Single Crystal Silicon Micromachined Capacitive Microphone

Tu Xiang Zheng

 

In 2006, the present author designed a Micro-Electro-Mechanical System, or MEMS microphone as shown in the above picture. The MEMS microphone is capacitive sensing device. In essence, it operates like a high frequency pressure sensor. It is comprised of two capacitor plates that, under the influence of the sound wave, vibrate with respect to each other. The resulted variation of the capacitance is then amplified by an interface circuit to produce either an analog or digital output signal.

The market for MEMS microphones has been growing steadily. It was reported to reach approximately $1.2 billion in 2015. Of the total market, almost $900 million is coming from smart phone, tablets, and wearable platforms. Additional applications include hearing aids, automotive, virtual reality, headsets, smart home, and internet of thing. As the quality of MEMS microphones continues to improve, new applications such as far field and directional audio are emerging.

Many efforts have been made to fabricate acoustic capacitive microphones. W. Kuhnel et al. have reported a micromachined subminiature capacitive microphone. The described capacitive microphone consists of a membrane chip and a back plate chip. The membrane chip has a silicon nitride thickness of 150 nm and a metallization layer thickness of 100 nm. The back plate chip has an electrode on a silicon bridge. Both the chips are fabricated respectively and then bonded together to form a capacitor.

J. J. Bernstein et al. have reported the fabrication and results of very high sensitivity acoustic transducers fabricated using surface and bulk silicon micro-machining techniques in a manufacturing environment. The silicon microphone described here is a capacitive microphone. The basic movable element is a thin (.about.3 micron thick) diaphragm made from p+ silicon. The p+ silicon is one side of an air gap capacitor. The p+ regions are formed using boron solid source diffusion at high temperatures. The other plate of the capacitor is a 20 micron thick perforated gold back plate formed using electroplating. The air gap is defined using a 2.2 micron thick sacrificial photoresist.

Altti Torkkeli et al. have reported a capacitive silicon microphone. The reported capacitive silicon microphone consists of two freestanding polysilicon membranes, a low-stress bending membrane and a high-stress back plate, which are separated by an air gap. A back chamber is arranged by encapsulation and static pressure changes are prevented with small equalization holes in the bending membrane. The device is fabricated combining bulk and surface micromachining techniques. Silicon substrates are etched in TMAH and sacrificial oxide between the membranes is etched in PSG-etch followed by freeze drying to prevent sticking.

The microphone design has gone through a number of iterations since the fabrication of the first batch of working devices. The most notable efforts have been made to reduce the thickness of the flexible plate and the air gap and lower the bias voltage of the capacitor.

However, it should be pointed out that difficulties have frequently been encountered with such efforts. In a thin plate there are two kinds of forces which resist deflection in response to acoustic signals. The first kind of force includes plate bending forces which are proportional to the thickness of the plate. These forces can be reduced by using a very thin plate. The second kind of force, which resists deflection, includes membrane forces which are proportional to the tension applied to the plate. In the case of a thin plate, tension is generally a result of the fabrication technique and of mismatches in thermal expansion coefficients between the plate and the particular means utilized to hold the plate in place. The thermal mismatched tension lowers the flatness of the plate. Reducing the thickness of the plate and air gap may mean the capacitor plates pulling together under a lower bias voltage.

The present design provided a single crystal silicon micromachined capacitive microphone whose capacitor structure comprises a single crystal silicon substrate, an acoustic cavity recessed from the back side of the substrate, a flexible single crystal silicon plate with the edge clamped to the inside of the substrate and the rear side facing the cavity, a single crystal silicon contained supporting frame having the top surface coated with a thin insulating layer, a stiff and perforated single crystal silicon plate supported at the edge by the supporting frame, an air gap sandwiched by the flexible plate and the stiff plate and surrounded by the supporting frame, and two electrodes disposed around the stiff and perforated plate and interconnecting to the flexible plate and the stiff and perforated plate, respectively.

Compared with the prior art capacitive microphone, it is easy to find that the single crystal silicon micromachined capacitive microphone has the following outstanding features:

Firstly, the single crystal silicon microphone is made from a three layer structure consisting of a single crystal silicon substrate, a thinner epitaxial single crystal silicon layer, and a thicker epitaxial single crystal layer and the prior art microphone is made from a five layer structure consisting of a single crystal silicon substrate, a thin insulating layer, a thin single crystal silicon layer, a thicker oxide layer, and a thicker polysilicon layer. The three layer structure of the single crystal silicon microphone is composed of a same kind of material. In this structure there is no thermal mismatched tension to reside therein. All thermal mismatched tension related problems are able to cancel forever. The five layer structure of the prior art microphone is composed of three different kinds of materials. Due to having different thermal expansion coefficient, thermal mismatched tension always exists between each two different material layers. As is well known, lower tension may result in lowering the sensitivity of the devices and higher tension may result in damage of the devices. Furthermore, a released thin plate with a strong tension often bucks up so that the achievable thickness of the flexible plate and the air gap of the microphone are severely limited.

Secondly, the acoustic cavity of the all single crystal silicon microphone has an opening area smaller than the area of the flexible plate and the acoustic cavity of the prior art microphone has an opening area larger than the area of the flexible plate. A small opening area means less losing mechanical strength and enables to further shrink the microphone size.

Thirdly, the epitaxial single crystal silicon layer for making the stiff and perforated plate has a rest portion with high quality, which can be used to fabricate an electronic circuit, such as a CMOS circuit for conditioning the electronic signals generated by the microphone. For the prior art microphone the top layer is a polysilicon layer that cannot be used to fabricate the CMOS circuit.

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.

Earliest Paper with Porous Silicon Based Micromachining Process

Tu Xiang Zheng

The present author published a paper with the title “Fabrication of Silicon Microstructures Based on Selective Formation and Etching of Porous Silicon” in J. Electrochem. Soc, Vol. 135, No. 8, in August 1988. It was the earliest paper that describes silicon microstructures formed based on porous silicon micromachining process.

The next paper with the title “Using porous silicon as a sacrificial layer” by P Steiner, A Richter and W Lang. was published in 1993, in Journal of Micromechanics and Microengineering  Volume 3, Number 1.

The silicon microstructures were tiny mechanical devices such as sensors, valves, gears, mirrors, and actuators embedded in silicon chips. Before porous silicon micromachining or 1988 year, all these devices were produced by bulk micromachining process or surface micromachining processing.

Silicon wafers can be anisotropically wet etched, forming highly regular structures. Wet etching typically uses alkaline liquid solvents, such as potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH) to dissolve silicon. These alkali solvents dissolve the silicon in a highly anisotropic way so as to produce V-shaped grooves.

Surface micromachining process builds microstructures by deposition and etching of different structural layers on top of the silicon wafer. Generally polysilicon is commonly used as one of the layers and silicon dioxide is used as a sacrificial layer which is removed or etched out to create the necessary void in the thickness direction. Added layers are generally very thin with their size varying from a few microns.

The earliest paper provided a new micromachining process for silicon microstructures formation. The process consists of selective anodization of silicon in concentrated HF solution to form porous silicon and etching of the porous silicon in dilute KOH solution to form desired microstructures. In the process a starting material was n-type silicon wafer having resistivity in the range of 3.2 - 4.8 Ω-cm. Proton implantation with post-implantation annealing was employed to produce a high donor concentration layer in the wafer. Then nitrogen implantation was performed to create highly resistive region in the high donor concentration layer. The un-implanted regions provided the entrance windows through which the anodic current was able to reach the underneath layer. Since the donor concentration in the wafer was much lower than that in the proton implanted layer, the anodic reaction could be stopped automatically at the interface between the high donor concentration layer and the un-implanted regions.

The porous silicon micromachining incorporates the advantages of both bulk and surface micromachining:

• The porous silicon layer as a sacrificial layer can be formed in the silicon wafer and processed from the front side.
• Porous silicon is rapidly etched in dilute hydroxide solutions at room temperature.
• Sacrificial layer formation can be patterned both by selective substrate doping, as porous silicon formation is highly selective with respect to different dopant types and concentrations, and by masking of the substrate.
• Deep channels can be formed in the silicon wafer removing a formed porous layer.
• Porous silicon provides a planar sacrificial surface and is formed much more quickly than thermally grown or chemically deposited sacrificial layers.
• It can also be oxidized to form thick sacrificial oxide layers, thick oxide layers for thermal isolation or for SOI applications.
• Using porous silicon as a sacrificial layer also greatly reduces processing time and complexity, as well as device area, over bulk micromachining.
• It is possible to manufacture free-standing structures of high mechanical and electrical quality since the mechanical structures may be constructed from single crystal silicon. 

Using porous silicon micromachining process the present author had developed several MEMS sensors and actuators. Among them are piezoresistive pressure sensors, thermal flow sensors, thermal conducting (vacuum) sensors, capacitive pressure sensors, and ink jet printer heads. 

Porous Silicon Membranes for Removing Carbon Dioxide From Natural Gas

Tu Xiang Zheng

30 years ago I was assigned to prepare porous silicon membranes for study of removing carbon dioxide from natural gas based on Knudsen diffusion. Natural gas mainly consists of 79-84 mol % methane and 5-8% mol % Carbon dioxide. To meet specifications carbon dioxide must be removed before a natural gas can be delivered to the pipeline.

Knudsen’s diffusion occurs in a porous membrane, whose pore sizes are smaller than the mean free path of the gas molecules. The mass flux of a gas through the porous membrane can be expressed as:

J_k = D_k (∂ρ/∂L)              (1)

Where J_k is mass flux of the gas through the porous membrane, D_k is the Knudsen coefficient, ρ is the density of the gas, and L is the thickness of the membrane.

The Knudsen coefficient is defined as

D_k = d_p /3 (8R_gT/πM_g)^1/2         (2)

Where d_g is the diameter of the pores, R is the gas constant (8.3144J/mol k in SI units), M_g is the molecular weight of the gas (in units of kg/mol) and T has units of k.

Hence, for Knudsen diffusion, the square root of the inverse ration of the molecular weights of the gases will determine the mass flux of the gases through the porous membrane. As shown in the following table, the square root of the ratio of the molecular weights between the methane and the carbon dioxide is 1.66 that represents the mass flux ration between the carbon dioxide and the methane through the porous silicon membrane.

The porous silicon was prepared by anodization of silicon wafers in concentrated HF solutions. The used silicon wafers were p-type silicon wafer (0.01-0.001 Ω-cm), polished on one side and oriented along the crystalline direction. The used HF solutions were composed of HF wt 49% and ethanol and the anodization took place in a double tank cell. The anodization was carried out in the dark at a constant current density from 18 to 36 mA/cm². The obtained porous silicon membrane had a pore size from 6 to 10 nm and a porosity of about 50 %.

In order to form a porous silicon membrane first, applied current density was 36mA/cm² and anodization time was 30 minutes. As a result, a porous silicon layer with a thickness of about 30 microns was created. Then, the applied current density was abruptly increased in order to enable the porous silicon layer detached from the silicon wafer. The abrupt increase applied current density led to a high porosity layer and high released gas pressure. So the porous silicon layer was easy being detached from the silicon wafer.

As an alternative, the porous silicon membranes can be obtained by combination of forming and etching of the porous silicon. First, a thick porous silicon layer was formed in a silicon wafer and then etched in a diluted KOH solution. Secondly, a thin porous silicon layer was formed by etching the leaved silicon layer in the silicon wafer.

After fabrication of the porous silicon membrane, the sample was rinsed gently with ethanol. Then a final rinse was carried out with hexane in order to minimize the possibility of shattering of the membranes due to strong capillary forces and thermal stresses exerted when ethanol evaporates from the pores. Finally, the membranes were dried in a nitrogen flow.

Low Power and Fast Response Carbon Dioxide Gas Sensors

Tu Xiang Zheng

Carbon dioxide can cause negative health affects to humans including drowsiness and at high enough concentrations suffocation. It has been recommended that the maximum time averaged exposure to the atmosphere containing 5,000 ppm carbon dioxide is not over eight hours. As such it is highly desirable to be able to measure carbon dioxide in order to control indoor air quality and in environmental monitoring.

The present author provides a MEMS carbon dioxide gas sensor as shown in the above figure. The sensor is based on a silicon wafer and fabricated utilizing CMOS technologies. Since the sensor is required to be operated at an elevated temperature a thermal insulating pad is formed in the silicon wafer which is used to support the sensor body. Both a resistor for heating and a thermopile for temperature sensing are formed on the thermal insulating pad. Then depositing an electrical insulating layer and laying a tin dioxide layer is formed thereon. By employing such device structure with good thermal insulation to the silicon wafer, the sensor presents a series of advantages such as miniaturized size, low power consumption, and fast response.

Reducing the size of the sensor is the most effective way to reduce overall power consumption. The size of the heater of the sensor can be reduced as small as 60 µm x 800 µm. This affords field operation on a single 9V battery for an acceptable time. In addition, shortening the thermal response time enables the sensor to be operated for a very brief period during measurement. This pulse heating provides a further reduction in the power consumption. All these capabilities make the sensors ideal for the applications ranging from low power wireless to cell phones and wearable and conformal sensing systems.

The thermopile of the sensor is not able to measure the absolute temperature, but generate an output voltage proportional to the temperature difference between the heater and the silicon wafer. This is suitable for the sensor to adapt a pulse width modulation (PWM) based temperature controlled circuit. It is a key to stabilize the temperature of the sensor since the response of the sensor increases and reaches the maximum at a certain temperature, and then decreases rapidly with increasing the temperature. In the modulation circuit a microcontroller is programmed to generate different modulating 8 bit digital signal which is converted to analog signal using digital to analog converter (DAC). The analog signal is then used to control a PWM based driver circuit which drives the heater of the metal oxide gas sensor.

The pulse width modulation (PWM) based temperature controlled circuit has been reported to have three advantages:

• A cyclic temperature variation can give a unique response for each gas as rate of reaction of the different analytic gases are different at different temperature;
• Low temperature may lead to the accumulation of incompletely oxidized contaminant, which may get removed during cyclic oscillating voltage; and
• Thermal cycling can lead to improvements in sensitivity because for each gas there is a heater voltage for which it shows maximum conductance-temperature characteristics.