Highly sensitive infrared temperature sensor using self-heating compensated microbolometers

Highly sensitive infrared temperature sensor using self-heating compensated microbolometers

Sensors and Actuators 79 Ž2000. 122–127 www.elsevier.nlrlocatersna Highly sensitive infrared temperature sensor using self-heating compensated microb...

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Sensors and Actuators 79 Ž2000. 122–127 www.elsevier.nlrlocatersna

Highly sensitive infrared temperature sensor using self-heating compensated microbolometers M.V.S. Ramakrishna a , G. Karunasiri a

a,)

, P. Neuzil b, U. Sridhar b, W.J. Zeng

b

Department of Electrical Engineering, Center for Optoelectronics, National UniÕersity of Singapore, Singapore, 119260, Singapore b Institute of Microelectronics, 11 Science Park Road, Singapore, 117685, Singapore Accepted 30 August 1999

Abstract This paper experimentally demonstrates a novel technique that drastically reduces the self-heating effect in microbolometers and which can be used to enhance the response due to infrared ŽIR.. This is accomplished by using two bolometers with tailored thermal parameters, in particular, similar thermal mass but different thermal conductivity. Test devices with responsivity of over 6000 VrW at bias voltage of 3 V have been fabricated. Results indicate that this method is robust even under considerable mismatch of device parameters. We believe that this technique would pave way for realization of relatively simple, low cost and sensitive IR detectors for use in thermometry, imaging and other IR applications. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Bolometer; Thermometer; Infrared; Titanium; Thermal conductance; Self-heating; Compensation

1. Introduction This decade has witnessed the emergence of siliconbased microbolometer thermal detectors as a suitable candidate for detection of infrared ŽIR. radiation w1–6x. Traditionally, photon detectors were preferred primarily due to their superior sensitivity w1x. However, the recent advances in micromachining technology have made it possible to fabricate highly sensitive thermal IR detectors w2x. In contrast to conventional photon detectors, thermal detectors can be operated at room temperature. This fact coupled with the possibility of monolithic integration of readout electronics with the microbolometer results in inexpensive large thermal detector arrays w2,3x. Three of the most common thermal detectors include the bolometers w1–4x, thermopiles w5,6x, and pyroelectric w7x detectors. Bolometers are generally easier to fabricate than pyroelectric detectors and have better responsivities than thermopiles w5–7x. However, their operation is significantly affected by generation of self-heating when current is passed through the device for readout w8,9x. This self-

)

Corresponding author. Tel.: q65-8742162; fax: q65-7791103; e-mail: [email protected]

heating effect fundamentally limits the use of bolometers in applications where high temperature sensitivity is required w10x. This has partially contributed to the success of thermopiles, which do not suffer from self-heating effect. The most common method employed to eliminate selfheating involves the use of a circuit configuration such as a Wheatstone bridge combined with pulsed bias techniques w10,11x. As the temperature of the detector increases at a rate determined by the thermal time constant, the amount of self-heating can be greatly reduced by biasing the detector for a time period smaller than the thermal time constant. This approach also allows the use of large voltage pulses, which significantly improve the responsivity of the sensors Žcompared to low voltage DC bias.. However, even with pulse bias the amount of self-heating is still significant in the case where feeble amount of radiation has to be detected. In order to overcome this problem, two matching bolometers with one of them covered so as to ‘block’ the radiation from reaching that device has been experimented w11x. In this case, by taking a differential output the self-heating signal, which is common to both the detectors can be eliminated w10,11x. The above technique is very effective but requires a configuration whereby one can selectively transmit radiation such that it falls on one of the detectors while

0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 2 8 0 - 0

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material, the self-heating power in each device is given by w14x, Ps

Vs 2

, Ž 2. 4R where Vs is the bias voltage across the bridge and R is the resistance of the bolometer. Thus, the output voltage, Vout ,

Fig. 1. Wheatstone bridge circuit and test setup used for the study of self-heating compensation.

shielding the second device. In addition, the shield and IR window should be properly aligned with the respective sensors. The devices therefore, have to be spaced considerably apart from each other thus imposing a stringent requirement on the fabrication of the sensor. Also, the above technique is not practical for the correction of self-heating effect in uncooled focal plane arrays ŽFPA. w12x. In such applications, the self-heating signal is not fully compensated thus decreasing the dynamic range of the amplifier used thereafter. The application of the proposed technique to FPAs would, however, warrant a careful study of the effects of additional noise introduced by the reference bolometer in comparison with using a load resistor. In particular, bolometers based on semiconducting materials are known to have higher 1rf noise than metal film bolometers used in this study w3,13x. In the following, we will describe a simple approach which allows for compensation of self-heating without the need to use IR shields. This is achieved by engineering the bolometers to have specific thermal properties as illustrated in Sections 2 and 3.

2. Theory of operation Consider the Wheatstone bridge circuit shown in Fig. 1 consisting of two fixed resistors and two microbolometers B1 and B 2 with specific thermal parameters. The heat balance equation is given by

H

dT dt

q GDT s P q f ,

Ž 1.

where H is the thermal mass, DT is the temperature of the microbolometer with respect to the heat sink, P is the self-heating power, f is the incident IR power and G is the thermal conductance. It can be shown that for a DT < 2, where a is the temperature coefficient of resistance ŽTCR. of the sensor

Fig. 2. Photographs of Ža. bolometer B1 Žlow thermal conductance. and Žb. bolometer B 2 Žhigh thermal conductance..

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of the circuit can be obtained as Vout s

Vs 3a 16 R

q

t t1

ž

Ž 1 y ey .

y

G1

Vs af 4R

t t2

Ž 1 y ey .

ž

G2

1

1 y

G1

G2

/

/

,

Ž 3.

where t 1 , t 2 represent the thermal time constants and G 1 , G 2 are the thermal conductance of the two bolometers, respectively. In the above derivation we assume that the incident IR signal is slowly varying compared to the time constants of the bolometers. It was also assumed that both the bolometers have the same TCR. The thermal time constant is determined from the following equation

ts

H G

,

Ž 4.

When the bridge is biased using a pulse voltage such that t < t 1 and t < t 2 we have, Vout s

Vs 3a t 16 R

ž

1

1 y

H1

H2

/

q

Vs af 4R

ž

1

1 y

G1

G2

/

,

Ž 5.

where H1 and H2 are the thermal masses of the two bolometers.

The first term in the above expression represents the self-heating effect while the second term represents the response due to incident IR. Thus, when the thermal masses of both the detectors is the same and G 2 < G 1 it can be seen that self-heating signal can be eliminated while the response of bolometer B 1 due to the IR radiation is not significantly affected. The equation also illustrates the relative trade offs in choosing a proper ratio of thermal conductances. Smaller ratio would mean that the IR response is also affected while very large ratio would affect the extent of self heating compensation as the conditions t < t 1 and t < t 2 may not be valid. 3. Experimental The photographs of the fabricated bolometers used in this study are shown in Fig. 2Ža. and Žb.. The thermal mass depends on the specific heat capacity, density and volume of the membrane material. Therefore, etch openings were designed such that the volume of the material is nearly the same for both the bolometers. The thermal conductance depends on the length to width ratio of the legs. The bolometer in Fig. 2Ža. has long legs and thus smaller thermal conductance while the one in Fig. 2Žb. has short legs and is expected to have larger thermal conductance. The first step of the fabrication process involves deposition and patterning of polysilicon sacrificial layer. This

Fig. 3. Measured output voltages for the two bolometers as a function of time at 294 K. Note that the line scales for the two bolometers are different. The extracted thermal parameters for B1 are G1 s 3.2 = 10y7 WrK and H1 s 4 = 10y8 JrK and for B 2 are G 2 s 2.8 = 10y6 WrK and H2 s 3.6 = 10y8 JrK.

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Fig. 4. Measured self-heating response of bolometer B1 at 2 and 3 V bias voltages across the bridge. The responses under IR illumination at f s 110 nW are also shown.

˚ of PECVD was followed by deposition of about 5000 A nitride. Subsequently, Al is sputtered and patterned for use as bond pads. In order to protect the sensor material during subsequent etching process, a thin layer of TiN was de˚ of sensor posited followed by sputtering of about 600 A material Ti and another thin layer of TiN. The entire structure was passivated using a layer of PECVD nitride.

Plasma etching was then used to create openings for subsequent micromachining, which was accomplished by using a solution of TMAH doped with silicon. The fabricated sensors were first tested for their electrical properties. The devices have dimensions of about 200 = 200 mm2 . Separate experiments w15x which involved a detailed study of the material properties of these devices,

Fig. 5. Measured self-heating response of bolometer B 2 at 2 and 3 V bias voltages across the bridge. The responses due to IR radiation are also shown and found to be much smaller compared to the bolometer B1 .

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Table 1 Comparison of parameters for bolometer B1 and bolometer B 2 Resistance TCR Žat 294 K. Thermal conductance Thermal mass Thermal time constant Responsivity Žat 3 V bias.

Bolometer B1

Bolometer B 2

26 k V 0.27%rK 3.2=10y7 WrK 4=10y8 JrK 115 ms 6000 VrW

24 k V 0.29%rK 2.8=10y6 WrK 3.6=10y8 JrK 12 ms 770 VrW

have shown that the TCR of these devices was around 0.3%rK while the nominal resistance at 294 K was about 26 k V. The experimental set-up used in the measurement is shown in Fig. 1. All the experiments were carried in vacuum Žat a pressure better than 10y5 Torr. and at a temperature of 294 K. At first, the thermal parameters for both the devices were obtained using the circuit in Fig. 1 Žwith one of the bolometers replaced by a metal film resistor.. The rationale behind this method was explained in a previously published paper w14x. Fig. 3 shows the response of both the bolometers to a step voltage input to the bridge. It is to be noted that the Wheatstone bridge has only one bolometer with the other three arms containing resistors whose resistance is the same as that of the bolometer. The thermal time constants were found to be around 115 ms for B 1 and 12 ms for B 2 . From the value of the output voltage for t 4 t the thermal conductance can be calculated from the following equation w14x Vsat s

Vs 3a 16 RG

.

Ž 6.

This gives the value of thermal conductance to be around 3.2 = 10y7 WrK for B 1 and 2.8 = 10y6 WrK for B 2 .

The thermal mass can be obtained from the variation of output voltage with time for t < t . This can be seen from Figs. 4 and 5 where the curves representing the self-heating are used to determine the thermal mass according to the following equation w14x Vout s

Vs 3a t 16 RH

.

Ž 7.

The thermal masses were thus found to be around 4 = 10y8 JrK and 3.6 = 10y8 JrK, respectively. Table 1 summarizes the properties of both the devices. The responsivity of the bolometer B 1 was estimated using input power to be around 6000 VrW at a bias voltage of 3 V. Figs. 4 and 5 also show the measured output voltages obtained under IR illumination for two different bias voltages. In the measurement, we have used one bolometer at a time Žwith the other three arms with equivalent resistors. to illustrate the effects of self-heating. It can be clearly seen that B1 is responsive to both self-heating and incident IR while B 2 is only responsive to the self-heating due to its larger thermal conductance. Also, a huge self-heating signal of about 8 mV was generated while an IR power of 110 nW generated only an output voltage of about 0.7 mV. Fig. 6 shows the output voltages obtained using both bolometers connected to the bridge as shown in Fig. 1 at a bias of 3 V. The set of curves are as a result of varying amount of IR from a blackbody source incident on the bolometers simultaneously. The curves labeled as ‘without compensation’ refer to the response obtained by using bolometer B 1 along with three equivalent resistors while those labeled as ‘with compensation’ refer to the response

Fig. 6. IR and self-heating response of the bridge consisting of two bolometers contrasted with response of the bolometer B1 at bias voltage of 3 V. The two bolometers were exposed to the same amount of IR from a blackbody source.

M.V.S. Ramakrishna et al.r Sensors and Actuators 79 (2000) 122–127

obtained by using the two bolometers B 1 and B 2 along with two equivalent resistors in the Wheatstone bridge circuit of Fig. 1. It can be seen that the self-heating signal is drastically reduced leaving only the desired output due to IR signal. This demonstrates the validity of the simple yet very effective approach employed in this study to remove the effect of self-heating. It can be seen that further reduction in the duration of pulse bias, for example, to about 100 ms would permit the use of much larger bias voltage thus further increasing the responsivity of the bolometer B 1. Optimization of these devices depends on a large number of factors such as the bias amplitude, bias duration, size of the pixel, the exact value and the ratio of thermal conductances, mismatches of thermal masses, resistance and TCR, etc. However, it was found that very good compensation could be obtained even in the case where there is considerable mismatch between the various parameters as can be seen from Table 1.

4. Conclusions By utilizing bolometers with similar thermal mass and different thermal conductivity, self-heating effect can be drastically reduced without severely affecting the response to IR signal. This new compensation scheme is robust even under considerable mismatch of device parameters. The bolometers can be easily fabricated without the need for complicated procedures that are currently utilized for compensating the self-heating effect. This concept can be easily extended to cover applications ranging from IR thermometry to IR imaging. We believe that successful engineering of thermal parameters is the key for realizing easy to fabricate, low cost, simple yet highly sensitive IR detectors.

Acknowledgements The authors would like to thank G. Chen, T. Mei and P.D. Foo for their invaluable contributions. The work is supported in part by NSTB grant GR6471.

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