U.S. patent application number 09/885833 was filed with the patent office on 2001-11-01 for josephson junction array device, and manufacture thereof.
This patent application is currently assigned to OXXEL OXIDE ELECTRONICS TECHNOLOGY GMBH. Invention is credited to Zehe, Alfred.
Application Number | 20010035524 09/885833 |
Document ID | / |
Family ID | 8223025 |
Filed Date | 2001-11-01 |
United States Patent
Application |
20010035524 |
Kind Code |
A1 |
Zehe, Alfred |
November 1, 2001 |
Josephson junction array device, and manufacture thereof
Abstract
A superconductive device is disclosed, which has specific
characteristics of a generator and/or detector of sub-millimeter
wavelength radiation, comprising a two-dimensional lateral array of
mesas (column-shaped elements) each containing vertically stacked
Josephson junctions on top of one another. This device is capable
of covering the entire frequency range between the microwave and
far infrared spectral regions, in a plurality of applications,
where radiation emission and detection is involved. According to
its various embodiments, thin columns (stacks) of Josephson
junctions are monolithically built between superconducting
electrical top and bottom contact layers. Mutually isolated
segments cut out of the contact layers allow for optimization of
circuit parameters such as impedance matching to load and
maximizing the output power. External electronic control allows
modulation of the radiation field and other operation modes of the
device. The specification also describes special applications of
the disclosed device.
Inventors: |
Zehe, Alfred; (Puebla,
MX) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Assignee: |
OXXEL OXIDE ELECTRONICS TECHNOLOGY
GMBH
Fahrenheitstr. 1, 28359
Bremen
DE
|
Family ID: |
8223025 |
Appl. No.: |
09/885833 |
Filed: |
June 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09885833 |
Jun 20, 2001 |
|
|
|
09230413 |
May 19, 1999 |
|
|
|
Current U.S.
Class: |
257/31 ; 257/32;
257/33; 257/E27.007 |
Current CPC
Class: |
H01L 27/18 20130101 |
Class at
Publication: |
257/31 ; 257/32;
257/33 |
International
Class: |
H01L 029/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 23, 1996 |
EP |
96111820.5 |
Jul 19, 1997 |
EP |
PCT/EP97/03891 |
Claims
1. A Josephson junction array device, comprised of a substrate, a
superconducting thin film, deposited on that substrate and
containing a sequence of layers typical in a series array of
vertically stacked Josephson junctions, comprising further a
superconducting mesa structure or structures on top of, or
fabricated out of said superconducting thin film, electrical bottom
and top contacts and means for applying an electrical voltage or
current to the electrodes, characterised in that the mesas are
monolithically integrated between superconducting electrical end
contacts on either side, and that means are provided for any
partial number of those superconducting mesas being combined in
groups for interconnection and control purposes.
2. A Josephson junction array device according to claim 1,
accomplished in epitaxially grown c-axis oriented high-T.sub.c
superconductor films containing intrinsic Josephson junction
stacks.
3. A Josephson junction array device according to claim 1, with
artificially fabricated Josephson junction stacks, achieved by
inserting barrier layers between high-T.sub.c superconductor layers
that serve as superconducting electrodes.
4. A Josephson junction array device according to claim 1, where
barrier layers are created by site-selective doping which reduces
the critical temperature of high-T.sub.c superconductor
materials.
5. A Josephson junction array device according to claim 1,
fabricated in a high-T.sub.c superconductor single crystal which
contains intrinsic Josephson junction stacks.
6. A Josephson junction array device according to claim 1,
accomplished in a superconducting metal-insulator-metal
superlattice structure, where each metal-insulator-metal unit is
designed to provide for the properties of a Josephson junction.
7. The method of producing a Josephson junction array device as
defined in claims 1 to 6, wherein the superconducting multilayer
film is epitaxially grown by Molecular Beam Epitaxy (MBE) and its
analogues, as e.g., Atomic Layer Epitaxy (ALE), Fine-focus Ion Beam
Epitaxy (FIBE), etc.
8. The method of producing a Josephson junction array device as
defined in claim 7, where the MBE-apparatus is provided with
spectroscopic means for interface inspection and control of the
growing films.
9. A method of forming a Josephson junction array device according
to claims 2, 3, and 7, comprising the following steps (a)
epitaxially grow a high-T.sub.c superconductor film, about 100-200
nm thick, on a suitable wafer, (b) cool down and deposit a
protective overlayer, 1-10 nm thick, on top of the high-T.sub.c
superconductor-film, (c) apply lithography for array definition on
the overlayer, and ion-mill through the overlayer and about halfway
through the deposited high-T.sub.c superconductor film, (d) deposit
an insulating film, and dissolve the photoresist on top of the
mesas, (e) apply a temperature cycle, and lift off the protective
overlayer in order to recreate the pristine top surface of the
columns (mesas), (f) deposit a second high-T.sub.c superconductor
film on top of the whole upper surface for top-contact definition,
(g) deposit metal contacts on the top and the bottom high-T.sub.c
superconductor electrodes.
10. A method for manufacturing a Josephson junction array device
according to claim 9, with additional fabrication of parallel
superconducting contacts between linear Josephson junction arrays
in mesa structures, comprised of the following steps: (a) deposit
the first high-T.sub.c superconductor film (HTS-1) and the
protective overlayer, (b) spin-on, expose, and develop the
photoresist for mesa etch, (c) etch or ion-mill to define mesa
structures, (d) deposit the insulator, (e) dissolve the photoresist
and lift-off the insulator above it, (f) remove the protective
overlayer by evaporation within the growth chamber, (g) deposit the
second high-T.sub.c superconductor layer (HTS-2), (h) spin-on,
expose, and develop photoresist for the segment definition-etch,
(i) etch or ion-mill to define the cluster structure, (j) dissolve
photoresist, (k) deposit metal contacts on the top and the bottom
high-T.sub.c superconductor electrodes.
11. A method of manufacturing a Josephson junction array device
according to claim 10, which enables the fabrication of both series
connections of mesas and parallel connections of mesas, comprised
of the following steps: (a) deposit the first high-T.sub.c
superconductor (HTS-1) and protective overlayer, etch to define
mesas, deposit insulator, and remove photoresist, (b) etch away to
form the trench separating the two clusters, deposit insulator, and
remove photoresist, (c) remove the protective overlayer and deposit
the second high-T.sub.c superconductor layer (HTS-2), (d) spin-on
photoresist, align carefully, expose, develop, and etch the
trenches in the high-T.sub.c superconductor (HTS-2) layer to
separate the segments, (e) deposit metal contacts on the top and
the bottom high-T.sub.c superconductor electrodes.
12. A method of manufacturing a Josephson junction array device
according to claims 6 and 7, which is comprised of the following
steps: (a) epitaxially grow a first metal film (M), at least one
superconducting coherence length thick, (b) epitaxially create a
thin insulator (I) on top of first metal film, (c) epitaxially grow
a second metal film, at least one super conducting coherence length
thick, on top of the insulator, (d) epitaxially grow a second thin
insulator on top of the second metal film, and repeat (a) through
(d) 2-100 times, (e) deposit a protective overlayer, (f) apply
lithography for array definition on the uppermost layer, and
ion-mill through the overlayer and halfway through the deposited
MIM-structure, (g) deposit an insulator film, and dissolve the
photoresist on top of the mesas, (e) apply a temperature cycle, and
lit off the protective overlayer in order to recreate the pristine
top surface of the columns (mesas), (f) deposit a thick metal layer
on top of the crystalline columns for top-contact definition.
13. A method of producing a Josephson junction array device as
claimed in any of the claims 1 to 6, wherein the superconducting
multilayer film is epitaxially grown by Chemical Vapor Deposition
(CVD) and its analogues.
14. A method of producing a Josephson junction array device as
claimed in any of the claims 1 to 6, wherein the superconducting
layer system is grown by Pulsed Laser Deposition (PLD) and its
analogues.
15. A method of producing a Josephson junction array device as
claimed in claim 14, where the PLD apparatus is provided with
spectroscopic means for interface inspection and control of the
growing films.
16. A method of producing a Josephson junction array device as
claimed in any of the claims 1 to 6, wherein the superconducting
layer system is grown by Sputter Deposition (SD) and its
analogues.
17. A method of producing a Josephson junction array device as
claimed in claim 16, where the SD apparatus is provided with
spectroscopic means for the control of the growing films.
18. A method of producing a Josephson junction array device as
claimed in any of the claims 1 to 6, wherein the high-T.sub.c
superconductor is grown as a bulk single crystal material.
19. A method of producing a Josephson junction array device as
claimed in any of the claims 1 to 6, where the device is fabricated
by use of of an array-defining micro-mask.
20. A Josephson junction array device as claimed in any of the
claims 1 to 6, comprising a linear or two-dimensional lateral array
of at least 2 to several thousand columns in parallel connection,
each of said columns containing 2 to 2000 vertically stacked
Josephson junctions, between common superconducting base and top
contacts, with means for connecting to a controllable current
source such that the Josephson junctions are driven into the ac
(emitting) regime.
21. A Josephson junction array device according to claim 20, where
the contact means provide for voltage measurements in the detector
regime.
22. A Josephson junction array device according to claim 20,
comprising superconducting top contacts split into two segments,
with means for connecting to a controllable current source such
that a series connection between the two segments, each containing
multiple parallel connected columns, is achieved.
23. A Josephson junction array device according to claim 20, where
the top contact is split in three or more segments with means for
connecting to a controllable current source such that a series
connection of three or more segments with parallel connected
columns is achieved.
24. A Josephson junction array device according to claims 20, 21,
and 23, where any available segment is accessible independently by
external means in order to affect or suppress the ac (emitting)
regime of certain segments.
25. A Josephson junction array device according to claims 20, 21
and 24, where any available segment is accessible independently by
external means in order to establish an integrated detector mode of
certain segments in parallel with the ac (emitting) regime of other
segments.
26. A Josephson junction array device according to claim 23, where
a distributed array of series connected Tunneltron segments is
formed, placed at wavelength intervals along a serpentine
microstrip transmission line for higher power output.
27. An emitter and/or a detector (sensor) of electromagnetic
radiation in the millimeter and sub-millimeter spectral region for
communication and data transfer purposes based on the Josephson
junction array device according to any of the preceding claims.
28. A monolithic emitter-detector device of electromagnetic
radiation for radar-applications, including panoramic observations,
based on the Josephson junction array device as claimed in any of
the claims 1 to 26.
29. An emitter and detector device with focus-shifting wave field
properties (to be used, for example, in holographic imaging) based
on the Josephson junction array device as claimed in any of the
claims 1 to 26.
30. An emitter device with beam sweeping properties for addressing
spatially separated receptors or deflectors, based on the Josephson
junction array device as claimed in any of the claims 1 to 26.
31. An emitter and detector device applied for satellite-based
synthetic aperture radar (SAR) based on the Josephson junction
array device as claimed in any of the claims 1 to 26.
32. A monolithic emitter-detector device for millimeter and
sub-millimeter electromagnetic radiation for multi-frequency
imaging microwave radiometer applications based on the Josephson
junction array device as claimed in any of the claims 1 to 26.
33. An emitter and detector device of electromagnetic radiation in
the millimeter and sub-millimeter spectral region used for coding
and enciphering by means of the Josephson junction array device as
claimed in any of the claims 1 to 26.
34. An emitter and detector device applied for a microwave
spectrometer based on the Josephson junction array device as
claimed in any of the claims 1 to 26.
35. An emitter device utilized as local oscillator in a heterodyne
mixer for astronomical exploration apparatus, based on the
Josephson junction array device as claimed in any of the claims 1
to 26.
Description
BACKGROUND OF THE INVENTION
[0001] (a) Field of the Invention
[0002] The invention relates to superconducting devices,
corresponding technologies and application fields, and more
specifically to a novel generator and detector of sub-millimeter
electromagnetic radiation, and its multiple applications.
[0003] (b) Description of the Related Art
[0004] 1. Arrays of Artificial Junctions
[0005] The first realization of potential usefulness of Josephson
junctions as tunable microwave sources and detectors can be traced
back to the earliest works of B. Josephson and S. Shapiro. It was
also understood very early that a single Josephson junction emits
with too little power and too broad linewidth to be useful as a
practical microwave source. These deficiencies can be removed by
using arrays of Josephson junctions [Jain et al. 1984; Bindslev
Hansen and Lindelof 1984; Lukens 1990]. If the coupling between the
junctions is strong enough, phase locking may occur between them;
in this cases, the array emits coherent radiation [Lukens 1990;
Konopka 1994]. Possible coupling mechanisms and coupling strengths
have been analyzed in detail [Jain et al. 1984; Lukens 1990]. It
has been understood that the linewidth of the electromagnetic
radiation emitted from an array of Josephson junctions decreases as
the number of junctions within the array is increased, and can
become very narrow in large arrays [Lukens 1990; Wiesenfeld et al.
1994; Konopka 1994].
[0006] Power of the emitted radiation also increases with the
number of junctions in the array, and in large arrays it can become
large enough (P.gtoreq.1 mW) for many practical applications
[Bindslev Hansen and Lindelof 1984; Jain et al. 1984, Konopka et
al. 1994; Wiesenfeld et al. 1994]. It is important here that a good
impedance matching is achieved with the load, because in the
opposite case most of the radiation is reflected back and
dissipated within the device itself [Jain et al. 1984, Bindslev
Hansen and Lindelof 1984; Konopka 1994].
[0007] Another concern are various junction parasitics; for
example, junction capacitances are a source of power reduction at
higher frequencies [Lukens 1990. Wiesenfeld et al 1994]. This
favors small-area Josephson junctions. Another argument pointing to
the same conclusion is increased noise and linewidth broadening in
large-area junctions [Kunkel and Siegel 1994; Konopka 1994].
Technically, for w.gtoreq.4.lambda..sub.j- , where w is the
junction width and .lambda..sub.j is the Josephson penetration
depth, the current flow becomes inhomogeneous [Kunkel and Siegel
1994]. It has been understood also that to achieve complete phase
locking in an array of coupled Josephson junctions, it is necessary
that all the junctions within the array have similar critical
currents (I.sub.c); in general, uniformity of .+-.5% or better is
required for linear arrays [see e.g. Konopka 1994].
[0008] It is possible to relax somewhat the above stringent
requirements by using a distributed arrays of equidistant Josephson
junctions (see FIG. 8), provided that the operating frequency is
adjusted in such a way to match the spacing between the junctions
with the wavelength of the emitted electromagnetic radiation
[Lukens 1990; Han et al. 1994]. This obviously reduces tunability
in frequency, while the power of the emitted radiation can be
increased significantly.
[0009] There have been numerous experimental studies of Josephson
junction arrays, and some remarkable results have been achieved.
Most of these were based on conventional (low-T.sub.c)
superconductors, e.g. using Nb/Al-AlO.sub.x/Nb trilayer junctions.
Complete phase locking has been demonstrated in a linear array of
100 such junctions [Han et al. 1993]. In some cases, a broad-band
antenna (for example, a bow-tie antenna, or a two-arm logarithmic
spiral antenna), was integrated on the chip, and off-chip radiation
was detected and measured. In other cases, another Josephson
junction was integrated on-chip and coupled via a transmission line
to the array. Some of the best results include the following ones.
Emission of P=50 .mu.W at .nu.=400-500 GHz was observed from a
distributed array of 500 Josephson junctions [Han et al. 1994]. In
another circuit design (10.times.10 array), radiation was generated
with a linewidth as small as .DELTA..nu.=10 kHz, tunable over a
broad range, .nu.=53-230 GHz [Booi and Benz 1994].
[0010] With the discovery of high-temperature superconductivity in
La-Ba-Cu-O by G. Bednorz and K. A. Muller in 1986, and subsequent
improvements of the critical temperature in related cuprate
compounds up to T.sub.c>160 K, great expectations have arosen
for superconductive electronics, operational at liquid nitrogen
temperature and even above it. Indeed, Josephson junctions have
been fabricated since 1987 in dozens of laboratories worldwide, by
a variety of techniques. Emission due to ac Josephson currents in
artificial high T.sub.c Josephson junctions was measured and
analyzed [Kunkel and Siegel 1994] In the same study, phase locking
of two step-edge junctions was demonstrated over a broad frequency
range of .nu.=80-500 GHz. In larger arrays, only partial (up to 4
junctions) and rather unstable phase locking was observed [Konopka
1994]. This was understood to originate from a generically large
non-uniformity of such step-edge high-T.sub.c Josephson junctions,
where critical current variations of .+-.50% are typical [Konopka
1990]. In another experiment five and ten-junction arrays, one next
to the other, were fabricated using step-edge HTS junctions [Kunkel
and Siegel 1994], again with only partial phase-locking and very
small output power.
[0011] Artificial high-T.sub.c Josephson junctions and stacks are
prerequisite in one embodiment of the present invention (see
section V). They have indeed been fabricated successfully already
[Bozovic et al. 1994, Bozovic and Eckstein 1995, 1996a,b; Eckstein
et al. 1992, 1995, Ono et al. 1995] using atomic-layer-by-layer
molecular beam epitaxy (ALL-MBE). A variety of barrier layers have
been explored, including Bi.sub.2Sr.sub.2CuO.sub.6 [Bozovic and
Eckstein 1996b], Bi.sub.2Sr.sub.2Dy.sub.xCa.sub.1-xCu.sub.2O.sub.8
[Bozovic and Eckstein 1996, 1995],
Bi.sub.2Sr.sub.2Dy.sub.xCa.sub.1-xCu.sub.8O.sub.20 and
BiSr.sub.2Dy.sub.xCa.sub.1-xCu.sub.8O.sub.19 [Iozovic and Eckstein
1996, Eckstein et al. 1995], etc. High-resolution cross-sectional
electron microscopy has shown virtually atomically perfect
interfaces between the barriers and the superconducting electrodes
[Bozovic et al. 1994b]. These multilayers were lithographically
processed into mesa structures for vertical transport devices
[Eckstein et al. 1992, Bozovic and Eckstein 1996b]. Both
proximity-effect (SNS) junctions [Bozovic and Eckstein 1996b, 1995]
and tunnel (SIS) junctions [Bozovic and Eckstein 1996a, Bozovic et
al. 1994] have been fabricated in this way. They have shown
remarkably high characteristic voltages, up to I.sub.cR.sub.N=10 mV
(which corresponds to .nu.=2.5 THz) and uniformity of better than
.+-.5% [Bozovic and Eckstein 1996a]. It was further demonstrated
that the barrier properties such as its normal state resistance
R.sub.N and critical current I.sub.c can be engineered over a very
broad range (four orders of magnitude) by varying the doping level
within the barrier, e.g., by varying x in the barrier layer
consisting of Bi.sub.2Sr.sub.2Dy.sub.xCa.sub.1-xCu.sub.8O.sub.20
[Bozovic and Eckstein 1996a,b, 1995, 1994a; Eckstein et al. 1992].
Finally, some short vertical stacks of such Josephson junctions
have already been fabricated and they showed perfect phase locking
[Bozovic and Eckstein 1996b, 1994a; Eckstein et al. 1995; Ono et
al. 1995]. In conclusion, every critical technological step related
to fabrication of artificial trilayer Josephson junctions, and
their vertical stacks, which we assumed to be feasible in Section V
(iv). below, has already been successfully demonstrated and reduced
to practice.
[0012] In many of the papers mentioned here, speculative statements
were made about promising future applications of arrays of
Josephson junctions. For example, applications are predicted as
generators and detectors of GHz and THz radiation [Jain et al.
1984], and even more specifically in radio-astronomy and
radio-spectroscopy of heavy molecules [Konopka 1994], in voltage
standards [Ono et al. 1995], etc. No such applications have been
realized (i.e., reduced to practice) so far, because of technical
difficulties expounded above. It is generally understood that for
useful off-chip spectroscopic applications, emitted power of more
than 0.1-1 mW is needed without sacrifice in tunability [Konopka et
al 1994], and in reality this milestone has not been reached so
far.
[0013] In the U.S. Pat. No. 3,725,213 to Pierce (1973) a
superconductive barrier device and its fabrication technology is
disclosed, which, besides other aims, provides for a generator or
detector of millimeter and sub-millimeter radiation, based on a
granular or particulate structure of the superconductor material.
While enhanced radiation emission or sensitivity is intended by the
summation of many Josephson junction-containing grains, there is
little control and reproducibility, no phase locking, and complete
load mismatch to vacuum. Although this device is capable of
switching between the superconducting and normal conductivity state
by means of a magnetic field, generated by an electrical pulse
through a layer adjacent to the Josephson junction, no intention
has been made to control the radiation emission frequency by virtue
of the effect the magnetic field might have on the energy gap of
the superconductor.
[0014] A superconducting device is disclosed in U.S. Pat. No.
4,837,604 to Faris (1989), which comprises a plurality of Josephson
junctions, stacked vertically on top of one another, with series
connection of stacks. It is tailored to a three terminal switch,
intended to replace singe junctions and lateral arrays of junctions
in analog and digital switching applications.
[0015] Radiation emission is not an aim of that device neither
would the chosen design suite such objective.
[0016] In U.S. Pat. No. 5,114,912 to Benz (1991), a high-frequency
oscillator based on a two-dimensional array of Josephson junctions
is described. It is excited by the dc control current from an
appropriate current source. The frequency of Josephson oscillations
can be tuned continuously by adjusting this dc bias current.
[0017] Impedance matching to load can be achieved by selecting the
appropriate number of Josephson junctions in the array or by adding
resistive shunts. The perceived application of the device is as a
tunable dc-to-ac converter at GHz and even THz frequencies.
[0018] One drawback of this device is that it is explicitly
restricted to two-dimensional planar arrangements of Josephson
junctions, which are placed next to one another on the chip. This
geometry introduces severe limitations on the maximum possible
number of junctions in such an array. Namely, the minimum area of a
single junction is around 1 .mu.m.sup.2 because of limitations of
photolithographic technology, uniformity requirements, the need to
have a substantial critical current (not less than 1 mA for optimum
power) and low-contact-resistance top lead (or leads). On the other
hand, the phase-locking requirement restricts the lateral dimension
of the array to about .lambda./4, which is about 75 .mu.m for
.nu.=1 THz. Allowing for some spacing between junctions etc., one
gets something like 1-2,000 for the maximum number of junctions in
a phase-locking array of this design. In practice, arrays of
10.times.10=100 junctions were made. As we will discuss in Section
V (vii) below, this design does not allow for out-of-chip power of
emitted microwave radiation at a level interesting for conceivable
applications i.e. at least 0.1-1 mW. We will argue that alternative
designs proposed here (in Section V) allow for much denser packing
of Josephson junctions, artificial or intrinsic, and open prospects
for sources with much higher emitted power, and by virtue of this
property, for a variety of novel applications which are not
possible with the planar array devices.
[0019] In the E.U. patent EP 446146 to Harada and Hozak (1987) a
trilayer Josephson junction is disclosed, comprised of top and
bottom superconducting electrodes made of
L.sub.yBa.sub.2Cu.sub.3O.sub.4, where L.sub.y is Y or a rare-earth
element, and 6<y.ltoreq.7, and a non-superconducting barrier
made of Bi.sub.2Y.sub.xSr.sub.yCu.sub.zO.sub.- w, where
0.ltoreq.x.ltoreq.2, 1.ltoreq.y.ltoreq.3, 1.ltoreq.z.ltoreq.3, and
6.ltoreq.w.ltoreq.13. In this patent, no information was provided
about the properties of such junctions (such as I.sub.c, R.sub.n,
I-V characteristics, microwave modulation properties, etc.). Nor is
there any mention of formation of arrays, vertical or lateral,
their expected properties, or applications.
[0020] A magnetic control mode for the emission frequency has been
proposed for a device disclosed in the European Patent 513,557 to
Schroder (1992), where the device of that invention contains stacks
of Josephson tunnel junctions, in a series superconducting
connection.
[0021] In between each pair of neighboring SIS Josephson junctions,
there is one more superconducting layer, which is insulated on both
sides from the neighboring Josephson junctions. This layer is
intended to be used as the control line: by running a current
laterally through this layer, as already proposed in U.S. Pat. No.
3,725,213, one should generate a magnetic field which is intended
to affect the SIS Josephson junctions by reducing the gap in the
superconducting layers.
[0022] This device has several drawbacks, some of which make its
reduction to practice impossible within the constraints of
currently known materials parameters and available microfabrication
technologies.
[0023] In particular, that patent description does not teach one
how to fabricate the (series) superconducting contacts between
superconducting electrodes of stacked SIS Josephson junctions,
which is the critical technological step required to fabricate the
device. It requires one to deposit superconducting pads of
dimensions of about 1 .mu.m.times.0.01 .mu.m, on two opposite
lateral sides of a vertical layered column structure.
[0024] There is no known technology today capable of performing
such a task.
[0025] A further concern is the thickness required for the
superconducting control line (S.sub.2 in FIG. 1 of EP 513.557 A2)
in order that it can generate a magnetic field strong enough to
reduce the gap in the superconducting electrodes. Take, for
example, a layer which is 10 nm thick, within a 1 .mu.m.sup.2
mesa.
[0026] Such small area mesas are required to keep the critical
current of Josephson junction's small enough for the desired
phase-locked operation, as will be expounded later. Asuming a very
high critical current density of j.sub.c=10.sup.6A/cm.sup.2 in this
layer, one gets I.sub.c=a.sup.2 j.sub.c=10.sup.6 A/cm.sup.2.times.1
.mu.m.times.10 nm=10.sup.6 A/cm.sup.2.times.10.sup.-4
cm.times.10.sup.-6 cm=10.sup.-4 A. At a distance of about 10 nm,
this current would produce a magnetic field of about B.apprxeq.0.01
Tesla. Such a field is several orders of magnitude too small to
significantly affect the critical temperature and the
superconductivity gap in the neighbouring superconducting
electrodes. In high-T.sub.c superconductor materials, such as
YBa.sub.2Cu.sub.3O.sub.7 or Bi.sub.2Sr.sub.2CaCu.sub.2O.sub.8,
magnetic fields of several Tesla are needed to get a measurable
effect on T.sub.c, and that only if the field is oriented
perpendicular to the CuO.sub.2 planes. In the geometry given in EP
513557 A2, the magnetic field would essentially be parallel to the
CuO.sub.2 planes.
[0027] In this unfavorable geometry, even the highest magnetic
fields available today (over 50 T) essentially do not affect
T.sub.c in high-T.sub.c superconductor materials, such as
Bi.sub.2Sr.sub.2CaCu.sub.2- O.sub.8. To overcome this difficulty,
one would have to use much thicker control-line superconducting
layers, say 1 .mu.m thick or even thicker (e.g., 6 .mu.m thick, as
in U.S. Pat. No. 3,725,213). That, however, clashes with the
current limitations for epitaxial growth of high-quality
high-T.sub.c superconductor films (no more than few hundred nm). To
fabricate a stack of several such devices would be even less
possible.
[0028] An alternative approach would be to use some conventional
(low-T.sub.c) superconducting materials with much lower critical
fields, i.e., much more sensitive to an applied magnetic field.
However, apart from losing the advantages of a high T.sub.c (such
as the possibility to avoid expensive and cumbersome very low
temperature cryogenic systems), in this hypothetical embodiment of
the discussed device, one would also lose the advantage of
extremely thin superconducting electrodes which are possible if one
employs the high-T.sub.c superconductor cuprate compounds. The
superconductivity coherence lengths are much larger in conventional
low-T.sub.c superconductors, and for that reason one would have to
use much thicker superconducting electrodes. This, in turn, will
also limit in practice the number of devices in a stack to only few
in contrast to what is assumed in the description of the device in
EP 513557 A2.
[0029] On the other hand, while it is clearly impractical to
modulate the gap and T.sub.c of the superconducting electrodes as
proposed in EP 513557 A2, it is possible to introduce vortices in
the barrier and control the critical current by an applied magnetic
field. We will actually make use of this later in Section V.
[0030] Another problem is that of the device "cross-talk". Imagine
that one could somehow resolve the problem of control lines and
make them of some material that can carry strong enough currents
and generate magnetic fields that can reduce the superconducting
gap in the neighboring high-T.sub.c superconductor electrodes. The
problem is that such a field would affect more than just one
Josephson junction. To begin with, according to the design in EP
513 557 A2, for each control line there are two equidistant
Josephson junctions, which should be equally affected. But since
the magnetic field in the geometry under discussion will fall off
slowly as a function of the distance from the control line, one
would actually expect that it would affect every Josephson junction
within that stack. So, individual Josephson junction control,
although the principal aim of that proposal, is impaired with such
a design.
[0031] A further major-drawback of the device design in EP 513 557
A2 is that no considerations were made of output power of the
electromagnetic radiation to be generated. In particular, load
matching to vacuum was not considered. As pointed out above, it
would be impractical to make even a few-junction stack within that
design. This would imply a substantial output-impedance load
mismatch. In this case, most of the microwave radiation power would
be reflected back and dissipated within the device itself. The
device would burn out before one is able to extract significant
microwave power out of it.
[0032] In fact, optimization of a high-T.sub.c superconductor
Josephson junction array for maximum output power requires in
general both series and parallel superconducting connections, as we
will show in detail in the upcoming section V.
[0033] To summarize, it is our conclusion that the device described
in EP 513557 A2 has not been reduced to practice because of several
drawbacks in its design, namely, (i) its fabrication requires the
deposition of superconducting contacts about 0.01 .mu.m wide on
lateral sides of mesas that contain Josephson junction stacks, for
which there is currently no known technology, (ii) there are no
known superconductors that can sustain currents large enough to
generate magnetic fields strong enough to reduce the
superconducting gap in the high-T.sub.c superconductor Josephson
junction electrodes, within the dimensional constraints of the
device, (iii) the magnetic fields if generated would affect more
than one Josephson junction (i.e., there would be inadmissible
`cross talk` between individual devices within the same stack), and
(iv) output power would be too low for the proposed system
applications, in part because the device design does not allow for
Josephson junction array circuit optimization.
[0034] It is the purpose of this patent to disclose a device of the
invention that overcomes all the drawbacks discussed above.
[0035] 2. Intrinsic Josephson Junction Stacks In Cuprate
Superconductors
[0036] In the very first paper on cuprate superconductors in 1986,
J. G. Bednorz and K. A. Muller expressed an opinion that La-Ba-Cu-O
is a quasi-two-dimensional superconductor, in view of its
pronounced layered structure. Subsequently, this hypothesis has
been confirmed by a variety of experimental findings on various
cuprate superconductors (see e.g. Bozovic 1991), the most direct of
which was the observation of high-T.sub.c superconductivity in
one-unit-cell thick layers of Bi.sub.2Sr.sub.2CaCu.sub.2O.sub.8
[Bozovic et al.1994a]. On the other hand, the critical current
along the c-axis (i.e., perpendicular to the CuO.sub.2 layers) is
much smaller than in a direction parallel to the CuO.sub.2 planes.
However, it is not zero, i.e., supercurrent can run in the c-axis
direction. This clearly implies that the planar, quasi-2D
superconducting slabs are weakly coupled by Josephson
tunneling.
[0037] In other words, cuprate superconductors can be viewed as
natural (native, intrinsic) stacks of Josephson junctions, spaced
at 6-25 .ANG.. A theoretical model for a stack of Josephson coupled
superconducting layers was introduced already 25 years ago
[Lawrence and Doniach 1971], and studied in much detail since then.
The predictions include nonlinear I-V characteristics,
microwave-radiation induced I-V (Shapiro) steps, and microwave
emission from a sample under dc voltage bias.
[0038] Indeed, all these signatures were observed in cuprate
superconductors, first in Bi.sub.2Sr.sub.2CaCu.sub.2O.sub.8
[Kleiner et al. 1992] and subsequently also in
(Pb.sub.yBi.sub.1-y).sub.2Sr.sub.2CaCu- .sub.2O.sub.8,
Tl.sub.2Ba.sub.2Ca.sub.2Cu.sub.3O.sub.10, etc. [Kleiner and Muller
1994, Muller 1994, 1996] and by several groups [Rgi et al. 1994,
Irie et al. 1994, Schmidl et al. 1995, Yasuda et al. 1996, Tanabe
et al. 1996, Yurgens et al. 1996, Seidel et al. 1996, Xiao et al.
1996]. Most of these results were obtained on small single
crystals, but some work was also done on mesas lithographically
defined on single crystals or thin films [(Schlenga et al. 1995,
Muller 1996, Schmidl et al. 1995]. In some cases, phase locking of
over 1,000 native junctions was observed [Schlenga et al. 1995]. In
general, phase locking was only partial, as evidenced by the
appearance of multiple branches in I-V characteristics (in all
works published so far). Namely, if the Josephson junctions that
belong to a stack are not all identical, i.e., if their critical
currents vary from one junction to another, they will not all
switch to the normal state at the same point as the bias current is
increased. Correspondingly, emission from such an array is expected
to be a superposition of coherent (narrow-band) and incoherent
(broad-band) radiation, and indeed this was observed [Schlenga et
al. 1995, Muller 1996]. These variations in Josephson junction
characteristics are believed to arise from imperfections in the
crystal growth and in the lithographic process of defining mesa
structures, such as variations in the mesa cross-section area. [We
will address this problem in Section V. (iv) below]. The highest
frequency of the emitted radiation detected so far was .nu.=95 GHz,
due to limited detection capabilities [Muller 1996]. The power of
detected radiation was minuscule, less than 1 pW, partly due to
gross load-impedance mismatch. No practical devices or applications
have been reported so far, although some speculations about
conceivable future applications for sub-millimeter radiation
sources were put forward [Schienga et al. 1995].
SUMMARY OF INVENTION (PROPERTIES)
[0039] It is therefore an object of the present invention to
provide means for avoiding some or all of the above
difficulties.
[0040] It is another object of this invention to provide a novel
generator and/or detector of sub-millimeter electromagnetic
radiation, which applies simultaneously a plurality of vertically
stacked Josephson junctions connected into a two-dimensional
network, so that the generation of microwave power is considerably
enhanced.
[0041] It is another object of this invention to provide a novel
sub-millimeter radiation device with an array impedance that allows
an impedance matching to the load, this providing maximum off-chip
emission power.
[0042] It is another object of this invention to provide a novel
sub-millimeter radiation device having a remarkably small emission
linewidth (less than one millionth of the radiation frequency
.nu.), within its sub-millimeter waveband up to the emission
frequency of several THz.
[0043] It is another object of this invention to provide a novel
sub-millimeter radiation device with an electronic control mode
which allows one to continuously vary the emission frequency and/or
to tune the detection frequency, over a broad spectral range.
[0044] It is another object of this invention to provide a novel
sub-millimeter radiation device, whose emitted microwave intensity
can be modulated electronically, including an on/off mode,
providing also a fast electronic switch for superconducting
electronic circuits.
[0045] It is another object of this invention to provide a novel
sub-millimeter radiation device of which the main emission
direction of the wave field (i.e., the propagation vector of the
plane wave) can electronically be rotated within the propagation
plane, providing for a sweeping property of the device in both the
emission and the detection mode.
[0046] It is another object of this invention to provide a novel
sub-millimeter radiation device, of which the emitted wave field
can electronically be split into two or more parts, and each can be
controlled separately, allowing among other features for focusing
and defocusing of the combined wave field.
[0047] It is another object of this invention to provide a novel
sub-millimeter radiation device capable of emitting and/or
detecting independently, and even simultaneously, at different
fixed (predetermined) or electronically varied and controlled
frequency channels.
[0048] It is another object of this invention to provide a novel
sub-millimeter radiation device of which the emission and detection
mode can be inverted by external electronic means.
[0049] It is another object of this invention to provide a novel
sub-millimeter radiation device, suitable for its incorporation
into superconductor/semiconductor hybrid systems.
[0050] These and other objects are achieved according to the
present invention by providing a two-dimensional network of linear
column-shaped superconducting elements, each of which contains a
series array of vertically stacked Josephson junctions, being
further this column-shaped superconducting elements grouped in a
pre-designed manner under electrical contact plates, carrying
additionally said contact plates means for an external electronic
control In addition to this the claims 1 to 35.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] FIG. 1: A device of invention: Basic design of a
two-dimensional lateral array of column-shaped elements (mesas) (a)
with stacked Josephson junctions (b), top contact (c), and bottom
contact (d). In (e), the electronic control unit is contained.
[0052] FIG. 2: Electrical connections to mesas (column-shaped
active elements). (A) Each Josephson junction array column may
carry its own electrical contact (1, 2, 3, 4, 5, 6, etc.).
[0053] (B) Parallel connection of several active elements. Top
superconducting contacts may unite several mesas (Josephson
junction array columns) in groups (segments), each of which is
controlled electrically.
[0054] (C) A device of invention: Short Josephson junction array
with two segments of Josephson junction array columns connected in
series by superconducting top and bottom electrodes (c) and (d),
respectively.
[0055] (D) A device of invention: A more complex circuit involving
both parallel and series superconducting connections of linear
Josephson junction arrays (contained in vertical column
structures). Segmented electrical contacts, top (c), and bottom (d)
for the series connection of n segments
(1.ltoreq.n.ltoreq.1000).
[0056] FIG. 3: (A) Single crystal of a high-T.sub.c
superconductor-etched into a mesa structure (M-metal contact
pads).
[0057] (B) Epitaxial thin film of high-T.sub.c superconductor
etched into a mesa structure.
[0058] FIG. 4: In-situ fabrication of the superconducting top
electrode (schematic representation, not to scale.)
[0059] (A) Deposit a high-T.sub.c superconductor film (HTS-1).
[0060] (B) Cool down and cover it with the protective
overlayer.
[0061] (C) Spin-on, expose and develop the photoresist (PR) with
openings for mesa definition.
[0062] (D) Ion mill through the protective overlayer and about
halfway through the high-T.sub.c superconductor (HTC-1) layer.
[0063] (E) Deposit insulating material I.
[0064] (F) Dissolve PR and lift-off insulator on top of the
mesas.
[0065] (G) Bring the film back to the growth chamber, heat it up so
that the protective overlayer evaporates away leaving a pristine
top surface of high-T.sub.c superconductor (HTS-1) mesas.
[0066] (H) Deposit a high-T.sub.c superconductor layer (HTS-2).
[0067] FIG. 5: (A) A fragment of the device in FIG. 4, showing
mesas containing a weak link Josephson junction (WL) at the
interface between two high-T.sub.c superconductor layers (HTS-1 and
HTS-2), and having a widened bottom part as the result of the
ion-milling process.
[0068] (B), the same for the case of wet etching, where the mesas
have constricted bottom parts as a result of under-etching.
Artificial Josephson junctions (AJJs) are placed only in the
central portion of the mesa, thus avoiding non-uniformities from
both ends.
[0069] FIG. 6 (1) and 6(2): Fabrication of parallel superconducting
contacts between three linear Josephson junction arrays in mesa
structures.
[0070] (A) Deposit the first high-T.sub.c superconductor film
(HTS-1) and protective overlayer, O.
[0071] (B) Spin-on, expose, and develop photoresist R for mesa
etch;
[0072] (C) Etch or ion-mill to define mesa structures.
[0073] (D) Deposit insulator I.
[0074] (E) Dissolve photoresist and lift-off insulator above
it.
[0075] (F) Remove the protective overlayer by evaporation within
the growth chamber.
[0076] (G) Deposit the second high-T.sub.c superconductor layer
(HTS-2).
[0077] (H) Spin-on, expose and develop photoresist for the cluster
definition etch.
[0078] (I) Etch or ion-mill to define the cluster structure.
[0079] (J) Dissolve photoresist.
[0080] (K) Deposit metal contacts on the top and the bottom
high-T.sub.c superconductor electrodes.
[0081] FIG. 7(1)-7(3):. Series connection of mesas or parallel
connections of mesas.
[0082] (A) Deposit high-T.sub.c superconductor (HTS-1) and
protective overlayer, etch to define mesas, deposit insulator, and
remove photoresist. (Side view).
[0083] (B) Top view of cluster structure with four mesas, after the
step A.
[0084] (C) Etch away the trench separating the two clusters,
deposit insulator, and remove photoresist. (Side view).
[0085] (D) Top view of the same wafer after the step C is
completed.
[0086] (E) Remove the protective overlayer and deposit the second
high-T.sub.c superconductor layer (HTS-2). Spin-off photoresist,
align carefully, expose and develop, and etch the trenches in the
high-T.sub.c superconductor (HTS-2) layer to separate the clusters.
(Side view). F) Top view after the step E.
[0087] (G) The current path in the structure shown in E.
[0088] (H) The equivalent circuit for the structure shown in E;
each box represents a series Josephson junction array contained
within one mesa.
[0089] (I) A cluster of mesas containing both parallel and series
connections.(Top view).
[0090] (J) The equivalent circuit for the Josephson junction array
network shown in (I).
[0091] FIG. 8 Distributed array of clusters of mesas, each
containing a stack of Josephson junctions, connected by a
transmission line. The distance between clusters is equal to
.lambda., the wavelength of the electromagnetic wave within the
structure.
[0092] FIG. 9: A schematic representation of four different working
configurations of the AA-Tunneltron. A two-dimensional array of
mesas is shown, each containing a linear stack of Josephson
junctions. Each mesa is controlled separately, where shaded circles
represent the inactive (switched-off) mesas and open circles
represent the active (emitting) mesas.
DETAILED DESCRIPTION OF THE DEVICE OF INVENTION
[0093] In FIGS. 1 and 2 Josephson junction arrays are shown
according to the invention. They are comprised of arrays of
vertical column-shaped superconducting elements (a), each of which
represents a series array of stacked Josephson junctions (b), built
up between a bottom superconductor (c) and a top superconductor
(d), both providing superconducting electrical contact areas. In
the case of FIG. 2.c., the top superconductor layer (d) is divided
into two or more segments, while in FIG. 2.d. additionally the
bottom superconductor (c) is divided into segments, too. These
segments can be electrically connected in different ways, thus
allowing for optimization of circuit parameters (such as load
impedance matching) as well as for various control modes selected
according to the intended use of the device as a microwave
radiation emitter or a radiation detector.
[0094] In the case of conventional (low-temperature)
superconductors, the superconductor and barrier layers that form
Josephson junctions are stacked alternately so that there is a
barrier layer between every pair of superconductor layers.
[0095] In the case of high-temperature superconductors mainly
chosen for this invention, Josephson junctions may also be formed
naturally, given a single crystal growth mode.
[0096] For the standard devices of FIGS. 1. and 2, the bottom
superconductor (c) is common for all active elements, while the top
superconductor (d) may be divided into two or more parts, with
every part (segment) of the upper contact being provided with its
own electrical contact pad. The number of such segments may be
smaller or equal to the number of columns which contain linear
arrays (stacks) of Josephson junctions.
[0097] For the devices of FIG. 2.D., the bottom superconductor (c)
is divided into two or more parts, where every part (segment) is
provided with its own electrical contact pad, too.
[0098] Although arrays with a small number of columns, which
contain stacks (series arrays) of Josephson junctions are shown in
the figures, any number of superconductive elements (columns) can
be chosen.
[0099] For the Josephson-junctions in every active column-shaped
superconducting element, very high numbers are achieved
particularly with high-T.sub.c superconductor materials. As each
junction is tightly coupled to the next, the entire stack will
phase-lock in the emission regime.
[0100] (i) As explained in the previous chapter IV,
high-temperature cuprate superconductors can be perceived as
natural superconducting superlattices of SIS . . . , SNS . . . , or
SINIS . . . type. A simple way to produce a linear array of such
native Josephson junctions is by fabricating a mesa structure such
as shown in FIG. 3.
[0101] One can use, for example. Bi.sub.2Sr.sub.2CaCu.sub.2O.sub.8,
Tl.sub.2Ba.sub.2Ca.sub.2Cu.sub.3O.sub.10,
HgBa.sub.2CaCu.sub.2O.sub.6.2, La.sub.1 85Sr.sub.0.15CuO.sub.4,
etc, in form of single crystals or epitaxial thin films. The mesa
structures can be fabricated by standard photo-lithographic
techniques including wet etching, ion milling, etc. Such mesa
structures have been fabricated by several groups [Kleiner and
Muller 1994, Muller 1994, Muller 1996, Regi et al. 1994, Schmidl et
al. 1995] and they indeed showed non-linear I-V characteristics
with multiple branches, switching between branches, and hysteresis,
as well as Shapiro steps induced by microwave radiation, and
microwave emission.
[0102] (ii) However, I-V characteristics and other properties of
these mesa structures have been far from those expected for an
ideal phase-locked linear Josephson junction array. In particular,
the observed emission powers were minuscule, in the pW-region, and
hence such Josephson junction arrays are of no technological
value.
[0103] This came from three principal reasons: (a) the junctions
were not uniform in I.sub.c and R.sub.N, (b) the junction areas
were large in general, typically about 30.times.30
.mu.m.sup.2.apprxeq.10.sup.-5 cm.sup.2, and (c) the circuits were
not optimized for out-of-chip microwave emission. In view of (a)
and (b), phase locking was imperfect, random, and unstable. Native
Josephson junctions in cuprates show relatively large c-axis
critical current densities (.apprxeq.10.sup.4-10.sup.6A/cm.sup.2),
so such junctions have way too large critical currents, 100 mA or
more. Such large Josephson junctions have many complex modes of
excitation, with supercurrents running in both directions, fluxon
motion, etc. While aware of these difficulties, researchers in the
mentioned groups were unable to significantly reduce the junction
area, because of contact resistance problems. With too large
contact resistance there is a substantial heating of the junction
stack from above, which introduces non-uniformity in critical
currents and obstructs phase locking. In extreme cases, the
Josephson junction array device burns out.
[0104] (iii) Our first improvement over the existing art is a
method to resolve the above contact resistance problem by
depositing in-situ superconducting (i.e., zero-resistance)
contacts. This method enables making native Josephson junction
arrays of very small area, e.g. 1 .mu.m.sup.2 and even smaller.
Specifically it also avoids forming of a weak link between the mesa
and the top superconducting electrode.
[0105] (Such a weak link could introduce a Josephson junction with
smaller critical current, in series with the native Josephson
junction arrays, which could dominate the response and diminish the
Josephson junction array performance). The process is described in
FIG. 4.
[0106] We start by (A) depositing an epitaxial thin-film of a
high-temperature superconductor (HTS-1), such as La.sub.1
85Sr.sub.0.15CuO.sub.4, Bi.sub.2Sr.sub.2CaCu.sub.2O.sub.8,
Tl.sub.2Ba.sub.2Ca.sub.2Cu.sub.3O.sub.10 or
HgBa.sub.2CaCu.sub.2O.sub.6.2- , for example, on a suitable
substrate such as SrTiO.sub.3, LaAlO.sub.3, MgO, etc., possibly
with one or more buffer layers (such as CeO.sub.2,
Bi.sub.2Sr.sub.2CuO.sub.6, etc.) to improve epitaxy. The deposition
method can be thermal evaporation (e.g. molecular beam epitaxy,
MBE), sputtering, pulsed laser deposition (PLD), chemical vapor
deposition (CVD), liquid-phase epitaxy (LPE), etc.
[0107] (B) Then we cool down the sample without removing it from
the growth chamber, and in this step we deposit a protective
overlayer (O) on top of the high-T.sub.c superconductor (HTS-1).
This compound should be chemically inert relative to the
superconductor material, i.e., there should be no chemical
interaction among these two compounds.
[0108] It should also be volatile, i.e., it should evaporate away
at some intermediate temperature T.sub.1 less than the growth
temperature T.sub.s used for the deposition of the high-T.sub.c
superconductor film (usually T.sub.s=600-750.degree. C.). Examples
of materials suitable for the protective overlayer are volatile
metals such as Pb, Sn, Bi, etc., organic molecules, such as
C.sub.60, etc.
[0109] (C) The film is then removed from the growth chamber and,
using standard photolithography, covered with photoresist which is
exposed via a suitable mask and developed to leave openings for
etching.
[0110] (D) The film is then brought into a chamber for ion milling,
and mesas are defined by ion milling through the protective
overlayer and approximately halfway through the high-T.sub.c
superconductor layer HTS-1. It is important that there remains
enough undamaged high-T.sub.c superconductor film at the bottom of
the trench (say 100 .ANG. or more) so that there is a good
superconducting contact from below (the bottom high-T.sub.c
superconductor electrode). Any other etching procedure, such as dry
or wet etching, can be used as well, as long as the etching rate
can be controlled, so that (etching can be stopped before it
reaches the substrate and disconnects the bottom electrode.
[0111] (E) Without removing the film from the processing chamber,
and using the same photoresist, we deposit the insulating layer
(I). Suitable materials include SiO.sub.2, CeO.sub.2, SrTiO.sub.3,
ZrO.sub.2 or YSZ (yttrium stabilized zirconia), etc. A suitable
method is, for example, electron-beam evaporation. Physical vapor
deposition under high vacuum conditions ensures directional
deposition and should make subsequent lift-off easier.
[0112] (F) Now we take the film out of the processing chamber and
remove the photoresist by an appropriate solvent (e.g., acetone),
lifting off the insulator above the photoresist.
[0113] (G) We bring the film back to the growth chamber and heat it
up to the standard high-T.sub.c superconductor deposition
temperature T.sub.s (say 600-750.degree. C.). Since
T.sub.s>T.sub.1, the protective overlayer evaporates away,
leaving pristine high-T.sub.c superconductor surfaces at the mesa
tops. In this step, it is possible to use different reactive gases
(e.g., fluorine) which interact with the overlayer material and
etch it away, but do not react with high-T.sub.c superconductor
compound.
[0114] (H) Finally, we deposit another layer of high-T.sub.c
superconductor (HTS-2) to act as the top high-T.sub.c
superconductor electrode. Additional processing may include
depositing thicker metal contacts on the bottom (HTS-1) and top
(HTS-2) electrodes, as it is shown in FIG. 3.
[0115] (iv) Our next improvement over the previous art, as well as
over the method described in section (iii) above, is to use
artificial rather then native Josephson junctions. For that
purpose, one can use the technique of doping selective unit-cell
layers such as described in the literature [Bozovic et al. 1994a,b,
1996a,b; Bozovic and Eckstein 1995, Eckstein et al. 1992, 1995].
For example, one can dope with Y.sup.3+ or Dy.sup.3+ onto Ca sites
in Bi.sub.2Sr.sub.2CaCu.sub.2O.sub.8, or dope with Ni, Zn, or Co on
Cu sites etc.
[0116] Another suitable choice in La.sub.2-xSr.sub.xCuO.sub.y
systems is to use layers with optimum doping levels
(x.apprxeq.0.15-0.2; y.apprxeq.4) as high-T.sub.c superconductor
layers, and layers with a non-optimum value of x (e.g., x=0) as
barrier layers. Each of this will generate one or more layers of a
high-T.sub.c superconductor with a reduced critical temperature
(T.sub.c) and critical current density (j.sub.c), and hence create
an artificial Josephson junction with the critical current
(I.sub.c) lower than that of the native Josephson junctions with
which it is in a series connection.
[0117] In this way, we can also fabricate a stack of artificial
Josephson junctions, since as long as the operating current is
smaller than the critical current of the native Josephson
junctions, the latter will simply act as superconducting
connections between the artificial Josephson junctions, and thus be
`invisible`. By varying the doping levels, we can control the
normal-state resistance (R.sub.N) of the artificial Josephson
junctions, as well as their I.sub.c. By varying their spacings
(i.e., the number of high-T.sub.c superconductor layers with
optimum T.sub.c and J.sub.c), we can vary the degree of interaction
and coupling between the two successive artificial Josephson
junctions.
[0118] Finally, this modification allows us to achieve improvements
with respect to three additional problems.
[0119] First, despite our technique of using the protective
overlayer and removing it in ultra high vacuum, there remains a
possibility that the surface exposed after the protective overlayer
is removed may be slightly damaged or modified in comparison with
the internal interfaces.
[0120] This could lead to a formation of a `weak-link` Josephson
junction, i.e., the very top Josephson junction in the mesa may be
weaker (i.e., have lower I.sub.c) than the others and hence be the
first one to go normal as the current bias is increased. We can
avoid this by introducing artificial Josephson junctions within
each mesa with even lower I.sub.c.
[0121] The second problem arises from the tendency of the ion
milling process to produce non-uniform trenches, as shown
schematically in FIG. 5 a.
[0122] The same is true for other etching processes, such as wet or
dry etching, etc., see FIG. 5 b. In some circumstances so-called
under-etching occurs, and the trench is wider at the bottom; in
other cases, it may be the other way around. This indeed affects
considerably the uniformity of native Josephson junction arrays, as
shown schematically in FIG. 3.
[0123] Here, we can avoid this problem by appropriately placing the
active, artificial Josephson junctions in the middle of the mesa
only, thus avoiding non-uniformities coming from changes in the
mesa width close to its base, as well as from the top contact.
[0124] Finally, by increasing R.sub.N of the artificial Josephson
junctions in comparison with the native Josephson junctions, we can
optimize the dimensions (i.e., cross-section area of the mesas) so
that they are technologically accessible, (e.g. .about.1-5 .mu.m
wide), by using standard photo-lithographic techniques.
[0125] (v) Connecting mesas into clusters: Parallel
connections.
[0126] In order to optimize the circuit parameters (for example, to
achieve impedance matching with vacuum and minimize the reflectance
of the electromagnetic radiation at the device/vacuum interface),
it is desirable to have the capability of electrically connecting
the mesa structures, as described in section (iv) above, in any
desired way. The connections should be superconducting and they
should not introduce any weak links that may act as a weaker
Josephson junction in series with the mesa Josephson junction
arrays; in other words, one has to obtain common high-T.sub.c
superconductor electrodes.
[0127] Both series and parallel connections are needed, in general.
In this subsection, we describe several practical methods of
achieving such superconducting connections.
[0128] The method of fabricating a parallel high-T.sub.c
superconductor connection between two or more mesas follows
directly from our method of fabricating the top high-T.sub.c
superconductor electrode, described in section (iii) above.
[0129] It is illustrated in FIG. 6.
[0130] As explained in this figure, and following the method
described in section (ii) above, the recipe essentially relies on
the ability to control the etching rate and depth, in order to
preserve the bottom electrode integrity. Removal of the protective
overlayer in high vacuum within the growth chamber and immediate
deposition of high-T.sub.c superconductor material to comprise the
top electrode is also crucial. This guarantees continuity between
HTS-1 and HTS-2 layers so that no weak links at the interface
occur.
[0131] Notice also that there are no difficult alignment steps in
this procedure; in particular, insulation and mesa-etching are
self-aligned and done in the same processing chamber as two
consecutive steps without moving the wafer and using the same
photoresist mask. This technique can be used without modifications
to fabricate a parallel array of an arbitrary number of mesas
carrying native or artificial Josephson junction arrays.
[0132] (vi) Connecting mesas into clusters: series connection.
[0133] In this subsection, we will describe a method of achieving a
series connection between individual mesas, or between clusters of
mesas already connected in parallel. The method is general, and it
involves only one sensitive alignment step which however is not
critical for junction uniformity. It is described in FIG. 7. Most
process steps are similar to the ones described in section (ii) and
section (v) above. In step C, the alignment is not critical;
neither is the etch rate control, since all that is needed is to
etch all the way through to the substrate, to isolate the two
clusters.
[0134] Simple wet etching can be used in this step, e.g., by using
dilute nitric acid, HNO.sub.3. However, in step E there is a
somewhat more sensitive alignment, since the trench separating the
two top HTS-2 electrodes must be placed in-between the two
mesas.
[0135] Given the need to pack the mesas as close to one another as
possible, one might assume that this separation may be about 1-2
.mu.m, so this trench has to be placed there with about 1 .mu.m
accuracy.
[0136] This is well within ordinary photo-lithographic alignment
capabilities.
[0137] In FIG. 7. G and 7. H, we have illustrated the current path
and the equivalent circuit showing that we have indeed achieved
series connections of mesas and Josephson junction arrays contained
therein with this method. Note that all the contacts are
superconducting and there are no weak links in this structure.
FIGS. 7. I and 7. J illustrate the possibility to fabricate more
complex networks which combine both parallel and series connections
thus allowing for substantial flexibility in optimizing the circuit
parameters.
[0138] (vii) Circuit optimnization.
[0139] In sections (v) and (vi) above, we have shown how to
fabricate Josephson junction networks of complex topologies,
involving both series and parallel superconducting connections.
This allows for a substantial flexibility in circuit design and
optimization of various parameters. We will illustrate this here by
considering some realistic cases.
[0140] Let us consider first a single mesa containing a linear
Josephson junction array (stack) of either native or artificial
Josephson junctions. The characteristic voltage in such junctions
may be about 25 mV for native Josephson junctions; the maximum
value reported for artificial Josephson junctions is I.sub.cR=10
mV, where R is the normal-state resistance of a single junction.
The maximum allowable critical current for a single junction is
about I.sub.c=1 mA; above this value one in general observes
distorted I-V characteristics due to flux flow. (In other words,
the junction behaves like a "long junction" with currents flowing
both forward and backward at different places within the
junctions). From I.sub.cR=10 mV and I.sub.c=1 mA, we get
R=10.OMEGA. as a typical lower limit for the resistance of a single
junction. Load-matching the device to vacuum (since we want to
extract maximum power out of the Josephson junction array, we have
to minimize the reflectance of microwave radiation at the
device/vacuum interface) dictates that R.sub.TOT=NR=300.OMEGA., so
for R=10.OMEGA. we get N=30. If I.sub.c were smaller than 1 mA, we
would have gotten an even larger value for R, and hence a smaller
value for N, so this may be an upper limit for a single,
load-matched linear Josephson junction array.
[0141] Actually, one could get a smaller R and hence a larger N
while not exceeding the I.sub.c.ltoreq.1 mA limit, at the expense
of having a smaller I.sub.cR product, which can be easily achieved,
but this actually does not increase the output power. This can be
seen by recasting the power in the form P.sub.MAX.sup.out=1/8
I.sub.c.sup.2 R.sub.TOT, where both R.sub.TOT and I.sub.c are
constrained (I.sub.c.sup.MAX=1 mA, R.sub.TOT=300.OMEGA.) and do not
depend on N.
[0142] Dimensions of the junctions are also constrained by these
values. If j.sub.c.sup..perp.=10.sup.4 A/cm.sup.2, from I.sub.c=1
mA we get that the junction area is equal to
A=I.sub.c/j.sub.c=10.sup.-3 A/10.sup.4 A/m.sup.2=10.sup.-7
cm.sup.2=10 .mu.m.sup.2.
[0143] Hence, one needs square cross-section Josephson junctions
with about a=3 .mu.m on the side, or circular cross-section
Josephson junctions with about 3.5 .mu.m diameter. These values
correspond to native Josephson junctions in very anisotropic
high-T.sub.c superconductor materials such as
Bi.sub.2Sr.sub.2CaCu.sub.2O.sub.8.
[0144] For less anisotropic high-T.sub.c superconductor compounds,
j.sub.c.sup..perp. may be higher (up to 10.sup.6 A/cm.sup.2 in
YBa.sub.2Cu.sub.3O.sub.7), and hence one would need even smaller
junctions. On the other hand, if artificial junctions are created
by doping (as expounded in section (ii) above), j.sub.c.sup..perp.
can be significantly smaller, and hence larger Josephson junctions
may be used.
[0145] However, the power output of such a simple linear Josephson
junction stack will be modest:
P.sub.MAX.sup.out=1/8NI.sub.c.sup.2R=(1/8)-
.times.30.times.10.sup.6.times.10A.sup.2.OMEGA.=5.times.10.sup.-4
mW.
[0146] Higher power output can be achieved in more complex arrays
that combine parallel and series connections. Essentially, the idea
is to increase the total current while not exceeding the
I.sub.c.sup.MAX=1 mA limit for a single junction, by using several
parallel linear arrays. Of course this would decrease the total
resistance, which can be brought back by increasing the number of
Josephson junctions in each linear strand.
[0147] So, for a parallel connection of M strands each of which
consists of N Josephson junctions connected in series one has
R.sub.TOT=(N/M)R; for R.sub.TOT=300.OMEGA. and R=10 .OMEGA., one
gets N/NM=30 or N=30 M.
[0148] The total current is equal to I=MI.sub.c, so that the total
power is P.sub.TOT.sup.out=(1/8) (MI.sub.c).sup.2R.sub.TOT.
[0149] Given that R.sub.TOT=300.OMEGA. and I.sub.c=1 mA, one gets
P.sub.TOT.sup.out=(1/8).times.10.sup.-6.times.300.times.M.sup.2A.sup.2.OM-
EGA..apprxeq.0.4.times.M.sup.2 mW.
[0150] One can see that the total output power scales with M.sup.2,
and it seems as if it could be increased indefinitely. However,
some further constraints are imposed here by the maximum acceptable
dimensions of individual Josephson junctions and the entire cluster
Josephson junction array.
[0151] Namely, for a lumped Josephson junction array, phase locking
can be achieved over a distance of approximately .lambda./4, where
.lambda. is the wavelength of the emitted electromagnetic
radiation. If the desired maximum operating frequency is .nu.=3
THz, then .lambda.=100 .mu.m; for .nu.=1 THz, one has .lambda.=300
.mu.m.
[0152] Hence, the total area occupied by a two-dimensional cluster
with parallel connection of M mesas can be about 25.times.25
.mu.m.sup.2 for 3 THz operation, and up to 75.times.75 .mu.m.sup.2
for 1 THz operation.
[0153] If one mesa occupies the area of 3.times.3 .mu.m.sup.2, one
gets M between 5.times.5=25 (for 3 THz operation) and
20.times.20=400 (for 1 THz operation). Allowing for some separation
(2 .mu.m) between the junctions with these values for M, one would
predict quite respectable power levels, sufficient for many
applications: P.apprxeq.10 mW at 3 THz, P.apprxeq.160 mW at 1 THz
(!).
[0154] However, it would be very difficult to achieve these values
with thin film technology, because of the load-matching constraint.
Namely, we have already established the requirement that because
R.sub.TOT=300.OMEGA. and R=10.OMEGA., one has N=30 M.
[0155] Thus for M=25 one gets N=750 and for M=400 one gets
N=12,000. These numbers are too large for a single mesa in a thin
film, even when native Josephson junctions are used in high-T.sub.c
superconductor compounds. The typical c-axis periodicity is about
10-20 .ANG., so we get the mesa height of about 7.500 .ANG.=0.75
.mu.m for the 3 THz device and about 15-30 .mu.m for the 1 THz
limiting device. These mesa heights are unrealistically large for
the present day thin film technology. Uniform mesas can be
fabricated today with state-of-the-art single-crystal thin film
growth (such as by MBE) with the maximum height of about 1,000
.ANG.. This can be conceivably increased by a factor of 2 or 3 in
the near future. Thus, N is limited to about 100 in thin films, and
this only for native Josephson junctions. In general, artificial
Josephson junctions will involve several molecular layers of the
undoped high-T.sub.c superconductor plus one (or more) of doped
layers, so N will be significantly smaller.
[0156] Indeed, it is possible to fabricate significantly taller
mesa structures, 1-10 .mu.m, by etching bulk single crystals of
high-T.sub.c superconductor compounds. So far, however, these did
not show ideal phase locking and significant power levels of
emitted electromagnetic radiation.
[0157] Therefore, our optimized circuit design, within the
constraints of using the thin film technology, combines series and
parallel mesa connections along the lines illustrated in FIG. 7.I
and 7.H.
[0158] As an example, for 3 THz operation one can use an array of
5.times.5 mesas, with M=5 (all mesas in one row connected in
parallel) and N=5.times.30=150, where 5 mesas with 30 Josephson
junctions each are connected in series. This could provide
P.sub.TOT.sup.out.apprxeq.0.4M mW=2 mW, which still is sufficient
for many applications.
[0159] For 1 THz operation we can afford a larger area with say
20.times.20 mesas.
[0160] If pairs of neighboring rows are connected in parallel, one
gets M=40. Load matching is achieved with 120 Josephson junctions
in each mesa, and 10 mesas connected in series, i.e., N=1,200, so
that N/M=30. Here P.sub.TOT.sup.out=0.4.times.40 mW=16 mW.
[0161] The above considerations were restricted to the case of a
single lumped Josephson junction array. Indeed, it is possible to
further increase the maximum output power of emitted
electromagnetic radiation using the distributed array of such
clusters, as described in [Lukens 1990, Han et al. 1992, 1994]. We
can place clusters as described above equidistantly, with spacings
.lambda. between them (say .lambda.=300 .mu.m for .nu.=1 THz
operation).
[0162] By covering the whole film with an insulator (such as
SiO.sub.2, MgO, CeO.sub.2, etc.) and a metallic layer (e.g., gold
or silver), one can fabricate a transmission line Jong which the
electromagnetic wave will propagate, as illustrated in FIG. 8.
[0163] Phase locking can be achieved in such structures of
considerable lengths, thus increasing the output power of emission
significantly (by the number of distributed clusters). However, the
drawback of such a device is that its operation is restricted to
the frequency where the wavelength of electromagnetic radiation is
similar to the distance between two consecutive clusters. Thus one
can achieve significantly higher output powers at the expense of
frequency tunability.
[0164] Other aspects of chip optimization include on-chip
integration of an antenna, where one can conveniently utilize the
existing superconducting film (HTS-1 and HTS-2) to fabricate an
antenna of a desired design.
[0165] (viii) Tunneltron microwave emission source.
[0166] Once we have fabricated a thin film chip containing one
cluster of mesas (with in general both parallel and serial
superconducting connections between them), and other standard
microwave circuit elements such as transmission lines and antennas,
it can be connected to a standard control electronics box which
should include a controllable current source (up to about 100 mA),
and appropriate controls and readouts for frequency and power of
the emitted radiation. This comprises a complete source of
narrow-band electromagnetic radiation, tunable over a broad
frequency range and up to frequencies as high as 5-10 THz.
[0167] We call this apparatus of the present invention the (basic)
Tunneltron It is schematically illustrated in FIG. 1.A.
[0168] (ix) Advanced Adaptable Tunneltron.
[0169] A more advanced extension of the apparatus of this
invention, as described in section (viii) above, consists of a
number of clusters of mesas, each of which contains a series array
of naturally or artificially stacked Josephson junctions, which are
connected by superconducting contacts as described in sections (v),
(vi), and (vii). Here, each such cluster has separate electrical
contacts for external control, and is controlled individually. This
provides the user with the possibility of switching individual
clusters on or off, and thus changing the effective geometrical
shape of the working part of the circuit. We call this apparatus of
the invention AA-Tunneltron, abbreviating for Advanced Adaptable
Tunneltron.
[0170] The principle of operation of an AA-Tunneltron is
illustrated in FIG. 9. For simplicity, we have shown the example in
which M=1 and N=30, with each cluster consisting of a single mesa,
i.e. a single linear stacked Josephson junction array. Each column
is assumed to be optimized for output power, i.e., load impedance
matched to vacuum.
[0171] Each column is further assumed to be controllable
electronically, separately, and from the outside, by means of a
standard electronics control circuit.
[0172] Overall, this situation corresponds to the one illustrated
schematically in FIG. 2.A. The individual columns will phase-lock
provided they are close enough (say within about .lambda./4, where
.lambda. is the wavelength of the emitted electromagnetic radiation
within the medium) and uniform enough (say, the spread in I.sub.c
and R values of individual Josephson junctions being no more than
.+-.5%.)
[0173] Notice that under phase-locking condition, the output power
scales as M.sup.2, where M is the number of individual columns.
[0174] In FIG. 9, we show several possible configurations of a 2D
array of columns (mesas) each of which is assumed to consist of a
linear array of stacked artificial or native Josephson junctions.
Each column is further assumed to be individually controlled
(contacts are not shown), and to be optimized for emission power
output.
[0175] All the Josephson junctions are assumed to be phase locked.
(This indeed introduces geometrical and physical constraints, as
explained in section (vii) above; for example, the lateral
dimensions of the array are limited to about 25-100 .mu.m insofar
that THz-operation is requested). By switching off some columns
while the others are emitting it is possible to change the
effective geometrical shape of the array from approximately
circular (FIG. 9.A) to elongated and rectangular (FIG. 9.B and 9.C)
to triangular (FIG. 9.D). This explains the idea of the
AA-Tunneltron: one can electronically change the geometry of the
emitted microwave beam, focus or defocus, sweep, etc.
[0176] (x) The second embodiment of the Tunneltron using bulk
single crystals of high-T.sub.c superconductor materials.
[0177] Indeed, as it will be obvious to one of average skill in the
art that the processes and apparatus described in sections
(ii)-(ix) above, can be also produced by fully analogous steps
involving high-quality single crystals of high-T.sub.c
superconductors. The same photolithographic techniques and steps,
including etching and ion-milling, as well as subsequent
overgrowths by another high-T.sub.c superconductor layer (HTS-2)
for the top conducting electrode, can also be employed in this
case. The main limiting factor here is the uniformity of the
Josephson junctions in one mesa and between different mesas.
Another is the limited flexibility in controlling the materials
parameters such as j.sub.c or R. But within these constraints, the
same ideas embodied in the present invention of the Tunneltron can
also be practiced here, thus constituting just another special
embodiment of the same invention.
[0178] (xi) The third embodiment of the Tunneltron using
conventional (low-temperature) superconducting materials.
[0179] Further fully analogous embodiments of the present apparatus
of invention can be realized by using conventional
(low-temperature) superconducting materials, for example
Nb/AlO.sub.x/Nb or NbCN/MgO/NbCN tunnel junctions. As it will be
obvious to one of average skill of the art, one can use completely
analogous thin-film growth techniques and photolithographic steps
to produce the apparatus of this invention also with these
superconducting materials. There will be in this case some
additional constraints coming from less favorable materials
parameters.
[0180] First, because of the lower critical temperatures
(T.sub.c=9.2 K in Nb and T.sub.c=16 K in NbN or NbCN, etc.) the gap
frequencies in these materials are also substantially lower
(.nu..sub.8=0.9 THz in Nb, and .nu..sub.g=1.7 THz in NbN) compared
to those in high-T.sub.c superconductor materials (up to
.nu..sub.g=5-10 THz). Hence, with low-temperature superconductors
one is limited to lower operating frequencies.
[0181] The second limitation comes from the superconducting
coherence length being much larger in conventional (low-T.sub.c)
superconductors. For this reason one must in this case use
significantly thicker superconducting electrodes, including
specifically the inner electrodes within one mesa or stack of
Josephson junctions. For the same total film thickness, one gets
much fewer Josephson junctions in a stack. In practice, it will be
difficult to fabricate more than 5-10 Josephson junctions stacked
vertically one on top of the other in this way.
[0182] The final restriction also comes from the lower T.sub.c: in
this case one requires a lower operating temperature (e.g. 4.2 K or
liquid helium) which in principle requires more complex and more
expensive cryo-cooler equipment.
[0183] However, within these restriction, it is possible to
fabricate the apparatus of this invention (Tunneltron) also by
using conventional low-T.sub.c superconductors, which is just
another special embodiment of the same invention.
[0184] Applications: Electronics components, subsystems and systems
based on the Tunneltron
[0185] The present device of invention is useable in virtually any
prior art equipment which formerly employed millimeter and
sub-millimeter microwave radiation sources and detectors.
Additionally, completely novel application fields are opened by use
of this device.
[0186] In what follows, some examples are given:
[0187] Some lasers and backward-wave tubes (carcinotrons) operate
in the sub-millimeter region, but they are bulky sources with large
power consumption. Solid-state oscillators, such as GUNN-diodes or
IMPATT diodes are limited to the millimeter wave range. Josephson
junctions, clustered into arrays, and load-matched for optimum
power output, can be voltage-controlled and can cover a broad
frequency band all the way into the Terahertz frequency region.
[0188] Quantum detection of electromagnetic radiation, which is a
familiar concept in the visible and near-infrared region of the
spectrum, has until recently been possible in the microwave and
millimeter-wave portions of the spectrum only within narrow
bandwidths centered at the resonant frequencies of a few MASER
amplifiers. The standard detection techniques covering this
frequency region employ nonlinear resistive elements, usually
Schottky barrier diodes as classical rectifiers and heterodyne
mixers. Their performance is based on the conversion of power
between frequencies rather than the conversion of photons to
carriers, as is the case with quantum detection.
[0189] The abrupt non-linearity of the dc I-V characteristic in the
single-particle tunneling of SIS tunnel barriers provides a tool
for resistive mixing.
[0190] Heterodyne receivers with Josephson junctions as such mixing
elements have demonstrated a sensitivity approaching the quantum
limit at frequencies up to several hundred GHz. The function of a
heterodyne receiver is to mix a weak incoming signal at frequency
.nu..sub.g with a large-amplitude local oscillator frequency
.nu..sub.LO, whereby an intermediate frequency output
.nu..sub.if=.vertline..nu..sub.g-.nu..sub.L- O.vertline. is
generated and used for further electronic processing. A stream of
photons with an arrival rate of one photon per nanosecond is a
typical value of detection power of such a device. Radio-astronomy
in the millimeter and sub-millimeter wave portion of the spectrum,
which is highly interesting due to the intense exploration
activities of the interstellar medium, has now the potential to
reach major advances in describing the structure of the
universe.
[0191] Investigations of the 115 GHz rotational emission from
interstellar carbon monoxide (CO) with a .lambda.=2.6 mm wave
receiver points also toward the potential which the proposed device
might have in earth-bound microwave spectroscopy of organic and
inorganic materials.
[0192] Spectroscopy as a generic term implies the study of the
emission and absorption of electromagnetic radiation, as related to
the radiation frequency of the excitation source. The Tunneltron
device provides as an outstanding and new feature the possibility
to be used for both the excitation of the object under study, and
the detection of its response by absorption or emission.
[0193] As an example, the Tunneltron will enable monitoring of
organic and inorganic compounds in vapors, liquids and solids, with
respect to the chemical composition, geometric and energetic
structures, as well as interaction processes, and all this as a
function of various external parameters and in a time-resolved
manner, thus opening a wide application field for the device of
invention. Indeed, there are many other related problems where one
could make use of spectrometric properties, as e.g., the detection
of exposed or hidden chemical compounds (drugs, plastics, . . . )
after corresponding excitation by appropriate sub-millimeter
electromagnetic radiation, quality control devices for specific
materials properties (e.g., water content or impurities in solid
materials).
[0194] These also contribute to a wide field of new applications of
the device of this invention.
[0195] The Tunneltron as a coherent and tunable radiation source,
and in particular in its embodiment as a Maser, provides wave
properties of the emitted radiation field (coherence,
monochromaticity), which enable its use in holography and
interferometry. Holography is in principle a means of creating a
unique photographic image of a coherently illuminated object, where
an undisturbed beam and the beam reflected of the observed object
are brought to interference on a detecting medium.
[0196] The reconstruction of this interference pattern delivers a
three-dimensional picture of the object.
[0197] The capability to vary the frequency of the coherent
Tunneltron source, as well as the possibility of propagation
through obstacles (which are opaque in the visible, but transparent
for the corresponding millimeter or sub-millimeter radiation) in
front of the object, opens prospects for many further
applications.
[0198] Communication and data transfer (e.g., image transmission)
is another field of application of the device proposed here,
leveraging on its potential for covering a frequency region far
beyond the frequency bands administered and coordinated presently
by the ITU (Internat. Telecomm. Union). The new frequency region
provides a considerably increased number of new channels for both
satellite and terrestrial communication. [Consider a channel width
of 20 MHz, then a frequency band 5 THz wide would contain about
250,000 channels. This compares e.g. to the 40 channels for
satellite communication, 19.18 MHz wide, with the frequency band of
11.7 . . . 12.5 GHz, which was destined by the ITU for Europe,
Africa and the former Soviet Union together (Region 1)]. Take a 4
kHz channel width for telephone communication; in this case one
would get almost 2 billion talk channels, riding on such a high
carrier frequency.
[0199] A high quality transmission requires digital systems with
pulse code modulation (PCM).
[0200] Frequency modulation would increase the noise power way up
the 3 pW/km (approx. 52 dB), recommended by the CCIR as an upper
limit.
[0201] Digital systems allow a regeneration of signals in
intermediate relay stations, avoiding accumulation of errors.
[0202] It might even be possible to build a completely wireless
terrestrial microwave-relay communications network, a
broad-bandwidth wireless link to the Internet and a broad-bandwidth
satellite communications link in the new frequency region with all
advantages of the high number of telephone channels, with the
additional benefit of the reduced load of dangerous radiation by
virtue of the smaller absorption depth of human skin for
sub-millimeter radiation.
[0203] Multifrequency Imaging Microwave Radiometers (MIMR) for
distant satellite or airborn inspection of the underlying air space
or the earth's surface has a high potential for practical use.
Future remote-sensing satellites, instead of carrying multispectral
instruments covering up to seven spectral bands as they do now,
will have hyperspectral instruments gathering imagery in many more
spectral bands. Relatively closed-up panoramic observation out of
flying helicopters as well as from airplanes approaching the
airport and landing, will have an impact on maneuverability and
general security. The same may be true for street-based traffic
(automotive electronics), and even for robotics applications under
corresponding environmental conditions. A basic requirement is a
powerful, high-frequency, tunable, nearly monochromatic, and very
fast sub-millimeter source and detector. Such a device is provided
by the present invention. The device of the present invention,
which may be an on-chip integrated emitter and detector of
millimeter and sub-millimeter radiation, is also suitable for Radar
applications in the most general sense: radio-location systems,
radio-navigation systems, radio-electronic reconnaissance and
radio-electronic countermeasures.
[0204] There is a growing need for compact low-power radar for both
civilian and military applications.
[0205] Since it can be used to determine the velocity of a moving
object, automatic collision-avoiding systems in automobiles may be
improved; the same is true also for intrusion alarms and traffic
monitors.
[0206] The Tunneltron may also act as an active sensor in synthetic
aperture radar (SAR) applications. A SAR systems beams microwave
radiation towards the object (the Earth when remote sensing from
satellites is considered) and detects the returned echoes. The fine
tuning of the emitter allows to position the frequencies within the
absorption minima of the atmosphere. The electronic scanning
capability of the advanced Tunneltron delivers an additional
advantage here.
[0207] Biological and medical applications of the device of
invention are also manifold and straight forward.
* * * * *