Smell Liquid MEMS Vaporizers

Tu Xiang Zheng

Smell Liquid MEMS Vaporizers are used for digital smell technology which senses transmits and receives scent-enabled digital media, such as web pages, video games, movies and music. A vaporizer mainly composes: a silicon substrate, a micro-channel array, a membrane suspending over the micro-channel array and supported by the silicon substrate, a resistance heater and a resistance temperature sensor are disposed on the membrane. Since the vaporizer is a silicon-based integrated actuator which provides advantages including small size, compact structure, lower power consumption, lower cost, increased reliability, higher precision, and more environmental friendliness.

Smell consists of many different molecules, e.g.: the aroma of coffee is made up of 20 various molecules. Nonetheless our nose perceives only 15 molecules which are enough to identify the smell as coffee. The physiology of smell in humans begins in the nasal cavity. There, a huge number of receptors (over 40 million) are located in the upper roof of the cavity. When the molecules reach the receptors, an electric impulse is sent directly to the brain and establishes a direct connection between our sense of smelling and our brain.

Smell is the sense closest linked to memory. Studies have shown that people can remember a scent with 65% accuracy after 1 year while visual memory sinks to 50% after only a few months.  The smells we experience play a crucial role in how we associate with memories and places. Have you ever come across a whiff of something that instantly takes you back to an old memory? Whether it reminds you of your mother’s cooking or a childhood trip to the ocean, a distinctive scent sinks into your brain and stays there.

Smells can evoke powerful emotions. The perfume industry is built around this connection, with perfumers developing fragrances that seek to convey a vast array of emotions and feelings; from desire to power, vitality to relaxation. It is likely that much of our emotional response to smell is governed by association, something which is borne out by the fact that different people can have completely different perceptions of the same smell. Take perfume for example; one person may find a particular brand ‘powerful’, ‘aromatic’ and ‘heady’, with another describing it as ‘overpowering’, ‘sickly’ and ‘nauseating’. Despite this, however, there are certain smells that all humans find repugnant, largely because they warn us of danger; the smell of smoke, for example, or of rotten food.  

Healthcare professionals are aware of the powerful impact of scent on patient feelings of well being. Studies have shown that scent can be used in many applications to positively affect the behaviors and emotions of patients, family, caretakers, and healthcare staff.

Vanilla, lavender and neutralizing scents (“Pure”) are popular recommendations. Vanilla can reduce claustrophobia in MRI facilities, calm pre-surgery and dental patients, and can reduce patient cancellations. Neutralizing unpleasant odors for patients with a heightened sense of smell will also soothe and comfort. Citrus uplifts and helps ease anxiety.

Clearly Better Scents offers custom fragrance development developed for the unique needs of your hospital, nursing home, doctor’s office, dentist’s office, outpatient surgery facility, laboratory physical therapy or chiropractic facility.

Senses of smell can even affect productivity in office environments. Specific smells have been found to increase alertness which in turn results in higher productivity rates. One study found that when lemon oil was diffused throughout a Japanese office building, productivity among data entry operators increased by 54%. Scents can also be used to ward off mid-afternoon brain fog by revving your concentration levels.

It is may be supplied that smells can influence our perception of time. In one of the studies, 20 separate participants were exposed to a baby powder aroma, a coffee aroma, and no aroma at all. While the coffee aroma produced a reduced perception of time, the baby powder aroma produced a longer perception of time. Likewise, pleasurable fragrances have been shown to create “dwell-time” in stores, increasing the likelihood of customers making purchases.

In summary with the smell liquid MEMS vaporizers the digital smell communication will soon influence your everyday life. 

Unknown dark matter and familiar positron annihilation

Xiang Zheng Tu 

  

I must admit that I knew nothing about dark matter, but I am familiar with positron annihilation. In 1986 I utilized positron annihilation measurement to study of vacancy defects in GaAs liquid phase epitaxial layers. In the following year the research result was published in “Journal of Applied Physics” which was titled as “Positron-annihilation study of vacancy defects in GaAs liquid-phase epitaxial layers”. The paper concluded that the defects observed to trap positrons in undoped GaAs liquid phase epitaxial layers are neutral arsenic vacancies. Systematic trends of the epitaxial growth temperature on positron lifetime are observed. The setup for the measurement is shown in the above figure. Positrons are emitted from a radioactive source. The positron is the antiparticle of the electron, and when a positron enters a GaAs liquid phase epitaxial layer, it will find abundant supply of electrons with which to annihilate. The energy release by the annihilation forms two highly energetic gamma rays, and if one assumes that the momenta of the positron and electron before the annihilation, the two gamma rays photos must in opposite directions in order to conserve momentum. These coincident gamma rays at 180 degrees provide a powerful tool for eliminating all gamma events which are not coincident at 180 degrees.

It is interesting to know that positron annihilation measurement has been used to find dark matter. Despite striking evidence for the existence of dark matter from astrophysical observations, dark matter has still escaped any direct or indirect detection until today. Therefore a proof for its existence and the revelation of its nature belongs to one of the most intriguing challenges of nowadays cosmology and particle physics. A lot of work has been done to investigate the nature of dark matter through indirect signatures from dark matter annihilation into electron-positron pairs. It is thought that the dark matter particles are thermal relics, then dark matter particles and antiparticles exist in equal amounts, and they could also annihilate or decay to standard model particles that can be detected. As a two-body process, the rate of annihilation is proportional to the square of the dark matter density, whereas single-body decay process is proportional to the dark matter density. The primary products of the annihilations or decays, i.e. cosmic ray protons, antiprotons, electrons, positrons, gamma-rays and neutrinos, could in principle be observed on or around the Earth, while secondary radiation like gamma-rays, and radio or microwaves from synchrotron could be detected.

Alpha Magnetic Spectrometer (AMS-02) is a powerful state-of-the-art particle physics detector. This detector was installed on the International Space Station and operated by an international team composed of 56 institutes from 16 countries and organized under United States Department of Energy (DOE) sponsorship. It has collected the antiproton-to-proton ratio stays constant which cannot be explained by the secondary antiprotons from collisions of ordinary cosmic rays with interstellar medium.  A new source such as astrophysical accelerators and annihilating or decaying dark matter was subjected.

Samuel C. C. Ting who awarded the Nobel Prize in Physics said that more high-energy positrons than expected are buzzing around the galaxy—has not impressed the doubters. That positron excess, which a European satellite found in the mid-2000s and the AMS confirmed, has sparked hundreds of theory papers connecting it to hypothetical dark matter particles. The mutual annihilation of those particles might create a half-and-half blend of electrons and positrons in a narrow energy range. The electrons would fade into a sea of electrons from other sources, but the rarer positrons might stand out. To Ting, the best explanation for the extra positrons is a dark matter particle with a mass of 1 million megavolts —about as much energy as a flying mosquito. 

Water Flow Sensors with Wide Range of laminar Flow Rate

Xiang Zheng Tu 

The thermal flow sensors provided by POSIFA Microsystems Company are based on the linear relationship between the fluid flow rate and temperature difference dependence voltage or thermoelectric effect. The criteria for the operation of the sensors are to create and maintain laminar flow through the sensitive surface of the sensors.

Osborne Reynolds in 1883 proposed a concept relating to fluid flow properties through pipes of different diameter as well as the determination of boundary layers where the transition between laminar flow to turbulent flow occurs. The regimes where laminar or turbulent flow prevails are prescribed by a dimensionless parameter known as the Reynolds number, defined as

{\displaystyle \mathrm {Re} ={\frac {\rho uL}{\mu }}={\frac {uL}{\nu }}}

R_e = ρ u L / μ = u L / υ      (1)

Where:

ρ is the density of the fluid (SI units: kg/m³)

u is the average velocity of the fluid with respect to the object (m/s)

L is a characteristic length (m)

μ is the dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/m·s)

ν is the kinematic viscosity of the fluid (m²/s).

The characteristic length can be calculated with the generic equation as

d_h = 4 A / p                            (2)

Where

d_h = hydraulic diameter (m)

A = area section of the duct or pipe (m2)

p = "wetted" perimeter of the duct or pipe (m)

Based on equation (2) the characteristic length of a circular duct can be expressed as:

d_h = 4 π r² / 2 π r  = 2 r                              (3)

Where

r = pipe or duct inside radius  (m)

d = pipe or duct inside diameter  (m)

As expect the hydraulic diameter of a standard circular tube or duct is the inner diameter or two times the inner radius.

Based on equation (2) the characteristic length of an annulus duct or tube with an inside duct or tube can be expressed as

dh = 4 (π r_o² - π r_i²) / (2 π r_o + 2 π r_i) = 2 (r_o - r_i)               (4)

Where

r_o = inside radius of the outside tube (m)

r_i = outside radius of the inside tube (m)

Based on equation (2) the characteristic length of a rectangular duct or pipe with very length width can be calculated as

d_h = 4 a b / 2 (a + b) = 2 a                                 (5)

Where

a = height of the duct (m)

b = width of the duct (m)

b >> a

Generally, laminar flow occurs at Reynolds numbers of less than 2,000, but in practice Reynolds numbers of less than 1,000 are used to ensure laminar flow under all conditions (e.g., viscosity and density variations with temperature).

As can be seen from the above equations:

• The laminar flow rate of the circular duct or tube is only limited by the Reynolds number.
• The laminar flow rate of the annular duct or tube can be extended by increasing the radius of inside tube and outside tube simultaneously without changing the Reynolds number.
• The laminar flow rate of the rectangular duct or tube can be extended by increasing the width of the rectangular duct or tube without changing the Reynolds number.

POSIFA Microsystems preferred rectangular duct or tube. The following water flow ranges are available:

• 0 – 10 mL / min for medical application
• 0 – 40 mL / min for medical application
• 0 – 120 mL / min for medical application
• 0 – 1000 mL / min for Coffee Makers
• 0 – 3000 mL / min for Water Dispensers
• 0 – 10 L / min for  Liquid Cooling CPU Systems

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.