Atomic Layer Deposition for LEDs and OLEDs

Over the past few years, the wafer-based semiconductor industry has been using the atomic layer deposition (ALD) thin film coating technique to develop a wide range of electronic products and components. For an improved level of system miniaturization and integration, thin films have to be uniform, dense and conformal, as well as free from pinholes, cracks and other defects.

When the preferred film thickness begins to approach nanometer scale, traditional thin film deposition methods such as PVD and CVD do not meet this requirement. In contrast, ALD forms excellent quality films even on the most complex nanoscale geometries. This can be attributed to its surface controlled and self-saturating film growth mechanism.

LED manufacturing is a wafer-based technology – similar to integrated circuit (IC) device manufacturing techniques. ALD is an optimal method that can be incorporated into current LED manufacturing processes and it can provide a wide range of benefits to the industry, either by introducing new manufacturing steps or replacing existing ones to extend the product lifetime, improve the device efficiency, or to save manufacturing costs.

Currently a number of LED manufacturers across the globe are using Picosun’s ALD technology in their production. For OLEDs, Picosun delivers excellent ALD solutions to protect the devices against moisture.

Benefits of the ALD Technique

The ALD method can be used to produce thin films which have excellent uniformity and conformality, as well as superior structural and chemical purity. In this method, chemical adsorption reactions occur between gases and solids over the surface. The reactions are self-limiting, self-controlled, and promote film growth by successive atomic layers.

Different types of materials including nitrides, oxides, fluorides, polymers, pure metals, sulphides, and graded, doped or mixed layers, can be deposited. In addition, multi-layered nanolaminates can also be deposited by modifying the properties of the layers on the molecular level.

In addition to coating silicon wafers, the ALD method can be used for coating 3D objects, porous, powder and high aspect ratio (HAR) samples. Given that films grow by sequential atomic layers, both thickness and chemical composition of the film can be controlled accurately.

The ALD process is repeatable, and a wide range of materials can be easily deposited at low temperatures, enabling also the use of papers, plastics, thin metal foils, and other sensitive substrates.

Benefits of LEDs and OLEDs over Conventional Lighting

LEDs provide a number of benefits when compared to traditional lighting devices such as fluorescent tubes and incandescent bulbs. These benefits include:

• 10 to 50X longer lifetime
• Light is brighter
• Generate less heat
• Consume less energy
• Provide a wide color spectrum
• Turn on instantly sans a warming period
• Offer great design flexibility, thanks to their small and compact size

However, larger scale LED utilization has certain drawbacks and these include high cost and low efficiency for a number of applications. Although most types of LEDs are efficient enough to be utilized in car lamps, flashlights, mobile device displays and keypads, projectors, signs, TV screens, and indoor lighting, LEDs with improved brightness are still required for applications such as LED-based street lighting (Figures 1 and 2).

Figure 1. LEDs can produce neutral white light or shine in all colors of the spectrum. Image Credit: Artography/Dreamstime.com and Demarco/Dreamstime.com

Figure 2. LEDs enable bright, long-lasting, energy-saving illumination for a variety of uses such as portable and mobile devices, decorative items, indoor lighting, TV screens, and car headlights. Image Credit: Pupkis/Dreamstime.com, Demarco/Dreamstime.com and Mircea Hotea/Dreamstime.com

OLEDs are a relatively new type of LEDs. They generate soft and diffuse light across a large area in contrast to LEDs’ high-intensity point-like source. In OLEDs, the light-generating layer is made of electrically active polymer material, while in LEDs it is based on inorganic semiconductors. Even the OLED substrate can be flexible and based on polymer, enabling applications such as retractable or bendable displays and lighting “sheets”.

However, OLED efficiency is considerably lower and their lifetime is also shorter when compared to inorganic LEDs. Research and development are currently ongoing to improve this potential lighting source.

The first OLED applications such as lighting panels and TV screens are already in the market, although manufacturing processes and materials have to be further developed in order to reduce the cost of products containing OLEDs.

ALD for LEDs and OLEDs

The method used to manufacture LED chips is similar to other semiconductor device manufacturing methods. The end product is grown in sequential layers on top of a wafer substrate. The heart of the LED stack includes an electrically active semiconductor layer and two electrodes on the bottom and top of the active layer which serve as connectors. The semiconductor layer produces visible light when connected to an electric circuit.

The LED fabrication method is typically a batch process on maximum of 200 mm diameter wafers. Thin, conformal, uniform and pinhole-free material layers deposited by ALD can increase the extraction efficiency of the LED either by accomplishing more efficient reflectors and higher transparency electrodes or by passivation and/or customizing the refractive properties of chosen layers in the LED stack.

OLEDs are usually produced on glass sheets and as such have to be protected from moisture in order to ensure product performance. The ALD method provides a simple and consistent way to encapsulate the sensitive layers by transparent, gas, and watertight barrier films.

Transparent conducting layers are a major component of the OLED stack. Picosun ALD technology can be used to produce these layers as well as the encapsulant layers on metal or plastic foils, thus making it possible to integrate the OLED technology e.g. into printed electronics. Additionally, excellent barrier coating results have been achieved using Picosun’s roll-to-roll ALD unit, which is suitable for OLED research and development on flexible substrates.

PICOSUN™ ALD systems are compatible with glove boxes and suitable for working with OLEDs as the sample handling can be carried out in an inert nitrogen atmosphere. In addition, Picosun offers a range of cluster systems that come with a number of vacuum loading options ideal for LED and OLED manufacturing.

ALD can create the preferred material layers with improved process reliability and film quality. The required film function can be obtained with a smaller amount of material, thanks to ALD films’ nanoscale uniformity, conformality, and defect-free quality.

Batch processing which is fast, reliable, fully automated, and consumes a minimal amount of material, can save a significant amount of costs and ultimately reduce the price of end products.

Figure 3. Picosun’s PICOPLATFORM™ 200 vacuum cluster system for wafers up to 200mm diameter.

Figure 4. PICOSUN™ R-200 ALD tool integrated with a glove box.

Figure 5. PICOSUN™ P-1000 batch ALD tool for ultra-high volume production for wafers up to 450 mm, large glass/metal sheets, and large batches of 3D objects.

Figure 6. PICOSUN™ P-300B batch ALD tool for high volume production for wafers up to 300 mm, large glass/metal sheets, large batches of 3D objects, and powder samples.

PICOSUN offers high throughput, high quality, and cost-efficient cluster and batch ALD tools for LED and OLED manufacturing and R&D (Figures 3-6).

Conclusion

Global LED manufacturers are already using the ALD process in their production. This process provides a wide range of solutions to modern lighting industries to achieve better device performance and longer product lifetimes.

Read Picosun’s article for LED Professional magazine here.

This information has been sourced, reviewed and adapted from materials provided by Picosun Oy.

For more information on this source, please visit Picosun Oy.