Radiation detectors made of silicon are cost effective
and have excellent position resolution. Therefore, they are widely used for
track finding and particle analysis in large high-energy physics experiments.
Exposure of the silicon material to particle radiation causes irreversible defects that deteriorate the performance of the silicon detectors. Normally an approach to improve the radiation hardness of the silicon material is to increased oxygen concentration in silicon material.
Exposure of the silicon material to particle radiation
Exposure of the silicon material to particle radiation causes irreversible
defects that diminish the performance of the silicon detectors. Thus, improving
the radiation hardness of the detectors is a very important research subject in
experimental particle physics. Silicon detectors are pn-junction devices
operating at full depletion. Radiation induced defects diminish the performance
of the silicon devices in two principal ways.
First, in the silicon material, the defects create generation-recombination centres that decrease the minority carrier diffusion length and increase the leakage current with relation to the irradiation dose. Thus, the electrical signal created by the particle traversing the detector becomes difficult to distinguish from the background noise.
Second, lattice defects change the effective resistivity of the silicon. Consequently, the operating voltage needed for the full depletion of the detector changes and gradually may exceed the breakdown voltage of the device. Additionally, as the irradiation dose increases, the defects eventually change the type of conductivity of the silicon from n-type to p-type.
The deterioration of the silicon detectors in particle radiation is the most serious limitation of the long-term performance of large modern particle physics experiments. Thus, all the results obtained in the improvement of radiation hardness of silicon detectors are directly applicable to the international community of high-energy physics, including future CERN experiments. In addition, the results can be exploited in space research since similar detectors are used to measure high particle radiation fields in space.
Fig 1. Radiation detector and test structures processed on a silicon wafer.
Why high resistivity Czochralski silicon?
Silicon detectors have traditionally been processed
using Float Zone silicon (Fz-Si) wafers. Fz-Si crystals are grown without quartz
crucibles acting as a source of impurities, i.e. oxygen, carbon, or boron. The
lack of impurities results in high purity and high resistivity of the bulk
silicon. Because of the high resistivity, the detector can be fully depleted at
reasonable operating voltages. However, Fz-Si has a low oxygen concentration.
The oxygen concentration can be increased by prolonged high temperature drive-in
oxygen diffusion. Unfortunately, some metallic contamination is always present
in quartz tube diffusion furnaces. At elevated temperatures, the diffusion
velocity of many harmful metals is much higher than that of oxygen. This in turn
may lead to an extensive process induced contamination in the silicon wafers,
unless a careful quartz tube cleaning procedure is practiced.
Silicon wafers grown using the Czochralski method intrinsically contain high level of oxygen, typically 10^17 – 10^18 cm-3. In addition, the depletion voltage of the Czochralski silicon (Cz-Si) detectors can be tailored either by adjusting the oxygen concentration in the silicon bulk during crystal growth or by manipulating the thermal history of wafers during the detector processing.
Furthermore, using Cz-Si as detector material may offer economical benefits. Cz-Si wafers are available up to a diameter of 450 mm while Fz-Si wafers are typically of diameter 150 mm or 200 mm. Thus, very large area silicon detectors could be manufactured, which in turn could result in significant savings in the costs related to front-end electronics, interconnection, and module assembly.
Crystal growth of Czochralski silicon
In the Czochralski method, polycrystalline silicon fragments are melted inside a quartz crucible.
During the process, argon gas continuously flushes the interior of the crucible and the surface of the silicon melt. Silicon single crystals are grown by slowly pulling a crystal seed up from the molten silicon, thus developing an ingot. Later, wafers are cut from the ingot. Oxygen concentration is one of the most important parameters of silicon wafers. For example, oxygen precipitates bind unwanted metallic impurities present during the processing of silicon devices. Furthermore, stress induced during high temperature processing can lead to the formation of slip defects in the wafer.
The presence of oxygen stabilizes the wafer, and thus Cz-Si wafers are less prone to slip than Fz-Si wafers. During crystal growth, oxygen is dissolved into silicon from the quartz crucible. Most of the oxygen is dissolved as silicon monoxide and is flushed away by argon gas. Furthermore, the resulting oxygen concentration depends on the velocity of the silicon melt flow as well as on the rate of oxygen evaporation from the surface of the melt. All these parameters can be influenced in order to get a desired oxygen concentration in silicon ingot. Since the silicon melt is an electrically conductive liquid, the magnetic field is an effective way to moderate and control the melt flow.
The Magnetic Czochralski (MCZ) method has several advantages, e.g. reducing the erosion of the silica crucible and thus reducing introduction of impurities during crystal growth. In order to grow n-type silicon ingots, phosphorous dopant is added to the silicon melt in order to create desired donor states in silicon. However, boron is a common element in nature and thus easily drifts from quartz crucible to silicon melt during crystal growth. Since boron acts as an acceptor in silicon, the amount of unwanted boron is an important limitation to the magnitude of resistivity in ntype silicon.
Fig 2. Monocrystalline boules created using the Czochralski process.
Oxygenated Czochralski silicon detectors were found to be appropriate for
particle detection, i.e. leakage currents were relatively low at full depletion,
depletion voltages were reasonable, and no breakdown occurred near the depletion
voltages. In particle beam tests, the particle
detection performance of the
Czochralski silicon detector, i.e. resolution, efficiency and signal-to-noise
ratio, was shown to be comparable with the existing silicon strip detectors. In
proton irradiations, Czochralski silicon was found to be more radiation hard
than standard or diffusion oxygenated Float Zone Silicon.
Oxygenated Czochralski silicon detectors were found to be appropriate for particle detection, i.e. leakage currents were relatively low at full depletion, depletion voltages were reasonable, and no breakdown occurred near the depletion voltages. In particle beam tests, the particle detection performance of the Czochralski silicon detector, i.e. resolution, efficiency and signal-to-noise ratio, was shown to be comparable with the existing silicon strip detectors. In proton irradiations, Czochralski silicon was found to be more radiation hard than standard or diffusion oxygenated Float Zone Silicon.
- Two growth techniques for mono-crystalline silicon: Czochralski vs Float Zone.