Schultheis, Markus (Fulda, DE)
Simons, Christoph (Biebergemünd, DE)
Weigert, Martin (Hanau, DE)

Plaque It!

10/223531

10/07/2008

08/19/2002

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W.C. Heraeus GmbH & Co., KG (Hanau, DE)

264/667

419/39, 419/30, 501/134, 204/192.22, 204/298.13, 264/681, 204/298.12, 204/192.28, 419/38, 204/192.26, 204/192.27

C23C14/35; B28B1/00; B22F1/00; C04B35/00

264/667, 419/39, 204/192.22, 264/681, 204/298.12, 204/192.12, 204/192.15, 204/192.26, 419/38, 501/134, 419/30, 264/603, 204/298.13, 204/192.28, 204/192.27

4497700	Method of coating a transparent substrate	February, 1985	Groth et al.	204/192.27
5110662	Coated glazing material	May, 1992	Depauw et al.	428/192
5279722	Method for manufacturing panes with high transmissivity in the visible range of the spectrum and with high reflectivity for thermal radiation	January, 1994	Szczyrbowski et al.	204/192.27
5403458	Sputter-coating target and method of use	April, 1995	Hartig et al.	204/192.15
5962115	Pane of transparent material having a low emissivity	October, 1999	Zmelty et al.	428/216
6018169	Solid-state image sensor and method of fabricating the same	January, 2000	Tohyama
6193856	Target and process for its production, and method for forming a film having a highly refractive index	February, 2001	Kida et al.	204/192.22
6387446	Method for depositing titanium oxide layers using soluble powders	May, 2002	Lobmann et al.	427/372.2
6761984	Article coated with photocatalyst film, method for preparing the article and sputtering target for use in coating with the film	July, 2004	Anzaki et al.	428/697
20010006148	INTERFERENCE LAYER SYSTEM	July, 2001	Szczyrbowski et al.

DE3802852	August, 1989
DE4135701	May, 1993
DE4211363	October, 1993
DE19520843	December, 1996
DE19644752	April, 1998
DE19845291	April, 2000
DE19858227	June, 2000
DE19922162	November, 2000
DE19958424	May, 2002
EP1068899	January, 2001			Multilayer structure and process for producing the same
JP4141577	May, 1992
JP6305888	November, 1994
JP07233469	May, 1995			TARGET, ITS PRODUCTION AND PRODUCTION OF HIGH-REFRACTIVE-INDEX FILM
JP7233469	September, 1995
WO/1997/025450	July, 1997			PROCESS FOR COATING A SUBSTRATE WITH TITANIUM DIOXIDE
WO/1997/025451	July, 1997			SPUTTERING TARGETS AND METHOD FOR THE PREPARATION THEREOF
WO/2001/046488	June, 2001			ARTICLE COATED WITH PHOTOCATALYST FILM, METHOD FOR PREPARING THE ARTICLE AND SPUTTERING TARGET FOR USE IN COATING WITH THE FILM

Stein, The Random House College Dictionary, 1982, p. 1379.
Machine Translation of JP 07-233469.
Article entitled “Temperable Low Emissivity Coating Based on Twin Magnetron Sputtered TiO2 and Si3N4 Layers” by Szczyrbowski, et al. appearing in 1999 Society of Vacuum Coaters, pp. 141-146.

Mcdonald, Rodney G.

Stoffel, Klaus P.
Wolff & Samson PC

We claim:

1. An electrically conducting sputter target comprising: titanium dioxide (TiO₂) with an electrical resistivity of less than 5 Ω-cm; and a doping agent added to the TiO₂ in an amount of less than 5 mole %, the doping agent being gallium oxide (Ga₂O₃), whereby the sputter target has a density of 3.4-4.1 g/cm³ and can produce coatings having an index of refraction of greater than 2.3.

2. A sputter target in accordance with claim 1, wherein the doping agent is present in an amount of 0.05 to 1.0 mole %.

3. A method for producing a titanium dioxide coating on a substrate so that the coating has an index of refraction greater than 2.3, comprising the steps of: providing an electrically conducting sputter target including titanium dioxide (TiO₂) with an electrical resistivity of less than 5 Ω-cm, and a doping agent added to the TiO₂ in an amount of less than 5 mole %, the doping agent being gallium oxide (Ga₂O₃); and coating the substrate by direct-current sputtering, the target having a density of 3.4-4.1 g/cm³.

4. A method as defined in claim 3, wherein the coating step includes sputtering with planar magnetron cathodes.

5. A method as defined in claim 3, wherein the coating step includes sputtering with tubular magnetron cathodes.

6. A process for producing a sputter target, the process comprising the steps of: mixing titanium dioxide having an electrical resistivity of less than 5 Ω-cm with gallium oxide (Ga₂O₃) as a doping agent; cold pressuring the mixture; and sintering the cold-pressed mixture whereby the sputter target has a density of 3.4-4.1 g/cm³.

7. A method for using an electrically conducting sputter target comprising titanium dioxide (TiO₂) with an electrical resistivity of less than 5 Ω-cm, and a doping agent added to the TiO₂ in an amount of less than 5 mole %, the doping agent being gallium oxide (Ga₂O₃), whereby the sputter target has a density of 3.4-4.1 g/cm³ and can produce coatings having an index of refraction greater than 2.3, for large-area glass coating in the production of architectural glass.

BACKGROUND OF THE INVENTION

The invention relates to a sputter target composed of titanium dioxide for producing optically active functional coatings made of titanium dioxide. Coatings of this type are used, for example, in combination with coatings composed of other materials, for large-area glass coating in the production of architectural glass.

Coating systems that reflect thermal radiation, which consist, for example, of a coating of silver as a functional coating and at least one other coating composed of a metal oxide as a dielectric antireflection coating, are described, for example, in DE 41 35 701 A1. The metal oxide coating, which is placed above the silver coating, has the additional purpose, for example, of preventing long-term damage to the silver coating by oxidation. One of the requirements placed on the metal oxide coating is that it must not too strongly reduce the transmission of the total coating system in the visible wavelength range of light (e.g., in the case of automotive windshields), a transmission factor of at least 75% is required (DE 198 58 227 C1).

The basic requirement for coating systems of this type is thus the highest possible transmission in the spectral range of solar radiation, i.e., in the wavelength range of 300-2,500 nm, and especially in the range of visible light of 380-780 nm, while in the range of thermal radiation, i.e., the wavelength range of 4,000-50,000 nm, an emissivity value as low as possible is required (DE 199 22 162 A1). Coating systems of this type are described in the literature as “low-e coatings” (low heat emissivity), e.g., Szczyrbowski et al., 1999, Society of Vacuum Coaters 505/856-7188, 42 ^nd Annual Technical Conference Proceedings (1999), pp. 141-146. In practice, dielectric coatings are applied as topcoats or undercoats over or under the silver coatings. Various metal oxides and multiple combinations may be used for this purpose.

The homogeneity and long-term stability requirements of the coatings produced in this way are becoming increasingly stringent due to the increasing use of ever-larger substrates (e.g., glass plates with dimensions of 3.21 m by 6.00 m are customary, DE 199 22 162 A1), and the chemical engineering process parameters must be stably controlled.

It should be pointed out in passing, for example, that with many metal oxide coatings that have the required property of being highly refractive and that are suitable for the applications described above, the metal oxides may occur in several modifications. However, in the production of these coating systems, the requirement of reproducibility of the index of refraction of the highly refractive coating absolutely must be satisfied. In the specific application referred to here, it is necessary to guarantee an index of refraction of greater than 2.3, since otherwise the desired optical function of the titanium dioxide coating is not obtained. For the example of titanium dioxide, which can occur in rutile, anatase, and brookite modifications, all of which have different mechanical and optical properties, this means that, depending of the crystal lattice structure, i.e., depending on the titanium dioxide modification, the index of refraction can vary from 1.9 to 2.6. Minor instabilities in the way the process is carried out can thus have a significant effect on the index of refraction of the coating.

The literature describes a large number of different processes for producing titanium dioxide coatings on various substrates, e.g., sputtering, vaporization, chemical vapor deposition, and metal-organic decomposition. DE 198 45 291 A1 describes, for example, a wet-chemical process via a sol-gel process, in which a soluble powder that contains titanium is first produced, which is applied to a substrate by a chemical process, and then the coating is produced by a subsequent tempering process.

In contrast to this process, in PVD (physical vapor deposition) processes, the coatings are produced on the surface of a substrate by converting the coating material to the gas phase. In these processes, the coating is usually produced with a metal or metal compound. The advantage of PVD processes lies in the fact that almost all metals, metal alloys and metallic compounds can be deposited on virtually all materials (the substrate), i.e., suitable substrates include, for example, glass, plastics, ceramics, metals, and metal alloys.

PVD processes include sputtering or cathode sputtering, in which the sputter gas (e.g., argon) is ionized to form a plasma, and, in a gas atmosphere, the noble gas ions formed in this way are directed at a material source (the target). The energy of the ions is so great that atoms of the target material are dislodged and become the coating material. The vaporized components of the target form the desired coating on the surface of the workpiece to be coated. The process parameters during the sputtering process are selected in such a way as to systematically control the properties of the coatings, such as hardness, microstructure, adhesion, electrical conductivity, optical properties, and chemical and thermal stability.

Electrically conducting materials can be sputtered with a direct-current source, whereas in the sputtering of dielectric materials, it is necessary, for example, to ignite a high-frequency plasma in front of the target, so that the voltage does not drop over the insulating target.

Compared to conventional vaporization, sputtering, i.e., cathode sputtering, has the advantage that there are significantly more combinations in the selection of the material to be coated and the material to be “vaporized”. It is not an insignificant advantage that, for example, the possibility of including reactions with the plasma in the so-called “vapor chamber” can be incorporated in the entire engineering process, i.e., “reactive sputtering” is used, from which other variation possibilities may then be developed.

Consistent with the desired process parameters with respect to the gas-discharge characteristics, primarily inert gases are used, especially argon or helium. Furthermore, reactive gases may also be used for reactive gas sputtering, such as oxygen, acetylene, or nitrogen. In both inert gas sputtering and reactive gas sputtering, the sputter target constitutes tie reservoir of material to be consumed. In the case of inert gas sputtering, the coating formed on the substrate is deposited exclusively from the sputter target, while in reactive gas sputtering, the coating formed on the substrate is a reaction product between the target material and the reaction gas. These then are the basic prior-art possibilities, which can be used to produce thin coatings of dielectric materials (i.e., in the case of the present invention, metal oxides, such as titanium dioxide) by sputtering. Either a target made of titanium is used, whose metallic component reacts with oxygen in the plasma to produce a titanium dioxide coating on the substrate, or a target that already consists of titanium dioxide is used, so that the titanium dioxide is vaporized and deposited as a coating on the substrate.

DE 42 11 363 A1 and DE 195 20 843 A1 describe, for example, processes such as these for producing coatings with high transmission in the visible spectral range and high reflection in the thermal radiation range with the use of reactive sputtering, and they describe the coating combinations produced by these processes with a titanium dioxide coating integrated in the coating system. In the well-known reactive sputtering processes, the target is the cathode. Alternatively, so-called double electrodes are also used, which alternate between acting as the anode and the cathode. Electric power is supplied to the electrodes either as direct current or alternating current, as is described, for example, in DE 38 02 852 A1.

If dielectric coatings, e.g., SiO ₂ , Al ₂ O ₃ , ZrO ₂ , TiO ₂ , and ZnO, are to be deposited, the target to be sputtered then consists of the corresponding metal that is present as an element in the specified compounds, and a plasma cloud, which consists, for example, of an ionized Ar/O ₂ or Ar/N ₂ mixture, strikes the metal of which the target is composed and causes it to sputter. The metal oxides (or nitrides) formed in this way are then deposited on the surface of the substrate.

It is known from DE 196 44 752 A1 that stable sputtering conditions for producing high-grade metal oxide coatings of high optical quality in a reproducible and industrially cost-effective way can be adjusted only with the aid of expensive process control equipment. The problem associated with the stable production of dielectric titanium dioxide coatings of this type in the reactive sputtering mode by the DC sputtering process used in the cited patent is based on the fact that the cathode characteristic curve of the DC-supplied, high-power cathode, in which the titanium target is integrated, has a hysteresis loop. Depending on the oxygen supply in the plasma reaction chamber, this can cause the sputter operating point to jump uncontrollably from the metallic sputtering state to the oxidic sputtering state.

The sputtering rate itself is increased by the use of at least two adjacent electrodes arranged together in the plasma chamber, which is important especially in the case of substrates that are to be coated in continuous operations. Further details on sputtering rates and coating growth rates compared to conventional DC sputtering and the use of a “TWIN-MAG” may be found in the publication cited earlier (Szczyrbowski et al., 1999, Society of Vacuum Coaters 505/856-7188, 42 ^nd Annual Technical Conference Proceedings (1999), pp. 141-146).

It is also well known, e.g., from U.S. Pat. No. 6,193,856 (WO 97/08,359), that the rate of coating growth in reactive sputtering is very low, which is a disadvantage, particularly when substrates with large surface areas are being coated, and results in low productivity and high costs. A well-known possibility for avoiding these problems is specified in above-cited U.S. Pat. No. 6,193,856, in which the reactive partial process is omitted, the dielectric metal oxide is used as the sputter target, and then, for example, a metal oxide film composed of titanium dioxide is deposited on the substrate. A disadvantage of these processes is that these metal oxides generally have no electrical conductivity or only extremely low electrical conductivity and are therefore inherently unsuitable for the direct-current sputtering process.

According to our own results, the “critical” value for the electrical resistivity of a sputter target composed of a suitable metal oxide for a direct-current sputtering process, preferably titanium dioxide, is about 1 Ω-cm. It must be clearly stated that this is not a precise mathematical value, but rather is intended only to indicate the order of magnitude. This “critical” value depends strongly on the system parameters and is variously specified in the patent literature. For example, in WO 97/25,450, the value of 1 Ω-cm is confirmed, but a preferred value of 0.1 Ω-cm is also specified. In WO 97/25,451, a value of 10 Ω-cm is identified as being clearly too high for a direct-current sputtering process, a value of 0.5 Ω-cm is recommended, and 0.02 Ω-cm is given as the best value.

In this connection, a substoichiometric TiO _x with x in the range of 1.55 to 1.95 is generally used, e.g., in accordance with WO 97/25,450 and WO 97/25,451. To produce multiple coating systems with specific photocatalytic properties, EP 1,068,899 A1 describes the addition of oxides of metals from the group comprising niobium, tantalum, vanadium, zirconium, tin, zinc, chromium, ad copper in an amount of about 1-10 wt. % to the principal component, titanium dioxide. The desired active coatings are produced in this way on, for example, glass substrates. To produce coatings from a mixture of titanium dioxide and the other oxides specified above, one uses an electrically conductive sintered target composed of this mixture.

Examples of processes for producing a titanium dioxide sputter target of this type with a substoichiometric oxygen content for use in a direct-current sputtering process include hot pressing in a nonoxidizing atmosphere, as described in JP-A 07[1995]-233,469, or a reducing plasma spray process, as described in WO 97/08,359. However, these two processes are very cost-intensive.

In regard to the use of titanium dioxide targets for direct-current sputtering, the patent literature, e.g., WO 97/25,450, gives a possible improvement in the sputtering rate by a factor of 10 compared to conventional reactive sputtering with a pure titanium target and oxygen supplied in the plasma.

However, titanium dioxide targets of this type are generally economically infeasible due to their high production costs.

SUMMARY AND DESCRIPTION OF THE INVENTION

Therefore, the object of the present invention is to produce a titanium dioxide sputter target in such a way that it is suitable for direct-current sputtering, i.e., the electrical resistivity should be less than 5 Ω-cm (but preferably even less than 1 Ω-cm). To this end, doping agents are added in small amounts to increase the electrical conductivity of the sputter target. However, the doping agents must not have any negative effects on the properties of the coating. It must be guaranteed that the index of refraction of the coating deposited on the substrate is greater than 2.3, and, in addition, the doping agents must not cause increased absorption. Finally, it must be possible to produce the sputter targets in large numbers by a process that is very cost-effective and easy to control from the chemical engineering standpoint.

In accordance with the invention, these goals are achieved by adding doping agents to the basic titanium dioxide target material and then processing this mixture by a simple ceramic processing technique, e.g., cold pressing and sintering. The doping agent or mixture of doping agents is selected from the group of oxides comprising indium oxide (In ₂ O ₃ ), zinc oxide (ZnO), bismuth oxide (Bi ₂ O ₃ ), aluminum oxide (Al ₂ O ₃ ), gallium oxide (Ga ₂ O ₃ ), antimony oxide (Sb ₂ O ₃ ), and zirconium oxide (ZrO ₂ ) and is added to the titanium dioxide in amounts of less than 5 mole %. It was found to be especially advantageous to limit the relative amount of doping agent to the range of 0.05 mole % to 1.0 mole %. Especially good results are obtained when indium oxide (In ₂ O ₃ ) is selected as the doping agent.

The individual components of the mixture of titanium dioxide powder and the powder of the selected doping agent are mixed, for example, in an asymmetric moved mixer or in a rotating mixing vessel with a contrarotating stirrer. A pressing mold is then filled with the mixture of powders, and the mixture is pressed. The pressing pressure may be about 0.5 t/cm ² to 2.0 t/cm ² . To produce homogeneously compressed compacts, it is especially advantageous if, after the first pressing operation, a secondary compression is carried out in an isostatic pressing process with a compacting pressure in the second operation in the range of 2.0 t/cm ² to 3.0 t/cm ² .

After the compacting process, the compacts are sintered at 1,300-1,600° C. and standard pressure. The densities produced in this way are 3.4-4.1 g/cm ³ .

Possible embodiments are listed in the table below.

TABLE OF EMBODIMENTS WITH COMPARATIVE VALUES
	Compacting	Sintering		Electrical
	Pressure	Temperature	Density	Resistivity
Powder Mixture	|t/cm 2 |	|° C.|	|g/cm 3 |	|Ω-cm|	Source
0.5 mol % ZrO 2 , Remainder TiO 2	0.7	1400	3.48	0.4	Our Own Test
1.0 mol % ZrO 2 , Remainder TiO 2	1.5	1450	3.80	0.15	Our Own Test
0.05 mol % In 2 O 3 , Remainder TiO 2	1.5	1450	4.01	0.09	Our Own Test
0.25 mol % In 2 O 3 , Remainder TiO 2	1.5	1450	3.90	0.04	Our Own Text
0.5 mol % In 2 O 3 , Remainder TiO 2	1.5	1450	3.79	0.06	Our Own Test
0.5 mol % Al 2 O 3 , Remainder TiO 2	1.5	1450	3.85	0.21	Our Own Test
0.5 mol % Bi 2 O 2 , Remainder TiO 2	1.8	1500	3.86	0.82	Our Own Test
0.5 mol % Al 2 O 3 + 0.05 mol %	1.5	1500	3.92	0.19	Our Own Test
In 2 O 3 + 0.05 mol % ZrO 2 ,
Remainder TiO 2
0.05 mol % ZnO + 0.05 mol % Al 2 O 3 ,	1.5	1450	3.78	0.18	Our Own Test
Remainder TiO 2
Comparative Value
TiO 2	1.0	1400	3.85	1000	Our Own Test
TiO 2		Hot-pressed at	3.90-4.17	0.12-0.35	Our Own Test
		1,100-1,400° C.
TiO 2		Plasma-sprayed		0.02-0.5