Papers

AVS2001 Session SC+SS+EL-WeA: Chemistry of Semiconductor Etching & Cleaning

Wednesday, October 31, 2001 2:00 PM in Room 111

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		SC+SS+EL-WeA-1 The Chemistry of Anisotropic Silicon Etching: Tackling an Old Problem with New Tricks M.A. Hines (Cornell University) Aqueous bases, such as KOH, TMAH, and NH₄F, are the most important class of industrial silicon etchants. The popularity of these etchants is driven in large part by their extreme anisotropy (i.e. their high face-specificity). Relatively little is known about the chemical origins of etchant anisotropy, though. The problem is simple. On an atomic scale, an anisotropic etchant must be highly defect selective, but the study of surface defect reactivity is notoriously difficult. In this talk, I will discuss two new approaches to studying these reactions. On an atomic scale, I will show how defect reactivity can be quantified using a combination of scanning tunneling microscopy (STM) experiments and atomistic kinetic Monte Carlo simulations. This combination yields very detailed information about site-specific reactivity. To complement these rather time-consuming studies, I will also describe a new technique, which uses microfabricated test patterns, to rapidly assay the reactivity of 180 silicon surfaces simultaneously. We use this technique to perform orientation-resolved chemical kinetics experiments. I will show that the orientation-dependence provides additional insights into chemical reactivity. New phenomena, such as orientation-dependent morphological transitions, will also be described.
2:40 PM		SC+SS+EL-WeA-3 In situ Infrared Spectroscopy of Wet Chemical Etching of Si and InP with Electrochemical Control O. Pluchery, S.B. Christman, Y.J. Chabal (Agere Systems) The fabrication of Integrated-Circuits requires many different steps, including growth or etching. Since device performance strongly depends on surface preparation and control at each step, we have developed in situ Fourier transform infrared (FTIR) spectroscopy using multiple internal reflections to monitor the nature of interfaces. At the heart of high speed silicon technology is the growth of ultra thin oxide layer on top of the Si substrate. We monitor here the structure of such thermally grown oxides by sequential etching in dilute HF. The analysis of the LO and TO vibrational phonon modes of the oxide at 1070 and 1270 cm^-1 respectively shows that the etching mechanism exhibits two kinetic regimes depending on whether the HF flow wets the surface in a static or dynamic way. For static wetting, the LO absorption of the oxide undergoes a dramatic distortion that can be related to the unusual nature of the diffuse layer in the vicinity of the oxide surface. The etching mechanism depends in effect on the competition between diffusion and kinetics in this layer. In contrast to silicon, the passivating oxides of InP substrates are rather intricate. We examine here InP wafers that are covered by a thin oxide typical of "epi-ready" wafers offered by vendors. Analysis of the FTIR spectra shows a coexistence of several phases, such In₂O₃ (850 cm^-1) and InPO₄ at higher frequencies along with mixed oxide phases at intermediate frequencies. These oxides can also be removed by etching in HCl (10 wt%). This leaves a clean but very reactive InP surface that quickly attracts contaminants so that in situ analysis is highly desirable. In addition to the in-situ spectroscopy, we have devised an electrochemical control of the surface that makes it possible to further modify the composition of the adlayer as well as the diffuse layer.
3:00 PM		SC+SS+EL-WeA-4 Atomic Hydrogen Etching in Hot Wire Chemical Vapor Deposition System of Silicon Thin Films O. Srivannavit (University of Michigan) Atomic hydrogen generated by the filament in Hot Wire Chemical Vapor Deposition System plays an important role in depositing of thin films. It is believed that the etching process by atomic hydrogen taking place during the deposition process is one of key mechanism to obtain high quality thin films. In order to see a clear effect of interaction of atomic hydrogen with the growth surface, we focused only on the etching process in this system. We used amorphous and crystalline silicon as the substrate and then monitored its etching rate. The etching rate increases with increases of filament temperature due to increasing amount of atomic hydrogen generated on the filament. When substrate temperature increases, the etching rate decreases. We believe this is due to the decrease of the surface coverage of hydrogen with the increase of substrate temperature. The etching rate increases initially with pressure increase and then remains constant with further pressure increase. This phenomenon indicates that there is the competition between the increase in the amount of the atomic hydrogen generated and decrease in the diffusion coefficient of atomic hydrogen to the etching surface when pressure increases. The etching rate increases in the order of amorphous Si > poly-Si > crystalline silicon. This effect plays a key role in the selective deposition in this system.¹ In addition, we found that there is preferential crystalline orientation etching (111) < (110) < (100) which can be used to explain the crystalline orientation during deposition of poly-Si films in this system.² This preferential etching among amorphous phase and crystalline phase with the different orientation can be explained by the amount of dangling bonds in the silicon films. ¹ S. Yu, E. Gulari and J. Kanicki, Appl.Phys. Lett., 68, 2681 (1996) ² S. Yu, S. Deshpande, E. Gulari and J. Kanicki, Mat. Res. Soc. Symp. Proc., 377, 69 (1995).
3:20 PM		SC+SS+EL-WeA-5 Single Ion Impact Effects on Semiconductor and Insulator Surfaces Induced by Slow, Very Highly Charged Ions T. Schenkel (Lawrence Berkeley National Laboratory); A.V. Hamza, J.W. McDonald, D.H. Schneider, A. Kraemer, A. Persaud (Lawrence Livermore National Laboratory) The interaction of slow (<5 keV/u), very highly charged ions, such as Xe⁴⁴⁺ and Au⁶⁹⁺, with solid surfaces is dominated by the deposition of potential energy, rather then the kinetic energy of the ions.^1,2 For Au⁶⁹⁺, the sum of the binding energies of the electrons that were removed when forming the ion is 170 keV. This energy is deposited into a nanometer scale area within about 10 fs when an Au⁶⁹⁺ ion impinges on a surface.³ In our presentation we will report on the characterization of undoped silicon after exposure to low doses (~10E11 cm^-2) slow, highly charged ions. We recently observed strong photoluminescence at ~565 nm from irradiated silicon surfaces.⁴ Possible microscopic mechanisms for this effect will be discussed. We will compare atomic force microscopy data from surface defects induced by single ion impacts on mica, self-assembled monolayers and silicon in light of model descriptions of the materials response to the impact of slow, highly charged ions. Acknowledgements: This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. Part of this work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48. ¹T. Schenkel et al, Prog. Surf. Sci. 61, 23 (1999) ²T. Schenkel, et al., Phys. Rev. Lett. 80, 4325 (1998) ³M. Hattass, et al., Phys. Rev. Lett. 82, 4795 (1999) ⁴M. W. Newman, et al., submitted for publication.
3:40 PM		SC+SS+EL-WeA-6 Ex Situ Removal of Carbon and Oxygen from a Gallium Nitride (0001) Surface F. Machuca, Z. Liu, Y Sun, R.F.W. Pease, W.E. Spicer (Stanford University); P. Pianetta (SSRL and Stanford University) We report on a chemical cleaning study of gallium nitride (GaN) using synchrotron radiation to probe the electronic structure of the semiconductor surface and the adsorbed impurities. We study sulfuric peroxide and sulfuric water chemistries for carbon and oxygen removal using the surface sensitive core level information using XPS. We report that a sulfuric peroxide wet treatment followed by a vacuum anneal at 700C reduces C and O concentrations to a few percent of a monolayer. Moreover, this is the first study achieving an atomically clean GaN surface well below the decomposition temperature by 200C. This is a direct result of a weaker form of carbon being chemisorbed to the GaN surface after the peroxide treatment and that is subsequently thermally desorbed. The chemical form is predominantly an oxide of carbon. Whereas the sulfuric water treatment leaves a residual refractive carbon on the surface of GaN in the form of hydrocarbons. These hydrocarbons persist up to the maximum annealing temperature of 740C tested. We also show that by treating the GaN surface with an aggressive oxidizing chemistry like sulfuric peroxide, there is only near monolayer coverages of oxygen. This is direct evidence for the existence of a suboxide on the GaN surface and demonstrates GaN (0001) is not an active surface for bulk oxidation. We also test the effectiveness of the annealing ambient during the thermal desorption portion of the cleaning by comparing vacuum to ammonia annealing. Our findings indicate ammonia is ineffective in aiding thermal desorption of C and O at temperatures at or below 740C, contrary to other reports. Lastly, we offer evidence for a novel oxynitride species on the GaN surface.
4:00 PM		SC+SS+EL-WeA-7 The Study of InP(100) Chemical Cleaning by Synchrotron Radiation Photoemission Spectroscopy Y Sun, F. Machuca, Z. Liu (Stanford University); P. Pianetta (Stanford Synchrotron Radiation Lab); W.E. Spicer (Stanford University) The activation process for GaAs negative electron affinity (NEA) photoemitters has been studied extensively. However, the surface chemistry of other NEA materials such as InP is sufficiently different from that of GaAs that additional study is warranted on all aspects of the process starting from the initial surface cleaning to the final activation step. This work will concentrate on the preactivation clean in which the the surface species will be quantified using photoelectron spectroscopy. The goal of this work is to develop clean starting surface that will be used in subsequent activation studies. The cleaning process has three steps, the first two taking place in an argon purged glove bag attached to the load lock of the vacuum system to eliminate atmospheric contamination. Synchrotron radiation is used for the photoemission in order to obtain the necessary surface sensitivity and resolution for the In 4d, P 2p, C 1s and O 1s core levels as well as the valence band. In our most effective cleaning process, the InP is first etched in 4:1:100 H₂SO₄:H₂O₂:H₂O and results in a surface with 0.5-1 monolayers of In and P oxides and 0.5-1 ML of C contamination. Note that this is in contrast with GaAs in which this same etching step leaves elemental As and Ga suboxide and thus only requires a subsequent heat treatment to achieve a clean surface. For InP, a second oxide etching step is therefore required. This can either use a 9% HCl or a 1:1 H₂SO₄:H₂O solution both of which result in a hydrophobic surface with 0.3 ML of elemental P, 0.1 ML of C and complete removal of both the P and In oxides,. The lack of any significant amounts of S or Cl on the surface leads us to postulate that this surface is P terminated. Finally, a 360°C anneal in UHV gives a stoichiometric InP surface with no elemental P and only 0.05 ML C. These surfaces are now suitable for similar detailed studies of the full NEA activation process.
4:20 PM		SC+SS+EL-WeA-8 Ion Irradiation Induced Spontaneous Nanoscale Corrugation on Silicate Glasses C.C. Umbach (Cornell University); R.L. Headrick (Cornell High Energy Synchrotron Source); K.-C. Chang, J.M. Blakely (Cornell University) Grazing incidence x-ray scattering was used to determine the temperature and ion-energy dependence of nanoscale corrugations that form on an amorphous SiO₂ surface eroded by Ar⁺ ions. The corrugations have wavelengths between 20 and 200 nm with amplitudes of 1 nm. The corrugation wavelength λ^* shows a nearly linear dependence on ion energy for ion energies between 0.5 and 2 keV. Between room temperature and ~300° C, λ^* depends weakly on temperature and above ~300° it shows an Arrhenius-like increase. Ion-assisted viscous relaxation in a thin surface layer is shown to be the dominant smoothing process during erosion;the rate of viscous smoothing scales as (λ^*)^-4. Similar ion-induced corrugations have also been observed on aluminoborosilicate glasses.