Patent Application
20140091220
The present disclosure relates generally to microbolometer infrared/thermal detectors. More particularly, the disclosure relates to compensating for the self-heating effect in microbolometers.
Microbolometer thermal detectors based on silicon read out circuit are used for the detection of infrared radiation (IR) and are typically used in thermal cameras. Recent advances in micromachining technology have made it possible to fabricate highly sensitive thermal IR detectors. In contrast to conventional photon-based thermal detectors, microbolometers can be operated at room temperature and do not require an expensive or bulky cooling apparatus. Un-cooled microbolometer thermal detectors can be monolithically integrated with readout electronics to create large and inexpensive thermal detector arrays. Microbolometers are relatively easier to fabricate and have a relatively decent response compared to other thermal detector technology.
A microbolometer measures the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. In order to read the resistance of the microbolometer, current is passed through the device during readout which also causes a self-heating effect that limits the sensitivity of the microbolometer. This self-heating artifact is often referred to as a DC pedestal in the output signal and the signal of interest represents a very small voltage above this DC pedestal.
This self heating phenomena is not common for other types of IR detectors. For example, the detector structure mentioned in U.S. Pat. No. 6,583,416 does not have a self heating issue because an infrared sensitive capacitor is used to temperature. There is no self-heating effect because there is no DC current applied to the pixel. The patent uses blind reference pixel that is shielded from the incident thermal/IR radiation to cancel out a thermal effect related to the temperature of the detector and fixed pattern signal noise. This type of thermal effect is a different concept from the self heating effect of microbolometers. Self heating is a per element heat due to current flow which is different from the overall detector temperature that is addressed by the '416 patent. To cancel out the detector temperature effect it does not matter how the blind pixels are positioned as is illustrated by the variety of configurations shown in the patent.
The '416 patent also presents a detector structure in which the active pixel signals are read out one-by-one by addressing each pixel. Each pixel signal is read out with a blind reference pixel signal to cancel the thermal effect. Because the number of active pixel is much larger than the blind pixels each blind pixel will be read multiple times in one frame read out period. This operation is ineffective to cancel self-heating because reading the blind pixel multiple times per active pixel will result in a different amount of self-heating between the active and blind pixels.
Attempts to alleviate the self-heating effect include applying a voltage pulse to the microbolometer rather than a steady voltage. The voltage pulse has a shorter time period than the thermal time constant of the microbolometer but the effects of self-heating are still present. Other attempts, such as that described in European Patent No. 798545, describe using an active microbolometer for detecting infrared radiation, a blind microbolometer to compensate for the self heating effect and a differential amplifier detection circuit. The blind microbolometer is shielded from infrared radiation so that any change in resistance of the blind microbolometer is not due to incident radiation but includes the self-heating effect. These blind microbolometers may also be referred to as IR blind. European Patent No. 798545 further describes using a Wheatstone bridge configuration to cancel out the self-heating effect between the active and blind microbolometer.
The problem with the approach described by European Patent No. 798545 is that every active microbolometer must have a corresponding blind microbolometer and both must be read simultaneously to cancel the self-heating effect. This requires a larger physical layout and provides a lower fill factor to accommodate the blind resistors (i.e. the layout area of the blind resistor limits the number of active microbolometers per unit of area). The active and blind pixels must be readout (i.e. activated by the same voltage signal) at the same time to produce the same self-heating effect in order to cancel this effect out between the active and blind resistor.
Accordingly, there is provided an infrared imaging device having a shielded radiation detector that is powered synchronously with multiple active radiation detectors to correct for the self-heating effect and improve upon some of the issues noted above.
In a first aspect, an infrared imaging device is provided comprised of two or more active radiation detectors, the active radiation detectors change in resistance in response to incident infrared radiation; a shielded radiation detector that is shielded from incident infrared radiation, the shielded radiation detector corresponding to the two or more active radiation detectors; and a power supply coupled to the two or more active radiation detectors and the shielded radiation detector, the power supply configured to simultaneously provide power during a readout period to the two or more active radiation detectors and the shielded radiation detector wherein self-heating affecting change in resistance caused by current from the power supply is correlated between the two or more active radiation detectors and the shielded radiation detector. In some aspects, the infrared imaging device can further comprise a readout module coupled to a corresponding one of the two or more active radiation detectors and the shielded radiation detector. In still further aspects, the readout module is configured to subtracts a reference signal received from the shielded radiation detector from an active signal received from each of the two or more active radiation detectors.
In a second aspect, a method for reading an infrared imaging device is provided, the infrared imaging device having a plurality of groups of active radiation detectors that are subject to a self-heating effect, each of the groups having at least one corresponding shielded radiation detector. The method comprises selecting a group from the plurality of groups and the at least one corresponding shielded radiation detector; applying power synchronously to the selected group and the at least one corresponding shielded radiation detector; and subtracting a reference signal received from the at least one corresponding shielded radiation detector from active signals received from each of the active radiation detectors of the selected group, wherein removing the reference signal cancels the self-heating effect for each of the active radiation detectors.
For a better understanding of the various embodiments described herein and to show more clearly how they may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings which show at least one exemplary embodiment, and in which:
It will be appreciated that for simplicity and clarity of illustration, where considered appropriate, numerous specific details are set forth in order to provide a thorough understanding of the exemplary embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the embodiments described herein. Furthermore, this description is not to be considered as limiting the scope of the embodiments described herein in any way, but rather provides exemplary embodiments that could be varied or modified by a person skilled in the art.
The terms “row” and “column” are used herein to refer to the manner of addressing infrared radiation detectors in a two-dimensional array. The terms “row” and “column” are not meant to be limiting in a geometrical manner with the exception that rows are understood to be perpendicular to columns. For example, both rows and columns can be either vertical or horizontal as the terms are used herein.
Referring first to
Active radiation detectors 104-106 are preferably implemented as microbolometers that contain a temperature sensitive resistor, and unlike other infrared detecting equipment do not require cooling. The term “active” is used to distinguish active radiation detectors 104-106 from shielded radiation detectors 114-116. Power supply 130 is coupled to infrared imaging array 102 and active radiation detectors 104-106. Power supply 130 provides the electrical current that is supplied to active radiation detectors 104-106 in order to measure change in resistance during readout. Operation of active radiation detectors 104-106 is significantly affected by generation of self-heating when current is passed through active radiation detectors 104-106 during readout.
Shielded radiation detectors 114-116 are provided to cancel out the self-heating effect. Shielded radiation detectors 114-116 are constructed similarly to active radiation detectors 104-106 but further include a radiation shielding element, such as a radiation shielding film, to prevent infrared radiation from effecting the resistance of shielded radiation detectors 114-116. Changes in resistance of shielded radiation detectors are thus not due to incident infrared radiation.
Each of shielded radiation detectors 114-116 corresponds to a group of two or more active radiation detectors 104-106. The shielded radiation detector and its corresponding group of active radiation detectors are coupled to the power supply during the readout period so that the shielded radiation detector and active radiation detectors receive the same current and are subject to the same self-heating effect. Readout module 120 is configured to subtract a reference signal from the shielded radiation detector from an active signal from each of the corresponding active radiation detectors in order to cancel out the self-heating effect in the active signal.
Shielded radiation detectors 114-116 can be provided within infrared imaging array 102 or positioned outside infrared imaging array 102. Preferred embodiments position shielded radiation detectors 114-116 outside or on the edges infrared imaging array 102 so that shielded radiation detectors do not affect the fill factor of infrared imaging array 102.
Grouping of active radiation detectors 104-106 with shielded radiation detectors can be based on positioning.
Other embodiments can employ other grouping that are not limited by the device geometry. In still other embodiments, multiple shielded radiation detectors can be grouped with active radiation detectors. For example, shielded radiation detectors can be positioned at each end of rows 124-126 adjacent infrared imaging array 102. Readout module 120 can either average the reference signal from multiple shielded radiation detectors or select the reference signal from one of the multiple radiation detectors based on median value or other criteria.
Grouping by rows 124-126 allows for readout module 120 to read each row of active radiation detectors 104-106 with their corresponding shielded radiation detectors 114-116 in sequence using shared column buses 151-154. The active radiation detectors and the shielded radiation detector of the row are simultaneously biased by power supply 130 and sampled by readout module 120. The active radiation detectors and the shielded radiation detector receive power synchronously during the readout period so that the self-heating affecting a change in resistance due to current from the power supply is correlated between the active radiation detectors and the shielded radiation detector. This allows the signal from the shielded radiation detector to be used as a reference signal to remove the self-heating effect from the active radiation detectors that are grouped and similarly powered with the shielded radiation detector. This architecture also provide strong power noise cancellation because power noise from power supply 130 coupled into active and shielded radiation detectors are correlated during readout.
Control module 140 provides control signals to power supply 130, infrared imaging array 102, and readout module 120 to synchronize reading shielded radiation detectors with their corresponding group of active radiation detectors. Control module 140 can provide a row selection signal to activate a particular row 124-126 to couple power supply 130 to the radiation detectors and the radiation detectors to their corresponding column bus 151-154. Control module 140 can also provide control signals to power supply 130 to initiate a voltage pulse to generate a current pulse across active and shielded radiation detectors. Using a voltage pulse biases the radiation detectors for a shorter period of time to help reduce the self-heating effect.
Referring now to
Infrared imaging device 200 further comprises control module 240 that provides row selection signals 244-246 to a power switch for each row. The power switch is implemented as a row selection switch for each of the active and shielded radiation detectors that couples the radiation detector to the power supply lines (labelled as Vdd). Control module 240 asserts row selection signals 244-246 sequentially to allow readout module 220 to read each row in parallel in sequence. For example, asserting row selection signal 244 couples shielded radiation detector 214 and active radiation detectors 204a-c to the power supply signal, Vdd. Readout module 220 can then sample the row activated by row selection signal 244 (i.e. shielded radiation detector 214 and active radiation detectors 204a-c) from column buses 251-254.
Readout module 220 comprises differential readout circuits 261-263 that are coupled to active radiation detectors via column buses 252-254. The reference signal from shielded radiation detectors 214-216 is shared between differential readout circuits 261-263 via column bus 251. Readout circuits 261-263 removes the reference signal provided by shielded radiation detectors 214-216 from the output sampled from active radiation detectors 204-206. Readout circuits 261-263 can be implemented using a differential amplifier configuration or a capacitive transimpedance amplifier. Input to differential readout circuits 261-263 can also be sampled from an impedance bridge configuration.
Readout module 220 can further include an analog-to-digital (ADC) converter circuit and a digital memory that serves as the image buffer. Embodiments can include either the ADC or the ADC and memory on chip with the infrared radiation detectors or in a separate readout integrated circuit.
Referring now to
During the application of power a reference signal is sampled from the one or more shielded radiation detectors that is subtracted from each of the active signals sampled from each of the active radiation detectors in the selected row at step 306. Steps 302, 304 and 306 can be repeated for each row in the infrared imaging device. Where more than one shielded radiation detector is used, the reference signal can be an average or median of the output from the multiple shielded radiation detectors. Readout module 120 can include additional logic to process the sampled signals from each shielded radiation detector to provide the reference signal.
In step 304 power can be applied by provide row selection signal 244-246 that couples active radiation detectors 204-206 and shielded radiation detectors 214-216 to power supply 130. The power can be supplied as a voltage pulse that biases both shielded and active radiation detectors at the same time and for the same time period.
While the exemplary embodiments have been described herein, it is to be understood that the invention is not limited to the disclosed embodiments. The invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and scope of the claims is to be accorded an interpretation that encompasses all such modifications and equivalent structures and functions.
