Basic Properties of Silicon Crystals
Basic Properties of Silicon Crystals
Basic Properties of Crystals Type of crystals for silicon: Single Crystal Polycrystalline Amorphous periodic small crystals no long range arrangements order between of atoms atoms Crystal lattice is described by a unit cell with a base vector(distance between atoms) Types of unit cells Z Cubic BCC FCC Body Centered Cube Face Centered Cube
Basic Properties of Crystals Type of crystals for silicon: Single Crystal Polycrystalline Amorphous periodic small crystals no long range p y arrangements order between of atoms atoms Crystal lattice is described by a unit cell with a base vector (distance between atoms) Types of unit cells Types of unit cells Body Centered Cube Face Centered Cube
Directions and Planes in Crystals Directions (vector components:a single direction is expressed as [a set of 3 integers], equivalent directions(family)are expressed as a set of 3 integers> Planes:a single plane is expressed as(a set of 3 integers hk/=Miler indices)and equivalent planes are expressed as fa set of 3 integers Miler Indices:take x,y,z(multiple of basic vectors ex.x=4a,y=3a,z=2a) reciprocals(1/4,1/3,1/2)>common denominator(3/12,4/12,6/12)>the smallest numerators(3 4 6) [h k I]crystal direction is perpendicular lattice constant to (h k I)plane Z (100)plane a (110)plane [111] (111)plane [1001 X
Directions and Planes in Cr ystals Directions (vector components: a single direction is expressed as [a set of 3 integers], equivalent directions (family) are expressed as y equivalent directions (family) are expressed as Planes: a single plane is expressed as (a set of 3 integers h k l = Miler indices) and equivalent planes are expressed as {a set of 3 integers} Miler Indices: take x, y, z (multiple of basic vectors ex. x=4 a, y=3 a, z=2 a) reciprocals (1/4, 1/3, 1/2) Æ common denominator (3/12, 4/12, 6/12) Æ the smallest ( ) lattice constant numerators (3 4 6) [h k l] crystal direction is perpendicular to (h k l) plane
Silicon Crystal Structure Diamond lattice (Si,Ge,GaAs) Two interpenetrating FCC structures shifted by a/4 in all three directions All atoms in both FCCs ○inside one FCC O from the second lattice covalent bonding (100)Si for devices (111)Si not used-oxide chargest
Silicon Crystal Structure Diamond lattice (Si, Ge, GaAs) Two interpenetrating FCC structures shifted by a/4 in all three directions All t i b th FCC All atoms in both FCCs Diamond covalent bonding inside one FCC from the second lattice (100) Si for devices (111) Si not used - oxide charges
Silicon Surface Orientation (100) Devices are built on surface>surface orientation affects the electrical and physical properties Two commonly used crystal orientation in silicon (111)crystal plane Largest number of atoms per cm2, Oxidize fast 111) Higher density of interface states (defects) (100)crystal plane Superior electrical properties of the Lowest number of atoms per cm2 (100)Si/SiO,interface makes (100) Oxidize slow silicon dominant in manufacturing. Low density of interface states
Silicon Surface Orientation Devices are built on surface Æ surface orientation affects the electrical and physical properties (100) Two commonly used crystal orientation in silicon (111) crystal plane • Largest number of atoms per cm 2 , • Oxidize fast • Hi h d it f i t f t t (111) Hi g her density o f inter face states (defects) (100) cr ystal plane (111) ( )y p • Lowest number of atoms per cm 2 • Oxidize slow • Low density of interface states Superior electrical properties of the (100) Si/SiO 2 interface makes (100) silicon dominant in manufacturing
Defects in Silicon Crystals Point Defects Stacking Fault Dislocation Precipitate Various types of defects can exist in crystal or can be created by processing steps. Point defects: 。 Impurity related defect Native point defect Vacancy (a missing atom),Interstitial(an extra atom) Equilibrium concentration increases with T:1012-1015cm3@1000C
Defects in Silicon Crystals Point Defects V Stacking Fault – I Dislocation Precipitate Point defects: Various types of defects can exist in crystal or can be created by processing steps. Point defects: • Impurity related defect • Native point defect Vacancy (a missing atom), Interstitial (an extra atom) Equilibrium concentration increases with T: 1012-1015cm-3 @ 1000 o C
Defects in Silicon Crystals Dislocations 0000000000000 00000000000 Intrinsic point defects in 00000000000 O0O000O0O000 a crystal N and N o0000o0000ooo increase with T 业 0000000000000 0000000000000 o0o0900888000oo00 oooO 00O 88808888088899008 00000O00O0000 ●●●●●●●●● Agglomeration of 0000000000000 000000000000o6008 00008000000000000 Interstitials 号 Extrinsic-type dislocation loop. 0000000009908 90980o00o9800 O000 HOOOO o088600008886 collapse 0000000000000 Sequence of intrinsic-type dislocation loop formation. One-dimensional defects (dislocations): 。 Edge dislocation,dislocation loop Macroscopic edge dislocations stress in silicon after high temperature processes (LOCOS);temperature gradients during processing Microscopic dislocation loops agglomeration of V I during a cooling process (not enough time for V&I recombination) Dislocations can move when subjected to stresses or when excess point defects are present
Defects in Silicon Crystals – Dislocations Intrinsic point defects in a crystal N v and NI increase with T Agglomeration of Interstitials collapse One-dimensional defects (dislocations): • Edge dislocation, dislocation loop • Macroscopic edge dislocations Macroscopic edge dislocations Å stress in silicon after high temperature processes stress in silicon after high temperature processes (LOCOS); temperature gradients during processing • Microscopic dislocation loops Å agglomeration of V & I during a cooling process (not enough time for V & I recombination) • Di l i h bj d h i d f islocations can move w hen subjecte d to stresses or w hen excess point d e fects are present
Propagation of Dislocations by Climb Climb motion in an edge dislocation 00000000000 00000000000 00000000000 00000000000 00000000000 00000000000 00000000000 00000000000 ● 0000099/000 00000000000 0000000000 b 00000●00000 Shift 00000100000 00000●00000 0000000000 。。00010.00。 0000000000 0000000000 00000000c0 0000000000 Negative climb by absorbing self-interstitials 00000000000 00000000000 00000000000 00000000000 00000000000 00000000000 00000000.000 0000000°，°00 00000006000 b 00 00000●6000 00000 00●00 Shift 00000●60000 0000000000 00000100000 0000000000 0000000000 0000000000 0000000000 .0000000000 Positive climb by capturing vacancies
Propagation of Dislocations by Climb Climb motion in an edge dislocation Shift Negative climb by absorbing self-interstitials Shift Positive climb by capturing vacancies
Motion of Dislocations by Glide Movement of a dislocation by glide in response to shear stress Shear Stress Easy motion of Stress induced by dislocations Mismatch of thermal expansion coefficients Temperature gradient AT g=aY△T
Motion of Dislocations by Glide Movement of a dislocation by glide in response to shear stress St i d d b Easy motion of dislocations Stress in duce d by • Mismatch of thermal expansion coefficients • Tem p g erature gradient ∆ T σ = αY∆ T
Defects in Silicon Crystals Stacking Faults 【111 [001 A A.B.C:three different a b [1i1 (111)planes B b' Perfect stacking c c B Stacking of(111)planes viewed along [110]in the diamond structure ESF Induced by oxidation ISF Missing(111)plane B [111】 001] SFs bound by →[i011 dislocations Two-dimensional defects(stacking faults): Along {111)planes Intrinsic:removal of part of a plane of atoms in {111)directions Extrinsic:addition of a partial plane of atoms in (111)directions Oxidation induced stacking faults(OISF):stacking faults grow during oxidation due to absorption of more I
Defects in Silicon Crystals – Stacking Faults A, B, C: three different Perfect stacking (111) planes Induced by oxidation Missing (111) plane SFs bound by dislocations Two-di i l d f t ( t ki f lt ) dimensional d e fects (stacking fault s): • Along {111} planes • Intrinsic: removal of part of a plane of atoms in {111} directions • Extrinsic: addition of a p p {} artial plane of atoms in {111} directions • Oxidation induced stacking faults (OISF): stacking faults grow during oxidation due to absorption of more I