Pyroelectric Detector Arrays For Ir Imaging

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VIVEK CHANDRA VENGALA EE560 MICRONANOELCTRONIC SYSTEM DESIGN PYROELECTRIC DETECTOR ARRAYS FOR IR IMAGING

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LISA O’REILLY

APPROX. 24 HOURS

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PYROELECTRIC DETECTOR ARRAYS FOR IR IMAGING

INTRODUCTION : The radiation maximum of a black body ay 300k is in the infrared (IR) regime at a wavelength of approx. 10micrometers. To obtain electronic images in the IR regime, two different physical principles are frequently employed: 1. quantum detectors turn absorbed photons into the generation of electrical carriers, 2. thermal detectors work on the temperature increase upon the absorption of radiation and a consequent electrical sensing of the temperature change.

Quantum detectors : Quantum detectors are based on the internal photoelectric effect, i.e. they are utilizing the electronhole pair generation by absorbing photons in the space charge region of a semiconductor junction. The photon energy hc/ λ (where h denotes planck’s constant, c is the velocity of light, and lamda the wavelengths) needs to exceed the characteristic energy barrier ∆Wb for the carrier generation. For instrinsic semi conductors, which are frequently employed in IR detectors, ∆Wb is the band gap. For extrinsic semiconductors, it would be ionization energy for dopant. Or it can be the band gap between two subbands of a multiquantum well system. The most important quantum IR detector material for ir imaging arrays is (Cd1-xHgx)Te. In this intrinsic compound semiconductor, the band gap energy and hence the spectral range of the sensitivity can be tuned between from 1.5eV(lam=0.8 µm) for x = 0(CdTe) to zero for x = 1(HgTe). While quantum IR detectors exhibit high temperature resolution and can be fabricated with sensitivities almost up to the theoretical quantum efficiency, they show two strong disadvantages: firstly, their production is very costly, and secondly, they need to be cooled during the operation. For this reason, their spread has been limited mainly to night-visionsystems in military applications. However , there is a large potential for civilian and commercial applications ranging from the fir detection, night-driving assistance, surveillance, and building diagnosis. For this market, thermal detectors based on pyroelectric detector arrays are perfectly suited. They operate without cryogenic cooling and can be fabricated by standard MEMS and microelectronic technology. Figure 1 shows a pyroelectric infrared image, while typical infrared imaging cameras for civilian and commercial applications are shown in Figure 2.

Pyroelectricity is an extraordinarily sensitive principle to detect temperature changes. Although the first pyroelectric material was discovered some 2300 years ago by the Greeks, it is not until the 1950’s when pyroelectricity was also employed for technical use. In the last decades, proelectric detectors have been mainly built as single element systems made from

small bulk single crystals of pyroelectric materials. These sensors react to very slight but sudden changes of the temperature within their sight field. They are typically applied for intruder and fire alarm systems and, for example, as contactless lighting switches in buildings. In addition, they are used as temperature sensors for air conditioning systems, for biomedical systems, flow meters, etc..

Figure 1 : IR image of a house with major thermal insulation damage.

Figure 2 : a general purpose camera (left),Commercial system for a police car (center), and auto motive driving aid (right).

OPERATION PRINCIPLE OF PYROELECTRIC IR DETECTORS: Pyroelectric Detectors :

Pyroelectric materials develop a charge in response to a change in temperature. Pyroelectric detectors are widely used in motion detection applications for home security and automation systems. These detectors are commonly available in single-, dual-, or quad-element versions. Pyroelectric detector arrays can also be obtained, but these are generally used for imaging applications. A single-element detector responds to any temperature changes in the environment and therefore must be thermally compensated to remove sensitivity to ambient temperature. Dual-element detectors have the inherent advantage that the output voltage is the difference between the voltages obtained from each of the elements of the detector, which subtracts out environmental effects.

The pyroelectric detector converts incident thermal radiation into an electrical signal. This conversion takes place in three steps—the incident thermal radiation results in a change in temperature, altering the charge density on the electrodes. An electrical signal is generated by a preamplifier or impedance converter. The current generated by incident radiation is proportional to the change in temperature as well as the area of the detector elements.13 If iP represents the current flowing through the device on which an incident thermal radiation causes a temperature change _TP, then iP = pAs dT/Pdt where p is the pyroelectric constant and depends on the material, and As is the area of the elements of the detector. Since detection of human motion is of interest to us, pyroelectric detectors seem ideal since they respond only to changes in the heat flux that is associated with motion. This also helps to cancel out ambient fluctuations. The important characteristics of the pyroelectric detector that we used are summarized in Table 1. The power radiated by a human body is radiated isotropically and the power at the detector is a function of the area of the detecting elements as well as the distance between the source and the detector. The maximum range at which the sensor will work can be determined using the noise equivalent power _NEP_ value from Table 1. This parameter indicates the power level at which the SNR is 1.

THE PYROELECTRIC RESPONSE : The pyroelectric charge deltaQ generated by a temperature change deltaT of a pyroelectric element(figure 3) with electrode surface A mounts to ∆Q = p.A.∆Q,

(1)

where p is the pyroelectric coefficient.For infrared radiation detection, deltaT is the result of the absorbed radiation power during a certain time interval. The thermal properties of the device have to be suitably optimized by desin and materials choice for obtaining a maximum deltaT at the required response frequency. Thin film devices exhibit small heat capacities. This has to be compensated by a good thermal insulation in order to obtain large enough thermal time constants.

Micromachining is the main tool to reduce thermal conduction. Micromachining pays off especially well in case of arrays, for which the thermal crosstalk can be very much reduced. If the device is not operated in vaccum, air convection to the surrounding walls is also important. The temperature change is obtained from the heat flows, as sketched in Figure 4. First of all, some fraction ή of the infrared radiation (power p) falling onto the element is absorbed, i.e., transformed into heat. The temperature change at a given heat input ή.P.∆t depends on the heat conductance G to the surrounding heat reservoir (heat sink) at temperature T0 and heat capacity H of the element. Denoting by Ѳ= T-To the temperature increase above the constant heat sink temperature T0 , the increase of the internal energy of the element by H.∆Ѳ is equal to the heat input (ή.P.∆t) reduced by the amount of heat flowing to the heat sink of G.Ѳ.∆t. This yields the following differential equation. G. Ѳ + h . ðѳ/ ðt = ή . P

(2)

In the case of stationary, sinusoidally modulated radiation power P= Pw . ejwt , the stationary solution is readily obtained, yielding a temperature modulation amplitude of: Ѳw = ή.Pw/G +iw.H ,

|Ѳw| = ή. Pw/√1 + w^2.Ĩ^2th

(3)

The thermal time constant t = H/G has been introduced. There are thus two frequency domains separated by the inverse thermal time constant. On the low frequency side, the the temperature modulation amplitude is governed by the heat loss to the surrounding; on the high frequency side the heat capacity determine the thermal behaviour. Combining Eqs. (1) and (3) , the pyroelectric current with frequency w is derived as: Jw = P . A . w. |Ѳ |

(4)

The current responsivity (i.e the specific response) is the current per P of radiation power falling onto the detector element: Rj (w) = Jw/Pw = p.ή.w.A/G√1 + W^2.T^2Tth

(5)

Alternatively, it is also possible to measure the voltage across a parallel resistor Rp. In bulk devices, a value of 10 GΩ is typically applied (Rp should not exceed the gate impedence of the amplifier.) In thin film devices, it might be possible to avoid this parallel resistor, because thin film capacitors exhibit larger leakage currents than bulk capacitors. In the following we will consider Rp as an effective value, composed of the film resistance and the eventually mounted external parallel resistor. At higher frequencies the dielectric loss tangent tanð comes also into play. The conductance Y of the complete element (thus including the parallel resistance) is obtained as: Y(w) = 1/Rp + wC . tan ð + i.wC, |Y(w)| = 1/Rp √(1 + wtel tanð)^2 + w^2 .Tel^2

(6)

Heat content, C

H∆T

Rp V

Heat loss; G(T-T0)

Heat sink, T = T0

Capacitance

Parallel resistance

The electrical time constant Tel = Rp. C has been introduced. The voltage response is proportional to the impedence Z = 1/Y . Above the angular frequency Tel , the impedence decreases as 1/w . The pyroelectric element thus works like a low pass filter , cutting off the high frequency response. Omitting the small term in tan ð the complete voltage responsivity is written as: Rv(w) = |Z(w)| . Rj(w) =

p . ή .w . A . Rp G.√1+w^2.T^2 . √1+ w^2.Tel^2

(7)

The generak trends of the responsivity functions can now easily be sketched as a function of the frequency, provided we know the relative size of the two time constants. In the bulk devices one generally encounters Tel < Tth. For thin film devices the opposite is true. The down scaling i.e. the transition from the bulk to the thin film element, consists essentially in the shrinkage of the thickness of the pyroelectric material. The parallel resistor is adapted to the input impedance of the amplifier and typically amounts to 10GΩ for thin film and bulk materials. The heat conduction to the heat sink is difficult to decrease by the same amount as the thickness, because bulk pyroelectric materials are good thermal insulators. So we may conclude that the ratio of the time constants scales roughly as: Tel / Tth = RpC / H /G ά C/H ά (TP)^-2

(8)

This strong dependence on the element thickness tp makes the ratio switch from less than one to larger to one when scaling down from a single crystal to a thin film. The resultant frequency behaviour is schematically shown in figure 5. The voltage response for thin film devices may look similar as for bulk sensors. However, the time constants have the opposite order (i.e., Te > Tth) . As one consequence, the voltage response in this intermediate region is determined by other parameters. Approximative formulas for this region are obtained by the appropriate approximations in the voltage responsitivity: Thin film :

bulk : Rv ( I I ) = ή . P. A/ C . G

Rv = ή . p . A . Rp/ H

(9)

In thin film devices the parallel resistance is thus not important for the voltage response. This is very advantageous, as the film resistivity is not very precisely controllable. The mounting of parallel resistors could thus be avoided. In case of the thin film sensor, Eq (9) can be further modified by observing that C = ε ε0A/tp: Rv(II) = ή . p . tp/ ε ε0 . G

(10)

Figure 5: Log-log scheme of current and voltage response,together with the impedance for the typical thin film situation where the thermal time constant is much shorter than the electrical one. Thermal wave length effects are neglected.

Figure 6: Voltage and current responsitivity os a linear array element measured in air and in vaccum. The curves are calculated, the points are measured. This shows that the responsivity is essentially independent of the surface A. (The same can also be concluded for the bulk behaviour, Since the heat capacity is proportional to A). An optimization of the voltage response requires rather thick films and a very good thermal insulation. The figure of merit of the pyroelectric thin film material for the voltage response is therefore: Fv = p/ε ε0

(11)

This assumes that the limiting capacitance is provided by the material and not the read-out circuit. In case of bulk detectors this value is often devided by the specific heat capacity: Fv = p/(cpεε0).

Although not relavent for thin films, it might be useful to use the latter figure of merit in order to be compatiable with the more frequently applied definition. The upper frequency region (III) appears to be the best for current detection. Applying the appropriate approximations for w > 1/Tth, and w >> 1/Te,the current response is obtained as: Rj(III) = ή . P. A/H

(12)

Here a small thickness,giving a small heat capacity, and a large surface seem to be the best choice. However, one has to keep in mind that the heat conduction has to be small enough for obeying w > 1/Tth. One observes again that the responsitivity does not depend on A, if the design is such that the volume defining the heat capacity has the same area as the absorbing layer. If a device is scaled down in asurface, the surface, the current responsitivity does not change. The impact of the heat conductance G in region II, and its unimportance in region III, according to Eqs 10 and 12, is nicely demonstrated in Figure 6, which depicts the response of a pyro electric detector in air and vaccum, i.e. once with air convection (large G), and once without air convection (small G). NUMERICAL VALUES : The calculated curves for the responses as a function of frequency follow quite well the experimental behaviour as long as the dielectric, pyroelectric, and thermal properties do not vary with the frequency. The numerically simulated thermal behaviour , as obtained from finite element calculations, correspond well to the thermal parameters H and G derived from the pyroelectric measurements (responsivity of Figure 6). The responsivities and the detectivity of a focal plane element can be predicted by scaling down heat capacity and the thermal conductance to the dimensions needed for thermal imaging. Below, an example is given with a 100 × 100 µm area and absorptivity of 1. The heat capacity and the heat loss have been estimated for an element on a membrane operated in air, according to the experimental values found for the responsitivities in Figure 6. For such a small surface, the air convection is less important. This results in a gigher voltage responsivity. Peak values of roughly 40,000 V/W, 50 µA/W and 6.4 ×10^8 cmHz^1/2/W ( see figure8 ).

Pyroelectric coefficient p = 500 µC/m^2/K

Pyroelectric Film Thickness tp = 1µm

Dielectric Constant

Heat capacity

H = 6.4×10^-8J/K

ε = 250

Dielectric loss

tan ð = 0.01

Heat conductance

G = 4×10^-6W/K

Specific resistivity

p = 1.0×10^9Ωm

Parallel resistor

Rp = 100 GΩ

Heat convection coeff

γ = 50 W/m2/K

Thermal time const

Tth = 16 Ms

Specific heat capacity

cv = 3.3 MJ/K/m3

Absorption coefficient

ή = 1.0

Capacity

C = 22pF

Electric time const

Te = 2.2 s

Table 1: Numerical values for a 0.01 mm2 model element ona thin ceramic membrane, and some cooling by air convection.

Figure 8 : Calculated responsivities and detectivity for a 0.01 mm2 large PLT element, taking typical published parameters, as given in Table 1.

Characterization of Focal Plane Arrays for Thermal Imaging : The detectivity D is used to characterize the sensitivity of an IR detector independently od size and detection bandwidth, within the limits described earlier. D is a poor descriptor for pyroelectric sensors. However, for thermal imaging applications, D alone does not sufficiently characterize the complete device, which also contains an optical part. The weakness is not only in the optical part of the formulation. The quantity of interest is the minimal temperature difference of a black body target with respect to the background temperature one still can detect. It is called the “ noise equivalent temperature difference” , NETD. In order to determine this quantity one has to measure the noise voltage Vn of the detector when the target temperature by ∆T and to measure the signal voltage Vs(∆T). The NETD is derived as: NETD = ∆T/Vs/Vn ∆T is equal to the NETD if the signal is just equal to the noise voltage. Of course, the NETD can be related to the NEP and D of the detector knowing the optical knowing the optical properties of the imaging system. The temperature increase ∆T has to be such that the increase of the power falling onto one pixel of the focal plane array equals the NEP. The power increase proportional to dP/dT∆T in the wavelength interval ∆λ detected, where P is the power density emitted by the target surface . It is further proportional to the FPA pixel area Ad, and the transmissivity t of the topics.

Realized Devices, Characterization, and Processing Issues : There are various fabrication concepts for pyroelectric devices. Si is not the best suited substrate for pyroelectric thin film growth. Still, Si is the best suited substrate for all other fabrication steps, such as semiconductor processing and micromachining. For this reason some concepts have been developed to grow the pyroelectric film on an other substrate and later glue it onto Si wafers. Because of the requirements on thermal insulation, the substrate material underneath the pyroeletric pixel is removed and cantilever or membrane is formed using micromachining techniques. There are two different approaches to micromachining.

Linear arrays and small 2-d arrays are often fabricated by bulk micromachining which means that the bulk of the Si wafer is removed by back side etching to isolate the pyroelectric thin film elements. This technique requires an aligned double side lithography. The processing steps are not very critical. Linear arrays with 16 to 64 elements are applied in infrared gas detectors arrays. They exhibit a very high sensitivity and are able to discriminate different gases, such as different hydrocarbons or CO2 and CO . such a product is commercialised by IR Microsystems. Bulk micromachining is less suitable for large 2-D arrays due to the brittleness of membrane. Additionally, no transistors of the read out circuit can be placed underneath the pixel cell, since there is no material left. For this reason, large arrays have to be manufactured by Surface micromachining techniques. This term comprises processing steps which act on the surface of the device side of the wafer, again to thermally isolate the pyroelectric elements.This approach allows to integrate the complete microelectronic read-out circuit into the wafer underneath the pyroelectric detector array. Although double side lithography is not required, surface machining processes are more challenging. For example, a protection of the devices during Si etching is necessary. This problem can be solved by the use of easily removable sacrificial layers. They can be an additional layer, as indicated in figure , or they can be obtained by a local transformation of Si to porousSi, which can be removed easily afterwards. With the sacrificial layer techniques, only small distances to the Si substrate are achieved, making the use of vaccum imperative.

Figure 10 : Si wafer cross section to demonstrate different micromachining techniques for thermal insulation of suspended structures: (a) bulk micromachining (b) surface micromachining by front side etching of the Si substrate (c) surface micromachining by sacrificial layers

Figure 11: Typical structure of pyro electric elements of a linear array on a thin membrane fabricated by micromachinig. The elements are contacted to pads in the other direction than the one seen in this circuit. EXAMPLE : Membrane structure obtained by bulk micromachining for 1-D arrays A stress compensated membrane layer of Si3N4/SiO2 or low stress nitride is coated on both sides os double side polished wafer. This coating fulfills the following functions: It serves as a mask for backside etching in KOH or in an equivalent base. Second it serves as a support of the pyroelectric elements exhibiting a low thermal conductivity. Bottom electrode and pyroelectric film are deposited by sputtering and CSD, respectively. The top electrode is deposited and patterned by a lift-ff technique before a quartz layer is sputter deposited for reduction of parasitic capacity below the contacts pads. Windows down to the top electrodes are opened by a CF4 reactive ion etching. The PZT elements on the membrane part are etched free in a HCL:HF solution, leaving only narrow bridges between the elements and the bulk Si part, as needed for seperation of bottom and top conductor. The platinum bottom electrode is removed between the elements by electromechanical etching. This etching technique does not attack the membrane material. After deposition and patterning of the conductor lines,pads and absorbing layer, the Si is removed below the elements by back side etching, as defined by a window in the back-side nitride layer, in order to obtain the result shown in figure 12. The 0.9µm thick membrane with a specific conductivity of 2 W/m/K gives a fairly good thermal insulation, Which allowed to obtain rather high voltage responses at 1 HZ of 800V/W in air. The membrane roughly doubles the heat capacity of the pyroelectric element. Rather long thermal time constants of 28ms in air and 104ms in vaccum have been obtained with such devices. Siemens has developed a 11×6 array for thermal imaging. Sputter deposited PZT 20/80 films were applied and D of 3×10^8 cmHz1/2/W is reported for 10 Hz. This Si cut allows fabrication of narrower Si bridges, which are needed in this design for carrying the electrical connection lines to the pads to which the read out electronics is connected.

Figure 12 : Top view on 50 element array with 200µm period obtained with bulk micromachining, membrane size : 2×11 mm. The black platinum absorbers, the Cr-Au contact lines, the membrane layers between the elements, and the Sio2 layer for reduction of parasitic capacitance are well visible.

Figure 13 : View on 11 × 6 array of Siemens.

Read-Out Integrated Circuits (ROIC) : If it is the primary goal to determine the slowly fluctuating temperature of an object, as, e.g., for detecting badly insulated parts of a house, a chopper needs to be installed. The chopper blades define the reference temperature, and the signal output is proportional to T4(object)-T4(chopper) . For security applications and imaging of moving objects the chopper might not be necessary. A suitable signal treatment may be helpful in this case. As concerns the point detector of figure employed, e.g., for intruder detection, the image of an intruder walking by is first projected onyo one and then onto the other element differ in the sign. The signal thus is first positive, then negative, or vice versa. The electronics catches this feature to avoid false alarms that could be otherwise caused by a heating up the of the environment. The preamplification stage of the read-out electronics is the most crucial part. Special components or designs need to be selected for lowering the electronic noise contribution to below the dielectric noise of the pyroelectric element. The classical solution with discrete electronics is the use of the junction field effect transistor (JFET) in a source follower circuit. JFET’S exhibit smaller noise levels than MOSFET’S at the low frequencies required. For voltage amplification, the effective parallel resistor in schematic needs to be smaller than the input resistor of the JFET, which amounts typically to 10^11Ω. The Rp of the thin film element is often much below this value and so the external Rpa can be omitted. For thermal imaging , the measurement of the pyroelectric current is more adequate than the measurement of the voltage, because the voltage decreases as 1/w in the frequency range of interest, whereas the current stays stable. The current can be measured by an operational amplifier operated like charge amplifierwith a feedback capacitor. The feedback resistor must be rather high in order to have the cut-off frequency 1/RfCf below the measuring frequency. In integrated circuits Rf is often replaced by a switch allowing resetting the integration of of charge. Modern CMOS designs combined with filtering techniques allow the use of CMOS technology. For arrays, the readout IC is also equipped with a sample and hold multiplexing unit, and analog to digital conversion. One of the designs developed for 2D arrays is known under the name LAMPER( Low-noise Arrays of Mosfets for Pyroelectric Array Read ). If a chopper is used, it is possible to improve the signal to noise ratio by applying the lock in technique, at the price of a lower response time. Optimizing the performance involves compromises between possible operating modes. Generally, thin-film ferroelectric devices are designed to operate poled, well below the ferroelectric transition temperature. This offers the possibility of temperature independent operation, and minimizes the risk that device leakage current will saturate the amplifier. The ROIC contributes a parasitic capacitance Cs that is large compared to detector capacitance Cd, which is usually less than 3PF. Cs is the effective capacitance of the integrating capacitor in the feedback loop of a high gain amplifier circuit. Its effective capacitance is large because it is modified by the Miller effect. This results in an attenuation factor of: As = Cd/Cd + Cs

The parameter As attenuates the detector signal, but not all noises are reduced. Temperature fluction noise, and detector Johnson noise attenuated, while system noise is not. Consequently, the selection of detector material , operating temperature and biasing mode is not straight forward. The ROIC output is synchronized with a 30HZ chopper to create a periodic temperature change. The chopper is an Archimedes spiral that approximates a square wave thermal input. The chopper also AC couples the detector to the scene. This allows for the real-time calibration of offset non-uniformly. Consequently, AC coupled sensors work well in dynamic thermal environments. In figure the cchopper is shown. One half of the chopper is transparent, and the other half opaque or diffusing which gives the chopper the Archimedes spiral look.

Figure 14 : Top view on a single element detector, mounted on TO-39 header. The dice is 4.5 × 4.5 mm large. Two rectangular pyroelectric elements are sitting on a transparent membrane. A JFET is glued onto the dice at the upper left corner.

Figure 15 : Typical concepts for read out electronics with voltage measurement (a) and current measurement (b). Cp and Rp stand for the capacity and resistance of the pyroelectric thin film element. Rpa is the external parallel resistor, Cf and Rf are feedback capacity and resistor respectively.

Figure 16 : Chopper (Archimedes spiral) for the IR light

Conclusion : This topic reported the status of research in pyroelectric thin films, of pyroelectric thin film devices and applications . The sensitivity limit of pyroelectric IR detectors was derived and several ways of improving the present performance for approaching the theoretical limits were pointed out. It has been shown that materials engineering combined with thin film processing skills help to improve the relevant figures of merit. This includes not only an increase of the pyroelectric coefficient, but also a decrease of dielectric constant and dielectric loss. On the device side. Integration technology is of paramount importance for achieving the optimal combination with the required thermal properties and needs for electrical read-out. Good solutions for integration issues are the keys for obtaining competitive pyroelectric devices. Pyrotype devices of pyroelectric thin film detectors show already now high quality IR images. This is demonstrated in Figure 17 showing IR images shot with a monolithic thin film 320 × 240 array.

Figure 17 : Images formed with first monolithic thin film ferroelectric 320 × 240 array installed in a palmIR commercial camera.

References : 1 2 3 4 5 6 7 8 9

S.G. porter, Ferroelectrics 33, 193 (1981) R.W. Whatmore, A. Patel, et al., Ferroelectrics 104,263 (1990) Nanoelectronics and information technology, rainer waser 2nd edition,831. A.J. Moulson and J.M Herbert , Electroceramics. London, Chapman & Hall, 1990. C.H .Kohli, P.E. Schmid, F.Levy, IMF-9, Seoul, 1997. P.W . Kruse, Infrared Physics & Technology 36, 869 (1995) P.W . Kruse, SPIE 2552,556 (1995) J.J. Simone, F. Bauer,L.Audaire, Ferroelectronics 171, 239(1995) http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel3/3834/11198/00541941.pdf?arnumb er=541941 10 http://www.spectrumdetector.com/products/pyroelectricDetectors/index.php?gclid=CLrZ _JOAnpICFQ5ZMAod3Bg96w.