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R F I D T E C H N O L O G Y

The idea of using radio frequency,infrared, ultrasound, or combina-tions of these technologies for in-door location sensing, or localiza-tion, has been around for quite some

time.1 Although industry and academia havedone a lot of research in this area, commerciallyavailable systems are still rare. One reason is that

many approaches have techni-cal drawbacks, such as insuffi-cient robustness in harsh envi-ronments and limitations on thenumber of items that can belocated simultaneously. Further-more, few economically feasible

application scenarios have emerged to justify thesubstantial infrastructure and systems integrationinvestments that localization would require.2

Against this background, we describe thedesign and implementation of a real-time local-ization solution that Infineon Technologies usesin its wafer fabrication facility in Villach, Aus-tria. The system combines active RFID, passiveRFID, and ultrasound sensors to track plasticwafer boxes and wafer cassettes in the company’schip-manufacturing process. It’s been in use foralmost two years and has proved to be reliableand efficient in localizing and communicatingwith a large number of moving physical objects.

Custom fabrication requirementsInfineon Technologies is one of the largest semi-

conductor manufacturers in the world, with

36,000 employees and annual net sales totaling€7.19 billion in 2004. Its strategic focus is on sep-arate business units for communications, auto-motive electronics, and memory products.

Like other companies in the industry, Infineonseeks to increase automation in its productionlogistics so that it can reduce stock and improvethe efficiency of its transport processes. Whileseveral highly automated transport system solu-tions exist, they mostly address the needs oflarge fabrication facilities (fabs) with relativelystatic processes, limited product range, andhigh-volume production. In contrast, fabs withstronger customer orientation require more flex-ible, operator-centric automation approaches tohandle the large number of varying productionprocesses and frequent rearrangements of thefab’s machinery and other technical equipment.

Infineon’s Villach facility exemplifies theserequirements. Headquarters for the company’sAutomotive and Industrial business group, theVillach unit develops and produces car compo-nents primarily for use in engine management,transmission control, comfort and safety man-agement, and “infotainment.” The fab operates24/7 with 2,000 employees and manufacturesabout 800 different products—a total volume of10 billion chips per year. Because of the enor-mous variety of processes, the operators trans-port production lots manually from one of its600 machines to another, rather than using con-veyor belts. For this purpose, lots are kept inplastic boxes, each containing a wafer cassette

A real-time identification and localization system uses RFID andultrasound sensor technologies to improve tracking visibility for inboundlogistics in a wafer fabrication cleanroom.

Frédéric Thiesse and Elgar FleischUniversity of St. Gallen

Markus Dierkes Intellion

LotTrack: RFID-BasedProcess Control in theSemiconductor Industry

holder for up to 25 silicon wafers (seefigure 1). These boxes are stored onmetal shelves throughout the cleanroombefore and after processing. In this sys-tem, an average of about 400 produc-tion steps occur per lot before the chipsare eventually tested.

A central manufacturing execution sys-tem controls the actual production pro-cedure in each machine. While the MESknows each and every production step inthe machine, the intermediate transportprocess wasn’t at all transparent. Analy-ses showed that improved transparencyof inbound logistics could decrease leadtimes and increase machine utilizationratios. Furthermore, the planned oper-ation sequence was hard for operatorsto follow if the next lot for processingwasn’t in place.

Infineon Technologies decided to startthe LotTrack project, equipping the fabwith a real-time identification and local-ization system that would make a lot’sentire production process visible andthus controllable. The project’s maingoals were decreased lead times, com-plete elimination of handling faults, anda reduction of non-value-adding activi-ties. Moreover, the project was alsoregarded as one important step towarda paperless fab, which would furtherreduce particles in the air.

Requirements analysisIn the first project phase, the project

team had to select a suitable identifica-tion and localization technology. Choos-ing the best technology requires preciseknowledge of the desired solution’s re-quirements. In the case of lot box track-ing in a chip fab’s cleanroom, the fol-lowing requirements and constraints hadto be taken into account:

• The total number of wafer boxes thatthe tracking system must localize inthe fab is greater than 1,000.

• Especially in places with many shelves,the number of wafer boxes can begreater than 100 boxes per 6 squaremeters.

• To control the process in real time, thetime lag between physical movementsin the real world and position updatesin the system should be significantlyless than one minute.

• The system’s location accuracy shouldbe less than 1 meter to make the searchfor a specific lot box as efficient as possible.

• Walls, floors, and ceilings in the fab aremade of metal that can complicate RFcommunications and generate electro-magnetic reflections.

• A direct line of sight between the boxand a reader device on a wall or on the

ceiling isn’t always present—the tech-nology should be able to tolerate vary-ing box positions and orientations onthe shelf.

• Energy consumption of the trackingdevices that are attached to a lot boxshould allow for a battery lifespan ofat least two years.

• In case of system failures or other crit-ical errors, a simple shutdown-and-system-restore procedure for the wholesystem should take no more than fiveminutes.

Furthermore, during the requirementsanalysis phase, Infineon decided that thesystem should not only collect data fromthe fab for planning purposes but alsoenable direct communication with oper-ators to ensure more control over clean-room operations. Thus, the system wasto completely replace traditional paper-based working plans with, for example,text messages on an LED screen. For thispurpose, it had to be able to transferinformation to operators within 1.5 sec-onds, on average, in parallel with itslocalization functionality.

While localization of lot boxes pri-marily aims to minimize search times,the need to decrease the risk of process-ing errors at each operation also requiresidentification on the level of single wafercassettes. Therefore, the system needed away to identify cassettes when an oper-ator places them into a machine. By thismeans, the system could detect acciden-tally missing production steps and pre-vent repeated steps from occurring.

Localization conceptThe limitations in various existing sys-

tems (see the sidebar “Related Work inLocation Systems”) made clear that onlya combination of RF and ultrasoundtechnologies could resolve the trade-offbetween high location accuracy and highcommunication rate together with alarge number of wafer boxes. The design

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Figure 1. (a) A lot box and (b) a wafer cassette holder. (photo courtesy of Entegris)

(a) (b)

JANUARY–MARCH 2006 PERVASIVEcomputing 49

team selected active RFID as the desiredsolution’s foundation mainly because ofthe battery requirements and communi-

cation performance. As the first testsdemonstrated, identification in itselfworked fine whereas localization based

on RF field strength failed to a largedegree because of insoluble problemswith reflections.

L ocalization is usually associated with GPS, which can achieve

localization accuracy up to about 3 meters. Unfortunately, we

can’t use this classic location technology inside buildings because

the radio signals sent by GPS satellites are too weak to penetrate

walls.1 However, previous work has demonstrated successful indoor

localization using infrared, ultrasound, or radio frequency technology:

• The Active Badge Location System is an example of an IR-based

solution.2 The system was originally developed for tracking peo-

ple in office buildings. In this scenario, a badge worn by a person

sends a unique IR signal every 10 seconds. Sensors placed at

known positions pick up the signals and relay them to a central

location manager. The line-of-sight requirement and short-range

signal transmission are two major limitations of this approach.

• Cricket is an ultrasound-based positioning system that uses

active beacons to send periodic chirps on an RF channel.3 Each

beacon also sends an ultrasonic signal at the same time as the

RF message. Passive listeners that are attached to a host device,

such as a laptop PC or a mobile handheld, receive both signals.

Thus, listeners can estimate their distance to the beacon from

the time difference between the arrival of the RF and the ultra-

sound signal. Active Bat uses an approach similar to Cricket’s

but instead equips mobile devices with emitters and installs lis-

teners on the ceiling.4

• Active RFID transponders are used by Landmarc, which requires

signal strength information from each tag to readers.5 Readers

detect RFID-equipped objects and forward all collected data to a

central lateration algorithm. Furthermore, the system uses sta-

tionary reference tags to improve its location accuracy. Other

RF-based alternatives to RFID are the use of signal strength

measurements from an existing wireless LAN infrastructure as

implemented in the RADAR system6 or time-of-flight calculations

of ultrawideband signals.7

Positioning and tracking systems are fundamentally different. As

the name suggests, LotTrack was meant to be an object-tracking

application—that is, an application in which a central system

observes objects. This contrasts with a positioning application, in

which the physical object determines its own location.8 In either

case, the simplest way to generate the actual location information

is by associating the object with the controller device—for exam-

ple, an RFID reader—that has detected the object and whose posi-

tion is known. If an application requires more finely grained informa-

tion, it must use lateration that works with distance measurements

between the physical object and one or more controller devices.9

Each of the related technologies we’ve mentioned has its own

strengths and weaknesses regarding location accuracy, flexibility,

costs, and so on. In the LotTrack project, the design team rejected a

solution based on passive RFID at the beginning because of the

metallic environment and the limited read range of passive transpon-

ders. Nor does an active RFID infrastructure work properly in metallic

environments: While the achievable communication rate for data

transfers to a transponder is satisfactory, reflections and overlaying

signals result in highly volatile distance measurements.

An IR-based solution was rejected because it requires line of sight

for object identification. Furthermore, location changes of shelves

or production equipment would have led to a costly readjustment

of installed IR sensors.

Ultrasound is very accurate in distance measurement but, again,

would have been unsuitable for tracking numerous objects simulta-

neously. A system like the Active Bat, for instance, doesn’t perform

well if many objects are located at the same place and are sending

lots of ultrasound signals to the same sensor. Furthermore, ultra-

sound can cause problems when factory noise in the relevant fre-

quency band reaches a sufficiently high level, especially when the

distance between sender and object exceeds 10 meters.

REFERENCES

1. G. Borriello et al., “Delivering Real-World Ubiquitous Location Systems,”Comm. ACM, vol. 48, no. 3, 2005, pp. 36–41.

2. R. Want et al., “The Active Badge Location System,” ACM Trans. Infor-mation Systems, vol. 10. no. 1, 1992, pp. 91–102.

3. A. Smith et al., “Tracking Moving Devices with the Cricket Location Sys-tem,” Proc. 2nd Int’l Conf. Mobile Systems, Applications, and Services(MobiSys 2004), ACM Press, 2004, pp. 190–202.

4. M. Addlesee et al., “Implementing a Sentient Computing System,”Computer, vol. 34, no. 8, 2001, pp. 50–56.

5. L.M. Ni et al., “Landmarc: Indoor Location Sensing Using Active RFID,”Wireless Networks, vol. 10, no. 1, 2004, pp. 701–710.

6. P. Bahl and V.N. Padmanabhan, “RADAR: An In-Building RF-Based UserLocation and Tracking System,” Proc. IEEE Infocom 2000, IEEE CS Press,2002, pp. 775–784.

7. R.J. Fontana and S.J. Gunderson, “Ultra-Wideband Precision Asset Loca-tion System,” Proc. IEEE Conf. Ultra Wideband Systems and Technologies(UWBST 2002), IEEE CS Press, 2002, pp. 147–150.

8. S. Helal et al., “Enabling Location-Aware Pervasive Computing Applica-tions for the Elderly,” Proc. 1st IEEE Int’l Conf. Pervasive Computing andCommunications (PerCom03), IEEE CS Press, 2003, pp. 531–538.

9. J. Indulska and P. Sutton, “Location Management in Pervasive Systems,”Proc. Australasian Information Security Workshop Conf. ACSW Frontiers,vol. 21, Australian Computer Soc., 2003, pp. 143–151.

Related Work in Location Systems

50 PERVASIVEcomputing www.computer.org/pervasive

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For this reason, LotTrack uses ultra-sound for fine-grained localization. Ultra-sound emitters on the ceiling periodicallysend a signal that is received by activeRFID transponders, which are equippedwith ultrasound sensors. The tags calcu-late the time-of-flight for all received sig-nals and store these values in theirread/write memory. RFID controllersread this data from the tags and transferit to a central server that generates loca-tion information for all objects in the sys-tem. To make the whole system work,every component must be fully synchro-nized. For this purpose, controllers pro-vide tags with a clock signal ensuring thatthey all operate on the same time basis.

The localization procedure is based ona 10-second cycle that is divided intothree phases:

1. Transponders listen to a predefinedpattern of signals from ultrasoundemitters in their environment. Sincethe transponder is in sync with con-trollers, it knows exactly when thesignal was sent and thus calculatesits distance from the emitter.

2. The controller device uses the RFID

protocol to scan its environment fortags and reads their collected dis-tance measurements.

3. In the remaining time slot, tags andcontrollers communicate as needed.

In this solution, RFID serves three dif-ferent purposes:

• The RFID protocol provides an effi-cient way to identify even a large num-ber of tags in the environment.

• The measured RFID signal strengthand the coarse-grained distance infor-mation that the system can derivefrom it serve as an additional plausi-bility check for the localization algo-rithm’s results.

• RFID enables reading distance mea-surements and all other tag accessoperations.

To minimize the amount of ultrasoundemitters and to simplify the laterationalgorithm, the design team regardedbilateration along a line as sufficient forlocalization in a chip fab. The fab’s lay-out contains no wide rooms but onlyinterconnected corridors, so a corridor

ID and a distance value is enough infor-mation to find any place in the fab.Besides, it’s much easier for fab opera-tors to orient themselves using this solu-tion than using 3D coordinates.

The resulting algorithm works as fol-lows:

1. Distance measurements define cir-cles around the coordinates of agiven ultrasound emitter ui. Thealgorithm calculates intersectionpoints pj for all possible combina-tions of two circles. In the examplein figure 2, we get six partially dupli-cate intersection points from fourdifferent distance measurements.

2. The algorithm then performs plau-sibility checks and eliminates all evi-dently incorrect positions. In ourexample, p4 is eliminated becauseit’s located behind a wall.

3. LotTrack repeatedly calculates theaverage value of all positions andeliminates the ones whose distancefrom this mean exceeds a config-urable threshold. Eventually, Lot-Track considers the average of allremaining positions as the object’sactual position. In our example, itselects only p1 because the objectseems to be at rest and all circlesintersect at the same location.

4. LotTrack projects the position ontothe centerline of all corridors inwhich the ultrasound emitters ui arelocated. Thus, a 2D position istransformed into one or more 1Dlocations lk. In our example, we getlA on corridor A’s centerline and lBon corridor B’s centerline. BecauselB is not within corridor B’s logicalborders, which are predefined in theLotTrack database, the algorithmselects lA as the final result.

One specific problem that results fromthis simplified localization technique isthe calculation of positions at the corner

u1 u2 u3

u4

p1

p2

p3

p4

Corridor A

Corridor B

lA

lB

Figure 2. Calculating lot box locationsusing the localization algorithm.

of T-crossings. As our example demon-strates, tags sometimes receive ultrasonicsignals from emitters in two corridors.These signals are processed and result inconflicting positions—some in corridorA, others in corridor B. Unfortunately,the set of signals that the tag receives isn’talways the same within every localiza-tion cycle even if the box is at rest. Some-times the tag receives a few more signalsfrom one or the other corridor, whicheventually leads the localization algo-rithm to conflicting decisions. The effectis an apparent “blinking”—that is, thebox seems to jump back and forth fromtime to time. LotTrack prevents this be-havior by comparing the algorithm’s setsof p-values from the current and previ-ous localization cycles.

Hardware componentsInfineon’s partner in this project was

Intellion, a Swiss-based automation solu-tion provider. Intellion was responsiblefor project management, systems design,development, and implementation of theLotTrack system. The foundation for theLotTrack-specific localization tags was along-range technology for active RFIDtransponders that could operate in theUHF band and reach a maximum readrange of up to 100 meters. The newlydeveloped localization transponder,called the DisTag, also features

• an ultrasound sensor, • a two-colored LED, • a mechanical “flipdot,”• a no-power display, and • four keys for user interaction.

So the DisTag is not only an active IDtag and a tracking device but also the sys-tem’s interface to the cleanroom opera-tors (see figure 3). The display replacesthe traditional paper checklists and pro-vides information on the lot ID numberand the next production step. The ultra-sound sensor and display contribute

most of the DisTag’s energy consumptionrate increase. Nevertheless, dependingon environmental conditions, the Dis-Tag’s lifespan is about two years beforeit requires a battery change. Although itsmany special features make the embed-ded device significantly more expensivecompared to a standard long-rangetransponder, the price of one single Dis-Tag is still relatively low compared to thevalue of a wafer carrier’s contents.

The second system component is thecontroller device. The RFID controlleris connected to four RF antennas and avariable number of ultrasound emitters.To reduce installation activities in thecleanroom to a minimum, one RFantenna and three ultrasound emitterswere integrated into each standardizedantenna module with preconfiguredcabling that can easily be mounted onthe ceiling. The design team chose thedistance between ultrasound emitters toensure that a DisTag always receives atleast three signals from different emit-ters for bilateration.

A central server controls the system,managing network connections to everyoperational LotTrack controller (see fig-ure 4). The server receives all the col-lected controller data that serves as a

basis for the localization algorithm. Theserver is responsible not only for bilater-ation calculations but also for plausibil-ity checks and other filtering proceduresthat guarantee high data quality. The Lot-Track server transfers the results to themessaging middleware that supports apublish/subscribe mechanism. The mid-dleware is also necessary to submit com-mands from client systems to LotTrack—for example, read/write operations on aDisTag’s display. LotTrack’s clientsinclude the central MES and the Fab-Viewer, a visualization tool that gives agraphic overview of all the fab’s waferboxes. Furthermore, LotTrack continu-ously records management indicators tomonitor the actual status of readers,antennas, and DisTags that can be ac-cessed from an administration console.

User functionalityWafer production processes are orga-

nized according to dispatch lists thatcontain all waiting lots that fab opera-tors must process in a specific type ofmachine ordered by priority. Operatorscan select the lot they wish to processnext directly on screen. The DisTag’s flip-dot indicates when an operator hasreserved a lot box. LotTrack provides

JANUARY–MARCH 2006 PERVASIVEcomputing 51

Figure 3. DisTag features.

LED

No-power display

Keys

Ultrasound sensor

Flipdot

52 PERVASIVEcomputing www.computer.org/pervasive

R F I D T E C H N O L O G Y

information on the box’s position, suchas “Corridor A, 2.7 m.” The operatorcan also make the LED blink, making iteasy to find the box. Finally, the systemdetects if a box is currently at rest ormoving, which helps an operator decidewhether to wait for a box to arrive oractively search for it.

The operator then picks up the waferbox and brings it to a machine. The infor-mation on the display indicates themachine to which the system has assignedthat lot. The operator opens the box, putsthe wafer cassette into the machine, andstarts the processing. To make sure thatthe right lot is processed, Infineon choseto equip wafer cassettes with standardpassive RFID tags as a complement tothe active DisTags on the box. A readerdevice in the machine identifies thesepassive tags. In this way, the system candetect duplicate and missed processsteps before they occur. When the pro-cessing has finished, the operator takesthe cassette out of the machine and putsit back into the box. The system auto-matically moves the lot to another dis-

patch list and updates the DisTag dis-play. Then the operator puts the box ona shelf where it waits to be transportedto another machine.

Lessons learnedOver the course of almost two years’

operations, LotTrack has proved its reli-ability and efficiency. The followingresults indicate the system’s capacities:

• Performance. The installation in Vil-lach comprises about 100 controllerdevices and more than 1,000 DisTags.The system is installed on a standardtwo-CPU server and receives about 3billion distance measurement mes-sages from DisTags per day. Processingthis huge amount of data leads to 270million positions. Because LotTrackonly publishes position changes thatexceed a configurable threshold, thenet total of positions delivered to itsclients is 500,000 per day.

• Location accuracy. The first systemprototype provided location infor-mation within an accuracy of 5 cm

under laboratory conditions. Theseresults are consistent with the char-acteristics of other ultrasound loca-tion systems. While the productionsystem still meets Infineon’s require-ments, its accuracy reaches only 30cm. This is due to the dynamic pro-duction environment with severalmoving people and objects that causeultrasonic reflections and slightlyincorrect distance measurements thatour plausibility checks can’t recog-nize. Other factors are backgroundnoise (some machines in the fab gen-erate noise in the ultrasound spec-trum) and discrepancies between ref-erence coordinates in the LotTrackdatabase and the real positions ofultrasound emitters, but these influ-ences are virtually negligible.

• Time lag. The time lag between phys-ical movement and the correspondingposition message is no more than 30seconds. Actually, the system performseven better and generates positionupdates after 10 seconds. After thefirst tests with operators, we’ve nev-

Databasefor archiving

Controller 1

Controller 2

Controller n

Locationcalculation

Controller selectionbased on

signal strength

TCPconnection

to everyoperationalRFID reader

XML

mes

sagi

ng m

iddl

ewar

e

Positionupdates

Every 10seconds

Publication

In-memorytag repository

Data#1

Periodicdatabaseupdates

Mes

sage

buf

fer

Data collectionfrom controller 1

Data collectionfrom controller 2

Data collectionfrom controller n

DisTag communication

ManufacturingExecution

System (MES)

Tag access operations

FabViewer

Figure 4. The LotTrack system architecture.

JANUARY–MARCH 2006 PERVASIVEcomputing 53

ertheless chosen to merge locationinformation from three consecutivelocalization cycles to make better deci-sions about whether an object is at restor moving. This takes more time fornew positions to become visible, butthe results are also more convenientfor the user.

• Communication operations. When-ever the server must transfer data toa DisTag, it starts a thread that triesto perform a tag access operation viathe closest RF antenna. In 80 percentof cases, the operation succeeds atfirst try. In the other 20 percent,which fail because of moving boxes,the operation always succeeds aftertwo retries.

All in all, location visibility of fab lotshas increased from 65 to nearly 100 per-cent in covered areas. Formerly manualtransport processes, such as moving lots,have become completely transparent.

On the basis of these experi-ences, Infineon extended theVillach installation to a sec-ond, larger fab in Regens-

burg, Germany. Other installations arein the planning phase. The solution’sprocess impact includes fewer handlingerrors and reduced lead times, whichresult in lower manufacturing andinventory costs. Furthermore, the fastand rich communication with opera-tors enhanced the flexibility of trans-port and handling processes. Last butnot least, LotTrack is an important partof the paperless fab in that the DisTagdisplays provide needed informationand so replace the traditional paper lottraveler.

To integrate the enriched informationquality of accurate, reliable lot positionswith the existing manufacturing systemrequired significant training and changemanagement. Operators had to learn how

to use LotTrack in their working envi-ronment. Optimizing logistics processesand adjusting the entire productionautomation system took a lot of time.Nevertheless, the full potential of Lot-Track and similar systems is still waitingto be leveraged. Over the long term, thesesystems can lead to a more strategic use ofproduction flexibility as well as enablingcustomer-driven production systems andmass customization concepts.

REFERENCES1. J. Hightower and G. Borriello, “Location

Systems for Ubiquitous Computing,” Com-puter, vol. 34, no. 8, 2001, pp. 57–66.

2. M. Hazas, J. Scott, and J. Krumm, “Loca-tion-Aware Computing Comes of Age,”Computer, vol. 37, no. 2, 2004, pp. 95–97.

For more information on this or any other comput-ing topic, please visit our Digital Library at www.computer.org/publications/dlib.

the AUTHORS

Frédéric Thiesse is associate director of the Auto-ID Lab at the University of St.Gallen and project manager of the M-Lab, a joint research project of the Universityof St. Gallen and the ETH (Swiss Federal Institute of Technology) Zürich. His researchinterests include RFID integration with corporate information systems, RFID’s role inoperations management, and the design of pervasive products and services. Thiessereceived his PhD in business administration from the University of St. Gallen. Con-tact him at the Inst. for Technology Management, Univ. of St. Gallen, Dufourstrasse40a, 9000 St. Gallen, Switzerland; frederic.thiesse@unisg.ch.

Markus Dierkes is chief technology officer of Intellion, a spin-off of the University ofSt. Gallen that specializes in RFID and location-based automation solutions for man-ufacturing and logistics. His activities focus on ubiquitous computing’s potential fortransforming automation concepts and business models. He received his PhD in busi-ness administration from the University of St. Gallen. Contact him at Intellion, Schup-pistrasse 8, 9016 St. Gallen, Switzerland; markus.dierkes@intellion.com.

Elgar Fleisch is a professor of technology management at the University of St.Gallen, a professor of information management at the ETH Zurich, and cochair ofthe international Auto-ID Lab Network. His research interests include ubiquitouscomputing’s impact on products, services, business processes, business models, andusers. He received his PhD in business administration from the Vienna University ofEconomics and Business Administration. Contact him at the Inst. for TechnologyManagement, Univ. of St. Gallen, Dufourstrasse 40a, 9000 St. Gallen, Switzerland;elgar.fleisch@unisg.ch.

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