Metrologies for characterization of flatness and thickness uniformity in temporarily bonded wafer stacks

In the past few decades, the semiconductor industry has grown very quickly and the technology has advanced as predicted by Moore´s law. In order to improve the cost effectiveness of semiconductor processes, the device size has been reduced, but the wafer diameter continues to increase. Furthermore, in order to overcome the barrier of Moore´s law, Three-Dimensional Stacked Integrated Circuits (3DS-IC) integration technology attracts more and more interests in the semiconductor industry. One of the key processes of 3DS-IC technology is the implementation of temporary bonding solution which gives the ability to handle and process thinned Si wafers. Flatness and thickness uniformity of a thin Si wafer is critical for product reliability and yield. As a result, not only total thickness variation (TTV) of the thin Si wafer but also TTV/flatness of the wafer stack matters. As the temporary bonding process evolves, glass carriers have become a promising solution because of flexibility and excellent attributes given by adjustability of composition. Substantial benefits are also realized from Corning´s optimized fusion forming process. It is, therefore, extremely important that the flatness and TTV of the carrier wafer are well characterized to ensure expected performance. Flatness and thickness characterization of a temporary bonded stack is challenging due to different aspects. Firstly, as the diameter increases, stiffness of the wafer decreases. Significant sag is observed when the wafer is measured on a conventional points mount fixture. The sag could be so large that removing gravity is non-trivial. Secondly, the absorption spectrums of glass and Si could be quite different. Non-contact optical metrology has to work for both materials. Thirdly, precise metrology which can provide high resolution (lateral and thickness) of the thickness uniformity map in order to fulfill requirements of downstream processes is needed. Corning® Tropel® MSP300® i- a novel distance measuring interferometer based on a frequency stepping laser that is well suited to characterize flatness and TTV of glass. This metrology tool implements a novel mounting solution which prevents gravity induced wafer sag. It also realizes a much higher lateral resolution compared to the commercial metrology tools which only scan wafers along distinct slices and interpolate data in regions between these slices.For comparison studies of different mounting strategies and scan techniques, we characterize the wafer stack not only by MSP300, but also by a scanning system, which integrates translational stages and a low coherence interferometer (LCI) operating at infrared wavelength. Commercial software is used to generate data in the region between distinct scan slices. The wafer stack is composed of glass carriers and Si wafers implemented by ZoneBOND® from Brewer Science. The thinning process is also carried out on the Si wafers after wafer bonding. Variation of attributes of glass carriers are utilized for correlation study. The impact of various attributes of the glass wafer and bonding process, as well as the importance to suitably characterize the carrier wafer (data density, resolution, and mounting technique) will be demonstrated.