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Excimer lasers provide unique processing ability

29 Sep 2006

An excimer laser's high-power ultraviolet emission enables applications in research, micromanufacturing and biomedicine. Rainer Paetzel and Ruediger Hack provide an introduction to the technology and review key factors that potential buyers should consider.

Excimer lasers are unique sources of ultraviolet (UV) light. They deliver significantly higher pulse energy and overall average power than any other UV laser type. They also produce a wider range of wavelengths than any other commonly available UV laser offering output deep into the UV.

These specialized characteristics enable excimers to accomplish tasks that no other laser can perform. As a result, excimer lasers today support a diverse range of applications. These span everything from marking and micromachining tasks, human vision correction procedures (PRK and LASIK), fibre Bragg grating writing, silicon annealing for flat panel-display-manufacture, pulsed laser deposition, laser-induced fluorescence to semiconductor microlithography, to name just a few.

Excimer laser basics

Excimers are gas phase lasers, however, their construction, operation and output characteristics differ quite significantly from that of most other gas lasers. This is primarily because the inherent gain of excimer media is so much higher than for other gas lasers and is also due to the unusual requirements for pumping these media.

An excimer plasma discharge tube uses a mixture of three different gases: a halogen (either F2 or HCl), a rare gas (such as Ar, Kr or Xe) and a "bath" gas (Ne or He). The plasma tube is filled to a much higher pressure (typically around 400 kPa) than in other laser types. Because this relatively high pressure translates into high electrical resistance, the gas is first pre-ionized to produce a smooth, homogeneous discharge.

A discharge is created by applying a high voltage pulse (up to about 40 kV) across two parallel electrodes that run the length of the plasma tube. The resultant plasma contains a high concentration of excited dimers (hence the name excimer), such as KrF, ArF, XeF and XeCl. These transient chemical species produce a short, intense pulse of stimulated UV radiation as they decay back to their components. The typical pulse duration of an excimer laser is in the 10–50 ns range and excimer lasers can generally be operated at repetition rates from a single shot to the few kilohertz level (see "A rough guide").

Because of their extremely high gain, excimer laser resonators are typically configured so that the beam only makes a few passes through the cavity. In fact, unstable resonator designs are not uncommon, and the output couplers of excimer lasers usually have a reflectivity of between 10 and 50%. Thus, the output beam shape of an excimer laser is essentially defined by the cross-section of the plasma, which is in turn determined by the configuration of the electrodes. The main design goal of the cavity optics is to use as much of the plasma volume as possible to maximize output pulse energy.

A typical excimer laser beam has a large, rectangular cross-section (about 8 × 20 mm), with a near-Gaussian profile in the short axis, and a "top hat" profile in the long axis. Top hat refers to a beam intensity with an extended central plateau that falls off near the edges. Beam divergence in the long axis is usually several milliradians, which is many times the diffraction limit. An excimer laser's linewidth is relatively large and the coherence is low, unless additional line narrowing elements are placed in the resonator.

Excimer laser advantages

The unique output characteristics of an excimer laser offer distinct advantages in many applications. For example, UV wavelengths enable processing at a higher spatial resolution than visible and IR lasers. This is because the smallest feature size that can be produced is limited by diffraction, and diffraction increases linearly with wavelength.

The UV photons produced by an excimer also interact differently with most solid materials (especially organics) compared with longer wavelength photons. The focused beam from a visible or infrared laser processes a material by heating it until some has boiled off or vaporized. Typically, this heating also affects the surrounding material that has not been directly irradiated, resulting in peripheral thermal damage and less precise process control. In contrast, the inherently high energy of UV photons causes them to directly break the atomic or molecular bonds within a material in a process called photoablation. With short laser pulses, this can be a relatively cold process with little or no effect on the surrounding material.

This ability to process with a very high spatial resolution is further enhanced by the fact that most solid materials have very high absorption in the UV. As a result, the laser light only penetrates a very shallow depth into the material. This, along with the short pulse duration, means that each pulse removes just a thin layer of material, thus providing excellent depth control.

Practical considerations

The combination of highly corrosive gases and a high voltage discharge makes the interior of an excimer laser resonator an inhospitable place. Because of this, early excimer lasers were plagued by corrosion problems and needed frequent gas re-filling, as well as optics cleaning and replacement. They also suffered from a limited tube lifetime.

However, this situation has changed over the past decade as excimer laser manufacturers have made tremendous advances in negating electrochemical corrosion effects and maximizing product lifetime. This has greatly extended the intervals between all regular laser maintenance functions such as gas fills, optics cleanings and, ultimately, laser-tube replacements, all of which dramatically lower the total cost of ownership. It is important to note that both the service intervals and lifetime of excimer lasers are measured by pulse count, rather than operating hours (although the static lifetime for the gas fill is measured in weeks).

The impact of these lifetime, reliability and maintenance improvements depends upon the type of application. Typically, most scientific and research applications will use a laser at a relatively low overall duty cycle. For example, a researcher who runs the laser at 10 Hz for five hours a day, five days a week, accumulates a total of 47 million pulses a year. Under these conditions, the first optics cleaning interval, which might occur at one billion pulses, will essentially never be reached. For this type of user, initial purchase price is probably a more significant factor than laser operating costs. Also, downtime for periodic gas refills probably has no real economic impact in this environment.

In contrast, an industrial user who operates a laser at 300 Hz for two eight-hour shifts, five days a week, accumulates one billion pulses in less than 12 weeks. Here, the cost of consumables (especially laser tubes) is important in the overall cost of ownership. In some industries, production-line downtime itself has a substantial cost. Industrial users should consider the time needed to perform various maintenance tasks when evaluating a laser. Other concerns include the laser's stability and output consistency, as variations in these may manifest themselves in the quality of the final product.

The particular output characteristics, reliability, lifetime and mean time to repair of an excimer laser all depend on the specifics of its design and construction. Because of the potential economic consequences of these factors, the buyer of an excimer laser should be more aware of how a particular product is made (and serviced) than for other laser types.

All metal–ceramic plasma tube construction provides the highest resistance to corrosion and therefore the longest overall tube lifetime. This has become standard in the industry and should be considered a prerequisite, even for low-duty-cycle users.

The particular pre-ionization method used is also important and there are several different schemes currently in use. Spark pre-ionization offers the highest pulse energy, but spark pin erosion limits tube lifetime and creates dust contamination. A substantial improvement to this approach is achieved through the ceramic sliding discharge method: a discharge is diffused across a ceramic bar instead of directly sparking on to pins. This delivers a smoother discharge and eliminates pin erosion as a failure mechanism. This method is currently used in many high-energy and mid-sized excimer lasers. Corona pre-ionization uses a dielectric material arranged parallel to the electrodes in order to avoid high peak currents. This delivers a high degree of beam homogeneity and is mostly found in lower-power lasers.

Tube lifetime and optics cleaning intervals can also be extended by gas circulation and filtration systems that remove chemical by-products and dust particles as soon as they are generated. In addition, some excimer lasers are configured to enable optics cleaning without opening the resonator interior, thus saving time and significantly extending the lifetime of the tube.

Some of the more sophisticated products on the market feature a high degree of closed-loop microprocessor control, automatically adjusting laser operating parameters to maintain a consistent output. For example, the laser computer may automatically adjust the laser voltage as the gas fill ages in order to deliver constant output power. In some industrial applications, processing is accomplished using a burst of pulses, and process consistency depends on accurately maintaining the total energy dosage. Typically, the leading pulses of each burst show a systematic deviation from the set point and this can be corrected by actively monitoring energy output and using software algorithms to provide feed-forward control.

In conclusion, choosing an excimer laser for a specific task requires an understanding of how they are manufactured and serviced. Specifically, consumers should familiarize themselves with lifetime, service, total-cost-of-ownership and ease-of-use considerations before making a purchase.

• This article originally appeared in the September 2006 issue of Optics & Laser Europe magazine.

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