Semiconductor radiation detector and manufacture thereof

Details Semiconductor radiation detector and manufacture thereof Semiconductor radiation detector and manufacture thereof

1999-12-16

2002-11-26

Berman, Jack (Department: 2881)

Radiant energy

Invisible radiant energy responsive electric signalling

Semiconductor system

C250S370090

Reexamination Certificate

active

06486476

ABSTRACT:

BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a semiconductor radiation detector suitable for a device for irradiating a specimen with a charged particle such as an electron beam and x-ray, detecting characteristic x-ray generated from the specimen and analyzing the elements of the specimen and a device for irradiating a specimen with x-ray, detecting x-ray which is transmitted through or reflected from the specimen and analyzing the structure of the specimen, the manufacturing method and an apparatus of radiation detection using the above semiconductor radiation detector.
A method of irradiating a specimen with a charged particle such as an electron beam or x-ray, detecting characteristic x-ray generated from the specimen or fluorescent x-ray and analyzing the specimen is known. For its typical example, a method called energy dispersive x-ray spectroscopy of irradiating a specimen with an electron beam in an electron microscope, detecting characteristic x-ray generated from the specimen and analyzing the elements of the specimen can be given.
As characteristic x-ray or fluorescent x-ray has energy peculiar to an element composing a specimen, the number of the generation of x-ray per unit time to analyze elements is required to be counted every energy of x-ray. The energy dispersive x-ray spectroscopy is a method of using a detector wherein an output signal having height proportional to the energy of incident x-ray is acquired, identifying the energy of x-ray by combining the detector with a pulse-height analyzing circuit and analyzing elements.
For an x-ray detector used for the above energy dispersive x-ray spectroscopy, there is a semiconductor radiation detector (hereinafter simply called a detector) using a semiconductor crystal such as silicon and germanium.
An apparatus using these detectors and having energy resolution of approximately 140 eV for x-ray having energy of 5.9 keV is known. For the structure of a semiconductor radiation detector, three types of a pin-type, a pn-type and Schottky-barrier type (or a surface-barrier type) are known.
For a detector according to this type of energy dispersion method heretofore used, there is a detector produced by dispersing lithium in a silicon crystal called a lithium-drifted silicon detector.
Of the above three types of detectors, for a first example, the typical contour of a pin-type detector is shown in FIG.
7
A.
FIG. 7A
shows a cross section viewed along a line A-A′ in
FIG. 7B
showing the appearance of the detector. The detector uses a p-type silicon crystal
101
and the outline is cylindrical and the detector has a concentric deep groove
6
.
The pin-type has structure in which an intrinsic semiconductor region (an i layer)
1
formed by dispersing lithium in a semiconductor substrate
101
is held between a p-type layer
2
and an n-type layer
3
respectively formed on opposite surfaces, gold is respectively deposited on the surfaces of the p-type layer
2
and the n-type layer
3
and electrodes
4
and
5
are formed.
Negative voltage is applied to the electrode
4
on the p-type side of the detector by a bias supply
50
(reverse bias voltage is applied). Normally, x-ray is made incident from the surface of the electrode
4
on the p-type side. When x-ray
10
is incident upon the intrinsic semiconductor region
1
, a secondary electron is generated and produces an electron-hole pair
20
and
21
, losing energy. The generated electron
20
moves to the electrode
5
on the n-type side by an electric field between the electrodes
4
and
5
.
The number of the generated electron-hole pairs is proportional to the energy of incident x-ray. The electron
20
which reaches the electrode
5
is converted to a voltage pulse
52
having height proportional to the number by an amplifier
51
and the energy of x-ray is identified by a pulse-height analyzer
53
.
Reverse bias voltage applied to the electrodes
4
and
5
is set to high voltage of approximately 1000 V to prevent an electric charge (the electron-hole pair
20
and
21
) generated in the intrinsic semiconductor region
1
from being recombined and from being annihilated.
To acquire high energy resolution for the ability of a detector, it is required to reduce leakage current which flows in a detector when reverse bias voltage is applied down to 100 fA or less and to reduce the capacitance of the detector. Therefore, a detector is housed in a vacuum container, is cooled by liquid nitrogen and others, leakage current thermicly generated is reduced by keeping the detector at low temperature and further, surface leakage current is reduced by the concentric deep groove
6
.
The capacitance of a detector is in inverse proportion to the thickness of the intrinsic semiconductor region
1
and is proportional to area S. The area S means the cross section of a part (the intrinsic semiconductor region
1
) inside each groove
6
and is a sensitive part to x-ray. The thickness of the intrinsic semiconductor region
1
is set to approximately 3 to 5 mm.
In the case of a silicon detector, characteristic x-ray having energy of approximately 20 keV with the above thickness can be detected efficiently. For area S, a silicon detector having area of 10 to 30 mm
2
is known. If area is further large, capacitance is increased and energy resolution required for analyzing elements is not acquired. If area is 20 mm
2
, that is, the diameter inside each groove
6
is approximately 5 mm, a silicon detector having the outside diameter of approximately 11 mm is known.
Next, in a pn-type detector for a second example, an n-type layer or a p-type layer of high density is formed on the surface including a p-type or an n-type semiconductor crystal in place of the above intrinsic semiconductor region
1
to produce pn junction and the above detector utilizes a depletion layer generated by applying voltage in a reverse direction. The high-density same-type layer is formed on each opposite surface and further, each electrode is formed on the layer.
When x-ray is incident on the depletion layer in a state in which reverse bias voltage is applied between these both electrodes and the depletion layer is generated in pn junction, an electron-hole pair
20
and
21
are generated and the electron
20
moves on the side of the electrode
5
by an electric field generated in the depletion layer as in the intrinsic semiconductor region
1
of the pin-type detector shown in FIG.
7
A.
Also, for a third example, a detector utilizing a depletion layer generated by applying voltage to Schottky barrier formed by forming a metal electrode on the surface of a semiconductor substrate by gold and others in a reverse direction is called Schottky-barrier type detector (or a surface-barrier type detector). The thickness of a depletion layer is proportional to the square root of applied voltage and is in inverse proportion to the square root of the density of impurities in a crystal. To acquire a depletion layer 3 mm thick by applied voltage of 1000 V, a crystal of higher purity by 3 or 4 digits is required, compared with a crystal used for producing a normal transistor and a normal integrated circuit.
As for Schottky-barrier type detector, to acquire a depletion layer 3 mm thick by applied voltage of 1200 V as an example of the numerical value, a crystal of the purity of approximately 5×10
11
pieces per 1 cm
3
in the density of impurities is required. As recent crystal manufacturing technology is developed, a crystal of high purity which meets the above specification can be manufactured and is actually utilized.
As for a heretofore used lithium-drifted silicon detector, as lithium is thermally diffused when the detector is left at room temperature for a long time and has a bad effect upon the characteristics of the detector such as capacitance increases, the detector is said to be always kept at low temperature, however, each type detector shown in these first to third examples using a high-purity crystal is not required to be always kept at low temperature.

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