Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1998-05-26
2001-03-06
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S332000, C250S338400
Reexamination Certificate
active
06198098
ABSTRACT:
FIELD OF INVENTION
The field of the invention is in a high sensitivity microstructure bolometer with a corrugated air bridge, which consists of at least a layer of temperature responsive resistive material for infrared detection, which provides large area, low infrared reflection and large infrared absorption so as to increase sensitivity and detectivity.
BACKGROUND OF THE INVENTION
An infrared energy detector is a device which produces an output signal which is a function of the amount of infrared energy that is impinging on an active region of the detector. There are two types of infrared detectors, photon detectors and thermal detectors.
Photon detectors function based upon the number of photons that are impinging on a transducer region of the detector. Photon detectors for infrared radiation are typically made of small band gap (about 0.1-0.2 eV) semiconductors such as HgCdTe and these detectors operate as photodiodes or photocapacitors by photon absorption to produce electron-hole pairs. Photon detectors are relatively sensitive and have a high response speed compared to thermal detectors. However, the small band gap is only about 4KT at room temperature and as a result dark current swamps any detectable signals. Photon detectors operate well only at low temperatures and therefore require refrigeration by liquid nitrogen (LN) to provide sensitive detection. Moreover, for wavelength greater than 20 &mgr;m, there is no satisfactory cooled detector technology above liquid helium (LHe) temperatures.
Thermal detectors function based upon a change in the temperature of the transducer region of the detector due to absorption of the infrared radiation. Thermal detectors provide an output signal that is proportional to the temperature of the transducer region. Since radiation absorption usually occurs over a wide range of wavelengths, thermal detectors are typically responsive over a wide range of wavelengths.
A bolometer is a thermal detector for infrared detection having a transducer region made of a material which has its resistivity change as the temperature of the material increases in response to the infrared energy impinging on, and absorbed by, the material Thus, in response to the change of resistance, by connecting the material to a constant voltage supply, the electrical current through the material will vary in accordance with the infrared energy sensed by the material or by connecting the material to a constant current supply, the electrical voltage across the material will vary in accordance with the infrared energy sensed by the material. Monolithic electronic circuitry connected to the material is used to produce an output signal representative of the infrared energy impinging on the material. By arranging an array of bolometers, together with its output electrical signals, and a processor fed by the output electrical signals can thus be used to provide an electronic image of the source of the infrared energy.
In such application, the infrared sensitive materials are deposited on and the integrated circuitry is fabricated on a substrate or a layer of semiconductor. Most of the infrared sensitive materials of bolometers are suspended on the elevated, air bridging surface member from the substrate by semiconductor micromachining technology to thereby increase its thermal isolation from the substrate. The increased thermal isolation thereby increases the sensitivity of the bolometer to the impinging infrared energy. For example, see U.S. Pat. No. 5,369,280 (Liddiard).
The voltage responsivity of a bolometer, R
v
, is defined as:
R
V
=
I
b
R
βη
(
G
(
1
+
ω
2
τ
2
)
)
1
/
2
where I
b
is the bias current, R is the dc resistance, &eegr; is the absorptivity, G is the thermal conductance between sensitive element and the substrate, &ohgr; is the angular modulation frequency of the incident radiation and &tgr; is the thermal response time, which is given by C/G. C is the heat capacity (thermal mass) of the sensitive element. &bgr; is the temperature coefficient of resistance (TCR) and defined as:
β
=
1
R
ⅆ
R
ⅆ
T
where T is the temperature. Therefore, for high responsivity, high dR/dT, low G and low &ohgr; (&ohgr;&tgr;<<1) are required. Solid state micromachining techniques can be employed to create an air bridge under the infrared sensitive element to provide low thermal conductance.
The detectivity D* is determined by the ratio of the responsivity R
v
to the noise voltage V
n
:
D
*
=
R
V
Δ
fA
Δ
V
n
where &Dgr;f is the amplifier frequency bandwidth, &Dgr;V
n
is the total noise voltage of the detector, and A is the area of the active region of the detector.
The bolometers can be operated under room temperature, however, these thermal detectors typically have a lower sensitivity and a slower response speed than photon detectors. Accordingly, the improvement in the present invention is directed to a structure which increases the bolometer sensitivity, detectivity and reduces the infrared radiation loss due to reflection.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved microstructure for bolometer.
It is also another object of the invention to provide a microstructure for bolometer having a corrugated air bridge which consists of at least a layer of infrared radiation sensitive material, such structure having improved infrared absorptivity and sensitivity.
It is a further object of the invention to provide a microstructure for bolometer having a corrugated air bridge which has improved structural integrity.
It is yet another object of the invention to provide a microstructure for bolometer having a corrugated air bridge which provides large active area.
These and other objects of the invention are attained generally by providing a corrugated air bridge microstructure which consists of at least a layer of infrared sensitive material formed on a semiconductor layer or substrate. An electrical device having this corrugated air bridge microstructure suspended over the substrate is connected to a monolithic electronic circuit or to a hybrid electronic circuit via electrical contacts. In a first preferred embodiment of the invention, the electrical device is a bolometer and the electronic circuit is a readout circuit for the device. The bolometer includes a surface member and two pillars. The surface member is suspended over the surface of the substrate by the two pillars as an air bridge structure. Preferably the surface member is corrugated. A layer of infrared sensitive material which has its resistivity change as the temperature of the material increases in response to the infrared energy impinging on, and absorbed by, the material is formed over the corrugated surface member. Thus, in response to the change of resistance, by connecting the infrared sensitive material to a constant voltage supply, the electrical current through the material varies in accordance with the infrared energy sensed by the material or by connecting the infrared sensitive material to a constant current supply, the electrical voltage across the material will vary in accordance with the infrared energy sensed by the material. The monolithic electronic readout circuit connected to the two electrical contacts at the proximate ends on the infrared sensitive material is used to produce an output signal representative of the infrared energy impinging on the material.
In a second preferred embodiment of the invention, the electrical device is a bolometer and the electronic circuit is a readout circuit for the device. The bolometer includes a surface member and two pillars. The surface member is suspended over the surface of the substrate by the two pillars as an air bridge structure. Preferably the surface member is corrugated. A first layer of infrared sensitive material which has its temperature change in response to the infrared energy absorbed by the material is formed over the corrugated surface member. A layer of electrical insulating
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