UV lasers fuel precision micromachining

High power, short pulse width, and higher repetition rate yield higher speed and quality

RAJESH PATEL, JAMES BOVATSEK, and ASHWINI TAMHANKAR

Mobile devices such as smartphones and tablets are evolving at a rapid pace. As the devices are getting smaller, faster, lighter, and cheaper, they are becoming increasingly capable yet more complex to manufacture, requiring miniaturization and precision manufacturing of components. For key components such as semiconductor chips, microelectronics packages, touch-screen displays, and printed circuit boards (PCBs), the industry continues to face challenges to drive up manufacturing yield and throughput while lowering cost. As a result, laser processes have increasingly been applied to advance mobile-device manufacturing. As the increasingly complex devices require more and more sophisticated manufacturing processes, advances in laser sources are also needed.

Lasers with shorter wavelength, shorter pulse width, and low M2 (beam quality) enhance micromachining processes by creating a tightly focused spot and minimizing heat-affected zone (HAZ). High energy absorption, particularly at ultraviolet (UV) wavelengths and short pulse widths, vaporizes material quickly, reducing HAZ and charring. The small focused beam spot enables machining smaller features with higher precision. Higher power, higher pulse repetition frequency (PRF), pulse shaping, and pulse splitting capabilities all can contribute to higher micromachining throughput. And consistent, higher pulse-to-pulse stability ensures process repeatability and helps achieve higher process yield.

Traditional UV Q-switched, diode-pumped solid-state (DPSS) lasers have performed reasonably well in fulfilling sophisticated manufacturing requirements, but they have limitations in achieving higher speeds and maintaining higher micromachining quality. A common approach to increasing the processing speed is by increasing the laser's PRF while holding other process parameters fixed. However, for a typical Q-switched DPSS laser, this is not possible. For these lasers, average power and pulse energy decrease quite rapidly as PRF increases. Also, laser pulse width and pulse-to-pulse energy fluctuations tend to increase significantly at higher PRF.

Recognizing the need for new laser technology to overcome these limitations, Spectra-Physics developed Quasar, a UV hybrid fiber laser with a unique combination of high power and short pulse width at high PRF. Introduced in 2013 at a 40W power level (250kHz, 355nm wavelength), it has been scaled in 2014 to 60W (200–300kHz), increasing both its average power and pulse energy. At the same time, its minimum pulse width has been decreased from 5 to 2ns and its maximum PRF increased from 500kHz to 3.5MHz. These output characteristics give engineers access to new regimes of laser process parameter space.

In this article, results are presented from applying this combination of high UV power at high PRF, independently adjustable pulse width, and advanced pulse manipulation capabilities to micromachining of various microelectronic materials, including silicon (applications in chip manufacturing), alumina (application in microelectronics packaging manufacturing), glass (applications in touch-panel display manufacturing), and copper (applications in PCB and microelectronic packaging manufacturing).

Silicon dicing in semiconductor fabrication

Laser dicing of silicon wafers is an alternative to conventional dicing with a precision saw. As wafers have become thinner and lasers have become more powerful, advantages over saw-based dicing increase dramatically. Achieving higher dicing speed and good cut quality are very important to compete against conventional saw processes.

Scribe depth vs. speed for silicon, illustrating the process optimization benefit possible using TimeShift technology
FIGURE 1. Scribe depth vs. speed for silicon, illustrating the process optimization benefit possible using TimeShift technology.

We have demonstrated scribes at high scribing speeds with minimal thermal damage to the material on ~100μm-thick, polished, single-crystal silicon wafers using Quasar lasers. In FIGURE 1, the curve (a) for single 25ns pulses at 200kHz establishes the basic trend that as scribe speed increases, scribe depth decreases. By taking advantage of higher power at higher repetition rate and TimeShift technology, which allows a wide range of software-settable pulse energies and pulse widths, we observed an almost-3X increase in the speed over a single 25ns pulse scribing condition for a 50μm-deep scribe.

FIGURE 2 shows debris and HAZ for scribes carried out using the same energy in a single pulse and using TimeShift to create a burst of pulses at 500mm/s and 200kHz. Scribes using this technology resulted in high ablation quality with less debris on the top surface, despite scribing a 25-percent greater depth than that achieved using single pulses.

Views of a scribe created using single-pulse TimeShift technology
FIGURE 2. Views of a scribe created using single-pulse TimeShift technology. The technology yielded scribe depths of 20μm (a) and 25μm (b), respectively.

Scribing alumina ceramic

Alumina (Al2O3) ceramic is used widely for microelectronic packaging due to its high dielectric property coupled with high strength, corrosion resistance, stability, and relatively low cost. In a typical manufacturing scenario, a large-size alumina sheet having multiple modules has to be separated or singulated into individual modules at the end of the processing cycle. In a common technique for singulation known as "scribe and break," a deep scribe in substrate is created using a laser and the substrate is then separated by using mechanical force. A UV laser with high power can provide a clean, precise way of creating scribes at a high speed.

Scribe depth vs. fluence for alumina, illustrating the throughput benefit of TimeShift technology
FIGURE 3. Scribe depth vs. fluence for alumina, illustrating the throughput benefit of TimeShift technology.

Similar to silicon scribing, we have demonstrated that the Quasar laser can be used to create scribes in alumina at a higher speed with a minimal thermal effects using higher power and TimeShift technology. FIGURE 3 shows the clear advantage of using double-pulse burst micromachining over single-pulse machining. By splitting the energy available in a single 20ns pulse into two sub-pulses, an increase in ablation depth of up to 78 percent can be achieved. Also, FIGURE 4 shows that double-pulse burst mode creates same depth scribe using 40 percent less energy than the single pulse and has less loose debris on the top surface.

Comparison of alumina scribing quality using TimeShift technology. The top view (a) of the scribe used the single-pulse mode at 170μJ/pulse, while the same view (b) using the double-pulse mode enabled 101μJ/pulse
FIGURE 4. Comparison of alumina scribing quality using TimeShift technology. The top view (a) of the scribe used the single-pulse mode at 170μJ/pulse, while the same view (b) using the double-pulse mode enabled 101μJ/pulse. Scribe depth is 4μm in both cases.

Glass cutting in flat-panel display

In the display manufacturing process, touch-screen and LCD modules require both straight cuts for singulating pieces of glass and curved cuts for creating features such as corners, holes, and slots. As glass substrates used in consumer electronics displays continue to become thinner and stronger (through chemical or thermal treatment), laser glass machining tools are showing great potential for providing high-quality cuts and high throughput while reducing yield losses associated with the conventional mechanical scribe-and-cleave process.

We have developed glass processing techniques utilizing the laser-material interaction effects created by the TimeShift technology. In our patent-pending process, tailoring of the individual laser pulses reduces thermal loading and the chipping or cracking it can cause in the material. This has yielded good cut quality at linear cutting speeds of over 1.5m/s in chemically strengthened glass such as Corning Gorilla, Asahi Dragontail, and Scott Xensation. Similar results have also been obtained in soda lime glass, advanced flexible glass such as Corning Willow, and process development work for machining sapphire is underway. FIGURE 5 shows results obtained in 0.7mm Gorilla Glass having depth of the chemically strengthened layer (DOL) of 40μm. It shows clean-cut edges with minimal chipping, and no visible micro-cracks.

Examples of straight line, curvilinear, and hole cuts in 0.7mm Gorilla Glass with DOL of 40μm, all obtained utilizing the Quasar laser's TimeShift technology
FIGURE 5. Examples of straight line, curvilinear, and hole cuts in 0.7mm Gorilla Glass with DOL of 40μm, all obtained utilizing the Quasar laser's TimeShift technology.

Copper cutting in advanced packaging and interconnect

A typical flex circuit singulation application involves clean and fast through-cutting of thin (10–20μm) copper layers on a polymer substrate. Also, via drilling in many PCB constructions involves ablation of a copper (Cu) layer of similar thickness. We have investigated the potential effects of a more subtle aspect of TimeShift technology in these applications by studying copper scribing using sub-pulse (burst) processing to enhance the depth of grooves created in bulk Cu.

The effects of TimeShift features in copper scribing. Variation in material removal rates with varying sub-pulse time separation (a), and varying number of sub-pulses (b)
FIGURE 6. The effects of TimeShift features in copper scribing. Variation in material removal rates with varying sub-pulse time separation (a), and varying number of sub-pulses (b). Total energy of each burst of sub-pulses was fixed at either 20 or 45μJ.

FIGURE 6a shows that 10 sub-pulses separated by 10ns machined deeper grooves than single pulse (0ns separation case) of the same energy. Increasing the pulse separation to 25ns, however, resulted in lower material removal rates than the single-pulse case. Effects such as these can be easily isolated utilizing the flexibility of TimeShift technology. This can give the development engineer insight into the laser-material interaction mechanisms that may dominate the machining results, permitting more rapid and full process optimization for speed and/or quality.

FIGURE 6b shows that for 5ns sub-pulse duration, dividing the total energy in the pulse into a greater number of sub-pulses results in higher material removal rates. Similar to the results shown for silicon in FIGURE 1 and for alumina in FIGURE 3, multiple sub-pulses also tended to produce cleaner cut edges with less debris.

Summary

In manufacturing processes for mobile consumer electronics devices, lasers are routinely used to micro-machine a variety of materials. We find that using the unique combination of high UV power at higher PRF, along with TimeShift programmable pulse shape technology delivered by the Quasar laser, significant advances in micromachining can be achieved.

The processing benefits of the UV laser have been demonstrated in several common microelectronic materials used in mass production, including silicon, ceramics, glass, and copper. We have shown that operating in new regimes of process parameter space (higher power at higher PRF), and utilizing the advanced pulse splitting and shaping features, both process speed and micromachining quality can be simultaneously improved. The results indicate that process recipe development is straightforward using this laser. With proper parameter optimization, high quality and high throughput can be achieved with this new UV nanosecond pulsed laser source, fueling the capabilities of today's laser micromachining processes to meet the challenges of manufacturing tomorrow's consumer electronics products.

ACKNOWLEDGEMENT

Quasar is a registered trademark of Spectra-Physics.


RAJESH PATEL (raj.patel@spectra-physics.com) is Director, Strategic Marketing & Applications, JAMES BOVATSEK is Applications Lab Manager, and ASHWINI TAMHANKAR is Senior Applications Engineer, all at Spectra-Physics, Santa Clara, CA.

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