Coherent light generators – Particular active media – Gas
Reexamination Certificate
2002-04-04
2004-02-24
Scott, Jr., Leon (Department: 3828)
Coherent light generators
Particular active media
Gas
C372S010000, C372S027000, C372S032000, C372S008000, C372S098000, C372S029012
Reexamination Certificate
active
06697408
ABSTRACT:
TECHNICAL FIELD
This invention relates to short pulse simultaneously Q-switched/cavity dumped and simultaneously super pulsed, Q-switched and cavity dumped CO
2
lasers and more particularly to such lasers in material processing
BACKGROUND
It has become well appreciated in the laser machining industry that machined feature quality is improved as one utilizes shorter laser pulse widths and higher laser peak intensity in drilling holes. More specifically, the geometry of holes drilled with lasers become more consistent, and exhibits minimal recast layers and heat-affected zone around the holes as the laser pulses become shorter and their peak intensity becomes higher (XiangLi Chen and Xinbing Liu;
Short Pulsed Laser Machining. How Short is Short Enough
, J. Laser Applications, Vol. 11, No. 6, December 1999, which is incorporated herein by reference).
It is desirable to have the highest quality at the lowest cost but often one must choose a compromise. High-machined feature quality means low recast layer and heat-affected zone thickness, small surface roughness, accurate and stable machined dimensions. Low cost of ownership means a quick return on the investment made in the purchase of the laser machining equipment. Low cost of ownership also involves low maintenance, low operational costs, and high process speeds and yields in addition to low equipment cost. The choice of the laser parameters such as wavelength (IR, near IR, visible or UV lasers) and operational pulse format (milliseconds, microseconds, tenths of microseconds, nanoseconds, picosecond or femtosecond duration pulses) depends on the particular process, material design tolerance, as well as cost of ownership of the laser system.
Moving from lasers that function in the IR region (i.e. CO
2
) to the near IR (i.e. YAG or YLF), to the visible (i.e. doubled YAG or YLF), to the near UV (i.e. tripled YAG, YLF or excimer lasers), the trend is toward higher equipment cost in terms of dollar per laser average output power and lower average power output (which are disadvantages) while also having a trend toward higher power density (w/cm
2
) because of the ability to focus shorter wavelengths to smaller spot sizes (which is an advantage).
Moving toward shorter pulsed widths, the laser costs and the peak power per pulse and therefore power density (w/cm
2
) both tend to increase, while the average power output tends to decrease which results in the cost in terms of dollars per laser output power to increase.
The recast layer and heat-affected zone thickness are greatly reduced when using nanosecond pulses over millisecond and microsecond wide laser pulses (XiangLi Chen and Xinbing Liu;
Short Pulsed Laser Machining: How Short is Short Enough
, J. Laser Applications, Vol. 11, No. 6, December 1999). These improvements result from the higher laser beam intensity associated with the higher peak powers that are obtained with shorter laser pulses that utilize Q-switching, mode locking and other associated techniques and the fact that the pulse duration is shorter than the thermal diffusion time. For example, the typical thermal diffusion time for a 250 micron diameter hole is approximately 0.1 millisecond. In spite of the lower energy per pulse, high drilling speeds can still be cost effectively obtained because of the high pulse repetition rate obtained with these technologies. The high laser beam intensity provided by short laser pulses technology results in vaporization-dominated material removal rather than the melt-expulsion-dominated mechanisms using millisecond wide laser pulses. It is also known that shorter pulse width yield more limited heat diffusion into the surrounding material during the laser pulse. Hole-to-hole dimensional stability is also improved because the hole is drilled by the material being nibbled away by tens to hundreds of laser pulses of smaller pulse energy but occurring at a high pulse repetition frequency rather than by a few high-energy pulses. For the same reason, thermal and mechanical shocks from nanosecond pulses are also reduced compared with millisecond pulses. These advantageous effects obtained with nanosecond laser pulses have been detected by observing fewer micocracks occurring when holes were drilled in brittle materials such as ceramic and glass when utilizing nanosecond laser pulses.
When the intensity is further increased through laser mode locking techniques to get down to the subnanosecond pulse width (i.e. picoseconds and femtosecond region), additional reductions in the recast and heat-affected zones are observed. Since a typical electron energy transfer time is in the order of several picoseconds, femtosecond laser pulse energy is deposited before any significant electron energy transfer occurs within the skin depth of the material. This forms a plasma that eventually explodes and evaporates the material leaving almost no melt or heat-affected zone. Due to the small energy per pulse (~1 mJ), any shock that is generated is weak resulting in no microcracks even in brittle ceramic alumna material. Femtosecond pulses are not presently obtainable with CO
2
lasers due to the narrow gain of the laser line. Femtosecond pulses are presently obtainable with solid-state lasers.
For the same total irradiated laser energy, femtosecond pulses remove two to three times more material than the nanosecond pulses. However, even “hero” type, one of a kind experimental, state of the art laser research and development systems that operate in the femtosecond range deliver only several watts of average power, while nanosecond lasers yield one or two order of magnitude higher power output. Consequently, femtosecond lasers are still too low in average power to deliver the required processing speeds for most commercial applications. It has been reported (XiangLi Chen and Xinbing Liu;
Short Pulsed Laser Machining: How Short is Short Enough
, J. Laser Applications, Vol. 11, No. 6, December 1999) that a 1W femtosecond laser requires more than a minute to drill a 1.0 mm deep hole of 0.1 mm diameter. Present femtosecond lasers have such high cost that their use is cost effective for only special high value applications that unfortunately have relative low unit volume market potential. For example, Lawerance Livermore National Lab has made use of the fact that femtosecond laser pulse energy is deposited essentially with no thermal transfer to cut and shape highly sensitive explosive materials without denotation.
It is well known that the trend for optical absorption in metals as a function of wavelength is toward lower absorption with increasing wavelengths as shown in FIG.
1
. Consequently, the near IR, visible and ultra violet wavelength regions are most effective in machining most metals. This advantage does not exist in plastic material. The data contained in
FIG. 1
is not relevant once a plasma is initiated on the metal surface because all of the laser energy is absorbed in the plasma, which in turn imparts the energy to the material. Once the plasma is initiated, the absorption as a function of wavelength variation for metals becomes essentially flat. Consequently, one can paint the surface of the metal for greater absorption at longer wavelengths and the higher absorption advantage of shorter laser wavelengths is effectively eliminated.
The electronics industry has needs to shrink the size of semiconductor and hybrid packages, and greatly increase the density of printed circuit boards because of the market desire for smaller cellular phones, paging systems, digital cameras, lap top and hand held computers, etc. These needs have resulted in interest in the use of lasers to form small vertical layer-to-layer electrical paths (via) in printed circuit boards. The short pulse CO
2
laser is particularly attractive for drilling via holes in printed circuit boards because of 1) the high absorption of the printed circuit board or hybrid circuits resin or ceramic material at the CO
2
wavelength when compared to YAG or YLF lasers which operate in the near IR and in the visible and UV wavelength regions with ha
Demaria Anthony J.
Fontanella Joel
Hart Richard A.
Henschke Robert
Kennedy John T.
Coherent Inc.
Jr. Leon Scott
Stallman & Pollock LLP
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