FOCUS: LASER MICRO PROCESSING
Laser Micro Processing of Semiconductors and Dielectrics How Lasers Present Solutions to Cutting, Drilling and Scribing of Semiconductors and Dielectrics. In 2007 the semiconductor manufacturing industry is expected to make more than $250 billion gross revenue and invest over $40 billion in new equipment. The demand for cheaper and increasingly powerful end products, like PCs, Laptops and MP3 Players is the driving force for further development of production equipment. Barrier breaking picosecond lasers for direct ablation of semiconductors are paving the way for the next generation of such machinery. The process flow in semiconductor manufacturing is commonly divided into the front-end and the back-end section. The front-end production line contains all processes for forming integrated circuits (ICs) directly on silicon wafers. Typical steps are depositioning, patterning, removal, doping and activation plus several inspection steps. Here, lasers are used for selectively patterning of a thin film by mask projection instead of direct laser ablation. An example of the re-
sult of these front-end processes is shown in Figure 1: A silicon wafer with several square or rectangular shaped integrated circuits, which are called “dies”. Innovation in the front-end process chain is driven by Moore’s Law, which predicts that the number of transistors on a chip roughly doubles every two years. To keep pace with scaling and IC density changes on the front end, technology advances lead to new trends in the back-end section. The back-end of the production line typically starts with the grinding process on the back side of the wafer. During the grinding process the thickness of the silicon wafer is reduced from about 600–700 µm to 100 µm or even thinner. Whereas a thicker silicon wafer is an advantage for the wafer stability in the front-end, it becomes a drawback for the upcoming packaging procedure. Packaging means separating and mounting the individual dies (chips), connecting the die to the pins or bumps of the package (i.e. by wire bonding) and encapsulating the
FIGURE 1: Silicon wafer with a matrix of quadratic (or rectangular) shaped integrated circuits using high power ps lasers for the separation process, enables for a high yield even for ultra thin wafers.
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THE AUTHOR SASCHA WEILER Dr. Sascha Weiler is working for TRUMPF Laser located in Schramberg (Germany) as Product Manager for the TruMicro Series 5000. He is also responsible for the business fields Semicon and Flat Panel Display.
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Dr. Sascha Weiler Product Management Business Fields Semicon and Flat Panel Display TRUMPF Laser GmbH + Co. KG Aichhalder Straße 39 78713 Schramberg, Germany Tel.: +49 (0) 74 22 / 515 - 8169 Fax: +49 (0) 74 22 / 515 - 175 E-mail:
[email protected] Website: www.trumpf-laser.com
FIGURE 2: Edge of a silicon wafer with a thickness of 300 µm cut with a high average power picosecond laser of the TruMicro Series 5000. No chipping and heat affected zone can be detected.
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
FOCUS: LASER MICRO PROCESSING
bonded die. These steps contain several driving forces to reduce the thickness of the silicon. Smartcards for example are thin pocket sized cards containing a chip with integrated circuits. These smartcards require thin IC chips and so do SD memory cards.
Wafer Dicing Separation of the individual dies can be done by blade saws, i.e. diamond blade saw. Here the silicon wafer is typically mounted on a Dye Attached Tape (DAT), a sticky tape in a metal frame that fixes the wafer. The separated dies (or dice, where the name comes from) can then be picked and placed i.e. into lead-frame packages. However, thin silicon wafers become a challenge for blade saw dicing machines. Due to the mechanical contact, wafer saws have to operate very carefully to avoid breaking the die or producing chips along the cutting edge. Using lasers as a non-contact tool for the die-singulation results in faster dicing speeds compared with the blade saw. A further advantage of using ultrashort pulsed lasers is the high cutting quality and the negligible heat affected zone. This leads to cutting edges with high tensile stress which is essential for the die to sustain mechanical load during the next processing steps. As laser beams can be focused to small spot sizes, the cutting kerfs can be reduced to less than 20 µm compared to a typical
value of 120 µm for the blade saw. Using this aspect, the cutting streets can be narrower which reduces the “dead space” on the wafer and increases the number of dies per wafer. A third big point for the laser results from a trend toward new dielectric materials that are used as insulators inside a chip, separating the conducting interconnections from each other. As structure sizes get smaller, and integrated devices such as transistors get closer, the insulating dielectrics need to get thinner. These thin inter-level dielectrics cause capacitive losses proportional to their dielectric constant k. Silicon Dioxide is the material that is commonly used, and has a comparably high dielectric constant of k=4. Using materials with the same thickness but a lower dielectric constant (“low-k materials”), results in a higher chip performance with less power consumption compared to silicon oxide. The low-k materials are typically more brittle and have a lower adhesion than Silicon Dioxide. For the wafer dicing process, this increases the risk of chipping and delamination of the low-k layers when using a blade saw. Using ultrashort pulsed lasers enables high speed and high quality scribing of these low-k layers without the drawbacks of a mechanical treatment. This low-k scribing can either be combined with a subsequent full laser wafer cut or with dicing by blade saw. All the above described points show that
the technological trends in chip manufacturing make ultrafast lasers with high average power and pulse energy an advantageous tool for wafer dicing. The quality that can be achieved with such a laser is shown in Figure 2, where the cutting edge of a silicon wafer with a thickness of 300 µm is shown. Even under SEM, no chipping or heat affected zone can be detected. This supreme cutting quality enables for a higher yield compared to the blade saw which is more than compensating the investment of the laser device.
Package Dicing After the described singulation, the die pads are connected to the pins on the package by wire bonding. The following encapsulation with mold compound seals the die. The result is a matrix leadframe with packaged chips, which have to be singulated either by blade saw or laser. Figure 3 shows a matrix leadframe with packaged Field Programmable Gate Arrays (FPGAs). The singulation was done with a high power picosecond laser of the TruMicro Series 5000. Here several materials, like FR4, copper interconnections and mold compound (MC) have to be cut. The advantage of picosecond lasers over dicing saws or longer pulsed lasers is the low heat affected zone combined with an excellent edge quality. The entire material mix can be cut in one process with superior edge quality.
THE COMPANY Trumpf Laser Schramberg, Germany TRUMPF Laser is a manufacturer of solid-state lasers for processing materials and is a market leader in this sector. At the Schramberg facility, about 600 highly qualified employees work in development, production, sales and service. They ensure highest product reliability and quality. Great emphasis is also placed on competent customer consulting and dependable field service. The solid-state lasers from TRUMPF are used primarily for welding, cutting, marking and micro-machining. In electrical and precision engineering and in medical technology, these lasers are tools of first choice just as they are in the automotive industry and its suppliers as well as in the aerospace industry.
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
FIGURE 3: Laser cut matrix of packaged FPGA chips
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FOCUS: LASER MICRO PROCESSING 3D Packaging – Through Silicon Vias Latest trends in packaging try to accommodate Moores Law by going into the z direction. This expansion means stacking two or more dies to reach higher performance per chip area. Established 3D packaging technology is using wire bonding to connect the stacked dies to the package. The only area to place connections on such a batch is on the peripherals of each die, which is a fundamental limitation for the area of the top dies. Crosstalk and speed losses due to the fact that wires used for bonding can act as antennas are further limitations of this technology. A way to overcome these obstacles is by making short, direct connections through the silicon substrate of each die. For this purpose blind holes, so called Through Silicon Vias (TSVs), are drilled into the silicon either prior to the front-end processes (via first) or after the front-end processes (via last). For via first, these blind holes are coated with an isolating layer and filled with conducting material. After the front-end steps are complete, the silicon wafer is thinned (ground) which opens the blind holes and forms the connections to the backside. Today, the most common process for generating TSVs is Deep Reactive Ion Etching (DRIE), which allows for high aspect ratio blind holes with low surface roughness. However, the DRIE process requires a vacuum environment and the use of expensive masks. Due to parallel processing of all TSVs on a wafer, the processing costs when using DRIE are always constant. Only for a very large number of vias per wafer DRIE does become economical. The advantage of using lasers for TSV drilling is the lower purchasing cost, because
FIGURE 4: Matrix of vias drilled in green ceramic tape (picture by courtesy of KMS Kemmer Technology Center).
neither vacuum nor lithography or masks are required. The lower running costs due to fewer consumables is another advantage of laser drilling. Furthermore, the laser process is very flexible when changes in TSV layout are required. The use of high average power and high pulse energy picosecond lasers enables for TSVs without heat affected zone. For these lasers, only a combination of sufficiently high pulse energy combined with high pulse repetition rate in the order of 200 to 500 kHz allows for a high throughput.
LTCC – Drilling of Green Ceramic Tapes Ceramic circuit boards are commonly used in harsh ambient conditions i.e. for high temperature cycles of the environment. Low Temperature Co-fired Ceramics (LTCC) is a technology to form multilayer ceramic circuit boards. Here, unfired ceramic tapes are structured, laminated and stacked. The following sintering process generates a ceramic multilayer circuit board out of the batch of ceramic tapes.
The connections between the layers are done by drilling vias into each individual green ceramic tape, which are then filled with a conductive paste. Via formation is commonly done either by mechanical drilling or by using a low power carbon (di-) oxide laser. However both methods exhibit limited via quality, minimum via diameter and throughput. The task of drilling vias with high edge quality, variable diameter and high throughput can be fully completed by using high average power picosecond lasers. Figure 4 shows a matrix of vias that have been drilled with the TruMicro 5050. The vias show a perfect roundness and no chipping or heat affection of the surrounding material. Here, the drilling rate allows for the generation of more than 1000 of these vias per second. Hence, using picosecond lasers with high average power and pulse energy enhances quality and throughput for via drilling of green ceramic tapes.
Conclusion Picosecond lasers with high average power and high pulse energy enable for micro machining of semiconductors and dielectrics resulting with a combination of high quality, yield and throughput. Hence, these lasers are necessary tools for the next generation of production equipment for the semiconductor industry.
Acknowledgement The author would like to thank Pat Grace for his help in the composition of this article.
BACKGROUND – MATERIAL PROCESSING WITH ULTRASHORT PULSES When is short not short enough? If, for example, no mechanical and thermal modification of material can be tolerated through laser processing, the interaction time between laser pulse and material has to be reduced. To this end, it is necessary to shorten the pulse duration to typically less than 10 ps (1 ps = 10–12 s). Material processing using picosecond pulses, compared to nanosecond or microsecond, is distinctive in that it provides smaller molten material volumes and a higher vapor pressure. Material removal is considered to be pure sublimation.
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Picosecond pulses are ideal for processing of materials where thermal influence must be minimized. Furthermore, for ps pulse durations, the technical approach to generating the pulses can be greatly simplified. Direct diode pumping and amplification (power scaling) without “Chirped Pulse Amplification“ (CPA) are necessary requirements for the success of ultra-short pulse technology in the industrial market. Added to this is the fact that for a cost-effective application in industrial micro processing, scaling the average output to values of 50 W and greater is necessary.
The diode-pumped disk lasers from the TruMicro 5000 series guarantee an average output power of 50 W with infrared and 25 W for green wavelength. With a pulse repetition rate this corresponds to pulse energies of 250 µJ (infrared) and 125 µJ (green). The laser exit beam is diffraction-limited and has a roundness of over 95 percent. Precise switching or attenuation of picosecond pulses is done by means of an external modulator. This series is used in applications where no burr or deposits are allowed to develop and no post processing is allowed.
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
