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For Glass and Silicon Wafer Cutting, Shorter Pulse Widths Yield Better Results

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For Glass and Silicon Wafer Cutting, Shorter Pulse Widths Yield Better Results For Glass and Silicon Wafer Cutting, Shorter Pulse Widths Yield Better Results BY DIRK MÜLLER, COHERENT INC. n manufacturing of semiconductor microelectronics, then explodes away because of Coulomb repulsion. When One limitation of ultrafast processing is that it pro- displays, medical devices and many other industries, using ultrafast pulses, the material is exposed to the laser vides lower material removal rates, and ultrafast lasers there is an increasing trend toward higher-precision energy for such a short time that the energy can’t be car- have been more costly than long pulse laser sources. As Iprocessing. This means cutting, drilling and marking a result, ultrafast processing is typically reserved for ap- ried beyond the area of impact, so the surrounding area parts with smaller feature sizes and greater accuracy, stays cold. Energy left over after the bond-breaking pro- plications that demand the greatest possible precision, superior edge quality and with a reduced effect on sur- cess is carried away with the expelled particles. Together, quality and smallest HAZ. rounding material. In the past, most precision laser- these effects result in an inherently colder process with based processing applications relied on nanosecond pulse significantly reduced HAZ. This is also a very clean pro- From sapphire cutting to LED dicing widths or ultraviolet output (or both). But traditional cess, leaving no recast material and thereby eliminating Because of their advantages, ultrafast processing la- sources cannot always service this new, demanding class the need for elaborate post-processing. sers have now been adopted in a number of demanding, of applications. As a result, some applications are now Another major advantage of ultrafast processing is that high-precision applications. These include LED dicing; turning to lasers with ultrafast — picosecond or femto- it is compatible with a very broad range of materials, in- sapphire cutting; drilling of automobile engine fuel second — regime pulse widths to accomplish these tasks. cluding several high-bandgap materials including glass, injection nozzles and engine cooling plates; hole drill- sapphire and certain fluorinated polymers that have low ing and structuring of biomedical filters; cutting and Ultrafast processing benefits linear optical absorption and are therefore difficult to drilling of FR-4 resin; cutting and drilling of both low- The goal of micromachining is the creation of micron- machine with existing commercially available lasers. temperature co-fired ceramics and high-temperature co- scale features, such as holes, grooves and marks, with More specifically, the technique is “wavelength neutral”; fired ceramics; and microprocessing of metals such as high-dimensional accuracy while avoiding peripheral that is, nonlinear absorption induces a light-matter inter- stainless steel and copper. More recently, picosecond la- thermal damage to surrounding material. In other words, action even if the material is normally transparent at the sers have also been used to mark integrated circuit pack- precise, clean cuts and marks with high surface quality laser wavelength. ages. The short pulses ameliorate potential damage to the and minimal heat-affected zone (HAZ). embedded die during marking. There are two basic mechanisms by which a laser Several Asian tool manufacturers have been using can precision drill, scribe, cut or mark a material. Many traditional applications rely on infrared and visible Q- switched lasers, which have pulse widths in the tens of nanoseconds range, and which remove material via a photothermal interaction. Here, the focused laser beam acts as a spatially confined, intense heat source. Targeted material is heated rapidly, eventually causing it to be va- porized — essentially boiled away. The advantage of this approach is that it enables rapid removal of relatively large amounts of target material. Furthermore, nanosecond laser technology is mature; these sources are highly reliable and have attractive cost Coherent of ownership characteristics. However, for the most de- Comparison of a 200-µm-diameter hole drilled in manding tasks, peripheral HAZ damage and/or the pres- stainless steel with a nanosecond laser (left) and ence of some recast material can present a limitation. a picosecond laser (right). The picosecond laser Suzhou Delphi Laser Co. This includes the delamination of surface coatings, mi- produced a cleaner hole with less recast material crocracking, or changes in the bulk material properties. and a smaller heat-affected zone (HAZ). One way of minimizing the size of the HAZ is to em- ploy a nanosecond laser having output in the UV, rather than in the visible or near infrared. UV light is strongly absorbed by most materials, limiting how far the laser light penetrates into the part and therefore reducing the HAZ. The second mechanism for laser material removal is based on photoablation, which involves directly break- ing the molecular or atomic bonds that hold the material together rather than simply heating it. This can be per- formed with ultrafast lasers because their short pulse- widths lead to very high peak powers (megawatts and above). These high peak fluences drive multiphoton ab- sorption, which strips electrons from the material and Suzhou Delphi Laser Co. Coherent Suzhou Delphi Laser Co. Cross sections of glass cut with a nanosecond laser Schematic illustrating the major differences between Results of stealth dicing a silicon carbide wafer with (top) and a picosecond laser (bottom). The picosec- ultrafast processing and processing with longer a picosecond laser, before mechanical separation ond laser produced a cut with fewer microcracks and pulse width lasers. of the individual dies. less residual debris. Reprinted from the January 2017 issue of Industrial PhotonicsTM Laurin Publishing Coherent RAPID series picosecond la- First, the picosecond laser produces a computers that incorporate touchscreens. microns deep. sers for several years now, specifically much smaller heat-affected zone than There are two important trends in touch- Traditional mechanical glass cut- for semiconductor wafer dicing and the nanosecond laser,” Zhao explained. screen display glass scribing. The first is ting can lead to microcracks and debris. glass cutting. Joshua Zhao, sales man- “This allows the cutting street to be re- a drive toward the use of thinner glass Laser glass cutting, based on CO2 and ager at Suzhou Delphi Laser Co. for the duced in size from 25 µm, down to 14 µm. substrates in order to minimize the total nanosecond solid-state lasers, has been Americas region, discussed how the la- Which, in turn, results in a higher yield weight of the display. The second is a in use for some time in the display indus- sers are employed and the benefits they — that is, one can pack more devices need to cut curved shapes in the glass, try. Both of these lasers produce dramat- have delivered. “Wafer dicing can ac- onto a given wafer. Also, we have fewer rather than simply straight lines, in order ically better results than mechanical cut- tually be accomplished in two different pieces that don’t separate properly than to allow rounded edges on the display, as ting, but can each have some limitations, ways. In the first, called laser grooving, before, generating less waste. As a result, well as to accommodate more complex especially for very thin glass (<300 µm the beam is focused onto the surface of we’re also able to run the process faster screen geometries. thickness). Zhao said, “[Picosecond laser the wafer in the street area (the empty than before. For example, the nano- As the display glass gets thinner, it cutting] yields a finished piece that has area between circuit components),” he second laser could process 15 wafers is critical that the finished product still greater edge strength, and is much more said. “The laser makes a scribe part way per hour, for a 10 mil × 23 mil chip size, retain the ability to withstand being resistant to breaking during use.” through the wafer, and the individual but the picosecond laser can process dropped, handled roughly and pressed A new generation of reliable, high- chips are subsequently singulated me- 23 wafers per hour. Plus, we can even upon (for touchscreens). A typical LCD power, industrial ultrafast lasers is en- chanically. The second method is called process thicker wafers; now we can dice touchscreen actually contains three or abling higher-precision microprocess- stealth dicing. Here the beam is focused 200-µm thick wafers, while our nanosec- four stacked layers of glass. The topmost ing in a variety of applications. Expect within the wafer itself, where it creates ond laser process couldn’t go above 100 µm (outer) sheet is often a 700-µm-thick pro- this technology to impact industries as a scribe within the material. Again, the thickness.” tective cover glass. To minimize the risk diverse as microelectronics fabrication, individual chips are then singulated me- of scratching and breakage, this outer medical device manufacturing and auto- chanically, typically by tape expansion. Picosecond laser glass cutting: layer of the topmost glass is chemically motive production. “Previously, we employed a 355-nm avoiding cracks and debris treated to produce a very tough surface nanosecond laser for laser grooving, but Another important picosecond pro- — Corning’s Gorilla Glass, Asahi’s Meet the author now we’ve switched to a 1064-nm pi- cessing application is glass cutting. This Dragontrail and Schott’s Xensation are Dirk Müller, Ph.D., is the director of Strate- cosecond laser and perform stealth dic- application is driven by the tremendous examples of this. The thickness of this gic Marketing at Coherent, Inc.; email: Dirk. ing. This has delivered several benefits. market growth for cellphones and tablet strengthened layer typically runs tens of [email protected]..
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