Applications That Will Benefit from Digital Capacitive Sensors

The variable distance that exists between the surface of a capacitance probe and a target source is a major determining factor of the electrical capacitance that will be formed between these two objects. To improve measurement problems that industrial operators often face, parallel plate capacitance sensors have offered numerous advantages as compared to when other sensing systems have been implemented, of which include:

  • No physical contact between the sensor and target required
  • Zero mechanical loading
  • Prevention of any wear to the probe or target
  • Prevention of any distortion on the target sample
  • Improved stability, precision, and resolution
  • Ability to work in harsh environments
  • Withstand temperatures: <1200 ºF (650 ºC) and <4 ºK
  • Can function well in extremely high magnetic fields (<2 Tesla)and vacuum conditions (<10-7 Torr)
  • Costs approximately one third of typical laser interferometers at a matched or higher functionality

MIT Instruments offers novel digital capacitance technologies that exceed the traditional measurement credentials of capacitance sensors. Of these technologies, MTI offers the Accumeasure™ D Series amplifiers that offer users:

  • 24-bit resolution
  • Exceeds properties of 16-bit A/D converters, of which include:
    • Precision improvements
    • Less noise
    • Reduced linearity errors
  • Eliminated drift problems:
    • Achieved through built-in digitally selectable filters that optimize resolution
    • Frequency response does not require the addition of an additional offset or drift to the full sensor
  • Extended digital range as compared to analog amplifiers
  • Multiple range extensions (as high as 10) can be added at time of ordering
  • Superior linearity and accuracy:
    • Achieved by digitally correcting the linearity at every point in the range, as compared to previous amplifiers that only correct linearity at certain points on the displacement curve
  • Linearity specifications of 0.01% full scale range
  • Compatible with the ‘Internet of Things’ with Ethernet and USB:
    • Allows users to incorporate multiple sensors or web-based applications
    • Can be displayed on smart phone screens

The following applications address a wide range of industry sectors in which the capacitance sensors offered by MTI Instruments exceed in performance as compared to other measurement systems at a fraction of the cost.

Consumer Electronics: Rapid Motion Control at a Sub-Micron Accuracy

Today’s electronic manufacturing process requires the rapid and precise positioning of the many miniature components that has shifted from an accuracy of the process control from the millimeter level to a micron, or even sub-micron, level.

To meet the growing demand of improving the overall precision of these extremely small device components, electronics manufacturing facilities typically require the piezoelectric positioning of actuators and/or voice coil motors that are equipped with a closed-loop feedback control.

Electronics manufacturing facilities previously utilized analog feedback systems to provide the closed-loop control of conventional positioning stage and extreme positioning accuracies using voice coil motors or piezoelectric stages. While useful for earlier production purposes, these systems required motion stage feedback to be obtained from a position sensor.

As compared to analog systems, a digital closed-loop control system provides users with an improved stability without causing the same noise production, interference, and/or distortion issues that were encountered following the previous use of analog systems.

MTI’s new generation of digital capacitance sensor systems include a digital capacitance probe and an Accumeasure capacitance amplifier. This system is specifically designed to ensure that a closed-loop feedback is achieved at an impressive precision rate that involves piezoelectric actuators or voice coil motors.

During its operation, the digital probe of the capacitance sensor system measures the piezo stage position in real time. The digital Accumeasure amplifier then sends the stage position to the programmable motion controller through an Ethernet output.

Additionally, each digital capacitance sensor system also comes with a motion controller that compares feedback signals to a command position set point. These signals then provide information to the controller that will then adjust the piezo stage with sub-micron precision as necessary.

If a change in the set point or disturbance in the process is detected, the controller performs a series of calculations that will return the motion stage to the exact commanded position. The overall performance of the capacitance sensor system varies depending on the response rate of the controller in conjunction with each of the positioning components and the feedback sensor.

Digital Benefits:

  • Built in superior 24-bit digital output:
    • Eliminates the use of a separate A/D system
  • Accommodates commercial off-the-shelf closed-loop servo software
  • Digitally adjustable filter:
    • Ensures highest loop response
    • Enhanced resolution
  • Very low linearity error:
    • Provides the highest precision of the system in absolute positioning
  • Multiple probes can be networked without any signal degradation
  • Ethernet output for remote locations of sensors

MTI digital capacitance amplifier with MTI ASP-500M- ILA capacitance probe, embedded in the Piezo System Jena PX-100 stage. The digital amplifier offers exceptional accuracy and linearity. Off-the-shelf control programs make it easy to use and tune digital feedback.

MTI digital capacitance amplifier with MTI ASP-500M- ILA capacitance probe, embedded in the Piezo System Jena PX-100 stage. The digital amplifier offers exceptional accuracy and linearity. Off-the-shelf control programs make it easy to use and tune digital feedback.

Semiconductor Manufacturing: 3D IC Construction

Semiconductor and microelectronics manufacturers have utilized three-dimensional integrated circuits equipped with vertically-stacked silicon wafers and diesto improve the overall performance of these devices.

To create a vertical orientation, device engineers exploit the Z-axis in their design, by adding wafers and dies in the design to avoid any potential latency and/or footprint penalties that can occur following the use of two dimensional processes. While simple in theory, the implementation of a vertical position requires that all coplanar surfaces of the device come in direct contact with all pins, pads, and pillars.

To determine the coplanarity of any device, device engineers often measure the gap present between the two planes. This is an important preliminary task to perform since most bonding tool actuators rely on these measurements to perform a process known as ‘active parallelism compensation’.

Active parallelism compensation is a process that involves acquiring all necessary adjustments of a given component, while also determining that all device pins and/or solder balls are on the same geometric plane to allow for the surfaces to bond together properly without causing any residual stress to the system.

The bonding of surfaces varies depending upon the precision of the angle and gap measurement; therefore, active parallelism compensation, while useful, in theory can be both challenging and costly to achieve, especially when considering that the resolution requirements of devices are leaning towards the sub-micron range.

To achieve a high-resolution position control, MTI Instruments offers ASP-50-ILA capacitive displacement probes, a D-300 Digital Accumeasure capacitive amplifier and actuator/control system, all of which function to monitor and adjust the gap/angle present between the bond plane, which is specifically achieved through the die bonding tool, as well as the ground plane (die or substrate).

The construction of this multi-component system involves mounting at least three capacitance probes to the die bonding tool, as well as the use of at least three actuators which can be used to adjust the die or substrate for coplanarity with the die bonding tool.

The D-300 Digital Accumeasure capacitive amplifier processes the gap measurements by interfacing with the control system. Thisis achieved through the use of either a 1000 base Ethernet output or USB, both of which can be used to monitor the coplanarity and provide position control to the actuators. Note that both the amplifier and control system can be used to produce a sample rate of 1,000 points/second for the planar position.

Digital Benefits:

  • Built in superior 24-bit digital output:
    • Does not require a separate A/D system
  • Accommodates commercial off-the-shelf closed-loop servo software
  • Digitally adjustable filter:
    • Provides the highest loop response and resolution
  • Extremely low linearity error:
    • Ensures extremely high precision rates in absolute positioning
  • Amplifier linearity: 0.01% (full scale range)
  • Resolution: 5 nm p-p (at loop position bandwidth speeds of 100 Hz
  • Drift: <100 ppm

Three gap measuring probes mount to the upper plane (die bonding tool). A control system monitors coplanarity of the tool to the lower plane (die or substrate) and provides position control to the actuators.

Three gap measuring probes mount to the upper plane (die bonding tool). A control system monitors coplanarity of the tool to the lower plane (die or substrate) and provides position control to the actuators.

Monitoring Gaps in the Wind Turbine Industry

To meet the growing demand for efficient renewable energy, the wind turbine industry has experienced a dramatic increase in both power outputs and the number of installations. Following this trend, the maintenance of wind turbines has become an increasingly critical task to achieve, particularly that which involves minimizing the downtime and potential catastrophic failure of these turbines as much as possible.

To meet this goal, wind turbine operators are interested in implementing sensor systems that can predict when maintenance will be required prior to the event of these adverse conditions occurring.

MTI Instruments initially began their involvement in addressing this concern with their own capacitance sensor systems when a leading manufacturer of wind turbine power generators approached the company.

The main objective of the manufacturer was to incorporate sensors into the industry that could maintain peak efficiency by ensuring an optimal gap, withstand harsh environments, last ten years or more and function in high magnetic fields. To achieve these specific requirements, long sensor cable lengths would be required for routing to the control panels.

MTI Instruments offers a three-component sensor system that is made up of a passive capacitive displacement probe, which is a special interconnect cable that is specifically designed to reduce magnetic loop pickup, and a Digital Accumeasure capacitive amplifier.

The capacitance probe, which is less than 500 micrometers (µm) in thickness, functions as one plate of a classic two-plate capacitive gap sensor of which the target, which is typically grounded, forms the second plate. The capacitance probe is a robust tool comprised of non-magnetic material is sealed with polyimide to avoid corrosion.

Additionally, the design of the capacitance probe complies with C4 ASTM environmental guidelines. The interconnect cables of this system are custom designed ensure that any magnetically induced currents are avoided to subsequently prevent any excess noise or burn out from occurring to the sensor cable.

The second component of MTI’s capacitance is the digital Accumeasure amplifier, which injects a current into the probe and then measures the impedance of the capacitive gap that is formed by the probe and target. The impedance factor is directly proportional to the gap, as demonstrated by the following formula:

Gap = (area of the probe x dielectric constant of air)/capacitance of the gap

Numerous gap monitoring sensor systems have been installed in the wind turbine industry in an effort to provide both a continuous and real-time measurement. In addition to this purpose, these sensor systems have also proven useful in other industries for the monitoring armature gaps, as well as for measuring small gaps in the energy, aviation and automotive devices and systems.

Digital Benefits:

  • Digital networking of gap sensors
  • Superior range with high accuracy because of the correction to digital linearity
  • A digital design that allows excellent thermal stability of the complete system
  • Field programmable amplifier bandwidths

Gap monitoring sensors from MTI Instruments help wind turbine operators predict impending maintenance to avoid catastrophic failure.

Gap monitoring sensors from MTI Instruments help wind turbine operators predict impending maintenance to avoid catastrophic failure.

Solar Technology: Monitor Ungrounded Targets

The current state of both the semiconductor and solar industries have required an increased production demand for higher density chips that exhibit smaller critical dimensions. To this end, wafer fabricators have become increasingly interested in obtaining a greater dimensional control of their silicon products; a challenge that has now been achieved through the use of non-contact capacitance sensors.

Capacitance sensor systems offer an exceptional precision, accuracy and speed that can be useful for measuring the flatness, thickness, and other critical dimensions of these products in a highly accurate manner.

MTI’s capacitance sensor systems utilize the capacitance probe sensor as one plate of a classical two-plate capacitive gap measurement scenario. In this design, the grounded target, which would be the silicon wafer in this case, forms the second plate. Unfortunately, the grounding process can cause scratches or other forms of damage to the fragile and expensive wafer.

Additionally, this process can also prohibit sensing scenarios during which the wafer must be moved to acquire all metrology measurements. To address these grounding challenges, parasitic capacitance coupling, a process which utilizes a second sensor working 180 degrees out of phase or a grounded chuck to support the wafer, can be used.

Despite the usefulness of this additional procedure, its effectiveness remains limited. MTI Instruments has therefore introduced a push-pull probe technology that is specifically designed for ungrounded targets.

More specifically, the design of the capacitance push-pull probe must be capable of addressing various scenarios that can occur at the PV level. For example, to measure the thickness and warp of 156 mm2 photovoltaic (PV) wafers as they pass at the rate of one wafer per second at an accuracy of < 1 µm, the probe sensor can be used to measure ungrounded semiconductor wafers.

Based on conventional capacitance measurement principles, the design of the push-pull probe incorporates two capacitance sensors that are built into one probe body. Each sensor is driven at the same AC voltage with a 180-degree phase that exists shift between signals.

This shift plays an important role in allowing the current to travel across the target surface rather than through the target to ground. This subsequently ensures that any potential inaccuracies that can be generated by poorly grounded targets are eliminated.

MTI’s AS-562-PP amplifier remains part of this capacitance sensor system, functioning to sum up the individual output signals. In doing so, the amplifier produces a single 0 to 10 VDC output that is proportional to the probe-to-wafer gap at a sub-micron accuracy which occurs at probe standoff distances up to two mm.

Digital Benefits:

  • Push-pull technology:
    • Passive
    • Extremely stable over a wide temperature range
    • Can be used on high bulk resistivity targets
    • Does not require recalibration for changes in the target material
    • The push-pull amplifier design cancels common mode electrical noise that may be induced in the target
  • MTI’s PV-1000 digital controller:
    • Accepts three capacitive thickness channels
    • Provides digital correction for very high linearity
    • Performs wafer sorting commands
    • Computes pass/fail metrology measurements

The Push-Pull probe is a unique version of MTI’s Accumeasure™ amplifier series. This special design provides accurate surface information for wafer bow and thickness.

The Push-Pull probe is a unique version of MTI’s Accumeasure™ amplifier series. This special design provides accurate surface information for wafer bow and thickness.

Automotive Technology: Brake Rotor Thickness Variation

Modern cars and trucks can achieve unprecedented fuel efficiencies and handling characteristics as a result of their exceptional lightweight features. Since all components of these vehicles are also reduced in their total weight, the advancing automotive technologies have also allowed for a significant reduction in the total stress and strain applied to the vehicle, thereby improving the overall mileage and performance.

The installation of cooling vents into the newest and thinnest brake rotors has further improved performance of these components. While useful, these changes to the brake system have also reduced the available braking surface, which has ultimately led automotive engineers to consider alternative materials and designs.

To keep the cooling components as part of the braking systems, since these vents have been shown to drastically improve the resiliency of these systems against rotor distortion and failure; the dynamic testing of these novel designs is imperative.

During these testing procedures, it is imperative to continuously obtain data on disk runout, thickness variation, coning, warping, and temperature to provide engineers with all the necessary information that can be used to accurately determine the response rate of prototype units to real life conditions.

Herein, MTI Instruments offers a novel multi-channel brake rotor measurement system known as the Accumeasure™ D Series. Within this system, the amplifier is equipped with advanced technology that changes a reliable capacitive electric field measurement into a highly accurate 24-bit digital reading.

The amplifier can therefore detect any potential errors that arise as a result of analog filtering to ensure they are eliminated, while also achieving an ideal linearization, range extension and summing of channels.

Both single-ended capacitance probes and ‘push/pull’ capacitance sensors can be used to monitor the distance between probe and rotor while spinning within the brake system. When a push/pull sensor is used, the two sensing elements of this single probe body eliminates any electrical grounding of the rotor. The push/pull sensor probe, which is composed of both Inconel and ceramic materials, can withstand temperatures as high as 1200 °F (650 °C).

Several major vehicle manufacturers have already utilized MTI’s Accumeasure high-temperature sensors and the Accumeasure™ D Series amplifier for their brake system testing procedures. In addition to brake testing, these sensor systems have also been successfully used to measure and monitor spindle and shaft run out, engine vibration, thermal expansion/contraction, and suspension travel within the automotive industry.

Digital Benefits:

  • The digital Accumeasure:
    • Compact unit
    • Runs off 24 VDC
    • Provides either four channels of single ended probes for a grounded target; or two push pull probes for ungrounded rotors
  • Digital linearity correction
    • Longer range probes can remain at a safe distance from the hot rotor surface without losing accuracy
  • Digital Accumeasure features include:
    • Linearity for accurate measurements
    • Ethernet and USB ports
    • Probe range extension for large stand-off distances.

MTI’s Accumeasure™ D Series amplifier provides up to four independent measurement channels in a rugged, compact amplifier package. Features include multiple unit synchronization, range extension, sub-micron resolution, and 0.01% error of full scale linearity. Push/pull probe design permits groundless, non-contact measurement of rotating targets.

MTI’s Accumeasure™ D Series amplifier provides up to four independent measurement channels in a rugged, compact amplifier package. Features include multiple unit synchronization, range extension, sub-micron resolution, and 0.01% error of full scale linearity. Push/pull probe design permits groundless, non-contact measurement of rotating targets.

Determining Water Contamination in Oil Systems

The adverse effects associated with the contamination of water in oil include:

  • Higher viscosity
  • Reduced load carrying ability
  • Hydrolysis through the formation of acids, sludge, and varnish
  • Formation of foam and subsequent air entrainment
  • Additive depletion
  • Corrosion on metal surfaces
  • Loss of lubrication film strength leading to increased wear
  • Cavitation
  • Filter plugging

The maintenance and repair of oil systems has become an increasing important task to achieve within the automotive industry. To this end, efforts toward achieving corrective action have been achieved through the use of in-line capacitance probes.

More specifically, capacitance probes offer the automotive industry both an automatic and continuous method of monitoring any water that enters lubrication systems. Whereas oil typically exhibits a dielectric constant in the two to five range, water instead will often exhibit a dielectric constant of 80; therefore, even extremely small amounts of water can have a significant impact on the water state reading.

A capacitance probe for oil contamination analysis involves the placement of the probe against a non-conductive section of piping that carries the oil supply to be monitored. A second reference probe is then placed against a sealed and non-conductive tube that contains a sample of the lubricating oil. For this purpose, both tubes must remain in thermal contact with each other. MTI offers flat flexible capacitance probes that can be easily bonded to the tubes.

Each capacitance probe that is directly bonded to the tubes must be capable of measuring the dielectric field present between the face of the probe and the grounded plate present on the opposite side of tube. A monitoring CPU is also incorporated into the system to create­ an alarm if the dielectric reading for the oil flow channel diverges from the sample reading.

By continuously measuring the capacitance of the oil ratio to oil/water mixture through the tube, the overall moisture level of the oil system is minimized. When the water present within an oil system is reduced, any potential corrosion effects also subside to ultimately improve the overall reliability of the system.

Digital Benefits of the Digital Accumeasure:

  • Equipped with a special calibration program built in to handle these types of dielectric calibrations
  • Can communicate with an off the shelf USB alarm module to make a complete alarm system

MTI’s flexible, off-the-shelf flat probes easily bond to the tube with epoxy to mold to the shape of a plastic/glass oil tube and continuously monitor the capacitance of the oil or oil/water mixture passing through. The company’s D200 Digital Accumeasure serves as the monitoring sensor signal conditioner. Immune to thermal drift and easily calibrated to the sensitivity of water in oil, it is available with a digital limit alarm module.

MTI’s flexible, off-the-shelf flat probes easily bond to the tube with epoxy to mold to the shape of a plastic/glass oil tube and continuously monitor the capacitance of the oil or oil/water mixture passing through. The company’s D200 Digital Accumeasure serves as the monitoring sensor signal conditioner. Immune to thermal drift and easily calibrated to the sensitivity of water in oil, it is available with a digital limit alarm module.

This information has been sourced, reviewed and adapted from materials provided by MTI Instruments Inc.

For more information on this source, please visit MTI Instruments Inc.

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