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FEATURE SENSORS & SENSING SYSTEMS PREVENTING SENSOR FAILURE


OKOS looks into the benefits of using Scanning Acoustic Microscopy (SAM) to test the physical sensing elements of sensors before they are assembled, helping prevent failure in the field


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lectromechanical sensors – such as pressure, flow and vacuum switches – are composed of electrical and mechanical


parts. These interact and transmit information or commands to other components of a larger, more complex, system. However, to keep the entire system functioning safely, sensor producers need to manufacture these devices to precisely deliver accurate measurements. In the past, if there was an issue with a sensor it was often found when the product failed in the field. Now, Scanning Acoustic Microscopy (SAM) and other sophisticated testing is being used to assess the physical sensing elements to determine the components are sound before they are assembled into sensors that will be used in critical applications.


WHAT IS SAM? SAM is a non-invasive and non-destructive ultrasonic testing method which is already the industry standard for 100% inspection of semiconductor components to identify defects such as voids, cracks and the delamination of different layers within microelectronic devices. This high performance failure analysis and quality testing is being applied to specialty metals and materials to detect subsurface flaws, dis-bonds, cracks and other irregularities. “Previously, sensor manufacturers had no


way to test the functionality of a sensor until it was in the field. If it gave incorrect results on a known good test, they called it a failure. They had no metrology to test the sensor elements during the manufacturing process,” said Hari Polu, president of OKOS, a manufacturer of SAM and industrial ultrasonic non-destructive (NDT) systems. OKOS is a wholly owned subsidiary of PVA TePla in Germany and offers both manual and automated inspection systems for flat panels, thin plates, circular discs, sputtering targets, and special alloys.


OIL FIELD SENSORS SAM can, for example, be used to ensure sensor quality in oil drilling equipment. These sensors are sensitive to vibrations or generate vibrations at a specific frequency. In addition, they also provide metrology attributes of fluid properties in real time. Electro-mechanical advanced sensors


are used in critical areas such as oilfield exploration and production processing to take pressure data and fluid samples from the bottom of high pressure and high temperature wells. The sensors establish the hold-up of fluids using the density and electrical properties of oil, gas and water. The characteristics of


10 DESIGN SOLUTIONS FEBRUARY 2023


the vibration can help to determine the density of the wellbore fluid mixture. “If a manufacturer is building defective


sensors and anything fails anywhere in the process upstream, that’s extremely costly for an oilfield application. If they find defective sensors in the field downstream, that’s also very expensive,” explained Polu. Oil drilling equipment manufacturers can test


tuning fork sensors’ piezoelectric crystals with SAM to determine if flaws exist before shipping. Since piezoelectric ceramics are fragile, sensitive components may present internal cracks undetectable to a visual inspection. Ceramics presenting cracks, even if internal and invisible, must be discarded to avoid the premature fault of ultrasonic transducers and converters in which they are mounted, and the resulting losses from repairs and technical assistance. Scanning acoustic microscopy works by


directing focused sound from a transducer at a small point on a target object. The sound hitting the object is either scattered, absorbed, reflected or transmitted. By detecting the direction of scattered pulses as well as the ‘time of flight’, the presence of a boundary or object can be determined as well as its distance. To produce an image, samples are scanned point by point and line by line. Scanning modes range from single layer views to tray scans and cross-sections. Multi-layer scans can include up to 50 independent layers. Depth-specific information can be extracted and applied to create two- and three-dimensional images without the need for time consuming tomographic scan procedures and more costly X-rays. The images are then analysed to detect and characterise flaws such as cracks, inclusions and voids. When high throughput is required for 100% inspection, ultra-fast single or dual gantry scanning systems are utilised along with 128 sensors for phased array scanning. Multiple transducers can also be used to simultaneously scan for higher throughput. Polu added: “We’re able to take the SAM


technology, which is good at identifying defects in metals, and apply it to detecting the critical parts in sensors, so manufacturers don’t release defective products into the field.”


LITHIUM NIOBATE WAFER QUALITY


Lithium niobate (LiNbO3) is one of the most versatile and well-developed active optical materials that is widely used in electro-optics, acousto-optics, nonlinear optics, waveguides and fibre optic gyroscopes (FOGs). One potential application is for oilfield sensors. Traditionally, when cutting, separating and


assembling lithium niobate wafers into a sensor housing, only a small percentage prove to be good in the field. The challenge is determining which wafers are defective before they are designed into products. For this type of application, the SAM VUE400


from OKOS detects bad wafers prior to use in an electro-mechanical device. The medium-sized SAM system, designed for both lab use or manufacturing floors, is traditionally used to detect voids, disbands, cracks, delamination and internal defects in semiconductor package failure analysis. With an ultrasonic digitizer and digital pulse receiver, the system can repeatably scan with an accuracy of ±0.5 micron. The SAM equipment can inspect various


items with unique product geometry or sizes – from crystal ingots, wafers and electronics packages, to miniature physical packaging, metal bar/rods/billets, turbine blades, etc. However, as important as the physical and mechanical aspects of conducting a scan, the software is the key to analysing the information to produce detailed scans. OKOS therefore chose a software-driven,


ecosystem-based, solution. Its ODIS Acoustic Microscopy software supports a wide range of transducer frequencies from 2.25 to 230 MHz. Multi-axis scan options enable A, B and


C-scans, contour following, off-line analysis, and virtual rescanning for metals, alloys and composites. This results in highly accurate internal and external inspection for defects and thickness measurement via the inspection software. Today, manufacturers have the potential to save


significant amounts of money per year in oilfield sensors or similar applications by detecting and eliminating defective lithium niobate wafers before use in high-value, electro-mechanical sensors. These savings stem from screening at the wafer level, which prevents packaging and shipping bad products. The total does not even account for substantially improved wafer yields.


OKOS www.okos.com


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