Strip detector

Details Strip detector Strip detector

1999-05-03

2001-02-06

Meier, Stephen D. (Department: 2822)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Electromagnetic or particle radiation

C257S336000

Reexamination Certificate

active

06184562

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a strip detector for detection of ionising particles and/or radiation, as well as to a method aiming at the manufacture of the detector. The strip detector comprises a silicon substrate which, at least on one substrate surface, provides n-doped regions spaced from each other as strips and voltage supply regions, as well as a p-doped insulating region between the n-doped region Moreover, the detector comprises a first insulating layer as well as metal strips on a substrate surface, which are disposed above the n-doped regions.
PRIOR ART
Silicon strip counters serve to detect ionising particles and radiation as part of physical experiments and in other detectors. The functional principle is based on diodes arranged in the form of strips which are capacitively coupled or connected directly to amplifiers sensitive to charges. The spacing of the strips and their width in combination with the selection of the read-out mode define the spatial resolution of the detector. The semiconductor substrate is very highly ohmic so that moderate voltages are sufficient to deplete the entire silicon wafer throughout its thickness. As a result a high conversion length is achieved for the incident particles or radiation and hence a high signal at a comparatively low detector capacitance.
The semiconductor substrate may be weakly n- or p-doped. The strips as such commonly consist of highly doped regions. They may be either p- or n-doped, with the rear large-area electrode being required to be of the respectively opposite doping type. In double-side strip counters the strips of an opposite type of doping. Using an inclination of the strips of both sides relative to each other, a spatial resolution in the x- and y-directions is obtained.
Another possibility of distinction for strip counters consists in the manner in which the signal is coupled to the electronic read-out system. In a simple form the diodes, arranged in strip form, are connected to the inputs of the electronic amplifiers, either directly or via externally interposed capacitors. They are therefore referred to as directly coupled strip counters. In a capacitively coupled strip counter the capacitors are integrated into the strip counter. To this end the capacitor is utilised which is formed by the highly doped strip-shaped region, an insulating layer as dielectric material (e.g. silicon oxide) and a metal strip. Such a typical capacitively coupled strip counter is illustrated as prior art in FIG.
2
. In p-doped strips such as those drawn as obliquely hatched regions at the upper substrate surface in
FIG. 2
, the electrical insulation is provided by positive oxide charges which are fixed substantially at the interface between the silicon substrate material—in the embodiment according to
FIG. 2
the substrate material is provided with an n-type doping—and the silicon dioxide (SiO
2
) serving as insulating layer, and which are created during the actual oxidation process. As a result, the formation of parasitic hole channels, which would short the strips, is suppressed. The strip detector, which is processed on two opposite sides according to
FIG. 2
, includes metal strips MS for signal take off, which are each disposed above correspondingly doped regions. Amplifiers A are connected to the metal strips. Voltage supply contacts Sp are provided for voltage supply and for maintaining appropriate electric fields within the detector structure, via which contacts the biasing voltage±Vbias is applied.
When, for instance, n-type strips also used in double-sided detectors—as is the case in the illustration according to
FIG. 2
, cf. lower substrate surface—the aforementioned positive oxide charge induces an electron layer which would short-circuit the n-regions, in the regions between the n-doped regions of the n-strips below the contacts ms, the supply voltage contacts intermediate rings MSS at the detector edge, for reducing the supply voltage in a direction towards the detector edge. In order to prevent this, the detector surface is p-doped in the intermediate regions (p-comp).
Two variants have been generally accepted. By means of a photoresist mask applied during implantation the p-layer may remain restricted to the regions between the n-type layers. The process is referred to as “blocking isolation” in English literature. This technique has been described already in the following contributions: J. Sedlmeir, diploma thesis submitted to the Technical University of Muenchen, 1985, and G. Lutz et al., MPI-PAE/Exp. El. 175, 1987, as well as G. Lutz, p, 195, New York: Plenum, 1988. Concerning this,
FIG. 3
a
illustrates a detector detail which is provided with n-doped regions in the substrate surface, with implantation of an insulating p-region
2
in the intervals therebetween so as to avoid short circuits. An insulator layer
3
, e.g. silicon oxide, is applied directly on the substrate surface, which includes the oxide charges. Metal strips
4
are provided for signal take off above the n-doped regions.
Another possibility consists in the implantation of the p-layer
2
without any mask, wherein it is over-compensated by the n-regions
1
which are more highly doped (cf. in this respect
FIG. 3
b
in the following where the same reference numerals as in
FIG. 3
a
are used). This is referred to as “spray isolation” and is described, for instance, in an article by J. Kemmer and G. Lutz in Nucl. Instr. and Meth. A 326, p. 209, 1993.
Both possibilities involve advantages and disadvantages. In accordance with their object, strip detectors are frequently employed in a radiation-loaded environment. lonising radiation and particles of high energy, however, damage the semiconductor material, so that the properties of the detector are degraded in the course of time.
Two types of damage can be roughly distinguished from each other.
An irradiation of the detector surface causes an increase of the defects in the zone of the interface between the silicon and the SiO
2
material, and hence an increase of the oxide charges. This effect begins already at comparatively low radiation doses. The size of the generated oxide charges reaches saturation, however, at higher doses. During irradiation with particles, defects are created even in silicon itself. The radiation induces a strongly increased leakage current and a change of the doping in the semiconductor substrate, with a change of the type of doping in the semiconductor from n to p (type inversion)., which creates an entirely new field distribution within the component. This effective p-type doping continues to increase in the course of irradiation and results in strongly increased depletion voltages and hence in higher operating voltages.
Both types of damage, i.e., the increase of the oxide charges and the inversion of he doping type to the p-type, cause an increase of the strength of the electric field in the zone
5
, identified in
FIG. 3
a
with “blocking isolation”. This means that the breakdown voltage is reduced as the irradiation is increased. Hence there is a conflict between the required increase of the operating voltage and the reduced breakdown voltage.
Another problem resides in the aspect that specifications and tests inevitably relate to non-irradiated detectors. As, however, the breakdown characteristics are deteriorated in the course of operation, a reliable statement about the suitability of the measured detector cannot be derived from measurements performed on a non-irradiated component. Moreover, in a system the replacement of defective detectors often incurs substantial expenses because of complicated assembly techniques. This problem does not arise with “spray isolation”. The maximum of the electric field is located in this case directly at the junction between the n-region and the p-insulating region. Cf. in this respect the zones
5
in
FIG. 3
b
. The height of this maximum is determined by the difference in concentration and by the steepness of the pn-junction (doping gradient) as well as by the applied voltage. Irradiation

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