In infrared detectors, the use of devices configured in the form of an array and capable of operating at ambient temperature, i.e. not requiring cooling to extremely low temperatures, is known—in contrast to detecting devices called “quantum detectors” which can only operate at extremely low temperature, typically that of liquid nitrogen.
These uncooled detectors traditionally use the variation in a physical unit of an appropriate material as a function of temperature at around 300 K. In the case of bolometric detectors, this physical unit is electrical resistivity.
Such an uncooled detector is generally associated with:
means of absorbing the infrared radiation and converting it into heat,
means of thermally isolating the detector so that its temperature can rise due to the effect of the infrared radiation,
thermometric means which, in the context of a bolometric detector, use a resistance element,
and means of reading electrical signals provided by the thermometric means.
Detectors intended for infrared imaging are conventionally produced as a one- or two-dimensional array of elementary detectors, said array being “monolithic” or mounted on a substrate generally made of silicon which incorporates means of sequentially addressing the elementary detectors and means of electrical excitation and of pre-processing the electrical signals generated by these elementary detectors. These means of sequential addressing, electrical excitation and pre-processing are thus produced on the substrate and constitute a readout circuit.
Although monolithic integration of the detectors with the corresponding readout circuit is advantageous in terms of manufacturing costs, it is nevertheless possible to hybridise an array of detectors on such a readout circuit.
The device comprising such an array of elementary detectors and an associated readout circuit is generally placed in a hermetically sealed package and is electrically and thermally connected to its external environment using classic technologies. The pressure inside such a package is reduced in order to limit the thermal losses of the picture elements (pixels) into the substrate. The package also has a window that is transparent to the radiation to be detected, in this case infrared radiation.
In order to capture a scene using this detector, the scene is projected through suitable optics onto the array of elementary detectors and clocked electrical stimuli are applied via the readout circuit to each of the elementary detectors or to each row of such detectors in order to obtain an electrical signal that constitutes an image of the temperature reached by each of said elementary detectors. This signal is then processed to a greater or lesser extent by the readout circuit and then, if applicable, by an electronic device outside the package in order to generate the thermal image of the observed scene.
The essential difficulty of using bolometric detectors is the extremely small relative variation in their electrical resistivity that is representative of the local temperature variations of an observed scene compared with the average value of these resistances. In fact, the physical laws of thermal emission in the infrared spectrum of the observed scene from 8 to 14 μm (equivalent to transparency band of the terrestrial atmosphere in which bolometric detectors are usually used) result in a differential power dP on the detector's focal plane of the order of 50 μW/cm2 when the temperature of the scene varies 1 K either side of 300 K. Determining this value is easily within the capabilities of those skilled in the art by applying the above-mentioned physical laws.
This estimate is valid for an f/1 optics, good transmission between the scene and detector and if the detector only receives a negligible amount of energy outside the specified wavelength band, for example and typically if the package has a window that is transparent in this range and opaque below and beyond the stated limits.
Consequently, the variation in temperature dT of a bolometer working at thermal equilibrium associated with an infrared power dP absorbed on its surface S is given by the following equation:dT=Rth·dP, where Rth is the thermal resistance between the sensitive part of the bolometer, the temperature of which rises due to the infrared radiation, and the isothermal substrate on which it is mounted, the temperature of which is constant or varies only very slowly.
Thus, for a bolometer of typical dimensions of the order of 30 μm×30 μm which represents a surface area of 9.10−4 cm2, the typical thermal resistance is of the order of 10 to 30 MK/W which results in an increase in the temperature of the bolometer of the order of 0.005 K to 0.015 K if the element of the scene observed by the bolometer varies by 1 K.
The resulting variation in resistance dR is expressed by the following equation:dR=TCR·dT where TCR is the relative coefficient of variation in resistance of the material that constitutes the sensitive part of the bolometer at around its operating temperature. For the usual materials in this field (vanadium oxides, amorphous silicon), this coefficient TCR is approximately 2% per K. In other words, the relative variation in resistance dR/R resulting from a difference of 1 K over the scene is therefore of the order of 0.02% where R is the electrical resistance across the two current input terminals on the sensitive bolometric material.
Nowadays, thermal imaging resolutions much better than 1 K, typically 0.05 K or even less are required. Such results can be obtained by producing structures that have very high thermal resistances Rth by using sophisticated techniques. However, there remains the need to measure minute relative variations in resistance, typically—as stated earlier—of the order of 10−5 in order to resolve temperature variations in time and space of just a few dozen millikelvins.
In order to explain the difficulty of analysing such a small variation, FIG. 1 shows a schematic view of simple hypothetical means of making repeated measurements, i.e. sampling an electrical resistance Rb, e.g. in a bolometric material. To achieve this, a bias voltage Vb is applied across the terminals of resistor Rb at a specific instant. The resulting current through Rb is integrated for a period Tint called the “integration time” in a capacitor Cint. The voltage Vs on the output of this integrator, an image of resistance Rb, is given by the equation:Vs=(Vb/Rb)·Tint/C assuming, for the sake of simplicity, that Rb varies little throughout integration period Tint.
At the end of the integration period, voltage Vs is used as a wanted imaging signal and then reset to zero by a reset switch RAZ before the next readout operation of Rb starts.
Thus, an array of N resistors (bolometers) can be read using this principle with the aid of simultaneous integration (by means of N integrators) or sequential integration (in an integrator at the end of a line or end of a column or even a single integrator for the array). If the array thus produced is illuminated by projecting an infrared scene, Vs will provide variations in space (obtained from each bolometer) representative of the scene. The reader is reminded that voltage Vs as stated previously consists largely of a component that is constant from one detector to the other (a signal called a common-mode signal) which therefore has no relevance in terms of imaging. Only the minute variations in Vs associated with local differences (from one bolometer to another) and variations in time (the scene varies as time passes) in the received radiant flux constitute the wanted signal for the observed scene.
The constraints inherent in microelectronic circuits in terms of voltage (only several volts), the values of bolometric resistance Rb that can be accessed and controlled (several dozen to several hundred kOhms) and the need to use integration periods sufficient to limit the integration time would result in the need to use capacitances Cint having extremely large values that would be incompatible with the surface area available on each detection picture element or pixel (of the order of the surface area of one bolometer) and, in practice, even incompatible with mounting this capacitor towards the edge or on the edge of the readout circuit where the surface area is not confined to that of the picture element. There is therefore a need to establish methods of reading that limit the current that is to be integrated to levels that are compatible with capacitances that can reasonably be implemented.
A known solution such as that described in the document entitled “LETI/LIR's amorphous silicon uncooled microbolometer development”—Infrared Detectors and Focal Plane Arrays V, 14-17/04/98, SPIE ORLANDO, SPIE Proceedings Vol. 3379 is shown schematically in FIG. 2.
This solution involves diverting most of the background or common-mode current (independent of illumination) flowing through each bolometer in the array through a special structure called a “compensation resistor” having a value Rc and ideally insensitive to illumination. This compensation resistor is located at the end of a column or the end of a line of the array, one of its terminals is biased by voltage Vc which is negative relative to the input potential of the integrator and the other terminal is connected to the input of the integrator.
Usually, an addressing device (not shown in order not to introduce unnecessary detail in FIG. 2) successively applies the current of each bolometer in the same column or same row to the input of the integrator. Voltage Vs on the output of the integrator at the end of integration is then expressed by the quotient:Vs=(Vb/Rb−Vc/Rc)·Tint/C. 
Current Vc/Rc is trimmed by adjusting voltage Vc and by the design choice to make Rc close to the average value (on the array) of Vb/Rb under normal operating conditions. In this way the level that is common to all the pixels, regardless of illumination, is eliminated from output signal Vs. This being so, the dynamic range of the integrator (Vs max.−Vs min.) can substantially be devoted to representing variations in resistances Rb in space and time, i.e. the temperature of each scene element.
Another solution also intended to substantially limit the common level has also been suggested, for example in the document entitled “On-chip compensation of self-heating effects in microbolometer infrared detector arrays”—Sensors and Actuators A 69 (1998) 92-96. This option involves using a resistance bridge that includes the bolometer in one of its legs and a compensation resistor, ideally insensitive to illumination, in its other leg as shown schematically in FIG. 3.
As in the previous solution, a compensation resistor is placed at the end of each column or each row and a switch system (not shown) successively connects each bolometer in the same column or same row to the resistance bridge. The unbalancing of the bridge produced by the rise in the temperature of bolometer Rb due to the effect of infrared illumination by the scene is applied to the column or row amplifier as a differential input.
These compensation resistors may be formed in the readout circuit—this gives them natural insensitivity to infrared illumination. Advantageously, regardless whether they are used for current subtraction or in a bridge configuration, they are made of the same material as the bolometric resistors in the array and preferably obtained simultaneously (i.e. during the same technology operation) so that the dR/RdT relative temperature variation characteristics of resistors Rb and Rc are as similar as possible and ideally identical. This arrangement makes it possible to compensate overall variations in the temperature of the readout circuit by constructional means because the active imaging element Rb and the compensation element Rc both have the same temperature coefficient TCR in this case and therefore vary together in the same direction and with the same relative variation when the temperature of the readout circuit fluctuates either side of its quiescent point.
The compensation element, with this advantageous method of construction using the same material, is typically placed at the end of a column or end of a row so as to limit the surface area of the pixel. The construction of this element in the pixel of the detector would imply a surface area set aside for this structure and therefore an ultimately larger pixel or, for a given surface area, using part of this surface area in order to insert the compensation element, this resulting in a loss of performance of active element Rb. However, the surface area of the detector is always crucial from an economic viewpoint in microelectronics, especially for imaging components and more especially for infrared imaging where the cost of optics relative to the surface area of the focal plane has a paramount impact on the final cost of the system.
This being so, optimising the structure of the compensation surface usually demands a non-negligible surface area beyond the optically active surface area and this has an economic impact on the number of detectors that can be produced collectively on the same substrate.
In addition, in order to use the above-mentioned advantageous layouts under conditions that are technically acceptable, the compensation structures and detection structures (actual bolometers) must be produced simultaneously, namely by the construction of suspended membranes obtained by thin-film deposition. One unavoidable consequence of this fabrication technology is that these compensation structures have a certain degree of thermal isolation that is not zero relative to the substrate that forms the readout circuit despite any design precautions that might be taken in order to minimise such thermal isolation. Even if they are located away from the optically active surface (compensation relocated to end of column or end of row), these structures are usually illuminated by the optics in the same way as the active array of bolometers. In fact, it is impossible to position them very far away for economic reasons: the substrate surface area this takes up, which defines the number of detectors per unit of substrate, and the size of the package in which the detection device has to be integrated. This results in variation in space and time during operation of resistors Rc (of compensation current) which degrades image quality and limits the performance of the component, especially in the event of strong local illumination of the compensation structures.
Also, the current that flows through these compensation structures tends to heat them because their thermal resistance, even though low, is not zero. The increase in temperature ΔT of a bolometer as a function of time during integration interval 0<t<Tint is given in a simplified but representative manner by the expression:ΔT#(Vb2/Rb)·Rthb(1−exp(−t/Rthb·Cthb))).where:
Vb2/Rb is the electric power developed through the bolometer during integration;
Rthb and Cthb are the resistance and heat capacity of the bolometer respectively;
the product Rthb·Cthb represents the time constant for return to equilibrium temperature in the absence of biasing (typically several milliseconds, i.e. much longer than the usual integration time of several microseconds).
As a result, the increase in temperature of the bolometer at the end of integration due to the effect of the readout current is given approximately by the expression:ΔTb#(Vb2/Rb)·Tint/Cthbwhich does not depend on the thermal resistance of the bolometer but rather on its heat capacity.
An equivalent relation is obtained for compensation resistance Rc:ΔTc#(Vc2/Rc)·Tint/Cthcin the general case where the time constant Rthc. Cthc of the compensation resistor is equally large compared with Tint.
If ΔTb differs from ΔTc, this difference causes a differential current that is independent of illumination of the active element and therefore an unwanted integrated signal that would dramatically reduce the dynamic temperature range of the scene of the readout circuit. In fact, the electric readout power is much higher than the infrared power to be detected: usually, the readout temperature increase is 0.5 to several degrees, i.e. the variation in the resistance Rb is easily of the order of several % compared with 0.001% which is equivalent to the variation in the temperature of the scene of several dozen millikelvins as evaluated earlier. As a result, it is crucial to eliminate most of the differential current associated with electric heating.
In the case of integration as disclosed in the above-mentioned LETI publication, this differential readout temperature increase is not a problem because it is sufficient to slightly modify one of the voltages Vb or Vc in order to compensate the additional temperature-rise current differences.
The second document cited discloses that differential integration using a resistance bridge makes it possible, if ΔTb#ΔTc, which implies Rc#Rb and Cthc#Cthb (since Vb=Vc in a balanced bridge circuit), to compensate not only the common-mode current but also the additional current associated with electric heating of the bolometer during the integration period.
For an array of n rows, the resistors Rc placed at the end of columns are excited n times more often than a bolometer in the array. Because of this they must have a return-to-equilibrium time that is sufficiently short to return to roughly the temperature of the substrate between two consecutive integration periods (the time to read one complete row). Since Cthc is set in the vicinity of Cthb, it follows that Rthc must therefore be relatively small. However, in order for the temperature rise of the active and compensation structures to remain comparable, Rthc cannot be very small. Finally, in the case of reading using a resistance bridge, the optical sensitivity of the detector at the level of the compensation structures cannot therefore be negligible.
FIGS. 4 and 5 schematically show a configuration in accordance with the first of the cited document according to the prior art. FIGS. 4 and 5 show an array of n rows and m columns of bolometric detectors identified by their respective coordinates as a function of their position.
The “active” surface includes all the resistors Rb made of a bolometric material and positioned in m rows and n columns. An example of such a detector is described in detail for example in relation to FIGS. 6 and 7.
Consequently, the compensation used in this example, located at the end of the column, comprises c rows and m columns (cf. FIG. 4). However, it is conceivable, as shown in FIG. 5, to use compensation areas at the end of each of the columns of the “active” detection area no longer consisting of differentiated elementary surfaces but of elongated column-shaped areas. The essential point is the effective, available compensation surface area for each of the columns in the “active” detection area. In contrast, each of the compensation columns is well differentiated from the adjacent columns.
An example of compensation areas in accordance with the prior art are shown in FIGS. 8 and 9.
In order to overcome the structural drawback associated with the non-negligible optical sensitivity of the detector at the level of the compensation structures, it has been proposed to insert an opaque screen between the active detection array and the compensation structures, typically inside the package in order to remain close to the focal plane, thereby limiting the shadowing effect on these structures. Such a screen is, for example, fitted in the space shown in FIGS. 4 and 5 by the broken line.
However, this solution has a considerable economic impact: fabrication is more complex, more accurate assembly is required, cost in terms of surface area of microelectronic circuitry and the volume of the package.
Another limitation of the technique described is the electrical noise added by the compensation structure. In fact, the materials usually used for microbolometry, especially amorphous materials, exhibit low-frequency noise at 1/f (f for frequency) which largely determines the performance of detectors. Experience shows that the current noise power relative to the actual current (Vb/Rb) is proportional to 1/b1/2 where b is the active volume of the bolometric material. The active volume relates to the volume of the bolometric materials through which the current lines effectively pass during operation of the structures. This relation applies to active bolometer's (Rb) and to the compensation element (Rc) because the latter is made of the same material and substantially the same current flows through it. The resulting overall noise on signal Vs is then proportional to:((1/b)+(1/c))1/2 where b and c are the active volumes of elements Rb and Rc respectively. It is apparent that one should maximise the volume of element Rc in order not to add excessive low-frequency noise to that which bolometer Rb would produce on its own.
In practice, the degraded performance inherently due to additional noise associated with element Rc is negligible if c is at least ten times greater than b. Since it is preferable to obtain the active and compensation structures during a single technology operation, the thickness of the bolometric material is structurally identical in both these types of structure and, consequently, it is the active surface (affected by the current lines) of elements Rc which must be ten times larger than that of elements Rb. This means that it would be advisable to typically add the equivalent of ten extra rows (for a compensation structure positioned at the end of a column) at the edge of the imaging array in order to accommodate compensation structures there. This overall-size constraint may not be very significant for arrays with a large number of points or pixels but needs to be taken into consideration in terms of the number of products per substrate in the case of detectors with a smaller surface area.
It must be emphasised that using large-area compensation structures, as recommended, implies that Cthc is much greater than Cthb and that the temperature rise during the readout pulse is much smaller. It was stated earlier that although this condition is not overriding in the context of the technical solution proposed in the LETI publication cited earlier, in contrast, in the context of using a bridge connection as described in the other publication mentioned (“On-chip compensation of self-heating effects in microbolometer infrared detector arrays”), it is crucial that Cthc#Cthb, in other words, the compensation structure must, in this case, necessarily have a volume c close to the volume b of the active element (since c and Cthc are closely linked) and, consequently, it is impossible to minimise the noise added by compensation.