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Sensors and Actuators A 171 (2011) 414– 420

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

j ourna l h o me pa ge: www.elsev ier .com/ locate /sna

wireless and passive wafer cleanliness monitoring unit via electromagneticoupling for semicondutcor/MEMS manufacturing facilities

u Zhang, Junseok Chaechool of Electrical Computer and Energy Engineering, Arizona State University, Tempe, AZ, USA

r t i c l e i n f o

rticle history:eceived 18 February 2011eceived in revised form 24 July 2011ccepted 9 August 2011vailable online 22 August 2011

eywords:lectromagnetic coupling

a b s t r a c t

This paper presents a wireless and passive chemical sensing system, in situ real time, via electromagnetic(EM) coupling, capable of monitoring wafer cleanliness during rinsing process at semiconductor/MEMSmanufacturing facilities. A MEMS chemical sensor is embedded in a wafer-form transponder to evaluatethe rinsing process in situ by measuring the conductivity of rinsing water inside micro-features formedby two interdigitated electrodes. All necessary power for the transponder is supplied from an externalinterrogator via the on-wafer transponder antenna. The modulated conductivity data is then emittedback from the transponder to the external interrogator in wireless and battery-free manner. The wireless

assive sensingnderwater sensingafer cleanliness monitoring

system has been implemented on a 4-inch glass wafer to maintain the wafer form factor, not disturbinghydrodynamics of the rinsing process. The working distance of the system was measured to be about25 cm, primarily limited by the coupled power to the transponder. Real time and in situ characterizationof system was performed with three different control solutions: hydrochloric acid (HCl), sulfuric acid(H2SO4), and sodium hydroxide (NaOH). Detection uncertainty of the system was observed to be lessthan 2% (2 ppb for a 100 ppb solution).

. Introduction

Expensive de-ionized (DI) water is overused in rinsing processeso ensure wafer cleanliness quality control at semiconduc-or/MEMS manufacturing facilities. This is because the rinsingrocesses are not monitored effectively. Rinsing quality is eval-ated by either examining the left-over contaminants or byeasuring conductivity of the rinsing water at the outlet of the

inse tank [1]. All these methods cannot provide in situ and realime information to monitor the progress of rinsing processes. Inrder to save the cost and develop effective rinsing recipes, it isrucial to monitor the rinsing water in situ and in real-time.

With feature sizes scaling down, contaminants inside micro-eatures formed by patterned wafers are hard to be removed duringhe rinsing process. It would be useful to monitor the rinsingrocesses inside micro-features rather than just measuring wateronductivity/concentration near the surface. In addition, such aonitoring system is preferred to be wireless and passive. A wiredonitoring method faces many challenges in the practical use, e.g.

rotection of contacts, good connectivity of wires, arrangement of

ires, etc. Using a battery in such a wireless system is also challeng-

ng. Battery needs to be well protected from water and chemicals,nd has limited lifetime.

E-mail address: xu.zhang.2@asu.edu (X. Zhang).

924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2011.08.005

© 2011 Elsevier B.V. All rights reserved.

All these challenges motivate our research: a wireless and pas-sive on-wafer monitoring unit. We have studied an in situ andreal-time monitoring system based on inductive coupling [2]. Anexternal interrogator inductor coupled the secondary inductorlocated on the transponder and a MEMS conductivity sensor mea-sured the conductivity of the rinsing water inside micro-featuresformed by the electrodes of the sensor. The micro-features aredesigned to mimic the trenches of patterned wafers. Thus, theconductivity measurement of the water inside the micro-featurescould reflect the rinsing progress of real wafer cleaning. The con-ductivity information was then modulated and transmitted backto the external interrogator. All the necessary power for transpon-der to function are absorbed from the coupled AC signals, which isconverted to DC by the on-wafer rectifier network. Using such mon-itoring system, potentially up to 50% of DI water could be saved [3].Considering a semiconductor/MEMS facility consumes a few mil-lion liters of DI water per day, much resources, energy and costcould be saved.

However, the inductive coupling system only had a workingdistance of less than 10 cm. Such short working distance was pri-marily due to the near field nature of inductive coupling (magneticfield degrades with distance3). In order to enhance the working

distance, we present an electromagnetic (EM) coupled system inthis paper, which improves the working distance to about 25 cmin our measurements, much relevant to practical applications. Wehave characterized the wafer-form wireless passive monitoring

X. Zhang, J. Chae / Sensors and Actuators A 171 (2011) 414– 420 415

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ig. 1. (a) Schematic of the real time in situ wireless/passive wafer cleanliness moquivalent circuit of the MEMS sensor. The sensor is designed to let r (sensitive to s

nit for three different chemicals, hydrochloric acid (HCl), sulfu-ic acid (H2SO4), and sodium hydroxide (NaOH), to mimic rinsingolutions. Though working in a passive and wireless fashion, theystem has about the same concentration accuracy compared totate-of-the-art conductivity sensors.

The paper is organized as follows. In Section 2, the systemchematic and working principles of the system are described,hich is followed by detailed discussions of design choices androcedures in Section 3. Fabrication process is also included inection 3. In Section 4, characterizations of the system with differ-nt concentrations are presented. The ability to in situ monitoringafer cleanliness during rinsing is demonstrated. Discussion and

ssessment of the data are also included in this section. Finally, theonclusion remarks are presented in Section 5.

. System schematic

The schematic of wireless and passive monitoring system viaM coupling is illustrated in Fig. 1(a). The monitoring systemonsists of three parts: an external sending antenna (with signalenerator), an external receiving antenna (with spectrum analyzer)nd an on-wafer transponder implemented on a glass wafer. Theending antenna transmits a carrier to the transponder and theeceiving antenna picks the modulated signal emitted back fromhe transponder. In all power/signal transmissions, no battery is

nvolved on the transponder. The transponder consists of a match-ng network, load modulation components (C and a switch), aectifier, a voltage regulator, a local oscillator and a MEMS sen-or. When the carrier arrives at the on-wafer antenna on the

ng system via electromagnetic coupling: battery-free underwater sensing and (b)n concentration) dominates the impedance.

transponder, the rectifier converts the RF signal into DC output.Then the DC output is further stabilized by a voltage regulatorto power the local oscillator. The voltage regulator minimizes theundesired oscillation frequency sensitivity to DC supply. A MEMSsensor is embedded in the oscillator (Fig. 1(b)). In the model, r isthe resistance of solution between the two electrodes. Rct is thecharge transfer resistance and Cd is double layer capacitance, bothof which describe solid–liquid interface chemistry. Cu and Cl are thecapacitors formed due to the dielectric layers above and below theelectrodes, respectively. Rsub, Rel, and Rb are resistances of the sub-strate, electrodes and water above the electrodes, respectively. Thesensor is designed to let r, the most sensitive element to solutionconcentration, dominate the impedance. Details of sensor designand parasitics analysis were presented in our previous work [2].The measured conductivity (dominated by resistance of r) from theMEMS sensor correlates the ionic concentration of the water andindicates the cleanliness of the rinsing water inside micro-featuresformed by interdigitated electrodes. Thus, the oscillator frequencychanges as the conductivity of solution inside the micro-features.The oscillator output is then mixed to the carrier by the modu-lation switch and is presented as sidebands from the carrier. Thesidebands are finally extracted and mapped to the ionic concentra-tion of the solution as the in situ and real time readout of how cleanthe rinsing water inside the micro-features is.

The choice of the carrier is a compromise between antenna

size and EM propagation loss. On one hand, energy propagatedand received by an antenna is inversely proportional to frequencysquare [4]. High frequency induces high energy loss through thepropagation media and produces a short working distance. On

4 d Actuators A 171 (2011) 414– 420

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16 X. Zhang, J. Chae / Sensors an

he other hand, as frequency lowers the size of antenna on theransponder becomes large. Since the transponder antenna shoulde implemented on the wafer, the size of the antenna shouldt within the footprint of wafer. Decreasing the size of antennaesults in degradation of antenna performance. Our choice of car-ier frequency was 433 MHz, one of the available bands for wirelesspplications among ISM (Industrial–Scientific–Medical) bands [5].t 433 MHz, we were able to implement the monitoring systemith a working distance of 10′s cm and fit the antenna on a 4-inafer with reasonable performance.

. System design and fabrication

The discussion of this section focuses on the design of then-wafer transponder; the sending/receiving interrogators arexternal components thus do not have many limitations of design.etails of MEMS sensor, rectifier and oscillator design were pre-

ented in [2]. The MEMS sensor consisted of two interdigitatedlectrodes covered by dielectrics. The impedance between the twolectrodes characterized the ionic concentration/conductivity ofhe water inside the micro-features formed by the electrodes. Cas-aded voltage doubler was used as the rectifier for the transponder.ajor difference from the previous approach, inductively coupling

ystem, is that the diodes used in the voltage doubler worked atuch higher frequency range (100 MHz–10 GHz for EM coupling)

han that for inductive coupling (less than 13.56 MHz). A RC oscil-ator scheme was implemented in the design. Impedance of theensor is translated into oscillator frequency in the system. As dis-ussed in [2], the oscillator frequency is:

(Z) = 1(2rCosc ln 2)

(1)

here r is the real part of the sensor impedance which is propor-ional to the solution conductivity and Cosc is an external capacitoro allow the oscillator operating at a few kHz. MEMS sensor changeshe RC constant to change the oscillator frequency in accord withhe ionic concentration of the water inside the micro-features.

The requirement for the voltage regulator includes low staticower consumption and low sensitivity to supply voltage. Wehose NCP585 (from ON semiconductor) which has a low quiescenturrent of 3.5 �A (estimated static power consumption of 5 �W)nd low output voltage sensitivity to supply voltage, 0.05%/V [6].he switch needs to have low static power as well. In our design,witch, ISL43L210 (from Intersil Americas Inc.), was used, whichas a supply current of 0.018 �A (estimated static power consump-ion of 0.2 �W) [7]. The modulation capacitor (C) in Fig. 1(a) needso satisfy high SNR (signal-to-noise ratio) to enhance working dis-ance, and we set it with 1 pF. The matching of the transponderircuit to the transponder antenna is crucial since it impacts the effi-iency of power transfer and the working distance of the system. An-section matching network consisted of an inductor and a capaci-or was implemented for better power efficiency [8]. The values ofhe inductance and capacitance were calculated by ADS (Advancedesign System, Agilent). The matching network was verified by aetwork analyzer.

The transponder antenna has a severe size limitation. In ordero maintain hydrodynamics of rinsing water the transponder needso be in the form of a wafer and the antenna must be integrated onhe wafer; in this study it is set as a 4-in wafer. The transponderntenna in our system was derived from an inverted-F antenna. Theadiation arm of the antenna was meandered to shrink the antennaize. Besides, the antenna was fed by coplanar feeds to simplify the

abrication. The matching of the antenna was tuned by the capaci-ive arm of the antenna as shown in Fig. 2(a). The antenna and theransponder circuit were covered by a PDMS layer to protect themgainst chemical attacks during the rinsing process, leaving only the

coplanar feed covered by a PDMS protection layer, (b) simulated gain (maximum2.4 dB) and the omni-directional radiation pattern of the antenna and (c) simulatedand measured S11 parameter of the antenna, −10 dB bandwidth of ∼80 MHz.

sensor exposed to the environment. The antenna was simulated,with the PDMS layer on, and showed a gain of about 2.5 dB with anomni-directional pattern (Fig. 2(b)). The simulated and measuredS11 parameters of the antenna are shown in Fig. 2(c), where simu-lation and measurement match well. The designed antenna has arelatively large bandwidth, −10 dB bandwidth of 80 MHz (20%).

The fabrication process of the transponder is illustrated in Fig. 3.Fabrication started with a 4-in glass wafer. The electrodes of sen-sor were first deposited and patterned by lift-off. The electrodesare made of evaporated Al and covered by evaporated glass. On

the other side of the wafer, the antenna interconnects and pads ofthe transponder were electroplated using Cu. The electroplatingensures the pads are robust enough to go through the subse-quent soldering process and also reduce the series resistance. Once

X. Zhang, J. Chae / Sensors and Actuators A 171 (2011) 414– 420 417

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concentration of the solution according to Eq. (1). Consequently,as the concentration varies from 60 ppb to 2000 ppb, the measuredoscillator frequency increases as shown in Fig. 5(b). The measure-

ig. 3. Fabrication process of the transponder: (a) MEMS sensor, made of evaporatedonnections are formed by electroplating Cu, (c) discrete components are soldered ond (e) a PDMS layer protects the components and leaves the sensor exposed.

he seed layer (Ti/Cu) was removed, discrete components of theransponder were soldered on the wafer. The sensor was thenonnected to the transponder circuit by through-wafer connec-ions, which are formed by drilling the wafer using a diamond drill0.9 mm in diameter) and filling the holes with solder. The padsor the through-wafer connections have a size of 5 mm × 5 mmo accommodate manual alignment during the drilling process.inally, the antenna, the interconnects and the transponder circuitere covered by a PDMS layer and cured at 70 ◦C for 20 min, while

he sensor on the backside of the wafer was not covered by theDMS layer. A fabricated wireless monitoring transponder is shownn Fig. 4.

. Results and discussions

The passive wireless monitoring system was characterized byipping the MEMS sensor in control solutions of HCl, H2SO4, and

aOH. An exemplar of wirelessly received signal was shown inig. 5(a). The oscillator frequency, f(Z), is the frequency difference�f) between the carrier and the first sideband peak. The mea-ured oscillator frequency vs. concentration of the three solutions is

ig. 4. Fabricated monitoring wafer on 4-in glass wafer. The antenna, circuit andnterconnects are protected by PDMS layer, leaving the sensor, located on the backide of the wafer, exposed to the environment.

d SiO2, is fabricated by lift-off on the backside of a glass wafer, (b) antenna, pads andafer, (d) MEMS sensor is connected to the oscillator by through-wafer connections,

shown in Fig. 5(b). The measurement uncertainty was less than 1%.As the resistance (real part of the impedance of the MEMS sensor)is inversely proportional to the conductivity/concentration of thesolution, the measured oscillator frequency is proportional to the

Fig. 5. (a) Exemplar spectrum of the detected signal, and (b) detected oscillator fre-quency as a function of concentration of different solutions. Different solutions havedifferent frequencies as the conductivity of the solutions is a function of speciesvalence, molar concentration, and molar ionic conductivity of each ion. Measure-ment uncertainty is less than 1%.

4 d Actuators A 171 (2011) 414– 420

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18 X. Zhang, J. Chae / Sensors an

ents show the oscillator output frequency is a linear function ofhe concentrations of hydrochloric acid (HCl) and sodium hydrox-de (NaOH) solutions. On the other hand, the measurement ofulfuric acid (H2SO4) shows a non-linear response. Such non-linearesponse is due to the contribution of the double layer capacitanceo the output frequency. The previous discussion of RC constant ofscillator frequency considered “R” is the only parameter changedy the concentration. In fact, the double layer capacitor Cd attributesart of “C” in the RC constant, which is also a function of the concen-ration and results in the non-linearity. The non-linearity increasess the range of the concentration increases as shown in Fig. 5(b).he non-linearity of sodium hydroxide is not obvious because theutput frequency is not very sensitive to the concentration changes that of sulfuric acid is. We also notice, different solutions showifferent frequencies even when their concentrations are the sames shown in Fig. 5(b). The conductivity of a solution could be esti-ated by the weighted sum of ionic conductivity of each ion in the

olution [9]:

=∑

i

zici�i (2)

here � is the solution conductivity, zi is the species valence, cis the molar concentration and �i is the molar ionic conductivityf each ion. Base on Eq. (2), the conductivity of HCl, H2SO4, andaOH could be calculated as 11.7 m, 10.4 m and 6.2 m, where m is

he mass concentration of the solution in ppb. At the same massoncentration, NaOH has the lowest oscillator frequency whereasCl and H2SO4 have similar oscillator frequencies, if the frequency

s dominated only by the resistance of the sensor.The oscillator frequency, however, is a function of many param-

ters including its DC supply, thus the detection uncertainty ofhe system is sensitive to the stability of the DC output of theoltage regulator that powers the oscillator. The output of the reg-lator has a finite sensitivity (0.05%/V) to its input DC, which alsohanges as working distance. The oscillator frequency vs. workingistance was measured to determine the detection uncertainty ofhe system. A 200 k� resistor was used, instead of the sensor, inhe measurement to exclude any interference from solution con-entration variation. Within the working distance, the oscillatorrequency fluctuation was well controlled within 2%, which also

arks the detection uncertainty of the system was about 2% (i.e., ppb of a 100 ppb solution). Thus, the DC supply sensitivity of thescillator may need to be improved to enhance the detection uncer-ainty where applications demand the uncertainty of less than%. The system measurement uncertainty is comparable to state-f-the-art battery-operated wired conductivity sensors, which isypically 1–3% [10,11]. However, our passive monitoring system

easures water cleanliness wirelessly inside cavities or trenchesormed by patterned wafers in situ and in real time.

The maximum working distance of the system is determined byhe power coupled to the transponder. The coupled power avail-ble to the transponder, a function of the working distance, coulde indirectly observed by DC output of the rectifier (supply forhe transponder circuits; 1.4 V is the minimum voltage to operatehe transponder). Measurements from Fig. 6 suggest the workingistance was about 25 cm, where the DC output of the rectifiereached 1.4 V. Theoretical estimation (dashed line) includes thenergy spreading, interface reflection and water absorption. Thenergy spreading characterizes the energy behavior as a functionf the distance between the two antennas:

P0�2G1G2

EM =

(4�R)2(3)

here P0 is the power transmitted from the carrier source, � ishe wavelength, G1 and G2 are the gains of the transmitting and

Fig. 6. DC voltage output of the rectifier as a function of working distance. Therectified DC needs to be greater than 1.4 V to power the system. Measurementuncertainty is less than 3%.

receiving antennas, respectively, and R is the working distance. Theloss due to water absorption of the wave propagation is:

L = exp(−˛R) (4)

where ̨ is the absorption coefficient of water. The loss due toair/water interface is estimated at approximately 7 dB [12], thoughthe reflection loss is a function of frequency and incident angle.The dots in Fig. 6 are from the experimental measurements withuncertainty of less than 3%, which match well with the theoreticalprediction except in near field (<0.1 m). The measurements devi-ate from the estimation in near field since equations used in theestimation are only valid for far field. It is difficult to estimate thenear field behavior of the antenna and experimental measurementis often considered to be the most accurate method to characterizethe near-field behavior [13].

Real time measurement of the system was performed byimmersing the transponder in a tank containing various concen-trations of chemicals: HCl, H2SO4, and NaOH. We let DI water flowconstantly to mimic a wafer rinsing process. A commercial con-ductivity sensor (SB80PC from VWR sympHony) was placed nearthe transponder sensor as a reference. All measurements startedwith a conductivity of 50 �S/cm. The wirelessly recorded data ofthe three solutions were plotted in Fig. 7. As the rinsing processcontinued these solutions were diluted to less than 60 ppb (�f of∼6.7 kHz) after 600 s. Both the commercial sensor and the MEMSsensor on the transponder showed similar trend as the rinsing pro-ceeded. However, the MEMS sensor needed more time to indicate“clean”. This result was expected as it takes more time to removethe impurities inside the micro-features that mimics the patternson manufacturing wafers.

In all three chemical rinsing experiments, we observed unde-sired frequency peaks (Fig. 7). These peaks result from thecapacitive part of the MEMS sensor. We have a 200–300 k� resis-tor (Rp) in parallel with the MEMS sensor to estimate impedanceof no liquid and monitor the drying process after the rinsing. As aresult, the impedance of the sensor, the parallel resistor and thecapacitor from the oscillator could be modeled as in Fig. 8, whichis equivalent to a series RC circuit (Req, Ceq, and Cosc). r is the resis-tance of the sensor and is a function of solution conductivity. Cd isthe double layer capacitor of the sensor [9,14], which increases asconcentration. Cosc is a discrete capacitor of the oscillator to set theoscillation at a few kHz (details of Cosc and the working principle ofthe oscillator are presented in [2]). The impedance of the parallelpart (sensor and R ) could be expressed as:

p

Z =(

r + 1jωCd

)‖Rp =

Rp + ω2C2d

Rpr (Rp + r)

1 + ω2C2d

(Rp + r)2+

ωCdR2p

j[

1 + ω2C2d

(Rp + r)2] (5)

X. Zhang, J. Chae / Sensors and Actuators A 171 (2011) 414– 420 419

Fig. 7. Real time and in situ rinsing monitoring of different solutions: (a) HCl, (b)H2SO4, and (c) NaOH. Transponder responses are the real time and in situ profileof wafer cleanliness. Undesired frequency peaks exist due to the resistor parallel tothe sensor and the capacitive impedance of the sensor. Though not a problem forpractical rinsing, the frequency peaks could be further optimized or removed.

Table 1Specification of EM coupling system.

Bandwidth of transponder antenna 80 MHzCarrier frequency 433 MHzOscillator frequency 6–11 kHzCalibrated concentration range 60–2000 ppbInput power at the sending antenna 20 dBmMaximum working distance 25 cm

Fig. 8. Equivalent model of the RC series circuit that determines the oscillator frequency,undesired frequency peaks in the real time in situ rinsing profiles.

Detection uncertainty <2%

Thus, the equivalent capacitor (Ceq) could be found by

Ceq =1 + ω2C2

d

(Rp + r

)2

ω2CdR2p

(6)

When the concentration is high (t = 0 in the Fig. 7), r is negligibleto Rp and the Ceq is dominated by Cd. Cd starts to decrease as rins-ing process continues to dilute the solution. So does Ceq. However,as the rinsing process continues the concentration decreases and rbecomes comparable to Rp. The change of Ceq is, now, dominatedby r. The RC constant, the product of Req and CoscCeq/(Cosc+Ceq) inFig. 8, exhibits a local minimum around 200–400 s, showing fre-quency peaks. Based on the model discussed in [9,14], the modelparameters of Fig. 8 were extracted as: Rp = 300 k�, r = 10′s k�,Cd = 1′s nF and Cosc = 1 nF. When r is comparable to Rp, e.g. 1/4 ofRp, the oscillator frequency is at approximately 10 kHz as demon-strated in Fig. 7. As shown in Fig. 7, there are frequency peaks ataround 9 kHz. However, the peaks are located far from “clean” state;thus it does not hurt the practical use of the system. If the systemneeds to operate at high concentrations, one can reduce Rp, mov-ing the peaks to higher frequency. However, since r and Rp are inparallel, change of Req caused by r (concentration change) becomessmall as Rp decreases. As a result, the system has less sensitivity andmore detection uncertainty in low concentration range. Alterna-tively, a very large Rp could be used to eliminate the peaks, movingthe peaks to lower frequency outside the oscillator frequency range.Yet the system may not be able to monitor the drying process asthe oscillator frequency could be too low to be extracted from thecarrier. As trade-off among detection uncertainty, concentrationrange and the ability to monitor the drying processing, we chose Rp

of 300 k� and located the peak at higher than the interested con-centration range. The key parameters of the system performanceare summarized and listed in Table 1.

Ceq decreases first and increases later as concentration increases, which causes the

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20 X. Zhang, J. Chae / Sensors an

. Conclusion

This paper presents the design of wireless and passive systemor in situ and real-time underwater sensing application. Systemchematic and design considerations are described in the paper.he designed system was calibrated between 60 and 2000 ppbith three different solutions: HCl, H2SO4 and NaOH. It is nec-

ssary to calibrate the monitoring system, however, as differentanufacturing facilities have different rinsing recipes. Measuredaximum working distance of the system was about 25 cm, making

he system practical for monitoring rinsing process in semiconduc-or/MEMS manufacturing facilities.

cknowledgement

Authors thank all help from NanoFab facilities at Arizona Stateniversity.

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