It just so happens that this phenomena is present when inspecting silicon (Si) and gallium arsenide (GaAs) – two of the most important materials in the printed circuit board (PCB) and integrated circuit (IC) production industry.
SWIR wavelengths have high-level transmittance through silicon beginning at around 1050 nm. Following this discovery and the apparent ability to look through silicon-based matter has given rise to the development of highly accurate defect inspection systems.
A powerful tool in the detection of many different types of defect, SWIR LEDs and microscopic imaging allows proper analysis to find the cause. Many circuit failures can be attributed to a broken or open circuit. The culprits of such failures are as diverse as metal impurities in the substrate, electrical overstress (EOS), or misaligned masking during the photolithography process.
While many defects may immediately render the layered IC useless, others may only become apparent once in the hands of the end-user. The ability to find and identify these problems can reveal issues in the production process. This can prevent serious financial losses and production downtime for manufacturers, as well as avoiding the recall of potentially all devices produced during the period that the defect manifested.
Why is Silicon Defect Inspection so important?
Semiconductor Devices Defect Inspection
Integrated circuits, more commonly known as microchips, are a collection of tiny components, such as diodes, microprocessors, and transistors that are electrically interconnected and built upon a silicon wafer substrate. Since their introduction in the 1960s, and the following advancements in miniaturization, ICs have completely revolutionized electronics and have become the centerpiece in the majority of modern society’s everyday tools. From cars to smartphones, washing machines to GPS satellites, it is now almost impossible to imagine the world without ICs.
Over time, the number of transistors (and other components) housed on a single microprocessor chip has rapidly grown. This allows manufacturers to create increasingly complex devices, while the size of the end product is actually decreasing! A side effect of this advancement is as simple as this – any system or construction has more chance of experiencing a defect with every additional component.
A popular example is the processing power of NASA’s famous Apollo 11 mission to the Moon, in 1969. The two Apollo Guidance Computers (AGCs) each contained 2,048 ICs, which were home to just a few tens of thousands of transistors. Considered compact in their day, the AGCs measured 60 x 30 x 15 cm, and weighed in at a staggering 30 kg! By comparison, we now carry a smartphone in our pocket that weighs under 200 g and contains more than 6 or 7 billion transistors. The processing power of a modern smartphone is hundreds of millions of times that of the Apollo Guidance Computers.
While the failure of a microchip may seem inconvenient at times, our society is now so dependent on them that it could actually escalate a dangerous situation. Now, imagine if just one of those transistors experienced a failure on the way to the moon… perhaps the famous words would be “one small defect for integrated circuits, one giant catastrophe for mankind”!
Photovoltaic Defect Inspection
One of the most useful SWIR defect inspection applications is the testing of individual solar cells or entire arrays that convert solar radiation into electrical energy. Constructed from semiconductor materials, the most common solar cells are silicon-based (Si). While there are other compound and organic materials that can be used for the same purpose, Si is one of the most accessible elements on Earth, which significantly boosts its popularity in this application.
This principle of energy conversion – also known as photovoltaics – is used to generate useable electricity and serves as the basis of photo-sensor technology. Silicon has a “forbidden band gap” of 1.12 electron Volts (eV), which means the photons hitting the solar cell must possess energy in excess of 1.12 eV in order to generate an electronic charge. Any slight imperfection in the production process can impede the ability of a cell to generate electricity efficiently.
When trying to harness the power of the Sun, the photovoltaic cells must be as efficient as possible and are expected to last up to 30 years in the outdoors. Defective production processes can cause serious efficiency problems immediately or lead to problems further down the line, such as gradual corrosion failures that cause a progressive decrease in power generation.
How are Ushio’s SWIR LEDs used in silicon defect inspection?
There are various types of silicon defect inspection, such as detection of cracks and internal defects in the wafers themselves, or the detection of defects in circuits formed with different materials on silicon substrates. All of these applications rely on the transmittance of SWIR wavelengths longer than 1100 nm through silicon.
Typically, defects are detected by acquiring a transmission image of the area to be measured. This could be an image of an entire wafer, or a small area measuring only a few microns. When the area of interest is that small, microscopic optics are an essential part of the detection system, while an InGaAs area sensor can be relied on to detect even the smallest defects.
SWIR LEDs illuminate the target area of the Si wafer and the transmitted image is captured by a photodetector, typically an InGaAs area sensor. In principle, any SWIR wavelength longer than 1100 nm will reliably transmit through silicon, but longer wavelengths may be used depending on the impurities in the silicon and the different materials that form the circuit.
Contact Ushio
If you would like to make an inquiry regarding the role Ushio’s SWIR LEDs in silicon defect inspection applications, contact Ushio’s regional experts via the following link:
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