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.

Micromachined Vertical Vibrating Gyroscopes

Tu Xiang Zheng

As well known, micromachined vibrating gyroscopes have gained popularity in recent years. A main application of these sensors is to determine the yaw (vehicle rotation about its vertical axis) or roll (vehicle rotation about its lengthwise horizontal axis) angle of the vehicle. These sensors also play a key role in consumer electronics for platform stabilization in camcorders and video-game headsets.

The above mentioned US patent describes a micromachined vertical vibrating gyroscope that can be used to counteract the rolling effect on a vehicle, and thus, are a preferred stabilization tool for vehicles such as airplanes, ships, and cars. The emergence of micromachining technology has generated the possibility to produce gyroscopes that present many advantages over their counterparts, such as lower cost, small size, higher performance, and lower power consumption. These advantages explain why the gyroscopes have been widely used in smart phones.

The present gyroscope is implemented by vibrating the proof mass in a direction parallel to the substrate, either lateral or rotational, and sensing vertical displacement or torsional motion due to Coriolis effect.

The gyroscope consists of three single crystal silicon assemblies: an outer single crystal silicon assembly, an intermediate single crystal silicon assembly, and an inner single crystal silicon assembly. The outer assembly includes a plurality of arc-shaped anchors arranged in a circle and extending from a single crystal silicon substrate coated with an insulating annulus thereon. The intermediate assembly is a suspended wheel concentric with the arc-shaped anchors. The inner assembly is a suspended hub concentric with the circle formed by the anchors and having no axle at its center. The three assemblies are connected to each other through several flexures. The intermediate suspended wheel is driven into rotational vibration by lateral comb capacitors. Input angular rates are measured by two vertical capacitors. The gyroscope is fabricated utilizing a bipolar-compatible process comprising steps of buried layer diffusion, selective epitaxial growth and lateral overgrowth, deep reactive ion etching, and porous silicon processing.

In operation of the gyroscope a voltage is applied to the lateral driving capacitors. The intermediate wheel is then stimulated into rotational vibration about the coordinate z-axis that is set to be vertical to the substrate plane. For the rotational vibration of the wheel, the flexures provide flexible mechanical support. As the rotational angular becomes too large the stops begin to abate the vibration so as to prevent the flexures from damaging. The lateral monitoring capacitors is used to measure the frequency and amplitude of the rotational vibration of the wheel. When the substrate experiences an angular rate about the coordinate x-axis that is set to be perpendicular to the flexures a Coriolis force is induced. The Coriolis force exerts on the inner vibrating hub and causes the hub to be rotationally vibrated about the coordinate y-axis.

In the balance state the two vertical capacitors and are designed to be completely equal. When the hub rotates about the coordinate y-axis, the two vertical capacitors are no longer equal. If the hub rotates counterclockwise, the capacitance of the vertical capacitor will increase and the capacitance of the vertical capacitor will decrease. If the rotation direction reverses, the difference of the capacitance also reverses. Since the difference of the capacitance of the two vertical capacitors depends upon the input angular rate, the input angular rate can be determined by measuring the difference of the capacitance of the two vertical capacitors.

The measurement circuit of the gyroscope can be adopted in open loop or in close loop. In open loop the amplitude of a carrier signal can be modulated by the difference of the capacitance of the two vertical capacitors. After demodulation with the carrier frequency and the driving signal frequency a DC voltage proportional to the input angular rate can be yielded as the output of the measurement circuit. In close loop the yielded signal is first fed to a rebalance circuit. The rebalance circuit then provides a rebalance voltage applying to the vertical capacitors to null the rotation of the inner vibrating hub about the coordinate y-axis. The rebalance voltage is proportional to the input angular rate. So the measurement circuit can be implemented as a Σ∆ interface circuit.