L.E.N. Lim

Optimal design of neural networks using the Taguchi method

Abstract In the last five years, many new learning algorithms have been designed and developed to train neural networks for solving complex problems in a wide variety of domains. One of the principal deficiencies with current neural network research is associated with the design of the neural networks. The design of a neural network involves the selection of an optimal set of design parameters to achieve fast convergence speed during training and the required accuracy during recall. These design parameters include both the micro-structural and macro-structural aspects of a neural network. This paper describes an innovative application of the Taguchi method for the determination of these parameters to meet the training speed and accuracy requirements. Using the Taguchi method, both the micro-structural and macro-structural aspects of the neural network design parameters can be considered concurrently. The feasibility of using this approach is demonstrated in this paper by optimizing the design parameters of a back-propagation neural network for determining operational policies for a manufacturing system. Results drawn from this research show that the Taguchi method provides an effective means to enhance the performance of the neural network in terms of the speed for learning and the accuracy for recall.

Plasmonic microzone plate: Superfocusing at visible regime

A silver film-based superlens, a plasmonic microzone plate, is put forth for the purpose of superfocusing at visible regime. The numerical analysis results reveal that it has unique focusing characteristics of longer focal length and depth of focus with resolving power beyond diffraction limit in comparison to the conventional zone plates. Working at near field, it increases working distance l at near field from l <lambda/10 to l approximate to lambda similar to 5 lambda. This feature makes it a promising superlens to be used as a probe for the optical systems with high resolution imaging and detection as well as feedback control system immune for autofocusing. (c) 2007 American Institute of Physics.

Fabrication of Si microstructures using focused ion beam implantation and reactive ion etching

Localized Ga implantation by means of a focused ion beam and subsequent deep reactive ion etching is used to fabricate microstructures or nanostructures in Si. It is found that the area irradiated with the focused ion beam acts as an etch stop in reactive ion etching. The etch rate ratio between the unimplanted and Ga-implanted area increases to 2.56 with an increasing Ga ion dose up to 1.59 × 1016 ions cm−2. However, the etch rate ratio decreases with further increase in ion dose and the surface of the etched Si microstructures is concave due to the effect of ion sputtering during focused ion irradiation. This technique is promising for the fabrication of 3D nanostructures in Si.

Optimization of laser deep-hole drilling of Inconel 718 using the Taguchi method

Abstract The use of a Nd:YAG laser to drill deep cooling holes in aerospace components is a stringent operation. Various characteristics pertaining to the input beam, focussing lens, and assist gas have to be optimized in order to produce a hole economically and of sufficiently good quality. This paper reports the use of the Taguchi technique of experimental design in optimizing the process parameters for drilling deep-holes in nickel-based superalloy, Inconel 718. The thickness of the material is 25.0 mm. Oxygen is the assist gas andthe focal length of the focussing lens is 300 mm. The effects of five process parameters — pulse energy, pulse duration, pulse shape, focal position, and assist gas pressure — have been explored. The various parameters are assigned to an L18 orthogonal array. The primary response under study is the drilling time. It is predicted that a minimum drilling time of 31.51 s is needed to drill a hole with a pulse energy of 30.0 J, a pulse duration of 1.8 ms, a “treble” pulse shape, a focal position of 0.0 mm into the material, and an oxygen pressure of 0.35 MPa. Confirmatory experiments have produced results that lay within the 95% confidence interval.

Angular dispersion compensation for acousto-optic devices used for ultrashort-pulsed laser micromachining

Ultrashort pulsed laser material processing is a new micromachining method that is gaining interest. Its capability of submicrometer machining has been proved. To obtain high speed and highly flexible beam steering, a two-axis acousto-optic deflector is employed. However, dispersion associated with acoustic-optic interaction will cause serious spatial deformation on the writing spot. The compensation for dispersion is proposed and studied. Experiments show promising results. An additional advantage of the proposed compensation method is that it can also precisely control the pulse number, and, hence improve the quality of ablation.

A review of the Nd: YAG laser marking of plastic and ceramic IC packages

Abstract The marking of integrated circuit (IC) packages with neodymium-doped yttrium aluminium garnet (Nd: YAG) lasers has become a widely accepted technique in the microelectronics industry. This paper presents a literature survey and discussion of the various methods and mechanisms of Nd: YAG laser marking of plastic and ceramic IC packages. The effects of the material properties (e.g. the absorptivity and the melting point) and the marking parameters (e.g. power density, the focal position and the marking speed) on the mark legibility characteristics (e.g. the mark contrast and the mark width) are reviewed. Economic advantages and technological limitations of thr marking of IC packages with Nd: YAG lasers are discussed also.

Femtosecond pulsed laser direct writing system

Laser writing using conventional lasers has found widespread application in the fabrication of electronic components and the repair of integrated circuits. However, due to large thermal damage associated with the laser ablation process, these lasers are not suitable for microelectronic components with submicron feature size. To address these issues, a novel system is introduced for laser direct writing using femtosecond lasers. Here, an acousto-optic deflector-based scanning system has been developed for steering the femtosecond laser beam with high positional accuracy, resolution, and scan speed. The capability of the system to machine complex features with submicron line widths has been proved. This system has profound potential application in the electronic and microfabrication industry.

A computer-aided framework for the selection and sequencing of orientating devices for the vibratory bowl feeder

Abstract Correct part orientation has been the key to many successful automation applications, and often vibratory bowls were used. Traditionally, the design and tooling of the vibratory bowls are limited to a group of skilled experts. In this paper a feature-based framework for the automatic selection and sequencing of orientating devices is presented. A direct link between part representation and the selection and sequencing of modular orientating devices is provided by a spatial technique, in place of traditional CAD representation. Also described is the classification of a parts orientating features and classification of orientating devices. These two classifications are integrated into a knowledge-based system that will automatically generate sequences of modular orientating devices.

Near-field behavior of zone-plate-like plasmonic nanostructures

The near-field behavior of a new plasmonic structure, the plasmonic micro-zone-plate (PMZP), is presented. The PMZP can realize superfocusing at a working distance on the micrometer scale and a resolving power beyond the diffraction limit. Compared with conventional Fresnel zone plates (CFZPs), its unique characteristics of a significantly elongated depth of focus (DOF) and focal length will make autofocusing easier for the relevant optical systems. These characteristics imply that it is possible to realize a free feedback control system for autofocusing systems in which probe scanning is performed with a constant working distance from the probe to the sample surface, provided that the flatness variation of the sample substrate is within the DOF. Moreover, unlike the CFZPs, there is no series of focal points appearing for beam propagation in the near-field region with a propagation distance ranging from lambda to 8 lambda or even longer. In addition, transmission properties in the near-field region are investigated by means of a computational simulation based on a finite-difference time-domain numerical algorithm. Peak transmission wavelength shifts were observed while the metal film thickness was changed. Focusing characteristics were analyzed for different numerical apertures of the PMZPs.

Application of Taguchi methods in the optimization of the laser-cutting process

Abstract In this paper, the use of an overall figure-of-merit (FOM) function which integrates the weighted effects of the various quality characteristics of a laser-cut specimen and the cost components of a production environment is proposed. Taguchi methods and statistical techniques are used to formulate the experimental layout, to analyze the effect of each control factor on the results, and to predict the optimum setting for each control factor. A set of confirmatory experiments is then conducted to verify the estimated response. A Trumpf 2000 laser machine which incorporated a Rofin Sinar RS500 laser was used to cut 4.5 mm thickness low-carbon steel sheets. Five control factors were used in a modified L8 orthogonal array design. A high FOM was obtained using the treatment conditions of: beam power of 415 W, oxygen pressure of 0.12 MPa, focal position of 1 4 , focal length of 63.5 mm, and cutting speed of 0.8 m/min. The confirmatory experiments returned an average FOM value that lay within the 95% confidence interval.