Laser Drilling and Pattern Processing for MCM-L Prototyping

Transcription

1 Laser Drilling and Pattern Processing for MCM-L Prototyping Laser Drilling and Pattern Processing for MCM-L Prototyping Zsolt Illyefalvi-Vitéz, Miklós Ruszinkó, and János Pinkola Department of Electronics Technology Technical University of Budapest Goldman t. 3., Budapest H-1111, Hungary Phone: Fax: Abstract A significant step has been taken for implementing lasers in microelectronics manufacturing by the realization and improvement of diodepumped Nd:YAG solid state lasers with high output power. Efficiency, beam properties, size and reliability of frequency multiplied lasers have reached remarkable qualities for the wavelength of 1064, 532, 355 and 266 nm. The short pulse duration, high repetition rate, and optional wavelength of these lasers have opened new dimensions for material processing and, in particular, for conductive pattern generation for MCM-L laminates by laser direct writing, and through board or blind via generation by controlled pulsed laser application. The paper describes theoretical aspects and practical approaches of the application of frequency multiplied Nd:YAG lasers in the field of Printed Wiring Boards and MCM-L substrates prototyping. Key words: Laser Direct Writing, Laser Drilling, Laser Patterning, Prototyping of MCM-Ls and PWBs, All-Solid-State Lasers, and Sensor Patterning. 1. Introduction: Driving Forces Toward Laser Direct Writing The other important driving force is the need for rapid and low cost prototyping in the field of MCMs. A prototyping process should fulfill the following requirements: rely basically on the same processing and materials that are applied in the production, fast turnaround, flexibility in modifying the interconnection layout, apply relatively low cost special methods, possibly avoid or reduce the high cost artwork and photo-mask count, and enable as low batch numbers as one piece. Miniaturization is a never-ending driving force in electronics manufacturing, especially in the production of circuit boards and substrates for multichip modules. The primary aim is to achieve greater wiring density. The improvement of pattern resolution, that is, the reduction of line and space widths, is certainly a proven method for increasing circuit density, however, without the reduction of the size of via holes and associated land pads, the possibility of miniaturization is limited. As a consequence, nowadays both micro via drilling and pattern generation are acquiring greater and greater importance The Laser Direct Writing Process Laser-induced direct patterning is a process where an intense laser beam is used to affect a controlled area, a pattern, on the surface of a target material. This process includes laser direct writing where the surface pattern is generated by the controlled movement of a focused laser spot. When a material surface is irradiated by a photon beam, the absorbed part of the energy of the beam can produce a mostly physical (non reactive, thermal) or a mostly chemical (reactive) effect, and a 349

2 Intl. Journal of Microcircuits and Electronic Packaging corresponding processing. The mechanism is determined by the parameters of the laser beam, the materials characteristics, and the processing conditions. Generally, the thermally induced physical effects dominate in such applications as surface patterning and drilling, as well as, in cutting, trimming, welding, alloying, annealing, among other applications. When the laser-matter interaction is governed by the thermal effect, the absorbed laser energy heats up a volume of the target material which can be transformed into liquid and/or vapor phase, and material removal can take place. This material departing process is also called ablation. The melted liquid is expelled from the interaction region through hydrodynamic motion where the effects of surface tension, capillary, drift toward cooler sites to find energy equilibrium, vapor pressure, vapor driven splashing, the pressure of the intense light, play part in the process 4. The absorption coefficient of the irradiated surface determines the efficiency of processing. The absorption highly depends on the wavelength of radiation. Figure 1 presents the absorption versus wavelength function for the three materials used in usual PWB laminates. Absorption UV Visible Glass Copper IR FR µm λ/4 λ/3 λ/2 λ (Nd:YAG) Wavelength Figure 1. Absorption curves of copper, glass and FR-4 epoxy. The volume of material ablated by a laser pulse has great importance for fine resolution machining. The laser ablated volume of material depends on the parameters of the radiation, the material surface characteristics, and the environmental conditions. The most significant factors are, the area of the focused laser spot, and the affected depth determined by the heat penetration resulted in temperature distribution. The area of the focused laser spot can be calculated from the d s spot diameter using Equation (1), where k is the wavelength of the laser radiation, f is the focal length of the objective lens, and D is the entering beam diameter. The absorption curves and Equation (1) show that the application of UV radiation is essential from the point of view of both uniform processing efficiency and fine resolution machining. On the other hand, theoretical analysis proves that the L penetration depth of heat can be determined by Equation (2), as follows, L = (4jt p ) 1/2 (2) where j = K/qC is the thermal diffusivity, t p is the pulsewidth, K is the specific heat conduction, q is the density, and C is the specific heat capacity of the sample. Equation (2) shows that heat penetration depth is smaller if shorter pulse is used. Laser pulses with uniform energies can be realized by shorter width of pulse accompanied by higher peak power, and by longer pulse duration with lower peak power. Then, the shorter pulse, with shallow penetration, affects smaller volume of material and causes higher temperature, while a longer pulse results in larger affected volume but lower temperature. As a consequence, in order to achieve fine resolution machining, short pulse duration, high peak power lasers should be used, which is the case by the application of frequency multiplied UV Nd:YAG lasers. In this way, the effect of the thermal mechanism, which ultimately limits the capability of lasers in many processing applications, can be reduced by eliminating the unwanted flow of melted material, which substantially degrades the edge quality or limits the minimum of the processable layer thickness. 3. Ultraviolet Nd:YAG Lasers for Direct Writing Since radiation of shorter wavelength, which usually accompanied by shorter pulse width, offers distinct advantages, with the invention of the UV excimer lasers in the mid-seventies, and frequency multiplied solid state lasers during the recent years, a host of new possibilities for materials processing emerged. In particular, the ability to remove organic materials, metals and glasses from a known depth with less thermal damage makes UV lasers very attractive for via generation and patter direct writing of MCM-L laminates. Three types of UV laser sources are in use in the electronics industry for general purpose micromachining and for the special need of via drilling (Table 1). ds = 4/π λ f/d (1) 350

3 Laser Drilling and Pattern Processing for MCM-L Prototyping Table 1. Characteristics of micromachining UV lasers. Laser type Wavelength (nm) Pulsewidth (ns) Pulse energy (mj) Repetition rate (Hz) Excimer CVL 271, , Nd:YAG 266, ,02-0, In the past decades, excimer lasers with their short UV wavelength, short pulse duration and very high fluence have enjoyed much success in producing fine surface structures. Due to the large cross section of the beam, mask projection techniques are in use instead Figure 2. Optical layout of an Nd:YAG laser with fourth of direct writing. However, the lack of flexibility of the mask projection method and the difficulty of handling corrosive gases mean harmonic generator option 9. drawbacks for excimer laser applications. Laser direct writing applying frequency multiplied UV Nd:YAG A frequency doubled copper vapor laser (CVL) has a perfect lasers with the above characteristics is a promising process for both wavelength to machine glasses, FR-4, and other polymers, while its the following functions, fundamental frequency is eminent for copper. Westwind declares via generation, and that with its high repetition rate, high peak power and short pulse fine resolution surface patterning duration CVL is the best technology for µm vias 5. The only of MCM-L boards. The process fulfills the requirements of fast and problem with CVLs is that when they are applied in mass production they need regular service for copper supply. low cost prototyping as well. The solid state Nd:YAG laser is probably the most common laser applied in the electronics industry. Its fundamental near IR (1064 mm) wavelength machines organic materials by melting or burning, which 4. Via Generation Techniques Using makes it unsuitable for most via generation applications. It is possible to triple or quadruple this laser frequency by adding nonlinear Laser Direct Writing crystals to the standard resonator configurations, which shifts the frequency into the ultraviolet spectrum at 355 or 266 nm, respectively, which are much more suitable for micromachining applications 6. Via generation by laser direct writing means that the small diameter spot of a focused laser beam is used to drill vias in PWB or For only a couple of years, the rapid developments in diode MCM-L laminates. As it may be deduced from the previous discussion, high repetition rate, short pulse width, high peak power, UV pumped, all-solid-state laser technology, have resulted in low-cost, compact, rugged, turn-key products of Q-switched Nd:YAG lasers, laser machining, and standard glass fiber reinforced FR-4 epoxy and as a consequence, they became the best choice for microelectronics direct writing applications. The advantages of diode pump- copper clad laminate samples are considered. The diameter of the laser spot when it is focused is about µm. ing over conventional lamp pumping arise mainly from the increased Through-hole vias can be generated by two processes; laser efficiency of the overall optical pumping process. Diode laser pump punching or laser trepanning 10. Laser punching is a process in which sources themselves are typically % efficient in terms of electrical-to-optical conversion, while a well-designed Nd:YAG laser the laser spot is positioned to the center of the machined hole, and the laser punches through the material thickness using multiple pulses. can have optical conversion efficiency as high as 55 %. Consequently, The diameter of the via is determined by the spot size of the focused the thermal problems associated with lamp pumping are largely eliminated, resulting in compact systems with excellent TEM 00 laser beam. The diameter as well as the via wall angle can be controlled by changing the focal plane of the laser beam. The peak power mode characteristics and very high stability 7-8. of the laser pulses, which distributing over the surface area gives the The ultraviolet region of the spectrum of an Nd:YAG laser can peak intensity and governs the ablation, is set higher than the copper be produced by frequency multiplying. The fundamental 1064 nm machining threshold. In this case, the pulses easily remove both the wavelength of the laser can be shifted to 355 nm by third harmonic glass and the epoxy resin materials, thus punches the via through generation, while the fourth harmonic generation results in 266 nm. the laminate. Although all harmonic generators utilizing the nonlinear optical effects of special crystals, they are different regarding constructions, Using the laser trepanning method, the through hole via is generated by steering the laser spot along a circular pattern around the their site in the laser, and their efficiency. For example in the Continuum HPO-1000 laser, Figure 2, the 266 nm wavelength is ob- center of the machined hole. Due to the shallow penetration depth of the short laser pulse, in a single pass the circular kerf can be machined untill a limited depth. Therefore, a number of passes are tained by quadrupling the frequency using two LBO (LiB 3 O 5 ) nonlinear, noncentrosymmetric crystals 9. needed to drill the via through. The peak power is set higher than the 351

4 Intl. Journal of Microcircuits and Electronic Packaging copper threshold. To increase the machining efficiency, the beam is steered along the circle by wondering the focusing lens. Copper clad laminate Copper clad laminate Blind via generation requires a two-step method, and both processes should be carried out with well controlled parameters. The two steps are laser spiraling and laser punching. Laser spiraling is Covering by laser processable mask Covering by laser processable mask similar to laser trepanning, but the laser spot is steered along a spiral instead of a circle. The spiral starts from the center of the machined Patterning by laser ablation Patterning by laser ablation hole and enters the contour circle, thus the whole area of the via is scanned. A number of passes are needed to cut through the outer Pattern etching Electroplating of etch-resistant mask copper layer and penetrate into the polymer layer. It is of vital importance not to drill through the dielectric layer, but leave some Mask stripping Mask stripping material there, otherwise the inner copper layer would be damaged. After this step, a defocused beam is used for the second punching process. The beam diameter is larger than that of the spiraled hole, Pattern etching and the peak power of the pulses is lower than the ablation threshold of copper. The UV beam cuts cleanly through the dielectric material. a.) b.) The UV laser generated blind vias are clean, free of carbonization, and can be easily plated. The surface of the inner copper layer Figure 3. The gap cutting and the line cutting processes. requires neither further cleaning nor pre-etching. The following etch-resistant layer materials have been tested: electroplated tin, electroplated gold, and a polymer (a conventional photoresist). In contrast with the former processing, in the course of the line cutting process laser engraving determines the conductor pattern by means of an auxiliary laser processable film. The process 5. Surface Patterning by Laser Direct steps are as follows (Figure 3b), Writing using FR-4 single-sided copper clad laminate as basic material, application of electroplating-resistant, laser processable mask coating onto the copper clad laminate surface, A new approach is being developed for prototyping MCM-Ls. It direct pattern transfer by cutting this film using laser ablation combines the advantages of laser surface patterning, electroplating, processing, and chemical etching In order to achieve finer pattern resolution electroplating of etch-resistant metallization coating into the and smaller via diameter by laser direct writing, it is essential to mask openings on copper clad laminate surface, apply UV laser light with its smaller focal spot feature and similar mask stripping, and absorption for most processed materials. Obtaining a suitable UV wet chemical etching of the copper film. laser system unfortunately takes a longer time, consequently for preliminary As it can be seen from the comparison of the Figures, the line research a general purpose IR Nd:YAG laser cutting process contains more steps than its gap cutting counterpart. micromachining system has been applied. The same polymer film materials, mainly different type of photoresists, Two different processing sequences have been examined, they were used for laser processable mask, as for the gap cutting are, process. Several etch-resistant electroplated metal layer systems have the gap cutting process, in which the laser kerfs determine the been tested, including tin, nickel, and gold over nickel structures. place of the spacings between interconnect lines, and Process optimization aimed at the following elements, the line cutting process, in which the conducting pattern itself choosing the best processing sequence that consists of only conventional is determined by laser engraving. PWB processing steps like electroplating 15, and chemical The steps of the gap cutting process when producing a single etching, and an additional laser direct writing ablation step, interconnection layer are as follows 12 (Figure 3a). choosing appropriate mask layer materials which can be laser using FR-4 single-sided copper clad laminate as basic material, patterned and which are resistive for electroplating and/or chemical application of etch-resistant, laser processable mask coating onto etching, and the copper clad laminate surface, choosing appropriate laser types and parameters for fine-line direct pattern transfer into the etch-resistant film using laser realization. ablation processing, The two main processing sequences, that is, the gap cutting and wet chemical etching of the copper film to obtain the conductive the line cutting processes, have been compared. At present, using an pattern, and IR laser with long pulse duration, the line cutting process seems to etch-resistant coating removal by chemical methods. be better, however the gap cutting process is also appropriate. The IR laser could process a special polymer layer very fine, while electroplated tin and gold-over-nickel pattern-plated etch-resistant layers showed similarly suitable features. 352

5 Laser Drilling and Pattern Processing for MCM-L Prototyping 6. Sensor Patterning Example During the past 20 years, a new group of organic polymers has been revealed with the ability to conduct electric current inherently. These electroactive conducting plastics (ECPs) are still under development for appropriate applications, such as rechargeable batteries, capacitors, field-effect transistors, enzymatic biosensors, and gas sensors. The aim of an interesting research initiated by TU Budapest in the frame of an international project is to find a practical exploitation of the special properties of these types of materials. ECPs can be synthesized and deposited onto conducting surfaces by a simple electrochemical polymerization method. Preliminary studies on these layers have shown that they exhibit fast and reversible response even at room temperatures, which cannot be expected with inorganic films ECPs can be used to sense gases and vapors by monitoring the change in the conductance of an ECP resistor element with the exposure of the polymer layer to the sample gas. For the preparation of a resistor sensing element, a conductive film pattern of two electrodes with an insulating gap is necessary (Figure 4). When the resistor film is electrochemically deposited, it grows through between the electrodes, it fills up the gap, and it forms the resistor. Current Gap Gas ECP film Conductive film Substrate The conductive film is realized by conventional thick film processing on alumina substrates. The narrow gap between the elec- lasers., Printed Circuit Fabrication, February D. Moser, Sights Set on Small Holes? How to get there with trodes should have a resolution which cannot be realized by con- ventional screen printing technology, therefore, a continuous film is screen printed first, and it is divided by a gap made by laser engraving. The direct writing laser processing technology was the same that was described above and are to be used for different purposes in MCM-L prototyping. The last step is the electrochemical deposition of the polypyrrole to from a gas sensitive resistor film through the gap. 7. Conclusions A significant step has been taken for implementing lasers in microelectronics manufacturing by the realization and improvement of diode-pumped, frequency multiplied, Nd:YAG solid state lasers with high output power. The short pulse duration, high repetition rate, and the alternative wavelengths of these lasers have opened new dimensions for material processing. The recent developments and results in this field set the trend for the microelectronic packaging industry and pointed out the direction of research and investment for these research projects as well 18. As a consequence of this trend, a new UV Nd:YAG laser was installed at the Department, and research projects were initiated to utilize the new possibilities, in particular, in via drilling of FR-4 laminates. A new approach was also established in fast and cheap prototyping of MCM-Ls. The method combines the advantages of etch-resistant metal film electroplating, wet chemical etching, and laser direct pattern transfer. The results indicate the resolution being in a far appropriate level, and the processing provides the required flexibility for low batch number manufacturing and prototyping. The high resolution processing capability of the laser will be used in many different layer patterning applications, such as, for the electrodes of resistive or capacitive type sensors. Polypyrolle film Filled gap Acknowledgments Conductive film Alumina substrate This research was supported by the European Commission in the frame of INCO Copernicus Projects Cheap MultiChip Modules No. IC15-CT (DG 12 SNRD), SIGMA No. ERBIC 15CT , as well as by the following projects in Hungary: MKM FKFP 0253/1997 and MKM FKFP 0254/1997. Figure 4. Sensor pattern realized by the use of laser direct writing. References 353

6 Intl. Journal of Microcircuits and Electronic Packaging 2. M. Owen, Roelants, E.Van Puymbroeck, J. Laser Drilling of Blind Holes in FR4/Glass. Circuit World, Vol. 24, No. 1, pp , S. Contini Hennink, Machining Lasers Find Niches by Solving Very Small Problems. Photonics Spectra, pp , November J. Wilson, J.F.B. Hawkes, Lasers. Principles and Applications. Prentice Hall, New York, Westwind Multilase: The Next Generation Microvia Technology. Westwind data sheet, A. Cable, Laser Micro-Via Drilling in Circuit Boards. Circuitree, December S. Jenny, Diode Pumping Expands Market for Solid-State Lasers. Laser Focus World, pp , June Millennia - A New Class of High-Power Diode-Pumped CW Visible Laser. Laser Forefront of Spectra-Physics, No. 6, HPO-1000 Diode pumped 1000 Hertz Nd:YAG laser. Continuum, User Manual, Laser Drilling Techniques. Application Note, Electro Scientific Industries, Inc., G. Harsányi, J. Pinkola, E. 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Proceedings of the 18th International Spring Seminar on Electronics Technology, Temesvar (Czech Republic), pp , June 26-30, G. Bidan, Electroconducting conjugated polymers: new sensitive matrices to build up chemical or electro-chemical sensors. A review. Sensors and Actuators B, Vol. 6, pp , Charlesworth, J. M. Partridge, A. C. Garrard, N. Mechanistic studies in the interactions between poly(pyrrole) and organic vapours, Journal of Physical Chemistry, Vol. 97, pp , P. Nemeth. Computer Aided Design, Manufacturing and Trimming of Thin Film Resistor Networks. Proceedings of the 21st International Spring Seminar on Electronics Technology, Neusiedl am See, Austria, pp , May 4-7, About the authors Dr. Zsolt Illyefalvi-Vitéz received his Diploma in Electronic Engineering and his Dr.Techn. Degree from BME, the Technical University of Budapest, Hungary, in 1965 and 1974, respectively, and the Candidate of Technical Science Degree (an equivalent of Ph.D.) from the Hungarian Academy of Science, in He joined BME- ETT, the Department of Electronics Technology of BME in 1965, where he is currently Associate Professor of components and circuit modules technology, and laser processing. He is also the Head of Department. The focus of his research activity has been on microelectronics manufacturing with the main interest in laser beam processing, trimming, and computer aided design of microelectronics components and circuit modules. In the field of education, he is responsible for teaching of topics on microelectronics, packaging, components and manufacturing technology. Dr. Illyefalvi-Vitéz is member of the IEEE (CPMT and ED), IMAPS (elected representative of IMAPS-Hungary in the European Liaison Committee), the Optical Society of America, and other national/international scientific societies. Dr. Miklós Ruszinkó received his Diploma in Electronic Engineering and his Dr.Techn. Degree from BME, the Technical University of Budapest, Hungary, in 1979 and 1987, respectively. He joined BME-ETT, the Department of Electronics Technology of BME in 1983, where he is currently Assistant Rrofessor of design and technology of electronic circuit modules, and laser processing. The focus of his research activity has been on microelectronics circuit design with the main interest in the control of laser systems and processing. In the field of education, he is responsible for teaching of topics on design and production of electronic circuit modules. Dr. Ruszinkó is member of the IEEE CPMT, IMAPS and other national scientific societies. Dr. János Pinkola received his Diploma in Electronic Engineering and his Dr.Techn. Degree from BME, the Technical University of Budapest, Hungary, in 1970 and 1987, respectively. He joined BME-ETT, the Department of Electronics Technology of BME in 1970, where he is currently Assistant Professor of printed wiring board technology, and laser processing. The focus of his research activity has been on microelectronics manufacturing with the main interest in laser beam processing for imaging and drilling processes of PWBs for multichip modules. In the field of education, he is responsible for teaching of topics on electronics technology. Dr. Pinkola is member of the IEEE CPMT, IMAPS and other national scientific societies. 354