Applying Motion Control
When Arthur C. Clarke remarked that any sufficiently advanced technology is indistinguishable from magic, he may have had semiconductor fabrication in mind, right up there with nuclear physics. Chips on the edge have features only a few atoms wide (some 65 nm with those as small as 45 nm in the works) and are fabricated with processes that are increasingly complex.
When Arthur C. Clarke remarked that any sufficiently advanced technology is indistinguishable from magic, he may have had semiconductor fabrication in mind, right up there with nuclear physics. Chips on the edge have features only a few atoms wide (some 65 nm with those as small as 45 nm in the works) and are fabricated with processes that are increasingly complex. The motion control systems used in the manufacture of many chips are required to provide repeatable motion with precision in the micrometer to nanometer range. At the other end of the scale, applications such as automated material handling systems (AMHS) used to move wafers between tools and around a factory are more easily met with more traditional products that can routinely handle application-specific requirements for smooth operation, speed, reliability, and productivity.
Control for a maturing market
While the semiconductor industry is more than 40 years old, it has only begun to mature, moving from a laboratory to a production mentality. Where semiconductor tools were once highly customized and relied heavily on home-grown solutions to meet even the most routine yet exacting tasks, toolmakers today are leaning toward partners who have “off-the-shelf” solutions and the expertise to help apply them.
Previously, an OEM would custom design a wafer lift mechanism for transferring wafers between processing chambers. Today, the same OEM wants to buy a wafer lift from a supplier who understands how to achieve the required levels of speed, acceleration, and vibration-free motion. In other words, as the industry moves out of the lab and onto the factory floor, companies are doing the same thing other industries have done: concentrating their engineering resources on the technologies that give them a clear market differentiation and partnering with suppliers who have the expertise to do the rest.
Tool designers usually begin by defining the kinematics of the machine and even the closure software routines to achieve the movement. Given the kinematics, the designer will typically choose the motor before the control package. Motors are often low power and must have a small form factor to fit the space constraints. While stepper, servo, and linear motors are standard fare, the industry also makes wide use of piezomotors and actuators. Relying as it does on solid-state crystalline structures that change shape under an applied voltage, piezotechnology can achieve sub-nanometer resolutions.
Drives and controls must be able to provide fast multi-axis movement, synchronization, repeatable precision, and stable dwells. While hardware is certainly important, software designed for the nanoworld is perhaps the distinguishing feature for semiconductor applications. OEMs typically wrote their own routines, which were heavily customized to the application. As with most software development, the process was long, tedious, and expensive.
Software for semiconductors
A new generation of controls offers “off-the-shelf” software with algorithms and control loops optimized for semiconductor applications. Unlike motion control for automotive or machine tool applications, controllers in this industry do not always use well-known and nearly universal IEC 61131 programming. One reason is the need to integrate the motion control tightly into other tool operations, including I/O modules, machine vision, and tool functions. The software is often supplied as C++ libraries used by the machine programmer. The OEM gets proven, reliable, and sophisticated software to ease integration, speed development time, and ultimately reduce time to market.
Tuning in semiconductor tools, which also presents challenges, can be overcome with a three-step tuning process. Autotuning ensures that each axis movement is reliable and free of oscillations. For the non-critical axes in a machine, autotuning is usually sufficient to maximize performance, but further optimization of the axes control loops can be achieved by using time-domain tuning for optimizing control and feed-forward parameters, and frequency-domain tuning for setting filters and analyzing the complete mechatronic loop. This tuning package also allows fast adjustments to be made for slight differences in machine mechanics.
Most engineers equate productive motion control with higher-speed movements. Sometimes, however, in a fab, ultraslow is the requirement. For example, the speed of a transmission electron microscope used in metrological inspection of a wafer or chip is 1 nm/s, about the rate a human hair grows. Slow, stable speed can be as challenging as a high one in terms of synchronization and resolution. The controller, for example, should be able to compensate for even small vibrations inherent in the machine’s operation and for thermal effects. Still, given the small areas being scanned, this speed is highly productive as measurement systems increase in bandwidth to carry out multiple measurements with fewer operations. Such ultra-precise positioning requires extremely tight integration between the mechanical stages and the motion controller.
Space is also a critical issue in semiconductor tools, with designers trying to fit more functionality in less space. The NYCe4000 control platform from Bosch Rexroth, for example, reflects trends in motion control form factors. Using a modular architecture to allow easy scaling for different applications, it combines high-speed motion control, drive, and I/O points into a compact package—smaller than an industrial PC—that also increases reliability and simplifies assembly.
The integration of a flexible, universal drive with the motion control system makes it suitable for low-power servo axes and stepper axes driven by motors up to 1 kW, which allows control of all motors in a machine with the same drive type. Since it is easy to integrate a customer-specific connector board into the system, no additional external connector panels are required because sensors used can be plugged into OEM-specified matching connectors in the motion-control system.
Flexible, safe, efficient
Semiconductor OEMs have to cope with a notoriously cyclical market that can stretch manufacturers to the limit during periods of strong growth and force them to resize in leaner times. A modular, scalable control platform offers OEMs ability to standardize on one system, while lowering costs, speeding development, and simplifying purchasing and inventory. Flexibility requires that all required machine-control architectures have the ability to be handled with only a few plug-in components. Scalability requires that these components can be combined in various ways to achieve operations from low to high axis counts.
Safety goes beyond personnel to embrace the protection and safety of high-value wafers and chips as well. Controls must meet SEMI S2 standards and accommodate interlocks. SEMI S2 defines minimum performance-based safety requirements that address a number of hazards, including chemical, electrical, fire, noise, radiation, mechanical, and seismic.
To protect wafers, motion control systems must provide vibration-free, shock-free movements. Even with accelerations as high as 200 m/s2 and 15 g and speeds of 10 m/s, motion must be smooth and precise. In addition, the controller should offer loop healing and flexible, configurable safety handling.
The cost of a fab is extremely high, with next-generation fabs for 450-mm wafers expected to run in the tens of billions of dollars. As with any maturing industry, the ability to achieve performance goals is followed by a focus on cost reduction.
Controls should help drive cost reduction through greater standardization, reduce development time through “off-the-shelf” solutions replacing custom approaches, and offer flexibility to meet diverse needs.
Picking the right controller can lower the total cost of ownership of a tool by providing better control with standardized components, saving space, speeding development, increasing tool productivity, and reducing waste.
Kevin Steele, semiconductor and medical product group manager, Bosch Rexroth Corp., www.boschrexroth-us.com
Higher performance through microscope/motion integration
As a maker of scanning and transmission electron microscopes for semiconductor metrology and research, FEI Co. knows the importance of extreme-precision motion control. For its new compact Strata dual-beam microscope, the company wanted to reduce costs and create a new modular motion control system for easier integration with their mechatronic systems.
Working closely with Nyquist, a semiconductor motion control company recently acquired by Bosch Rexroth, FEI created such a system using Bosch-Rexroth’s NYCe 4000 motion-control system for ultra-low-speed movements with extremely high accuracy and repeatability. The mechanical specimen-manipulator stages of the FEI microscope must provide thermal compensation and vibration resistance to allow resolution on the nanometer level. The Strata system uses ultrasonic piezoelectric motors for movements.
“Achieving a breakthrough in cost reduction is the main driver,” says Wim Wondergem, FEI’s motion and mechanics manager. “But factors such as ease of use, high reproducibility of manipulations, and high throughput of samples all play an increasing role in our business.”
The NYCe system helps reduce costs with an integrated motion control and drive in a single space-saving package that’s capable of controlling up to 10 axes. In addition, the NYCe can be supplied with an application-specific breakout panel to simplify cabling. Drives are modular, making it possible to remove and replace them for maintenance or repair. The result is a drive and control system that, even beyond performance improvements, helped FEI lower costs by providing easier integration and assembly, low heat generation to alleviate thermal concerns, space savings, and the convenience of easy servicing. “Compared to previous motion control solutions, the integration of drives and motion control in a single unit will help us save 50% on costs,” says Wondergem.
Software is important in achieving precise motion control. The NYCe software was created for semiconductor demands, which include piezo control, nonlinear compensation, and user-defined algorithms. Development tools make tuning fast and easy in frequency or time domains.
The software tools allow machine builders to raise machine performance, provide early fault tracing, and save development time. User-definable control saves time for developing powerful control loops and controlling complex mechanical systems. Designers can quickly configure complex control loops using pre-defined control blocks while extending control algorithms across axes. The software allows dynamic control for complex nonlinear motion control. It also provides for custom algorithm development and user-defined control algorithm code that is automatically generated and downloadable to any controller.
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