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.

30 Years of Pressure-Sensing Solutions

After 30 years of designing and manufacturing pressure sensors, we at Merit Sensor are happy to see our products used in so many applications. In the past year we were fortunate to support the surge in global demand for mechanical ventilators (read more here), while we continued to support other medical applications, such as blood-pressure transducers and diagnostic equipment. But our pressure sensors are used far beyond the medical field. They have been integrated for use in aerospace, automotive, consumer, and industrial fields. And more and more we see our products used in novel ways within these industries.

Merit Sensor 30-Year Anniversary

There are a couple of main factors that have helped us to flourish during the past 30 years. One has been the solid support of our parent company, Merit Medical. The other has been our involvement in the different aforementioned industries. The following discussion of our history and expertise will shed some light on these factors.

Our History

In 1991 Fred Lampropoulos, Chairman and CEO of Merit Medical Systems, founded Sentir Semiconductor. Sentir Semiconductor became a reliable pressure-sensor supplier for Merit Medical’s digital inflation syringe. In 1999 Sentir Semiconductor was vertically integrated with Merit Medical, and in 2002 the name Sentir Semiconductor was changed to Merit Sensor Systems.

In 2004 the building of a new and improved facility on the Merit Medical campus in South Jordan, Utah, commenced. This construction included a class 100 (ISO 5) cleanroom for wafer fabrication, commonly known in the industry as a wafer fab or simply a fab.

Merit Campus in South Jordan, Utah

Merit Medical Campus in South Jordan, Utah

Our Expertise

Owning and operating a fab allows us to develop unique technologies and keep them in house and also to closely monitor and control our processes. In addition to owning and operating a fab, we also have very specialized testing and calibration equipment for characterizing the performance of our pressure sensors.

Not only do we have the equipment, but we also have the expertise gained from years of experience working with technical experts in different industries. With our experience as a guide we have been able to design MEMS silicon die and pressure-sensor packages that offer the bridge configurations, pressure types (e.g., absolute, gauge, vacuum, and differential), sensitivity, stability, accuracy, media compatibility (e.g., for wet and harsh environments), temperature tolerance, and output options that provide the most value to the end application and to the customer.

Today we continue to provide pressure sensors to our parent company, while also serving many other special customers from various industries around the world.

We look forward to another 30 years of growth and success!

Three Common Types of Pressure-Sensor Packages

At the heart of every MEMS pressure sensor is a MEMS silicon die. Merit Sensor owns and operates a wafer fab, where it produces all of its own MEMS die. Packaging a MEMS die requires specialized equipment and skills to handle the small and sensitive die and to perform delicate wire bonding. Therefore, many customers purchase pressure-sensor packages, in which the die have already been mounted and wire bonded. This article will discuss Merit Sensor’s three types of packages: uncompensated, passively compensated, and fully compensated.

Uncompensated

The most basic pressure-sensor package is uncompensated. In an uncompensated package the MEMS die has been mounted to a ceramic substrate with a special die-bond material, wire bonded to electrical traces on the ceramic, and covered with a protective cap or gel. Since each silicon die is inherently unique, the output for each one will be unique. Fortunately, silicon die have outputs that are very repeatable. This means the output can be compensated.

PMD Series Pressure Sensor Internal Components

PMD Series Pressure Sensor – Uncompensated

To get an accurate output, the customer will need to perform some degree of compensation. Certain applications lend themselves to compensation performed by the customer. Factors that will often determine whether the compensation is performed by Merit Sensor or the customer include the following:

  • Cost
  • Accuracy
  • Size
  • Output signal

Passively Compensated

A more ready-to-use version of a pressure-sensor package, especially for use at room temperature, is one with passive compensation. In this case the pressure-sensor package is basically the same as an uncompensated package; however, the thick-film resistors on the ceramic substrate have been laser trimmed, providing adequate compensation of the die’s output in operating temperatures between 10 °C and 40 °C.

Laser-Trimmed Thick-Film Resistor on AP Series Pressure Sensor

AP Series Pressure Sensor – Passively Compensated

For applications, such as invasive blood-pressure monitoring in a hospital room, compensation in this temperature range is sufficient. Other benefits of passive compensation are the pure analog signal with practically infinite resolution and frequency response times in microseconds.

Fully Compensated

In a fully compensated pressure-sensor package, signal conditioning (an on-board ASIC) is used to compensate the die’s output across a wide temperature span. A MEMS silicon die does not know the difference between pressure and temperature, so this level of compensation is especially critical in applications where the temperature of the sensing environment fluctuates drastically or reaches extremes highs or lows. Compensation through signal conditioning can provide a linear output and make that output as accurate as ±1 percent of the full-scale output (±1 %FS total error band) in operating temperatures between -40 °C and 150 °C.

Wheatstone Bridge with ASIC

Wheatstone Bridge on a MEMS Die with an On-Board ASIC

If we use fuel pumps in airplanes and fuel rails in vehicles for examples, it is typical for pressure sensors to be exposed to extreme temperatures; nevertheless, it is essential that these pressure sensors offer an accurate output. A fully compensated pressure sensor would be the appropriate solution.

TVC Series Pressure Sensor Internal Components

TVC Series Pressure Sensor – Fully Compensated

It is important to emphasize that each pressure sensor will require individual compensation, as each one will have a unique output inherent to its MEMS die. Many customers simply do not have the time or equipment to do this logistically or economically to each unit passing through their assembly line.

Merit Sensor has the experience and equipment to handle this necessary step for the customer. Furthermore, it often, although not always, makes sense for compensation to be done before the part leaves our facility. Nevertheless, we have left options for those customers who choose to do their own compensation. As always, our sales managers and technical team will be happy to answer any related questions.

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.

Blood-Pressure Monitoring During the COVID-19 Pandemic

As a result of the COVID-19 pandemic, many people have been in hospitals under critical care this year. These patients require beat-to-beat blood-pressure monitoring, which helps clinicians see important vital signs about the patient in real time over the length of the critical-care treatment and make clinical decisions accordingly. Therefore, continuous, reliable monitoring of a patient’s blood pressure is as important as ever.

Merit Sensor has been a supplier of blood-pressure sensors since 2010. Our parent company, Merit Medical, to whom we supply blood-pressure sensors, is one of the global leaders in blood-pressure transducers. Check out their newest transducer, the Meritrans DTXPlus. Through our experience and that of our parent company, we have learned what matters most to clinicians using invasive blood-pressure devices.

Meritrans DTXPlus Blood Pressure Transducer

Merit Medical’s Meritrans DTXPlus

One important factor is that the fluid column from the needle to the sensor should be free of bubbles. Bubbles dampen the pressure pulses in the fluid column and, therefore, degrade the conversion of the pressure signal. To prevent bubbles, clinicians routinely tap or whack the blood-pressure transducer with a hemostat or forceps. This certainly poses a risk to the integrity of the pressure sensor and can cause even greater issues than just dampened signals. However, due to the insight provided by our parent company and our in-house expertise, we have improved the BP Series design over the years and have produced a robust pressure sensor that is able to withstand this common debubbling practice.

Merit Sensor's BP Series pressure sensor

Merit Sensor’s BP Series

Another common-enough issue that could destroy a pressure sensor is the inadvertent opening of the transducer’s stopcock to an undesired pressure source. For example, when a clinician injects medicine or contrast through the line, there is a pressure spike of around 300 psi in the line. This is considerably greater than the typical pressure of blood pressure, which is around 2 psi. In order that the pressure sensor be accurate at such a low pressure, it must contain a very thin MEMS diaphragm. At Merit Sensor we have designed a blood-pressure-sensor package that provides accuracy at low pressure yet robustness when exposed to overpressure. The BP Series has a typical burst pressure of > 800 psi. As long as the overpressure does not damage the MEMS diaphragm or the sensor package, the sensor will return to its specified performance once it is again within its operating pressure range of −30 to 300 mmHg.

In addition to providing a robust pressure sensor that can withstand forceful tapping and high overpressure, Merit Sensor has complete control over its manufacturing processes and supply chain. We own and operate a wafer fab in South Jordan, Utah (USA). Our on-site wafer fab enables us to monitor production closely and ensure high quality with everything we produce. It also gives us the flexibility to meet unique requirements of our customers, who might have a unique application for a blood-pressure sensor. With Merit Sensor you get reliability as well as flexibility.

To learn more about the advantages of owning and operating a wafer fab, read this interview published by AZoSensors.

Pressure Measurement for Ventilators and Respirators

Merit Sensor has a long history of supplying products to the medical-device industry. We are owned by Merit Medical, a leading manufacturer and marketer of disposable medical devices used in interventional, diagnostic, and therapeutic procedures.

A couple of the products we supply for medical applications are the BP Series for monitoring blood pressure, where accuracy and reliability are extremely important, and the AP Series for angioplasty, where knowing the precise pressure of the catheter balloon is essential.

The current demand for mechanical ventilators and respirators due to the COVID-19 pandemic presents us with another opportunity to fill a need in the medical field. The use of pressure sensors in ventilators and respirators is similar to their use in CPAP and BiPAP. Essentially, when the lungs need assistance taking in air, a machine blows the required amount of air into the lungs. But how does the medical professional monitor and control the required amount of air, or positive airway pressure? This is where pressure sensors play a critical role.

LP Series pressure sensor for mechanical ventilators

Merit Sensor’s LP Series is the ideal pressure sensor for these applications. The LP Series can measure pressure as low as 1 inH₂O (250 Pa) with resolution better than 0.001 inH₂O (<0.1 Pa). It was designed to measure differential or gauge pressure, depending on the application. It contains two pressure ports to which tubing can be connected, one tube directing pressure to the topside of the MEMS sensing element and the other tube directing pressure to the backside of the MEMS sensing element. It comes in several calibrated pressure ranges, functions with a 3.3- or 5-volt supply, and offers I²C or analog output.

Cross-section image 2 of LP Series pressure sensor

The LP Series, along with all of Merit Sensor’s other products, is made in our on-site wafer fab and assembly areas in Salt Lake City, Utah, USA. Manufacturing our products in our own facility ensures that we have control over product quality and supply chain. It also allows us to customize products to meet the unique demands of our various customers and markets.

We are proud to be a reliable supplier for life-enhancing and life-saving devices, and we hope that our products can play an important role in helping humanity, especially at this time.

Merit Sensor is taking precautionary steps to minimize the effect of COVID-19 on its operations

Merit Sensor, a subsidiary of Merit Medical, has been closely monitoring the impact of COVID-19 and the effects on our operations, supply chain, and personnel.  We want to assure you that we are taking every precautionary step to ensure that you receive uninterrupted support from our company. Merit has formed a COVID-19 operations task force that meets daily to stay on top of local, national, and international news and related changes in order to direct the organization appropriately. Below we have listed the most important factors related to our business continuity.

Personnel

Merit Sensor has instituted several steps to ensure the health and safety of its employees. Non-essential employees have been asked to work from home. Additionally, the temperature of every employee who enters the manufacturing site is being taken while the employee remains in his/her car. We have also asked that all meetings be conducted via video conferencing and that social distancing be implemented during breaks and lunches. The cleaning of the premises has also been stepped up to prevent the spread of the virus among the essential employees required to be onsite. Merit does employ a Chief Medical Officer and has a clinic onsite to assist any employee who may feel unwell.

Supply Chain & Logistics

Merit is not currently suffering from any shortage of materials as a result of COVID-19. We always try to mitigate our risk by having more than one supplier or requiring suppliers to produce at multiple sites. We also maintain several months of raw materials to aid with such occurrences. Presently, COVID-19 has not caused any disruptions to our supply, operations, or delivery of products. We are taking every precaution to ensure that manufacturing operations and delivery remain uninterrupted, and that ample product supply is available to continue to meet our customers’ needs.

You can be assured that Merit is doing everything in its power to ensure that our customers are supported in every way possible. We will continue to provide you with ongoing updates as the situation changes.  If you have any questions or comments, please feel free to contact us directly.

Sincerely yours,

Rick Russell, President

Trade Show Updates

We would like to inform you of our latest trade show updates. They are as follows:

The MEORGA MSR-Spezialmesse that was scheduled for March 25th in Frankfurt, Germany, has been cancelled. Our distributor in Germany, Delta Regeltechnik, plans to represent Merit Sensor at the next MEORGA MSR-Spezialmesse that is scheduled for May 27th in Leverkusen, Germany.

The Mostra Convegno Expocomfort (MCE) that was scheduled for March 17th through 20th in Milan, Italy, has been postponed until September 8th through 11th. Many thanks to GVZ Components, one of our distributors in Italy, for their hard work and patience to make this event a success for their company and for Merit Sensor.

That being said, we fully understand that trade shows in general are on standby. We are thankful to these organizations that are acting responsibly and proactively to curb the spread of the coronavirus. We look forward to when the time is right for people to attend these trade shows.

To stay up to date on our company news and events, please watch our website and follow us on LinkedIn and Twitter.

All the best from Merit Sensor.

Slides

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Merit Sensor Demonstrates Pressure Measurement in a Diesel-Particulate Filter

We take pride in our pressure sensors and like to show what they can do. Recently at the Sensor+Test trade show in Nuremberg, Germany, we used a diesel-particulate filter (DPF) to demonstrate how our sensors can be used to indicate when a DPF is clogged with particulate and needs to be regenerated.

DPF demo Sensor+Test

Visitors at our Sensor+Test booth viewing our DPF demo

A couple of our senior engineers integrated one of our pressure sensors into a modern-day DPF. This engineering work included designing and 3D printing a coupler to connect a fan to the input end of the DPF, simulating the flow of exhaust; 3D printing a custom fan guard and generic housing for the pressure sensor; and programming a microprocessor to vary the fan speed automatically.

Our TR Series is an ideal choice for the DPF because it is impervious to harsh media, such as exhaust, and contains a MEMS die that ranges from 0 to 500 psi. For this application the MEMS die in the TR Series would be produced in a gage configuration, enabling it to measure pressure from the backside as well as the top side. The TR Series would then be calibrated for a differential output.

DPF diagram TR Series

Diagram of the TR Series measuring differential pressure, or ΔP, in a DPF

The main purpose of this demonstration was to show another one of our solutions to a real-world application. The DPF is just one of many applications for which our products are designed. We thrive on understanding our customers’ applications and providing the best solution for each one.