By: Justin Lackey, Product Manager, Systems — Bosch Rexroth Corp.
Precision linear modules are compact, well-engineered mechatronic devices with a range of sizes and motion technology — profiled rail, linear bushings, and ball screws — that offers different levels of precision, stability, and endpoint accuracy.
One industry with the highest levels of precise, stable, and accurate motion in manufacturing tools is the semiconductor industry. Semiconductor fabs, and the tool builder OEMs that supply them, have a very special set of automation needs. This includes mechatronic modules designed for smooth motion during wafer handling, noncontact material transport through the process chambers, and nanometer-scale positioning accuracies.
The recent global supply chain impact of COVID-19 has placed great pressure on this industry to rapidly expand production capacity. However, semiconductor fabrication requires thousands of process steps that manipulate chip structures and create transistors that are just a few nanometers in size.
Thus, while the ramp-up in production capacity has never been more critical for the world’s digital economy, fab operators and tool builders both recognize the essential need to use the finest linear motion technology to ensure that the new production lines operate with the highest levels of quality and productivity.
Understanding several key design and performance characteristics that distinguish precision linear modules can help machine and system designers select the proper precision linear modules to satisfy these demanding requirements.
The critical impact of precise motion
Ultraprecise and delicate motion requires extremely stable, nearly vibration-free motion during travel. Reaching the targeted endpoint with the utmost accuracy is the primary goal.
Stable motion is often critical to protecting fragile materials from damage or degradation caused by linear travel. Semiconductor wafers are a perfect example: they are extremely delicate, and a finished wafer may contain chips potentially worth millions of dollars, depending on the size.
However, each wafer must be transported through hundreds of process steps — and each time it’s moved from one step to another, vibration in the linear module risks damaging the in-process wafer, reducing its final value — the less vibration, the lower the risk.
Endpoint accuracy is equally critical for productivity purposes. If a tray of electronic parts moves through a high-speed automated assembly process, maximum throughput productivity is achieved when the linear module delivers the tray into the assembly tool with micron-level accuracy.
At the same time, stable motion and endpoint accuracy must be repeatable through thousands of motion cycles every day. If it takes several milliseconds to fine-tune the positioning of the parts, those milliseconds add up to hours of additional production time, reducing throughput and potentially inflating costs, and impacting delivery schedules.
Modular elasticity and materials of construction
Aluminum or steel is most often used to create linear module housings. Aluminum housings are typically used in more standard linear modules as they can be cost-effectively extruded to satisfy a wider range of module size and length.
However, for precision linear modules used in semiconductor fabrication applications, modules with steel machined housings offer advantages. These housings typically exhibit much less modular elasticity and deviation from the desired travel path than aluminum-based modules.
Modular elasticity can cause the module to reflect any slight deviations in a machine’s frame at the linear module’s mounting point. If there is some deviation, like a twist or bow where the module is attached, it can reflect that deviation.
Machined steel housings prevent that kind of deviation, helping ensure precise“flat” travel through a motion sequence. This contributes to vibration reduction, endpoint accuracy, and locational repeatability.
Motion and linear guide components
Precise and stable motion is also a product of the motion components that drive the linear module. For precision linear modules, ball screw drives provide the optimum solution.
Ball screw drives are ultra-efficient in converting rotary motion into linear motion. As mechanical drive elements, they can be installed in X-Y-Z orientations and perform movements with the precision and repeatability needed.
Ball screws with full contact seals offer a unique combination of high rigidity, high precision, and respectable speed, making them useful in various precision motion applications. Specifically, their ability to handle substantial axial loading often makes them a better choice than linear motors, particularly in metal-, wood-, and stone-cutting applications.
Equally important is the design of the linear guides in precision modules. Accuracy in linear guides depends on many factors: the trueness of the rail on which the runner block or bearing travels, the raceways inside the bearing through which the balls or rollers travel, and the flatness of the rail mounting surface.
One of the most important areas to assess is the smoothness of ball recirculation inside the runner block as it travels along the rail. Applications at the high end of the accuracy spectrum can be adversely affected by even minute movement of the balls in the recirculation chamber or by just a slight pivoting of the rail system about its axis.
Any deflection or clearance reduces accuracy, and any roughness in the recirculation of the balls can cause inaccuracy. To address this issue, leading linear module suppliers incorporate guides that optimize recirculation at crucial transition points, providing smooth motion consistently as the balls circulate in the bearing raceways.
Lubrication and sealing considerations
No matter how well-designed and engineered, linear modules need proper lubrication throughout their life cycle to maintain precise and stable motion. For the cleanroom conditions common in semiconductor plants, choose precision modules with more advanced LSS standards and LSC cleanroom lubricants.
Linear modules require lubrication because they have moving parts — but moving parts can generate tiny airborne particles if the module is not properly sealed (lubricants themselves can also disperse into the air). Look for a precision ball screw nut and linear carriage assembly that’s sealed from both sides with ball screw assembly seals. This design significantly reduces the risk of lubricants escaping outside.
Configuration and technical support
One final element to consider when selecting precision linear modules is the level of technical support available from the supplier to help machine builders choose, specify, configure, and order the modules they need.
Choosing a linear module supplier with easy-to-use, step-by-step online sizing and configuration tools can help machine designers quickly configure and order the right modules when needed. Some companies also enable selection and sizing for the mechanics, motor, and drive combination with a single tool.
When direct help is required, it also makes sense to work with linear module suppliers who have engineering staff with deep backgrounds in linear motion technologies and applications. In many cases, when machine builders are unsure about specific sizing and performance requirements of their applications, these experts will have solved those kinds of challenges in the past.
As the semiconductor industry and the tool OEMs supplying it engage in a major expansion cycle to meet surging demand, achieving critically needed throughput levels shouldn’t require sacrificing quality for speed. The right precision linear module technologies, offering ultraprecise linear motion, vibration-free transport, and tight endpoint accuracy, can play an essential role in meeting these goals.
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