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.
Indium 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.
The 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.
In 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.
Material quality is important in all semiconductor applications. High material quality at the start of wafer processing is critical to reducing waste. Dislocation defects in the material can be very detrimental to a semiconductor device. Users can now image these defects by passing shortwave infrared light through a semiconductor wafer and imaging it with an uncooled InGaAs camera. The light diffracts ever so slightly at these large defects, allowing the user to image the location and determine the number of these defects in the wafer. The quality control inspectors are then able to prevent poor quality wafers from being processed. Figure 2 shows how the wafer is transparent to SWIR light (left) versus visible light (right) where the wafer is opaque.
Many semiconductor devices are processed using both sides of the wafer. Alignment difficulties may occur because the wafer is opaque to visible light. Since the wafers become transparent at longer wavelengths, SWIR cameras are able to see through them, while using standard glass optic microscopes. The ability to quickly inspect the alignment of the backside pattern to the front side is a noted benefit to using SWIR imaging. This is in addition to being able to observe processing defects in the semiconductor that can be otherwise obscured by the metal layers on the top surface of the integrated circuit. Another advantage is that InGaAs-SWIR cameras utilize glass optics instead of more expensive Germanium or other OR-optimized optics, making them simple to integrate into existing wafer processing fabs.
Silicon, GaAs, InP and other semiconductor materials are not only being used in the microelectronics industry but also in the optoelectronics industry. Silicon waveguides are being utilized in telecommunication applications and future work will include the microelectronics integrated with the optical control. These waveguides direct light through the material to various points but the waveguides do have losses. InGaAs cameras can image the light being used in the waveguides, typically 1.3 and 1.5μm, easily allowing the user to image the losses as well as the total output. Mating the waveguides with light emitters by viewing through the waveguide permits active, automated alignment, improving yield and driving down costs. SWIR cameras and photodiode arrays, developed with indium gallium arsenide technology from SUI, Goodrich Corporation (Fig. 3) are becoming increasingly important diagnostic tools for both the telecom and semiconductor manufacturing markets.
InGaAs cameras over the last couple of years have become more sophisticated with more resolution, sensitivity, and greater capability (processing power) in a smaller, lighter form factor. This has allowed the semiconductor industry the ability to capitalize on this new technology and to integrate it into its testing, inspection, and quality control systems. As the cameras become more integrated in this industry they will find even more applications.
This article was written by Martin H. Ettenberg, Ph.D., Director of Imaging Products, and Douglas S. Malchow, Manager of Commercial Business Development, at Sensors Unlimited Inc. (Princeton, NJ). For more information, contact Mr. Ettenberg at