Sealed linear encoder delivers sub-micron positioning resolution

Increasing global demands in the highly-challenging prototyping and production of silicon wafer and related substrates have played directly into the hands of Loadpoint. The company has carved a niche in manufacturing bespoke equipment for cutting, dicing, grinding, polishing, slitting and drilling components used in the semiconductor, printer head, optical, ultrasound scanning and medical diagnostic sectors.

The MicroAce 66 model 150 x 150 x 10mm capacity micro dicing machine is the latest example of how Loadpoint meets the challenges posed by bespoke high-precision equipment: the machine had to obtain - and maintain - an axis positioning resolution of 0.1 micron to consistently achieve a pitch accuracy of better than three microns when dicing across components.

To achieve this goal, the MicroAce 66 has a closed-loop feedback system for ultra-precise control over the programmed cutting process, which can involve dicing wheels as large as 75mm diameter and slitting wheels as thin as 15 microns.

By working very closely with Heidenhain (GB), Loadpoint's choice of Heidenhain LF481C sealed, compact, incremental linear encoders not only provided the required level of positioning accuracy and repeatability but also, according to Clive Bond, Chief Engineer: "Use of the Heidenhain brand effectively provides the customer with a renowned reference standard."

Customer confidence

With almost 90 per cent of Loadpoint equipment being exported, Bond reflects how one European customer challenged the company's ability to build a machine that would be able to provide such precise levels of pitch accuracy for slotting silicon wafers. He says: "Our response was to contact Heidenhain and, by utilising the Heidenhain VM182 comparator system, we put the customer totally at ease."

The VM 182 system incorporates a steel scale with two co-ordinate phase gratings and a scanning head connected to the scale by an auxiliary carriage. The scale is clamped to the table and the scanning head to the machine spindle. Bond explains: "This qualified, without any doubt and according to ISO 230-2, both linear and non-linear error curves as well as any reverse error in each of the machine axes. So successful was the trial that we now take this reference to customer sites when we check and recalibrate machines."

Custom machinery

Based in Cricklade near Swindon, Loadpoint has progressively built its business over 40 years to provide a micro-machining sub-contract operation as well as a machine, process development and prototyping facility that is said to be one of the most advanced facilities of its type in the UK. The company has a turnover of some 3million and employs 25 people. High on its agenda is the continuous development of bespoke machinery capable of working in micron and sub-micron applications.

It enjoys a long relationship with Heidenhain, having utilised and capitalised on Heidenhain linear scales 35 years ago while its competitors were still relying on open-loop stepping motors. This move became the foundation stone in its ability to achieve higher orders of precision and repeatability, and it inevitably led to a particular focus on special applications.

Loadpoint's development laboratory has seen various special machines and processes created for machining silicon, silicon carbide, glass, quartz, gallium arsenide, germanium, lithium niobate, lithium tantalate, sapphire, lead zirconate and titanate (PZT), as well as equipment for processing components such as wafers, substrates, plates, laminates, rods, blocks, tubes and assemblies.

With such a wide range of different and challenging applications, Loadpoint has become largely self-sufficient in order to meet the precise demands of the development of customised equipment - it even has its own nano control system featuring high-level diagnostics and remote communication.

Proprietary software

In addition, Loadpoint develops its own software and such is this level of expertise that in-house programs are able to monitor process quality by including the company's own Kerf Check for measuring the width of cut, size and area of chipping, while also identifying and correcting alignment offset errors. The software also monitors and display spindle current, blade wear, chuck vacuum and the co-ordinates for each of the required machine axes.

The latest MicroAce 66 machine (launched in January 2010) has X and Y axis travels of 160 and 164mm, respectively, and a Z axis able to accept components up to 10mm thick when using the maximum size slitting wheel of 76.2mm.

Each axis is fitted with the Heidenhain LF481C linear scale for position feedback, which also includes Heidenhain's mounting spar; this means that, in the event of an encoder change, there is no requirement for drawn-out clocking procedures to establish datum positions.

Sealed encoder

The linear encoder is fully protected from dust and other contaminant, and an aluminium housing and elastic sealing lips protect the scale, scanning carriage and guideway. Indeed, the significant benefit of the LF481C (pictured right) being sealed was another major factor in Heidenhain securing this business against a supplier of exposed scales.

The 1.2kW air bearing spindle has a top speed of 80,000rpm (the optional alternative is 2.4kW and 60,000rpm) and this works in conjunction with a theta-axis bearing rotary table with vacuum chuck. The table has a high-resolution, direct-drive torque motor with an axis resolution of 0.0004 degrees. A four-direction coolant delivery system has outlets for main the blade jet, side wash bars, wafer fan jets and flood jets.

Also incorporated in the full-screen display vision and alignment system is monocular video alignment that incorporates pattern recognition with a Z axis autofocus to set up the alignment of the image. Through-the-lens LED illumination and external high-intensity LED fibre ring illumination are also included.

About the Heidenhain LF481C linear encoder

Heidenhain's LF481C is one of a comprehensive range available from the company. Sealed linear encoders are available with full-size scale housings for high resistance to vibration up to measuring lengths of 30m, as well as slimline scale housings for measuring lengths up to 2040mm in applications where installation space is limited.

Follow the link for more information about Heidenhain's portfolio of linear encoders and length gauges.