= Solution
\b[Deposit hydrogenated <silicon nitride> by <plasma-enhanced chemical vapor deposition>.] A plasma activates a silicon-containing precursor and a nitrogen source, enabling a conformal thin film at a lower substrate temperature than a purely thermal process. Tune gas composition, power and time to control thickness, refractive index, stress and hydrogen content.
The film serves as an <antireflection coating> and can also provide chemical and field-effect <surface passivation>. As a simple optical check, a quarter-wave thickness is $d\simeq\lambda_{\rm design}/(4n)$; $d\simeq75\,\mathrm{nm}$ and $n\simeq2$ correspond to a design wavelength near $600\,\mathrm{nm}$. The textured surface and broadband illumination require a fuller optical optimization, so this check is an order-of-magnitude explanation rather than an exact reflectance calculation.
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