Hot Lot Research Articles

A study on the lot production management in a thin-film-transistor liquid-crystal display fab

Because of their higher processing priority, hot lots often interrupt the production of regular lots in a Thin-Film-Transistor Liquid-Crystal Display (TFT-LCD) fab. As a result, how to manage the production of hot lots so that their production demand can be met and their negative effects on regular lots minimized is a very important issue. In this paper, we identity three problems – the lot selection of interbay AGVs, the lot selection of intrabay machines, and the photo bay selection of lots – that can affect the production of hot lots and regular lots and propose methods for them. A fuzzy-based dynamic bidding (FBDB) method is proposed for the first two problems. The bidding functions in this FBDB method consider several attributes of the current system to obtain the true values of bids. An earliest possible time (EPT) method that also considers several attributes of the current system is proposed for the third problem. These two methods are compared with methods used by a Taiwanese TFT-LCD fab through computer simulations. The effects of hot lot ratios on the performance of these methods are also analyzed. Six performance measures are adopted to measure the throughput and tardiness performance of all lots, regular lots and hot lots. The simulation results show applying the FBDB and EPT methods to the three problems studied here can result in very good results in all performance measures.

A parameterized-dispatching rule for a Logic IC sort in a wafer fabrication

In this paper, a parameterized dispatching rule (PDR) using the response surface method is presented for a Logic IC sort in a wafer fabrication. A broad product mix, variable lot sizes and yields, long and variable set-up times, and limited test equipment capacity can characterize the operations in this wafer probe centre. The objective of this study is to simultaneously optimize the desirability function that is the combination of multiple attributes of the products such as the hot lot or normal lot, critical ratio with good on-time delivery performance, processing time, set-up time, waiting time, and machine flexibility. This is accomplished by combining the multiple attributes together using a linear combination with relative weights. Also, multiple performance measures are considered in this study. Discrete event simulation models based on eM-Plant are developed to implement the proposed rule. The comparison is also made with other rules such as shortest set-up time (SST), shortest processing time (SPT), shortest set-up and processing time (SSPT), smallest critical ratio (SCR), first in first out (FIFO) and least machine flexibility (LMF). Here, two combined performance values are used as the performance measure that both are combined with the multiple performance measures to a single value using the desirability function. The simulation results show that the four proposed PDRs outperform other rules in many different operational conditions.

VRP-Based Model and Algorithm for Hot Rolling Lot Planning

According to the process programs of hot rolling strips production in an iron & steel enterprise, a VRP (Vehicle Routing Problem) based model for hot rolling lot planning is presented, an ACO (Ant Colony Optimization) algorithm is designed to solve it. Computational study for practical instances has been made, the hot rolling lot plans obtained with the new model are compared with that made by senior planner through human-machine coordination, results show that the former is more effective and efficient.

An interactive scheduler for a wafer probe centre in semiconductor manufacturing

This paper analyses production operations and scheduling constraints on a wafer probe centre in semiconductor manufacturing. Such actions as lot split and hot lot preemption are discussed. Due to the combinatorial difficulties of scheduling problems with sequence-dependent setup costs and multiple criteria, a humancomputer interactive scheduler named the 'interactive computer aided scheduling system' (ICASS) is proposed. This ICASS can search for schedules to achieve a specific performance level set by a human. An experiment was conducted on a wafer probe centre to compare the performance of ICASS with that of a manual approach and six priority rules. The results indicate that ICASS spent less scheduling time and provided better schedules than the manual approach. All the six priority rules yielded only poor schedules with unbalanced performance in three kinds of criteria, although they were beneficial in computation time.

Sterile hypopyon following intraocular lens surgery.

Energy-dispersive x-ray analysis (EDXA) was used to study nine intraocular lenses from one manufacturer. Residual polishing compound was found on two of three dry-sterilized lenses from a "hot lot" associated with a high incidence of sterile hypopyon. Similar material was not present on normal lenses. This polishing compound was apparently fused to the lens surface, preventing its removal by ultrasonic cleaning. In addition, three wet-sterilized lenses were found to be covered with small crystals containing barium. These crystals apparently resulted from leaching of the glass storage vial by the caustic storage fluid. It is suggested that some cases of sterile hypopyon following intraocular lens surgery may be caused by residual polishing compound present on the lens surface.