CVD Technology - Evolution

Transcription

1 CVD Technology - Evolution NEW DEVELOPMENTS IN ON-LINE CVD TECHNOLOGY FOR THE GLASS INDUSTRY On-line chemical vapor deposition (CVD) technology has been used, within the glass industry, for many years. The advances to this technology from the earliest versions to the recently launched systems are astonishing. It is now a well established technology for high volume on-line coating. Until recently, CVD technology was effectively available only to large international glass companies who had developed the technology in-house. However, for the first time, it has now become possible for any glass company to purchase CVD technology as a result of Stewart Engineers launching an independent on-line CVD system, the AcuraCoat System. COATING TECHNOLOGY SELECTION A variety of techniques are available to deposit thin films on flat glass. The most widely used for producing high quality functional coatings can be subdivided into two classes: physical vapor deposition (PVD) and chemical vapor deposition (CVD). PVD processes include a number of approaches. Sputtering is one. It is the one most widely used for glass. Sputtered coatings are generally referred to as soft coated. Pyrolytic coatings are applied using CVD methods and are often referred to as hard coated. Both types of coating offer their own advantages and disadvantages. When considering which system is right for your operation, you should consider the performance and handling factors that best meet your product and manufacturing needs. The flow chart on the following page highlights the differences between the two coating systems. CVD FLOAT LINE INTEGRATION To properly integrate a CVD coating system into the float glass production line, many challenges have to be overcome. The glass line speed can be up to 1000 m/hr, the glass ribbon is typically over three meters wide, and typical required film thicknesses can be several hundreds of nanometers (nm). Such film thicknesses (at certain line speeds) require growth rates of tens of nm/s, and can reach 100 nm/s. In some cases more than one coater head is required to achieve final target film thickness, even for one layer. Selection of the coating integration position on the float line is a critical decision as shown in Figure 1. As a general rule, the further upstream (and therefore the hotter the glass), the harder and more durable the film produced, and the faster the growth rate achieved. However, the higher temperatures result in significant challenges for engineering and precursor chemistry. Most CVD coating is undertaken in the cooler part of the bath, the Lehr gap, or in the hotter part of the Lehr 1. LOAD RAW MATERIALS ~1550 C 1050 C 600 C 40 C 20 C MELTING FLOATING COOLING CUTTING Figure 1: Possible positions for incorporating a CVD coating system in a float glass production plant 1

2 Glass Coatings Pyrolytic CVD Technology Hard Coatings Sputtered PVD Technology Soft Coatings Material Properties Deposited at higher temperatures (650ºC) using CVD methods Coating strongly adheres to the glass Production Benefits and Costs + On-line CVD processing + Excellent lead times + High durability + Unlimited shelf life + Process done at atmospheric pressure + Stable to tempering + Consistent appearance between annealed and tempered glass Material Properties Deposited at lower temperatures (ambient) using batch-sputtering techniques Adheres multiple layers of metal oxides to glass Production Benefits and Costs + Traditional technique used to apply coatings + Wide variety of target materials available to form coatings + Well established process to apply and handle coatings + In some cases, superior optical property performance Performance properties are not as good as sputtered coatings for certain applications Requires fast deposition rate, limiting layer thickness Limited amount of chemistries Until recently CVD technology, for on-line use was not readily available on attractive commercial terms Consumer Benefits and Costs + Outperform sputtered coatings in heating dominated climates + Can be used in a single-pane, single-glaze window + New CVD methods developed with lower Solar Heat Gain Coefficient allowing pyrolytic glass to be used in cooling-dominated climates + High visible light transmittance reduces load on lighting systems Off-line processing Increased lead times necessary Special handling required to avoid scratches Sensitive to moisture, limiting shelf life Tempering can cause visual differences with annealed glass Edge deletion is necessary Color variability between batches Consumer Benefits and Costs + Superior performance properties in coolingdominated climates + Lower Solar Heat Gain Coefficient than pyrolytic Cannot be used in a single-pane or single-glaze windows Typically residential windows in coolingdominated climates are single-pane, single-glaze windows sputtered coating not used in cooling climates

3 CVD Production Technology A number of variants of CVD coating have been used commercially over the years. First, the CVD coating can be undertaken on-line (actually on the glass production line) or off-line. Second, the mechanism for precursor delivery can be liquid spray and powder spray. The on-line coating technology discussed will concentrate on true vapor CVD where the precursors are vaporized fully prior to delivery to the coater head. This CVD technology is the most widely employed in current float glass manufacturing plants. In order to explore the development stages of CVD coating, three illustrative phases are highlighted, in their chronological order. The key differentiating characteristics are: Laminar or turbulent flow Pre-mixed or separately delivered film precursor chemicals Bath or Lehr location Generation 1: Unidirectional laminar flow technology This Generation 1 beam type is one of the earliest laminar flow designs dating from the late 1970s. It is normally located in the float bath and has a water-cooled beam as shown in Figure 2. The design and operation are the simplest; however, the beam can handle only a limited range of coating materials. Such beams have been used to deposit silica, silicon and SiCO coatings. GAS DISTRIBUTION GAS EXTRACTION GLASS FLOW Figure 2: A unidirectional beam design 2 Generation 2: Turbulent flow technology Where the proposed precursors cannot be mixed together due to an unacceptable level of pre-reaction in the gas phase, it is necessary to deliver them separately to the reaction zone as shown in Figure 3. To achieve this, the beam has two separate delivery inlet slots. Rapid gas mixing is essential and a turbulent mixing and flow regime is employed. The beam is water-cooled. This type of chemistry and beam design is capable of the highest CVD growth rates leading to rates of over 100 nm/s. The consequence of turbulent operation in this design, however, is low precursor efficiency (typically below 10%) and very high gas volumes leading to significant waste gas handling and scrubbing requirements. This inefficiency leads to higher overall chemical cost when compared to a Generation 3 system. This type of beam has been used to deposit F-SnO 2 from, for example, tin tetrachloride and water 3.

4 GAS DISTRIBUTION GAS DISTRIBUTION GAS EXTRACTION GLASS FLOW Figure 3: A turbulent beam design 3 Generation 3: Multi directional, laminar flow technology This beam technology was introduced around the mid 1980s onward. It is a laminar flow regime and handles premixed gases only. These premixed gases split into an upstream and downstream flow as shown in Figure 4. This approach offers potential for excellent film uniformity and high precursor efficiencies (30-50% is achievable, in certain cases). In combination with oil heating and cooling, the beam gives the capability for the widest range of coating materials. The beam can be sensitive to set-up and operation and needs careful process control. Many variants of this basic design approach have been proposed in particular to achieve improved control of chemical reaction and increased growth rates. Designs involving multiple gas inlets and exhausts have also been developed. However, care needs to be taken, as increasing complexity can lead to difficulties in achieving process control and therefore, acceptable yields. GAS DISTRIBUTION GAS EXTRACTION GAS EXTRACTION GLASS FLOW Figure 4: An example of dual flow beam technology 4 It is notable that all three coating beam technologies are still in high volume production use. In many cases more than one beam technology is used in tandem, to produce a multi-layer product.

5 CVD Production Processes Key Process issues Apart from coating head design related issues, for high volume CVD coating, a number of critical process engineering related issues must be solved, including: a. Selection of an appropriate precursor system b. Controlled precursor delivery (handling and vaporizing chemicals) c. Temperature control (of coater heads on-line) d. Controlled gas distribution and maintenance of a gas flow regime compatible with achieving target film uniformity targets. e. Integration into the float bath furnace f. Waste gas handling g. Process control (high on-line yield is critical to avoid glass loss) It is the correct combination of technologies to achieve all of the above that defines the ultimate performance characteristics of the CVD process. Recent Developments The complexity and multi-disciplinary nature of CVD has made developing a complete turnkey CVD system a major barrier to wider diffusion of on-line CVD technology. The recent launch of the AcuraCoat system offers a breakthrough in access to this on-line CVD technology. Stewart Engineers and its partners (who have extensive research and industrial experience in thin-films and applications in the float glass industry) have developed the most technically advanced on-line CVD system available to the float glass manufacturing industry. The design is based on a 3 rd generation concept and uses an oil cooled, multi-directional, laminar flow beam technology to be located within the float bath furnace. Key Features Line Characteristics: Width of glass: 3.4m (typical) Width of coating: > 3.0 m (typical) Speed: m/hr Process Specification: Run time (the time between extended clean up): average 8 hrs, > 12 hrs possible Coating output: > 20% of total production Coating process yield: > 80% Start up time: 30 minutes Shut down time: immediately GAS 1 GAS 2 GAS 3 FLOAT BATH MIXING CHAMBER UNDER COATING HEAD WATER OIL POLLUTION PLANT TOP COATING HEAD WATER OIL Figure 5: AcuraCoat System Flow Diagram. TOP COATING HEAD WATER OIL MIXING CHAMBER GAS 1 GAS 2 GAS 3 GAS 4 GAS 5 EVAPORATOR CHEMICAL 1 CHEMICAL 2 CHEMICAL 3 The system is located onto a carriage which can be inserted and removed readily within the bath. Engineering modifications to the bath are undertaken and it is possible to introduce the system either as a new build or as a retrospective upgrade.

6 Figure 6: AcuraCoat System cross-section indicating carriage and support members. As a consequence of this advanced design, a wide range of products and product properties can be achieved. Stewart s AcuraCoat System produces a series of glass products delivering energy efficiency that will meet or exceed energy code requirements. This system is capable of producing reflective, low-e, solar control and self-clean coatings. This newly available technology has the potential to catalyze and support increasing diffusion of on-line CVD coating technology within the glass industry world-wide. References 1. Large Area Glass coatings, Hans Joachim Glaser, ISBN R Berry US patent 4,857,097 (1989) 3. B Grundy, E Hargreaves, P.J. Whitfield GB 2, , (1989) 4. Patent W/O 96/11802