Figure Process flow from starting material to polished wafer.

Transcription

1 Figure Process flow from starting material to polished wafer. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 1 Starting material: silicon dioxide (SiO ): pure form of sand (quartzite) Metallurgical-grade silicon; purity (98%): the sand is placed in a furnace with various form of carbon: SiC(solid)+SiO (solid) Si (solid) + SiO(gas)+CO(gas) The silicon is pulverized and treated with HCl to form trichlorosilane (SiHCl 3 ) which is liquid at room temperature (boiling point=3 C): Si(solid)+3HCl(gas) 300 C SiHCl 3 (liquid)+h (gas) Fractional distillation of the liquid to remove impurities Reduction reaction of the purified SiHCl 3 SiHCl 3 (gas)+h (gas) Si (solid)+3hcl(gas) this reaction takes place in a reactor containing a resistance-heated silicon rod which serves as the nucleation point for the deposition of silicon Electronic grade silicon (EGS): polycristalline, high purity (10-9 ) 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI

2 Figure.8. Simplified schematic drawing of the Czochralski puller. Clockwise (CW), counterclockwise (CCW). 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 3 Czochralski-grown silicon single crystal In crystal growth, a known amount of dopant is added to the melt to obtain the desired doping concentration in the grown crystal. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 4

3 Equilibrium segregation coefficient : Dopant conc. in solid C k 0 = = Dopant conc. in liquid int erface k 0 <1 during growth the dopant are rejected into the melt the melt becomes progressively enriched with the dopant s l M 0 =initial weight of melt C 0 =initial doping concentration C s =doping concentration in the crystal C l =doping concentration in the liquid S=amount of doping remaining in the melt ds = - Cs dm; C l =S/(M 0 -M) Dopant B P As k type p n n C s = k 0 C 0 1 M M 0 k 0 1 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 5 Figure Curves for growth from the melt showing the doping concentration in a solid (C s /C 0 ) as a function of the fraction solidified (M/M 0 ) for several segregation coefficients (k 0 =C s /C l ). 4 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 6

4 While the crystal is growing, dopants are constantly being rejected into the melt, a concentration gradient will develop at the interface Segregation coefficient: k 0 =C s /C l (0) Effective segregation coefficient k e =C s /C l Usually: k e > k 0 To get uniform doping concentration (k e 1) high pull rate low rotation speed Add ultrapure polycristalline silicon continuously to the melt so that the initial doping concentration is maintained Figure Doping distribution near the solid-melt interface. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 7 Figure 10.6a. Float-zone process; Schematic setup. Float zone process to grow Si with lower contamination than Czochralski technique. Materials with higher resistivities No contamination from the crucible Float zone technique is used for high power, high voltage device. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 8

5 Figure Curves for the float-zone process showing doping concentration in the solid as a function of solidified zone lengths. 4 L is the length of the molten zone at a distance x along the rod k e =effectivesegregation coefficient 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 9 Figure Relative impurity concentration versus zone length for a number of passes. L denotes the zone length. 4 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 10

6 To obtain homogeneous distribution of dopants, a float zone silicon slice with a low doping concentration is irradiated with thermal neutrons: Si14 + neutron Si14 + γ ray P15 + β ray Half life of Si 31 is.6 hours Figure (a) Typical lateral resistivity distribution in a conventionally doped silicon. (b) Silicon doped by neutron irradiation. 5 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 11 Wafer Shaping Grinding of the surface to define the diameter of the material Flat regions along the length of the ingot to mark the specific crystal orientation and the conductivity type The ingot is sliced by diamond saw into wafers: surface orientation, hickness, taper(wafer thickness variation from one end to another), bow (surface curvature of the wafer). The wafer is lapped using a mixture of Al O 3 and glycerine to produce a typical flatness uniformity around µm. The damaged and contaminated regionscan be removed by chemical etching to provide a smooth and specular surface. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 1

7 Figure Identifying flats on a semiconductor wafer. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 13 CRYSTAL DEFECTS: POINT DEFECTS any foreigh atom incorporated into the lattice at a substitutional site or interstital site A missing atom in the lattice creates a vacancy Frenkel defect: a host atom which is situated between regular lattice sites and adjacent to a vacancy 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 14

8 CRYSTAL DEFECTS: LINE DEFECTS or DISLOCATIONS Edge dislocation: there is an extra plane of atoms AB inserted into the lattice Screw dislocation: produced by cutting the crystal partway and pushing the upper part one lattice spacing over Edge dislocation in a cubic lattice Screw dislocation in a cubic lattice 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 15 CRYSTAL DEFECTS: AREA DEFECTS Twins: a change in the crystal orientation across a plane Grain boundary: a transition between crystals having no particular orientational relationship to one another Appear during the crystal growth Stacking fault: the stacking sequence of atomic layer is interrupted 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 16

9 Figure Stacking fault in semiconductor. (a) Intrinsic stacking fault. (b) Extrinsic stacking fault. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 17 CRYSTAL DEFECTS: VOLUME DEFECTS Precipitates of impurities or dopant atoms because of the inherent solubility of the impurity in the host lattice. The solubility of most impurities decreases with decreasing temperature. If an impurity is introduced to the max.. Concentration allowed by its solubility and the crystal is then cooled, an equilibrium state is achieved by precipitating the impurity atoms in excess of the solubility level. The volume mismatch between the host lattice and the precipitates results in dislocations. Semiconductor Devices, /E by S. M. S Copyright 00 John Wiley & Sons. Inc. All righ reserve Figure Solid solubilities of impurity elements in silicon. 11 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 18

10 Property MATERIAL PROPERTIES: CzochralskiFloat zone ULSI ρ (P) n-type (Ω cm) and up 4-40 and up ρ(b) p-type (Ω cm) and up 4-40 and up ρ gradient (%) <1 τ(µs) Oxygen (ppma) 5-5 not detected Carbon (ppma) <0.1 Dislocation (per cm ) <500 <500 <1 Diameter (mm) up to 00 up to 100 up to 300 Slice bow (µm) <5 <5 <5 Slice taper (µm) <15 <15 <5 Surface flatness (µm) <5 <5 <1 Heavy metal impurity (ppma) <1 <0.01 < /11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 19 Dissolution of oxygen from the silica crucible and transport of carbon to the melt from the graphite susceptor during crystal growth. Carbon atoms in silicon occupy substitutional lattice sites. Formation of defects Oxygen act as donor, distorting the resistivity -> unintentional doping Oxygen in an interstitial lattice site can increase the yield strength of silicon Gettering -> thermal treatment-> oxygen evaporates-> lowers the oxygen content at the surface (denuded zone). Further thermal cycles to promote the formation of oxygen precipitates in the interior of the wafer for gettering impurities. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 0

11 Figure Denuded zone width for two sets of processing conditions. Inset shows a schematic of the denuded zone and gettering sites in a wafer cross section. 1 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 1 Figure Three common susceptors (graphite) for chemical vapor disposition Mechanism of CVD: The reactants are transported to the substrate region Transfer to the substrate surface where they are absorbed A chemical reaction occurs, catalyzed at the surface, followed by growth of the epitaxial layer The gaseous products are desorbed into the main gas stream The reaction products are transported out of the reaction chamber Pancake susceptor Horizontal susceptor Barrel susceptor 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI

12 Sources used for silicon CVD (or VPE) growth: Silicon tetrachloride SiCl 4 ; dichlorosilane SiH Cl ; trichlorosilane SiHCl 3 ; silane SiH 4. Main reaction (temperature 100 C) : SiCl Additional competing SiCl 4 4 (gas) + H (gas) (gas) + Si(solid) Si(solid) + 4HCl(gas) reaction : SiCl (gas) If the SiCl 4 concentration is too high, etching rather than growth of silicon will take part. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 3 Figure Effect of SiCl 4 concentration on silicon epitaxial growth. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 4

13 Sources used for silicon doping: P-type: diborane (B H 6 ) N-type: phosphine (PH 3 ) and arsine (AsH 3 ) Diluent gas: hydrogen High temperature are needed to give sufficient mobility to adsorbed atoms for finding their proper position 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 5 Figure Schematic illustration of (a) lattice-matched, (b) strained, and (c) related heteroepitaxial structures. 19 Homoepitaxy is structurally identical to the lattice-matched heteroepitaxy. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 6

14 Figure Illustration of the elements and formation of a strained-layer superlattice. 17 Arrows show the direction of the strain. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 7 Figure Schematic cross section of a metal-oxide-semiconductor fieldeffect transistor (MOSFET). 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 8

15 Figure 11.. Schematic cross section of a resistance-heated oxidation furnace. Oxidation temperature : C; gas flow rate = 1000 sccm 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 9 Si(solid) + O (gas) Si(solid) + H O SiO SiO (solid) (solid) + H (gas) Figure Growth of silicon dioxide by thermal oxidation. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 30

16 For SiO thickness = 100 nm what is the Si thickness being consumed A Si =8.9 g/mole ; ρ Si =.33 g/cm 3 ; A SiO =60.8 g/mole ; ρ Si =.1 g/cm 3 ; Molar volume: V si =8.9/.33 cm 3 /mole =1.06 cm 3 /mole; V sio =60.8/.1 cm 3 /mole =7.18 cm 3 /mole; 1 mole of Si is converted in 1 mole of SiO (Si thickness) x area (SiO thickness) x area (Si thickness) = (SiO thickness) (Si molar volume) = (SiO molar volume) = = A silicon layer 44 nm is consumed 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 31 Basic structural unit of silicon dioxide Two-dimensional representation of a quartz crystal lattice. Two-dimensional representation of the amorphous structure of SiO (silica). ρ silica =.1 g/cm 3 ;ρ quartz =.65 g/cm 3 The silica structure is quite open because only 43% of the space is occupied by SiO molecules; this accounts for the lower density and allows impurities (e.g. Na) to enter and diffuse 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 3

17 Oxide thickness : x D C 0 k (t + τ) = 1+ 1 k D C1 C 0 =surface conc. of oxidants F 1 =flux of oxidants through SiO F =flux of oxidants through Si C 1 =conc. Of oxidants in the oxide Early stages: x varies linearly with time; surface reaction is rate limiting B x = (t + τ) A As the oxide layer becomes thicker, the reaction becomes diffusion limited x = B (t + τ) B x = (t + τ) A Figure Basic model for the thermal oxidation of silicon. 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 33 Figure Linear rate constant versus temperature. Dependence on crystal orientation Figure Parabolic rate constant versus temperature. YES NO Thin oxide (gate oxide) dry oxidation Thick oxide (field oxide) wet oxidation 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 34

18 Figure Experimental results of silicon dioxide thickness as a function of reaction time and temperature for two substrate orienta-tions. (a) Growth in dry oxygen. (b) Growth in steam. 3 1/11/003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XI 35