Measuring Carbon Dioxide Concentration in Humidity Air

Tu Xiang Zheng

POSIFA’s thermopile thermal conductivity sensors are manufactured by MEMS and CMOS technologies which offer real "best-in-class" performance for drift accuracy, linearity and repeatable performance, as well as lower cost-of-ownership. It can be widely used for determining the gas concentrations of humid binary mixtures of gases. It can also be calibrated to measure a single component of a multi-component gas mixture.

As a MEMS device, a POSIFA’s thermopile thermal conductivity sensor comprises a hot plate for minimizing its power consumption and an integrated thermopile for measuring the temperature difference between the hot plate and the body of the sensor. The hot plate is crated in a silicon substrate and suspended over a cavity recessed in the substrate so as to have its three edges supported by the body of the substrate and the rest edge free. In order to make the hot plate being hot a resistive resistor is positioned near its free edge, which is heated by applying a voltage. After heating a temperature difference is established between the heater and the body of the substrate since the heat transfer from the heater to the body is resistance by the thin film structure of the hot plate. The thermopile is configured to have its hot junctions along one side of the heater and its cold junctions on the edges of the body and so the temperature difference can be measured by the thermopile.

In operation, a measurement sensor and a reference sensor are typically connected in a differential amplifier circuit and generally operated at a constant voltage. When the sensors are all running in dry air, the sensors loose heat at a similar rate resulting in a zero differential signal between the two sensors providing to the amplifier. When the measurement sensor is exposed to the gas mixture of dry air and carbon dioxide, the atmosphere around the sensor changes, resulting in a lesser amount of heat being lost from the sensor, leading to an increase in the temperature difference of the thermopiles. The increase is dependent on the mixture thermal conductivity being less than the thermal conductivity of the dry gas. The reference sensor being sealed does not show this effect.

The output voltage of each thermopile is proportional to the total thermal conductance. Neglecting radiation effects, the total thermal conductance is equal to the sum of the thermal conductance of the thin film structure, the thermal conductance through the gas and the heat loss by convection. Since the thin film structure conductivity and the heat loss by convection are identical in both sensors and keeping both sensors at the same temperature, the output voltage difference of the two thermopiles depends only on the thermal conductance difference between the measurement gas mixture and the reference dry air.

The output voltage difference ΔV of the two thermopiles can be expressed as

ΔV = NS (T_h -T_d) (λ_m –λ_a)                                                                 (1)

Where N is the number of the thermocouples, S is the Seebeck coefficient of the thermocouple, T_h is the temperature of the heater of the sensor, T_b is the temperature of the body of the sensor, λ_m is the thermal conductivity of the gas mixture, λ_a is the thermal conductivity of the dry air.

The thermal conductivity of the gas mixture can be calculated by

λ_m  = λ_a n_a + λ_CO2 n_CO2 = λ_a (1- n_CO2 )+ λ_CO2 n_CO2                               (2)

Where n_a is the volume fraction of the dry air, n_CO2 is the volume fraction of carbon dioxide gas, and λ_CO2 is the thermal conductivity of carbon dioxide gas.

The thermal conductivities of carbon dioxide gas and dry air can be calculated by

λ_CO2 = -2.400 x 10^-5 + 2.16 x 10^-7 T – 3.244 x 10^-11 T²                      (3)

λ_a = kA₀ + kA₁ T+kA₂T² + kA₃T³ +kA₄T⁴ +kA₅T⁵                                             (4)

Where KA0 = 2.276501 x 10³, = 1.2598485 x 10⁴, KA2 = 1.4815235 x 10⁷,

KA3 = 1.73550646 x 10¹⁰, KA4 = 1.066657 x 10¹³ and KA5 = 2.47663035 x 10¹⁷.

With equation (1) and expressions (2), (3) and (4) the concentration of the carbon dioxide gas in dry air can be calculated based on the measured differential voltages of the thermopiles of the two POSIFA’s thermopile thermal conductivity sensors. 

Carbon dioxide gas is usually mixed with humid air instead of dry air. Tsilingiris suggests the following expression, which was original proposed by Wassiljewa, as the basis of the calculation of the thermal conductivity of a humid air.

λ_mh = { [1-RH (p_sv/p₀)]λ_a } / { [1-RH (p_sv/p₀)]+[RH (p_sv/p₀)Φ_av] }+

{ [RH (p_sv/p₀)λ_v] } / { RH (p_sv/p₀)+[1-RH (p_sv/p₀)]Φ_va }                   (5)

Where

RH is the relative humidity of the humid air,

P_sv is the saturated vapor pressure of water,

p₀ is the total atmospheric pressure,

λ_v is the thermal conductivity of water vapor,

Φ_av is the interaction parameter between air and water vapor and

Φ_va is the molecular interaction parameter between water vapor and air.

The saturated vapor pressure of water was expressed as

Psv = E₀ + E₁ t + E₂ t ² + E₃ t ³ + E₄ t ⁴                                          (6)

Where E₀ = 0.7073034146, E₁ = 2.703615165 x 10², E₂ = 4.36088211 x 10³,

E₃ = 4.662575642 x 10⁵ and E₄ = 1.034693708 x 10⁶.

The molecular interaction parameters were expressed as

Φ_av = 2^1/2 (1 + M_a/M_v)^-1/2[ 1 +(μ_a/μ_v)^1/2(M_v/M_a)^1/4 ]²                       (7)

Φ_va = 2^1/2 (1 + M_v/M_a)^-1/2[ 1 +(μ_v/μ_a)^1/2(M_a/M_v)^1/4 ]²                       (8)

Where M_a and M_v are the molecular weights of air and water vapor, μ_a and μ_v are the viscosity of the air and water vapor.

The viscosity of dry air and water vapor are offered by the following correlations

μ_a = MA₀ + MA₁ T+ MA₂  T²+ MA₃ T³+ MA₄ T⁴                            (9)

Where MA₀ =-9.8601 x 10^-1, MA₁= 9.080125 x 10^-2, MA₂ = -1.17635575 x 10^-4,

MA₃ =1.2349703 x 10^-7and MA₄ = -5.7971299 x 10^-11.

μ_v = MV₀ + MV₁T                                                                          (10)

Where MV₀ = 8.058131868 x 10¹ and MV₁ = 4.000549451 x 10^-1.

The thermal conductivity of water vapor was expressed as

λ_v = KV₀ + KV₁ T + KV₂ T²                                                             (11)

Where KV₀ = 1.761758242 x 10¹, KV₁ = 5.558941059 x 10^-2 and KV₂ = 1.663336663 x 10^-4.

The unknown parameters in the equation (5) are the relative humidity RH and the temperature T which are commonly measured by using a relative humidity sensor and a temperature sensor. Then the concentration of the carbon dioxide gas in the humid air can also be determined based on the date provided by the POSIFA’s thermopile thermal conductivity sensors. 

Optimized Design of Water Laminar Flow Sensor Tubes

Xiang Zheng Tu

POSIFA’s water laminar flow sensors use the thermal properties of water to measure the flow of water flowing in a tube. The water laminar flow sensors are structured to comprise of a thermal insulated base created in a silicon substrate, a long stripe polysilicon resistor as a heater disposed at the central region of the base, and two thermopiles with their hot junctions along the two sides of the heater and cold junctions disposed on the base surrounding region of the silicon substrate. A constant amount of heat is applied to the heater of the sensor. Some of this heat is lost to the flowing water. As flow rate increases, more heat is lost. The amount of heat lost is sensed by the thermopiles of the sensor. The output signal of the thermopiles is used to determine water flow rate based on the convection heat of the water flowing over the sensor.

A sensor chip is assumed to be mounted on the wall of a tube in such way that the water flowing perpendicularly to the long stripe polysilicon resistor and one thermopile is located up stream and the other thermopile is located down stream. The water flow is required to be completely developed which means the laminar flow can be considered as the relative motion of a set of concentric cylinders of fluid, the outside one fixed at the surface of the sensor chip and the others moving at increasing speeds as the centre of the tube is approached. The resulted forced convection heat transfer can be described by Newton’s Law of Cooling as

 Ǭ = hA(T_s – T_f )                               (1)

The rate of heat Ǭ transferred to the surrounding water is propotional to the sensor chip exposed area A, and the difference between the polysilicon resistor surface temperature T_s and the water free stream temperature T_w. The constant of proportionality h is termed the convection heat transfer coefficient, which is given by:

h = N_u κ_w / l                                      (2)

    

Where l is the characteristic length, Nu is the Nusselt number and κ_w is the thermal conductivity coefficient of water. l is the effective diameter of the tube which is defined as:

l = 4A/P                                             (3)

with A the flow cross sectional area, and P the perimeter, respectively. The Nusselt number has been calculated by Gianchandan et al as

N_u = 0.664 Re^1/2 Pr^1/3                          (4)

Where Re is Reynolds number and Pr is Prandtl number. Since Re= lv/υ and Pr = υ/α, the equation (4) can be expressed as

N_u = 0.664(lv/υ)^1/2(υ/α)^1/3                    (5)

Where v is the average velocity of water flow, υ is the kinematic viscosity of water and α is the thermal diffusivity of water.

 Replacing equations (2) and (5) into equation (1) results in the expression as

Ǭ = 0.664(lv/υ)^1/2(υ/α)^1/3A(T_s–T_f )/l     (6)                 

In this expression Ǭ can be measured by the thermopiles of the sensor as output voltage, v is required to be determined by the measured output voltage and all other parameters can be obtained from available physical and chemical data base.

A laminar water flow needs some length of tube to fully develop the velocity profile after passing through components like bends, valves, pumps, and turbines or similar. The entrance length can be expressed with the dimensionless Entrance Length Number as

El = le / d                                              (7)

Where

El = Entrance Length number

L_e = length to fully developed velocity profile (m, ft)

d = tube or duct diameter (m, ft)

The Entrance length number correlates with the Reynolds Number  and for laminar flow the relation can be expressed as:

El_laminar = 0.06 Re                                  (8)

The water flow is laminar when R_e < 2300. Reynolds Number and Entrance Length for one liter of water at approximately 20⁰ C flowing through tubes of different dimensions:

Note that the water viscosity varies with temperature. The kinematic viscosity of water at 20⁰C used to calculate the table above is 1.004 x10⁶m²/s.

At 0⁰C the kinematic viscosity is 1.787 x 10^-6m²/s the Reynolds values in the table above must be multiplicated with 1.004/1.787 = 0.56. At 100⁰C the kinematic viscosity is 0.29x10^-6m₂/s the values in the table above must be multiplicated with 1.004/0.29 =3.46.

As shown in the table above, the long entrance lengths are not accepted for the most applications of the POSIFA’s water laminar flow sensors, such as coffee makers and drinking water dispensers. In order to short the entrance lengths a laminar flow device (element) is required to place in the entrance region of the tube. The laminar flow device creates the flow of water to be laminar or restricts the water to be flow as laminar flow before flowing into the laminar flow developed region of the tube. The device typically utilizes a material which has randomly-arranged Capillaries for dividing the velocity components of the incoming fluid stream into smaller components. Some of the velocity components cancel each other thereby presenting a more uniform velocity profile, reducing the turbulence of the fluid, and allowing laminar flow at higher flow rates than would otherwise be possible.

The developed laminar flow region of the tube comprises a plurality of narrow passageways along the flow path. The flow sensor is incorporated one of the narrow passageways. It is proffered that the narrow passageways have two plane parallel surfaces where the width is much greater than the space between the plates than the characteristic dimension is equal to the distance between the plates. In this way the main flow is split among all of them obtaining, as a result, a reduced Reynolds number. To help in this reduction, very often the sum of the cross-sectional area of all capillaries is larger than the main tube cross-sectional area.

To facilitate measurement and control of larger flow rates, a bypass version of the water flow sensor was developed. The bypass flow sensor is comprised of a capillary sensor tube connected to the main flow tube as a shunt line. The sensor tube usually has inside diameter less than 3 mm so that the sensor is operated in the laminar flow region over its full operating range. The flow in the main tube is inferred by measuring the flow in a small bypass tube using the flow sensor. The main flow in the large tube can be estimated from the previously determined ratio of main flow to bypass flow. The diameter of the main tube can be as larger as 20 or 30 mm. In these cases the water flow will transition from laminar to turbulent flow. In turbulent flow the speed of the fluid at a point is continuously undergoing changes in both magnitude and direction but the average velocity is still in flow direction. 

Anti - Sound Wave Interference Thermal MEMS Motion Sensors

Xiang Zheng Tu

  

It has been reported that a research team of University of Michigan used a $5 speaker and precisely tuned acoustic tones to deceive 15 different models of accelerometers into registering movement that never occurred. The approach served as a backdoor into the devices - enabling the team to control other aspects of the system. This research calls into question the longstanding computer science belief that software can automatically trust hardware sensors, which feed autonomous systems with fundamental data they need to make decisions.

In this research the accelerometers are capacitive MEMS devices which are typically structured with a diaphragm acting as a mass that undergoes flexure in the presence of acceleration. As shown in the above figure two fixed plates sandwich the diaphragm, creating two capacitors, each with an individual fixed plate and each sharing the diaphragm as a movable plate. The flexure causes a capacitance shift by altering the distance between two parallel plates, the diaphragm itself being one of the plates. Under zero net force the two capacitors are equal but a change in force will cause the moveable plate to shift closer to one of the fixed plates, increasing the capacitance, and further away from the other fixed reducing that capacitance. This difference in capacitance is detected and amplified to produce a voltage proportional to the acceleration. The dimensions of the structure are of the order of microns.

It is not surprise that the diaphragm of the accelerometer is also sensitive to acoustic pressure and works like a capacitive MEMS microphone. A capacitive microphone is commonly formed by a movable membrane and a rigid back plate, forming a structure with a plate capacitor. The movable membrane responds and changes its position when the acoustic pressure hit its surface, producing a capacitance variation between the back plate and the membrane, which in turns produces a current flow proportional to the distance variation between the membrane and the back plate.

There is no essence difference between these two capacitive MEMS devices. It is true that said Kevin Fu, U-M associate professor of computer science and engineering, the fundamental physics of the hardware allowed us to trick capacitive accelerometers into delivering a false reality to the microprocessor. And their findings resonantly upend widely held assumptions about the security of the underlying hardware.

However POSIFA’s thermal MEMS motion sensors can make these things total different. Using the POSIFA’s thermal MEMS motion sensors sound waves are no longer allowed to hack everything from phones to fitness trackers. Reference to the above figure a POSIFA’s thermal motion sensor comprises a thermal isolated plate created in a silicon substrate, a resistive heater, and two thermopiles both are formed on the surface of the plate. The laws of physics teaches that the temperature field generated by a moving heat source is asymmetry and able to be measured. In steady state, the vertical cross-sectional temperature field is a sequence of symmetry concentric circles each representing an isotherm on the lateral plane. When the heat source moves the vertical cross-sectional temperature field will be skewed towards down motion direction. The skewed lateral cross-sectional temperature field consists of a contracted half plane and an expended half plane both are divided by a line perpendicular to the motion direction. Since two thermopiles sensors are placed on the plane around the heat source, all isotherms can be reconstructed. A lot of useful information including the direction and velocity of the moving heat source can be extracted form the reconstructed plane isotherms.

Acceleration is used to measure the change in velocity, or speed divided by time. For example, a car accelerating from a standstill to 60 mph in six seconds is determined to have an acceleration of 10 mph per second (60 divided by 6). So with several accelerometers on your smart phone, you can determine if the smart phone is moving uphill, whether it will fall over if it tilts any more, or whether it’s flying horizontally or angling downward. And you know how to tilt your smart phone it can rotate their display between portrait and landscape mode accordingly.

The thermopile flow sensors can replace capacitive accelerometers for measuring the speeds of any moving objects including smart phones. The working principle is based on the fact that a moving object experiences an apparent wind that is the wind in relation to the moving object. Suppose the object is a riding bicycle on a day when there is no wind. Although the wind speed is zero, the rider will feel a breeze on the bicycle due to the bicycle is moving through the air. This is the apparent wind. On the windless day, the measured apparent wind will always be directly in front and equal in speed to the speed of the bicycle. It is very clear that it is impossible for the thermal motion sensors to response sound wave because there is no sound wave sensing mechanism to take place. 

Thermopile Temperature Sensors

Xiang Zheng Tu

POSIFA’s thermopile sensors are based on the technologies of silicon micromachining and CMOS manufacturing. In the thermal sensor fabrication a part of silicon substrate is removed by a porous silicon etching process, leaving on top only a thin sandwich layer membrane of PECVD SiO₂/Si₃N₄, which has low thermal conductivity. On the membrane a resistor and two thermopiles are formed there which are used as the main device elements of the sensor. The resistor is located along the central line of the membrane and made of a deposited thin polysilicon film. Two thermopiles are located on the two opposite sides of the resistor respectively. The thermopiles have alternate hot junctions disposed near the resistor and alternate cold junctions expending out of the membrane and ended on the bulk part of the silicon substrate. The junctions are formed by both a deposited thin polysilicon film and a deposited thin aluminum film.

The thermopile sensors can be used as a lot of different thermal sensors with a little modification which include fluid flow sensors, gas thermal conductivity sensors, vacuum sensors and temperature sensors. In working of the temperature sensors the polysilicon resistor is used as a heater and one thermopile is used for temperature different sensing.

Since the membrane is heated by the heater the temperature different generates between the membrane and the bulk part of the silicon substrate. It is need to know that the low thermal conductivity of the membrane is beneficial to the temperature different maintain.

The power dissipated in the polysilicon resistor causes its temperature to rise above a room temperature by:

T_PR-T_room=(V²/R_PR)θ                                       (1)

Where

T_PR=the raised temperature of polysilicon resistor due to internal power dissipation

T_room  =the reference temperature of polysilicon resistor

V=the driving voltage of polysilicon resistor in V

R_PR=the value of the polysilicon resistor in ohms at T_PR; and

θ=the self-heating polysilicon resistor ting effect in °C/mW.

For an ideal thermocouple, the open-circuit voltage obtained is proportional to the temperature difference between the hot junctions and cold junctions which are constructed of polysilicon film and aluminum film,

  

∆V=S(T_PR-T_room)                                                (2)

where S is the relative Seebeck coefficient, expressed in µV/K. The relative Seebeck coefficient of a junction can be calculated as the absolute value of the each Seebeck coefficient of polysilicon and aluminum; that is,  

S=S_Poly−S_Al                                                                      (3)

Because a voltage is produced when a temperature difference exists between the two junctions of the thermocouple junction pair, the thermocouple can be used as a detector of incident radiation. In open-circuit operation the output voltage produced is usually low, on the order of a tenth of a microvolt per degree celsius of temperature difference for a single junction pair. In order to increase the output voltage, several junction pairs may be connected in series. The responsivity is then increased by n if n thermocouple junction pairs are placed in series; that is,

∆V=n(S_Poly−S_Al) (T_PR-T_room)                                  (4)

Such a device is called a thermopile.

Combining equation (1) and (5) results the equation as:

∆V=n(S_Poly−S_Al ) (V²/R_PR)θ                                     (5)

The resistance of a polysilicon resistor is specified at a room temperature, any other resistance at another temperature is determined by:

R_PR=R_room[1+α(T_PR-T_room)]                                     (6)

Where
R_room=the resistance at a room temperature in ohm
α=the temperature coefficient of the resistance

After replacing equation (6) into equation (5) an equation is obtained as

∆V=n(S_Poly−S_Al ) V²θ / R_room[1+α(T_PR-T_room)]        (7)

From the equation (7) it can be seen that the output voltage ∆V of the thermopile is inversely proportional to the resistance R_room at a room temperature.

An example of a resistance and temperature coefficient of polysilicon resistors with temperature are shown in the following figure.

 

These polysilicon resistors were made of a 700 nm thick polysilicon films that were grown at 580^◦C in furnace by low-pressure chemical vapor deposition (LPCVD). The polysilicon films were partially implanted by boron (p-type) and phosphorus (n-type) from 4×10¹⁵ to 10×10¹⁵ at cm⁻² doses. The implant energy for each group doses was 70-80kev. Afterward, the doped polysilicon films were annealed in furnace at 1000^◦C for 30 min to activate dopants and obtain a uniform doping profile through the whole thickness and repair the defects in the crystalline structure.

As shown in the above figure the resistance of the polysilicon resistor varies with temperature. If the room temperature T_room rises from 15⁰C to 30⁰C the resistance will change about 200Ω for a polysilicon resistor with 15800Ω at 20⁰C. Since the output voltage ΔV of the thermopile is inversely proportional to the resistance of the polysilicon resistor (as a heater) a specified room temperature can be determined from the measured output voltage of the thermopile.

Off cause polysilicon resistors without thermopiles can also be used as temperature sensors. But common types of resistor temperature sensors are made from platinum instead of polysilicon. Platinum has temperature coefficient α = 0.003925 Ω/(Ω·°C) and polysilicon with sheet resistance ranging from 25 to 150Ω/□ has temperature coefficient α =1x10^-3 Ω/(Ω·°C). So the sensitivity of the platinum resistor temperature sensors is much higher than the polysilicon resistor temperature sensors.

Thanks to CMOS manufacturing technology it allows POSIFA to integrate up to 40 pair of thermocouples in each thermopile. That means the responsivity of the thermopile temperature sensors is increased by 40 which is comparable with platinum resistor temperature sensors.

There are three additional advantages that justify the use of MEMS and CMOS manufacturing technologies.

1. CMOS offer the thermopile structure materials with higher Seebeck coefficient.

2. CMOS beneficial to tune the main characteristics of the thermopiles by doping.

3. MEMS allow the thermal capacity of the thermopile to be reduced effectively by miniaturization.

Thus POSIFA’s thermopile temperature sensors have the best potential to satisfy the demands on miniaturization and mass production. 

Smart Air Supply Anti-Haze masks

Xiang Zheng Tu 

A smart air supply anti-haze mask, as shown in the above figure, comprises a sealed mask, a pollution filter, a micro-electric fan and a POSIFA thermal flow sensor based microcontroller. The mask is sealed to the face during inhalation and creates a breathing space by resting far away from the face. Two one - way valves are connected to the mask which are used to direct air flow in and out respectively. The pollution filter is made of multiple porous membranes and blocks against haze PM2.5 particles in the suctioned air.

The micro-electric fan moves enough filtered air to the mask through the in air flow valve.

The air is required to deliver to the mask according to an air flow waveform that is restored in the microcontroller. In order to do so the micro-electric fan is driven by a PWM signal that is send from the microcontroller. The PWM signal is generated by modulating an air flow rate signal measured by a thermal flow sensor. The thermal flow sensor can be installed in two ways. One is installed on the bask surface of the filter. In the first way the air flow rate is measured by the sensor immediately after passing the filter. In the secondly way the air flow rate is measured by the sensor immediately after the fan blowing.

In the second way the thermal flow sensor is installed in a laminar flow restrictor. Reference to the above figure, the restrictor is positioned in an air flow tube that is located between the micro-electric fan and the in one-way valve. The micro-electric fan produces a turbulent flow to the air flow tube. The restrictor consists of a plurality of collimated channels which are used in dividing the velocity components of the incoming flow stream into smaller components. Some of the velocity components cancel each other thereby presenting a more uniform velocity profile, reducing the turbulence of the flow, and allowing laminar flow passing through the channels.

As well known, laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion. For air flow in a channel, the Reynolds number is defined as

Re = (ρvD_H)/μ  = (QD_H) / (NυA)                        (1)

where:

Re is 2300 for air flow.

D_H is the hydraulic diameter of the channels (m).

Q is the volumetric air flow rate (m³/s).

A is the channel's cross-sectional area (m²).

N is the number of channels.

v is the mean velocity of air flow (m/s).

μ is the dynamic viscosity of air equaling to 1.983x10^-5 Pa·s.

ν is the kinematic viscosity of air equaling to 15.11x10^-6m²/s.

ρ is the density air equaling to 1.2754 kg/m³

The hydraulic diameter of the channels can be found by

D_H = 4A/P                                                             (2)

Where A is the cross-sectional area of the channel and P is the total perimeter of all channel walls that contact with the air flow. It should be noted that the length of the channel exposed to the flow is not included in the Reynolds number.

In the second way the air flow passes through a porous material such as a pollution filter. In this case Darcy‟s law is applicable which is stated as

Q = - (κAΔp) / (μL)                                         (3)

Where:

Q (m³/s) is the total discharge,

κ (m²) is the intrinsic permeability of the porous material,

A (m²) is the cross-sectional area to flow,

Δp (Pa) is the total pressure drop,

μ (Pa·s)  is the viscosity, and

L (m) is the length. 

Darcy‟s law is only applied for Re < 1, although it is sometimes not easy to define the pore diameter in a stringent way. Darcy’s law assumes laminar or viscous flow (creep velocity) and it does not involve the inertia term. Darcy’s law also assumes that in a porous material a large surface area is exposed to flow, hence the viscous resistance will greatly exceed acceleration forces in the flow.

So for a smart air supply anti-haze mask the thermal flow sensor is not necessary to install in a laminar flow restrictor. Instead it may directly install on the surface of the filter of the mask because the out flow of the filter is an air laminar flow. The laminar flow tends to flow without lateral mixing, and adjacent layers slide past one another. There are no cross-currents perpendicular to the direction of flow, nor eddies or swirls of flows.

As shown in the Darcy‟s law the total pressure drop (Δp) represents the viscous resistance to the flow. That is why the air flow is reduced while one wears a normal mask. There is no doubt that you want to protect you from air pollution and guard your health you will sacrifice some comfort or even though feel overly suffocating. But when wear a smart air supply anti-haze mask you will feel comfortable as usual and absolutely nothing will happen, because there is a micro electric-fan that can supply enough filtered air to you.

High Reliability of  POSIFA’s Thermal Water Flow Sensors

Xiang Zheeng Tu

 

A POSIFA thermal water flow sensor is fabricated in a silicon substrate. A combination of a heater and two thermopiles is used as sensing element of the sensor. A porous silicon layer is formed in the substrate for thermal insulation between the sensing element and the substrate, while the top layer is made of a SiO₂/Si₃N₄ stack thin film. The mechanism of water flow detection mainly depends on measuring the change in the electrical voltage of the thermopiles, associated with the heat convection transfer caused by the water flow.  In operation the sensor is heated by applying an electric voltage pulse to the heater. The pulse can be rectangular with pulse width 20ms, repeat frequency 1Hz resulting in 1.8mw power consumption. It has been measured that the Instantaneous peak temperature of the sensor is lower than 50⁰C.

It has been proved that Arrhenius' equation can be used for calculation of the failure rate of a semiconductor. The equation is expressed as

L = A exp (Ea / k T)                                                  (1)

Where

L is the lifetime of a semiconductor device

T is the absolute temperature (in kelvin)

A is the pre-exponential factor, a constant for each semiconductor device

Ea is the activation energy for each failure mechanism (in Joules mol-1)

R is the universal gas constant

Activation energy refers to the minimum amount of energy required to trigger a temperature-accelerated failure mechanism. The following table shows some activation energy values obtained for various failure mechanisms commonly encountered in the semiconductor devices.

Failure Mechanism	Accelerating Factors	Activation Energy
Oxide Film Defect	Electric Field, Temperature	0.3- 1.1 eV
Al Wire Corrosion	Humidity, Temperature, Voltage	0.7 - 0.9 eV
Al Wire Electromigration	Temperature, Current Density	0.5 – 0.7 eV

Since the sensors normally driven with electric voltage pulses the mean current density is very low. So Al wire electromigration for the sensor failure can be ignored and the main failure mechanism is Al wire correction. If voltage is applied, the leakage current between Al conductors will be added as a factor for Al corrosion. Al corrosion reaction proceeds as follows:

(a) Reaction on anode side

Under the normal ambient conditions, since the surface of “Al” is covered with oxide film, “Al” is in the passive state and exists stably. At the bias voltage application status, if the surface of the anode side adsorbs the Cl- ions diffused from the inside of the sealed resin, the Al wire protected by the passive state gibbsite may react and finally melt as:

At first, the hydroxide on the surface reacts with the Cl- ions to generate fusible salt.

Al(OH)3 + Cl- → Al(OH)2Cl + OH-                          (2)

The substrate Al exposed by this reaction reacts with the Cl- ions.

Al + 4Cl- → AlCl4 - + 3e-                                          (3)

In addition, when the sealed resin absorbs moisture, reaction with the moisture may start.

AlCl4 - + 3H2O → Al(OH)3 + 3H+ + 4Cl-              (4)

Finally Al(OH)₃ will be generated. Different from the protective oxide film, the generated Al(OH)₃ is not soluble, but has a high enough cubic expansion rate to cause cracking on the protective oxide film. So the generated Al(OH)₃ promotes corrosion.

(b) Reaction on cathode side

As the sealed resin absorbs moisture, the hydroxide ion concentration will be increased near the electrode due to oxygen reduction by application of bias and reaction generates hydrogen as

O2 + 2H2O + 4e- → 4(OH)-                                   (5)

H2O + e- → (OH)- + (1/2)H2                                 (6)

The OH- ions generated by the above reaction are diffused from the defect such as pinhole, void, crack, etc. on the Al protective oxide film to the substrate Al, and then react as:

Al + 3(OH)- → Al(OH)3 + 3e-                                (7)

The reaction on the cathode side also generates aluminum hydroxide.

The graph in above figure shows the relationship between the lifetime and the operation temperature of semiconductor devices. The red slash line is the activity energy of 0.7 eV, which represents Al wire corrosion mechanism and the red vertical line represents the typical operation temperature of the water thermal flow sensors. This means that the lifetime of the sensors is expected to be very high when compare with other semiconductor devices which need to be operated at least at 125⁰C.

Optical Coherence Topography with Tunable Cavity Surface Emitting Laser

Tu Xiang Zheng

  

US Patent 6,602,427 issued to the present author describes a micromachined optical mechanical modulator based WDM transmitter/receiver module. The Fabry-Perot cavity of the mechanical modulator is structured from a three-polysilicon-layer stack formed on the surface of a single crystalline silicon substrate. The polysilicon membrane and its supporting polysilicon beams of the cavity are cut from the top polysilicon layer of the stack and are released by selective etching of their underlying polysilicon. The etched underlying polysilicon layer is heavily doped and then converted into porous polysilicon by anodization in HF solution. The polysilicon membrane and its supporting polysilicon are finally released using a reactive ion etch process to avoid stiction often generated in a wet etch process. A conic hole is formed on the backside of the single crystalline silicon substrate for receiving an optical fiber that can be passively aligned with the Fabry-Perot cavity.

Optical coherence tomography (OCT) is a non-invasive imaging test that uses light waves to take cross-section pictures of your retina, the light-sensitive tissue lining the back of the eye. With OCT, each of the retina’s distinctive layers can be seen, allowing your ophthalmologist to map and measure their thickness. These measurements help with diagnosis and provide treatment guidance for glaucoma and retinal diseases, such as age-related macular degeneration and diabetic eye disease. OCT can also be used for intravascular imaging of plaque to assess heart disease, cancer biopsy imaging, developmental biology research, art preservation, and industrial inspection.

As shown in the above figure, a called swept-source OCT uses a wavelength-swept laser light source, that is, one whose emission sweeps back and forth across a range of wavelengths. A detector and a high speed analog-to-digital (A/D) converter complete the imaging system. The OCT has several fundamental advantages including ultrahigh imaging speeds, deep tissue penetration, Doppler OCT flow analysis, and long imaging range. With such a compact, high-performance, low-cost swept source for OCT it is possible to achieve a combination of ultrahigh sweep speeds, wide spectral tuning range, adjustability in sweep trajectory, and extremely long coherence length.

Wavelength tuning of the micromachined cavity is accomplished by applying a voltage between the top membrane and bottom membrane, across the air gap. A reverse bias voltage is used to provide the electrostatic force, which attracts the top membrane downward to the bottom membrane and shortens the air gap, thus tuning the laser wavelength toward a shorter wavelength (blue shift). It has been shown that the cavity using electrostatic force follows a 1/3 gap size rule. As the voltage is applied, the top membrane is attracted downwards with a displacement approximately equaling to 1/3 gap size. As increases further, the attractive force cannot be balanced by the mechanical spring force, and the membrane collapse onto the bottom membrane. Increasing voltage further at this point results either no movement or capacitor discharge. The top membrane can be brought back to its original position when the voltage is removed if an appropriate mechanical design is used.

The incident light to the micromachined cavity is emitted by a vertical cavity surface emitting laser. The micromachined cavity transmits a narrow band of wavelengths and rejects wavelengths outside of that band. The cavity will resonate when the following condition is met:

nd cosθ = mλ/2                         (1)

where θ is the incident light angle normal to the mirror, λ is wavelength, d is the micromachined cavity length, n is the refractive index of the medium, and m is the fringe order number. For normal incident light, with air as the medium (n = 1), the resonating micromachined cavity equals multiples of a half wavelength.

By driving the micromachined cavity with specially shaped voltage, the wavelength can be swept in time as required for swept source OCT. In classical physics, where the speeds of the top membrane of the micromachined cavity relative to the bottom membrane are lower than the velocity of laser light, the relationship between observed micromachined cavity transmitted light frequency f and the incident light frequency f₀ is expressed as

f = [(c+υ_r)/(c + υ_s)] *f₀                     (2)

  

Where c is the velocity of light, υ_r is the velocity of the top membrane relative to bottom membrane or air and υ_s is the velocity of the incident light relative to air. It can be seem that the transmitted light frequency or wavelength is decreased if two membranes of the cavity is moving away from the other.

It has been reported that the micromachined cavity can be move very fast, allowing the micron-scale cavity length to be tuned rapidly. It has demonstrated a fundamental repetition rate of 600 kHz, which for OCT purposes allows its individual scans to be acquired at rates as high as 1.2 MHz through the use of both forwards and backwards sweeps.