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Calcium fluoride offers unique optical properties

21 Jun 2007

The rapid progress that is being made in lithography and many other areas of technology owes a lot to calcium fluoride crystals. Peter Maushake, product manager of German firm SCHOTT AG, looks at the optical properties that have made this material so successful.

Calcium fluoride (CaF2) is a unique optical substance that occurs naturally as the mineral fluorite. The material has a number of important optical properties including excellent ultraviolet (UV) transmittance; a high laser-damage threshold; low axial and radial-stress birefringence; and high refractive-index homogeneity.

The extremely high laser-damage threshold of CaF2 has made it the material of choice for use in excimer laser optics. It has also become very important as a UV optical material for integrated circuit lithography in beam delivery, illumination and projection systems that now operate down to the 45 nm node.

As well as microlithography, this single-crystal material is often used in a variety of infrared (IR) and UV applications because it is highly transmissive in these wavelength regions and has an extremely low birefringence. CaF2 is both physically stable and chemically inert, which means that optical windows made from it are highly resistant against any impact.

Out of the fluoride crystal family of materials (including barium fluoride, magnesium fluoride and lithium fluoride), CaF2 is known to be one of the hardest. This is yet another reason why it is particularly well suited for use in manufacturing a wide range of different lenses, prisms and mirror substrates. CaF2 is commonly manufactured in different qualities or grades, i.e. for visible, IR and UV applications. UV-grade CaF2 requires a much higher level of material purity compared with IR and is therefore considerably more difficult to produce. The UV-grade version of the material is manufactured using a complex precipitation process and is mainly used in advanced optical parts for microlithography and laser applications.

From a powder to a crystal
CaF2 blanks are manufactured in a variety of sizes to meet a customer's exact specifications. The starting material is a refined CaF2 powder that has to be as pure as possible. The first step is to melt the powder in a furnace. Next is a crystallization process and the resulting crystal is then tempered down to room temperature.

At high temperature, CaF2 crystals are weak and very sensitive to stress. For this reason, the temperature ramp-down must be carried out very slowly to minimize thermal gradients. As the crystal begins to cool and gain strength, the tempering rate can be increased. The entire process is rather complex and takes several weeks but annealing is an important, albeit time-consuming, step. To further decrease the stress level, a secondary tempering cycle is often applied using a separate annealing furnace once the blank has been cut from the boule. This can easily add several weeks to the overall manufacturing process.

Because leading crystal manufacturers design and build their own furnaces, much of the proprietary information on CaF2 crystals resides in furnace design and process recipe. The latest generation of vacuum furnaces can currently fabricate CaF2 blanks at diameters of up to 300 mm with highest transmission rates at 193 nm and below.

A polished CaF2 surface can be expected to withstand several years of exposure to normal atmospheric conditions. Low solubility and wide transmission make CaF2 extremely useful for a variety of different applications, including mirror substrates for UV laser systems, windows, lenses and prisms for UV and IR applications.

CaF2 is also able to withstand maximum temperatures of up to 800 °C in dry atmosphere. In fact, this material is used quite commonly in cryogenically cooled thermal-imaging systems. Due to its high transmission, it is also used in spectroscopic windows and lenses.

Minimizing absorption is the key to reducing the loss of energy and lens heating. Thanks in particular to their low absorption, CaF2 crystals are frequently used in high-power laser optics. Their polished surfaces remain stable and last for several years under working conditions.

At 193 nm, the best grades of CaF2 approach an absorption per centimetre thickness of <0.0005cm-1 (base 10). Depending on laser parameters (such as fluence), long-term tests reveal no significant change in absorption, even after billions of pulses. Because CaF2 is a crystalline material, it is unlikely to suffer from the compaction and rarefaction that is seen in fused silica.

Homogeneity of refractive index is yet another important parameter for a lens material. Here, the best grades of CaF2 meet the requirements of less than 1 part-per-million of index variation.

Lithography leads the way
Due to its unique characteristics, CaF2 undoubtedly ranks as the material of choice for producing lithography lenses. As the relentless march to higher-productivity semiconductors continues, optical technologists are pursuing shorter wavelength exposure sources in an effort to achieve smaller line widths and faster and smaller devices.

Wafer exposure systems initially started with an exposure-source wavelength of 436 nm. As technology advanced, the exposing wavelength rapidly moved to 405 nm, and then to 365 nm, the very limit of what the human eye can see. The push for shorter wavelengths continued beyond visible limits to 248 nm using a krypton-fluoride laser and finally to 193 nm with an argon-fluoride laser.

For a time, there was a significant market for CaF2 as only CaF2 lenses can be used in conjunction with fluorine laser technology at 157 nm. However, this step has now been removed from the roadmap and 193 nm technology forms the main application.

Semiconductor exposure systems at 193 nm use CaF2 for a small number of elements. However, this percentage will increase as the numerical aperture of the lens increases. The demand for CaF2 193 nm lenses is set to increase dramatically as new high-numerical-aperture lenses are introduced.

Today's steppers require lens blanks of up to 10 inches in diameter. This places a strong demand on yield because defects generally limit the number of large blanks that can be cut from a single boule.

It's not just UV-based applications that benefit from the optical properties of CaF2. In the IR and visible wavelength ranges, CaF2 optics are finding uses as zoom lenses in astronomy and photography, as well as in microscopy.

Lenses used in commercial television cameras have now become one of the strongest optical markets for CaF2 crystals. Although this application is less demanding in terms of absorption at short wavelengths, it requires a substantial volume of material and thus competes with semiconductor applications.

CaF2 is also used extensively for laser windows at UV wavelengths. This generally requires smaller sizes than the lenses used in microlithography.

Looking forward
Today, it is extremely important that the various sectors of the semiconductor industry, such as device makers, tool suppliers, material suppliers, laser manufacturers, universities and national labs, share their understanding of CaF2 issues and solutions, so that each company does not have to define issues and invent solutions on its own. Customers are well advised to discuss their exact needs and technical requirements with the crystal manufacturer and all other parties involved at a very early stage.

The industry has already made significant progress on CaF2, yet the material still offers immense potential for further advancements. Thanks to increased furnace capacities and capabilities, material manufacturers will be in a much better position to come to a consensus on, and to address with confidence, the actual demand for CaF2 crystals. Improving yields and quality to an even higher level will remain the key challenge in the future. Leading crystal manufacturers already have programs in place aimed at upgrading the yields and should be able to meet the demands of toolmakers in the future.

• This article originally appeared in the June 2007 issue of Optics & Laser Europe magazine.

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