Hari Polu, president, OKOS
Electromechanical sensors, such as pressure, flow and vacuum switches, composed of electrical and mechanical parts, interact and transmit information or commands to other components of a larger, more complex system. To keep the entire system functioning safely, sensor producers need to manufacture these devices to precisely deliver accurate measurements. Traditionally, however, if there was an issue with a sensor, it was often found when the product failed in the field. This was because sensor manufacturers had no metrology method to test the functionality of sensor elements during the production process.
Now, sophisticated testing such as scanning acoustic microscopy (SAM) 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.
SAM is a non-invasive and non-destructive ultrasonic testing method. The testing is already the industry standard for 100 percent inspection of semiconductor components to identify defects such as voids, cracks and the delamination of different layers within microelectronic devices. Now, the same rigor of failure analysis and quality testing is being applied to specialty metals and materials to detect subsurface flaws, disbonds, cracks and other irregularities.
OKOS’ MACROVUE 1000-P scanning acoustic microscope, which has a scan area of 2,500 x 1,500 mm.
Oilfield sensors
As an example, SAM can be used to ensure sensor quality in oil drilling equipment. These sensors are sensitive to vibrations or generate vibrations at a specific frequency. They provide metrology attributes of fluid properties in real time.
Electromechanical 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 the vibration can help to determine the density of the wellbore fluid mixture.
If sensors in an oil drilling application fail, be it upstream or downstream, it is extremely costly.
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.
SAM 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 (ToF) the presence of a boundary or object and its distance can be determined.
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 percent 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.
Ensuring lithium niobate wafer quality
Lithium niobate (LiNbO₃) is one of the most versatile and well-developed active optical materials and is widely used in electro-optics, acousto-optics, nonlinear optics, waveguides and fibre optic gyroscopes (FOGs).
One potential application is oilfield sensors. Traditionally, in this application, when cutting, separating and assembling LiNbO₃ wafers into sensor housing, only a small percentage prove to be good in the field. The challenge is determining which wafers are defective before incorporation into products.
For this type of application, the SAM VUE400 from OKOS is ideal for detecting bad wafers prior to use in an electromechanical device. It is a medium-sized SAM system, designed for lab use or manufacturing floors and traditionally used to detect voids, disbands, cracks, delamination and internal defects in semiconductor package failure analysis. Featuring an ultrasonic digitiser and digital pulse receiver, the system can repeatably scan with an accuracy of +/-0.5 µm.
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 are, the software is key to analysing the information to produce detailed scans.
OKOS' 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.
Today, manufacturers have the potential to save significant amounts of money per year in oilfield sensors or similar applications by detecting and eliminating defective LiNbO₃ wafers before use in high-value electromechanical sensors. These savings stem from screening at the wafer level, which prevents the packaging and shipping of bad products. The total does not even account for substantially improved wafer yields.
The use of SAM takes a well-established testing paradigm in the semiconductor industry for wafers and applies it to very thin metals, thus preventing the costly failures of a variety of electromechanical sensors in the field.
OKOS’ VUE 400-P scanning acoustic microscope, which has a scan area of 380 x 360 mm.
OKOS
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