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.