Q-switched CO2 laser for material processing

Abstract

A simultaneously super pulsed Q-switched CO2 laser system for material processing comprises sealed-off folded waveguides with folded mirrors that are thin film coated to select the output wavelength of the laser. The system also comprises a plurality of reflective devices defining a cavity; a gain medium positioned within the cavity for generating a laser beam; a cavity loss modulator for modulating the laser beam, generating thereby one or more laser pulses; a pulsed signal generation system connected to the cavity loss modulator for delivering pulsed signals to the cavity loss modulator thereby controlling the state of optical loss within the cavity; a control unit connected to the pulsed signal generation system for controlling the pulsed signal generation system; and a pulse clipping circuit receptive of a portion of the laser beam and connected to the pulsed signal generation system for truncating a part of the laser pulses.

Description

TECHNICAL FIELD OF THE INVENTION This invention relates to short pulse Q-switched and simultaneously super pulsed and Q-switched CO2 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 he...

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Benefits of technology

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.

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 ml), any shock that is generated is weak resulting in no microcracks even in brittle ceramic alumna material. Femtosecond pulses are not presently obtainable with CO2 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 CO2 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 CO2 wavelength when compared to YAG or YLF lasers which operate in the near IR and in the visible and UV wavelength regions with harmonic generating technique; 2. the lower cost per watts associated with CO2 lasers when compared to YAG lasers, and 3. because of the high reflectivity of copper at CO2 wavelengths, which enables CO2 laser via hole drilling equipment to drill through the resin layer down to the copper layer where the drilling is stopped because of the high reflectivity of the copper interconnect material at the CO2 laser wavelengths. These are called “blind via,” which connect the outer layer of a circuit to the underlying inner layer within the multi layer board. The major disadvantages of CO2 lasers in via hole drilling is the larger spot size obtainable with its 10.6 micron wavelength when compared to shorter wavelength laser. Another disadvantage is that pulse widths below several nanoseconds are difficult to obtain with CO2 lasers. The major advantages of CO2 Q-switched lasers are: they offer lower cost per watt of laser output when compared with solid state lasers, higher absorption of their radiation by resin and ceramic board materials, their ability to operate at high PRF, their ability to generate substantial output power under Q-switched operation, and their ability to stop drilling when the radiation gets to the copper layer.

Problems solved by technology

Moving toward shorter pulsed widths, the laser costs and the peak power per pulse and therefore power density (W / cm2) 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.

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.

Femtosecond pulses are not presently obtainable with CO2 lasers due to the narrow gain of the laser line.

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.

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.

The major disadvantages of CO2 lasers in via hole drilling is the larger spot size obtainable with its 10.6 micron wavelength when compared to shorter wavelength laser.

Another disadvantage is that pulse widths below several nanoseconds are difficult to obtain with CO2 lasers.

In most cases, this tail is detrimental to a hole drilling process.

To the present time, Q-switched CO2 lasers have not found extensive commercial application, as have solid-state lasers (whose upper state life times are measured in seconds instead of tenths of seconds as for the CO2 laser).

Some of the reasons for the lack of interest in commercial CO2 Q-switched lasers are high cost of the electro-optic crystal (namely CdTe), limited suppliers for the electro-optic (EO) crystals, large performance variation between different optical paths within an EO crystal and large performance variation between different crystals.

There is also difficulty in obtaining good anti reflection thin-film coatings on CdTe crystals.

In addition, electro-optic modulators cannot be easily replaced by acousto optic modulators in the IR because they have higher attenuation and poorer extinction performance than in the visible region, as well as larger thermal distortion and poorer reliability.

Q-switched CO2 lasers were also considered to have poorer reliability than the Q-switched solid state laser which was mostly caused by the CdTe crystals.

Mechanically Q-switched CO2 laser have also been utilized but they do not have the pulsing flexibility of electronically Q-switched lasers.

TEA lasers have also been used to date, but they suffer from higher time jitter from pulse to pulse, higher pulsed voltage requirements along with associate acoustic shock noise and non-sealed off laser operation which requires gas flow.

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