Norbert Ludwig, managing director, Aerotech
A sensor with a limited field of view (FoV) can often be required to measure a surface that is much larger. This can be remedied by positioning systems that move either the sensor or the sample. The positioning and running accuracy of the movement system are crucial for the measurement results. Another fundamental prerequisite is for the sensor feedback and positioning system feedback to be in precise synchronisation. Aerotech, a manufacturer of high-performance motion control and positioning systems, offers a range of solutions for surface measurement technology applications, from the motion component level to motion subsystem platform, to turnkey machinery solutions.
Most optical sensors for measuring the surface properties of objects have only a limited FoV. When measuring the surface outside the FoV, either the sensor or the object must be moved or repositioned. It is important to distinguish between the position of the object being measured and the sensor movement, whereby the positioning itself should have as little influence as possible on the measurement result. The higher the sensor resolution and accuracy, the more precise the motion system must be. In addition to precision, another important requirement in many applications is the degree of automation.
Precise positioning and monitored control
Surface analysis is often integrated directly into the production line as a quality assurance process. As is often the case in semiconductor manufacturing, for example, wafer inspection or flat panel production, quality is assessed after almost every process step, necessitating fully automated in-process inspection with object loading, measurement and subsequent further processing. Control of the motion systems and synchronisation with sensors can generally be described as motion control.
Aerotech offers customised solutions for automated surface measurement technology where high throughput and maximum accuracy are required. Among other things, users benefit from high-performance linear and rotary stages as well as controllers with hardware-based, real-time, position-synchronised triggering of sensors. This article provides an overview of the surface measurement technology solutions provided by Aerotech.
Surface measurement motion platform (SMP)
The SMP, shown in figure 1, is a multi-axis system made up of various rotary and linear actuators but has a very compact, space-saving design. It is particularly suitable for the optical measurement of spherical, aspherical and cylindrical surfaces.
Figure 1: The surface measurement motion platform (SMP) is a multi-axis system comprising rotary and linear actuators. It offers more flexibility in 2D and 3D contouring, nanometre resolution and therefore higher precision, and up to 40 percent reduced machining time and up to 60 percent smaller footprint than Cartesian coordinate systems.
The core component is an air-bearing precision rotary table, on which the measurement object is attached using either a vacuum chuck or a clamping device. The sensor itself can be linearly adjusted in the Y and Z planes and can also be tilted with another axis of rotation so that it is always aligned perpendicular to the tangential plane of the target.
Various surface sensors can be integrated via the control software. The controller offers both digital and analogue inputs for this. By selecting the actuators, surfaces can be measured down to the submicron range. This is particularly useful for the precise measurement of optical components in applications with mirrors or lens optics.
ABRX air-bearing rotary stages
The ABRX series of air-bearing rotary axes is especially suitable for surface measurement in nanometres. ABRX rotary axes are available in diameters of 100 and 150 mm, as shown in figure 2, or 250 mm, depending on the size of the object and the load capacity. They are operated directly and reach rotational speeds of up to 300 rpm.
Figure 2: The ABRX series of air-bearing rotary axes are particularly suitable for surface measurements in the nanometre range and as an extension to the SMP measuring platform.
A special feature of the ABRX series is the air-bearings for minimising radial and axial errors, all of which are in a range of less than 25 nm. Errors can be divided into two types, namely synchronous and asynchronous. The errors that occur periodically with the axis rotation are synchronous. At the factory, Aerotech is able to measure the synchronous errors and compensate for them via moving either a Z-axis in the axial direction or X-axis in the radial direction and correcting any measured values accordingly.
Asynchronous errors, on the other hand, are to be considered more as position noise and cannot be easily compensated for. By compensating for the synchronous errors, however, the radial and axial errors can be reduced well below 10 nm. The ABRX series is particularly suitable for surface measurements in the nanometer range and of course also as an extension to the SMP measuring platform.
Automation1 software-based machine controller (iSMC)
The Automation1 iSMC, shown in figure 3, required several years of development time due to its complexity. As a digital control platform, it affords high measurement optimisation potential relating to faster movement and settling times, enhanced position stability, and increased contour accuracy.
Figure 3: The Automation1 software-based machine controller (iSMC) has integrated digital and analogue input/outputs (I/Os), allowing for easy integration of complex movements and the associated control of process tools.
The Automation1 iSMC has an exceptional setpoint generation rate and this is particularly helpful for surface measurement technology. The update rate of the position can be increased from 8 to 20 kHz, enabling more rapid and accurate positioning of the sensor head or measurement object.
In addition, faster input/output (I/O) signals guarantee almost instantaneous triggering of measuring cycles. The position synchronised output (PSO) allows the measuring cycle to be started as soon as the defined position is reached. Continuous measurement can also be initiated, whereby the measured values are synchronised with the position data. This results in considerable advantages over time-based triggering, the accelerations or decelerations of which can be neglected.
The Automation1 iSMC affords enhanced signal-to-noise ratio of the amplifiers (whether analogue or digital), which has significantly improved in-position stability. This results in lower position noise, which is particularly relevant for measuring tasks in the submicron range. In addition, a Hyperwire fibre-optic interface ensures very rapid signal transmission, even over long distances. As an optical signal, it is not susceptible to electromagnetic interference. As an added advantage, an intuitive user interface significantly simplifies human-machine interaction.
Integration of measuring systems into Aerotech positioning systems
Aerotech has an application team dedicated to helping customers integrate measuring and positioning systems. An example is the integration of the Keyence VHX digital microscope, which produces superior surface images but only accepts part sizes less than 100 mm2.
Aerotech stages, such as the one shown in figure 4, are used alongside software to extend the measurement volume of these microscopes. Common measurement area sizes enabled range from 300 x 300 mm up to 1.5 x 1.5 m. The dedicated product development behind this Aerotech offering makes the integration easy for users of the microscope. The positioning system comes in a variety of mechanical configurations based on the part that is being imaged. Furthermore, the image acquisition process can be automated and large FoV images can be stitched together to cover more surface area with minimal operator intervention.
Figure 4: Application example of an integrated surface metrology positioning system from Peak Metrology with a Keyence VHX series digital microscope.
Such forms of integration can now also be realised for complete sets of testing equipment. In a recent application-specific implementation, a laser line scanner has been used as a measurement sensor. The measurement data is recorded and compared with the CAD data using software. Special test dimensions can be visualised directly in the evaluation. The application is TÜV SÜD-approved1 and guarantees compliance with all machine directives as well as national and international safety requirements.
Aerotech offers integration solutions to measuring system manufacturers that cannot offer a complete solution. Expertise is rooted in microscope and measurement sensor integration. Digital microscopes, laser microscopes, pattern projection profilometers, area sensors and line sensors are the main measurement devices supported.
Aerotech
Reference
1TÜV SÜD certification marks and certificates represent third-party endorsement by a globally renowned organisation.