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Choosing and Using the Right TFT Manufacturing Method

The FPD industry evolved from semiconductor technology, and as such, FPD manufacturing methods have had the tendency to follow those used for semiconductors. Namely, FPD fabrication often involves “batch” or “cluster” type coaters, which utilize static deposition. However, as FPD substrate sizes increase, it’s easy to assume that lessons from other adjacent markets, such as architectural glass coating, could be adapted to improve the build process. The architectural glass industry may hold particular appeal in this regard because of its highly developed techniques for large-area coating.

Although borrowing from adjacent industries has definite benefits, it’s equally important to keep in mind the unique requirements of FPD devices such as TFTs (thin-film transistors). It’s true that FPD can take advantage of many architectural glass coating methods. However, certain techniques that are used with success for architectural glass coating are completely inappropriate for TFT manufacturing. Keeping this idea in mind may help you choose and operate your manufacturing method with the greatest success.

The following sections explain the importance of the static deposition method to TFT manufacturing. They also compare this method to the dynamic deposition process, which is used quite successfully to coat large architectural glass substrates. With this information and understanding, you will be able to further optimize your FPD manufacturing system.

Static Deposition: Reduced Particle Generation

Why does the FPD TFT industry utilize a static deposition process? The reason lies with the high material content of the resultant display, the higher technology associated with TFTs, and the corresponding importance of high yield.

Compared to TFT manufacturing, glass coating has a relatively high tolerance for particles, which often cause film defects. Because of this high tolerance, architectural glass has adopted a “dynamic” process, where the substrate moves continuously in front of the cathodes during deposition. During dynamic deposition, the giant carriers that move the substrate in front of the cathodes generate a significant number of particles due to vibration and contact with the guide.

However, TFTs, like semiconductors, are highly vulnerable to particle contamination, which can devastate process yield and throughput. Therefore, dynamic deposition’s high level of particle generation eliminates it as a viable method for TFT manufacturing. Instead, TFT manufacturing must use a static deposition method, in which the substrate remains “parked” in front of the cathodes. There is no substrate movement during deposition, and it is inherently a much more controlled environment. Therefore, fewer particles are created. This immediately impacts yield by eliminating a significant cause of film defects.

Static Strategies: Optimizing Your Manufacturing Operation

Although static systems are considered the norm for TFT manufacturing, it is still beneficial to study certain benefits that are inherent to dynamic systems. This information helps identify opportunities to optimize your static system with the use of more advanced-technology equipment and other strategies.

Film Quality

In dynamic systems, each cathode creates a different layer of sputtered material. If a defect such as a pinhole appears in one layer, it is possible that the subsequent layers may “repair” it by covering it up. In a static system, however, there is only one layer, so a pinhole in the substrate coating causes irrevocable damage. Because pinholes can be caused by arcs, a strong arc-management strategy is more beneficial in a static system than in a dynamic system.

Preventing Arc Damage

Your arc-management strategy should include a highly responsive and capable power supply, a short power supply-to-cathode cable length, and high cable quality.

Power Supply Response—Choose a power supply with highly effective arc-management technology. A high-quality power supply detects arcs quickly. The time between arc detection and response should be nearly instantaneous. The power supply should immediately remove energy from the arc, turning off just long enough to quench it completely. It must then turn power back on rapidly so that deposition can continue with minimal interruption. Additionally, having the ability to vary the power supply’s arc management features will allow further process enhancement.

The arc-management technology featured in AE’s power supplies is highly developed and engineered with data from real-world manufacturing conditions. Unlike competing power supplies that may provide excessive and therefore damaging energy to arcs, AE products detect and extinguish them with accuracy and speed.

Power Supply Capability—Even with the best power supply featuring top-notch arc management, some energy gets through before the arc is extinguished. The amount of energy that is provided to an arc depends on your power supply’s capabilities as well as the energy stored within your cable (see Cable Quality and Length below). Delivered arc energy is proportional to the process power. As the industry moves toward higher and higher power levels, delivered arc energy therefore becomes even more critical because the aggregate energy delivered during an arc event can become too great for the process to tolerate. Power supplies must have minimal stored energy (expressed as mJ per kW) after the turn-off following arc detection. A power supply with lower stored energy provides less energy to the arc before it is extinguished. Therefore, the arc causes less damage.

AE power supplies have the lowest stored energy commercially available. Pinnacle^® power supplies store less than 2 mJ per 1 kW of output, while the Summit^® power supply delivers less than 1 mJ per 1 kW. For the best total arc management solution, AE power supplies provide superior arc management, plus minimal stored energy to further mitigate the damage caused by arcs.

Cable Quality and Length—Energy is stored inductively in cabling, and cables have a certain amount of inductance per meter. Decreasing cable length and using a low-inductance cable reduce the stored energy in the power supply-cable-cathode system. Therefore, use the shortest, lowest-inductance cable possible between the power supply and cathode.

Process Productivity and Uptime

In a dynamic system, if a cathode goes down due to an equipment failure, manufacturing can continue, although at a slower rate. For example, in a system with four cathodes, such as the one shown in Figure 1, the substrate is coated with four individual layers of the same material, one per cathode, as it passes through. If one cathode goes down, the remaining three cathodes lay down three layers instead of four. By slowing down the speed of the substrate as it passes through the system, the process can compensate for the “missing” layer by thickening those that remain. As the substrate moves, the remaining cathodes “take up the slack” caused by the failure, and ultimately, although productivity is negatively affected by the manufacturing slowdown, the uniformity of the final product does not change significantly.

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On the other hand, the failure of even one cathode during static deposition brings the entire system to a halt. Productivity is not just slowed down; it is immediately reduced to zero. Because the substrate remains stationary, only one layer is created during static deposition, with each cathode corresponding to a specific area of the substrate. When a cathode goes down, the corresponding area receives significantly less material, creating a large valley in the film (Figure 2). Production must be shut down completely until the cathode is restored. In contrast, during dynamic deposition, each cathode corresponds to a specific layer over the entire surface of the substrate, and the tool can compensate for the effects of a downed cathode.

Maintaining High Productivity

The catastrophic effect of cathode failure in a static system increases the importance of equipment reliability. This, as well as the ability to repair or replace your equipment with speed and ease, is key to maintaining productivity and product quality. AE designs are ideal for static systems because of their extremely high reliability, which minimizes the chance that a cathode would go down due to power supply failure. In the event that a repair is needed, our power supplies are easy to access, service, and replace compared to some competing products. Further, AE’s global support infrastructure enables fast support and repair anywhere in the world. Therefore, in the rare event that service is necessary, downtime is reduced to an absolute minimum.

Conclusion

Although static deposition systems present unique issues during process set up and operation, an array of tools and technologies exist to help you easily resolve them. This enables you to fully benefit from the low particle generation levels that static systems provide.

Please contact us with any questions about setting up or optimizing your static deposition system.

Ask the FPD Experts!

Are you struggling to squeeze more profit out of your FPD process?

Bruce Fries, AE's global segment manager for flat panel display business, and Ken Nauman, AE’s FPD global segment engineer, answer some of your difficult questions. Submit your question or comment to FPDapplications@aei.com.

1. How do I determine if pulsed DC is a good fit for my FPD process?
2. With pulsed DC, does the lack of sputtering during the voltage reversal affect my sputter rate?
3. Are there any technologies that can extend OLED lifetime by improving the quality of the encapsulation layer?
4. Where can I get help developing OLED and other advanced processes?
5. What existing product technologies can benefit FPD?

1. How do I determine if pulsed DC is a good fit for my FPD process?
   Answer: If you have processes that are very sensitive to damaging arc events, then pulsed DC can surely help. Charge buildup on dielectric surfaces is inherent to every target. Pulsed DC serves to prevent damaging arcs from happening in PVD processes by periodically reversing the voltage and neutralizing this buildup.
   
   Pulsed DC almost always creates better film quality, cost savings, yield, and throughput than straight DC. It reduces the occurrence of pinhole defects and improves electrical properties by reducing resistivity. It can also reduce material costs by both improving target utilization and enabling the use of less expensive targets, with no negative effects on film quality. This dramatically increases process productivity and throughput.
   
   For existing DC-powered PVD processes, it’s relatively easy to add this valuable pulsing feature by integrating an accessory, such as AE’s Pulsar®, into your system.
   
   
2. With pulsed DC, does the lack of sputtering during the voltage reversal affect my sputter rate?
   Answer: Only slightly. AE’s unique pulsed-DC topology allows for energy storage during the voltage reversal step. This energy is then released during the subsequent sputter step. In essence, the average power delivered is therefore equal to similar DC-sputtering processes.
   
   That said, sputtering rate is complex and influenced by many variables, including:
   
   • Chamber geometry and cathode/anode design
   • Operating pressure
   • Gas mix
   • Target cooling
   • Target thickness
   • Magnetic strength
   • Operating power
   • Target-to-substrate distance

   
   Optimizing your sputtering system is both an art and a science—a balance among cost, sputtering rate, and film quality. The real key is to know and understand your chamber and sputtering process intimately. To fully understand how pulsed DC affects your process, perform initial rate runs at longer times than your actual process run to learn the personality of your chamber and process. To learn what to expect during the real process, you can try these initial runs at lower powers, and slowly raise the power each time as a method of system characterization.
   
   

3. Are there any technologies that can extend OLED lifetime by improving the quality of the encapsulation layer?
   Answer: Thin-film encapsulation significantly improves OLED lifetime sustainability by creating a barrier against air and moisture. This layer may be especially beneficial to flexible displays because the various substrates being considered, such as flexible polymers, can be penetrated by liquids and gases. Poor film quality can allow water and air to contaminate the organic layers by diffusion through the substrate.
   
   In order to create this barrier, it’s critical to have the appropriate film properties for your application, including an absence of pinholes, as well as your desired level of film density and crystallinity. Various plasma processes allow you to control energy to enable the improved film characteristics that effective encapsulation requires.
   
   AE has products that can attain the appropriate energy levels, as well as the arc-management capabilities that prevent arc-caused pinholes. AE’s diverse portfolio features DC, pulsed DC, and RF products that are designed to solve the challenges posed by such leading-edge applications. Please contact us at FPDapplications@aei.com for additional information.
   
   
4. Where can I get help developing OLED and other advanced processes?
   Answer: Expertise in adjacent thin-film markets is extremely useful to FPD process innovation. The push for better luminous efficiency, and the introduction of new devices such as flexible displays (OLEDs) and digital signage, create the need for more advanced manufacturing processes that enable end-product cost reduction. The following table draws general parallels between tomorrow’s FPD manufacturing and today’s adjacent thin-film processes.
   
   

   FPD Application	Adjacent Thin-Film Application	Commonalities
   All next-generation FPD devices	Semiconductors	Extremely precise processes
   Flexible displays	Web coating	Flexible substrates Very high throughput Low-temperature processes
   Large-scale displays	Architectural glass	Large-area substrates Equipment sourcing strategies Increasing power requirements
   OLEDs	Photovoltaics	Manufacturing operation design[1] Technology innovation

   [1] Photovoltaics convert light to electricity, while OLEDs perform a reverse operation, converting electricity to light. Therefore, the two applications have extremely similar materials, equipment, processes, and procedures. Some examples of these commonalities include transparent conductive oxide, conductor, and encapsulation layers. See Question 3 above for details on encapsulation.

   
   
   So, where can you find expertise that encompasses all of these thin-film industries? AE has been innovating technologies that enable precise plasma processes for over 25 years. With experience in all of the adjacent thin-film applications listed above, we can be a valuable partner in your process development efforts^[2].
   
   Once process design is complete, AE can assist with on-site system integration. We can also perform extensive in-situ tests to help ensure the success of your new design. This can become critical, given the trend of limiting initial acceptance testing (IAT) and performing only final acceptance testing (FAT) at the end-user site^[2].
   
   If you have questions about your specific application development efforts, we’d be happy to answer. Please contact us at FPDapplications@aei.com.
   
   

   [2] Please check with your equipment supplier to see what AE support options apply to you.

   
   
   
5. What existing product technologies can benefit FPD?
   Answer: In terms of manufacturing technology, today’s FPD market actually has an advantage over the early semiconductor industry. While semiconductor development had no technology base to start from, FPD was derived from semiconductor equipment and methods. Therefore, it started out with strong, highly developed manufacturing techniques. This has also enabled more rapid advancements compared to other industries. As the FPD market matures, existing technologies from other markets will continue to offer benefits.
   
   Technologies that offer benefits to FPD manufacturing include:
   
   

   Technology	Benefits
   Arc management	Reduces substrate damage (pinholes) Improves yield Allows higher power levels for increased throughput
   Flow control	Increases process stability Enables faster process transitions and shorter process steps
   Match network technology	Improves power-delivery accuracy and efficiency, for better film quality and yield
   Precise power delivery	Improves yield
   Precise subsystem control and monitoring functions	Eases process manipulation and innovation Enhances process productivity and yield Increases uptime
   Pulsed DC	Improves film quality and yield Reduces material cost

   
   

   AE has a wide selection of high-performing products that have been developed over our 25 years in the industry. Please contact us at FPDapplications@aei.com to find out how the technologies listed above can benefit your specific process.