Hundred-micron-sized
all-solid-state Li secondary battery arrays embedded
in a Si substrate
K. Kushida, K. Kuriyama,
a)
and T. Nozaki
b)
College of Engineering and Research Center of Ion Beam Technology, Hosei University, Koganei, Tokyo
184-8584, Japan
͑Received 3 September 2002; accepted 28 October 2002͒
Fabrication and charge/discharge behaviors of all-solid-state Li secondary battery arrays
embedded into three Si trenches of 200ϫ100
m
2
in area and 0.5ϳ2.0
m in depth are presented.
The battery arrays in a Si wafer, consisting of multiple layers ͑Al current
collector/Li/SiO
2
–15 at.%P
2
O
5
/LiMn
2
O
4
/polycrystalline silicon current collector͒, are prepared
by combining a sol-gel spin-coating method and Si very-large-scale integration technologies.
Porous spin-on glass (SiO
2
–15 at.%P
2
O
5
) is adopted as an electrolyte layer, in which spatial paths
for Li
ϩ
ions are artificially introduced into the glass. Each active battery area is isolated with double
insulating walls (Si
3
N
4
/SiO
2
). The battery arrays demonstrate a constant capacity of
ϳ9.2
Ahcm
Ϫ2
at ϳ3.6 V up to 100 cycles. © 2002 American Institute of Physics.
͓DOI: 10.1063/1.1531220͔
Mounting electric energy sources on semiconductor tips
is highly attractive because of the rapid growth of electrical
and mechanical integration technologies such as wireless
telecommunications, emerging integrated optoelectronic cir-
cuits and rapidly growing microelectromechanical systems
͑MEMS͒ in recent years. Since Shokoohi, Tarascon, and
Wilkens reported their work,
1
much research ͑Refs. 1–8͒ has
been performed on battery component materials ͑cathode,
electrolyte and anode͒ and thin-film Li batteries. Among
such research, there have been some reports on the fabrica-
tion of a mesa-type of all-solid-state Li battery using spinel
LiMn
2
O
4
5
or layered-rocksalt LiCoO
2
͑Refs. 4, 6, 8͒ cath-
odes using thin-film technologies such as a radio-frequency
sputtering method. In these works, sputtered Lipon films
(Li
3
PO
4
:N) have been adopted as an electrolyte layer. Li
microbatteries must be considered as a local power supply
for one or several active electric circuit element͑s͒ in Si very-
large-scale integration ͑VLSI͒, rather than as a power gen-
erator delivering electric energy to a whole electronic chip
such as a MEMS circuit, since, simply, battery dimensions
limit the capacity of batteries. Furthermore, from the view-
point of industry, simpler and more Si VLSI-matched tech-
nologies would be required for the fabrication of such Li
microbatteries. Accordingly, we focused on the embedded
battery configuration instead of the mesa structure ͑Refs.
4–6, 8. In this letter, we demonstrate 100
m sized all-solid-
state Li secondary battery arrays, consisting of multiple lay-
ers ͑Al current collector/Li/SiO
2
–15 at. %P
2
O
5
/LiMn
2
O
4
/
polycrystalline silicon current collector͒ prepared by com-
bining a sol-gel spin-coating method ͑Refs. 9–11͒ and Si
VLSI technologies, as a first step in the research on local
power suppliers in Si VLSI circuits. Our present study con-
cerns the glassy electrolyte (SiO
2
–15 at. %P
2
O
5
), which is
prepared as a porous structure and which conducts lithium
ions by means of diffusion down the pores and grain bound-
aries.
All-solid-state Li secondary battery arrays were embed-
ded into a Si substrate, as shown schematically in Fig. 1.
First, a polycrystalline silicon layer (ϳ300 ⍀) with a thick-
ness of 460 nm was deposited onto a ͗100͘ Si substrate as a
current collector for the cathode material, spinel LiMn
2
O
4
.A
double insulating layer consisting of SiO
2
͑600 nm in thick-
ness͒ and Si
3
N
4
͑1600 nm͒ layers was also deposited onto
the polycrystalline silicon layer to isolate each battery elec-
trically, before embedding the cathode material into the sub-
strate. Hundred-micron-sized trenches ͑100
m in width
ϫ200
m in lengthϫ0.5–2.0
m in depth in each battery͒
were etched with a dry etcher, retaining the polycrystalline
silicon layer ͓see Fig. 1͑b͔͒. The cathode material, spinel
LiMn
2
O
4
, was deposited over the entire surface of the Si
substrate by a sol-gel spin-coating method ͑6000 rpm rotat-
ing speed͒͑Refs. 9–11͒ with subsequent annealing at 550°C
for 30 min. The sol solution for spinel LiMn
2
O
4
was ob-
tained from Li and Mn acetates ͑99% pure͒ with a small
amount of citric acid as a chelating agent. Excess spin-coated
LiMn
2
O
4
was eliminated by a mechanical polishing tech-
nique, retaining the cathode material embedded into the
trenches. The SiO
2
–15 at. %P
2
O
5
glass, which has usually
been used in VLSI as a spin-on passivation film, was adopted
as an electrolyte, since Li
ϩ
ions are introduced into and
move through the oxide glass even at room temperature by
means of the diffusion of Li
ϩ
ions, which occurs because of
the external electric field applied during the charge and ow-
ing to the difference in chemical potential between the
LiMn
2
O
4
cathode and the Li anode during the discharge.
This is analogous to the fact that alkali ions such as Na, K,
and Li in SiO
2
are mobile when electric fields are present.
12
The Si substrates with the embedded cathodes were coated
with a commercially available liquid precursor of the spin-on
glass ͑P–Si–film: P-48316-SG, Tokyo Ohka Kogyo Ltd.,
Co.͒ using a spin coater ͑4000 rpm rotating speed͒. The
a͒
Author to whom correspondence should be addressed; electronic mail:
kuri@ionbeam.hosei.ac.jp
b͒
Permanent address: Technical Laboratory, Citizen Watch Co. Ltd., Toko-
rozawa, Saitama 359-8511, Japan.
APPLIED PHYSICS LETTERS VOLUME 81, NUMBER 26 23 DECEMBER 2002
50660003-6951/2002/81(26)/5066/3/$19.00 © 2002 American Institute of Physics