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 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 oxide d ox d Si 20 x 10 8 cm 0 1 cm 2 x10 6 So if a 20 dox is scaled to 0 13 mm sheet of paper thickness the Si thickness scales to 3 1 d Si d ox 65 m 6 4 x10 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 ox2 Ad ox B t 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 i2 Ad i B Note that d ox A 2 1 t A2 4B 1 So for A2 and t 4B t d ox2 and in this thickness time regime d t1 2 Bt parabolic growth For t d ox A2 4 B B t A and in this thickness time regime we have linear growth B parabolic rate constant describes oxidant diffusing through SiO2 3 2 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 Wet Dry T oC 1200 1100 1200 1100 B m2 hr 0 720 0 287 0 045 0 012 B A m hr 14 4 1 27 1 12 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 resistivity 3 3 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 depletion region back contact Vox s gate metal SiO2 Si VGB 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 4 3 2 2 Energy Band Diagram For the ideal device described above the energy band diagram is as follows 3 25 eV EF 9 eV M SiO2 1 1 eV Si 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 VGB Vox S c Thus when the applied voltage VGB is 0 Vox S c If there is no voltage drop across the oxide or the semiconductor we have what is called the flatband condition FB then Vox 0 in which case VGB VFB c This value of VGB is called the flatband voltage S 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 encounter the metal semiconductor work function difference wms which is 3 5 wmetal wSi wms It is also customary to define a potential from this energy difference …