Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
2000-06-01
2004-03-16
Christensen, Andrew (Department: 2615)
Television
Camera, system and detail
Solid-state image sensor
C348S309000, C250S2140RC
Reexamination Certificate
active
06707497
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to the field of electromagnetic radiation, in particular visible or infrared radiation, detector networks. The field is limited more particularly to image sensors (ref. [1], [2] of the appended list) composed of a detector circuit interconnected to a read circuit. It relates to a device and method of biasing such photodetectors.
TECHNOLOGICAL BACKGROUND
Overview of Photodetection
A photodiode is a semiconductor device, which, when illuminated by sufficient energy radiation, outputs a photocurrent by generating electron-hole pairs. Two categories of photodiodes can be produced, according to the type of junction and substrate doping, the N-type junction on P-type semiconductor material and its equivalent, a P junction on N material.
Only the case of N photodiodes on P will be discussed hereafter. All the principles that will be presented are easily transposed to type-P photodiodes on N substrate by a person skilled in the art.
The detector circuit is generally formed of an arrangement of elementary photodiodes implanted with regular spacing according to a matrix of m lines by n columns wherein all junctions are coplanar. Each photodiode is coupled to a preamplifier implanted on the read circuit providing the conversion of the photocurrent output by the detector into a physical quantity compatible with analog processing systems achievable in integrated circuits (current, load, or voltage). The functions implanted on the read circuit also enable multiplexing the information output by each photodiode to a limited number of video outputs. The information output by each photodiode and conditioned by the read circuit analog system corresponds to a picture element or pixel.
The detector circuit can be illuminated either on the side where junctions are made or on the opposite side. The detector circuit is interconnected to the read circuit by means of an adequate method, e.g. microspheres in the case of sensors made by means of a hybrid detector circuit reversed on a read circuit (ref. [3]).
E.g., the principle of photovoltaic detection enables the production of image sensors operating in the visible region spectrum band, or the infrared one (thermal imaging). Spectrum band selectivity is obtained by producing photodiode junctions on a semiconductor material the forbidden bandwidth of which fits the wavelength to be detected.
State-of-the-art of Photodiode Biasing
The invention relates to the method used for biasing the photodiodes of the detector circuit. Hereafter, the review of the state-of-the-art will focus on the issue of biasing photodiodes of such sensors.
First of all, the operating principle of the sensor will be recalled, then the impact of the material, whereon the detector circuit is produced, on controllability—i.e. the capacity of imposing a level, here to apply a voltage source—photodiode electric nodes.
Sensor Operating Principle
On the one hand,
FIG. 1
a
represents a look-through cross-section of a junction between a P-type semiconductor substrate
1
and an N-type area
2
producing an N/P junction. The symbolic representation is composed of the symbolic representation of a diode
3
, the anode
4
of which is located above cathode
5
, so as to show that it is the substrate that is P-type.
FIG. 1
b
represents the same elements, however, this time, it is substrate
1
that is N-type. A P-type area
2
′ is implanted on this substrate
1
′ producing a P/N junction. Symbolically, this junction is represented by a diode
3
′, the anode
4
′ of which is located above cathode
5
′, so, as to show that it is substrate
1
′ that is N-type.
The current-voltage characteristic of such a junction is represented in FIG.
2
. On curve (a), the non-linear characteristic of the ideal junction with zero illumination can be seen: low dynamic impedance when the diode is forward biased, with anode voltage being greater than cathode voltage, and on the contrary, high dynamic impedance when the photodiode is reverse biased with an anode voltage less than the cathode voltage. When the photodiode is illuminated, the current-voltage characteristic, represented by curve (b) is translated vertically by an amount I
p
equal to the photocurrent generated by the photodiode. It should be noted that conventionally, the photodiode current-voltage characteristics are represented in conventional quadrants and not with the actual current and voltage signs.
The schematic diagram of a sensor is represented in
FIG. 3
, it corresponds to the cross-section of a matrix sensor, normal to the layer planes, following one of the directions of the lines and columns of the sensor matrix.
This diagram illustrates the case of a hybrid detector circuit
17
, reversed on a read circuit
20
as mentioned, e.g., in document [
2
]. The N photodiodes of the row corresponding to the cross-sectional plane are marked D
1
to D
N
, their anodes A
1
to A
N
and their cathodes K
1
to K
N
. The photodiode anodes of detector circuit
17
are connected to the inputs E
1
. . . E
N
of read circuit
20
. Continuity between detector
17
and read
20
circuits is provided by a vertical connection, e.g. of the indium microsphere type
21
.
The read circuit preamplifiers
20
are numbered from PA
1
to PA
N
.
The imaging process of this pixel row is the following one:
1. biasing the photodiode during image sensing so that it delivers a photocurrent;
2. processing the current output by the photodiode by means of preamplifiers;
3. multiplexing the output signal of each preamplifier to a video output.
The process is repeated at frame rate.
In practice, each photodiode is biased in the reverse part of its characteristic, at a voltage where the intensity of its current with zero illumination is relatively low in comparison with its photocurrent intensity. Controlling the difference of potential between the anode and cathode of each photodiode is therefore decisive for the operation of the detector circuit.
Controlling the potential of each anode is provided by the preamplifier input (e.g., virtual ground of a differential amplifier). On the other hand, the cathodes of each photodiode cannot be controlled individually. In fact, they are short-circuited by the semiconductor material where the junctions are made. Therefore, the cathodes K
1
to K
N
can only be controlled indirectly, via a single electric node of the detector circuit identified as K
C
—for common cathode.
Impact of the Detector Circuit Substrata
The electric characteristics of the layers composing the slice whereon the detector circuit is produced will determine the resistor for accessing the cathode of each photodiode. A schematic cross-section of these slices is represented in FIG.
4
. We can distinguish between three categories of slices:
1. the so-called solid substrate ones, represented in
FIG. 4-A
;
2. the so-called epitaxial substrate ones, represented in
FIG. 4-B
;
3. the so-called insulating substrate ones, represented in
FIG. 4-C
.
The solid substrate of
FIG. 4-A
is composed of a single layer
4
for the whole slice thickness. Slice resistivity &rgr;
1
is uniform and suitable for realizing high-performance photodetector junctions.
The epitaxial substrate,
FIG. 4-B
, is a dual layer one
7
,
8
. The photodiode junctions are made in the upper layer
7
of reduced thickness and resistivity &rgr;
2
suitable for producing photodiodes. The bottom layer
8
is made of the same material. It is very thick and its resistivity &rgr;
3
is very low, for minimizing the resistor accessing the junction cathodes.
The top layer
9
of an insulating substrate
10
,
FIG. 4-C
, has thickness and resistivity characteristics that are close to that of the epitaxial substrate. The base
10
thereof is also very thick. It can be produced by stacking up various materials, but at any rate, it acts as an electric insulator.
The detector circuits operating in the visible spectral range are produced on solid or epitaxial, or even insulating (ref. &l
Hamelin Régis
Martin Jean-Luc
Paltrier Sylvain
Pantigny Philippe
Christensen Andrew
Commissariat a l'Energie Atomique
Thelen Reid & Priest LLP
Wu Dorothy
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