OLED microdisplays: how the technology develops and where it is used

How microdisplays of a new generation are produced – with ultra-high resolution and low power consumption – tell the employees of the Central Research Institute “Cyclone” (Ruselectronics)

How the production of OLED microdisplays works

OLED displays (Organic Light Emitting Diode – organic light-emitting diodes that independently emit light when an electric current passes through them) are created in several stages by deposition of organic layers on silicon wafers.

In production, two variants of plates are used. To create test LEDs, pure (empty) silicon ones are taken, and for the final product – microdisplays – silicon wafers with crystals (chips) with an active-matrix control scheme. Active Matrix Organic Light-Emitting Diode (AMOLED) is a display technology that uses organic light emitting diodes as light-emitting elements and an active matrix of transistors to control them.

Process cleanliness

Before entering the production hall, it is necessary to put on a protective suit and go through the process of “dedusting” in the airlock before entering. The entire process of creating OLED microdisplays takes place in cleanrooms. Dust is the main enemy of future microdisplays. One tiny particle adhering to the plate during the deposition process of the organic layer can lead to a short circuit or loss of hermetic properties of the film. The ingress of dust will affect both the quality of color reproduction and the life of the microdisplay – it will quickly fail. Therefore, until the production process begins, the plates are stored separately – in vacuum locks with constant air evacuation, where they are not exposed to external influences of oxygen and moisture, which can be absorbed on the surface.

Automation

The main production work is carried out with the help of machines, but they work under the supervision of an operator-technologist. All commands are given using the control panel. All production processes take place in a vacuum, even simple movement: a robotic arm transfers the plates from chamber to chamber. Recalculation of each plate, thickness control down to one nanometer and other tests are also performed automatically.

Display manufacturing steps

In the spray recipe (the unique composition of the organic layer) for each plate, the sequence of application is prescribed. According to it, the robotic arm picks up the silicon wafer and guides it to different stages.

Cleaning. First of all, the plate is sent for plasma cleaning of the surface so that all impurities are removed from it.

First layer is the organic structure necessary to correct the emission spectrum. Here, organic electroluminescent materials are used, with the help of which an organic LED is obtained. The electro-optical properties and current-voltage characteristics (dependence of electrical voltage on the current strength in the electrical circuit or its individual elements) are checked, which at the output affect the image quality.

Spraying. The quality of an OLED microdisplay depends not only on the organics, but also on the next stage – metal deposition, which gives conduction. This is the electrical contact that connects the entire organic structure and covers the LED.

Sealing. After an organic structure is created and a translucent electrode is applied, there is a multi-stage sealing, which is very important for the OLED itself. Oxygen and moisture are so detrimental that if a structure that has just been deposited is taken into the atmosphere, it will actually live for hours or even minutes.

Tests. Before the next step, the OLED microdisplay is tested and checked for manufacturing errors. Finished displays are checked at the probe station. So that no microvibration interferes with the test, the equipment is placed on a massive granite platform. Here, first of all, the chromaticity coordinates, brightness, lighting parameters of the organic structure, which was obtained after the application and sealing process, are checked.

Assembly. After the plate has passed the sealing stage of the OLED structure, it can be removed from the vacuum without fear that it will quickly degrade. Then the assembly of the display begins, the glass is glued: either a conventional display glass with anti-reflective coatings, or glass with color filters if a full color image is required. Gluing glass is necessary so that the thin-film sealing formed by sputtering is not broken. That is, glass performs not only optical functions, but also increases the protective and mechanical properties.

Separation into light-emitting matrices. When the glass is glued, the organic coated plates are cut into pieces. To do this, mark the reference points that are responsible for alignment, and the actual cutting points. Then the machine builds the map itself. The plate is mounted on a hoop so that the crystals do not scatter during cutting, and is sawn. During the cutting process, water is supplied – otherwise the blade will collapse almost immediately. Rotating, the circular saw gradually cuts through the “sandwich” of silicon with glass.

Creating a Display. The split light-emitting matrices become full-fledged displays at this stage, when they are installed on the interface boards from which all signals are output. In fact, this is an adapter, with the help of which the control signal and the video image itself enter the silicon chip.

Characteristics of OLED microdisplays

A micro display is a display with a resolution of 1280 by 1024 pixels, while the diagonal is only 2 centimeters, that is, it turns out 12 microns per pixel – this is 20 times thinner than a human hair. Each pixel is made up of three sub-pixels to make a full color image. The sizes of these subpixels are 3 by 11 microns.

Where are OLED microdisplays used?

The level of detail that is achieved in OLED microdisplays is importantnot only for smartphones, laptops, but also for professional equipment, for example, vision devices in the infrared range: surveillance, sighting equipment, security systems. Add here industrial thermal imaging, IT-sphere, industrial control and even housing and communal services.

Another big area of ​​application is VR and augmented reality. In this case, displays can be used not only for entertainment purposes, but, for example, when assembling complex technological equipment, or controlling processes in a room where human presence is undesirable. Such systems are already being tested at Rostec and Rosatom.

Competitiveness and the future of the manufactured product

At the output, an OLED microdisplay costs about ₽100–120 thousand. This is cheaper than from European manufacturers, and slightly more expensive than from Chinese ones.