Method of making infrared sensor with a thermoelectric...

Details Method of making infrared sensor with a thermoelectric... Method of making infrared sensor with a thermoelectric...

2001-09-28

2003-04-01

Chaudhari, Chandra (Department: 2813)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Thermally responsive

C438S074000, C438S280000

Reexamination Certificate

active

06541298

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This patent application claims benefit of and priority to Jap. Pat. Application No. P2000-298277 filed Sep. 29, 2000; the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an infrared sensor and method of fabricating it and, more particularly, to a low-cost, high-sensitivity, uncooled infrared sensor and method of fabricating it.
2. Description of the Related Art
Infrared imaging makes it possible to image objects night and day. Also, infrared radiation has a feature that it has higher permeability into smoke and fog than visible light. In addition, infrared imaging can obtain information about the temperature of the subject. Therefore, infrared imaging finds extensive use as monitor cameras and fire detection cameras, as well as use in military defense applications.
Quantum-type solid-state infrared imagers as mainstream devices have a drawback that they must be operated at cryogenic temperatures and thus a cooling mechanism is necessary. In recent years, uncooled solid-state infrared imaging devices free of this drawback have been developed rigorously. In the uncooled, i.e., thermal type, solid-state infrared imager, incident infrared radiation having a wavelength of about 10 &mgr;m is converted into heat by an absorption mechanism. This heat causes a change in the temperature of the heat-sensitive portion. This temperature change is converted into an electrical signal by a thermoelectric conversion, and the electrical signal is read out. In this way, infrared image information is obtained.
Methods for improving the sensitivity of such an uncooled infrared sensor are classified into the following three major categories.
One method for improving the sensitivity is to improve the ratio of the infrared power, dP, incident on the infrared detection portion to the variation, dTs, of the temperature of the target, i.e., dP/dTs. In this method, the sensitivity improvement is mainly achieved by optics. That is, an infrared lens having a larger diameter is used. An antireflective film is coated. A low-absorption lens material is used. The infrared absorptivity of the infrared detection portion is improved. The infrared absorption area is increased. As uncooled infrared sensors have been equipped with an increasing number of pixels in recent years, most unit cells have come to use a size of approximately 40 &mgr;m×40 &mgr;m. Of the aforementioned items, improvement of the IR absorption area of the infrared detection portion remains a relatively important issue. However, it was reported that the IR absorption area has been successfully improved up to about 90% of the pixel area by forming an IR absorption layer on top of the pixels (Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p. 556, 1999). It will be difficult to achieve a higher sensitivity improvement by optical improvements.
Another method for improving the sensitivity is to improve the ratio of the variation, dT
d
, of the temperature of the infrared detection portion to the power, dP, of the incident infrared radiation, i.e., dT
d
/dP. This method is a thermal method, while the method previously described is an optical procedure. Generally, in an uncooled infrared sensor mounted in a vacuum package, heat conduction via support structures for supporting the infrared detection portion above a hollow structure inside the support substrate is currently prevalent in the transportation of heat from the infrared detection portion to the support substrate. Accordingly, leglike support structures made of a material having a low coefficient of thermal conduction are laid out such that they are made as thin and long as the design permits (e.g., Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p. 556, 1999).
An infrared sensor having leglike support structures is described.
FIG. 22
is a cross-sectional view showing a cross-sectional structure of infrared detection pixels in the infrared sensor having the prior art support leg structures. As shown in this figure, an SOI (silicon-on-insulator) substrate is formed by a silicon substrate
506
, a buried oxide film
508
, and a single-crystal silicon film
509
. An infrared detection portion is formed on the patterned single-crystal silicon film
509
on this SOI substrate. This infrared detection portion utilizes a silicon pn junction described later. The single-crystal silicon film
509
under the single-crystal silicon substrate
506
is partially etched away to form a hollow structure
507
. A dielectric film
510
is formed on the silicon substrate
506
. A laminate structure consisting of a reflective layer
501
, a dielectric layer
502
, and an infrared absorber layer
503
is formed on the single-crystal silicon film
509
. Infrared radiation is absorbed and converted into heat in this laminate structure. The produced heat is transmitted to the infrared detection portion of the single-crystal silicon film
509
. A temperature variation due to heat is converted into a voltage change. An electrical signal caused by the voltage change is transmitted to conductive interconnects
517
in peripheral circuitry via a conductive interconnect
516
. In
FIG. 22
, support leg structures include the conductive interconnect
516
and the dielectric film
510
surrounding the leg structures, and support the single-crystal silicon film pattern
509
above the substrate.
While the pixel size has been reduced to about 40 &mgr;m×40&mgr;m, microprocessing at the silicon LSI process level has been already accomplished. Therefore, it is difficult to improve the sensitivity further by devising improved layouts of the support structures. Similarly, it is difficult to further reduce the coefficient of thermal conduction which is one of characteristics of the material of the support structures. Indeed, with respect to conductive interconnect for sending out an electrical signal from the infrared detection portion, two conflicting requirements are imposed, i.e. electrical conduction and thermal conduction which are similar in mechanism. Consequently, it will be difficult to improve sensitivity by further material improvement.
Another method for improving the sensitivity is to improve the ratio of the variation dS in the electrical signal produced by a thermoelectric converter to the variation dT
d
in the temperature of the infrared detection portion, i.e., dS/dT
d
. This method is an electrical method. It is important in this method, unlike the other two methods, that various electrical noises produced simultaneously be reduced. Various thermoelectric converter means have been reported.
For example, thermopiles for converting a temperature difference into an electric potential by the Seebeck effect (e.g., Toshio Kanno, et al., Proc. SPIE Vol. 2269, pp. 450-459, 1994), bolometers for converting a temperature change into a resistance change by a temperature variation of a resistor (e.g., A. Wood, Proc. IEDM, pp. 175-177, 1993), pyroelectric devices for converting temperature variations into electric charge by the pyroelectric effect (e.g., Charles Hanson, et al., Proc. SPIE Vol. 2020, pp. 330-339, 1993), and a silicon pn junction for converting a temperature change into a voltage change by a constant forward electric current (e.g., Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p. 556, 1999) have been reported.
Of these devices, the infrared detection device making use of a silicon pn junction is described in further detail in
FIG. 23
which is a perspective view showing the structure of infrared detection pixels using the lateral pn junction. As shown in
FIG. 23
, a silicon layer pattern
609
is formed on a laminate structure comprising a silicon substrate
607
and a dielectric film
608
. A pn junction is formed in each silicon layer pattern
609
. Conductive interconnects
617
are formed between the silicon layer patterns
609
to connect the pn junctions of the silicon layer patterns
609
in series. This structure can obtain a larger voltage cha

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