Passivation layer assembly on a substrate and display substrate having the same

A display substrate comprising:

• a substrate;
• a thin film transistor on the substrate, the thin film transistor including a source electrode, a drain electrode and a gate electrode each containing a metal;
• and
• a passivation layer assembly formed on the substrate, the passivation layer having a contact hole through which the drain electrode is partially exposed, the passivation layer assembly including:
• a first silicon nitride layer
• a second silicon nitride layer;
• and
• a third silicon nitride layer,
• wherein each of the silicon nitride layers have a different nitrogen composition:
• wherein a contact hole through which the drain electrode is exposed is formed at the first, second, and third silicon nitride layers:
• and
• wherein an upper portion and a lower portion of the contact hole has a width greater than the width of a middle portion of the contact hole.