Advanced UV Lasers Enable Precision Processing

Transcription

1 Advanced UV Lasers Enable Precision Processing UV Lasers exhibit high spatial resolution and minimal thermal damage to surrounding material Lasers with ultraviolet (UV) output offer unique benefits for processing a wide range of materials. In particular, the short wavelength, high energy UV photons experience less diffraction than longer visible or infrared wavelengths, which enables materials processing at higher spatial resolution. In addition, UV lasers micromachine primarily by non-thermal means, thus minimizing the heat affected zone due to processing. As a result, UV lasers are now employed for many demanding, high precision tasks in semiconductor fabrication, display manufacture, medical device production, therapeutic medical procedures and scientific research. This article explores some of these applications as well as recent advances in the laser technology that has been developed to service them. Stereolithography Stereolithography is a technique for rapidly creating complex, three dimensional parts out of a photopolymer resin directly from a CAD file, without the use of conventional machining. In stereolithography, a complex shape is built up as a series of thin layers. Specifically, a UV laser is scanned over a bath of the photopolymer, tracing out a cross section of the desired shape. The laser radiation causes the photopolymer to solidify and adhere to the layer below it. After a layer is traced, the part is lowered slightly in the photopolymer bath, and the laser traces the next layer. Finally, the completed part is cleaned in a chemical bath and cured in a UV oven. Originally, stereolithography was used for rapid production of prototype parts, mostly to check the mechanical fit of complex assemblies. However, over the past few years, advances in resin materials, the development of dual-spot writing technology, and the availability of more efficient and cost effective lasers have improved the eco- THE AUTHORS TORSTEN RAUCH Dr. Torsten Rauch studied physics and graduated in the field of nonlinear optics. After some years in sales for Coherent he joined Coherent Luebeck as product line manager. Currently he is serving Coherent as a staff product line manager for commercial UV lasers and as head of service, field integration and applications lab. Dr. Torsten Rauch Seelandstrasse Luebeck, Germany Torsten.Rauch@coherent.com VOLKER PFEUFER Dr. Volker Pfeufer studied physics and graduated in the field of laser physics. After several years of research work in Germany and USA her entered industries. He was active as head of application, project manager and marketing director he joined Coherent GmbH Luebeck, Germany as Product Line Manager for diode-pumped solid state lasers. Dr. Volker Pfeufer Seelandstrasse Luebeck, Germany Volker.Pfeufer@coherent.com RALPH DELMDAHL Dr. Ralph Delmdahl is Product Marketing Manager at in Göttingen, Germany. His current responsibilities include providing strategic directions for the company s excimer business unit. He holds a PhD in Laser Physics from Braunschweig Technical University and Master degrees in Economics and Business Administration from the Open University in Hagen. Dr. Ralph Delmdahl Product Marketing Manager Hans-Böckler-Str Göttingen, Germany Ralph.Delmdahl@coherent.com MARK MONDRY Mark Mondry is the Product Line Manager for the AVIA product line. His six years of experience at Coherent includes roles in both product management and engineering. Previously, Mr. Mondry has held product marketing and engineering positions at Philips Lumileds, JDS Uniphase and SDL. He holds a Masters degree in Electrical Engineering from the University of California at Santa Barbara. Mark Mondry Coherent, Inc Patrick Henry Drive Mark.mondry@coherent.com Website: 20 LTJ May 2009 No WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 Modulator Paladin Laser Scan Optics Direction of Motion FIGURE 1: The Coherent Matrix UV series are short pulsed, diode-pumped, solidstate lasers specifically designed and constructed to deliver superior beam characteristics and pulsing control, together with exceptional reliability. Panel FIGURE 2: In LDI, a UV laser writes the desired circuit pattern directly on to the board, eliminating the traditional photo tool and contact printing. THE COMPANY Coherent Inc. Coherent designs and manufactures a broad selection of lasers and supplies electro-optic instruments for laser test and measurement. The company s products include laser diodes and laser diode systems, carbon dioxide (CO 2 ) lasers, excimer lasers, ion lasers, CW and Q- switched DPSS lasers and systems, ultrafast lasers and amplifiers. The company provides worldwide service and applications support. Contact: Petra Wallenta, PR Manager Europe Phone: +49 (0)89/ Petra.Wallenta@coherent.com Website: nomics and mechanical properties of parts manufactured with stereolithography. As a result, this process is being increasingly used for small batch production of fully functional parts in applications such as telecommunications, white goods, medical devices, electrical products, and even in motor racing. Coherent developed the Matrix UV series of diode-pumped, solid-state, Q-switched lasers (Figure 1) specifically to meet the needs of demanding, yet cost sensitive processing applications such as stereolithography. Output powers from 0.5 W to 2.0 W at 355 nm, and a pulsewidth in the 10 to 30 ns range are available. These lasers incorporate a number of technical innovations in order to deliver the ultimate in reliability and performance, thus enabling system builders to take full advantage of the inherent benefits of short pulse, UV laser light. Matrix UV lasers are constructed using Coherent s unique PermAlign technology. In this approach, a series of low thermal expansion ceramic bases is rigidly attached to the laser base plate; each cavity optic is optimally aligned and then directly fixed on to its base using solder. The advantage of this monolithic approach is that cavity components will not shift due to ambient vibrations or shocks since there are no separate parts to move. In addition, this technique uses no plastics or rubber that could outgas. Once the laser has been factory aligned, the head is permanently closed, sealing the cavity from the effects of dust, vapor and moisture. PermAlign construction also lends itself to high volume robotic assembly; the optics are placed and soldered based on feedback from beam analysis instrumentation, including a beam profiler and power meter. Permalign construction, combined with other advances in resonator design and drive electronics, enable Matrix UV lasers to deliver a high quality, TEM 00 beam, with excellent pointing stability, low noise, and with complete control over every laser pulse: both energy and timing. The exceptional optical performance of Matrix UV lasers directly impacts the quality of parts stereolithography can deliver, as well as its economics. This is because the maximum horizontal writing resolution is directly dependent upon the laser s beam quality and pointing stability (since these determine focused spot size and the accuracy of spot placement). High beam quality also extends the life of the photopolymer, thus lowering process costs. This is because a poor quality beam contains some light outside of the central writing spot. This stray light produces partial curing of the photopolymer, thus necessitating replacement of the expensive material earlier than otherwise required WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim LTJ 21

3 FIGURE 3: The Coherent Paladin Advanced is a mode locked, DPSS laser that offers 16 W of quasi-cw output at 355 nm for increased throughput in LDI. Laser Direct Imaging The demand for greater functionality and decreased size in hand held electronic devices, such as cell phones, PDAs and MP3 players, has led to increased use of High- Density Interconnect (HDI) circuit boards because these enable manufacturers to fit more circuitry into a smaller space. Over the past few years, Laser Direct Imaging (LDI) has become a key technology in the production of HDI components. The traditional method for manufacturing circuit boards is to prepare a phototool containing the desired circuit pattern, and then to contact print this pattern on to a photoresist-coated panel. In LDI, a laser is used to image a pattern directly on to the photoresist-coated panel, completely eliminating the production and use of a traditional phototool. In the most common LDI implementation, the front end CAM system is used to modulate a focused laser beam that is raster scanned across the panel. The desired image pattern is built up line by line, analogous to the way in which an image is formed on a CRT display. After imaging is completed on one side of a panel, the panel is flipped and the second side is imaged (Figure 2). The most obvious benefits of LDI are the time and costs savings associated with the creation, use, handling and storage of phototools. In addition, LDI avoids any quality problems associated with film related defects. LDI also delivers significantly better registration than traditional contact printing fabrication methods, and this improvement can increase process yields, especially for very high density boards. In traditional contact printing, registration errors occur when there are dimensional changes in either the phototool or the panel. These dimensional changes happen because the materials used for the mask and panel (such as FR4 and Teflon) vary in size as a function of temperature and humidity. Furthermore, variations in both phototool film and PCB substrates are typically anisotropic. The net result of all this is that it is generally not possible to apply a single scaling factor to the artwork in order to correct registration issues. LDI avoids part of this problem altogether, since the dimensions of the laser produced pattern are not affected by environmental conditions. In addition, the inherent flexibility of LDI enables the size, orientation and shape of the written pattern to be varied as needed. Early LDI systems were based on argon ion lasers, typically with 4W of output at multiple wavelengths in the 351 to 364 nm range. However, these lasers have numerous practical disadvantages due to their very low conversion efficiency; the result is that they require 480V, 3-phase power, and a continuous, high volume supply of cooling water. The Coherent Paladin laser (Figure 3) was developed to meet the needs of LDI and other applications that need a reliable, highpower UV laser source with reduced operating costs. The Paladin is a mode locked, diode-pumped, solid-state (DPSS) laser with frequency-tripled output at 355 nm. Paladin s all-solid-state construction produces ruggedness, high reliability and long lifetime, and yields an output beam with excellent mode quality and extremely good pointing, power stability and noise characteristics. While the output of a mode locked laser is quasi-cw (fast pulsed), the extremely high repetition rate of Paladin lasers (typically 80 MHz to 120 MHz) makes them appear essentially CW in applications such as LDI. The increased efficiency of an all solid state laser substantially reduces its power requirements and the amount of waste heat generated. This enables the Paladin to operate from standard 110/220V input, and to be cooled by a small, closed loop chiller, thus eliminating the need for a high flow rate of water, making it far more practical for LDI than an ion laser, and reducing the carbon footprint. Recently, Coherent has been able to extend the output of this product line to 16W (the Paladin Advanced ) which is the highest commercial power available in this product class today. This is quite significant, since the 16 W output level makes it possible to use less expensive dry films than usual for LDI, while still maintaining adequate process throughput rates. Wafer Singulation Increasing demand for miniature electronics devices also creates a need for logic wafers incorporating low-κ dielectrics to improve power consumption. Wafers incorporating low-κ dielectrics present significant challenges for traditional die singulation using saws. In particular, low-κ dielectrics exhibit high porosity, physical softness and poor adhesion making traditional saw cutting problematic. Consequently, mechanical sawing can cause edge de-lamination and other edge damage. Damaged dies and fragile dies translate directly into lower yield. Moreover, these newer, softer materials tend to clog the blade, which also impacts throughput. These issues can be slightly reduced Circuitry Street Low-k Layers Four Laser Beams Thin Saw Blade Silicon Tape FIGURE 4: Schematic of one type of laser half-cutting, in which both a UV laser and a conventional saw are used for singulation. 22 LTJ May 2009 No WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

4 A B C D GaN Bonding Layers New Substrate Sapphire Substrate FIGURE 5: The AVIA is a q-switched DPSS laser, specifically optimized for wafer scribing. move to narrower streets for many products, the laser is used to make a single cut along the center of the street. This laser cut is wider than the narrow saw blade which then singulates in a single step (Figure 4). Currently, q-switched, DPSS UV (355 nm) lasers are the source of choice for singulation applications. The short wavelength produces a small Heat Affected Zone (HAZ), and the short pulsewidth of q-switched laby slowing the cut speed and thus reducing throughput, but laser scribing avoids this tradeoff. As a result, laser scribing has become the enabling technology for low-κ dielectrics. At present, the dominant laser process for low-κ dielectrics is called half-cutting. There are several variations on this approach, but the common principle is to cut through the soft epi layers with a laser, thereby isolating these layers on the die and leaving their edges relatively clean and undamaged. This enables the wafer to then be mechanically sawed without the blade ever coming into contact with these layers. With wider streets (the blank area between individual chips), the laser is sometimes used to create a narrow groove down each side of the street acting as a type of crack-stop. In these cases, the sawing often is in two steps; first a wider blade is used to clear out the street and then a narrow blade is employed to quickly cut through the silicon down to the support tape. With the Excimer Laser Beam FIGURE 6: Laser lift-off overview. A) GaN LEDs are grown on a sapphire substrate. B) A second substrate is bonded to the top side of the completed LEDs. C) The laser is used to decompose the bond between the LED epitaxial layers and the original sapphire substrate. D) The LEDs are now on the second substrate. Crystal Laser Systems Single frequency CW Laser in the deep UV CryLaS GmbH Berlin successfully developed further models of its already existing FQCW266-laser series. Laser systems of the FQCW-266-series consists of a diode-pumped solid-state laser with a resonant frequency conversion stage (emitting a fixed wavelength of 266nm), plus a control unit. The laser head is contained in a sealed aluminium housing, which allows operation in a wide range of environmental conditions. Heat removal from the laser head is done via thermal conduction through a base plate or by air convection. Due to the single longitudinal mode operation, the laser delivers an extremely small bandwidth and a large coherence length. The laser beam has a near Gaussian shape (TEM00) with nearly diffraction limited parameters. Compared to other CW lasers at 266nm, the FQCW266 system has a low power consumption of typical W (200W maximum) and starts with a small footprint of only 271 x 190 mm. The laser heads of the new models with 50 or 100mW output power are slightly larger compared to the low power models, whereas the control unit is exactly the same size (363 x 325 x 115mm). Also the average power consumption is only marginally increased from 70W to 100W. In addition to the serial (RS232) interface the control unit now is equipped also with an USB interface for remote control operation. The optical properties of the FQCW are very close to the low power models: M2<1.3, beam divergence <0.8mrad, intensity noise below 0.5% RMS (100Hz 10MHz) and power stability below 1% RMS in 8 hours. The coherence length has been determined to be larger than 1000m. At the moment four models of CryLaS FQCW266 series with an output power of 5, 10, 50, or 100mW, are available. zünd precision optics A new dimension Perfection in laser and optical technology Swiss quality products for laser technology, telecommunications and measurement technology. Beam splitting and beam bending prisms in sizes from 1 mm to 40 mm. Mühlackerstr. 72 CH-9436 Balgach Switzerland Tel Fax For more details please check or contact us direct by mail info@crylas.de WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim LTJ 23

5 sers (tens of nanoseconds) means that the thermal energy from each pulse is further minimized and can dissipate by conduction before the next pulse arrives. The high repetition rate and high power also reduce Cost Of Ownership (COO), thus driving cost of manufacturing down. The most important recent technological development in singulation lasers has been the introduction of models with higher power and greater operational flexibility, in terms of their pulsing characteristics. An example of this is the Coherent AVIA which has been specifically designed and optimized for wafer scribing (Figure 5). This laser produces over 8 Watts of ultraviolet with a repetition rate as high as 250 khz, which is currently the highest repetition rate/ power combination available in this type of laser. This high repetition rate and relatively high output power supports fast scribing (up to 150 mm/sec) requiring only a few passes of the laser, while ensuring excellent pulse to pulse overlap, resulting in smooth, rather than serrated edge profile. Operational flexibility is achieved though the use of a feedback mechanism called PulseEQ, which ensures constant pulse energy and high pulse to pulse stability over the laser s entire 100 khz to 250 khz repetition rate range. The advantage of this is that it allows the same laser to perform optimally for both scribing (half-cutting) and dicing (full-cutting). Specifically, the Avia can be set to produce relatively low energy pulses at a high repetition rate, in order to scribe through the top layer of circuitry, which is typically about 10 µm thick. Alternately, the laser can quickly switch to much higher energy pulses to actually cut through the silicon (in conjunction with additional post processing). Laser Lift-Off UV lasers are also playing a key role in the production of high brightness LEDs based on GaN material. These LEDs have already found use in automotive headlights and other solid state lighting applications. GaN LEDs are most commonly fabricated on sapphire substrates because this material provides a good lattice match for the growth of GaN crystals. However, the use of sapphire as a substrate limits LED output power because of its poor electrical and thermal conductivity, which prevents efficient heat dissipation. Laser lift-off (LLO) provides a way to produce GaN LEDs on substrates with more FIGURE 7: The Coherent VarioLas UV material processing system, is a turnkey solution for laser microprocessing, available with the common excimer wavelengths of 193 nm, 248 nm and 308 nm. desirable thermal and electrical characteristics than sapphire. In LLO, the LEDs are first grown epitaxially on sapphire substrates using standard processes. Then, the topside of the LED wafer is bonded to the second substrate, which is usually silicon or a metallic material. Finally, the laser is directed through the sapphire, which is transparent at UV wavelengths (down to about 190 nm). But the GaN absorbs strongly below about 350 nm; thus, if there is sufficient laser power, photo-thermal decomposition occurs at the interface between GaN and sapphire layers resulting in debonding. The end result is that the LED devices are left bonded solely (topside down) to the second substrate (Figure 6). In terms of laser power, a fluence of at least 400 mj/cm 2, somewhere in the 190 nm to 350 nm wavelength range, in required order to decompose the GaN. The most practical way to achieve this is with an excimer laser operating at either 248 nm or 193 nm. Coherent has developed the VarioLas family of UV material processing systems (Figure 7), based on the COMPexPro 100 excimer laser, specifically for applications of this type. In the VarioLas UV, a mask projection system is used to deliver a homogenized, square (2 mm x 2 mm) beam on to the substrate. The advantage of this approach is that a beam of this size can cover several LED dies at once (typical die size is about 400 µm square). In addition, the use of a large, highly homogenous beam avoids the creation of thermal gradients across the dies which might produce nonuniform delamination. Diffusion or cracking is prevented by placing the beam edges and overlaps of the illuminated areas in the trench regions between the dies. Conclusion The high fluence (1 J/cm 2 at 193 nm or 3.5 J/ cm 2 at 248 nm) achieved with today`s high power excimer lasers enables precise, high yield LLO, with fast processing speeds. The use of ceramic preionization, together with Coherent s NovaTube metal-ceramic tube technology, results in both long tube lifetime and excellent pulse-to-pulse stability at all excimer laser wavelengths. This delivers the flexibility, uptime, cost of ownership and process consistency characteristics required for advanced UV micromachining applications. In conclusion, high power UV excimer laser light is a unique and useful tool for a wide variety of processing tasks which require high spatial resolution and minimal thermal damage to surrounding material. As the applications for UV lasers diversify, laser manufacturers are responding with a broader array of tools that are specialized and optimized to meet the exact needs of each task. Furthermore, laser producers continue to improve both performance and reliability, so as to deliver more favorable cost of ownership characteristics, and to support greener manufacturing. 24 LTJ May 2009 No WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim