It is expected that thin-film superconductors can be used as detectors or sources of infrared and microwave radiation, as magnetometers, as voltage standards, and for voltage and current measurements, for electronic signal processing, and in digital circuitry

Kirschman, R. K.

Mercereau, J. E.

Notarys, H. A.

English

NASA Technical Reports Server

Jtne 1975
NASA TECH BRIEF 
S	 NASA Pasadena Office
B75-10120 
]A-
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the TecIr Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Microelectronic Fabrication of Superconducting Devices and Circuits 
. 
.
A proximity-effect superconducting device is a type 
of weak-link superconductor which exhibits 
Josephson-like effects, but operates on a different 
principle. The device is a thin-film superconductor 
with a short section that has been weakened in the 
sense that its pair density is lower than that of the rest 
of the film. It is expected that such devices can be 
used as detectors or sources of infrared and 
microwave radiation, as magnetometers, as voltage 
standards, and for voltage and current measure-
ments, for electronic signal processing, and in digital 
circuitry.
Enlarged View 
of Weakened 
Section 
1m
Bonding Pads 
Niobium Over 
Tantalum Over 
Zirconium Film 
Weakened 
Section 
I	 1 m 
Inductor 
Figure 1. A Typical Design for a Superconducting Microcircuit
ioo-A Anodized ioo-X 
Tantalum 
Film
j (Weakened) 
Section
Niobium 
/	 ,." Film 
Sapphire
__
Zirconium 
Film 
Dev 
sup
Figure 2. Example of Superconducting Films and Anodizations 
To date, proximity-effect structures have been 
fabricated by a series of manual-processing steps. 
These methods require too much time and labor for 
large-scale production. To overcome the problem, a 
fabrication technology has been developed in which 
the manual processing is replaced with industrial 
microelectronic fabrication methods of the type used 
for semiconductor microcircuits. Operating prototype 
devices have already been made completely using this 
technology. 
There are major differences between the present 
microcircuitry and this new projected form, such as 
the use of different materials, different processing 
techniques, and a different operating environment. 
For example, refractory superconducting films are 
used instead of silicon or germanium, anodization is 
used instead of doping to make active areas in the 
film, and the circuits operate at cryogenic tempera-
tures instead of at room temperatures. 
An example of the new technique is the following 
sequence of steps used to fabricate a batch of super-
conducting microcircuits containing proximity-effect 
structures. A typical microcircuit design is shown in 
Figure 1. The first step is to make a layered thin-film 
parent material as shown in Figure 2. A sapphire 
(continued overleaf) 
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substrate [typically 1.5 in. (3.8 cm) in diameter by 10 
to 20 mils (0.03 to 0.05 cm) thick] is sequentially 
coated with a base film of zirconium (or tungsten) 
about 100 A thick followed by a middle film of 
tantalum also about 100 A thick and a top film of 
niobium of the same thickness. These metals have 
superconducting critical temperatures of approxi-
mately 0.5, 4, and 9 K, respectively. The films are 
deposited in a single processing operation by the 
evaporative metal deposition technique under a high 
vacuum, on the order of 104 torr or higher. The 
sapphire wafer is heated to about 400° C prior to and 
during deposition. 
The next step is to form the weakened sections 
needed for the proximity-effect structures. This is 
done in the following manner. The surface of the top 
metal film is covered with a positive working type of 
photoresist material, which is spun on to produce a 
uniform thin coating on the surface. The usual baking 
step to harden the photoresist is omitted; instead the 
coating is allowed to dry at room temperature. 
Baffling and yellow light are used as precautions to 
avoid undesired exposure to stray light. 
The wafer is then placed in a mask aliner in which a 
mask has been installed. This mask is provided with 
multiple slot openings in the proper positions to 
produce the weakened sections (the slot openings in 
the masks are typically about 1;Am wide by about 
100 m long). The resist is then exposed and 
developed, and the wafer is then rinsed with deionized 
water and is blown dry with dry nitrogen gas. 
As mentioned, the weakened sections are produced 
by anodizing. The anodizing process does not actually 
remove the metal but converts the exposed metal to its 
oxide. Anodizing is performed by applying a dc 
potential which is typically about 10 volts, for a period 
which is typically 0.1 second. 
Following anodization, the photoresist is removed 
using an appropriate solvent, and the wafer is 
cleansed. A second photoresist is then applied. 
Additional masks to define the conductor leads and 
bonding pads are then placed in the mask alineF, 
alined with the first pattern; and the photoresist is 
exposed, developed in the appropriate developer, 
rinsed, and dried as before.
All of the metal of the films except that in the 
masked areas is then removed by plasma etching. 
Carbon tetrafluoride (CF4) gas at a pressure of 0.7 
torr with an RF power density of about 0.05 
watts/cm 3
 applied for a period of one-half hour 
produces a plasma which completely removes the 
exposed metal film areas. The remaining photoresist 
is removed with the appropriate solvent, and the wafer 
is cleansed, scribed, and broken into chips each of 
which contains a microcircuit. The chips are then 
mounted in integrated-circuit packages using epoxy 
cement. However, the cement is not baked but is 
allowed to cure at room temperature. Electrical 
connections between the pads on the chip and 
package are made with aluminum wire using 
ultrasonic bonding. 
The sequence of steps given above is typical; 
variations of the procedure are possible. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP75-10120 
Patent status: 
NASA has decided not to apply for a patent. 
Source: Randall K. Kirschman,

James E. Mercereau, and

Harris A. Notarys of
Caltech/JPL
under contract to

NASA Pasadena Office

(NPO-13419)
S 
Categories: 01 (Electronics - Components 
and Circuitry) 
08 (Fabrication Technology) 
03 (Physical Sciences)
S 
B75-10120


Microelectronic fabrication of superconducting devices and circuits

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