Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2002-03-15
2004-09-28
Gagliardi, Albert (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
Reexamination Certificate
active
06797957
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an infrared detection element using the temperature dependence of spontaneous polarization of ferroelectrics. As an infrared detection element generally used for consumer appliances, there is a pyroelectric infrared detection element using the characteristics of changing of spontaneous polarization due to temperature changes.
In the pyroelectric infrared detection element, infrared light supplied to an infrared detection element periodically interrupted by a chopper. According to temperature dependence of the infrared detection element synchronized with the interruption frequency by the chopper, the infrared detection element is charged. The charge is amplified by an amplifier and read as a current or a voltage, thus infrared light is detected.
An example of a conventional pyroelectric infrared detection element already put into practical use is shown in
FIG. 1. A
pyroelectric element is made of a ceramics sintered substance
31
of barium strontium titanate (hereinafter, abbreviated to BST) having a heat-insulating structure, in which the element is stuck onto a Si IC substrate
32
for reading via metallic bumps
33
so as to improve the sensitivity and response speed. In the pyroelectric element of the kind using ceramics, there is a limit to decrease a size of each element, since it is extremely difficult to control the thickness and horizontal length of the element to several tens &mgr;m or less. There is also a problem particularly in the response speed because a heat capacity of the element is large, thereby making it difficult to provide a small and light sensor.
Further, to solve such problems of the infrared detection element using a pyroelectric substance, production of an infrared detection element using a thin pyroelectric film has been tried. An example of a conventional infrared detection element using a thin pyroelectric film is shown in FIG.
2
. As shown in the drawing, an infrared detection element
44
composed of a thin polycrystalline BST film held by upper and lower electrodes
43
is formed on the surface of an Si substrate
41
via a thin amorphous SiN (silicon nitride) support film
42
. To realize the heat-insulating structure, an open space
45
by anisotropic etching is formed at the back of the Si substrate
41
.
However, a pyroelectric property of such a thin polycrystalline BST film, particularly ferroelectricity that is a basis for development of the pyroelectricity, is far inferior to that of BST ceramics sintered at a high temperature. Moreover, the thinner the film is made, the more degraded are the ferroelectricity and the pyroelectric property in the prior art. Namely, when a thin polycrystalline pyroelectric film is used, the infrared detection element can be made thin and small. There is, however, a problem arises that the detection sensitivity is greatly reduced.
To make rapid improvement in the ferroelectric property and the pyroelectric property of such a thin pyroelectric film, a method for using a thin single-crystalline pyroelectric film by epitaxial growth has been proposed (see Japanese Patent No. 3094753).
A manufacturing process of a conventional infrared detection element using the thin single-crystalline pyroelectric film is exemplary shown in FIG.
3
. As shown in the drawing, a thin single-crystalline film
52
of lantern-added lead titanate (hereinafter, abbreviated to PLT) by epitaxial growth is prepared and patterned on a single-crystalline MgO substrate
51
(A). A first polyimide film
53
for supporting, a lower electrode
54
, and a second polyimide film
55
are sequentially deposited and patterned (B). The single-crystalline MgO substrate
51
is inversely bonded on an alumina substrate
56
processed beforehand via a metallic bump
57
. On the alumina substrate
56
, a lead wire
58
is formed(C). The single-crystalline MgO substrate
51
is removed by etching (D). A photo detector electrode
59
is formed on the exposed surface of the thin PLT film
52
. Here, the first polyimide film
53
is partially removed, and the lower electrode
54
is exposed. Conductive paste
60
and the lead wire
58
are, then, electrically connected on the surface thereof, and the thin PLT film
52
is bonded onto the alumina substrate
56
(E).
The infrared detector composed of a thin single-crystalline PZT film with the heat resistant support, which is manufactured through such a complicated process as described, proved that the sensitivity is far higher than that of a detector composed of a thin polycrystalline film. However, there are problems in the infrared detector that the manufacturing process is complicated, that the yield rate is low, and that the cost is high.
Having made a number of experiments and theoretical analyses to improve the performance of an infrared detector using such a thin single-crystalline pyroelectric film, the inventors have found for the first time limits or problems on use of the thin single-crystalline pyroelectric film supported in the air. The discovered problems will be described hereunder in detail.
BaTiO
3
, one of the ferroelectrics for example, shows a primary phase transition in a following manner as temperature rises from a low temperature; rhombohedral crystal rhombic crystal→tetragonal crystal→cubic crystal. Further, when the temperature becomes lower from a high temperature, a primary phase transition, from cubic crystal to tetragonal crystal occurs inversely. In the course of the phase transition from the cubic crystal to the tetragonal crystal due to the temperature decrease, a and b axes shrink by about 0.005 Å and c axis extends by 0.01 Å, from their original length of 4.01 Å at the curie temperature. With the phase transition, spontaneous polarization of 18 &mgr;C/cm
2
is generated in the crystal.
This discontinuous change of the lattice constant in the phase transition of the ferroelectrics brings about a discontinuous change in the temperature property of the spontaneous polarization. The discontinuous changes of the spontaneous polarization with the temperature in the neighborhood of the Curie temperature make accurate temperature measurement unable in the neighborhood of the Curie temperature.
SUMMARY OF THE INVENTION
The present invention provides an infrared detection element, which eliminates a discontinuous primary phase transition of ferroelectrics based on the fact described, and enables the accurate temperature measurement even in the neighborhood of the Curie temperature.
Further, the present invention is made to provide an infrared detection device incorporating the infrared detection element.
The infrared detection element according to the present invention has a single-crystalline base layer with a thickness of 50 nm to 10 &mgr;m having a principal surface, a first electrode layer formed on the principal surface of the single-crystalline layer, a ferroelectric layer which is formed on the first electrode layer and composed of a single-crystalline layer or a unidirectionally oriented layer with which distortion in a direction within a surface parallel to the principal surface of the single-crystalline base layer is elastically constrained, and a second electrode layer formed on the ferroelectric layer, wherein an amount of charge of the ferroelectric layer of the infrared detection element is detected as an electric signal from the first and second electrode layer.
In the infrared detection element according to the present invention, the elastic restriction on the ferroelectric layer is not sufficient when the thickness of the single-crystalline base layer is less than 50 nm. However, when the thickness of the single-crystalline base layer is more than 10 &mgr;m, heat capacity becomes large and the response to infrared light thus becomes slow.
Further, in the infrared detection element of according to the present invention, it is preferable that the ferroelectric layer has the perovskite structure so that the amount of charge in the ferroelectric layer may change depending on the
Abe Kazuhide
Kawakubo Takashi
Sano Kenya
Gagliardi Albert
Kabushiki Kaisha Toshiba
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