Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2000-06-14
2002-10-15
Sherry, Michael (Department: 2829)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C438S056000
Reexamination Certificate
active
06465857
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to devices for detecting atomic or nuclear particles and, more particularly, to semiconductor particle detectors.
BACKGROUND OF THE INVENTION
During the investigation of nuclear reactions brought about in the laboratory, it is necessary to detect and to identify the products of nuclear reactions, which may be ions of atoms or nuclear fission fragments with higher or less high atomic numbers, or alpha particles. Similar requirements generally arise during analysis of radioactive phenomena with the emission of high-energy particles, as in cosmic-ray analysis.
In order to identify a particle, it is necessary to measure various quantities such as electric charge, kinetic energy, vectorial momentum, and atomic number. A known method of measuring charge and energy provides for the use of two superimposed semiconductor detectors, of which one is relatively thick (300-400&mgr;m) and one is relatively thin (from a few &mgr;m to a few tens of &mgr;m). An incident particle interacts first with the thin detector, losing only some of its energy (&Dgr;E) and then with the thick detector to which it yields all of its residual energy (E−&Dgr;E), where E represents the energy of the particle before impact with the thin detector.
The manufacture of &Dgr;E detectors is quite difficult, particularly when detectors with a relatively extensive active area are required as is the case for position detectors, that is, detectors which can also provide information on the spatial distribution of the incident particles. In these cases, it is necessary to form plates of semiconductor material with an area of as much as a few hundred square centimeters and a thickness of only about ten &mgr;m, which would therefore be extremely fragile and difficult if not impossible to handle.
Various techniques have been proposed for the manufacture of detectors of this type. One of these (described in an article by G. Thungstrom et al., in Nuclear Instr. and Meth. in Ph. Res., A 391 (1997) 315-328) provides for joining of two superimposed silicon wafers with the use of a metal silicide as a joining and interface layer, reduction of the thickness of one of the wafers to achieve the desired thickness for the &Dgr;E detector and passivation of the free surfaces of the wafers thus joined, opening of windows for the active areas of the detector and for the connection to the metal silicide interface layer, and implantation and diffusion of doping impurities to form junctions.
The resulting structure is shown schematically and in section in FIG.
1
. It includes a first, n-type monocrystalline silicon wafer
10
with a low concentration of impurities (n−−), for example, phosphorus, having a surface layer
11
with a higher concentration of n-type impurities (for example, arsenic) and by a second silicon wafer
9
, which is also n-type with a low concentration of impurities (phosphorus), and which is joined to the first wafer by a layer
12
of cobalt silicide. Two planar p-type regions
13
and
14
are formed on the free surfaces of the two wafers and metal contacts
15
and
16
which form two electrodes of the detector are formed thereon. A third electrode is formed by a contact
17
on the cobalt silicide layer
12
. From an electrical point of view, the structure is equivalent to a pair of diodes having a common cathode (the layer
12
). The upper, thin diode forms the &Dgr;E detector and the lower, thick diode forms the &Dgr;-E detector.
In operation, two voltages, −V
1
and −V
2
relative to a common terminal represented by the ground symbol in the drawing, are applied between the upper and lower electrodes
15
and
16
, respectively, and the intermediate electrode
17
, the voltages being of a sign such as to bias the two diodes in reverse. Two depletion regions are thus formed between the two p regions and the two n layers adjacent thereto, which form the active portions of the two detectors. A particle to be detected, represented by an arrow with a broken line, strikes the front surface of the detector and, as it passes through the two depletion regions, brings about the formation of electron-hole pairs which move towards the electrodes, giving rise to pulsed currents. These currents are collected and amplified by suitable circuits, generally indicated
18
and
19
, and are then measured and displayed by a suitable electronic device, indicated
7
, to give an indication of the quantities &Dgr;E and E−&Dgr;E and hence of the masses of the incident particles.
With the known detector described above, the portion which detects &Dgr;E can be made very thin while avoiding the problems of fragility indicated above because it is processed when it is fixed to the much thicker portion which detects E−&Dgr;E. However, this detector is not suitable for mass production because it requires processing steps which are not provided for in normal processes for the manufacture of monolithic integrated circuits. Moreover, it cannot function as a position detector.
A structure which can be produced by standard techniques and which can be used as a position detector is described in patent publication EP-A-0730304 and is shown schematically in FIG.
2
. As can be seen in the drawing, the detector is formed in a single monocrystalline silicon chip
20
which comprises three superimposed layers: two n-type layers, that is, an upper layer
22
and a lower layer
23
, and one p-type layer
21
which is strongly doped and is therefore marked P+, and which is “buried”, that is, interposed between the two n-type layers, and extends to a certain distance from the lateral surface of the chip. In order to contact the p+layer
21
electrically, a region
24
, which is also a strongly doped p-type region is provided, extending from the upper surface of the chip to the p+ layer
21
and having, on the surface, a contact element in the form of a metal strip
27
, for example, an aluminium strip. Seen in plan, the p+ contact region
24
and the metal strip
27
are typically in the form of square or rectangular frames.
The upper layer
22
and the lower layer
23
are contacted electrically by two n-type surface regions which are strongly doped and are therefore marked N+ and indicated
31
and
32
, respectively, and by two metal layers
25
and
26
, respectively. The latter, together with the metal strip
27
, form the terminal electrodes of the detector and serve to connect the detector to biasing voltage sources and to circuit amplifiers, processors and indicators similar to those already described briefly with reference to FIG.
1
.
The above-described structure is produced by the usual manufacturing processes for planar technology. This enables the detector to be mass produced, although with some difficulties and with results which are not always completely satisfactory. In particular, it is the formation of the buried layer which creates some problems. It can in fact be produced by high-energy implantation of boron ions directly at the desired depth in the substrate, or by surface doping of the substrate followed by the formation of the n layer
22
by epitaxial growth. In the first case, the high-energy implantation is a fairly critical and potentially harmful operation and, in the second case, when operating with the usual monocrystalline silicon substrates with <111> crystallographic orientation and with boron as the dopant, phenomena of epitaxial shift (epi-shift), self-doping and disappearance of the alignment marks for identifying the limits of the buried geometrical arrangements occur. This leads to an enlargement of the buried layer which is difficult to control.
The structure of
FIG. 2
can be modified, as shown in
FIG. 3
, to operate as a position detector. The modification includes the formation of elemental cathode electrodes
31
a
-
31
d
distributed over the active surface of the &Dgr;E detector and of the connection of each elemental cathode electrode to the detection circuit by a respective metal contac
Patti Davide
Valvo Giuseppina
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Pert Evan
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