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SPIE Professional July 2011

6D Nanometrology

Motion-system design and manufacturing techniques need 6D precision.

By Bill Hennessey

Nanometrology is the science of calibrating and using measuring equipment to quantify the physical size of, or distance from, any given object at the nanometer-precision level. Inspection is a critical step in product development and quality control. This is even more important to the manufacturing process today considering the trend of miniaturization, nanoprecision processes, and precision manufacturing that has challenged legacy metrology motion systems and sensors.

While it's not possible to be perfect in the real world, attaining 6D precision for motion systems is a must, and it begins with taking the right approach to the problem. Many engineers do not realize it's possible to have excellent repeatability to a planar position but still experience 'point-in-space' misalignment because of the degrees of freedom in an axis.

But some engineers are on the forefront of nanoprecision metrology with innovative thinking that has resulted in advancements in manufacturing techniques and metrology sensors to solve the challenge of reducing errors.

6-D Nano Precision
Legacy metrology systems

Modern measurement equipment such as hand tools, CMM (Coordinate-Measurement Machines), and optical comparators are some of the more common tools currently in use. A CMM is based on computer numerical control technology to automate measurement of Cartesian coordinates typically from physical contact with the part although they may have laser- and vision-based sensors. Optical comparators are used when the part can't physically be touched. Today, optical 3D (laser) scanners are becoming more common.

By using a light-sensitive detector (e.g., digital camera) and a light source (laser, line projector), the triangulation principle is employed to generate 3D data, which is evaluated in order to compare the measurements against nominal geometries.

These systems, in particular CMMs, have used motion systems that deliver mostly 2D precision. They look at repeatability, accuracy, and resolution, which describe only part of the motion performance, since repeatability and accuracy is relative only to a plane in space, not to the point required by nanometrology sensors.

Equipment using legacy metrology motion systems with new technology sensors often measure the motion system and not the actual object.

There is a great need for motion systems and for kinematic models of modern measurement systems to evolve to a still higher precision level. However, basing the approach on legacy 2D positioning technology often fails spectacularly, as the measurement-tool dot size is much smaller than a traditional 2D motion system's ability to follow a programmed path.

The 6D visualization model

The basic principle of nanometrology is a six-dimensional visualization of the point in space that you wish to measure versus the conventional 2D thinking of a plane somewhere along the path of measurement. Motion-system design and manufacturing techniques need to be radically changed to meet these 6D precision demands from sensors and precisely manufactured products.

One type of sensor for nanometrology is a white light laser interferometer. These sensors and others based on lasers have pushed the metrology motion systems requirements with less than 100-nm accuracies and a 5-micron dot size or smaller data measurement range.

ALIO typically uses < 1-nm to 5-nm resolution non-contact optical-position-feedback encoders that can move meters per second but has used position-feedback systems that can achieve 38-picometer resolution.

 Assumed errors in the 2D and 4D world are only a fraction of the real-error story. When all six dimensions are included, the error you get is a lot larger.Assumed errors in the 2D and 4D world are only a fraction of the real-error story. When all six dimensions are included, the error you get is a lot larger.
Assumed errors in the 2D and 4D world are only a fraction of the real-error story. When all six dimensions are included, the error you get is a lot larger. See the "real error" presentation (PDF).

The motion systems and kinematic structure of the modern measurement systems need to have better precision than the sensors, thus 6 degrees of error for each axis of motion needs to be reduced from microns to nanometers.

To better understand 6D precision, we must understand where the point in space we are measuring is because the flatness, straightness, pitch, yaw, and roll of the motion system are generally more important errors to account for than 2D repeatability and accuracy in a plane.

The 2D visualization of a plane does not account for where the point in space is since the biggest potential errors of straightness and flatness are not factored in but assumed. As illustrated above, the true precision in the real world is much larger than the assumed errors in the 2D world. Thus the data collected will need sophisticated algorithm manipulation and many more data points collected to approximate the part's full dimensional quality.

Real-world applications

There are several applications in the photonics industry that can benefit from this point precision, especially when working with optics and steering mirrors. Trying to measure the surface of a concave or convex surface with a 5-micron dot-size sensor moving in a serpentine motion multiple times back and forth across the surface assumes that the motion system straightness and flatness are perfectly parallel with no bi-directional motion errors. But this is not the case with legacy motion systems that can have 20 to 50 microns of errors.

Six-dimensional nanoprecision motion systems reduce these errors below the 5-micron measuring sensor, thus allowing the optics to be measured and not the motion system.

6D motion systems

Now that we understand the challenges of measurement with new nanoprecision-sensor technology, we need to focus on the motion system and how to reduce all the errors of each axis.

Nanoprecision motion systems begin with the design of a new product from concept to component selection, paying particular attention to assembly. Precision and quality are foremost when creating a revolutionary motion system. Motion-system errors need to be minimized during design, manufacture, and assembly. The following is a list of critical areas that need to be thought out and focused on for success. These points, along with an understanding of the true meaning and results of repeatability and accuracy of a system are key for success in the world of 6D nanometrology.

• Design of the motion-system structure is the critical foundation for 6D precision. Legacy designs and past design practices need to be replaced with an "outside-the-box" look at the design while factoring in environment, stiffness, thermal growth, assembly, and quality.

• Bearing selection is determined per the application needs. Ultra-precise crossed rollers or air bearings are the current precision standards.

• Drive systems include amps and controllers, which must work together with the position feedback system to not only move precisely to a point, but hold position without dither and minimize velocity deviation during continuous data capture.

• Position-feedback systems are based on optical encoders, laser interferometers, and laser scales and are application- and environment-dependent.

• Material selection is dependent on the application, but 6061 aluminium is still the most cost-effective material if it can be machined to high tolerance without deforming the structure. Proper machining technique is the foundation of building a 6D nanometrology system.

• Assembly is critical since even the most precise structure can be deformed by poor assembly techniques.

• Testing with laser interferometers and other NIST-traceable metrology solutions are the only way to verify that all the previous steps have been completed successfully.

Bill HennesseyBill Hennessey is founder and CEO of ALIO Industries . He has some 30 years experience as an entrepreneur, mechanical engineer, and marketing professional in the robotics, lasers, and automation fields. ALIO Industries is based in Colorado (USA) and has been focused on 6D nanopositioning systems for semiconductor, metrology, ink-jet deposition, PV, medical, and other photonics-enabled industries for more than 10 years. The company holds several patents and pending patents based on 6D nanopositioning.

All the world's a stage

In addition to metrology systems, ALIO Industries designs and builds precision automation stages, or platforms, for nano-level manufacturing applications, including microlithography, fiber optics, medical devices, micro-machine tools, photovoltaics, and semiconductors.

In lithography, for instance, the stages are precise enough to project images on substrates without the need for a photomask.

In medical applications, images are stitched together precisely, thus reducing the need for software to "best fit" the images together.

The role of the stage is critical to the stitching, as it controls the alignment of the exposures. Also, if the substrate is curved, a 6D motion system is the only way to control pitching up or down.

ALIO's products are 6-D Nano Positioning™ motion systems in serial and parallel kinematic platforms.

Have a question or comment about this article? Write to us at spieprofessional@spie.org.

DOI: 10.1117/2.4201110.11

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