calcium-fluoride caf2 Infra-Red, UV, VUV, Eximer, Raman

Calcium Fluoride (CaF2) Windows

Below are just some of our CaF2 substrates that we have in stock:

Link to optec

• (100), 10x10x 0.5 mm 2 Sides polished
• (100), 10x10x 0.5 mm 1 Side polished
• (100), 10x10x 1.0mm , 1 Side polished
  
• (100), 10x10x 1.0mm , 2 Side polished
  
• (111), 10x10x 0.5mm , 2 Side polished
  
• (111), 10x10x 1.0mm , 1 Side polished
  
• (111), 10x10x 1.0mm , 2 Side polished
  
• (111), 1"x1.0 mm , 2 Side polished
  
• single crystal for evaporation, purity >99.995%, 5x5x5 mm as cut
  

Sputtering Depostion

Clients use the following CaF2 specs to sputter in the TIFR lab.

Calcium Fluoride Crystal Substrates

• CaF2, (100), 10x10x 0.5 mm 1 Side polished QTY : 10 no
• CaF2, (111), 10x10x 0.5mm , 1 Side polished QTY : 10  no
• CaF2, (100), 2 inch dia x 0.5 mm 1 Side polished  QTY : 1 no
• CaF2, (111), 2 inch dia x 0.5mm , 1 Side polished  QTY : 1 no

CaF2 for FTIR

Researchers use CaF2 for Fourier-transform infrared spectroscopy (FTIR). This method obtains the infrared spectrum of absorption or emission of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high-spectral-resolution data over a wide spectral range.

Material Safety Data

MSDS avaialable upon request.

Can Fajan's Rules Explain Why Calcium Fluoride Has a Lower Melting Point Than Calcium Oxide?

Can Fajan's rules explain why Calcium Fluoride has a lower melting point than calcium oxide? The answer is yes, but the process of comparison is not straightforward. The difference in the compounds is due to the charge distribution. In general, a compound with a higher charge has a higher melting temperature than a compound with a lower charge.

According to Fajan's rules, a molecule's covalent character depends on its electrostatic force and effective nuclear charge. The latter, which is the most important factor, is related to the size and relative charges of the cation and the anion. As the size increases, the effect of oppositely charged ions is lessened. Thus, the melting point of CaF2 is lower than that of its cousin, iodide.

An electronegativity difference between a cation and an anion is a good predictor of the type of chemical bond. A greater difference in electronegativity between two molecules means that the bonds are more polar. Therefore, Linus Pauling proposed an empirical relationship between percent ionic character and the difference in electronegativity between the two molecules. This is shown in the red curve below.

Can Fajan's rules explain why CaO has a lower melting point than CaF2? This question is controversial but is a common question among chemists. For example, the difference between the two compounds in terms of their molecular structure is the primary factor behind the difference between their melting points. If a substance has a low molecular weight, it will have a lower melting point than one with a high molecular mass. If it is larger, it will be more polar.

Can Fajan's rules explain why CaO has a lower melting point than CaF2? In addition to being covalent, CaF2 is also an ionic compound. This means that it contains a cation that is conjugated with a base, while a cation that has a high charge has a lower charge than a cation.

The rules based on the Effective Nuclear Charge and Electronegativity can explain why CaF2 has a lower melt point than CaO. By comparing the ionic nature of the two molecules, they can understand why the former is warmer than the latter. And the answer to this question can be found in a number of other ways. For example, an ionic compound contains more potassium than CaF2.

As a weak acid, CaF2 has a lower melting point compared to its sister, the latter has a higher melting point than the former. An ionic compound is an acid that has a smaller melting point than CaO. An ionic compound can have a lower melting temperature than a cation with a higher charge.

The chemical properties of CaF2 are a result of its ionic charge. In contrast to calcium, strontium ions are larger than calcium and fluorine. This means that $ceCaF2$ will have a higher bond strength than CaO. A higher bond strength means more energy is required to break it. It is thus essential to understand the differences between the two compounds in order to understand how they differ.

In general, ionic charge can be determined through the electrochemical equation. Hence, the electronegativity of a compound will help you determine its ionic character. The greater the difference, the higher the degree of polarity of the bond. If the difference is greater, it indicates that the bond is less ionic. If it is smaller, the chemical reaction will not occur.

An ionic compound is an ionic compound. If the ionic compounds were liquids, they would not be polar. This is because ionic compounds have a low melting point. The opposite is true when the ionic charge of a compound is large. This results in a higher polarization. Moreover, the difference between the ionic charge and the ionic capacity of a substance is smaller.

What is Polycrystalline Calcium Fluoride?

You may be wondering what is Polycrystalline Calcium fluoride (PCF)? This article will provide you with the basics of this important material. You will learn about the uses for PCF, as well as how it is manufactured. The benefits of PCF are also discussed. This mineral can be used in the manufacture of a number of different products. For example, it is used to create a wide range of plastics.

Spectroscopic CaF2 Windows

Spectroscopic CaF2 windows have excellent transmission properties from 130nm to 10um and have numerous applications in a variety of fields, including white light generation. These windows are highly efficient at transmitting light, and their thickness ranges from 0.5 to 100 mm. They are available in three different grades, including regent grade, Eximer grade, and Raman grade, and are designed to work at specific wavelengths.

Spectroscopic CaF2 windows offer several advantages over other materials. Because they exhibit excellent transmission in the visible and ultraviolet, they can also be used in laser applications. Their low density, wide transmission range, and excellent machinability make them ideal for optical components. Furthermore, their low thermal conductivity make them a highly cost-effective material for a variety of applications.

Spectroscopic CaF2 windows were created by scientists at Sandia National Laboratories, a multi-mission laboratory run by the U.S. Department of Energy and the National Nuclear Security Administration. These windows provide an ideal optical window through which researchers can study irradiation processes. Detailed information about the irradiation path of CaF2 has now been published.

Using spectroscopic techniques, researchers have studied the effects of 100 keV Tb ion implantation on polycrystalline CaF2. They observed that the ionic state of Tb is not affected by coexistence of the secondary phases. This results in improved antireflective coatings and down-conversion of light. The spectral range of the conversion is also expanded, leading to greater conversion efficiency.

Another method for making optical windows of polycrystalline calcium fluoride is through the use of ion beams. By using a grazing incidence, 100 MeV ions irradiated on CaF2 can create novel ion-tracks. This allows researchers to track the forces to the surface in an atomized state. The resulting ion-tracks, which are visible under spectroscopy, reveal three distinct parts. During the first stage of the process, a fast heavy ion opens a groove that spans 100aEUR

Vacuum Ultraviolet Scintillator

A new spectrophotometer for measuring the transmittance of Polycrystalline Calcium fluoride crystals has been developed. This new device offers several advantages over the conventional method of direct measurement using F2 or excimer lasers. Among these benefits, its ease of use, accuracy, and low cost make it an excellent choice for internal transmittance measurements. This technology is applicable for measuring light wavelengths and transmittance of many materials, including polycrystalline Calcium fluoride.

The light emitting element of the present invention is a metal fluoride crystal represented by the chemical formula LiM1M2F6. Li includes six Li and M1 and M2 represent alkaline earth metals and metal elements. Eu2+ is at least 0.02% by mole in this material. The resulting product efficiently volatilizes without remaining trapped in the polycrystalline Calcium fluoride crystal.

This chemical compound can be highly sensitive to moisture above 500degC. Because of this, barium fluoride is not ideal for a vacuum environment. However, it is a good candidate for electrochromic filters. It is also highly resistant to X-rays and is used in a wide variety of applications. It is useful in research and medical applications and can be used in a wide range of environments.

The chemical composition of calcium fluoride crystals is an important factor in the quality of the material. The material can be produced in many ways, including by hot-pressing. When the polycrystalline material is heated, it recrystallizes and retains its optical properties. However, the heat and force involved in manufacturing it makes it prone to absorption bands and may not be suited for certain applications.

Lead Conversion Into Polycrystalline Calcium Fluoride (CaF2)

Polycrystalline calcium fluoride is an important component of a variety of ceramic materials, including ceramic tiles, sand, and marble. The process of lead conversion into polycrystalline calcium fluoride involves the chemical reaction of lead and calcium fluoride. During this process, the lead is oxidized to a neutral salt, which is called calcium fluoride. CaF2 nanoparticles contain Yb,Er, and Tm.

Fluoride exposure affects the physiochemical and structural properties of bone. It also impacts calcium control in rabbits. These are just a few of the detrimental effects of fluoride, but the effects of Fluoride on bone are well documented. The findings from the present study will help guide future research into the toxic effects of fluoride on human health. Calcium supplements, in particular, are essential for a number of reasons.

Light Transmittance

In order to investigate the light transmittance of Polycrystalline Calcium fluorides, the researchers irradiated the samples with a F2 laser and then measured their spectral transmittance. To accomplish this, the crystals were optically ground with diamond grains to prepare them for the F2 laser. The wavelength of the laser used in this study was 890 nm. As such, the calcium fluoride crystals showed very high light transmittance.

In order to improve the light transmittance of the Polycrystalline Calcium fluoride crystal, the material is heat treated to reduce the thermal stress. This crystal is then cut and processed to form an optical member. The light transmitted by the optical member is approximately 82%. It should be noted that the optical performance of this material decreases with increasing distance from the light source. This is because it loses 1% of its intensity with every 10 mm of length.

The internal transmittance of Polycrystalline Calcium fluorides was comparatively high, although the spectroradiometer's accuracy was poor. The light transmitted through the material decreased with decreasing thickness, and the internal reflectance was 99.5% versus 100%. Both samples showed very similar internal transmittance, and the difference was only 0.1% for the Comparative Example. This resulted in a highly inconclusive comparison.

Crystaltechno Ltd. is a company that manufactures crystals and optical components for use in a variety of applications, including medical technology, laser research, and aviation. Its products are used in special equipment, and their light transmittance spectra are extremely broad. The company is currently developing a new range of optical components, which include laser lenses and other optical components. This allows it to serve as a versatile, low-cost alternative to other types of materials.

What is Calcium Flouride's Melting Point?

The process of making Polycrystalline Calcium fluoride begins with a preprocessing furnace. The furnace is maintained at a vacuum of 10-3 to 10-5 Pa and gradually increases the temperature until it reaches the melting point of calcium fluoride. Then, the material is placed into the growth crucible, where a single crystal is grown. As the growth furnace heats up, the desorbed gas in the preprocessed material is mixed with the growing crystal.

This process requires a vacuum furnace. The pressure inside the furnace is slowly increased to maintain the high vacuum. The melting point of polycrystalline Calcium fluoride is around 700 degrees Celsius. At a temperature higher than this, the carbon compounds on the surface of the powder begin to decompose. The upper limit of the furnace is degrees C. During this process, the temperature is monitored to ensure that the final product is stable.

The spectral absorption coefficients of calcium fluoride were determined using a single-beam ir spectroradiometric system. The wavelength range was 2-12 microns, and the temperature was changed from 500 degrees C to 600 degrees C. The finalized ceramics were characterized by X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy.

The final temperature of polycrystalline calcium fluoride is approximately 100 degrees above the decomposition temperature. The crystallization process is a complicated process. A mixture of calcium fluoride powder and a scavenger is mixed with the starting material. Then, the material is placed in the growth furnace. The temperature inside the furnace gradually rises until the desired melting point is reached. The crystals are then formed.

What are The Benefits of Calcium Fluoride Wafers?

Choosing the right dental care for your teeth has never been easier thanks to the variety of fluoride products that are available in the market. These products come in the form of wafers, gels, and mouthwashes. Some are cheaper than others, and some are more stable than others.

Calcium fluoride is cheaper than natural fluorite

Using calcium fluoride in water supplies has been a controversial practice. There are many arguments as to why this is a bad idea. Some argue that the calcium ions in the water are toxic and should be removed. Others claim that the fluoride is harmless and useful for teeth.

The truth is that fluoride is a mineral that is used in a variety of industries. It is also used for jewelry and ornamental carvings. It is also used in glass manufacture. The manufacturing of these materials requires a high purity of calcium fluoride.

There are two types of fluoride; natural and industrial fluoride. Natural fluoride is naturally occurring and is found in some water supplies. Industrial fluorides are made from fluorine and fluoride. Industrial fluorides are fully absorbed by the body. The difference between natural and industrial fluoride is that the natural types contain a natural metal cation. Industrial fluorides contain a synthetic fluoride ion.

Industrial fluoride is used for drinking water and in many industrial applications. It is also used for dental treatment. The crystalline form of fluorite contains a calcium ion and is a valuable mineral in many industrial applications.

Fluorite is also known as murrina in the Latin language. It is found in various areas of the world including China, Argentina, Tanzania, South Africa, Canada, and the United Kingdom. The mineral has a variety of colors, including reddish purple, white, and brown. The color is determined by the substitution of calcium for other elements in the crystalline structure.

Calcium fluoride is often used as a window material for infrared and ultraviolet wavelengths. It is also used for semiconductor manufacturing. It has a non-linear refractive index at high power densities. It also has deep coloration due to impurities. It is used to make optical elements that help reduce chromatic aberration.

Natural fluorite has a variety of uses including smelting flux and optical components. It is used as a precursor for HF. It can also be used to make glass and enamels. It is not used widely as a semiprecious stone.

The production costs of mining fluorite are much higher than those of naturally occurring fluorite. Most of the mining took place in underground mines at depths of more than one thousand feet.

Calcium fluoride is more stable than its counterparts

Unlike other fluorides, calcium fluoride is relatively stable and durable in normal atmospheric conditions. This is beneficial for applications in photovoltaic cells and lasers. Its optical quality, high transmission range, and low laser damage threshold makes it a good choice for optical components.

Calcium fluoride is a white insoluble mineral. It is produced by adding hydrogen fluoride to calcium carbonate. It has a refractive index of 1.361.42, a high dielectric constant, and a high band gap. It is a common substance used to make optical components and electronic devices. Calcium fluoride can be produced in thin films that are transparent over a wide range of frequencies. It is an excellent choice for infrared applications. Its crystal thin film has high dielectric strength and a high band gap.

In addition, calcium fluoride is useful in producing photovoltaic panels. The regent grade has excellent IR characteristics. It is resistant to laser damage, but its refractive index changes with time. The refractive index of the regent grade depends on the temperature of the substrate.

It is difficult to make high-purity calcium fluoride with the methods used in prior art wastewater treatment systems. Although this process is able to reduce by-produced 2.4% H2SiF6, it results in a 10 to 15% loss of cost when compared to the pure component. Moreover, the process does not produce uniform results.

A better recovery technique is needed to achieve high purity. The process must also be able to achieve reactivity percentages that are almost 100%. This requires a long time to complete the reaction, which increases the equipment scale.

The calcium fluoride system consists of a reaction tank R1 and a thickener R2. The thickener separates the liquid phase from the solid phase. The solid phase is then treated with 5% hydrofluoric acid. The liquid phase part of the etchant is then removed and the powder calcium carbonate is added to the residual fluorine. The process is repeated until the desired purity is achieved. The results are shown in Tables 3 and 4. The reactivity percentage of the regent grade is very high.

Calcium fluoride wafers take long times to settle

Compared with other fluoride materials, Calcium fluoride wafers are relatively inexpensive to produce and have excellent lifetime stability. They are used in laser applications as well as in photovoltaic cells. Calcium fluoride is particularly useful in the DUV range of wavelengths. It is also durable in normal atmospheres. It is highly transparent, with high transmission ranges and band gaps. It is used for prisms and lenses.

Calcium fluoride is also a good candidate for the construction of laser optics, because it has a wide range of transmission. It has a high damage threshold, and is stable in both dry and wet environments. Its refractive index is also high. It can be used in a range of wavelengths from 0.15 um to 9 um.

It is a relatively cheap material to produce, and it is very durable at high temperatures. It is a good choice for high power laser optics, especially in the IR less than 6 microns. It can also be used for excimer systems, since it has a high dielectric constant and a low absorbtion.

The purity of Calcium fluoride can be improved by adding fluorosilicic acid, which is produced during the etching process. A high purity Calcium fluoride wafer can be made using a counterflow technique.

Counterflow is a method that requires the simultaneous flow of a calcium carbonate and etchant. This technique can achieve 100% recovery and high purification of the calcium fluoride. However, this method involves large scale equipment. Its advantages are that it eliminates waste and allows for the reuse of fluoride. It is also effective in producing high purity fluorite, which is cheaper and more reliable.

In addition to the recovery process, there are several other processes that can be used to improve the purity of the calcium fluoride. These include the addition of a chemical equivalent, such as a fluorosilicic acid, and an etching assistant, such as acetic acid. Adding a surfactant can also help.

In addition, it is important to minimize the amount of unreacted calcium carbonate. This can be achieved by using a counterflow HF5% hydrofluoric acid treatment. The ratio of the recovered calcium fluoride to the treatment amount of solution can be 4 to 10/100.

Calcium fluoride causes skeletal fluorosis

Symptoms of crippling skeletal fluorosis are severe, painful, and affect mobility and muscle wasting. This type of bone disease is a result of the accumulation of excess fluoride in the body. It can cause painful calcification of ligaments and joints. It can also lead to neurological damage due to spinal cord compression.

Fluoride is a naturally occurring mineral that is found in rocks and water. It is also present in certain foods. The amount of fluoride present in the body depends on how much fluoride is ingested. Fluoride can be detrimental to the health of both adults and children. In order to prevent skeletal fluorosis, it is important to eliminate sources of fluoride from the diet.

Several studies have shown that high doses of fluoride can cause gastrointestinal irritation, stress fractures, and abnormal bone mineralization. The US Environmental Protection Agency (EPA) recommends a maximum of two milligrams per liter (mg/L) for children. However, this recommendation is not enforceable by law.

The extent of long-term fluoride toxicity depends on the age, gender, and sex of the individual. In addition, the amount of calcium ingested is also a factor in the outcome of fluorosis.

The present study investigated the effects of fluoride toxicity on hormonal parameters, bone microstructure, and histological parameters. Fluoride-induced bone lesions are characterized by increased bone mass, osteosclerosis, and exostosis formation.

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Fluoride toxicity can be prevented by taking a fluoride-free diet, eliminating sources of fluoride, and avoiding water with high fluoride levels. Moreover, exercise can help to reduce toxins from the body. A fluoride-free diet should also include organic foods. This diet should include animal products, fresh fruits, and vegetables.

There are several therapies that can help to reduce bone loss. Nevertheless, fluoride therapy probably cannot restore bone resorption. Calcium supplementation has also been found to be effective in ameliorating some of the hazardous effects of fluoride.

A low fluoride dose has been found to reduce the incidence of fractures. However, the use of fluoride therapy may be less effective in patients with substantial trabecular connectivity. Moreover, fluoride may affect bone mineral metabolism.

CaF2 Windows UV Grade

Calcium fluoride (CaF2) windows are transparent optical elements made from single-crystal calcium fluoride they are used in a variety of applications, including:

1. UV spectroscopy: CaF2 windows are used as UV-transmitting windows in spectrometers and other optical instruments. They are beneficial for applications in the UV-VIS-NIR region, where they have high transmission and low absorption.
2. Laser optics: CaF2 windows are used as protective windows in laser systems, where they can withstand high power densities and high repetition rates.
3. Infrared (IR) imaging: CaF2 windows are used in IR imaging systems as they have high transmission in the mid-IR region and low absorption.
4. X-ray imaging: CaF2 windows are used in x-ray imaging systems because they have high transmission and low absorption in the soft x-ray region.
5. Astronomy: CaF2 windows are used in telescopes and other astronomical instruments as they have high transmission in the near-IR region and low scattering.

UV-grade CaF2 windows are made from high-purity calcium fluoride and are carefully polished to minimize surface defects and scattering. They are designed for use in UV applications where high transmission and low absorption are critical.

CaF2 Windows IR Grade

Calcium fluoride (CaF2) windows are transparent optical elements made from single-crystal calcium fluoride and are used in a variety of applications, including:

1. Infrared (IR) spectroscopy: CaF2 windows are used as IR-transmitting windows in spectrometers and other optical instruments. They are useful for applications in the mid-IR region, where they have high transmission and low absorption.
2. Laser optics: CaF2 windows are used as protective windows in laser systems, where they can withstand high power densities and high repetition rates.
3. Infrared (IR) imaging: CaF2 windows are used in IR imaging systems as they have high transmission in the mid-IR region and low absorption.
4. X-ray imaging: CaF2 windows are used in x-ray imaging systems because they have high transmission and low absorption in the soft x-ray region.
5. Astronomy: CaF2 windows are used in telescopes and other astronomical instruments as they have high transmission in the near-IR region and low scattering.

IR Grade CaF2 Protective Windows for Spectroscopy Applications

Calcium fluoride (CaF2) windows are transparent optical elements made from single-crystal calcium fluoride they are used in a variety of applications, including:

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 12/603,046 filed on Oct. 21, now U.S. Pat. No. 8,252,208, the content of which is relied upon and incorporated herein by reference in its entirety, and the benefit of priority under 35 U.S.C. §120 is hereby claimed.

FIELD

The invention is directed to calcium fluoride crystals and optics made therefrom with improved laser durability that can be used for the transmission of below 250 nanometer (nm) electromagnetic radiation.

BACKGROUND

Excimer lasers are the illumination sources of choice for the microlithographic industry. The use of high power lasers, for example, those with pulse energy densities (fluence) above 20 mJ/cm2, with pulse wavelengths below 250 nm (for example, 193 nm and below) can degrade the optics used in laser lithography systems. T. M. Stephen et al., in their article &#;Degradation of Vacuum Exposed SiO2 Laser Windows,&#; SPIE Vol. , pp. 106-109 (), report on the surface degradation of fused silica in an Ar-ion laser. More recently, it has been noticed that there is optical degradation in high peak and average power 193 nm excimer lasers using materials made from substances other than silica.

Ionic materials such as crystals of MgF2, BaF2 and CaF2 are the materials of choice for excimer optical components due to their ultraviolet transparencies and their large band gap energies. Of these three materials, CaF2 is the preferred material due to its cubic crystal structure, performance, quality, cost, and relative abundance. However, the polished but uncoated surfaces of CaF2 optics are susceptible to degradation when exposed to powerful excimer lasers operating in the deep ultraviolet (&#;DUV&#;) range, for example at 248 and 193 nm and the vacuum ultraviolet (&#;VUV&#;) range, for example at 157 nm. For lasers operating at 193 nm, 2-9 KHz, with pulse energy densities of 20-80 mJ/cm2, the surfaces of the optical elements made from these ionic materials are known to fail after only a few million laser pulses. In other applications, for example medical lasers, alternate operating parameters could exist such as 193 nm laser fluences of 200 mJ/cm2- mJ/cm2 (very high fluences) and very low repetition rate (for example 10-100 Hz) that may also result in the accelerated failure of such optical elements. The laser damage is thought to be the result of fluorine migration from the crystal optic interior or bulk to the surface where the fluorine is lost to the atmosphere. The loss of fluorine from the CaF2 crystal optic results in the formation of F centers which can then combine to form Ca colloids near the surface and within the bulk. These Ca colloids subsequently increase scatter and heating of the optical element, with eventual catastrophic failure. U.S. Pat. No. 6,466,365 (the &#;365 patent) describes a method of protecting metal fluoride surfaces, such as of CaF2 optics, from surface degradation by use of a vacuum deposited coating, for example, a silicon oxyfluoride material. While coatings may be sufficient to address surface damage, the microlithographic industry constantly demands greater performance from excimer sources, and consequently from optical components used in connection with excimer laser based systems. Therefore, the laser durability of the bulk material, CaF2, must also be improved by limiting the formation of Ca colloids that result in the eventual failure of the optical element. The solution presented herein will either eliminate the problem or greatly extend the bulk durability and consequently the length of time that existing and future optical elements can be used without having to be replaced.

Solutions to the issue of optical element lifetime involving the use of other optical materials, such as MgF2, have been considered. However, it is believed that such materials will also experience degradation similar to that of CaF2 with time, leading to the same requirement; i.e. that the expensive windows be replaced. It is further believed that the degradation problems of CaF2, MgF2, and other fluoride optical materials will be exacerbated with the advent of laser systems operating at wavelengths below 193 nm. Thus, identifying a method to increase the laser durability of the CaF2 bulk appears to be the most straightforward method of achieving the industry demands for improved laser performance.

SUMMARY

In one aspect the invention is directed to doped CaF2 crystals, and optics made therefrom, that can be used in below 250 nm laser systems, including laser microlithographic systems. The optics are made from crystal CaF2 material that has been doped with a selected amount of dopant material, for example without limitation, magnesium (Mg). In a one embodiment the amount of dopant is less than ppm. In another embodiment the amount of dopant is >0 and &#; ppm. In a further embodiment the amount of dopant is >0 and &#;500 ppm. In yet another embodiment the amount of dopant is >0 and &#;200 ppm.

In one aspect, the invention is directed to a laser optic having improved laser durability, said optic comprising a CaF2 crystal material doped with a selected amount of a selected dopant, and said optic having a ratio of 515/380 nm transmission loss of less than 0.3 after exposure to greater than 2.8 MRads of γ-radiation. In one embodiment the dopant and amount is selected from the group consisting of >0.3- ppm Mg, >0.3-200 ppm Sr, >0.3-200 ppm Ba. In another embodiment the dopants are selected from the group consisting of Ce and Mn in an amount of less than <0.5 ppm of the selected dopant. In a further embodiment the dopant and amount is 2-500 ppm Mg. In a different embodiment the dopant and amount is 10-100 ppm Mg. In an additional embodiment the ratio of 515/380 nm transmission loss is less than 0.2 after exposure to greater than 2.8 MRads of γ-radiation. In a further embodiment the ratio of 515/380 nm transmission loss of less or equal to 0.1 after exposure to greater than 2.8 MRads of γ-radiation. The laser optic can also have a coating thereon, the coating being at least one material selected the group consisting of SiO2.F, Al2O3, MgF2, BaF2, CaF2, SrF2, NaF, LiF, AlF3, LaF3, GdF3, NdF3, DyF3, YF3 and ScF3.

In another embodiment the invention is directed to a laser optic having improved laser durability, the optic comprising a CaF2 single crystal material doped with 20-100 ppm Mg, and optic having a ratio of 515/380 nm transmission loss of less than or equal to 0.2 after exposure to greater than 2.8 MRads of γ-radiation. In one embodiment the ratio of 515/380 nm transmission loss is less than or equal to 0.1 after exposure to greater than 2.8 MRads of γ-radiation. In a further embodiment the optic has a coating thereon, said coating being at least one material selected the group consisting of SiO2.F, Al2O3, MgF2, BaF2, CaF2, SrF2, NaF, LiF, AlF3, LaF3, GdF3, NdF3, DyF3, YF3 and ScF3.

The invention is also directed to a doped CaF2 crystal suitable for making laser optics having improved laser durability, said crystal consisting of CaF2 as the major component, and at least one dopant selected from the group consisting of the group consisting of >0.3- ppm Mg, >0.3-200 ppm Sr, >0.3-200 ppm Ba. In one embodiment the dopant and amount is 2-500 ppm Mg. In another embodiment the dopant and amount is 10-100 ppm Mg.

In another embodiment the crystal has a ratio of 515/380 nm transmission loss of less than 0.3 after exposure to greater than 2.8 MRads of γ-radiation. In an additional embodiment the crystal has a ratio of 515/380 nm transmission loss of less than 0.2 after exposure to greater than 2.8 MRads of γ-radiation. In a further embodiment the crystal has a ratio of 515/380 nm transmission loss of less than or equal to 0.1 after exposure to greater than 2.8 MRads of γ-radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (prior art) illustrates a crystal growth crucible having a seed crystal reservoir and the axial orientation direction of the seed crystal.

FIG. 2 (prior art) illustrates the growth crucible of FIG. 1 loaded with doped CaF2 feedstock.

FIG. 3 illustrates the crucible of FIG. 2 contained within the upper zone of a two zone furnace, the feedstock and the upper part of the part of the seed crystal having been melted.

FIG. 4 illustrates the change in the ratio of 515/380 nm transmission loss for un-doped and Mg-doped CaF2.

FIG. 5 is the Raman spectrum illustrating the formation of colloids in un-doped CaF2 crystal.

FIG. 6 is a graph illustrating the transmission loss ratio R (Δ515 nm/Δ380 nm) versus measured Mg content

FIG. 7 is a graph illustrating transmission loss slope (% loss/Bp) versus R ratio.

FIG. 8 is a graph illustrating the transmission loss at 193 nm versus the number of laser pulses for CaF2 samples having different ppm levels of Mg.

DETAILED DESCRIPTION

As used herein the terms &#;calcium fluoride crystal&#; and &#;calcium fluoride optic&#; means a calcium fluoride crystal, or optic made therefrom, containing at least one dopant as specified herein and in an amount within the range given for each dopant as specified herein. The crystal can be a single crystal such as is grown by the Bridgman method, the Bridgman-Stockbarger method and other methods known in the art, or it can be a crystal formed by heating a calcium fluoride powder or plurality of small crystals under pressure at a temperature such that the powder or plurality of crystals fuse to form a calcium fluoride crystal as is also known in the art. These processes are typically conducted under vacuum, in an inert or fluorinating atmosphere, or under conditions containing only minor amounts of oxygen. Examples of crystals of alkaline earth metal fluorides grown using the Bridgman, Bridgman-Stockbarger, and Czochralski methods, or variations thereof, can be found in, for example without limitation, U.S. Pat. Nos. 7,033,433, 6,989,060, 6,929,694, 6,702,891, 6, 704,159, 6,806,039, 6,309,461 and 6,123,764. The crystals can be made into optics by methods well known in the art.

As used herein the terms &#;calcium fluoride single crystal&#;, &#;calcium fluoride single crystal optic&#;, and similar terms including the word &#;doped&#;, mean a single crystal of calcium fluoride, or optic made therefrom, containing at least one dopant as specified herein and in an amount within the range given for each dopant as described herein. Dopant amounts are given in parts-per-million (ppm) by weight of the dopant metal ion in the crystal.

Further, it is to be recognized that the CaF2 crystals can contain, in addition to the intentional metal dopant described herein, very low levels of other &#;contaminants&#;, for example without limitation, contaminants such as those specified herein. All such contaminants are to be deemed as due to the inability to absolutely eliminate such materials from the feedstock or processing environment, and are not to be deemed as being intentionally present or affecting the durability of the doped CaF2 crystals and optics of the invention. In the art cited above for making CaF2 crystals it was preferred that the doped calcium fluoride feedstock is such that the final crystal optic product has impurity levels, by weight as measured by ion-coupled plasma mass spectroscopy (ICP-MS) or other appropriate method known in the art, of less than 0.1 ppm Li, less than 4 ppm Na, less than 3 ppm K, less than 0.2 ppm Sc, less than 0.2 ppm Y, less than 0.2 ppm La, less than or equal to 0.2 ppm Gd, less than 0.2 ppm Yb, less than 0.2 ppm Ti, less than 0.1 ppm Cr, less than 0.5 ppm Mn, less than 0.4 ppm Fe, less than 0.2 ppm Co, less than 0.2 ppm Ni, and less than or equal to 0.3 ppm Cu. Preferably the calcium fluoride raw material has less than or equal to 0.5 ppm Na and 0.5 ppm K. The total of such contaminants is generally less than 50 ppm.

The dopants can be added to the CaF2 feedstock used to make the CaF2 crystal as a fluoride, oxide, carbonate, or finely powdered metal. The mixture of CaF2 powder and dopant is treated with an oxygen scavenger such as CF4, SnF2 or PbF2 to remove oxygen. When a metal powder is used as the dopant, the scavenger treatment also converts the metal to metal ions as well as removes oxygen. Similarly, the scavenger helps in removing oxygen from metal oxide dopant thereby converting it to a metal fluoride.

The doped CaF2 crystals used in the γ-ray tests described below were grown using a crystal growth and annealing apparatus as described in the '461 patent. Summarizing, the apparatus as described in the '461 patent has a primary heating system mounted near the top and sides of the crystal and a secondary heating system mounted near the bottom of the crystal. This secondary heating system may or may not be used during the production of the doped crystals. The method, generally, of the '461 patent used to make the crystals described herein has steps of (1) forming a liquid of crystal material, including the dopant, in a crucible by heating the crystal material using heat from the primary heating system; (2) lowering the crucible out of the primary heating system so that successive portions of the liquid crystal material cool to a temperature suitable for crystal formation; (3) reducing the temperature of the primary heating system; (4) raising the crucible into the primary heating system and supplying heat from the secondary heating system; and (5) reducing the heat output of the primary and secondary heating systems so that the average temperature of the crystal is cooled over time. It is especially important to maintain a low temperature gradient during the initial phases of cooling, when the hot crystal has relatively low yield strength. Cooling times in the range of 20-40 days are described in the '461 patent. However, in the preferred case cooling times may be on the order of 10 to 25 days.

The growth of crystals of selected orientation, for example, a <111>, <110> or <100> crystal can be done by using a crucible having a reservoir in its bottom, as illustrated in FIGS. 1 and 2, into which, for example, a <111> seed crystal is placed. After the doped CaF2 has been prepared it can be annealed to reduce stresses within the crystal and the birefringence that may result from such stress. Such annealing methods have been described in the art; for example, in U.S. Pat. No. 6,806,039.

The doped crystals of the invention can also be grown using the method described in the '039 patent. FIGS. 1-3 herein illustrates some of features of the crystal growth process described in the '039 patent and briefly summarized as follows. Lead fluoride was used as an oxygen scavenger.

FIG. 1 shows a crystal growth crucible 62 for growing doped crystal having a crystal growth chamber and a seed crystal orientation receiver 64 for receiving and orienting a seed crystal 60 in relation to the adjoining above crystal growth chamber (designated herein 90). Arrow 92 shows the preferred crystal axis direction of the seed crystal. FIG. 2 shows the growth crucible loaded with the seed crystal 60 and the CaF2 feedstock 70 containing the selected dopants as described herein. In the preferred case, a seed crystal may not be used during the crystal growth process. The optical crystal is later removed from the large bulk crystal in a manner that provides an optical element whose surfaces have the desired crystallographic orientation. The machining techniques used to produce this optical element with the desired crystallographic surface orientations are known in the art. FIG. 3 shows the crystal growth crucible 62, with lid 63 thereon, containing the doped feedstock as a melt 66 with an upper portion of seed crystal 60 melted. The doped feedstock was melted in the upper hot melt zone of controlled atmosphere vacuum furnace 110. Controlled atmosphere/vacuum furnace 110 was heated by resistive graphite heating elements 8. An insulating furnace baffle 14 preferably separates the upper and lower heating elements to isolate the lower cool anneal zone (below the baffle) from the upper hot melt zone (above the baffle) and forms there between a crystal growth temperature gradient. The partially melted crystal seed 60 and melted doped feedstock 66 is progressively moved through the crystal growth temperature gradient to grow a seeded oriented doped CaF2 crystal. After the single crystal is fully grown it can be cooled as described herein or elsewhere in the art within the lower portion of the growth furnace or it can be cooled and moved to a separate annealing furnace according to the schedule given above or other annealing schedules known in the art.

It is recognized to those skilled in the art that the local concentration of a specific dopant may vary axially throughout the crystal. The degree of dopant variation is dependent upon the segregation coefficient of the dopant within the material, the rate of crystal growth, the diffusivity of the dopant within the molten material, and the convective state of the molten material during growth. Careful measurements made using ICP-MS have been used to identify the amount of dopant present in the optical elements tested. It is the actual measured dopant concentration values which are discussed herein.

As stated above, it is known that polished but uncoated surfaces of CaF2 are susceptible to degradation when exposed to powerful lasers operating in the DUV and VUV ranges. For example, when using 193 nm lasers operating at 2-9 KHz with pulse densities of 20-80 mJ/cm2, the surfaces or the optical elements made from these ionic materials are known to fail after only a few million laser pulses. R. Bennewitz et al, &#;Bulk and surface processes in low-energy-electron induced deposition of CaF2&#;, Amer. Physical Society, Physical Review B, Vol. 59, No. 12 (), pages -, suggest that the cause of the damage is thought to be fluorine diffusion from the bulk of the crystal to the surface. Bennewitz et al indicate that metal (Ca) formation was observed on the surface of the crystal and that &#;Colloid formation [in the crystal] results from aggregation of F centers, a process favored in CaF2 by the good match between the lattice structure and atomic spacing of calcium metal and the Ca2+ sublattice in CaF2.&#; FIG. 5 shows the Raman spectrum of CaF2 before and after exposure to 193 nm laser radiation. The change in the Raman spectra demonstrates the existence of Ca colloids in CaF2 after exposure spectrum to 193 nm laser radiation. U.S. Pat. No. 6,466,365 (the &#;365 patent) describes a method of protecting metal fluoride surfaces, such as CaF2, from degradation by use of a vacuum deposition, of a silicon oxyfluoride coating/material. While for the moment this is a reasonable solution, the microlithographic industry constantly demands more performance from excimer sources, and consequently from optical components used in connection with excimer laser based systems. In particular, the industry would prefer to use uncoated CaF2 optics because of the reduced costs, better transmission, and the general outlook that the less complex the optic, the less likely it is that something will go wrong. The lithographic industry is currently seeking optics that can survive as many as 50 billion pulses of 20-80 mJ/cm2 with an acceptably low level of degradation over this period. Coating the optics, by itself, is believed insufficient to reach this goal without improvements in the laser durability of the bulk material.

Disclosed herein are optics made of single crystal CaF2 doped with one or more dopant materials in specific amounts selected from the group consisting of Mg, Sr and Ba (&#;dopant&#;) in order to extend the lifetime of the CaF2 optic when it is used in high power laser systems; for example, lasers operating at 193 nm, 2-9 KHz, with pulse energy densities of 20-80 mJ/cm2. The amount of each dopant selected to add to CaF2 is from within the following ranges; >0.3- ppm Mg, >0.3-200 ppm Sr, and >0.3-200 ppm Ba. Each of these dopants form solid solutions with CaF2 within the given concentration ranges. Each dopant also has an atomic radius that differs from the Ca ion within the crystal lattice. The ionic radii values (Pauli, in Angstroms) are Mg=0.69, Ca=0.99, Sr=1.13 and Ba=1.45. This difference in atomic radii distorts the crystal lattice in a manner that reduces the time required for the recombination of excitons created with the CaF2 structure by exposure to laser irradiation. While the addition of one or more dopants reduces the exciton lifetime, it does not prevent the formation of all lattice defects caused by exposure to radiation. However, the addition of one or more dopants does appear to inhibit the formation of Ca colloids that are typically associated with laser damage in CaF2 single crystals

In one embodiment, the present invention is directed to an alkaline earth crystal consisting of CaF2 as the major component and at least one dopant selected from the group consisting of >0.3- ppm Mg, >0.3-200 ppm Sr, >0.3-200 ppm Ba. In another embodiment the dopants are selected from the group consisting of Ce and Mn in an amount of less than <0.5 ppm of the selected dopant. In another embodiment the alkaline earth single crystal consists of CaF2 as the major component and at least one dopant selected from the group consisting of >2-500 ppm Mg, >2-100 ppm Sr, >2-100 ppm Ba. In a further embodiment the invention consists of CaF2 as the major component and at least one dopant selected from the group consisting of >10-100 ppm Mg, 5-50 ppm Sr, >2-10 ppm Ba. In an additional embodiment the alkaline earth single crystal consists of CaF2 as the major component and at least one dopant selected from the group consisting of >20-100 ppm Mg, 1.0-200 ppm Sr, and >1.0-200 ppm Ba. In a further embodiment CaF2 is the major component and the dopant is 20-60 ppm Mg.

Mixed alkaline earth metal fluorides have been described in the both the patent and technical literature. For example, U.S. Pat. Nos. 6,806,039, 6,630,117, 6,649,326, and U.S. Patent Publication No. /, describe making mixed alkaline earth fluoride single crystals of general formula M1xM2(1-x)F2 where x is in the range of 0.1-0.9; such mixed metal crystals all containing greater than 10,000 ppm of the lesser of the two alkaline earth metal ions. V. Denks et al., &#;Excitonic processes in pure and doped CaF2,&#; J, Phys. Condens. Matter, Vol. 11 (), pages -, investigated CaF2 doped with Mg, Mn, Na and Li ions. The authors investigated CaF2 crystals doped with (a) Mg ions in amounts in the range of 0.01-0.1% (page ) or 0.2% Mn ions (page ). In their conclusion on page , regarding impurities [dopants], they stated &#;None of the impurities (Mg or Mn) described in the present paper led to an improvement of the radiation stability of CaF2.&#; This conclusion was based upon their fluorescence measurements and is contrary to concepts and information put forth herein. In addition, Denks et al. state, without specification, that they did find an impurity which might raise the radiation resistance of CaF2. In a subsequent paper, V. Denks et al., &#;Impurity-Related Excitonic Processes in CaF2&#;Sr&#;, Phys. Stat. Sol. (a), Vol. 191. No. 2, (), pp. 628-632 describes a CaF2:Sr single crystals in which Sr ranges from 0.05 to 4 mol % (0.05 mol %=˜561 ppm or 0.6 wt. % Sr). In this subsequent paper, Denks et al. conclude that doping CaF2 with Sr at this high level may impart increased durability to radiation exposure. In some patents, for example, U.S. Pat. No. 6,999,408, Mg, Sr and Ba were regarded as impurities in CaF2 and were kept to level below 0.5 ppm Mg, 19 ppm Sr and 5 ppm Ba. Neither do these patents recognize the ability of these specific metallic ions at specific dopant levels to impart increased laser durability to CaF2.

It is also highly desirable to have an accelerated test by which doped single crystal CaF2 optics can be laser durability tested. Presently, the accelerated test methods use a very high power excimer laser and can last anywhere from a few days to several weeks. This method of testing is both expensive and time consuming. Other methods (for example, laser fluorescence as cited above in Denks et al.) have been investigated to determine whether they could accurately indicate a CaF2 optic's laser durability; however, these methods have only met with limited success. Currently, the only viable method to &#;rapidly&#; evaluate the improved laser durability of doped CaF2 optical elements was suggested by T. D. Henson et al. in &#;Space radiation testing of radiations resistant glasses and crystals&#;, Proc. SPIE. V (), pp. 54-65. Henson et al. suggest that transmission testing after exposure to γ-radiation serves as a viable test method of the durability of CaF2 optics. Therefore, this method was employed to evaluate doped CaF2 samples as described in this disclosure. Samples of doped and un-doped CaF2 optics having a thickness of 7 mm were exposed to a dose of between 28.3 and 28.7 kGy (2.83-2.87 MRad) using a gamma-ray [γ-ray] source. The transmission spectra from 200 to nm of the samples were tested before exposure and again at 25, 100, 430 and 600 hours after γ-radiation exposure. It was found that the doped CaF2 crystals with improved laser durability had a lower ratio of 515/380 transmission loss than undoped CaF2 material had. The 515/380 transmission loss ratio is defined as the decrease in transmission at 515 nm after exposure compared to before exposure divided by the similar loss in 380 nm transmission after exposure compared to before exposure. These particular wavelengths are compared because the presence of Ca colloids results in absorption at around 515 nm while F center presence results in absorption at around 380 nm (an F center is a fluoride ion vacancy with one electron is in the vacancy). During the course of the evaluation of the irradiated doped and undoped (D and UD) samples, it was found that while both the D and UD samples have F centers (decreased 380 nm transmission), the D samples do not appear to making colloids whereas the UD do make colloids (decreased 515 transmission). This result is particularly impressive since the precursor to colloid formation is the presence of F centers. Apparently, at low concentrations of a dopant such as Mg as used in the present sample optics, the dopant inhibits colloid formation which in turn improves laser lifetime.

Generally, it was found that un-doped CaF2 optic samples (UD) had a loss ratio after exposure greater than 0.4 and that the ratio increased on the order of 25% as transmission recovery after exposure increased, though the increase was at a gradually leveling-out rate. In contrast, the doped CaF2 optic samples (D) has a loss ratio of less than 0.3 throughout the entire evaluation period indicating less colloid formation for given amount of F center formation. In some embodiments the loss ratio of the D optic samples was less than 0.2. In the example shown in FIG. 4 the loss ratio was less than or equal to 0.1. The D optics contained were Mg-doped in the range of 10-100 ppm, preferably in the range of 20-80 ppm.

Thus, in one embodiment the invention is directed to a laser optic comprising a CaF2 crystal material doped with a selected amount of a selected dopant whose purpose is to inhibit the formation of Ca colloids and thereby impart improved laser durability to the optical element. The purpose of the selected dopant is to inhibit the formation of Ca colloids and thereby impart improved laser durability to the optical element. In one embodiment the colloid inhibiting dopant and amount is one selected from the group consisting of >0.3- ppm Mg, >0.3-200 ppm Sr, >0.3-200 ppm Ba is added to inhibit the formation of Ca colloids. In another embodiment the colloid inhibiting dopant is Mg in an amount in the range of 2-500 ppm. In further embodiment the colloid inhibiting dopant is Mg in an amount in the range of 10-100 ppm. The foregoing laser optics have a ratio of 515/380 nm transmission loss of less than 0.3 after exposure to greater than 2.8 MRads of γ-radiation. In one embodiment the ratio of 515/380 nm transmission loss of less than 0.2 after exposure to greater than 2.8 MRads of γ-radiation. In another embodiment the ratio of 515/380 nm transmission loss of less than or equal to 0.1 after exposure to greater than 2.8 MRads of γ-radiation.

The doped CaF2 optics according to the invention can be coated or uncoated. The coating materials can be a materials selected from the groups consisting of fluoride, oxide and fluorinated oxide films which are applied to the surfaces of the optic using advanced plasma techniques known in the art. Example of such coating materials and the techniques for coating optics can be found in the commonly-owned U.S. Pat. No. 7,242,843 and citations therein whose teachings are incorporated by reference. The coating material can be applied directly to the optic. Coating materials include SiO2.F, Al2O3, MgF2, BaF2, CaF2, SrF2, NaF, LiF, AlF2, LaF3, GdF3, NdF3, DyF3, YF3 and ScF3. The optics to be coated include prisms, windows and lenses, and can further include mirrors made of CaF2.

An embodiment of the present application is directed to doped CaF2 crystals and optics made therefrom doped with a selected amount of Mg, the amount of Mg being in the range of 13 ppm to 250 ppm. In another embodiment the Mg dopant is in the range of 15 ppm to 250 ppm. In an additional embodiment the Mg dopant is in the range of 20 ppm to 250 ppm. As an example, the Mg doped CaF2 crystals can be made by the addition and thorough mixing of MgF2 powder with CaF2 powder prior to melting the materials to form a Mg doped CaF2 crystal using the methods described in this application. In order to carefully control the amount of Mg dopant in the final crystal the Mg content of the CaF2 powder is carefully analyzed prior to the additional of the dopant.

Commercial high purity CaF2 containing 1 ppm or less of Mg was obtained and analyzed for Mg content. Twenty-seven samples of CaF2 powder were analyzed and the highest Mg content of the as-obtained powders was 0.300 ppm Mg. The average Mg concentration of the twenty-seven samples was 0.079 ppm Mg. The CaF2 powders were then doped with an Mg source to selected dopant levels, formed into crystals and optics. The Mg content of the formed crystal and/or optic was then analyzed using inductively coupled plasma mass spectroscopy (ICP-MS) to determine the level of Mg in the crystal.

As has been mentioned above, single crystal CaF2 is the material of choice for making the optics used in powerful excimer lasers operating in the deep ultraviolet (&#;DUV&#;) range, for example at 248 and 193 nm and the vacuum ultraviolet (&#;VUV&#;) range, for example at 157 nm. However, the CaF2 single crystal material and optics made therefrom are susceptible to degradation when exposed to such radiation, with the loss of transmission. The data presented in the following paragraphs show that doping CaF2 with Mg in an amount greater than 13 ppm results in a crystal having greatly improved transmission properties and durability. Samples of CaF2 single crystals doped with various amount of Mg as described below were tested using a 10 Watt (10 W) laser operating at 193 nm and 3,000 Hz (3 kHz) with an energy pulse of 3.5 mJ/pulse. The groups of samples consisted of:

(A) Standard CaF2 crystals prepared using as-received CaF2 and no added Mg.

(B) Samples doped to a Mg content in the range of 4-5 ppm Mg.

(C) Samples doped to a Mg content in the range of 7-12 ppm Mg.

(D) Samples doped to a Mg content in the range of 13-20 ppm Mg.

(E) Samples doped to a Mg content of >20 ppm Mg.

The foregoing were then subjected to 0.5 to 1.6 billion ((0.5&#;1.6)×109) 193 nm laser pulses at a peak fluence of 240 mJ/cm2 for accelerated laser damage testing (ALDT). The ALDT fluence exceeded the typical use fluence of approximately 30 mJ/cm2 for a 60 W laser or approximately 50 mJ/cm2 for a 90 W laser. This is very aggressive test that will show laser damage. The (B), (C), (D) and (E) sample groups are illustrated in FIG. 8 as &#;wedges&#; extending from 0 Bp tp 1 Bp.

The 193 nm transmission is determined by measurements of power collected by a Molectron Powermax detector. Every 30 minutes, during which time the sample is exposed to 5.4 million pulses, the sample is moved to an unexposed portion for 10 seconds to record the change in wattage. There is about 10 seconds of averaging done at each location to arrive at the wattage levels used for comparison. This transmission measurement, which is well known in the art, was continued until the sample was removed from laser testing after being exposed to a minimum of 0.53 billion pulses.

After a particular sample had completed its ALDT testing, the slope of the 193 nm transmission loss per billion laser pulses was calculated from the in situ transmission measurements.

FIG. 8 is a plot of the transmission loss versus the number of pulses up to 1.0 billion pulses. In FIG. 8 &#;N&#; is the number of samples in each group, &#;R&#; represents the transmission ratio Δ515 nm/Δ380 nm after exposure to at least 2.8 MRads (million rads) of γ-radiation, which will be discussed in detail below, and &#;Bp&#; is &#;billion pulses.&#; The horizontal lines labeled L0, L1 and L2 represent the top of the individual transmission loss wedges at 1 Bp for the samples of groups (C), (D) and (E). Looking at the line for each group, one can determine when each group above it reaches the same transmission loss. For example, for group (E) containing >20 ppm Mg, the transmission loss after 1 billion pulses can be determined to be approximately 0.15% by following line L2 from the right to left axis for transmission loss. One can also see that the same transmission loss is reached by groups (B), (C) and (D) at approximately 0.08, 0.24 and 0.28 Bp, respectively. Thus one can say that the transmission loss rate for groups (B), (C) and (D) is 12 times, 4 times and 3.52 times greater, respectively, than that of group E. For group (D) and (C) the transmission loss at 1 billion pulses is approximately 0.5%. Groups (B) and (C) reach the same transmission at approximately 0.24 and 0.94 Bp, respectively. For group (C) the transmission loss a 1 billion pulses is approximately 0.55%, and group B reaches the same transmission loss at approximately 0.28 billion pulses. The R value for standard undoped CaF2 crystal material is >0.40.

The data in FIG. 8 thus illustrates that as the level of Mg dopant in the CaF2 is increased the durability of the CaF2 crystal is increased and lower transmissions loses are encountered with increasing Mg content. The data also indicates that the decrease in transmission loss is not linear with increasing Mg content. Note that in FIG. 8 as the Mg content increases the R value decreases.

FIG. 6 is a graph illustrating the transmission loss ratio R=Δ515 nm/Δ380 nm for a large number of different samples that have been exposed to at least 2.8 MRads (million rads) of γ-radiation. These &#;R&#; values were obtained on approximately 170 samples of known Mg content of which a subset were also evaluated using the ALDT described in this specification. The graph indicates that at some Mg concentration between 10 ppm and 15 ppm Mg there is a break in the rate at which the transmission ratio decreases and the effect of continuing to add Mg begins to level out as the Mg level increases to at least 200-250 ppm. The rate of transmission ratio loss continues to decrease after approximately 40 ppm Mg, but at a slower rate than between 13-40 ppm Mg, as the Mg level in the CaF2 increases until at about 140-160 ppm. After approximately 160 ppm Mg the rate of the transmission ratio further continues to decrease, at a much slower rate than before, out to at least 200-250 ppm Mg.

FIG. 7 is a graph of the transmission loss slope (% Loss/Bp) versus the R ratio Δ515 nm/Δ380 nm. The graph combines the finding of graphs 6 and 8, and clearly illustrates that the ALDT and γ-radiation test results are consistent with one another and that both show the dependence of the transmission loss and the rate of its increase on the Mg content in the doped CaF2 crystal.

The present application is directed to a laser optic having improved laser durability, the optic consisting essentially of a CaF2 crystal material doped with Mg in an amount in the range of 13 ppm to 250 ppm, said optic having a ratio of 515/380 nm transmission loss of less than 0.3 after exposure to greater than 2.8 MRads of γ-radiation. In an embodiment the amount of Mg dopant (MW=24.312) is in the range of 15 ppm to 250 ppm. In another embodiment the Mg dopant is in the range of 20 ppm to 250 ppm. Further, in an embodiment the ratio of 515/380 nm transmission loss is less than 0.2 after exposure to greater than 2.8 MRads of γ-radiation. In another embodiment the ratio of 515/380 nm transmission loss is less or equal to 0.1 after exposure to greater than 2.8 MRads of γ-radiation. The laser optic also has an ALDT transmission loss of less than 1% after exposure to 1 billion pulses from a 193 nm laser operating at 10 W, 3 kHz and energy pulse of 3.5 mJ/pulse. In one embodiment the ALDT transmission loss is less than 0.5% after exposure to 1 billion pulses from a 193 nm laser operating 10 W, 3 kHz and energy pulse of 3.5 mJ/pulse. In another embodiment the ALDT transmission loss is less than 0.25% after exposure to 1 billion pulses from a 193 nm laser operating 10 W, 3 kHz and energy pulse of 3.5 mJ/pulse.

The present application is also directed to a doped CaF2 crystal suitable for making laser optics having improved laser durability, said crystal consisting of CaF2 as the major component and 13-250 ppm Mg, wherein the doped single crystal has a ratio of 515/380 nm transmission loss of less than 0.3 after exposure to greater than 2.8 MRads of γ-radiation. In an embodiment the dopant in the single crystal is 15-250 ppm Mg. In an embodiment the Mg dopant is in the range of 20 ppm to 250 ppm. In another embodiment the single crystal has a ratio of 515/380 nm transmission loss of less than 0.3 after exposure to greater than 2.8 MRads of γ-radiation. In a further embodiment the single crystal has a ratio of 515/380 nm transmission loss of less than 0.2 after exposure to greater than 2.8 MRads of γ-radiation. In an additional embodiment the single crystal has a ratio of 515/380 nm transmission loss of less than or equal to 0.1 after exposure to greater than 2.8 MRads of γ-radiation. Further, when laser tested the crystal was found to have an ALDT transmission loss of less than 1% after exposure to 1 billion pulses from a 193 nm laser operating 10 W, 3 kHz and an energy pulse of 3.5 mJ/pulse. In one embodiment the ALDT transmission loss was less than 0.5% after exposure to 1 billion pulses from a 193 nm laser operating 10 W, 3 kHz and an energy pulse of 3.5 mJ/pulse. In another embodiment the ALDT transmission loss was less than 0.25% after exposure to 1 billion pulses from a 193 nm laser operating 10 W, 3 kHz and energy pulse of 3.5 mJ/pulse.

It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

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