metrology LEDs
layers, blocking layers and other layers built up on the surface of the die.
Making matters worse, thermal cycling, which is inevitable when operating an LED bulb, can cause these structural gaps to expand and become more effective thermal blockers or electrical insulators.
Although LED modules are typically tested for voltage and light output, they are rarely scrutinised for internal structural defects, which can play havoc with production yield and device lifetime. Widely used techniques in the fabs, such as X-ray and infrared microscopy are unable to expose these imperfections. However, they are easily revealed with acoustic micro imaging systems based on VHF or UHF ultrasound. At Sonoscan of Elk Grove Village, IL, we have recently developed an instrument with this capability, the C-SAM system. It can be used for process control, revealing defects during production, and it can be a valuable aid for failure analysis.
Acoustic imaging may be carried out on an undiced LED wafer, or on LED arrays during assembly. In both cases, our system’s scanning transducer needs a flat surface into which to pulse the ultrasound signals. Wafers and the heat sink surface of an assembled LED are both flat; as is, at some points in production, the face of the LED. To probe an LED incorporating a lens, it may be necessary during failure analysis to grind this optical element flat to permit acoustic imaging.
The ultrasonic transducer emits pulses into the surface of the undiced wafer or individual chip, and a few millionths of a second later it reads the return echoes, resulting from reflections at interfaces. Boundaries between two solid materials tend to produce echoes of medium amplitude, while the highest amplitude echoes are returned by the solid-to-gap interfaces encountered at internal defects. The transducer carries out its pulse-echo role thousands of times a second while traversing across the sample at speeds well in excess of 1 m/s.
October 2012
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