- Email: [email protected]
A pyroelectric polymer infrared sensor array with a charge amplifier readout
Sensors and Actuators 76 Ž1999. 145–151 www.elsevier.nlrlocatersna
A pyroelectric polymer infrared sensor array with a charge amplifier readout D. Setiadi ) , H. Weller, T.D. Binnie Department of Electronic and Electrical Engineering, Napier UniÕersity, 219 Colinton Road, Edinburgh EH14 1DJ, UK Received in revised form 24 November 1998; accepted 4 December 1998
Abstract This paper presents a new pixel architecture for an infrared sensor array based on a pyroelectric polymer integrated with a CMOS charge amplifier. The fill factor of the sensor is optimised by placing the amplifier structure directly below the sensing area. The maximum responsivity is 1000 VrW @ 1 Hz and the specific detectivity is 6.84 = 10 6 cm 6HzrW @ 100 Hz. These results are presented in comparison with those of a single MOSFET with an external 10 9 V bias resistor. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Charge amplifier; Infrared sensor; PVDF; VDFrTrFE copolymer; Ferroelectric polymer
1. Introduction The development of low-cost pyroelectric sensors has gained a great deal of interest due to the large number of potential applications. An economical sensor is achieved by integrating the pyroelectric material with low-cost integrated electronics. Ferroelectric polymers are preferred over ceramics and single crystals, due to their cost-effectiveness, their low thermal conductivity and their fabrication being compatible with semiconductor technology. Conventionally for this type of sensors w1,2x, the pyroelectrically generated charge is supplied to the gate of a single FET. The impedance of the sensing element is extremely high and it cannot be used for biasing the gate of the FET to supply the gate leakage current. Therefore, a large gate bias resistance is necessary. Values of 100 M V or greater are required to achieve good responsivity. Although on-chip resistors are available, their accuracy, repeatability and linearity in CMOS process technology remain poor, and large values cannot be realistically implemented. A radiation-damaged diode for biasing the MOSFET is suggested by von Munch and Thiemann w3x. The bias ¨
) Corresponding author. Tel.: q44-131-444-22-66; Fax: q44-131455-42-31; E-mail: [email protected]
resistor obtained from this process is in the order of 10 10 V. It requires additional process steps over the standard CMOS technique and is not readily available commercially. Hammes w4x suggested use of a dc feedback loop. The bias point of the sensor element is measured by a dc amplifier and compared to a reference value. If necessary, the feedback loop can be used to re-adjust the biasing. The gate of the readout FET is biased by means of a current source instead of a diode. However, the result of this approach is inconclusive. In this paper we describe a fully integrated low noise charge amplifier with no external bias resistors for pyroelectric sensor readout. This charge amplifier is implemented in the standard CMOS process technology. Pyroelectric sensors based on PVDF and VDFrTrFE copolymer with the charge amplifier have been realized. The performance of both sensors is presented. These results are then compared with those of a conventional single MOSFET pre-amplifier with an external bias resistor.
2. Charge amplifier Charge amplification is possible since the sensing element presents a sufficiently large internal capacitance. The amplifier voltage gain can be defined as the ratio of equivalent input capacitance over the feedback capaci-
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 8 . 0 0 3 7 2 - 0
146
D. Setiadi et al.r Sensors and Actuators 76 (1999) 145–151
tance. An equivalent electrical circuit for sensor and charge amplifier combination is shown in Fig. 1. R d is the ferroelectric polymer equivalent electrical resistance. Cd is the sum of the ferroelectric polymer electrical capacitance and the bottom electrode to substrate capacitance while Cf is the feedback capacitance. A core amplifier has been realized by using a singleended, folded, cascade amplifier. This structure is extensively employed for the purpose of charge amplification w5–8x. It presents the advantages of having a high dc gain and large frequency bandwidth. The dc operating point of the input transistor is set by means of resistive feedback using a MOS transistor. The charge amplifier is implemented in the double metal, double polysilicon, MIETEC 2.4 mm CMOS technology. The feedback capacitor of 1 pF is attained using two polysilicon layers. The specification of the charge amplifier is given in Table 1, and Fig. 2 shows a microphotograph of the chip. The chip contains 2 matrices of 2 = 2 pixel elements; each element has an area of 240 mm = 240 mm. The matrix configurations are: single MOSFET, and charge amplifier. The charge amplifier is fabricated underneath the area of the second metal layer which acts as the bottom electrode for the pyroelectric sensor. This is done by reserving the first metal layer in the process for the amplifier interconnect, and using the second metal layer to implement the pixels. This affords a large area. Low-noise input transistors are therefore possible without sacrificing pixel fill factor. It is a generally accepted assumption that the total amplifier noise is dominated by the input stage of the charge amplifier. Noise of the input transistor contains three independent noise components. These are the channel thermal noise, 1rf noise and shot noise. It can be shown that the 1rf noise in a CMOS process sets the lowest limit of the charge amplifier w6x. In order to achieve the best noise performance, one must choose an optimal input transistor dimension and a maximal dc bias level. The optimal transistor dimension is
Table 1 Charge amplifier specification Properties
Value
Unit
DC voltage gain Bandwidth Input referred noise Ž@100 Hz. Feedback capacitance Power supply Area
23 )1 57 1 "2.5 245=200
MHz nVr6Hz pF V mm2
determined by the minimal transistor gate-length L and the average of optimal gate-width W for which the channel thermal noise and 1rf noise are minimal. The higher the dc bias current, the lower the thermal noise contribution will be. The maximal dc bias current is mainly limited by the available power supply, VDD and dc bias voltages of the MOS transistors.
3. Technology For the pyroelectric sensor based on PVDF film, a 5 mm-thick non conducting adhesive ŽUV curing acrylic. was applied to the silicon substrate containing the readout electronics and 9 mm pre-poled PVDF pressed into place. The PVDF film is metalised only on the top surface; the bottom electrode was removed after polarization. The thermal conductivity of the capacitive adhesive film is lower than that of PVDF film. Therefore, this layer acts beneficially as a thermal insulator. For the copolymer sensor, a filtered 10 wt.% concentration 65r35 VDFrTrFE copolymer is deposited on the silicon substrate by using spin-coating, resulting in a 1 mm-thick film with a measured uniformity of 5%. The sample is annealed as follows: first, the sample is kept for 24 h at 258C. The annealing temperature is then increased to 1008C. The sample is kept at this temperature for 6 h, after which annealing for 10 min at 1608C occurs. Finally, the annealing temperature is slowly decreased to 258C over 5 h. A 100 nm aluminum top electrode that is also used as a mask for etching the copolymer is deposited directly by thin film evaporation using a shadow mask. The VDFrTrFE copolymer is removed from the bonding pads by selective etching. A post anneal is carried out for 1 h at 1008C prior to the etching process. Finally, the VDFrTrFE copolymer is poled by on-chip step-wise poling w9x.
4. Measurement results
Fig. 1. Sensor and charge amplifier equivalent circuit.
The experimental apparatus used to measure the sensitivity and frequency response of the ferroelectric polymer pyroelectric sensors is shown diagramatically in Fig. 3. A
D. Setiadi et al.r Sensors and Actuators 76 (1999) 145–151
147
Fig. 2. Microphotograph of the three 2 = 2 pyroelectric sensor arrays with their respective pre-amplifiers and some test structures. From left to right: test structures, 2 = 2 matrix with single MOSFET’s, voltage amplifiers, and charge amplifiers.
calibrated low temperature black body source Žglobal element. with maximum emission at 8.6 mm was used to illuminate the sensors. The radiative power density is 65 Wrm2 at the sensor. A mechanical chopper modulates the
radiation to provide the required time varying signal for the sensors and a reference signal for the lock-in amplifier. The mechanical chopper and the lock-in amplifier ŽSR830 DSP Stanford Research Systems. are controlled by the
Fig. 3. Experimental apparatus for sensor response measurements.
148
D. Setiadi et al.r Sensors and Actuators 76 (1999) 145–151
Fig. 4. Responsivity of pyroelectric sensors based on PVDF and VDFrTrFE with the charge amplifier and the single MOSFET with an external 10 9 V bias resistor.
computer. A germanium filter was used for screening ambient visible light from the sensor. For noise measurements, the dynamic signal analyzer HP35660A has been used. Fig. 4 shows the responsivity curves for the PVDF and VDFrTrFE copolymer sensors with the charge amplifier and the single MOSFET with an external 10 9 V bias resistor. Generally, the responsivity of PVDF pyroelectric
sensors is higher than that of the VDFrTrFE copolymer pyroelectric sensors. Nevertheless, a relatively linear responsivity curve is observed for the VDFrTrFE copolymer pyroelectric sensor with the charge amplifier readout. The maximum responsivities of 1000 VrW and 100 VrW are obtained for the PVDF pyroelectric sensor and the VDFrTrFE copolymer pyroelectric sensor, respectively. These results also imply that the input resistance of the
Fig. 5. Voltage noise of the charge amplifier and the single MOSFET with an external 10 9 V bias resistor.
D. Setiadi et al.r Sensors and Actuators 76 (1999) 145–151
149
Fig. 6. Noise equivalent power of PVDF and VDFrTrFE pyroelectric sensor with the charge amplifier readout.
charge amplifier is higher than the 10 9 V of the externally biased MOSFET. Noise sources in the pyroelectric sensor can be modelled by two sets of equivalent noise sources, a detector noise of the ferroelectric polymer, and charge amplifier noise. The total noise of the sensor is the quadratic sum of the various contributions of the individual noise sources. The detector noise of the ferroelectric polymer is very small compared with the noise from the charge amplifier.
Hence, the noise of the pyroelectric sensors will dominantly be the charge amplifier noise, which remains constant for both PVDF and VDFrTrFE sensors. On the contrary, the noise of the single MOSFET is dominated by the thermal noise of the external 10 9 V bias resistor. Fig. 5 shows the output voltage noise of the pyroelectric sensors with the charge amplifier and the single MOSFET including the external 10 9 V bias resistor. The output voltage noise of 1.61 mVr6Hz and 6.70 mVr6Hz at 100
Fig. 7. Specific detectivity of PVDF and VDFrTrFE pyroelectric sensor with the charge amplifier readout.
D. Setiadi et al.r Sensors and Actuators 76 (1999) 145–151
150
Table 2 Performance of pyroelectric sensors with a charge amplifier and a single MOSFET No. 1 2 3 4 5
Parameter Ž@100 Hz.
Charge amplifier PVDF
Responsivity Noise voltage NEP DU NETD
402 1.61 3.51 6.84 = 10 6 0.09
Unit
VDFrTrFE
Single MOSFET Ž10 9 V bias resistor. PVDF VDFrTrFE
97.8 1.61 12.8 1.88 = 10 6 0.33
496 6.70 13.5 1.76 = 10 6 0.35
VrW mVr6Hz nWr6Hz cm 6HzrW K
Hz for the sensor with the charge amplifier and the single MOSFET are obtained, respectively. The relative performance of the integrated pyroelectric sensor is assessed in terms of the Noise Equivalent Power ŽNEP. or the specific detectivity Ž DU . which is independent of the sensor area. The noise equivalent power is the incident radiation power required to produce an electrical output equal to the noise, and is expressed as NEP s
Vn SV
'A NEP
A is the area of the pyroelectric sensor and NEP the noise equivalent power. Since the sensor area is 240 = 240 mm2 , the specific detectivities of integrated PVDF and VDFrTrFE copolymer pyroelectric sensors are 6.84 = 10 6 cm 6HzrW and 1.88 = 10 6 cm 6HzrW at 100 Hz, as shown in Fig. 7. The noise equivalent temperature difference ŽNETD. is recognised as a useful figure of merit when comparing the resolution of a thermal sensor. The NETD is defined as the scene temperature difference, DT Žblack body with mean temperature of 300 K. which causes a signal to noise ratio of unity at the output w10x: NETD s NEPU
4F2
ž /
14m m
H8m m
A pyroelectric polymer sensor with an integrated charge amplifier, implemented in the standard CMOS process, suitable for array implementation has been realized. The charge amplifier lies directly under the sensing element. Hence it maximises pixel fill factor. Responsivity and noise equivalent power ŽNEP. of this sensor are compared with those of the single MOSFET with a 10 9 V bias resistor. The responsivities of pyroelectric sensors with the charge amplifier and the single MOSFET are of the same order. The noise of the single MOSFET with the 10 9 V bias resistor is higher than that of the charge amplifier. Thus the NEP of pyroelectric sensors with the charge amplifier is beneficially lower than that with the single MOSFET.
Acknowledgements This work is supported by the UK Engineering and Physical Sciences Research Council ŽEPSRC grant award GRrK17224..
References
´ tGA
where Gs
the energy current emitted per wavelength per solid angle V , and 1rF is the aperture of the sensor optic. By assuming ´ t s 1 and 1rF s 1, the minimum of the NETD for the PVDF and VDFrTrFE pyroelectric sensors are 0.09 K and 0.33 K. Table 2 shows the performance of both sensors.
5. Conclusions
Vn denotes the total noise of the pyroelectric sensor and S V the responsivity of the pyroelectric sensor. Noise equivalent powers of 3.51 nWr6Hz and 12.8 nWr6Hz at 100 Hz have been achieved for the pyroelectric sensors based on PVDF and VDFrTrFE, respectively. The NEP is shown as a function of chopper modulation frequency in Fig. 6. The specific detectivity is given by DU s
110 6.70 68.4 0.35 = 10 6 1.81
E PV Ž T , l . ET
dl
´ denotes the emissivity of the source, t the transmittivity of the space between the source and sensor Žfor example atmosphere, lenses., and A is defined as previously. PV is
w1x D. Setiadi, Integrated VDFrTrFE copolymer-on-silicon pyroelectric sensors, PhD Thesis, Univ. of Twente, 1995. w2x S. Bauer, B. Ploss, A simple technique to interface pyroelectric materials with silicon substrates for infrared detection, Ferroelectrics Letters 9 Ž1989. 155–160. w3x W. von Munch, U. Thiemann, Pyroelectric detector array with ¨ PVDF on silicon integrated circuit, Sensors and Actuators A 25–27 Ž1991. 167–172. w4x P.C.A. Hammes, Infrared matrix sensor using PVDF on silicon:
D. Setiadi et al.r Sensors and Actuators 76 (1999) 145–151
w5x
w6x
w7x
w8x
w9x w10x
theory, design, fabrication and testing of pyroelectric sensor using PVDF, PhD Thesis, Delft Univ. of Technology, 1994. A. Simoni, G. Torelli, F. Maloberti, A. Sartori, S.E. Plevridis, A.N. Birbas, A single-chip optical sensor with analog memory for motion detection, IEEE Journal of Solid State Circuits 30 Ž1995. 800–805. S. Tedja, J. van der Spiegel, H.H. Williams, A CMOS low noise and low power charge sampling integrated circuit for capacitive detectorrsensor interfaces, IEEE Journal of Solid State Circuits 30 Ž1995. 110–119. Y. Hu, E. Nygard, A new design of a low noise, low power consumption CMOS charge amplifier, Nucl. Instr. and Meth. A 365 Ž1995. 193–197. W.M.C. Sansen, Z.Y. Chang, Limits of low noise performance of detector readout front ends in CMOS technology, IEEE Transactions on Circuits and Systems 37 Ž1990. 1375–1382. D. Setiadi, M. Wubbenhorst, P.P.L. Regtien, The step-wise poling of ¨ VDFrTrFE copolymers, Ferroelectrics 186 Ž1996. 255–258. H. Schopf, P. Wurfel, W. Ruppel, Integrated pyroelectric sensor ¨ array of NaNO 2 on silicon, Sensors and Materials 3 Ž1991. 25–40.
Dadi Setiadi was born in Palembang, Indonesia, in 1965. He received his M.Sc. degree in electrical engineering in 1991 from the Delft University of Technology. In 1995, he received the PhD degree in electrical engineering at the University of Twente, his thesis dealing with integrated VDFrTrFE copolymers-on-silicon pyroelectric sensors. In 1996, he became a post-doctoral fellow in the Sensor and Instrumentation Group, Napier University, United Kingdom.
151
Harald J. Weller was born in St Quentin, France, in 1973. He received the M.Sc degree in electrical engineering in 1997 from Napier University. In the summer of 1996, he worked at Racal-MESL, Edinburgh, as a student engineer. He is currently a PhD candidate in electrical engineering at Napier University, Edinburgh. His research interests include the design and evaluation of low-noise, low-power CMOS preamplifiers for on-silicon Infra-Red focal plane arrays. T.D. Binnie obtained a PhD in solid state physics from Heriot Watt University before employment as a Development Engineer for Hughes Microelectronics Europe Since joining Napier University as a lecturer in electronic engineering in 1986 he has worked on many research and development projects. In 1991 Dr Binnie set up the Sensor Systems Group at Napier University to work on a variety of academic and commercial contracts. The group consists of four academic staff and three full time research staff. Dr. Binnie is a co-holder of SERC Grant Award. ŽGRrH81016. ‘Traffic Representation by Artificial Neural Systems’, and program leader of EPSRC grant award ŽGRrK17224. ‘Infrared Sensor Arrays’. He has also held commercial research contracts with Shell Exploration and Production ŽUK., Shell Research and Hewlett Packard Research Laboratories and is a consultant to Vision Group . Dr. Binnie was appointed Reader in Sensor Systems in 1996 and is a member of the EPSRC College of Peers for Control and Instrumentation. He was joint winner of the Enterprise Oil—Heriot Watt University Environmental Award, in 1995.
A pyroelectric polymer infrared sensor array with a charge amplifier readout D. Setiadi ) , H. Weller, T.D. Binnie Department of Electronic and Electrical Engineering, Napier UniÕersity, 219 Colinton Road, Edinburgh EH14 1DJ, UK Received in revised form 24 November 1998; accepted 4 December 1998
Abstract This paper presents a new pixel architecture for an infrared sensor array based on a pyroelectric polymer integrated with a CMOS charge amplifier. The fill factor of the sensor is optimised by placing the amplifier structure directly below the sensing area. The maximum responsivity is 1000 VrW @ 1 Hz and the specific detectivity is 6.84 = 10 6 cm 6HzrW @ 100 Hz. These results are presented in comparison with those of a single MOSFET with an external 10 9 V bias resistor. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Charge amplifier; Infrared sensor; PVDF; VDFrTrFE copolymer; Ferroelectric polymer
1. Introduction The development of low-cost pyroelectric sensors has gained a great deal of interest due to the large number of potential applications. An economical sensor is achieved by integrating the pyroelectric material with low-cost integrated electronics. Ferroelectric polymers are preferred over ceramics and single crystals, due to their cost-effectiveness, their low thermal conductivity and their fabrication being compatible with semiconductor technology. Conventionally for this type of sensors w1,2x, the pyroelectrically generated charge is supplied to the gate of a single FET. The impedance of the sensing element is extremely high and it cannot be used for biasing the gate of the FET to supply the gate leakage current. Therefore, a large gate bias resistance is necessary. Values of 100 M V or greater are required to achieve good responsivity. Although on-chip resistors are available, their accuracy, repeatability and linearity in CMOS process technology remain poor, and large values cannot be realistically implemented. A radiation-damaged diode for biasing the MOSFET is suggested by von Munch and Thiemann w3x. The bias ¨
) Corresponding author. Tel.: q44-131-444-22-66; Fax: q44-131455-42-31; E-mail: [email protected]
resistor obtained from this process is in the order of 10 10 V. It requires additional process steps over the standard CMOS technique and is not readily available commercially. Hammes w4x suggested use of a dc feedback loop. The bias point of the sensor element is measured by a dc amplifier and compared to a reference value. If necessary, the feedback loop can be used to re-adjust the biasing. The gate of the readout FET is biased by means of a current source instead of a diode. However, the result of this approach is inconclusive. In this paper we describe a fully integrated low noise charge amplifier with no external bias resistors for pyroelectric sensor readout. This charge amplifier is implemented in the standard CMOS process technology. Pyroelectric sensors based on PVDF and VDFrTrFE copolymer with the charge amplifier have been realized. The performance of both sensors is presented. These results are then compared with those of a conventional single MOSFET pre-amplifier with an external bias resistor.
2. Charge amplifier Charge amplification is possible since the sensing element presents a sufficiently large internal capacitance. The amplifier voltage gain can be defined as the ratio of equivalent input capacitance over the feedback capaci-
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 8 . 0 0 3 7 2 - 0
146
D. Setiadi et al.r Sensors and Actuators 76 (1999) 145–151
tance. An equivalent electrical circuit for sensor and charge amplifier combination is shown in Fig. 1. R d is the ferroelectric polymer equivalent electrical resistance. Cd is the sum of the ferroelectric polymer electrical capacitance and the bottom electrode to substrate capacitance while Cf is the feedback capacitance. A core amplifier has been realized by using a singleended, folded, cascade amplifier. This structure is extensively employed for the purpose of charge amplification w5–8x. It presents the advantages of having a high dc gain and large frequency bandwidth. The dc operating point of the input transistor is set by means of resistive feedback using a MOS transistor. The charge amplifier is implemented in the double metal, double polysilicon, MIETEC 2.4 mm CMOS technology. The feedback capacitor of 1 pF is attained using two polysilicon layers. The specification of the charge amplifier is given in Table 1, and Fig. 2 shows a microphotograph of the chip. The chip contains 2 matrices of 2 = 2 pixel elements; each element has an area of 240 mm = 240 mm. The matrix configurations are: single MOSFET, and charge amplifier. The charge amplifier is fabricated underneath the area of the second metal layer which acts as the bottom electrode for the pyroelectric sensor. This is done by reserving the first metal layer in the process for the amplifier interconnect, and using the second metal layer to implement the pixels. This affords a large area. Low-noise input transistors are therefore possible without sacrificing pixel fill factor. It is a generally accepted assumption that the total amplifier noise is dominated by the input stage of the charge amplifier. Noise of the input transistor contains three independent noise components. These are the channel thermal noise, 1rf noise and shot noise. It can be shown that the 1rf noise in a CMOS process sets the lowest limit of the charge amplifier w6x. In order to achieve the best noise performance, one must choose an optimal input transistor dimension and a maximal dc bias level. The optimal transistor dimension is
Table 1 Charge amplifier specification Properties
Value
Unit
DC voltage gain Bandwidth Input referred noise Ž@100 Hz. Feedback capacitance Power supply Area
23 )1 57 1 "2.5 245=200
MHz nVr6Hz pF V mm2
determined by the minimal transistor gate-length L and the average of optimal gate-width W for which the channel thermal noise and 1rf noise are minimal. The higher the dc bias current, the lower the thermal noise contribution will be. The maximal dc bias current is mainly limited by the available power supply, VDD and dc bias voltages of the MOS transistors.
3. Technology For the pyroelectric sensor based on PVDF film, a 5 mm-thick non conducting adhesive ŽUV curing acrylic. was applied to the silicon substrate containing the readout electronics and 9 mm pre-poled PVDF pressed into place. The PVDF film is metalised only on the top surface; the bottom electrode was removed after polarization. The thermal conductivity of the capacitive adhesive film is lower than that of PVDF film. Therefore, this layer acts beneficially as a thermal insulator. For the copolymer sensor, a filtered 10 wt.% concentration 65r35 VDFrTrFE copolymer is deposited on the silicon substrate by using spin-coating, resulting in a 1 mm-thick film with a measured uniformity of 5%. The sample is annealed as follows: first, the sample is kept for 24 h at 258C. The annealing temperature is then increased to 1008C. The sample is kept at this temperature for 6 h, after which annealing for 10 min at 1608C occurs. Finally, the annealing temperature is slowly decreased to 258C over 5 h. A 100 nm aluminum top electrode that is also used as a mask for etching the copolymer is deposited directly by thin film evaporation using a shadow mask. The VDFrTrFE copolymer is removed from the bonding pads by selective etching. A post anneal is carried out for 1 h at 1008C prior to the etching process. Finally, the VDFrTrFE copolymer is poled by on-chip step-wise poling w9x.
4. Measurement results
Fig. 1. Sensor and charge amplifier equivalent circuit.
The experimental apparatus used to measure the sensitivity and frequency response of the ferroelectric polymer pyroelectric sensors is shown diagramatically in Fig. 3. A
D. Setiadi et al.r Sensors and Actuators 76 (1999) 145–151
147
Fig. 2. Microphotograph of the three 2 = 2 pyroelectric sensor arrays with their respective pre-amplifiers and some test structures. From left to right: test structures, 2 = 2 matrix with single MOSFET’s, voltage amplifiers, and charge amplifiers.
calibrated low temperature black body source Žglobal element. with maximum emission at 8.6 mm was used to illuminate the sensors. The radiative power density is 65 Wrm2 at the sensor. A mechanical chopper modulates the
radiation to provide the required time varying signal for the sensors and a reference signal for the lock-in amplifier. The mechanical chopper and the lock-in amplifier ŽSR830 DSP Stanford Research Systems. are controlled by the
Fig. 3. Experimental apparatus for sensor response measurements.
148
D. Setiadi et al.r Sensors and Actuators 76 (1999) 145–151
Fig. 4. Responsivity of pyroelectric sensors based on PVDF and VDFrTrFE with the charge amplifier and the single MOSFET with an external 10 9 V bias resistor.
computer. A germanium filter was used for screening ambient visible light from the sensor. For noise measurements, the dynamic signal analyzer HP35660A has been used. Fig. 4 shows the responsivity curves for the PVDF and VDFrTrFE copolymer sensors with the charge amplifier and the single MOSFET with an external 10 9 V bias resistor. Generally, the responsivity of PVDF pyroelectric
sensors is higher than that of the VDFrTrFE copolymer pyroelectric sensors. Nevertheless, a relatively linear responsivity curve is observed for the VDFrTrFE copolymer pyroelectric sensor with the charge amplifier readout. The maximum responsivities of 1000 VrW and 100 VrW are obtained for the PVDF pyroelectric sensor and the VDFrTrFE copolymer pyroelectric sensor, respectively. These results also imply that the input resistance of the
Fig. 5. Voltage noise of the charge amplifier and the single MOSFET with an external 10 9 V bias resistor.
D. Setiadi et al.r Sensors and Actuators 76 (1999) 145–151
149
Fig. 6. Noise equivalent power of PVDF and VDFrTrFE pyroelectric sensor with the charge amplifier readout.
charge amplifier is higher than the 10 9 V of the externally biased MOSFET. Noise sources in the pyroelectric sensor can be modelled by two sets of equivalent noise sources, a detector noise of the ferroelectric polymer, and charge amplifier noise. The total noise of the sensor is the quadratic sum of the various contributions of the individual noise sources. The detector noise of the ferroelectric polymer is very small compared with the noise from the charge amplifier.
Hence, the noise of the pyroelectric sensors will dominantly be the charge amplifier noise, which remains constant for both PVDF and VDFrTrFE sensors. On the contrary, the noise of the single MOSFET is dominated by the thermal noise of the external 10 9 V bias resistor. Fig. 5 shows the output voltage noise of the pyroelectric sensors with the charge amplifier and the single MOSFET including the external 10 9 V bias resistor. The output voltage noise of 1.61 mVr6Hz and 6.70 mVr6Hz at 100
Fig. 7. Specific detectivity of PVDF and VDFrTrFE pyroelectric sensor with the charge amplifier readout.
D. Setiadi et al.r Sensors and Actuators 76 (1999) 145–151
150
Table 2 Performance of pyroelectric sensors with a charge amplifier and a single MOSFET No. 1 2 3 4 5
Parameter Ž@100 Hz.
Charge amplifier PVDF
Responsivity Noise voltage NEP DU NETD
402 1.61 3.51 6.84 = 10 6 0.09
Unit
VDFrTrFE
Single MOSFET Ž10 9 V bias resistor. PVDF VDFrTrFE
97.8 1.61 12.8 1.88 = 10 6 0.33
496 6.70 13.5 1.76 = 10 6 0.35
VrW mVr6Hz nWr6Hz cm 6HzrW K
Hz for the sensor with the charge amplifier and the single MOSFET are obtained, respectively. The relative performance of the integrated pyroelectric sensor is assessed in terms of the Noise Equivalent Power ŽNEP. or the specific detectivity Ž DU . which is independent of the sensor area. The noise equivalent power is the incident radiation power required to produce an electrical output equal to the noise, and is expressed as NEP s
Vn SV
'A NEP
A is the area of the pyroelectric sensor and NEP the noise equivalent power. Since the sensor area is 240 = 240 mm2 , the specific detectivities of integrated PVDF and VDFrTrFE copolymer pyroelectric sensors are 6.84 = 10 6 cm 6HzrW and 1.88 = 10 6 cm 6HzrW at 100 Hz, as shown in Fig. 7. The noise equivalent temperature difference ŽNETD. is recognised as a useful figure of merit when comparing the resolution of a thermal sensor. The NETD is defined as the scene temperature difference, DT Žblack body with mean temperature of 300 K. which causes a signal to noise ratio of unity at the output w10x: NETD s NEPU
4F2
ž /
14m m
H8m m
A pyroelectric polymer sensor with an integrated charge amplifier, implemented in the standard CMOS process, suitable for array implementation has been realized. The charge amplifier lies directly under the sensing element. Hence it maximises pixel fill factor. Responsivity and noise equivalent power ŽNEP. of this sensor are compared with those of the single MOSFET with a 10 9 V bias resistor. The responsivities of pyroelectric sensors with the charge amplifier and the single MOSFET are of the same order. The noise of the single MOSFET with the 10 9 V bias resistor is higher than that of the charge amplifier. Thus the NEP of pyroelectric sensors with the charge amplifier is beneficially lower than that with the single MOSFET.
Acknowledgements This work is supported by the UK Engineering and Physical Sciences Research Council ŽEPSRC grant award GRrK17224..
References
´ tGA
where Gs
the energy current emitted per wavelength per solid angle V , and 1rF is the aperture of the sensor optic. By assuming ´ t s 1 and 1rF s 1, the minimum of the NETD for the PVDF and VDFrTrFE pyroelectric sensors are 0.09 K and 0.33 K. Table 2 shows the performance of both sensors.
5. Conclusions
Vn denotes the total noise of the pyroelectric sensor and S V the responsivity of the pyroelectric sensor. Noise equivalent powers of 3.51 nWr6Hz and 12.8 nWr6Hz at 100 Hz have been achieved for the pyroelectric sensors based on PVDF and VDFrTrFE, respectively. The NEP is shown as a function of chopper modulation frequency in Fig. 6. The specific detectivity is given by DU s
110 6.70 68.4 0.35 = 10 6 1.81
E PV Ž T , l . ET
dl
´ denotes the emissivity of the source, t the transmittivity of the space between the source and sensor Žfor example atmosphere, lenses., and A is defined as previously. PV is
w1x D. Setiadi, Integrated VDFrTrFE copolymer-on-silicon pyroelectric sensors, PhD Thesis, Univ. of Twente, 1995. w2x S. Bauer, B. Ploss, A simple technique to interface pyroelectric materials with silicon substrates for infrared detection, Ferroelectrics Letters 9 Ž1989. 155–160. w3x W. von Munch, U. Thiemann, Pyroelectric detector array with ¨ PVDF on silicon integrated circuit, Sensors and Actuators A 25–27 Ž1991. 167–172. w4x P.C.A. Hammes, Infrared matrix sensor using PVDF on silicon:
D. Setiadi et al.r Sensors and Actuators 76 (1999) 145–151
w5x
w6x
w7x
w8x
w9x w10x
theory, design, fabrication and testing of pyroelectric sensor using PVDF, PhD Thesis, Delft Univ. of Technology, 1994. A. Simoni, G. Torelli, F. Maloberti, A. Sartori, S.E. Plevridis, A.N. Birbas, A single-chip optical sensor with analog memory for motion detection, IEEE Journal of Solid State Circuits 30 Ž1995. 800–805. S. Tedja, J. van der Spiegel, H.H. Williams, A CMOS low noise and low power charge sampling integrated circuit for capacitive detectorrsensor interfaces, IEEE Journal of Solid State Circuits 30 Ž1995. 110–119. Y. Hu, E. Nygard, A new design of a low noise, low power consumption CMOS charge amplifier, Nucl. Instr. and Meth. A 365 Ž1995. 193–197. W.M.C. Sansen, Z.Y. Chang, Limits of low noise performance of detector readout front ends in CMOS technology, IEEE Transactions on Circuits and Systems 37 Ž1990. 1375–1382. D. Setiadi, M. Wubbenhorst, P.P.L. Regtien, The step-wise poling of ¨ VDFrTrFE copolymers, Ferroelectrics 186 Ž1996. 255–258. H. Schopf, P. Wurfel, W. Ruppel, Integrated pyroelectric sensor ¨ array of NaNO 2 on silicon, Sensors and Materials 3 Ž1991. 25–40.
Dadi Setiadi was born in Palembang, Indonesia, in 1965. He received his M.Sc. degree in electrical engineering in 1991 from the Delft University of Technology. In 1995, he received the PhD degree in electrical engineering at the University of Twente, his thesis dealing with integrated VDFrTrFE copolymers-on-silicon pyroelectric sensors. In 1996, he became a post-doctoral fellow in the Sensor and Instrumentation Group, Napier University, United Kingdom.
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Harald J. Weller was born in St Quentin, France, in 1973. He received the M.Sc degree in electrical engineering in 1997 from Napier University. In the summer of 1996, he worked at Racal-MESL, Edinburgh, as a student engineer. He is currently a PhD candidate in electrical engineering at Napier University, Edinburgh. His research interests include the design and evaluation of low-noise, low-power CMOS preamplifiers for on-silicon Infra-Red focal plane arrays. T.D. Binnie obtained a PhD in solid state physics from Heriot Watt University before employment as a Development Engineer for Hughes Microelectronics Europe Since joining Napier University as a lecturer in electronic engineering in 1986 he has worked on many research and development projects. In 1991 Dr Binnie set up the Sensor Systems Group at Napier University to work on a variety of academic and commercial contracts. The group consists of four academic staff and three full time research staff. Dr. Binnie is a co-holder of SERC Grant Award. ŽGRrH81016. ‘Traffic Representation by Artificial Neural Systems’, and program leader of EPSRC grant award ŽGRrK17224. ‘Infrared Sensor Arrays’. He has also held commercial research contracts with Shell Exploration and Production ŽUK., Shell Research and Hewlett Packard Research Laboratories and is a consultant to Vision Group . Dr. Binnie was appointed Reader in Sensor Systems in 1996 and is a member of the EPSRC College of Peers for Control and Instrumentation. He was joint winner of the Enterprise Oil—Heriot Watt University Environmental Award, in 1995.