Flexible Sputter Solutions

December 1, 2008
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The main applications for large area glass coatings (LAGCs) can be found in the architectural, automotive, display and photovoltaics (PV) industries. In all of these application fields, there is a strong evolution toward customization. Application-specific setup and process tuning are key elements in enabling manufacturers to produce unique products.

A series of flexible and high-performing sputter hardware components has been developed for rotating cylindrical magnetron applications. Using these key components, a customized and complete sputter solution can be developed for specific applications. Whether it concerns an existing coater in an upgrade/rebuild project or a new coater in a green field situation, maximum flexibility helps implement all of the critical components that are needed for sustaining a controlled process in the manufacture of high-end products.

Vacuum Coating Benefits

Vacuum coating by sputtering has established a leading position for the deposition of thin films because of its relatively easy scalability to high volumes and sizes, its good control of most layer characteristics, and its wide variety of available materials and possible coating stacks. The technology is commonly accepted and widely used in the architectural and automotive industries. In display and PV applications, however, the use of rotatable technology has been introduced more recently.

Within the large area coating business, the rotating cylindrical magnetron concept has proven to offer superior properties relative to the planar concept and to satisfy most industrial requirements. The geometry and sputter performance of rotating cylindrical sputter targets result in several advantages relative to planar sputter targets, including:
  • A larger useful target material inventory and increased target material utilization, leading to reduced machine down-time.
  • Increased process stability for reactive depositions (reduced arc sensitivity).
  • Enhanced target cooling, leading to the ability to use higher power density and thus obtaining a higher deposition rate.
  • A more focused ejection of particles.
  • Enhanced anode functionality during AC sputtering.

Figure 1.

Total Solution Approach

Coater customization has become crucial in order to accommodate the stringent demands of the cutting-edge products in these different industries. Customization possibilities go far beyond the level of integrating hardware; a total solution approach provides maximum flexibility.

The starting point of the total solution approach is a specific application need. This could involve increasing the coater throughput or the stand times of the targets, or optimizing the total target material use for a number of cathodes. It can also be as complex as a complete redesign of a coating module in an existing coater.

How the coating solution is implemented largely depends on whether it is part of a new coater in a green field project or an existing coater in an upgrade project. The main difference between a green field and an upgrade project lies in the number of degrees of freedom and the more stringent boundary conditions for upgrade projects. A total cost of ownership (TCO) calculation is used to work out in detail the most optimal configuration with respect to cathode layout, type and number of cathodes, type and number of targets, integration, gas distribution, etc.

The TCO method allows the optimization of the coater performance by decreasing the total cost of ownership. Furthermore, the TCO calculation is a powerful tool to make a quantitative assessment of the benefits offered by new or enhanced investment hardware or consumables, or to find out the critical parameters or components of the coating process.

The model used for the TCO calculation takes into account the investment costs, depreciation, power consumption, labor cost, maintenance costs, consumables, etc. The main outcome of the model is the coating capacity, the cost per year and the cost per square meter of coated substrate.

Starting from a stack consisting of different layers, the number of targets of a specific material, the power density per material and the possible glass speed needed to deposit the required layer, thicknesses are calculated for a specific coater configuration. The calculation is based on a database of material properties and on operating specific performance and costing parameters. This way, one can take into account the enhancements of introducing the rotatable technology and its inherent advantages (e.g., higher power densities and increased target utilization by new evolutions in magnet bar design).

Following are two examples that illustrate how a TCO calculation can be used to optimize coater configuration for a particular coating stack while decreasing the total cost of ownership. Figure 1 depicts a stack for the LAGC industry. The desired requirement is to at least double the line speed in an existing coater without changing the campaign length.

Table 1. Comparison of TCO results before and after optimization.

The result of the TCO calculation is shown in Table 1. By replacing the single planar Sn cathodes with dual rotatable Sn cathodes and using two instead of one Ag cathode, the line speed can be increased from 1 to 2.3 m/min with a similar campaign length. The ability to increase the line speed in a large area glass coater is an aspect of manufacturing that makes it possible to realize supplementary cost reductions, resulting in an added competitive advantage.

In this particular case, it follows from the TCO calculation that the total cost per square meter of coated substrate can be reduced to less than half by switching to rotatable technology. It should also be mentioned that in the rotatable configuration, a spare cathode position becomes available as well.

Table 2. Deposition of 500 nm ITO with planar and rotatable ITO targets.

The second example focuses on a thin film PV application with a deposition of 500 nm, ITO on 600 mm x 1200 mm glass panes in an existing coater. The requirements include the coating of at least 32 glass panes per hour and a continuous production of six days. The result of the TCO calculation is shown in Table 2.

For the deposition of 500 nm under the above conditions, six rotatable ITO targets are required. Each target has a minimum lifetime of six and one-half days, allowing six days of continuous production as required. For the planar case, 12 targets are required to deposit the 500 nm layer with the requested substrate speed. The continuous production requirement of six days will not be met by using 12 planar targets, however, since target replacement will be necessary after four and one-half days.

The TCO model can be extended to a value creation model, including a complete business plan incorporating payback times or other financial parameters. Optimizing the complete coater design for rotatable technology can offer several advantages. In the case of an FPD coater, for example, a typical coater door may contain double the number of rotatable magnetrons compared to planar magnetrons. Moreover, as indicated in the PV example, the maximum power that can be applied on each individual target is higher for rotatable targets.

Taking this and all the other advantages of rotatable targets into account, equipping a PV or FPD coater with rotatable technology instead of planar magnetrons could enable manufacturers to reduce the number of coat zones (by up to half) or to obtain more complex coating stacks (when the same number of coat zones is used).

Figure 2. Total solution approach for horizontal and vertical applications.

Horizontal and Vertical Applications

Once the coating solution is worked out completely, a customized product can be built from standard components (see Figure 2). Sputter module solutions consist of the magnetron (end blocks, compact end block, or axial magnetron and adjustable magnet bar), electronics (DC or AC), gas control (gas manifold, pumps, plenum and process control), integration of the solution (PLC control, safety interlocks, custom designed automation, etc.) and targets.

In the traditionally known double-ended magnetron configuration, the rotatable target is mechanically supported and powered by two end blocks. The end blocks ensure operational reliability over long and uninterrupted production cycles. The latest development integrates advanced techniques to respond to the requirements and challenges of high-power AC sputtering (up to 400 A AC) and the need for improved robustness under a variety of operating conditions.

These new features complement the double dynamic seal configuration and enable manufacturers to proactively detect seal deterioration without the need to interrupt a running coating campaign. By making the end blocks even more robust, higher mean times between failures are reached, further increasing the coater uptime.

The compact end block system provides a retrofit solution for existing smaller and vertical coaters without modifying the coater chamber itself. Since the complete coater configuration and surroundings are fixed, the end block approach is the easiest and most straightforward way to introduce rotatables in the coater. The compact end block system also offers the flexibility to adjust the target-substrate distance (since for existing coaters, the substrate is often further away from the coater opening). On the other hand, the traditional end block configuration takes up some space in the coater, leading to a decreased target length (and possibly a decreased uniform coating width).

In order to maintain the desired uniformity level, the compact end block system has been designed to occupy a minimal space that results in a maximal target length. All functionalities are located at one side so only one compact end block is needed. The compact end block can be used in DC or in AC at a maximum current of 200 A at 40 kHz, or at lower amperages at higher frequencies. Targets up to 3.7 m can be used. Support at the opposite side is recommended for target lengths over 1 m. The compact end block can be used at any angle and in a hanging or standing position. The size of the compact end block is less than 4 in. (compared to standard end blocks, which need two times 8 in.) and takes up a minimal amount of space in the vacuum chamber.

For new-generation vertical coaters, the axial magnetron system provides optimal integration during coater design. In the case of the axial magnetron, all functionalities are located at the atmospheric side. Like the compact end block, the axial magnetron offers the possibility for top or bottom mounting. The mounting flange has been kept small in order to offer optimal flexibility, which allows the mounting of two axial magnetrons side by side with a target axis spacing below 8 in. (e.g., in AC sputtering). The axial magnetron can be used in DC or in AC at a maximal current of 300 A at 40 kHz. Targets up to 4 m can be applied in a vertical position.

Both the horizontal and the vertical sputter module solutions can be equipped with full control of the gas flow, including (local) gas supply as well as pumping. A coating module with integrated pumps offers distinct advantages, including:
  • The specific process, and thus the coating, is determined to the largest extent by the coating module and no longer by the coater or the specific coater environment.
  • No separate pump sections are needed, so the coater length can be limited.
  • The gas distribution can be tuned to the pump capacity.
  • The coater can be used with more coating and less pumping zones, so more complex coating stacks become possible.
  • A user-friendly solution is offered in which pumps, cathodes and all other important functionalities can be removed from the coater as a whole.
To complete the total solution approach, an extended series of rotating cylindrical sputter targets includes low- and high-melting-point metallic targets, as well as a range of high-end ceramic materials. Special targets have also been developed to cope with manufacturers’ specific requests (special compositions, length dimensions, etc.).

Electronics Issues

In addition to the design and construction of an optimal mechanical setup, the use of adequate electronics for controlling the plasma process is very important. Tight output regulation combined with low output inductance can result in low energy dissipation when a short-circuit (arc) occurs, preventing target violation. During the delay after an arc event (before the output of the power supply is stopped), there is only a very small current rise and the re-ignition of the plasma is done in a smooth way to prevent further arcing.

Arc suppressing units are based on the same enhanced technology as the DC power supply. When an arc occurs, the working voltage drops due to the arc’s low impedance. Reaction time on arc initiation can be set from a few µs onward to enable a cleaning effect in the area where the arc originated. For persistent arcs, arc handling electronics can keep track of the arc-suppressing events and adjust the timings automatically, if necessary.

AC switching (a few 10s of kHz) between two full-sized magnetrons has become a standard in large area glass processing for the deposition of isolating layers. AC modules can be used in combination with a standard DC power supply to guarantee a stable deposition rate and uniform coating over long periods of time (preventing arcing and anode contamination), particularly during the deposition of insulating layers. Depending on the model, frequency range, current capacity and output, waveform (block or sine) may vary.

Both arc suppressing units and switching units can be integrated in the rotatable lid of the cathode, and both can be linked up with other commercially available power supplies to create a system with superior performance. The integration of the switching unit in the rotatable cathode lid itself implies that the AC signal is only formed very close to the cathode, simplifying the situation with respect to electromagnetic interference and radiation, skin effect, etc. while achieving faster reaction times and limited transient effects. The same accounts for the arc suppressing unit being able to detect and react very closely to the magnetrons and plasma.

Flexible Customization

In the current coating business environment, customization has become essential to ensure a competitive advantage. Starting from specific application parameters, the development of a total coating solution involves using a total cost of ownership calculation and a value creation model to optimize the coater performance in all of its aspects. In addition, high-performing sputter hardware components and targets enable flexibility in the customization of existing coaters in upgrade projects, as well as in new coaters in green field projects, for both horizontal and vertical applications.

For more information regarding flexible rotating cylindrical sputter hardware solutions, contact Bekaert Advanced Coatings, E-3 Laan 75-79, 9800 Deinze, Belgium; (32) 9381-6161; fax (32) 9381-6 86; e-mail infobac@bekaert.com; or visit www.bekaert.com/bac.


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