Mid Sweden University

As a result of the ever increasing energetic UV radiation doses and the need for more radiation hard devices, damage and degradation testing of optical sensors have become very imperative. In this report, results describing the effect of prolonged UV beam irradiation on the performance of a p+n duo-lateral position sensitive detector (lPSD) are reported. The results include the use of a simple method to visualize in 3-dimensional graphs, the effect of radiation damage on the lPSD sensitivity and position detection deviation over the entire active area. The results also show that the ionization damage effects at the Silicon-Silicon oxide interface result in decrease in sensitivity, increase in position detection deviation, and increase in leakage and shot noise current.

There has been an increase in the use of duo‐lateral position sensitive detectors inpractically every radiation and beam detection application. These devices unlike other light detection system utilize the effect of the lateral division of the generated photocurrent to measure the position of the integral focus of an incoming lightsignal. The performance of a PSD is impaired or strengthened by a number of events caused by parameters such as interface states and recombination introduced during the fabrication of the detector and/or its absorption of ionizing particles. This thesis show the results from the successful implementation of alternative characterization methods of these effects and parameters using scanning electronmicroscopy and UV beam profiling techniques on duo‐lateral position sensitive detectors (LPSDs). To help create the groundwork for the research content of this thesis, different technical reviews of previous studies on interface states, surface recombination velocity and radiation damage due to continuous absorption of ionizing irradiation on detectors are investigated. The thesis also examines published theoretical and measurement techniques used to characterize these surface/interface phenomena. The PSDs used in this research were developed using silicon technology and the various methodologies put into the fabrication of the detectors (n+p and p+n structures) were fashioned after the simulated models. The various steps associated with the clean room fabrication and the prior simulation steps are highlighted in the content of the thesis. Also discussed are the measurement techniques used incharacterizing the fixed oxide charge, surface recombination and the position deviation error of the LPSDs in a high vacuum environment of a scanning electron microscope SEM chamber. Using this method, the effects of interface states and surface recombination velocity on the responsivity of differently doped LPSDs were investigated. By lithographically patterning grid‐like structures used as scaleon n+p doped LPSD and using sweeping electrons from the SEM microscope, a very high linearity over the two‐dimensions of the LPSD total active area was observed. An improved responsivity for low energetic electrons was also achievedby the introduced n+p structure. The lithographically patterned grids helped eliminate further external measurement errors and uncertainties from the use of other typical movable measurement devices such as actuators and two dimensional adjusters which would normally be difficult to install in a remote vacuum chamber. In a similar vein, field plate and field rings were patterned around an array ofthe PSDs used as pixel detector(s). By studying the interpixel resistance and breakdown characteristics, the most effective structural arrangement of the field plate and field rings used to curb induced inversion channel between the n+ doped regions of the pixel‐detector is observed. By using UV beam profiling after the irradiation of UV (193 nm or 253 nm) beam on n+p and p+n doped PSDs, the degree of radiation damage was also investigated. The results obtained help to illustrate how prolonged UV radiation can impact on the linearity and the position deviation/error of UV detectors. The results in this thesis are most relevant in spectroscopic and microscopic applications where low energy electrons and medium UV (MUV) radiation are used.

We report experimental results on the degree of radiation damage in two duo-lateral position sensitive detectors (LPSDs) exposed to 193 nm and253 nm ultraviolet (UV) beam. One of the detectors was an in-house fabricated n(+) p LPSD and the other was a commercially available p(+) n LPSD. We report that at both wavelengths, the degradation damage from the UV photons absorption caused a much more significant deterioration in responsivity in the p(+) n LPSD than in the n(+) p LPSD. By employing a simple method, we were able to visualize the radiation damage on the active area of the LPSDs using 3-dimensional graphs. We were also able to characterize the impact of radiation damage on the linearity and position error of the detectors.

Silicon detectors made on p-substrates are expected to have a better radiation hardness as compared todetectors made on n-substrates. However, the fixed positive oxide charges induce an inversion layer ofelectrons in the substrate, which connects the pixels. The common means of solving this problem isby using a p-spray, individual p-stops or a combination of the two. Here, we investigate the use offield plates to suppress the fixed positive charges and to prevent the formation of an inversion layer.The fabricated detector shows a high breakdown voltage and low interpixel leakage current for astructure using biased field plates with a width of 20 μm. By using a spice model for simulation of thepreamplifier, a cross talk of about 1.6 % is achieved with this detector structure. The cross talk iscaused by capacitive and resistive coupling between the pixels

Characterizing a position sensitive detector in a vacuum environment without beam position monitoring devices can bechallenging and expensive. With this in mind, we have designed and fabricated a duo-lateral position sensitive detector (PSD) incorporatedwith simple and inexpensive surface features. It was evaluated using scanning electron microscopy. To assist in pinpointing precise positioningas well as acting as path guide during the sweeping of electrons, multiple grids were lithographically patterned on the top layer of the duolateralPSD. By sweeping electrons along two axes of the detector, the position detection error of both axes was determined from the signalsrecorded using a transimpedance amplification circuit. We were able to characterize the linearity over the x- and y-axis of the PSD and theresults show a very high linearity over two-dimensions of the PSD’s active area and that accurate beam monitoring for spectroscopic measurement without additional beam position monitoring devices is possible.

This paper explores an alternative to the standard method of studying the responsivities (the input—output gain) and other behaviours of detectors at low electron energy. The research does not aim to compare the results of differently doped n⁺p detectors; its purpose is to provide an alternative characterization method (using scanning electron microscopy) to those used in previous studies on the responsivity of n⁺p doped detectors as a function of the electron radiation energy and other interface parameters.

When a material is irradiated, it becomes more electrically conductive due to the absorption of the electromagnetic radiation. As a result, the number of free electrons and holes changes and raises its electrical conductivity. A simple but interesting phenomenon to characterise a fabricated n+p photodetector in order to determine its linearity (photoresponse) and photoconductance was employed. Using the transient decay when the irradiation source is switched off, the minority carrier concentration, effective lifetime and surface recombination velocity present at the surface of the detector were measured. © 2012 Chinese Institute of Electronics.

Different methods exist in relation to probing and investigating thephysical and structural composition of materials especially detectors whoseusage have become an integral part of radiation detection. The use of thescanning electron microscopy is just one of such exploratory methods. Thistechnique uses a focused beam of high-energy electrons to generate a varietyof signals at the surface of the device under investigationThis thesis presents the results derived from signals from electron beamsampleinteractions, revealing information about the different cleanroomfabricated electron detectors used. This information includes the detector’sexternal morphology and texture, surface recombination, fixed oxide chargeand the behavioral characteristic in the form of its position detection accuracyand linearity.An electron detector with a high ionization factor and which has a 10nmSilicon Oxide passivating layer was fabricated. Results from using the scanningelectron microscopy showed that its maximum responsivity wasapproximately 0.25 A/W from a possible 0.27 A/W. In conjunction withsimulations, results also showed the significance of the effect of the minoritycarrier's surface recombination velocity on the responsivity of the detectors.In addition, measurements were conducted to ascertain the performancevariance of these electron detectors with respect to their surfacerecombination velocity and fixed oxide charge when the doping profile isaltered.By incorporating special features on a fabricated duo-lateral positionsensitive detector (PSD), a position sensing resolution of the PSD using theelectron microscopic method was also evaluated. The evaluation showed avery high linearity over two-dimensions for 77% of the PSD’s active area.The results in this thesis offer a significant improvement in electrondetectors for applications such as gas chromatography detection of traceamounts of chemical compounds in a sample as well as applications involvingposition sensitive detection.

Electron detection at low energy range for scanning electron microscope (SEM), electron capture detector and electron probe micro-analysis (EPMA) applications, require detectors with high sensitivity and accuracy for low energy range. Such detectors must therefore have a thin entrance window and low recombination at the Si-SiO2 interface. An electron detector with 100 photons to electron-hole pair production rate having a 10 nm SiO2 passivating layer reveals a responsivity of approximately 0.25 A/W when irradiated. Simulations results showing the responsivity of electron interaction between detectors of varied interface fixed oxide charge density Qf show that there is an appreciable difference with the responsivity of a p +n detector and that of an n+p. The simulation results also show the significance of the effect of the minority carriers transport velocity Sn,p on the responsivity of the detector. © 2011 IOP Publishing Ltd and SISSA.

Lateral position sensitive devices (PSD) are important for triangulation, alignment and surface measurements as well as for angle measurements. Large PSDs show a delay on rising and falling edges when irradiated with near infra-red light [1]. This delay is also dependent on the spot position relative to the electrodes. It is however desirable in most applications to have a fast response. We investigated the responsiveness of a Sitek PSD in a mixed mode simulation of a two dimensional full sized detector. For simulation and measurement purposes focused light pulses with awavelength of 850 nm, duration of 1 µs and spot size of 280 µm were used. The cause for the slopes of rise and fall time is due to time constants of the device capacitance as well as the photo- generation mechanism itself [1]. To support the simulated results, we conducted measurements of rise and fall times on a physical device. Additionally, we quantified the homogeneity of the device by repositioning a spot of light from a pulsed ir-laser diode on the surface area.