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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 intertwined 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 1

such as NIST . The certified properties of these standards that make them

In the past, significant confusion and disagreement in measurement science terminology has contributed to an inability to accurately compare data from multiple sources. Due to their universal nature, standards play a key role in defining such terminology on an international scale for a variety of industries.

Terms such as accuracy, precision, repeatability, reproducibility, random and systematic error pervade metrology activities in an often confusing and argumentative manner. Worse yet, there have been significant differences in the statistical treatment of metrology data in the various nations that participate in international commerce.

Consistent application of metrology standards plays a role in semiconductor manufacturing yield, as the effective use of yield data by multi-national companies depends on a cohesive and consistent understanding of metrology technology and standards worldwide.

suitable for instrument calibration are often determined through the use of

Resolving measurement uncertainty

standard test methods that are written

In 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,

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In the process of physical standards certification, the result of the certified measurement is called the measurand. The measurand consists of the value of the property (for example, film thickness or defect size) determined by certified measurement and the degree of uncertainty. In a successful calibration process, the instrument being calibrated reports a measurement result that is within the range of the uncertainties of the calibration standard.

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Figure 1. A taxonomy of standards.

resulted in significantly greater order in the use of terminology and uniformity in the treatment of measurement data. By defining a consistent method for reporting the results of measurements, the protocol forms the foundation for the internationally accepted definition of traceability in measurements. As defined by the International Vocabulary of Basic and General Terms in Metrology (VIM; 1993), traceability is:

One simplistic way to view the concept of uncertainty is to consider a measurement process which consists of a series of repeated trials where the arithmetic average (mean) of the measurements is recorded for each of the trials. The dispersion of these mean values characterizes the uncertainty of that measurement process. Uncertainty should not be confused with error, as it is an expression of the statistical nature of the measurement process.

The value of measurement data in establishing acceptable yield parameters depends on calibration with a low uncertainty. Consider, for example, a 4 nm gate oxide which has a process tolerance of 0.2 nm.

In order to have a 4:1 ratio of measurement capability to process tolerance, calibration standards must have an uncertainty of less than 0.05 nm. This is approximately one tenth of the spacing of silicon atoms (the lattice constant).

“The property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties.�

Traceability is of value to the semiconductor industry as it provides a tangible benchmark for measurement from an impartial third party arbiter of high level technical capability. An IC manufacturer can invoke the use of traceable standards in the process of acceptance testing a new metrology or process tool. Similarly, an IC manufacturer can use traceable metrology data to certify the quality of the products that he ships.

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Calibration challenges

“The semiconductor world is shrinking!” This is the preamble to virtually every presentation today concerning semiconductor manufacturing, but consider how true this statement is: • Gate oxides are approaching 4 nm, and are forecast to be perhaps 2 nm before a material change becomes necessary — this puts thin film growth and measurement dimensions in the realm of a few atomic layers.

• Particle and defect detection are often done optically at dimensions far below the wavelength of light employed by the detection tool, but how can we identify the source of the particle or deduce its size?

standards models will be used for calibration of all types of advanced metrology and inspection tools. As achieving acceptable semiconductor yield levels continues to become increasingly dependent on highly accurate metrology and inspection, such calibration standards will play a correspondingly significant role

in our future world of atomic dimensions.

1. The National Institute of Standards and Technology, Gaithersburg, MD, USA. 2. International Organization for Standardization, Geneva, Switzerland.

4 out of 5 Perfectionists Insist On VLSI’s Thin Film Metrology Standards.

Clearly, these rapidly accelerating changes continue to demonstrate the need for accurate, precise and repeatable measurements. With the international nature of the semiconductor industry, such measurements must be traceable to reliable and universal standards.

What the future holds

The semiconductor industry is now focused on, among other things, an organized, international, cooperative forecasting of our technical needs for the future and likely solutions, formulated into industry-wide roadmaps. This international effort provides an opportunity to understand the needs for both advanced metrology tools and the calibration standards to verify them. In addition, the underlying need for advanced education of measurement science technologists continues to be clear.

A shared vision among technologists around the world is emerging, where a combination of physical standards and consensus-based

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The Measurement Standards for the Industry.

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