Bonding of 2 mm thick silicon wafers using LTCC as an intermediate layer

Abstract

For the fabrication of a micro fluidic high pressure oil sensor (400 bar) based on an infrared transmission measuring principle the bonding of 2 mm silicon wafers is necessary. Conventional bonding techniques such as silicon fusion bonding or anodic bonding are not suitable for bonding thick and inflexible silicon wafers, because these techniques can not compensate for the wafer bow. We present a new bonding procedure for silicon substrates thicker than 1 mm using a silicon adapted LTCC tape as an intermediate leveling layer. The wafers are preprocessed by etching a nano structured silicon surface on the internal side. The silicon wafers are aligned and stacked with pre-structured green LTCC tapes by an optical stacking unit. During the hot isostatic lamination at 55 bar the structured LTCC tape is adjusted to the silicon. A subsequent pressure assisted sintering leads to a wafer bonding strength up to 5000 N/cm2. With the bonding technique it is possible to create cavities and channels between the thick wafers by the use of punched and laser cut LTCC. The fabrication steps of the sandwich build-up especially the sequential lamination and the optical adjusting procedure of the flexible (LTCC) and inflexible (2 mm Wafer) substrates will be explained in detail. A method to reduce the shrinkage and distortion of the green LTCC during handling is demonstrated. The distribution of the bonding and bursting strength of the single fluidic systems on a complete sandwich substrate is analyzed.