Calcium Fluoride Optics Rise to the Deep UV Challenge

Traditional methods for producing calcium fluoride material and components must be modified to meet microlithographic requirements.

By: David Collier and Wayne Pantley, Alpine Research Optics

Short-wavelength applications such as microlithography are creating a growing demand for high precision optical components that operate in the deep ultraviolet (DUV) spectral region (below 250 nm). Some of this need is being met with components fabricated from calcium fluoride (CaF₂), a crystalline material that transmits from 130 nm to 10 µm. Historically, CaF₂ has been used mostly for infrared (IR) applications. To meet the demanding requirements of short-wavelength applications, both bulk material and component-level CaF₂ manufacturers must modify their methods techniques (see Figure 1).

FIGURE 1: Calcium fluoride components must meet demanding performance specifications, both on a bulk materials level and on a component fabrication level (Alpine Research Optics).

Lithographic requirements
In microlithography, an optical system projects a high-reduction-ratio image of a circuit pattern onto a photoresist-covered silicon wafer. State-of-the-art lithographic steppers use krypton fluoride (KrF) excimer lasers operating at 248 nm. To meet the demand for ever-smaller features, manufacturers are already working to develop systems for 193 nm, and even 157 nm. Although fused silica components are adequate for 248 nm, refractive optical systems at 193 nm often use at least some CaF₂; at 157 nm, CaF₂ is virtually the only useful transmissive material available.

Although systems currently print 0.25-µm features, the goal of 193- and 157-nm systems will to be produce features smaller than 0.1 µm. To create a sufficiently high-resolution image over the widest possible field, a typical lithographic objective contains over 25 elements, some more than 1 foot in diameter. In fact, it is not uncommon for a projection lens to weigh over 100 lb, and cost in the neighborhood of $1 million.

Short wavelength applications put very stringent requirements on materials and fabrication. To minimize susceptibility to optical damage, as well as scattering and absorption losses that degrade throughput, bulk CaF₂ must be extremely pure and homogeneous. To reduce image degradation stemming from optical fabrication errors, the component surfaces must be flat and smooth to the level of tens or hundreds of angstroms rms.

Projection optics typically experience fluences of between 0.1 mJ/cm² to 1 mJ/cm² per pulse. Other system optics are subjected to fluences between 1 mJ/cm² to several mJ/cm² per pulse. Since a production line system might involve several million pulses per day, the cost of replacing the optics, not to mention the astronomical cost of lost production time, means that the optics must be able to withstand several billion pulses over their lifetime.

Improvements via process control
To meet the size, quality, and quantity requirements of microlithography, material producers are working feverishly to improve CaF₂. For example, Bicron (Solon, OH) has specifically focused their development efforts for excimer grade CaF₂ on the following areas:

· Improved DUV transmission
· Higher damage threshold
· Lower fluorescence
· Better index homogeneity
· Reduced stress birefringence
· Increased physical size
· Higher-quantity production

The naturally occurring, mined CaF₂ material that is acceptable for most IR applications is completely inadequate in regard to these parameters. Instead, synthetic CaF₂ is typically produced using the Stockbarger method. In this approach, a crucible containing molten material is slowly lowered through a "freeze-zone" where crystallization takes place. The entire process typically takes 6 to 8 weeks for a batch (see Figure 2).

FIGURE 2: In the Stockbarger fabrication method, molten material in a crucible is lowered through a temperature-controlled zone while it crystallizes.

Bicron has improved the Stockbarger technique, modifying its furnaces to create a freeze-zone that allows tighter control over crystal growth, and adding sophisticated process control instrumentation for better regulation of temperature, vacuum, and the annealing process. These latter factors are particularly important, as they govern material homogeneity and stress birefringence.

The results have been quite dramatic. Over approximately the past year, stress birefringence of production parts has been reduced from a common value of 20 nm/cm to just 1 nm/cm. These process innovations have also enabled Bicron to increase the physical scale of the material. The company now routinely produces 6 in.long CaF₂ ingots with 24 in. diameters, with the capability to reach 36 in. diameters (see Figure 3).

FIGURE 3: Process refinements have allowed manufacturers to produce CaF₂ ingots with diameters of 13 in. and 24 in. (Bicron, Inc.).

The primary factor affecting the transmission, damage threshold, color-center formation and fluorescence of the final crystal is the purity of the materials used. To this end, Bicron utilizes a variety of testing methodologies (spectrophotometry, fluorescence spectroscopy, etc.) to detect about 50 different contaminants during production of the precursor materials from which the CaF₂ is created. At each process step, specific impurities are targeted for chemical elimination. Ongoing development and rigorous application of these techniques at Bicron over the past year has lowered typical contaminant levels from 10 ppm to under 0.5 ppm.

Further material purification is also performed during the crystal growth process. Chemical scavengers placed in the crucible during crystallization react with any remaining impurities. In addition to the traditionally-used scavengers, Bicron has developed a number of new chemicals that are designed to react and produce gaseous by-products that can then be removed from the process by vacuum pumps.

Fabrication challenges
Meeting the performance demands of short-wavelength optics has also required development in optical polishing techniques. At DUV wavelengths, mid- and high-spatial-frequency surface roughness significantly degrades image resolution and contrast. Component flatness and adherence to optical prescription are also critical to performance—typical component tolerances for flat excimer laser optics are l/10 wave flatness at 193 nm, and 10-5 surface quality.

The challenge of meeting these specifications in CaF₂ is made even more difficult by the fact that it is an inherently difficult material to process. CaF₂ is hygroscopic, soft and very prone to chipping, particularly at the edges. Material that chips off during polishing may then be dragged over the soft surface of the component, leading to small surface scratches known as sleeks. As a result, fabrication time for CaF₂ is typically 50% to 100% longer than for glass.

Fabrication improvements
Manufacturers involved in polishing CaF₂ have each developed their own techniques for working with the material. At Alpine Research Optics (Boulder, CO), we have environmentally isolated the polishing area for CaF₂ from the rest of our production area. This prevents small airborne particulates from our glass and silica polishing operations from settling on the optics and causing scratches. Fine debris from CaF₂ polishing (mostly salt crystals from fluoride material) must also be continuously evacuated from the production area. The edge bevels of CaF₂ must even be smoothed or polished to minimize edge-chipping effects.

We use dedicated polishing equipment for CaF₂ because the material requires different polishing compounds than glass and silica. Diamond and sapphire grits are used at the initial stage because their hardness speeds material removal. Unfortunately, these compounds also cause sleeking, so their use requires a delicate timing balance.

Finally, we tend to work with pad-polishing rather than conventional pitch laps. This approach speeds up the process because polishing compound stays on the surface of the polyurethane pads, whereas it becomes embedded in pitch laps.

Performance testing
A group at MIT's Lincoln Laboratory (Lexington, MA), led by Mordechai Rothschild and Vladimir Liberman and funded by Sematech (San Jose, CA), is at the forefront of the effort to improve the testing of excimer-grade CaF₂ optics, especially under the conditions encountered in microlithography. In the Lincoln Lab test setup, the combined output from two 193-nm excimer lasers is split into 12 separate beams (see Figure 4). Each beam is then sent through three 80-mm-long samples; both CaF₂ and fused silica are tested simultaneously. The long path length helps accentuates bulk material effects, making them easier to separate from surface effects.

FIGURE 4: Twelve-beam test setup exposes optics to typical microlithographic fluences for extended periods of time, as part of a project to better understand material lifetimes and performance.

Each laser operates at 400 Hz, but by combining their output, an overall repetition rate of 800 Hz is attained. Approximately 70 million pulses are delivered over the course of a day, with a total exposure of about one billion pulses per month. Exposure fluences range from 0.3 mJ/cm² to 3 mJ/cm² per pulse along various beam lines.

One of the most interesting results obtained so far is that transmission actually increases after an initial period of irradiation. This occurs because the excimer laser is actually ablating some of the contaminants left on the surface from the polishing process (hydrocarbons adhere well to the surface of CaF₂ , making it harder to clean than glass).

Testing conducted over the past two years has helped manufacturers to achieve a steady improvement in material quality. For current excimer-grade CaF₂, transmission degradation at fluences below 1 mJ/cm² per pulse is not a major problem. At higher fluences, results are less clear. Because the testing conditions cannot be accelerated, the group has yet to accurately quantify total lifetime performance.

The most important conclusion drawn from the testing to date is that initial material measurements do not necessarily correlate with extended-use performance. Consequently, the consumer cannot simply purchase components for excimer laser applications based on manufacturer's nominal specifications. The long-term characteristics of the product must be established.

Microlithography advances have created an increasing demand for high volumes of large, high quality excimer laser optics. Process innovation in bulk-material and component-level fabrication, combined with comprehensive testing will ensure that CaF₂ manufacturers will be equal to the challenge.

About the authors…
David Collier is president and Wayne Pantley is sales manager of Alpine Research Optics, 3180 Sterling Circle, Boulder, CO 80301. Phone: 303-444-3420; Fax: 303-444-1686.