Precision Glass Processing with Pico-second Laser Pulses. Chemically Strengthened Glasses. Structure

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1 Precision Glass Processing with Pico-second Laser Pulses Mathew Rekow, Yun Zhou and Nicolas Falletto Mechanical scribe and break, grinding and sawing have been mainstays of glass processing for centuries. Even with the microelectronics revolution, these age old techniques have continued to be the dominate glass processing techniques. When high power lasers came on the scene in 1960s (1), glass cutting quickly became a target application. Many innovative techniques were developed but even so the economics and complexity of a laser based system compared to a saw and a blade prevented universal adoption in industrial processes. In the 2000 s the introduction of chemically strengthened glasses that defy traditional cutting techniques has tilted the playing field in favor of the laser. The result has been an explosion of new lasers and laser techniques for processing glass. Some of these techniques such as Diamond Blaze by ESI produce very fast linear cuts with excellent quality. However in today s microelectronics markets curved corners and internal curved features for home buttons, speaker holes and so on require a different technique. For example at Eolite we have developed a proprietary process that produces both internal and external curved and linear cuts with very high yield in highly chemically strengthened glasses. The force of air jet or even just gravity is enough to remove the cut parts and the cut quality is comparable a ground edge reducing or eliminating the need for post processing to remove damage. Chemically Strengthened Glasses Structure Chemically strengthened glass has its origins in the 1960 s with the development of a glass by Corning called Chemcor (2) that found use in the automotive, aerospace and pharmaceutical industries but apparently never became a mainstay in the industry. However in the early 2000 s the technology was revived and improved under the trade name Gorilla Glass and rapidly became the dominate coverglass for smart phones and tablet computers. By the end of May 2013 it is estimated that over 1.5 billion devices had been sold that utilized Gorilla Glass (3). The fundamental process of creating Chemcor is still the fundamental process used to make Gorilla Glass. A sodium glass is immersed in an alkaline bath that induces the sodium ions to diffuse out from the glass and larger potassium ions from the bath diffuse in (4). Because the potassium ions are larger than the sodium ions the surface of the glass is left with a large residual compressive stress as shown in Figure 1. This compressive stress in turn prevents cracks from opening thereby limiting the stress at the tip of any cracks that are introduced, reducing their ability to propagate and induce failure in the component. The result is a glass many times more damage resistant than unstrengthened glasses and that can stand up to the daily stress and strain a typically smart phone will encounter.

2 Figure 1: Cross section of glass sheet illustrating the structure of the resulting stress zones after chemical strengthening. This compressive surface stress has obvious benefits for the mechanical performance of the glass but this same enhanced toughness comes with a down side, traditional scribing and sawing processes become more problematic. For the highest strength glasses, cutting by traditional means is literally impossible. The very same feature that makes the glass so strong makes it very difficult to cut mechanically. First of all a scratch has to be very deep and the applied force must be very high to break a material. At the same time once the scratch is deep enough to reach the tensile zone in the middle, crack tips are under tensile stress, causing rapid, spontaneous and chaotic propagation of cracks in the tensile center layer. The resulting uncontrolled and irregular crack propagation through the center layer often results in the destruction of the entire part. A new cutting methodology is needed. Laser Technology Development One of the first patents specific to laser processing of glass was issued in 1969 and several others follow shortly thereafter (1) (5) (6). Since then, these laser techniques for glass cutting have become common place. However, for large industrial operations sawing, scribing and breaking, and abrasive cutting are the dominant techniques (7). For the first generation of chemically strengthened glass these traditional techniques were sufficient, but with the increasing stress levels of the newest generations of glasses they have become unviable. A new generation of laser processes such as the Diamond Blaze process from ESI (8) have come to the forefront to make high quality and fast straight cuts. In these processes a controlled amount of damage is introduced in the tensile layer and/or the compressive layers. The very high internal stress causes cracks to propagate along the laser processed line and the components spontaneously self-separate along the laser treatment line. Generally however these techniques are limited to straight cuts. Other techniques have been developed that can, to some degree, achieve curved cuts but since there is effectively zero kerf width, separation of the components can be a problem because the cut parts are effectively locked inside the hole they were cut from. The laser glass cutting technique we have developed at Eoliteutilizes a picosecond pulse laser to produce microscopic chips in the glass, producing more and more chips until the material is effectively cut through. While this results in relatively modest cutting speeds, the process is universal for all types and thicknesses of chemically strengthened glass and is inherently scalable with laser power. Furthermore kerf walls are nearly vertical, components singulate with negligible applied force, and the surface finish and chip size are comparable to fine grinding.

3 Process Setup and Equipment, A 30 Watt, 515 nm picosecond fiber laser, the Hegoa TM, was utilized to develop this process. This laser produces between 15 and 50 ps pulses at repetition rates up to 3 MHz. The collimated laser beam was expanded to about a 12 mm beam diameter to yield about an 8 um diameter spot at the focus of a 100 mm focal length F-theta telecentric scan lens. The scanner was then used to move the focus spot as required to produce the various cut geometries reported. The scanner was mounted on an automated z- axis to control the depth of the focus spot on the work piece. For this work the maximum pulse energy used was 20 µj. Glass Cutting Process The 515 nm laser beam is focused to about an 8 um spot size and a galvanometer scanner is used to move the laser spot on the work surface at speeds up to 10 meters per second. The combination of a small focus and a very short picosecond pulse duration results in a strong non-linear interaction at the focal point. This in turn allows the unfocused beam to freely pass through the glass without deleterious heating and thermal lens effects, enabling the focal interaction spot to be placed at the far (usually bottom or back) surface of the material. Every laser pulse then removes a tiny chip of glass with geometry similar to the spot size (Figure 2-Left)Figure 2. The a-thermal nature of the picosecond interaction insures that most of the excess heat energy is carried away with the glass chip and residual heating of the substrate is minimal. As the laser beam is then translated along the scan direction a shallow groove is cut in the surface of the glass. Figure 2: Left- Schematic of glass chip removal by individual laser pulses. Right - schematic of kerf development by placing individual laser paths adjacent to one another. The focal spot is then repeatedly scanned along a desired path with offsets added at each repetition of the path as needed to generate a kerf of desired width (Figure 2 Right). Continued repetition of these paths, with occasional adjustments in the vertical axis to keep the laser interaction zone at the evolving cut surface, rapidly cuts through the entire part. The process can be terminated before penetration of the strengthened layer to create a surface trench as shown in Figure 3. Provided the compressively stressed layer is not breached, such trenches are stable and will not cause failure of the part.

4 Figure 3: Left - SEM of developing kerf In the Hegoa glass cutting process. Right High magnification SEM plainly showing the adjacent passes that make up the developing kerf. In this picture the total kerf width is about 200 micron across consisting of 15 adjacent passes. Process Physics At first glance it is not clear how this method of cutting avoids chaotic and spontaneous crack propagation as the laser cut approaches the tensile inner layer. However we have utilized this methodology to make hundreds of cuts in the highest center tension (C.T.) and thinnest (400um) strengthened glasses in the marketplace with yields > 99%. We believe that the key feature comes from the basic principles of fracture mechanics. First of all, fracture mechanics indicates that there is a critical crack length above which a crack propagates uncontrollably with very little energy input. For glass with a C.T. of 91 MPa, Griffith s criterion can be used to calculate that the critical crack length is about 5 um (Equation 1). Comparing this 5 micron critical crack length to the processing spot size of about 8 um gives insight on how this process avoids catastrophic spontaneous crack propagation, namely no cracks are introduced that are Equation 1: Griffith s criterion for the critical crack length in glass where E is Young s modulus, is the surface energy density for glass and is the applied stress. a f 2 E 2 f chaotic fracture of the material. larger than the critical crack length. Furthermore we have observed that arresting the cutting process within the center layer (the one under tensile stress) results in fracture after some length of time, indicating the machining process proceeds faster than the rate of spontaneous crack growth in that layer. This simple combination of small chip size with reasonably rapid cutting rate appears to explain how this process avoids

5 Process Results This bottom up cutting process creates clean cuts with nearly vertical side wall and maximum top and bottom edge chip sizes approximately equal to the processing spot size of 8 um. We have found that the same basic process parameters are sufficient for cutting glasses from 0.4 to 2 mm in thickness and from zero up to 91 MPa C.T. with process yields greater than 99%. Figure 4 shows the cut edge both top and bottom for an internal cut feature in 700 micron thick C.T. = 40 MPa glass. The maximum observed chip size is approximately 10 um for both the top and the bottom of the cut. The dark edge is shadow and is not indicative of any actual discoloration of the glass. The taper was measured to be about a 5 degree. Figure 5 shows the Hegoa process applied create 10 holes in a 40x60 mm sheet of 400 micron thick C.T.91 glass. The yield over a sample of 20 sheets each with 10 square holes was 100% (200 total internal cuts). Figure 5-Right shows the cut process result for square and round hole 10 mm in diameter, a round hole 2 mm in diameter, and external rounded corner cuts. 2 mm radius 2 mm radius Bottom surface Top surface Figure 4: Optical Microscopy Indicating the Small Chip Size on Both the Top and the Bottom Surface of the Cut. Note that the dark edge is shadow, not discoloration of the glass. Figure 5: Left - Ten square button holes cut into a 40 x 60 mm, 400 um thick piece of CT91 demonstrating the high yield and repeatability of the process. Right Selection of both internal and external cuts.

6 Process Speed In order to compensate for the relatively small amount of material removed per laser shot, the scanner is operated at its maximum velocity and the laser at high pulse repetition frequency (PRF). Even so the velocity of the galvanometer scanner is the limiting factor in determining process speed for the Hegoa G30 laser (515nm and 30 Watts maximum). Figure 6 shows the result of a scalability experiment that demonstrates that the cutting process in 1.0 mm thick glass material is linearly proportional to the laser power. Proportionally higher speeds are achieved on thinner glasses. In processing the various shapes of Figure 5, smaller corner radii limited the scan speed to 5 m/s hence the real process speeds for this work were about 2.5 mm/s second utilizing only about 14 Watt of the available laser power. Figure 6: Scalability of the Hegoa glass cutting process with average laser power for 1.0 mm thick glass substrates Conclusion We have demonstrated a simple cutting operation with a ps fiber laser that is capable of making both internal and external cuts in chemically strengthened glasses such as Gorilla glass at the highest center tension values and thinnest form factors available in the market place. Furthermore this process has been shown to be nearly universal across all tested glass types, requiring only a change in the total number of passes depending only upon the glass thickness. Process speeds were demonstrated up to 4.0 mm per second for 1.0 mm thick material with linear scalability with glass thickness. With process optimization cutting speeds from 6 to 8 mm/s are achievable for 0.4 mm thick C.T. = 91 glasses. The Hegoa glass process can be paired with high speed linear cutting processes to enable both rapid singulation of large glass panels and efficient production of internal and external curved features.

7 Mathew Rekow is Applications Manager Yun Zhouis an applications engineer and Nicolas Falletto is Product Manager all at Eolite Lasers, a subsidiary of ESI, Inc. location? References 1. Hafner, W. Method of Working Glass with Absorbent by a Laser Beam. 3,453,097 USA, July 1, Pogue, David. Gorilla Glass, the Smartphone's Unsung Hero. December Corning Incorporate. Corning Inc. News Releases. [Online] October [Cited: January 16, 2014.] 4. Corning Incorporated. How It's Made. corninggorillaglass.com. [Online] [Cited: Jan 16, 2014.] 5. Grove, Francis John. Cutting of Glass with a Laser Beam. 3,543,979 USA, December 1, Verheyen, Willy. Glass Cutting. 3,932,726 USA, January 13, Gaebler, Frank. Laser glass cutting in flat panel display production. s.l. : PenWell Corporation, ESI Inc. ESI Announces the DiamondBlaze Product Line for Strengthened Glass Singulation. [Online] January 11,