Wedge drives precision elevator

A new elevator stage, the PZA 25 from Primatics, Corvallis, Oreg. (www. primatics.com), uses a precision-machined wedge and recirculating bearings to position with 25-nm resolution. The wedge, an inclined plane, converts horizontal ball-screw movement into vertical elevation of the table top (up to 25 mm), with position feedback provided by either an internal linear or rotary encoder. Vertical-position resolution using a 25-nm linear encoder is 0.025 mm.

“The inclined-plane approach offers superior performance to traditional Z-axis stages,” says Alan Petersen, Primatics' president. “The PZA offers a lower profile and easy access to all sides of the load.” Using a frameless motor eliminates a motor coupler and external motor, which reduces the table footprint by half.

Signals connect through an external cable instead of a connector on the table, letting the PZA fit into tight spaces. The table brings machine designers vertical-positioning capability in a package designed for semiconductor inspection, fiber-optics assembly, and machine-vision applications.

Traditional vertical-motion systems mount a standard linear stage vertically on a Z-bracket. This approach has several disadvantages. Putting a load like a probe or fluid-dispensing head on a vertical stage induces pitch and yaw moment loads on the carriage stage. The moment loads deflect the stage and change the angular relationship between the load and horizontal plane. Deflection increases as the load increases or the load's center of gravity moves farther away from the stage. The result is that vertical motion isn't quite vertical. Using a Z-stage such as the PZA25 lets the load sit directly over the point of lifting, inducing no angular moment loads and creating truer vertical motion.

This issue is important in the semiconductor industry because 300-mm wafers are becoming common. The L-bracket necessary to hold a 300-mm wafer on a traditional vertical stage would be so large that it would deflect significantly under its own weight, adding to the deflection resulting from the mounting of the stage itself. With the PZA25, that same 300-mm wafer would be centered on the carriage, giving the wafer plenty of support, and ensuring that it can be positioned with minimal deflection.

Another disadvantage of mounting a traditional linear stage vertically is wasted space. Linear stages mounted vertically require a lot more vertical headroom to accommodate the stage and motor. Inclined-plane Z-stages take up a lot less vertical space. For example, a typical compact linear stage might extend as much as 240 mm. In comparison, the Primatics vertical stage expands and contracts as required, with a 55-mm retracted height rising to 80 mm at its extreme vertical extension.

Finally, in the semiconductor industry, linear stages mounted vertically can contribute to particulate contamination. Typical clean-room laminar air flowing past vertically mounted linear stages can blow microscopic contaminants off the stage and onto a wafer. Because the PZA is selfcontained and typically mounts under the work in process, it generates less particulate contamination. Any particles generated are carried away by the laminar flow before they have a chance to migrate up to the critical process areas.