Titania coatings by plasma cvd at atmospheric pressure

ES 2 289 331 T3

DESCRIPTION

Titanium oxide coatings using atmospheric pressure plasma CVD.

Titanium oxide is a widely used material in thin-film coatings. It has been used, for example, as a hard coating and as a transparent dielectric material in optical stacks. In recent years the photocatalytic properties of titanium oxide have been of increasing interest (for example Paz & luo, J.Mat. Res Vol 10, no 11, Nov 1995). Titanium oxide layers have been deposited by many techniques (eg evaporation, e-beam, sputtering, sol gel, and CVD).

Chemical Vapor Deposition (CVD) has been widely used for many years across a wide range of industrial applications to produce thin film coatings. In such a procedure a reactive gaseous mixture is introduced into the coating region and an energy source is applied to initiate (or accelerate) a chemical reaction, which results in the development of a coating on the target substrate.

Power sources (for CVD) are typically thermal or plasma. However other sources (such as laser, arc, UV, etc.) have been used in special application areas. The choice of thermal or plasma activation is determined by a number of factors - such as required film properties, growth rates, products of integration into the process, economic considerations, etc. However, a critical deciding factor is often the operating temperature defined by the maximum allowable temperature for the chosen substrate. Atmospheric pressure CVD (APCVD) has become increasingly established in recent years as a subset of technologically and commercially attractive CVD coatings. In particular it has been used successfully in high performance continuous or semi-continuous coating processes. The APCVD method 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 through many of the deposit activation methods are quite similar, there are in certain cases important differences leading to potentially more "differentiated" characteristics of the approach to the procedures.

Such combinations of advantages have led to AP thermal CVD being used in a wide range of industrial applications such as in-line glass coating, tool coating, ion barrier depositing, anti-corrosion and metal adhesion coatings, metal coatings, scratches on bottles, etc. An example of AP thermal CVD applied to a continuous process is described in patent No. WO 00/705087.

As mentioned above, a major limitation to the scope of thermal APCVD application has to date been the substrate temperature required to achieve target growth rates and target thin sheet properties. Typically for AP thermal CVD these can be above 500 ° C and can reach over 1000 ° C in some applications. Several applications (APCVD) are known that use temperatures below 500 ° C, for example in the growth of certain II-VI materials by organic metal CVD. However they are generally limited in scope of application, and apart from that cited above, they tend to produce films with properties sufficient for the purpose, but not optimized (due to low temperature needs). Typically in situations where CVD is the preferred method, but where substrate temperatures are limited below 500 ° C and sometimes need to be below 100 ° C, then a plasma method is frequently selected. This need for reduced substrate temperatures is also manifested in systems where diffusion processes, which are intensified at higher temperatures, lead to degradation of the material or device. However, to date such plasmas, which are used in industrial CVD coating applications, have been based on a vacuum method.

Such a vacuum method, while capable of achieving lower target substrate temperatures, has significant technological limitations for certain applications. Vacuum systems are typically of significant capital cost, and can result in lower growth rates (relative to APCVD) due to lower concentrations of reactive species in a vacuum. Furthermore, vacuum-based procedures are more difficult to integrate into a high-throughput procedure that requires for example complex and expensive handling methods (for example load closure) between the introduction of the substrate (from the open air) to the system. empty. For continuous tapes, films or sheets this can be a major limitation, and although some solutions (based on differential pumping) have been proposed this is rarely applied due to cost and complexity.

Background of the invention

The disclosed invention addresses the low temperature deposition of titanium oxide and also optionally a process devised to achieve / retain a degree of photocatalytic activity. This procedure is aimed, in part, at filling the significant technological "gap" between current AP thermal CVD and vacuum plasma CVD for reservoir. The invention describes a route to achieve the low substrate temperatures associated with plasma CVD while avoiding the cost and process design constraints of a vacuum system. The invention also enables titanium oxide growth rates to be achieved much faster than normally possible with vacuum plasma CVD.

ES 2 289 331 T3

Although atmospheric pressure glow discharge plasmas (APGDP) have been known for some time, the application of such plasmas has been largely limited to surface treatment, for example pre-treatment of plastics before second stage printing or coating.

In recent years a number of literature publications, and more recently patents, have covered the area of ​​APGDP, its generation and applications. Patents such as (US 5,938,854 and US 6221268) discuss the application to surface treatment. A small number of literature publications cover the topic of using APGD plasmas to produce coatings on a surface. These primarily consider deposition of "plasma polymerized" films, that is, films that have significant organic content or exhibit characteristics that would not normally be considered inorganic. (eg Goosens, Dekempeneer et al, Surface and Coatings Techn, 2001, and DE 19955880). Several patents deal with inorganic film depositing (eg US 6235647). However, the materials considered and the suggested methods are not optimized for industrial exploitation.

JP 2000 313962 A describes an atmospheric pressure plasma CVD process using a plasma based on a gas mixture of 0.005-5% by volume of titanium tetraisopropoxide, 3% by volume or less of O₂, 60-97% by volume of Ar and 3-40% by volume of N₂. Plasma is an AC plasma that has a frequency of 15-50 kHz. The substrate is placed between two electrodes. The procedure is suitable for polymeric substrates with lower melting points. It is important to avoid that the humidity concentration is too high. The desirable upper limit is 500 ppm.

JP 08 253322 A teaches an atmospheric glow discharge plasma CVD process using a titanium alkoxide for depositing a TiO layer₂. The substrate temperature can reach up to 200 ° C. The apparatus is suitable for providing a laminar flow of precursor gas.

US 2003/072891 A1 describes CVD layer deposition of atmospheric plasma. In the examples, TiO2 is deposited on cellulose ester films. It is mentioned that the electrodes are made of stainless steel and can adjust the temperature by circulating water. Plasma density is 0.03 W / cm².

WO 02 48428 A relates to a method for producing anti-reflection coatings on organic films by plasma CVD at atmospheric pressure. The coating can be a TiO2 film.

US-A-5 993 916, US 2001/025205 A1, US-B1-6 329 237 and US 2002/086476 refer to low pressure CVD of TiN films, Ta₂OR₅ and (Ba, Sr) TiO₃ for applications in microelectronics. The films are subsequently treated in a plasma.

The invention described herein addresses these limitations and defines a process or method particularly compatible with establishing an industrially viable process for the deposition of functional titanium oxide coatings.

According to the invention there is provided a method according to claim 1.

The method particularly addresses the need for high growth rates of titanium oxide at temperatures lower than those normally used in APCVD procedures. The procedure also identifies the importance of controlling plasma conditions and gas phase concentrations to achieve the target compositional, physical, and functional properties.

In order to achieve the target low temperature operation and process characteristics to be achieved, the needs of the plasma type and operation details need to be carefully selected. There are many different types of plasma. However, a glow discharge plasma is desirable because it can be operated as a non-thermal plasma. A number of power sources and configurations can generate such plasmas, however we have found that the use of low frequency AC plasmas gives proper performance.

In such a case, when a suitable plasma is used the thermal temperature of the plasma is much lower than the electronic temperature. The preferred frequency range for this type of atmospheric pressure plasma is different from that normally used for vacuum plasma generation. This can be understood in terms of the generation and entrapment of sufficient plasma species, within the plasma coating zone, which will be moderated by the densities of gaseous molecules at AP. For example, diffusion rates, active species lifetimes, and charge accumulation will become markedly different with higher pressure operation. The frequency range below 100 KHz is proposed, and a number of publications use frequencies around 20 KHz or lower. The optimal frequency will depend on a number of factors including reactor design, materials used, plasma gases used, additive concentrations, voltage and power levels used.

The gases used to support GD plasma are normally selected from helium, argon and nitrogen (or their mixtures), although it is possible to introduce additional gases as minor components to achieve particular plasma characteristics (eg oxidizing properties).

ES 2 289 331 T3

To achieve good quality optical and mechanical properties in titanium oxide films grown using this method, and to also achieve photoactive titanium oxide, we have found it necessary to carefully control the plasma and chemical reactions that occur. An example of this is with levels of water vapor in the reaction chamber to avoid unwanted reactions. Careful control of the oxidizing source (typically oxygen gas, but alternative oxygen-containing species can be used, eg, organic oxygen-containing species) is necessary to achieve optimum performance in terms of film properties. Main innovative aspects

The use of an APGD plasma CVD method to deposit inorganic films is an innovative method. Getting good film quality and a viable procedure is new.

We define a procedure in which the gas flow through the reaction zone is as laminar as possible. This requires that a manifold be used for the introduction of gases and, preferably, that an additional manifold be used in the extraction region. Furthermore, it is necessary that all the reactive gases to be introduced are premixed before their introduction into the reaction zone. It is necessary to properly choose the plasma type, power, frequency and plasma gas (s). We have used a power source frequency of 10-25KHz, power levels from less than 1 to around 10 watts per cm² (5-10 W / cm² are not part of the invention). The applied plasma gases have been helium, argon and nitrogen. Helium gives the most stable and flexible configured plasma systems and generally the best film quality. However, other gases can be used successfully if design limitations and film property goals are flexible enough. Reactive precursors and oxidizing gases have also been carefully selected for optimal performance. In our titanium oxide growth work we have used titanium tetrachloride and titanium oxide alkoxides.

Careful control of plasma and gaseous composition conditions, during film growth, can be important in achieving gradual control of the stoichiometric control of the layer composition that is highly desirable to achieve desired functional film properties. Achieving this is a balance of having sufficient reaction (e.g. plasma energy, time, sources of oxidation, concentrations of reactive species, etc.) to achieve desired chemical and structural properties while avoiding undesirable reactions (e.g. pre-reaction and heightened homogeneous reactions).

In our work the inventors have observed that to achieve maximum physical and photocatalytic properties a post-treatment of film growth in an APGD plasma produces controllable changes in properties, which are very beneficial. Such changes are believed to be due to bombardment of the growth film with reactive plasma species, which densifies, and further reacts chemically with the film. Surprisingly, we have observed -under certain conditions- a degree of crystallinity or an increase in the degree of crystallinity of the films during this post-treatment stage, which is not only useful for the physical properties (adhesion, hardness, scratch resistance , etc.), but has a significant impact on the photocatalytic properties. The post-treatment step can be carried out in-situ (for example by closing the precursor flow and maintaining a plasma - possibly of different characteristics than those used for growth). However, it should be noted that the films produced by this method are photoactive even when they do not show measurable crystallinity. Applications

While not wishing to limit the scope, we illustrate the potential of the invention by example.

For coating continuous substrates moving under or through the coating region and where coating at a temperature below that normally applied in thermal CVD is desirable and atmospheric pressure operation is desired. This can include coating of plastic components of plastic films, continuous or semi-continuous sheets (for example of glass, metal, or plastic like window profiles) and fibers.

Experimental examples

The following examples are understood to illustrate the scope and potential, and are not in themselves limiting.

Typical experimental conditions applied:

Frequency: 10-25 KHz (variable).

Power: 100 W to 1 KW.

Configuration: parallel plates with one or both surfaces covered by a dielectric barrier (eg glass, ceramic or plastic film or sheet).

Electrode spacing (measured from the inner surface of either the metal or dielectric electrode) variable, between approximately 1 and 15 mm. The optimal region was between 2 and 6 mm.

ES 2 289 331 T3

Power densities: 0.1-10 Watts cm² (generally 0.5-2 watts cm²; 5-10 W / cm² do not belong to the invention).

Support (i.e. plasma) helium gas.

Support gas temperature (released) - ambient to about 50 ° C (to ensure volatility of the precursor).

Plasma gas temperature approximately 50-100 ° C (measured by contact probe).

Support gas flow 1-2 liters min.

Titanium oxide precursor pre-volatilized (for example in a bubbler) and released in concentration ranges below 1% and most often in the region of 0.1%

Oxygen level less than 1% and typically less than 0.1%.

Measurement of photocatalytic activity

The activity of the films produced has been measured using a number of techniques including the rate of stearic acid removal, reduction of optical scattering, and surface energy measurements. The most commonly applied literature technique is stearic acid removal and the procedures are described in an article by Heller and Paz, J Mat Res vol. 12, no 10, Oct 1997.

Briefly, a film of stearic acid is deposited on the coating to be tested and, under selected UV irradiation, the reduction of a selected IR peak of stearic acid is area controlled as it progressively reduces. In our work we use an FTIR spectrometer to order this peak.

Analytical instrumentation

Scanning electron microscopy (SEM) images were obtained using a Philips XL30 with Phoenix X-ray energy dispersion analysis (EDAX) spectrometer. X-ray photoelectron spectra (XPS) were recorded on Kratos Axis 165 or Amicus spectrometers while the X-ray diffraction (XRD) data was recorded on a Philips PW1130 diffractometer. Rutherford backscatter measurements (RBS) were performed using a 2 MeV accelerator and He + analyzer beam at normal incidence and 168 ° scattering angle in IBM geometry. The RBS data was compared with simulation data from a model using the Quark software package.

Experimental example 1

Demonstration of achievable growth rate

A film of titanium oxide was grown from titanium tetrachloride (0.1%) in a helium carrier gas. The substrate used was glass. Both uncoated glass and glass that was pre-coated with a silica blocking layer were used. The plasma was started in nominally 100% helium, then the premixed reagent gas mixture was introduced. The total gas flow was approximately 2 liters per minute. The power level was 100 Watts over an area of ​​approximately 150 cm². The configuration was a parallel plate with a separation of 4 mm and both electrodes covered by a 2 mm thick glass dielectric. The gas introduction temperature was ambient.

The movie was grown over 10 seconds. The gas outlet temperature was approximately 80 ° C. The film grew to a thickness equivalent to a growth rate greater than 100 nm per second.

The film was measured for photoactivity and found to be photocatalytically active to stearic acid and contact angle effects. The activity rate (measured spectroscopically by reduction of the MIR peak area) was evaluated at 2.5 x 10^-2 cm^-1 min^-1 for a film thickness of approximately 100 nm. The level of activity varied with thickness, (as described in the literature), and significantly higher levels of activity were recorded with increasing film thicknesses.

Experiment 2

Demonstration of optical properties

The films produced in Example 1 were visually transparent. They were measured on a UV / Vis spectrometer and were found to have high transmission levels through the visible region. Transmission varied with thickness as predicted from reflection-related interference effects. Reflection levels of around 12% to 20% were when the film was thickest. Absorption levels were typically low from several percentage units to less than 1.

ES 2 289 331 T3

Experimental example 3

Demonstration of the effect of post-treatment on properties

Two films were produced, one according to Example 1 and a second produced according to Example 1 but then "post-treated" by holding it in GD plasma for 60 seconds after the coating had finished. The resulting films differed markedly in properties. Post-treated films were much more adherent and durable (for example by crosslinking resistance tests and soaking in water).

Experimental example 4

Demonstration of potential to coat plastics

The conditions applied to Example 1 were then used to coat plastic substrates. The plastics chosen included Perspex, PET, and poly (propylene). In each case the plastic substrate was pre-treated with the plasma for approximately 20 seconds. This pre-treatment was found to enhance adherence. The films grew similarly to those on glass, and were also found to be photoactive. The films were adherent (for crosslinking) and resistant to simulating abrasion.

Experiment 5

Use of alternative plasma gases

The system used in Example 1 was tested for use with alternative plasma gases (helium). Argon and nitrogen were used as examples. Argon was the best of the two.

The alternative gases were able to produce a GD plasma, but better quality discharges were seen with reduced electrode gaps and thinner dielectrics.

Experimental example 6

Effect of oxygen level

The conditions of Example 1 were again operated by varying the oxygen levels. Oxygen was fixed at 1%, 0.1%, and 0.025%.

The experiments were repeated on both glass and plastic substrates. At 1% oxygen the film grew rapidly, but with a degree of haze and pre-reaction that was detrimental to the optical properties of the produced film. At 0.1% the growth rate was comparatively high and the pre-reaction was significantly reduced. Towards the rear end of the reactor some more hazy film was observed. If the total gas flow was increased (eg doubled) it was significantly reduced, but at the expense of reduced growth rate and precursor efficacy. At 0.25% the growth rate was reduced by approximately 25-50%, but the films were optically high quality and had good adhesion (cross-linking test and after immersion in water for 2 hours).

The results for glass and plastic showed similar trends.

Experimental example 7 Effect of bottom water

Helium was introduced into the reactor with humidity levels deliberately set at about 1% and 0.05%. At the highest level the plasma was unstable and of reduced quality as determined by visual inspection of the plasma region. No film growth was attempted. At the lower level a significant degree of gaseous pre-reaction was observed. The resulting film was hazy and of low adhesion. When the humidity levels were returned to the conditions of Example 1 (ie, a few tens of ppm) the films became of good quality.

Experimental example 8

Use of alternative precursor

As an example of an alternative precursor (to titanium tetrachloride), titanium tetraisopropoxide was tried. The conditions of Example 1 were used, except that no oxygen was used because the precursor contains sufficient oxygen for the reaction. The precursor concentration was set at the same level as titanium tetrachloride. In the example, a titanium oxide film grew rapidly and of good quality again.