We present
here a novel, low-cost uncooled parylene bolometer. The device is made
of two layers of pyrolyzed parylene and a metal layer for interconnections.
We demonstrate that high responsivity can be achieved by tailoring the
electrical conductivity and the temperature coefficient of resistance
(TCR) using different pyrolysis conditions for each parylene layer.
Figure (1) shows a typical resistive uncooled bolometer: a free-standing
temperature-sensitive element is linked to a substrate by low thermal
conductance legs. Eq (1) shows the expression of the d.c. responsivity
(in Volts per incident Watt) for such a device. The key parameters to
obtain good performance are: high pixel-TCR and low pixel-to-substrate
thermal conductance. Most uncooled bolometers use vanadium oxide [1]
or amorphous silicon [2] as temperature-sensitive material, reaching
a TCR of about 1.5% to 3%. The suspension legs are usually made of silicon
nitride or polysilicon.
Recent work in our group showed that the electrical conductivity of
pyrolyzed parylene can be adjusted over a very wide range (from insulating
down to ≈10-2_.cm) depending on the pyrolysis conditions. We studied
the temperature dependence of the resistance of pyrolyzed parylene films.
Figure (3) shows the temperature dependence of a sample having a resistivity
of 1.9*103_.cm at room temperature. As can be seen on the temperature
follows an Arrehnius dependence. As for most materials, the TCR of pyrolyzed
parylene increases with resistivity. There is therefore a trade-off
between responsivity and signal-to-noise ratio. The measured TCR was
-4%/K for films having ~107_.cm resistivity down to -0.3%/K for films
having 10-2_.cm resistivity. In short, the large TCR of pyrolyzed parylene
enables the construction of an all-parylene bolometer as reported here.
Our device process, Figure (4), begins with a 5000Å oxide growth
and patterning. A 3_m-thick parylene-C layer is then deposited and pyrolyzed
in a nitrogen atmosphere. The temperature is raised to 800°C at
10°C/min then cooled down to room temperature at 2°C/min. The
film is patterned with O2 plasma to define the suspension legs. A second
layer of parylene (0.8_m) is deposited, then pyrolyzed at 660°C
for two hours (with the same ramping parameters as previously) and patterned
with O2 plasma to define the pixel area. Next, a Ti/Au interconnection
layer (60Å/2000Å) is evaporated and patterned. Finally,
the bolometers are released by XeF2 etching. Figure (5) shows a fabricated
free-standing device.
Table (1) shows different characteristics of interest for the two pyrolyzed-parylene
layers. The TCR of the second parylene layer at room temperature was
measured to be -1.63%/K. Current-Voltage characteristics of the bolometers
were measured in vacuum to calculate the pixel-to-substrate thermal
conductance by measuring self-heating. Figure (5) shows the resistance
and temperature rise as a function of input power for a 50*50_m2 bolometer
with two 5_m*170_m suspensions beams. The temperature rise is calculated
from the resistance change and TCR. The corresponding thermal conductance
is 5.43*10-8W.K-1. Knowing the dimensions of the legs, we can estimate
the thermal conductivity of the first layer of pyrolyzed parylene to
be 1.5 W.m-1.K-1. This is significantly less than values reported for
silicon nitride [3-4] with the added advantage of electrical conductivity,
and one order of magnitude lower than polysilicon. From the TCR and
the thermal conductance, we can calculate the responsivity to be (for
a 1 Volt bias), where _ is the absorptivity of the pixel and believed
to be close to 1 because of the carbon-like property of pyrolzed parylene.
We have successfully fabricated uncooled infrared sensors with a simple
two-layer pyrolyzed-parylene process. Electrothermal study shows that
with identical dimensions, a better thermal insulation can be achieved
while keeping high TCR. IR optical characterization as well as dynamic
behavior is currently underway.
References.
[1] P.E.
Howard et al Proc.SPIE Vol. 3698 131 (1999)
[2] Tissot JL, Infrared Phyhsics & Technology 43 (3-5) 223-228 Jun-Oct
2002
[3] M. Von Arx, O.Paul and H. Baltes JMEMS Vol 9. No.1 March 2000 136-145
[4] S. Hafizovic, O. Paul, Sensors and Actuators A 97-98 (2002) 246-252
