Looking at the Role of Lasers in Semiconductor Manufacturing

Semiconductors are an integral part of the inner workings of medical devices, assisting in the conductivity between a non-conductor and a conductor to control electricity flow. In turn, the assembly process to create the perfect semiconductor is incredibly detailed, especially now that devices are getting smaller and smaller. With the rapid miniaturization of semiconductors to fit into these smaller devices, the role of lasers in semiconductor manufacturing has adapted.

Laser manufacturing uses

Laser technology is frequently used in semiconductor manufacturing for multiple reasons including cutting, welding, removing a coating, and marking because of its thin, precise, versatile, and powerful beam.

Cutting/Scribing

In the production of semiconductors, there are various cutting steps, including cutting wafers out of crystal blocks and stencils out of films. The use of lasers for cutting ensures chips are cut cleanly so they fit correctly into the final device. Semiconductors can be cut into many shapes and patterns using lasers that are unattainable using other cutting methods. Cutting wafers using this method reduces tool wear, loss of material, and, instead, returns a higher yield, according to the Fu Foundation School of Engineering and Applied Science at Columbia University.

“The advantages of laser cutting include lack of tool wear, reduced loss of material around the cut, higher yield because of reduced breakage, and rapid turnaround because of the ease of fixturing,” according to Columbia learning material on the laser machining of semiconductors.

An alternative to cutting is scribing — a series of closely spaced or overlapping blind holes drilled partway through the material. This is a widely used method for applications in semiconductor manufacturing like dicing alumina substrates into chip carriers or separating silicon wafers into chips. Of note, the kind of laser needed for scribing is dependent on what material is being used.

“The scribing of alumina uses carbon dioxide lasers and the scribing of silicon is done with Nd:YAG lasers, because the absorption of the different materials is high at the different wavelengths,” according to the university.

The motivation to use scribing versus cutting comes down to how fast the actions take place on the manufacturing floor. “For alumina around 0.025 in. thick, the material may be scribed at rates around 10 in./sec with an intermediate-power CO2 laser, whereas cutting rates may be a few tenths of an inch per second for a similar laser,” University staff wrote. “Scribing also offers the advantage that a substrate may be scribed before processing is completed and then easily separated into chips after processing.”

Welding

Laser welding or laser diode welding is a process of melting adjoined sections of semiconductor components together, like attaching a wafer to a supporting plate. For a supporting plate, like a lead frame, to be ready for adhesion, a laser marks the frame with an identifying mark, then roughens the surface to ensure the two parts are securely together. Once together, the laser marker removes the burrs created during the roughening process.

Coating removal

Making sure a semiconductor is clean and clear of defects is part of the manufacturing processes called coating removal. Using a laser, typically Nd:YAG, unwanted coatings can be deburred like resin or copper, and removed like gold plating or thin film coatings. For deburring, the laser uses its thin and precise beam to remove excess material without causing damage to the product. Defects can be analyzed more clearly when a coating is removed, taking away the need to disassemble for inspection, which can cause product damage.

Marking

Laser-marked semiconductors are important for product traceability and readability, meaning the laser must be legible in very small print. A products traceability means that it can be tracked through the many steps of production and, finally, through distribution. This makes finding specific classes of defects much easier to find and separate.

The marked chips must also be readable as the markings are a useful way to tell which product is suitable for an application. “Lasers not only cut into the surface of a wafer, but they can also re-arrange the surface particles, creating marks that are minutely shallow yet easily readable,” according to Wafer World.

There are two types of marks used on semiconductors, etched markings and annealed markings. Etched markings are when a laser is used to remove thin layers of material, leaving behind a textured mark of about 12 to 25 microns deep. These are usually called “hard marks” because there is a visible change in the surface layers

Annealed markings, on the other hand, use a laser set to a lower power level so that the molecules are re-arranged instead of etched. This creates contrast on the chip surface that is visible when it reflects light.

Laser types

Currently, companies mostly use solid-state lasers for chip manufacturing as its known for high power and uses ores for the laser medium. The ore medium is commonly made up of yttrium, aluminum, garnet, or yttrium vanadate crystal. Nd:YAG lasers, for example, use neodymium-doped yttrium-aluminum garnet crystals as its medium. The laser beam is created using an oscillator that irritates the medium with light from a laser diode.

One solid-state laser used for chip marking, engraving, and cutting is a fiber laser. The high-speed laser uses “fiber as a resonator and creates an overlapping structure with fiber cladding dopped with Yb ions,” according to Keyence, noting its fiber laser is call the MD-F Series 3-Axis Fiber Laser. “Some uses for fiber lasers include removing burr in the preproduction process, marking traceability codes, and removing resin to analyze defects.”

Excimer lasers are also used for semiconductor manufacturing. These are deep ultraviolet (UV) lasers with a 126 nm to 351 nm wavelength and are mostly used for polymer micro-processing. Compared to solid-state, UV laser beams are much shorter, making them suitable for any type of material, including very fragile and delicate, and can operate in very small precise areas with reduced action spots. When used for marking, UV lasers modify the product material’s structure at the molecular level without producing heat in the surrounding areas.

Laser innovation

Currently, solid-state and excimer lasers are seen as the main choices when using laser manufacturing for semiconductor production. However, a new option might soon be available that rivals the classics. A recent Nature study by a group of researchers at Kyoto University, led by Susumu Noda, wrote that they have taken steps to overcome limitations of semiconductor laser brightness by changing the structure of photonic-crystal surface-emitting lasers (PCSELs). Brightness is a merit that encompasses how well a beam of light can be focused or how little it diverges, according to the Institute of Electrical and Electronics Engineers. PCSELs, while seen as attractive options for high-brightness lasers, previously had a history of not being scalable for large operations due to challenges the lasers size and brightness.

Usually, the problem with PCSELs stem from wanting to expand their emission area, meaning there is room for light to oscillate in the direction of emission as well as laterally. “These lateral oscillations, called higher-order modes, can ruin the beam’s quality,” IEEE wrote. “Furthermore, if the laser is operating continuously, heat inside the laser changes the device’s refractive index, causing the beam’s quality to deteriorate even further.”

In the Nature study, the researchers used photonic crystals embedded in the laser, along with “adjustments to an internal reflector, to allow single-mode oscillation over a wider area and to compensate for thermal disruption.” These changes allowed the laser to maintain its high beam quality whole operating continuously.

Researchers were able to develop a PCSEL in the study with a diameter of 3 mm, a 10x jump from previous 1 mm-diameter PCSEL devices.

“A [continuous-wave] output power exceeding 50 W with purely single-mode oscillation and an exceptionally narrow beam divergence of 0.05° has been achieved for photonic-crystal surface-emitting lasers with a large resonant diameter of 3 mm, corresponding to over 10,000 wavelengths in the material,” researchers wrote in the study. The brightness… reaches 1 GW cm−2 sr−1, which rivals those of existing bulky lasers.”

Of note, by “bulky lasers”, the researchers are referring to the solid-state and excimer lasers currently used in for semiconductor laser manufacturing.

As part of the process to establish a 1,000-square-metter Center of Excellence for Photonic-Crystal Surface-Emitting Lasers at Kyoto University, Noda and his group of researchers also made the switch from manufacturing the photonic crystal using electron-beam lithography to making it with nano-imprint lithography.

“Electron-beam lithography is precise, but usually too slow for mass manufacturing,” according to IEEE. “Nano-imprint lithography, which basically stamps a pattern into a semiconductor, is useful in creating very regular patterns quickly.”

The next step, according to the study, is to continue scaling the diameter of the laser from 3 mm to 10 mm — a size that reportedly could produce 1 kilowatt of output power.