Infrared imager using room temperature capacitance sensor

Details Infrared imager using room temperature capacitance sensor Infrared imager using room temperature capacitance sensor

1999-04-08

2001-06-19

Epps, Georgia (Department: 2873)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

C250S336100

Reexamination Certificate

active

06249001

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to room temperature capacitance sensor, and more particularly to a low-cost manufacturable infrared imager that operates at room temperature and has substantially improved performance approaching the theoretical background limited performance limit.
BACKGROUND OF THE INVENTION
Instruments for the measurement of infrared (IR) radiation are becoming increasingly important for a variety of commercial and non-commercial applications. Research into the development of uncooled sensors with response throughout the infrared spectrum has been particularly important due to the limitation on the operation of cooling systems. Uncooled infrared sensors would have important applications for space-based remote-sensing of thermal sources, night vision, target identification, thermal mapping, event detection, motion detection, and others. The limitations of the performance of the existing uncooled sensors often are the primary constraints to the performance of infrared imaging systems for many applications. As a result, there has been considerable investment in the development of uncooled infrared sensors.
A broad assortment of infrared detectors has been developed over the last 40 years. In most cases, they may be classified as either quantum or thermal detectors, depending upon whether the incoming radiation is converted to excitations which are collected, or is converted to heat and detected through changes in temperature. In general, a quantum detector which operates at detector temperatures Td is usually superior to a thermal detector at the same temperatures for infrared frequencies in which hv>>k
B
T
d
, where h is Plaiick's constant and k
B
is Boltzmann's constant. However, for infrared frequencies in which hv<<k
B
T
d
. thermal detectors represent the only functional technology. The operation of quantum detectors is limited by the availability of efficient photon conversion mechanisms, while the operation of thermal detectors is limited by the availability of sensitive thermometers. Only thermal infrared sensors operate in the mid-to-far infrared range (&lgr;>10 &mgr;m) at room temperature.
The pneumatic infrared detector, which was originally developed by Golay, is classified as a thermal detector. Golay's detector consists of a small cavity filled with gas at room temperature. The cavity is separated from the surroundings by a window and a thin, flexible membrane. The membrane is coated on one side with a thin metallic film, which has significant absorption throughout the infrared spectrum whenever the sheet resistance of the film is approximately half of the impedance of free space. The trapped gas in the Golay cell is heated by contact with the membrane and expanded thermally, which forces the membrane to deflect outward. This deflection is usually detected with optical or capacitive displacement transducers. At present, these detectors are bulky, fragile, difficult to fabricate, and expensive. Nevertheless, they have been widely used, primarily because of their improvement in sensitivity over all other room-temperature detectors in the mid-to-far infrared range. Attempts to miniaturize the Golay cell for incorporation into focal plane arrays have been unsuccessful because of scaling laws which relate the sensitivity of conventional displacement transducers and their active area. The need for focal-plane arrays of uncooled detectors stimulated the development of pyroelectric detector arrays, the best of which are 5-10 times less sensitive than the Golay cell.
Current state-of-the-art uncooled IR focal plane arrays use many different thermal detection mechanisms such a bolometric (sensor resistance is modulated by temperature), pyroelectric (dielectric constant is modulated by temperature), and thermoelectric effects. As discussed above, thermo-mechanical effects have been explored using modifications of the Golay cell. The performance of IR imagers based on these technologies is limited compared with imagers based on direct photon conversion, such as PtSi detectors operated 77 K, and also is considerably worse than the theoretical background limited performance. In all approaches, the fundamental limits to the performance are controlled by the ability to thermally isolate the detector from its surroundings, the detector sensitivity to a change in temperature, and the introduction of extraneous noise sources. One of the reasons for degraded performance is the parasitic thermal resistance paths inherent in the supporting structures of the sensing elements. Another reason is the electronic noise present in the readout scanning circuitry.
With the above considerations in mind, the present invention is based on the development of an IR capacitance structure that deflects the position of a plate in response to temperature changes.
SUMMARY OF THE INVENTON
The present invention provides a high-performance infrared imager that operates at room temperature. More specifically, this invention uses an infrared (IR) capacitance structure to sense changes in temperature. Thermal energy deforms the structure of the present invention resulting in a deflection that determines a capacitance which is then sensed.
The present invention provides an infrared capacitance sensor composed of a bi-material strip which changes the position of one plate of a sensing capacitor in response to temperature changes due to absorbed incident thermal radiation. The physical structure of this capacitance sensor provides high thermal radiation resistance and high thermal sensitivity by utilizing a bi-material strip composed of two materials with a large difference in thermal expansion coefficients (e.g., Si
3
N
4
and Al) mechanically supported by a long strip of material with high thermal resistance (e.g., Si
3
N
4
).
Additional embodiments within the scope of this invention are also possible. These embodiments are extensions of the basic IR capacitance structure and include (1) a bridge structure with a bi-material element for increased structural stability, (2) a bridge structure without a bi-material element, relying only on the thermal expansion and the “beam buckling concept” in which the two ends of the structure are pinned for increased process simplicity, and (3) variations where the support arms may be parallel or co-linear with the bi-material element.
Another aspect of this invention is the design and operation of a readout multiplexer for a focal plane imager made up of an array of these capacitance sensors.
Another aspect of this invention is the use of a correlated double sampling (CDS) circuit to reduce the 1/f noise and dc offset of the pixel amplifiers.
Another aspect of this invention is the use of 2× over-sampling for both the reference and signal samples in the CDS readout circuit so that the mechanical resonant frequency of the capacitance sensor is at the Nyquist frequency of the samples.
Another aspect of this invention is that the readout method does not remove the signal which is stored as a change in capacitance.
The foregoing and other aspects of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.


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