How to turn an accident into a great experiment Chapter 6 OXIDATION How to turn an accident into a great experiment
1955 - Development of Oxide Masking “… Bell Labs researchers encountered a major problem with pitting on the surface of silicon wafers during high-temperature diffusion. This problem was overcome by chemist Carl Frosch during a serendipitous accident in which the hydrogen gas carrying impurities through the diffusion furnace briefly caught fire, introducing water vapor into the chamber. The resulting “wet-ambient” diffusion method had covered the silicon surface with a layer of glassy silicon-dioxide (SiO2).” From http://www.computerhistory.org/semiconductor/timeline/1955-Oxide.html
Introduction Si is unique in that its surface can be passivated with an native oxide Layers are easily grown thermally It has few defects It is stable over time Its mechanical and electrical properties are almost ideal Good adhesion Prevents the penetration (in-diffusion) of dopants Resistant to most chemicals used in fabrication Easily patterned and etched with specific chemicals or dry etched with plasmas
Thermal oxide properties Amorphous lattice constant of quartz ~0.5nm Density: 2.27g/cm3 Dielectric constant: 3.9 DC resistivity @ 25C: 1016-cm Energy gap: 9eV Thermal conductivity: 1.3W/m-C Refractive index: 1.46 Melting point: 1700C Molecular weight: 60.08g/cm3 Molecules: 2.3x1022/cm3 Specific heat: 1 J/g-C Film stress: Compressive 0.2-0.4GPa Etch rate: 6:1 BOE 100nm/min
http://www. enigmatic-consulting
Introduction Oxide is used for several applications Gate dielectric Mask against ion implantation or diffusion For isolation of various lateral regions As insulator between various metal layers
Introduction
NTRS Roadmap
Introduction Si will oxidize at room temperature 0.5 - 1 nm (5- 10 Å) thick layer forms in about 5 minutes The reactions slows and stops after a few hours when the layer is 1 – 2 nm thick Properties of the oxide depends on the surface treatment and crystalline structure Which plane of the unit cell is exposed
Introduction The most critical application is the gate oxide 2.5 – 5 nm thick Projected to be < 1 nm in a few years Will sustain about 50% of the theoretical breakdown voltage of the bulk material of 10 – 15 MV/cm Thicknesses must be controlled to about 1 atomic layer
Introduction Amorphous arrangement of atoms means that there is a possibility that multiple Si atoms will be connected Thin ‘wires’ of Si within SiO2 layer enables leakage currents to flow When films get this thin, quantum mechanical effects including tunneling become important Finite probability that an electron can penetrate through an energy barrier Tunneling is usually undesirable, but some devices are now built using this phenomenon (nonvolatile memory)
Basic Concepts Oxide grows by diffusion of oxygen/H2O through the oxide to the Si/SiO2 interface Thus, a new interface is continuously growing and moving into the Si wafer The process is known as: Dry oxidation when oxygen only is used. Wet oxidation when water vapor (with or without oxygen) is used.
Basic Concepts
Basic Concepts The process involves an expansion the density of an equal volume of Si occupies less space than a volume of oxide containing the same number of Si atoms Nominally, the oxide would like to expand by 30% in all directions; but it cannot expand sideways because it is constrained by the Si atoms Thus, there is a 2.2 expansion in the vertical direction In figure 6-4, note the growth of the LOCOS (Local Oxidation of Silicon) oxide above the surface Also note the “bird’s beak” of oxide under the nitride layer – a stress-induced rapid growth of oxide
Basic Concepts
Basic Concepts
Basic Concepts If there are shaped surfaces where oxide must grow, this expansion may not be so easily accommodated The oxide layers are amorphous (i.e., there is only short range order among the atoms) There are no crystallographic forms of SiO2 that match the Si lattice The time required for transformation to a crystalline form at device temperatures is very very long
Basic Concepts The oxide that grows is in compressive stress This stress can be relieved at temperatures above 1000oC by viscous flow There is a large difference in the TCE (thermal coefficient of expansion) between Si and SiO2 This increases the compressive stresses in the oxide and results in tensile stresses in the Si near its surface Si is very thick while the oxide is very thin Si can usually sustain the stress Since the wafer oxidizes on both sides, the wafer remains flat; if you remove the oxide from the back side, you will see a warping of the wafer The stress can be measured by measuring the warp of the wafer
Basic Concepts The electrical properties of the Si/SiO2 interface have been extensively studied To first order, the interface is perfect The densities of defects are 109 – 1011 /cm2 as compared to Si atom density of 1015 /cm2 Most defects are associated with incompletely oxidized Si Deal (1980) suggested a nomenclature that is now used to describe the various defects
Defect Nomenclature
Defect Nomenclature There are four type of defects Qf is the fixed oxide charge. It is very close (< 2 nm) to the Si/SiO2 interface Surface concentration of 109 –1011/cm2 Related to the transition from Si to SiO2 Incompletely oxidized Si atoms Positively charged and does not change under normal conditions
Defect Nomenclature Qit is the interface trapped charge Appears to incompletely oxidized Si with dangling bonds Located very close to the interface Charge may be positive, neutral, or negative Charge state may change during device operation due to the trapping of electrons or holes Energy levels associated with these traps are distributed throughout the forbidden band, but there seem to be more near the valence and conductions bands Density of traps is 109—1011 cm-2 eV-1
Defect Nomenclature Qm is the mobile oxide charge It is not so important today but was very serious in the 1960’s It results from mobile Na+ and K+ in the oxide Shift in VTH is inversely proportional to COX and thus, as oxides become thinner, we can tolerate more impurity
Defect Nomenclature Qot is charge trapped anywhere in the oxide Broken Si-O bonds in the bulk oxide well away from the interface by ionizing radiation or by some processing steps such as plasma etching or ion implantation Metal ions from surface of Si or introduced during growth Fe, Mn, Cr, Cu Normally repaired by a high-temperature anneal They can trap electrons or holes This is becoming more important as the electric field in the gate oxide is increased They result in shifts in VTH
Defect Nomenclature All four types of defects have deleterious effects on the operation of devices High temperature anneals in Ar or N2 near the end of process flow plus an anneal in H2 or forming gas at the end of process flow are used to reduce their effect
Manufacturing Methods Furnace capable of 600 – 1200 oC with a uniform zone large enough to hold several wafers Gas distribution system to provide O2 and H2O Generally, H2 is burnt with O2 at the entrance of the furnace to create water vapor TCA or HCl may be used to remove metal ions Control system that holds the temperatures and gas flows to tight tolerances (0.5 C)
PRODUCTION FURNACES Commercial furnace showing the furnace with wafers (left) and gas control system (right).
PRODUCTION FURNACES Close-up of furnace with wafers.
PRODUCTION FURNACES
Models The first major model is that of Deal and Grove (1965) This lead to the linear/parabolic model Note that this model cannot explain the effect of oxidation of the diffusion rate the oxidation of shaped surfaces the oxidation of very thin oxides in mixed ambients The model is an excellent starting place for the other more complicated models
CHEMICAL REACTIONS Process for dry oxygen Si + O2 SiO2 Process for water vapor Si + 2H2O SiO2 + 2H2
OXIDE GROWTH Si is consumed as oxide grows and oxide expands. The Si surface moves into the wafer. Original surface 54% SiO2 46% Silicon wafer
MODEL OF OXIDATION Oxygen must reach silicon interface Simple model assumes O2 diffuses through SiO2 Assumes no O2 accumulation in SiO2 Assumes the rate of arrival of H2O or O2 at the oxide surface is so fast that it can be ignored Reaction rate limited, not diffusion rate limited
Deal-Grove Model of Oxidation Fick’s First Law of diffusion states that the particle flow per unit area, J (particle flux), is directly proportional to the concentration gradient of the particle. We assume that oxygen flux passing through the oxide is constant everywhere. F1 is the flux, CG is the concentration in the gas flow, CS is the concentration at the surface of the wafer, and hG is the mass transfer coefficient
J Distance from surface, x N No Ni Silicon dioxide SiO2 Si Xo
Deal-Grove Model of Oxidation Assume the oxidation rate at Si-SiO2 interface is proportional to the O2 concentration: Growth rate is given by the oxidizing flux divided by the number of molecules, M, of the oxidizing species that are incorporated into a unit volume of the resulting oxide:
Deal-Grove Model of Oxidation The boundary condition is The solution of differential equation is
Deal-Grove Model of Oxidation xox : final oxide thickness xi : initial oxide thickness B/A : linear rate constant B : parabolic rate constant
There are two limiting cases: Very long oxidation times, t >> xox2 = B t Oxide growth in this parabolic regime is diffusion controlled. Very short oxidation times, (t + ) << A2/4B xox = B/A ( t + ) Oxide growth in this linear regime is reaction-rate limited.
Deal-Grove Model of Oxidation