Electric heating – Metal heating – By arc
Reexamination Certificate
2001-03-26
2003-02-04
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121700
Reexamination Certificate
active
06515257
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
(NONE)
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
(NONE)
REFERENCE TO A MICROFICHE APPENDIX
(NONE)
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a high-speed laser-based via generation system for producing through-substrate and blind vias to make interconnects in high-density microelectronics systems. More particularly, this invention relates to an opto-mechanical system which delivers controlled pulses of laser energy to a large number of program-selectable via sites simultaneously at very high optical efficiency regardless of the number or density of vias. The system delivers the full energy of the laser among the vias being generated through the use of a high-speed opto-mechanical beam-steering system and a specialized energy recycling illumination system.
(2) Description of Related Art
Vias, the small diameter holes through one or more substrate layers, play an important role in electronic manufacturing because they provide the interconnections between layers in electronic modules. Electronic manufacturing has evolved to provide denser, faster, and more complex packaging technologies which rely heavily on a large number of micro-vias—vias with diameters less than about 150 &mgr;m—to provide the increasing number of required interconnections. The generation of these vias in electronic substrates has become a throughput-limiting and crucial step in the fabrication of advanced electronic modules. Via numbers and densities in these modules have increased to keep pace with the increasing complexity of electronic devices, but current via generation technologies cannot keep up with this growth and are fundamentally limited with respect to achievable throughput rates. This invention seeks to address this problem.
Depending on the application, the vias may need to be generated in regular, periodic patterns, such as in chip carriers, or in non-periodic patterns, such as on high density interconnect printed wiring boards. Often the vias need to be drilled through one or several substrate layers. Almost as often, the vias need to be generated such that they terminate or bottom out at a particular layer or depth—these are called blind vias. The vias need to be generated in a wide range of electronic substrate materials. These materials are most commonly dielectric polymers or glass-filled epoxies, but can also be more mechanically durable materials such as ceramics and metals. The wide range of substrate material properties poses a challenge for via generation systems.
Several technologies currently exist for via generation in electronic substrates: conventional mechanical drilling using metal or ceramic bits; lithographic patterning of the via pattern followed by chemical or plasma etching of the substrates; and direct laser photo-ablation. The mechanical technologies cannot produce micro-vias at high throughputs or with fine dimensions due to breakage of the bits and minimum diameter limitations. Lithographic and etching technologies are expensive, require multiple processing steps, are difficult to reconfigure, and are hard to control in depth for blind via applications. Optical technologies using laser photo-ablation show the most promise for rapid, clean production of micro-vias in a variety of substrates, but current systems are too slow to keep up with current production demands. This is mainly due to the serial nature of the via generation process and the power limitations of current frequency multiplied solid-state laser systems.
Laser via generation through photo-ablation provides many advantages: it allows a multitude of materials to be drilled without generation of harmful debris; it allows flexibility in via diameter, depth, and placement; and it typically does not require additional process steps. Photo-ablation is the process by which material is broken down to its smaller molecular or elemental components by being irradiated by a very high-fluence beam of ultraviolet radiation. For most materials photo-ablation is most efficient for radiation in the UV-region of the spectrum since most materials absorb energy very strongly at these wavelengths. For each material and laser wavelength there is a photo-ablation threshold fluence above which ablation occurs efficiently, and there is a photo-etch rate that characterizes the rate at which the material is ablated away. For most dielectric materials used in electronics manufacturing the ablation threshold fluence can range from about 100 mJ/cm
2
to several Joules/cm
2
at wavelengths near 300 nm, and etch-rates for 1 J/cm
2
fluences at these wavelengths are typically 0.5 &mgr;m/pulse. For this reason, powerful UV lasers are required to generate vias efficiently. Currently, only excimer lasers such as XeCl and certain frequency-multiplied solid-state lasers such as Nd:YAG can deliver the required fluences. Excimer lasers are attractive because they are the most powerful of these, whereas frequency-multiplied solid-state lasers are used due to their high repetition rates and focusability.
Systems which use solid-state laser sources typically focus the low average power, frequency-multiplied beam into a very small spot—typically about 20 &mgr;m in diameter—to achieve the high fluences required for photo-ablation. They generate the required via patterns by a combination of galvo-mirror beam deflection to move the spot on the substrate and automated planar (X-Y) stage motion to present different areas of the substrate to the beam. The throughput of these systems depends on how quickly the focused laser spot can be moved from one via site to the next, by the dwell time required at via site, and by the average power of the laser source. Although this process can be easily programmed and is, thus, very flexible, it is essentially a serial process, and therefore, has an inherently low throughput. In addition, via diameters are limited to a minimum size of about 20 &mgr;m.
Systems using scanning mask projection and high-power excimer lasers for massively parallel via generation have been described in Jain, U.S. Pat. No. 5,285,236. Such systems capitalize on the much greater powers delivered by excimer lasers to generate the required fluences over large areas of the substrate. In these systems a large-area, uniform beam is produced by a specialized illumination system, as described in Jain, U.S. Pat. No. 5,059,013, and Farmiga, U.S. Pat. No. 5,828,505. This beam illuminates a via pattern on a mask which is projected onto the substrate by a projection imaging system. In such a system, all the vias in the illuminated region are generated simultaneously, the throughput being limited only by the etch-rate of the material and not the number of vias. For very dense via density applications, this type of system can achieve extremely high throughputs, especially when the illumination system incorporates energy recycling as described in Hoffman and Jain, U.S. Pat. No. 5,473,408. For low via densities, however, such a large-area projection system can be slow and inefficient. In addition, such a system is not very flexible in that it requires expensive masks to be generated for each required via pattern.
Current electronic manufacturing demands via generation systems with the programmability of the serial solid-state laser-based systems and the high throughputs of the massively parallel excimer laser-based systems. Highly desirable features are high-speed via generation; full via pattern programmability—including via diameter, position, and depth; capability to drill high-threshold photo-ablation substrates; and full and efficient utilization of available laser energy. The invention described below provides all these features. It makes full and efficient use of the power available from excimer lasers and provides full programmability of the via pattern.
BRIEF SUMMARY OF THE INVENTION
This invention is a via generation system which produces vias in a variety of microelectronic substrate materials by the process of laser photo-ablation. This system optim
Farmiga Nestor O.
Jain Kanti
Anvik Corporation
Elve M. Alexandra
Kling Carl C.
Tran Len
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