Inter-pixel isolation is required for devices with n+ readout electrodes within a p-type
bulk because of the formation of a permanent electron accumulation layer at the silicon to silicon dioxide interface regions. This electron accumulation layer is a by-product of the processing techniques used and is caused by the dielectric layer on top of the
silicon trapping positive ions during the detector processing steps. This effect is also be caused by ionising radiation and made worse because more static positive charge builds up in the dielectric, until saturation is reached at approximately 2·1012 ions
cm−3. The positively charged oxide attracts electrons at the interface with the silicon,
forming the accumulation layer. This electron accumulation layer connects all the readout implants together, essentially creating a diode. In contrast, if the readout electrodes are p+ implants then this accumulation layer would act to isolate the readout electrodes
[140].
To isolate the n+ readout electrodes from each other, p-type doping is implanted between then+ readout electrodes to create a static negative charge that removes the
electrons at the interface. Choosing the isolation geometry and the implant parameters involves a compromise between strong isolation and lower breakdown voltage. The higher the isolation implant doping concentration, the stronger the isolation is. However, high concentrations of p-type implants can cause high field regions at their edge. This can cause early device breakdown if the electric fields at the reaches ∼ 300 kV/cm [142]. There are three commonly used methods for isolating n+ implants in n-in-n and n-in-p
devices, they are: homogeneous p-spray, p-stop and moderated p-spray. These three methods are illustrated in Figure 7.5.
Figure 7.5: Three common forms of inter-pixel isolations: p-spray, p-stop and moderated p-spray. The n-type pixels are marked in blue and the p-type isolation is coloured in red [143].
The homogeneous p-spray option uses an unstructured implant with a relatively low concentration over the whole sensor area. This is the easiest to implement during processing, but it leads to a direct contact between the n-type pixel implant and the p-spray implant. This can cause sensor break down during normal sensor operation if the concentration of the p-spray is too high. P-stop isolation is achieved by implanting distinct p-type implants between the n+ readout implants. These implants can either
be separate or linked together. This method requires an extra processing step during production as thep+ implants require their own mask stage. The p-stop isolation implant
uses a higher doping concentration compared to the p-spray method. Since the doping concentration is high the placement of thep+ implants must be very accurate. If the p+ isolation implant and the n+ readout implant touch or overlap a p-n-junction would be
formed, creating a high electric field.
The moderated p-spray method is a hybrid method that features the advantages of the homogeneous p-spray and p-stop methods and requires the same amount of processing steps as the p-stop method. A central p-stop is implanted into a p-spray region. This has the advantage of having good isolation, but since it is surrounded by p-spray the electric fields produced at the interfaces are severely reduced [142].
The Effects of Radiation Damage on
Detector Performance
This section discusses the effects of irradiation on silicon detectors. Particles passing through the detectors can interact with the electrons in the material via ionising energy loss, as discussed in Section 6.2, and by non-ionising energy loss by scattering off lattice atoms. Both effects take place in the silicon bulk but also in the dielectric (often SiO2)
used to electrically passivate the sensor surface. Both bulk and surface effects must be considered when investigating the effects of radiation on device performance [143]. This section will focus on the electrical effects of irradiation damage specific to n-in-n and
n-in-p sensor geometries.
8.1 Surface Effects
Surface effects occurs in the dielectric (oxide and/or nitride layers) used on the surface of the silicon crystal, and in the silicon-dielectric interface. For silicon detectors the most commonly used dielectrics are silicon oxide (SiO2, about 100 nm thick) or silicon nitride
(SiN3, >100nmthick) layers. There are two types of surface effects:
Effects within the volume of the SiO2 or SiN3layer: Radiation passing through
the oxide creates electron-hole pairs. These electron-hole pairs typically recombine, but some of the more mobile electrons can escape the dielectric under the influence of electric fields leaving behind the less mobile ions. These trapped ions create a fixed positive charge in the dielectric [145].
Defects at the interface between silicon and the dielectric layer: Radiation can cause the introduction of new energy states at the interface between silicon and insulator. These states can be occupied by electrons or holes affecting the surface charge [131, 146].
8.1.1 Implications of Surface Damage
The positive charge in the insulating layers attracts negative charge carriers forming a conductive accumulation layer of electrons. In n+ readout detectors this increased accumulation layer impacts on the inter-electrode isolation. this can partially compensate the negative space charge introduced via the p-stop, p-spray or moderated p-spray implants discussed in Section 7.5. It has been experimentally observed that the oxide charge saturates at relatively low irradiation doses, (2−3) ·1012e−cm2[131, 145] and
does not exceed a value of about 2·1012cm−3.