WO2007015235A1

WO2007015235A1 - Circuitry for balancing a differential type focal plane array of bolometer based infra-red detectors

Info

Publication number: WO2007015235A1
Application number: PCT/IL2006/000884
Authority: WO
Inventors: Amnon Adin
Original assignee: Semi-Conductor Devices-An Elbit Systems-Rafael Partnership
Priority date: 2005-08-04
Filing date: 2006-07-31
Publication date: 2007-02-08

Abstract

The present invention relates to a bolometer type focal plane array system with circuitry for adjusting its differential measuring pixel detectors during reading, which comprises: (a) an array of i x j pixel detectors of the bolometer type for sensing scenery radiation, wherein each of said detectors provides a differential measurement of the scenery radiation, and comprises at least one resistor which is sensitive to IR radiation coming from the scenery, and at least one detector reference resistor which is masked from the scenery; (b) j column current sources (100) that comprises at least one source reference resistor (Rb) having a same TCR as of said at least one detector reference resistor, and which is thermally shorted to said substrate, wherein said current source provides to a selected detector a corresponding compensation current (Jc) which is inversely proportional to said at least one source reference resistor; (c) row selector for sequentially selecting a row n within said FPA, while connecting respectively each column current source to a corresponding pixel detector within the selected row thereby to provide a compensation current by each of said current sources to the corresponding detector; and (d) reading circuitry for reading indication for the scenery radiation as sensed by each of the i x j detectors of the array.

Description

METHOP AND CIRCUITRY FOR BALANCING A

DIFFERENTIAL TYPE FOCAL PLANE ARRAY OF BOLOMETER

BASED INFRA-RED DETECTORS

Field of the Invention

The field of the invention relates to devices for sensing light radiation. More particularly, the present invention relates to a method and circuitry for compensating for variations in the substrate temperature of a bolometer type focal plane array.

Background of the Invention

Bolometers are widely used for sensing low radiation of light, generally in the IR band. In most conventional cases, the bolometers are provided in a form of a focal plane array (FPA), wherein the array comprises a plurality of individual sensing elements (hereinafter also referred to as "pixels" or "pixel detectors"). A significant advantage of the bolometer type detectors is their reduced weight and power consumption, particularly due to the fact that they do not require cryogenic cooling. In addition, they are generally much less expensive in comparison with cooled focal plane arrays. However, the typical sensitivity of bolometer type detectors is significantly lower than of cooled-type detectors. Moreover, as bolometer type detectors are very sensitive to temperature variation, they require special means for stabilizing the temperature of the array (FPA) substrate, and for compensating each individual detector for said temperature variations.

VOx (Vanadium Oxide) resistors are widely used in typical bolometers, as the VOx has a relatively large TCR (temperature coefficient of resistance), and low 1/f noise contribution. Typical bolometer FPAs are required to detect radiation with a resolution in the order of 50°mK of the scenery temperature. The temperature variations at the bolometer due to such heat variations within the scenery are in the order of 0.1°mK. It should be noted that in order to bring the detector to its operational point, it is required to heat the active resistor (the resistor which is exposed to the scenery) of the detector by a temperature in the order of few degrees. Said necessity to provide a sensitivity and resolution in the order of 40000 times less than the heating of the active bolometer resistor enforces the use of a differential measurement. The most common and simple circuitry that applies differential measurement is the Wheatstone bridge, and a circuitry which includes Wheatstone bridge is commonly used in bolometer-type FPAs, as will be described in detail later.

^■However, even though a Wheatstone bridge performs a differential measurement, the output signal must be amplified several hundred times before it can be provided to the display. Therefore, even a small imbalance of the bridge (which is practically unavoidable due to production tolerances) causes saturation at the amplifier output and special compensation circuitry should be used. The outcome of the compensation is the convergence of the amplifier output at the calibration point into a specified range, e.g. ±100 mV (an additional procedure, known as Residual Non-Uniformity Correction — NUC - is carried out external to the FPA in order to get a much better uniform display of a uniform background). Yet, the prior art compensated uncooled bolometer-type FPAs are still very sensitive to variations in the temperature of the substrate, which should be highly stabilized (better than ± 10 m°C in the prior art high end FPA's).

Therefore, special compensation circuitries are required for compensating in the FPA pixel level for all said variations of temperature. More particularly, special circuitry is required to compensate for the non-uniformity of the detectors (i.e., to compensate for their different offset and gain), and to further compensate for the nonuniform effect of the change of the substrate temperature on each detector.

The readout from an i rows x j columns bolometer- type detectors matrix is typically made in a row by row manner by means of j column amplifiers. However, before reading from each differential detector, for example the Wheatstone bridge, it is mandatory to assure that the differential detector is balanced. The imbalance of a bridge evolves, among other reasons, from the non-uniformity of the VOx resistors forming the bridge, and also from the variations of the substrate temperature. It should be noted that typically there are special control means for maintaining a constant and stable substrate temperature. However, these control means cannot assure absolute temperature stability, and even a very small variation in the temperature immediately affects the detector balance. The effect of non-uniformity of the VOx resistors on the balance of the bridges can be corrected in a coarse manner relatively easily. This is typically done by performing a pre-measurement procedure for a specific substrate temperature that results in one matrix of correction data for same selected substrate temperature, that includes for each pixel bridge a level and polarity of compensation current that has to be supplied to the bridge before reading in order to assure balance. However, each such matrix with fixed correction data cannot account for the dynamic variations in the substrate temperature. Moreover, the larger the expected variations are, and the larger the non-uniformity of the components is, a higher relative resolution is required from the current sources that provide the compensation balancing current.

It is an object of the present invention to provide a circuitry that dynamically adjusts the value of the various current sources upon variation in the substrate temperature.

It is still another object of the present invention to provide a circuitry that will loosen the substrate temperature stability requirement in comparison with the prior art.

It is still another object of the present invention to provide a circuitry that will substantially increase the period between consecutive calibration procedures, each of which involves closure of a shutter which temporary eliminates the acquisition of an image.

It is still another object of the present invention to provide a circuitry that will improve the image that is obtained from the FPA.

These and other objects and advantages of the present invention will become apparent as the description proceeds.

Summary of the Invention

The present invention relates to a bolometer type focal plane array system with circuitry for adjusting its differential measuring pixel detectors during reading, which comprises: (a) an array of i x j pixel detectors of the bolometer type for sensing scenery radiation, wherein each of said detectors provides a differential measurement of the scenery radiation, and comprises at least one resistor which is sensitive to IR radiation coming from the scenery, and at least one detector reference resistor which is masked from the scenery; (b) j column current sources that comprises at least one source reference resistor having a same TCR as of said at least one detector reference resistor, and which is thermally shorted to said substrate, wherein said current source provides to a selected detector a corresponding compensation current which is inversely proportional to said at least one source reference resistor; (c) row selector for sequentially selecting a row n within said FPA, while connecting respectively each column current source to a corresponding pixel detector within the selected row thereby to provide a compensation current by each of said current sources to the corresponding detector; and (d) reading circuitry for reading indication for the scenery radiation as sensed by each of the i xj detectors of the array.

Preferably, each of the detectors is connected in a bridge-like structure.

Preferably, each of the detectors is connected in a Wheatstone bridge structure.

Preferably, each bridge detector comprises one resistor which is exposed to scenery radiation and is thermally coupled to the substrate through very high thermal resistance (this kind of coupling will be referred hereon for brevity as "almost isolated"), one resistor which is masked from scenery radiation and is thermally almost isolated from the substrate, and two reference resistors that are thermally shorted to the substrate, and wherein all said resistors having a same TCR. Therefore, said four resistors follow the substrate temperature variations, while the one exposed to the scenery develops an additional temperature difference which depends upon the scenery temperature.

Preferably, said masked and reference resistors are common to more than one bridge.

Preferably, all the detector resistors, and the source reference resistors are VOx resistors.

Preferably, said current sources provide to each decoder a corresponding compensation current, as listed in a pre-measurement matrix.

Preferably, each current source comprises a reference current stage that comprises said reference source resistor, and a plurality of positive and negative polarity current mirroring stages, for selecting a current, the amplitude of which is multiplicity of the current produced by said reference current stage. In a preferable case, one reference current stage is common to all the column current sources of the array. In one embodiment of the invention, each of the two reference resistors is replaced by a constant current source.

Brief Description of the Drawings

In the drawings:

Fig. 1 illustrates the general structure of a typical prior art array of bolometer-type detectors;

Fig. 2 illustrates a typical structure of one bolometer-type detector having a structure of a Wheatstone bridge;

- Fig. 3 is an example for the structure of a typical 2X2 pixels FPA according to the prior art;

- Fig. 4 illustrates the structure of one bolometer-type detector having a structure of a Wheatstone bridge, with a compensation current source;

Fig. 5 illustrates the structure of a compensation, mirroring-type current source according to one embodiment of the invention; and Fig. 6 illustrates the general structure of a system for providing compensation current to a bolometer-type FPA.

Detailed Description of Preferred Embodiments

The general structure of a typical FPA 1 of the bolometer type is shown in Fig. 1. The FPA 1 has i rows and j columns, therefore comprising i xj bolometer-type matrix of pixel detectors. The readout from the FPA is performed by selecting a full row of pixel detectors by means of row selector 2. When a row is selected, all the detectors of the selected row are simultaneously sensed, and the readouts from all the pixel detectors of the selected row are provided into the inputs of corresponding; column amplifiers Ai — Aj. As will be elaborated hereinafter, when a row is selected, the reference signal for the differential amplification is common to all the column amplifiers. Fig. 2 shows the general structure of one of said i x j pixel detectors of the FPA of Fig. 1. As shown, all resistors of the detector are arranged in a form of Wheatstone bridge, which provides a differential measurement. The resistor Rp is the "active" resistor which is specific to each pixel detector. All the resistors R_P are thermally almost isolated from the substrate and are exposed to the scenery. The resistor Rr is thermally almost isolated from the substrate, and is common to all the pixel detectors of each row, and which is referred to herein as a "blind" resistor, as it is totally masked from the scenery. Resistor Rm is one resistor which is common to all the pixel detectors of the FPA, said resistor Rm is thermally shorted to the substrate of the FPA. Resistor Rc is one resistor per column, which is common to all the pixel detectors within each column, and which is also thermally shorted to the substrate. Therefore, in the exemplary FPA of Fig. 1, there are i x j resistors R_P, j resistors Rc, i resistors Rr, and one resistor R_m. All the resistors are preferably of VOx type, and should preferably have as identical properties as possible, more particularly, as identical as possible electrical resistance in pairs (Rp = Rr, Rc = Rm), same thermal coefficient of resistance (TCR), and Rp and Rr should have same thermal capacitance and resistance. It should be noted that the resistors R_r and Rm may be advantageously made of several resistors that are connected in parallel, provided that . Whenever a pixel is selected,

the differential readout is conveyed into the corresponding column amplifier Ai-Aj.

It should also be noted that while all the active resistors R_P are constantly irradiated, the readout is streamed out one row at a time, using a single amplifier per column. The "blind" resistor Rr is used for compensating against the dynamic thermal behavior of the active resistor R_p during the readout. Therefore, it is located at a thermally almost isolated location which is masked from the scenery radiation. Each said resistor Rr is electrically connected only during the readout of the specific row. The other resistors Rc and Rm are repetitively connected during the readout of each one of the rows. These resistors are thermally shorted to the substrate in order to prevent their destruction due to excessive heating. Thus, when each specific bridge is active, there are formed two branches, where the differential voltage Vo is a function of the scenery radiation which is applied over Rp only.

Fig. 3 illustrates the structure of a typical FPA 1. For the sake of brevity, the FPA was reduced to the size of 2 x 2 pixel detectors. The row select signals that are produced by the row selector 2 (of Fig.l), enable the selection of rows in a sequential order. When a row is selected, the voltage at the junction between the resistor Rm, (which is one resistor common to all the FPA pixel detectors) and the selected Er is simultaneously provided to a first of the two inputs of each column amplifier as a reference signal. The voltage over each corresponding resistor R_P within the selected row is provided to the second input of the corresponding amplifier. It can be easily seen that the structure is of a Wheatstone bridge where Rm and the various resistors Rc and Rr complete the bridges, and therefore the radiation measurement within each detector is differential.

The "natural" provision in order to account for the non-uniformity of the various bridge resistors, which cause imbalance of the bridge, would be connecting additional series and/or parallel resistors. Yet, the resolution and switching requirements make it impractical and a current source may be used instead for providing a balancing current during the reading from the bridge. It should be noted that the imbalance of the various bridges, if not properly corrected, may result in a saturation at the output of the operational amplifier A[i,s,.. j] (see Fig. 1). Fig. 4 shows a typical prior art structure of a bridge detector, with a compensating current source 100. As the reading from the array is performed in a row by row manner, there are needed j such corresponding current sources, i.e., Ici to Jc_/. The value of the current and its direction (polarity) 70 or 70' is specific to each bridge detector, and is determined in a pre- measurement procedure. The pre-measurement procedure provides for each selected substrate temperature a matrix which indicates for each specific pixel detector the value and direction of the corresponding compensating current Ici to Lj. However, as each such matrix is suitable for only one substrate temperature, several such matrices are necessary for several substrate temperatures. Still, there are substrate temperatures (between those temperatures for which matrices are provided) in which the compensation currents only partially compensate for the substrate temperature variations. As will be shown, one advantage of the present invention is that the current sources of the present invention are positioned on the substrate of the FPA and change in accordance with the substrate temperature.

An amplifier stable operation point is essential and all elements of the bridge should have an identical response to substrate temperature variations. The problem evolves from the fact that each bridge includes components of somewhat different behavior. In view of the above compensation requirements, even a rough compensation requires quite a high resolution in the supply of the current, in the order of 10 digital bits (1:1024).

In typical high performance un-cooled imagers the substrate temperature stabilization is in the order of 10 m°K. It has been noted by the inventors that a significant relief in the temperature stabilization requirement can be obtained when the temperature dependency behavior of the compensating current source Ic resembles inversely a temperature dependency of a bolometer-type resistor, identical to the behavior of the resistors of the pixel bridge.

Looking at Fig. 4, if the four bridge resistors have same TCRs but exhibit

a resistance miss-mach, resulting i R y /C, ≠ R v /l , then, the output

voltage of a non-radiated bridge before compensation is :

With a steady supply voltage and assuming that all four bolometers have the same temperature coefficient, Vo(O) depends only upon the resistances ratio and does not change with the substrate temperature. The balancing voltage contribution of the compensating current source should be:

P[Rp I \RcJ = -Vo(O) which should remain unchanged at any substrate temperature in spite of the resistances variation. This implies that in order to keep the bridge balanced, the compensation current Ic has to change inversely with respect to the resistance change of the VOx resistors of the bridge. Therefore, according to the present invention the current source (CS) which produces the current Ic is based on a VOx resistor, which is positioned on, and is thermally shorted to the substrate and the voltage upon which is constant. Thus the current Ic that this current source supplies inversely changes with respect to the variation of the bridge resistances with substrate temperature. Of course, said current source has to be capable of producing a selective two-way current direction.

Fig. 5 illustrates a circuitry 100 for providing compensation current Ic to a selective bridge pixel. The circuitry 100 is essentially a current source, and therefore it is also indicated in the application and drawings as CS. According to the invention the resistor Rb is a same type resistor with same TCR (Temperature Coefficient of Resistance) as of the resistors of the pixel bridges. As the resistors of the bridges are typically made of VOx, the resistor Rb is also preferably made of VOx, to match the TCR characteristic of the bridges resistors. The resistor Rb is also thermally shorted to the substrate. OpA is an operational amplifier, and Vcc is a stabilized power supply. Vr is a constant reference voltage, which may be produced from Vcc, optionally by on-chip voltage divider resistors, all having same temperature coefficients. Ma is a MOS transistor in the amplifier feedback loop. The voltage across Rb is equal to Vr due to the

high gain feedback loop. Therefore, changes with temperature

inversely with respect to the resistance change of the VOx resistor (Rb) as required for compensation. I serves as a reference current, which is scaled, or inverted by means of circuitry 35, which may be, for example, a binary scaled current mirroring circuit well known in the art, as shown. A typical current mirroring circuit receives a reference current, and scales this current by a plurality of selective scaling units. In the general mirroring circuit 35 of Fig. 5, units Ma, Mb, Mc and Md provide reference currents / to the twin polarity mirror circuits MOa to Mna and MOb to Mnb. MOa and MOb, each outputs current having a magnitude equal to I, Mlα and MIb, each outputs current having magnitude of 2*1,

and so on, while Mnα and Mnb each outputs current equals to 2ⁿ*J. Each

time when a specific configuration 38a or 38b of switches is selected, a corresponding current Ic is with the desired amplitude and polarity is produced. It should be noted that the selection is performed either from the switches 38a or switches 38b, so that in this manner the polarity of the desired Ic is selected. The one or more individual currents that are produced by the selected Mo to M_n current units (α or b) are then summed at line 97 forming the compensation currents -Ic or +I_C respectively. Of course, there may be 2ⁿ⁺¹ optional values for L, and same number for -L all being multiples of I. The values of land n are determined according to the required current resolution. More particularly, once I is determined during the design phase, n is selected during the calibration process. It should be noted that upon selection of a specific row, each of the column compensating current sources 100 compensates its corresponding column bridge. It should be further noted that preferably, the reference section 135 of Fig. 5 is a single reference circuit which is common to all the columns, while there are j mirroring sections 35, one for each column amplifier. Furthermore, it should be noted that the appropriate magnitude and polarity of the current (and therefore the selected combination of switches) which is suitable for each specific bridge is obtained from said predetermined matrix 101 (see Fig. 6) which is obtained during a pre-measurement step.

Fig. 6 is a general block diagram further illustrating the structure of the invention. Matrix 101 is the pre-measurement compensation matrix, which contains indications for the necessary compensation currents Ic specific to each of the i x j sensors of the array. When row selector 2 selects a row, indications 120 for the corresponding compensation currents Ic(i,2,..j) axe conveyed to corresponding switch selector units 121 SEL(i,2,...j). Based on the corresponding compensation current, each selector unit determines and conveys the corresponding combination of switches, while the polarity is determined by the selection of set 38o or set 386 of the switches (see Fig. 5). These are the various compensation currents Ic required from each of the sources 100).

EXAMPLE

The following example illustrates how the Vo in a bridge such as in Fig. 2, which is not perfectly balanced and is not exposed to the scenery, is maintained unchanged even when the temperature changes significantly, and how a circuit such as shown in Fig. 4 and Fig. 5 compensates it automatically.

Assuming that the initial temperature of 300⁰K, has changed to Ts; each of the resistors of the bridge changes by a same factor a (the factor a is identical to all the bridge resistors, as they are all made from FOx, have the same TCR, and are all thermally coupled to the same substrate).

Looking at Fig. 2 we obtain:

Rmn _(T2)

Adding the current source Ic (100 in Fig. 4) and selecting the switch combination 38a or 38b (Fig. 5) which compensates for Vo at 300⁰K:

where:

" — I Vindicating the current multiplication factor

of the reference current I.

At the Ts temperature, assuming that Rb of Fig.5 has the factor a as all the bridge resistors, then keeping same said switch combination (same K) as at 300⁰K, the compensating current becomes:

and the compensation contribution of Ic(τs) is:

j _:

It can therefore be concluded that the use of the compensating current source circuitry of the invention, which comprises a resistor having the same TCR as of the bridges resistors and which is also thermally shorted to the substrate, provides a very high compensation stability with substrate temperature variations, which is limited only by the dispersion of the TCRs of the various VOx resistors. Furthermore, the operation of a typical high end (high performance) FPAs is limited to a discreet and highly stabilized substrate temperature. When the ambient and case temperatures change, stabilizing the former exact substrate temperature results in high power consumption. In order to reduce this power consumption, the stabilized substrate temperature may be selected to be close to the ambient temperature. Thus, several pre-measurement matrices 101 corresponding to several substrate temperature operating points should be prepared. Also, as the bolometer resistance changes extremely over a specified substrate range (e.g. +700% to -50% of the 27°C value for -40⁰C to +7O⁰C substrate range), the "constant" compensating current value should also be adjusted accordingly, requiring an elaborate circuitry (such as a switching circuitry for selection from several reference resistors). The present invention overcomes this difficulty as the compensating current adjusts itself automatically. Furthermore, while prior art high end FPAs require stabilizing the substrate temperature to better than ± 10 m°C, the present invention enables much looser stabilization, limited only by the dispersion of the VOx resistors TCRs.

Experiment

An un-cooled FPA having a compensation circuitry according to the invention was used in ambient temperature varying between -4O⁰C to +7O⁰C, where the substrate temperature has been stabilized to the ambient one. It has been found that a full compensation was obtained in all this range of temperatures where the compensation reference current has adjusted itself automatically without any hardware change (of course, the mirrored current switch combination was adjusted according to the substrate temperature).

It should be noted that the performance of the compensation arrangement of the present invention is limited by only the dispersion of the TCRs of the various bridges resistors and of the

involved. Experimental results showed that after compensation and NUC calibrations at a specific substrate temperature, there was only minor uniformity degradation (equivalent to 50 m°C in the scenery) when the substrate temperature was changed by ± 50 m°C (compared to the more tight t3φical prior requirement of ± 10 m°C substrate temperature stability requirement). In other words, the requirement for substrate temperature stability is significantly loosened.

It should be noted that the invention and examples given above related to a bridge having four resistors. However, resistors R_m and R_c can be replaced by two identical current sources without affecting the principle invention. In that case, the bridge contains only the resistors Rp and Rr. While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried into practice with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.

Claims

1. A bolometer type focal plane array system with circuitry for adjusting its differential measuring pixel detectors during reading, which comprises: a. an array of i x j pixel detectors of the bolometer type for sensing scenery radiation, wherein each of said detectors provides a differential measurement of the scenery radiation, and comprises at least one resistor which is sensitive to IR radiation coming from the scenery, and at least one detector reference resistor which is masked from the scenery; b. j column current sources that comprise at least one current source reference resistor having a same TCR as of said at least one detector reference resistor, and which is thermally shorted to said substrate, wherein said current source provides to a selected detector a corresponding compensation current which is inversely proportional to said at least one source reference resistor; c. row selector for sequentially selecting a row n within said FPA, while connecting respectively each column current source to a corresponding pixel detector within the selected row thereby to provide a compensation current by each of said current sources to the corresponding detector; and d. reading circuitry for reading indication for the scenery radiation as sensed by each of the i x j detectors of the array.

2. A focal plane array according to claim 1, wherein each of the detectors is connected in a bridge-like structure.

3. A focal plane array according to claim 2, wherein each of the detectors is connected in a Wheatstone bridge structure.

4. A focal plane array according to claim 3, wherein each bridge detector comprises one resistor which is exposed to scenery radiation and is thermally almost isolated from the substrate, one resistor which is masked from scenery radiation and is thermally almost isolated from the substrate, and two reference resistors that are thermally shorted to the substrate, and wherein all said resistors having a same TCR.

5. A focal plane array according to claim 4, wherein said masked and reference resistors are common to more than one bridge.

6. A focal plane array according to claim 1, wherein all the detector resistors, and the source reference resistors are VOx resistors.

7. A focal plane array according to claim 1, wherein said current sources provide to each decoder a corresponding compensation current, as listed in a pre-measurement matrix.

8. A focal plane array according to claim 7, wherein each current source comprises a reference current stage that comprises said reference source resistor, and a plurality of positive and negative polarity current mirroring stages, for selecting a current, the amplitude of which is multiplicity of the current produced by said reference current stage.

9. A focal plane array according to claim 8, comprising one reference current stage which is common to all the column current sources of the array.

10. A focal plane array according to claim 4 wherein each of said two reference resistors is replaced by a constant current source.