Pyroelectric sensor

• US 20070108385A1
• Filed: 02/20/2004
• Published: 05/17/2007
• Est. Priority Date: 02/21/2003
• Status: Active Grant

• Current Assignee: Aptiv Technologies AG (Aptiv PLC), Symetrix Corporation
• Original Assignee: Symetrix Corporation
• Inventors: Celinska, Jolanta, McMillan, Larry, De Araujo, Carlos, Schubring, Norman, Mantese, Joseph, Micheli, Adolph
• Granted Patent: US 7,564,021 B2
• Time in Patent Office: Days
• US Class Current: 250/338.300
• CPC Class Codes: G01J 5/34 using capacitors, e.g. pyroelectric capacitors

Abstract

A ferroelectric/pyroelectric sensor employs a technique for determining a charge output of a ferroelectric scene element of the sensor by measuring the hysteresis loop output of the scene element several times during a particular time frame for the same temperature. An external AC signal is applied to the ferroelectric scene element to cause the hysteresis loop output from the element to switch polarization. Charge integration circuitry, such as a combination output capacitor and operational amplifier, is employed to measure the charge from the scene element. Preferably, the ferroelectric of the scene element is made of an economical and responsive strontium bismuth tantalate, SBT, or derivative thereof, disposed directly between top and bottom electrodes. Because of the frequency characteristics of the sensor, created by the external AC signal, the element need not be thermally isolated from the silicon substrate by a traditional air bridge, which is difficult to manufacture, and instead is preferably thermally isolated by spin-on-glass, SOG. To prevent saturation of an output signal voltage of the sensor by excessive charge accumulation in an output capacitor, the sensor preferably has a reference element configured electrically in parallel with the scene element. When the voltage of the AC signal is negative the output capacitor is discharged by flowing current through the reference element thus interrogating the polarization of the reference element which is compared to and subtracted from the polarization of the scene element for each cycle. The polarization difference measured for each cycle over a set time period are summed by an integrating amplifier to produce a signal output voltage.

Claims

1. A pyroelectric sensor system comprising:

an alternating voltage source generating an alternating voltage at a predetermined frequency;

a pyroelectric scene element responsive to the alternating voltage, said scene element generating a hysteresis loop charge output in response to the alternating voltages, said alternating voltage driving the scene element over at least a portion of the hysteresis loop output of the scene element, said scene element being constructed and arranged so that the alternating voltage causes a polarization reversal of dipoles in the scene element in response to positive and negative changes of the alternating voltage;

a charge integration system responsive to the charge output from the scene element, said integration system determining the area within the hysteresis loop defined by the hysteresis loop output and generating a signal indicative of the charge output from the scene element; and

a ferroelectric layer of the scene element being selected from a group consisting of strontium-bismuth-tantalate (SBT), barium-bismuth-tantalate, and lanthium-bismuth-tantalate.

2. The pyroelectric sensor system set forth in claim 1 wherein the system is an infrared imaging system and the scene element has an infrared absorber that absorbs infrared radiation directed at the scene element from a scene.

3. The pyroelectric sensor system set forth in claim 1 comprising:

the ferroelectric layer being disposed between a first and a second capacitive plate of the scene element;

wherein the alternating voltage is applied to the first capacitive plate so that the alternating voltage causes a polarization reversal of dipoles in the scene element in response to positive and negative changes of the alternating voltage; and

wherein the charge integration system is responsive to the charge output from the second capacitive plate of the scene element.

4. The pyroelectric sensor system set forth in claim 3 comprising:

a silicon substrate structure acting as a thermal heat sink; and

a continuous thermal barrier coating attached directly between the second capacitive plate and the silicon substrate, wherein the thermal barrier does not have an air bridge.

5. The pyroelectric sensor system set forth in claim 1 comprising:

a ferroelectric reference element configured electrically parallel to the scene element;

wherein the reference element is responsive to the alternating voltage and generates a hysteresis loop charge output in response to the alternating voltage which drives the reference element over at least a portion of the hysteresis loop output of the reference element, the element being constructed and arranged so that the alternating voltage causes a polarization reversal of dipoles in the reference element in response to positive a negative changes of the alternating voltage; and

the charge integration system being responsive to the charge output from the reference element, the integration system determining the area within the hysteresis loop output of the reference element defined by the hysteresis loop output of the reference element and generating a signal voltage output indicative of a difference between the hysteresis loop outputs of the scene and reference elements.

6. The pyroelectric sensor system set forth in claim 5 comprising:

an operation amplifier of the charge integration system; and

wherein the signal voltage output of the charge integration system is indicative of a plurality of summed differences between the hysteresis loop outputs.

7. The pyroelectric sensor system set forth in claim 6 wherein the number of the plurality of summed differences is equal to the product of a predetermined time period of scene measurement times the frequency of the external AC signal.

8. The pyroelectric sensor system set forth in claim 7 wherein the charge integration system receives a current from the scene element when the external AC signal voltage is positive and returns a current to the reference element for interrogation of the reference polarization when the external AC signal voltage is negative.

9. The pyroelectric sensor system set forth in claim 5 comprising:

an output capacitor of the charge integration system;

an operation amplifier of the charge integration system configured in parallel with the output capacitor and acting as a summer of a plurality of summed differences between the hysteresis loop outputs of the scene and reference elements which is indicative of the signal voltage output;

a scene diode detector for flowing current from the scene element to the charge integration system for charging the output capacitor when the external alternating voltage is positive; and

a reference diode for flowing current from the charge integration system to the reference element for discharging the output capacitor and interrogating the reference element polarization when the external alternating voltage is negative.

10. A pyroelectric sensor system comprising:

an alternating voltage source generating an alternating voltage at a predetermined frequency;

a pyroelectric scene element responsive to the alternating voltage, said scene element generating a hysteresis loop charge output in response to the alternating voltages, said alternating voltage driving the scene element over at least a portion of the hysteresis loop output of the scene element, said scene element having a ferroelectric layer disposed between a first capacitive plate and a second capacitive plate wherein the alternating voltage is applied to the first capacitive plate so that the alternating voltage causes a polarization reversal of dipoles in the scene element in response to positive and negative changes of the alternating voltage;

a charge integration system responsive to the charge output from the second capacitive plate of the scene element, said integration system determining the area within the hysteresis loop defined by the hysteresis loop output and generating a signal indicative of the charge output from the scene element;

a silicon substrate structure acting as a thermal heat sink; and

a continuous thermal barrier coated directly between the second capacitive plate and the silicon substrate, wherein the thermal barrier does not have an air bridge.

11. The pyroelectric sensor system set forth in claim 10 wherein the thermal barrier is spin-on-glass (SOG).

12. The pyroelectric sensor system set forth in claim 10 wherein the thermal barrier is yttria stabilized zirconia (YSZ).

13. The pyroelectric sensor system set forth in claim 10 wherein the system is an infrared imaging system and the scene element has an infrared absorber that absorbs infrared radiation directed at the scene element from a scene.

14. The pyroelectric sensor system set forth in claim 10 wherein the ferroelectric layer of the scene element is selected from a group consisting of strontium-bismuth-tantalate (SBT), barium-bismuth-tantalate, and lanthium-bismuth-tantalate.

15. The pyroelectric sensor system set forth in claim 10 comprising:

a ferroelectric reference element configured electrically parallel to the scene element;

wherein the reference element is responsive to the alternating voltage and generates a hysteresis loop charge output in response to the alternating voltage which drives the reference element over at least a portion of the hysteresis loop output of the reference element, the element being constructed and arranged so that the alternating voltage causes a polarization reversal of dipoles in the reference element in response to positive a negative changes of the alternating voltage; and

the charge integration system being responsive to the charge output from the reference element, the integration system determining the area within the hysteresis loop output of the reference element defined by the hysteresis loop output of the reference element and generating a signal voltage output indicative of a difference between the hysteresis loop outputs of the scene and reference elements.

16. The pyroelectric sensor system set forth in claim 15 comprising:

an operation amplifier of the charge integration system; and

wherein the signal voltage output of the charge integration system is indicative of a plurality of summed differences between the hysteresis loop outputs.

17. The pyroelectric sensor system set forth in claim 16 wherein the number of the plurality of summed differences is equal to the product of a predetermined time period of scene measurement times the frequency of the external AC signal.

18. The pyroelectric sensor system set forth in claim 17 wherein the charge integration system receives a current from the scene element when the external AC signal voltage is positive and returns a current to the reference element for interrogation of the reference polarization when the external AC signal voltage is negative.

19. The pyroelectric sensor system set forth in claim 15 comprising:

an output capacitor of the charge integration system;

an operation amplifier of the charge integration system configured in parallel with the output capacitor and acting as a summer of a plurality of summed differences between the hysteresis loop outputs of the scene and reference elements which is indicative of the signal voltage output;

a scene diode detector for flowing current from the scene element to the charge integration system for charging the output capacitor when the external alternating voltage is positive; and

a reference diode for flowing current from the charge integration system to the reference element for discharging the output capacitor and interrogating the reference element polarization when the external alternating voltage is negative.

20. A pyroelectric sensor system comprising:

an alternating voltage source generating an alternating voltage at a predetermined frequency;

a pyroelectric scene element;

a ferroelectric reference element configured electrically parallel to the pyroelectric scene element;

wherein the scene and reference elements are responsive to the alternating voltage, said elements generating a hysteresis loop charge output in response to the alternating voltages, said alternating voltage driving the elements over at least a portion of the hysteresis loop outputs of the respective elements, said elements being constructed and arranged so that the alternating voltage causes a polarization reversal of dipoles in the elements in response to positive and negative changes of the alternating voltage; and

a charge integration system responsive to the charge output from the scene and reference elements, said integration system determining the area within the hysteresis loops defined by the hysteresis loop outputs and generating a signal voltage output indicative of a difference between the hysteresis loop outputs of the elements.

21. The pyroelectric sensor system set forth in claim 20 comprising:

an operation amplifier of the charge integration system; and

wherein the signal voltage output of the charge integration system is indicative of a plurality of summed differences between the hysteresis loop outputs.

22. The pyroelectric sensor system set forth in claim 21 wherein the number of the plurality of summed differences is equal to the product of a predetermined time period of scene measurement times the frequency of the external AC signal.

23. The pyroelectric sensor system set forth in claim 22 wherein the charge integration system receives a current from the scene element when the external AC signal voltage is positive and returns a current to the reference element for interrogation of the reference polarization when the external AC signal voltage is negative.

24. The pyroelectric sensor system set forth in claim 20 comprising:

an output capacitor of the charge integration system;

an operation amplifier of the charge integration system configured in parallel with the output capacitor and acting as a summer of a plurality of summed differences between the hysteresis loop outputs of the scene and reference elements which is indicative of the signal voltage output;

a scene diode detector for flowing current from the scene element to the charge integration system for charging the output capacitor when the external alternating voltage is positive; and

a reference diode for flowing current from the charge integration system to the reference element for discharging the output capacitor and interrogating the reference element polarization when the external alternating voltage is negative.

25. The pyroelectric sensor system set forth in claim 20 wherein ferroelectric layers of the scene and reference elements are selected from a group consisting of strontium-bismuth-tantalate (SBT), barium-bismuth-tantalate, and lanthium-bismuth-tantalate.

26. The pyroelectric sensor system set forth in claim 20 comprising:

the scene and reference elements each having a ferroelectric layer disposed between a first capacitive plate and a second capacitive plate wherein the alternating voltage is applied to the first capacitive plates so that the alternating voltage causes a polarization reversal of dipoles in the scene and reference elements in response to positive and negative changes of the alternating voltage;

a silicon substrate structure acting as a thermal heat sink; and

a continuous thermal barrier coating attached directly between the second capacitive plate of the scene element and the silicon substrate, wherein the thermal barrier does not have an air bridge.