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InGaAs SWIR Imagers Optimize Semiconductor Inspection

As the volume of consumer electronics increases, semiconductor fabrication plants are manufacturing larger and larger wafers to handle the demand (e.g., 300 mm substrates). The escalating value of these larger wafers is driving the industry to employ more advanced imaging technologies for quality control. Inspecting the raw material substrate for flaws before processing and detecting defects during processing is critical to keeping costs down. New and improved inspection techniques save the semiconductor manufacturing industry hundreds of millions of dollars each year.

altIndium gallium arsenide (InGaAs) cameras, operating in the SWIR (short-wave infrared) wavelength band from 0.9 μm to 1.7 μm, allow users to image through semiconductor materials such as silicon (Si), gallium arsenide (GaAs), indium phosphide (InP) and others. This is possible because the light that InGaAs detects is lower energy than the bandgap of the material being inspected. The ability to image through these semiconductor materials provides a non-destructive inspection technique that offers great benefits to manufacturing process control.

altThe largest application for SWIR imaging in this industry is for a technique named emission microscopy. Silicon, gallium arsenide, indium phosphide and other semiconductor materials actually emit a small number of photons at the band edge. This very low light level is detectable by cooled SWIR cameras. In emission microscopy, GaAs, Si, or InP circuits are activated and shortwave IR cameras are used to observe where design or manufacturing defects have occurred by observing the emissions in the circuit when they occur. Figure1(left to right) shows 3 images of a GaAs circuit imaged with a SWIR-InGaAs camera from SUI, Goodrich Corporation. On the left, the circuit is seen with only ambient light. The middle photo shows the circuit “on” or activated under the same ambient lighting conditions. The right photo is the GaAs circuit with no illumination in the room and only the emission is seen (the small white spot) to indicate the defective junction on the GaAs circuit.

altIn more sophisticated applications of emission microscopy, manufacturers use special single-element photodiodes to observe when the actual emission occurs while the circuit is on or operational. This allows the user to determine when a gate is being activated. The emission from the gates is on the order of just a few photons. Single-photon counting systems are used to capture these events and the time they occur. This was previously accomplished with either very expensive, specialized photodiodes that had to be cooled to temperatures of liquid nitrogen or below, or with cooled, photo-multiplier tubes. Now, more affordable InGaAs avalanche photodiodes (APDs) cooled with solid-state, 3-stage coolers are being implemented to handle these tasks. These newer, specialized indium gallium arsenide APDs are used as photon counters because of their extreme sensitivity, making them ideal for emission microscopy applications.