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Double layer detection of solar wafers in wafer production

Wafers are light and thin, and stick easily, especially during the cleaning process. Stuck wafers must be detected quickly, as they will likely break and damage others. SICK’s WLL180T fiber-optic sensor coupled with wide bandwidth fibers (LL3), use a 1450 nm infrared beam to penetrate the wafer, instantly detecting stacked wafers.

Task

Ultra-thin silicon wafers are used in the production of solar cells and modules. Unwanted stacking can happen during all the production steps. When two or more wafers are stuck together they will likely break in the next process steps. The wafers fragments, remaining on the machine usually cause the breakage of other wafers.

Implementation

SICK offers two different sensing principles to detect stacked wafers.

High-power infrared, typically operating at 1450 nanometres can travel through silicon. Part of the light energy is absorbed by the wafer. Stacked wafers absorb more light than a single wafer. The sensor switching points should be set half way between the energy levels measured with one and two wafers. High-power infrared is the recommended solution from the wafer slicing up to the metallization process. After the metallization process, the detection principle can not be used as infrared light is blocked by the metal coating on the wafers.

The Ultrasonic double sheet sensor is an alternative solution which can be used after the metallization process. The sensor works based on ultrasonic dampening and distinguishes automatically between none, one or two layers of wafers. This is done extremely reliably whenever an air gap exists between stacked wafers. Therefore this is the recommended solution on dry wafers and after the metallization process.

SICK offers ultrasonic double wafer detection in two form factors. The UM18 double sheet detector is a two-piece version requiring the mounting of a sender and a receiver and requires no settings. The UF3 ultrasonic fork sensor is a compact solution with a fixed 3 mm air gap and manual settings.

Customer Benefits

  • Reduce waste as stacked wafers are a major cause of breakage

  • Following product families can be used
    Product family LL3
    A wide variety of solutions to your most challenging applications: SICK's fiber-optic cables
    • Very large selection of plastic and glass fiber-optic cables.
    • Fiber-optic cables resistant to chemicals and high temperature
    • Threaded and smooth sleeves, bands of light (array), 90° reflection versions available
    • Focused optics
    • Proximity and through-beam versions available
    • Plastic , protective metal or Teflon sheathing available
    Product family WLL180T
    High-performance fiber-optic sensor with world’s fastest response time
    • Selectable response time up to 16 µs
    • Sensing range up to 20 m (through-beam system); up to 1,400 mm (proximity system)
    • Bus-compatible with anti-interference
    • 2 x 4-digit digital display
    • Adjustable hysteresis
    • Rotatable display screen
    • High-resolution signal processing
    • Programmable time delays
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