Of particular technological importance are the chemical reactions that occur at solid surfaces during the processing of semiconductor wafers. For example, dry processes such as reactive ion etching (RIE) and chemical vapor deposition (CVD) are routinely employed in the manufacture of microelectronic devices. Although much is known about the conditions necessary for making device structures, the mechanistic details of the surface reactions that occur during these procedures are largely unknown. By determining the underlying physics and chemistry behind these reactions, device manufacturers will be better able to design new processes.

Our research program is unique in that we employ surface science techniques to look at fundamental aspects of RIE and CVD. Semiconductor surfaces are studied after reaction with model gas-phase precursor species in UHV. A variety of surface analysis techniques are employed to probe the surfaces, both in our laboratory and at synchrotron radiation (SR) facilities. Systems are studied in the submonolayer regime, where the interaction between the bare substrate atoms and the reactant is quantified, and in the high exposure regime typical of a 'real' process. Both the geometric and electronic structures of a sample are probed, and the results are correlated into a detailed understanding of the properties of a system that is not available from the use of a single technique.

An example of this type of research is our work on the XeF₂/silicon etching reaction. XeF₂ is a convenient source of atomic fluorine, which allows one of the most important interactions in semiconductor processing to be probed on an atomic scale. Soft x-ray photoemission (SXPS) and photon stimulated desorption (PSD), which are both SR-based techniques, were employed to study Si surfaces after etching with XeF₂. A reaction layer, composed of intermediate fluorosilyl species, forms on the surface as a result of the etching process as indicated in the figure. We showed (1) that the fluorosilyl species have a particular arrangement on the surface which is critical to the etching process [1], (2) that the evolution of the reaction layer geometry is a direct result of the formation of trivalent Si defects in the lattice [2], and (3) how the electronic structure of the surface region affects the reaction dynamics [3].

The etching of III-V semiconductor surfaces by halogens is important in the manufacture of optoelectronic devices [4].  The surface reactions of these materials present a more complex problem than for elemental semiconductors, however, as there are competing reaction pathways.  For example, it has been known that ordered structures form when halogens adsorb onto certain III-V semiconductor surfaces, while other surfaces become disordered and etch, but a physical picture that explains these observations was lacking.  We have found that the initial surface structure determines the pathways that are followed by any given reaction, as some of these systems will spontaneously etch at low temperature, while others will form stable ordered overlayers [5-11].  From our most recent work on chlorine and iodine adsorption on InAs(001) and InSb(001), we have developed a mechanistic understanding of these reactions [12-14].  This mechanism is based on the ideas that (1) surface group III elements are initially more reactive to incoming halogen molecules due to their empty surface orbitals, and (2) steric limitations imposed by halogen-halogen interactions on a surface can inhibit the formation of dihalide species.  This understanding of the fundamental surface interactions is critical in designing processes to be used for binary materials.

An area that is analogous to etching is deposition.  Atomic layer epitaxy (ALE) is a chemical deposition technique that can produce layer-by-layer epitaxial growth of Si with monolayer control.  We studied ALE processes on Si and Ge using alternating cycles of SiH₂Cl₂ and H₂, and were the first to identify the surface intermediate reaction products for this system [15,16]. We have also looked at the growth of insulating films of GaF₃ on GaAs surfaces via reaction with XeF₂ [17-19].  These 10 eV bandgap films have potential for use in III-V microelectronic devices, as this method of growth involves a simple chemical reaction that makes it easily amenable to current manufacturing techniques.

Our future plans for semiconductor processing studies involve the use of energetic, rather than thermal, reactants.  The motivation here is to better understand the mechanisms involved in plasma processing in order to provide better control of the structures that are produced, with the ultimate goal being the reproducible fabrication of uniform nanoscale features.  We will use controlled molecular beams of fast halogen atoms and low energy beams of halogen ions to etch semiconductor surfaces.  We will then investigate the resulting surfaces with our array of surface chemical and structural probes.
 

1. C.W. Lo, D.K. Shuh, V. Chakarian, T.D. Durbin, P.R. Varekamp and J.A. Yarmoff, "XeF₂ Etching of Si(111): The Geometric Structure of the Reaction Layer", Phys. Rev. B 47, 15648-15659 (1993).
2. C.W. Lo, P.R. Varekamp, D.K. Shuh, T.D. Durbin, V. Chakarian and J.A. Yarmoff, "Substrate Disorder Induced by a Surface Chemical Reaction: The Fluorine-Silicon Interaction", Surf. Sci. 292, 171-181 (1993).
3. C.W. Lo, D.K. Shuh and J.A. Yarmoff, "The Influence of Electronic Structure on XeF₂ Etching of Silicon", J. Vac. Sci. Technol. A 11, 2054-2058 (1993).
4. W.C. Simpson and J.A. Yarmoff, "Fundamental Studies of Halogen Reactions with III-V Semiconductor Surfaces", Ann. Rev. Phys. Chem. 47, 527-554 (1996).
5. D.K. Shuh, C.W. Lo, J.A. Yarmoff, A. Santoni, L.J. Terminello and F.R. McFeely, "Chlorine Chemisorption on and the Onset of Etching of Cleaved GaAs(110) at Room Temperature", Surf. Sci. 303, 89-100 (1994).
6. W.C. Simpson, W.M. Tong, C.B. Weare, D.K. Shuh and J.A. Yarmoff, "The Temperature Dependence of the Cl₂/GaAs(110) Surface Product Distribution", J. Chem. Phys. 104, 320-325 (1996).
7. W.C. Simpson, D.K. Shuh, W.H. Hung, M.C. Hansson, J. Kanski, U.O. Karlsson and J.A. Yarmoff, "The Role of Surface Stoichiometry in the Cl₂/GaAs(001) Reaction", J. Vac. Sci. Technol. A 14, 1815-1821 (1996).
8. W.C. Simpson, D.K. Shuh and J.A. Yarmoff, "Room Temperature Chlorination of As-Rich GaAs(110)", J. Vac. Sci. Technol. B 14, 2909-2913 (1996).
9. P.R. Varekamp, M.C. Hansson, J. Kanski, B.J. Kowalski, L. Olsson, L. Ilver, Z.Q. He, J.A. Yarmoff and U.O. Karlsson, "Angle-resolved photoemission spectroscopy of the 1x1 ordered overlayers on iodine-saturated GaAs(001) and InAs(001)", Surf. Sci. 352/354, 387-390 (1996).
10. P.R. Varekamp, M.C. Hansson, J. Kanski, D.K. Shuh, M. Bjrkqvist, M. Gthelid, W.C. Simpson, U.O. Karlsson and J.A. Yarmoff, "Reaction of I₂ with the (001) surfaces of GaAs, InAs, and InSb; I. Chemical interaction with the substrate", Phys. Rev. B 54, 2101-2113 (1996).
11. P.R. Varekamp, M.C. Hkansson, J. Kanski, M. Bjrkqvist, M. Gthelid, B.J. Kowalski, Z.Q. He, D.K. Shuh, J.A. Yarmoff and U.O. Karlsson, "Reaction of I₂ with the (001) surfaces of GaAs, InAs, and InSb; II. Ordering of the iodine overlayer", Phys. Rev. B 54, 2114-2120 (1996).
12. W.K. Wang, W.C. Simpson and J.A. Yarmoff, "Passivation versus etching:  Adsorption of I₂ on InAs(001)", Phys. Rev. Lett. 81, 1465-1468 (1998).
13. W.K. Wang, W.C. Simpson and J.A. Yarmoff, "Reactions of I₂ and Cl₂ with In- and As-terminated InAs(001)", Phys. Rev. B 61, 2164-2172 (2000).
14. W.K. Wang, S.R. Qiu, B. Corbitt, S.T. Riggs and J.A. Yarmoff, "Chemisorption of iodine on In- and Sb-terminated InSb(001)", Surf. Sci., 462, 211-221 (2000).
15. J.A. Yarmoff, D.K. Shuh, T.D. Durbin, C.W. Lo, D.A. Lapiano-Smith, F.R. McFeely and F.J. Himpsel, "Atomic Layer Epitaxy of Silicon by Dichlorosilane Studied with Core Level Spectroscopy", J. Vac. Sci. Technol. A 10, 2303-2307 (1992).
16. T.D. Durbin, D.A. Lapiano-Smith, F.R. McFeely, F.J. Himpsel and J.A. Yarmoff, "The Chemisorption and Reaction of Dichlorosilane on Ge(100) and Ge(111) Surfaces", Surf. Sci. 330, 147-155 (1995).
17. P.R. Varekamp, W.C. Simpson, D.K. Shuh, T.D. Durbin, V. Chakarian and J.A. Yarmoff, "Electronic Stucture of GaF₃ Films Grown on GaAs via Exposure to XeF₂", Phys. Rev. B 50, 14267-14276 (1994).
18. W.C. Simpson, T.D. Durbin, P.R. Varekamp and J.A. Yarmoff, "The Growth of GaF₃ Films on GaAs(110) at Elevated Temperatures Studied with Soft X-ray Photoelectron Spectroscopy", J. Appl. Phys. 77, 2751-2758 (1995).
19. W.C. Simpson, P.R. Varekamp, D.K. Shuh and J.A. Yarmoff, "Soft X-Ray Photoelectron Spectroscopy Study of the Reaction of XeF₂ with GaAs", J. Vac. Sci. Technol. A 13, 1709-1713 (1995).