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Construction, properties and application of pyroelectric single-element detectors and 128-element CCD linear arrays
Sensors and Actuators A, 25-27
413
(1991) 413 416
Construction, Properties and Application of Pyroelectic Single-element Detectors and 12%Element CCD Linear Arrays G. HOFMANN, Twhnische
L. WALTHER,
Uniuersitiit
J. SCHIEFERDECKER,
Dresden, Institut fir
N. NEUMANN,
FestkCrperelektronik,
Abstract Pyroelectric detectors based on LiNb03 and L-alanine doped triglycine sulfate (DTGS:L-A) are described. Single-element detectors with ion-beam milled pyroelectrlc chips have D* (500K, 10Hz, 25 “C) values of 6 x 10’ and 2 x 10’ cm G/W respectively. 128-element CCD linear arrays with NEP (500 K, 40 Hz, 25 “C) values of 4 and 1 nW respectively were also realized.
Introduction Pyroelectric detectors are thermal radiation sensors that can operate at room temperature without cooling. Millions of such detectors are produced every year throughout the world. Of great importance are detectors with one or two sensitive elements. In the last few years, pyroelectric solid-state multi-element detectors with integrated readout circuits have been developed, which may be used in compact thermal imaging systems. A short overview of some developments of pyroelectric single- and multi-element detectors at the Dresden University of Technology is given here.
Construction The pyroelectric materials used are either lithium niobate (LiNb03) or deuterated and L-alanine doped triglycine sulfate (DTGS: L-A). Both materials allow the production of detectors with high long-term stability and 09244247/91/%3.50
V. NORKUS,
Mommsenstrasse
M. KRAUSS and H. BUDZIER
13. 8027 Dresden (Germany)
good reproducibility. The realization of a sufficient responsivity with a very small temperature dependence is possible using LiNbO,, whereas very high specific detectivities may be achieved with detectors based on DTGS:L-A. Table 1 lists the values of some material parameters of the crystals used. The detector-specific material parameters Mu and MI are given by Mu = ~I(c;s,)
(1)
Mr = ~lcb
(2)
where p is the pyroelectric coefficient, cb is the volume specific heat and E, is the dielectric constant. The thermal diffusivity is denoted by ap. Pyroelectric single-element detectors contain the pyroelectric chip, a high-megohm resistor and a low-noise FET, which are mounted in a TO-transistor package with an IR window (KRS 5, CaF,, coated Ge or Si). By means of mechanical or chemical grinding, lapping and polishing processes, chips with a thickness dr, of about 20 pm are produced. By ion-beam milling the thickness of the pyroelectric chips can be reduced in the region of the sensitive area to values of 4=2-5pm (LiNb03) and dp= 10,nm (DTGS:L-A) respectively. The evaporated NiCr electrodes are photolithographically TABLE 1. Material parameters at 25 “C Material
LiNbO,
DTGS:L-A
M,,(lO-‘°Ccm/J) M,( lo-* C cm/J) a,(cm’/s)
0.85 0.25 0.014
5.5 I.0 0.003
0
Elsevier Sequoia/Printed
in The Netherlands
414
structured. To increase the absorption, it is possible to evaporate an Ag-black coating. For the integrated signal-processing components a thick-film high-megohm resistor (24 x 10” Q) and a JFET are commonly used. For the improvement of the thermal dc. operating point stability, it is advantageous to apply a poly-Si high-megohm resistor (sheet resistance at 25 “C, 2 x 10” R/O; exponential temperature dependence of the resistance, reduction of about a factor of two per 10 K). It seems to be possible to realize a simple readout circuit using the poly-Si resistor and a low-noise MOSFET (e.g., a MOSFET with a gate area of 180 000 pm2: volta e noise of 52 nV/@ at 10 Hz, 6.5 nV/ ,B Hz at 10 kHz). The high-megohm resistor and FET are operated in a common-drain transistor circuit by external coupling with a bipolar-transistor constant current source [I]. The components of the pyroelectic linear array are mounted together in a 24 pin DIL ceramic package with an IR window of coated germanium. The pyroelectric chip contains 128 sensitive elements (element size 90 x 100 pm2 on 100 pm pitch). The typical element thickness d,, is about 20 pm, but it may be also reduced by ion-beam milling. Especially in the case of LiNb03, insulation of the elements from each other by ionmilling slots is advantageous. The Si readout circuit contains a FILL-SPILL gate modulation input for every detector element, a common 1; phase BCCD shift register and a floating diffusion as well as a floating gate output. A novel integrated CCD difference technique yields the detector signal difference of two successive chopping phases. By means of a special simultaneous interconnecting technology, the pyroelectric elements and the CCD input structures are connected [2].
Properties
The responsivity of the pyroelectric singleelement detectors for a sinusoidal modulated incident radiation of at least f= lo- 1000 Hz chopping frequency is
ii;
W~“(LIh
Sv = & = 27cfi,A,( 1 + C@&.)
(3)
C, = s,s,Asldp (4) where & is the r.m.s. value of the incident radiation flux, tii is the r.m.s. value of the signal voltage at the preamplifier output, As is the sensitive area, tr is the optical window transmittance, CIis the absorption coefficient, 1T, 1is the normalized current responsivity [ 31 of the detector element, z+ and C,, are the amplification factor and the input capacitance of the preamplifier and E, is the permittivity of free space. With a sensitive area of 2 x 2 mm2 and using rF = 0.72 (KRS 5-window), uv = 0.99 and C,, = 2 pF the calculated responsivity of a LiNb03 chip (d, = 220 pm, o!= 0.65, IT,1 = 1) is Sv (500 K, 10 Hz, 25 “C) = 175 V/W. Typical measured values are near 180 V/W. Detector elements with an additional black coating (d, = 20 pm, a)TR\= 0.95) have a responsivity according to eqn. (3) of 250 V/W. The measured value is 265 V/W. The temperature dependence of the responsivity of LiNbO, single-element detectors is very low (]A&/& (25 ‘C)] < 1.5% at T = 15-70 “C). Calculated and measured Sv (500 K, 10 Hz, 25 “C) values of DTGS: L-A single-element detectors without a black coating (d, = lo-30 pm, tl = 0.9, IT,1 = 1) reach 1500 VW. Responsivity variations of both LiNbO, and DTGS:L-A detectors over a period of three years remain within the measurement accuracy of 3%. Figures 1 and 2 show the specific detectivity D* = 1/As S,/i&
(5) as a function of the chopping frequency and element thickness, where fik, is the r.m.s. value of the normalized noise voltage at the preamplifier output. In the given frequency range, tan 6 noise [3] dominates. Measured D* (500 K, 10 Hz, 25 “C) peak values of thin ion-beam milled detector elements reach 6 x 1O’cm e/W for LiNb03 and 2 x lo9 cm e/W for DTGSL-A. The main applications of the described single-element detectors have up to now been in radiation pyrometers, gas analysers and Fourier interferometers.
415
a value of 130 000 V/W (predicted value: 185 000 V/W) has been measured. The measured DTGS:L-A linear arrays (d, = lo30 pm, c1= 0.9, lTRl = 1) have responsivities 40 Hz, (500 K, 25 “C) = 470 OOO& 700 000 V/W. Equation (6) yields theoretical values of 500000-980000 V/W. The measured responsivity variations among the 128 elements are typically 2-5%. The measured noise equivalent power
cmHz” W
NEP = i&/S,
Fig. I. Specific detectivity D* (500 K, J LiNbO, single-element detectors.
parameter
108& 10
25 “C) of
: dp
at 500 K, 40 Hz and 25 “C yields 4-7 nW ( LiNb03) and 1.O- 1.7 nW (DTGSL-A). Here u”kis the r.m.s. value of the output noise voltage. The dominating contributions to the modulation transfer function MTFs are the geometrical MTF (MTF,), the thermal MTF resulting from the thermal coupling between the detector elements (MTF,,,) and the capacitive MTF arising from the capacitive coupling between the detector elements (MTF,): MTFs = MTF, MTF,,, MTFc
(8)
MTFo = sin(n&r) /(n&z)
(9)
MTF,,, =f/(a,n2R2)tanh(a,x2R2/f) 102 f-
HZ
103
Fig. 2. Specific detectivity D* (500 K, A 25 “C) of DTGS:L-A single-element detectors.
The responsivity of the linear array for a rectangular modulated incident radiation of at least lo-130 Hz is
where Hu is the voltage transfer factor, C,, is the parasitic capacitance, CEG is the capacitance of the input gate and nSSVEis a factor characterizing the signal processing of the CCD multiplexer. Using rF = 0.8, nSSVE= 4, Hu = 3 and C,,, + CEG= 130 fF, eqn. (6) gives a responsivity Sv (500 K, 40 Hz, 25 “C) = 105 000 V/W for the LiNb03 linear array (d, = 20 pm, DT= 0.7, IT,1 = 1). Typical measured values are about 90 000 V/W. For a linear array with a thin ion-beam milled LiNbO, chip (d, = 4 pm, with insulating slots)
(7)
[4]
(10)
where R is the spatial frequency and a is the element length (a = 90 pm). The thermal MTF following eqn. (10) is dependent on the modulation frequency. It is only important for chips without insultating slots between the detector elements. The terms MTFc and MTFc are independent of the modulation frequency and limiting MTF, at high modulation frequencies. The calculation of MTF, is based on a numerical approximation using given geometrical and electrical values of the detector elements. The good agreement between the theoretical and measured MTFs values of LiNb03 and DTGS:L-A linear arrays without insulating slots is shown in Figs. 3 and 4. A LiNb03 linear array containing insulating slots yields an improved MTFs value of about 0.45 (3 lp/mm). The described linear arrays have been used in IR line scanners [5] or thermal imaging systems for the contactless pick up of one-dimensional or two-dimensional temperature radiation distributions respectively.
416
parameter: f
1
a5
1
MT%
3
b/mm
Fig. 3. Modulation transfer function MTF, of LiNbO, linear arrays.
Acknowledgements The authors are grateful for support in the development of pyroelectric linear arrays from their colleagues at the WF Berlin GmbH.
0
2 R-
Fig. 4. Modulation transfer DTGS:L-A linear arrays.
3
function
lplmm
MTF,
of
J. Schieferdecker, L. Walther, G. Hofmann, G. Heine, V. Norkus, M. Krauss, H. Budzier, W. Titel and F. Banse, Pyroelektrische Mehrelementsensoren mit CCD-Ausleseschaltung, Radio Fernsehen Elektronik, 39 (1990) 221-226. S. T. Liu and D. Long, Pyroelectric detectors and materials, Proc. IEEE, 66 (1978) 14-26.
R. M. Logan and T. P. McLean, Analysis of thermal spread in a pyroelectric imaging system, Infrared
References 1 G. Hofmann, Vorverstarker fur pyroelektrische Sensoren, Feingeriitetechnik, 32 (1983) 499-502.
Phys., 3 (1973) 15-24.
M. Blaschke, B. Gutschwager and U. Kienitz, Infrarot-Zeilenkamera mit pyroelektrischem Zeilensensormodul, Radio Fernsehen Elektronik, 39 (1990) 218-221.
413
(1991) 413 416
Construction, Properties and Application of Pyroelectic Single-element Detectors and 12%Element CCD Linear Arrays G. HOFMANN, Twhnische
L. WALTHER,
Uniuersitiit
J. SCHIEFERDECKER,
Dresden, Institut fir
N. NEUMANN,
FestkCrperelektronik,
Abstract Pyroelectric detectors based on LiNb03 and L-alanine doped triglycine sulfate (DTGS:L-A) are described. Single-element detectors with ion-beam milled pyroelectrlc chips have D* (500K, 10Hz, 25 “C) values of 6 x 10’ and 2 x 10’ cm G/W respectively. 128-element CCD linear arrays with NEP (500 K, 40 Hz, 25 “C) values of 4 and 1 nW respectively were also realized.
Introduction Pyroelectric detectors are thermal radiation sensors that can operate at room temperature without cooling. Millions of such detectors are produced every year throughout the world. Of great importance are detectors with one or two sensitive elements. In the last few years, pyroelectric solid-state multi-element detectors with integrated readout circuits have been developed, which may be used in compact thermal imaging systems. A short overview of some developments of pyroelectric single- and multi-element detectors at the Dresden University of Technology is given here.
Construction The pyroelectric materials used are either lithium niobate (LiNb03) or deuterated and L-alanine doped triglycine sulfate (DTGS: L-A). Both materials allow the production of detectors with high long-term stability and 09244247/91/%3.50
V. NORKUS,
Mommsenstrasse
M. KRAUSS and H. BUDZIER
13. 8027 Dresden (Germany)
good reproducibility. The realization of a sufficient responsivity with a very small temperature dependence is possible using LiNbO,, whereas very high specific detectivities may be achieved with detectors based on DTGS:L-A. Table 1 lists the values of some material parameters of the crystals used. The detector-specific material parameters Mu and MI are given by Mu = ~I(c;s,)
(1)
Mr = ~lcb
(2)
where p is the pyroelectric coefficient, cb is the volume specific heat and E, is the dielectric constant. The thermal diffusivity is denoted by ap. Pyroelectric single-element detectors contain the pyroelectric chip, a high-megohm resistor and a low-noise FET, which are mounted in a TO-transistor package with an IR window (KRS 5, CaF,, coated Ge or Si). By means of mechanical or chemical grinding, lapping and polishing processes, chips with a thickness dr, of about 20 pm are produced. By ion-beam milling the thickness of the pyroelectric chips can be reduced in the region of the sensitive area to values of 4=2-5pm (LiNb03) and dp= 10,nm (DTGS:L-A) respectively. The evaporated NiCr electrodes are photolithographically TABLE 1. Material parameters at 25 “C Material
LiNbO,
DTGS:L-A
M,,(lO-‘°Ccm/J) M,( lo-* C cm/J) a,(cm’/s)
0.85 0.25 0.014
5.5 I.0 0.003
0
Elsevier Sequoia/Printed
in The Netherlands
414
structured. To increase the absorption, it is possible to evaporate an Ag-black coating. For the integrated signal-processing components a thick-film high-megohm resistor (24 x 10” Q) and a JFET are commonly used. For the improvement of the thermal dc. operating point stability, it is advantageous to apply a poly-Si high-megohm resistor (sheet resistance at 25 “C, 2 x 10” R/O; exponential temperature dependence of the resistance, reduction of about a factor of two per 10 K). It seems to be possible to realize a simple readout circuit using the poly-Si resistor and a low-noise MOSFET (e.g., a MOSFET with a gate area of 180 000 pm2: volta e noise of 52 nV/@ at 10 Hz, 6.5 nV/ ,B Hz at 10 kHz). The high-megohm resistor and FET are operated in a common-drain transistor circuit by external coupling with a bipolar-transistor constant current source [I]. The components of the pyroelectic linear array are mounted together in a 24 pin DIL ceramic package with an IR window of coated germanium. The pyroelectric chip contains 128 sensitive elements (element size 90 x 100 pm2 on 100 pm pitch). The typical element thickness d,, is about 20 pm, but it may be also reduced by ion-beam milling. Especially in the case of LiNb03, insulation of the elements from each other by ionmilling slots is advantageous. The Si readout circuit contains a FILL-SPILL gate modulation input for every detector element, a common 1; phase BCCD shift register and a floating diffusion as well as a floating gate output. A novel integrated CCD difference technique yields the detector signal difference of two successive chopping phases. By means of a special simultaneous interconnecting technology, the pyroelectric elements and the CCD input structures are connected [2].
Properties
The responsivity of the pyroelectric singleelement detectors for a sinusoidal modulated incident radiation of at least f= lo- 1000 Hz chopping frequency is
ii;
W~“(LIh
Sv = & = 27cfi,A,( 1 + C@&.)
(3)
C, = s,s,Asldp (4) where & is the r.m.s. value of the incident radiation flux, tii is the r.m.s. value of the signal voltage at the preamplifier output, As is the sensitive area, tr is the optical window transmittance, CIis the absorption coefficient, 1T, 1is the normalized current responsivity [ 31 of the detector element, z+ and C,, are the amplification factor and the input capacitance of the preamplifier and E, is the permittivity of free space. With a sensitive area of 2 x 2 mm2 and using rF = 0.72 (KRS 5-window), uv = 0.99 and C,, = 2 pF the calculated responsivity of a LiNb03 chip (d, = 220 pm, o!= 0.65, IT,1 = 1) is Sv (500 K, 10 Hz, 25 “C) = 175 V/W. Typical measured values are near 180 V/W. Detector elements with an additional black coating (d, = 20 pm, a)TR\= 0.95) have a responsivity according to eqn. (3) of 250 V/W. The measured value is 265 V/W. The temperature dependence of the responsivity of LiNbO, single-element detectors is very low (]A&/& (25 ‘C)] < 1.5% at T = 15-70 “C). Calculated and measured Sv (500 K, 10 Hz, 25 “C) values of DTGS: L-A single-element detectors without a black coating (d, = lo-30 pm, tl = 0.9, IT,1 = 1) reach 1500 VW. Responsivity variations of both LiNbO, and DTGS:L-A detectors over a period of three years remain within the measurement accuracy of 3%. Figures 1 and 2 show the specific detectivity D* = 1/As S,/i&
(5) as a function of the chopping frequency and element thickness, where fik, is the r.m.s. value of the normalized noise voltage at the preamplifier output. In the given frequency range, tan 6 noise [3] dominates. Measured D* (500 K, 10 Hz, 25 “C) peak values of thin ion-beam milled detector elements reach 6 x 1O’cm e/W for LiNb03 and 2 x lo9 cm e/W for DTGSL-A. The main applications of the described single-element detectors have up to now been in radiation pyrometers, gas analysers and Fourier interferometers.
415
a value of 130 000 V/W (predicted value: 185 000 V/W) has been measured. The measured DTGS:L-A linear arrays (d, = lo30 pm, c1= 0.9, lTRl = 1) have responsivities 40 Hz, (500 K, 25 “C) = 470 OOO& 700 000 V/W. Equation (6) yields theoretical values of 500000-980000 V/W. The measured responsivity variations among the 128 elements are typically 2-5%. The measured noise equivalent power
cmHz” W
NEP = i&/S,
Fig. I. Specific detectivity D* (500 K, J LiNbO, single-element detectors.
parameter
108& 10
25 “C) of
: dp
at 500 K, 40 Hz and 25 “C yields 4-7 nW ( LiNb03) and 1.O- 1.7 nW (DTGSL-A). Here u”kis the r.m.s. value of the output noise voltage. The dominating contributions to the modulation transfer function MTFs are the geometrical MTF (MTF,), the thermal MTF resulting from the thermal coupling between the detector elements (MTF,,,) and the capacitive MTF arising from the capacitive coupling between the detector elements (MTF,): MTFs = MTF, MTF,,, MTFc
(8)
MTFo = sin(n&r) /(n&z)
(9)
MTF,,, =f/(a,n2R2)tanh(a,x2R2/f) 102 f-
HZ
103
Fig. 2. Specific detectivity D* (500 K, A 25 “C) of DTGS:L-A single-element detectors.
The responsivity of the linear array for a rectangular modulated incident radiation of at least lo-130 Hz is
where Hu is the voltage transfer factor, C,, is the parasitic capacitance, CEG is the capacitance of the input gate and nSSVEis a factor characterizing the signal processing of the CCD multiplexer. Using rF = 0.8, nSSVE= 4, Hu = 3 and C,,, + CEG= 130 fF, eqn. (6) gives a responsivity Sv (500 K, 40 Hz, 25 “C) = 105 000 V/W for the LiNb03 linear array (d, = 20 pm, DT= 0.7, IT,1 = 1). Typical measured values are about 90 000 V/W. For a linear array with a thin ion-beam milled LiNbO, chip (d, = 4 pm, with insulating slots)
(7)
[4]
(10)
where R is the spatial frequency and a is the element length (a = 90 pm). The thermal MTF following eqn. (10) is dependent on the modulation frequency. It is only important for chips without insultating slots between the detector elements. The terms MTFc and MTFc are independent of the modulation frequency and limiting MTF, at high modulation frequencies. The calculation of MTF, is based on a numerical approximation using given geometrical and electrical values of the detector elements. The good agreement between the theoretical and measured MTFs values of LiNb03 and DTGS:L-A linear arrays without insulating slots is shown in Figs. 3 and 4. A LiNb03 linear array containing insulating slots yields an improved MTFs value of about 0.45 (3 lp/mm). The described linear arrays have been used in IR line scanners [5] or thermal imaging systems for the contactless pick up of one-dimensional or two-dimensional temperature radiation distributions respectively.
416
parameter: f
1
a5
1
MT%
3
b/mm
Fig. 3. Modulation transfer function MTF, of LiNbO, linear arrays.
Acknowledgements The authors are grateful for support in the development of pyroelectric linear arrays from their colleagues at the WF Berlin GmbH.
0
2 R-
Fig. 4. Modulation transfer DTGS:L-A linear arrays.
3
function
lplmm
MTF,
of
J. Schieferdecker, L. Walther, G. Hofmann, G. Heine, V. Norkus, M. Krauss, H. Budzier, W. Titel and F. Banse, Pyroelektrische Mehrelementsensoren mit CCD-Ausleseschaltung, Radio Fernsehen Elektronik, 39 (1990) 221-226. S. T. Liu and D. Long, Pyroelectric detectors and materials, Proc. IEEE, 66 (1978) 14-26.
R. M. Logan and T. P. McLean, Analysis of thermal spread in a pyroelectric imaging system, Infrared
References 1 G. Hofmann, Vorverstarker fur pyroelektrische Sensoren, Feingeriitetechnik, 32 (1983) 499-502.
Phys., 3 (1973) 15-24.
M. Blaschke, B. Gutschwager and U. Kienitz, Infrarot-Zeilenkamera mit pyroelektrischem Zeilensensormodul, Radio Fernsehen Elektronik, 39 (1990) 218-221.