Carrier techniques for thin wafer processing

Three different types of carrier techniques have been investigated and developed: thermal release tapes, solvable thermoplastic glue layer and mobile electrostatic carrier. These carriers were applied for manufacture of ultra-thin RFID chips, 12 μm thin CMOS image sensors and to a new process sequence that enables the formation of solder balls at the front side of an already thinned device wafer. Technical capabilities of different carrier techniques are compared with respect to allowed temperature range, type of bonding and de-bonding mechanism and their compatibility with typical wafer fab processes. Mobile electrostatic carriers were used to perform solder ball bumping at 55 μm thin silicon wafers. The process sequence demonstrates the capability of electrostatic carrier technology to enable thin wafer processing at elevated temperatures. INTRODUCTION The increasing demand for thin semiconductor devices have required more and more sophisticated solutions for their manufacturing processes. Appropriate support systems are a basic need for secure handling and processing of very thin semiconductor device wafers. Generally this is accomplished by temporarily bonding a rigid carrier substrate onto the front side of a device wafer before thinning. Well known techniques use polymeric bonding agents like wax, solvable glues or thermally releasable adhesive tapes [1]. Further manufacture concepts are based on bonding materials that can be released after UV laser irradiation through a transparent glass carrier [2]. Application of polymer based bonding techniques is limited to the temperature range below 200 °C. Further increased temperature stability is required in order to allow process steps like sintering of backside metal or plasma etching of dielectric layers. In contrary to adhesive bonding electrostatic attraction has the potential to enable thin wafer processing at elevated temperatures [3]. Recent development work at the authors’ institute has lead to a new type of mobile electrostatic carriers based on silicon wafer substrates which were successfully applied to process steps like photo-lithography and plasma etching [4]. Next sections will describe following different types of carrier techniques: thermal release tapes, thermoplastic glue layers and mobile electrostatic carriers. THERMAL RELEASE TAPES Application of thermal release tapes has become a wide spread method in order to support wafers with low topographies during thinning processes. A carrier is attached to the process wafer by means of a double-sided adhesive tape with one side thermal releasable. Subsequently, backside grinding and etching down to 10μm can be performed. Removal of the carrier is performed by a heating treatment between 90 and 150°C. This method was also applied for wafers with high surface topography. In this case, device topography was embedded by an additional tape. Furthermore this method can be combined with the Dicing-by-Thinning technology (DbyT) as illustrated in Fig. 1. Dicing grooves are prepared at wafer front side with the trench depth corresponding to the projected chip thickness. Preparation of these chip grooves can be accomplished by means of a wafer saw or by silicon dry etching. After mounting the trenched device wafer to the carrier substrate the wafer pair is thinned from its backside until the chip grooves are opened.