MICROCHIP MANUFACTURING by S. Wolf

Transcription

1 MICROCHIP MANUFACTURING by S. Wolf Chapter 13: THERMAL- OXIDATION of SILICON 2004 by LATTICE PRESS

2 Chapter 13: THERMAL-OXIDATION of SILICON n CHAPTER CONTENTS Applications of Thermal Silicon-Dioxide Physical Properties of Silicon-Dioxide Modeling the Growth of Silicon-Dioxide Films The Deal-Grove Model Secondary Effects that Impact the Oxide Growth Process Growth of Thin Oxide Films The Si/SiO 2 Interface Dopant-Redistribution during Oxidation Oxidation Systems Horizontal Furnaces Vertical Furnaces RTP Systems Oxide Film-Thickness Characterization Summary of Key Concepts MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-2

3 Chapter 13: THERMAL-OXIDATION OF SILICON n APPLICATIONS OF THERMAL SiO 2 Thermally-grown SiO 2 films are employed in IC applications in the thickness range of nm. These films are used as: Gate Oxides in MOSFETs Capacitor Dielectrics in DRAMS Passivation layers on silicon devices Tunnel-oxides in non-volatile memories Field oxides in LOCOS structures As MOSFETs are scaled the gate-oxide thickness must be reduced! MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-3

4 n THE PHYSICAL PROPERTIES OF THERMAL SiO 2 SiO 2 can exist in crystalline or amorphous form. Thermally-grown SiO 2 (fused-silica) on Si-wafers is amorphous. Fused-silica has the following properties: Density = 2.2 g/cm 3 Dielectric constant = 3.9 Melting point ~1700 C Dielectric strength = 10x10 6 V/cm The microscopic structure of SiO 2 is a tetrahedron, with a Si atom surrounded by 4 oxygen atoms.these tetrahedra are joined to each other by bridging oxygen atoms. However, in fusedsilica, some tetrahedra are not joined, resulting in non-bridging oxygens. Impurities in SiO 2 cause more non-bridging oxygens. Atomistic structure of thermal SiO 2 Model of impurity incorporation in fused-silica MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-4

5 n MODELING THE GROWTH OF SILICON DIOXIDE FILMS T he basic chemical reaction of silicon & oxygen is: Si (solid) + O 2 (vapor) --> SiO 2 (solid) (Dry Oxidation) Another way to grow silicon dioxide is to react Si & water vapor: Si (solid) + 2H 2 O (vapor) --> SiO 2 (solid) + 2H 2 (Wet Oxidation) The reaction occurs at the silicon/silicon-oxide interface, which means that the oxidizing species must diffuse thru the existing oxide film to reach this interface, and react there. The thickness of silicon that is consumed by the formation of the growing oxide is 44% of the oxide film-thickness. For example, if a 1000-Å-thick layer of SiO 2 is grown on a Si-substrate, this will consume 440Å of the silicon-substrate. MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-5

6 n MODELING THE GROWTH OF SILICON DIOXIDE FILMS: THE DEAL-GROVE (or LINEAR-PARABOLIC) MODEL The Deal-Grove Model accurately predicts oxide growth rates of oxide-films of any thickness (except very-thin films) It assumes that three fluxes are involved in the process: F 1 = h g (C G - C o ) F 2 = D N/ x = D (C o - C i /x ox ) F 3 = k s C i Note: x ox = oxide thickness Under steady-state conditions, F 1 = F 2 = F 3, so: C i = C*/ (1 + [k s /h] + [k s x ox C*/ (1 + [k s x ox /D]) C o = C*(1 + k s x ox /D)/(1 + [k s /h] + [k s x ox C* Note that a simplification is made in deriving the above expressions. That is, F 1 is neglected (which turns out to be a very good approximation). MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-6

7 n MODELING THE GROWTH OF SILICON DIOXIDE FILMS: THE DEAL-GROVE MODEL (Continued) Combining the two previous equations we get: dx ox /dt = F/N 1 = DC * x ox /N 1 (D + k s x ox ) Integrating this equation (see Appendix A in text), results in the Deal-Grove Model: x ox 2 +Ax ox = B (t + t) where: B = 2DC * /N 1 (parabolicand rate constant) C * k S /N 1 (linear-rate and constant) t = x 2 i + Ax i /B where x i = thickness of any oxide present at the start The Deal-Grove Model can also be written as: x ox = A/2 { 1 + (t + t)/[a 2 /4B] - 1} MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-7

8 n MODELING THE GROWTH OF SILICON-DIOXIDE FILMS: THE DEAL-GROVE MODEL (Continued): Linear and Parabolic Growth-Rate Regimes: For thin oxides B /A(t + t) For thick oxides B (t + t) The rate constants B and B/A have physical meaning (oxidant diffusion-rate and interface reaction-rate, respectively). B = C1 exp (-E1/kT) and B/A = C2 exp (-E2/kT) By using these two relations - and the data in the figure shown here - the values of B and B/A at any tempplots of B and B/A using the values erature can be calculated. given in the Table above MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-8

9 n MODELING GROWTH OF SILICON DIOXIDE FILMS: THE DEAL-GROVE MODEL (Continued): The values of B and B/A are also often tabulated for specific temperatures - as is done in Chap. 13 of the text. In any case, using the values of B and B/A in the Deal- Grove Model - together with the appropriate temperatures and times - the resulting film-thickness of SiO 2 is calculated Calculated oxide thickness for (100)-Si in dry-o 2, based on Deal-Grove Model Calculated oxide thickness for (100)-Si in H 2 O, based on the Deal-Grove Model MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-9

10 n MODELING GROWTH OF SILICON DIOXIDE FILMS: THE DEAL-GROVE MODEL (Continued): Factors That Impact Oxide Growth Rates There are two groups: primary - (the most important) and secondary (less important, but still significant) Primary: Temperature - Time - Wet or Dry (Wet oxidation is faster) Secondary: Doping in Si substrate Higher doping in Si increases growth rate Oxygen pressure Higher pressure gives faster growth Dilution by nitrogen or argon is slower Si surface orientation <111>-Si > <100>-Si Chlorine in the oxidizing ambient increases growth rate Oxidation rateconstants for dry-oxidation as a function of phosphorus doping level at 900 C. Oxidation rate as a function of oxygen partial-pressure in dry O 2 -Ar mixtures. MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-10

11 n MODELING GROWTH OF SILICON-DIOXIDE FILMS: THE DEAL-GROVE MODEL (Continued): Modeling the Growth of Very-Thin Oxide Films The Deal-Grove Model fails to accurately predict the growth of oxide films less than about 250Å-thick (25-nm-thick) when they are grown in dry oxygen Since gate-oxides of MOSFETs with gate-lengths smaller than 0.5-microns are less than 10-nm-thick, this is a serious limitation To overcome this problem, a modification to the Deal-Grove Model was made by Massoud. A term is added to the Deal- Grove Model which works well: dx ox /dt = {B / [2x ox + A]} + C exp (-x ox / L) The second term gives a higher dx ox /dt during initial growth 7-nm. Thus, the second term vanishes for thicker oxides, reverting the equation to the original Deal-Grove Model. Because this fix is simply implemented along with the Deal-Grove Model, this is the approach used in processsimulation programs (such as SUPREM) to model very-thin oxide growth. Good agreement with experimental results is found. MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-11

12 n THE Si/SiO 2 INTERFACE Besides the characteristics of the SiO 2 -bulk, the properties of the Si/SiO 2 -interface also play a crucial role when the SiO 2 functions as the gate-dielectric of a MOSFET. Four types of charges are associated with the Si/SiO 2 interface: Mobile charge (due to Na ions) Interface-trap charge Fixed-oxide charge Oxide-trap charge MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-12

13 n DOPANT REDISTRIBUTION DURING OXIDATION The thermal-oxidation process impacts the doping concentration in the Si-substrate near the Si surface. This phenomenon is called dopant redistribution. Dopant redistribution is determined by two factors: The segregation coefficient, m The dopant-diffusivity in the Si and in the oxide Four cases occur: m < 1 & diffusion in SiO 2 is slow m < 1 & diffusion in SiO 2 is fast m > 1 & diffusion in SiO 2 is slow m > 1 & diffusion in SiO 2 is fast MICROCHIP MANUFACTURING The effect of thermal oxidation on the impurity-segregation at the Si/SiO 2 interface by LATTICE PRESS Sunset Beach CA 13-13

14 n PRODUCTION SYSTEMS FOR GROWING SiO2 FILMS: FURNACES Oxide films have traditionally been grown in oxidation furnaces. These are batch-systems that can process over 100 wafers at a time. Their subsystems include: - Furnace Cabinet - Heating Elements - Fused-Silica Tubes - Measuring & Control Thermocouples - Load-Station - Fused-Silica Paddles & Boats - Temperature Control System - Source Gas Cabinet & Gas Delivery System Horizontal Furnace MICROCHIP MANUFACTURING Vertical Furnace 2004 by LATTICE PRESS Sunset Beach CA 13-14

15 n PRODUCTION SYSTEMS FOR GROWING SiO2 FILMS: RAPID-THERMAL PROCESSING (RTP) SYSTEMS Rapid-Thermal Processing (RTP) systems are single-wafer tools in which the process-temperature is ramped up and down very rapidly (e.g., C/sec). Thus, RTP can heat a wafer from room-temperature to 1100 C in a few seconds. Left: Drawings of various RTP reaction-chambers Right: RTP reactionchamber heated with a honeycomb array of lamps. Photo courtesy of Applied Materials. MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-15

16 n SILICON DIOXIDE FILM THICKNESS MEASUREMENTS Color-chart for thermal SiO2 films Using reflected-light and interferenceeffects to measure oxide film thickness An ELLIPSOMETER is an instrument used to measure very-thin films of SiO2 A high-resolution TEM photograph of an 2.4-nm thick SiO2 film grown on a Si substrate MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-16

17 n SUMMARY OF KEY CONCEPTS Thermal oxidation has been a key element of silicon technology since its inception. Silicon-doxide is one of magical materials that has enabled silicon technology to dominate ICs. Thermally, chemically, mechanically, and electrically-stable SiO 2 layers on silicon distinguish silicon from other possible semiconductors The Deal-Grove Model accurately predicts the growth process of SiO 2 films (except for very-thin films). Very-thin gate-oxide films are critical for fabricating deepsubmicron MOSFETs. Such films must have very uniform thickness and high gate-oxide integrity (GOI). The simple Deal-Grove Model has been extended to include 2D-effects, high dopant concentrations, mixed ambients (e.g., Cl in the growth ambient), and thin oxides. Process simulators (such as SUPREM) today include these physical effects, and are quite powerful in predicting oxidation geometry. Silicon-dioxide films are grown today in vertical furnaces and RTP systems. Such tools enable the oxide films needed in deep-submicron ICs to be reliably fabricated. MICROCHIP MANUFACTURING 2004 by LATTICE PRESS Sunset Beach CA 13-17