Revolutionary Gas Flow Sensor Technology

Wouter Sparreboom, System Architect Embedded Systems at Bronkhorst High-Tech’s R&D department, was involved in the early stage of product development of the revolutionary product line called FLEXI-FLOW. Wouter explains the process of product development to an actual marketable new product.

Mr. Sparreboom, why did you start this new product development?

Wouter Sparreboom / System Architect / Bronkhorst High-Tech

Surface Channel Technology (SCT) was developed at the University of Twente around 2007. This technology has been a typical example of the use of semiconductor processes to minimize systems – flow sensors in our case. It was not until 2013 that I delved deeper into this matter, with the intention to make a marketable product out of this unique technology.

At that time the most popular instruments in our product portfolio were the EL-FLOW series. We were convinced that if we were able to combine the proven technology of this EL-FLOW series with the benefits of ‘micro technology’, we could develop a faster flow instrument with more functions integrated (such as pressure sensors), yet at the same compact footprint.
We already gained experience with ‘micro technology’ when introducing the IQ+FLOW anemometric MEMS (Micro-Electro-Mechanical System) chip thermal flow sensor in 2004. Our first product using ‘micro technology’.

In 2020, I started a development project with several R&D colleagues. Our team had access to the Nanolab of the University of Twente, involved in the manufacturing of the through chip sensors. In cooperation with PHIX Photonics Assembly, we were able to ‘pack’ the chips into a robust sensor module.

What is Surface Channel Technology?

Surface Channel Technology consists of surface channels etched into a silicon substrate using high-density plasma. Subsequently the inner surfaces of these channels are coated in a controlled way with a dense silicon-rich silicon nitride layer of about 1 micron uniform thickness using a Low Pressure Chemical Vapor Deposition (LPCVD) process. The coated channels are converted into free hanging silicon nitride capillaries by etching away the silicon that surrounds the silicon nitride coating.

The LPCVD process occurs at a high temperature of about 800 °C. After cooling down to room temperature, silicon nitride, which has a thermal expansion coefficient somewhat higher than silicon, shrinks more than silicon and enters a tensioned state, improving the mechanical properties of the free hanging capillaries. Furthermore, silicon nitride is highly chemically resistant which contributes to the robustness and versatility of the sensor. Since silicon nitride is an electrical insulator, the sputtered metal resistors on top of the capillaries are galvanically separated from the gaseous media at the inside.
This technology was used to develop the capillary flow sensor.

Can you tell us something about the capillary flow sensor?

The heart of the FLEXI-FLOW mass flow instrument is a capillary flow sensor, mounted as a bypass to the main gas flow channel. Essentially, this sensor consists of two straight silicon nitride capillaries, each with a diameter of 100 micron and a wall thickness of 1 micron. Temperature-dependent metal resistors on top act as heaters and temperature sensors.

During operation, gas enters the capillaries, is heated by the heaters, and subsequently the temperature of the gas is measured at a defined position downstream. Flowing gas transports this heat, and in the laminar flow regime inside the capillaries, the temperature difference (or its voltage difference analogy) is a direct measure of the gas flow rate.

This flow sensor is patented by Bronkhorst and is called Trough Chip Sensor – TCS technology. It offers a fast and stable flow sensor applied in a proven bypass construction for a reliable and accurate flow measurement. It replaces the traditional capillary tube sensor. The slogan of the FLEXI-FLOW refers to this as well; swift & stable, proven & precise.

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We develop and manufacture smart, sustainable and customer-specific low flow fluidics handling solutions from our vibrant base in Ruurlo in the Achterhoek region of the Netherlands. Our precise and reliable mass flow meters and controllers for liquids and gases find their way in laboratories, test-benches, machinery and a wide variety of industries. Bronkhorst ...