1. Nanometer Positioning Performance

GENERAL

High-end semiconductors inspection systems require very high positioning resolution and accuracy, fast movement and settling, and very low jitter in standstill state. The SPiiPlus motion controller provides special features highly suited to the extreme performance demands of these types of applications.

HIGH POSITIONING ACCURACY

The SPiiPlus can optionally be provided with up to eight SIN-COS encoder multipliers, each with a programmable factor in the range of x4 - x65,536.

When a SIN-COS encoder is used, the maximum velocity is 250,000 sine periods/second (higher rates are available upon request) and the maximum acceleration is 108 sine periods/second2. The selected multiplication factor does not affect the maximum velocity and acceleration.

As an example, assume a linear axis is provided with 500 line per mm SIN-COS encoder (2µm resolution). Using a multiplier factor of x8,192, the feedback resolution is 0.24 nanometer. The maximum velocity and acceleration are 0.5 meter/second ( 2.048*109 counts / second) and 200 meter/second2 ( 8.192*1011 counts/second2) respectively.

Field tests using the encoder in the example above achieve a jitter of ±1nanometer.

ADVANCED PROFILE GENERATION & SOPHISTICATED SERVO CONTROL ALGORITHM

The SPiiPlus generates a third order motion profile (S-curve) based on the desired position, velocity, acceleration, and jerk. The profile generation rate is programmable: 0.5, 1 (default) or 2kHz. The profile is generated using 64-bit floating-point calculations. On-the-fly position, velocity, and acceleration changes can be done during any stage of the profile.

The control loops are executed at an uncompromising 20kHz rate, with calculation accuracy of 48 bits. High position and velocity loop bandwidths provide responsive servo with exceptional dynamic tracking, fast settling time, and outstanding smoothness at low velocities.

MEASURED PERFORMANCE

The SPiiPlus was tested with a high-end wafer inspection XY table. The SIN-COS encoder resolution was 500 lines per millimeter and the SIN-COS internal multiplier factor wasx8,192. The controller's data collection capability was used to gather the test results, later to be analyzed using an Excel spreadsheet. Data can also be examined using the ACScope that is provided as part of the SPiiPlus MMI software tool.

Two type of movements were measured:

  • 25mm movement with a settling window of +/-50nm.
  • 5µm movement with a settling window of +/-4nm.

RESULTS

[ Table shows results of testing at the nanometer level. ]

CONCLUSIONS

The SPiiPlus advanced controller with its internal SIN-COS multipliers answers the most demanding needs of state of the art wafer inspection systems, achieving sub-nanometer resolution with outstanding move and settle performance. It is accomplished using off-theshelf incremental SIN-COS encoders, which are much less expensive than the alternative laser interferometers sensors.

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APPENDIX: MOTION PLOTS

[ Graph shows 230 msec settle time on a 25 nanometer move. ]

[ Graph shows 51msec settle time on a 5 nanometer move. ]

[ Graph shows +/-0.8 nanometer standing still positioning error. ]

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2. OPTICAL MICROSCOPE AUTO FOCUS CONTROL USING THE SPiiPlus
　


GENERAL

A wafer inspection machine has a microscope used to detect and analyze flaws during the process of chip fabrication. The microscope focus is controlled by a motor according to one or combination of two feedbacks:

  • Incremental encoder: 8000 counts per revolution. 1 encoder count = 15nm.
  • Auto-focus (AF) sensor: Transmits a beam towards the wafer surface that is received by two detectors. In proportion to the intensity of the received beams, two analog signals are generated: SUM and DIF. The SUM signal is the sum of the two beams? intensities. The DIF signal is the difference of the two beams? intensities.

REQUIREMENTS

    1. Settling time <0.5 second for a distance of 10,000 counts.
    2. ?Smooth? switching from AF mode to position mode and vice versa.
    3. Robust AF algorithm and safe implementation.

IMPLEMENTATION

Three focus control modes were defined:

    1. Manual only - user controls the focus manually with a joystick - control loop is based on encoder feedback.
    2. Manual with AF - user controls the focus manually with a joystick as in manual-only mode. Once the SUM signal is greater than a predefined threshold (2V), the AF is activated automatically (on-the-fly). In this state the position control loop is switched to be based on the DIF analog signal while the velocity loop remains based on the
    encoder feedback.
    3. Auto with AF - servo motion is generated based on predefined motion parameters. Once the SUM signal is greater than a predefined threshold (2V), the AF is activated automatically on-the-fly as in section 2 above.

SUM and DIF signal characteristics:

The SUM signal voltage increases when the focus improves. The SUM signal range is 0V to 10V, converted to 14 bits.
The DIF signal range is -10 to 10V, converted to 14 bits. Maximum signal (10 V) = 8192 counts, relates to a distance of 200um. Thus, 1 count of the DIF signal relates to ~24nm.

[ Figure 2 - SUM and DIF signals plots ]

Gain and Polarity:

Since the scaling of the encode signal and the DIF signal are different (15nm vs. 24nm), a scaling factor of 1.6 is used to calculate the position gain in AF control (AF gain). During AF
activation, the DIF signal multiplied by the AF gain is fed to the position loop instead of the position error signal.
The AF gain polarity depends on the direction of the motor.
For example, when the motor moves in the positive direction, the DIF signal stays positive and increases. In this case, the position error is negative. Thus, the AF gain also becomes negative.

RESULTS

1. When the motor was moved +10000 counts from the focus point and the AF was activated, a settling time of 0.3 sec was achieved.

[ Figure 3: SPiiPlus soft scope display of position error (DIF signal) during autofocus activation. ]

2. A smooth transfer between the two feedback types was achieved in all three focus control modes.
3. The robustness of the AF implementation was proven by simulating edge scenarios, such as where one or both of the beams were blocked. The AF algorithm was found to be stable and safe in every simulation.

CONCLUSIONS

The SPiiPlus controller provides enhanced functionality required for complex and demanding applications. Advanced features like the AF can be easily and quickly implemented. The unique structure of the SPii (Servo Processor - DSP) enables on-the-fly switching between different types of feedback source (such as encoder and analog signal) without compromising the performance and smoothness of the motion.

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