Purpose of the Study

The specific purpose of this dissertation is to contribute understanding on the control of cycle time and cycle time variability in a front end semiconductor production line. Many different sources of variability can be identified in a production line. Therefore, a conversation was engaged with semiconductor factory managers to identify the dominant sources. Batching and tool availability have emerged as the main disturbances in a modern front-end semiconductor line. This study will focus on those two factors and the control of their negative impact on the line‘s productivity. In collaboration with

these factory managers, three objectives (1 to 3) were identified to fulfill this purpose. Finally, the lessons learnt from previous simulation and modeling process should be applied to produce and develop a new release policy.

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1.3.1 Objectives

Factory managers pointed out that the variability created by process factors has for origin specific toolsets or operations. A metric is essential to investigate operation individually and identify sources of process variability in the production line.

Objective 1: Develop a metric to measure the amount of variability created by an operation.

Many different factors of variability can be identified in a production line (Section 2.3). Nevertheless, factory managers again pointed out that for a modern front-end semiconductor factory three factors (as justified in section2.13) are preponderant: tool availability, batching and re-entrant lines. These three factors cannot be studied simultaneously. Too many variables would considerably increase the difficulty. The scope of this study will be limited to the detailed analyses of tool availability and batching only. Then the results obtained will be tested in a re-entrant environment. Interesting areas for policy, practice and future research for the academic community, will be highlighted.

Objective 2: Understand and explain the impact of tool availability and batching on cycle time and cycle time variability.

Managers also pointed out that some operations have high output variability, but they do not have the highest impact on the overall production line. Thus, the interactions between operations are keys to the reduction of process flow variability. The circumstances easing or emphasizing variability in the flow have to be exposed.

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Objective 3: Determine how the variability in the flow of items is affected by the interaction between operations.

The utilization of an appropriate release strategy appears as a promising solution to control cycle time and cycle time variability (Section 2.6). Therefore, the results obtained from the three previous objectives will be used to develop a release strategy adapted to front end semiconductor production lines.

Objective 4: Develop a new release strategy to take advantage of the interaction between operation and control batching and tool availability disturbances.

1.3.2 Significance of the Study

This study supplements existing literature on operation management by shedding new light on process control. It summarizes the knowledge available on process variability and scheduling policies. It identifies a range of mechanisms affecting cycle time and cycle time variability. In particular, it develops the concept of critical availability and provides a novel release strategy enhancing the performances of the production line.

Thus, the study brings an innovative reference to managers of front end semiconductor factories and helps the development of their scheduling policy. For the academic community, it highlights useful areas for policy, practice and future research.

1.4 Approach

A well developed methodology is useful in demonstrating what was done in the research process, and to articulate how research practices transformed observations into data, results, findings, and insights

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1.4.1 Overview of Methodology

Two different approaches were adopted in this study.

First, a descriptive study was undertaken to better comprehend the nature of the problem. A front end semiconductor devices manufacturer was contacted to get access to the production data. The analysis of the data was performed using several statistical calculations. This preliminary work was needed to gain familiarity with the phenomenon in the situation and understand what is occurring. Then a model could be developed and a rigorous design was setup for comprehensive investigation.

Second, an explanatory study was undertaken to clarify the causes, the context, and consequences of the observed phenomenon. A simulation model was developed using Extend Simulation software (www.extendsim.com). The advantage of simulation is the facility to monitor and control the entire production system without doing any changes in the real environment, saving time, efforts and money. Parameters can be modified many times to determine the optimum configuration.

1.4.2 Limitations

In the exploratory study, the absence of control on the content of the data was a seriously limiting factor. The data were giving indication as to the ―when‖, ―how many‖ and ―how often‖ something occurs, but they couldn‘t tell us ―why‖ or ―how‖. The

research could not describe what caused a situation. Moreover hypothesis and theory couldn‘t be put under the test as no experiment could be run.

With simulations, the fidelity and validity of the simulations outcomes is dependent on the acquisition of valid source of information, the relevant selection of key

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characteristics and behaviors, and the appropriate use of simplifying approximations and assumptions within the simulation. Moreover simulation results are hypothetical. Ideas and theories may be tested with simulations, but the transfer to real environment is not 100% sure and outcomes may differ slightly from predictions, particularly for complex systems as semiconductor processing lines.