According to IPC,a microvia is a hole with a diameter of < 0.15mm.
It can be a through viahole (with all respect to a aspect ratio),
But we normally see them as blind vias between 2 layers.
Mostly drilled by laser,but some pcb manufactuers are also drilling mircorvias
with a mechanical drill bit.
It is slower,but the holes have a clean and nice cut.
The microvia copper fill process is an electrochemical depossition
Process applied in the manufacturing of multilayer process,
Also called capped vias.
The process is complex,copper filling of microvias is available from most
PCB manufacturers,that are capable of producing HDI PCB Boards.