High Resistivity N-type Silicon Research Articles

History, status and prospects of producing silicon sensors for HEP experiments at Infineon Technologies

Within this project we are developing single-sided AC-coupled silicon strip sensors and DC-coupled hexagonal pad sensors for applications in High Energy Physics (HEP) experiments. The project started with a feasibility study to re-produce the state-of-the art of silicon sensors of that time, which were based on high-resistivity n-type float-zone 6-inch silicon wafers. Later, the first silicon strip sensors on 8-inch p-type wafers were produced within this project. This influenced the decision of another project to build a silicon sensor based sampling calorimeter. This paper presents the history, current status, and prospects of this sensor development project, jointly conducted by the Institute for High Energy Physics (HEPHY) of the Austrian Academy of Sciences and Infineon Technologies AG.

Surface damage characterization of FBK devices for High Luminosity LHC (HL-LHC) operations

The very high fluences (e.g. up to 2×1016 1 MeV neq/cm2) and total ionising doses (TID) of the order of 1 Grad, expected at the High Luminosity LHC (HL-LHC), impose new challenges for the design of effective, radiation resistant detectors. Ionising energy loss is the dominant effect for what concerns SiO2 and SiO2/Si interface radiation damage. In particular, surface damage can create a positive charge layer near the SiO2/Si interface and interface traps along the SiO2/Si interface, which strongly influence the breakdown voltage, the inter-electrode isolation and capacitance, and might also impact the charge collection properties of silicon sensors. To better understand in a comprehensive framework the complex and articulated phenomena related to surface damage at these very high doses, measurements on test structures have been carried out in this work (e.g. C–V and I–V). In particular, we have studied the properties of the SiO2 layer and of the SiO2/Si interface, using MOS capacitors, gated diodes (GD) and MOSFETs manufactured by FBK on high-resistivity n-type and p-type silicon, before and after irradiation with X-rays in the range from 50 krad(SiO2) to 20 Mrad(SiO2). Relevant parameters have been determined for all the tested devices, converging in the oxide charge density NOX, the surface generation velocity s0 and the integrated interface-trap density NIT dose-dependent values. These parameters have been extracted to both characterize the technology as a function of the dose and to be used in TCAD simulations for the surface damage effect modeling and the analysis and optimization of different classes of detectors for the next HEP experiments.

Determination of Dopant Density Profiles of Heavily Boron-Doped Silicon From Low Temperature Microphotoluminescence Spectroscopy

Low temperature microphotoluminescence spectroscopy ( μ -PLS) is employed to determine the inhomogeneous doping profiles for heavily boron-doped regions on silicon wafers. Samples having various Gaussian function doping profiles, in terms of surface dopant density and depth factor, are prepared via two-step thermal boron diffusion on high resistivity n-type silicon wafers. Measured PL spectra are normalized to the Si band–band luminescence peak, and PL components of undiffused Si are subtracted to resolve the doping peak. We show that the wavelength of the doping peak has a reliable and simple linear relationship with the measured surface dopant density on a semilog plot, and so establish a calibration curve which can be applied to estimate the surface dopant density. A second calibration curve for estimating the depth factor is also established after correcting the measured doping peak intensity to account for incomplete dopant ionization. We show the effectiveness of this method by reconstructing independently measured doping profiles using the surface dopant density and depth factors estimated from the PL spectra. Furthermore, by performing two-dimensional mapping, with μ -PLS measurements at 2 μ m spatial resolution, we are able to map the surface dopant density and diffusion depth factor of micron-scale, locally diffused features.

Robust bidirectional continuous electrowetting based on metal–semiconductor (M–S) diodes

We demonstrate bidirectional continuous electrowetting by embedding metal–semiconductor diodes in the electrowetting substrate. Unlike conventional electrowetting on dielectric, bidirectional continuous electrowetting uses a single electrode pair to actuate a droplet through long distances. As long as the voltage potential is maintained between two end electrodes, the droplet moves toward the electrode with the higher potential. However, previously reported material systems had limited success in repeated actuation. In this work, diodes based on Schottky barriers were fabricated by forming metal–semiconductor junctions between titanium and high-resistivity n-type silicon. The performance enhancements were evaluated using current–voltage measurements of interface pairs. When the titanium is coated with gold to limit electrochemical reactions, the Schottky diodes achieved superior performance compared to electrochemical diodes previously studied. Droplet speed range from 8 to 240 mm/s is reported. Under repeated actuation, the speed of the droplet showed no degradation for up to 2000 cycles (experiment duration).

Characterization of Al2O3 Thin Films Prepared by Thermal ALD

Thermal Atomic Layer Deposition was used to deposit Al2O3 layers with thickness ranging from 2 to 100nm for surface passivation of silicon solar cells. Various characterization techniques were used to evaluate the chemical, physical and optical properties of the layers and interfaces. Minority carrier lifetime around 2ms was measured for an optimal thickness of 15nm for as-deposited layers on high resistivity n-type silicon substrate. An annealing step at 400°Cincreases lifetime up to 5.7ms for the same structure.

Study of the design optimization of AC-coupled single-sided silicon strip sensors

The high-intensity rare-isotope accelerator, RAON, will be constructed for nuclear physics research in Korea. AC-coupled single-sided silicon strip detectors (SSSDs) are being investigated for use in the Si-CsI detector of a large acceptance multi-purpose spectrometer to measure the energies of various isotopes. To determine the optimal design, four SSSD design parameters were examined in this study, namely the ratio of p+ implant width to strip pitch (I/P), the width of the metal layer, the presence of an n+-edge field shaper (FS), and the distance between the guard-ring and sensor edge (DGS). The designed detectors were fabricated on high resistivity n-type silicon wafers of 500 μm thickness. The SSSDs had the strip pitch of 730 μm and 32 readout strips in each, and the size of the sensors was 40.0 × 25.5 mm2. In terms of the leakage current and production yield, the noise improved by up to 30%when the I/P ratio was 0.4, the metal layer was wider than the p+ implantation, and the DGS with n+-edge FS was twice the sensor thickness. The signal-to-noise ratio of the SSSD with the design parameters that provided the optimal leakage current and coupling capacitance was measured to be 29.1 using a 90Sr radioactive source and commercial electronics.

A High Frame Rate Hybrid X-Ray Image Sensor

This paper describes a solid-state image sensor for high-speed X-ray imaging. The sensor is made up of a light sensitive detector layer bump-bonded to a readout integrated circuit (ROIC). The detector layer is high resistivity n-type silicon and is fully depleted in operation. The p-implanted islands are used to define pixel regions with 100-μm × 100-μm area. The detector layer contains 852 × 209 pixels indium bump-bonded to four identical CMOS ROICs. Each ROIC contains 213 × 209 pixels and is fabricated using a 0.25-μm CMOS process. The ROIC utilizes a capacitive transimpedance amplifier-type front-end coupled to a switched capacitor analog memory. This architecture allows storage of eight frames for high-speed burst imaging of up to a million frames per second, followed by a slower multiplexed readout. Details of the sensor design and operation are presented together with characterization results.

DMAPS: a fully depleted monolithic active pixel sensor—analog performance characterization

Monolithic Active Pixel Sensors (MAPS) have been developed since the late 1990s based on silicon substrates with a thin epitaxial layer (thickness of 10–15 μm) in which charge is collected on an electrode, albeit by disordered and slow diffusion rather than by drift in a directed electric field. As a consequence, the signal of these conventional MAPS is small (≈1000 e−) and the radiation tolerance is limited. In this paper, the development of a fully Depleted Monolithic Active Pixel Sensors (DMAPS) based on a high resistivity substrate allowing the creation of a fully depleted detection volume is presented. This concept overcomes the inherent limitations of charge collection by diffusion in the standard MAPS designs. We present results from a prototype chip EPCB01 designed in a commercial 150 nm CMOS technology. The technology provides a thin (≈50 μm) high resistivity n-type silicon substrate as well as an additional deep p-well which allows to integrate full CMOS circuitry inside the pixel. Different matrix types with several variants of collection electrodes and pixel electronics have been implemented. Measurements of the analog performance of this first implementation of DMAPS pixels are presented.

Imaging of free carriers in semiconductors via optical feedback in terahertz quantum cascade lasers

To monitor the density of photo-generated charge carriers on a semiconductor surface, we demonstrate a detectorless imaging system based on the analysis of the optical feedback in terahertz quantum cascade lasers. Photo-excited free electron carriers are created in high resistivity n-type silicon wafers via low power (≅40 mW/cm2) continuous wave pump laser in the near infrared spectral range. A spatial light modulator allows to directly reconfigure and control the photo-patterned intensity and the associated free-carrier density distribution. The experimental results are in good agreement with the numerical simulations.

Effect of magnetic field on the formation of macroporous silicon: structural and optical properties

ABSTRACTThe conventional method to fabricate porous silicon with n-type substrates requires light assisted generation of holes used in the electrochemical reaction. Recently, two different methods have been proposed to fabricate some similar structures: Hall effect [1] and lateral electrical field [2]. Hall effect assisted etching involves the application of a perpendicular electric and magnetic field to achieve the concentration of holes at the HF/silicon interface to assist the electrochemical reaction, while the other involves the application of a lateral electrical field across the silicon wafer. In this work, the electrochemical etching of high resistivity n-type silicon wafers under the combined effect of magnetic and lateral electrical field to produce photoluminescent macroporous structures under dark conditions, is reported. A lateral gradient in pore sizes as well as in light emission is observed. Optical and structural properties were studied for their possible applications as a biosensor.

Study of radiation damage induced by 12 keV X-rays in MOS structures built on high-resistivityn-type silicon

Imaging experiments at the European X-ray Free Electron Laser (XFEL) require silicon pixel sensors with extraordinary performance specifications: doses of up to 1 GGy of 12 keV photons, up to 10(5) 12 keV photons per 200 µm × 200 µm pixel arriving within less than 100 fs, and a time interval between XFEL pulses of 220 ns. To address these challenges, in particular the question of radiation damage, the properties of the SiO(2) layer and of the Si-SiO(2) interface, using MOS (metal-oxide-semiconductor) capacitors manufactured on high-resistivity n-type silicon irradiated to X-ray doses between 10 kGy and 1 GGy, have been studied. Measurements of capacitance/conductance-voltage (C/G-V) at different frequencies, as well as of thermal dielectric relaxation current (TDRC), have been performed. The data can be described by a dose-dependent oxide charge density and three dominant radiation-induced interface states with Gaussian-like energy distributions in the silicon band gap. It is found that the densities of the fixed oxide charges and of the three interface states increase up to dose values of approximately 10 MGy and then saturate or even decrease. The shapes and the frequency dependences of the C/G-V measurements can be quantitatively described by a simple model using the parameters extracted from the TDRC measurements.

Device Design for a 12.3-Megapixel, Fully Depleted, Back-Illuminated, High-Voltage Compatible Charge-Coupled Device

A 12.3-megapixel charge-coupled device (CCD) that can be operated at high substrate-bias voltages has been developed in support of a proposal to study dark energy. The pixel size is 10.5 mum, and the format is 3512 rows by 3508 columns. The CCD is nominally 200 mum thick and is fabricated on high-resistivity n-type silicon that allows for fully depleted operation with the application of a substrate-bias voltage. The CCD is required to have high quantum efficiency (QE) at near-infrared wavelengths, low noise and dark current, and an rms spatial resolution of less than 4 mum. In order to optimize the spatial resolution and QE, requirements that have conflicting dependences on the substrate thickness, it is necessary to operate the CCD at large substrate-bias voltages. In this paper, we describe the features of the CCD, summarize the performance, and discuss in detail the device-design techniques used to realize 200-mum-thick CCDs that can be operated at substrate-bias voltages in excess of 100 V.

Test beam results of heavily irradiated magnetic Czochralski silicon (MCz-Si) strip detectors

Strip detectors with an area of 16 cm 2 were processed on high resistivity n-type magnetic Czochralski silicon. In addition, detectors were processed on high resistivity Float Zone wafers with the same mask set for comparison. The detectors were irradiated to several different fluences up to the fluence of 3 × 10 15 1 MeV n eq / cm 2 with protons or with mixed protons and neutrons. The detectors were fully characterized with CV- and IV-measurements prior to and after the irradiation. The beam test was carried out at the CERN H2 beam line using a silicon beam telescope that determines the tracks of the incoming particles and hence provides a reference measurement for the detector characterization. The n-type MCz-Si strip detectors have an acceptable S/N at least up to the fluence of 1 × 10 15 n eq / cm 2 and thus, they are a feasible option for the strip detector layers in the SLHC tracking systems.

Test beam results of a heavily irradiated Current Injected Detector (CID)

A heavily irradiated (3×10 15 1 MeV n eq/cm 2) Current Injected Detector (CID) was tested with 225 GeV muon beam at CERN H2 beam line. In the CID concept the current is limited by the space charge. The injected carriers will be trapped by the deep levels and this induces a stable electric field through the entire bulk regardless of the irradiation fluence the detector has been exposed to. The steady-state density of the trapped charge is defined by the balance between the trapping and the emission rates of charge carriers (detrapping). Thus, the amount of charge injection needed for the electric field stabilization depends on the temperature. AC-coupled 16 cm 2 detector was processed on high resistivity n-type magnetic Czochralski silicon, and it had 768 strips, 50 μm pitch, 10 μm strip width and 3.9 cm strip length. The beam test was carried out using a silicon beam telescope that is based on the CMS detector readout prototype components, APV25 readout chips, and eight strip sensors made by Hamamatsu having 60 μm pitch and intermediate strips. The tested CID detector was bonded to the APV25 readout, and it was operated at temperatures ranging from −40 to −53 °C. The CID detector irradiated at 3×10 15 1 MeV n eq/cm 2 fluence shows about 40% relative Charge Collection Efficiency with respect to the non-irradiated reference plane sensors.

Electrical crosstalk in front-illuminated photodiode array with different guard ring designs for medical CT applications

Abstract This paper presents electrical crosstalk studies on front-illuminated photodiode arrays for medical computed tomography (CT) applications. Crosstalk is an important factor to the system noise and image quality. The electrical crosstalk depends on silicon substrate properties and photodiode structures. The photodiode samples employed in this paper are planar processed on high-resistivity n-type silicon substrate, resulting in a p+/n−/n+ diode structure. Two types of guard ring structures are designed and applied to the same geometry of two-dimensional photodiode arrays. One structure is an n guard ring in the gap area between pixels, and the other structure is an additional p+ guard ring around each pixel together with the n guard ring. A 10 μm light spot with wavelength of 525 nm is used to scan across the surface of the photodiode array in the electrical crosstalk measurements. The electrical currents of two neighbor pixels are measured and the results are compared between two guard ring designs. The design with the p+ guard ring structure gives better electrical crosstalk suppression. Moreover, the measurement results show much smaller influence on surrounding pixels with the p+ guard ring structure in the case of disconnected pixel. Besides the electrical crosstalk, the light sensitivity within the gap area is also discussed between two guard ring designs.

Fast pore etching on high resistivity n-type silicon via photoelectrochemistry

In this paper, five factors, namely the HF (hydrofluoric acid) concentration, field strength, illumination intensity as well as the oxidizing-power and conductivity of electrolytes were found to strongly affect the fast pore etching. The oxidizing power of aqueous HF electrolyte of different concentrations was especially measured and analysed. A positive correlation between optimal bias and HF concentration was generally observed and the relationship was semi-quantitatively interpreted. Pore density notably increased with enhanced HF-concentration or bias even on patterned substrates where 2D (two-dimensional) nuclei were densely pre-textured. The etch rate can reach 400 μm/h and the aspect ratio of pores can be readily driven up to 250.

TCT and test beam results of irradiated magnetic Czochralski silicon (MCz-Si) detectors

Pad and strip detectors processed on high resistivity n-type magnetic Czochralski silicon (MCz-Si) were irradiated to several different fluences with protons. The pad detectors were characterized with the transient current technique (TCT) and the full-size strip detectors with a reference beam telescope and a 225 GeV muon beam. The TCT measurements indicate a double junction structure and space charge sign inversion in MCz-Si detectors after 6 × 10 14 1 MeV n eq / cm 2 fluence. In the beam test a signal-to-noise ( S/ N) ratio of 50 was measured for a non-irradiated MCz-Si sensor, and a S/ N ratio of 20 for the sensors irradiated to the fluences of 1 × 10 14 1 and 5 × 10 14 1 MeV n eq / cm 2 .

Reactions of interstitial carbon with impurities in silicon particle detectors

We present deep level transient spectroscopy (DLTS) data measured on very high resistivity n-type float-zone silicon detectors after irradiation with 6MeV electrons. The carbon interstitial annealing kinetics is investigated as a function of depth in the detector structure and related to the inhomogeneous depth distribution of oxygen and carbon impurities in the devices. We compare our results with data published in previous works and point out some possible misinterpretation of DLTS data due to detector processing induced inhomogeneous distribution of impurities. Finally, we present a method to determine the absolute concentration of the oxygen and carbon impurities as functions of depth in devices by carefully analyzing the carbon interstitital annealing kinetics.

Hydrogen-plasma-induced thermal donors in high resistivity n-type magnetic Czochralski-grown silicon

In this work, the formation of donors in n-type high resistivity magnetic Czochralski-grown silicon wafers, directly exposed to a hydrogen plasma, is investigated by a combination of capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements. C-V analysis demonstrates diffusionlike concentration profiles close to the surface, pointing to the formation of hydrogen-related shallow donors in silicon during the hydrogenation. In addition, oxygen thermal donors are created during a subsequent annealing (20min) performed at 350–450°C, as demonstrated by DLTS. It is shown that the hydrogen-related shallow donors are the dominant donors in as-hydrogenated samples, while hydrogen acts as a catalyst during the formation of oxygen thermal donors in the temperature range of 350–450°C. It is finally shown that the formation of both kinds of donors is Fermi-level dependent.

Integrating Silicon detector with segmentation for scanning transmission X-ray microscopy

Scanning transmission X-ray microscopes require detectors with high quantum efficiency and wide dynamic range. While large area detectors provide absorption contrast, the addition of spatial segmentation adds phase contrast imaging capabilities. We describe a charge integrating Silicon detector for use at energies from 200–1000 eV. The detector uses patterned rectifying junctions on high-resistivity n-type Silicon, with separate current readout for each segment. The detector has been subdivided into eight regions arranged in a circular geometry according to the beam profile in a scanning X-ray microscope. The uncooled chip is fully depleted by a positive bias voltage applied at the ohmic contact on the back side. X-rays are collected on the radiation-hard back side with very high efficiency ( > 75 % for 250 eV X-rays), and compact, low-noise electronics integrate the current from the detector segments. The RMS noise of the combined system is about 500 electrons/channel for a 1 ms integration time, which is equivalent to about five photons per channel at 360 eV X-ray energy.