Study of commercially available CCDs is required to build a model for the
radiation damage effects, which can be used for estimation of the
device lifetime in the expected environment. Radiation hardness studies of
CCDs can provide valuable information on the preferable device
architecture, chip geometry and operating conditions,
which ultimately determine
the vertex detector design, performance and cost. Much of this study was
concentrated on the device application at near-room
temperature because of
the advantages of this operation. Operation at low temperatures is usually
required to suppress the radiation damage effects
in CCDs .
By avoiding cooling to cryogenic temperatures it is possible to decrease the
thermal distortions of the supporting CCD ladders and to simplify their design,
which results in better detector geometry and increased measurement precision.
Although attractive from designer's point of view, operation at elevated
temperatures faces problems from radiation damage effects, which are addressed
in this study.
Both surface and bulk damage effects are expected to take place in the CCD sensors because of the type and the energy spectrum of the radiation background. Ionizing radiation creates electron-hole pairs in silicon dioxide, which is used as gate and field dielectric in CCDs. Charge carriers drift in the electric field (externally applied or built-in) to the corresponding electrode. Electrons quickly reach the positive electrode, but some of the holes remain trapped in the oxide and give rise to radiation-induced trapped positive oxide charge, which can be stable for long time. At any Si-SiO2 interface there are a number of interface traps, which result from the strained or dangling silicon bonds at the boundary between the two materials. Ionizing radiation causes the density of these traps to increase, generating radiation-induced interface traps. The formation of radiation-induced trapped oxide charge (or flat band voltage shift) and interface traps is referred under the term surface damage.
Radiation with sufficiently high energy can displace Si atoms from their
lattice positions, creating displacement damage.
This process affects
the properties of the bulk semiconductor and is known as bulk
Low energy electrons and X-rays can deliver only small
energy to the recoil Si atom
and mainly isolated displacements, or point defects, can be created.
other hand, heavier particles, such as protons and neutrons can knock out
silicon atoms which have sufficient energy to displace other atoms in the
The secondary displacements form defect clusters, which have high
local defect density and can be tens of nanometers wide.
Defect clusters often
have complicated behavior and more damaging effect on the properties of
semiconductor devices than point defects.
The energy threshold for displacement
of a Si atom has been estimated to be about 260 keV for electrons and 190 eV
for neutrons, therefore in the expected radiation environment bulk defects can
be generated by both particles.
Cluster damage is also expected, because the
threshold for cluster production is known to be 5 MeV and
15 keV for electrons and neutrons ,
Trapped holes in the oxides change the parameters of MOS structures in a way identical to applying an external voltage to the gates. Small flat band voltage shifts, in the order of few volts, can be accommodated by adjustment of the amplitude of the gate bias and drive voltages. A limitation may arise from the maximum allowed power dissipation in the gate drivers and in the CCD chip, because the dissipated power is proportional to the voltage amplitude squared. If the flat band voltage shifts are higher, the device can stop to function properly because of parasitic charge injection from the input structures, incomplete reset of the output node or distortion of the shape of the potential wells . However, as long as no parasitic effects from the increased pulse amplitudes appear, the shifts are not a limitation for the device operation.
Interface traps are the dominant source of dark current in modern CCDs, because the generation rate at the Si-SiO2 interface is usually higher than that in the epitaxial bulk silicon. Increase of the surface dark current is the main effect expected from radiation-induced interface defects.
Radiation-induced bulk defects cause the dark current
in CCDs to increase, which
is well known phenomenon  .
Dark current is an issue only for near-room temperature operation or long
integration times, because it can be reduced to negligible values by cooling.
Irradiation with heavy particles (e.g. protons, neutrons) often creates large
non-uniformities in the dark current spatial distribution in the CCDs
These non-uniformities, also known as ``dark current
spikes'' or ``hot pixels'' manifest themselves as pixels with much higher dark
current than the average value for the CCD.
Their presence has been connected
with the high electric fields caused by the device architecture and
field-enhanced emission, the cluster nature of the radiation damage and crystal
strains in the silicon material.
Dark current spikes have big consequences for
high temperature applications. Additionally, some of them show random
fluctuations of the generated current, or Random Telegraph Signals (RTS)
Another important bulk damage effect is the loss of signal charge during
transfer, or Charge-Transfer Inefficiency (CTI).
Charge losses occur when bulk
defects capture electrons and emit them at a later moment,
so that the released
charge cannot join the original signal packet from which it has been trapped.
The basic mechanism can be explained
by the different time constants and temperature dependencies of the electron
capture and emission processes.
In the presence of bulk defects electrons are trapped with a capture time
and consequently released with an emission time constant
For a defect at energy position Et below the conduction
band the Shockley-Read-Hall theory gives
|=||electron capture cross section,|
|Xn||=||entropy change factor by electron emission,|
|vth||=||thermal velocity for electrons,|
|Nc||=||density of states in the conduction band,|
|ns||=||density of signal charge.|
The capture time constant is typically of the order of several hundred nanoseconds and has weak temperature dependence, whereas the emission time constant changes many orders of magnitude because of the exponential temperature behavior. At low temperature the defects can be considered almost permanently occupied with electrons, because the emission time constant of the defects can be very large, of the order of seconds. The defects cannot capture signal electrons and the CTI at low temperature is small. At high temperature the emission time constant becomes small and comparable with the charge shift time. Trapped electrons are able to join their signal packet, because most of them are emitted already during the charge shift time, and charge losses are small. At temperatures between the two extremes these is a peak of the CTI value.
The mechanism of charge transfer losses is illustrated on Fig. 8.19. When the signal packet encounters traps, some of its electrons are captured and later released. Those electrons, which are released in the trailing pixels do not join their original signal packet and account for the CTI. If a charge packet enters a pixel, in which part of the traps are occupied, less signal electrons can be trapped and therefore less charge can be lost.
We are often interested not only in the CTI value, but in the total
losses the charge suffers after all the transfers
it takes to reach the output.
For a generated charge Cgen, transferred n times, the output charge
Cout is given by