ECE_220A_Lecture-14_Nov_8_2011-slides - = 1 2 3 4 Clusters...

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Unformatted text preview: = 1. 2. 3. 4. Clusters of atoms nucleate to form islands Some islands are ‘unstable’. Stable islands increase in size. Growing islands meet at grain boundaries. Is this film structure stable? What happens if you put additional energy into the film? Zone 1: amorphous, porous Zone 2: larger grain size, columnar T: smaller grains Zone 3: large 3D grains Effect of deposition conditions on film structure From Microelectronic Materials, CRM Grovenor HW-3 Discussion Position 1 Positions 2 and 3 Aluminum only • small change in R after sintering • resistivity 10x ‘quoted’ values • 500 microns by 2.6 mm How much of a change in Al resistivity would you expect to see, after sintering? Suppose resistivity started at 10 -5 ohm-cm Decreased by a factor of 5 to 5 x 10-6 ohm-cm Aluminum only • small change in R after sintering • resistivity 10x ‘quoted’ values • 500 microns by 2.6 mm Sintering: for grain growth 450 C, 15-30 minutes = Sintering: what happens at the interface? 450 C, 15-30 minutes A method to prevent excessive diffusion of Si into Al? 450 C, 30 min. in H2 Sze, VLSI Technology 2nd Edition, p. 406 Discuss PR profiles What are the key elements of this ‘new’ fabrication technology? • • 1. 2. 3. 4. 5. Making a pattern (template) Transferring that template into your material Lithography Metalization and making contact to the outside world Defining local electronic behavior: doping Isolating electronic regions: oxidation Carving out different regions of the material: etching Wet Chemical Etches: provide different etched profiles Isotropic etch Crystallographic etch Figure 11.7 (100) silicon wafers after directional etching in KOH, isopropyl alcohol, and water. The upper photo shows a 50-µm-deep etch. The lower photographs are of 80-µm-deep trenches etched at 10-µm pitch on (110) and 10° off (110) (after Bean [21], ©1978 IEEE). Fabrication Engineering at the Micro- and Nanoscale Campbell Copyright © 2009 by Oxford University Press, Inc. 3-component Etch of Silicon Etching generally a combination of oxidation (breaking bonds) and making the oxidized form soluble in solution Si + 2 h+ -> Si 2+ Where do we get the holes? HNO3 + HNO2 -> N2O4 + H2O N2O4 = 2 NO2 2NO2 = 2 NO2- + 2h+ 2 NO2- + 2H+ = 2HNO2 Where do we get H+? H2O = OH- + H+ Total: Si2+ + 2 (OH)- -> Si (OH)2 Si(OH)2 -> SiO2 + H2 How do we make the oxide soluble? SiO2 + 6HF -> H2SiF6 + 2H2O Si + HNO3 + 6HF → H2SiF6 + HNO2 + H2 + H2O Etch Rate of Silicon in HF/HNO3/dilutent HNO3:HF:CH3COOH:2:6:2 Figure 11.5 Intersection lines imposed on Figure 11.4A to show etch rate. A Schematic View of the Etch Process Etch behavior with diffusion characteristics Etch behavior with a reaction characteristics time dependence Howes & Morgan Diffusion-limited etching Material-transport limited Can provide mirror-like surfaces Howes & Morgan Gallium Atsenide: Material, Devices and Circuits Diffusion-limited, effects of mass transport seen in etch profiles Now to gas-phase etching Etching of GaSb in 2% Br solution Tan et al., Diffusion Limited Chemical Etching Effects in Semiconductors, Solid State Electronics, 38, p 17 (1995) Simulated etched profile GaAs etch Figure 11.6 The etch rate of GaAs in H2SO4,H2O2, and H2O. The right leg is the concentration of H2SO4, the bottom leg is H2O, and the left leg is H2O2. All scales increase in the clockwise direction (after Iida and Ito, reprinted by permission of the publisher, The Electrochemical Society Inc.). Selectivities possible with chemical etchants According to material According to Doping Runyan & Bean, Semiconductor Integrated Circuit Processing Dubrovko Babic Dry Etching Etch rate of Si, using Ar+ AND XeF2 MUCH GREATER than sum of etch rates using either Ar+ or XeF2 alone. Coburn & Winters, J. Appl. Phys. 50, 3189 (1979) Chlorine greatly enhances etch rate of Silicon in Ar+ Fluorine RETARDS etch rate of Al in Ar+ What Can an Ion Do to a Substrate? Mechanism of Etching 1. Adsorption or chemisorption 3. Desorption of SiF4 (volatile) F+ 2. Reaction of F+ and Si How do ions enhance the chemical etch rate? An Etch Mechanism Figure 11.10 Species concentration in a CF4 plasma as a function of the amount of oxygen in the feed gas (after Smolinsky and Flamm, reprinted by permission, AIP). Figure 11.11 Schematic diagram of a high pressure anisotropic etch showing the formation of sidewall passivating films. Selective Etching Figure 11.12 Etch rate of Si and SiO2 in (A) CF4/O2 plasma (after Mogab et al. [59], reprinted by permission, AIP), and (B) CF4/H2 plasma (after Ephrath and Petrillo [100], reprinted by permission of the publisher, The Electrochemical Society Inc.). Figure 11.13 Etch profiles of SiO2 with increasing concentration NF3 (after Donnelly et al., reprinted by permission, AIP). ...
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