A combined interposer (120) includes multiple constituent interposers (120.i), each with its own substrate (120.iS) and with a circuit layer (e.g. redistribution layer) on top and/or bottom of the substrate. The top circuit layers can be part of a common circuit layer (120R.T) which can interconnect different interposers. Likewise, the bottom circuit layers can be part of a common circuit layer (120R.B). The constituent interposer substrates (120.iS) are initially part of a common wafer, and the common top circuit layer is fabricated before separation of the constituent interposer substrates from the wafer. Use of separated substrates reduces stress compared to use of a single large substrate. Other features are also provided.