Flow sensor by surface channel technology

Swift & Stable, Proven & Precise

On April 5, 2022, Bronkhorst launched a new and unique product: the FLEXI-FLOW. This multi-parameter instrument can measure and control both gas flow and upstream/downstream pressure while also measuring temperature. This combination in a single compact instrument is unique. To develop this instrument, ‘surface channel technology’ is used, resulting in a chip-based capillary flow sensor.

What makes this technology so special, and how did Bronkhorst develop this new technology into a marketable product? Wouter Sparreboom is System Architect Embedded Systems at Bronkhorst R&D. He was involved in the early stage of product development of this revolutionary product line called FLEXI-FLOW. Wouter explains the process of product development to an actual marketable new product:

capillary flow sensor for surface channel technology

Why did you start this new product development?

“Surface channel technology 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.”

Jarno Groenesteijn (left) & Jack van Putten (right) at the Nanolab of the University of Twente

“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.

And on April 5 there it was… Bronkhorst introduced the FLEXI-FLOW to the world; a successful example of a unique academic technology converted into a marketable product.”

Let’s have a look at the technology…

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.”

Laminar Flow Element and TCS-technology flow sensor in bypass configuration

“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 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.”

How can users benefit from an instrument using this flow sensor?

Fast response time

“Swiftness (speed) is an important advantage of this new technology. Traditionally in thermal mass flow sensors with the bypass principle, small steel tubes are used with a response time of 1 second or longer.''

New Gas flow controller: FLEXI-FLOW instrument (right) is very compact

The new FLEXI-FLOW instrument (right) is very compact

''Due to the material properties as well as the dimensions, the tiny silicon nitride capillaries in the FLEXI-FLOW have a very low thermal mass, which means that the capillaries are heated and cooled very fast. This allows for fast response times in the order of milliseconds.”

Swift & stable, proven & precise

“MEMS technology is used to fabricate these capillary flow sensors. A fast and stable ‘through chip sensor’ (TCS) applied in our proven bypass construction replaces the traditional metal tube sensor. This results in a reliable and highly precise flow measurement.”

Cooperation with University of Twente

“In the nineteen eighties, the Sensors and Actuators Research Unit of the University of Twente laid a solid foundation for ‘micro technology’. Miniaturization had already become an important trend for lab applications, and flow sensors were no exception. In 1990, Theo Lammerink successfully concluded his PhD research project on MEMS thermal flow sensors at the University of Twente – laying the basis for the eventual development of surface channel technology.

About the same time, the first contacts between the university and Bronkhorst arose. Joost Lötters and Wybren Jouwsma visited the university and started talking about this matter.
The start of developing a marketable product… FLEXI-FLOW.”

Revolutionary flow sensor by surface channel technology