The present invention relates to packaging of microelectronic devices and interposer structures, especially conductive via structures and methods of forming such via structures in semiconductor and interposer packages. 
Microelectronic elements generally comprise a thin slab of a semiconductor material, such as silicon or gallium arsenide, commonly called a die or a semiconductor chip.  Semiconductor chips are commonly provided as individual, prepackaged units.  In some unit designs, the semiconductor chip is mounted to a substrate or chip carrier, which is in turn mounted on a circuit panel, such as a printed circuit board. 
The active circuitry is fabricated in a first face of the semiconductor chip (e.g., a second surface).  To facilitate electrical connection to the active circuitry, the chip is provided with bond pads on the same face.  The bond pads are typically placed in a regular array either around the edges of the die or, for many memory devices, in the die center.  The bond pads are generally made of a conductive metal, such as copper, or aluminum, around 0.5 μm thick.  The bond pads could include a single layer or multiple layers of metal.  The size of the bond pads will vary with the device type but will typically measure tens to hundreds of microns on a side. 
Through-silicon vias (TSVs) are used to connect the bond pads with a second face of the semiconductor chip opposite the first face (e.g., a first surface).  A conventional via includes a hole penetrating through the semiconductor chip and a conductive material extending through the hole from the first face to the second face.  The bond pads may be electrically connected to vias to allow communication between the bond pads and conductive elements on the second face of the semiconductor chip. 
Conventional TSV holes may reduce the portion of the first face that can be used to contain the active circuitry.  Such a reduction in the available space on the first face that can be used for active circuitry may increase the amount of silicon required to produce each semiconductor chip, thereby potentially increasing the cost of each chip. 
Conventional vias may have reliability challenges because of a non-optimal stress distribution radiating from the vias and a mismatch of the coefficient of thermal expansion (CTE) between a semiconductor chip, for example, and the structure to which the chip is bonded.  For example, when conductive vias within a semiconductor chip are insulated by a relatively thin and stiff dielectric material, significant stresses may be present within the vias due to CTE mismatch between the conductive material of the via and the material of the substrate.  In addition, when the semiconductor chip is bonded to conductive elements of a polymeric substrate, the electrical connections between the chip and the higher CTE structure of the substrate will be under stress due to CTE mismatch. 
Size is a significant consideration in any physical arrangement of chips.  The demand for more compact physical arrangements of chips has become even more intense with the rapid progress of portable electronic devices.  Merely by way of example, devices commonly referred to as “smart phones” integrate the functions of a cellular telephone with powerful data processors, memory and ancillary devices such as global positioning system receivers, electronic cameras, and local area network connections along with high-resolution displays and associated image processing chips.  Such devices can provide capabilities such as full internet connectivity, entertainment including full-resolution video, navigation, electronic banking and more, all in a pocket-size device.  Complex portable devices require packing numerous chips into a small space.  Moreover, some of the chips have many input and output connections, commonly referred to as “I/Os.”  These I/Os must be interconnected with the I/Os of other chips.  The interconnections should be short and should have low impedance to minimize signal propagation delays.  The components which form the interconnections should not greatly increase the size of the assembly.  Similar needs arise in other applications as, for example, in data servers such as those used in internet search engines.  For example, structures which provide numerous short, low-impedance interconnects between complex chips can increase the bandwidth of the search engine and reduce its power consumption. 
Despite the advances that have been made in semiconductor via and interposer via formation and interconnection, there is still a need for improvements in order to minimize the size of semiconductor chips and interposer structures, while enhancing electrical interconnection reliability.  These attributes of the present invention may be achieved by the construction of the components and the methods of fabricating components as described hereinafter.