MANUFACTURING, SYSTEMS & SOFTWARE

It is a unique benchmark that more than2850 different types of semiconductordevices are being concurrently fabricatedwith a total volume of over 1 million piecesof 8” equivalent wafers every month in theleading foundry. If including active devicesnot making deliveries within the sameaccounting periods as well, this numberwould be magnified a few times more. Inaddition, this benchmark is being constant-ly rewritten since the rebound from thefinancial crisis in 2009.

With so many different products beingfabricated at the same time, it is a formida-ble logistical nightmare for the processcontrol engineers attending to the dailyoperations orchestrated by myriad compo-nents of the control hierarchy and its infra-structure. Still, offering the same servicelevels of advanced process controls (APC)is of paramount importance in qualityassurances to every customer, big andsmall. Achieving this operational objectivenecessarily imposes on the foundry servic-es providers some unique problems notencountered by integrated device makers(IDM). In summary detail below, we exam-ine four critical aspects: high-mix, low-volume, run-length, group-size, in thesearch of practicable solutions, the unique-ly proven sets of pillars in foundry APC.

After two decades of rapid evolutions,what sets foundry services apart from IDMin semiconductor manufacturing today liesin its widely diverse scopes of product cov-erage, instead of on the levels of techno-logical sophistications, as it once was. As it currently stands, the fundamental diff-erence in scope coverage constitutes asevere logistical problem to foundry servic-es. In the pursuit of its own goals of premi-um revenue and sustainable development, afoundry operates to satisfy five constraintsfrom its customer expectations: promptresponse, ample capacity, flexible capabili-ty, excellent quality and minimum cost.Subsequently, effective management of the highly severe logistical problem bringsabout a tremendous amount of complica-tions and unique difficulties regarding qual-ity performances of APC in foundry.

Quantification of the operational differ-ences starts with the numbers of activedevices concurrently being fabricated(Figure 1). As the general trend of assetreshuffling away from in-house manufactur-ing keeps accelerating in the semiconduc-tor business, the total number of devicesand combined volumes of delivered waferskeep increasing in the leading foundry. Thiscurve was compiled from historic recordsover the last decade and it closely reflected

The Devil inFoundry APC

Keung Hui, Jason MouTSMC

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this business movement. In contrast, priorreports from IDMs listed the number ofactive devices at around 150,[1] less than 6 percent of the current benchmark of2850 products. Such a huge gap in thenumber of products necessitates a struc-tural change in the optimal designs of thecontrol hierarchy and its infrastructures. A successful solution to this high-mix prob-lem holds the key to satisfy the customerexpectation of prompt responses.

The second aspect of the operationaldifferences penetrates into the distributionprofiles of the product-volume composi-tions (Figure 2). The foundry business

model enables extensive realizations oframpant innovations of IC designers andintellectual properties (IP) providers, in allsizes of economic scales. Subsequently,volume of each product comes in drasti-cally differing scales, ranging from a fewdozens to thousands of pieces of wafers(Figure 2). Tracking the chronological evo-lutions of the product-volume composi-tions over a span of accounting periods,the distributions roughly maintain thesame silhouettes. While there are a numberof devices finding widespread applications,the absolute majority of all productions areof low-volume types: Around 75 percent of

Figure 1. Distributions of Delivered Volumes Against the Number of Products in the Leading Foundry(active devices without delivery within sampling period were not counted)

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all products bring about a final volume of less than 150 pieces of 8” equivalentwafers (six lots at 25 pieces). Only lessthan 3 percent of all products fill up suffi-cient volumes to those levels fabricated by IDM. The immediate implication of thisstructural difference is that foundrieseffectively operate in continual transientstates, instead of reaching some dynamicforms of “steady states” as with IDMs. Thelatter condition plays a fundamental rolefor most process controllers to functionproperly; subsequently, methods of realiz-ing APC other than the conventionaldesign philosophies are needed for cost-effective process controls.

Another cross section of the product-volume compositions is the volume-per-centage distribution of the devices. The

top 100 products alone contributed over52 percent of all the historic volumes ofwafers ever fabricated. Two distinct modesof delivery profiles are observed: large vol-umes concentrated in short periods, andlow volumes intermittently spread overlong accounting spans. Clearly, the high-rate products are for applications in fash-ion and the low-rate products are fordurables of long life spans. The formerproducts temporarily drive the foundry,such as that of the IDMs, but the majorityof intermittent deliveries set the norm ofoperations in foundries. Being capable of switching between operation modessmoothly becomes crucial to meet thecustomer expectation of ample capacities.

The third difference exposes the wideranges of the semiconductor devices

The Devil in Foundry APC

Figure 2. Chronological Evolutions of Product-Volume Compositions With Low-Volume Dominance

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intended for diverse spectra of analogueand digital applications: clock rates, fre-quency bands, voltage ratings, power con-sumptions and other parameters of impor-tance. Subsequently, many more types ofprocessing tools with different engineeringcapabilities need to be installed than mostIDMs need, as the latter are likely to focuson some market niches rather than com-prehensively manufacturing all-encom-passing products. Compounded with thecomplexities of reentrant flows of differentproducts, utilizations of the processingtools vary greatly, with non-uniform runlengths as dictated by the high-mix, low-volume compositions.

Snapshots of run lengths of productsprocessed by different tools are pulledover selected periods (Figure 3). Even for

high-volume devices, the number of toolsavailable to process an operation step inthe lengthy sequences depends criticallyon the tool constraints, among other fac-tors. Worse still, this situation of severelynon-uniform loadings occurs to tool setseven though they may belong to the sametool groups. Subsequently, the APC hierar-chy must inherit multiple sets of tacticsresponding to non-uniform loadings andyet maintain uniform behaviors to guaran-tee performances. Obviously, non-uniformoperating conditions are highly undesirablefrom the viewpoint of throughputs; never-theless, having achieved the economies of scale complements the engineeringrequirements among the tool sets and creates significant imprints on the cus-tomer expectation of flexible capabilities.

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Figure 3. Non-uniform Run Lengths of Control-Threads for High-Volume Products

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The fourth aspect fundamentally differ-entiates the tool-level realizations of APCin foundries from those in IDMs, renderingmany of the conventional practices pio-neered by the latter no longer optimal inthe former. With high levels of work-in-progress to maximize utilizations forthroughputs, metrology delays normallytake up to around six lots before measure-ments become available for feedback tothe process controller. Given compositionprofiles of products shown in Figure 2,clearly, methods other than the conven-tional thread controls[2] employed byIDMs must be adopted for over 75 percentof those devices having a volume of lessthan six lots (Figure 4).

Practical constraints render sampledmeasurements useful only to long runlengths of threaded controls in convention-al feedback loops. For raging transientsdue to the high-mix, low-volume situation,some other forms of data feedback or usesof the metrology measurements are need-ed for these 75 percent of short run-lengthproducts, whether or not the control isthreaded. A primary and logical strategy to simultaneously resolve the three issuesof non-uniform tool loadings, short run-lengths and sparse metrology feedbackstries to superimpose grouping controls of tools, products or a mixture of both.Optimizing the management practices andcontrols of the functional groups proves as

The Devil in Foundry APC

Figure 4. Application Ranges of Thread-Controls Using Conventional Process Controllers

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the most cost-effective to efficiently fulfillthe customer expectation of excellentqualities.

Lastly, the concerted efforts in con-structing the optimization hierarchy withinherent characteristics of high-mix pro-ductions, the infrastructure to slide intooperation modes accommodating loadingsurges, the comprehensiveness of engi-neering diversity in catching technologicaldemands, and the optimal realizations ofprocess control systems in escorting quali-ty assurances aligns synergistically towardthe business goal of premium revenues tothe foundry and the customer expectationof minimum costs.

The construction process, however, isfull of details with too many uncharted ter-ritories where the devil looms. At the lowerspectrum in realizing APC in foundries andfor which gargantuan volumes of processand tool data are crunched at high fre-quencies, the most hideous pitfall is theassumption of ergodicity, one detail toooften enshrouded behind thick veils andharmfully taken for granted by most dataanalysts in the designs of APC applica-tions.[3]

In conclusion, we tried to quantify theoperational differences between IDMs andsemiconductor foundries over APC appli-cations from four aspects in relation to thefive customer expectations. The originalarticle was motivated by an exposition ontemptations of elaborate statistical con-structions without cautious alerts to theunderlying assumptions for APC applica-tions to function properly. The revelationhopes to draw the attention of practition-ers on building more robust process andequipment models for quality control pur-

poses. Otherwise, the devil of foundry APClurks in the absence of ergodicity in thesystem dynamics, rendering the logisticalproblem not controllable in effective andefficient manners.

References1. John Schmitz, “View on Advanced

Control Techniques from a Wafer FabManager’s Perspective,” 3rd EuropeanAEC/APC Conference, Dresden,Germany (2002)

2. C.A. Bode, J. Wang, Q.P. He, T.F. Edgar,“Run-to-run control and state estima-tion in high-mix semiconductor manu-facturing,” Annual Reviews in Control v. 31, pp. 241-253 (2007)

3. K. Hui, J.I. Mou, “The Devil in FoundryAPC,” AEC/APC Symposium XXII,Austin, Texas (2010)

About the Authors

Keung HuiKeung Hui is a technical manager with

TSMC. He holds a Ph.D. in control engi-neering with interests in modeling, simula-tions and optimizations. Dr. Hui is the pro-gram chair of many process control sym-posia in Taiwan.

Jason Mou Jason Mou is a deputy director respon-

sible for the development of APC solutionsin the Manufacturing Technology Center ofTSMC. He holds a Ph.D. in engineering andhas held various educational and executivepositions. �

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