Piezoelectric Motors Control Set-and-Hold Semiconductor Applications

Dave Arnone, Aaron Van Pelt and Kathy Li Dessau

With the shift to tighter integrated circuit manufacturing specs and 157-nm lithography, motion control increasingly demands high resolution and long-term stability not found with conventional motors.

Motion control applications in the semiconductor manufacturing industry range from mask and wafer alignment to laser beam steering and active pointing stabilization. Because these applications are “set and hold,” with position first optimized and then maintained for a certain length of time, they demand high resolution and long-term stability. Positioning accuracy, the traditional emphasis of motion control systems, is less important in this environment.

This increasing emphasis on resolution and stability will only continue with the trend toward manufacturing of tighter-geometry integrated circuits and the shift from 248-nm lithography (with a KrF excimer laser) to 193-nm (with an ArF excimer laser) and now to 157-nm processing (with an F₂ excimer laser).

Processing at shorter wavelengths also dictates that motion control systems be exceptionally clean to eliminate contamination of the optics, wafer and mask. Any inorganic and organic compounds introduced into the air from the mechanical or optical components are broken down by the high-energy UV beam and deposited on the optics surfaces.

The UV beam polymerizes any surface contaminants, introducing a high-heat-capacity site that in turn burns the optical coating. A cleanliness benchmark, where the outgassing level of organics and inorganics is compatible with the UV beam environment, is vacuum compatibility to 10^—9 t, although 10^—6t may be adequate in some instances.

Today’s semiconductor-targeted systems must deliver vacuum compatibility as well as excellent resolution and superior long-term stability. For electron-beam mask-writing systems where magnetic elements steer the beam, the motion control systems also must be highly nonmagnetic. Manufacturers must build them using materials such as titanium and molybdenum to minimize the magnetic properties.

Figure 1. Stick-slip piezo-based motion control systems can be a good fit for semiconductor applications.

Until now, stepper motors, DC servomotors and ferroelectric actuators (i.e., piezoelectric or electrostrictive) have dominated the traditional motion control market. Designed for the industrial manufacturing environment, these devices provide the repeatability and accurate automation required for processes such as cutting and welding. Most, however, do not offer good long-term stability and/or cannot be made vacuum-compatible or nonmagnetic. Thus, a better fit for semiconductor applications may be piezo-based systems. These systems can hold their position with no applied voltage or brake, deliver high stiffness (thus, long-term stability) and provide step sizes in the range of tens of nanometers. They also can be manufactured cost-effectively to be nonmagnetic and vacuum-compatible.

Piezo-based motors

Piezoelectric motion control devices have been around for many years. In the simplest case, the piezoelectric material simply expands and contracts under applied electrical voltage to provide the positioning mechanism. Such a device is very fast and provides extremely fine control but has a limited travel range. Often it is integrated in-line with a screw to boost the range of motion (Figure 2). The problem is that material expansion is a direct measure of applied voltage, and constant power must be applied to hold the adjusted position. Piezo systems also typically suffer from hysteresis and creep (also known as low stiffness), and require a clean, low-noise power supply for accurate positioning.

Figure 2. A conventional piezo-driven actuator’s extension is the sum of the extensions of the piezoelectric material and the micrometer.

Even though they can be both vacuum-compatible and nonmagnetic, their limited travel range, poor long-term stability and need for constant applied voltage have made them less desirable for precision set-and-hold applications such as those in the semiconductor industry.

The piezoelectric transducer also is seen in other configurations where movement does not rely solely upon expansion and contraction for positioning. These stick-slip motors rely on the basic difference between static and dynamic friction. One example of this principle is the tablecloth trick in which a quick pull leaves the dishes on the table (low dynamic friction), while a slow pull of the tablecloth carries the dishes with it (high static friction). These nontraditional piezo motors use slow motion to move, turn or slide along objects, and fast motion to return to their original position, thereby providing both high resolution and long travel. Many can be nonmagnetic and vacuum-compatible as well.

One of the first examples of stick-slip piezo motors uses multiple piezo elements to move a shaft in a manner similar to that of an inchworm. These devices have an extension actuator and two clamps that move in a synchronized clamp-extend/clamp-retract cycle. They deliver nanometer resolution, long travel ranges and fairly high velocities of a few millimeters per second. However, because the piezo is used to clamp the shaft, holding position requires that the power be on, which could compromise long-term stability. Furthermore, because the piezo is the holding element, it does not deliver the stiffness often required in semiconductor applications.

Another nontraditional piezo-based configuration applies an ultrasonic frequency to a piezo beam, generating a planar elliptical path of the piezo tip. By using a spring to hold these piezo vibrators against a ceramic strip fixed to a base, these vibrators will “walk” with respect to the base. Often these motors are embedded in a sliding stage.

Because they combine high speed —up to 250 mm/s — with very high resolution, the devices are best-suited for applications requiring fast motion. An advantage is that they can be designed with enough force to allow wobble-free positioning when not energized. However, the holding force between the piezo beam and the ceramic strip limits long-term stability, and stiffness is relatively low because the piezo is the holding element.

Finally, a third stick-slip design, called the Picomotor, uses a piezo element to turn the screw. The piezo is positioned between two jaws to provide the torque to turn the screw (Figure 3). Here, the piezo is used only to turn the screw and not to hold the adjusted position. Thus, the mechanical stability of the motor is identical to a nonmotorized screw and nut set, delivering high stiffness and long-term stability.

Figure 3. In a stick-slip motor that uses piezos to turn a screw, two jaws grasp an 80-pitch screw (2a), and a piezoelectric transducer slides them in opposite directions. Slow action of the piezos causes a screw rotation (2b), while fast action causes no rotation (2c) because of inertia.

The working principle behind this stick-slip motor is similar to how a person turns a screw. Two jaws, connected to either end of a piezoelectric transducer, grasp an 80-pitch screw. A slow-rising electrical signal applied to the piezo slowly changes its length, causing the jaws to move in directions opposite and tangential to the screw, just as a thumb and forefinger would. This slow motion makes the screw turn by static friction. 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 remains motionless, holding position. Simply reversing the order of fast and slow pulses reverses the direction of the motor.

The main advantages of such a device include high stiffness and resolution, as well as long travel ranges. Because of the slip-stick nature of the motion, though, the device tends to be slower than the other types of actuators. Nevertheless, the system can be vacuum-compatible (10^—6 t) and nonmagnetic. Because the screw-based stick-slip motor relies upon the mechanical properties of the screw, it can optimize set-and-hold applications, providing a combination of submicron resolution and long-term stability.

Semiconductor manufacturing

One set-and-hold application using a compact stick-slip motor involves the Z/tip/tilt platform of a scanning X-Y stage for semiconductor wafer inspection, processing and microscopy. A fast X-Y stepper or servo system scans the wafer or sample under inspection. Then three compact stick-slip motors provide the final precision adjustment to align orientation and focal height (Figure 4). These three adjustment actuators sit on the scanning X-Y stage and are periodically updated, then held constant over significant time intervals.

Figure 4. The high-speed translation stage uses three Picomotor products to adjust the height, tip and tilt.

Because of the critical alignment needs, these actuators must keep their positions over the scan. Thus, this particular semiconductor application requires compact, stiff, stable high-resolution actuators capable of setting and maintaining position to tens of nanometers even with no applied power. In addition, the motors must have low mass because they must sit directly on the high-speed moving translation system. DC servomotors and steppers are too large, while traditional piezo motors have too much creep and hysteresis. In this electron-beam application, though, the motors had to be custom-made to be vacuum-compatible and nonmagnetic.

Compact stick-slip motors also can counteract laser drift in industrial metrology applications. Actively stabilizing the laser can significantly improve system metrology. In this example, the laser stabilization technique continuously stabilized a UV laser beam’s location and pointing in an advanced reticle contamination inspection station. Here, the beam left the laser and went through two motorized mirror mounts prior to reaching the work site (Figure 5). Constant beam pointing was possible with the adjustment of two mirror mounts. P1 and P2, the two points in space through which the laser beam will propagate, were imaged on quadrant detectors called QD1 and QD2, respectively. In turn, the system used the detected signals to servo the beam’s location and pointing. By keeping P1 sufficiently close to a location called M2 and far from P2, the system allowed treating the sampled legs independently.

Figure 5. In this feedback system, points P1 and P2 define the two points in space through which the laser beam will propagate.

Because the adjustments were made only occasionally, this manufacturer required devices with superior long-term stability. Moreover, the optomechanical modules that were used to steer the UV beam had to provide high flatness, stability and reliability. New Focus engineers designed them to conform to these critical specifications, allowing development of a custom bonding process that could withstand the exceptionally clean environment yet not induce optical distortion.

Meet the authors

Dave Arnone is mechanical engineering manager, and Aaron Van Pelt and Kathy Li Dessau are product managers at New Focus Inc. in San Jose, Calif.