Eamonn Ahearne

Ultraprecision grinding technologies in silicon semiconductor processing

Abstract The production of silicon substrates for integrated circuits continues to set standards in levels of precision form and finish tolerances required of surface generation processes. Extreme tolerances are specified for a range of parameters such as total thickness variation, global and local planarity, and surface finish over substrate dimensions of up to 300mm in diameter (current-generation silicon wafer). These tolerances are related to the ‘design rule’ for each generation of microprocessor and memory unit. The economic and technological environment of an industry that demands such precision is reviewed. The general production process is then described with particular reference to surface grinding as an enabling technology. The context of developments in ultraprecision machine tool technology is delineated, requirements for assuring the indicated tolerances are set out and machine solutions representing the ‘state-of-the-art’ and ‘next-generation’ machine technologies reported.

The performance of polycrystalline cubic boron nitride tools in continuous, semi-interrupted, and interrupted hard machining

Abstract Polycrystalline cubic boron nitride (PCBN) cutting tools have enabled large-scale industrial hard machining owing to their high hot hardness and wear resistance. Experience clearly shows that tool requirements vary depending on the presence and severity of interrupts in the workpiece. The interrelationships between workpiece interruption parameters and tool wear and performance are assessed using a programme of continuous, semi-interrupted, and interrupted hard machining tests. A hypothesis for observed variations in wear behaviour between different PCBN grades and test conditions is developed on the basis of detailed tool wear scar analyses.

Electromechanical modelling for piezoelectric flextensional actuators

The piezoelectric flextensional actuator investigated in this paper comprises three pre-stressed piezoceramic lead zirconate titanate (PZT) stacks and an external, flexure-hinged, mechanical amplifier configuration. An electromechanical model is used to relate the electrical and mechanical domains, comprising the PZT stacks and the flexure mechanism, with the dynamic characteristics of the latter represented by a multiple degree-of-freedom dynamic model. The Maxwell resistive capacitive model is used to describe the nonlinear relationship between charge and voltage within the PZT stacks. The actuator model parameters and the electromechanical couplings of the PZT stacks, which describe the energy transfer between the electrical and mechanical domains, are experimentally identified without disassembling the embedded piezoceramic stacks. To verify the electromechanical model, displacement and frequency experiments are performed. There was good agreement between modelled and experimental results, with less than 1.5% displacement error. This work outlines a general process by which other pre-stressed piezoelectric flextensional actuators can be characterized, modelled and identified in a non-destructive way.

An Investigation of Force Components in Orthogonal Cutting of Medical Grade Cobalt Chromium Alloy (ASTM F1537)

An ageing population, increased physical activity and obesity are identified as lifestyle changes that are contributing to the ongoing growth in the use of in-vivo prosthetics for total hip and knee arthroplasty. Cobalt–chromium–molybdenum (Co-Cr-Mo) alloys, due to their mechanical properties and excellent biocompatibility, qualify as a class of materials that meet the stringent functional requirements of these devices. To cost effectively assure the required dimensional and geometric tolerances, manufacturers rely on high-precision machining. However, a comprehensive literature review has shown that there has been limited research into the fundamental mechanisms in mechanical cutting of these alloys. This article reports on the determination of the basic cutting-force coefficients in orthogonal cutting of medical grade Co-Cr-Mo alloy ASTM F1537 over an extended range of cutting speeds ( v c ) and levels of undeformed chip thickness ( h m ). A detailed characterisation of the segmented chip morphology over this range is also reported, allowing for an estimation of the shear plane angle and, overall, providing a basis for macro-mechanic modelling of more complex cutting processes. The results are compared with a baseline medical grade titanium alloy, Ti-6Al-4V ASTM F136, and it is shown that the tangential and thrust-force components generated were, respectively, ≈35% and ≈84% higher, depending primarily on undeformed chip thickness but with some influence of the cutting speed.

Modelling of Piezoelectric Actuator (PEA) for Advanced Process Control in Chemical Mechanical Polishing (CMP)

Lead zirconate titanate (PZT) stacks are commonly used for submicron resolution actuation, fast response times and high sensitivity. They are usually modeled as expansion generators without external load. This paper proposes an electromechanical model for a commercially available micro-piezoelectric actuator (PEA) which comprises pre-stressed PZT stacks and external amplifier flexure frame for closed loop force control. The proposed model avoids the need to measure the piezoelectric charge which is usually required in conventional electromechanical models. The mechanical part of the PEA was modeled as a linear, lumped, double mass-spring-damper system and the related parameters were experimentally identified. The PEA system was characterised under load-free and load-applied conditions, and the electromechanical coupling ratios which describe the energy transfer from the electrical domain (voltage) to the mechanical domain (endpoint displacement/force) were experimentally determined.

Piezoelectric drive systems in ultraprecision machines: a review of the state-of-the-art

The production of ultraprecision and micromachined components has been enabled by the development of a range of drive system technologies with commensurate resolution. In particular, systems based on piezoelectric actuators have provided an alternative to mechanical drive systems. These actuators have been incorporated in drive systems for high resolution positioning of atomic force microscopes, high frequency vibration compensation and high speed valves for diesel injection. The performance of systems based on piezo actuators is characterised by high electromechanical advantage, submillisecond response times and nanometric resolution. In many cases the inherent limited range of travel has been compensated by the design of flexure systems. When combined, these provide precision control free of friction and backlash, otherwise unattainable with conventional mechanical drives. The objective of this paper is to give an overview of the current state-of-the-art. It will highlight the novel application of these alternative technologies. The advantages of piezoelectric technology and their limitations will be discussed.