Abstract
An increase in the radiation levels during the high-luminosity operation of the Large Hadron Collider calls for the development of silicon-based pixel detectors that are used for particle tracking and vertex reconstruction. Unlike the conventionally used conductively coupled (DC-coupled) detectors that are prone to an increment in leakage currents due to radiation, capacitively coupled (AC-coupled) detectors are anticipated to be in operation in future collider experiments suitable for tracking purposes. The implementation of AC-coupling to micro-scale pixel sensor areas enables one to provide an enhanced isolation of radiation-induced leakage currents. The motivation of this study is the development of new generation capacitively coupled (AC-coupled) pixel sensors with coupling insulators having good dielectric strength and radiation hardness simultaneously. The AC-coupling insulator thin films were aluminum oxide (Al2O3) and hafnium oxide (HfO2) grown by the atomic layer deposition (ALD) method. A comparison study was performed based on the dielectric material used in MOS, MOSFET, and AC-coupled pixel prototypes processed on high resistivity p-type Magnetic Czochralski silicon (MCz-Si) substrates. Post-irradiation studies with 10 MeV protons up to a fluence of 1015 protons/cm2 suggest HfO2 to be a better candidate as it provides higher sensitivity with negative charge accumulation on irradiation. Furthermore, even though the nature of the dielectric does not affect the electric field within the AC-coupled pixel sensor, samples with HfO2 are comparatively less susceptible to undergo an early breakdown due to irradiation. Edge-transient current technique (e-TCT) measurements show a prominent double-junction effect as expected in heavily irradiated p-type detectors, in accordance with the simulation studies.
Highlights
The Phase-2 Upgrade of the LHC to high-luminosity LHC (HL-LHC) in 2027 is expected to increase the instantaneous luminosity by a factor of 5–7, along with a goal of delivering an increase in the integrated luminosity from 400 fb−1 to 3000–4,000 fb−1
The physics behind this can be explained as a consequence of increasing negative charge accumulation at the oxide–silicon interface that causes the bands to be pulled down farther in equilibrium
Radiation-induced threshold voltage shift due to charges trapped in the gate oxide as well as the oxide–silicon interface is dependent on the irradiation dose that the MOSFET device is exposed to
Summary
The Phase-2 Upgrade of the LHC to high-luminosity LHC (HL-LHC) in 2027 is expected to increase the instantaneous luminosity by a factor of 5–7, along with a goal of delivering an increase in the integrated luminosity from 400 fb−1 to 3000–4,000 fb−1 (an increase of a factor of ten compared to the expected dataset at that time). With the use of silicon dioxide, characteristically a positive oxide charge, the dielectric layer results in surface electron accumulation near the interface of the insulating layer and the p-bulk This would lead to the formation of a short circuiting channel between the n+-segments, thereby degrading the spatial resolution of the detector. The segmented implants can be electrically discrete from one another by utilizing thin films of a field insulator with negative oxide charge, such as hafnium oxide (HfO2) and aluminum oxide (Al2O3), for an improved surface current termination strategy Both Al2O3 and HfO2 have a high negative oxide charge of the order of 1011–1013 cm−2 and possess very good dielectric constants, thereby yielding a higher oxide capacitance (Härkönen et al, 2016a,b). This in turn provides a higher capacitive coupling along with a better insulation and improved radiation hardness (Tsui et al, 2013)
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