WO2003025262A1

WO2003025262A1 - Method for producing break-proof calcium fluoride monocrystals and their use

Info

Publication number: WO2003025262A1
Application number: PCT/DE2002/003190
Authority: WO
Inventors: Burkhard Speit; Gunther Wehrhan
Original assignee: Schott Glas; Carl-Zeiss-Stiftung Trading As Schott Glas; Carl-Zeiss-Stiftung
Priority date: 2001-08-31
Filing date: 2002-08-30
Publication date: 2003-03-27
Also published as: DE10142652A1; DE10142652B4

Abstract

The invention relates to a method for producing break-proof, large-size calcium fluoride monocrystals, which are suited for optical components used in the low UV range. The CaF2 starting material is firstly melted. Afterwards, the melt is solidified while cooling it to form a monocrystal. In order to increase the breaking strength, Th, Ba, W, Sb, Y and/or rare earths are added by doping to the CaF2 starting material that can contain small amounts of impurities.

Description

METHOD FOR PRODUCING BREAK-RESISTANT CALCILWLUORIDE SINGLE CRYSTALS AND USE THEREOF

description

The invention relates to a method for producing unbreakable calcium fluoride single crystals, and to the use of the crystals obtained therewith.

Single crystals of calcium fluoride are required, among other things, because of their optical properties, that is, in particular because of their permeability to ultraviolet radiation, as a starting material for optical components in DUV photolithography, in which deep ultraviolet (DUV = Deep UV) with a wavelength λ smaller 250 nm radiating light sources such as excimer lasers, fine patterns can be applied to semiconductor wafers coated with photoresist and the like. For this it is necessary that the components such as lenses or prisms made of calcium fluoride have a very high optical homogeneity. Errors in the calcium fluoride crystal disturb the optical homogeneity; the calcium fluoride then also exhibits stress birefringence. Calcium fluoride crystals with defects and especially those with stress birefringence are of course not suitable for producing optical components from them. Such crystal defects usually originate from foreign atoms, ie from impurities, which are built into the crystal lattice and thus its homogeneous disrupt nity. The aim is therefore to produce single crystals for optical elements from the purest possible material.

For example, JP-A-10-059799 describes the effects of strontium in calcium fluoride crystals. The strontium content should then be kept below 1 x 10 ¹⁸ atoms / cm ³ , so that the optical properties of the calcium fluoride do not deteriorate. On the other hand, JP-A-09-315815 states that a strontium content of 1 - 600 ppm with a lanthanum and yttrium contamination of less than 1 and 10 ppm, respectively, should prevent the permeability of the calcium fluoride crystal when irradiated with intense laser light in the UV -Range, as is the case with the use of these crystals described above, decreases significantly.

Calcium fluoride crystals are produced as one of the usual crystal growing processes, such as the Stockbarger-Bridgman process or the VGF process (VGF = Vertical Gradient Freezing, cooling in the vertical temperature gradient) as relatively large single crystals with dimensions in the decimeter range with very high purity. These large starting crystals are then cut, sawed, ground, etc. into the shape required for the optical components and finally the finished optical components are produced therefrom by special treatments such as surface finishing and the like.

It has now been shown that the fracture frequency of the crystals during processing is relatively high, especially in the case of highly pure calcium fluoride crystals. It is not only annoying when an almost finished component breaks into several parts during one of the last processing steps, but it is sufficient that in the crystal during the processing tion dislocations and microcracks occur, which can ultimately lead to the finished component having a reduced optical transmission.

The fracture energy of the calcium fluoride crystals along the 111 surfaces is only 490 mJ / m ² , which is extremely low. Slate has similarly low values; in comparison, the fracture energy in quartz, for example, is 4,300 mJ / m ² .

The raw crystal is cut, ground and polished to produce optical components such as lenses. These steps act mechanically on the crystal structure of the crystal. With calcium fluoride, the optical axis of the lenses runs along the surface normal of the <111> plane. Every time the curved lens surface is machined, strong shear forces occur which act along the <111> plane. As a result, the crystal suffers microscopic and macroscopic damage in the form of dislocations and cracks.

Attempts have already been made to make the optical axis of lenses deviate from the <111> surface normal in a somewhat offset orientation. If the optical axis deviates from the <111> orientation, however, the hardness anisotropy becomes more apparent in the machining, which only increases the problems during machining. So there were z. B. in the case of larger deviations, additional problems with voltage birefringence.

The object of the invention is therefore to provide a method for producing a calcium fluoride single crystal with high breaking strength. This object is achieved according to claim 1 in that the calcium fluoride raw material from which the single crystal is grown, a strength agent selected from Th, Ba, W, Sb and / or rare earths is added in an amount between 10 and 15,000 ppm ,

Advantageous embodiments of the method according to the invention are mentioned in the subclaims.

According to the invention, it was surprisingly found that a small addition of Th, Ba, W, Sb and / or possibly rare earths to the crystal bulk causes the desired increase in breaking strength. Of the rare earths Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, Y and Yb are particularly preferred. The strength agents are added in the form of their salts, in particular halides or also as oxides. A particularly preferred salt is fluoride. The strength agents are particularly preferably doped in the form of their lowest oxidation state.

It has surprisingly been found that even a small addition in the range of 1, especially at least 10 ppm, preferably at least 20 ppm, causes an increase in the breaking strength. The maximum amount to be used according to the invention is expediently at most 15,000 ppm, with at most 10,000 ppm, in particular at most 8,000 ppm, being preferred. In many cases, a maximum of 5,000 ppm is completely sufficient.

If the strength agent according to the invention is added in the form of an oxide, the amount is preferably 100-10,000 ppm, with 200-8,000 ppm being particularly preferred.

If the strength agents used according to the invention are added in the form of their salts, in particular their fluorides, amounts of 20-500 ppm, in particular 45-200 ppm, have proven to be expedient.

In a preferred embodiment, the strength agents according to the invention are used together with strontium, i.e. H. Strontium salts and / or oxide used. The amount of strontium is usually 1 to 250 ppm, preferably 10 to 200 or 150 ppm, with amounts of 80 to 140 ppm being particularly preferred.

Surprisingly, it was also found that in the production of single crystals, by adding the strength agents according to the invention, it is even possible to dispense with scavengers or that they only have to be added in a smaller amount than is customary. Scavengers to be used according to the invention are PbF ₂ , ZnF ₂ , SnF ₂ , CdF ₂ and / or CoF ₂ . These scavengers, like the strengthening agents which act as scavengers according to the invention, have the ability to remove oxygen-containing impurities such as e.g. B. H ₂ 0, OH ^" and O ^2" and deliver them to the environment in the gaseous state. Preferred scavengers are those whose higher-quality oxides have a lower vapor or sublimation point than the lower oxide used according to the invention.

Of the rare earths, those are particularly preferred which do not generate any fluorescence in the crystal at the wavelengths to be used later. According to the invention, it has also been found that the small addition of the strength agents surprisingly does not adversely affect the optical properties of the calcium fluoride single crystal.

When introducing additional contaminants, such as. B. the strength agent used in the calcium fluoride single crystal is actually to be expected that the number of defects and defects in the crystal lattice increases and thus deteriorate the properties of the crystal. Instead, exactly the opposite occurs when the strength agents are added, namely a significant reduction in the susceptibility of the crystal to breakage and the associated stabilization of the optical properties of the crystal. With the method according to the invention, it is readily possible to achieve the breaking strength by at least 20 mJ / m ² , usually at least 40 mJ / m ² . Usually an increase of at least 50 mJ / m ² can be achieved without any problems.

The raw material for the calcium fluoride crystal can contain a total of up to about 100 ppm of foreign atoms such as Na, K, Li and Mg as impurities. Other impurities are essentially below 1 ppm. The first-mentioned impurities can also be reduced to less than 1 ppm per element by a special procedure in crystal growing, in which these impurities evaporate. This also applies to sodium. However, sodium is an element that humans release in relatively large quantities through the skin. In a way, people are always surrounded by a sodium vapor. When processing calcium fluoride, where it is inevitably always in the vicinity of people, the Na concentration increases, for. B. on the surface again to 2 ppm and more. Interestingly, adding sodium or increasing the sodium content in the calcium fluoride crystal doped with strontium, as mentioned above, hardly changes the breaking energy. On the other hand, strontium compensates for the negative effects of sodium contamination on the radiation properties.

The calcium fluoride raw material for the production of the single crystal can therefore be present as an impurity between about 1 and 10 ppm sodium, e.g. T. but also considerably more.

The increase in the mechanical strength of the calcium fluoride crystal due to the extremely small amount of strength agent of less than 15,000 ppm, preferably less than 10,000 ppm, is therefore all the more surprising since normally such effects on crystals only with doping of several percent, in usually at least 2 - 3%. Another important fact which favors the use of the strength agents is that they do not accumulate in the crystal, but remain homogeneously distributed during the crystallization. This homogeneous distribution is absolutely necessary to increase the mechanical strength, i. H. the distribution coefficient should be as close as possible to 1.

The invention is described in more detail using the following embodiment.

A large-format single crystal made of CaF ₂ with a diameter of 25 cm is produced in a tubular crucible, which is closed with a lid. tionssymmetrisch.es temperature profile and the crucible are moved relative to each other in the direction of the crucible bottom to the crucible opening, so that the raw CaF ₂ material, together with the strength agent from which the single crystal is melted, first melts and then solidifies into a single crystal in the falling part of the temperature profile, as described for example in PCT / DEOl / 00788. The single crystal is for the manufacture of optical components for the

DUV range (λ <250 nm; DUV = Deep UV) provided.

CaF ₂ single crystals are generally produced in a tubular drawing furnace under vacuum at 10 ^"4 to 10 ^{" 7} mbar by the Stockbarger-Bridgman or the Vertical Gradient Freeze (VGF) process. The crucible filled with the raw material for the crystal is subjected to a temperature gradient along its length in the vertical and (rotationally symmetrical) in the horizontal direction in order to first melt the material and then allow it to solidify at a low temperature gradient. The vertical gradient is intended to ensure an essentially flat crystallization front. There is usually a seed crystal on the bottom of the crucible. The temperature is lowest there. Crystallization begins at the seed crystal when the temperature of the CaF melt falls below the melting point. According to Stockbarger-Bridgman, the crucible is moved mechanically by a temperature profile impressed on the drawing furnace, according to the newer VGF method, the temperature profile of a gradient tube furnace is moved electrically (at a speed of about 1 mm / hour) past the stationary crucible. The production time for a CaF ₂ single crystal with a height of 200 - 400 mm and a diameter of about 250 mm for optical components is several weeks. Only the most exact observance of the temperature of around 1400 ° C in the area of the crystallization front (permitted temperature fluctuations <1 ° C in the axial and <5 ° C in the radial direction) and a sufficient residence time at this temperature guarantee the formation of a highly homogeneous single crystal with refractive index fluctuations Δn < 1 x 10 ^"6. Disturbances of the homogeneity also show up in a voltage birefringence, which makes the crystal unusable for the desired optical purposes.

The components made from the CaF ₂ single crystals - these are mainly lenses, prisms and optical windows - are used, for example, in optical devices for DUV photolithography such as steppers and excimer lasers.

Steppers are devices with which the structures of integrated circuits are optically mapped onto photoresist-coated semiconductor wafers. In order to be able to image the required fine structures (the structure widths are currently around 0.25 μm), the optical component must have a high degree of homogeneity - the refractive index or the refractive index, as mentioned, must be constant over the entire component to Δn <10 ^"6 Excimer lasers with wavelengths of 193 nm or 157 nm are used as the light source.

In order to keep the exposure times when exposing the photoresist-coated semiconductor wafers short, high-intensity radiation and high light transmission of the optical components are required. Short exposure times mean less time is spent in the manufacture of the semiconductor chips and enable the systems for chip production to be used to a high degree. The permeability of the CaF ₂ single crystals in the DUV range can also change subsequently. If microscopic errors are introduced into the crystal, for example during processing, the transmission behavior of the crystal deteriorates.

Any disturbance in the regular crystal structure such as defects, dislocations and foreign atoms in the crystal lattice also increases the sensitivity to radiation damage caused by hard radiation, as the intense UV light from excimer lasers represents. This creates light-absorbing color centers in the crystal that reduce its permeability.

Foreign atoms such as Na, K, Li and Mg can occur in total up to a content of 100 ppm in CaF ₂ . Through special measures, such as. B. the cleaning of the starting substances and / or the use of scavenger in crystal growth, the impurity content can be reduced to less than 1 ppm. The production of a CaF ₂ single crystal goes through six phases:

1. The crucible with the CaF ₂ raw material is slowly heated to the desorption temperature of the water from about 400 ° C to 600 ° C and left at this temperature for some time to dewater the raw material.

2. The temperature is then raised to about 1450 ° C over a period of about 20 hours; in this phase the oxygen is removed from the raw material. For this purpose, fluorides such as PbF ₂ , SnF ₂ or ZnF ₂ are mixed into the raw material, which have a high affinity for oxygen and those with the oxygen contained in the raw material react to the corresponding volatile oxide. The unused fluorides finally also evaporate completely at this temperature. This process is called the scavenger process.

3. The foreign atomic compounds are then evaporated together with CaF ₂ for about a week at 1450 ° C., the total mass of the vaporized substances determining the freedom from foreign atoms of the single crystal. For each crystal growing system, depending on the raw material used and the structure of the system, there is a minimum mass of material to be evaporated in order to obtain a single crystal with the required purity.

4. The actual crystal growth then takes place in an approximately two-week phase, during which the temperature is gradually reduced to approximately 1200 ° C.

5. and 6. The resulting crystal is then cooled to room temperature in two hours.

The strength agent is preferably added to the raw material as a fluoride and possibly also as an oxide. It is particularly preferred according to the invention to use the agent in its lowest occurring oxidized form or oxidation state, i. H. z. B. in the case of Ce or Yb as 3+ instead of 4+.

Even a comparatively high contamination of the CaF ₂ single crystal, particularly in the form of a high Na content between about 1 and 10 ppm, then no longer has any influence on the properties of the crystal. The manufacturing steps sawing / cutting, grinding / lapping and polishing are carried out to produce lenses etc. from the raw crystal. These steps involve strong mechanical effects on the crystal structure of the CaF ₂ with its low breaking or surface energy of approx. 490 mJ / m ² . The CaF ₂ preferably cleaves along the 111 surfaces, which are also identical to the surfaces of the lenses, so that mechanical forces act parallel to the surfaces during each processing. This means that when the 111 surfaces are machined, they easily split and thus the material fails.

By adding the strength agents according to the invention, the breaking strength of the crystal is now increased, usually by at least 4 or 5%. Usually the strength is increased by 5 - 20%, usually by 10 - 15%.

The increase in strength parallel to the 111 surface is determined as follows: The surfaces are ground and polished on cubes with an edge length of 15 mm. The top and bottom of the cube are 111 faces. The cubes are fixed on a table to prevent them from moving and laterally a stamp is moved against the upper half of the side surface until a break occurs.

The invention also relates to the use of such single crystals obtained according to the invention for the production of optical elements for DUV lithography and for the production of wafers coated with photoresist and thus for the production of electronic devices. The invention therefore also relates to the use of the single crystals obtained by means of the method according to the invention and / or in the device according to the invention for the production of lenses, prisms, light guide rods, optical windows and optical devices for DUV lithography, in particular for the production of steppers and excimer lasers and thus also for the production of integrated circuits and electronic devices such as computers containing computer chips and other electronic devices which contain chip-like integrated circuits.

* * *

Claims

claims

1. A method for producing unbreakable large-format calcium fluoride single crystals for optical components by melting CaF ₂ raw material and then cooling with solidification of the melt to a single crystal, characterized in that a strength agent is added to the calcium fluoride raw material to increase the breaking strength of the crystal from Th, Ba, W, Sb in an amount between 10 and 15,000 ppm.

2. The method according to claim 1, characterized in that the strength agent is added in its lowest oxidation level.

3. The method according to any one of the preceding claims, characterized in that the strength agent is added as a salt and / or oxide.

4. The method according to any one of the preceding claims, characterized in that the oxide in an amount of

100 - 10,000 ppm is used.

5. The method according to any one of the preceding claims, characterized in that the strength agent is a fluoride.

6. The method according to any one of the preceding claims, characterized in that the fluoride in an amount of

20 - 500 ppm is added.

7. The method according to any one of the preceding claims, characterized in that it contains strontium as a further strength agent.

8. The method according to any one of the preceding claims, characterized in that a scavenger is added to the calcium fluoride raw material before or during the melting.

9. The method according to any one of the preceding claims, characterized in that the raw CaF ₂ material contains as an impurity between 1 and 10 ppm sodium.

10. The method according to any one of the preceding claims, characterized in that the raw CaF ₂ material contains up to 100 ppm of further impurities such as K, Li and Mg foreign atoms.

11. Use of calcium fluoride single crystal obtained according to one of the preceding claims for the production of lenses, prisms, light guide rods, optical windows and optical components for DUV photolithography, steppers, excimer lasers, wafers, computer chips, as well as integrated circuits and electronic devices, such Circuits and chips included.

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