A device includes: a first substrate; a second substrate; interconnects bonding the first substrate to the second substrate; and a polymer brush-based underfill layer in a gap between the first substrate and the second substrate. A method includes: attaching initiator molecules to one or more surfaces in a gap between a first substrate and a second substrate of a bonded structure, where the first substrate and the second substrate are bonded by interconnects; growing polymer chains from the initiator molecules; and annealing the bonded structure to form an underfill layer from the polymer chains in the gap.