METHOD TO MODULATE THE SENSITIVITY OF A BOLOMETER VIA NEGATIVE INTERFERENCE

Abstract

A semiconductor sensor system, in particular a bolometer, includes a substrate, an electrode supported by the substrate, an absorber spaced apart from the substrate, a voltage source, and a current source. The electrode can include a mirror, or the system may include a mirror separate from the electrode. Radiation absorption efficiency of the absorber is based on a minimum gap distance between the absorber and mirror. The current source applies a DC current across the absorber structure to produce a signal indicative of radiation absorbed by the absorber structure. The voltage source powers the electrode to produce a modulated electrostatic field acting on the absorber to modulate the minimum gap distance. The electrostatic field includes a DC component to adjust the absorption efficiency, and an AC component that cyclically drives the absorber to negatively interfere with noise in the signal.

Claims

1. A micro-electromechanical systems (MEMS) bolometer system, comprising: a substrate; a first absorber structure spaced apart from the substrate by a first gap; a first electrode supported by the substrate and spaced apart from the first absorber structure; and a voltage source operatively coupled to the first electrode and configured to generate a first modulated electrostatic force on the first absorber structure using the first electrode such that a minimum height of the first gap above the substrate is modulated by the first modulated electrostatic force.

2. The MEMS bolometer system of claim 1, wherein: the first modulated electrostatic force includes a DC component and an AC component.

3. The MEMS bolometer system of claim 2, further comprising: a mirror supported by the substrate at a location aligned with at least a portion of the first absorber structure.

4. The MEMS bolometer system of claim 3, wherein the mirror is located between the first electrode and the at least a portion of the first absorber structure.

5. The MEMS bolometer system of claim 2, wherein the first electrode comprises a mirror supported by the substrate at a location aligned with at least a portion of the first absorber structure.

6. The MEMS bolometer system of claim 5, further comprising: a second absorber structure spaced apart from the substrate by a second gap; a second electrode supported by the substrate and spaced apart from the second absorber structure; and an AC inverter having an output operatively coupled to the second electrode, wherein the voltage source is operatively coupled to the second electrode through the AC inverter and configured to generate a second modulated electrostatic force on the second absorber structure using the second electrode such that a minimum height of the second gap is modulated by the second modulated electrostatic force, and a phase of the first modulated electrostatic force is shifted by 180 degrees from a phase of the second modulated electrostatic force.

7. The MEMS bolometer system of claim 2, wherein: the first absorber structure has a mechanical reaction time; the first absorber structure has a thermal time constant; the AC component has a maximum frequency; and the maximum frequency is based upon the mechanical reaction time and the thermal time constant.

8. The MEMS bolometer system of claim 2, further comprising: a high pass filter operatively connected to the absorber structure; and a demodulator operatively connected to an output of the high pass filter.

9. A method of operating a micro-electromechanical systems (MEMS) bolometer system, comprising: spacing a first absorber structure apart from a substrate by a first gap; generating a first modulated electrostatic force on the first absorber structure using a first electrode supported by the substrate and spaced apart from the first absorber structure; and modulating a minimum height of the first gap above the substrate using the first modulated electrostatic force.

10. The method of claim 9, wherein generating the first modulated electrostatic force comprises: generating the first modulated electrostatic force using a DC component and an AC component.

11. The method of claim 10, wherein spacing the first absorber structure apart from the substrate by the first gap comprises: selecting a distance associated with a local maximum or minimum of a mean normalized absorption of the first absorber structure for a wavelength of interest; and spacing the first absorber structure apart from the substrate by the selected distance.

12. The method of claim 11, wherein modulating the minimum height of the first gap comprises: using the DC component to bias the first absorber structure from a first location associated with the selected distance to a second location closer to the substrate such that a first variation of the mean normalized absorption of the first absorber structure for a given change in the minimum height of the first gap at the second location is greater than a second variation of the mean normalized absorption of the first absorber structure for the given change in the minimum height of the first gap at the first location.

13. The method of claim 12, wherein spacing the first absorber structure apart from the substrate by the first gap comprises: spacing the first absorber structure apart from a mirror supported by the substrate at a location aligned with the mirror.

14. The method of claim 13, further comprising: positioning the mirror at a location between the first electrode and the at least a portion of the first absorber structure.

15. The method of claim 12, wherein the first electrode comprises a mirror supported by the substrate at a location aligned with at least a portion of the first absorber structure.

16. The method of claim 10, further comprising: spacing a second absorber structure apart from the substrate by a second gap; generating a second modulated electrostatic force on the second absorber structure using a second electrode supported by the substrate and spaced apart from the second absorber structure, the generating using the DC component and a further AC component that is phase shifted by 180 degrees relative to the AC component; and modulating a minimum height of the second gap above the substrate using the second modulated electrostatic force.

17. The method of claim 10, wherein: the first absorber structure has a mechanical reaction time; the first absorber structure has a thermal time constant; the AC component has a maximum frequency; and the maximum frequency is based upon the mechanical reaction time and the thermal time constant.

18. The method of claim 10, further comprising: isolating a signal indicative of radiation absorbed by the first absorber structure from a voltage in the first absorber structure via a high pass filter operatively connected to the absorber structure; and demodulating the signal via a demodulator operatively connected to an output of the filter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIGS. 1A-1C are graphs that illustrate a known modulation scheme including a noise component for filtering noise from a signal.

[0048] FIG. 2 depicts a side view of a bolometer with an absorber that provides the function of a thermistor in accordance with principles of this disclosure.

[0049] FIG. 3A depicts a side plan view of the bolometer of FIG. 2 along with a partial electrical schematic of the bolometer.

[0050] FIG. 3B depicts an electrical schematic of the bolometer of FIG. 2.

[0051] FIG. 4 depicts a side plan view of the bolometer depicted in FIG. 2, wherein the absorber is in a modulated position.

[0052] FIG. 5 is a graph of simulation data illustrative of absorption vs. a varying gap between the absorber and the mirror in the bolometer depicted in FIG. 2.

[0053] FIG. 6 depicts an electronic schematic of a multi-sensor bolometer according to this disclosure.

DETAILED DESCRIPTION

[0054] For the purposes of promoting an understanding of the principles of the embodiments described herein, reference is now made to the drawings and descriptions in the following written specification. No limitation to the scope of the subject matter is intended by the references. This disclosure also includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the described embodiments as would normally occur to one skilled in the art to which this document pertains.

[0055] FIG. 2 depicts a side plan view of a semiconductor sensor 100 which in this embodiment is a bolometer. The sensor 100 includes a substrate 102, a mirror, 104 an absorber 106, and suspension legs 108 and 110.

[0056] The substrate 102 may be a complementary metal oxide semiconductor (CMOS) substrate or any other acceptable type of substrate. In this embodiment, the substrate 102 is a silicon wafer. While FIG. 2 illustrates only one sensor 100 formed on the substrate 102, the substrate 102 may include any acceptable number of sensors 100, and may include electronic circuitry usable to access an output of the sensor 100.

[0057] The mirror 104 is disposed on the substrate 102, and may be, for example, a metal reflector or a multilayer dielectric reflector. The absorber 106 is suspended over the mirror 104 by suspension legs 108 and 110 to form a gap between the absorber 106 and mirror 104. In this embodiment, the minimum height of the gap between the mirror 104 and the absorber 106 is about 2.5 μm. Since the efficiency of the absorber 106 for absorbing different wavelengths of radiation is related to the gap, the gap is selected to optimize absorption in a wavelength region to be sensed. In this embodiment, the 2.5 μm height of the gap is associated with the long-wavelength infrared region.

[0058] The absorber 106, in addition to absorbing energy from incident photons, is selected to provide a good noise-equivalent temperature difference (NETD). In order for the absorber 106 to have a good NETD, the material selected to form the absorber 106 should exhibit a high temperature coefficient of resistance while exhibiting low excess noise (1/f noise). Semiconductor materials such as vanadium oxide are common in micro-machined bolometers due to their high temperature coefficient of resistance. While metals have a lower temperature coefficient of resistance than some semiconductor materials, such as vanadium oxide, metals typically have much lower excess noise than many semiconductor materials.

[0059] Accordingly, in this embodiment the absorber 106 comprises metal. Titanium and Platinum are two metals which exhibit desired characteristics. Titanium, for example, exhibits a bulk resistivity of about 7*10-7 Ohm. Using a bulk resistivity of 7*10-7 Ohm, the thickness of the absorber 106 to match the impedance of free-space Y (377 Ohm/square meter) should be about 1.9 nm. The resistivity of materials formed to a thickness less than about 50 nm, however, can be several times higher than the bulk value. Accordingly, depending on process parameters, the thickness of the absorber 106, if made from titanium, is preferably about 10 nm. Impurities can also be introduced into the absorber 106 during formation in order to tune the resistivity if needed.

[0060] Consequently, the thickness of the absorber 106 in this embodiment is about 10 nm and the length of the absorber 106 from the suspension leg 108 to the suspension leg 110 is about 25 μm. This configuration provides a ratio between the thickness of the absorber 106 and the length of the absorber 106 in the order of 1/1000 and the ratio of the thickness of the absorber 106 to the gap height of about 1/100.

[0061] Other aspects of a bolometer device such as the embodiment illustrated in FIG. 2 are described in U.S. Pat. No. 7,842,533, granted Nov. 30, 2010, the disclosure of which is incorporated herein by reference in its entirety. Where a definition or use of a term in a reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0062] In the embodiment illustrated in FIGS. 2-4, the mirror 104 is additionally configured as an electrode, although in some embodiments, a separate electrode is provided, either above or below the mirror 104.

[0063] As illustrated in FIGS. 3A and 3B, the electronic schematic 300 of the sensor 100 includes a DC current source 112, and an output voltage 114. The DC current source 112 supplies the absorber 106 with a current, in particular, a probe current. The output voltage 114 reflects a resistance change in the absorber 106 due to absorption of incident radiation, and thus corresponds to a detector signal of the sensor 100.

[0064] The detector signal of the sensor 100 is modulated by the mirror/electrode 104 in order to compensate for noise components embedded in the sensor 100 using the electronic schematic 300. The mirror/electrode 104 is configured to create an electric potential between the electrode 104 and the absorber 106 which exerts an electrostatic force that repositions the absorber 106. A voltage source 116 powers the mirror/electrode 104, and thus drives the electric potential. The voltage source 116 has a DC component configured to modulate or modify a position of the absorber 106 and an AC component configured to drive the absorber 106 with a carrier frequency.

[0065] FIG. 4 depicts a side plan view of the sensor 100, in which the voltage source 116 is powering the mirror/electrode 104 with the DC component. The DC component drives an electric potential between the mirror/electrode 104 and the absorber 106. The electric potential exerts an electrostatic force that repositions the absorber 106 such that the absorber 106 is in a modified position relative to a rest position 118. In other words, the electrostatic force acts on the absorber 106 to change a minimum distance of a gap 120 between the absorber 106 and the mirror/electrode 104. The rest position 118 corresponds to a position of the absorber 106 when the DC component of the voltage source 116 is 0 V.

[0066] While the electrostatic force acts on the absorber 106, a spring force, i.e. a resiliency, of the absorber 106 counteracts the electrostatic force, such that actuation of the absorber 106 is based upon the electrostatic force and a mechanical transfer function of a structure of the absorber 106 and the mirror/electrode 104. In other words, an extent to which the absorber 106 is repositioned is based at least in part upon the voltage source 116 that powers the mirror/electrode 104, but also upon mechanical properties of the absorber 106, the mirror/electrode 104, and other structure of the sensor 100.

[0067] For different minimum heights of the gap 120, the sensor 100 exhibits different absorption efficiencies for various wavelengths of radiation. Thus, an absorption efficiency of the absorber 106 is modulated by modifying the minimum distance of the gap 120 using the mirror/electrode 104.

[0068] FIG. 5 illustrates a graph 502 of simulation data of a mean normalized absorption of radiation by the absorber 106 due to variation of the gap 120. The rest position 118 is selected to correspond to a point on the graph 502 that is close to a local maximum or minimum of the mean normalized absorption. In one embodiment, the minimum distance between the mirror and the absorber in the rest position is about 5 μm, corresponding to a local minimum 504, or, in another embodiment, the minimum distance between the mirror and the absorber in the rest position is about 7 μm, corresponding to a local maximum 506.

[0069] When the rest position 118 is at a point on the graph close to a local minimum or maximum, the DC component of the voltage source 116 modulates the absorption efficiency of the absorber 106 to a point on the graph 502 having a higher slope relative to a local minimum or maximum. In one embodiment, the DC component modulates a minimum height of the gap 120 from about 7 μm, corresponding to the local maximum 506, to a height of about 6 μm corresponding to the point 508. In another embodiment, the DC component modulates a minimum height of the gap 120 from about 5 μm, corresponding to the local minimum 504, to a height of about 4 μm corresponding to the point 510.

[0070] Because the points 508 and 510 on the graph 502 have a high slope relative to a local minimum or maximum, the carrier frequency of the AC component, when overlaid on the DC component, will have a greater effect on the absorption efficiency of the absorber 106. The output signal resulting from the probe current of the DC current source 112 will exhibit an AC component resulting from the AC component of the voltage source 116. A maximum actuation frequency for the carrier frequency depends at least in part upon a thermal time constant of the absorber 106 and a mechanical reaction time of the absorber 106. The resulting output of the sensor 100 can be filtered using a high pass filter in a manner similar to the modulation described with regard to the middle graph of FIGS. 1A-C above.

[0071] FIG. 6 depicts a circuit diagram of a sensor system 600 which, in this embodiment is a multi-sensor bolometer. The system 600 includes a plurality of sensors, in this embodiment a first sensor 602 and a second sensor 604, a DC current source 606, a voltage source 608, an inverter 610, and a filter circuit 614, where each sensor 602 and 604 includes an absorber structure and electrode.

[0072] The DC current source 606 supplies the first sensor 602 and the second sensor 604 with a DC current, in particular, a probe current. The voltage source 608 supplies each of the first sensor 602 and the second sensor 604 with a DC current component, supplies the first sensor 602 with a first AC current component, and supplies the second sensor 604 with a second AC current component that is phase shifted 180 degrees by the inverter 610.

[0073] Outputs of the first sensor 602 and second sensor 604 are compared to form an output voltage 612. The first sensor 602 and the second sensor 604 thus form a differential pair. The output voltage 612 is modulated by a carrier frequency of the voltage source 608 and modified by mechanical transfer functions of the first sensor 602 and the second sensor 604.

[0074] The filter circuit 614 acts on the output voltage, and includes a high pass filter and a demodulator, which operate in a manner similar to the modulation described with regard to the middle graph of FIGS. 1A-C above to isolate components of the detection signal that are indicative of the radiation absorbed by the absorber material and to filter out low frequency noise and thermal drift and return the resulting filtered signal back to base band frequency through demodulation.

[0075] While the above embodiments have been described with reference to detection of infrared radiation using a bolometer, the reader should appreciate that the above-described tool is not limited to infrared radiation. A bolometer according to the present disclosure can be configured for other types of detection, for example detecting particles, gravity waves, microwaves, and far-infrared radiation. Additionally, while various embodiments have been described above, the present disclosure is not limited to such embodiments. Other embodiments include one or more features described above.

[0076] It will be appreciated that variants of the above-described and other features and functions, or alternatives thereof, may be desirably combined into many other different systems, applications or methods. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements may be subsequently made by those skilled in the art that are also intended to be encompassed by the foregoing disclosure.