Bolometric heat detector

Details Bolometric heat detector Bolometric heat detector

1998-12-16

2001-05-08

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

Reexamination Certificate

active

06229144

ABSTRACT:

TECHNICAL DOMAIN AND PRIOR ART
The invention relates to bolometric heat detectors, for example to detect infrared and/or nuclear radiation (X-rays, &ggr; rays, neutrons, etc.)
Infrared detectors usually comprise a sensitive element that can be heated by infrared radiation in band III (8 to 12 &mgr;m), characteristic of the temperature and emissivity of observed bodies. The temperature increase in the sensitive element causes a variation in an electrical property of the sensitive material; electrical charges appear by the pyro-electric effects, or a capacitance variation due to a change in the dielectric constant, or a variation in the resistance of a semiconducting or metallic material.
These detectors usually operate at high performance as a result of a low calorific mass of the sensitive material, good thermal insulation between the active layer and its support (these first two conditions require the use of thin layers) and finally high sensitivity of the conversion effect from a temperature rise to an electrical signal (TCR for a resistance).
Monolithic infrared imagers operating at ambient temperature are made by directly connecting a matrix of sensitive elements to a CMOS or CCD type silicon multiplexing circuit.
The thermal detector may be encapsulated under a vacuum or under a gas that is a poor conductor of heat, to improve performance. The housing then includes a window transparent in band III.
In resistive type bolometric detectors, the absorbed incident radiation causes an increase in the temperature of the detector, that in turn induces a variation in the electrical resistance. These variations of the resistance cause variations in the voltage or current at the detector terminals, forming the signal output by the sensor.
Application FR-95 07151 describes a resistive type bolometric structure using a semiconducting material. The sensitive material may be a slightly or strongly resistive p-type or n-type polycrystalline or amorphous silicon. It may also be a vanadium oxide (V205, V02) generated in a semiconducting phase.
Usually, the sensitive material is placed on an insulating support (SiO
2
, SiO, SiN, ZnS, etc.) that provides the mechanical stiffness of the bolometric structure. It may also be completely encapsulated with one of these insulating materials.
There are several electrode configurations that can be used to make the value of the resistance compatible with the characteristics of the read circuit. These configurations can be classified in two main categories:
detectors with coplanar electrodes (as in application FR-96 10005),
detectors with electrodes facing each other (sandwich).
This invention relates to the first category of detectors.
For this type of device, there is a problem in obtaining a good resistive contact with the doped semiconductor. Doping of the semiconductor reduces its temperature coefficient TCR.
To obtain a resistive contact, so that the current can pass in both directions with a negligible potential drop in the contact region, an attempt is made to:
reduce the height of the barrier as much as possible by a judicious choice of the metal used
reduce the thickness of the space charge area (or desertion or depletion) by strongly doping the semiconductor close to the contact.
In order to make bolometric detectors with a high temperature coefficient, the expert in the subject conventionally uses a material that is only slightly doped, in which resistive contacts are obtained by local overdoping achieved using implantation or diffusion techniques. These techniques are firstly relatively expensive to use, and secondly require heat treatments at high temperature incompatible with a bolometric technology assembled on a CMOS or CCD type silicon multiplexing circuit.
One alternative is to use a double slightly doped material/doped material type of layer, the contact regions being defined by etching. However, this technique is very difficult to use due to the lack of selectivity between the two layers, and since their thicknesses are very small.
DESCRIPTION OF THE INVENTION
The purpose of the invention is a bolometric heat detector and a bolometric detection device with a good resistive contact and a good temperature coefficient (TCR).
Another purpose of the invention is to make a imple low cost bolometric type detection structure that guarantees very good resistive contacts and therefore generates very little low frequency noise.
In order to maintain good resistive contact and a good TCR, the invention proposes to depopulate carriers in the inter-electrodes area by using a semiconducting doping layer of the type opposed to that used in the inter-electrodes layer.
Therefore the bolometric heat sensor or detector according to the invention includes an active part composed of at least two coplanar electrodes in electrical contact with a first thin semiconducting layer doped by a first doping agent with a first type of conductivity, a second thin semiconducting layer doped like the first layer, or undoped, in electrical contact with the electrodes, and a third thin semiconducting layer doped by a second doping agent of a second type of conductivity opposed to the first, the second layer being placed between the first and the third layers.
Electrically, the potential difference applied between the electrodes makes a current circulate in the inter-electrodes area defined by the first and second layers (if the second layer is doped), the third layer being at a floating potential.
Therefore, the purpose of the invention is a bolometric heat detector with coplanar electrodes, and insertion of an active layer in this type of detector or bolometer to modify its operation. Its main advantage is that it can be used to obtain materials with a high temperature coefficient, starting from relatively conducting layers, and therefore more easily seable from a technological point of view.
The presence of two opposite types of semiconductors makes the structure of the invention different from traditional structures. In prior art, the bolometric material may be encapsulated between two layers of insulating material such as SiN, Si
3
N
4
or SiO. However, these layers do not modify the properties of the bolometric material in a controlled manner.
The position of the Fermi level in the semiconducting materials gap is a parameter that controls the conductivity properties of these materials. Thus, the current passing through the structure is limited by the region with the highest resistance, like a depopulated region. The nature of the contacts, or the creation of a region depopulated by an appropriate structure, may modify the initial conductivity of these materials. Thus, the invention proposes to generate regions depopulated of carriers for this purpose, by encapsulating the bolometric material on at least one of its surfaces, with a material that can generate a space charge zone (semiconducting material with the opposite type of conductivity, charged oxide, metallic material).
The invention is intended particularly for use with bolometric detectors made on a read circuit. These bolometers are usually made at low temperature (typically <450° C.). The result is that the materials used are in amorphous, microcrystalline or polycrystalline form and are thin, in order to reduce the heat capacity.
For example, the detector according to the invention may be a detector of infrared type radiation.


REFERENCES:
patent: 4679063 (1987-07-01), White
patent: 5021663 (1991-06-01), Hornbeck
patent: 5288649 (1994-02-01), Keenan
patent: 5369280 (1994-11-01), Liddiard
patent: 5912464 (1999-06-01), Vilain et al.
patent: 0 354 369 (1990-02-01), None
patent: 0 749 007 (1996-12-01), None
patent: 2752299 (1998-02-01), None
Patent abstracts of Japan, vol. 011, No. 017 (E-471), Jan. 17, 1987, JP 61 187380, Aug. 21, 1986.

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