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Understanding Common Specifications for MEMS Silicon Dies

To evaluate the quality and performance of a MEMS silicon die, customers must rely on specifications, at least until they can test parts for themselves. This article will discuss the most common specifications related to these pressure-sensor dies.

The primary thing to understand about MEMS dies is that when they are exposed to either pressure or temperature, they will produce a corresponding output, which will be in millivolts, provided that an input voltage, or excitation voltage, has been supplied. The millivolt output from the MEMS die is essentially the pressure value. Therefore, the general characteristic to look for in any MEMS die is a stable and repeatable output when the die is tested under various conditions.

This article discusses common specifications used to characterize a pressure-sensor die’s performance under different operating conditions.

The first groups of specifications we will discuss are commonly used to characterize how the MEMS die will perform at room temperature (25 °C).

Bridge resistance (or impedance): This indicates the resistance (from Ohm’s Law the voltage divided by the current) measured across the bridge. Due to our Wheatstone bridge design along with our Sentium® and MeritUltra™ processes, the input resistance (+E to -E) and the output resistance (+O to -O) on all of our dies are the same.

Offset (or zero-pressure output voltage): This indicates the difference, at zero pressure, between zero output and the actual output of the MEMS die. With absolutely no offset, at zero pressure the output would be 0 mV/V. However, with an offset of ±10 mV/V, the difference with 5 volts of excitation could be ±50 mV. Refer to the image of the transfer function below.

Sensitivity (or span): Sensitivity and span are, in general, synonymous. The two terms are used to indicate the electrical output, or the response, of the MEMS die to an applied pressure and supply voltage. It is typically represented by the slope of a line on a graph with output on one axis and pressure (for a given supply voltage) on the other axis. Refer to the image of the transfer function below. Sensitivity is generally stated in terms of microvolts per volt per psi (µV/V/psi).

Transfer Function Graph for S Series 15 psi MEMS Die

Non-linearity (or linearity): This shows how linear/non-linear the output is. The ideal output is perfectly linear. For example, at a constant 5-volt supply, for every pound per square inch that the pressure were to increase, the output in millivolts would increase linearly, as shown in the image of the transfer function above. The pressure non-linearity is calculated by measuring—at the mid-point of the pressure range—either of two differences: One is between the actual output and the best-fit straight line (BFSL) or the other is between the actual output and the invisible line that connects the two endpoints of the actual output. This line is called the end-point line or terminal base. Refer to the image below. The actual output shown in this image has been exaggerated for illustration. Whether the pressure non-linearity is based on the BFSL or end-point line, it is expressed as a percentage of the full-scale output (FSO).

MEMS Die Pressure Non-Linearity Example

Pressure hysteresis: This shows the delta, or difference, of the output at zero pressure and then up to full-scale pressure and back to zero pressure. It would be ideal to have no pressure hysteresis, meaning the output would be the exact same every time the pressure returned to zero. This specification will give you one indication of the die’s repeatability. Pressure hysteresis is expressed as a percentage of full-scale output (FSO).

The next three specifications indicate how a part will behave over a specified temperature range. At Merit Sensor all MEMS dies are tested over a temperature range from -40 to 150 °C.  These three specifications are first-order effects.

Temperature coefficient of offset (TCO): This is also known as temperature coefficient at zero pressure (TCZ). This indicates the offset changes at zero pressure as temperature changes.

Temperature coefficient of resistance (TCR): This indicates how the resistance changes at zero pressure as temperature changes. The bridge resistance does change significantly over temperature.

Temperature coefficient of sensitivity (TCS): This is also known as temperature coefficient of span. It indicates the deviation in full-scale output as temperature changes. As the temperature increases, sensitivity decreases. So at room temperature you might get a 100 mV output, but at 150 °C the output will decrease to around 75 mV.

The great news is that all the errors listed above are repeatable and consistent, which means they respond well to compensation. In addition to manufacturing MEMS dies, Merit Sensor also builds pressure-sensor packages and performs calibration over various temperature ranges.

The following two specifications, however, deal with errors that cannot be compensated: thermal hysteresis and long-term drift. Therefore, if you are trying to decide which MEMS die to buy, you will want to find a supplier that produces parts with good specs in these two areas. We, at Merit Sensor, know that our customers do not want their parts, which contain our MEMS dies, to fail in their customers’ applications; therefore, we take pride in producing MEMS dies with excellent thermal hysteresis values and long-term stability.

Thermal hysteresis: This is typically performed at zero pressure and shows the difference between the output when the temperature is at room temperature and then increased to 150 °C and then returned to room temperature and then decreased to -40 °C and then returned again to room temperature and so on. This testing characterizes the repeatability of the die over numerous thermal cycles. It would be ideal to get the same output every time the temperature returned to a given value.

S Series MEMS Die Accuracy with Thermal Hysteresis - Solid White Background

Long-term stability (or long-term drift): This specification indicates how stable the output of the die will remain, or, in other words, how little the offset will drift, over time and sustained temperature. We have tested parts, for example, at 150 °C for 300 hours.

S Series MEMS Die Long-Term Stability - Solid White Background

One thing to watch for is a data sheet advertising a MEMS die with an accuracy of ±0.25 %. Here’s the catch: That accuracy refers only to non-linearity at room temperature; it does not take the other errors that have been discussed into consideration. Hopefully this article has helped you to better understand the different performance characteristics of MEMS silicon dies and the specifications that are used to quantify the dies’ performance.

Finally, if you would like to learn more about the technology and performance of MEMS dies, we invite you to watch our recently broadcast webinar, which is now on demand.

Four Characteristics of Our Newest MEMS Sensing Element

Merit Sensor has owned and operated a wafer fab from its beginnings. Fabricating our own MEMS (micro-electro-mechanical systems) sensing elements, or die, is something that sets us apart from other pressure sensor manufacturers, many of whom source their MEMS die from foundries or suppliers. Producing our own wafers, which are diced into individual MEMS sensing elements, allows us to control our own technologies, development, and supply chain. To learn more about the advantages, read this AZoSensors interview with our director of engineering.

Since we continue to see interest worldwide in these MEMS sensing elements, we continue to develop MEMS die with superior performance at competitive cost. Our newest MEMS product on the market is the S Series, offering optimal size, sensitivity, and stability. Perhaps best of all is its excellent performance in regards to thermal hysteresis. Each of these characteristics will be discussed below.

Size

One remarkable feature of the S Series is its solid performance at a very small size: 1.5 mm x 1.5 mm x 0.9 mm. This size also makes it is possible to optimize the amount of die produced on each 150 mm (6 inch) wafer. The end result is a lower-cost die for the customer without any loss of superior performance.

S Series MEMS Die Dimensions

S Series MEMS Die Dimensions

Sensitivity

Silicon, which is the raw material of MEMS wafers, has piezoresistive properties, which means when pressure is applied, it is strained and its resistance changes accordingly. An output is based on the changes in resistance. Merit Sensor uses Wheatstone bridge technology to optimize the linearity of the output. It is challenging, however, to obtain an adequate output when the pressure is low. Nevertheless, through Merit Sensor’s proprietary MeritUltra technology the S Series provides a typical output at 5 psi / 34 kPa / 345 mbar of 100 millivolts (mV).

Stability

A stable part will remain accurate, i.e. it will not drift, over time and sustained temperature. The S Series data sheet specifies a long-term stability of ± 0.2 % of the full-scale output (% FSO). The chart below shows how stable and accurate the part has proven to be, demonstrating a typical offset drift of <0.05 % FSO at 300 hours.

S Series MEMS Die Long-Term Stability

Long-Term Stability of the S Series

Thermal Hysteresis

The characteristic we are really talking about here, once again, is accuracy. In addition to remaining accurate over time and sustained temperature, the S Series displays exceptional accuracy when exposed to thermal cycling. A MEMS sensing element is inherently sensitive to temperature. Its resistance and output will change when temperature changes. Fortunately, changes that are consistent are simple to compensate. The S Series die exhibits very consistent, accurate output when it is exposed to extreme temperatures and returned to room temperature. In thermal cycling tests it demonstrated a typical thermal hysteresis offset of <0.05 % FSO.

S Series MEMS Die Accuracy with Thermal Hysteresis

Accuracy of S Series with Thermal Hysteresis

If you have any questions about using the S Series in your application, contact one of our sales managers. You might also find the application note “Handling of Mounting of Pressure Die” useful.