Silicon micromachining of high aspect ratio, high-density through-wafer electrical interconnects for 3-D multichip packaging

This paper presents a novel silicon micromachining method, which combines tetra methyl ammonium hydroxide (TMAH) etching and deep-reactive ion etching (DRIE) along with bottom-up copper electroplating, to fabricate high-density and high-aspect ratio through-wafer electrical interconnects (TWEIs) for three-dimensional multichip packaging. The silicon wafer was locally etched with TMAH from the backside until the desired membrane thickness was reached, and then DRIE was performed on the membrane until the holes were etched through. TMAH etching preserved large areas of the wafers at the original thickness, thus, ensuring relatively strong mechanical strength and manipulability. DRIE made it possible to realize high-aspect ratio holes with minimized wafer area consumption. A new bottom-up copper electroplating technique was developed to fill the high-aspect ratio through-wafer holes. This method can avoid seams and voids while achieving attractive electrical features. Through-wafer holes, as small as 5 μm in diameter, have been realized by using the combination of TMAH and DRIE, and have been completely and uniformly filled by using bottom-up copper electroplating.