Aaron Van Pelt, New Focus Inc.
Traditional motion-control devices, including stepper motors and DC servomotors, were designed for the industrial manufacturing environment and brought repeatability and accurate automation to processes such as cutting and welding. However, most of them are not appropriate for motion-control vacuum applications or in ultraclean, nitrogen-purged environments because they do not offer good long-term stability and/or cannot be made compatible with the vacuum environment.
In recent years, the move to shorter wavelengths in semiconductor manufacturing has led to an increase in demand of vacuum-compatible and ultraclean motion-control systems, which benefits not only the semiconductor industry, but also scientific markets. These systems must provide low-outgassing and low particle contamination, high resolution, long-term stability and high stiffness. Today there are a variety of choices and vacuum-compatibility levels ranging from 1026 to 1029 t, as well as ultraclean components.
Some examples of in vacuo motion control are adjusting slits in x-ray monochromators, positioning samples in vacuum chambers, manipulating in vacuo components at synchrotron facilities and making adjustments within electron-beam reticle inspection systems.
A semiconductor excimer beam delivery system is an example of a system using motion control in an ultraclean, nitrogen-purged environment. Ultraclean, nitrogen-purged applications often set a cleanliness benchmark at a vacuum-compatibility of 10–9 t, although in many cases, 10–6 t is adequate.
The main concern is that of contaminating the optics, wafer or mask. Of particular concern are organic compounds that can break down in the high-energy UV beam, allowing them to become deposited on the optical surfaces and greatly reducing the efficiency of the optics. Further, the UV beam polymerizes the contaminants on the surfaces, introducing a high-heat-capacity site that in turn can burn the optical coating.
Thus, for the scientific vacuum and ultraclean semiconductor applications, motion-control systems must offer low-outgassing and low particle contamination in addition to more typical motion requirements of resolution, stability and stiffness. For electron-beam environments, nonmagnetic operation also is useful.
New Focus’ Picomotor “stick-slip” motor design employs a piezo element positioned between two jaws to provide the torque to turn the screw (Figure 1). The jaws, each one connected to opposite ends of a piezoelectric transducer, grasp an 80-pitch screw. A slowly rising electrical signal applied to the piezo gradually changes its length, causing the jaws to move in directions opposite and tangential to the screw, just as a thumb and forefinger do as a person turns a screw. This slow motion makes the screw turn by static friction.
Figure 1. A “stick-slip” motor uses piezos to turn a screw. Two jaws grasp an 80-pitch screw (A), and a piezoelectric transducer slides the jaws in opposite directions. Slow action of the piezos causes a screw rotation (B); because of inertia, fast action causes no rotation (C).
At the end of the transducer motion, a fast-rising electrical signal quickly returns the jaws to their starting positions. Because of the screw’s inertia and low dynamic friction, it holds its position. Reversing the order of the fast and slow pulses reverses the motor’s direction. The piezo is used only to turn the screw and not to hold the adjusted position, so the mechanical stability of the motor is identical to a nonmotorized screw and nut set. Thus, the motor delivers high stiffness and excellent long-term stability, and is ideal for set-and-hold applications. It also can provide nanometer-scale resolution.
When selecting optomechanics or motion-control components to be used in ultrahigh-vacuum environments or clean-critical, nitrogen-purged environments, select components that outgas extremely low levels of volatile organic compounds. One way to measure this is by characterizing vacuum components with standard gas chromatography mass spectrometry analysis (Figure 2). A typical measurement would identify total mass loss in parts per million at 85 °C over three hours.
Figure 2. This chart shows typical outgassing rates of several vacuum-compatible mounts and acuators in parts per million volatile mass loss at 85° C over three hours using standard gas chromatography mass spectrum analysis.
Although this accurately measures outgassing components and their concentrations, it also is important — particularly for clean-critical applications in the semiconductor industry — to minimize the number of particles that may be generated by the motion-control system.
For example, the model 8330 Picomotor actuator Ultra contains all of its moving parts within a sealed volume; its motion couples through a vacuum bellows so that no particles can escape into the clean environment. In addition, the actuator comprises materials that have very low outgassing characteristics, making it appropriate for use in VUV/EUV environments and in ultrahigh-vacuum environments down to the 10–9 t level.
Figure 3. The Picomotor actuator Ultra is designed to minimize contamination and outgassing in clean-critical environments, including vacuum-ultraviolet and extreme-ultraviolet applications. A sealed bellow ensures the cleanliness of the actuators through its travel range.
For x-ray or other high-radiation vacuum applications, wire insulation is another concern. Vacuum-compatible devices typically use Teflon-coated wires. Teflon is compatible for use in vacuum environments, but it is not recommended for use in high-radiation applications where, with repeated exposure to x-rays and other radiation, the coating can become brittle and fall off, possibly leading to electrical shorts. For these applications, Kapton-coated wires are recommended. An alternative for electrically insulating wires is to thread ceramic beads onto the wires in high-radiation, vacuum environments.
Meet the author
Aaron Van Pelt is a product marketing engineer at New Focus Inc. in San Jose, Calif.