Improving Imaging Systems by Reducing Coating Defects

A2623 — EODAS Nodule Defect Reduction

Objective

Multilayer vapor deposition coatings are used in imaging systems to reduce reflection
and increase transmission of light into the detector. Small defects in coatings can
create a scatter site for entering light, or completely block one or more detector
elements. Coating defects can arise from several sources. Imbedded particles which
have been over-coated will result in a spherical cap nodule defect on the surface of
the wafer. Debris on the surface of the wafer may result from improper handling,
storage, or tooling.
Preventing or removing nodule defects and debris on coatings could significantly help
improve the yield of detector systems. This project had a dual-methods approach:
to reduce the formation of nodule defects through an improved coating process,
and to develop a laser ablation process to efficiently remove nodule defects without
damaging the remainder of the wafer.

Payoff

When nodule defects occur, focal-plane array (FPA) performance can be degraded,
often through the creation of cluster defects (several adjacent non-functioning pixels).
The current mitigation strategy of polishing flatness into the filter wafer can result in
scuffing, which can degrade all or most of the FPA wafer.
The project significantly reduced the potential for the imaging systems to malfunction
due to multilayer coating issues. The payoff is measured as improved yield of FPAs.
For F-35, estimated cost savings of $3,864 per aircraft were implemented in 2017
from yielded improvements in filter wafers, reduced labor for wafer repair, and
improved yield in FPA downstream processing.

Implementation

The primary transition platform for this project is the F-35 Electro-Optical Distributed
Aperture System (EO / DAS) sensor system. The EO / DAS consists of six infrared
sensors mounted around the F-35 structure. The infrared sensors are one of the
primary cost drivers for the EO / DAS. Coating process improvements demonstrated
on this project were communicated to the supply chain and implemented in 2017.
The technique of laser ablation for nodule defect mitigation was demonstrated as
a viable and cost-effective option for nodule defect mitigation. This process can be
implemented in the future on F-35 with a capital investment, if needed, or on any
multilayer coating.