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Journal of Non-Crystalline Solids 338–340 (2004) 744–748
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IR bolometers based on amorphous silicon germanium alloys
M. Garc�ıa *, R. Ambrosio, A. Torres, A. Kosarev
Instituto Nacional de Astrof�ısica, �Optica y Electr�onica, INAOE, P.O. Box 51&216, Puebla Z.P. 72000, M�exico
Abstract
In this work, we report the fabrication and characterization of bolometers based on amorphous silicon germanium alloys (a-
Si1� xGex:H,F). The fabrication of microbolometers with thermally isolated a-SiGe thin film as sensing layer, is described. Mem-
brane-supported and bridge-supported detectors have been fabricated. Sensing layer is deposited by low frequency plasma enhanced
chemical vapor deposition technique. Since this deposition is carried out at relative low temperature TD ¼ 573 K, the read-out IC
fabricated on silicon substrate is not affected, providing compatibility with silicon IC technologies. Responsivity and detectivity were
measured under illumination at different bias voltages. The thermal time constant, the thermal conductance and current noise are
determined by means of the voltage–time curves. Responsivity and detectivity are observed to depend on the physical properties of
the materials forming the device and on the device geometry. Obtained results of pixel resistance, thermal resistance and responsivity
demonstrate a-SiGe:H,F like a material suitable for realizing high-performance microbolometers.
� 2004 Elsevier B.V. All rights reserved.
PACS: 73.61.Jc; 85.60.)q; 85.60.Dw; 85.85.+j
1. Introduction
The use of micromachining techniques has greatly
increased the detectivity of thermal infrared (IR)
detectors like bolometers. These techniques enable the
thermal insulation of the device to be increased, maxi-
mizing the temperature increase due to the absorption of
IR radiation. Bolometers present several advantages
over semiconductor diodes, such as uncooled operationclose to ambient temperature and providing cheap
detector technologies in compact systems.
Resistive bolometers are formed by a temperature-
dependent resistor and an IR absorber. The resistor
should have a large temperature coefficient of resistance,
which is defined as TCR ¼ ð1=RÞðdR=dT Þ, so a small
temperature increase results in a significant change in
the bolometer resistance. In order to increase sensitivity,a good thermal insulation of the detector is needed.
Membrane-supported detector and bridge-supported
detectors have been developed in order to get such
thermal insulation. Typical membrane material are sili-
con nitride or silicon oxide. Metals like platinum and
* Corresponding author.
E-mail address: [email protected] (M. Garc�ıa).
0022-3093/$ - see front matter � 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jnoncrysol.2004.03.082
nickel, semiconductors like vanadium oxide, supercon-ductors and non-crystalline material have been used as
sensing element. However metals present a very low
TCR, vanadium oxide is not a standard material in IC
technology and superconductors need special tempera-
ture conditions.
For room temperature operation, amorphous silicon
(a-Si:H) has been used in commercial applications [1–3],
however it presents a very high resistivity. In order toreduced it, boron doping is used but this results in lower
activation energy (Ea) and consequently in lower TCR
[4]. Previous studies of a-Si1�xGex:H,F (x ¼ 0; . . . ; 1)thick films have demonstrated this material as a good
candidate as the active material of uncooled micro-
bolometer, due to its relatively high activation energy,
Ea ¼ 0:4 eV, and therefore, a high TCR and mod-
erate resistivity at room temperature, rRT ¼ 2:5 · 10�3
X�1 cm�1 [5,6,9]. Additionally, the complete process of
fabrication of a bolometer using these films is performed
at low temperature (T ¼ 573 K). This temperature is
compatible with standard processes of silicon IC fabri-
cation, which makes this process an excellent candidate
for the fabrication of 2-D arrays of microbolometers.
In this work, the process of fabrication of membrane-
supported and bridge-supported microbolometers using
M. Garc�ıa et al. / Journal of Non-Crystalline Solids 338–340 (2004) 744–748 745
a-Si1�xGex:H,F (x ¼ 0; . . . ; 1) as the sensing material isdescribed. Characterization of the fabricated structure is
presented and the measured characteristics are com-
pared with other recent published results.
Fig. 1. Diagrams of the membrane-supported detector structure: (a)
Back-side view through SiO2 membrane on a Si substrate. There are
two structures of different size on the diaphragm. (b) Top-view of an
a-Ge:H,F microbolometer on the membrane. (c) Cross-section of the
membrane-supported microbolometer.
Fig. 2. Diagrams of the bridge-supported detector structure: (a) Top-
view of an a-SiGe bridge-supported microbolometer. (b) Cross-section
of the microbolometer.
2. Fabrication process description
Two different configurations for thermal isolationwere used in fabricated devices: membrane-supported
structure and bridge-supported structure. The fabrica-
tion process of both structures is briefly summarized in
the following. For the membrane-supported structure,
the initial substrate is a p-type crystalline silicon wafer,
in which a 1 lm-thick SiO2 layer was thermally grown.
Later, a 0.1 lm-thick a-Si3N4 layer was deposited on the
polished side of substrate by mean of low-pressurechemical vapor deposition (LPCVD). A mask is used to
define the diaphragm by removing the c-silicon by wet
chemical etching in a bulk-micromachining process.
Once the diaphragm is formed, aluminum (Al) is e-beam
deposited and etched for patterning the connection lines,
pads and contacts for the sensing layer. On top of this,
the active element material deposition is performed. The
sensing layer itself is made by depositing 0.1 lm of a-Ge:H,F by low frequency (LF) PECVD at temperature
of TD ¼ 573 K, a pressure PD ¼ 0:6 Torr, a power
W ¼ 350 W. A mixture of GeF4 and H2 was used for the
deposition of a-Ge:H,F. After this, a 0.19 lm thick a-
Si3N4 layer was deposited by LF-PECVD on top of the
sensing layer. Finally, the sensing and the a-Si3N4 layers
were etched to define the active area of the device. Fig.
1(a) and (b) show the top and bottom view of the fab-ricated diaphragm-supported structure and the cross
section is depicted in Fig. 1(c).
The fabrication process of bridge-supported detector
consists of depositing the active and supporting layer
onto a sacrificial layer, which is etched away to form a
bridge. First, a 1 lm-thick SiO2 layer was deposited on
the p-type substrate. Deposition of an Al sacrificial layer
of 2.5 lm thick is performed. A lithographic step todefine the bumps which will define the bridge dimension
is made. Later, a deposition of a 0.7 lm-thick layer of a-
Si3N4:H by PECVD is made, this will be the mechanical
support and will provide thermal isolation to the sensing
layer. Reactive ion etching (RIE) was performed on a-
Si3N4:H for patterning the bridge and the sacrificial
layer was removed by wet etching. At this point the
bridge structure is formed. Active layer was formed bydeposition of a 0.6 lm-thick layer of a-Si1� xGexH,F
(x ¼ 0:9) by LF-PECVD at temperature TD ¼ 573 K, at
pressure PD ¼ 0:6 Torr, and a frequency fD ¼ 110 kHz.
Deposition was made from a mixture of SiH4 +GeH4.
The activation energy of sensing layer is Ea ¼ 0:29 eV,
and the electrical conductivity is rRT ¼ 2 · 10�3
X�1 cm�1. A 0.13 lm-thick a-Si3N4:H layer was depos-
ited by LF-PECVD on top of the sensing layer. Etching
the a-SiGe:H,F/Si3N4 by RIE was the following step
to define the active area of the sensor. The next step was
the opening of contacts through the a-Si3N4 layer by
RIE. Finally Al was e-beam deposited and patterned on
the active area of the sensor forming the electrical con-
tacts to the sensor. The top view of a device is shown inFig. 2(a). The active area of the sensing layers was
100 · 100 lm2, as shown in Fig. 2(a). A sketch of the
fabricated bridge structure is shown in Fig. 2(b).
746 M. Garc�ıa et al. / Journal of Non-Crystalline Solids 338–340 (2004) 744–748
3. Characterization of the bolometers
Among the figures of merit which define the perfor-
mance of bolometers are the responsivity and the de-
tectivity. The responsivity is defined as the signal
generated per unit incident power. For the case of bo-
lometers, the generated signal is an electrical signal,
voltage or current, and the incident power (Pinc) was
taken as the input signal. The output signal, when deviceis voltage-biased, was obtained from the difference in
current produced by the incident power on the device:
Iout ¼ Iir � Idark; ð1Þand responsivity is expressed as
RI ¼IoutPinc
: ð2Þ
In this way, responsivity of four devices of both struc-
tures were calculated. The values of responsivity of the
devices at different voltages are presented in Fig. 3.
Thermal behavior of a bolometer is characterized by
its thermal conductance (G) and its thermal capacitance
(C), and both parameters determine the response time
through the thermal time constant (s) of the bolometer,which is defined as the following relationship:
s ¼ CG: ð3Þ
Measurements of voltage–time curves for determining
these thermal characteristics were performed in vacuum
at pressure Pm ¼ 3 mTorr. Thermal time constant was
measured by applying a current pulse to the bolometer
0.1
1
10
Res
pons
ivity
R (
mA
/ W
)
8765432Voltage V ( V )
7654321
Noise I noise ( pA
)
Responsivity Inoise
12
46
102
46
100
Res
pons
ivity
R (
mA
/ W
)
20151050Voltage V ( V )
70605040302010
Noise I noise ( pA
)
ResponsivityI noise
(a)
(b)
Fig. 3. Responsivity of several devices in function of applied voltage.
The IR incident power Pinc ¼ 107 nW from a black body radiator
at T ¼ 1123 K. (a) Responsivity of membrane-supported detector.
(b) Responsivity of bridge-supported detector.
and measuring the voltage drop using a oscilloscope anddetermining the time to reach the steady state. The smeasured for the membrane-supported bolometer was
500–800 ms, and that one for the bridge-supported
structure was 300–500 ms.
The thermal conductance was measured by applying
a current pulse. The voltage drop on the bolometer (Vb)and the current (Ib) flowing through it were measured. A
power W ¼ IbVb is dissipated in the bolometer. For abias pulse of duration much longer than the s, the steadystate is reached, and then W and G are related by
W ¼ GðTb � T0Þ; ð4Þwhere Tb is the bolometer temperature at the steady state
and T0 is the room temperature. Tb is estimated from [7]
Tb ¼T0
1þ kT0Ea
ln RðTbÞRðT0Þ
h i : ð5Þ
The resistance of the bolometer RðT0Þ is calculated as
Vb=Ib, when the bias pulse is very narrow, in such a waythat the structure is not heated, and RðTbÞ is calculatedin the same way when steady state is reached. G was
estimated from equation (4). The thermal conductance
measured in this way was G ¼ 2 · 10�7 W/K for the
membrane-supported structure and G ¼ 5 · 10�7 W/K
for the bridge-supported structure. Since equation (3),
thermal capacitance can be estimated from calculated
values of s and G. Then, for the membrane-supported
106
2
3
456
107
Det
ectiv
ity D
* ( c
m H
z1 / 2
/ W
)
8765432
104
2
4
68
105
Det
ectiv
ity D
* ( cm
Hz1
/ 2 /
W )
20151050
Voltage V ( V )
Voltage V ( V )(a)
(b)
Fig. 4. Detectivity as a function of applied voltage for several a-SiGe
bolometers: (a) Detectivity of membrane-supported detector. (b) De-
tectivity of bridge-supported detector.
Table 1
Comparison of uncooled microbolometers
Characteristics Units Syllaios et al. [1] Iborra et al. [9] Liang et al. [10] Present work
Membrane-
supported
structure
Bridge-supported
structure
Temperature-sensing
material
a-Si:H,B GexSi1� xOy Poly-Si–Ge a-Si1�xGex:H,F
x ¼ 1 (a-Ge)
a-Si1�xGex:H
x ¼ 0:9
Activation energy, Ea eV 0.22 0.185–0.316 )0.145 0.4 0.29
TCR, a K�1 0.028 0.024–0.04 0.014–0.022 0.051 0.037
Pixel area, Ab lm2 48 · 48 100· 100 40 · 40 60 · 60 100· 100Pixel resistance, Rb X 3 · 107 (1.2–6.4)· 105 3.5 · 105 109 3· 107Thermal resistance KW�1 4 · 107 5 · 105 – 5 · 106 2· 106Thermal time constant, sth ms 11 1.8–2 16.6 100–800 300–500
Responsivity, RV VW�1 106 50–380 (at 100
nA bias)
1.6 · 104 4.2 · 106 1.9· 106 at
I ¼ 550 nA
Responsivity, RI AW�1 – – – 6.2 · 10�3 8.6· 10�2 at
V ¼ 7 V
Spectral response lm 5–14 – – 2–14 2–14
Detectivity, D� cmHz1=2 W�1 – 4.8 · 1075.6 · 106
8.3 · 108 2.6 · 106 1· 105
M. Garc�ıa et al. / Journal of Non-Crystalline Solids 338–340 (2004) 744–748 747
structure C ¼ 1:2 · 10�7 J/K, and for the bridge-sup-
ported structure C ¼ 2 · 10�7 J/K.
Detectivity relates several parameters of detectors in
order to compare performances of different devices, and
it is defined as
D� ¼ R� ffiffiffiffiffiAd
p
Inoise=ffiffiffiffiffiffiDf
p ; ð6Þ
where R is the detector responsivity, Ad is the detector
area (60 · 60 lm2 for membrane-supported structure
and 100 · 100 lm2 for the bridge-supported structure),
Inoise is the noise current, and Df represents the band-
width frequency. The calculated detectivity, from exper-imental responsivity measurements for each one of the
structures at different biasing voltage, is presented in
Fig. 4.
The noise current depends on the voltage bias and
has values ranging from 1–5 pA for a voltage range
from 2 to 8 V for the membrane-supported. Noise
current of the bridge-supported structures is one order
magnitude higher than that of the membrane-supportedstructure. The obtained characteristics are compared
with those presented by Syllaos et al. [1], Iborra et al.
[7] and Liang et al. [8], the comparison is presented in
Table 1.
Pixel resistance of membrane-supported detector has
not been optimized, but its TCR is the highest of the
compared devices. Higher conductivity is observed when
doping the a-Si:H sensitive layer, but it also yields alower TCR. Our a-Si1�xGex bridge-supported structure
overcomes this disadvantage, it presents the same pixel
resistance as in [1], and a high TCR since doping is not
used. Additionally, the obtained thermal resistance of
both structures is comparable to the one reported in
other papers. Detectivity of studied microbolometers is
lower than the reported values, this is due to the high
noise of our structure.
At this point of the work we have shown that the
sensing material here proposed will offer a better alter-
native for the fabrication of 2-D monolithic arrays for
IR image detection than that used for comparison in this
work. A future work is the reduction of the noise and
resistance of the pixel by improving the quality of theohmic contacts to our materials.
4. Conclusions
Microbolometers with thermo-sensing layer made of
a-SiGe-based films have been fabricated in two different
configurations: the membrane-supported structure andthe bridge-supported structure. Activation energy and
conductivity of sensing layer provide high TCR. The
process of fabrication is fully compatible with the IC
standard process of fabrication. Performance charac-
teristics of the fabricated devices have been studied and
present values comparable with those reported for sim-
ilar materials, which present a-SiGe based films as a
good candidates for IR uncooled microbolometers.Work in order to reduce noise and improve thermal
isolation is needed.
Acknowledgements
The authors would like to thank Mr Mauro Landa
for his help during the technical preparation of the
samples. M.G. also acknowledges CONACYT for sup-
port granted through scholarship #128942.
748 M. Garc�ıa et al. / Journal of Non-Crystalline Solids 338–340 (2004) 744–748
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