VDF/TrFe pyroelectric sensors have now definitely reached the level of a product. Based on a bidimensional staring array, it can be considered as a whole system with a monolithic technology processed on a silicon substrate provided with the integrated read out circuit. The paper will describe the main procedure dealing with the elaboration of a 32 x 32 focal plane array developed, in the context of the PROMETHEUS PROCHIP European Program (EUREKA), as a passive infrared obstacle detection applied to automotive. Additional experimental data suggest that this microsystem could operate in space environment

Audaire, L.

Bauer, F.

Bauer, Ph.

Simonne, J. J.

English

NASA Technical Reports Server

PYROELECTRIC APPLICATIONS
OF THE VDF-TrFE COPOLYMER
/
"2: :" ._..:
J.J.Simonne 1, Ph.Bauer 2, L.Audaire 3, F.Bauer 4
1 Laboratoire d'Analyse et d'Architecture des Syst_mes LAAS-CNRS, 7, Avenue du
Colonel Roche, F-31077 Toulouse Cedex, France
2 SOFRADIR, 43/47 rue Camille Pelletan, F-92290 Chatenay-Malabry, France
3 CEA/DTA/LETI/DOPT/S-LIR, 17 rue des Martyrs, F-38054 Grenoble Cedex, France
4 Institut de Recherche Franco-Allemand de St Louis, ISL, BP 34, F-68301 Saint Louis,
France
ABSTRACT
VDF/TrFE pyroelectric sensors have now definitely reached the level of a product. Based
on a bidimensional staring array, it can be considered as a whole system with a
monolithic technology processed on a silicon substrate provided with the integrated read
out circuit.
The paper will describe the main procedures dealing with the elaboration of a 32x32
Focal Plane Array developed, in the context of the PROMETHEUS PROCHIP European
Program (EUREKA), as a passive infrared obstacle detection (1) applied to automotive.
Additional experimental data suggest that this microsystem could operate in space
environment.
INTRODUCTION
A few years after the emergence of copolymers as pyroelectric elements, the high grade
material currently commercially available (2) and its capability to be processed on a
silicon substrate in a way similar to any microelectronic system, offer a real chance of
industrial development to these sensors. Large sectors of industry dedicated to mass
production like automotive, but also, medecine, agronomy, space.., are concerned, and
new developments of the Microelectromechanical system (MEMS) which can be adapted
to the technology of these sensors deserve to pay attention to a material, which appears as
a true challenger of 'high tech - low cost' infrared detectors already present or arriving on
the market.
incident radiation
( Radiati°n Absofoing Layeri _i_iVOF-TrFE
__ Thermal Insulator NiCr
( Silicon Substrate )
detector: P(VDF-TrFE)
Rcticul_ion
absorber
electrode
electrode ]
insulator ]polyimid¢
read out circuit
passivation
Fig.1 Basic Sensor Structure Fig.2 Detector array - Pixel cross-section
https://ntrs.nasa.gov/search.jsp?R=19960054117 2020-06-16T03:43:22+00:00Z
This paperintendsto describethe latestdevelopmentachievedin the designand the
technologyof a32x32copolymerVDF-TrFEstaringarray,associatedwith a focalplane,
andcoupledwithasignalprocessingcircuit integratedinasiliconsubstrate.
Theheterostructure(Fig.1)basicallyincludes:
- a copolymer layer (VDF,0.7 - TrFE, 0.3) allowing the pyroelectricdetectionafter
absorptionof the choppedincident infrared radiation; convertedinto a temperature
variation,it will induceapyroelectricsignalbetweentheelectrodesdepositedaboveand
underneaththecopolymer.Thismaterialis providedon topwith athinabsorberfilm.
- a silicon substratein which the readout circuit is processedin a 1.21amC.MOS
technology.
- a thermalinsulator(polyimide),sandwichedin betweencopolymerandsilicon, which
improvesthethermalsensitivityof thepixels.
ELABORATIONOFTHE STARINGARRAY
A schematicpartialcross-sectionof thearrayis displayedin Fig.2,showingthe different
layers of the heterostructure. The 'via' allows to connect electrically copolymer and signal
processing circuits.
0,0
a)
0.|
Fig.3
b)
Cross-talk 3D map of a pixel (pitch 100ttm )
a) when the whole area is covered
by the copolymer layer
b) when the copolymer is reticulated
Three main steps were successfully completed to validate this approach:
- the copolymer, which cristallizes directly in a beta polar phase, is available under a
liquid form and deposited by spin coating upon its support (polyimide+silicon). The real
difficulty is to perform the poling procedure across the whole heterostructure already
provided with the read out circuit. This has been achieved without any damage of the
electronic circuit. The method used, first developed by F.BAUER (3), consists in the
application of a very low frequency electric field (0.1Hz) at room temperature, allowing
to generate many times the hysteresis loop while the magnitude of the field is increased
continuously up to a limit slightly under the breakdown voltage of the material
(3xl08V/m). Reproducible values for the remanent polarization, equal or higher than
8_C/cm ? are commonly reached.
Fig.4 Dry etching for reticulation of the sensor array
- The low thermal conductivity of (VDF-TrFE) should allow to avoid any reticulation of
the copolymer layer in the matrix array; However, according to Broadhurst et al (4) and
confirmed by our experiments, a large part of pyroelectricity in this material is mostly due
to a coupling between piezoelectricity and thermal dilatation of the material rather than to
the variation of the spontaneous polarisation in the crystalline phase at constant volume.
For this reason and for its beneficial impact on the thermal cross-talk as well, the
reticulation by oxygen plasma etching of the different layers constituing the active part of
the detector - metal, absorber, copolymer - has been successfully tested; this procedure
allows a maximum of deformation under the effect of a temperature variation, improves
the pyroelectric coefficient, and a minimum thermal cross-talk has been also checked after
this procedure (fig.3). A SEM picture of the reticulation is shown in figure 4.
- The signal processing silicon circuit has a direct impact on the sensitivity of the sensor.
A first system (IRPY1) elaborated has demonstrated the necessity to reduce the fixed
pattern noise by a self calibration integrated at the level of each unit pixel. Therefore, a
second generation (IRPY2) has been designed, including a reduction of the read out
capacitance and of the, access resistances, allowing a significant decrease of the whole
area by a factor 2 (see the picture of both sensors on their headers).
In order to optimize the performance of the sensor, the theoretical analysis of the
response of the array has been investigated through the thermal diffusion model adapted
to a general heterostructure by Shu-Yau Wu (5), and through a second model based on a
network of distributed thermal resistances and capacitances associated to each layer, to
include the effect of the 'via' on the thermal behaviour of the sensor.
As a result, a thickness of 17p.m for the copolymer, and of 101.tm for the polyimide have
been selected in the project.(4). The thermal cut off frequency has been optimized below
50Hz for the frame rate application of 10Hz.
The read out circuit is a 32 lines x 32 columns, 100btm pitch staring array. The pixel
signals are addressed by lines and the internal data bus buffers are multiplexed to the
video amplifier. Fig.5 gives a schematic view of the array architecture whose
characteristics are detailed in Ref.(6).
Fig.5 Copolymer Focal Plane Array: Architecture
of the read out circuit
SENSORPERFORMANCE
Interest of (VDF-TrFE) in the presentapplicationdoesnot rely on its pyroelectric
coefficient,fairly low whencomparedtoothertraditionalpyroelectricmaterials(seeTable
1). Other parameterslike dielectric constant (fig.6) and loss tangent (fig.7) have
hopefullyadirectimpactonthesensorperformanceandallowthecopolymertoexhibit a
goodvoltagemerit factor.In addition,thematerialdoesnotrequirecostlypreparation,is
insensitiveto humidity,is chemicallyinert,stable,and,asalreadysaid,availableundera
liquid form,propertieswhichmakeit attractivein IR imagery.
Improvementon twoparticularperformanceshasbeen emphasized: sensitivity and noise
properties.
Material Form Curie point
*C
Pyroelectric
coefficient
Permittivity
F/m
Thcr'mal
diffusion
Voltage readout
merit factor
Charge readout
merit factor
nCb/cm2/K cm/*C
TGS Crystal 49 35 50 72 7 4.9
..L.iTaOj Crystal 620 17 43 4 2.6
PbTiOj Ceramic 490 30 200 140 1.5 2. l
PZT Ceramic > 300 35 300 110 1.2 2
P(VFz-TrFE ) Copolymer 125 4 7 20 5.7 1.5
Pyro.-electric materials : properties, performances and ability to readout.
Table 1
32x32 pyroelectric copolymer arrays
61'
0
1
Frequency (Hz)
JoB _-8.2 _C/cm2
----c>--- Pr- 6.8 _?./cm2
-----'----- Pr - 5.8_C/cm2
---o--- Pr -4._ l_C./cm2
Fig.6 tg5 versus f for several remanent polarisation Pr
(tg5 is minimum between 10 and 200Hz)
_8
-.- 6
E 5
_- 4
tkl
_q 2
u
1
0 | !
1 100 10000 1000000
Frequency (Hz)
Pr - 8.2 jlC/cm2
Pr -, 6.8 laC./cm2
Pr = 5.8 laC./cm2
Pr = 4.8 _C/cm2
Fig.7 _;r versus f for several remanent polarisation Pr
Chopper I
Integration --
Reset
ReBd out
_ollower output
oulput \
II Chopper [
SIMPLE DEC (IRPYI)
Integration --
output
DOUBLE DEC ORPY2)
Fig.8
concerning the sensitivity, a double correlated sampling (double DEC), using the
subtraction of the signals generated in both initial positions of the chopper inducing in a
pixel +AT (aperture), then -AT (shutting), allows to double the response (fig.8).
- concerning noise properties, the fixed pattern noise is ruled out by the double DEC
procedure, while other noise sources have also been shrunk by optimization of the
connexion lengths, leading to the circuit lay out of the second generation. Figure 9 and
figure 10 illustrate the improvements of the responsivity and of the Noise Equivalent
Temperature Difference (NETD) with both technologies IRPY 1 and IRPY2, including
different upper electrodes.
100E + 07
100E ÷06
l .OOE* 05
1 10 100
Chopper frequency (Hz)
1000
• IR.PYI DIS]R3 92-2
o IRPYI D[SPO 92-16
• [R.PYI DISPO 93-0_
• [P.FYI DISPO 93-04
• II_PY2 D_ 94-41
o _ DISPO 94-:36
• 13tPY2D_ 94-44
Fig.9 Evolution of the responsivity with the improvement
of the technology.
nature of the upper electrode: 92-2 thin Cr layer;
92-16 graphite spray; 93-05 thin Cr layer;
93-04 thin Cr layer; 94-41 absorber; 94-36 thin Cr layer;
94-44 reticulated absorber.
3.60
3.00
2.50
2.00
1.00
0.60
0.00
IIIII
III11
I!III
Iiill
llllJ 
llb 
I IIU
-  tT[!
r TTTTT
10 100
Chopper frequency (El[z)
-----o.---=u,v=i,,_ _==u,_cj
ll/,Yl _i.16 {.i_ ulc')
Fig.10 Noise properties for different technologies
(se fig.9 for the nature of the upper layer)
The NETD histograms presented in figure 11, with an ideal optic transmission and a F/1
numerical aperture, give a mean measured value of 0AOK perfo_Tned on all pixels of the
IRPY2 array, which could be even decreased to 0.16K as noticed through simulation on
the same figure.
im
ido
14o
0 0.$ I I .J I, 2 $ 4 • ,,J 1 j 4
(K) (K) (K)
NEDT IRPY2 9442 (Absorber) NEDT IRPY2 9441 (Absorber) NEDT IRPY2 943& (Tibia Cr electr_le)
mean value : 0.48K mean value : B.40K mean value : 0.75K
variance : 0.12K variance : 0.16K variance : 0.1$K
Optimum simulation NEDT : 0.16K
bandwidth : 8-14j.tm, optics f/l, transmission : 1
Fig.ll NETD Histograms of the IRPY2 devices
COMMENTS
This project has demonstrated the capability of VDF-TrFE to be processed as a
pyroelectric Focal Plane Array dedicated to infrared imagery, with latest performances
able to reach those presented in recent alternative solutions, namely the microbridge
resistive bolometer array developed by Honeywell (7), and the hybrid dielectric
bolometer processed by Texas Instruments (8). This copolymer thin film heterostructure
and a micromachined linear array also implemented elsewhere (9) make clear that the
Microelectromechanical systems (MEMS) will be the most predictable approach selected
for 'high tech-low cost' IR imagers.
If automotive has been the driving force of this application, this Conference is a good
opportunity to examine how space can be also considered as a potential field for this
material. Tests experimented at different temperatures have demonstrated a working
stability between -50°C and +50°C before unfolding, and in the range'-150°C/+150°C,
when operating outside. Stability with time in another useful factor in favour of
copolymer, property which emphasizes the role which could be played by this system in
the payload of a satellite: a 10 years storage does not disturb the response of the active
material. Furthermore, we must keep in mind the tremendous characteristics of (VDF-
TrFE) in piezoelectricity, demonstrated in shock compression (10) and accelerometer
applications (11). Some early successful tests have been made to examine the behaviour
in a heavy ion ambient and more investigation is likely needed to check its properties
versus other kinds of charged particles; nevertheless, in a period where the space
technology is going to know a huge transformation, capabilities of the pyroelectric and
piezoelectric copolymer cannot be ignored, even if additional data must be provided to
state whether they appear as a useful approach to the space field.
ACKNOWLEDGEMENTS
We would like to acknowledgethetechnologyteamsof LETI/LIR, LAAS and ISL for
their contributionto thiswork, andPROMETHEUS-PROCHIPfor financialsupportof
this program.
REFERENCES
(1) EUREKA/PROMETHEUS/PROCHIP,roject2211
seealso D.Esteve,P.A.Rolland,J.J.Simonneand G.Vialaret, SPIE,2470, Infrared
ImagingSystems:Design,Analysis,Modeling,andTestingVI, 386(1995)
(2) Supplier:Piezotech/ISL,BP34,F-68301,Saintlouis, France
(3) F.Bauer,Frenchpatent8221025;US patent4611260(4) M.G.Broadhurst, G.T.Davis, J.E.McKinney, R.E.Collins, Journal of Applied
Physics,49, N°10,4992,Oct.1978
and LJ.Simonne, Ph.Bauer, L.Audaire, F.Bauer, PROC. Workshop on the Technology
of Ferroelectric Polymers, in press, Albuquerque NM, USA, Oct. 17-19 (1995)
(5) Shu-Yau Wu, IEEE Trans. on ED, 1, 88 (1980)
(6) Ph.Bauer, L.Audaire, Ph.Rambaud, M.Pirot, F.Bauer and J.J.Simonne, PROC. of
the ESSCIRC'95 Conf., Lille, F, 330 (1995)
(7) R.A.Wood, SPIE, 2020 Infrared Technology XIX, 34 (1993)
(8) Ch.Hanson, H.Beratan, R.Owen, M.Corbin and S.McKenney, SPIE, 1735, Infrared
Detector: State-of-the-Art, 17 (1992)
(9) N.Neumann, R.Kohler and G.Hofmann, PROC.Ferroelectric Polymer Shock and
Dynamic Sensor workshop, St Louis, F, R126/94, 23.1 (1994)
(I0) F.Bauer, R.A.Graham, M.U.Anderson, H.LeFebvre, L.M.Lee, and R.P.Reed,
PROC 8th IEEE Int.Symp.on Application of Ferroelectrics ISAF'92, Greenville SC,
USA, 273 (1992)
(11) B.Andre, J.Clot, E.Partouche, J.J.Simonne and F.Bauer, Sensors and Actuators A,
33, 111 (1992)



Pyroelectric Applications of the VDF-TrFE Copolymer

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