The present invention relates to improved sputtering targets and, in particular to sputtering targets of titanium dioxide, and to a method for the preparation thereof.
Sputtered coatings of various oxides (e.g. silica) and nitrides (e.g. silicon nitride) are used to form optical coatings showing interesting properties on a number of substrates. Known applications include low emissivity films on window glasses, cold mirrors on reflectors, enhanced mirrors for photocopiers and antireflective coatings on picture glass or TV screens. These coatings are usually made of stacks of several different layers with different refractive indices, preferably of low and high refractive index, to produce optical filters. For antireflective coatings it is preferred to combine two materials showing the highest and the lowest possible refractive indices. Such materials are titania and silica. Another advantage of these materials is their durability.
Titanium dioxide coatings have a high refractive index and can thus be used to provide coatings of a high refractive index or to provide the high refractive index coatings in optical stacks. The existing process for producing titanium dioxide coatings comprises using titanium metal as the sputtering target and using oxygen as a component of the plasma gas. The titanium is thus converted to titanium dioxide during the sputtering process. Although satisfactory coatings of titanium dioxide can be produced, the rate of production is very slow and much slower than coating with silica.
As a substitute for titanium dioxide it has been suggested to use alternative materials such as niobium oxide. Whilst it is possible to coat a substrate with niobium oxide using a niobium metal target at slightly higher speeds than the equivalent process using titanium, niobium is very expensive.
Thus, there is a need for an improved process for coating titanium dioxide onto substrate materials. We have now surprisingly discovered that titanium dioxide can be sputtered from a target comprising sub-stoichiometric titanium dioxide to provide coatings on a substrate either of sub-stoichiometric titanium dioxide, or titanium dioxide, depending upon the sputtering conditions.
Accordingly, the present invention provides a sputtering target which comprises sub-stoichiometric titanium dioxide, TiOx, where x is below 2. Sub-stoichiometric titanium dioxide, TiOx, where x is below 2 and generally is in the range of from 1.55 to 1.95 is known in the art. It may be produced by the reduction of stoichiometric TiO2. It is a form of titanium dioxide which is conductive.
The sputtering target of the present invention may comprise sub-stoichiometric titanium dioxide, TiOx coated onto a target base. such as a backing tube or plate, for example a target base of an electrically conductive material, for example stainless steel or titanium metal. The target may be of any type known in the art, for example a rotatable target or a flat magnetron target.
The sputtering target of the present invention may be prepared by plasma spraying titanium dioxide onto a target base. During the plasma spraying process, the titanium dioxide loses some oxygen atoms from its lattice and is converted into the sub-stoichiometric form. The primary plasma gas used for the plasma spraying is preferably argon, with hydrogen as the secondary plasma gas. The titanium dioxide which is subjected to plasma spraying preferably has a particle size in the range of from 1 to 60 micrometers. It is also important to cool the sputtering target during the plasma spraying in order to quench the titanium dioxide in sub-stoichiometric form and to improve the conductivity thereof. It also important to use a certain amount of hydrogen in the plasma gas in order to produce a high temperature plasma and to assist in the reduction.
The present invention also provides in another aspect a process for coating a substrate surface with titanium dioxide, which process comprises using as a sputtering target a target comprising sub-stoichiometric titanium dioxide, TiOx. The sputtering from the target is preferably carried out using as the plasma gas argon, a mixture of argon and oxygen, a mixture of nitrogen and argon, or a mixture of nitrogen and oxygen. If the plasma gas does not contain oxygen, e.g. if pure argon is used, then the coating will comprise sub-stoichiometric titanium dioxide. The coating is not completely transparent and possesses some conductivity. If, however, the plasma gas contains oxygen then the sub-stoichiometric form of titanium dioxide is converted into the transparent form which is stoichiometric or substantially stoichiometric. The degree of transparency will depend upon the amount of oxygen contained in the plasma gas. A preferred gas mixture to form transparent titanium dioxide as the coating comprises 70-90% by volume argon and 30-10% by volume of oxygen.
The substrate which is coated according to this process may comprise, for example, optical glass, the screen of a cathode ray tube, such as a TV screen, cold mirrors, low-emissivity glasses, architectural glasses, antireflective panels, flexible films or oxygen barrier films. In this case the coating process will be carried out under conditions such that the sub-stoichiometric titanium dioxide is converted into the stoichiometric form.
In a further aspect the present invention also provides a process for the preparation of sub-stoichiometric titanium dioxide, TiOx, where x is below 2 which process comprises subjecting titanium dioxide to a plasma flame. In carrying out this process the titanium dioxide is preferably sprayed through a plasma flame, for example a plasma flame using a mixture of argon and hydrogen as the plasma gas.
The main advantage of the present invention is that from the sub-stoichiometric titanium dioxide targets used in the present invention the rate of sputtering is increased by a factor of about ten as compared to sputtering from a titanium metal target, thus making the process industrially attractive.
The present invention will be further described with reference to the following Examples.