SAM Increases Electro-Mechanical Sensor Reliability

Scanning Acoustic Microscopy testing ensures that oilfield and optical sensors are built without essential material defects

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 the field.

Now, sophisticated testing like 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% 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, 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 sensors elements during the manufacturing process," says Hari Polu, President of OKOS, a Virginia-based manufacturer of SAM and industrial ultrasonic non-destructive (NDT) systems. OKOS is a wholly owned subsidiary of PVA TePla AG, Germany and offers both manual and automated inspection systems for flat panels, thin plates, circular discs, sputtering targets, and special alloys.

Oil Field 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. These sensors provide metrology attributes of fluid properties in real time.

Electro-mechanical advanced sensors are used in critical areas such as the 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 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," says 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

analyzed to detect and characterize flaws such as cracks, inclusions, and voids.

When high throughput is required for 100% inspection, ultra-fast single or dual gantry

scanning systems are utilized along with 128 sensors for phased array scanning. Multiple

transducers can also be used to simultaneously scan for higher throughput.

"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," says Polu.

Ensuring 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 fiber optic gyroscopes (FOGs).

One potential application is for oilfield sensors. Traditionally, in this application, when cutting, separating, and assembling lithium niobate 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 to detect bad

wafers prior to use in an electro-mechanical device. The medium-sized SAM system

designed for 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 analyzing the

information to produce detailed scans.

For this reason, "OKOS decided early on to deliver a software-driven, ecosystem-based

solution," says Polu. The company's 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.

"The use of SAM takes a well-established testing paradigm in the semiconductor industry

for wafers and applies it to very thin metals. This prevents the costly failures of a variety

of electro-mechanical sensors in the field," concludes Polu.

For more information, contact OKOS at info@okos.com or visit www.okos.com.

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