SPFM Pre-Cleaning for Formation of Silicon Interfaces by Wafer Bonding
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The aim of this project was to investigate surface preparation procedures capable of enhancing room temperature bondability as well as giving good electrical characteristics of bonded Si/Si junctions. Many of the common cleaning procedures, e.g. APM, HPM or SPM, create a highly hydrophilic chemical oxide at the surface. Hydrophilic surfaces bond quickly and show relatively strong adhesion already at room temperature [3]. They also have less tendency to attract particles from air and chemicals, which decreases the particle contamination probability at the interface. On the other hand, HF-dipped (H-terminated) hydrophobic surfaces, which are oxide-free and thus preferable for electronic applications, are characterized by a slower contact wave as well as weaker adhesion [4]. The reason for this is weaker attraction forces between the terminating atoms. In the Sulfuric Peroxide Hydrofluoric Mixture (SPFM) consisting of both oxidizing and oxide etching agents there is a competition between oxidation and oxide etching [5]. The SPFM cleaning process produces a hydrophilic (OH terminated) surface, and hence a strong room temperature bond [6]. Using SPFM instead of other hydrophilizing cleaning processes has the advantage that the interfacial oxide can be kept at a minimum, ensuring good electrical properties of the bonded Si/Si junction. The SPFM cleaned silicon surfaces were characterized using XPS (X-ray Photoelectron Spectroscopy) and AFM (Atomic Force Microscopy). Electrical characterization using current vs voltage measurements and spreading resistance technique was performed on bonded Si/Si junctions. SIMS (Secondary Ion Mass Spectroscopy) was used to map the presence of impurities in the vicinity of the bonded interface. Both the electrical characterization and the SIMS analysis were done on samples prepared using different cleaning procedures prior to wafer bonding to compare the results after SPFM cleaning with results after other cleaning techniques.
267 Mat. Res. Soc. Symp. Proc. Vol. 477 0 1997 Materials Research Society
EXPERIMENTS AND RESULTS The wafers used were n-type, 6-9 Qlcm, FZ wafers of crystallographic orientation (1-0-0). After dipping in 2% HF:H 20, and rinsing in water, the wafers were cleaned for 2 minutes in H2SO 4:H20 2 4:1 (SPM), to which 10 to 1000 ppm of HF had been added. Surface analysis using XPS and AFM was used to investigate the surface properties. The wafer bonding properties were investigated by contacting the wafers immediately after cleaning and drying. The velocity of the growth of the bonded area (the contact wave velocity) and the surface energy of the formed interface were monitored as measures of the bondability of the surfaces. To investigate the influence of cleaning procedures on the electrical properties of the Si/Si junctions, silicon wafers were bonded after surface preparation using 2% HF, SPFM(1000 ppm), SPFM(10 ppm), APM/HPM or HNO 3. The wafers were dipped in 2% HF:H 20 and rinsed in water before the different subsequent cleaning steps. The bonded pairs were anneal
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