3-1 3. THE MOS CAPACITOR (MOSC) The MOSC is a two terminal MOS device. We begin with this device since it is relatively simple to analyze and contains the important physics of the MOSFET channel. 3.1 FABRICATION We review the basic steps in the fabrication of a MOSC. We assume that a uniformly doped Si wafer is on hand and that a “gate-quality” oxide is to be formed. 3.1.1 Wafer Clean 1. Remove surface contaminants: metallic, organic, ionic... 2. Remove “native oxide” 3. Leave surface “smooth” The “RCA” clean used to be an industry standard for accomplishing these steps. In recent years there has been much research on more effective cleaning procedures for VLSI/ULSI. 3.1.2 Oxide Growth/Deposition Conventional (resistance heated) Rapid Thermal Oxidation (RTO) Deposited oxides (used for thick oxides, e.g., field and inter-metal, and for metal oxides (e.g. HfO2)) 3.1.3 Oxide Thickness The equivalent oxide thickness in 45 nm MOSFETs is approximately 2 nm (~ 20 Å). (We say “equivalent” because in some processes alternative oxides, such as HfO2 or HfSiO2, are used. Their electrically equivalent SiO2 thickness is 2 nm). We can get a numerical perspective on dox by considering the thickness of a state-of-the-art oxide on a more convenient scale. Si wafer thickness dSi ~ 1 mm so for a 20 Å oxide... 681021.01020xcmcmxddSiox So if a 20 Å dox is scaled to 0.13 mm (sheet of paper thickness), the Si thickness scales to3-2 mxddoxSi651046 i.e., about the height of a 10-story building. 3.1.4 Oxidation Kinetics In 1965 Deal and Grove presented a phenomenological model describing oxidation of SiO2. We review the results briefly here. Thickness equation is: d Ad B tox ox2( ) A, B = parameters related to diffusion of oxidant through existing oxide and reaction at interface = parameter accounting for initial oxide thickness di, as follows: d AdBi i2 Note that dAtA Box//21412 So for tABand t24, d Btox2 and in this thickness/time regime, d ~ t1/2 parabolic growth. For t A B24/ dBAtox~ ( ) and in this thickness/time regime we have linear growth. B = parabolic rate constant (describes oxidant diffusing through SiO2)3-3 B/A = linear rate constant (describes oxidant reacting at interface) More detailed discussion of Deal - Grove model can be found in most texts on processing (see S. Sze, "VLSI Technology", McGraw Hill, 1983. 3.1.5 Oxidation Ambients Water/Steam Oxidation proceeds relatively quickly. Oxide film contains water related species including H2O, Si - OH, Si - H. These have implications for charge trapping and oxide reliability, since hydrogen creates electron traps (more on this later). Dry Oxidation proceeds slowly and film contains fewer water related species. Note that H is abundant, so we can never have films containing no water-related species. Comparison of Deal - Grove rate constants (from S. Sze, “VLSI Technology”) T (oC) B ( m2/hr) B/A ( m/hr) Wet 1200 0.720 14.4 1100 0.287 1.27 Dry 1200 0.045 1.12 1100 0.012 0.071 Note that the equilibrium concentration of water in SiO2 (C*) is higher than that for O2; this explains the higher oxide growth rate in water-containing ambients, since B, B/A C*. For example at 1000 oC, C*(O2) = 5.2 x 1016 cm-3, C*(H2O) = 3.0 x 1019 cm-3. Pressure Steam pressure to ~ 20 atm. has been used to generate thick field oxides at low T. Dry pressure to 500 atm. has been used to grow gate oxides; however because the trend has been to lower thickness this is not a necessary technology. Henry’s Law C* PG so concentration of oxidant in oxide depends on ambient pressure. Review of Si oxidation models E. A. Lewis and E. A. Irene, “Models for the Oxidation of Si”, J. Vac. Sci. Technol. A4 (3), 916 (1986) 3.1.6 Metallization Materials Al is popular for research devices doped poly Si is more compatible with modern processing metal-silicides and refractory metals are being used more and more for their low resistivity3-4 These notes (and most device texts) will assume Al gate metal unless otherwise noted. 3.2 MOSC ELECTRICAL PROPERTIES References Tsividis, Operation and Modeling of the MOS Transistor S. Wolf, Sections 3.1 - 3.4 3.2.1 System Definition SiO2Sidepletion regionback contactgate metalVGBVox s Depletion region is the volume in which mobile carriers are not present (within the depletion approximation). Depletion region width is w, which is a function of applied voltage. Ideal Device We define an ideal device as one for which (i) there is no charge in the oxide, or at the Si-SiO2 interface, and (ii) the contact potential difference is 0. In real devices, oxide and interface defects exist, and there is a non-zero contact potential. We examine these effects later.3-5 3.2.2 Energy Band Diagram For the ideal device described above, the energy band diagram is as follows. 3.2.3 Contact Potential and Work Function Difference Two non-identical materials in contact with one another will develop a contact potential, which arises because electrons in the material with the higher average electron energy will move to the material with the lower energy. As an example, we considered the pn junction diode, which displays a contact potential difference (or “built - in potential”) Vo. For more than two materials connected in series, we can show that only the first and last enter into determination of the contact potential. Thus, in the MOSC we have a contact potential c, which we define as the voltage drop encountered in going from the metal gate, through a “short” to the substrate contact, and into the Si substrate. The contact potential results in band bending in the device even if the gate and substrate are shorted together. We may ask what gate voltage is necessary to “cancel” the contact potential, i.e., to cause the potential difference between gate and Si substrate to be 0. Evidently we need to apply a voltage equal in magnitude but opposite in sign to c. From Kirchhoff's voltage law, cSoxGBVVThus when the applied voltage VGB is 0, cSoxV. If there is no voltage drop across the oxide or the semiconductor, we have what is called the “flatband condition (FB)”; then 0SoxV in which case VGB = VFB = -c. This value of VGB is called the flatband voltage, VFB; application of VFB to the gate will result in no voltage drop across the oxide/semiconductor, and the energy bands in the semiconductor will be “flat”. In the device literature we often