Reactive Ion Beam Etching of Surface Relief Gratings for AR- & MR-Devices

Augmented Reality (AR) describes a technology where computer-generated objects and information are displayed in real-life surroundings. The technology is used for several applications in business, like service and surgery assistance, as well as for consumer market applications, like navigation. State of the art augmented reality as well as mixed reality (MR) devices use optical waveguides with diffraction gratings in order to guide light from a display by total internal reflection (TIR) into the view of the human eye.

Figure 1 shows schematic the optical gratings used as light couplers. The smaller input grating diffracts light coming from a display into the waveguide. At the out-coupler grating, light is diffracted in direction of the viewer. For a uniform illumination of those output gratings, the grating depth increases with traveled distance of light in the waveguide, also named “leaky gratings”.

Different approaches for the design of coupling gratings exist. One type is the so called Surface Relief Grating (SRG). Within this group the gratings are divided into blazed gratings, slanted gratings, binary gratings and analog surface relief gratings.

 

Processing of Surface Relief Gratings (SRG)

In order to manufacture those gratings, in the first step photo lithography is used to create a mask or pattern on a glass substrate. In the next step this mask will be used to structure an intermediate metal mask or the glass directly. For this the widely used dry etching technology reactive ion etching (RIE) is not suitable, as here ions are always perpendicularly accelerated to the surface. In contrast to RIE the reactive ion beam etching (RIBE) process offers unique advantages. Due to the ion beam source, that extracts an ion beam in which the substrate can be freely tilted, slant angles as flat as 60 degree (from substrate normal) can be etched (see Fig. 2 and 3).

Instead of direct etching of glass substrates another opportunity is using nano-imprint lithography (NIL). For NIL, a master stamp is structured by RIBE, which will be pressed into a polymer spin coated substrate for replication of the pattern. However, polymers do not offer refractive indices in the range of high refractive index glasses, which has disadvantages regarding the field of view (FOV) of the final device which will be explained in the following.

Crucial parameters for the RIBE process are selectivity between mask and substrate and control of geometry between side wall angle and bottom angle. By using different etching gases, like CHF3, O2 or Cl2, isotropic or anisotropic etching behaviors can be controlled, which finally defines the resulting geometry of the gratings. Reactive gases can either be introduced into the ion beam source or into the chamber (see Fig. 4).The ability to independently control ion energy and ion current is additionally used to adjust slant geometry and selectivity.

As stated before, another reason to choose RIBE processing for manufacturing of surface relief gratings is, that for the final AR/MR product, a wide field of view (FOV) is needed. The critical angle for total internal reflection limits the shallowest angle under which light may travel within the waveguide. For high refractive index materials, the critical angle is smaller and the FOV bigger. Such high refractive materials (refractive indices higher 2) might not be etched with standard gases like used for quartz etching but with adapted mixtures and recipes. So, the unique capabilities to control gas mixture, ion energy and ion current, substrate angle and even substrate temperature makes RIBE the preferred technology for etching of slanted relief gratings.

 

 

 

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Related Information

Related Products: scia Mill 150 & scia Mill 200 & scia Mill 300

  • Full surface etching with superior homogeneity
  • Reactive gas compatibility in RIBE and CAIBE processing, including fluorine / chlorine gases
  • Substrate holder featuring rotation and tilt for arbitrary angles of incidence
  • Thermal control of wafers during the process over a wide temperature range
  • Ion beam source with high stability, adjustable ion energy and ion current density
  • Complete software integration and automated processes via recipe