Improved QE in CMOS image sensors with nano-black antireflection layer

A novel anti-reflection process is demonstrated
which improves the quantum efficiency (QE) of a CMOS image
sensor, with particular benefits at the ultraviolet (UV) and near
infrared (NIR) ends of the electromagnetic spectrum. Also, the
dark current and photoresponse non-uniformity (PRNU) were
reduced to about 33% and 55%, respectively, of the values for a
conventional control sensor. The nano-black anti-reflection
layer was made using a reactive-ion-etch technique to form
nano-scale spikes at the surface which greatly reduce the
reflectivity of the surface, which has a matt-black appearance.
The sensor used, a CIS115 from Teledyne-e2v, is a back-side-
illuminated (BSI) device with ≈10 um active silicon thickness
and 2000 X 1504 pinned photodiode pixels with a pitch of 7 um.
The improved QE is most impressive at UV wavelengths, below
400 nm, where the QE increases towards 100%, although no
correction was made for an increased electron generation rate,
as this is not easily quantified. This high QE result is compared
with a conventional antireflection (AR) coating which shows a
steep drop in QE below 400 nm. There is also an improvement
in QE in the NIR (from 700 nm to 1100 nm) for the nano-black
sensor, and this is despite the approx. 1 um thinning of the
silicon by the etching process, which would normally reduce the
QE. Some of the QE improvement may be the result of increased
scattering of the incident light, which is supported by the
reduced PRNU.