Setting the stage for clearer images

Precision metrology equipment from Thermo Electron Corp., Waltham, Mass. (www.thermoelectron.com), checks thin-film layers deposited on silicon wafers for thickness and irregularities. Wafers are held by an electrostatic chuck mounted to a Z-axis stage. This assembly sits on a platform that steps (X axis) and scans (Y axis) the wafer beneath an X-ray fluorescence metrology microscope.

Clear scanned images rely on low jitter levels at the chuck. For example, an imaging sensor collecting data at 1 kHz with the Y stage scanning at 10 mm/sec theoretically gives 100-mm resolution. Measuring geometrical features smaller than this resolution requires jitter-free constant velocity. In reality, jitter of the Y stage erodes constant velocity and has two major additive sources; from the linear bearings and from the drive mechanism.

When TEC updated its 200-mm-diameter wafer machines to process 300-mm wafers, it also wanted to improve smoothness of motion, boost throughput, and up repeatability to 2 µm. But the original recirculating-ball-bearing leadscrew stages used for stepping and scanning didn't operate with the required smoothness and constant velocity. And larger versions of these stages (for extending stroke length) wouldn't fit the existing footprint.

One approach replaces the Y-axis leadscrew with a more compact, jitter-free linear motor. High Accuracy Ultra Linear Motor Stages from Bayside Motion Group use noncontact, frictionless, ironless linear motors supported on crossed-roller-bearing stages. Crossed-roller stages produce about one-tenth the jitter of recirculated-ball stages that shuttle balls in and out of grooves. Lowering jitter in the Y axis also cuts induced Z-axis jitter, all of which improves image quality. Smoothness of motion was tested to be 0.03%.

The X axis receives a Luge LM Positioning Stage, also from Bayside. Here, motors mount directly onto ball-screw shafts to eliminate compliance. Integral design also helps reduce the high preload levels common in some high-accuracy leadscrews, and the associated stick-slip action. Stick-slip happens when a control system commands a motor to move a small amount but friction prevents it. In response, control-loop integral gain builds to lower position error. When the stage finally does move it tends to overshoot the intended target. Stages with better structural stiffness and less torsional wind-up allow high acceleration rates and short settling time without excessive preload. This boosts throughput and reduces component wear.

Z-axis focusing motion is handled by a Bayside Z Wedge Stage. The use of crossed-roller bearings and an integrated motor raise stiffness and lower Abbe offset and error. Abbe offset equals the linear distance from the work surface -- the chuck, in this case -- to the position feedback and bearing mechanism of the stage. Abbe error is simply Abbe offset multiplied by stage pitch error in radians. Here, stages fitted with cross rollers have about half the pitch error of those with recirculating balls. The result of all this is high positioning accuracy and position-holding stability, and a wide, stable mounting platform for the wafer chuck.