5 Semiconductor Applications Where Direct Drive Linear Motor Stages Excel
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Precision positioning has always been important to the semiconductor industry and is increasingly important as microchips become smaller and their capabilities increase.
MISUMI has been supplying precision positioning solutions to US manufacturers for over a decade, but until recently it had been limited to more traditional leadscrew-driven stages.
While the leadscrew-driven stages have been capable of performing most of the tasks needed for the semiconductor industry, direct drive linear motor stages have taken their place for more precise fabrication applications.
Leadscrew-Driven vs Direct Drive
A linear motor stage is a high-precision actuator that uses a linear motor to achieve motion. This differs from motorized leadscrew linear stages that use a traditional rotary motor and leadscrew mechanism to achieve motion.
To understand the key difference, let’s look at a linear motor compared to a traditional rotary motor. One way to think about a linear motor is that it’s a traditional motor that has been unwound and laid flat.
The same basic components are present in both devices, and the same laws of physics apply.
The diagram below illustrates the structure of both:
Direct-drive linear motor stages derive many of their advantages from removing the leadscrew itself and using the physics of an electric motor to achieve fast, smooth, and precise motion. This is useful for several industries, and the semiconductor industry is one of the largest beneficiaries.
Here are five applications where direct-drive linear motor stages excel over their leadscrew-driven predecessors.
1. Photolithography
Mapping the structure of a microchip is one of the first steps a wafer goes through during fabrication. It’s no surprise that positioning of the photomask, projection lens, and wafer in respect to each other would need higher accuracy and resolution as microchips reduced in size.
While many traditional screw-driven stages are able to meet accuracy requirements for this, backlash would become a growing problem. The natural solution to that would be to use a positioning stage with zero backlash, which is where direct drive stages shine.
Backlash happens because the gap between a ball screw and lead threads is not zero. Once the stage is sent to its desired location, it is still capable of moving a small amount equal to the gap between the gears/balls and lead threads. There are methods, such as preloading, that can reduce backlash, but there always tends to be a small amount.
Direct drive motors, linear or otherwise, don’t use a lead screw and therefore don’t have any backlash. This is advantageous for photolithography, as any drift in position will affect the quality of the projection.
2. Wafer Inspection
Wafer inspection ensures quality by finding defects in the wafer or pattern deviations after a process the wafer went through. This is another area where backlash could be detrimental. In addition, wafer inspection is a textbook “step and settle” type of application.
In summary, step and settle applications consist of an incremental movement (step) and then waiting for the sample to stabilize (settle). Linear motor stages excel in this process due to their high speed, high acceleration, and low vibration.
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Using a mechanical leadscrew creates mechanical vibration, which can increase settling time in any application where stabilization is necessary. Direct drive motors are air gapped and rely on electromagnetism for motion. This removes friction and noise from the system, resulting in quicker settling times.
The magnetic system can also step at a faster pace with better accuracy, further reducing wafer inspection times without sacrificing quality. Increased throughput is a strong motivator!?
3. Dicing
Taking a thin wafer of 100mm to 300mm diameter and dicing it into small squares or rectangles requires finesse. For one, wafers can be brittle. Also, a miscut can be costly as it would render a die, or group of dies, worthless.
The combination of precision, accuracy, and smooth motion makes direct drive linear motor stages ideal for the delicate touch required in this application.
Automation of delicate processes was one of the reasons motorized precision stages were created to begin with. Vibration produced by moving mechanical components like a leadscrew would be an issue for smaller chip sizes.
Also, higher precision and accuracy help with the throughput and total output of dies that can be produced from one wafer.
4. Die Bonding and Wire Bonding
These two processes are where the chip itself begins to look more like the chips we’re all familiar with. Die bonding sets the semiconductor chip onto a package or substrate.
Wire bonding connects the chip to external conductors. Fine precision is required for each task. Linear motors with linear encoders excel in this area, as they are often designed to achieve sub-micron-level resolution.?
Traditional stages could also work here, but linear motor stages are trusted to be more reliable performers for smaller chip sets due to the fine resolution of linear encoders. This is also another area where backlash could be detrimental, and step/settle times are better off being short.
5. Chip Level Inspection
Perhaps the last inspection before testing the chip. A finished chip is inspected using various methods to reduce the risk of a defective chip making into a finished product. Inspection methods include visual inspection, machine vision, scanning technologies, and other methods that could fill their own articles.
What they all have in common is the need for high accuracy and repeatability and quick step and settle times to increase throughput. Any astute reader already knows why direct drive linear motor stages would excel here, and it bears repeating.
While leadscrew stages have been used for chip level inspection, direct drive linear motor stages are being used more and more often as chip sizes get smaller and the priorities of the application shift from cost-saving to high precision and quicker throughput.
Direct drive stages with high resolution, zero backlash, and smooth motion for quick step and settle times meet these criteria.
The Advantages of Choosing Direct Drive Linear Motor Stages
We have explored only a few applications where direct drive linear motor stages excel in semiconductor. The factors that allow these stages to excel over traditional leadscrew-type stages include:
While leadscrew-actuated stages are here to stay for various reasons, direct drive stages present advantages to several semiconductor applications, simply by removing the leadscrew itself and unrolling the motor. They will be useful tools for decades to come.
This article was first published on MISUMIUSA.com