Microelectronic elements with rear contacts connected with via first or via middle structures

A method of fabricating a micro-electronic unit, comprising:

• providing a microelectronic element including a semiconductor region of monocrystalline form, the microelectronic element having a front surface, a rear surface remote from the front surface, a front contact exposed at the front surface, an active circuit element adjacent to a front monocrystalline semiconductor region surface, a region of polycrystalline semiconductor material disposed in a trench insulated from the monocrystalline semiconductor region extending towards the rear surface, and a via disposed entirely between the front and rear surfaces of the microelectronic element, the front contact being electrically coupled with the region of polycrystalline semiconductor material through the via;
• forming an opening extending from the rear surface partially through the monocrystalline semiconductor region and exposing the region of polycrystalline semiconductor material;
• removing at least a portion of the polycrystalline semiconductor material through the opening;
• forming a conductive region at least partially replacing the removed polycrystalline semiconductor material;
• and
• forming a rear contact electrically coupled with the conductive region and exposed at the rear surface for electrical connection with a circuit element.