Electric lamp and discharge devices – Discharge devices having a thermionic or emissive cathode
Reexamination Certificate
1999-06-23
2004-11-09
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Discharge devices having a thermionic or emissive cathode
C313S3460DC, C313S349000, C313S337000
Reexamination Certificate
active
06815876
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cathode for use in electron beam projection lithography and a method for making the cathode, and more particularly, to a cathode with an improved work function and a method for making the improved work function cathode.
2. Description of the Related Art
Projection electron beam lithography, such as Scattering Angular Limitation Projection Electron Beam Lithograph (SCALPEL™), utilizes electron beam radiation projected onto a patterned mask to transfer an image of that pattern into a layer of energy sensitive material formed on a substrate. That image is developed and used in subsequent processing to form devices such as integrated circuits.
The SCALPEL™ mask has a membrane of a low atomic number material on which is formed a layer of high atomic number material. The layer of high atomic number material has a pattern delineated therein. Both the low atomic number membrane material and the high atomic number patterned layer of material are transparent to the electrons projected thereon (i.e., electrons with an energy of about 100 keV). However, the low atomic number membrane materials scatters the electrons weakly and at small angles. The high atomic number patterned layer of material scatters the electrons strongly and at large angles. Thus, the electrons transmitted through the high atomic number patterned material have a larger scattering angle than the electrons transmitted through the membrane. This difference in scattering angle provides a contrast between the electrons transmitted through the membrane alone and the electrons transmitted through the layer of patterned material formed on the membrane.
This contrast is exploited to transfer an image of the pattern from the mask and into a layer of energy sensitive material by using a back focal plane filter in the projection optics between the mask and the layer of energy sensitive material. The back focal pane filter has an aperture therein. The weakly scattered electrons are transmitted through the aperture while the strongly scattered electrons are blocked by the back focal plane filter. Thus, the image of the pattern defined in the weakly scattered electrons is transmitted through the aperture and into the layer of energy sensitive material.
FIG. 1
is a schematic diagram illustrating the concept of a conventional SCALPEL™ system. A beam B of electrons
10
is directed towards a scattering mask
9
including a thin membrane
11
having a thickness between about 1,000 Å and about 20,000 Å (0.1 &mgr;m and about 2 &mgr;m thick.) The membrane
11
is composed of a material which is virtually transparent to the electron beam B composed of electrons
10
. That is to say that electrons
10
in beam B pass through membrane
11
freely in the absence of any other object providing an obstruction to the path of electrons
10
in the beam B as they pass from the source of the beam through the membrane
11
.
Formed on the side of the membrane
11
facing the beam
10
, is a pattern of high density scattering elements
12
to provide a contrast mechanism that enables reproduction of the mask pattern at the target surface. The scattering elements
12
are patterned in the composite shape which is to be exposed upon a work piece
17
(usually a silicon wafer) which is coated with E-beam sensitive resist, which as shown in
FIG. 1
has been processed into pattern elements
18
. The electrons
10
from the E-beam B which pass through the mask
9
are shown by beams
14
which pass through electromagnetic lens
15
which focuses the beams
14
through an aperture
16
′ into an otherwise opaque back focal plane filter
16
. The aperture
16
′ permits only electrons scattered at small angles to pass through to the work piece
17
.
A conventional SCALPEL™ exposure tool is illustrated in FIG.
2
. The exposure tool
20
includes a source
22
(usually an electron gun), a mask stage
24
, imaging optics
26
, and a wafer stage
28
. The mask stage
24
and the wafer stage
28
are mounted to the top and bottom of a block of aluminum, referred to as the metrology plate
30
. The metrology plate
30
, which is on the order of 3000 lbs., serves as a thermal and mechanical stabilizer for the entire exposure tool
20
.
FIG. 3
illustrates a prior art source
22
in more detail. The source
22
includes a cathode
42
, an anode
43
, a grid electrode
44
, focusing plates
45
, and a filament
46
. Each of the cathode
42
, anode
43
, grid electrode
44
, and focusing plates
45
exhibit substantial circular and radial symmetry about an imaginary line of focus
50
. In the prior art systems in U.S. Pat. No. 5,426,686, the cathode
42
is made of gallium arsenide (GaAs), bialkali cathode materials, cesium antimonide (Cs
3
Sb), or a pure material having a low work function, such as tantalum (Ta), samarium (Sm), or nickel (Ni). In other prior art systems disclosed in U.S. Pat. No. 5,426,686, the material of photocathode
42
is made of a metal added to a semiconductor material by mixing or by depositing through sputtering or annealing. The metal is preferably tantalum (Ta), copper (Cu), silver (Ag), aluminum (Al), or gold (Au), or oxides or halides of these metals. One such example of a prior art photocathode is constructed from tantalum (Ta) annealed on the surface of nickel (Ni).
Most e-beam lithography systems (direct e-beam writing machines, etc.) require essentially point electron sources with high current densities. Conventional thermoionic cathodes, such as pure metal (tungsten or tantalum), lanthanum hexaboride (LaB
6
), etc. cathodes are sufficient for these applications.
In contrast, SCALPEL™ systems require a 1 mm
2
approximately parallel electron beam with a cross-sectional current density variation of within 2%. Conventional thermoionic cathodes have work function variations across the emitting surface substantially greater than 2%, for example 5-10%. However, as noted on page 3769 of “High emittance electron gun for projection lithography,” W. Devore et al., 1998 American Vacuum Society, J. Vac. Sci. Technol. B 14(6), November/December 1996, pp. 3764-3769, the SCALPEL™ process requires a thermoionic cathode with a work function variation less than 2%.
The conventional cathode which meets the SCALPEL™ requirements for other parameters, such as emission uniformity, low work function, low evaporation rate, high voltage operating environment, and vacuum contamination resilience is a tantalum (Ta) cathode having a disk shape. The disk-shaped tantalum (Ta) cathode is made from cold or hot rolled tantalum (Ta) foils which are hot pressed into a micro-polycrystalline material. Because of its polycrystalline nature, the grains are substantially misoriented with each other (on the order of 5-20°). The conventional Ta cathode also has an uncontrolled grain size distribution between 5-400 &mgr;m. Due to the sensitivity of tantalum's work function to the crystallographic orientation, the conventional polycrystalline Ta cathode work function distribution is “patchy” (also on the order of 5-400 &mgr;m), varying from grain-to-grain (because of differing orientations) and resulting in an unacceptably patchy or non-uniform emission pattern. Growth of the misoriented and differing sized grains at a high operating temperature further aggravates the patchiness problem. The non-uniformities caused by grain misorientation, uncontrolled large grain sizes, and grain growth on the cathode surface at the high operating temperature are transferred by the SCALPEL™ electron optics down to the shaping aperture (the object plane) and eventually to the wafer surface (the imaging plane).
When used as a SCALPEL™ cathode, the conventional polycrystalline cathode materials experience grain growth and rough texture development (together termed “recrystallization”) at the SCALPEL™ high operating temperatures (1200-2000° C.) and extended time period (greater than 1000 hours). Although the total emission current is satisfactory, structural developments at the ca
Jin Sung-ho
Katsap Victor
Waskiewicz Warren K.
Zhu Wei
Agere Systems Inc.
Patel Ashok
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