With a few significant details, users can determine the life-cycle cost of
owning a manufacturing tool. While the examples shown below are based on
semiconductor manufacturing almost all manufacturing processes are faced with
these same issues.
At A Glance:
Using cost-of-ownership (COO) has significant benefits for the end user. It
is neither complex nor hard to do. With a few significant details about
purchase, operation, utilization, and performance, users can determine the
life-cycle cost of owning a semiconductor tool. Over the life of the system,
equipment reliability, utilization, and yield factors may have a greater
impact on COO than initial purchase costs. COO was developed for wafer
fabrications tools but can easily be extended to other applications. These
new applications are broadening the impact of cost modeling analysis and
providing a metric for improvement for the semiconductor industry.
With equipment costs rising every generation and manufacturers increasingly
sensitive to the cost per wafer, SEMATECH began developing a
cost-of-ownership (COO) model in 1990. Since then, COO standards have been
developed with Semiconductor Equipment and Materials International
(SEMI), and a commercial COO model has been prepared through a joint
Historically, purchase decisions have been based on initial purchase and
installation costs. However, purchase costs do not consider the effect of
equipment reliability, utilization, and yield. Over the life of the system,
these factors may have a greater impact on cost-of-ownership than initial
purchase costs. Lifetime cost-of-ownership per good device or wafer is
generally sensitive to production throughput rates, overall tool reliability,
and yield. It is relatively insensitive to initial equipment purchase price.
While initial COO models were developed for wafer fabrication tools, COO can
easily be extended to other applications.
A COO model has several benefits for the end user. First, the model can
provide a clear estimate of the cost-of-ownership. The program can also
highlight details that might be overlooked. COO provides an objective
analysis method for evaluating decisions. Both suppliers and manufacturers
can work from hard data to support a purchase plan. The model can also be
used to evaluate process and tool design. Finally, the COO model provides
communication between equipment suppliers and users. They are able to speak
the same language - comparing similar data and costs using the same
algorithms and equations.
Basic COO Algorithm
Estimating a tool's cost-of-ownership is neither complex nor hard. With a
few significant details, users can determine the life-cycle cost of owning a
semiconductor tool. The basic cost-of-ownership algorithm is described by:
CW = (CF + CV + CY)/(TPT * Y * U)
CW = Cost per wafer
CF = Fixed cost
CV = Variable cost
CY = Cost due to yield loss
TPT = Throughput
Y = Composite Yield
U = Utilization
Fixed costs include purchase, installation, and facilities costs that are
normally amortized over the life of the equipment. Variable costs such as
material, labor, repair, utility and overhead expenses are costs incurred
during equipment operation. Throughput is based on the time to meet a
process requirement such as depositing or etching a nominal film thickness.
Composite yield is the operational yield of the tool and may include
breakage and misprocessing. Utilization is the ratio of production time
compared to total available time.
Yield loss cost is a measure of the value of wafers lost through operational
losses and defects. Yield models are used in COO models for estimating the
relationship between contamination and yield loss or scrap. These models
relate integrated circuit yield to circuit and process parameters such as
device geometry and particle density.
A complete COO model requires information from many sources. To facilitate
data collection, the SEMATECH model has input fields for nearly 100 different
categories. By referencing the basic COO equations, these input categories
may be aggregated into a few significant inputs for initial COO estimates.
The Texas A&M Center of Excellence in Manufacturing Systems Research has
suggested the following grouping of COO inputs:
Annual operating cost
Process scrap yield
Die scrap yield
Value of wafer at process step
Value of completed wafer.
Equipment cost includes all fixed costs such as equipment purchase,
installation, and facilities support costs that are normally amortized over
the life of the equipment. Annual operating cost includes all of the
variable costs such as material, labor, repair, utility and overhead expenses
due to equipment operation. Process scrap yield, also referred to as
mechanical throughput yield, is the operational yield of the tool. Die scrap
yield is the defect limited yield that is recognized at wafer test or probe.
Downtime is the non-production time lost due to scheduled maintenance,
engineering usage, standby, and repair. Repair time is estimated from mean
time between failures (MTBF) and mean time to repair (MTTR).
Metrology & Inspection Tools
The COO model was developed to address the economic and productive
performance of a fabrication tool or specific semiconductor process step.
But COO for non-process equipment such as metrology tools is also needed.
Equipment cost, annual operating cost, downtime, and wafer values for the
metrology tool are considered the same way process tools are. Scrap costs
must be more carefully considered. Scrap caused by the inspection methods,
such as destructive testing, is part of COO. Scrap identified by the
inspection, but caused by previous processing, is part of process tool COO.
Thus yield losses caused by the process tool must be clearly separated from
yield losses caused by inspection.
Two additional costs of inspection are the cost of discarding a good device
and the cost of shipping a bad device. These costs are functions of the
inspection sampling plan and metrology characteristics. Minimizing the cost
of shipping a bad device is one purpose of inspection. However, if either
the inspection sampling plan or the metrology tool are insufficient, bad
devices will be shipped. But if the inspection specifications are too
restrictive, then good devices may be rejected or reworked. These costs are
due to the inspection step and are part of inspection COO.
Cluster Tools and Work Cells
Estimating the cost-of-ownership of cluster tools and work cells requires
more than just considering the COO of each component. The interactions among
the cluster tool or work cell components must also be considered. A cluster
tool is an integrated system of process, transport, and cassette modules
mechanically linked together. A work cell is a group of resources that are
treated as a single entity.
Cluster tools and work cells can be configured in serial, parallel, or a
combination of serial and parallel. For a simple serial system, component
failure rates may be added to estimate system failure rate. However, a
cluster tool is not a simple serial system. In a multiple chamber cluster
tool with a single wafer handling platform, the platform is serial with each
chamber. Each chamber has a serial failure mode(interrupts other chambers)
and a parallel failure mode (no interruption of other chambers). Each
chamber is independent of the other chambers but repair may require
interruption of the platform or common process supplies for safety reasons.
The simplification of the cluster tool as a serial system allows a first
estimate of COO. However, better estimates require more complex analysis
such as simulation models.
Throughput rate depends on whether the components of the system are
configured in series or parallel. If four tools were configured in series,
then product would move from tool A to tool B to tool C and finally to tool
D, completing the sequence. In this case the system throughput rate is
defined by the bottleneck chamber, but if the tool were configured in
parallel where each chamber performed the same task, the system throughput
rate is defined by the sum of the chamber throughput rates. If subsequent
COO analysis shows that throughput is a significant factor, then better
estimates may be obtained using discrete event simulation tools.
Test & Assembly
A major difference between a wafer fab COO model and a test and assembly
modelis the unit produced. Thus, by thinking in terms of cost per good unit
(die or circuit) instead of cost per good wafer, test and assembly COO can be
The COO conditions for metrology and inspection also apply to electrical and
mechanical test. Scrap identified by test but caused by the previous
processes is part of process COO and must be clearly separated from yield
losses caused by test. The cost of discarding a good device and the cost of
shipping a bad device is one purpose of test. However, if either the test
methods or specification guard bands are insufficient, bad devices will be
shipped. But if specifications are too restrictive, then good devices may be
rejected. Many test procedures accept a higher probability of rejecting good
devices to reduce the shipment of bad devices.
Waste disposal COO includes both disposal costs and the COO of equipment
required for waste treatment, disposal or reclamation. Equipment may range
from simple storage containers to complex chemical reprocessing facilities.
For complex systems, a complete COO model of the waste disposal system
should be completed. Waste disposal COO is part of the total life-cycle COO
of the process tool generating the waste. Thus waste disposal COO can be
strongly influenced by design changes in the process tool and waste disposal
COO strongly influences process tool COO.
1. S.Venkatesh, D.T.Phillips, "The SEMATECH Cost of Ownership Model: an
Analysis and Critique," SEMATECH/SRC Contract No.91-MC-506 Final Report
÷Appendices: Cost Analysis Background Material, Reporting Period: 9/1/91 to
9/1/92, Texas SCOE, Texas A&M University.