Operational Models for Evaluating the Impact of Process Changes on Cluster Tool Performance

This thesis describes operational models that integrate process models to expedite process change decisions for cluster tool performance improvement. The process engineer attempting a process change needs to wait for the industrial engineer to approve the change after making sure it does not degrade cluster tool performance. Having a model that integrates process parameters into the operational model of the tool helps the process engineer quantify the impact of process changes on tool performance.This makes the process change decision faster. Two integrated models for understanding cluster tool behavior have been developed here. One is a network model that evaluates the total time needed to process a lot of wafers for a given sequence of activities involved in the process. Including a manufacturing process model (in the form of a Response Surface Model) gives an integrated network model that relates the total lot processing time to process parameters like temperature and pressure and to process times. The second model developed is an integrated simulation model that can be used when the sequence of wafer moves is not given but is determined by a scheduling rule. The model can be used to quantify the impact of changes to process parameters and product characteristics like deposition thickness on total lot processing time. The thesis contains examples that illustrate the types of insights that one can gain into cluster tool behavior from using these integrated models

Optimum cost analysis of batch service retrial queuing system with server failure, threshold and multiple vacations

The aim of this paper is to analyze the queuing model entitled to cost optimization in bulk arrival and a batch service retrial queuing system with threshold, server failure, non-disruptive service, and multiple vacations. When bulk arrival of customers find the server is busy, then all customers will join in the orbit. On the other hand, if the server is free, then batch service will be provided according to the general bulk service rule. Batch size varies from a minimum of one and a maximum of ‘b’ number of customers. Customers in the orbit seek service one by one through constant retrial policy whenever the server is in idle state. The server may encounter failure during service. If the server fails, then ‘renewal of service station’ will be considered with probability . If there is no server failure with probability in the service completion or after the renewal process and if the orbit is empty, the server then leaves for multiple vacations. The server stays on vacation until the orbit size reaches the value N. For this proposed queuing model, a probability generating function of the orbit size will be obtained by using the supplementary variable technique and various performance measures will be presented with suitable numerical illustrations. A real time application is also discussed for this system. Additionally, a cost effective model is developed for this queuing model

Evaluating the Impact of Process Changes on Cluster Tool Performance

Cluster tools are highly integrated machines that can perform a sequence of semiconductor manufacturing processes. Their integrated nature can complicate analysis when evaluating how process changes affect the overall tool performance. This paper presents two integrated models for understanding cluster tool behavior. The first model is a network model that evaluates the total lot processing time for a given sequence of activities. By including a manufacturing process model (in the form of a response surface model, or RSM), the model calculates the total lot processing time as a function of the process parameter values and other operation times. This model allows one to quantify the sensitivity of total lot processing time with respect to process parameters and times.In addition, we present an integrated simulation model that includes a process model. For a given scheduling rule that the cluster tool uses to sequence wafer movements, one can use the simulation to evaluate the impact of process changes including changes to product characteristics and changes to process parameter values. In addition, one can construct an integrated network model to quantify the sensitivity of total lot processing time with respect to process times and process parameters in a specific scenario.The examples presented here illustrate the types of insights that one can gain from using such methods. Namely, the total lot processing time is a function not simply of each operation's process time, but specifically of the chosen process parameter values. Modifying the process parameter values may have significant impacts on the manufacturing system performance, a consequence of importance which is not readily obvious to a process engineer when tuning a process (though in some cases, reducing process times may not change the total lot processing time much). Additionally, since the cluster tool's maximum throughput depends upon the process parameters, the tradeoffs between process performance and throughput should be considered when evaluating potential process changes and their manufacturing impact

Immunological corollary of the pulmonary mycobiome in bronchiectasis:The Cameb study

Understanding the composition and clinical importance of the fungal mycobiome was recently identified as a key topic in a “research priorities” consensus statement for bronchiectasis. Patients were recruited as part of the CAMEB study: an international multicentre cross-sectional Cohort of Asian and Matched European Bronchiectasis patients. The mycobiome was determined in 238 patients by targeted amplicon shotgun sequencing of the 18S–28S rRNA internally transcribed spacer regions ITS1 and ITS2. Specific quantitative PCR for detection of and conidial quantification for a range of airway Aspergillus species was performed. Sputum galactomannan, Aspergillus specific IgE, IgG and TARC (thymus and activation regulated chemokine) levels were measured systemically and associated to clinical outcomes. The bronchiectasis mycobiome is distinct and characterised by specific fungal genera, including Aspergillus, Cryptococcus and Clavispora. Aspergillus fumigatus (in Singapore/Kuala Lumpur) and Aspergillus terreus (in Dundee) dominated profiles, the latter associating with exacerbations. High frequencies of Aspergillus-associated disease including sensitisation and allergic bronchopulmonary aspergillosis were detected. Each revealed distinct mycobiome profiles, and associated with more severe disease, poorer pulmonary function and increased exacerbations. The pulmonary mycobiome is of clinical relevance in bronchiectasis. Screening for Aspergillus-associated disease should be considered even in apparently stable patients.MOE (Min. of Education, S’pore)NMRC (Natl Medical Research Council, S’pore)Published versio

Generation of Feature-Based Models

Often there can be a number of ways to perform machining operations on a stock to produce a part, and only some of them are cost- and quality-effective. To determine the best machining sequence, it is necessary that each sequence of machining operations be evaluated and the best sequence be chosen. A feature-based model is a sequence of machining operations that when performed on a stock produces a part. This paper describes a way to generate all the feature-based models for a design of a part so that the designer can choose the best sequence of machining operations by evaluating each of the feature-based models.A part could be prismatic, sculptured or rotational, but the scope of this project is restricted to rotational parts (i.e. parts that are rotationally symmetric, about the horizontal axis). A rotational part can be reduced to two dimensions, without loss of information, by taking a cross section going through its rotational axis. The purpose of the project was to develop an algorithm that operates on the two-dimensional representation of a rotational part and generates feature-based models.There are drafting packages which have provisions for describing three-dimentional parts and some of them also support tolerance and surface finish descriptions. Most of the packages however, do not support feature extraction and generation of feature-based models. The system developed in this project provides a simple X-window user interface for describing a part and a specifying the features, and generates the feature-based models for the part