GB2524238A - Anti-reflection coating for use in insulation glazing applications - Google Patents

Abstract

A thin film layer grown on a coated substrate that introduces an anti-reflection property consists of a film with an average refractive index that is lower than that of the underlying coating on the substrate, wherein the film has a non-uniform refractive index through its depth and a part of the film near an air interface has a refractive index that is lower than the average refractive index of the film. The film layer is preferably non-structured silica or silica based. The film preferably has a controlled degree of porosity to achieve a graded refractive index. Also disclosed is an atmospheric pressure chemical vapour deposition based process using a flame as a reaction energy source for depositing an anti-reflective film layer onto a substrate or coated substrate, wherein a reactive gas mixture including a film precursor impinges on the substrate and the size and concentration of particulates within the mixture is controlled to achieve target optical properties of the layer.

Description

Anti-reflection coatinE for use in insulation zlaziiw applications Key words CVD,añti-reflection coating, Chemical Vapour Deposition, atmospheric pressure, coating, flame assisted, low emissivity.

Introduction

Chemical Vapour Deposition (CVD) has been widely used for many years across a wide range of industrial applications, to produce thin film coatings. In such a process a reactive gas mixture is introduced in the coating region, and a source of energy applied to initiate (or accelerate) a chemical reaction, resulting in the growth of a coating on the target substrate.

Atmospheric pressure CVD (APCVD) has established itself increasingly in recent years, as a technologically and commercially attractive sub set of CVD coating. It has béeñ particularly successfully employed in high throughput continuous or semi-continuous coating processes. The APCVD approach has also found application in smaller volume processes where its lower overall costs can be decisive. Furthermore, although in many cases the film properties of CVD coatings across many of the deposition activation approaches are broadly similar, there are in certain cases important differences leading to potentially further "differentiated" characteristics of the processes approach.

Such combinations of advantages has lead to AP thermal CVD being used in a wide range of industrial applications such a on-line glass coating, tool coating, ion barrier layer deposition, anti-corrosion and adhesion layers on metals, scratch coatings on bottles etc. An example of large scale AP thermal CVD applied to a continuous process is described in patent no WO 00/75087.

Typically for AP thermal CYD these can be over 500°C and can reach over 1000°C in some applications. Such high temperatures are typically applied to achieve desired properties of hardness, durability and structure.

A number of coating materials have been deposited by such large scale processes (tin oxide, aluminium oxide, silicon oxide titanium oxide).

One major application of this technology is in the coating of glass to impart a low emissivity property (Low E) to enhance window energy efficiency. Introduced several decades ago, such coatings are now widely used. In recent years it has become more common to use multiple glazing unit structures, to further enhance the insulation property. Such glazing structures can, for example, use 3 glass panes and 2 gas insulation zones, along with 2 or more Low E coatings. Such Low B films can be deposited by a range of coating processes such as CVD and sputtering.

Whilst the examples above refer to Low E coatings, the invention could equally be used to reduce reflection in other coating structures (such as solar control coated glass) and reference to Low in the ongoing text, encompasses this interpretation.

One major drawback of such Low B coatings is that they reduce the transmissikrn of light due to (multiple surface) reflection. The RI of the Low E coatings are generally higher than the underlying glass substrate and thus increase the reflection losses. This leads to both cosmetic (reflection and multiple image) issues and also reduces the integrated overall efficiency of the energy balance into the building. Furthermore, the cosmetic impact of the reflections can be further strengthened due to colour being introduced into the reflection by an interference effect.

Whilst anti-reflection layers have been proposed for such structures previously, the performance, and cost of such coatings, has been an issue and widespread application has not been developed.

The invention described herein addresses these current limitation, and defines a film design and a process or a method particularly compatible with the establishment of an industrially viable process for the deposition of fi.inctional and structure controlled coatings onto, for example, metal based materials and transparent substrates such as glass.

A variation of APCVD is where a flame is used to supply some or all of the energy required to activate the CVD deposition process. There are two basic variants of this process: combustion CVD (where the precursor and!or it's carrier solvent is flammable (Georgia Tech Institute Surface and Coating J, 94-95, 1997, 13-20)). and thus contributes (normally significantly) to the flame energy, or flame assisted CYD where either little or no energy is provided by the precursor (Transition metal oxides, Blectrochemical Soc, EuroCVD 14, 2003, vol.1, pg 557-564)) or carrier solvent (although it may contribute to oxidation source). In such reports the application related functional properties proposed include bather, anti-corrosion, anti-reflection and adhesion.

An anti-reflection (AR) layer normally consist of one or more layers of thin films which are tuned to produced a reflection compensation effect compared to the surface they are deposited on. The standard approach is that the thin film layer (the top layer) is of lower refractive index (RI) than the substrate. Additional layers can be introduced between this low RJ top layer (to produce a "stack" of layers) and the substrate surface to achieve improved AR performance and br to adjust the residual reflected colour.

Generally, the lower the RI of the final layer, (for any given stack design) gives lower overall reflection. Thus a process which produced low RI layers would be highly advantageous.

However, it is known that graded RI films can be particularly effective at producing a AR surfice. However such layers have traditionally been very hard to achieve at an industrial scale.

Main innovative aspects In this invention we address many or all of the current technology limitations noted above.

The proposed invention refers to the uses of graded refractive index layers deposited onto a surface to achieve optimised high anti-reflection (AR) performance. This is achieved by producing, firstly a controlled degree of porosity in the film. Secondly it defines a degree of gradient to the refractive index (where the top of the film has a lower RI than the avenge bulk of the film. Furthermore, carefUl control of such RI grading can achieve very high AR performance and importantly a very neutral reflection colour (due to the wide active AR wavelength range).

At the same time, the chosen process to deposit the film (a specific variant of FACVD) can produce the films with a highly attractive industrially compatible process. The process is easily integrated into existing processing and is compatible with many existing handling equipment designs in industrial use.

The coating is also toughenable /annealable which is a significant benefit which many AR layers struggle to achieve, indeed such annealing can even further enhance the FACYD film properties by partially sintering the structure.

A thrther, surprising (and important for glazing applications) aspect of the invention, is that the coatings are of very low scatter. One previous drawback of some porous and/or graded RI designs was that they introduced an undesirable, or even unacceptable level, of light scatter (for some applications). Furthermore, depositing on top of an under coating (which already has a degree of roughness) has been reported to magni1 this detrimental scatter effect. The proposed design and method, surprisingly, reduces or overcomes this prior disadvantage to a significant extent. It is furthermore, possible with the proposed process to "tune" the balance between AR property and scatter. Thus whilst the best AR properties have a low degree of haze, this could (if required by application) be further tuned to an even lower level by reducing the AR effect, but whilst still retaining a significant degree of AR affect.

As noted above, a critical aspect for any over-coating is how it grows on the underlying surface or film. This is particularly important when the substrate is not a smooth surface. Whereas a glass surface may be considered relatively smooth (with a rms roughness perhaps of the order lnm), a Low E film may be up to several nms roughness, and in the case of CVD Low B may be 5 or more mns roughness. We have found that the proposed FACVD grown film grows (if the process conditions are carefully chosen) onto such rougher surfaces with a structure highly desirable for application as an AR film. In such situation a desired (surface roughness related) property (e.g. of minimise scatter, cleanability, handleability) is understood to be achievable normally by reducing the surface roughness ("planarising the surface").

Whilst we do not wish to be limited, we propose that in our work we have seen that whilst the average roughness can in some cases be increased, the surthce feature size has reduced (or is some cases not increased to the same extent as anticipated) and this imparts better overall properties than would have been expected, such as minimized scatter. Furthermore, the substrate surface roughness, when combined with the inventive process steps, enhances the graded refractive index property achievable.

Applications Whilst not wishing to limit the scope, we illustrate the potential of the invention by example of it's application to coating surfaces used for energy efficiency glazings e.g. those made with CVD or sputter coatings.

For coating continuous (or continuously fed) substrates which move under or through the coating region the substrates can optionally be pre-heated to the fIrther control the desired reaction temperatures. Alternatively, the FACVD process can be integrated into a glass production line where the glass is already heated (and may also have been pre-coated with a Low E coating further up the line (in a higher temperature zone).

Experimental Preferably low toxicity and low cost precursors and solvents are employed and we have employed organo-silicon compounds (and preferably) pre-vapourised into a carrier gas flow and mixed with the chosen combustable gas prior to introduction into the flame.

Careful control of the process set up parameters and the gas composition mixes, enhances control of the porosity and the graded RI structure. The graded RI structure has also been further enhanced by the use of 2 or more FACVD heads (used in-line in ours laboratories) to give facile capability to deposit sequential layers. Each head having either/or both different gas compositions or process set up.

The gas composition variations explored include different organo-siicon compounds, different gas phase concentrations of the organo-silicon compound, different combustable gases, different combustable gas ratios (to carrier gas) and different oxygen levels (relative to carrier gas). Within process set up variations which we have demonstrated as important in control of properties, are head temperature (where which we control), extraction, Height of the head above the substrate and the coater head design.

The examples below are meant to illustrate scope, and potential, and are not in themselves meant to be limiting.

Typical experimental conditions applied: Flame support gas: propane (but methane, butane also tested).

Temperature of substrate: between room temperature and 500C (variable with precursor and process conditions e.g. oxidant, and metal nano-structure desired) has been explored, however most of the work has been based on substrate temperatures between 150 and 200C. Higher temperatures have indicated that more durable films can be produced, but depending on application, the lower substrate temperatures are acceptable and easier to use.

Precursors used: silica from TEOS (tretraorthoxysilicate) -bubbler setting typically 110 °C.

(note: DM50, ehloro silanes, silicon tetrachloride were also tested). The silica precursor is normally pre-vapourised (either by a bubbler approach or a flash evaporator in our work). We have also explored a "fme mist" delivery approach (where the liquid precursor is atomised and carried into the flame by the carrier gas.

This approach also works and can lead to another mechanism for particulate size control -however we prefer the pre-vapourised approach as this is easier to process engineer (and then control particulate size by other process mechanisms).

Carrier gas and Oxidant source, in this work we have used air or simulated air (i.e. by mixing oxygen and nitrogen) but a variety of other oxygen sources could be used (including oxygen, N20, and organics containing oxygen e.g. alcohols).

Carrier gas-nitrogen/oxygen or air (flow rates per 100 substrate width between 2 and 1 min-l). Ratios of oxygen to nitrogen have been explored above the ratio found air.

Carrier and other gas temperatures typically between pre-heated 60 and 130°C Substrate type and structure tested: Glass and steel, silicon wafer Substrate preparation: typically the substrate is washed in a soap or similar cleaning/degreasing solution, fully rinsed several times in de-ionized water, and then solvent (e.g. iso-propanol) rinsed and dried in an oven.

The burner heads employed include 2 in house designed heads and 4' commercially sourced burner heads. Whilst all of these heads produced films, there are significant variations depending on head design. Furthermore, the set up of the process is influenced by the head design. However, the range of conditions defined within this document, are compatible with achieving the targeted film structures with optimisation.

Configuration: All the gas components are premixed, and delivered at raised temperature to a FACVD burner. The head is temperature controlled. The head internal distributor spreads the gasts across the width of the coating target zone/substrate and allows for essentially laminar flow to the flame zone. A flammable gas (e.g. propane) was introduced along with an approximately stoichiometric (for fill oxidation of the precursors) oxygen via air. Varying the oxygen/gas ratio impacts on the growth conditions and can be sub or super stochiometric by small degrees (few to few tens of %) to adjust the properties of the grown film.

Film thicknesses produced: silica, from approx 20 nm to several hundred nms -depending on optical specification targeted. The film thickness defines the region spectral region the AR films works best in (where a thicker film works at a higher wavelength). In this work, we have focused on the visible region and the filth thicknesses have been mainly within the region 50 to just over lo0nm. This film thickness range allows for adjustment to the (residual) reflected colour and also impacts on overall scatter levels (thinner films having lower scatter) for given process growth conditions. The film is primarily silica based, but can be non stoichiometric (i.e. has an oxygen ration which is not Si 02) and may have small amounts of components derived from the precursor or the carrier gases (such as carbon and water). It is also possible to add additional components to the gas stream which whilst not changing the basic optical properties and film structure, may impart some benefit to properties, including examples such as further enhanced durability (e.g. controlled carbon levels), anti bacterial/fungicidal properties andlor modifS' surface energy.

Film thicknesses was measured using a step profiler and producing a step in the AR film and has shown that we are depositing approximately 1 Onm a pass under the flame head we used with the settings However, increasing the size of the head would increase this figure (and we demonstrated this in our work by lengthening the coating zone (in the direction of movement of the substrate) and saw an increase in growth rate per pass (although in this design example, the effect was not fully linear (i.e. increase in growth rate was less than the increase in head dimension). The thickness can also be controlled either by through speed (i.e. residence time under the coating head), the number of passes (or by increasing number of installed coater heads as an alternative approach), or by varying the precursor concentration in the flame. In this work, we have retained the precursor concentration relatively low and controlled thickness by varying passes. Enhanced refractive index gradient control was achieved by varying the particulate size and concentration within the flame gas phase.

The higher the precursor concentration, the greater the size and concentration of particulate in the flame. In most of our work we targeted particulate size (measured by AFM or SEM of the final film where the particles could be seen embedded) at below 250nm, and preferably below lOOnm diameter. Particle size and incorporation levels could also be controlled by varying height of the flame above the substrate, the temperature of the flame, the oxygen level within the flame, the design of the head, the temperature the head was controlled at, and the temperature of the substrate during coating. The use of more than one head in-line (i.e. sequential coating) also allowed for different coating conditions for each head which could be used to further control and enhancement of the graded RI property. In the work undertaken here, we used up to 2 FACYD heads in-line. We also used 4 different coater head designs (one built to in-house design, one commercial bought in burner head, and two which were built for

us but to our design specification.

Flame zone optimization Flame zone optimization is required for effective anti-reflective coatings growth by FACVD, to control the graded RI properties. The key zone (see Fig 10) is at the tip of the flame front extending into the flame. Increased uniformity is noted from passing the substrate through the flame front in order to cut through the flame. This effectively increases the tip size by dispersing it across the surface. This zone is the honest part of the flame and superior AR films are shown from materials deposited in this region. If the sample is moved away from the flame front, an increase in powdery deposits and optical haze is shown. Placing the sample too high into the flame causes disruption to the flame stability reducing film uniformity and optical performance.

Optimum samples placement is proposed to be approx within the -25% to 75% of the flame length.

Experimental Set-up The coater head is an in house design/specified head. The coater set up includes a heatable susceptor on which the substrate is placed. This susceptor sits on a motorised drive which allows the substrate to be scanned under the flame head, which is pointing downward. We have also operated the system with head underneath the substrate.

The lab coater employed has a variable speed (traverse under the fixed coating head) of up to approximately 72m/hr. The residence time within the frill flame zone was generally approximately Isec. Including the wider "spread out" zone of the flame would approximately double this. Wider (flame zone) or multiple heads can be used to increase the effective dynamic growth rate.

Coating Conditions for examples quoted: Temperatures Susceptor (to heat substrate)-200°C Glass -3 and 4mm Float (borosilicate also used) Gas feed Lines -200°C Bubbler set temp -100-125°C Precursor temp (measured inside bubbler) -100°c Precursor Delivery typically between 3. x 104mo1/min (70°C @ 0.2 L/min) to 2.x 102mo1/min for optimum AR performance Flows Air-19L/min Propane -1 L/min Nitrogen -0.4L/min Flame head width 100mm (up to 200mm also testedfor initial scale up feasibility,).

Set-up Gap (FACVD Head to Susceptor) -8mm (but was varied between 3 and over 10mm in R&D work partly to adapt for different subs (rate thicknesses).

Extraction -vacuum of 0.05 inches water gage (with the substrate and Susceptor under head) Speed of substrate carrier-70 m hr-I Pre-clean of substrate Wash -400m1 Distilled Water + 0.5m1 anionic surfactant.

Coating Process: the flame coating head is temperature controlled and the waste gases are extracted away froth the coating zone.

E'canwle resultc Anti-reflection coating on Fluorine doped tin oxide film: Figs I and 2 (Tables) illustrate the effect of increasing film thickness, and figs 3 and 4 show the resultant trend in reflection and transmission across the visible spectnim.

The gains in transmission are close to the reduction in reflection indicating that the gain is coming primarily from an anti-reflection effect and also that the absorption and scatter associated with the AR film are low.

Maximum anti-reflection effect is seen around 1 OOnm film thickness, however the chosen final thickness would take into account the colour of the film (fig. 5) and other trade-oft's such as scatter (if this was a particularly sensitive parameter)r. For the

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achieved best AR coating performance, scatter was measured at under 0.2% (and could be reduced further by trading the AR effect).

Colour Fig 5 shows the change in refl*,ted colour as the AR film increases in thickness. The colour changes are modest and acceptable for many applications. If required, the thickness of the under coat layers (e.g. colour suppression or anti-iridescence layers) can be adjusted to compensate and return the colour into an acceptable region.

Roughness Fig 6 illustrates a major advantage of the proposed approach. The surface roughness increases only slightly compared to the starting substrate. Indeed, in this example, the reference 300nm tin oxide Rnis of 15.7nm falls with AR coating initially and only rises to 16,6 nm alter coating.

Thermal stability Samples of the AR films (on float glass and borosilicate glass) were heated to 550C and soaked at this temperature for 30 minutes. The films were measured for optical properties before and after this treatment. Changes in optical properties (reflection and scatter) were small and generally close to the measurement error of the instruments. The purpose of this test.was to indicate thermal stability and in particular, potential for toughening or annealing of the substrates after coating.

Extension to ITO The AR coatings were also deposited onto a glass substrate coated with ITO (prepared by vacuum sputtering). Figs 7 and 8 show the gains made in transmission and (reduction in) reflection. The differences between reflection and transmission changes again indicate the low absorption and low scatter achievable.

Claims (13)

1. Claims I. A thin film layer grown on a coated substrate where such layer introduces an anti-reflection (AR.) property and such layer consisting of a film with an average refractive index (RI) which is lower than the underlying coating on the substrate and fulfilling the following criteria a. A film which has a non uniform RI structure through its depth b. A film where the upper part of the AR film (near the air interface surface) is of lower RI than the average of the AR film RI.
2. 2. A thin film according to claim I where the AR film has a graded refractive index and where the variation in RI is at least 0.1 from highest to lowest region of the AR film, and more preferably 0.2 or more.
3. 3. A thin film according to claim 1 were the AR film exhibits a low level of optical scatter, below 2%, preferably below 1% and more preferably below 0.5%.
4. 4. A method according to claim 1 fur depositing a layer onto a substrate or coated substrate, the method fulfilling the following criteria i. an atmospheric pressure based process * ii. using a flame as a reaction energy source iii. a chemical vapour deposition (CVD) based reaction where the reactive gas mixture including the film precursor (from within the flame) impinges onto the substrate iv. where the size and concentratiàn of particulates within the reactive gas mixture impinging on the coated substrate is controlled to achieve target optical properties in the AR. film.
5. 5. A method according to claim 4 where the coating region, relative to the flame, is controlled to within a region approximately the -25% to 75% of the flame length as illustrated in fig 10.
6. 6. A method according to claim 1 where the AR. coating is deposited onto a substrate which is glass and the glass substrate optionally has a Low Emissivity (low B) thin film coating on it.
7. 7. A thin film according to claim 6 where the visible reflection of the low E surflice is reduced by at least 2% and preferably by 4% and more preferably by 5% or more.
8. 8. A coating according to claim 1 where the chemical composition of the film is nano-structured silica or primarily silica based.
9. 9. A coating according to claim I where the film can be thermally toughened or annealed without significant change to its optical properties.
10. 10. A method according to claim 4 where the film can, by selection of coating set up conditions, be grown onto a coated substrate in such a way as to reduce the overall measured feature size of the surface.
11. Ii. A method according to claim 3 where the level of haze/scatter of light is adjustable by varying the targeted degree of anti-reflection property.
12. 12. A method according to claim 4 where the coating is deposited from a precursor mixture containing pre-vapourised precursors.
13. 13. A method according to claim 4 where the FACVD process can be integrated in-line to a substrate manufacturing or processing line where other processing is undertaken including deposition of the low E underlayer coating.