The computer in manufacturing-reduction of scrap by computer monitoring

Transcription

1 The cmputer in manufacturing-reductin f scrap by cmputer mnitring by P. E. GOBER Westinghuse Semicnductr Divisin Yungwd, Pennsylvania ABSTRACT A cmputer furnace mnitring system was implemented as the first stage f a cmputer prcess mnitring system designed t prvide better cntrl f the prcess used t manufacture high pwer semicnductr devices. The purpse f the furnace mnitring system is t reduce scrap resulting frm furnace malfunctins that are therwise nt detected in time t salvage the prduct. The system als imprves reprducibility by maintaining a tight cntrl f the elevated furnace temperatures (±2 C at 12 C). Additinal results f the system are greatly imprved peratin visibility during the run, increased furnace utilizatin as a result f cmputer assisted scheduling, and imprved crrelatin f results amng different furnaces. The@ 20 cmputer prvides the prcess 110 necessary t mnitr furnace temperatures as measured by thermcuples, sund alarms when deviatins frm the spec ccur, stre infrmatin fr later analysis, plt furnace behavir, and assist in scheduling by calculating cycle times. It als prvides the furnace peratrs with the ability t quickly and accurately determine furnace temperature at any time during the cycle. The real time fregrundl backgrund perating system f cmputer, based n a strict pririty system, allws data analysis. p'r~r~ms ~ ~n withljut msturbingille re~ll time mnitring f physicai parameters such as temperature. The furnace peratr's interface with the cmputer, a set f related prgrams accessed frm a teletype by a single cmmand, is als described. The system is user-riented, and emplys a cnversatinal questin and answer frmat that guides the peratr thrugh the varius prcedures. The system als incrprates errr detectin and crrectin methds t prevent mistakes frm imprperly entered data. The cmputer is an effective tl fr the reductin f cst resulting frm scrap. It als prvides the basis fr an integrated mnitring system encmpassing the entire manufacturing prcess. INTRODUCTION The cmputer has lng been used by industry t handle payrlls, accunting, and varius recrds, and by scientists fr mathematical analysis, mdeling, and simulatin. A less expired area, hwever, is that f the cmputer as a manufacturing tl: the cmputer can play an imprtant rle in day-t-day peratins in a manufacturing envirnment, where features such as memry technlgy, innvative architecture, and language syntax must pale in significance when cmpared t the manufacturing user's cncern with the cmputer's effect n his prduct quality, amunt and cst f defective material prduced, and cnsequences f missing a prductin schedule. The Westinghuse Semicnductr Divisin in Y ungwd, Pennsylvania, is such a manufacturing facility. The prductin f high pwer semicnductr devices invlves prcessing raw silicn in rd frm thrugh several diffusin, ally, metallizatin, and passivatin peratins befre the fabricatin is cmplete, and the device is tested, packaged, and ready fr use in its final frm. The mst advanced technlgy, hwever, cannt make a gd prduct unless the many parameters which define the prcess recipe are accurately measured and cntrlled. Parameters such as furnace temperatures, gas flws, belt speeds, and humidity are critical t the manufacturing prcess, and even the mst sphisticated gauges are useless unless smene can cntinuusly bserve them..t\uer. w.v.es'i~atia~,.ie.e&:.al -u~~e6... ~ we ~ purchase a minicmputer t help reduce D.A. (defective apparatus) by mnitring sme f these vital parameters. The mnitring f diffusin furnace temperatures was selected as the initial applicatin because f relative ease f implementatin (reading vltages generated by thermcuples) and the large ptential return n investment. The furnace mnitring system wuld be the first step in establishing a real-time and histrical data base f prduct characteristics and perfrmance. Other imprtant parameters, such as gas flws, culd be added t the system, and the multi-tasking capability f the cmputer chsen wuld permit the executin f data analysis and design prgrams in a backgrund mdel while real-time mnitring cntinued in the fregrund. The cmputer selected was a 20. Imprtant features include well-develped prcess-riented hardware that can be fully cntrlled under a high level language (ORTRAN IV), and a multitasking perating rm the cllectin f the Cmputer Histry Museum (

2 0 Natinal Cmputer Cnference, system which prvides the fregrund/backgrund capability mentined abve. The cmputer was installed in Octber,, and has been expanded t its present cnfiguratin f 6K cre, 3.5 millin (l6-bit) wrds disk strage, incremental pltter, line printer, card reader, CRT cmputer cnsle, TTY furnace system cnsle, and prcess hardware cnsisting f a 0-pint-per-secnd analg input system, external interrupt system, cntact clsure input system, and cntact clsure utput system. Apprximately 16K f cre is reserved fr the furnace mnitring sftware, and the cmputer perating system ccupies anther 12K. The rest f cre is available fr ther real time and batch prgrams. The cmputer is capable f running 1,02 tasks n a pririty basis. The name chsen fr the cmputer, "DARIN," siands fr "Defective Apparatus Reductin and Infrmatin," and reflects the cmputer's purpse: D.A. reductin and imprved prduct by prviding infrmatin that was therwise either unavailable r difficult t btain. THE DIUSION PROCESS The first step f the manufacturing prcess is the diffusin f dpants int the sliced silicn t impart the desired electrical characteristics. rm three t six separate diffusin peratins are required fr each device, and errrs in this step f the prcess are usually irreversible. Diffusin peratins are carried ut in large furnaces at elevated. temperatures (1100 t I 2 C). The temperature must be maintained within a tlerance f ±2.5 C fr extended perids (tw t 0 hurs). This sak cycle is fllwed by a six hur slw cl t quench the diffusin. Each furnace is equipped with a timer set t maintain the peak temperature fr the prescribed time and then switch ver t a prgrammed slw cl. If the timer r furnace cntrller malfunctins, the junctin may be driven t deep. A run that is damaged in this manner cannt be salvaged. Since a run cntains frm 0 t 1000 slices, such furnace malfunctins are very cstly, nt nly in terms f scrap generated, but als in prductin time lst when a replacement run must be started frm the beginning f the fur t six week prcess. A clsely related prblem is that f scheduling the furnaces. It is very difficult fr the freman t keep track f the cnditins f 51 furnaces, used fr 11 prcesses, each with different behavir characteristics, and very few set fr the same time cycles. COMPUTER IMPLEMENTATION Cmputer mnitring was instituted fr half the furnaces in June, 5. It has since been expanded t include all the furnaces. The system was designed nt nly t flag furnace malfunctins, but als t be a useful tl fr the furnace peratrs. As an example, ne f the simplest features f the system, that f temperature reading, has prven extremely valuable. Befre the cmputer was installed, furnace temperatures were checked by a slw and ften inaccurate prcedure: a thermcuple, attached t a chart recrder, was inserted int the center zne f the furnace, and allwed t stabilize fr apprximately minutes, befre the temperature was read as a vltage, which was then cnverted t degrees by a table. This methd culd nly be used with empty furnaces; there was n way t read the furnace temperature while a run was laded in the furnace. In additin, the chart recrders, althugh calibrated weekly, were prne t drift, and culd drift as much as C withut detectin. Such errrs were unknwingly passed n t the furnaces prfiled with thse recrders, and were an additinal surce f D.A. SYSTEM DESCRIPTION The furnace mnitring system is centered arund a set f tables ccupying apprximately ne-fifth f the cre reserved fr the system. The tables describe the real time state f each furnace, as well as define temperature specificatins and furnace characteristics. Each time a furnace is laded, the tables are updated t reflect cycle infrmatin such as the times the run shuld enter cl dwn, be unladed, and a prjectin f the time the furnace will be reheated and ready fr a new run. The system is cntrlled by a master scheduling rutine, MAST, which references the cmputer's internal 60 Hz clck and issues calls t tasks perfrming the fllwing functins: I. Calibrate analg-t-digital cnversin system every secnds. 2. Read and stre furnace temperatures every secnds. 3. Cmpare temperatures t specs every 60 secnds.. Recrd ut-f-spec data n disk every 60 secnds. 5. Sund alarms as they ccur. TEMPERATURE MEASUREMENT urnace temperatures are measured with a type S (PlatinumlPlatinum-lO% Rhdium) thermcuple lcated n the utside f the liner f each furnace. This cnfiguratin prvides accurate detectin f temperature behavir (±0.5 C), minimizes thermcuple expsure t crrsive elements in the furnace, and des nt interfere with furnace lading r unlading. The thermcuples are cnnected t the cmputer by screw terminals in a thermally insulated cmpartment used as a cld junctin bx (CJB). A resistance temperature detectr (RTD) munted within the CJB measures rm temperature. This temperature is cnverted t millivlts and is added t the millivlt measurement f the type S thermcuple befre the thermcuple measurement is cnverted t degrees centigrade. The relatinship between temperature and vltage fr a rm the cllectin f the Cmputer Histry Museum (

3 The Cmputer in Manufacturing 1 type S thermcuple is linear in the ranges 0 _30 C and looo C t 1300 C. The diffusin furnaces have sak temperatures between 11 C and 12 C. Therefre, an equatin f the frm Y=mX+b can be used t cnvert rm temperature, as measured by the RTD, t millivlts n a type S thermcuple scale. A secnd equatin f the same frm is used t cnvert millivlts, as measured by the thermcuple with reference t the 0 established by the RTD, t degrees centigrade. The equatins are detailed belw. RTdC=(RTDmV/O.11)+25 (1) RTm V =(RTdC*O.OO6l) (2) TdC=(TmV+RTmV)* (3) RTdC = Rm temperature in C RTDmV =(milli-) Vltage detected by RTD RTm V = Rm temperature in millivlts (fr type S thermcuple) TdC =urnace temperature in C Tm V = urnace temperature measured in millivlts (type S thermcuple) The cnstants in equatin (1) are specific t the Mdel S RTD used. The cnstants in equatins (2) and (3) are derived frm a least squares fit f temperature versus vltage, using tables frm the Natinal Bureau f Standards (1). Althugh temperature readings are cnstantly updated, they are nly cmpared t spec temperatures while the furnace is laded. A set f flags infrms the cmparisn prgram, CHEK, f the status f each furnace and, therefre, f the actin t be taken. The pssible furnace states are: 1. Empty 2. Shut dwn (fr maintenance r cleaning) 3. Laded, in sak cycle, and in spec. Laded, in cl dwn cycle, and in spec 5. Ready t be unladed 6: ixmdedanct 'nt f spec CHEK examines the status flag f each furnace and then perfrms the apprpriate check. N check is made fr cnditins 1 and 2. urnaces in sak (3) are checked fr temperature within ±2.5 C f spec, fr time t tum ff gas flws, and fr the beginning f cl dwn. urnaces in cl () are mnitred t maintain a cling rate f at least ne degree C per minute until 00 is reached. When a furnace ges ut f spec (6), a timer is started. If it returns t spec within five minutes, nrmal mnitring cntinues; if it remains ut f spec fr five minutes, an alarm is sunded and a message is printed n the TTY, ntifying the furnace peratrs f the furnace, its temperature, and the spec. If crrective actin des nt bring the furnace back in spec within ten minutes, the alarm is rung again. The alarm is als sunded if a furnace des nt enter the cl dwn cycle within ten minutes f the prescribed time. While a furnace is ut f spec, the data is lgged n a disk file nce a minute. This prvides a recrd f furnace behavir that can be used t determine crrective actin, interpret results, r study furnace characteristics. Any furnace, regardless f cnditin, can be flagged t lg data in this manner, prviding a means f studying reheat cycles and recvery times. This data is summarized nce a day in a table shwing furnace number, the time it went ut f spec, hw lng it remained ut, the spec temperature, and the minimum, maximum, and average temperature during that perid. If mre detail is needed, the data can be printed as a simple chrnlgical list r displayed n the x-y pltter as a graph f temperature versus time. URNACE OPERATOR'S INTERACE The furnace peratrs interact with the cmputer thrugh a cnversatinal task, CONI, which runs n the TTY at the peratr's lading statin. The peratr specifies the desired actin, such as lading a run, and the task calls in the apprpriate prgram r subrutine. All interactin is in the frm f questins frm the cmputer and answers frm the peratr. Errr detectin and crrectin methds are included in the prgrams. The prgrams were designed t be user-riented, and t make the cmputer a useful tl fr the furnace peratrs, requiring a minimum f peratr respnse. The peratrs use the cmputer t lad and unlad runs, t read furnace temperatures, and t check furnace availability. The alarms and printed messages alert them t prblems and help them determine the necessary crrective actin. Sample interactins are shwn in igures 1 and PROCESS? PHOS URNACE? 0 "''f~ l'f1mp-~ RPEf'M.l... PHOS-DEP TIME? 2.0 TIME? 5. VERIY DATA PROCESS = PHOS STARTED AT 22 COOLOOWN AT RUN ENDS CE A V AILABLE URNACE = SOAK TIME = 53 1 (DATE-HR-MIN) (DATE-HR-MIN) (DATE-HR-MIN) PHOS O AT (DATE-HR-MIN) I DATA IS CORRECT, TYPE 0 TO START RUN. I NOT, TYPE 1 TO RE-ENTER DATA. igure 1 rm the cllectin f the Cmputer Histry Museum (

4 2 Natinal Cmputer Cnference, TIME TO TURN O WATER IN URNACE 3 CLOCK = : RUN IN URNACE 3 IS DONE-CLOCK = : 05 0 URNACE? 3 05 BY URNACES = 0 BY OPERATIONS = I PROCESS? PHOS PROC. = PHOS URNACE NO. NOW. SPEC = LATER OUT O SPEC. RUN ENDED IN URNACE 3 AT :5 URNACE? URNACE 26 READS 12.5 URNACE? 2 URNACE 2 READS URNACE? 2 URNACE 2 READS x J~ igure 3-5: 22- :5 22- : :2 22-: 22-16:2 22-1: 2 - :36 22-: : 0-0: 0 x x URNACE? 3 URNACE 3 READS 10.1 URNACE? URNACE READS URNACE? URNACE READS URNACE? 00 RESULTS igure 2 The results f the furnace mnitring system have exceeded the initial gal f reducing D.A. by catching furnaces that fail t g int cl. It was prjected that, t be cst-effective, the cmputer shuld catch at least ne furnace malfunctin f this type a mnth. The actual savings have been 3 t furnace lads a mnth, a cnsiderable amunt f prduct. The cmputer als prvides instantaneus and accurate temperature readings, which nt nly saves time ver the previus manual methd, but als enables the peratrs t determine temperatures at any stage in the furnace cycle instead f nly while the furnace is empty. The peratrs and freman are able t btain upt-date infrmatin n furnace status, including prjectins f next available furnace fr a given prcess. Sample displays f this infrmatin are shwn in igures 3 and. The cmputer's ability t track furnace behavir has been used t study the reheat time f furnaces, which resulted in increasing thrughput by 3-5 hurs in sme cases, and als prvides data used t determine which furnaces shuld be replaced. Ptential benefits are even greater. The cmputer prvides mre extensive and current infrmatin n the furnace system than was available befre. We are currently expanding the system t include frmatin f a data base that will be used t crrelate furnace behavir, device characteristics determined by the diffusin peratins, and perfrmance at final test. Data is als being cllected n the frequency and type f maintenance required by each furnace, with the gal f scheduling regular preventive maintenance. Wrk-in-prcess inventry in the diffusin area is als being tracked by the cmputer, and prvides the freman with mre current infrmatin than was btainable with the previus cumbersme handcunt methd. CONCLUSIONS The small cmputer has becme a valuable prductin tl in the manufacturing envirnment f Yungwd. The imprved prcess cntrl, cmbined with the cmputer's versatility in the areas f data gathering and analysis, ffer almst unlimited ptential. In rder t be effective, hwever, such a system must be designed t fit the manufacturing prcess. It must be specific enugh t meet the peculiar needs f each applicatin, yet the prgrams must be structured t allw fr changes in the envirnment; in ur system, a furnace may be cnverted frm ne prcess t anther, experimental runs may require a nn-standard temperature, etc. rm the cllectin f the Cmputer Histry Museum (

5 The Cmputer in Manufacturing 3 URN LAG LODTYP TEMP SPEC COOL ROUR MIN DONE ROUR MIN ALRM II 12 l ~ u STMDRI ALGABN ALSEAL OXIDAT PDPDRI PDPDRI PDPDRI OXIDAT OXIDAT l , luq () :5 1 igure Mst imprtant, the system must be riented tward the user-in this case, hurly emplyees, supervisrs, and engineers wh are nt cmputer peratrs r prgrammers. The cnversatinal questin-and-answer methd has been very successful at Yungwd, allwing the user t "cnverse" with the cmputer in familiar terms. The user's feedback shuld als be used t imprve the system. r example, initially the cmputer nly alerted peratrs t prblems, such as a temperature ut f spec. After becming familiar with the cmputer and with entering data n the TTY, the peratrs asked if the cmputer wuld be able t smehw print a message when it was time fr them t tum ff gas flws. Such a mdificatin f the riginal system was well wrth the prgram changes because it cntributed nt nly t the bjective f reducing D.A. in an area that had nt been riginally cnsidered, but als made the rm the cllectin f the Cmputer Histry Museum (

6 Natinal Cmputer Cnference, cmputer a mre useful tl, nt just a "glrified thermmeter" r an expensive new gadget that is mre truble than it is wrth. T be truly effective, as a manufacturing tl, the cmputer system must be carefully designed, and simplicity f interface with the end user, regardless f his backgrund, must nt be dismissed as an unnecessary frill. The furnace mnitring system was a test case fr the cmputer at the Semicnductr Divisin. It has been successful, and the experience gained in slving the prblems f implementatin is being applied t the expansin f the system t include mnitring f ther prcesses. The cmputer is becming even mre imprtant as a manufacturing tl in areas nt riginally cnsidered, such as wrkin-prcess inventry and device design, and we are lking frward t cntinued expansin int ther areas that affect the daily dllars and cents cncerns f the manufacturing plant. rm the cllectin f the Cmputer Histry Museum (