pd and Al may still be used in combination with chemical coating, Pd on the chemically p-doped CNT and Al on the chemically n-doped CNT. Ti, for example, has an intermediate work function (about 4.33 eV) and physical and electrical contact of a CNT with Ti creates a Schottky barrier that is about equal for holes and electrons.

The angle α formed by the crossing of CNTs 105 and 120 is greater then zero degrees and no greater than 90°.

One example fabrication of CNT diode 100 A is as follows:

(1) Form first catalytic nano-particle 110 on insulating layer 135 ;

(2) Grow first CNT 115 by CVD with the reactant gases flowing in a first lengthwise direction;

(3) Form second catalytic nano-particle 125 on insulating layer 135 ;

(4) Grow first CNT 115 by CVD with the reactant gases flowing in the a lengthwise direction (the angle between the first and second lengthwise directions being a;

(5) Form first contact 115 ; and

(6) Form second contact 130 .

Optionally, after step (2) CNT 115 may be doped by chemical coating, in which case it may be advantageous to form a removable protective coating over CNT 115 prior to step (3). Optionally, after step (4) CNT 120 may be doped by chemical coating, in which case it may be advantageous to form a removable protective coating over CNT 115 prior to forming the doping chemical coating. Optionally, after step (2) CNT 105 may be doped by a first chemical coating and after step (4) CNT 120 may be doped by second chemical coating, in which case it may be advantageous to form a removable protective coating over CNT 115 prior to step (3) and remove a portion of the protective coating where second CNT 120 is expected to cross first CNT 105 prior to forming the second doping chemical coating.

In fabricating CNT diodes 100 D of FIG. 4 , CNT array of FIG. 5 and CNT diode 100 E of FIGS. 6A and 6B , which include multiple p-type and multiple n-type CNTs and are described infra, optional steps to remove any purely conductive (as opposed to semi-conductive CNTs) may be added after steps (2) and (4). In a first example, conductive CNTs may be removed by passing a high current (e.g. about 109 A/cm2) through the CNTs. In a second example, conductive CNTs may be removed by subjecting the CNTs to a (e.g. methane) plasma. Additional steps, to protect the CNTs formed in step (2) may be required.

In one example insulating layer 135 is SiO 2 and substrate 140 comprises single-crystal silicon. In a first example, insulating layer 135 and substrate 140 may be a included in an integrated circuit chip which includes devices such as CMOS FETs, bipolar transistors, PN junction and Schottky diodes, thin film and trench capacitors, FET and polysilicon resistors and metal inductors which are wired together along with CNT diode 105 A (or any of CNT diodes 100 B, 100 C, 110 D and 100 E and CNT diode array 150 , all described infra) to form integrated circuits. In a second example, insulating layer 135 and substrate 140 may be included in an integrated circuit chip which includes devices such as CNTFETs, CNT diodes, CNT capacitors, CNT resistors and CNT inductors which are wired together along with CNT diode 105 A (or any of the CNT diodes 100 B, 100 C, 110 D and 100 E and CNT diode array 150 , all described infra) to form integrated circuits. In a third example, insulating layer 135 and substrate 140 may be included in an integrated circuit chip which includes devices such as CMOS FETs, bipolar transistors, PN junction and Schottky diodes, thin film and trench capacitors, FET and polysilicon resistors and metal inductors, CNTFETs, CNT diodes, CNT capacitors, CNT resistors and CNT inductors which are wired together along with CNT diode 105 A (or any of the CNT diodes 100 B, 100 C, 110 D and 100 E and CNT diode array 150 , all described infra) to form integrated circuits. The wiring in either the first, second or third examples may comprise conventional semiconductor wiring techniques such as damascene wires and contacts, CNT wires or combinations thereof.

FIG. 2A is a top view and FIG. 2B is a section view through line 2 B- 2 B of FIG. 2A of a second CNT diode is according to the present invention. In FIGS. 2A and 2B , a CNT diode 100 B is similar to CNT diode 100 A of FIGS. 1A and 1B , except instead of CNTs 105 and 110 crossing each other at an included angle α, they are parallel to each other (i.e. α=0°) with CNT 120 on top of CNT 105 and they are in electrical contact along a portion of the length of each CNT. In FIGS. 2A and 2B , CNTs 105 and 120 lie in a same plane that is perpendicular to the plane defined by the top surface of an insulating layer 135 .

It will be noted that contact 130 contacts CNT 120 in a middle region of CNT between its opposite ends. Contact 130 may contact CNT 120 over the end of the CNT over catalytic nano-particle 125 . Fabrication of CNT diode 100 B is similar to fabrication of CNT diode 100 A described supra.

FIG. 3 is a top view of a third CNT diode is according to the present invention. In FIG. 3A , a CNT diode 100 C is similar to CNT diode 100 A of FIGS. 2A and 2B , except instead of CNT 120 being on top of CNT 105 , they are next to each other and the respective ends opposite catalytic nano-particles 110 and 125 have been bent away from each other. In FIG. 3 , CNTs 105 and 120 lie in a same plane that is parallel to the plane defined by the top surface of an insulating layer 135 . In one example, bending is performed using an atomic force microscope prior to formation of contacts 115 and 130 , Though not shown in FIG. 3 , only one of CNTs 105 and 120 needs to be bent, the other may remain straight. Otherwise, fabrication of CNT diode 100 C is similar to fabrication of CNT diode 100 A described supra.

FIG. 4 is a top view of a fourth CNT diode is according to the present invention. In FIG. 4 , a CNT diode 110 D is similar to CNT diode of FIGS. 1A and 1B , except there are a multiplicity of substantially parallel first CNTs 105 and a multiplicity of substantially parallel second CNTs 120 forming a multiplicity of diodes D arranged in an array of columns and rows wired in parallel because a single contact 115 electrically contacts every CNT 105 and a single contact 130 electrically contacts every CNT 120 . In one example, CNTs 105 are about perpendicular to CNTs 120 . The number of CNTs 105 can be as small as one provided the number of CNTs 120 is at least two and vice versa.

FIG. 5 is a top view of a CNT diode array according to the present invention. In FIG. 5 , a CNT diode array 150 is similar to CNT diode 100 D of FIG. 4 , except each CNT 105 is electrically connected to a single contact pad 115 and each CNT 120 is electrically connected to a single contact pad 130 . This allows various diode circuits to be easily wired together into circuits, including ESD protection circuits. Additionally, CNT diode array 150 may be tested and defective individual diodes, rows or columns “marked” as defective and not wired into circuits.

FIG. 6A is a top plan view and FIG. 6B is a section view through line 6 B- 6 B of FIG. 6A of a fifth CNT diode according to the present invention. In FIGS. 6A and 6B a CNT diode 100 E is similar to CNT diode 100 B of FIGS. 2A and 2B except CNT diode 100 E includes a multiplicity CNTs 105 and 120 . CNTs 105 extend in a lengthwise direction parallel to each other in a first layer that is parallel to the plane defined by the top surface of insulating layer 135 (see FIG. 2B ). CNTs 120 extend in the same lengthwise direction as CNTs 105 parallel to each other in a second layer that is parallel to the plane defined by the top surface of insulating layer 135 (see FIG. 2B ). Each CNT 105 in the first layer is in electrical contact with one or two CNTs 120 in the second layer (forming a CNT 105 / 120 group) along a portion of the length of each adjacent CNT 105 and 120 in the group.

FIG. 6C is a cross-sectional view of an alternative arrangement of CNTs illustrated in FIGS. 6A and 6B . In FIG. 6C , an alternative arrangement of CNTs 105 and 120 is illustrated where adjacent CNTs 105 are in electrical contact with each other along a portion of the length of each adjacent CNT 105 and adjacent CNTs 120 are in physical and electrical contact with each other along a portion of the length of each adjacent CNT 120 .

FIG. 7 is a first CNT diode ESD circuit according to the present invention. In FIG. 7 an ESD protection circuit 200 includes a first CNT diode D 1 and a second CNT diode D 2 . The cathode of first CNT diode D 1 is connected to VDD and to the anode of second CNT diode D 2 is connected to VSS (e.g. ground). An input pad is connected to the anode of CNT diode D 1 and the cathode of CNT diode D 2 . Integrated circuits to be protected are connected to the input pad.

Diode arrays DA 1 and DA 2 are independently selected from the group of diodes arrays consisting of CNT diode array 100 D and CNT diode array 150 described supra.

FIG. 8 is a second CNT diode ESD circuit according to the present invention. In FIG. 8 an ESD protection circuit 205 includes a first CNT diode array DA 1 and a second CNT diode array DA 2 . The cathodes of each diode of first CNT diode array DA 1 are connected to VDD and the anodes each diode of second CNT diode array DA 2 are connected to VSS (e.g. ground). An input pad is connected to each anode of each diode of CNT diode array DA 1 and to the cathode of each diode of CNT diode array DA 2 . Integrated circuits to be protected are connected to the input pad. Diode D 1 and D 2 are independently selected from the group of diodes consisting of CNT diode 100 A, CNT diode 100 B and CNT diode 100 C described supra.

The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.