Radiant energy – Ion generation – Field ionization type
Reexamination Certificate
2000-05-01
2003-01-28
Anderson, Bruce (Department: 2881)
Radiant energy
Ion generation
Field ionization type
C250S398000
Reexamination Certificate
active
06512235
ABSTRACT:
FIELD OF THE INVENTION
This invention is generally in the field of electron emission based techniques, such as electron microscopy (EM), specifically scanning electron microscopy (SEM), and electron beam lithography (EBL), utilizing a nanotube-based electron emission device, with high electron optical quality, specifically high brightness and low energy spread. The electron emission device of the present invention can be used with any electron beam column or other system that requires such properties.
BACKGROUND OF THE INVENTION
It is the common goal of various applications utilizing an electron beam source, such as SEM and EBL, to have a simple and stable electron beam source, in particular, a source capable of being installed in a miniature device and having high electron optical quality. The simplicity of the electron source is defined by its working conditions, such as operating vacuum and temperature parameters. As for the optical quality, it is predominantly determined by the brightness (i.e., current density per solid angle) and the energy spread of the electron beam. Both the brightness and energy spread determine the amount of current that can be focused on a small spot on the surface of a sample,
SEM and EBL are known techniques widely used in various applications, such as the manufacture of semiconductor devices. Electron sources conventionally used in SEM and EBL tools are typically one of three kinds: thermal sources, cold field emitters (CFE), and thermal field emitters or “Schottky-emitters”.
An electron beam generated by a thermal source has a wide energy spread, i.e., about 2 eV for tungsten filaments and 1.2 eV for LaB
6
, and low brightness, i.e., about 10
5
-10
6
A/cm
2
sr. Consequently, the electron sources of this kind require the complicated construction of an electron beam column, as well as high acceleration voltage, to achieve the resolution of a 1-10 nm.
CFE are characterized by a higher brightness (about 10
8
-10
9
A/cm
2
sr) and a narrower energy spread (about 0.3-0.4 eV), as compared to that of thermal sources. It is usually made of single crystal tungsten. Such a cathode-electrode requires “flashing” at a high temperature (more than 1800 K) to clean and reform its surface. Moreover, CFE suffer from ultra-high vacuum requirements (e.g., 10
−9
-10
−10
Torr), which is a major factor in the cost of a CFE-based device. Due to the unavoidable adsorption of molecules on the tip-like cathode-electrode, it is complicated to operate CFE in a stable manner.
A TFE source utilizes a compromise concept between those underlying the implementation of the electron sources of the above two kinds. TFE are made of tungsten coated by zirconium oxide, aimed at lowering the work function. These sources, as compared to CFE, are more stable, require lower vacuum (about 10
−8
-10
−9
Torr), and have a comparable high brightness (about 10
7
-10
8
A/cm
2
sr). A TFE 'source still requires an ultra high vacuum. A further drawback is associated with the need to stabilize the system at a high working temperature, i.e., about 1800° C. The voltage required to achieve a desirable high resolution is typically high. This may damage the sample and/or cause undesirable charging thereof To solve this problem, beam deceleration may be employed. However, this complicates the construction of an electron beam column, and enhances chromatic aberrations. All the conventional electron sources use a very small fraction of the current that is emitted from the electron source. A state of the art Schottty source emits about 100 &mgr;A/sr with only 10 nA of the source current in a TFE gun can actually be used as a probe current.
As a result of the above disadvantages of the conventional electron sources, their use in EM and EBL tools make these tools expensive and bulky. This impedes their application as integrated tools. Indeed, when using SEM for the inspection of workpieces on a production line, for example, in the manufacture of semiconductor devices, SEM is typically a stand-alone machine accommodated outside the production line. Accordingly, workpieces to be inspected are removed from the production line and brought to the SEM. This slows the production. Moreover, any unnecessary handling of such delicate workpieces as semiconductor wafers is undesirable. Thus, it is highly desirable to use a miniaturized SEM that can be brought to the sample to be inspected, rather than bringing the sample to the SEM. The miniaturized SEM technology is known as “Micro-columns”. As for the lithography tools, it is a core challenge of the semiconductor industry, to go beyond optical resolution, which currently limits the minimal feature size of the active elements of a semiconductor device.
Electron beam lithography is not limited by the optical diffraction limit, but by the throughput of the electron beam apparatus. There are three main approaches to this problem. First, the use of a miniature electron beam source device that can be utilized in an arrayed operation (“micro-columns”); second, direct writing in “proximity focus”; and third, the SCALPEL (Scattering with Angular Limitation Projection Electron Beam Lithography). Although these technologies have been known for several years, none of them is used in commercial applications. This is due to the following reasons: the miniaturized arrayed operation is limited by the absence of an adequate electron source, which has the desired electron optical quality (high brightness and low energy spread), which is compatible with miniaturization and with silicon technologies, and which can be produced with sufficient alignment to the optical axis. Currently micro-columns utilize a TFE. The high temperature of the TFE sources places additional limitation on the micro-columns. The proximity focus electron-beam writing was not utilized due to the absence of an electron source that can be patterned in the sub-100 nm scale and that emits electrons with the required electron-optical quality, specifically, a sufficiently narrow angular distribution. The SCALPEL technology is limited by several factors, notably, the small area that can be uniformly exposed by the currently available electron sources.
Attempts have been made to develop electron beam sources with improved electron-optical quality and operating vacuum parameters, so far with no success. Concurrently, carbon-based nanotubes have been developed and studied as field emitters. Their main properties and advantageous features are disclosed, for example, in the following publications:
Shoushan Fan et al., “
Self-Oriented Regular Arrays of Carbon Nanotubes and Their Field Emission Properties
”, Science, Vol. 283, p. 512-514, January 1999;
O. H. Wang et al., “
Field Emission from nanotube Bundle Emitters al Low Fields
”, Appl. Phys., Lett., 70 (24), pp. 3308-3309, June 1997;
J. M. Bonard et al. “
Field Emission Induced Luminescence from Carbon Nanotubes
” Phys. Rev. Lett., 81, 1441, 1998;
Phillip G. Collins and A. Zetti “
A Simple and Robust Electron Beam Source from Carbon Nanotubes
”, Appl. Phys. Lett., 69 (13), pp. 1969-1971, September 1996;
Walt A. de Heer et al., “
A Carbon Nanotube Field Emission Electron Source
”, Science, Vol. 270, November 1995;
O. G. Wang et al. “
A Nanotube-Based Field-Emission Flat Panel Display
”, Appl. Phys. Lea., Vol. 72, No. 22pp. 2912-2913, 1998;
WO 98/11588; WO 96/42101; EP 0913508; U.S. 5,973,444; WO 98/05920; and U.S. Pat. No. 5,872,422;
Various molecular morphologies can be grown, known as MWNT (Multi Wall Nano Tubes) and SWNT (Single Wall Nano Tubes). MWNT may be produced as capped or open, and SWNT can appear also as tight bundles. The various nanotubes have been grown with diameters down to a few nanometers.
Although nanotubes are known to have exceptionally good field emission properties (high current at low applied voltage, as well as low energy spread) they did not find their application in EM or EBL. This is largely because the brightness of “bare” carbon nanotubes is essentially low as compared to that of a TFE source. Thus, it is essentia
Eitan Guy
Rosenblatt David
Zik Ory
Anderson Bruce
El-Mul Technologies Ltd.
Juneau Todd L.
Nath Gary M.
Nath & Associates PLLC
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