Process for the deposition of aluminium oxide coatings

European).

18 / P21916PL00

EP 1 730 087 B1

The subject of the invention are new methods of depositing an alumina coating on the surface of a substrate, which in a preferred embodiment is a glass substrate, and in an even more preferred solution is a continuous glass tape produced in the production process of fleet glass, for substrates, especially glass substrates which have an alumina coating on which at least one of its surfaces and coated substrates, especially coated glass substrates, which have a multilayer coating on their at least one surface, wherein at least one layer in said multilayer coating comprises alumina.

Alumina coatings are known to be useful in a variety of applications including the electrical and optical industries. The coatings are abrasion resistant and can be used as the sole layer in a coating or as a top layer in a multilayer coating to take advantage of these abrasion resistance properties. Alumina coatings have been deposited on a variety of substrates, including metals, semiconductor materials, and glass. Coatings can be deposited using a variety of methods, such as chemical vapor deposition, spray pyrolysis and magnetron sputtering.

There are proposals for depositing alumina coating on glass using a chemical vapor deposition method (here, for convenience of CVD). USP 4308316 proposes the use of trimethylaluminum as a precursor for the deposition of an amorphous alumina layer. WO 97/20963 proposes the use of dialkyl aluminum alkoxide as a precursor for depositing an alumina coating on a silica or glass substrate. USP 6037003 proposes the use of alkyl aluminum alkoxide as a precursor for the deposition of an alumina coating. USP 5217753 proposes the use of triethylaluminum or diethylaluminium chloride as a precursor in the process of depositing an alumina coating on a coated glass substrate that has a first layer of titanium oxide and a second layer of silicon complex on which aluminum oxide is deposited.

CVD processes at atmospheric pressure are used to deposit various coatings on the surface of the glass tape during the fleet glass production process. The coating can be deposited on the glass strip when it is at a temperature that is sufficient to drive the deposition reaction. The coating apparatus through which the vapors are brought into contact with the surface of the glass strip can be placed in a fleet bath, in a gap between the fleet bath and the tunnel compressor or in the tunnel compressor. There is a continuing need for processes in which coated glass can be produced with consistent quality over a longer period of time, thereby improving the cost-effectiveness of the production process.

We have now discovered that the deposition of alumina coatings using the CVD process at atmospheric pressure occurs quickly and smoothly when the carboxylic acid ester is in vapors that contact the glass. The introduction of the vapor ester in this way provides a process in which an alumina coating of uniform quality is deposited, with a higher deposition rate, which allows the coating to be deposited on a continuous glass belt during the fleet glass production process over a prolonged period of time. We have also found that the coatings obtained in some of these new deposition methods exhibit new and better properties, and these new coated substrates, and especially coated glass substrates are a second aspect of the present invention.

In a first aspect of the present invention, there is provided a method of depositing an alumina coating on a surface of a substrate that has an elevated temperature, which consists in contacting the surface of said substrate with a mixture of fluids containing aluminum precursor and carboxylic acid ester.

The aluminum precursor can be any aluminum compound that can be evaporated at atmospheric pressure at a temperature that is lower than the temperature at which it reacts with the ester or any co-oxidant that may be present. Any of the aluminum compounds previously proposed for use in alumina deposition CVD processes is potentially useful in the methods of the present invention. Examples of organoaluminium compounds that can be used as a precursor include trialkylaluminum compounds, such as trimethylaluminium and triethylaluminum, alkylaluminum alkoxides, for example those having the general formula RnAl2 (OR¹) 6-n, in which R and R¹, which can be the same or different, are alkyl groups having from 1 to 5 carbon atoms, and n is an integer that has a value from 1 to 5, dialkyl aluminum alkoxides, for example those having the general formula R2AlOR¹in which R and R¹which can be the same or different are alkyl groups which have from 1 to 5 carbon atoms, alkyl aluminum halides which have the general formula RnAlX3-n in which R is an alkyl group containing from 1 to 5 carbon atoms, X is an atom halogen, total, preferably compounds which has a number Preferred in the methods chlorine atom and an value of 1 or 2. the organoaluminium to be used according to the present invention are dimethylaluminium chloride and diethylaluminum chloride.

In particularly preferred embodiments, the aluminum precursor is an inorganic aluminum compound. Examples of inorganic aluminum compounds that can be used as a precursor in the methods of the present invention include aluminum trihalides, which have the general formula AlX3 in which X is halogen, most preferably chlorine.

Carboxylic acid esters that are useful in the methods of the present invention are compounds of the general formula RC (O) -OC- (X) (X¹) -C (Y) (Y¹) -R¹in which R and R¹which can be the same or different are hydrogen atoms or alkyl groups that have from 1 to 8 carbon atoms and X, X¹, Y and Y¹ are alkyl groups having 1 to 4 carbon atoms or hydrogen atoms, preferably at least one of Y or Y¹ is hydrogen. The ester preferably contains from 3 to 18 carbon atoms, in particular from 3 to 6 carbon atoms.

Preferred esters for use in the methods of the present invention include ethyl formate, ethyl acetate, ethyl propionate, ethyl butyrate, n-propyl formate, n-propyl acetate, n-propyl propionate, n-propyl butyrate, isopropyl formate, isopropyl acetate, propionate isopropyl, isopropyl butyrate, n-butyl formate, n-butyl acetate, sec-butyl acetate and tert-butyl acetate.

The most preferred esters for use in the methods of the present invention are ethyl esters, especially ethyl formate, ethyl acetate and ethyl propionate, with ethyl acetate being the most preferred.

The methods of the present invention require the preparation of a fluid mixture containing an aluminum source and a carboxylic acid ester. The mixture should be kept at a temperature that is lower than the temperature at which the aluminum source reacts to form alumina. The mixture is preferably kept at a temperature that is below the decomposition temperature of the ester.

Where the fluid mixture is a vapor, the aluminum source and ester are preferably compounds that can evaporate at a suitably low temperature. Preferred sources of aluminum are those that are liquid at room temperature, or those that can dissolve in a solvent to form a solution that can evaporate.

The fluid mixture is preferably a gas mixture. The components of the gas mixture can be combined at the surface or at the surface of the substrate, whereby they are normally combined to form a precursor gas mixture, which is then brought into contact with the surface of the substrate. The gas mixture generally contains a gaseous carrier, helium ester decomposition is maintained. A mixture than a temperature such as nitrogen or at a temperature lower and high enough to keep the components in the vapor phase.

The precursor gas mixture is introduced into the space directly above the surface of the substrate. This surface should be at a temperature that is elevated high enough to initiate the reaction between the aluminum precursor and the alumina carboxylic acid ester and lead to the degradation of the coating of the substrate surface. The substrate temperature is generally from 350^aboutC to 700^aboutC, preferably from 550 to 700^aboutC, especially from 600 to 650^aboutC.

The process of embedding techniques and devices.

can be carried out using the known preferred CVD is laminar flow CVD, although turbulent flow CVD can also be used.

The substrate is generally a substrate that has a flat surface on which the coating is deposited, the preferred substrate being a glass substrate. The substrate may be a glass plate, with the substrate preferably being a continuous glass tape produced as part of the fleet glass production process.

The temperature of the tape produced in the production of fleet glass decreases as it flows out of the furnace. The glass temperature is from about 1100^aboutC at the supply end of the fleet bath to around 200^aboutC at the point where the belt exits the tunnel compressor. CVD processes are generally carried out at the point where the glass tape is dimensionally stable and its temperature is in the range of 800^aboutC. The exit temperature of the fleet bath is generally around 600^aboutC, and the temperature at the entrance to the tunnel compressor is 580^aboutC.

The method of the present invention can be carried out in a fleet bath, in an interval between the fleet bath and the tunnel compressor, or in stress relieving. In preferred embodiments, the coating is carried out in a fleet bath.

CVD processes for the deposition of metal oxide or silicon oxide coatings on the surface of a glass tape in the production of fleet glass are well known and widely used in the production of coated glass. The CVD methods of the present invention can be carried out using known devices and techniques.

The flow rate of the fluid mixture should be adjusted to provide the desired deposition rate and coating of the required thickness and quality. The optimal speed is influenced by a number of factors, including the nature and temperature of the substrate, the surface area of ​​the substrate, the linear speed of the strip in the production of fleet glass and the rate at which the waste gas is removed from the coating apparatus.

The aluminum precursor and carboxylic acid ester will generally contain from 1.0 to 15.0%, in particular from 2.0 to 8.0% by volume of the precursor gas mixture. The molar ratio of aluminum precursor to carboxylic acid ester will preferably be from 0.1: 1 to 0.5: 1.0, in particular from 0.2: 1.0 to 0.4: 1.0. The balance of the fluid mixture may include an inert carrier gas, which is preferably nitrogen or helium.

The carboxylic acid ester is normally the only oxygen-containing individual in a fluid mixture, however, minor proportions of another oxygen-containing compound, especially oxygen (or air) can be used.

The methods of the present invention provide faster deposition rates than methods of prior art, i.e. methods that do not use carboxylic acid ester. The methods of the present invention may result in the deposition of an alumina coating that is less dusty than coatings obtained using methods of prior art.

The alumina coating will generally have a stoichiometry corresponding to the stoichiometry of alumina, i.e. Al2O3. Coatings that have a slight oxygen deficiency can also be produced and useful.

The coating can be deposited directly onto clean glass or glass onto which one or more coatings have previously been deposited. Such multilayer coatings can be deposited on glass tape in the production of fleet glass by arranging a series of coating stations along the strip.

One type of coating that can be deposited on the glass before the alumina layer is the alkali metal blocking layer. Such layers are known in the art. The preferred blocking layer in this invention is a silicon oxide blocking layer. Silicon oxide may be silica, i.e. it may have SiO2 stoichiometry or may contain other elements such as carbon (such layers are commonly called silicon oxycarbides and are deposited, for example, as described in GB 2199848) or nitrogen (such layers are called nitride nitrides of silicon).

Another type of coating that can be deposited on the glass before the alumina layer is an opalescent layer or layers such as those described in USP 4377613. Such a lower layer typically contains a tin oxide film deposited directly on the glass and a silica film deposited on top of the tin oxide .

The coating thickness of aluminum oxide is normally from

300 A to 2000 A, preferably from 500 A to 1500 A, in particular from 600 A to 1200 A. The alumina coating produced using the methods of the present invention has a refractive index of 1.5 to 1.7, especially approximately 1 , 6, these values ​​being the mathematical average of measurements carried out at a wavelength of 340-770 nm. The refractive index means that the alumina layer can be used to replace existing bottom coatings that provide color suppression properties, such as silicon oxycarbide coatings or silica / tin oxide coatings.

The alumina coatings of the present invention may be smoother than existing coatings, usually silicon oxycarbide coatings. Alumina coatings may have a surface which has an Rms index of 1.0 to 5.0 nm. Coatings like this, which is relatively smooth, can be used as fogging layers by depositing them on top of a coating that is relatively known rough. Haze-reducing coatings have been described in, for example, GB 2136316 and EP 782975. An example of a relatively rough coating on which an alumina coating may be usefully deposited as a fog-reducing coating using the methods of the present invention is a tin oxide coating, and especially a tin oxide coating. which was embedded using the CVD process. Coated glass that has a tin oxide coating and an alumina coating deposited thereon as a fogging layer is another embodiment of the present invention. Such coated glasses are preferably made by depositing an oxide coating directly on the glass using the CVD process.

The alumina coatings of the present invention have a hardness that at 20^aboutC is typically from 8 to 10 on the Mohs hardness scale.

The methods of the present invention can also be used to make mixed alumina and another metal. Such mixed oxides preferably have a larger proportion of aluminum and a smaller proportion of other metal. The metal content of these mixed oxides is typically at least 90 mol% aluminum and 10 mol% or less of other metal or metals. Such mixed oxides can be produced by introducing a suitable amount of an additional metal source into the fluid mixture. Examples of additional metals that can be usefully introduced include tin and zinc. A number of tin sources, for example monobutyl tin trichloride, are used in CVD processes and can be incorporated into a fluid mixture. Similar sources of zinc, such as diethyl zinc, are available and can be used in the methods of the present invention. The presence of these additional metals may increase the refractive index of the layer and this may be useful in some applications. The presence of additional metal can give the coating electrical conductivity.

The alumina coating may further be doped if desired, e.g. with fluorine. Such fluorine doping may affect some optical solid coatings and / or may impart certain properties to the outer surface of the coating, e.g. hydrophobicity, etc.

Fig. 1 schematically illustrates an example of a static reactor for chemical vapor deposition and a gas supply system useful in carrying out the methods of the present invention and used in Examples 1-7.

In Fig. 1, the static chemical vapor deposition reactor and gas supply system, generally designated 1, includes a reactor 3 which has an outlet line 5 and an inlet line 7, both of which can be wrapped and heated with heating tape, thereby reducing condensation probability on these lines. Line 7 connects to the four-way valve 9. Other connections to valve 9 are line 11, which connects to the source of purified gas, line 13, which connects to the waste gas furnace, and line 15, which connects to bubblers 17, 19 and 21, and to motorized heated syringes 23 and 25. Lines 27, 29 and 31 supply vapor generated in the bubblers to line 15. Lines 33 and 35 deliver liquids injected from the driving members of the syringe to line 15. Line 37 connects to the nitrogen source.

Unless otherwise stated, all gas volumes are measured at standard temperature and pressure. The thickness values ​​reported for the layers were determined by high resolution scanning electron microscopy and optical modeling of reflection and through spectra of coated glass. Coating thickness was measured with an uncertainty of about 5%. The surface roughness of the coatings was determined using a Tapping Mode AFM instrument.

The process conditions in Examples 1 to 7 are summarized in Table 1.

Table 1

<td>Example</td><td> 1</td><td> 2</td><td> 3</td><td> 4</td><td> 5</td><td> 6</td><td> 7</td>

<td>Precursor Al</td><td>DMAC (1.0M) in hexane</td><td>DMAC (1.0M) in hexane</td><td>TEDA</td><td>DMAC (1.0M) in hexane</td><td>DMAC 97%</td><td>DMAC 97%</td><td>DMAC 97%</td>

<td>Subsoil</td><td>Glass/ SiO2</td><td>Glass/ SiO2</td><td>Glass/ SiO 2 / SnO2</td><td>Glass/ SiO2</td><td>Glass/ SiO2</td><td>Glass/ SiO 2 / SnO2</td><td>Glass/ SiO 2 / SnO2</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Syringe 1</td><td>DMAC</td><td>DMAC</td><td>TEDA</td><td>DMAC</td><td></td><td></td><td></td>

<td>Speed flow</td><td>1.25 ml / min</td><td>1.25 ml / min</td><td>0.4 ml / min</td><td>0.4 ml / min</td><td></td><td></td><td></td>

<td>Temp. ^aboutC</td><td> 90</td><td> 90</td><td> 150</td><td> 150</td><td></td><td></td><td></td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Syringe 2</td><td></td><td>MBTC</td><td>EtOAc</td><td></td><td></td><td></td><td>H₂ABOUT</td>

<td>Speed flow</td><td></td><td>1.12 ml / min</td><td>0.6 ml / min</td><td></td><td></td><td></td><td>0.32 ml / min</td>

<td>Temp. ^aboutC</td><td></td><td>On</td><td> 150</td><td></td><td></td><td></td><td> 120</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>sparger 1</td><td>EtOAc</td><td>EtOAc</td><td></td><td>EtOAc</td><td>EtOAc</td><td>EtOAc</td><td>EtOAc</td>

<td>Temp. ^aboutC</td><td> 40</td><td> 42</td><td></td><td> 56</td><td> 52</td><td> 43</td><td> 46</td>

<td>N2 carrier scam</td><td> 400</td><td> 400</td><td></td><td> 350</td><td> 450</td><td> 450</td><td> 450</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>sparger 2</td><td></td><td></td><td></td><td>DEZ 15% w</td><td>DMAC</td><td>DMAC</td><td>DMAC</td>

<td>Temp. ^aboutC</td><td></td><td></td><td></td><td> 66</td><td> 27</td><td> 28</td><td> 29</td>

<td>Carrier scam</td><td></td><td></td><td></td><td> 800</td><td> 230</td><td> 250</td><td> 250</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Oxygen flow slm</td><td> 0,15</td><td> 0,15</td><td> 0,06</td><td></td><td></td><td> 0,15</td><td></td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>N2, balance flow slm</td><td> 10</td><td> 8</td><td> 4</td><td> 5</td><td> 5</td><td> 7</td><td> 7</td>

<td>Glass temp. ^aboutC</td><td> 525</td><td> 600</td><td> 600</td><td> 625</td><td> 650</td><td> 650</td><td> 650</td>

DMAC - dimethyl aluminum chloride

TEDA - triethylaluminium tri-sec-butoxide

DEZ - diethyl zinc

MBTC - monobutyl tin chloride.

The following experimental conditions are used in

Examples 8 to 13. A laboratory oven, which has a conveyor that allows glass panes to move through the oven, includes a single (25.4 cm) wide bidirectional coating device. The coating apparatus is adapted to transfer evaporated components to the surface of the glass pane. The glass pane is preheated to 632^aboutC.

The vapor streams fed into the coating apparatus are generated using source chambers known as bubblers, which are maintained at specific temperatures. An inert gas, such as helium or nitrogen, is introduced into the bubblers at an adjustable speed, so as to pull the reagent with it into the kitty cat and transfer it to the coating apparatus and then to the glass surface.

Table 2

<td>Example</td><td> 8</td><td> 9</td><td> 10</td><td> 11</td><td> 12</td><td> 13</td>

<td>Precursor Al</td><td>DEAC clear</td><td>DEAC clear</td><td>DEAC clear</td><td>DEAC clear</td><td>DEAC clear</td><td>DEAC clear</td>

<td>Substrate</td><td>Glass</td><td>Glass/ SICO</td><td>Glass/ SiO2 SnO2</td><td>Glass/ SiO 2 / SnO2</td><td>Glass/ SiO2</td><td>Glass/ SICO</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Babbler 1</td><td>DE AC</td><td>DE AC</td><td>DE AC</td><td>DE AC</td><td>DE AC</td><td>DE AC</td>

<td>Temp. ^aboutC</td><td> 160</td><td> 160</td><td> 160</td><td> 160</td><td> 160</td><td> 160</td>

<td>Carrier slm</td><td> 1</td><td> 1</td><td> 1</td><td> 1,4</td><td> 1,4</td><td> 1,4</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Babbler 2</td><td>EtOAc</td><td>EtOAc</td><td>EtOAc</td><td>EtOAc</td><td>EtOAc</td><td>EtOAc</td>

<td>Temp. ^aboutC</td><td> 60</td><td> 60</td><td> 60</td><td> 60</td><td> 60</td><td> 60</td>

<td>Carrier slm</td><td> 1</td><td> 1</td><td> 1,4</td><td> 2</td><td> 2</td><td> 2</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>He flow balance slm</td><td> 35</td><td> 35</td><td> 35</td><td> 35</td><td> 35</td><td> 35</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Speed conveyor inch / min</td><td> 125</td><td> 125</td><td> 125</td><td> 125</td><td> 300</td><td> 300</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Glass Temp. ^aboutC</td><td> 632</td><td> 632</td><td> 632</td><td> 632</td><td> 632</td><td> 632</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Thickness coatings, A.</td><td> 355</td><td> 625</td><td> 757</td><td> 1088</td><td> 828</td><td> 1067</td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Light refractive index</td><td></td><td></td><td></td><td></td><td> 1,606</td><td></td>

<td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Ra nm</td><td></td><td> 1,4</td><td></td><td> 7,4</td><td> 0,4</td><td> 1,9</td>

The following experimental conditions apply to Examples 14-21 in which the aluminum precursor is an inorganic aluminum compound. The laboratory furnace, which has a movable conveyor for transferring glass or glass panes through said furnace at a speed of 381 cm / min (150 inches / min), also contained a single, 25.4 cm (10 inch) wide bidirectional coating device, with which coating apparatus is suitable for transferring evaporated reagents to the surface of glass panes to form a film or film assembly by chemical vapor deposition.

The glass panes were heated to approximately 632^aboutC, while the coating apparatus, at the frontal surface of the reactor, i.e. the part closest to the glass surface, had a temperature of approximately 260^aboutC.

The total gas flow was essentially 35 standard liters per minute (slm) in all examples except Example 19 where it was 32 slm.

The preparation of various precursor materials was carried out by using multiple source chambers known as "bubblers", one for each of ethyl acetate (EtOAc) and aluminum trichloride (AlCl3), which were maintained at specific temperatures. Helium gas was introduced into the bubbler at a particular flow rate. Alternatively, EtOAC could be fed using a heated thin film evaporator.

It was found that the obtained alumina coatings produced according to the invention have refractive index values ​​in the range of 1.5-1.65, which allows the desired optical effects to be obtained especially when using coating layers.

Table 3 summarizes the deposition conditions and film thickness for examples. The thickness of these in combination with the other obtained film was optically determined by chemical etching, for Examples 14-18.

while for Examples 19-21, then profilometry was used

Table 3

<td>Example</td><td>% AlCl3</td><td>% EtOAc</td><td>ABOUT %</td><td>% HF</td><td>Thickness (AND</td><td>Growth rate (A / s)</td>

<td> 14</td><td> 3</td><td> 9</td><td> 15</td><td> 0</td><td> 1018</td><td> 254,5</td>

<td> 15</td><td> 1</td><td> 6</td><td> 15</td><td> 0</td><td> 1013</td><td> 253,3</td>

<td> 16</td><td> 1</td><td> 3</td><td> 0</td><td> 0</td><td> 1285</td><td> 321,3</td>

<td> 17</td><td> 3</td><td> 6</td><td> 7,5</td><td> 0</td><td> 1991</td><td> 497,8</td>

<td> 18</td><td> 1</td><td> 9</td><td> 15</td><td> 0</td><td> 1017</td><td> 254,3</td>

<td> 19</td><td> 7</td><td> 9</td><td> 0</td><td> 0</td><td> 1788</td><td> 447</td>

<td> 20</td><td> 2</td><td> 6</td><td> 0</td><td> 1,4</td><td> 851</td><td> 212,8</td>

<td> 21</td><td> 2</td><td> 6</td><td> 0</td><td> 2,9</td><td> 949</td><td> 237,3</td>

The various reagents described above were combined in a coating apparatus to deposit an alumina coating, in Examples 19-21, on a clear soda-silica glass pane on which an SiO layer₂ 200 A.

deposition was carried out on glass previously deposited

In Examples 14-18 Pilkington Energy

Advantage ™, in which the top layer is SnO2: F. As you can see the information shown in Table 3, the connection

AlCl3 and EtOAc without other components, when combined, deposit the method described herein, give alumina films of useful thicknesses due to very satisfactory growth rates. The addition of molecular oxygen appears to significantly increase film growth rate and film thickness under certain circumstances.

The addition of dopants, such as fluorine, does not appear to increase the growth rate or film thickness, but may provide other benefits.

Alumina coatings are preferably deposited at a rate of over 200A / sec. High deposition rates may be important when substrates are coated in the production process. This is especially true for a direct fleet glass manufacturing process in which the glass tape travels at a specific linear speed of the order of several hundred inches per minute and in which a specific coating thickness is required. The deposition rates obtained in preferred embodiments of the present invention may be two or more times higher than the deposition rate by other known methods for depositing alumina coatings. Particularly high alumina deposition rates can be obtained in the present invention using an ester-containing precursor mixture that contains 3 to 6 carbon atoms.

The deposition rate depends on the particular ester used and the concentration of both the ester and aluminum chloride as well as the glass temperature. In the case of a particular combination of compounds, the optimal concentrations (and especially the optimum ratio of ester to aluminum chloride) and flow rates for rapid deposition of coatings can be determined by a simple test. At the same time, however, it is estimated that the use of higher reagent concentrations and high gas flow rates probably results in a less efficient overall conversion of the reagents into a coating, so that the optimal industrial operation condition may differ from the conditions that provide the highest deposition rates.

For preferred solutions using an inorganic aluminum precursor, the gaseous reagent mixture that is fed to the surface of the hot glass substrate according to the invention preferably contains (all in mole percent) from about 0 to about 40% oxygen, from about 1.5% to about 25% organic ester and from about 0.5% to about 10% aluminum halide, and especially contains from about 0% to about 25% oxygen, from about 3% to about 15% organic ester and from about 1% to about 5% aluminum halide.