Design and Implementation of Test Wafer Management System

Test Wafer Management System

I. Description

A leading semiconductor manufacturing Fab had a very high usage of test wafers with a test wafer to production wafer ratio of 2.3 to 1. The Fab spent approximately $1.1M per quarter on test wafers. The Fab did not have any system to manage and track test wafer usage.

II. Objectives

• Design and implement a test wafer management system that would:
• Lower test wafer costs by at least 30%.
• Reduce the test wafer to production wafer ratio to 1:1.
• Ensure tools are not idle due to an insufficient supply of test wafers.
• Forecast future test wafer purchases based upon existing test wafer inventory and consumption.
• Ensure test wafers are used and reused to their full potential.

III. Methodology

• The overall Project Methodology (Figure 1) involved five major phases: data collection, data analysis, model design, simulation and model results, and sensitivity analysis and recommendations.

Overall project methodology 

• Tefen’s TWMS (Test Wafer Management System) software package was selected as the modeling tool due to its user-friendliness (Figure 2), its capability to capture a realistic test wafer flow (Figure 3), and its flexibility in performing various whatif scenarios.


Flow of tests in TWMS

• A simple system was set up in the Fab to identify test wafer types and to help
operators internally recycle test wafers.
• A Test Wafer Controller position was added to manage test wafer inventories and ensure tools were never idle due to a lack of test wafers.

VI. Results

• Test wafer costs decreased by 50% in just 6 months.
• The ratio of test wafers to production wafers was reduced to approximately 1:1.
• The Fab now has a software package that forecasts and manages their test wafer usage.
• Test wafer inventories were actively managed to continuously support production.
• Test Wafer Controllers took nonvalue- added activities away from the operators.
Consequently, the operators could devote more time to value-added activities (such as moving WIP).

Ratio of test wafers to production wafers    



    Quarterly test wafer cost