Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1998-10-21
2001-08-21
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370140
Reexamination Certificate
active
06278119
ABSTRACT:
BACKGROUND
The invention relates to using a delta-doped charge-coupled device (CCD) to determine the energy of a low-energy particle.
A charge-coupled device (CCD) includes an array of coupled electronic gates, such as metal-oxide-semiconductor field effect transistors (MOSFETs), that together convert optical or particle energy into an electronic signal. CCDs are used in a wide variety of applications, including digital imaging systems such as digital cameras.
FIG. 1A
shows the general structure of a thinned CCD
10
, which commonly is used for ultraviolet light detection. The CCD
10
includes a semiconductor core
12
comprising, e.g., lightly doped silicon, onto which a thin insulating layer
14
, e.g., a layer of silicon oxide, is formed. An oppositely charged dopant layer
15
, e.g., n-type dopant, may be implanted at the front surface
24
of the semiconductor core
12
to form a “buried channel” CCD, which is described below. A conductive gate
16
is formed on the front surface
18
of the insulating layer
14
to apply an electric potential to the device. Typically, the back surface
20
of the semiconductor substrate
12
includes a thin, insulating native oxide layer
22
that forms naturally on the semiconductor's back surface
25
.
In operation, the conductive gate
16
is biased with respect to the back surface
20
of the semiconductor substrate
12
by a voltage supply V. As photons or particles strike the device
10
through its back surface, electron-hole pairs form in the substrate core
12
. Depending on whether the semiconductor is p-type or n-type, the electrons or the holes migrate toward the semiconductor-oxide surface
24
, where they accumulate in a “collection well”
26
(
FIG. 1B
) that develops in the semiconductor
12
near the semiconductor-oxide surface
24
. The implanted layer
15
creates a buried channel where collected charge accumulates in the semiconductor core
12
a given distance below the insulating layer
14
.
Incident energy from photons or particles is converted into charge in the semiconductor core, and the charge accumulates in the collection well during a given integration period. The amount of charge collected in the well
26
during the integration period is generally proportional to the total energy of the particles penetrating the semiconductor
12
during the integration period. The efficiency of the conversion of energy to charge depends on the energy-dependant interaction of photons or particles in the CCD structure. Therefore, different CCD structures can have markedly different efficiencies. The CCD
10
generates an output signal by serial measurement of the charge collected in each pixel during the integration period.
The thickness of the semiconductor core
12
in a typical thinned CCD
10
is 8-15 &mgr;m, which allows the thinned CCD to detect some particles striking its back surface
25
. For example, a typical thinned CCD can detect electrons having kinetic energies greater than about 10 keV. The sensitivity of the thinned CCD to these low-energy particles is limited, however, by a “dead layer” caused by the presence of a “potential well”
28
(FIG.
1
B), which forms near the substrate's back surface
25
as a result of charge trapped in the native oxide layer
22
. Particles moving with kinetic energies below a certain level do not penetrate far enough into the CCD to overcome the potential well.
Backside surface treatment technology has been used to alter the CCD structure and thus to reduce the effects of the potential well
28
. These techniques include UV-induced adsorption of negative ions on the native oxide surface, deposition of a conductive layer over the oxide, and introduction of a thin p+layer by ion implantation. Backside treatment has improved the particle detection capabilities of CCDs, but the utility of CCDs as particle detectors is limited by the CCD structures. For example, using conventional detectors, detection of electrons is limited to particles with energies above 1 keV, and detection of protons is limited to particles with energies above 10 keV.
SUMMARY
The inventors have recognized that a backside-thinned delta-doped CCD can be used to detect very-low-energy particles, including electrons with energy levels less than 1 keV and as low as 50 eV, and protons with energies less than 10 keV and as low as 1.2 keV. The delta-doped CCD also can be used to determine the energies of individual particles striking the CCD. The delta-doped CCD exhibits a gain of approximately 170 for 1 keV electrons, which represents more than 200% improvement over conventional backside-treated, thinned CCDs, such as biased flash-gate CCDs. This discovery allows for the use of CCDs in more sophisticated, low-energy particle applications, such as detecting and imaging electrons in low-energy electron diffraction (LEED) spectroscopy, low-energy reflection electron energy loss spectroscopy (REELS), and low-energy plasma detection.
The invention relates to using a backside-thinned, delta-doped CCD to determine the energy of a low-energy particle, such as a proton having energy below 10 keV. A system implementing the invention exposes the back surface of the CCD to the low-energy particle and measures the amount of electric charge collected in the CCD as a result of the penetration of the particle into the CCD. The system then applies a conversion factor to the measured charge to calculate the energy of the particle.
In some implementations, conversion factor is calculated by measuring an amount of charge collected as a result of the penetration of a particle having a known energy. Low-energy particles may include particles that penetrate no deeper than 1.0 nm and as little as 0.5 nm into the CCD, e.g., electrons having energies as low as 50 eV and protons having energies as low as 1.2 keV.
Other embodiments and advantages will become apparent from the following description and from the claims.
REFERENCES:
patent: 4822748 (1989-04-01), Janesick et al.
patent: 5122669 (1992-06-01), Herring et al.
patent: 5376810 (1994-12-01), Hoenk et al.
patent: 5399863 (1995-03-01), Carron et al.
patent: 5574284 (1996-11-01), Farr
patent: 5670817 (1997-09-01), Robinson
patent: 5969368 (1999-10-01), Thompson et al.
Croley Donald R.
Murphy Gerald B.
Nikzad Shouleh
California Institute of Technology
Fish & Richardson P.C.
Gagliardi Albert
Hannaher Constantine
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