DOPING BY DIFFUSION AND IMPLANTATION

DOPING BY DIFFUSION AND IMPLANTATION 7 th Indo-German Winter Academy-2008 Presented by Niranjan Patil IIT Bombay, Course -3 Tutor: Prof N. Dasgupta IIT Madras Doping by Diffusion & Implantation 1

Outline 1. Diffusion Introduction Experimental Techniques Mathematics Electric field enhancement Oxidation Enhancement Temperature dependence 2. Implantation Introduction Modern Implanters Ion stopping Implantation Profile Damage Channeling Annealing Doping by Diffusion & Implantation 2

What is Doping and Diffusion Doping Addition of a small percentage of foreign atoms such as Sb, Ge, As Dramatic change in electrical properties Production of n type and p type semiconductors. Mostly used semiconductors are doped Diffusion Mass transfer phenomena Transport of molecules under concentration gradient Explained by Ficks laws Influence of temperature Influence of defects Doping by Diffusion & Implantation 3

Doping by diffusion Sources used: 1.Solid Sources: BN, AlAsO 4 2.Liquid Sources: BBr 3,POCl 3 3.Gas Source: AsH 3, PH 3, B 2 H 6 4. Spin on glass: SiO 2 +dopant oxide Carried out by chemical process Appropriate dopant mixture is passed over high temperature (900 1200 o C ) semiconductor wafer in a closed or open tube. High temperatures implies high diffusion rates Yields good reproducibility and most common technology in IC industry Doping by Diffusion & Implantation 4

Diffusion Experimental Techniques Open furnace Boron Liquid source system Boron has a high solubility in silicon Reactions: 4BBr 3 + 3O 2 2B 2 O 3 + 6Br 2 2 B 2 O 3 +3 Si 4B + 3SiO 2 For different semiconductors different types of furnaces are used Doping by Diffusion & Implantation 5

Fick s 1 st Law Flow is proportional to concentration gradient Effect is proportional to cause F : Diffusion flux (Atoms per unit area per unit time) C : Concentration of solute D : Diffusivity (or Diffusion constant or Diffusion Co efficient) Doping by Diffusion & Implantation 6

Fick s II nd Law Non steady state diffusion How diffusion causes the concentration field to change with time Development of C x and C t profiles Solution depends on boundary conditions Doping by Diffusion & Implantation 7

Doping by diffusion Doping by diffusion is a 2 step process: 1. Predeposition: Sample is heated in the impurity gas for a short time (about 15 minutes), molecules in gas diffuse into sample but remain near surface. 2. Drive-in: Sample is annealed (held at high temperature) for a longer time (about 1-2 hours), dopants diffuse deeper into sample. Doping by Diffusion & Implantation 8

Predeposition Constant Surface Concentration Diffusion from an infinite source of dopant Boundary conditions: 1. 2. Solution would be, Doping by Diffusion & Implantation 9

Drive in Diffusion from a Limited Source Also called Gaussian profile Constant amount of dopant boundary conditions: B Solution becomes Doping by Diffusion & Implantation 10

Effect Of Successive Diffusions Multiple diffusion steps in the IC Process Temperature T 1 for time t 1 == Diffusivity D 1 Temperature T 2 for time t 2 == Diffusivity D 2 Pre deposition Drive in Time Doping by Diffusion & Implantation 11

When the doping is higher than n i, electric-field effects become important Electric field induced by higher mobility of electrons compared with dopant ions. Effect of Electric field External force acting on the diffusing species in addition to normal diffusion, then there is a additional flux or flow of species For high concentrations h 2 Doping by Diffusion & Implantation 12

Equations F is the total flux Ψ is potential in doped semiconductor µis mobility, it relates to diffusivity v is velocity in external field v E E x C Ftotal F F' D Cv x C C C D v t x x x Doping by Diffusion & Implantation 13

Atomic scale diffusion mechanisms Many effects OED and TED are not explains by macroscopic models Most of the diffusion process are non Fickian reason being defects Point defects are linked to atomic scale Different dopant different mechanisms Vacancy Assisted Mechanism Kick-out Interstitial(cy) Doping by Diffusion & Implantation 14

Oxidization enhanced diffusion (OED) O 2 Surface Recombination G R Bulk Recombination * I Inert Diffusion Buried Dopant Marker Layer OED Stacking Faults Grow Both point Interstitial and vacancy defects are important in silicon Oxidation provides an I injection source. Nitridation provides a V injection source. Stacking faults serve as "detectors" as do dopant which diffuse Doping by Diffusion & Implantation 15

Influence of point defects Thus dopant diffusion can be enhanced or retarded by changes in the point defect concentrations Oxidation injects interstitials, raises C 1 /C * 1 reduces C V /C * V through I V recombination in the bulk silicon. Nitridation does exactly the opposite. B, P enhanced by oxidation Sb retarded by oxidation. f I f V Silicon 0.6 0.4 Boron 1.0 0 Phosphor 1.0 0 us Arsenic 0.4 0.6 Antimony 0.02 0.98 D A D A * C f I I * C f V I C V C V * f I +f v =1 Doping by Diffusion & Implantation 16

Effect of Temperature D 0 : A constant called frequency factor related to the frequency of the atomic vibration E A : Diffusion activation energy Si B In As Sb P Units D o 560 1.0 1.2 9.17 4.58 4.7 cm 2 sec 1 E A 4.76 3.5 3.5 3.99 3.88 3.68 ev Doping by Diffusion & Implantation 17

Diffusion limitations Dopant profiles are not uniform In most of the cases profiles do not follow Fick s law reason being defects High temperature is required to carry diffusion process It is very difficult to determine profile Limited solid solubility of dopant. Doping by Diffusion & Implantation 18

Doping by Ion Implantation Doping by Diffusion & Implantation 19

Introduction Energetic ion beam of dopant atoms used into surface layers of solids Ions are accelerated through electric field Ion range (penetration) depends on their KE (of the order kev) It is a non equilibrium process while diffusion is under thermal equilibrium Temperature around 600 900 0 C Wide range of dopant products Doping by Diffusion & Implantation 20

Advantages High Purity of dopants Short process times, good homogeneity Exact control of the amount of implanted ions Relatively low temperatures during the process Wide variety of dopants e g oxide, nitride, metals, and resist Implantation through thin layers (e.g. SiO 2 or Si 3 N 4 ) is possible Low penetration depth of the implanted ions. Optimization of the dopant profiles Doping by Diffusion & Implantation 21

Ion Implantation Process 2 3 4 5 1 Doping by Diffusion & Implantation 22

Mass analyzer Ions extracted from source are analyzed in a magnetic field Path becomes curved because of Lorentz force 2 F total qvb Mv / R Radius of curvature depends on mass Leads to separation Selected beam Heavier Ions Doping by Diffusion & Implantation 23

What happens inside a material Ions are stopped by 2 mechanism 1. nuclear stopping 2. electronic stopping Stopping power due to inelastic electron stopping Stopping power due to nuclear elastic scattering Ion range in target: N = Target atom density S i (E) is Stopping power Doping by Diffusion & Implantation 24

Nuclear stopping Interaction with electric field of nuclei of target Coulomb scattering (elastic) At 100 kev an ion of 15 amu as velocity, v ion 10 6 m/s +So ion is far past nucleus before nucleus can displace in response to Coulomb force nuclear scattering is not strong at high ion velocity; only significant when ion slows down. Doping by Diffusion & Implantation 25

Electronic stopping 1. Non local local: ion experiences drag due to free or polarizable electrons: incident ion attracts electron polarization Ion velocity=> charge separation, drag 2. Local: passing ion causes internal electronic transitions Because electrons can follow fields up to optical frequencies (high velociries) electronic losses dominate at higher ion velocities. Effect is like a motion through fluid. Doping by Diffusion & Implantation 26

Implantation range & deviation incident ion Range (R P ) is a function of ion energy mass and atomic number of ion mass and atomic number target material. R R p R p R l Doping by Diffusion & Implantation 27

Doping by Diffusion & Implantation 28

The Gaussian profile log (ion concentration) straggle = standard deviation Gaussian distribution R R P lateral straggle straggle ion beam y Doping by Diffusion & Implantation 29

Implantation Profile equations N x exp x R p N p 2 2 Rp 2 Dose = Q 0 Ndx 2 N p R p N x Q x Rp exp 2 R 2 R p p 2 2 More precise profiles are given by Pearson IV profiles Doping by Diffusion & Implantation 30

Dopant profiles Different dopants in implantation Different energies in implantation Doping by Diffusion & Implantation 31

Damage Process Implanted ions transfer energy to lattice atoms Atoms to break free Freed atoms collide with other lattice atoms Free more lattice atoms Damage continues until all freed atoms stop One energetic ion can cause thousands of displacements of lattice atoms Doping by Diffusion & Implantation 32

Implantation damage light ion Damage tree disordered regions heavy ion collision cascade spike light ion transfers a small amount of energy each collision deflected through a large scattering angle displaced target atom will have little energy imparted to it electronic stopping relatively little damage heavy ion large amount of energy transferred incident ion deflected through a small scattering angle displaced target atoms can produce a large number of displacements nuclear stopping considerable lattice damage in a relatively small volume Doping by Diffusion & Implantation 34

Channeling Ions can penetrate deeper into the crystal along crystal axes or planes Leads to non uniform profile Multiple collisions Channeling Model of a silicon crystal seen along the 110 direction. Doping by Diffusion & Implantation 35

Ways to avoid channeling effect implant through an amorphous oxide layer misorient the beam direction to all crystal axes 7 0 along <110> for Si) predamage on the crystal surface channeling can be reduced but not avoided Doping by Diffusion & Implantation 36

Annealing Goals: Remove primary damage created by the implant and activate the dopants. Restore silicon lattice to its perfect crystalline state. Restore the electron and hole mobility. Do this without appreciable dopant redistribution. Put dopant atoms in substitutional sites where they will be electrically active. One of two anneals is usually used One of two anneals is usually used: 1) Furnace anneal (typically 30 minutes at 800ºC) 2) Rapid thermal anneal (typically 10 seconds at 1100ºC) Doping by Diffusion & Implantation 37

Profile Evaluation during annealing Annealing is carried out at high temperature 800 0 C D is diffusivity D constant Doping by Diffusion & Implantation 38

Annealing Conventional furnace Heat large batch of wafers Time= 30 minutes Annealing in an oxidizing (O 2 ) or inert (N2) atmosphere Temperatures about 700 to 1100 0 C If the substrate is amorphous, it can regrow by SPE Layer by layer epitaxial realignment from amorphous/crystalline surface The amorphous / crystalline interface migrates toward the surface at a fixed velocity Doping by Diffusion & Implantation 39

RTA Rapid thermal annealing(i) Using optical energy, heat small area, 1200 0 C Use light box of Halogen Heat Lamps Raises temperature of whole surface in seconds Rate =(50 100 0 C/s) activation energy of 5 ev, hence high temperatures Temperature uniformity. Water cool back of target As only heat surface (not whole wafer) cools quickly Doping by Diffusion & Implantation 40

RTA Rapid thermal annealing(ii) Applied Materials 300 mm RTP System Furnace & RTA profiles Doping by Diffusion & Implantation 41

Disadvantages Damage of the substrate is caused by the implanted ions The change of material properties is restricted to the substrate domains close to the surface Sometimes lead to change in chemical properties Channeling and diffusion make it difficult to achieve very shallow profiles and to theoretically predict the exact profile shapes. Additional cost of annealing Doping by Diffusion & Implantation 42

Difference Diffusion Simple Mechanism High temperature Isotropic dopant profile Cannot independently control of the dopant concentration and junction depth Batch process Ion Implantation Complex Mechanism Low temperature Anisotropic dopant profile Can independently control of the dopant concentration and junction depth Both Batch and single wafer process Doping by Diffusion & Implantation 43

Implantation gives better control Doping by Diffusion & Implantation 44

References Books S.M. Sze: Physics of Semiconductor Devices, John Wiley & Sons, New York, 1981 River, New Jersey, 2001 Ion implantation Science and technology by J.F.Ziegler first edition, 1980 Ion Implantation to semiconductors by G Carter and W A grant, 1976 VLSI technology by S.M.Sze, second edition, 2001 Silicon VLSI Technology Fundamentals, Practice And Modeling By Plummer, Deal and Griffin, 2000 Shaw, D., Atomic Diffusion in Semiconductors, Plenum (1973). Web http://www.casetechnology.com/links.html www.gs68.de/tutorials/implant.pdf wins.engr.wisc.edu/research/scw/intl/sciem III/SCW Corros UW.ppt http://users.ece.gatech.edu/~alan/ece6450/lectures/ece6450l5 I%20Implantation.pdf http://www.ee.iitb.ac.in/uma/~hits/print/06%5b1%5d.ion%20implantation.pdf http://web.utk.edu/~prack/thin%20films/implant-1.pdf Doping by Diffusion & Implantation 45

Thank You! Doping by Diffusion & Implantation 46