Electric heating – Metal heating – By arc
Reexamination Certificate
2002-05-09
2004-08-31
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121690
Reexamination Certificate
active
06784399
ABSTRACT:
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
COPYRIGHT NOTICE
© 2002 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR §1.71(d).
TECHNICAL FIELD
The invention relates to laser micromachining and, in particular, to micromachining layered or multimaterial substrates or devices, such as reinforced or unreinforced epoxy resin, FR4 or green ceramic, with a Q-switched CO
2
laser.
BACKGROUND OF THE INVENTION
Q-switched solid-state lasers are well known and have been demonstrated successfully for many laser micromachining applications. However, micromachining parameters for Q-switched lasers, including their wavelengths (ranging from near infrared to deep ultraviolet), pulsewidths, pulse energies, and pulse repetition rates, have still not been perfected for certain classes of layered organic, inorganic, and metallic microelectronic material constructions with respect to throughput and machining quality, such as cleanness, side wall taper, roundness, and repeatability.
One such class of materials, commonly used in the printed wiring board (PWB) industry, includes glass cloth impregnated with one or more organic polymer resins that is sandwiched between conductive metal layers, typically copper. This material configuration is known as “FR4 .”
Another class, commonly used as packaging materials for high-performance integrated circuits, includes unfired, “green” ceramic materials. These ceramic substrates are formed by high-pressure pressing of powders of common ceramics such as aluminum oxide (Al
2
O
3
) or aluminum nitride (AlN). The micron-(&mgr;m) or submicron-scale particles are held together with organic “binders” that provide sufficient mechanical integrity for machining operations such as via drilling to be carried out. Afterward, the green material is fired at high temperature, driving off the binders and fusing or sintering the microparticles together into an extremely strong, durable, high-temperature substrate.
U.S. Pat. Nos. 5,593,606 and 5,841,099 of Owen et al. describe techniques and advantages for employing Q-switched UV laser systems to generate laser output pulses within advantageous parameters to form through-hole or blind vias through at least two different types of layers in multilayer devices, including FR4. These patents discuss these devices and the lasers and parameters for machining them. These parameters generally include nonexcimer output pulses having temporal pulse widths of shorter than 100 nanoseconds (ns), spot areas with spot diameters of less than 100 &mgr;m, and average intensities or irradiances of greater than 100 milliwatts (mW) over the spot areas at repetition rates of greater than 200 hertz (Hz).
CO
2
lasers have also been employed for drilling microvias in multilayered materials, including FR4. Because the 9- to 11-&mgr;m wavelength of all CO
2
lasers is highly reflected by metals such as copper, it is very difficult to use CO
2
lasers to drill substrates that feature overlying metallic layers. Consequently, CO
2
lasers are preferably applied to layered substrates with either no overlying copper or an overlying copper layer in which via openings have been previously etched by standard chemical means. Such “pre-etched” or “conformal mask” multilayered substrates also typically have no woven reinforcements such as glass fibers in the dielectric layers and are commonly produced in the printed wiring board industry for compatibility with CO
2
laser microvia-drilling operations. Common CO
2
microvia-drilling lasers include RF-excited lasers, transversely excited atmospheric (TEA) lasers, and fast-axial-flow lasers.
RF-excited CO
2
lasers are the most common type of CO
2
laser employed for microvia-drilling applications. These lasers employ a radio-frequency (RF) electrical discharge to provide the excitation or “pump” energy that causes stimulated emission from the CO
2
molecules. The CO
2
molecules, mixed with other gases, are sealed in a tube at pressures well below atmospheric (typically less than 100 torr), and the RF discharge is typically applied across electrodes oriented perpendicularly to the axis of the laser cavity. RF-excited CO
2
lasers produce pulses with relatively long pulsewidths, such as 3-50 microseconds (&mgr;s), at moderate pulse repetition rates or pulse repetition frequencies (PRFs), such as 2-10 kHz with pulse energies in the 1- to 30-millijoule (mJ) range. The instantaneous power levels of these lasers are low to moderate, typically 1 kW or below, although leading-edge designs are approaching 2 kW.
TEA CO
2
lasers employ higher gas pressure (near or above atmospheric) and a DC electrical pump discharge that, as in RF-excited lasers, is applied across electrodes oriented transversely to the laser cavity axis. The main advantage of the TEA CO
2
laser design is the short pulsewidth spike and the corresponding high instantaneous power that these lasers can generate. The high power is produced in a 100- to 150-ns spike followed by a low-power “tail” lasting up to several microseconds. Pulse energies in the hundreds of millijoules are typical. This combination of pulse energy and pulse spike duration results in extremely high peak instantaneous power, on the order of megawatts. However, because the laser beams exhibit many spatial transverse electromagnetic “modes,” TEA lasers are not highly focusable like the other types of CO
2
lasers, so much of the available pulse energy is either masked in the beam-delivery system or by the substrate itself, or both. Nevertheless, TEA lasers can produce vias of excellent quality in FR4. Such vias exhibit glass fiber ends that are cleanly vaporized and flush with the via wall, and little or no over-etching of the surrounding polymer resin. Despite the advantages in via quality, TEA CO
2
lasers suffer from the disadvantage of operating at low PRFs, typically 0.2-0.5 kHz. As a result, drilling speed and throughput are limited.
Fast-axial-flow CO
2
lasers have seen less application in commercial microvia-drilling applications. Unlike the RF-excited and TEA designs, the tube containing the CO
2
gas mixture is not sealed. Rather, the gas mixture flows rapidly through the tube, along the laser cavity axis. Although the gas is collected and recirculated, the need for an external gas reservoir is disadvantageous. Pumping excitation is accomplished by either DC or RF discharge and is usually longitudinal for DC discharge and transverse for RF excitation. A high flow speed is needed to avoid significant heat buildup in the gas while in the discharge region, so this design requires additional gas-pumping equipment that is not needed in the sealed-tube designs.
Despite the added complexity, fast-axial-flow CO
2
lasers have become the most common industrial CO
2
laser design for applications that require high average power (0.5-10 kW), such as metal cutting or welding. Application to via drilling is limited by the large size and complexity of the laser. Most via-drilling work with these lasers has been aimed toward utilizing the high average power that they generate and obtaining short pulses, such as 1-10 &mgr;s, at moderate PRFs through the use of an external modulation device such as a shutter.
For both RF-excited and fast-axial-flow CO
2
lasers, the external modulator is required to obtain short pulsewidths (less than several hundred microseconds) needed for microvia-drilling operations. In the externally modulated configuration, the instantaneous power of these lasers is equal to their average power, which is relatively low.
On the other hand, Q-switched CO
2
lasers have been used for some time in military imaging radars but have not been applied to material processing until recently. In “A High-Power Q-switche
Dunsky Corey M.
Harris Richard S.
Kennedy John T.
Matsumoto Hisashi
Newman Leon
Electro Scientific Industries Inc.
Heinrich Samuel M.
Stoel Rives LLP
LandOfFree
Micromachining with high-energy, intra-cavity Q-switched CO2... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Micromachining with high-energy, intra-cavity Q-switched CO2..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Micromachining with high-energy, intra-cavity Q-switched CO2... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3317283