Spring99 therole

Spring 1999 Yield Management Solutions46

Standards

In the past, significant confusion and disagreement inmeasurement science terminology has contributed to aninability to accurately compare data from multiplesources. Due to their universal nature, standards play akey role in defining such terminology on an interna-tional scale for a variety of industries.

Terms such as accuracy, precision, repeatability, repro-ducibility, random and systematic error pervade metrol-ogy activities in an often confusing and argumentativemanner. Worse yet, there have been significant differences in the statistical treatment of metrologydata in the various nations that participate in interna-tional commerce.

Consistent application of metrology standards plays arole in semiconductor manufacturing yield, as the effec-tive use of yield data by multi-national companiesdepends on a cohesive and consistent understanding ofmetrology technology and standards worldwide.

Resolving measurement uncertaintyIn the early 1990’s, worldwide adoption of an ISO2

protocol, “Guide to the Expression of Uncertainty in Measurement”, began to address this issue. This protocol, developed by an international working group,

F E A T U R E S

The Role of Standards In Yield Management

by Jim Greed, President, VLSI Standards

Metrology plays a significant role in the management of yield; many measurements of wafer and reticle attributes can be correlated with ultimate device electrical performance, and are therefore used to maintain process control in the fab. Calibration of metrology and inspection tools has assumed increasing importance due to both the requirements of contemporary quality systems and the demands of consistent worldwide multi-site manufacturing. Throughout the process,standards provide the enabling technology to perform these tasks.

An Overview of Standards

The term standard can mean either a

physical artifact such as a reference

material used to calibrate a metrology

tool, or a documented procedure or list

of attributes used to qualify a product

(e.g. a product safety standard,). In

the field of measurement science, the

uses of this term are usually inter-

twined as shown in figure 1, which

delineates some of the most basic types

of standards.

Physical standards have one or more

well established properties, and are

often traceable to a national authority

such as NIST1. The certified properties

of these standards that make them

suitable for instrument calibration are

often determined through the use of

standard test methods that are written

rather than physical standards.

In the process of physical standardscertification, the result of the certi-fied measurement is called the measurand. The measurand consistsof the value of the property (forexample, film thickness or defectsize) determined by certified mea-surement and the degree of uncer-tainty. In a successful calibrationprocess, the instrument being cali-brated reports a measurementresult that is within the range ofthe uncertainties of the calibration standard.

The value of measurement data inestablishing acceptable yield para-meters depends on calibration witha low uncertainty. Consider, forexample, a 4 nm gate oxide whichhas a process tolerance of 0.2 nm.In order to have a 4:1 ratio of mea-surement capability to process tol-erance, calibration standards musthave an uncertainty of less than0.05 nm. This is approximately one tenth of the spacing of siliconatoms (the lattice constant).

One simplistic way to view the concept of uncertainty is to considera measurement process which consists of a series of repeated trialswhere the arithmetic average (mean)of the measurements is recorded foreach of the trials. The dispersion ofthese mean values characterizes theuncertainty of that measurementprocess. Uncertainty should not beconfused with error, as it is anexpression of the statistical natureof the measurement process.

resulted in significantly greaterorder in the use of terminology anduniformity in the treatment of mea-surement data. By defining a con-sistent method for reporting theresults of measurements, the proto-col forms the foundation for theinternationally accepted definitionof traceability in measurements. Asdefined by the InternationalVocabulary of Basic and GeneralTerms in Metrology (VIM; 1993),traceability is:

“The property of the result of a measure-ment or the value of a standard wherebyit can be related to stated references,usually national or international standards, through an unbroken chainof comparisons all having stated uncertainties.”

Traceability is of value to the semi-conductor industry as it provides atangible benchmark for measure-ment from an impartial third partyarbiter of high level technical capa-bility. An IC manufacturer caninvoke the use of traceable stan-dards in the process of acceptancetesting a new metrology or processtool. Similarly, an IC manufacturercan use traceable metrology data tocertify the quality of the productsthat he ships.

Spring 1999 Yield Management Solutions 47

F E A T U R E S

Figure 1. A taxonomy of standards.

Standards

International Regional orNational

Physical andCertifiable

Written

Weights andMeasures

Properties ofMaterials

ProductQuality/Safety

Compulsory Voluntary

48

F E A T U R E S

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The Measurement Standards for the Industry.

Calibration challenges“The semiconductor world isshrinking!” This is the preamble tovirtually every presentation todayconcerning semiconductor manufac-turing, but consider how true thisstatement is:

• Gate oxides are approaching4 nm, and are forecast to be per-

haps 2 nm before a material changebecomes necessary — this puts thinfilm growth and measurementdimensions in the realm of a fewatomic layers.

• Particle and defect detection areoften done optically at dimensionsfar below the wavelength of lightemployed by the detection tool, buthow can we identify the source ofthe particle or deduce its size?

Clearly, these rapidly acceleratingchanges continue to demonstratethe need for accurate, precise andrepeatable measurements. With theinternational nature of the semicon-ductor industry, such measurementsmust be traceable to reliable anduniversal standards.

What the future holdsThe semiconductor industry is nowfocused on, among other things, anorganized, international, cooperativeforecasting of our technical needsfor the future and likely solutions,formulated into industry-wideroadmaps. This international effortprovides an opportunity to under-stand the needs for both advancedmetrology tools and the calibrationstandards to verify them. In addi-tion, the underlying need foradvanced education of measurementscience technologists continues tobe clear.

A shared vision among technolo-gists around the world is emerging,where a combination of physicalstandards and consensus-based

standards models will be used forcalibration of all types of advancedmetrology and inspection tools. Asachieving acceptable semiconductoryield levels continues to becomeincreasingly dependent on highlyaccurate metrology and inspection,such calibration standards will playa correspondingly significant role

in our future world of atomicdimensions.

1. The National Institute of Standards andTechnology, Gaithersburg, MD, USA.

2. International Organization for Standardization,Geneva, Switzerland.