Photonic radiation sensor of large dimensions

Details Photonic radiation sensor of large dimensions Photonic radiation sensor of large dimensions

1998-12-28

2001-02-13

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Semiconductor system

C250S352000, C250S370090

Reexamination Certificate

active

06188070

ABSTRACT:

TECHNICAL FIELD
This invention relates to a large dimension photonic radiation detector.
STATE OF PRIOR ART
The most frequently used high performance infrared detectors at the present time are based on a technique of hybridizing a detection chip on a read chip through a network of micro-balls (for example based on indium) providing the electrical and mechanical interconnection between the two chips.
The read circuit that reads the signals detected by each pixel and multiplexes them on a single output or a small number of outputs, is an analogue silicon based circuit. The complexity of these circuits (size, density, analog nature) makes it impossible to reasonably consider any other technology for making them.
The detection chip is usually composed of a large number of pixels placed according to a two-dimensional or mosaic structure with ixj pixels (for example 128×128, 256×256, 640×680 pixels) or possibly according to a single-dimensional or quasi-single dimensional structure with ixj pixels where i=1 or I<<j (for example 288×4 or 480×4 pixels). In order to make high performance components, the most frequently used detection material at the present time is a Cd
x
Hg
1−x
Te alloy (0≦x≦1) epitaxed by any technique (LPE, MBE, MOVPE, ISOVPE, etc.) on an Cd
1−z
Zn
z
Te substrate (0≦z≦1) adapted as mesh parameter. The elementary photosensitive component consists of an N/P diode formed in the CdHgTe layer.
There are several technological variants for making these junctions. The most mature technique at the present time is based on a planar process using an ionic implantation to form the junction as described in document references [1] or [2] at the end of this description. Note that if the contact point on the N area is at the level of each pixel, the contact point on the P area is usually common to all elementary diodes making up the device. This device offers many advantages compared with other technologies, mainly related to its relative simplicity that leads to a very good reproducibility/reliability; lack of etching to define active surfaces, simple metallurgical stacking (this structure requires a single layer, whereas at least two layers are necessary for hetero-junction structures) lower sensitivity of the detector to low frequency noise when the device is polarized. The robustness and reliability of this technology are demonstrated by the fact that components made in this way were the first to be industrially available. Furthermore, it was recently shown that the same dark current can be obtained in these structures in which an N material junction is formed on P material, as shown in
FIGS. 1A and 1B
and described in document reference [3], as is obtained in hetero-junction structures in which the P area is above the N area as shown in FIG.
2
.
FIGS. 1A and 1B
show a schematic view of a hybridized two-dimensional infra-red radiation detector on its read circuit and an enlargement of the pixel showing a section through a detection element, respectively.
FIG. 1A
also shows a detecting chip
10
, a silicon circuit
11
, and metal connections (for example indium micro-balls). The video signal is obtained at
13
.
FIG. 1B
corresponds to a magnification of part reference
14
in FIG.
1
A. This FIG. shows a CdZnTe substrate
20
, an CdHgTe layer
21
, an N/P diode
22
formed in this layer
21
, a passivation layer
23
, an anti-reflection layer
24
, a contact
25
, an indium micro-ball
12
, a silicon circuit
27
, the received radiation
28
being an infra-red illumination.
FIG. 2
illustrates a “P on N” hetero-junction detector comprising a CdZnTe substrate
30
on top of which there is a type N detecting layer
31
made of Cd
x
Hg
1−x
Te, a P type detecting layer
32
made of Cd
y
Hg
1−y
Te where y>x, and a passivation layer
33
.
FIG. 2
also shows a pixel contact
34
, and a “substrate” contact
35
common to all pixels located at the end of the mosaic.
At the present time there is an increasingly pressing need for two-dimensional circuits with a very large number of pixels, for example more than 256×256 elements. With these components, when an attempt is made to detect a strong signal, as is the case for example for a thermal camera mode operating in the 8-12 &mgr;m band and when observing a scene at a temperature of close to 300K, a depolarization phenomenon may be observed in the diodes located far from contact points, called “substrate contacts” in the rest of this description, usually located at the edge of the matrix and which are common to all pixels in the matrix as described above, particularly in N on P planar structures. This phenomenon can be very troublesome when attempting to make the device operate and can even make it unusable. It is related to the value of the access resistance to these diodes from the center of the component. In the case of N on P planar structures, this access resistance depends on the transport properties from the P area in the CdHgTe layer
21
(typical doping within the range of a few 10
16
cm
31 −3
, typical mobility of carriers (holes) within the range 300-500 cm
2
/V/s), and obviously by the size of the component.
Some “obvious” solutions may be suggested to solve this problem. They include a substrate contact (i.e. in the case of planar structures on the P layer in the CdHgTe layer
21
) close to each diode or each group of diodes, of sufficiently small size to prevent any depolarization problems. However, this implies a reduction in the mosaic filling factor and therefore a drop in its performance, either at each pixel or in each group of the pixels, and increased technological complexity (connections between all substrate contacts). Another solution is to increase the thickness of the CdHgTe layer
21
compared with the normally used layer (thickness e
0
) and therefore the thickness of the P area that remains after ionic implantation of N areas, in the case of N on P planar structures. Once again the performance of the device is degraded; either the quantum efficiency reduces (for example if e
0
is greater than the diffusion wave length of minority carriers), or the dark current increases (if e
0
is less than the diffusion wave length of minority carriers), or both (if e
0
is less than the diffusion length of minority carriers and if the final thickness of the layer exceeds the diffusion length of minority carriers). In all cases the ratio between the photonic current and the dark current reduces.
This access resistance problem occurs at a much lower level in hetero-junction structures that use a P on N structure as shown in
FIG. 2
, and which benefit from excellent conducting properties of N type layers related to the very high mobility of electrons in these layers; in these structures, this N type layer forms the substrate contact common to all diodes. However, this technology has serious disadvantages compared with N on P “planar” technology.
The purpose of the invention is to overcome the high series resistance problem encountered on diodes made using the planar technology described above and located at the center of large two dimensional mosaics with ixj pixels (where i and j are large, for example greater than 128) designed to detect a high signal level while maintaining the electro-optical characteristics of detectors.
DISCLOSURE OF THE INVENTION
This invention relates to a large two-dimensional photonic radiation detector based on a technique of hybridizing a detection chip onto a read chip through a micro-balls network making the electrical and mechanical inter-connection between two chips, the detection chip being composed of a two-dimensional structure of ixj pixels, an active layer being epitaxed onto a substrate, each elementary photosensitive component consisting of an N/P or P/N diode formed in the active layer, the contact point on the N or P area being made at each pixel, the contact point on the other P or N area being common to all diodes, characterized in that it comprises a

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