Semiconductors are an integral part of the inner workings of medical devices, contributing to the conductivity between non-conductors and conductors to control the current. In turn, the assembly process to make the perfect semiconductor is very detailed, especially now that devices are becoming smaller and smaller. As semiconductors are rapidly miniaturized to fit into these smaller devices, the role of lasers in semiconductor manufacturing has adapted.
Laser technology is often used in semiconductor manufacturing for its thin, precise, versatile and powerful beams for a variety of reasons, including cutting, welding, coating removal and marking.
Cutting / Scribing
In the production of semiconductors, there are various dicing steps, including cutting wafers out of crystal blocks and templates out of thin films. Dicing with a laser ensures that chips are cleanly cut so that they fit properly into the final device. Using lasers allows semiconductors to be cut into many shapes and patterns that are not possible using other dicing methods. According to Columbia University's Fu Foundation School of Engineering and Applied Science, cutting wafers using this method reduces tool wear and material loss and results in higher yields.
Columbia's study material on semiconductor laser processing states that "advantages of laser cutting include less tool wear, reduced material loss around the cut, higher yields due to less breakage, and faster turnaround due to ease of fixturing."
Another option for cutting is scribing - drilling a series of closely spaced or overlapping blind holes halfway through the material. This is a method widely used in semiconductor manufacturing applications, such as cutting aluminum oxide substrates into chip carriers or separating silicon wafers into chips. It is worth noting that the type of laser required for scribing depends on the material used.
The university says, "Aluminum oxide scribing uses CO2 lasers, while silicon scribing uses Nd:YAG lasers because different materials have different absorption rates at different wavelengths."
The motivation for using scribing versus cutting depends on the speed at which the action occurs in the fabrication shop. "For aluminum oxide, which is about 0.025 inches thick, the material can be scribed at a rate of about 10 inches per second using a medium-power CO2 laser, whereas for a similar laser, the cutting rate may be fractions of an inch per second," writes the university staff. "Scribing also offers the advantage of being able to scribe the substrate before processing is complete and then easily separate it into chips after processing."
Soldering
Laser soldering or laser diode welding is the process of melting adjacent parts of a semiconductor component together, much like securing a wafer to a support board. For support boards that are ready to be bonded, such as lead frames, the laser places an identification mark on the frame and then roughens the surface to ensure that the two parts are securely bonded together. Once bonded together, the laser marking machine removes the burrs created by the roughening process.
Coating Removal
Ensuring that the semiconductor is clean and free of defects is part of a manufacturing process called coating removal. Using a laser (usually Nd:YAG), unwanted coatings can be removed as if they were resin or copper, and as if they were gold or thin film coatings. For deburring, the laser utilizes its fine, precise beam to remove excess material without causing damage to the product. Removal of coatings allows defects to be more clearly analyzed, eliminating the need for disassembly for inspection, which could result in product damage.
Marking
Laser marking of semiconductors is important for product traceability and readability, which means that the laser must be clearly legible in very small prints. Product traceability means that the product can be tracked through the multiple steps of production as well as final distribution. This makes it easier to find and isolate specific categories of defects.
Marked chips must also be readable, as marking is a useful way to determine which product is suitable for an application. According to Wafer World, "The laser not only cuts into the surface of the wafer, but also rearranges the surface particles to create extremely shallow but easy-to-read markings."
There are two types of markers used on semiconductors: etch markers and annealed markers. Etch markers are thin layers of material that are removed using a laser, leaving a textured mark about 12 to 25 microns deep. These are often referred to as "hard marks" because there is a visible change in the surface layer.
Annealing marks, on the other hand, use a laser set to a lower power level to rearrange the molecules rather than etching them. This creates a contrast on the chip surface that is visible when light is reflected.
Types of lasers
Currently, companies mostly use solid-state lasers for chip fabrication because they are known for their high power and use ore as the laser medium. Ore media typically consists of yttrium, aluminum, garnet, or yttrium vanadate crystals. For example, Nd:YAG lasers use neodymium-doped yttrium aluminum garnet crystals as the medium. The laser beam is generated using an oscillator that stimulates the medium with light from a laser diode.
One type of solid-state laser used for chip marking, engraving, and dicing is the fiber laser, Keyence says, adding that the high-speed lasers use "optical fibers as resonators and create overlapping structures through Yb-ion doped fiber cladding," noting that its fiber lasers are known as the MD-F series of 3-axis fiber lasers. "Some of the uses of fiber lasers include removing burrs from pre-production processes, marking traceability codes, and removing resin for defect analysis."
Excimer lasers are also used in semiconductor manufacturing. These are deep ultraviolet (UV) lasers with wavelengths ranging from 126 nm to 351 nm that are primarily used for polymer micromachining. The shorter UV laser beams compared to solid state make them suitable for any type of material, including very fragile and delicate materials, and allow them to work in a very small precise area with a reduced point of action. When used for marking, UV lasers alter the structure of the product material at the molecular level without generating heat in the surrounding area.
Laser Innovations
Currently, solid-state and excimer lasers are seen as the main options when using laser manufacturing for semiconductor production. However, a new option that can rival the classics may soon be available. In a recent study published in the journal Nature, a team of researchers from Kyoto University led by Susumu Noda wrote that they have taken steps to overcome the limitations of semiconductor laser brightness by changing the structure of photonic crystal surface emitting lasers (PCSELs). According to the Institute of Electrical and Electronics Engineers, brightness is an advantage that includes the degree of focusing or divergence of a beam of light.PCSELs, while seen as an attractive option for high-brightness lasers, have previously been unscalable for use in large-scale operations due to challenges with the size and brightness of the lasers.
Often, the problem with PCSELs stems from the desire to expand their emitting area, which means that there is room for the light to oscillate in the direction of emission and in the transverse direction. "These transverse oscillations are known as higher-order modes and can destroy the quality of the beam," the IEEE writes. "In addition, if the laser is operated continuously, the heat inside the laser can change the refractive index of the device, leading to further deterioration of the beam quality."
In the Nature study, the researchers used photonic crystals embedded in the laser and "adapted the internal reflector to allow single-mode oscillations over a wider area and to compensate for thermal damage." These changes allowed the laser to maintain high beam quality throughout continuous operation.
The researchers developed a 3-mm-diameter PCSEL in their study, a 10-fold jump from the previous 1-mm-diameter PCSEL device.
"For a photonic crystal surface-emitting laser with a large resonant diameter of 3 mm, [continuous-wave] output powers of more than 50 W, pure single-mode oscillations, and an extremely narrow beam divergence of 0.05°, corresponding to more than 10,000 wavelengths in the material, have been achieved," the researchers wrote in the study. The brightness ...... reaches 1 GW cm-2 sr-1, comparable to existing large lasers."
It is worth noting that by "large-volume lasers," the researchers mean the solid-state and excimer lasers currently used in semiconductor laser manufacturing.
As part of the process of establishing a 1,000-square-meter center of excellence for surface-emitting lasers for photonic crystals at Kyoto University, Noda and his team have also shifted from manufacturing photonic crystals using electron-beam lithography to fabricating them with nanoimprint lithography.
"E-beam lithography is precise, but usually too slow for large-scale manufacturing," says the IEEE. "Nanoimprint lithography basically embosses patterns onto semiconductors and is useful for creating very regular patterns quickly."
The next step, according to the study, is to continue to expand the diameter of the laser from 3 to 10 millimeters - a size that reportedly produces 1 kilowatt of output power.
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