A tremendous development in the field of imaging radiation detectors has taken place in the last decade.
Conventional X-ray film has been replaced by digital X-ray imaging systems in a number of ways. Such systems mainly
consist of silicon charge coupled devices (CCDs) where incident photons create electron-hole pairs in the thin silicon
absorption layer near the surface. In contrast to visible light, which is absorbed within a 2 µm layer of silicon, the
penetration of X-ray is much deeper due to higher photon energy. This disadvantage is often circumvented by the use of
a scintillator absorption layer. Due to scattering of the low energy fluorescence photons, resolution and contrast of the
X-ray images decrease. In order to eliminate these disadvantages, hybrid detectors consisting of direct converting
semiconductors and readout electronics parts are fabricated.
For this configuration, it is advantageous that both parts can be optimized separately and different materials can
be used. Because of the well developed technology, the readout chip is fabricated out of silicon. As absorbing material,
silicon is less suitable. In a silicon substrate of 500 µm thickness, only 15% of a 30 keV radiation is absorbed and
converted into charges. In order to increase the absorption, materials with a higher atomic mass have to be used. Several
compound semiconductors can be used for this purpose. One of them is GaAs, which is available as high quality semi-insulating
wafer material.
For detector optimization, GaAs wafers from several manufacturers with different properties were investigated.
Test structures with Schottky and PIN diodes were fabricated. The I/V curves of the diodes, the spectral response from 5
up to 150 keV, the carrier concentration, and the carrier mobility were measured and compared. A survey of the results
and the criteria for material selection resulting from these measurements will be provided in the paper.
This paper will discuss opportunities and future trends in non-destructive evaluation (NDE) and health monitoring based
on new sensor principles and advanced microelectronics. Conventional NDE is based on single probe approaches, where
systems are portable and used for local inspection, or on multi probe techniques with the support personnel in workshops
using complex NDE systems with multiple sensors. Miniaturization of electronics and new sensor approaches allow
realizing local probe techniques. This denotes that the probe and data acquisition are located at the structure to be
monitored or inspected. Hence, this approach can be used for continuous monitoring as well as for periodic inspections
and will increase the efficiency of inspection procedures and reduce inspection time.
For the last decade a tremendous development in the field of imaging radiation detectors has taken place. Conventional X-ray film has been replaced by digital X-ray imaging systems in a number of ways. Such systems mainly consist of silicon charge coupled devices (CCDs) where incident photons create electron-hole pairs in the thin silicon
absorption layer near the surface. In contrast to visible light, which is absorbed within a 2 μm layer of silicon, the penetration of X-ray is much deeper due to higher photon energy. This disadvantage is often circumvented by the use of a scintillator absorption layer. Due to scattering of the low energy fluorescence photons, resolution and contrast of the X-ray images decrease. In order to eliminate these disadvantages, hybrid detectors consisting of direct converting semiconductors and readout electronics parts are fabricated. For this configuration, it is advantageous that both parts can be optimized separately and different materials can be used. Because of the well developed technology, the readout chip is fabricated out of silicon. As absorbing material, silicon is less suitable. In a silicon substrate of 500 μm thickness, only 15% of a 30 keV radiation is absorbed and converted into charges. In order to increase the absorption, materials with a higher atomic mass have to be used. Several compound semiconductors can be used for this purpose. One of them is GaAs, which is available as high quality semiinsulating wafer material. For detector optimization, GaAs wafers from several manufacturers with different properties were investigated. Test structures with Schottky and PIN diodes were fabricated. The I/V curves of the diodes, the spectral response from 5 up to 150 keV, the carrier concentration, and the carrier mobility were measured and compared. A survey of the results and the criteria for material selection resulting from these measurements will be provided in the paper.
KEYWORDS: Structural health monitoring, Sensors, Signal processing, Nondestructive evaluation, Inspection, Reliability, Safety, Data communications, Digital signal processing, Wind energy
Safety and availability of ageing infrastructures require periodic or continuous monitoring of the structure’s integrity. Innovative design criteria for new infrastructure components may allow material and energy conservation if components are continuously monitored by using advanced sensor systems. This concept for recurring Structural Health Monitoring will replace a significant part of conventional NDE by new maintenance concepts. The goal consists in sensor networks based on advanced principles of testing technology with integrated signal/data processing and data communication. NDE modeling is required for the quantification of measurement results. Finally, a decision on the integrity of the structure based on sensor results requires detailed knowledge about material behavior and modeling capacity for materials and components. IZFP has developed sensor concepts for complex solutions applicable to Structural Health Monitoring for different applications. These applications include railroad inspection, aircraft inspection, inspection of wind energy systems, power electric switches and micro gas valves. Basic concepts and applications of sensor networks will be presented.
A method has been developed by which, after removal of the bulk silicon in a substrate transfer process, the backside of a wafer can be processed with the same lithography as the front side of the wafer. To achieve an accurate front-to-backwafer alignment accuracy, mirror symmetric alignment markers for an ASML PAS5000 waferstepper have been developed and applied in a Silicon-on- Anything process. In this manner minimum dimension low-ohmic contacts were fabricated on the backwafer. The mirror symmetric alignment markers are used in combination with standard overlay test procedures to characterize the front-to backwafer overlay accuracy. The measured overlay errors are divided up in non- mirror symmetric lens distortions and wafer distortion as a result of the substrate transfer process. The practical minimum device feature that can be realized on the backwafer is limited to 0.9-1.2 micrometers as a result of front-to-backwafer overlay errors.
A new process for the fabrication of piezoelectric quartz thin films on silicon is investigated. With this process, new silicon-implemented acoustic wave delay lines for sensor applications can be realized. An acoustic-wave delay-line consists of two interdigital thin film metal transducers fabricated on a piezoelectric crystal. In order to realize acoustic-wave devices on (non-piezoelectric) silicon, the use of piezoelectric thin films such as zinc oxide, aluminum nitride or PZT has been reported. However, these films often exhibit stress, aging, pinholes, or poor reproducibility which affects the performance of the device. The bonding of piezoelectric quartz (with its known and fixed mechanical and piezoelectric properties) to silicon improves the performance of silicon-implemented acoustic-wave devices. The process used, consists of a wet chemical treatment after which the wafers are prebonded at room temperature. Annealing at 140 degree(s)C for 3 hours yields a sufficient high bond strength.
In this paper we present a silicon micromachined wet cell for use with a Love-wave liquid sensor. The Love-wave sensor is composed of an electronic amplifier and an acoustic Love- wave delay-line on a piezoelectric substrate. Together they form an oscillator. Liquid is placed in intimate contact with the Love-wave sensor; corresponding to its viscosity the acoustic wave velocity changes, which is observed through a change in the oscillation frequency. An issue that arises in a sensor of this type is that the input impedance of the interdigital transducers (IDTs) of the delay-line changes dramatically due to the dielectric properties of the liquid above them. This adds electrical load to the amplifier and affects the oscillator's performance by reducing its resolution and sensitivity. The electric loading of the IDTs by the liquid also leads to unwanted sensitivity with respect to the electrical properties of the liquid. The wet cell was designed to overcome this disadvantage. By virtue of this cell the liquid is directed only over the wave propagation path, and so the transducers are protected from the liquid's influence. In designing the cell, bubble formation in the liquid, chemical inertness, bonding aspects and temperature effects were all considered. The design utilizes a silicon micromachined channel that guides the liquid between the transducers. Furthermore a heater for controlling the temperature of the liquid has been incorporated. Experiments have shown that placing thin side walls of a silicon micromachined channel in the propagation path of the wave adds little to the insertion loss. Losses of only 6 dB or less were recorded, which confirms the suitability of this configuration. In addition to viscosity sensors this design can be applied to a broad range of Love-wave liquid sensors, including those in the biochemical area.
KEYWORDS: Chemical vapor deposition, Low pressure chemical vapor deposition, Surface roughness, Temperature metrology, Plasma enhanced chemical vapor deposition, Oxides, Silicon
The influences of surface characteristics, including adsorptive states led by different chemical treatments and surface roughness, on direct bonding between dissimilar CVD materials were investigated. The bonding procedures were carried out at temperature lower than 400 degrees Celsius. In this temperature range, LPCVD poly-silicon, PECVD oxide, and LPCVD silicon-nitride showed highly process dependent bonding behaviors, i.e., bondable or not bondable to another material under certain experimental conditions. Based on these facts, a selective bonding conception for Si-based CVD material is proposed and applied to fabricate new fluid structures and devices.
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