The Impact of SRG Manufacturing Quality on AR Headsets

4 June 2020

A Bottleneck in Augmented Reality Technology

Experiencing fully immersive augmented reality requires a major leap in several technologies. From 5G to microdisplays, nanofabrication is at the heart of this challenge.

In addition, AR headsets need to combine the largest amount of high-end technology at the smallest form factor.

To make a headset, you need:

• a microdisplay to generate the virtual image
• complex optics to guide the light to the user’s eyes
• sensors to capture both the environment and the user activity
  • ambient light sensors
  • gesture recognition
  • eye tracking
  • head position tracking
  • 3D sensing (and more)
• powerful processing unit to handle the large amount of data being produced
• a computer interface
• a battery

And that's just to name a few! 

Microsoft Hololens headset

Amongst these many technical challenges, one key challenge is to produce a virtual image bright enough that it comfortably blends with the real-world indoor and outdoor.

To do so, combiners are used to “combine” a virtual image to the real world. A virtual image is projected to the user’s eye whilst transmitting a real image.  

Optical combiner technology. Credit: PwC

Optical Waveguides

Of the numerous technologies developed for combiners, the optical waveguide approach is a promising technology. Waveguide combiners can achieve high field-of-view at low form factor.  How seamless the virtual image appears will then partly result from the display brightness in Nit, the efficiency of the combiner and the lens transparency.

Webinar: Slanted Etching of High RI Materials

In diffractive waveguide based on surface relief grating (SRG), the incident light waves flow into the waveguide at an angle set by the first grating, in-coupler. This angle is set to allow for Total Internal Reflection through the waveguide and the light is finally extracted to exit into the pupil via a second grating the out-coupler.

SRG-based waveguide.
Credit: Karl Guttag

 

Optical combiner.
Credit: Karl Guttag

 

The in- and out-couplers are SRG which are nano metric grooves. The geometry of the structures directly defines the coupling efficiency. More light is collimated into the waveguide when using slanted features which then results in increased efficiency. This is critical to ensure the digital image appear with sufficient definition and brightness to the human eye.

White Paper: Profile Control for Slanted Etching in Augmented Reality

Surface Relief Gratings for AR

Surface relief gratings are generally replicated from a master mould. Ion beam etching (IBE) is a technique well suited to fabricating the master mould for slanted gratings.

Since the geometry of the structure partly defines the coupling efficiency, precise definition of the dimensions of the master mould is required. However, there are many challenges to producing SRGs on wafer scale with satisfying yield:

• Poor mask definition at small dimensions: Typical dimensions are from λ/2 to λ which results in groove around 100 to 200nm.
• Control the slanted angle and groove parallelism: Due to the slanted structure, the process is highly subject to ion deflection which results in asymmetric etching and therefore loss of control over the angle.
• Etching depth non-uniformity: Dry etching at an angle leads to non-uniform ion distribution. The etching depth will then tend to be higher in the area closer to the ion sources.

Defined SRG on Si master mould

 

Non-parallel SRG on master mould

 

If you would like to learn more on processing slanted features, take a look at our AR Solutions.