Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1998-09-18
2001-07-10
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370010, C257S428000
Reexamination Certificate
active
06259099
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention relates to an ultra-thin detector of ionizing radiation such as, for example, UV photons, X photons and &agr; or &bgr; particles, and to processes for manufacturing such detector.
Detectors in accordance with this invention find numerous applications for the detection of charged particles or neutrons in media having strong gamma background noise, for the measurement of flight time of heavy ions or, further, for the on-line measurement of flows such as flows of X radiation or of protons.
The invention also finds applications for the production of so-called <<intelligent>> detectors formed by a stack of several detectors whose thickness is adjusted to various types of radiation to be detected simultaneously.
2. State of the Prior Art
Semiconductor radiation detectors operate along the principle of the collection of charges, formed by electron-hole pairs, and generated by interaction of ionizing radiation with matter. When crossing through the semiconductor material, such radiation transfers to the latter all or part of its energy and causes the generation of electron-hole pairs. For example, energy in the region of 3.6 eV is required to generate an electron-hole pair in silicon.
The diagram in
FIG. 1
illustrates the operation of a silicon detector. When ionizing radiation shown by arrow
10
crosses through a silicon block
12
, electron-hole pairs
14
are generated in the latter. Electrodes
16
and
18
, respectively covering opposites surfaces of the silicon block, are connected to a voltage source
20
and earth respectively to create an electric field E within silicon block
12
. Under the effect of this field, the charges, that is to say the electrons and holes, migrate towards electrodes
16
and
18
and may be collected on the detector terminals, for example terminal
22
, to produce an electric signal which may be recorded.
If electric field E is constant throughout all the detector volume, for each photon or each incident particle
10
, regardless of their interaction volume with the semiconductor, it is possible to record an electric impulse at the detector terminals that is proportional to the energy that the photon or particle has transferred to the silicon. However, the electron-hole pairs have a natural propensity to recombine with one another, and the signal is only equal to the transferred energy if the distance of free movement of the electrons and holes before their recombination is greater than the thickness of the detector.
The transfer of radiation energy to matter is related to the type and energy of incidental radiation. This transfer takes place according to different processes and with different energy losses. As an illustration, table I below gives approximate thicknesses of silicon (e) required to absorb 90% of the energy of incidental radiation from different types of radiation and charged particles.
TABLE I
Ra-
Visible
UV
X
X
X
Gamma
alpha
alpha 5
electron
diation
600 nm
200 nm
1 keV
10 keV
100 keV
1 MeV
1 MeV
MeV
50 keV
e(&mgr;m)
2
0.02
7
300
2.3.10
5
3.6.10
5
3
25
20
Radiation energy losses nevertheless have a statistical character and the values given in table I correspond to average thicknesses that are required for the absorption of 90% of radiation. For example, a gamma photon of 500 keV crossing through a silicon detector 300 &mgr;m thick (standard detector), has a reaction probability of 7×10
−4
in the detector, leading an energy transfer to the silicon of 150 to 200 keV. In the event of a strong flow of gamma photons in the region of 10
5
photons per second, 70 electric impulses per second are recorded at the terminals of the detector, corresponding to a photon energy loss in the detector of 150 to 200 keV. In fact, an entire series of impulses are recorded, corresponding to energy losses of between 0 and 500 keV.
In a certain number of applications of semiconductor detectors, it would be desirable for such detectors to have a thickness of between 0.1 &mgr;m and a few tens of micrometres.
As an example, if the radiation comprises several types of radiation corresponding to very different absorption thicknesses in the silicon, and it is required only to measure one type of radiation, for example that corresponding to the smallest absorption thickness, it would be desirable to use a detector whose thickness is precisely equal or close to this absorption thickness.
For example, to detect alpha particles of 1 MeV in a medium having a strong gamma radiation interference signal, the alpha signal to gamma signal ratio is optimum for a silicon detector approximately 4 &mgr;m thick, corresponding to the absorption thickness of alpha particles. For such detector, all the alpha particles lose all their energy in the detector. A very small proportion of gamma photons (<1/1000) undergoes interaction with the detector and the gamma photons which interact with the silicon lose a very small quantity of energy. Since the electric signal is directly proportional to the energy lost by radiation in the silicon, the alpha signal is therefore far superior to the gamma signal.
A further application requiring the availability of thin detectors is the measurement of X radiation, gamma radiation and/or on-line particles, when the radiation loses its energy in the silicon in virtually uniform manner.
For example, for the on-line measurement of the flow of alpha particles of 5.5 MeV, the detector needed is one such that the energy losses are in the region of 100 keV, so that particle counting is possible without the particle energy loss being too high. In particular, in the case of this example, a silicon detector having a thickness of approximately 0.5 &mgr;m would be suitable.
At the current time, surface barrier silicon detectors that are commercially available have a thickness of approximately 10 &mgr;m or over.
These detectors are generally obtained by mechanical thinning of silicon wafers that are several hundred micrometers thick. For reasons related to mechanical resistance and control over the remaining thickness, it is very difficult to produce detectors by mechanical thinning having a thickness of less than 10 &mgr;m.
Also, wafer thickness is only controlled to a precision of approximately ±1.5 &mgr;m. These detectors are therefore not adapted to the above-mentioned applications.
Document (1) referenced at the end of this disclosure concerns a radiation detector with a detection diode made in a thick layer of silicon. This detector is associated with an electronic circuit fabricated in a superficial layer of silicon insulated from the thick silicon layer by an insulating oxide layer. The thickness of the layer (100 &mgr;m ) eliminates this type of detector from the field of application under consideration which concerns thin detectors. Document (2) also referenced at the end of this disclosure describes a heat insulated radiation detector using a diode with a structure of MESA type. With the technique described in this document it is not possible either to produce thin detectors. Finally, document (3) concerns photodiodes made in the thin layer of a SOI substrate. These diodes are not however adapted to the detection and spectrometry of ionizing radiation.
Finally, one purpose of the present invention is to describe a thin detector having a thickness of preferably between 0.1 and 10 &mgr;m. Another purpose is to make available a detector whose thickness can be controlled with great precision. A further purpose of the invention is also to make available an ultra-thin detector which can be used not only to conduct counting of the number of photons or charged particles received, but also to perform energy spectroscopy of this radiation. The invention also sets out to describe simple processes for producing such detectors. The invention also proposes a process of for manufacturing thin detectors having low capacity and obscurity current compared with those obtained using conventional techniques.
DESCRIPTION OF THE DISCLOSURE
To reach these objectives, the object of the
Babadjian Lionel
Foulon François
Spirkovitch Serge
Burns Doane Swecker & Mathis
Commissariat A l'Energie Atomique
Epps Georgia
Hanig Richard
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