This paper presents an uncooled, room-temperature, all-parylene bolometer. The device is made of two layers of pyrolyzed (or "carbonized") parylene and a metal layer for interconnections. We demonstrated 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

Konishi, Satoshi

Liger, Matthieu

Tai, Yu-Chong

Caltech Authors - Main

UNCOOLED ALL-PARYLENE BOLOMETER 
Muiihieu Liger, ’Saioshi Konishi, ’Yu- Chong Tui 
‘California Institute of Technology MC 136-93 
Pasadena, CA91 125 USA 
E-mail: matt@mems.caltech.edu 
2Department of Mechanical Engineering, Faculty of Science and Engineering, Ritsumeikan University, 
Kusatsu, Shiga JAPAN 
I 
ABSTRACT 
This paper presents an uncooled, room-temeprature, all- 
parylene bolometer. The device is made of two layers of 
pyrolyzed (or “carbonized”) parylene and a metal layer for 
interconnections. We demonstrated that high responsivity 
can he achieved by tailoring the electrical conductivity and 
the temperature coefficient of resistance (TCR) using 
different pyrolysis conditions for each parylene layer. 
1. INTRODUCTION 
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. 
(1) AV aH*Jl %(KP-’)  = -= -> 
P G  
where a is the TCR of the sensing element. R is the 
bolometer resistance, G is the pixel-to-substrate thermal 
conductance and q is the bolometer absorptance. The thermal 
time constant is given by: 
- 
where C is the thermal capacitance. The key parameters to 
obtain good responsivity are: high pixel-TCR and low pixel- 
to-substrate thermal conductance. However, as we try to 
decrease the thermal conductance, we must be able to 
decrease the thermal conductance. Most uncooled bolometers 
use vanadium oxide [l] or amorphous silicon [2 ]  as 
Suspension leg Incoming Radiation Resistive temperafllre Sensor 
R, with TCR a and 
absorptivity q 
with thermal P(W) 
F&re 1: SfundurdResfsfive Eolomefer Sfmcfure 
0:7803-8265-x104/$17.00 02004 IEEE , 593 
temperature-sensitive material, reaching a TCR of about 
1.5% to 3%. Another possible material is YBaCuO [3].The 
suspension legs are usually made of silicon nitride or 
polysilicon. In the case of silicon nitride legs, it is necessary 
to have another layer for electrical conduction. Here, we 
propose a bolometer using pyrolyzed parylene both for the 
temperature-sensing element and for the suspension legs. 
2. THERMAL PROPERTIES OF PYROLYZED- 
PARYLENE 
Pyrolyzed parylene as a MEMS “sacrificial” material was 
first reported by Hui et al in 1998 [4], in which carbonized 
parylene is subsequently burned away in O2 environment at 
high temperature. Other than that, It is also known that 
pyrolysis of polymers can lead to electrically conductive or 
partial conductive films [SI. Interestingly, our recent work 
showed that the electrical conductivity of pyrolyzed parylene 
can he adjusted over a very wide range (from insulating 
down to -1O~’S2.cm) depending on the pyrolysis conditions 
[6].  We studied the temperature dependence of the resistance 
of pyrolyzed parylene films. Figure (2) shows the 
temperature dependence of the resistance of a sample having 
a resistivity of 1.9*103S2.cm at room temperature. As can be 
seen on this figure, the conductivity follows an Arrehnius 
dependence: 
oao, e , (2 )  * -EJkT 
where E, is the activation energy. 
2.6 
2.4 
2.2 
- 2.0 
2183.6rT-4.921 
5 
3 1.8 
1.6 
1.4 
0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 
1lT (K-‘) 
Figure 2: Resisfance Temperature Dependence 
for pYro/yredPaylene 
For a resistivity having an Arrehnius dependence, the TCR is 
given by, 
(3) 
4 
L-P ’ 
a = -- 
Like other materials often used in uncooled infrared sensors 
(vanadium oxide and amorphous silicon), the TCR of 
pyrolyzed parylene increases with resistivity. Figure (4) 
shows the TCR of various films having different resistivities 
(obtained by pyrolysis at different temperatures). The TCR 
of pyrolyzed-parylene does show a logarithmic dependence 
on the resistivity. The measured TCR was -4%K for films 
having -1O’Q.cm resistivity down to -0.3%K for films 
having -lO.’R.cm resistivity. The corresponding activation 
energies are 0.023eV and 0.3 l e v  respectively. Because 
higher bolometer resistance leads to higher thermal noise, 
there is a trade-off between high responsivity (given by high 
CR) and signal-to-noise ratio (given by low resistance). 
I 
, 
$ 
2 1- 
-- 3 -  
2-  
L L .  
- 
. . . . . . . . . . . . . . .  ..................... .................. 
4 ;  
......... I ...... ~; ..................... 
............. ,.... . . . . . . .  , ........ 
I 
.................. rn’ i 
I l l 1  : 
........ ..... ......................... ~ : ~ 
0 , :  , I , I , . , I ,  : I  
10” IO-’ IO’ i o3  io5 10’ io9 
RoomTernperature Resistivity (n.cm) 
F&re 3: TCR V~Y. resisiiviQ for various 
air, it was found that the-resistivity of pyrolyzed parylene is 
sensitive to moisture. Therefore, the TCR measurements 
were performed in a vacuum chamber. Figure (4) shows the 
resistance drop of a pyrolyzed-parylene film when exposed 
to air after being stabilized in vacuum. This resistance 
change is reversible. However, this sensitivity to moisture is 
not a problem for our application since uncooled bolometers 
operate in vacuum (for thermal insulation purposes). 
1.04 I 
Exposure to air (min) 
Figure 6 Resistance drop when aposed 
to air ajer vacuum 
2. DESIGN 
Since our ultimate goal is the build an uncooled IR focal- 
plane array, the bolometer design is similar to that shown on 
figure (I) .  The pixel size was chosen to be SOx50pm2, a 
standard size allowing acceptable resolution with a chip- 
sized array. For this geometry, the total pixel resistance is 
given by: 
= 2 4  + (4) 
assuming we can neglect contact resistances. Upon incoming 
radiation, most of the temperature increase occurs on the 
pixel. Therefore, to maximize the relative change in total 
resistance, the resistance of the pixel must he dominant over 
the resistance of the suspension legs. However, with such a 
layout, it is obvious that the number of electrical “squares” of 
the suspension legs is greater than that of the pixel (=I). For 
that reason, the sheet resistance of the suspensions legs must 
he much smaller than the sheet resistance of the pixel. The 
process-tunahlity of the properties of pyrolyzed-parylene 
allows us to fabricate a bolometer meeting this requirement 
with a simple two-layers process. To allow comparison with 
VOX-based bolometers the target TCR was set around -2%, 
which according to figure (3) would he obtained for a 
resistivity in the order of I O 2  f l a n .  The width of the 
suspensions legs was designed to be 5pm and their lengths 
vaned from 50pm to 17Opm, corresponding to a number of 
resistor squares varying from 10 to 34. Therefore, for the 
total bolometer resistance to be dominated by the pixel 
resistance, the resistivity of the suspension legs should he on 
the order of IO.’ to 10.’ R a n  (if the thicknesses are 
comparable). 
3. FABRICATION 
Process Flow 
Our device process, figure (5), begins with a 5000k, oxide 
growth and patterning. A 3pm-thick parylene-C layer is then 
deposited and pyrolyzed in a nitrogen atmosphere. The 
temperature is raised to 800°C at 10”Cimin then cooled 
down to room temperature at 2Wmin. The resulting film is 
patterned to define the suspension legs. The pyrolyzed- 
parylene etching is done in a PE11 plasma etcher with 400W, 
200mT of 0 2  using a photoresist mask. The etching rate of 
pyrolyzed-parylene was found to be comparable to that of 
parylene (-18008Jmin) for these same settings. A second 
layer ofparylene (0.8pm) is deposited, and then pyrolyzed at 
660°C (with the same ramping parameters as previously). 
For better repeatability, the samples are being kept at the 
pyrolysis temperature for 2 hours. The second layer of 
pyrolyzed parylene is patterned define the pixel area. Next, a 
TVAu interconnection layer (608J2000A) is evaporated and 
patterned. Finally, the bolometers are released by XeF2 gas- 
phase etching. Figure (6) shows a fabricated free-standing 
device. 
Processing Issues 
Typically, parylene depositions involve a prior coating of 
A174 for adhesion promotion [7]. However, it was found that 
594 
5000A SiOz growth and patterning; I"  
parylene deposition (3pm) 
I I 
1'' Paqlene pyrolysis at 800°C 
(shnnks to 0 6pm), patterning 
_1 ?-----------; 7 
2d Parylene deposition (O.Spm), pyrolysis 
at 660°C; patterning 
TdAu Evaporation (60&2000A) and 
patterning, XeF, release 
0 Palylene pyrolyzed at TUAu 
650°C 
Figure I. Processflow 
this procedure leads to poor adhesion of the films after 
pyrolysis. On the other hand, the adhesion of pyrolyzed 
parylene on Si02/Si wafers that have not been coated with 
A174 was excellent. This is also part of the reasons why we 
chose to pattern the parylene after pyrolysis rather than the 
opposite. It was also observed that pyrolysis of pattern 
parylene leads to undesirable border effects, probably due to 
the pyrolysis-induced stress. Finally, due to the isotropic 
etching, patterning the parylene after it has been shrunk (4 to 
5 times) by pyrolysis minimizes undercut. 
4. CHARACTERIZATION 
Sheet resistance and contact resistance were measured by 
Greek-cross structures. The contact between the pyrolyzed 
parylene layers and Ti/Au was found to he ohmic with a 
specific contact resistance of 3 . 5 ~ 1 0 ~ ~  Q a n 2  for first layer 
and 3.5 Q.cm2 for the second layer. The contact between the 
first and the second layer of pyrolyzed parylene was also 
ohmic with a specific contact resistance of 1.57 C l a n z  The 
TCR of the second parylene layer around room temperature 
was measured to he -1.63%K. Table (1) shows different 
characteristics of interest for the two pyrolyzed-parylene 
layers. The ratio between the sheet resistance of the first and 
the second layer is 8 . 7 ~ 1 0 ~ .  Therefore, for the chosen 
Figure 6. Fabricaiedbolomeier 
geometries, the total resistance of the bolometers is indeed 
dominated by the temperature-sensing element obtained from 
the second layer of pyrolyzed parylene. 
Table 1: Patyfene Lyvers Properiia 
After wire-bonding and packaging, the bolometers were 
placed in a vacuum chamber (<lmT) and stabilized for 
several hours to eliminate any potential moisture-related 
drift. Current-Voltage characteristics of the bolometers were 
measured with an HP4145 working as a voltage 
sourceicurrent monitor. A hold time of 1 second was used at 
each bias to ensure thermal steady-state. Figure (7) shows the 
IV curve for a 50x50pm2 bolometer with two 5pmxl70pm 
suspensions beams. The upward curvature seen on this figure 
indicates self-heating (the TCR of pyrolyzed-parylene being 
negative). Unreleased bridges do not exhibit this self-heating. 
Figure (8) shows the resistance and temperature rise as a 
function of input power. The temperature rise is calculated 
from the resistance change and TCR. The corresponding 
thermal conductance is 5 . 4 3 ~ 1 0 ~ ~ W . h ' .  Knowing the 
dimensions of the legs, we can estimate the thermal 
0.0 1 , , , , , , , , , , ~ I 
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 
V(V0lt) 
Figure % Bolometer ZVcharacieristic 
595 
conductivity of the first layer of pyrolyzed parylene to be 
K ~ ~ = I  .5 W.m.'.K-'. Similar calculations on other bolometers 
having different geometries (smaller suspensions legs) lead 
to lower thermal conductivities 1.1 W d . K ~ ' < ~ , , < 1 . 5  W.ni  
'.K~'. This is comparable to values reported for PECVD 
silicon nitride [8-91 and lower than LPCVD nitride, with the 
major advantage of providing electrical conductivity. It is 
also one order of magnitude lower than polysilicon [IO]. 
From the TCR and the thermal conductance, we can expect 
the responsivity to be R=3*16 *q (for a 1 Volt bias, and 
5pmx170pm suspension legs), where q is the absorptance of 
the pixel. Although the sensitivity to incoming radiation was 
qualitatively observed, the absorptance of pyrolyzed parylene 
thin films for the wavelengths of interest (e.g. 8pm to 14pm 
for thermal imaging) still needs to be characterized. 
[31 
141 
r51 
Input Power (W) 
.Figure 8.. PkelResistance andtemperatwe rise 
as ajmciion of inputpower 
However, because of the carbon-like properties of pyrolyzed 
parylene, we can expect the absorptance to be close to I. 
CONCLUSIONS 
We have successfully fabricated uncooled infrared sensors 
with a simple two-layer pyrolyzed-parylene process. 
Electrothermal study shows that pyrolyzed-parylene is a 
promising candidate to replace silicon nitride and polysilicon 
for the thermal insulation, while it can also be used to 
achieve high-TCR thin films for the pixel. IR optical 
characterization as well as dynamic behavior is currently 
underway. Future work also includes the development of a 
purely surface-micromachining process, and the fabrication 
of a bolometer array. 
ACKNOWLEDGEMENTS 
This work was supported by the NSF Center for 
Neuromorphic Engineering (CNSE) at Caltech. The authors 
would like to thank Trevor Roper, Tanya Owen and Ted 
Harder for their assistance. 
A Semiconductor YBaCuO Microbolometer for 
Room Temperature IR Imaging,'' A. Jahanzeb,C. M. 
Travers, Z. Celik-Butler, D. P. Butler, and S. Tan, 
ZEEE Transactions on Electron Devices,vol. 44, pp. 
1795-1801, 1997 
S. Ranganathan, R. McCreery, S.M. Maji, and M. 
Madou, "Photoresist-derived for 
microelectromechanical Systems and 
Electrochemical applications", Journa/ of The 
E/ect~oche~;ca/Socie~, l47(I), pp.277-282,2000 
E. E. Hui, C. G. Keller, and R. T. Howe, 
"Carbonized parylene as a conformal sacrificial 
layer", 8th SoMSfate Sensor and Achrator 
Worhhop TechnicalDigest, Hilton Head, 1998, pp. 
256-260 
S .  Konisbi M. Liger, T. A. Harder and Y.C. Tai, to 
be published in ZEEEMEMSV4 TechnicalDigest 
Product Specifications, A-174' Slane Promotion, 
Specialty Coating Systems, Inc., Indianapolis, IN, 
Phone: (800) 356-8260. 
M. Von Am, 0.Paul and H. Baltes JA4EMSVol9. 
No.1 March 2000 136-145 
S. Hafizovic, 0. Paul, Sensors andktuators A 97- 
98 (2002) 246-252 
Angela D. McConnell, Srinivasan Uma,, and 
Kenneth E. Goodson, "Thermal conductivity of 
doped polysilicon layers", Joz/rna/ of 
~rooe/ec~romechanica/ Systems, Vol 1 0 (3), 
Sep 2001 360-369. 
REFERENCES 
[l] P.E. Howard etu/Proc.SPIE Vol. 3698 131 (1999) 
[2] Tissot JL, Infrared Phyhsics & Technology 43 (3-5) 
223-228 Jun-Oct 2002 
596 


Uncooled all-parylene bolometer

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Uncooled all-parylene bolometer

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