Electric lamp and discharge devices: systems – Cathode ray tube circuits – Combined cathode ray tube and circuit element structure
Reexamination Certificate
2003-08-23
2004-10-12
Nelms, David (Department: 2818)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Combined cathode ray tube and circuit element structure
C313S293000, C313S309000, C313S311000, C313S495000, C315S005180, C315S005190
Reexamination Certificate
active
06803725
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to microwave vacuum tube devices and, in particular, to microscale vacuum tubes (microtubes).
BACKGROUND OF THE INVENTION
The modem communications industry began with the development of gridded vacuum tube amplifiers. Microwave vacuum tube devices, such as power amplifiers, are essential components of microwave systems including telecommunications, radar, electronic warfare and navigation systems. While semiconductor microwave amplifiers are available, they lack the power capabilities required by most microwave systems. Vacuum tube amplifiers, in contrast, can provide microwave power which is higher by orders of magnitude. The higher power levels are because electrons can travel faster in vacuum with fewer collisions than in semiconductor material. The higher speeds permit larger structures with the same transit time which, in turn, produce greater power output.
In a typical microwave tube device an input signal interacts with a beam of electrons. The output signal is derived from the thus-modulated beam. See, e.g., A. S. Gilmour, Jr.,
Microwave Tubes
, Artech House, 1986, 191-313. Microwave tube devices include gridded tubes (e.g., triodes, tetrodes, pentodes, and klystrodes), klystrons, traveling wave tubes, crossed-field amplifiers and gyrotrons. All contain a cathode structure including a source of electrons for the beam (cathode), an interaction structure (grid or gate), and an output structure (anode). The grid is used to induce or modulate the beam.
Conventional vacuum tube devices are typically fabricated by mechanical assembly of the individual components. The components are made separately and then they are secured on a supporting structure. Unfortunately, such assembly is not efficient or cost-effective, and it inevitably introduces some misalignment and asymmetry into the device. Some attempts to address these problems have led to use of sacrificial layers in a rigid structure, i.e., a structure is rigidly built with layers or regions that are removed in order to expose or free the components of the device. See, e.g., U.S. Pat. No. 5,637,539 and I. Brodie and C. Spindt, “Vacuum microelectronics,”
Advances in Electronics and Electron Physics
, Vol. 83 (1992). These rigid structures present improvements, but still encounter formidable fabrication problems.
The usual source of beam electrons is a thermionic emission cathode. The emission cathode is typically formed from tungsten that is either coated with barium or barium oxide, or mixed with thorium oxide. Thermionic emission cathodes must be heated to temperatures around 1000 degrees C. to produce sufficient thermionic electron emission current, e.g., on the order of amperes per square centimeter. The necessity of heating thermionic cathodes to such high temperatures creates several problems. For example, the heating limits the lifetime of the cathodes, introduces warm-up delays, requires bulky auxiliary equipment for cooling, and tends to interfere with high-speed modulation of emission in gridded tubes.
While transistors have been miniaturized to micron scale dimensions, it has been much more difficult to miniaturize reliable vacuum tube devices. This difficulty arises in part because the conventional approach to fabricating vacuum tubes becomes increasingly difficult as component size is reduced. The difficulties are further aggravated because the high temperature thermionic emission cathodes used with conventional vacuum tubes present increasingly serious heat and reliability problems in miniaturized tubes.
A promising new approach to microminiaturizing vacuum tubes is the use of surface micromachining to make microscale triode arrays using cold cathode emitters such as carbon nanotubes. See Bower et al.,
Applied Physics Letters
, Vol. 80, p. 3820 (May 20, 2002). This approach forms tiny hinged cathode, grid and anode structures on a substrate surface and then releases them from the surface to lock into proper positions for a triode.
FIGS. 1A and 1B
illustrate the formation of a triode microtube using this approach. FIG.
1
(
a
) shows the microtube components formed on a substrate
1
before release. The components include surface precursors for a cathode
2
, a gate
3
and an anode
4
, all releasably hinged to the substrate
1
. The cathode
2
can comprise carbon nanotube emitters
5
grown on a region of polysilicon. The gate
3
can be a region of polysilicon provided with apertures
6
, and the anode
4
can be a third region of polysilicon. The polysilicon regions can be lithographically patterned in a polysilicon film disposed on a silicon substrate. The carbon nanotubes can be grown from patterned catalyst islands in accordance with techniques well known in the art. The high aspect ratio of the nanotubes (>1000) and their small tip radii of curvature (~1 to 30 nm), coupled with their high mechanical strength and chemical stability, make them particularly attractive as electron emitters.
FIG. 1B
shows the components after the release step which is typically manual. Release aligns the gate
3
between the cathode
2
and the anode
4
in triode configuration.
The term “flexural member” includes any structure that induces or allows movement of a structural region into its desired configuration in the device. “Pop-up” indicates that the structural region is induced to move upon release, without the need for external force. “Hinge mechanism” indicates one or more flexural members, e.g., a hinge, that allows the component to be moved, e.g., rotated, by applying external force. The cathode structure contains a cathode and one or more grids. The input structure is where the microwave signal to be amplified is introduced (in some configurations, the input structure is a grid of the cathode structure). The interaction structure is where the electron beam interacts with the microwave signal to be amplified. The output structure is where the amplified microwave power is removed, and the collection structure is where the electron beam is collected after the amplified microwave power has been removed.
FIG. 2
, which is useful in illustrating a problem to which the present invention is directed, is a scanning electron microphoto which shows an exemplary surface micromachined triode device. On the surface of the device substrate
10
, e.g., a silicon nitride surface on a silicon wafer, are formed a cathode electrode
12
attached to the device substrate
10
surface by a hinge mechanism
13
and a spring
11
, a grid
14
attached to the device substrate
10
surface by a hinge mechanism
15
, and an anode
16
attached to the device substrate
10
by a hinge mechanism
17
. Also on the substrate
10
surface are contacts
18
electrically connected to the cathode electrode
12
, grid
14
, and anode
16
. The contacts
18
and connective wiring are typically polysilicon coated with gold, although other materials are possible. Design of the connective wiring should take into account the subsequent rotation of the cathode electrode
12
, grid
14
, and anode
16
, to avoid breakage and/or reliability problems. The substrate
10
also has three locking mechanisms
24
,
26
,
28
, which secure the cathode
12
, grid
14
, and anode
16
in an upright position, as discussed below. All these components, including the hinges, are formed by a surface micromachining process. The inset is a magnified view of the aligned and patterned carbon nanotubes
19
(deposited on the cathode
12
), placed against the MEMS gate electrode (grid
14
).
The cathode electrode
12
, with attached emitters
19
, the grid
14
, and the anode
16
, are surface micromachined and then mechanically rotated on their hinges,
13
,
15
,
17
and brought to an upright position—substantially perpendicular to the surface of the device substrate
10
. The locking mechanisms
24
,
26
,
28
are then rotated on their hinges to secure the cathode electrode
12
, grid
14
, and anode
16
in these upright positions.
In the structure of
FIG. 2
, the cathode electrode, the grid, and the anode are arrang
Lowenstein & Sandler PC
Nelms David
Nguyen Dao H.
The Regents of the University of California
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