Considerations for Design of ASIC of Thermal Flow Sensors

Xiang Zheng Tu

Thermal mass flow sensors are manufactured using MEMS (Micro-Electro-Mechanical Systems) technology. The sensor chip comprises of a thermally isolated pad supporting a heater and one or two thermopile(s), all integrated in a silicon substrate. Heater temperature is typically controlled to be several degrees above ambient temperature. Thermal flow sensors operate on the principles of heat transfer across the surface of the sensing element. The upstream sense thermopile is cooled, the downstream sense thermopile is heated, and the combined differential electrical signal is proportional to flow.

The thermal flow sensors enable the ASIC to fulfill the basic market requirement for the thermal flow sensors: low-power, low-cost, able to run on battery, and with automated meter reading. The main attraction of this design is that the flow sensing module of the ASIC keeps running even when the ASIC goes into low-power mode. Since the ASIC is in low-power mode for most of the time, it reduces the power consumption.

The thermal flow sensors allow the ASIC to support a battery driven power supply and be capable of time keeping. It senses the signals from the flow sensor, calculates the flow and then accumulates it. The total flow accumulated and the month wise profile of the flow are stored and updated in the memory. The user key available on the board can be used to display the flow accumulated in a month and the date on the LCD. The ASIC also supports wireless communication with another handheld device. Thus, the ASIC supports a user deriving the flow readings using a handheld device from a distance.

For operating the thermal flow sensor a voltage reference is needed. The voltage reference is a DAC output of the ASIC, which is generated by a modulated bandgap voltage reference. The heater of the sensor is heated by an additional DAC output of the ASIC which is generated by modulating a regulated voltage. So calibration and correction of the sensor can be achieved by varying the offset and gain of a programmable-gain-amplifier and by varying the sensor heater excitation current or voltage.

The offset of a CMOS amplifier is usually in the order of 1mV and can be reduced only by increasing the area of the CMOS devices. Almost the same is true for the 1/f noise of the amplifier. It is preferred to utilize dynamic offset cancellation techniques, such as Auto-zeroing and chopping. This technique can reduce the offset to the microvolt level, while also removing 1/f noise. The offset cancellation is done in two phases a sampling phase and an amplification phase. During phase 1 the input signal is disconnected and the input of amplifier is connected to ground. So during the amplification phase the offset is subtracted, resulting in an output voltage free from offset.

A US Patent Issued to the Present Author Was Published in December 15, 2015 

Tu Xiang Zheng

The present author is happy to speak out that a US patent issued to me was published in December 15, 2015. The title and the patent number of the US patent are “Vacuum cavity-insulated flow sensors” and 9,212,940, respectively. This invention is related to thermal mass flow sensors, which are for sensing the mass flow rate of fluid flow. In the same field the present author already hold two US patents. One is the US patent 6,139,758 with a title as “Method of manufacturing a micromachined thermal flow meter” and the other is the US patent 6,378,365 with a title as “Micromachined thermal flow meter having heating element disposed in a silicon island”. All these US patents utilized the porous silicon micromachining technology proposed in 1988 and since then continuously improved by the present author.

Flow can be measured in a variety of ways. One way is thermal mass flow sensors.

Thermal mass flow sensors generally use combinations of heated elements and temperature sensors to measure the difference between static and flowing heat transfer to a fluid and infer its flow with the fluid's specific heat and density. If the density and specific heat of the fluid are constant, the sensor can provide direct mass flow readouts, and does not need any additional pressure and temperature compensation over their specified range.

With the powerful porous silicon micromachining technology MEMS thermal mass flow sensors have been explored extensively for their simple structure and implementation in POSIFA Microsystems. The micromachining technology is amenable to creating micro-heaters and thermal sensors with no moving parts, thus simplifying fabrication and operational requirements. Other advantages of thermal mass flow sensors is small size, short response time, low power consumption, higher sensitivity to low flow rates.

POSIFA thermal mass flow sensors can be used to measure the flow of gases in a growing range of applications, such as chemical reactions or thermal transfer applications that are difficult for other flow measuring technologies. This is because thermal mass flow sensors monitor variations in one or more of the thermal characteristics (temperature, thermal conductivity, and/or specific heat) of gaseous media to define the mass flow rate.

POSIFA thermal mass flow sensors can satisfy many industrial and laboratory applications that require the detection or precise measurements of liquid flows. Commercially available liquid flow sensors, mostly are constituted by turbines equipped of an optical or magnetic pick-up, are generally very expensive devices, especially if reasonable precision and reliability are requested. Other factors that limit the extensive use of flow sensors for liquids are the difficulty of matching low flow measurement ranges with low insertion loss, the compatibility with corrosive or unfiltered liquids and the possibility to plug the sensors directly on the conducts. Such requirements are typical of the biomedical and environmental monitoring fields where the cost is also a crucial factor.

Another application of the POSIFA thermal mass flow sensors is for micro-pump controllers. Micro-pumps are the essential components in the liquid handling system, micro analytical instrumentation, genetic engineering, protein synthesis, portable sampling systems, environmental monitoring and drug delivery. Various mechanical micro-pumps with different actuating principles have been developed, such as thermo-pneumatic, electrostatic, shape memory alloy (SMA), electromagnetic as well as piezoelectric. All micro-pump controller needs to incorporate a high quality flow sensor for sensing any malfunctions that lead to an accuracy loss or accident to take place. The malfunctions generally include bubble, leakage, degradation, and failure. The small size, low power consumption, good reliability, and fast response of the POSIFA thermal mass flow sensors are preferred for this application.

First commercial application of Porous Silicon Based Micromachining

Tu Xiang Zheng

In the middle of 1988, the present author was first proposed a new process called as porous silicon based micromachining. Using this process, silicon membranes and silicon cantilever beams were successfully fabricated. This process included selective etching of silicon in concentrated HF solution to form porous silicon and selective removing of the porous silicon in dilute alkaline solution to obtain desired silicon microstructures.

In order to do so, proton implantation with post-implant annealing was employed to produce a thicker high donor concentration layer in lightly doped n-type silicon substrates, and nitrogen ion implantation was employed to create thinner highly resistive islands in the formed high donor concentration layer. The etching and the removing were exactly restricted within the region defined by the proton implantation and the nitrogen ion implantation, respectively. The donor states produced by the implanted protons and the radiation damage created by the implanted nitrogen ions were eliminated by annealing at 1000⁰C.

The first commercial application of the porous silicon micromachining was the piezoresistive pressure sensors. The present author was granted a US patent 5,242,863 with title as “Silicon diaphragm piezoresistive pressure sensor and fabrication method of the same” in 1993. The fabrication of the sensor uses porous silicon as a sacrificial layer which is formed in a silicon substrate.

The sensing principle of the present sensor is based on piezoelectric effect of silicon. The sensor is composed of a diaphragm, certain resistors on the diaphragm, a cavity buried under the diaphragm, and a silicon substrate. When a pressure is applied on the sensor, the diaphragm will deform and induce bending stresses that leads to increase of the resistance of the resistors.

According the patent 5,242,863, a silicon diaphragm piezoresistive pressure sensor comprises a diaphragm formed by a single-sided fabrication method. The pressure sensor is made up of a substrate on which there is a diaphragm at or near the surface of the substrate with a chamber under the diaphragm. The pressure sensor is fabricated by undercutting a silicon substrate to form a diaphragm and a cavity within the bulk of the substrate under the diaphragm. The fabricating steps including a) forming a buried low resistive layer under a predetermined diaphragm region; b) converting the low resistance layer into porous silicon by etching of silicon in a concentrated hydrofluoric acid solution; c) removing the porous silicon by selective etching; d) filling the openings formed in the etching of porous silicon with a deposited material to form a sealed reference chamber, and c) Adding appropriate means to the exterior of the diaphragm and substrate to detect changes in pressure between the reference chamber and the surface of the substrate.

The present method enables the sensor to be fabricated by what is called a single-sided processing method wherein all the processing steps are conducted solely on the upper side of the silicon substrate. Accordingly, the diaphragm and the reference pressure chamber are all formed by processing the substrate from one side. This greatly simplifies the manufacturing method compared to the conventional method which, and among other things, leads to a substantial reduction in production costs.

With the present method it is possible to form a diaphragm with a high degree of accuracy and avoid the problems caused by a lack of uniformity of thickness of the silicon substrate, a perennial problem with the present double-sided manufacturing method. Consequently, it is possible to fabricate diaphragms of relatively small and highly accurate dimensions in reference to a predetermined crystal plain dimension of the substrate. The ability to form a diaphragm of a predetermined and reduced thickness and dimension with high accuracy allows the production of sensors of much higher sensitivity and accuracy as compared to those made by current manufacturing methods.

Since the reference pressure chamber is formed within the bulk of the silicon substrate from one side, an absolute pressure sensor is formed with an air tight seal all which can be done by integrated circuit fabrication techniques. The conventional manufacturing method has a persistent problem in providing for air tight bonding between the diaphragm and the base material a serious obstacle to effective and efficient mass production. Obviously with the technique as described herein with its simplified fabrication process the actual cost of manufacturing accurate and small pressure sensors can be substantially reduced.

The present method also enables the silicon pressure sensor to be formed by integrated circuit manufacturing techniques. This is possible because all of the processing steps are conducted by a one-sided processing method as described herein, consequently it is easy to design and treat the silicon pressure sensor itself as one element of an integrated circuit because the techniques of both manufacturing integrated circuits and the pressure sensor as described herein are substantially the same techniques. This allows for manufacture of combined pressure sensor and integrated circuits of predetermined signal processing characteristics with appropriate circuits, amplification and whatever addition devices are necessary for the use of the pressure sensor. 

It is clear that using the porous silicon micromachining many sensors and actuators can be processed with good parameters and a good yield. The sensors include accelerometers, gyroscopes, pressure sensors, humidity sensors, and microphones, The actuators include ink jet printer heads, Fabry–Pérot interferometers, and vaporizers.