Motion
XTRA
Fine-tuning Semiconductor Production
Two linear motion elements redesigned to meet industry
parameters
Specialized machines for semiconductor industry
tasks, ranging from wafer polishing to wire bonding, must meet demanding
performance specifications and require particularly high reliability, accuracy
and repeatability. Modular components used in these machines, such as those
employed within integrated linear motion systems, are designed to be compact
and cost-effective, while offering greater endurance and load capacities.
Combating Cross-roller Cage Creep Issues
As an example, precision rail guide systems with cross-rollers usually
are employed in X/Y positioning applications where accuracy, high precision,
and speed are essential. These assemblies can be found in wafer inspection
and lithography machines, metrology instruments for inspecting wafer flatness,
wire bonding equipment, and in many other applications where multiple-axis
positioning tables or stages come into play.
Cross-roller systems are robust linear bearings with high load-carrying
capability suitable for a wide range of linear bearing arrangements with
limited travel. But end-users of these systems more often than not can attest
to a potentially persistent problem during a machine's nonstop use: migration
of the cage and roller assembly. This migration is so common that it has
been dubbed by the industry as "cage creep."
Close-up reveals Anti-Creep System - See at
Semicon West
Cage creep can be especially noticeable when cross-roller systems are
subjected to high speeds, high acceleration, uneven loads, and vertical
orientations. In most cases, the system's cage gradually migrates toward
one end of the travel. As the cage moves farther from its centered position,
the system friction increases at the stroke limits. The cage then hits the
end stops and causes the rollers to skid. The machine either has to be shut
down for cage repositioning or it must be equipped with larger (and more
costly) motors to deliver the necessary power to reset the cages.
Designers have sought to reduce cage creep by incorporating wires and
pulleys into assemblies, but these attempted solutions have achieved only
limited success and have added unwanted costs and taken up premium space.
The ultimate goal has been to eliminate cage creep altogether, and a
patented Anti-Creep System (ACS) has been designed to provide this capability.
The ACS keeps the cage permanently centered within a cross-roller system
and, as users in the semiconductor industry can attest, the cage does not
migrate and never needs to be reset. In an industry where size and space
matter, the ACS (which is an internal drop-in component) works easily within
the design envelope of a standard cross-roller system, maintaining dimensional
integrity of an assembly.
Roller Screws with Fine Leads
Another example of innovation in linear motion is the evolution of roller
screws to meet specific performance requirements. Roller screws transfer
the load from the nut to the screw shaft through a number of threaded or
grooved rollers rather than balls. Their greater number of contact points
enables a higher load-carrying capacity and a much longer life than similarly
sized ball screws (which also are bulkier due to larger diameters). But
the industry "wish list" for roller screws has included a desire
for even finer leads, which would promote high positioning accuracy, repeatability,
and rigidity.
Recirculating roller screws incorporate grooved
rollers - See at Semicon West
Designers have responded by developing recirculating roller screws (with
grooved rollers), which feature unprecedentedly small leads (1mm). The design
of these screws promotes fine resolution, minimum drive torque, and high
mechanical advantage to minimize input torque and increase resolution. The
screws' roller contacts make assemblies simple, robust, and reliable, all
in a relatively smaller package; and their many strong contact points contribute
to heavy load-carrying capacity, high rigidity, and long life. These screws
usually are specified for precision-positioning applications, such as wafer
polishing and testing equipment, as well as for many other types of machines.
--SG
For more information:
Circle 701 - SKF Motion Technologies, or connect directly
to their website via the Online Reader Service Program at http://www.OneRS.net/106df-701
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