Non-Conductive Material Measurement with Capacitance Sensors
Capacitance sensors are non-contact devices that can be used to obtain position, thickness, and dynamic measurements with a high degree of precision, accuracy, and resolution. Capacitive measurement is used with electrically-conductive materials such as metals but can also be used to measure non-conductive materials such as glass, sapphire, and many plastics. Even if a non-conductive material is relatively inexpensive (such as closed cell foam), manufacturers want to avoid errors that can result in significant waste across high volumes.
Importantly, capacitance measurement is also used with semi-insulating materials such as gallium arsenide (GaAs) and silicon nitride, both of which are used in the semiconductor industry. Capacitance gauges support quality checks before wafers are patterned and value is added. Applications include backgrinding, or lapping, for thickness measurements. Additional applications include the qualification and calibration of lithography tools and lapping equipment, measuring unpatterned wafers prior to shipping, and inspecting these wafers upon receipt.
Figure 1: Measuring the Thickness of a Non-Conductive Material
Capacitance Sensors vs. Other Measurement Technologies
Capacitance-based measurement systems include probe-based capacitive sensors, fixtures, an amplifier, and system components such as probe holders and cables. Capacitance, the ratio of the change in an electric charge to the corresponding change in its electrical potential (voltage), is formed between a probe and a grounded target and varies as a function of the distance (gap) between them. With parallel plate capacitive measurement, the probe and the target are the conductors. To measure non-conductive or semi-insulating materials, a metal ground plate can be used as one of the targets.
Precision measurements can also be obtained with laser interferometers, but these high-end devices cost significantly more than capacitance gauges. Plus, because laser interferometers provide relative readings, an operator must calibrate the device to a known thickness before use. By contrast, capacitance gauges provide absolute measurements instead of relative ones for greater absolute accuracy. Laser sensors also have a tendency to drift respective to temperature, thermal heating, and other factors. Capacitance circuits tend to be more stable and combine low drift with high accuracy.
Other measurement technologies cannot match the advantages of capacitance either. For example, eddy current probes have diverging sensing fields (bigger target requirements) and are sensitive to the target material type. Capacitance sensing is fast and can measure non-conductive materials, including very thin substrates. Additionally, capacitive sensors do not require in-situ calibration and can be used with any conductive target once they are calibrated. Capacitance sensors also offers advantages over linear variable differential transformers (LVDTs), contact-based electromechanical devices that are affected by temperature variations and also apply a slight force on the target.
Figure 2: Capacitance Sensors and Target
Figure 3: Types of Capacitance Probes
Capacitance Measurement and Dielectric Constants
With capacitance measurement, the sensing element in the capacitive probe emits an electric field that passes through the non-conductive, or insulating, material to the ground plate. The thickness of this insulating material can be determined from its dielectric constant, a numerical value that describes a material’s ability to store electrical energy in an electrical field. The dielectric constant of the air between the probe and the non-conductive material is slightly greater than 1. The dielectric constants for glass and plastic materials have significantly higher values.
The dielectric constant of the non-conductive material under measurement affects the capacitance between the two conductors: the capacitive probe and the plate. Therefore, putting a sheet of glass or plastic between the probe and plate causes a measurable change in capacitance. Different thicknesses of the same non-conductive material produce differences in capacitance that can be measured with submicron precision. Thickness measurements work best with materials that have good control over their dielectric content and do not experience significant batch to batch variation.
Figure 4: Dielectric constants for non-conductive materials
Figure 5: Measuring Non-Conductive Substrates
Capacitance Amplifiers: Analog and Digital
Capacitance sensors send signals to capacitance amplifiers that output a voltage proportional to the thickness of the measured material. The connection between the capacitance probe and capacitance amplifier can be wired or wireless. The amplifier itself can be analog or digital. Wireless capacitance probes are often used in hazardous locations or hard-to-service environments such as rotating machinery. They require digital capacitance amplifiers, support Bluetooth communications, and won’t interfere with other wireless probes in the same environment. Both analog and digital amplifiers can be connected to computers and used with software for data monitoring, reporting, and analysis.
Analog capacitance amplifiers require wired connections, provide nanometer-level accuracy, and cost less than digital amplifiers. However, errors may occur because of analog filtering, linearization, range extension and the summing of channels. Plus, whenever low-level analog signals are transmitted over wires, signal degradation can cause noise and electrical interference. Digital amplifiers are more expensive but protect data integrity in noisy environments such as a processing facility or factory. They also eliminate the need for analog-to-digital conversion and can achieve extremely high resolution when when they use low-noise cables and have pre-amplifiers close to the probe.
Figure 6: Analog capacitance amplifier
Figure 7: Digital capacitance amplifer
Complete Capacitive-Based Precision Measurement System
MTI Instruments of Albany, New York (USA) offers a complete capacitance-based measurement system that includes a digital amplifier with a pair of dielectric probes and optional accessories such as low-noise probe extension cables, dielectric fixture, 24 VDC power supply, and micro USB cable. The Accumeasure D includes measurement software to support data logging and analysis and comes with MTI Basic, LabVIEW, .NET and DLL drivers for use with other closed loop systems. The Accumeasure D also includes a DIN rail mount kit so the amplifier can be installed inside an equipment rack and supports standard Ethernet and USB connections to networked computers.
As an in-line capacitive-based measurement system, Digital Accumeasure technology is ideal for obtaining position, thickness, or dynamic measurements for non-conductive materials on a production line. This digitally-controlled system has a selectable frequency filter of 0.1 Hz to 5 KHz and a maximum sample rate of 20 kHz. MTI’s Digital Accumeasure comes standard with 24-bit USB/Ethernet digital output but can also provide analog outputs for systems that require them. This true direct digital technology has up to 0.01% FSR linearity and sub-nanometer resolution.
To learn more, contact MTI Instruments.