In-plane sensing in microLED displays - A new performance and manufacturing paradigm for optical sensing in consumer and automotive electronic devices

Traditionally, front-facing sensors in mobile devices perform functions such as display brightness control, white balancing and face authentication. These sensors have often been integrated in cut-outs in the display (bezels). The trend toward bezel-less displays means sensor manufacturers have developed sensors that can be integrated behind the display. 

In OLED displays and “behind OLED” sensing, an area where ams OSRAM is a technology pioneer, the sensors must be placed behind a display plane densely populated with OLEDs. This behind OLED sensing technology presents some implementation challenges due to the need for tight synchronization between the display system and the sensor system to avoid interference between the OLEDs and the sensors. The BOLED sensors must offer very high sensitivity to provide sufficient optical signal strength when operating behind an OLED display that has very low transmissivity (typically less than 10%). Also stacking a system with separate planes – one for the display, another for sensing – increases the overall height of the sensing display. 

MicroLEDs, the next technological advance, offer an exciting combination of very small dimensions and very high brightness. To date, there is no industry-standard specification for the mechanical features of a microLED: while some manufacturers have classed devices with sides as long as 50µm as ‘microLEDs’, ams OSRAM believes such devices are too large to have a disruptive market effect. Instead, the company is using its miniaturization expertise to create chip edge lengths of 10µm and less. At this size, a microLED is so small that it is not visible to the naked eye.

In fact, RGB microLEDs are so small and can be driven at such high current that they can be widely spaced while still maintaining a very high-resolution output and high brightness. Display engineers refer to this as a low ‘fill factor’ – the ratio of total microLED area to total pixel area. A low fill factor means that there is free area around the microLEDs. While these areas are imperceptible to the human eye, they are large enough to accommodate sensor components such as micro-photodiodes (microPDs) and near infrared (NIR) microLEDs (see Figure 1). These are key components for optical sensors, where the microPDs either measure the incident light or the invisible light that was emitted by the NIR microLED and reflected by an object such as a finger touching the surface of the display.

Aside from making the sensor elements practically invisible and removing the need for the stacking of display and sensors, there is another strong advantage to the in-plane display sensing architecture that microLEDs enable. By placing the sensor components in the same plane as the RGB microLED, they gain an unimpeded line of sight through the cover glass. This can simplify the implementation of sensors that are already found in OLED displays. 

Additionally, the pixelation of the optical sensor function in a microLED display (see Figure 2) enables new use cases and sensors. 

A microLED sensing display could enable new and improved sensor functions such as:

• Display local brightness control, where the ambient light sensing is distributed over the full display
• Camera and display white balancing
• Proximity sensing
• Vital signs monitoring
• Fingerprint recognition, which eliminates sub-systems such as ultrasonic sensing used for fingerprint recognition in today’s mobile phones. This could increase the fingerprint sensing area and reduce the height of the total sensing/display stack.
• Touch sensing
• Gesture recognition, by adding directionality to the sensing elements combined with computational imaging.

The vision for in-plane display sensing enabled by microLEDs is a simpler system architecture with enhanced sensor functionality, lower power, and lower sensor costs.  

There are still essential technical development steps to be made before this vision becomes real and in-plane display sensing is available in the first products. When sensing is integrated in-plane within the display, the optical signal chain requires a complex set of interfaces that provide connections between the RGB and NIR micro-emitters, their microIC pixel-drivers, the microPDs and their read-out circuit, signal processing circuitry, other display elements, and the device’s SoC (see Figure 3).

ams OSRAM is working to address the challenge that in-plane sensing brings by working together with technology suppliers, display manufacturers, display driver and system-on-chip (SoC) manufacturers, OEMs, customers and other stakeholders. ams OSRAM is developing the alliances, standards and specifications required to pull together the complex new supply chain that can bring in-plane sensing to the market.