CA1147139A - Deposition method - Google Patents

Abstract

Abstract of the Disclosure This disclosure describes an improvement in an apparatus for forming a continuous coating of progressively changing composition on heated glass over the length of a processing path through which said glass passes said apparatus comprising, in addition to means to support said glass and means to move it continuously along said processing path, the improvement wherein there is provided a reaction zone the interior surfaces of which are formed primarily by said moving glass and a heat-transfer-medium moderated, temperature-controlled wall opposed to said glass said zone having, at one end thereof, a port means at a first station to introduce and distribute a gaseous reaction mixture comprising reactants which form reaction products that deposit on said glass at different rates, a gas flow path within said reaction zone wherein said gaseous reaction mixture flows along said glass surface, and said temperature controlled wall to port means at a second station at the opposite end of said zone to remove residual gaseous reaction mixture, wherein said second station being relatively positioned along said reaction zone with respect to said first station that said heated glass wall of said reaction zone forms means to provide sufficient energy to said gaseous reaction mixture to achieve a substantial difference in composition deposited on said glass between the reaction gas mixture composition, as it passes between said first station and second station and wherein said temperature-controlled wall is maintained at a temperature low enough to avoid deposits of reaction products thereon. A particular advantage of the invention is that it can be used to make glass having a coating with a layer of continuous-ly varying refractive index between the glass and the coating preventing the observation of iridescence. The apparatus is particularly efficacious when applied to clear and lightly tinted glasses wherein the problem of iridescent color has had its greatest commercial impact.

Description

~7~3~

This application is divided out of our copending parent application Serial No. 320,878 filed on February 6~ 1979. The parent application relates to glass structures bearing a thin~ functional, inorgtmic coating ~e.g. a coating of tin oxide forming means to promote reflectivity of in~ra-red light) which structures have improved appearance as a consequence of reduced iridescence historically associated with said thin coatings, and methods for achieving thè
aforesaid structures.
The invention of this application relates to an apparatus suitable for making such glass structures.
Glass and other transparent materials can be coated with transparent semiconductor films such as tin oxide, indium oxide or cadmium stannate, in order to reflect infra-red radiation. Such materials are useful in providing windows with enhanced insulating value (lower heat transport), e.g. for use in ovens, architectural windows, etc. Coa~ings of these same materials also conduct electricity, and are employed as resistance heaters to heat windows in vehicles in order to remove fog or ice.
One objectionable feature of these coated windows is that they show interference colors ~iridescence) in reflected light, and, to a lesser extent, in transmitted light. This iridescence has been a serious barrier to widespread use of these coated windows (see, for example, American Institute of Physics Conference Proceeding No. 25, New York, 1975, Page 288).
In some circumstances, i.e. when ~he glass is quite dark in tone (say, having a light transmittance of less than about 25%) this iridescence is muted and can be tolerated. However, in most architectural wall and window ap-plications, the iridescent effect normally associated with coatings of less than about 0075 microns is aesthetically unaccep~able to many people. (See~ for ex ample~ United States Patent ~,710,07~ to Stewart.) ':

3~

Iridescent colors are quite a general phenomenon in transparent ilms in the thickness range of about 0.1 to 1 micron, cspecially at thicknesses below about 0.85 micron. Unfortunately, it is precisely this range o-f thickness which is of practical importance in most commercial applications. Semiconductor coat-ings thinner than about 0.1 micron do not show interference colors, but such thin coatings have a markedly inferior reflectance of infra-red light, and a markedly reduced capacity to conduct electricity.
Coatings thicker than about 1 micron also do not show visible irides-cence in daylight illumination, but such thick coa*ings are much more expensive to make, since larger amounts of coating materials are required, and the time necessary to deposit the coating is correspondingly longer. Furthermore, films thicker than 1 micron have a tendency to show ha~e, which arises from light scattering from surface irregularities, which are larger on such films. Also, such films show a greater tendency to crack, under thermal stress, because of differential thermal expansion.
As a result of these technical and economic constraintsJ almost all present commercial production of such coated glass articles comprise films ln the thickness range of about 0.1 to 0.3 microns, which display pronounced iri-descent colors. Almost no architectural use of this coated glass is made at present, despite the act that it would be cost-effective in conserving energy to do so. For example, heat loss by infra-red radiation through the glass areas of a heated building can approximate about one-half of the heat loss through uncoated windows. The presence of iridescent colors on these coated glass ; products is a major reason for the failure to employ these coatings.
Co-pending application, Serial No. 313,8~3, filed on October 20, 1978, ~ discloses means to reduce this iridescence to unobservably small values, by ; means of an additional layer or layers placed in register with the main coating, ~ 2 -~7~3~

including a gradient-type coating. The present disclosure is directed primarily toward an apparatus for forming such a gradient-type anti-iridescent layer.
Summary of thé Invention An object of this invention is to provide novel apparatus which is suitable for making the above identified novel products and, indeed, which is suitable for use in making coatings of gradually changing compositions and from gaseous reactants whether or not such coating be on glass or some other substrate and whether or not such coatings comprise a maximum amount of one component with-in or at an extre~ity of the depth of the coating structure.
Other objects of the invention will be obvious to those skilled in the art on reading the instant invention.
According to one aspect of the invention of the parent application there is provided a process for continuous coating of a substrate by a film formed of reactive components of a gas mixture in which properties of the coating vary continuously through the thickness of said film, said process comprising the steps of (a) flowing said gas mixture through a reactioll zone defined by a flow path for said reactive components contiguous to, parallel with and bound by, a surface of the substrate (b) depositing, preferentially, a reaction product derived from a more reactive component of said mixture on a portion of said surface exposed earlier to said gas mixture~
~c) depositing, preferentially, a reaction product derived from a less reactive component of said mixture on a portion of said sur~ace exposed later to said gas mixture, and (d) moving said substrate surface through said reaction zone in a direc-tion parallel to said flow path to obtain a change in the composition of said 7~39 coating throughout the ~hickness of said coating, as said substrate emerges ~rom said reaction zone.
According to a further aspect of the invention of the paren~ applica-tion thera is provided a process for forming on a substrate, a thin, coating, which has a changing composition from a predominantly first coating composition nearest the substrate to a predominantly second coating composition more remote from the substrate, said process comprising the steps of ~ 1) introducing into a first end o~ a reaction chamber and out the other end of said chamber a mixture of a first reactant gas from which said first coating compound is formed, a second reactant gas from which said second coating compound is formed, and a thild gas which forms means to react with each of said reactant gases to form said coating compounds, wherein said first reactant gas reacts at a substantially different rate with said third gaa, than does said second reactant gas, the different rate of reaction with said third gas forming means to provide a difference in relative concentration of said reactant gases from one end of said chamber to the other and to provide different quantities of said coating compounds from one end to another; and

(2) continuously passing a substrate to be coated through said reaction chamber from said first end of the chamber to said other end of the chamber; and ~3) coating said substrate with a progressively changing composition as it moves through said chamber, said composition formed by depositing said coating compounds said compositions indicative of the relative reactivity and concentra-tion of said reactant gases along said chamber.
According to another aspect of the invention of the parent application there is provided a transparent glass product substantially free of iridescent appearance, having a glass substrate bearing a coating which is substantially uniform across the surface area thereof, said coating consisting of

3~

(a~ a lower coating zone comprising a material formed of at least two components which is characterized by a gradual change from a first composition proximate to said substrate having a relatively high proportion of a -~irst com-ponent to a second composition more remote from said substrate having a relative-ly large proportion of said second component; and (b) an upper coating zone having a refractive index substantially the same as that of said second component.
According to the present in~ention there is provided in an apparatus for forming a continuous coating of progressively changing composition on heated glass over the length of a processing path through which said glass passes said apparatus comprising, in addition to means to support said glass and means to move it continuously along said processing path, the improvement wherein there is provided a reaction zone the interior surfaces of which are formed primarily by said moving glass and a heat-transfer-medium moderated, temperature-controlled wall opposed to said glass said zone having, at one end thereof~ a port means at a first station to introduce and distrihute a gaseous reaction mixture compris-ing reactants which form reaction products that deposit on said glass at dif-ferent rates, a gas flow path within said reaction zone wherein said gaseous re-action mixture flows along said glass surface, and said temperature controlled wall to port means at a second station at the opposite end of said zone to remove residual gaseous reaction mixture, wherein said second station being relatively positioned along said reaction zone with respect to said first station that said heated glass wall of said reaction zone forms means to provide sufficient energy `~ to said gaseous reaction mixture to achieve a substantial difference in composi-tion deposited on said glass between the reaction gas mîxture composition, as it passes between said first station and second station and wherein said tempera-ture-controlled wall is maintained at a temperature low enough to avoid deposits ~: - 5 -3~

of reaction products thereon.
The parent invention utilizes the formation of layers of transparent material between the glass and the semiconductor film. These layers have re-factive indices intermediate between those of the glass and the semiconductor film. With suitable choices of thickness and refractive index values of these intermedlate layers, it has been discovered that the iridescent colors can be made too faint for most human observers to detect, and certainly too faint to interfere with widespread commercial use even in architectural applications.
Suitable materials for these intermediate layers are also disclosed herein, as well as processes for the formation of these layers.
In the preferred form of the parent invention, these intermediate layers blend together continuously to form a graded layer in which the refrac-tive index varies, preferably in a smooth transition, as one moves through the layer away from the glass toward the semiconductor coating, from a value at the glass surface matching the index of the glass, to a refractive index value match-ing that of the overlying semiconductor film, at a point proximate to that over-lying film.
A coating with refractive index varying through its thickness may be produced by a novel method of the parent invention, in which a gas mixture with 2~ components of different reactivities, flows along the surface of a moving glass substrate.
In the Drawings Figure 1 is a graph illustrating the variation of calculated color intensity of various colors with semiconductor Eilm thickness.
Figure 2 illustrates, schematically and in section, a non iridescent coated glass constructed according to the parent invention, with an anti-iridescent interlayer of continuously-varying composition according to the ~7~3g parent invention.
Figure 3 is a graph indicative of a typical gradient o~ refractive indices, idealized, and represen-ting the gradual transition from 100% SiO2 to 100% SnO2.
Figure 4 is a section, somewhat simplified to facilitate the descrip-tion thereof of, a novel apparatus of the present invention of the type conveni-ent for use in the process of the parent invention.
Figure 5 illustrates the experimental measurement of the gradient in chemical composition of a silica-tin oxide gradient zone prepared according to the parent invention.
Figure 6 shows an observed variation o~ the refractive index of the initial deposit of SiO2-SnO2 at the glass surface~ as a function of gas composi-tion.
Methods and Assumptions It is believed desirable, because of the subjective nature of color perception~ to provide a discussion of the methods and ass~ptions which have been used to evaluate the parent invention and this invention. It should be realized that the application of much of the theory discussed below is retro-spective in nature because the information necessarily is being provided in hindsight, i.e. by one having a knowledge of the subject matter disclosed herein.
In order to make a suitable quantitative evaluation of various possible constructions which suppress iridescent colors, the intensities of such colors were calculated using optical data and color perception data. In this discus-sion~ film layers~are assumed to be planar, ~ith uniform thickness and uniorm refractive index within each layer. The refractive index changes are taken to be abrupt at the planar interfaces bet~een adjacent film layers. ~ continuously varying refractive index may be modelled as a sequence of a very large number of .

~1~73L3~31 very thin layers with closely spaced refractive indices. Real refractive in-dices are used~ corresponding to negligible absorption losses within the layers.
The reflection coefficients are evaluatéd for normally incident plane waves of unpolarized light.
Using the above asswmptions, the amplitudes for reflection and trans-mission from each interface are calculated from Fresnel's formulae. Then these amplitudes are summed, taking into account the phase differences produced by propagation through the relevant layers. These results have been found to be equivalent to the Airy formulae ~See, for example, Optics of Thin Films, by F. Knittl, Wiley and Sons, New York, 1976) for multiple reflection and inter-ference in thin films, when those formulae were applied to the same cases.
The calculated intensity of reflected light has been observed to vary with wavelength, and thus is enhanced in certain colors more than in others.
To calculate the reflected color seen by an observer, it is desirable first to specify the spectral distribution of the incident light. For the purpose, one may use the International Commission on Illumination Standard Illuminant C, which approximates normal daylight illwmination. The spectral distribution of the reflected light is the product of the calculatecl reflection coefficicnt and the spectrum of Illuminant C. The color hue and color saturation as seen in reflection by a human observer, are then calculated from this reflected spectrum, using the uniform color scales such as those known to the art. One useful scale is that disclosed by ~lunter in ood Technology, Vol. 21, pages 100 - 105, 1967.
This scale has been used in deriving the relationship now to be disclosed.
The results of calculations, for each combination of refractive indices and thicknesses of the layers, are a pair of nwnbers, i.e. "a" and "b". "a"
represents red ~if positive) or green ~if negative) color hue, while "b" de-scribes a yellow (if positive) or blue (if negative) hue. These color hue ~h~7~

results are useful ln checking the calculations against the ohservable colors of samples including those of the invention. A single number, "c", represents the "color saturation": c = ~a2~b2)1/2. This color saturation index, "c"~ is directly related to the ability of the eye to detect the troublesome iridescent color hues. When the saturation index is below a certain value, one is not able to see any color in the reflected light. The numerical value of this threshold saturation of observability depends on the particular uniform color scale used, and on the viewing conditions and level of illumination ~see, for example, R.S~ H~mter, The Measurement of Appearance, Wiley and Sons, New York, 1975, for 1~ a review of numerical color scales.) In order to establish a basis for comparison of structures a first series of calculations was carried out to simulate a single semiconductor layer on glass. The refractive index of the'semiconductor layer was taken at 2.0, which is a value approximating tin oxide, indium oxide, or cadmium stannate films. The value 1~52 was used for the glass substrate; this is a value typical of commercial window glass. The calculated color saturation values are plotted in Figure 1 as a function of the semiconductor film thickness. The color satura-tion is found to be high for reflections from films in the thickness range 0.1 to 0.5 microns. For films thickner than 0.5 micron, the color saturation de-creases with increasing thickness. These results are in accord with qua'litative observations of actual films. The pronounced oscillations are due to the vary-ing sensitivity of the eye to different spectral wavelengths. Each of the peaks corresponds to a particular color, as marked on the curve ~R=red, Y=yellow, G=green, B=blue).
Using these results, the minimum observable value of color saturation was established by the following experiment: Tin oxide films with continuously varying thickness, up to about 1.5 microns, were deposited on glass plates, by _ g _ ~7~3~1 the oxidation of tetramethyltin vapor. The ~hickness profile was established by a temperature variation from about 450C to 500C across the glass surface. The thickncss profile was then measured by observing the interference Eringes under monochromatic light. When observed under diffused daylight, the films showed interference colors at the correct positions shown in Figure 1. The portions of the films with thicknesses greater than 0.85 micron showed no observable interference colors in diffused daylight. The green peak calculated ~o lie at a thickness of 0.88 micron could not be seen. Therefore, the threshold of observability is above ~ of these color units. Likewise, the calculated blue peak at 0.03 micron could not be seen~ so the~ threshold is above 11 color units, the calculated value for this peak. However, a faint red peak at 0.81 micron could be seen under good viewing conditions, e.g. using a black velvet background and no colored objects in the field of view being reflected, so the threshold is below the 13 color units calculated for this color. We conclude Erom these studies that the threshlold for observation oE reflected color is between 11 and 13 color units on this scale, and therefore we have adopted a value of 12 units to represent the threshold Eor observability of reflected color under daylight viewing conditions. In other words, a color saturation of morc than 12 units appears as a visibly colored iridescence, while a color saturation of less than 12 units is seen as neutral.
It is believed that there will be little objection to commercialization of products having color saturation values of 13 or below. However, it is much preferred that the value be 12 or below and, as will appear in more detail herein-after, there appears to be no practical reason why the most advantageous products according to the parent invention, e.g. those characterized by wholly color-free surfaces, i.e. below about 8, cannot be made economically.
A value of 12 or less is indicative of a reflection which does not ~7~3~

distort the color of a reElected image in an observable way. This threshold value of 12 units is taken to be a quantitative standard with which one can evaluate the success or failure of various multilayer designs, in suppressing the iridescen~e colors.
Coatings with a thickness of 0.85 micron or greater have color satura-tion values less than this threshold of l2, as may be seen in Figure 1. Experi-ments confirm that these thicker coatings do not show objectionable iridescente colors in daylight illumination.
Use of an Interlayer of Graduated Refractive Index It has been discovered that a film intermediate between the glass sub-strate and a semiconductor layer can be built up of a graded composition, e.g.
gradually changing from a silica film to a tin oxide film. Such a film may be pictured as one comprising a very large number of intermediate layers. Calcula-tions have been made of reflected color saturation for a variety of refractive index profiles between glass of refractive index n=1.52 and semiconductor coat-ings of refractive index n=2Ø For transition layers thicker than about 0.15 micron, the calculated color saturation index is usually below 12, i.e. n0utral to the eye, and, for, transitions more than about 0.3 microns the color is al-ways undetectable. The exact shape of the refractive index profile has very little effect on these results, provided only that the change is gradual through the graded layer.
What Materials can be Used A wide range o:E transparent materials are among those which can be selected to make products meeting the aforesaid criteria by forming anti-iridescent undercoat layers. Various metal oxides and nitrides, and their mi~-tures have the correct optical properties of transparency and refrac~ive index.
Table A lists some mixtures which have the correct refractive index range between - 11 - .
;

- .

-L3~

glass and a tin oxide or indium oxide film. The wcight percents necessary can be taken from measured re~ractive index versus composition curves, or calculated from the usual Lorentz-Lorenz law for refractive indices o~ mixtures (Z. Knittl, Optics of Thin Films, Wiley and Sons, New York, 976, page ~73), uslng measured refractive indices for the pure films. This mixing law generally gives suffici-ently accurate interpolations for optical work, although the calculated refac-tive indices are sometimes slightly lower thcm the measured values. Film re-fractive indices also vary somewhat with deposition method and conditions ém-ployed.
Figure 3 gives a typical curve of refractive index versus composition or the important case of silicon dioxide-tin dioxide mixtures.
Table A.
Some combinations of compounds yielding transparent mixtures whose refractive indices span the range from 1.5 to 2Ø

SiO2 SnO2 SiO2 i3N~
SiO2 TiO2 SiO2 In203 SiO2 Cd2SnO4 Process for Forming Films Films can be formed by simultaneous vacuum evaporation of the appropri-ate materials of an appropriate mixture. For coating of large areas, such a window glass, chemical vapor deposition (CVD) at normal atmospheric pressure is more convenient and less expensive. However, the CVD method requires suitable volatile compounds for forming each material. Silicon dioxide can be deposited by CVD from gases such as silane, SiH4, dimethylsilane (CH3~2SiH2, etc. Liquids which are sufficiently volatile at room temperature are almost as convenient as ~7~L~9 gases; tetramethyltin is such a source for CVD of tin compounds, while (C2H5)2SiH2 and SiC14 are volatile liquid sources for silicon.
A continuously graded layer of mixed silicon-tin oxide may be built up during a continuous CVD cocating process on a continuous ribbon of glass by the following novel procedure. A gas mixture is caused to flow in a direction paral-lel to the glass flow, under ~or over) the ribbon of hot glass, as shown, for example, in Figure 4. The gas mixture contains an oxidizable silicon compound, an oxidizable tin compound, and oxygen or other oxidizing gas. The compounds are chosen so that the silicon compound is somewhat more quickly oxidized than is the tin compound, so that the oxide deposited on the glass where the gas mixture first strikes the hot glass surface, is mainly composed of silicon dioxide, with only a small percentage of tin dioxide~ The proportions of silicon and tin com-pounds in the vapor phase are adjusted so that this initially deposited material has a refractive index which closely matches that of the glass i-tself. Then, as the gas continues in contact with the glass surface, the proportion of tin oxide in the deposited film increases, until at the exhaust end of the deposition reglon, the silicon compound has been nearly completely depleted in -the gas mix-ture, and the deposit formed there is nearly p~lre tin oxide. Since the glass is also continually advancing from the relatively silicon-rich (initial) deposi-tion region to a relatively tin-rich ~final) region, the glass receives a coating with a graded refractive index varying continuously through the coating thick-ness, starting at the glass surface with a value matching that of glass, and ending at its outer surface, with a value matching that of tin oxide. Subsequent deposition regions, indicated in Figure 3, can then be used to build up further layers of pure tin oxide, or layers of tin oxide doped, for example~ with fluorine.
- A suitable gas mixture for this purpose, preferably includes the oxidiz-, . ..

~' ' ' ' " '. ' "' ' ~
, . . .. .

~7~3~

able silicon compounds, l,1,2,2,tetramethyldisilane (l~e2SiSiMe2ll); 1,1,2,-trimethyldisilane H2MeSiSiMe2H, and/or 1,2,dimethyldisilane ~H2MeSiSiMeH2) along with tetramethyltin (Me4Sn). It has been found that the initially deposit-ed ilm is silicon-rich, and has a refractive index close to that of glass, while the later part of the deposit is almost pure tin oxide.
The Si-H bonds in the above-disclosed silicon compounds are highly useful in the process, since compounds without Si.H bonds, such as tetramethyl-silane Me4Si, or hexamethyldisilane Me3SiSiMe3, are oxidized more slowly than is tetramethyltin, and the initial deposit is mainly tin oxide, and the latter part of the deposit is mainly silicon dioxide. In such a case, i.e. when one is using compounds such as Me~Si, one may flow the gas and glass in ~ site direc-tions in order to achieve the desired gradation of refractive index, provided the gas flow is faster than the glass flow. However, the preferred embodiment is to use the more easily oxidizable silicon compounds, and concurrent gas and glass flow directions.
It is also desirable, in forming coat-ings wherein the composition varies monotonically with distance from the substrate, that the silicon compounds have a Si-Si bond as well as the Si-H bond. For example, a compound containing Si-H
but not SiSi bonds, dimethylsilane Me2SiH2, along with tetramethyltin, produces an initial deposit of nearly pure tin oxide, which then becomes silicon-rich at an intermedia~e time and finally becomes tin-rich still later in the deposition.
Although .~pplicant does not wish to be bound by the theory, it is believed tha-t the Si-Si-H arrangement facilitates rapid oxidation by an initial thermally in-duced deoompositio1l in which the hydrogen migrates to the neighboring silicon HMe2Si-SiMe2H-~Me2SiH2~Me2Si. The reactive dimethylsilylene Me2Si species is then rapidly oxidized, releasing free radicals such as hydroxyl ~OH), which then rapidly abstract hydrogen from the Si-H bonds, thus creating more reactive silyl-. .. .

ene radicals, forming a chain reaction. The tetramethyltin is less reactive to these radicals, and thus mainly enters into the later stages of the oxidation.
The Me2SiH2 lacks the rapid initial decomposition step, and thus, cannot begin oxidation until after some tetramethyltin has decomposed to form radicals (CH3, 0~1, O, etc.) which then preferentially attack the Me2SiH2, at intermediate times, until the Me2SiH2 is consumed, after which stage the oxidation of tetra-methyltin becomes dominant again.
; It is preferred to have at least two methyl groups in the disilané
compound~ since the disilanes with one or no methyl substituents are spontaneous-ly flammable in air, and thus must be pre-mixed with an inert gas such as nitro-gen.
Other hydrocarbon radicals, such as ethyl, propyl, etc., may replacc methyl in the above compounds, but the methyl ones are more volatile and are preferred.
Higher partially alkylated polysilanes, such as polyalkyl-substituted trisilanes or tetrasilanes, function in a way similar to the disilanes. HoweverJ
the higher polysilanes are harder to synthesize, and less volatile than the di-silanes, which are therefore preferred.
When the initial deposition of the silica-tin oxide films contain less than about ~0% of tin oxide, there will be little or no haze created at the in-terEace of the glass substrate and the coating thereover. If it, for some reason, is desired to start the gradient above about 30% of tin oxide, it is preferable to have the glass coated with a haze-inhibiting layer~ i.eO silicon dioxide. Such a haze-inhibiting layer may be very thin, e.g. in the nature of 25 to lOO angstroms.
~igure ~ illustrates a section o~ a lehr in a float glass line. The structure of the lehr itself is not shown for purposes o~ clarity. The hot ~7~3~

glass 10, e.g. about 500 - 600C, is carried on rollers 12, 14, and 16 through the lehr. ~etween rollers 12 and 14 is positioned gas duct assembly 18 which comprises a gas inlet duct 20 and a gas outlet duct 22. Between ducts 22 and 20 and separated therefrom by heat exchanging wall members 24 is a duct 25 form-ing means to carry a heat exchange fluid, which, in turn forms means to cool gas exhaust from duct 22 and to heat gas flowing through duct 20. The tempera-ture of the heat exchange fluid is maintained at a sufficiently low temperature so that coating does not take place on the surface of the inlet duct.
Gas entering inlet 20 travels through a slit-like opening 28, thence along a reaction zone formed by the top surface 30 of duct assembly 18 and the lower surface of glass sheet 10. Upon reaching a second slit-like opening 32, the remaining gas is exhausted through duct 22. It is during the passage of the gas along the lower surface of glass sheet lO that a gradient coating is formed by the selective depletion of one of the reactants at different points along thelength of the deposition zone between rollers 12 and 14.
In the apparatus of Figure 4 a second gas duct assembly 38 is used to complete the deposition of a coating, e.g. by adding a fluoride-doped tin oxide - coating to the pre-deposited gradient coating Again, it is convenient to have gas enter the upstream port 28a and leave the downstream port 32a.
The ducting is suitably formed of corrosion resistant steel alloys and comprises a jacket 50 of thermal insulation.
Illustra~ive Examples In thls application and accompanying drawings there is shown and de-scribed a preferred embodlment of this and the parent invention and suggested various alternatives and modifications thereof, but it is to be understood that these are not intended to be exhaustive and that other changes and modificationscan be made within the scope of the invention~ These suggestions herein are ~7~39 selected and included for purposes of illustration in order that others skilled in the art will more fully understand the invention and the principles thereof and will be able to modify it and embody it in a variety of forms, each as may be best suited in the condition of a particular case.
Example 1 Glass heated to about 580C is moved at a rate of ]0 cm/sec across the apparatus sho~n in Figure 4. The temperature of the gas inlet duct is main-tained at a temperature of about 300C) by blowing appropriataly heated or cooled air through the temperature control duct. The first deposition region reached by the glass is supplied with a gas mixture of the following composition (in mole percent):
1,1,2,2 tetramethyldisilane 0.7%
tetramethyltin 1.~%
bromotrifluoromethane 2.0%
dry air balance The second deposition region is supplied with a gas mixture of the following composition ~in mole percent):
tetramethyltin 1.6%
bromotrifluoromethane 3.0%
dry air balance The flow rates of these gas mixtures are adjusted so that the average duration of contact between a given element of the gas mixture and the glass surface is about 0.2 ~econds.
The resulting coated glass is color-neutral in appearance, in reflected daylight. It has a visible reflectivity of 15%, and no visible haze. The infra-red reflectivity is 90% at a 10 micron wavelength. The electrical resistance is measured to be 5 ohms per square. The coa-ting is about 0.5 microns ~hick.

7~

Example 2 The deposition described in Example 1 is repeated, the only difference being the composition of the gas mixture supplied to the first deposition region:
1,2 dimethyldisilane 0.4%
1,1,2 trimethyldisilane 0.3%
1,1,2,2 tetramethyldisilane about 0.02%
~etramethyltin 1.5%
bromotrifluoromethane 2.0%
dry air balance The properties of the resulting product are indistinguishable from those of Example 1.
Samples of these coated glasses have been subject to Auger chemical analysis of the coating composition along with ion sputter-etching to reveal their chemical composition versus thickness. Figure 5 shows the resulting chemi-cal composition profile of the deposit over the region in which it varies. Near the glass surface the deposit is mainly silicon dioxide, with about one silicon atom out of eight being replaced by tin. As the distance away from the glass surface increases, the tin concentration increases and the silicon concentration- 20 decreases, so thak by distances greater than 0.18 micron from the glass surface, the deposit becomes tin oxide, with about 1.5 percent of the oxygen replaced by the fluorine. Vsing Figure 3, the silicon-tin composition profile is converted to a refractive index versus distance profile, which is also plotted in Figure 5.
These results confirm the abllity of the disclosed process to produce the desired variation of refractive index through the thickness of the deposited film.
~ 3 A tin oxide coating is placed on a glass substrate at di~ferent thick-3~3 nesses ~the glass substrate is first coated with an ultra-thin film of silicon dioxide to provide an amorphous haze-inhibiting surface.~
Thickness of Tin Oxide Iridescence Visibility 0.3 micron strong 0.6 micron distinct, but weaker 0.9 micron barely detectable except in fluorescent light 1.3 micron weak, even in fluorescent light The latter two materials are not aesthetically objectionable for architectural use, confirming the visual color saturation scale used to evaluate the designs.
In order to provide the most effective suppression of iridescent color, it is desirable that the refractive index of the initial deposit match closely that of the glass substrate, preferably to within ~.04, or more preferably to within +.02 refractive index units. In order to achieve this match, one varies the parameters of the deposition, particularly the ratio of tin to silicon atomsin the inlet gas. As an exampie of such variation, Figure 6 shows the variation of refractive index in the initial deposit from tetramethyltin plus 1,172,2 tetramethyldisilane gas mixtures, as a function of gas composition. The other parameters for these depositions were fixed as in Example 1. Figure 6 shows, forexample~ that an initial deposit of refractive index 1.52 (appropriate to match usual window glass refractive indices) is produced by a gas composition of equalnumbers of silicon and tin atoms. Matching to 1.52 ~.02 is achieved when the gas composition is kept between 47 and 52 atomic per cent o~ tin. While these exact numbers may differ somewhat in other conditions of deposition such as other temperatures or other compounds, it is a matter of routine experimentation to establish calibration curves such as Figure 6, in order to produce a suitable match of refractive indices between the substrate and the initially deposited ; ' ' 3~

coating composition.
It is to be noted that the reflection of light from the surface of the coated produc~s of Example 3 is about 16 to 17%, i.e. about 10% higher than that from the coated glass in Examples 1 and 2 which do have a graded undercoat ac-cording to the invention.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which might be said to fall therebetween.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In an apparatus for forming a continuous coating of progressively changing composition on heated glass over the length of a processing path through which said glass passes said apparatus comprising, in addition to means to sup-port said glass and means to move it continuously along said processing path, the improvement wherein there is provided a reaction zone the interior surfaces of which are formed primarily by said moving glass and a heat-transfer-medium moderated, temperature-controlled wall opposed to said glass said zone having, at one end thereof, a port means at a first station to introduce and distribute a gaseous reaction mixture comprising reactants which form reaction products that deposit on said glass at different rates, a gas flow path within said reaction zone wherein said gaseous reaction mixture flows along said glass surface, and said temperature controlled wall to port means at a second station at the oppo-site end of said zone to remove residual gaseous reaction mixture, wherein said second station being relatively positioned along said reaction zone with respect to said first station that said heated glass wall of said reaction zone forms means to provide sufficient energy to said gaseous reaction mixture to achieve a substantial difference in composition deposited on said glass between the re-action gas mixture composition, as it passes between said first station and second station and wherein said temperature-controlled wall is maintained at a temperature low enough to avoid deposits of reaction products thereon.

2. Apparatus as defined in Claim 1 wherein said ports are mounted below said glass, and said glass forms the upper boundary of said processing path.

3. Apparatus as defined in Claim 1 wherein said ports are slit-like openings arranged parallel to one another and substantially normal to the flow path of said glass.

4. Apparatus as defined in Claim 1 wherein said inlet and outlet ports communicate with inlet and outlet ducts, adapted to carry a heat-transfer medium which stabilizes and controls the temperature of the apparatus, and which forms a cooling means for gas in the outlet duct and heating means for gas in the in-let duct, all said ducts forming an integral unit adapted to fit between rolls of a lehr in a floor glass line and beneath a glass substrate carried on said rolls.

5. Apparatus as defined in Claim 4 comprising additionally, a second coat-ing apparatus mounted in series, within said lehr, said second apparatus forming means to provide an additional coating to said coating of progressively-changing composition.

6. Apparatus as defined in Claim 1 wherein said inlet and outlet ports communicate directly with inlet and outlet ducts; within said temperature-controlled wall within which said inlet and outlet ports abut a common tempera-ture control duct which is adapted to carry a heat-transfer medium, and which forms means to stabilize and control the temperature of the apparatus and which forms a heating means for gas in the inlet duct.