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Minimize Vibration, Improve Motion Control Performance
Understanding the various methods to reduce the impact of vibration is essential in semiconductor industry systems
Boaz Kramer, Contributing Writer -- Design News, September 25, 2006
Designers of motion control systems for the semiconductor industry are consistently confronted with increasing demands for higher performance. To produce more semiconductors faster and at a lower cost, semiconductor manufacturers rely on continuous improvement in motion control systems.
In the semiconductor manufacturing process, inspection usually involves positioning the silicon wafer relative to optical (or other) components by placing the wafer on a high-speed XY stage. Higher velocities and higher accelerations are typically required to increase the machine throughput. These are usually restricted by the voltage and current limitations of the motors and drives. Nevertheless, an aggressive motion profile would not necessarily yield higher throughput. In many cases, the practical velocity and acceleration are reduced well below their maximum achievable values. The reason for this is that profile duration, the commanded move time, is not important by itself. The important factor is the total sum of the move time and the settling time (see side bar, next page). The settling time is determined by various dynamic effects and can significantly increase with a very aggressive profile.
The settling time problem will manifest more severely when higher resolution is required and the settling widow is tightened. It should be noted that the resolution of instruments in the semiconductor industry approaches, and in some cases goes below, 1 nm. Instruments with this type of resolution are inevitably sensitive to even the smallest vibration or disturbance. Vibrations that might be excited by the motion profile itself require special considerations. The most common practice is to modify the motion profile by means of trial and error, to keep the entire move and settle duration as short as possible. In many cases, this method does not provide the desired results.
Passive and Active Isolation Techniques
Passive or active isolation systems are typically used to isolate the system from disturbances that may enter the system from the floor. Passive isolation systems employ a seismic mass supported on a soft spring. The spring can be made of air, metal or rubber materials. The spring and associated damping absorb vibrations above the resonance of the spring. To increase the effective isolation range, makers of passive isolation systems usually try to lower the resonant frequency. As a result, most passive isolation systems are very soft. This softness becomes a problem with a fast-moving stage.
When a servo force is applied to the load to generate a desired motion, it also acts on the isolated stationary base, causing it to vibrate. Since the frequency is low (usually below 1 Hz and up to 10 Hz) and the damping is very light, the isolation system vibrates long after the motion profile has ended. This vibration acts as a disturbance to the servo system. The vibration introduces position error and extends the settling time, preventing the inspection instrument from making any measurements.
The phenomenon is much less severe in active isolation systems. These systems detect vibration levels through sensors that send signals into a feedback or feedforward controller, which triggers actuators to counteract the forces.
In a feedback system, a sensor measures vibrations affecting the isolated base and the system reacts to reduce the vibrations level. This means the system is not only absorbing energy entering from the floor, it can also effectively absorb vibrations from the moving stage. In addition, typical active systems do not use air, so they are inherently stiffer than an air tables. In this case, the resonant frequency also depends on the tuning of the isolation system servo, and it is typically significantly higher (greater than 10 Hz) than that of the passive systems and much better damped. As a result, the vibration is suppressed better and faster.
Most active vibration isolation systems are relatively complex and costly. They are also more difficult to set up and their support electronics often require adjustment.
Command Feedforward
A simpler and less expensive approach for active damping is the command feedforward method. This technique is useful in cases where there is a known force applied to the isolated base and a signal proportional to that force is available. Command feedforward can be performed with sophisticated motion controllers, since the applied force depends on the stage acceleration calculated by the motion controller.
An analog signal proportional to the commanded acceleration in a certain direction is sent by the motion controller to an actuator, which in turn, produces a force equal in size but in the opposite direction.
Typically, additional analog signals indicating the commanded position are sent as well because of twist couplings. For example, the isolated base will twist clockwise if there is an X-acceleration when the stage is in the full -Y position, but counterclockwise when the stage is in the full +Y position.
The advantage of the command feedforward method is clear: it is significantly more cost-effective than the sensor-based isolation systems. In addition, the feedforward method cannot become unstable, and it can produce a significant improvement in settling performance. However, this approach also requires setup and proper tuning of the feedforward gains.
Active isolation systems, even when they are well-tuned, cannot eliminate the vibration entirely. As previously mentioned, semiconductor inspection machines can use very high resolution feedback devices. Consequently, they are very sensitive to even the slightest disturbance that can extend the settling time to nanometer or sub-nanometer settling windows. Two possible solutions to the problem are:
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Increased disturbance rejection of the servo system.
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Special motion profiles that don't excite disturbing vibrations.
Improved disturbance rejection can be achieved by increasing the servo bandwidth. This especially helps in air table systems where the resonance frequency is relatively low, well inside the bandwidth of the servo. In many cases, a standard PID or PIV control algorithm is not good enough and additional hi-order filters and special non-linear algorithms are needed to further enhance the disturbance rejection of the servo (so-called "disturbance rejection algorithms").
One method to improve the servo disturbance rejection is the "frame acceleration compensation." The performance of a stage with a passive isolation system can be upgraded by using two accelerometers in XY directions. The acceleration of the base is measured in each direction and a compensation force ML×ais applied to the drive command, such that the resulting position error is significantly reduced (ML - moving load mass).
It is important to note that improved servo disturbance rejection will not prevent vibration of the isolation system, but will minimize the relative move between the base and the load. This is the goal since the inspection instrument is installed on the base.
Application Examples
The "frame acceleration compensation" method was successfully implemented using an ACS Motion Control SPiiPlus series motion controller on a system with a passive isolation, 0.0625 µm resolution, and 1 µm settling window. The isolation system in this case did not use air and its resonance was relatively high (greater than 10 Hz). After using the technique, a significant improvement in settling time was observed.
Like active isolation systems, this method is less effective when the required settling window is a few nanometers or less. It is not possible to compensate for the vibration entirely and the settling time is still affected. The system may also be sensitive to noises resulting from accelerometers. In these instances, appropriate filtering should be applied.
Another example involves the settling performance in a system, where the desired move was a few millimeters and the settling window was only 1 nm. ACS Motion Control SPiiPlus series motion controller was used to control this system. In this case, the encoder's basic 2 µm resolution was enhanced by an on-board SIN-COS multiplier, achieving a final resolution of 0.49 nm. Implementing a non-linear "disturbance rejection" algorithm essentially cut the settling time in half.
Preventing the isolation system from vibrating is quite desirable, but only if it does not have a severe impact on the move time. In all other cases, isolation system vibration can be tolerated as long as its effect on the position error is minimized as much as possible.
Motion control system designers should be aware of the various methods that significantly improve system performance and throughput. Modern high-end motion controllers provide many of these improvements.
| ADDTIONAL RESOURCES | |
 The top right figure shows two different motion profiles for the same moving distance. The blue profile is more aggressive, while the red profile has lower acceleration and velocity. The top left graph presents the servo position error for these two motion profiles. Even though the blue profile has a significantly shorter duration, it generates a higher position error and results in a longer settling time. The total move and settle time is much shorter with the less aggressive profile. |
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 Move Time vs. Settling TimeFor applications that require point-to-point moves, machine throughput is usually translated into terms of move and settle. Move time relates to the duration of the motion profile. It is dominated by the commanded motion parameters, such as velocity and acceleration. Settling time relates to the duration from the time the profile ends until the system reaches and stays within a certain window of the target. In this particular example, a settling window of ±1 count is specified and the settling time is larger than the move time. |
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 Position error with (red) and without (blue) isolation system vibration. The blue graph shows the servo position error with isolation system vibration excluded, and the red graph shows it taking the vibration into account. Settling performance deteriorates severely with vibration. |
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 The frame acceleration compensation control scheme for a single direction uses feedback and a single accelerometer to reduce position error. |
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 For settling into a 1 nm window, the black graph shows position error using a standard PIV algorithm compared to the red graph’s position error using a special algorithm introducing enhanced disturbance rejection. |
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 Effect of isolation system dynamics on the Bode plot of a system The figure shows the open loop Bode plot of a system controlled by a PIV filter [2]. The blue plot describes the system without the isolation system dynamics. The red plot shows the effect of the isolation system dynamics. The additional resonance and anti-resonance are clearly observed with the introduction of phase lead. Note that the effect in the frequency domain is not significant; however, the effect on time domain performance can be crucial. More Details |
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 The effect of the isolation system vibration can be described using a basic model [1], illustrated in the figure below. It shows a direct drive system on a base with limited mass and stiffness. More Details |
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| Get more information on the Frame Acceleration Compensation | |
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Get more information on the "Move and Settle Performance" |
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