Fabrication of a Micromachined Two-Dimensional

Fabrication of a Micromachined Two-Dimensional Wind Sensor by Au–Au Wafer Bonding Technology Ziqiang Dong,Jingjing Chen,Yukun Qin,Ming Qin,Member,IEEE,and Qing-An Huang,Senior Member,IEEE

Abstract—The design,fabrication,and performance of a micro-machined2-D wind sensor are presented.The sensor operates based on the detection of temperature and?ow-dependent heat distribution on a hot sensing surface.It consists of a silicon sensing chip and a ceramic packaging substrate,in which the sensing chip is bonded to the front side of the ceramic packaging substrate through wafer-level gold bumps.The backside of the ceramic sub-strate provides a smooth surface for the sensor exposed to the wind ?ow.A silicon diaphragm was fabricated by wet etching to mini-mize its heat capacity,resulting in the improvement of the power consumption,response time,and resolutions.Experimental results show that the measurement of wind?ow speed is demonstrated in the range from0.5to40m/s with the sensitivity more than 2.73mW/ms?1.The sensor requires only2mW initial heating power,and in constant-temperature difference mode,the response time less than1.4s is obtained.By measuring temperature differ-ence in two directions perpendicular to each other,the detection of direction in a full range of360?has been achieved.The errors in the measured wind speed and direction after calibration are±4% and±2?,respectively.[2011-0219] Index Terms—Gold–gold bonding,thermal wind sensor,wafer-level packaging.

I.I NTRODUCTION

S ILICON?ow sensors based on a thermal principle are widely studied for industrial process control,environ-mental monitoring,indoor air conditioning system,etc.Since 1974[1],many efforts have been made to fabricate advanced and well operating?ow sensor[2],[3].Solid-state silicon thermal?ow sensors based on microelectromechanical systems (MEMS)technology have undergone acceptance as products for more than10years,and been considered a promising technology because of many features such as compact size,low power dissipation,high sensitivity,and easy compensation for ambient temperature changes.In order to improve the sensi-tivity of the?ow sensor,micromembranes and bridges were fabricated using anisotropic etching of silicon substrate from either front side or backside[4]–[10].Even more,the thermal element of the sensor was elevated from the substrate in order to make it be away from the bottom of the velocity boundary layer, so it can experience greater?ow velocity and exhibit better sensitivity[11].

Manuscript received July23,2011;revised November20,2011;accepted November24,2011.Date of publication January9,2012;date of current version April4,2012.This work was supported by the National Natural Science Foundation of China under Grant No.61076071.Subject Editor G.Stemme. The authors are with the Key Laboratory of MEMS of the Ministry of Educa-tion,Southeast University,Nanjing210096,China(e-mail:hqa@https://www.360docs.net/doc/c614093550.html,). Color versions of one or more of the?gures in this paper are available online at https://www.360docs.net/doc/c614093550.html,.

Digital Object Identi?er

10.1109/JMEMS.2011.2179014

Fig.1.Schematic view of a sensor.(a)Velocity pro?le and temperature pro?le

above the designed sensor.(b)Heat transfer of a traditional thermal?ow sensor.

As shown in Fig.1,a schematic view of velocity pro?les

and temperature pro?les above a thermal?ow sensor,as well

as heat?ow,are given.The gray element,blue element,red

element,and dark red element represent the ceramic substrate,

electrical lead-out pad,heaters,and sensing parts,respectively.

The green arrows represent the heat convection effect,and the

pink arrows represent the heat conduction effect.To put the

sensor,particularly wind sensor which is generally exposed to

outdoors environment,into practical applications,packaging is

essential.For precision measurement,the surface of the sensor

must be smooth enough and has not any block around it.The

sensor also needs to be thermal contact with the air?ow but

against mechanical contact and undesired media.In view of

these points,ceramic is considered as a proper material for the

packaging of the thermal?ow sensors because of its moderate

heat conduction,smooth surface,and stable mechanical prop-

erties.One of the packaging types is that a silicon chip with

heaters and detectors was glued to one side of a thin ceramic

carrier,while the other side of the ceramic carrier was exposed

to the air?ow[12]–[14].If the chip is not placed in the center of

a ceramic carrier,however,it will distort the output signals.The

compensation of this thermal asymmetry has been studied[15].

Flip-chip technology is another method adopted in the packag-

ing of the thermal?ow sensors[16],in which the front side of a

sensor chip was?ip-chip mounted on the ceramic substrate,and

an opening window was aligned with the sensor that enables

free convection.The on-chip integrated circuit was protected in

this packaging,but the sensor portion was still exposed to the

air?ow.

1057-7157/$31.00?2012IEEE

Fig.2.Photographs of a thermal wind sensor fabricated directly on ceramic

substrate.

Fig.3.Resistance deviations of sensing parts fabricated on one ceramic wafer.

Our group makes a large effort to study the thermal wind sensors.We have adopted two packaging techniques.One is that the front side of the silicon wind sensor was ?ip-chip packaged on the ceramic substrate using a copper pillar bumps technology [17].The thermal interactions between the sensor and the air?ow were achieved via the copper pillar bumps and the thin ceramic substrate.However,the single die packaging and assemble process can cause nonideal offsets such as thermal asymmetry in the static circumstance.The other is that the sensor was fabricated directly on ceramic wafers [18],[19]or glass wafers [20].

Sensing parts of thermal wind sensors are the most important elements for the measurement of wind speed and wind direc-tion,which determine the performance of the thermal wind sensors,such as accuracy,sensitivity,and consistency.In the past few years,efforts were made for the design and fabrication of the thermal wind sensors on ceramic substrate directly.Unfortunately,the sensing parts are composed of both narrow and long resistive lines.The quality of meander pattering of the sensing parts fabricated on the ceramic substrate was not very good,as shown in Fig.2,and the consistency of the sensing parts was dif?cult to be controlled across the wafer.Two main kinds of deviations of the sensing parts fabricated directly on the ceramic substrate result from the resistance and its temperature coef?cient.

100sensing parts fabricated directly on one ceramic wafer were tested as the testing samples.Fig.3shows the resistance deviations of the 100sensing parts at room temperature.The resistance of the sensing parts is in the range of 320Ωto 404Ω,with 360Ωbeing designed value.The resistance deviation is in

the range from ?11.2%to +17.9%,and most of the resistance deviation of the sensing parts are in the range from ?10%to +10%.20samples from the 100sensing parts were randomly chosen for the average resistance-temperature coef?cient test.The results of thermal characteristic of the sensing parts are from ?30?C to +100?C.The average resistance-temperature coef?cient of the 20sensing parts is from ?30?C to +100?C.Results show that the maximum average resistance-temperature coef?cient of the sensing parts is 2556ppm /?C,and the min-imum one is 2323ppm /?C.Most of the average resistance-temperature coef?cient of the 20sensing parts are in the range from 2360ppm /?C to 2460ppm /?C.The deviation of the average resistance-temperature coef?cient of the sensing parts is about ±2.1%.The deviation of the resistance and its tempera-ture coef?cient will cause the inconsistency of the wind sensors,which results in the low calibration ef?ciency.For example,for the 20sensing parts,if the heating power of the chip is constantly set to 100mW at the wind speed from 0m/s to 5m/s,the wind speed response sensitivity of the sensing parts is in the range from 3.1Ω/(m/s )to 3.7Ω/(m/s ).The deviation of the wind speed response sensitivity of the sensing parts reaches about ±9.7%.

Consequently,if the sensing parts are fabricated directly on ceramic substrate,unacceptable deviations of the sensing parts will be introduced.Inconsistency of the resistance and its temperature coef?cient greatly increases the calibration cost.Therefore,in order to improve the consistency of the wind sensors,a standard silicon process are used for the fabrication of sensing parts.

Generally,if both the sensing parts and heaters are fabricated on silicon substrate by using a standard IC fabrication process [9],wet etching is usually used to undercut the substrate so that heaters are fabricated on a suspended ?lm to minimum the heat capacity.In this case,however,the fragile structure is dif?cult to be packaged.Therefore,taking into account fabrication consis-tency,packaging reliability,and minimizing the heat capacity,a new type of thermal wind sensor with a two-part construction including a sensing chip and a packaging chip is reported in this paper.The sensor was fabricated by wafer-level packaging,thus the offsets caused by the packaging and assemble process can be eliminated.The heaters are fabricated on packaging chip (ceramic substrate)by using lift-off process,and the sensing parts are fabricated on sensing chip (silicon substrate)by using a standard IC process.Gold bumps were fabricated on both of the sensing chip and packaging chip.Wafer-level Au–Au bonding process was used to bond the sensing chip on the front side of the packaging chip.One kind of the gold bumps are used for thermal connection between sensing chip and packaging chip,and the other kind of gold bumps are used for electrical connection between them.In order to minimize the heat capacity of the sensor,wet etching process was used to etch the substrate of the sensing chip to fabricate it as a 46μm silicon diaphragm,resulting in the improvement of the power consumption,response time,and resolutions.In Section II,the design of the sensor is introduced.In Section III,the fabrication and packaging processes are presented.In Section IV,the experimental results as well as discussions are given.Finally,some conclusions are drawn.

DONG et al.:FABRICATION OF A MICROMACHINED TWO-DIMENSIONAL WIND SENSOR BY Au–Au WAFER BONDING TECHNOLOGY 469

II.S ENSOR D ESIGN

For a thermal wind ?ow sensor,the ?ow speed and direction can be measured by detecting temperature and ?ow-dependent heat distribution on its hot surface.Because the sensor to be designed measures air?ow by detecting transferred heat through the air,heat convection is the dominant factor of heat transfer.A.Power Dissipation

Heat radiation is too small to be considered when the temperature of a thermal ?ow sensor is lower than 300?C [21].The power dissipation of the sensor can be divided into P cond caused by heat conduction and P conv caused by heat convection,which is given by

P tot =P cond +P conv

(1)

where P tot is the total power dissipation of the chip.Following the boundary theory of a heated plate for a ?rst-order approxi-mation,it yields [21]

P cond ≈λL 2ΔT (2)P conv ≈hL 2ΔT

(3)

where ΔT is the overheat with respect to the air?ow,L is the chip size,λ=kc/D is the heat conduction coef?cient of the sensor,kc and D are the heat conductivity and the thickness of the ceramic substrate,respectively.The h is the convection coef?cient on the surface,which is given by [22]

h =0.664k f α?1/3ν?1/6L ?1/2U 1/2

(4)

where U ,ν,k f ,and αare the speed,the kinematic viscosity,

the heat conductivity,and the thermal diffusivity of the air?ow,respectively.Hence,P tot can be rewritten as

P tot = λL 2+0.664k f

L 3/2

α1/3ν1/6U 1/2 ΔT.(5)

It is shown that the thermal ?ow sensor obeys the King’s law in the traditional anemometer,i.e.,

P tot =(A +BU 1/2)ΔT

(6)

where A and B represent the constants depending on the chip geometry,the chip material properties,and the air?ow.B.Temperature Distribution

If a sensor chip is represented by a thin,square surface element with lateral dimensions L ×L and thickness D ,it is assumed to be thermally insulated from its substrate.There is no heat loss at the edges or the lower surface.If the value of D is much smaller than L ,the variation of the temperature across the thickness of the sensor can be omitted.When the overheat temperature is higher than the air?ow is constant (ΔT ),the value of the temperature difference (ΔT 0)between two ?xed points can be expressed as [23]

ΔT 0ΔT =c k f L k c D

(τw L 2/να)1/3G (φ)=F (U )G (φ)

(7)

Fig.4.(a)Design patterns.(b)The cross-sectional view of the fabricated sensor.(c)The left bump provides heat transfer,and the right bump provides electrical connection between the sensing chip and the packaging chip.

where c is the constant which equals to 0.08076when the temperature detectors located on the backside of the chip,τw ≈U 3/2,de?ned as the constant value of the wall shear stress τof the air?ow at the surface,G (φ)is the sensitivity function of ?ow direction,F (U )is the sensitivity function of ?ow speed,which is given by [23]

F (U )=

ΔT 0ΔT =c k f L k c D

(L 2/να)1/3√U.(8)

F (U )is proportional to L/D ,and inverse proportional to the

kc .Therefore,increasing chip size,reducing the thickness of chip and reducing lateral heat conductivity of the chip are the ways to enhance the performance of the sensor.

A 2-D direction sensitivity is obtained by measuring temper-ature difference in north-south direction (denoted as ΔT NS =ΔT 0sin Φ,corresponding to sensor output voltages V NS )and the temperature difference in east-west direction (denoted as ΔT EW =ΔT 0cos Φ,corresponding to sensor output voltages V EW ).The ?ow direction Φis independent of the ?ow speed and is determined by [23]

φ=tan ?1(V NS /V EW ).

(9)

C.Design Consideration

As shown in Fig.4,the sensor consists of a sensing chip (silicon)and a packaging chip (ceramic),in which the sensing chip is bonded to the front side of the packaging chip (ceramic)through wafer-level gold bumps.To reduce the effect of heat conduction,a silicon diaphragm is fabricated on the backside of the sensor by thinning out the silicon substrate so as to reduce its heat capacity.In addition,a silicon dioxide (SiO 2)layer,which has low heat conductivity,is formed on the silicon

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diaphragm.The heat conduction normal to the sensor may be reduced to some degree.Furthermore,the structure of the designed sensor is symmetrical,so that the heat conduction to all of the sensing parts is the same.The?ow passes over the backside of the ceramic packaging chip.The sensing chip makes thermal contact with the air?ow while still being pro-tected from direct mechanical contact.Four heaters,which are located on the backside of the ceramic plate,are used to heat the packaging chip to a constant temperature above the?ow, thus the asymmetrically cooling of?ow can induce a?ow-dependent heat distribution on its surface.The sensing chip includes ten sensing parts,two of which are located at the center and eight are located symmetrically around the center parts.The eight sensing parts form two Wheatstone full-bridges for the wind direction measurement,resulting in about50%increase of the sensitivity with respect to four sensing parts.A thermal insulation trench is located between the center pats and the others.Gold bumps are chosen here for thermal connection and electrical connection between the sensing chip and packaging chip.Three main features are considered.The?rst is that the bumps can be fabricated with different shapes and will not deform after bonded.The second is that heat transfer between the two chips can be guaranteed due to the good heat conduction properties of gold.The third is that the gold bumps can be directly fabricated on the passivation layer,which makes it pos-sible to fabricate simultaneously the thermal connection bumps (with passivation layer between the gold bumps and sensing parts)and electrical connection bumps(without passivation layer between the gold bumps and pads).

The speci?c two-part construction of the wind sensor results from the consistency of the sensing parts and the packaging reliability in batch production.Moreover,the heaters fabricated on the packaging chip will minimize the heat capacity of the sensor.Based on our previous simulation[20],all structural dimensions of the sensor have been obtained.

III.F ABRICATION AND P ACKAGING

A designed wind?ow sensor has four heaters on the pack-aging chip.A sensing chip consists of ten sensing parts on a silicon diaphragm.5μm thick gold bumps were fabricated on the packaging chip and sensing chip and then bonded together for thermal connection and electrical connection.The chip dimensions are6.0mm×6.0mm.The pattern widths of the heater and sensing parts are15μm and4μm,re-spectively.The silicon diaphragm thickness is46μm after processing.N-type,single-crystal(100)Si wafers(thickness= 400μm,diameter=100mm)were used for the fabrication of the designed sensing chip.Double-side,polished,ceramic wafers(thickness=300μm,diameter=100mm,?atness= 0.0127mm/mm,maximum surface?nish=0.0127μm.)were used for the fabrication of the designed packaging chip.

The fabrication process is shown in Fig.5.First,after clean-ing a silicon wafer,a1μm thick SiO2layer was thermally grown on the Si substrate through oxidation in order to reduce heat conduction effect.0.5μm thick aluminum was sputtered on the SiO2layer and patterned to form the thermistors as the sensing parts.Then,a passivation layer(0.1μm SiO2

+Fig.5.Fabrication process.(a)The sensing chip and the packaging chip.

(b)The wafer bonding process and TMAH etching.

0.7μm Si3N4)was made on the thermistors and patterned to form through-holes for electrical lead-out.An under bump metal(Au)was sputtered and pattered on the passivation layer, and then5μm thick gold bumps were fabricated on the silicon wafer using electroplating process,the SiO2layer acts as a blocking layer for the diffusion of the gold into the silicon substrate.Finally,dry etching process was used to remove the passivation layer,the SiO2layer,and silicon substrate so as to fabricate50μm deep trench in order to make a thermal insulation.The fabrication process of the packaging chip is as follows.3000?thick Pt was sputtered on a ceramic wafer for the fabrication of heaters.In order to improve Pt adhesion,Ti was used as an interlayer(1000?).Pt was patterned by a lift-off process.N2annealing was conducted at600?C after the lift-off process.Then,5μm thick gold bumps were fabricated on the ceramic wafer using electroplating process.The two wafers were bonded together using Au–Au bonding technology in a wafer bonding system(EVG-501)at2MPa,380?C,and30min for wafer-level packaging.The bonding area is7.4×10?4m2, and the tool force is1500N.Possible asymmetries may be introduced by the bonding.Because the sensors were fabricated by wafer-level Au–Au bonding,the asymmetries of the sensors across the wafer are the same.The asymmetries can be cali-brated simply through adjusting the heating power distribution on the chip.After the wafer-level bonding,none of defects and obviously bending of the bonded wafer has been found.Then,a silicon diaphragm was fabricated by TMAH etching at90?C to minimize heat capacity,and simultaneously resulting in the electrical feed-through in wafer-level packaging.The wet etching was performed on the backside of the bonded wafer, and a wafer holder was used to protect the rest of the structures during TMAH etching.For the protection of the sensing parts, a silicon-doped TMAH solution was adopted.

DONG et al.:FABRICATION OF A MICROMACHINED TWO-DIMENSIONAL WIND SENSOR BY Au–Au WAFER BONDING TECHNOLOGY

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Fig.6.Photograph of the sensing chip without thermal bumps fabricated on silicon substrate by using a standard IC

process.

Fig.7.Photographs of the fabricated wind sensor,sensing chip,packaging chip,and sensor chip after processing.

Fig.6shows a photograph of the sensing chip without ther-mal bumps fabricated on silicon substrate by using a standard IC process.The photographs of a sensing chip,a packaging chip,and a fabricated wind ?ow sensor are displayed in Fig.7.The thickness of the silicon diaphragm was measured to be 46μm by using a pro?le projector (Veeko NT1100).An air gap between the sensing chip and the packaging chip is about 10μm.The resistances of the fabricated heaters,the center sensing parts,and the other sensing parts are 400Ω,100Ω,and 166Ωat room temperature,respectively.Photographs are shown in Fig.8after wet etching,dicing,and an installed ?ow sensor on a PCB in order to connect to an outer electric circuit for tests.The wind ?ow sensor made use of direct chip attachment packaging [24],[25]to achieve the

electrical

Fig.8.Photographs after wet etching,dicing,and an installed ?ow sensor on a PCB.

connection with the PCB.A metal frame was glued to the FR4substrate for protection.

After the fabrication of the two-part construction thermal wind sensor,the consistency and deviation of resistance,average resistance-temperature coef?cient,and wind speed re-sponse sensitivity have been tested.It is found that the con-sistency of the sensing parts fabricated on silicon substrate by using a standard IC process has been improved greatly.The resistance deviations of the sensing parts have been con-trolled to less than ±0.5%.Its average resistance-temperature coef?cient is about 2300ppm /?C.The deviation of average resistance temperature coef?cient is about ±0.09%.By testing in a wind tunnel at the wind speed from 0m/s to 5m/s and at 100mW heating power,the wind speed response sensitivity of the sensing parts is about 3Ω/(m/s ),and its deviation is about ±1.7%.Table I shows the comparisons of the two-part construction with one-element approach.

IV .S ENSOR S YSTEM

Fig.9shows a block diagram of a wind sensor system for measuring ?ow speed and direction.The system consists of three parts:a sensor part,a detecting part for ?ow speed and direction,and a sampling and processing part.The circuits with low-offset instrument ampli?ers,Wheatstone-bridge circuits,and OP amps are designed to amplify and modify the output signals.A microprocessor is used to control the operation of the sensor system.The output analog signal is digitalized by the analog-digital converter.

When air?ow passes over the sensor,the heating power is increased to compensate the lost heat transferred to the air?ow,so the heating power is proportional to the ?ow speed.The constant-temperature difference (CTD)control circuit is shown in Fig.10,which is used to maintain the sensor at a con-stant temperature above ambient temperature.The CTD mode ensures that the heating power has a simple square-root law relationship with ?ow speed.An off-chip ambient temperature sensor (AD590)is used to compensate the in?uence of the ambient temperature.

When air?ow encounters the hot surface of the sensor,heat transfer from the surface to the air?ow is largest at the up-stream side.The upstream temperature will become slightly lower and the downstream temperature slightly higher than the average temperature of the chip.A small temperature

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TABLE I

C OMPARISONS OF C ONSISTENCY AN

D D EVIATIONS OF S ENSING P ARTS F ABRICATED ON C ERAMIC W AFER AND S ILICON W

AFER

Fig.9.Block diagram of the wind ?ow sensor

system.

Fig.10.Constant-temperature difference control circuit for power control and ?ow speed detection.

gradient (ΔT NS and ΔT EW )over the sensor is induced due to this nonuniform cooling effect.The temperature difference can be measured by detecting the eight sensing parts formed as two Wheatstone-bridge circuits.The temperature difference increases with the ?ow speed,and it is also sensitive to the ?ow direction.In calorimetric system,two instrument ampli?ers are used to amplify the output voltages (ΔV NS and ΔV EW )of two Wheatstone-bridges,which have a linear law relationship with the two orthogonal comments (ΔT NS and ΔT EW )of the ?ow-induced temperature gradient.According to (9),the ampli?ed voltage V NS and V EW can be used to calculate the

?ow direction.Moreover,the V 2NS

+V 2

EW has a square law relationship with ?ow speed.

V .R ESULTS AND D ISCUSSION

As shown in Fig.11,a fabricated sensor was installed in a laminar ?ow wind-tunnel covering ?ow velocity up to 40m/s and a full range of 360?for ?ow direction measurements.

At

Fig.11.

Experimental setup and the wind

tunnel.

Fig.12.Temperature characteristic of sensing parts on a silicon chip at the ambient temperature ranging from ?30?C to +60?C.

the location of the sensor,the cross-section area of the wind-tunnel is 25×25cm 2.The ?ow velocity was measured using a pitot tube,with accuracy better than 1.0%of the full scale in the range from 0.5to 40m/s.The temperature characteristics of sensing parts fabricated on a sensing chip are shown in Fig.12.The R detectors NSEW are located on the sensing chip around the R detectors C,and R detectors C are located in the center of the sensing chip.R detectors NSEW are used to detect the temperature difference between the upstream and downstream,and R detectors C are used to detect the center temperature of the chip.

Their temperature coef?cients are linear at the ambient tem-perature ranging from ?30?C to +60?C,which is about

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473

Fig.13.(a)Flow speed as a function of heating power at different overheat temperature in CTD mode.(b)Relationship of the heating power with the square root of?ow speed.ΔT is the overheat temperature and P0is the initial heating power.

2300ppm.The consistency of the temperature characteristics of these sensing parts indicates that the bonded gold bumps can ensure good thermal connection and electrical connection between the two chips.Therefore,Au–Au bonding technology may be considered as a promising method for electrical con-nections and thermal connections in the wafer-level packaging of the thermal?ow sensors.Fig.13shows the heating power as a function of?ow speed.Results show that the heating power keeps a linear relationship with the square root of?ow speed. The measurement of the?ow speed ranging from0.5to40m/s has an accuracy of±4%after calibration by using a look-up table.

When the overheat temperature is ranging from6?C to 45?C,the initial heating power will be ranging from2mW to85mW.With2mW initial heating power(overheat temperature=6?C),the sensor sensitivity for?ow speed in the CTD mode is better than2.73mW/ms?1for the full testing range.When the overheat temperature is ranging from6?C to45?C,the heating power is ranging from111.2mW to 452.6mW at the?ow speed of40m/s.The sensitivity can reach9mW/ms?1when the initial heating power is set to be 85mW.Fig.14shows the output voltage(the root of V2NS+ V2EW)of the calorimetric system,which operates in the CP (100mW constant heating power)and CTD(2mW initial heating power)mode,respectively,as a function of?ow

speed.Fig.14.Flow speed characteristic of calorimetric system operating in CP and CTD

modes.

Fig.15.Sensor response for?ow speed from5to10m/s and from10 to5m/s.

The output sensitivity in the CP mode is larger than that in the CTD mode at the?ow speed up to19.5m/s,while the output signals in the CP mode tend to saturate when the?ow speed is higher than20m/s.In the CTD mode,the output voltages keep the sensitivity(7.85mV/ms?1)for the?ow speed ranging from0.5to40m/s.Therefore,it is considered as a promising mode for?ow direction measurement in high?ow speed circumstances.

By detecting the output voltages of an ampli?er used to amplify the heating voltages,the sensor response to?ow speed change can be measured when an abrupt change of the?ow speed occurs.As shown in Fig.15,the response time is de?ned here as the90%value of the time cost for the change of the output voltages.It shows that the response time is less than1.4s and1.2s from5m/s to10m/s and10m/s to5m/s,respectively. The raw output voltages of the calorimetric system at different ?ow speeds are shown in Fig.16.The output voltages of the calorimetric system are very close to the cosine(sine)function of?ow direction.The measurement of?ow direction in a full range of360?has an accuracy of±2?over the full range of?ow speed.

VI.C ONCLUSION

A micromachined2-D thermal wind?ow sensor by wafer-level packaging through Au–Au bonding technology has been

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2012

Fig.16.Output voltages of calorimetric system for?ow direction measure-ment at different?ow speeds.

presented,fabricated,and tested.Gold bumps were used for thermal connection and electrical connection.The wafer-level packaging process reduces the offsets caused by the separate packaging and assembly.A silicon diaphragm was fabricated by wet etching,resulting in the improvement of the power consumption,response time,and resolutions.The measured ?ow speed reaches40m/s with only2mW initial heating power in CTD mode.The?ow direction in a full range of360?may be measured.The errors in the measured?ow speed and direction are±4%and±2?after calibration,https://www.360docs.net/doc/c614093550.html,pared with other MEMS-based wind sensor,the present sensor can measure?ow speed as high as40m/s with low heating power.

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1–3.

Ziqiang Dong was born in Hebei,China,in1982.

He received the B.S.and M.S.degrees in elec-

tronic engineering from Hebei University of Sci-

ence and Technology,Hebei,China,in2004and

2007,respectively.He is currently working toward

the Ph.D.degree at Southeast University,Nanjing,

China.

His current research focuses on the design,fabri-

cation,and test of wind/?ow

sensors.

Jingjing Chen was born in1988.She received the

B.S.degree in electronic engineering from Anhui

University,Hefei,China,in2009.She is currently

working toward the M.S.degree in the Key Lab-

oratory of MEMS of the Ministry of Education,

Southeast University,Nanjing,China.

Her main research interests are in the design and

simulation of MEMS

sensors.

Yukun Qin was born in1987.He received the B.S.

degree in electronic engineering from Anhui Univer-

sity,Hefei,China,in2009.He is currently work-

ing toward the M.S.degree in the Key Laboratory

of MEMS of the Ministry of Education,Southeast

University,Nanjing,China.

His main research interests is signal processing of

MEMS sensors.

DONG et al.:FABRICATION OF A MICROMACHINED TWO-DIMENSIONAL WIND SENSOR BY Au–Au WAFER BONDING TECHNOLOGY

475

Ming Qin(M’07)was born in China in1967.He

received the B.S.degree in electrical engineering

from Wuxi Institute of Technology,Jiangsu,China,

in1988,and the M.S.and Ph.D.degrees in electri-

cal engineering from Southeast University,Nanjing,

China,in1994and1997,respectively.

After graduation,he became an Assistant Profes-

sor in the Department of Electronic Engineering,

Southeast University,where he became an Associate

Professor in1999and a Full Professor in2003.

He was a Postdoctoral Researcher at Hong Kong University of Science and Technology in1999.He has served as Director of the CMOS MEMS Branch of the Key Laboratory of MEMS of the Ministry of Education in China.He has published over30peer-reviewed international journal/conference papers and holds more than20Chinese

patents.

Qing-An Huang(S’89–M’91–SM’95)received the

B.S.degree in electronic engineering from Hefei

University of Technology,Hefei,China,in1983,the

M.S.degree in electronic engineering from Xidian

University,Xi’an,China,in1987,and the Ph.D.

degree in electronic engineering from Southeast Uni-

versity,Nanjing,China,in1991.His Ph.D.research

focused on micromachined GaAs piezoelectric

sensors.

After graduation,he joined the faculty of the De-

partment of Electronic Engineering,Southeast Uni-versity,as an Assistant Professor,where he became an Associate Professor in 1993,a Full Professor in1996,and was specially appointed to Professor for the Chang-Jiang Scholar by the Ministry of Education in2004.He was a Visiting Scholar at Hong Kong University of Science and Technology,Hong Kong,from 1997to1998.He is currently the founding Director of the Key Laboratory of MEMS of the Ministry of Education,Southeast University.He has authored a book entitled Silicon Micromachining Technology(Science Press,1996),and published over100peer-reviewed international journals/conference papers.He is the holder of30Chinese patents.

Dr.Huang has served as Editor-in-Chief of the Chinese Journal of Sensors and Actuators.He was a Conference Cochair for the SPIE-Microfabrication and Micromachining Process Technology and Devices(Conference Proc. SPIE,vol.4601,November2001),Technical Program Cochair for the 7th IEEE International Conference on NEMS,and a TPC Member of TRANSDUCERS’09&’11as well as the IEEE Sensors Conference through 2002to2010.He has served as a Steering Committee Member of The Asia-Paci?c Conference of Transducers and Micro/Nano Technology(APCOT) (2008–2012).He served as the founding Chairman of the IEEE ED/SSC Joint Chapter(Nanjing).He received the National Outstanding Youth Science Foundation Award of China in2003.