Analog Encoders Reduce Pre-alignment Time
How do you double semiconductor wafer throughput, decrease wafer pre-alignment time fivefold (to 600 ms), and improve in positional repeatability fivefold (to 5 nm), while at the same time reducing cost and increasing reliability?
The competitive nature of the semiconductor industry places increasing importance on performance characteristics and integration ease for motion controllers used in today’s semiconductor manufacturing systems. New processes demand finer position resolutions and better repeatability, while delivering higher throughput. At the same time, increasing process complexities require more integration flexibility of the motion control components chosen for new machine projects. When semiconductor manufacturing equipment developer Micro Precision Automation (MPA) developed a new metrology stage to address these trends, they turned to Agile Systems’ microMAX R distributed motion control system for key ingredients to help them achieve the new stage’s performance goals.
|Digital and analog encoder waveforms.|
The system incorporates a fully integrated motion controller, servo amplifier and high-speed network for a single axis of motion. A high-speed network ties multiple separate motion axes into a full-featured, high-performance system.
Conventional quadrature, or incremental, encoders sense rotation with two pickups monitoring two encoder tracks. Each track consists of equal-size segments imprinted with a means to switch its pickup’s output between a high and low state. Motion registers as state changes.
For example, when the sensing is optical, the track segments alternate between black and white colors. The pickups are optical reflectometers (basically a light source paired with a photodetector). When the pickup moves from a black segment to a white segment, the output state switches from low to high. Further movement by one segment length provides another transition as the pickup moves from the white segment to the next black segment. Constant movement causes the output to become a square wave whose frequency is proportional to the motion speed.
A single track/pickup channel cannot, however, indicate the direction of motion. In fact, a back-and-forth vibration across one segment transition looks exactly like constant motion—in either direction.
Adding a second channel arranged to switch 90igh. Each negative transition will occur when B is low.
In that case, motion toward the left will pair positive channel A transitions with low channel B states and negative channel A transitions with high channel B states. A simple logic circuit can decode these paired encoder signals into up/down counter signals to increment/decrement a digital counter whose value then always corresponds to the monitored position. By monitoring channel B positions and incrementing/decrementing according to the then-current channel A state, the quadrature encoder system can achieve resolution equal to one half encoder segment length.
To achieve even better precision, MPA employed analog encoders developed by Heidenhain Corp. of Schaumburg, IL, which employ sinusoidal signals on both pickup channels instead of square waves. This allows electronic interpolation to obtain resolution substantially better than can be obtained from conventional square-wave encoders.
Higher encoder resolution does not, however, guarantee similarly higher position accuracy because of limited current control resolution of the servo amplifiers. The servo amplifier’s output current determines the torque the connected servomotor generates. High current produces high motor torque; low current produces proportionally lower motor torque.
The servo system controls the motion of the servomotor shaft by varying the drive current. To take advantage of higher encoder resolution, the servo drive system has to move the servomotor shaft in increments as fine as the higher encoder resolution. This requires a much higher effective current resolution than is available from conventional systems.
|Conventional motion-control configurations require relatively large electrical enclosures to house separate power supplies, servo amplifiers, motion controllers and signal converters.|
Agile Systems’ motion controller provides 14-bit effective current resolution, achieving position resolution of a few nanometers when applied to an appropriately designed mechanism. Previous generations of semiconductor metrology tools achieved positioning repeatability of 25 nm. MPA’s new stage features position repeatability of 5 nm with double the throughput.
The pre-alignment process takes up a considerable portion of the total tool-cycle time. It locates the center of the 300 mm wafer and locates the reference notch on its periphery. In most tools, the notch is found during a high-speed rotation and precisely located during a precision rescan. The process typically takes up to 3 seconds in conventional motion-control configurations.
MPA’s new system can reduce pre-align time to 0.6 second through careful collection of a laser-based prealignment signal. The system was designed using three motion controllers on a single backplane board, which allowed drastically improved signals over a conventional design. A high-speed Firewire B network connects the controllers to each other and to the central process controller for the metrology tool. Real-time axis position from axis encoders as well as digitized analog and digital signals can pass directly to the central process control through the high-speed network. This allowed MPA to collect clean high-speed prealignment data, which made it possible to eliminate the rescan step.
Conventional motion-control configurations require relatively large electrical enclosures to house separate power supplies, servo amplifiers, motion controllers and signal converters. Since metrology tools are tightly integrated with other semiconductor processing equipment, it is desirable to mount the motion-control hardware within the tool itself and eliminate the separate electrical enclosure found in conventional motion-control systems. Trying to find space for such a control configuration inside the tool proved extremely difficult.
MPA solved this space problem by choosing a motion control system that combines controller, servo amplifier, signal conversion and high-speed network in a single, compact package approximately, only 3 in long, 3
|Combining motion controller, servo amplifier, signal conversion, and high-speed network in a single compact package reduces motion-controller footprint.|
Reduced wiring was another key ingredient to support tight integration of motion control components and semiconductor tools. Combining the motion controller and servo amplifier in a single, fully integrated package eliminated a substantial amount of system wiring. Bringing all signal and interface connections out through compact interface connectors to a backplane further reduced cabling and wiring.
These features gave MPA tool designers ultimate flexibility in laying out the motion-control installation scheme. They created a custom PCB board containing the required interface signal connections to the motion controllers in a shape most appropriate for a given tool configuration.
More precise and finer motor current control is a prerequisite for designing tools with higher position accuracy and repeatability. High-speed networks facilitate reductions in wafer pre-align times, which translate into improved productivity for tool users. Compact construction and space saving backplane designs ease integration complexities as well.
|Edited by C.G. Masi, with information supplied by Agile Systems. C.G. Masi is a senior editor at Control Engineering. Contact him at email@example.com .|