Atomic Layer Deposition

Atomic Layer Deposition

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By Tiago Belé / November 16, 2022

Why Your AMOLED Screen Can't Survive Without ALD

Over the past few decades, display technologies have evolved rapidly, from 19" cathode ray tubes to micro-LEDs with 1 μm pitches. Consumers demand technologies with higher resolution, lower power use, better dynamic color range and curved or even flexing screens.

Aiming for client satisfaction, the development of microdisplays has prompted a great deal of interest in organic light-emitting diodes (OLEDs). OLED displays have gained popularity because they deliver outstanding performance. 

AMOLED, a version with an active-matrix organic light-emitting diode system, is the most popular variant. Several major technology companies use AMOLED displays in their high-end phones, including Samsung, Apple, LG, Xiaomi, and Motorola. These screens have been in high demand, with over 500 million displays produced annually. 

OLED technologies are used not only for mobile phones, but also for lighting and even transparent touch screen displays:

ALD in Your Phone - Part 1: Why Your AMOLED Screen Can't Survive Without ALD

 

To understand their advantages, we will explain OLED and AMOLED display secrets in more detail:

 

Why OLED Screens Are Better Than Other Displays


OLED panels emit their own light, whereas LCD displays, which use a color filter, need a separate backlight.

Traditional LCD screens have uniform backlights (called cold-cathode fluorescent lights, or CCFLs). In LCD, the panel is lit with precisely the same brightness regardless of whether the image is black or white.   This nice YouTube video explains the difference in working principle between LCD and OLED light emission displays.

 

ALD in Your Phone - Part 1: Why Your AMOLED Screen Can't Survive Without ALD
Image: Schematics on LCD and OLED structure and operation

 

In OLED displays, light is produced by the pixels. When black needs to be produced, the pixel can be turned off. This can save a lot of energy compared to LCD.  

 

How Does an OLED display work?

 

The  'organic' part in OLED refers to the small organic or polymer film inside the panel. The OLED panel makes this kind of display stand out compared to other types of panels. 

In terms of doping and structure, OLEDs are fundamentally different from LEDs. By changing the conductivity of the host semiconductor, doping creates p- and n- regions in LEDs, while in OLEDs, there is no p-n structure. 

An OLED consists of several layers:

 

ALD in Your Phone - Part 1: Why Your AMOLED Screen Can't Survive Without ALD

Encapsulation - This (ALD) layer protects the organic films from damaging humidity and oxygen.

Cathode -  This low-work function conductive material injects electrons when a current flows through the device. This layer may be opaque (barium metal) or transparent (LiF/Al).

Emissive layer - This layer is made of organic plastic molecules that transport electrons from the cathode, where light is made. An example polymer used in the emissive layer is polyfluorene. Different color emissions can be created using doping, which alters the bandgap.

Conducting layer - This layer is made of organic plastic molecules transporting "holes" from the anode. One conducting polymer used in OLEDs is polyaniline.

Anode (transparent) - The anode removes electrons (adds electron "holes") when a current flows through the device.

Substrate (clear plastic, glass, foil) - The substrate supports the OLED.

Light in OLEDs is produced as follows:

  1. When a DC power source is connected, the cathode receives electrons, and the anode receives holes
  2. The added electrons make the emissive layer negatively charged (similar to the n-type layer in a junction diode), while the conductive layer becomes positively charged (similar to p-type material).
  3. Positive holes are more mobile than negative electrons, so they jump across the boundary from the conductive layer to the emissive layer. When a hole (a lack of electron) meets an electron, they recombine and release a brief burst of energy in the form of a particle of light—a photon.

By modifying the quantum-mechanical optical recombination rate, doping can increase the radiative efficiency of OLEDs. Moreover, doping is one factor in determining the wavelength (color) of photon emission.


What About AMOLED?


OLED displays can have a passive matrix (PMOLED) or an active one (AMOLED). 'AMOLED' stands for active-matrix organic light-emitting diode, the most popular type.

In the AMOLED, a thin film transistor and capacitor (the active matrix) are attached to each LED to activate each pixel. The transistor and capacitor store energy to more quickly and precisely control how pixels are changed, providing a faster refresh rate and greater power efficiency. 

As with OLED, in addition to excellent contrast, AMOLED screens allow for true blacks by dimming or turning off the relevant pixels when needed. In addition to displaying a wide range of colors, AMOLED screens also have a large color gamut, which can cause images to appear very vivid.

AMOLED screens can also be made transparent or flexible. As a result, they are ideal for curved handsets, making them perfect for cell phones. Typically, AMOLED touchscreens have an extra touch-sensitive layer on top. 

 

The advantages of OLED over LCD are the following:

 

  1. The plastic, organic layers of an OLED are thinner, lighter, and more flexible than the crystalline layers in an LED or LCD.
  2. Because the light-emitting layers of an OLED are lighter, the substrate of an OLED can be flexible instead of rigid. OLED substrates can be plastic rather than glass used for LEDs and LCDs.
  3. OLEDs are brighter than LEDs. Because the organic layers of an OLED are much thinner than the corresponding inorganic crystal layers of an LED, the conductive and emissive layers of an OLED can be multi-layered. 
  4. Because OLEDs do not require backlighting, they consume much less power than LCDs (most of the LCD power goes to the backlighting). This is especially important for battery-operated devices such as cell phones.
  5. OLEDs are easier to produce and can be made into larger sizes. Because OLEDs are essentially plastics, they can be made into large, thin sheets. It is much more challenging to grow and lay down liquid crystals.
  6. OLEDs have large fields of view, about 170 degrees. Because LCDs work by blocking light, they have an inherent viewing obstacle from certain angles. OLEDs produce their own light and have a much wider viewing range.
  7. Fast Response Speed: OLED display response times are around 1,000 times faster than LCD displays; it is around 10 0 μs (0.01 ms). In contrast, LCD displays perform poorly at low temperatures, which does not affect OLED display performance.

Problems with OLED


OLED seems to be the perfect technology for all types of displays, but it also has some problems:

  1. Lifetime - While red and green OLED films have longer lifetimes (46,000 to 230,000 hours), blue organics currently have much shorter lifetimes (up to around 14,000 hours).
  2. Moisture and oxygen can easily damage OLEDs.

ALD can solve the second problem!


Oxygen and Moisture sensitivity of OLED and AMOLED


Since OLED light-emitting electroluminescent materials are sensitive to oxygen and moisture, proper encapsulation is essential for extending their lifespan. The electroluminescent materials in OLEDs oxidize when exposed to moisture or oxygen, leading to black spots and decreased light output. The electrode layer can also delaminate, eventually resulting in a complete failure of the panel. Moisture causes degradation three orders of magnitude faster than oxygen.

 

ALD in Your Phone - Part 1: Why Your AMOLED Screen Can't Survive Without ALD

Image from https://www.leti-cea.com/


In creating the trench for the pixels, dry etching creates dangling bonds and defects on the sidewalls of the pixels. Once moisture has penetrated the encapsulation surface, dark spots or "dead pixels" will appear due to the hydrolyzation of electrodes or electron injection layers. Atomic Layer Deposition is a powerful way to prevent these defects.

 

How ALD Coatings Help to Preserve Your OLED Display

 

An encapsulating material is used to protect the organic layers. The so-called water vapor transmission rate (WVTR, sometimes also referred to as MVTR, moisture vapor transmission rate), is the standard measure for quantifying moisture transmission. The lower this value, the better the protection. 
The unit is typically in grams per square meter per day (g/m2d). This means that we verify a coating by measuring how much water gets through an area of coating in certain conditions in 24 hours.

It is possible to encapsulate by applying parylene coating, laminating a glass sheet with epoxy glue and desiccant, and vacuum degassing or using thin-film encapsulation. Epoxy protection was the most used form to protect pixels. However, epoxy is brittle compared to thin film coatings. 

Parylene is also a popular encapsulating material. Nonetheless, the WVTR of Al2O3 is billions of times smaller than typical organic coatings like parylene and epoxy. This is exemplified by the fact that even though aluminum metal is very reactive, it still doesn't rust in the air: its 1 nm native oxide (Al2O3) completely passivates it! 

The Al2O3 coating deposited by ALD is identical in properties to the native oxide of aluminum metal: amorphous and pinhole free. The amorphous nature of ALD Al2O3 prevents pathways of gas molecules. In semicrystalline coatings, molecules like oxygen and water can travel along grain boundaries. The amorphous nature of ALD Al2O3 prevents this. Al2O3 deposited by other coating technologies, such as sputtering, evaporation, and CVD, also lack the qualities of Al2O3 deposited by ALD.  The pinhole-free, density, stoichiometric, amorphous, and well-adhering properties of ALD Al2O3 are remarkable for blocking water molecules!

A comparison of the WVTR of various barriers can be seen below, as well as the device requirements. Only “Advanced” barriers such as ALD Al2O3 can reach OLED requirements. 

 

ALD in Your Phone - Part 1: Why Your AMOLED Screen Can't Survive Without ALD

Image from https://www.leti-cea.com/

MLD-ALD nanolaminates

 

In some cases, combinations of organic and inorganic layers are deposited with MLD and ALD. Molecular layer deposition (MLD) is used for depositing organic layers, and Atomic Layer Deposition (ALD) is for depositing inorganic layers. An example is the application of a protective layer made of alucone (MLD) and Al2O3  (ALD). An example is shown in the figure below. Here the Al2O3 ALD layers, only several nm thick, are alternated with organic layers made by molecular layer deposition (MLD). The organic layers give the barrier some flexibility and robustness. 

In the case of a crack in the more brittle Al2O3 layer, the vapors have to form a long, tortuous path through the more flexible MLD layers, maintaining good barrier properties. An example of the organic layer grown by MLD is M-Tosh (where M stands for Metal), or AL-Tosh when Aluminum is used for the metal.

 

ALD in Your Phone - Part 1: Why Your AMOLED Screen Can't Survive Without ALD

OLED barrier nanolaminate, consisting of organic Al-Tosh made by MLD and Al2O3 made by ALD. From a webinar by Picosun

ALD is a very effective passivation technique because of its excellent uniformity, high-quality film density, and control of thickness on an angstrom level.

ALD has been proven to give the following benefits to OLED display and lighting technology:

- Endurance
- Sustained performance in harsh environments.
- relative immune to dust particles (ALD can coat below dust, avoiding pinholes)
- Reproducible
- 100% conformal.
- Pristine conformality in high-aspect-ratio structures 
- An improved lifetime of OLEDs 
- Increased quantum efficiencies for micro-LEDs

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ALD Companies and OLED/AMOLED

 

Many of the ALD companies listed on our ALD system pages make equipment for OLED barriers: Encapsulix, Beneq, Picosun, and many more. You can ask our team any questions on linkedin messenger, by clicking the Connect & Chat button on our website!

 

Further reading: 

 

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