LCD is a transmissive technology, i.e. the function of the pixels is to block or pass light that is generated behind them. A voltage applied to a thin layer of liquid crystal held between two polarizers facilitates the on/off function.
OLED and microLED are emissive technologies where each pixel is a small light source. OLED pixels are based on light emitting organic molecules and microLED pixels are semiconductor LEDs. While OLED molecules are deposited directly on the display backplane, microLED chips are manufactured on a semiconduction wafer and need to be transferred.
While OLED is already widely adopted in the market, microLED technology still lags even though it offers several advantages. Presently, the key challenge in adopting microLED technology is the process of transferring the individual microLED pixels from the semiconductor wafer onto to the backplane of the final display product.
The following table shows several differences between these technologies.
|
LCD |
OLED |
microLED |
Response Time |
ms |
us |
ns |
Viewing Angle |
Narrow |
High |
High |
Power Efficiency |
Medium |
Medium |
High |
Flexible Display |
No |
Yes |
Yes |
Brightness |
Medium |
Low-Medium |
High |
The process of making an OLED display is generally divided into three stages:
- Deposition and crystallization of amorphous silicon making the conductive wiring of the pixels
- Formation of pixels. Red, green, and blue emitting OLED molecules are deposited using vapor deposition or ink-jet printing
- Encapsulation and lift off.
- Encapsulation. OLED molecules are very sensitive to moisture. The display is encapsulated between thin layers of polymer or glass.
- Lift-off. OLED displays are manufactured on transparent polymer sheets. During the fabrication process, the display rests on a glass substate that supports it. When the display is ready, it is 'peeled' off the glass in what is called 'lift off'
The following manufacturing step utilize lasers:
- Laser annealing – transforming of amorphous silicon into polycrystalline silicon. The energy imparted by the laser beam enable the silicon atoms to move around until they settle in a stable crystallized form.
- Laser liftoff – separating the OLED display from the glass substrate. The laser beam is formed into a narrow thin line that scans across the OLED display and is absorbed in the interface between the OLED display the glass interface and separates the two. Cutting and drilling of glass and polymer parts.
- Cutting and drilling with lasers is done by evaporating material rather than removing material, for example, using a drill. This allows processing thin and fragile materials without cracking them.
microLED pixels are manufactured in a 'classic' semiconductor process and need to be transferred onto the microLED display backplane. This is currently the most challenging step in microLED manufacturing. For a microLED TV screen, millions of micron sized microLED's need to be transferred from the wafer onto the display backplane with high accuracy and speed. To add to the complexity, the microLEDs are much denser on the semiconductor wafer than they are on the display, making it impossible to transfer groups of microLEDs simultaneously.
Several companies are working on technologies for making this transfer step more efficient. One such company, Uniqarta, developed a technology that uses a rapidly moving laser beam to selectively release microLED chips from a polymer carrier onto the display backplane. The release of the microLED chip is achieved by local heating of the polymer carrier causing it to expand and create a 'bump' that pushes the microLED chip away.