Electro-optical sensor arrays with reduced sensitivity to...

Details Electro-optical sensor arrays with reduced sensitivity to... Electro-optical sensor arrays with reduced sensitivity to...

2001-10-04

2003-07-15

Allen, Stephone B. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C348S246000

Reexamination Certificate

active

06593562

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to electro-optical sensor arrays.
BACKGROUND
Electro-optical sensors are used in many systems where it is required to sense a portion of the electromagnetic spectrum. These systems include fiber-optics, telecommunications devices, electronic cameras, and machine vision equipment, as well as many other commercial and military systems. The electro-optical sensor components that allow these systems to sense electromagnetic radiation can be critical in determining the performance, sensitivity, cost, and dynamic range for the entire system.
Many modern electro-optical sensors contain two primary functional elements: a detector element or array of detector elements, and a read-out circuit. The term “detector element” is used herein to refer to an individual light detector or to the smallest individual light detecting regions in a detector array. The detector elements receive electromagnetic radiation and convert it into electrical signals. The read-out circuit, frequently an integrated circuit known as a read-out integrated circuit (ROIC), processes the electrical signals produced by one or more detector elements into a signal that is useful for the particular system in which the sensor is employed.
One common type of detector element is the photovoltaic junction detector element.
FIG. 1A
is a circuit diagram and
FIG. 1B
is a cross-sectional side view of a typical pn junction photovoltaic detector element
2
. In the example of
FIG. 1A
, detector element
2
is a diode structure including an anode
4
and a cathode
6
. A terminal
8
is electrically coupled to anode
4
, and a terminal
10
is electrically coupled to cathode
6
. Detector element
2
may be fabricated by diffusing a p-type region
12
into an n-type semiconductor
14
, thereby forming a pn junction as shown in FIG.
1
B. Since detector element
2
is a diode structure that is responsive to illumination, detector element
2
is also called a photodiode.
An electro-optical sensor may be used to spatially sample an electro-magnetic image in discrete sections referred to as pixels (picture elements). The term “detector pixel” is used herein to refer to one or more detector elements electrically coupled to provide a signal corresponding to an individual pixel in an image. In most conventional electro-optical sensors a detector pixel includes only one detector element. For example, the single detector element
2
illustrated in
FIG. 1A
may be used to sample a single pixel in an image. To sample the image of, for example, a line, the single detector element
2
may be scanned across the line (or the line scanned across the detector element). The electromagnetic radiation received at the detector element
2
is collected sequentially in time as the detector element moves relative to the line.
Alternatively, an image of a line may be sampled (without scanning) with a conventional linear array of detector elements each of which samples a pixel of the image of the line. In typical linear arrays, individual detector elements are fabricated next to each other in close proximity and in the necessary quantity to support the system application.
FIG. 2A
is a circuit diagram and
FIG. 2B
is a perspective view of a conventional pn junction photovoltaic detector array
16
. In the example of
FIG. 2A
, detector array
16
includes four detector elements
2
, each with terminals
8
and
10
. In
FIG. 2B
, four p-type regions
12
(one for each detector element
2
) are shown arranged in a line and diffused into n-type semiconductor
14
. A linear array
18
having 72 closely spaced detector elements
2
is illustrated in FIG.
3
. Typical linear arrays contain as many as 512 or more of such closely spaced detector elements.
For many applications one-dimensional image sampling with a linear array is adequate to provide the necessary information for the system. Spectrometers are an example of this type of application. For applications requiring two-dimensional image information, the image to be sampled may be scanned across the linear array and sampled sequentially in time to capture the image. Scanning mirrors and scanning mechanisms are typically used to provide this capability.
Sampling of a two-dimensional image without scanning may be accomplished with a conventional two-dimensional array of detector elements (also called a staring array) such as detector array
20
illustrated in FIG.
4
. Although array
20
includes 1024 detector elements
2
in a 32 by 32 arrangement, it is typical to find two-dimensional arrays containing as many as 1024×1024 detector elements. Array
20
can acquire a 32 by 32 pixel two-dimensional image without the use of a scanning mirror or scanning mechanism if each detector element samples a pixel of the image.
In electro-optical sensors, each detector element in the detector array is electrically connected to the read-out circuitry. In the case of a detector that has a single detector element
2
, as in
FIG. 1A
, it is reasonable to consider electrically connecting to the detector by means of wires and/or printed circuit board traces. However, in one-dimensional linear arrays, such as arrays
16
(
FIGS. 2A and 2B
) and
18
(FIG.
3
), or in two-dimensional staring arrays, such as array
20
(FIG.
4
), it becomes unrealistic to interface to the detectors using these methods. These arrays may contain from 512 detector elements to over one million detector elements, thus requiring from 512 to over one million electrical connections respectively. Furthermore, the detector elements are typically small, having widths of, e.g., 25 &mgr;m, and are closely spaced within the array. Thus, for example, 1 by 72 linear array
18
(
FIG. 3
) may have a total width of less than 2 mm. To allow for the small size of the detector elements and array, it is desirable to electrically interface the detectors directly to the readout circuit. It is also desirable to have the readout circuit elements in close physical and electrical proximity to the detector elements due to noise and manufacturing considerations. To meet these requirements for the electrical connection, integrated circuit wire bonding techniques and bump bonding technologies are employed to electrically connect detector elements to the read-out circuit.
FIG. 5
illustrates an electro-optical sensor
22
including a detector array
24
(linear or two dimensional) interfaced to an ROIC
26
through wire bonding or bump bonding techniques. Detector array
24
is in direct electrical contact with ROIC
26
and signals from each of the detector elements in the array are connected to interface electronics in ROIC
26
. Each bump bond connects a detector element to a corresponding set of interface electronics, often called the unit cell, which is located directly under the detector elements. The interface electronics, or unit cell, of the ROIC often provides the functions of biasing the detectors, integrating signal from the detectors, and multiplexing the integrated signals to the periphery of the array and to the system.
ROICs are typically formed in silicon using Complementary Metal Oxide Semiconductor (CMOS) technology. For electro-optical sensors that detect electromagnetic radiation in the visible spectrum and/or in the infrared spectrum up to a wavelength of approximately 1.0 &mgr;m, silicon can be used to form the detector as well as the readout circuit. For optical sensor components that operate at significantly shorter or longer wavelengths, alternate detector materials may be selected to provide the appropriate sensitivity for the desired region of the electromagnetic spectrum. For example, electro-optical sensors that detect infrared radiation may employ Indium Gallium Arsenide (InGaAs) or Indium Antimonide (InSb) as the detector materials. In such cases, the material used for the detector may be different from the silicon CMOS material technology that is preferred for use in the ROIC.
A problem with electro-optical sensors is that defects that can degrade sensor performance may occur in the

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