A ‘micro-light source’ is a light source with a side measuring less than 100 microns (1/10th of a millimetre). Examples include short-cavity lasers and micro-LEDs.
These devices offer a number of performance advantages, such as higher definition, smaller size, and lighter weight, and as a result these micro-light sources are considered essential to next-generation automotive displays, wearable smart glasses, communication equipment, and medical devices.
The market for micro-LED chips alone is expected to reach $2.7 billion by 2026, a compounded annual growth rate (CAGR) of about 241%.2
GaN-based light-source devices, both micro-LED and laser, have typically been fabricated on sapphire and GaN substrates. Conventional processes involve forming a thin GaN device layer for the light source directly onto the sapphire substrate by heating it to a high temperature (1,000 degrees C or more) in a controlled gas atmosphere. The device layer is then removed, or ‘peeled,’ from the substrate to create a GaN-based micro-light-source device.
Despite rising demand for smaller devices, however, technical challenges threaten the ability of this process to achieve miniaturisation targets in the near future: As devices become smaller, for example, the technical challenge of this peeling process can result in unacceptably low yield.
Fabrication of micro-light sources is also problematic because device layers must be deposited onto sapphire, silicon, or other materials with crystal structures that differ from that of the device layer. This creates high defect density and inherent quality control challenges.
GaN and sapphire substrates are also very expensive materials. Although silicon substrates cost less than sapphire, separating the device layer from a silicon substrate is extremely difficult.
In response to these various technical challenges, Kyocera has successfully developed the new process technology at the company's Research Institute for Advanced Materials and Devices in Kyoto, Japan.
First, a GaN layer is grown on a Si substrate which is available in high volumes at a low cost. The GaN layer is then masked with a non-growing material that features an opening in the centre. After this, when a GaN layer is formed on the Si substrate, a GaN nuclei is grown over the opening in the mask. The GaN layer, which is a growing nucleus, has many defects at the initial stage of growth; but, by forming the GaN layer laterally, high-quality GaN layers with low defect density can be created, and devices can be fabricated successfully from this low-defect region of the GaN layer.
Masking the GaN layer with a material that does not grow suppresses bonding between the Si substrate and the GaN layer, greatly simplifying the peeling process and since Kyocera’s process can deposit low-defect GaN over a wider area than before, consistent fabrication of high-quality device layers is now possible.
Kyocera’s new method facilitates successful and reliable separation of the GaN device layer from the relatively inexpensive Si substrate, which will greatly reduce manufacturing costs.
In the future, the advent of autonomous driving will create demand for displays that are brighter, higher-definition, power-efficient, more highly transparent, and lower cost, while the market for micro-light sources used in augmented reality (AR) and virtual reality (VR) is expected to expand rapidly.
Smart glasses and other products are being developed to facilitate the creation of virtual spaces through the metaverse in VR and ‘de-smartphoning’ in AR.
While conventional semiconductor lasers for AR have been miniaturised to as small as 300 microns in length, Kyocera is the first in the world to achieve a size of just 100 microns.
This so-called ‘novel cleaving method’ results in a size reduction of about 67% and helps minimise power consumption. Semiconductor lasers with lower power consumption make it possible to reduce the size and weight of the battery.
According to Kyocera it will be looking to offer a wide range of platform, substrate, and process technologies to bring high-quality, low-cost micro-light sources to market in the near future.
