CARBON NANOTUBE FIELD EMISSION DEVICE WITH OVERHANGING GATE
A carbon nanotube field emission device with overhanging gate fabricated by a double silicon-on-insulator process. Other embodiments are described and claimed.
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This application claims the benefit of U.S. Provisional Application No. 60/936,049, filed 18 Jun. 2007; and U.S. Provisional Application No. 61/008,034, filed 18 Dec. 2007.
GOVERNMENT INTERESTThe invention claimed herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title.
FIELDThe present invention relates to electronic devices and fabrication technology, and more particularly to field emission devices with carbon nanotubes.
BACKGROUNDField emission devices (cathodes) serve as a source of electrons, and are of use in various scientific and medical fields. Such applications may include sterilization systems, gas ionization systems (e.g., mass spectrometry), klystrons, flat-panel displays, cathode ray tubes, scanning electron microscopes, transmission electron microscopes, and energy dispersive x-ray analysis, to name a few. As another example, by radiating a target with an accelerated electron beam, field emission devices may be used as a source of x-rays. In many applications, it is desirable that field emission devices provide a relatively high electron flux with relatively low power consumption, are stable with a long life, and which may be efficiently manufactured.
In the description that follows, the scope of the term “some embodiments” is not to be so limited as to mean more than one embodiment, but rather, the scope may include one embodiment, more than one embodiment, or perhaps all embodiments.
For some embodiments, the gate diameter (the diameter of the hole at layer 114) is in the neighborhood of 25 microns, the depth of buried insulator layer 108 is in the neighborhood of 0.5 microns, the depth of buried insulator layer 110 is in the neighborhood of 1 microns, the depth of silicon device layer 112 is in the neighborhood of 9 microns, and the depth of silicon device layer 114 is in the neighborhood of 2 microns. Although not shown in
Although it is difficult to see from the perspective of
These dimensions cited above are provided only as an example of an embodiment, where other embodiments may have different sized layers, gates, undercuts, and carbon nanotube intra bundle spacing and size.
In practice, wafer 106 is much larger than the hole illustrated in
In step 2, photoresist layer 302 has been formed and patterned on silicon device layer 114 by well-known lithographic techniques. Silicon layer 114 is etched by deep reactive ion etching (DRIE), and photoresist layer 302 is removed, resulting in step 3. In step 4, buried insulator layer 110 has been etched by a hydrofluoric (HF) acid solution. The wafer is then thermally oxidized to passivate all exposed silicon surfaces with silicon dioxide layer 303, including the exposed sidewalls, as indicated in step 5.
As illustrated in step 6, photoresist layer 304 has been applied and patterned by well-known lithographic techniques, and a hole is opened into thermal silicon dioxide layer 303 by reactive ion etching (RIE) as shown in step 7. Silicon device layer 112 is then etched by DRIE as illustrated in step 8. The wafer is then etched with xenon difluoride (XeF2), as illustrated in step 9. XeF2 reacts only with exposed silicon and does not damage other materials such as photoresist and SiO2. As shown in step 9, the silicon etch front propagates laterally. A portion of buried insulator layer 108 is removed by RIE as shown in step 10. As shown in step 11, photo resist layer 304 is removed and the thermal silicon dioxide layer grown in step 5 is removed by HF.
As indicated in step 12, photo resist layer 305 is applied, and patterned at the bottom of the hole on silicon substrate 104 as shown. This pattern allows a catalyst dot array pattern 306 to be applied to the bottom of the hole on silicon substrate 104 as shown in step 13, where photoresist layer 305 has been removed. For some embodiments, the catalyst may be iron (Fe), with a thickness of 25 Angstroms. Under the Fe, there is a 30 Angstrom thick Aluminum oxide layer that serves as a diffusion barrier. Both Fe and Si inter-diffuse at the elevated temperature used for carbon nanotube growth. Carbon nanotubes 102 are then grown on the catalyst dot array 306 as shown in step 14.
Some embodiments may use drain holes formed near the gate electrodes (the portion of silicon device layer 114 that overhangs sidewall silicon device layer 112 at the top of the etched hole) to allow excess photoresist at the bottom of the hole to be spun out by centrifugal force. An embodiment is illustrated in
When photoresist material is applied to the bottom of the hole, as in step 12 of
A particular photoresist material that may be used is known as AZ4330RS, a product of Clariant Corporation, AZ Electronic Materials, with U.S. headquarters in Somerville, N.J.
Various modifications may be made to the described embodiments without departing from the scope of the invention as claimed below.
Claims
1. A field emission device comprising:
- a wafer having a hole and comprising a silicon substrate; a first insulator layer on the silicon substrate; a first silicon layer on the first insulator layer; a second insulator layer on the first silicon layer; and a second silicon layer on the second insulator layer, where the hole has a bottom at the silicon substrate; and
- at least one carbon nanotube formed at the bottom of the hole.
2. The field emission device as set forth in claim 1, wherein the first and second insulator layers comprise silicon dioxide.
3. The field emission device as set forth in claim 1, wherein the second silicon layer forms a gate with respect to the at least one carbon nanotube.
4. The field emission device as set forth in claim 1, the hole having a top and a sidewall, the second silicon layer forming at least a portion of the sidewall, wherein the second silicon layer at the top of the hole overhangs the sidewall.
5. A semiconductor process applied to a wafer, the wafer comprising a silicon substrate; a first insulator layer on the silicon substrate; a first silicon layer on the first insulator layer; a second insulator layer on the first silicon layer; and a second silicon layer on the second insulator layer; the semiconductor process comprising:
- etching through the second silicon layer, through the second insulator layer, and through the first silicon layer to the first insulator layer to define a hole;
- lateral etching in the first silicon layer to laterally enlarge the hole at the first silicon layer;
- etching through the first insulator layer to the substrate, so that the hole has a bottom that includes the substrate; and
- forming an array of carbon nanotubes at the bottom of the hole on the substrate.
6. The semiconductor process as set forth in claim 5, where forming the array of carbon nanotubes at the bottom of the hole on the substrate comprises:
- depositing photoresist material on the substrate at the bottom of the hole.
7. A semiconductor process as set forth in claim 6, where forming the array of carbon nanotubes at the bottom of the hole on the substrate further comprises:
- patterning the photoresist material to provide a pattern for a catalyst;
- depositing the catalyst on the substrate at the bottom of the hole; and
- growing the array of carbon nanotubes on the catalyst.
8. The semiconductor process as set forth in claim 5, where forming the array of carbon nanotubes at the bottom of the hole on the substrate comprises:
- depositing photoresist material on the wafer so that the photoresist material bridges the opening of the hole.
9. The semiconductor process as set forth in claim 8, where forming the array of carbon nanotubes at the bottom of the hole on the substrate further comprises:
- patterning the photoresist material to provide a pattern for a catalyst;
- depositing the catalyst on the substrate at the bottom of the hole; and
- growing the array of carbon nanotubes on the catalyst.
Type: Application
Filed: Jun 17, 2008
Publication Date: Mar 10, 2011
Applicant: CALIFORNIA INSTITUTE OF TECHNOLOGY (Pasadena, CA)
Inventors: Risaku Toda (Pasadena, CA), Michael J. Bronikowski (Altadena, CA), Edward M. Luong (Mission Viejo, CA), Harish Manohara (Arcadia, CA)
Application Number: 12/140,564
International Classification: H01L 29/41 (20060101); H01L 21/302 (20060101); B82Y 20/00 (20110101); B82Y 99/00 (20110101);