Wafer Lifter Detection Using Motor Current Monitoring

Lifter Mechanism and Abnormality Detection

During wafer transfer, the wafer can occasionally stick to the table at the moment it is lifted.
Without adding a separate sensor, changes in motor current alone can be used to identify this abnormal condition. This article explains how the lifter operates and how motor current monitoring can detect wafer sticking, supported by diagrams.

Lifter Structure: Fixed Table and Three Moving Pins

The table remains stationary while only the three lifter pins move up and down in synchronization. Each pin is driven by a DD motor and lead screw. The pins rise through the table and lift the wafer.

Lifter mechanism with three lifter pins rising through a fixed table, shown as an assembly photograph on the left and a cross-sectional diagram on the right
Figure 1: Lifter mechanism structure, with the assembly shown on the left and a cross-sectional view on the right. Three lifter pins rise through the fixed table to lift the wafer. Each pin is driven by a DD motor and lead screw, and the current of each motor is monitored.

Motor Used

MDH(6)-3018-108KECompact direct drive motor with a large hollow shaftOuter diameter: 30 mm
Hollow-shaft diameter: 6 mm		Resolution: 324,000 counts/rev
Peak torque: 0.4 N·m

What Changes Between Normal and Abnormal Operation?

The table remains fixed while the three pins extend through it to lift the wafer. During normal operation, the sequence progresses from (1) the lowered position to (2) upward movement and then (3) the fully raised position. During abnormal operation, the wafer does not separate from the table, and the pins continue pushing against it, creating excessive load.

Lifter-pin operation. The table remains fixed while only the wafer height and exposed pin length change. The upper sequence shows normal operation from the lowered position through upward movement to the fully raised position. The lower illustrations show wafer sticking and excessive load conditions.
Figure 2: Lifter-pin operation. The table height remains fixed, and only the wafer height and exposed pin length change. The upper sequence shows normal operation from (1) the lowered position to (2) upward movement and then (3) the fully raised position, where the wafer has been lifted. The lower illustrations show abnormal conditions, in which wafer sticking or excessive load applies additional force to the pins.

The condition in which “the pins continue pushing even though the wafer does not rise” is the key abnormality indicator. The additional force on the pins causes the drive motor current to increase.

Why Motor Current Reveals Wafer Sticking

When wafer sticking occurs, the pins continue pushing against the wafer and place additional load on the motors. As motor load increases, motor current also increases. This relationship makes it possible to detect the abnormal condition without adding a separate sensor. The motor current for each pin is monitored in real time.

0.00.30.50.81.0Current [A]Time → Measured current, including noise EWMA-filtered current Threshold, example: 0.5 A Normal: approx. 0.3 A Wafer sticking: approx. 0.8 A
Figure 3: Example motor-current response. The measured signal, shown by the thin line, fluctuates because of noise. Wafer sticking is detected when the EWMA-filtered signal, shown by the thick line, exceeds the threshold. Current remains stable at approximately 0.3 A during normal operation and rises to approximately 0.8 A when sticking occurs.

Motor Current Noise Filtering Using EWMA

Motor current contains small fluctuations and spike noise while the motor is operating. Comparing the raw current directly with the threshold can cause false detection from a momentary noise spike. To prevent this, the system uses the Exponentially Weighted Moving Average (EWMA).

EWMAn = α In + (1 − α) EWMAn−1
Contribution of the latest sample Retention of the historical trend
Figure 4a: The first term represents the latest current sample, while the second term retains the previously filtered value and its underlying trend.
SymbolDescription
EWMAnThe filtered current value calculated for the current sample. This output is used for abnormality detection.
InThe raw motor-current value acquired in real time for the current sample.
EWMAn−1The previously calculated filtered value, which retains the historical trend.
αThe smoothing factor, with a value between 0 and 1. It determines the balance between response speed and noise immunity.

By continuously combining the latest sample with the historical value at a fixed ratio, EWMA suppresses momentary spikes while reliably capturing the sustained load increase caused by wafer sticking.

Effect of the Smoothing Factor α

Large α, close to 1
The latest measured value In has a stronger influence. The response is faster, but the signal is more sensitive to noise.

Small α, close to 0
The historical average has a stronger influence. The waveform is smoother, but its response to sudden changes is slightly slower.

0.00.30.50.81.0Current [A]Time → Raw signal Iₙ EWMA (α=0.3) EWMA (α=0.1)
Figure 4b: Comparison of the raw signal and EWMA with different α values. A smaller α produces a smoother signal that is less affected by spikes, but its response to the rising current is slightly slower.

Wafer Sticking Detection Process Flow

Current SamplingReal Time
→
EWMA FilteringNoise Reduction
→
Threshold ComparisonAbove Set Value?
YES→
Wafer Sticking DetectedAbnormal Load
→
Machine StopAlarm Notification

Wafer sticking is detected only when the EWMA-filtered value exceeds the specified threshold, and the system then safely stops all axes. Momentary noise does not stop the machine, which helps prevent nuisance stops while reliably detecting actual abnormal conditions.

Detectable Abnormalities and System Benefits

Examples of Detectable Abnormalities

  • Wafer sticking
  • Wafer jamming
  • Mechanical interference of the lifter pins
  • Abnormal friction
  • Pin seizure or operating failure

System Benefits

  • Simple configuration without additional sensors
  • Reduced risk of wafer damage
  • Earlier detection of transfer abnormalities
  • Improved equipment safety and reliability

Summary

The table remains fixed while three lifter pins rise through it to lift the wafer. When the wafer sticks to the table, it does not rise as intended. This places excessive load on the pins and causes the motor current to increase.

By filtering and monitoring the motor current with EWMA, the system can avoid nuisance stops caused by noise while reliably detecting wafer sticking and transfer abnormalities. This supports safer and more reliable wafer handling.

Motor Used

MDH(6)-30 Series

MDH(6)-30 Series compact hollow-shaft direct drive motor

Driver Used

MC-200-7220 Series

MC-200-7220 Series compact servo driver

Case Study

Wafer Lifter Pins That Reduce Required Space by Half

Wafer lifter assembly