School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Course Tutor Dr R E Hurley Northern Ireland Semiconductor Research Centre School of Electrical & Electronic Engineering The Queen’s University of Belfast

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland 2. Silicon – properties and preparation

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Atomic no. 14. All levels filled to 3p. 3p has 2 electrons in levels with capacity 6. Hence Si shares with 4 nearest neighbours to satisfy unfilled 3p energy levels.

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Si is atomic number 14 and shares its outer electrons with the 4 nearest neighbour atoms forming covalent bonds

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland The diamond (C, Si, Ge) unit cell Bond to nearest neighbours All atoms in crystal are bonded and form one giant molecule!

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland s2p2 electrons → sp3 → shared electron pairs → crystal formation Hybridization (the electron clouds repel each other)

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Silicon unit cell facts Lattice constant, a = nm Unit cell has 8 atoms 1 atom corner, 3 atoms face, 4 atoms at ¼, ¼, ¼ lattice points Unit cell volume is 1.6 × cm × 1021 unit cells/ cm 3 = 5 × atoms/cm 3 Nearest neighbour distance in nm Covalent radius is nm → 27% packing ratio

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Silicon crystal for integrated circuits (ICs) (111) is the cheapest. Used for bipolars (110) is difficult to produce (experimental) (100) have best surface properties and used for MOST ICs.

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Effect of (100), (110), (100) on surface properties (100) – 4 dangling surface bonds per unit cell (110) - 8 dangling (+parallel) bonds per unit cell (111) - 4 surface bonds per unit cell

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Allowed energy levels of 1 atom overlap and form bands when atoms form solid The outer 2 levels → valence and conduction band At 0 degrees Kelvin, conduction band is empty, valence band is full. Forbidden energy gap exists Band structure of semiconductor

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Silicon band structure at K

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Silicon band structure Semiconductors. E g = eV Insulators. E g = several eV Conductors. Overlapping valence and conduction bands For silicon, E g (T) is given by T is in 0 K and 1.17 = E g for Si at 0 0 K.

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Silicon energy gap v. Temperature

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Intrinsic conductivity Holes and electrons

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Electrons excited by thermal energy jump to conduction band and become mobile intrinsic carriers with density, n i : k is Boltzmann’s constant Intrinsic conductivity

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Si intrinsic carrier concentration v. temperature

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Doping silicon Group III → excess +ve carriers = holes = p-type Group V → excess –ve carriers = electrons = n-type (Excess = excess of intrinsic concentration) Carriers can be majority or minority

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Ionisation energies of dopants in silicon

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Law of Mass Action In equilibrium: Majority carrier density x Minority carrier density = constant for material and temperature i.e. n h x n e = constant = n i 2 n i = 1.4 x cm 3 for silicon Why? Generation rate is constant, f(T) Recombination rate is f(n h x n e, T)

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Mobility Average velocity of carriers = carrier drift velocity, v d = E, where µ is the mobility = qτ/m*, E is the field. (τ is meant time between scattering collisions, m* is effective mass) This is not valid at very high fields when scattering processes are non-uniform

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Mobility v. doping concentration

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Resistivity Standard well-known formula For a doped semiconductor, n is carrier concentration

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Resistivity of silicon v. impurity concentration at 300K

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland The Fermi Level for Silicon The position of the Fermi level relative to the top of the valence band for intrinsic silicon is: M dh = , m dc = , M c = number of equivalent minima in conduction band. (Approximately at the centre of energy gap)

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Fermi Level for extrinsic silicon n or p >> n i, E i is the Fermi level for intrinsic silicon

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Fermi level v. temperature and doping

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Defects in crystals In a crystal point defect concentration depends on thermal fluctuations and vapour pressure In silicon only thermal fluctuations important Defects will affect electronic properties Defect concentration depends on energy of formation and equilibrium temperature. Processes can produce defects and affect performance

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Vacancy defects in silicon

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Ways to form defects in silicon In silicon unit cell there are 5 interstitial positions (½, ½, ½,),(¼, ¼, ¼),(¼,¾, ¼), and (¾, ¾, ¼) 3 ways to make point defects: 1.Schottky defect – silicon atom jumps to interstitial position and diffuses to surface, leaving vacancy. 2.Frenkel defect – silicon atom jumps to an interstitial, creating vacancy/interstitial pair. 3.Surface generation – surface atoms move to interstitial sites within lattice.

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland surface

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland No. of Frenkel defects per unit volume

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Energy to form Schottky defect = energy to remove an atom from lattice and out of crystal = 2.3 eV. Using statistics and basic thermodynamics can be shown that no. of defects, C s is given by: (N = atoms/cm 3 )

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Defects can be charged Vacancies or interstitials can capture or release an electron: Energy levels for + and – ve charged vacancies exist at 0.35 and -0.57eV in silicon energy gap, but double_charged –ves can also exist.

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Interstitial defects in silicon

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Processes causing defects: Thermal oxidation Thermal nitriding Ion implantation Exposure to radiation Effect of silicon processing

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Extended defects Silicon processing may produce extended defects: 1.Dislocations 2.Twinning 3.Stacking faults

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Dislocations in silicon Strain, a distortion of the crystal lattice is produced by stress. Sources of stress are: 1.Mechanical forces (high temperatures 2.High dopant levels (dopants with size mismatch to silicon) 3.Thermal gradients: Thermal gradient strain, S = α Y ΔT α = coefficient of expansion, Y = Youngs Modulus, ΔT is temperature difference between centre and edge of wafer. If Stress > yield strength of silicon → dislocation

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Formation of edge dislocation

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Twinning (2 regions at differing orientations.) Usually means many dislocations, micro-twinning can be produced by ion implantation

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Stacking faults (oxidation, implantation, epitaxy) Excess atoms in one region

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Production of electronic grade polysilicon from SiO 2 Arc furnace with coal or wood reduces SiO 2 making 98% pure solid silicon. Si powdered with HCl in fluid bed to form SiHCl 3. The process starts with sand from Australian beaches

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Reduced with high purity hydrogen to yield high purity silicon.. Then distilled to high purity. This is one large polycrystal Making silicon

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Impurities in silicon

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Making high purity silicon Czochralski technique: Small seed crystal dipped into molten Si bath and slowly withdrawn. The bath and growing “boule” are rotated in opposite directions. Argon atmosphere, impurities may be added for doping.

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Czochralski crystal puller

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Summary of wafer making processes Pull crystal under argon, rotating. Forms ingot. Grind ingot to fixed diameter. Saw off ends of ingot (waste) Saw into wafers (0.5 to 0.8 mm thick) Edge grind to remove sharp edges Lap wafers to flatten and ensure faces are parallel Wet etch to remove damage from lapping Polish to mirror finish Final clean (removes contamination)

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Silicon wafer manufacturing process

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland The best sand comes from the beaches of Australia. Pulling the crystal boule

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Diamond saw Polishing Ingots Sawing and polishing

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Standard wafer sizes

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Removing impurities For high quality VLSI impurities can be reduced to a low level from device regions by gettering: 1.Impurities released into solid solution 2.Impurities diffused through silicon 3.Impurities trapped by dislocations or precipitates Extrinsic gettering creates damage or defects on backside of wafer. After annealing → dislocations Intrinsic gettering – precipitate supersaturated oxygen (out of wafer) into clusters → stress.

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Extrinsic gettering Abrasion, grooving, sandblasting → anneal ( → dislocations. May produce microcracks!) Diffuse P → 1. Si-P precipitates (remove Ni impurities). 2. P vacancies → trap Au. Laser scanning damage → thermal shock → dislocation ‘nests’. High energy ion bombardment → stress Polysilicon layer → grain boundaries → traps.

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Intrinsic gettering Precipitate supersaturated oxygen out of silicon wafer → growing clusters → stress Stress → relief by dislocation loops, faults → trapping sites for impurities Starting wafers must have 15 – 19 ppma O. 1.Only heating required 2.Large volume of impurity sink. 3.Gettering is close to device regions. Highly doped boron regions will getter Fe (B may be incorporated or high-doped B substrate + epitaxial layer)

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Oxygen internal gettering (for CMOS) Buy high oxygen wafers (5E17 to 7.5E17 cm -3 ) Remove oxygen from device area (surface) (3 to 4 hrs at 1,100 0 C in 1% HCl in O 2 ) [> 950C diffusion preferred to nucleation] Nucleate oxygen – 1 hr at C. Form oxygen 5 nm precipitates at ~ C. (Precipitates create volume mismatch → dislocations)

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Oxygen-free zone > deepest active device area. Oxygen < 2E17 cm -3. (Oxygen can degrade device!) e.g. For CMOS with 250 nm feature size, oxygen-free zone >> 1.5 µm. (Use 2 hr steam cycle at C). Oxygen internal gettering (for CMOS)

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Oxygen internal gettering (for CMOS)

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Oxygen nucleation rate and denuded (oxygen-free) zone depth

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Oxygen profile through the denuded zone

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland More defect problems and COPs COPs = Crystal Originated Particles = defect that appears to be a particle to a laser-scanner. COPs are small voids in Si formed by vacancy clusters during crystal growth. COPs will degrade gate oxide layers in CMOS. During crystal growth, if V =crystal growth rate: V/G > 1.2 µm 2 /min K, vacancies predominate V/G < 1.2 µm 2 /min K, interstitials predominate G = axial temperature gradient near interface.

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland More defect problems and COPs During crystal growth if V =crystal growth rate: V/G > 1.2 µm 2 /min K, vacancies predominate V/G < 1.2 µm 2 /min K, interstitials predominate G = axial temperature gradient near interface. Hence slow cool silicon during crystal growth in temperature range ~ C. Also, high temperature annealing (see graph)

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Number of LPDs v. depth of anneal (defects or particles)

School of Electrical and Electronic Engineering Queen’s University Belfast, N.Ireland Light point defects (LPDs), as in Figure. LPDs may be COPs or actual particles for 150 mm dia (100) Si after various anneals. [Surface and depth(by polishing into crystal)] For 180 nm the COP goal was < 38COPs/200mm wafer or < 21COPs/150mm wafer. Thus, slow cooling and annealing to reduce COP levels become very critical as linewidths become smaller.