DAMAGE RESISTANT OPTICS FOR EXCIMER BEAM DELIVERY

Long life, damage resistant coatings are improving the economics of commercial excimer applications.

By David Collier and Wayne Pantley, Alpine Research Optics

David Collier is President and Wayne Pantley is Sales Manager of Alpine Research Optics, 3180 Sterling Circle, Boulder, CO 80301, 303-444-3420, FAX 303-444-1686, E-mail AROcorp@AROcorp.com, http://www.optics.org/arocorp/

Introduction

The excimer laser's unique combination of output characteristics make it an ideal choice for a wide variety of precision materials processing and biomedical applications. Advances in excimer laser reliability and economy over the past few years have now made it a practical source for many of these applications; in fact, excimers represent one of the fastest growth areas in the entire laser marketplace. However, until recently, optical damage to the beam delivery optics, and to the coatings in particular, has limited excimer based system reliability. In this article, we will examine how optics manufacturers have addressed this problem, extending typical coating lifetimes by over an order of magnitude. We will also see how new research programs should lead to even further improvements in damage resistance.

Excimer Applications

To understand the economic impact of optics failure, let's consider the two fastest growing applications areas for excimer lasers - photorefractive keratotomy (PRK) and microlithography.

In PRK, beam uniformity and pulse energy must be tightly controlled to ensure a successful surgical outcome. These parameters are directly affected by any degradation in the multilayer dielectric coatings on the mirrors and beamsplitters used in the beam delivery system. Until recently, many systems in the field required several optics to be replaced during scheduled service and maintenance, with a typical service interval of 4-6 months. Also, the total cost of the system optics (up to $10K) is quite significant, compared for example, to the typical service contract costs of around $40-50K/year.

George Caudle is a Staff Laser Scientist at Visx, a leading supplier of PRK systems. He explains, "At first, optics lifetimes definitely represented a "weak link" in the system, that we felt could be improved upon by working with vendors of these optics."

After years of using successively shorter mercury arc lamp wavelengths, microlithography is now turning to excimer lasers to provide the resolution for the next generation of integrated circuits (IC's). The retooling of IC manufacturers with 248nm krypton fluoride systems is now well under way with sales of several hundred systems predicted for 1997. In addition, prototype systems based on 193nm have already been developed and are expected to arrive on the production floor within 3-5 years.

Microlithography uses lower fluences (1-50mJ/cm2/pulse) than PRK (>100mJ/cm2/pulse). Yet optics failure is much more of a problem for this application because of the economics of continuous equipment operation. Simply stated, these machines are expected to operate virtually around the clock, and even routine (monthly) scheduled maintenance time is minimized. As a result, the total number of laser pulses is astronomical; a 1kHz laser delivers over 1 million pulses in less than 20 minutes.

Estimates for total downtime costs for an IC manufacturer range up to several hundred thousand dollars/hour for a 248nm system. These systems cost over $6 million each, whereas 193nm systems are expected to cost up to $15 million. Operational economics will be similarly scaled up, and downtime is therefore projected to cost over $1 million/hour!

Defining Optics Failure

In the early 1990's a typical 193nm turning mirror used in a PRK beam delivery system had a useful lifetime of 0.5 million pulses or less. What do we mean by useful lifetime? With high energy optics used for pulsed Nd:YAG lasers at 1064nm, optics failure has usually meant catastrophic failure - the coating is pitted, burned or partially blown off the substrate. In the case of short wavelength excimer photons, damage mechanisms are different, and performance deterioration may occur a long time before catastrophic failure. For a high reflector, failure has therefore been defined by a decrease in reflectivity to some arbitrary level.

For both PRK and microlithography, a 5% decrease in reflectivity often constitutes the definition of failure. In the case of PRK, a decrease in coating uniformity of more than a few percent would also constitute failure. The 5% value derives from the large number of optics which comprise a system. A typical lithography system may use as many as 12 fold mirrors just for the laser projection section - and this doesn't even include those used for remote laser delivery. A 5% drop in reflectivity on 12 mirrors used serially results in a 50% decrease in delivered laser energy. In lithography, decreased pulse energy slows the exposure process, costing the end user time and money.

In addition, excimer laser damage is an accelerated process - a 5% drop in reflectivity will quickly become a 10% decrease. End user economics dictate that the optics be replaced as soon as they begin to show noticeable (-5%) signs of deterioration - and certainly long before catastrophic failure is even a possibility.

Improving Coatings

In 1994, we began a program at Alpine Research Optics to develop longer life coatings for both 193nm and 248nm. The first obstacle we encountered was a lack of information and basic understanding about both the mechanism(s) leading to damage and failure, as well as reliable testing protocols to certify batches of optics. It was soon apparent that what we knew about producing long-life laser mirrors for the visible and near-IR didn't apply in the case of the deep UV.

For example, with pulsed Nd:YAG lasers at 1064nm and 532nm, it is well known that coating damage is related to peak power. Indeed, lamp-pumped Nd:YAG lasers with beam hot spots are notorious for quickly producing burns and pits in both coatings and substrates. Thermomechanical stress, choice of materials, and poor substrate preparation or cleaning techniques, all play a role in this type of catastrophic damage.

In contrast, excimer laser damage appears to correlate more closely with total accumulated flux. The operating atmosphere also plays an important role. Visx's Caudle notes that "We used to see significant optics lifetime differences between individual medical facilities. We suspected this was due to differences in air quality. This was confirmed by enclosing the optics in a closed filtered environment, which significantly reduced these site-specific performance anomalies."

Possible mechanisms for decreased coating reflectivity include photooxidation induced refractive index changes and/or absorption increases, problems with the coating/substrate interface, and solarization of the coating layers. Fortunately, research programs have now been funded to provide definitive answers (see side-bar) to the damage question.

In the area of testing, the qualifying specification for excimer optics used to be resistance to several high energy pulses (2J/cm2). This is perfectly acceptable for Nd:YAG optics but fairly useless for 248 and 193nm excimers. Indeed, as our work progressed we found that coating designs could be independently optimized for either high peak power resistance or longer life.

With no rapid testing method and no definitive information about failure mechanisms, we had to partner with our OEM customers. Over a two year period, we systematically refined coating designs and chamber conditions in an iterative manner - relying on feedback from these end users about the actual performance of each batch of coatings.

Improving the 248nm coatings proved to be a much simpler proposition than those for 193nm. The principal reason is that hafnium oxide is a well understood coating material that can be used for the high index layers. But HfO2 begins absorbing at l<226nm, so this couldn't be used at 193nm. Instead, we had to develop a unique, proprietary design based on a combination of fluorides for both the high and low index layers. However, we decided not to use thorium fluoride, a material sometimes used by others because of its desirable optical properties. We avoided thorium fluoride because it is a toxic, radioactive material which requires EPA and OSHA approved handling and disposal.

We also found that one of the most critical parameters in producing stable coatings is the substrate temperature during deposition. If the temperature of the optic is too low, the fluorides deposit poorly with unpredictable refractive index and shortened lifetime. On the other hand, if the temperature is too high, the coatings tend to craze as the optics are cooled.

Finally, it may surprise some readers to learn that we use a conventional e-beam to evaporate the fluoride coating materials. Several techniques, most notably ion-assisted deposition, have been developed in recent years, with the goal of improving layer hardness and consistency. But in the case of the fluorides used for deep UV mirrors, ion-assisted deposition typically increases the absorption at these wavelengths, thereby actually reducing the lifetime of the optic.

The other advantage of e-beam evaporation is that good material distribution and consistency can be obtained throughout the coating chamber. By ensuring similar uniformity of substrate temperature, we are able to produce excimer beam delivery optics in batches of 1-100 with diameters up to 8".

Results

Continuous incremental improvements to our coating process have now increased 193nm mirror lifetime from 0.5 million to 5 million pulses at 200mJ/cm2, as certified by Spica Technologies. As Visx's Caudle notes, "We have been shipping our latest model PRK system for about 18 months. In that time we've seen a dramatic reduction in optics failure."

In addition to the improvements summarized, we have found that these coatings can be optimized for high peak power usage or extreme lifetime. This bodes well for 193nm lithography, given that this economically important application will use 10X lower pulse energies than PRK.

Of course, longer life, more reliable optics will now benefit all excimer users, from plasma physicists to atmospheric chemists to micromachining job shops. After years of the commercial sector being serviced by trickle-down products from research and aerospace applications, we believe that this reverse process is representative of an important trend - indicating the continuing maturation of the electro-optics industry.

Damage Research and Testing

Producing long-life high reflectors for excimer applications has been hampered by the lack of both definitive information about failure mechanisms, and lifetest data and models. Work now being conducted in these areas should soon lead to further significant improvements in the reliability of these optics.

Companies such as Spica Technologies Inc. (Nashua, NH) now offer rigorous life testing of optics as a service for both manufacturers and end users. According to Mike Thomas, Chief Scientist and President of Spica, "At present, true lifetesting of these optics is essential. In the past we have avoided this type of testing because it is very time consuming and requires several dedicated excimer lasers. A recent cooperative agreement with J.P. Sercel Associates, Inc. (Hollis, NH) however, combines their excimer resources and expertise with our knowledge and experience of testing optics. This has enabled us to offer the first commercial life testing of excimer optics from 157nm - 308nm.

"As we collect additional data, we hope the results of these experiments will lead to the development of a life test model. This may allow us to reliably extrapolate shorter test data, which will be to everyone's advantage."

The testing and manufacturing of these optics should be further simplified once the damage mechanisms are completely understood. One of several research programs aimed at this problem was recently funded by SEMATECH at MIT Lincoln Laboratory. This group plans to use a variety of optics techniques to monitor the changes in microscopic and macroscopic properties of single and multilayer coatings. "Most of the existing research on excimer damage of optics has focused on the bulk materials," according to Lincoln Lab's Mordechai Rothschild. "With multielement lithography lenses for 193nm costing several million dollars each, naturally attention has been directed on finding long life substrates for the lenses. But now, the importance of prolonging the life of their AR coatings and the reflective optics has been fully recognized."