Basic information on EXCIMER LASERS

Excimer lasers are pulsed gas discharge lasers which produce optical output in the ultraviolet region of the spectrum. There are four commonly used excimer wavelengths, the wavelength output depends upon the active gas fill of the laser, the four wavelengths are:-

Wavelength          Active Gas              Relative Power

193nm                 Argon Fluoride                 60

248nm                 Krypton Fluoride             100

308nm                 Xenon Chloride                50

351nm                 Xenon Fluoride                 45

The wavelength output of an excimer laser can be changed simply by changing the gas mixture, the laser mirrors may have to be exchanged to obtain maximum output.

Eximer lasers are pulsed and the energy contained in a single optical pulse is measure in millijoules, (thousandth of a joule). Typical energy output from excimer lasers ranges from a few mJ’s to 1000mJ. To obtain useful power from excimer lasers the laser is pulsed at some number of pulse per second, this is know as the repetition rate and is specified in pulses per second (PPS) or Hertz (Hz). The average output power in Watts from an excimer laser is simply the product of the energy per pulse and the repetition rate divided by 1000.

Av. Power (W) = Energy (mJ) X Rep.Rate (Hz)/1000

For example a 20mJ excimer laser operating at 500Hz produces 10W of average power.

SPECTRAL PROPERTIES OF EXCIMERS

The spectral output of excimer lasers depends upon the specific gas fill and the selective properties of the laser resonator. In the simplest case of a free running excimer laser with a stable resonator, the spectral output is simply a property of the emission band of the excimer molecule. For Arf the output is a single peak with a width of approximately 500pm. KrF shows a main peak at 248.4nm approximately 350pm wide and two weaker, narrower bands near 250nm.

Excimer lasers can operate on a number of other wavelengths in addition to the commonly used ones. The table below is a partial list of other wavelengths which can be obtained:-

157nm*            Molecular Fluorine           10

222nm             Krypton Chloride             25

337nm             Molecular Nitrogen           5

428nm             Ionized Nitrogen               3

675-750nm       Atomic Fluorine                5

Excimer or 157nm Fluorine lasers can cut any solid material, from Diamond to the cornea of the eye. The rate of most excimer laser machining processes is determined by the material, the laser wavelength and the average power and /or the repetition rate of the laser, energy is usually not a determining factor.

Excimer lasers are typically used in machining materials which are hard to machine with other types of lasers, or where very high precision is required. Excimer lasers are also useful for cutting biological tissue where a clean cut is required without thermal damage to the surrounding tissue. Excimer lasers used in biological applications are often described as "cold lasers" for this reason.

The pulse length of excimer laser typically ranges from a few nanoseconds (nS) to just over 100nS full width half maximum (FWHM). This is a relatively short pulse length and leads to high peak power output from excimer lasers. For example a 50mJ laser with a 15nS pulse has an approximate peak power of 333kW.

The peak power density is a more relevant parameter for most applications. The peak power density is the peak power divided by the cross section of the laser beam. The peak power density depends upon the size of the laser beam at some focus. For example an 0.8cm X 0.3cm (unfocussed) beam cross section with the previously calculated 333kW has a peak power density of 1.4MW/cm² The same beam focussed to a 200 micron by 400 micron spot has a peak power density of 41GW/cm²

Two kinds of laser gas lifetime are defined. The static gas lifetime which is defined as the period of time the laser will drop to 50% of it’s specified energy when used infrequently (up to 30 minutes per day). The value depends upon the laser model and the active laser gas. A typical value for Krypton Fluoride in an EX50 laser is 12 weeks.

The dynamic gas lifetime is the number of pulses which can be obtained from the laser when operated in the constant energy mode at 50% rated power at maximum repetition rate, on a single gas fill. A typical value for KrF in an EX50 laser is 30 million pulses.