WO2004076353A1

WO2004076353A1 - Process for producing calcium fluoride

Info

Publication number: WO2004076353A1
Application number: PCT/EP2004/002035
Authority: WO
Inventors: Thomas Scholten; Harald Heuduk; Michael Theissen
Original assignee: Honeywell Specialty Chemicals Seelze Gmbh
Priority date: 2003-02-28
Filing date: 2004-03-01
Publication date: 2004-09-10

Abstract

A process for preparing calcium fluoride, comprising step (i): (i) addition of at least one aqueous solution of a calcium salt and at least one aqueous solution containing fluoride ions to a solution containing an acid which is preferably essentially free of fluoride, wherein calcium fluoride is formed.

Description

Process for Producing Calcium Fluoride

The present invention relates to a process for preparing calcium fluoride wherein an aqueous solution of at least one calcium salt and an aqueous solution containing fluoride ions are added to a solution of an acid which is free of fluoride. According to an especially preferred embodiment, high purity calcium fluoride is prepared in the process according to the invention, which is different from known calcium fluoride. The present invention also relates to the calcium fluoride obtainable by the process according to the invention and to the use of the calcium fluoride for producing single crystals, in particular for litographic processes. In particular, the invention relates to optical items, especially to lenses, mirrors and optical items having plane surfaces, such as prisms or cubes consisting primarily of the calcium fluoride according to the present invention.

According to the processes of the prior art, calcium fluoride is normally produced by the so-called carbonate process. Especially concerning the production of high purity calcium fluoride this process, however, has the disadvantage that the calcium carbonate which is not dissolved in the aqueous systems often used does not react completely when reacted with e.g. hydrogen fluoride, and that the final product includes carbonate as impurity. Tempering the product may lead to the formation of calcium oxide which is an impurity as well. All impurities as well as the physical properties of calcium fluoride have a considerable impact on the y application such as microelectronic applications or litographic processes using optical items made from calcium fluoride. A summary of processes of how to produce calcium fluoride via the carbonate process and of other production methods can be found, e. g., in „Gmelins Handbuch der anorganischen Chemie - Calcium Teil B - Lieferung 2", Verlag Chemie GmbH, Weinheim / Bergstr., 8th edition (1957) pp 387-391 and in „Gmelins Handbuch der anorganischen Chemie - Calcium Teil B - Lieferung 1", Verlag Chemie GmbH, Weinheim / Bergstr., 8th edition (1956) pp 70-71. It was an object of the present invention to provide a process which does not show the above mentioned disadvantages.

Therefore, the invention relates to a process for preparing calcium fluoride, comprising step (i):

(i) addition of at least one aqueous solution of at least one calcium salt and at least one aqueous solution containing fluoride ions to a solution containing at least one acid which is essentially free of fluoride, wherein calcium fluoride is formed.

In a preferred embodiment of the process according to the invention the at least one acid which is free of fluoride is soluble in water. In an even more preferred embodiment the solution to which the solution of the at least one calcium salt and the solution containing fluoride ions are added is an aqueous solution of an acid which is soluble in water.

In general, there is no special limitation as to the selection of the at least one acid which is essentially free of fluoride. In the present invention hydrochloric acid, nitric acid, formic acid and acetic acid or a mixture of two or more thereof are preferably used.

Therefore, the present invention relates to a process as described above characterized in that the acid which is essentially free of fluoride is preferably selected from the group consisting of hydrochloric acid, nitric acid, formic acid, acetic acid and a mixture of two or more thereof.

Acids are particularly preferred which, when used, allow the pH value of the solution in which the aqueous solution containing the dissolved calcium salt and fluoride ions are contacted to be adjusted to have a value of 1 or below.

Therefore, the present invention also relates to a process as described above characterized in that the solution containing the at least one acid which is essentially free of fluo- ride is an aqueous solution having a pH value of 1 or below. If the aqueous solutions containing on the one hand the at least one calcium salt and on the other hand fluoride ions are brought together in the acidic solution, the pH value of the new resulting mixture may change. According to the inventive process it is possible in this case to regulate the pH value of the mixture so that it remains constant or that it is changed in a desirable way. According to one embodiment the pH value of the mixture can be regulated in such a way that it is in a range of 1 or below thereby remaining constant or varying, independent of the addition of the aqueous solutions containing the at least one calcium salt on the one hand and fluoride ions on the other. Controlling and regulating the pH value can be achieved by methods known in the art. In one of the pre- ferred embodiments the pH value of the acidic solution is adjusted prior to the addition of the other aqueous solutions so that it remains in the desired range even without regulation after the addition of said solutions.

In general, there are no limitations as to the use of the at least one calcium salt which is dissolved in one of the aqueous solutions is used. As especially preferred calcium salts of the present invention, calcium chloride, calcium nitrate, calcium formate, calcium acetate or a mixture of two or more thereof is used.

Therefore, the present invention also relates to a process as described above character- ized in that the at least one calcium salt is selected from the group consisting of calcium chloride, calcium nitrate, calcium formate, calcium acetate and a mixture of two or more thereof.

As far as the at least one aqueous solution containing fluoride ions is concerned, solu- tion are preferred which contain hydrogen fluoride dissolved in water or ammonium fluoride dissolved in water or hydrogen fluoride as well as ammonium fluoride.

Therefore, the present invention also relates to a process as described above characterized in that the at least one aqueous solution containing fluoride ions is an aqueous so- lution of hydrogen fluoride, ammonium fluoride or a mixture thereof.

Especially preferred combinations of the aqueous solutions are: aqueous solution containing calcium chloride, aqueous solution containing hydrogen fluoride, aqueous solution containing hydrogen chloride; aqueous solution containing calcium nitrate, aqueous solution containing hydrogen fluoride, aqueous solution containing nitric acid; - aqueous solution containing calcium acetate, aqueous solution containing hydrogen fluoride, aqueous solution containing hydrogen chloride.

In general, the aqueous solutions may be added to the acidic solution in arbitrary order. One possibility, e.g., is to add the solution of the calcium salt to the acidic solution first and then to add the solution containing fluoride ions to the resulting mixture. Another possibility, e.g., is to add the solution containing fluoride ions to the acidic solution first and then to add the solution of the calcium salt to the resulting mixture. If two or more solutions containing fluoride ions or two or more solutions containing calcium salt or two or more solutions containing fluoride ions as well as two or more solutions con- taining calcium salt are used, all possibilities with respect to the order of adding the respective solutions may be used.

According to an especially preferred embodiment of the inventive process the solution containing the at least one calcium salt and the solution containing fluoride ions are added to the acidic solution synchronously.

The term synchronous addition" as used in the context of the present application defines processes in which dependent on the concentration of the at least one solution containing the at least one calcium salt and the concentration of the at least one solution containing fluoride ions are added to the acidic solution in such a way that calcium ions and fluoride ions are added to the acidic solution with a stoichiometric ratio of essentially 1 : 2.

Therefore, the present invention also relates to a process as described above wherein the at least one aqueous solution of the at least one calcium salt and the at least one aqueous solution containing fluoride ions are added synchronously to the at least one solution containing an acid which is essentially free of fluoride. According to another preferred embodiment of the inventive process a certain amount of hydrogen fluoride and / or ammonium fluoride is added to the preferably aqueous, acidic solution prior to the synchronous addition of the aqueous solutions in order to avoid a surplus of calcium ions. There is generally no limitation as to the amount of said added hydrogen fluoride and / or ammonium fluoride, and the amount may be adjusted to the amounts of calcium ions and fluoride ions which are synchronously added to the acidic solutions per time unit.

In the present invention the synchronous addition can be achieved by all possible processes. Among others, the concentrations of the aqueous solutions added to the acidic solution and the addition rate of said solutions may be coordinated in such a way that the desired amount of calcium ions and fluoride ions are brought together per time unit in the acidic solution. In particular, the concentrations of the aqueous solutions may and the addition rate may be adjusted to the experimental volume restrictions of the reactor. In general, e.g. concentrated solutions of calcium salt and fluoride ions or solutions with a calcium salt or fluoride ion content of 60 wt-%, 40 w -% or less can be brought together. The concentrations of the different aqueous solutions which are brought together can be different from each other. Diluted solutions are generally preferred because of the precision of measuring out the exact doses.

Furthermore, the addition of the aqueous solutions can be carried out continuously or in discrete steps (partial charges). When adding continuously and synchronously, calcium ions and fluoride ions are added in a stoichiometric ratio of essentially 1 : 2 per time unit to the acidic solution. When adding in discrete steps and synchronously, calcium ions and fluoride ions are added in a stoichiometric ratio of essentially 1 : 2 essentially at the same time in each addition step to the acidic solution.

The addition [of the aqueous solution] can be carried out continuously or in discrete, particularly small steps, preferably synchronously. The concentration of the calcium salt containing aqueous solution with respect to calcium salt generally is in the range of from 5 to 55wt.-%, preferably of from 20 to 45 wt.-% and especially preferably of from 30 to 40 wt-%.

The concentration of the fluoride ions containing aqueous solution with respect to fluoride ions generally is in the range of from 5 to 65 wt-%, preferably of from 10 to 60 wt- % and especially preferably of from 20 to 50 wt-%.

While adding the aqueous solutions to the acidic solution the resulting mixture in which the calcium fluoride is formed is kept homogeneous by one or more methods wherein one only has to take into account that keeping the mixture homogeneous does not prevent the calcium fluoride from being formed. A preferred method is stirring the mixture, and stirring can be carried out by every suitable method.

As to the temperatures the addition of the solutions to the acidic solution is carried out at, no special restrictions exist as long as calcium fluoride is formed in the acidic solutions.

According to an especially preferred embodiment of the inventive process the aqueous solutions are added to the acidic solution at temperatures above room temperature. More preferably the addition takes place at temperatures of from 50 to 135 °C, more preferably of from 60 to 130 °C, and most preferably of from 80 to 125 °C.

Therefore the present invention also relates to a process as described above wherein the at least one aqueous solution of the at least one calcium salt and the at least one aqueous solution containing fluoride ions are added to the at least one solution containing an acid which is essentially free of fluoride at temperatures of from 80 to 125 °C.

In this context it is possible to pre-heat one or more solutions to be added to the acidic solution prior to the addition. The temperatures of the solutions to be added and the acidic solution may be chosen prior to the addition or the additions in such a way that the temperature of the mixture resulting from the addition is within the preferred ranges described above. The temperatures of the solutions to be added may be different from the preferred temperature ranges described above or may be within these ranges. Furthermore it is possible to bring only the acidic solution to a certain temperature above room temperature, e.g. in the ranges described above, prior to the addition. The tem- perature of the mixture resulting from the addition can be kept constant or may vary, e.g. in the preferred temperature ranges described above, during addition of the solutions by methods known in the art.

Once calcium fluoride is formed in step (i) of the inventive process in the desired amount, it is isolated from the mixture, preferably after stopping the addition of the aqueous solutions.

For this, a batch process or- a continuous process can be used, e. g. in a cascade mixer, a batch reactor with continuous discharge or a reaction tube.

Prior to the actual isolation of the calcium fluoride it is preferred to cool down the reaction mixture containing the calcium fluoride wherein the process of cooling down may take place in one, two or more steps. For instance, a first cooling step may take place in a first vessel like, e.g., the reaction vessel the calcium fluoride is formed in, and in a second cooling step the whole reaction mixture or a part of it may be cooled down in a second vessel. It maybe desirable to subject the solution containing the calcium fluoride to the separation step for obtaining calcium fluoride without cooling down significantly.

During isolation the calcium fluoride formed is separated from the reaction solution by methods known in the art like, e.g., decanting washing, extraction by suction, filtration or a combination of two or more of these methods wherein the same or different methods may be carried out several times one after the other. It is especially preferred to wash the product several times with water. The isolated product is then subjected to a thermal treatment in a further step.

Therefore the present invention also relates to a process as described above, additionally comprising steps (ii) and (iii): (ii) isolation of the calcium fluoride formed in (i);

(iii) thermal treatment of the calcium fluoride isolated in (ii).

According to a preferred embodiment the thermal treatment is carried out in at least two steps (a) and (b).

According to an especially preferred embodiment the washed product is subjected at least once to a drying treatment in a step (a). It is possible to dry the product according to (a) several times wherein the drying temperatures may be equal or different.

The drying temperatures for the product are in arrange up to 200 °C. The drying periods and temperatures may be adjusted to the moisture content of the product to be dried.

According to a preferred embodiment the dried product is subjected to at least one fur- ther thermal treatment in a step (b) wherein the temperature the dried product is subjected to is preferably higher than the drying temperature(s). The temperature for this thermal treatment is in the range of 700 °C or below.

Therefore the present invention also relates to a process as described above character- ized in that according to (iii) the isolated product is subjected to at least one drying step (a), followed by at least one further thermal treatment in a step (b) wherein the temperature with regard to (b) is higher than the temperature(s) with regard to (a) and are in the range of 700 °C or below.

In an especially preferred embodiment the thermal treatment according to (b) is carried out at a temperature of from 400 to 700 °C, more preferably of from 450 to 700 °C and even more preferably of from 500 to 700 °C.

Therefore, the present invention also relates to a process as described above character- ized in that the thermal treatment (iii) comprises at least one step (b) in which the calcium fluoride is subjected to temperatures of from 500 to 700 °C. According to another aspect, the present invention relates to the calcium fluoride as such, which is obtainable by the process described above. In particular, the present invention relates to calcium fluoride, obtainable by a process as described above wherein the calcium fluoride formed in the acidic solution is isolated according to (ii) and ther- mally treated according to (iii).

In an especially preferred embodiment the inventive process is used to produce high purity calcium fluoride, which is novel as such and represents a further subject of the invention. The term „high purity" as used in the context of the present application indi- cates that the degree of purity with respect to a certain kind of impurity of a solution, a suspension, a chemical compound such as a salt, a gas or a liquid is in the range of 250 ppm or below, preferably of 150 ppm or below, more preferably of 1 ppm or below, preferably of 100 ppb or below, more preferably of 10 ppb or below and especially preferably of 1 ppb or below. In the context of the present application a salt, for in- stance, is defined as a high purity salt if it has a degree of purity x with respect to an impurity (X) and a degree of purity y with respect to another impurity (Y) wherein x is equal to or different from y, and x and y are within one of the above-mentioned ranges.

If the calcium fluoride to be produced is a high-purity calcium fluoride it is generally advantageous to use starting materials introduced in the process according to (i) which are sufficiently pure.

Impurities of the calcium fluoride are for example metals like aluminum, antimony, arsenic, barium, beryllium, lead, cadmium, chromium, iron, gallium, germanium, gold, indium, potassium, cobalt, copper, lithium, magnesium, manganese, molybdenum, sodium, nickel, platinum, silver, silicon, strontium, thallium, titanium, vanadium, bismuth, zinc, tin or zirconium. Other impurities are halogens such as chloride, bromide or iodide and, e.g. in the tempered product, oxygen. Water, carbonate, nitrate, nitrogen, rare earths, alkali metals and alkaline earth metals are further impurities largely missing in the calcium fluoride of the invention. The new calcium fluoride, according to the invention is characterized by the following maximum content of the relevant elements:

(a) barium content: < 0.5 ppm (using ICP-OES or ICP-MS)

(b) cobalt content: < 0.1 ppm (using graphite tube-AAS or ICP-MS) (c) lead content: < 0.05 ppm (using ICP-OES)

(d) strontium content: < 1 ppm (using ICP-OES)

(e) carbon content: < 10 ppm (using LECO Element Analyser)

(f) oxygen content: < 20 ppm (using LECO Element Analyser)

OES: Optical emission spectroscopy

MS: Mass spectroscopy

AAS: Atomic absorption spectroscopy

ICP: Inductively coupled plasma

Furthermore, the calcium product according to the invention can be distinguished by its crystalline free- flowing character with a high bulk density and - due to its crystallinity - a homogenous closed particle distribution. (The particle distribution is measured by using a Horiba Granulometer LA 920).

In order to produce high purity calcium fluoride it is especially preferred to use apparatus and tools which are, on the one hand, easy to clean and, on the other hand, inert at temperatures they are subjected to in the process. With respect to the preparation of high purity calcium fluoride being, e.g., highly pure with regard to metallic impurities, plastic materials are preferred. Examples for these materials are, among others, polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), co-polymer of ethylene and tetrafluoro ethylene (ETFE), co-polymer of perfluoro ethylene and alkyl ether (PFA), polytetrafluoro ethylene (PTFE) or a fluorinated co-polymer of ethylene and propylene (FEP). Fluoroplastic materials such as PVDF, PTFE or PFA are preferred.

In an especially preferred embodiment of the inventive process, the resulting calcium fluoride has a packed weight of greater than or equal to 1.20 g/ml or above, preferably of greater than or equal to 1.30 g/ml and especially preferably of greater than or equal to 1.40 g/ml or above. The packed weight is determined by caring out a graduated cylinder which is arbitrarily filled with (a) substance and weighed.

Therefore, the present invention also relates to calcium fluoride as described above, having a packed weight of greater than or equal to 1.20 g/ml or above, preferably of greater than or equal to 1.30 g/ml and especially preferably of greater than or equal to 1.40 g/ml or above.

The present invention also describes a process as described above wherein the calcium fluoride resulting from step (iii) has a packed weight as described above.

The calcium fluoride prepared according to the invention may be used in all possible fields in which calcium fluoride is used. Among others, litographic applications are a preferred field of usage. In many systems producing semiconductors litographic tech- niques are used to produce semiconductor wafers. In order to be able to produce even smallest structures on the wafer, one has to use UV radiation with wavelengths below 200 nm which is generated by an excimer laser based on noble gas-halogene or by a halogene laser based on F₂. The optical lenses used therein must have a high transparency for the UV radiation. Usable raw materials for the lenses are fluorides and espe- cially calcium fluoride. Generally the material is used as a single crystal or as layers in which the calcium ions and fluoride ions are present without bad spots.

In order to comply with the necessary specifications the calcium fluoride must have high purity quality and, e.g., an extremely low content of oxygen and metals.

Therefore, the present applications also relates to the use of calcium fluoride, obtainable by a process as described above or of calcium fluoride as such, as described above, for producing single crystals.

The present invention also relates to the use as described above for producing optical items, particularly lenses, mirrors and optical items having plane boundaries, such as prisms and cubes consisting primarily of the calcium fluoride according to the present invention, especially for the use in litographic processes. Thus, a further subject of the invention are optical items, particularly lenses, mirrors and optical items having plane boundaries, such as prisms and cubes consisting primarily of the calcium fluoride according to the present invention.

In the following example the present invention is explained in more detail.

Example

As starting material in the reaction 30 g 37% hydrochloric acid (quality: SLSI Puranal) dissolved in 300 g H₂O dest were used. As aqueous solutions 301,5 g calcium chloride- 2-hydrate (quality: calcium chloride-2-hydrate, recrystallized) dissolved in 275 H O dest and 215,7 g 40% hydrofluoric acid (quality: SLSI Puranal) in 215 g H₂O dest were used.

The equipment used was a reaction vessel consisting of a cooler and a stirring device with a stirrer motor. The system is closed with exception of the cooler, and all parts contacting the product and internal parts are made of PFA or Teflon.

Hydrochloric acid was put in the reaction vessel and heated to 100 °C. When the (desired) temperature was reached, the HF-dosing was started. After addition of 20 ml HF solution, the calcium chloride solution was added. Both solutions were added at a constant dosing rate within 4.5 hrs, followed by post-stirring at 100 °C for 1.5 hrs.

After cooling down to room temperature, the supernatant was decanted and the solid was washed three times in a decanting way with 900 ml H O dest and the isolated solid was washed with 1500 ml H₂O dest in total.

The product was dried at 120 °C overnight, followed by tempering. During tempering the product was heated for 4 hrs. at temperatures up to 650 °C. The product was obtained as a white crystalline solid having a packed weight of 1.42 g/ml. The oxygen content was less or equal to 30 ppm.

Claims

1. A process for preparing calcium fluoride, comprising step (i):

(i) addition of at least one aqueous solution of at least one calcium salt and at least one aqueous solution containing fluoride ions to a solution containing at least one acid which is preferably essentially free of fluoride, wherein calcium fluoride is formed.

2. A process according to claim 1 wherein the acid which is essentially free of fluoride is selected from the group consisting of hydrochloric acid, nitric acid, formic acid, acetic acid and a mixture of two or more thereof.

3. A process according to claim 1 or 2 wherein the solution containing the at least one acid which is essentially free of fluoride is an aqueous solution having a pH value of smaller than or equal to 1.

4. A process according to any one of claims 1 to 3 wherein the at least one calcium salt is selected from the group consisting of calcium chloride, calcium nitrate, calcium formate, calcium acetate and a mixture of two or more thereof.

5. A process according to any one of claims 1 to 4 wherein the at least one aqueous solution containing fluoride ions is an aqueous solution of hydrogen fluoride, ammonium fluoride or a mixture thereof.

6. A process according to any one of claims 1 to 5 wherein the at least one aqueous solution of the at least one calcium salt and the at least one aqueous solution containing fluoride ions are added synchronously to the at least one solution contain- ing an acid which is essentially free of fluoride.

7. A process according to any one of claims 1 to 6 wherein the at least one aqueous solution of the at least one calcium salt and the at least one aqueous solution containing fluoride ions are added to the at least one solution containing an acid which is essentially free of fluoride at temperatures of from 80 to 105 °C.

8. A process according to any of claims 1 to 7, additionally comprising steps (ii) and (iii):

(ii) isolation of the calcium fluoride formed in (i);

(iii) thermal treatment of the calcium fluoride isolated in (ii).

9. A process according to claim 8 wherein the thermal treatment (iii) comprises at least one step (b) in which the calcium fluoride is subjected to temperatures of from 400 to 700 °C.

10. Calcium fluoride, obtainable by a process according to claim 8 or 9.

11. Calcium fluoride preferably according to claim 10, characterized by one or more, preferably by all of the following features:

(a) strontium content: < 1 ppm (using ICP-OES) (b) carbon content: < 10 ppm (using LECO Element Analyser)

(c) oxygen content: < 20 ppm (using LECO Element Analyser).

12. Calcium fluoride according to claims 10 or 11, characterized by

(a) a barium content of < 0.5 ppm (using ICP-OES or ICP-MS) and/or (b) a cobalt content of < 0.1 ppm (using graphite tube-AAS or ICP-MS) and/or

(c) a lead content of < 0.05 ppm (using ICP-MS), preferably having the three features (a), (b) and (c).

13. Calcium fluoride, according to claims 10 or 11, characterized in that it has a packed weight greater than or equal to 1.20 g/ml, preferably of greater than or equal to 1.30 g/ml and especially preferably of greater than or equal to 1.40 g/ml or above.

14. Calcium fluoride according to claims 10 or 11, characterized in that it is crystalline and free-flowing and that it has a high bulk density and a homogenous and closed particle distribution.

15. Optical item, especially a lense, a mirror or an optical item having plane boundaries, preferably a prism or a cubus, consisting primarily of calcium fluoride, preferably of a monocrystalline calcium fluoride according to one of the claims with regard to calcium fluoride.