Creating a sensor that saves lives

Little Fiona is in the hospital. She is only three months old – and has become infected with RSV. She has had to undergo artificial respiration for a few days already. Her ribcage rises and falls rhythmically, controlled by a ventilator that monitors breathing and automatically regulates the correct dosage of oxygen. The heart of the ventilator is a gas flow sensor on a chip, just a few millimeters in size: this chip is manufactured by Sensirion. But how is a sensor like this one created? We took a tour of the company’s production facility in Stäfa on the banks of Lake Zurich.

It all starts with a thin silicon disc, which is used billions of times for electronic components in industry around the globe. It is called a wafer and in our case has a diameter of 200 mm and a thickness of less than 1 mm. At Sensirion in Stäfa, these wafers are turned into finished sensors. But first things first. 

The MEMS FAB 
We head to the first floor of the production building and enter the MEMS clean room. It’s easily recognizable by the yellow light throughout. This is where electronic circuits are supplemented with a sensor element. This requires a range of production stages to separate and structure layers.  
Plasma processes are used to separate layers. A light-sensitive coating is then applied to the wafer by a coater in a lithographyprocess. This photoresist is exposed to ultraviolet light in a mask aligner. A mask specifies the structures to be created, whereby only selected areas of the wafer are exposed. A mask specifies the structures to be created, whereby only selected areas of the wafer are exposed. That’s why yellow light is used in this room, so that wafers don’t undergo further exposure by mistake.  
Now the coating is processed in the developer: The exposed areas are removed, the unexposed areas remain. The wafer then once again enters a plasma process: The structured wafers are etched and then the coating on the wafer is again removed. This reveals the original surface. 
A multimeter weighs each wafer, a robot aligns it and loads it into the system, where structure sizes and layer thicknesses are measured. The entire process is fully automated. Now the next layer can be applied, and the process starts from the beginning again. Employees monitor the machines, transport the wafers in boxes to the next system, load the wafers into the system and start the corresponding recipe on the system. They are supported by an electronic system where the current processing status of the wafers can be seen at any time. Employees also document the stages they have taken on an operation card. 

The wafer backend 
Together with the wafers, we leave the MEMS FAB. Production employees transfer the wafers via a goods airlock to the next production stage: the wafer backend. During wafer probing, the chips are tested and calibrated for the first time while still in the form of wafers. Each wafer is loaded into the prober and connected to a probe card that can test up to 256 chips at the same time – an extremely efficient testing process. This is controlled by a Sensirion measuring computer with a prober script specially developed by the company’s in-house software group and process engineering. 

The wafer now goes into wafer dicing. First, it is mounted on a substrate with the help of an adhesive film. This prevents the chips from coming loose during cutting. Cutting is carried out with a precision in the micrometer range, and separates the chips. 

«The wafer now goes into wafer dicing. First, it is mounted on a substrate with the help of an adhesive film. This prevents the chips from coming loose during cutting. Cutting is carried out with a precision in the micrometer range, and separates the chips.»

_{Patroklos Alexopoulos, Process Integration Manager at Sensirion}

In the subsequent die bonding process, the chips are picked from the cut wafer and glued to the lead frame. Here, too, placement accuracy is in the micrometer range. The greatest challenges of this process are the different wetting properties of the surfaces and solid placement of the chips. After die bonding, the adhesive is hardened in an oven.

Now it’s time for wire bonding: The chips are connected for functionality. Very thin wires of just 20 to 30 μm diameter made of gold or aluminum are used for this purpose. These are applied with pressure, ultrasound and temperature. They establish the connection between the bond pad on the chip and the lead on the lead frame. The finished sensor will later be electrically contacted and controlled via these leads. 

The lead frame with the glued chips and mounted gold wires is passed to molding. Here, the sensors are sprayed with black, glass-rich epoxy resin. The cured epoxy protects the sensor against light, moisture and mechanical influences that could damage the fine wires or microstructures during use.

Component and module finishing  
We leave the molding area and accompany the lead frames through the final component production stage. The chips (sensors) are still arranged on a lead frame; they must be separated for further processing. To do this, an adhesive film is again used to set the lead frames on a mount, so the sensors do not come loose during separation. 

«I create shift plans, lead the team, organize the required materials and ensure the quality of our products and the safe and clean operation of our machines.»

_{Ivan Svilar, Supervisor at Sensirion}

The next step is lead frame dicing: Cutting wheels, fitted with abrasive grains made of synthetic diamonds, break down the metal lead frame into the individual components – the sensors. The balance between blade type, blade rotation, blade height and feed speed ensures flawless cutting quality. 

We accompany the sawn lead frames on their way to the next department and take a look at the fully automated inspection and testing area at Sensirion. This is where various sensors are automatically brought together, visually inspected, calibrated and packaged. 

This is all very exciting, complex and requires a great deal of special expertise in production. In all this, we almost lost sight of the gas flow sensor we need for the ventilator. This is assembled on the top floor in SensorManufacture & Industrialization. So back into the elevator. Upstairs, we meet the module assembly employees. Highly specific components and standard mass-produced goods are manufactured either entirely by hand or in a partly automated process, depending on the product and volume. The requirements for flow sensors in the semiconductor industry are particularly high, which is why a good operator is absolutely crucial for product quality. 

«Our team improves processes and carries out repairs on mechanical, pneumatic and electronic parts for the production machines. I also plan our team’s work. What I particularly like about Sensirion is its very welfare-focused and employee-friendly attitude.” 

_{Philipp Eggstein, Equipment Specialist at Sensirion}

Voilà. Our sensor is finished. So everything is ready for logistics. In intralogistics in Stäfa, logistics specialists package the sensors and prepare them for central logistics – this is the center for all our logistics activities, based in Nänikon on Lake Greifen, around 20 km from Stäfa. Sensirion operates a 3,500 m2 warehouse here, with space for 1,200 pallets and around 8,000 high-bay racks. Sensors are transported between Stäfa and Nänikon twice a day. 

«In our warehouse in Nänikon, we enter the finished sensors in the system and store them on ESD shelves. We can later retrieve them from here, pick them, pack them and ship them to our international customers.»

_{Ayla Pereira, Logistikerin at Sensirion}

Fiona has just opened her eyes in her crib. After an examination of all relevant indicators and blood levels, and an assessment by the senior physician, it becomes clear that the baby is through the worst of her illness. In two weeks’ time, at most, her parents will finally be able to cradle her in their arms again at home. The ventilator did an impeccable job and contributed its vital part to little Fiona’s recovery.