International Forum for Green Asia 2020

Surface Study of Aluminum Films Grown by Atomic Layer Deposition
for Superconducting Tunnel Junction
Sameh Okasha,1&2 Yuichi Harada1
1 Global Innovation Center, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan
2 Molecular and material science department, Interdisciplinary Graduate School of Engineering
Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, JapanAbstract
The Technology of atomic layer deposition (ALD) can deposit an ultra-thin layer of different
metals and metals oxide down to monolayer depending on the number of cycles at a relatively
lower temperature. It is a promising technique to replace some current techniques such as
shadow evaporation which is used in a small
Josephson junction (J.J.) fabrication. J.J. is
composed of a very thin tunnel barrier (~2nm)
sandwiched by superconducting electrodes with astandard junction size of 100 x 100 nm2 such as
Al/Al2O3/Al. The shadow evaporation technique with
Figure 1. A schematic view ofa suspended bridge is a standard method used forJosephson tunnel junction structure.
fabricating small Josephson junctions. Although it iswell known, however, it has many disadvantages, such as the complexity of scaling up many
junctions and limiting circuit layouts. Our work is to deposit aluminum thin film using ALD by
the precursor of dimethylethylamine alane (DMEAA) for the fabrication of small Josephson
junctions. Scanning electron microscope (SEM) was used to characterizes the surface
morphology and thickness of the aluminum film, while X-rays diffraction (XRD) explored its
contamination and crystal orientation. The results study showed the aluminum grains
morphology, pure aluminum thin film success deposition, and alane thin film deposition alongwith aluminum.
1. Introduction
The atomic layer deposition (ALD)
technique can deposit a conformal single
layer on each growth cycle at a rather low
temperature and also give a sharp interface
between different materials.1 This feature of
the ALD technique is very promising tofabricate a small Josephson junction which iscomposed of a very thin tunnel barrier(~2nm) sandwiched by superconducting
electrodes with a typical junction size of 100
x 100 nm2, such as aluminum/aluminum
oxide
Al/Al2O3/Al.2,3Theshadow
evaporation technique with a suspended
bridge is commonly used fabricating small
Josephson junctions as the standard.4,5,6 This
technique is well established, however, it has
several drawbacks, such as difficulty to scale
up to several junctions and restrictions ofcircuit layouts.
We propose an ALD technique to use forthe fabrication of small Josephson junctions.
Al2O3 thin films are commonly grown by
ALD, but so far few reports exist for thegrowth of aluminum thin films by ALD. In
this study, we grew aluminum thin films by
the precursor of dimethylethylamine alane(DMEAA) and studied its surface
morphology as the first step of the fabricationof small Josephson junctions.
2. Experimental and characterization
Di-Methyl-EthylAmine-Alane(DMEAA)7 precursor is selected among
other precursors, such as TMA1 and DMAH8.
DMEAA has several advantages. One ofthose is a low decomposition temperature asfollows:27

International Forum for Green Asia 2020
the white part is related to the existence ofalane (AlH3),10-12 which contributes toincreasing the size of the gains due to its large
cell parameters comparing to aluminumenhancing Stranski–Krastanow growth13. An
X-ray diffraction (XRD) analysis shows thedominanceofbeta-alane(β-AlH3)
contamination of (311) orientation. It exists
because of incomplete dehydration reaction
at 150 degrees Celsius on Si surface 9,12.

150 °C[(CH3)2C2H5N] AlH3 (L)(CH3)2C2H5N (g) + AlH3 (ads)
AlH3 (ads)

150 °CAl (ads) + 1.5H2 (g)The growth of aluminum thin films on
the silicon substrate by DMEAA is carriedout in the following ways. At first, the silicon
substrate is cleaned by the standard degrease
process, while the surface oxide layer andadsorbed materials are removed by Semicoclean 23. Then, the silicon substrate isimmediately loaded into the e-gun evaporator
and a 1.5 nm thick of titanium film isdeposited on the substrate. We found out thatthe growth of aluminum films by DMEAA
has to have a metallic layer. After that, thesubstrate is loaded into the ALD reaction
chamber with a temperature of 150 degrees
Celsius. The growth of aluminum films wasperformed with 0.3 seconds of pulsing timeand 3 seconds of purging time for threehundred cycles.
Its surface morphology and thickness of thealuminum film are characterized by SEM,
while its contaminations, as well as crystal
orientation, is examined by XRD. Process
imaging software is used for the analysis ofgrain size and film thickness statistically.

(a)

(b)

Figure 2. (a) White and (b) mirror parts in thealuminum thin film.

Intensity (a.u.)

3. Results and discussion
Due to the non-uniformity of gas flow,
its surface has a mirror and white parts asshown in Fig. 2. The mirror part is composedof smaller grains and shows the (111)orientation. However, the white part iscomposed of larger grains and shows moreroughness of the same orientation. By using
the image processing analysis of SEM images
by GATAN software, it is estimated that theaverage thickness of both mirror and whiteparts is (220) nm and (760) nm, respectively.
Furthermore, the white part contains betaalane (β-AlH3) from the X-ray diffraction(XRD) analysis as shown in Fig. 3. We
observed that the aluminum film has mainly(111) orientation with a small amount of(100) due to chemical bond deposition ofchemical reaction9. The surface roughness of2ϴ ⁰: 2Theta (degree)
Figure 3. XRD of β-alane contaminant inwhite part.

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International Forum for Green Asia 2020(a)

Intensity (a.u.)

(b)

(c)

2ϴ ⁰: 2Theta (degree)
Figure 5. XRD of β-alane contaminant attrails samples.
(d)Grains size.

5. Conclusion
We studied aluminum thin films grown by
ALD using DEMAA precursor as the first step torealize Josephson tunnel junctions. We foundthat a conductive layer is necessary to initiate
successfully the thin film growth of aluminumby DEMAA precursor. The grown aluminum
films show (111) orientation regardless thesubstrates. Although we obtained a mirror
surface the white surface was also observed,
which is due to the large grain size. We speculatethat the large grain size in white aluminum films
is related to the existence of alane.

SamplesFigure 4. Aluminum grain morphologies (a) top
view of grains distribution. (b) grainmorphologies of triangle, droplet and square.
(c) single grain of aluminum consists of twotriangles above each other. (d) average grains

Acknowledgment
The author is grateful for the financial
assistance provided by the green asia
program at Kyushu University.

size using underlayer of TMA or water.

It reveals that the new deposited relativity cold
surface layer of precursor could prevent theoccurrence of a complete dehydrating reaction of(AlH3) to aluminum which required temperaturefrom 97–197 ⁰C.12 We also tried to initiate thegrowth of aluminum by TMA with different cycles
in sample (A), (B)&(D), and water in sample (C)instead of the metallic layer. All samples didn’t
show thin film but grains. The shape of grains
shows mainly triangles, but the square shape isobserved due to the combination of two triangles asshown in Fig. 4. We observed partial growth ofaluminum and alane by XRD as shown in Fig. 5.

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Email: samehokasha@gmail.com

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