Integrating Organic Molecules to Silicon Surfaces
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H-terminated Si8. In addition, there have been extensive studies on the surface functionalization of porous silicon. 9- 2 These assembly processes have various advantages and shortcomings. For example, the cyclo-addition reaction can yield well-ordered organic layers but requires clean Si(100) in ultra-high vacuum (UHV) environment. The use of organolithium or Grignard reagent may not be compatible with some semiconductor processes where metal contamination of the surface should be avoided. The hydrosilation process yields dense organic layers but it only works in the solution phase and reaction rates are relatively slow. Here, we report a novel approach for the efficient assembly of organic molecules on silicon using Si-N and Si-O linkages. It is achieved by reacting amine or alcohol with chlorinated silicon surfaces to form an organic layer. The former is shown schematically in fig. 1. The alcohol assembly process is similar, but each molecule is anchored to the surface via one Si-O bond. These reactions are sufficiently general for the assemble of a variety of functional organic molecules on silicon surfaces in both vacuum environment and solution phases. In essence, these reactions are the reverse of the siloxane self-assembled monolayer (SAM) process based on alkylchlorosilanes and -OH terminated surfaces. Ideas for the assembly chemistry presented here can be traced to well known surface reactions in chemical vapor deposition,"3 particularly the atomic layer growth of silicon oxide or nitride from SiCl4 and H20 or NH 3.14
177 Mat. Res. Soc. Symp. Proc. Vol. 576 © 1999 Materials Research Society
EXPERIMENTAL The assembly processes were either gas-surface reactions on Si(100) in vacuum environment or solution phase reactions on H-terminated porous silicon and Si( 111). In this report, we mainly present results on the gas-surface assembly process which was carried out in an ultrahigh vacuum (UHV) system. The UHV system (base pressure 10"' torr) was equipped with an X-ray photoelectron spectrometer (XPS, VG) for surface analysis and a quadrupole mass spectrometer (QMS) for gas analysis. The Si(100) sample was a slice (18 x 10 x 0.4 mm) of an n-type wafer (0.01 Qcm, Wafernet). After thorough degreasing, we mounted the sample on a sample manipulator using two Ta clips following the procedure described by Yates and coworkers.' 5 A type C thermocouple was attached to the sample via a small Ta clip for temperature measurement. The sample was heated resistively. After system bake out, the sample was first degassed overnight in UHV at 900 K, followed by desorbing the native oxide layer in UHV environment to - 1300 K. The monochloride Si(lO0)-(2xl):CI surface was prepared by a saturation exposure to Cl 2 at a surface temperature of 300 K.' 3 The Cl terminated sample was subsequently transferred to a high vacuum reactor (base pressure 10-8 torr) attached to the UHV chamber and exposed to amine molecules at a vapor pressure of lx 102 torr and a surface temperature of 450 K. A typical reaction time of two hours converts
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