CMOSens® - Core of Sensirion's Sensor Solutions
The technology
Humidity sensors with CMOSens®
Mass Flow Control and differential pressure with CMOSens®
CMOSens® in Liquid Flow Measurement
Tes & Calibration
The technology
Sensirion sensor products guarantee outstanding performance and correspondingly high customer utility. They are all based on our innovative CMOSens® technology, which enables the sensor component to be combined with the analog and digital signal processing circuitry on a tiny CMOS silicon chip. This is implemented in advanced semiconductor technology, using specific microsystem processing steps to produce the microsensor structures on specially developed and patented semiconductor parts. The resulting sensor chips enable precise and reliable sensing of the desired physical parameters, such as relative humidity, temperature, or mass flow. CMOSens® accordingly represents a guarantee of high precision, reliability, and functionality combined with cost effectiveness.
Precision and Reliability
Combining the sensor with the signal processing circuitry ensures that the weak analog sensor signals can be amplified and digitized with high precision directly where they are generated, thereby making them resistant to interference. This provides the basis for the extremely high measurement accuracy of CMOSens® sensors. With the elimination of fault-prone solder joints, users also benefit from outstanding long-term stability, as has been proven by extensive field tests. Furthermore, CMOSens® technology is based on proven and reliable standard processes used in semiconductor fabrication, which also contribute to the high reliability and reproducibility of Sensirion sensor solutions.
Functionality
CMOSens® chips are intelligent microsensor systems with a high degree of integration and functionality. This is provided by analog and digital signal processing, which is integrated on the semiconductor chip along with the sensor component. Our chips are further enhanced by additional intelligence for self-tests, low-power operation, and built-in linearization, digitization and temperature compensation (the latter using an integrated temperature sensor). The present high level of functionality and intelligence results from many years of development work, as can be seen from the evolutionary stages leading up to the present chip generation:
1st generation: miniaturized sensor component
2nd generation: 1st generation plus amplifier and A/D converter
3rd generation: 2nd generation plus intelligence for linearization and temperature compensation
4th generation: 3rd generation plus memory to hold calibration data
Cost Effectiveness
As a standard semiconductor fabrication process, CMOS is optimized for reliable mass production. Sensors fabricated in CMOSens® technology are based on this cost-optimized standard process, which assures customers of an optimal price/performance ratio. Merging the sensor components with the signal processing circuitry on a single small chip allows system costs and package sizes to be reduced even further compared with conventional sensors. As a result, users obtain extremely high reliability and quality at a lower price.
Humidity sensors with CMOSens®
Capacitive humidity sensor principle
In addition to the resistive method, the capacitive measurement principle, above all, established and proved itself as a standard in the past. For this principle, the sensor element is built out of a capacitor. The dielectric is a polymer which absorbs or releases water proportional to the relative environmental humidity, and thus changes the capacitance of the capacitor. This change in capacitance can be measured by an electronic circuit. This allows the relative air humidity to be determined.
For humidity sensors with CMOSens® technology, a "micro-machined" finger electrode system with different protective and polymer cover layers forms the capacitance for the sensor chip, and, in addition to providing the sensor property, simultaneously protects the sensor from interference in ways previously not achieved.
Humidity, temperature and dewpoint
The temperature sensor and the humidity sensor together form a single unit. This also enables an accurate and point-precise determination of the dew point, without incurring errors due to temperature gradients between the humidity and the temperature sensors. Through the unique linkage of these sensor elements with the signal amplifier unit, the analog-to-digital converter, the calibration data memory, as well as the digital, bus-ready interface, all on a surface area of a few square millimeters.
Highest stability and performance
Total coverage with condensation or even immersion in liquid present no problems whatsoever. Over two years of hardness tests have already shown this result.
The signal amplification near the sensor allows the polymer layers to be optimized not for the signal strengths, but rather for long-term stability, which is significant for numerous applications. The analog-to-digital conversion, which is also performed "in place," makes the signal extremely insensitive to noise. A checksum generated by the chip itself is used for additional reliability. Last but not least, the calibration data loaded on the sensor chip guarantees that Sensirion humidity sensors have identical specifications and thus they can be replaced 100%. Other obvious advantages are the very short response times (4 sec at 1/e), high precision (±2% to ±5% according to configuration), as well as very low power consumption (< 3mA standby).
Digital Interface
Humidity and temperature sensors with CMOSens® technology can be connected directly to any microprocessor system by means of the digital 2-wire interface, which minimizes the system development times, saves costs, and leads to a significant advantage, especially for high-volume applications.
Mass Flow Control with CMOSens®
Principle of CMOSens®
Mass Flow Controllers & sensors
For more than 30 years, thermal mass flow measurements using coils around a steel capillary have been the standard in the precise measurement and dosing of mass flow rates. The new CMOSens® technology integrates this underlying, successful physical measuring principle in a miniaturized thermal sensor with all of the high-precision signal-conditioning circuitry on a single CMOS microchip.
Simultaneously, the two most important parameters for gas-flow measurement, speed and accuracy, are improved many times over.
Combined with specially developed sensor packaging, a system can be produced at a lower cost with a 10x higher control speed (150 ms) and a significantly higher accuracy (0.8% of the measured value over 10%-100% FS) , which represents an actual quantum leap in mass-flow measurement for the integration of a thermal flow sensor on a silicon chip, CMOSens® uses gas-flow sensors of an expensive but robust approach: a pressure-stabilized membrane, which has a glass-passivation layer and which is closed from the front, is etched into the silicon chip from below. The flat glass surface prevents the settling of contaminants. Simultaneously, the rear air cushion allows the pressure-tight membrane to be used even if there are strong vibrations.
A controllable heater element is mounted in the middle of this pressure-stable membrane and temperature sensors are mounted symmetrically upstream and downstream from this heater element in the direction of flow. Any flow over this membrane causes a transfer of heat and thus generates a precise measurable signal. Thanks to the low thermal mass of the membrane, the sensor reacts to changes of the gas flow within only 1.7 ms (1/e).
Readout circuitry integrated on same chip
The patented CMOS evaluation circuitry integrated on the same chip allows programmable, highly precise amplification and evaluation of the generated analog sensor signal. Typically, a CMOSens® gas-flow sensor measures a sensor voltage of only 500 nV with long-term stability and without noise. Two similarly integrated 16-bit A/D converters digitize the signals of the flow sensor and the additional temperature sensor into packets of 0.7 ms.
The integrated, digital 20-bit linearization unit connected to the output corrects each measurement packet for the non-linearity of the particular flow sensor and compensates for possible temperature effects with the help of the temperature signal. Then the linearized packets are averaged over a programmable period. This produces a very fast and highly precise sensor signal. The CMOSens® chip can be operated with a digital or analog output according to need
Fast settling time
A conclusive factor in performance for thermal mass flow controllers is the control speed. For conventional thermal mass flow controllers (MFC), the sensor element typically has a reaction time of a few seconds. Thus, to accelerate the control time for good MFCs, the reaction of the sensor is analyzed before the signal change and the possible final value is estimated in advance with the help of additional electronics. This produces faster control times on the order of almost one second at the price of higher system costs and lower control stability.
Because a CMOSens® mass-flow sensor reacts thermally about 1'000 times faster and the generated signal is linearized and temperature compensated on the CMOSens® chip every 0.5 ms, direct and much faster control can be realized. A CMOSens® MFC achieves settling times of less than 150 ms. The graphic below shows a comparison of settling times for a CMOSens® Mass Flow Controller and for an instrument with conventional construction.
High accuracy and repeatability
The second important feature of a Massflow Controller is its accuracy and the fundamental repeatability. To this end, the stability and resistance of the signal-conditioning circuitry to disturbances and the sensor's offset flexibility are especially important. Through the symmetry of the sensor element and the offset-compensated evaluation circuit integrated on the sensor chip, CMOSens® gas-flow sensors typically achieve an offset stability of <0.01% FS/y.
According to demand, CMOSens® MFCs can achieve an accuracy of 0.8% of reading (setpoint) or even more in the range of 10%-100% fullscale.
This high dynamic range could change the decision on the selection and use of a corresponding instrument. The accuracy of the controller is indicated in percentages of the setpoint or reading (%SP) instead of percentages of the fullscale (%FS). This means that the same MFC can be used for 400 sccm and 40 sccm, each with an accuracy of 0.8% of setpoint. Previously, MFCs of conventional technology required separate instruments calibrated correspondingly for each range.
Reliable and safe
Two weaknesses for flow sensors integrated on the silicon have been pressure resistance and the tightness of the sensor housing (packaging). Therefore, in parallel with the CMOSens® flow sensors, a stainless steel housing with integrated flow channel and vacuum-tight glass feeding throughs was developed for electrical contacts. The technology of glass feeding throughs has already proven to be best in vacuum technology for very tight and inert housings. Now gas-flow sensors made of silicon can be completely sealed in stainless steel. As sealing materials, only glass and gold-plated pins are used.
The reliability of an electronic instrument is essentially determined by the number of electrical contacts. The electrical contact of a poor solder point can become drastically worse over time. In particular, weak solder points can all of a sudden lead to the total failure of the instrument. All of the analog signal processing is performed on the same chip for CMOSens® sensors. This has the advantage of eliminating noise-susceptible solder points for small analog signals. This also explains the very high reliability of the CMOSens® humidity sensors even under very harsh operating conditions.
CMOSens® in Liquid Flow Measurement
For more than 30 years, thermal mass flow measurements using coils around a steel capillary have been the standard in the precise measurement and dosing of mass flow rates.
The new CMOSens® Technology integrates this basic physical measuring principle in an extremely fast, miniaturized thermal sensor with all of the high-precision signal-conditioning circuitry on a single CMOS microchip. A heating element on the microchip adds a minimal amount of heat to the medium for the thermal flow measurement. Two temperature sensors, symmetrically positioned above and below the source of the heat, detect even the slightest temperature differences, thus providing the basic information about the spread of the heat, which itself is directly related to the flow rate. Integration on a single chip ensures that the sensitive analog sensor signals can be amplified with high precision, digitalized and further processed. Semiconductor technology also allows small, battery-operated sensor modules to run very well.
Digital signal processing and the stored calibration data together provide repeatability of < 0.2% off the measurement value and allow for the output of a linear, completely calibrated, temperature-compensated flow measurement signal with speeds up to 200 values per second. As this type of sensor has a completely digital internal function, output signals may be provided as either analog or digital outputs.
In addition to interesting solutions for gas applications, the new CMOSens® Technology allows mass flow measurements for the smallest liquid flows. This is realized totally media isolated without further dead volume. Sensors based on this new principle are extremely fast, small, lightweight, and can be produced in large quantities. The accurate measurement of volume flow within the range of several milliliters per minute, and even down to the sub-nanoliter range, poses no problem.
Just a straight capillary
In the most diverse application areas, a liquid medium must be completely separated from its environment, and pressure resistance must be ensured. To this end, Sensirion has come up with a surprising solution, which has been patented in the meantime: Special packaging means the highly sensitive microchips, media-separated by plastic, steel or glass capillaries, can measure mass flows in nanoliters, microliters, or milliliters per minute directly with the greatest accuracy. The sensor for the fluid system is a simple, straight capillary without seals, dead volumes or restrictions. According to the flow range, the internal diameter can be several millimeters or micrometers.
A digital sensor for liquid flow based on this new technology is already up to 100 times faster, 10 times smaller and 25 times lighter than previous conventionally designed flow meters, which means optimization potential is not exhausted and battery-operated variants are possible.
Testing & Calibration
An important technical benefit for customers of Sensirion products is that our digital sensor systems are fully calibrated. This means that expensive and difficult calibration procedures no longer have to be performed by the customer.
Our in-house sensor calibration and testing infrastructure allows effective testing and calibration procedures according to established quality standards.
The results are high quality, fully interchangeable multi sensor modules at unbeatable price-performance ratios.
