Gerald Weigert

A comparison of mip-based decomposition techniques and VNS approaches for batch scheduling problems

This research is motivated by a scheduling problem found in the diffusion and oxidation areas of semiconductor wafer fabrication facilities. With respect to some practical motivated process constraints, like equipment dedication and unequal batch-sizes, we model the problem as unrelated parallel batch machines problem with incompatible job families and unequal ready times of the jobs. Our objective is to minimize the total weighted tardiness (TWT) of the jobs. Given that the problem is NP-hard, we propose two different solution approaches. The first approach works with a time window-based mixed integer programming (MIP) decomposition. The second approach uses a variable neighbourhood search (VNS). Using randomly generated test instances, we show that the proposed algorithms outperform common dispatching rules that cannot deal with the given constraints effectively.

Just-in-sequence material supply—a simulation based solution in electronics production

Abstract Just-in-time and just-in-sequence (JIS) concepts have been presented by the automotive industries in the past years. Today, where customer-oriented manufacturing is more and more demanded, electronics production is in a similar situation. The integral organizational means at Diehl AKO Nuremberg have been realized to achieve JIS material supply in the electronics sector. The right cooperation of software solutions from SAP (ERP), LES (logistics) to BOFOS (manufacturing planning) results in a set of improvements in material flow, scheduling, and document flow. The following paper shows the concepts, realizations, and achievements of the partners Diehl AKO and ETL. The main interest lies on the general approach, not on a particular detail.

Simulation based scheduling system in a semiconductor backend facility

The semiconductor manufacturing process is usually divided in two parts: frontend and backend. In contrast to the frontend, where the manufacturing process is dominated by cluster-tools and cyclic routes, the backend has a predominant linear structure. In contrast to the frontend flow which is mostly controlled by dispatch rules, the backend process is suitable for real scheduling. A scheduling system for the backend of Infineon Technologies Dresden based on a discrete event simulation (DES) system was developed and tested in the real industrial environment. The simulation model is automatically generated from the databases of the manufacturer. The system is used for short term scheduling - from one shift up to one week. The paper will focus on the aspect of optimizing the process flow and calculating exact release dates for lots. The basic principles are applicable not only in the semiconductor industry but also in other industrial sectors

Design and validation of heuristic algorithms for simulation-based scheduling of a semiconductor Backend facility

A simulation-based scheduling system is discussed which was developed for a semiconductor Backend facility. Apart from usual dispatching rules it uses heuristic search strategies for the optimisation of the operating sequences. In practice hereby multiple objectives have to be considered, e.g. concurrent minimisation of mean cycle time, maximisation of throughput and due date compliance. Because the simulation model is very complex and simulation time itself is not negligible, we emphasise to increase the convergence of heuristic optimisation methods, consequentially reducing the number of necessary iterations. Several realised strategies are presented.

Simulations-based and solver-based optimization approaches for batch processes in semiconductor manufacturing

Scheduling is one of the key factors for semiconductor fabrication productivity. Objectives like lot cycle time and throughput must be optimized to push the technological development and secure the existence on the rapid growing global market. But especially in the frontend the manufacturing process is dominated by cluster-tools and reentrance flows which makes a production planning and optimization very hard. The workflow here is mostly controlled only by dispatch rules. To get a further improvement in manufacturing planning strategies, there is an increasing request of exact or simulation-based solution methods for specified work centers or bottleneck machine groups. One example of this is the semiconductor oven process. Here, complex batch processes with a lot of restrictions have to be scheduled. A reduction of cycle time in this section by optimized manufacturing strategies has a great influence on all global optimization objectives. Two approaches are investigated in this paper.

Optimization of manufacturing processes by distributed simulation

The paper describes a simulation-based optimization system for manufacturing processes. The principle suitability of simple Threshold Accepting and Great Deluge Algorithms for optimizing practical manufacturing systems is shown, as well as the possibility of parallelizing of these algorithms. A special IP Multicast architecture is introduced which allows the simulation model to be extended to multiple clients and so the efficiency of the optimization algorithms to be increased by concurrent simulation. Each client can log on or log out at an optimization server at any time so that it should be possible to use the idle computer capacity of a single production department or a whole company. The optimization server is implemented as an experimental option in the simulation system simcron MODELLER®, which is particularly used in electronics and semiconductor manufacturing.

A multistage mathematical programming based scheduling approach for the photolithography area in semiconductor manufacturing

Facilities for wafer fabrication are one of the most complex manufacturing systems. Typically, the bottleneck of such facilities is the photolithography area because of its highly expensive tools and complex resource constraints. In this research, a multistage mixed integer programming based optimization approach for planning of such an area is presented. Thereby, several existing process constraints like equipment dedications, resist allocation, vertical dedications, mask availability are taken into account on the basis of different granularity levels. Altogether eleven different optimization models are presented within four different decomposition stages. Thereby, objected goals are the maximization of throughput, the minimization of setup costs and a balancing of machine utilization. On the basis of real manufacturing data the benefit of the proposed approach is evaluated within a first prototype.

Optimisation approaches for batch scheduling in semiconductor manufacturing

In this research, several approaches for the optimisation of batch processes are investigated and compared. The motivation for this is the analysis of an oven work centre in a semiconductor wafer fab. Different objectives like keeping operational due dates as well as the minimisation of lot completion time will be optimised with regard to existing process constraints. First, the problem is modelled from two different viewpoints: as scheduling problem and with the help of a simulation system. For this, it used mixed integer programming (MIP) and a discrete event simulation system respectively. As a next step, a MIP time window decomposition approach combining simulation and mathematical programming is discussed. This reduces the complexity of the given problem by solving local sub-problems. Experiments on real manufacturing data determine the optimisation potential for the work centre using the different approaches. This allows, among others, a comparison to dispatching strategies as they are used in the manufacturing line. [Received: 01 July 2009; Revised: 09 November 2009; Accepted: 01 February 2010].

Simulation-based scheduling of assembly operations

The production of large-sized machines or facilities is fundamentally different from the usual manufacturing processes. Mostly it is a fixed-site production, dominated by typical assembly processes instead of linear operation sequences in the known flow shop or job shop problems. To describe these processes, several graph methods were developed in the past, i.e. AND/OR-graphs or modified Petri nets, but most of them are static methods and they are not suitable for scheduling tasks. This paper demonstrates how Timed AND/OR-graphs can be modelled with a common discrete event simulation (DES) system. The objective is a simulation-based scheduling system for assembly processes which is able to improve the prediction of due date keeping as well as to optimise the workflow of the assembly systems.

Scheduling preventive maintenance tasks with synchronization constraints for human resources by a CP modeling approach

This paper presents an approach for scheduling different types of preventive maintenances (PMs) for a work center of a semiconductor manufacturing facility. The PM scheduling problem includes time-dependent synchronization constraints and is implemented in a constraint programming model. A mix of periodic and workload-specific maintenances is scheduled considering the synchronization to available engineers which have individual shift schedules and skills that define the range of feasible maintenances. This also comprises maintenances having process durations covering multiple shifts, which requires a continuous availability of sufficiently skilled engineers. Additionally to the PMs, also handling and maintaining of unscheduled downs is considered in the model. Multiple objectives are investigated and used for optimization and tested on realistic data. To compare the results an additional simulation model is built up.