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Partitioning Responses of a Thermoplastic Temporary Bonding Material in a Thermal Compression Bonder
Keywords: thermoplastic, bonding, parameters
Three-dimensional integrated circuitry (3D-IC) continues to be an evolving market in the semiconductor world. Companies on the leading edge of the 3D-IC industry are striving to create standardized production methodologies while faced with continually changing materials, equipment, and processes. In order to support the stability of a strong 3D-IC industry, all stages of the bonding process should be clearly established through systematic classification and publication. Developing a deeper understanding of the limitations and tolerances of the bonding process is a fundamental necessity as 3D-IC fabrication matures. In the typical 3D-IC bonding process, the substrate must be thinned to create through silicon vias (TSV) for stacking. Bonding to a carrier wafer with a temporary adhesive material is a commonly used method to provide structural support for the fragile device wafer during backside processing and thinning. Although both thermoplastic and thermoset materials can be used for this bonding, thermoplastic materials have definite advantages. Thermoplastic temporary bonding materials are removable with solvent cleaning, resulting in a low-residue device post-debond, while thermoset materials must be removed by peeling the bonding materials away from the substrate which can leave residue on the device topography. Thermoplastic temporary bonding materials are also rigid at room temperature, allowing for greater uniformity during backside thinning than can be achieved with thermoset materials, which must be soft enough after crosslinking to be removed by mechanical peeling. While carrier bonding is not a new process for the 3D-IC industry, the significance of each process parameter in the bonding recipe is not always clearly understood. To develop a fuller understanding of the significance of the process parameters, a thermoplastic temporary bonding material, bonded in a thermal compression bonder, was characterized for bond quality by partitioning four main parameters: time, temperature, force, and slew rate. The standard best-known method (BKM) was used as the control. Each parameter was individually adjusted to provide the highest resolution in the results. Blank silicon wafers were used as mock device wafers and were bonded to silicon and glass carriers for assessment. The device and carrier wafer bonds must exhibit good adhesion with no voids to survive the rigors of processing, as well as possess low total thickness variation (TTV) for precision during thinning. The bonded pairs were evaluated using confocal scanning acoustic microscopy (CSAM) and spectroscopic ellipsometry to identify voids or defects in the bondline and to determine TTV of the material, respectively. This experimental method provides insight into the significance of the bonding parameters and their correlation to a quality bonding process for this specific thermoplastic temporary bonding material. Because different thermoplastic temporary bonding materials have different properties, the key bonding parameters may shift depending on the material.
Juni Myers, Associate Applications Engineer
Brewer Science, Inc.
Rolla, Mo
USA


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