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. So this combination in a single compact instrument is unique. To develop this laboratory instrument, surface channel technology is used, resulting in a chip-based capillary flow sensor.
Wouter Sparreboom is System Architect for Embedded Systems at Bronkhorst R&D. And he was involved in the early stage of product development of this revolutionary product line called FLEXI-FLOW.
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 this case. But it was not until 2013 that Bronkhorst delved deeper into this technology, with the intention to make a marketable product.
At that time, the most popular instruments in our product portfolio were the EL-FLOW series. And Bronkhorst was convinced that they could combine the proven technology of the EL-FLOW series with the benefits of microtechnology. In doing so, Bronkhorst developed a faster flow instrument with more functions integrated, such as pressure sensors, and with the same compact footprint.
In 2004, Bronkhorst had already gained experience with micro-technology when introducing the IQ+FLOW, an anemometric MEMS (Micro-Electro-Mechanical System) chip thermal flow sensor. And this was Bronkhorst’s first product using microtechnology.
In 2020, Bronkhorst started a development project with several R&D colleagues. So their team had access to the Nanolab of the University of Twente, which was involved in the manufacturing of the through chip sensor. And in cooperation with PHIX Photonics Assembly, Bronkhorst was able to pack the chips into a robust sensor module. Then, by April 5, Bronkhorst introduced the FLEXI-FLOW to the world. Truly, this was a successful example of a unique academic technology converted into a commercial product.
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 LPCVD process. Then, a dense, silicon-rich silicon nitride layer of about 1 micron uniform thickness is applied in this Low Pressure Chemical Vapor Deposition (LPCVD) process. After that, 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. Silicon nitride has a thermal expansion coefficient somewhat higher than silicon. So after cooling to room temperature, the silicon nitride shrinks more than the silicon and enters a tensioned state. And this improves 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 inside.
The heart of the FLEXI-FLOW mass flow instrument is a capillary flow sensor that’s mounted as a bypass to the main gas flow channel. Essentially, this sensor consists of two straight silicon nitride capillaries. Each capillary has a diameter of 100 micron and a wall thickness of 1 micron. And temperature-dependent metal resistors on top act as heaters and temperature sensors.
During operation, gas enters the capillaries and is heated. Subsequently, the temperature of the gas is measured at a defined position downstream. Since flowing gas transports this heat, and the heat 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 Through Chip Sensor – TCS technology. Thus, it offers a fast and stable flow sensor applied in a proven bypass construction for a reliable and accurate flow measurement. And it replaces the traditional capillary tube sensor. The slogan of the FLEXI-FLOW refers to this as well: swift & stable, proven & precise.
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. Additionally, 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. So this means that the capillaries are heated and cooled very fast, which allows for fast response times – in the order of milliseconds.
MEMS technology is used to fabricate these capillary flow sensors. A fast and stable Through Chip Sensor (TCS) applied in Bronkhorst’s proven bypass construction replaces the traditional metal tube sensor. And this results in a reliable and highly precise flow measurement.
In the 1980s, the Sensors and Actuators Research Unit of the University of Twente laid a solid foundation for microtechnology. Miniaturization had already become an important trend for lab applications, and flow sensors were no exception. Then in 1990, Theo Lammerink successfully concluded his PhD research project on MEMS thermal flow sensors at the University of Twente. So this laid the basis for the eventual development of surface channel technology.
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