Doping by Diffusion and Implantation. Uma Parthavi M Dept. of Electrical Engineering, Indian Institute of Technology Delhi. Tutor: Prof.

Transcription

1 Uma Parthavi M Dept. of Electrical Engineering, Indian Institute of Technology Delhi. Tutor: Prof. N Dasgupta

2 2 Contents Doping Two step doping process Diffusion equipment & sources Diffusion-Microscopic & Macroscopic point of view Fick s Laws solutions Diffusivity Influence of Electric Field, Defects Oxidation Enhanced Diffusion Ion Implantation Implantation Basics Ion implanter Implantation profiles Channeling Damage annealing Comparison between diffusion and ion implantation References Contents

3 Doping Silicon Diffusion : The spread of particles through random motion from regions of higher concentration to regions of lower concentration Ion implantation Bombarding the substrate with ions accelerated to high velocities 3 Introduction

4 Creating Doped regions Step1 : Pre-deposition Controllably introduce desired dopant atoms Methods: Solid phase diffusion from glass layers Gas phase diffusions Ion Implantation Step2 : Drive-in The introduced dopants are driven deeper into the wafer without further introduction of dopant atoms Two step process for producing a junction 4 Diffusion

5 Diffusion sources 5 Diffusion

6 Diffusion- Equipment 6 Diffusion Equipment(showing predep. Of BSG)[2] Diffusion

7 Diffusion Microscopic Point of View : Considers the motion of dopant at atomic scale Computationally expensive and used in simulation tools More accurate Macroscopic Point of View : Considers overall motion of dopant profile Fick s Laws Considering the macroscopic point of view is important because it gives a sufficiently accurate first hand picture 7 Diffusion

8 Microscopic Point of View Vacancy Assisted Diffusion Interstitial Assisted diffusion Impurity atom 8 Diffusion

9 Fick s First Law Diffusive flux has a magnitude proportional to spatial concentration gradient C F D x F is flux(atoms/cm 2 sec); D is the diffusivity(cm 2 sec -1 ); C x is the concentration gradient. Flow is opposite to the direction of concentration gradient F D C 9 Diffusion

10 Fick s Second Law Increase in the concentration in a cross section of unit area with time is simply the difference between the flux into the volume and the flux out of the volume. what goes in and doesn t go out stays there C F Fin Fout t x x Flux in and out of a volume element If D is a constant, C t. F.( D C) 10 Diffusion

11 Solutions to Fick s Equations Steady state linear Limited source in infinite medium - Gaussian Limited source at surface - Gaussian Infinite source Error function 11 Diffusion

12 Steady state Steady state dopant concentration in constant with time 2 C D 0 2 x Solving for the above gives, C=a+bx 12 Diffusion

13 Limited source in infinite medium Boundary conditions: C 0 C as as t t 0 0 for for x 0 x 0 A constant dose of dopants introduced in an infinite medium C ( x, t) dx Q -ve Dopants Si Wafer +ve 13 Diffusion

14 Consequences Time evolution of Gaussian profile[1] Solution has an evolving Gaussian form Symmetric about the origin Peak concentration decreases by 1/ t given by C(0,t) Diffusion length = 2 It is an approximate and is Dt measure of how much the dopant has diffused 14 Diffusion

15 Limited source at the surface Virtual medium Virtual Dopants Real Dopants Si Wafer Dopants introduced at the surface Dopant dose Q introduced at the surface Can be treated as an effective dose of 2Q being introduced in a virtual infinite medium 15 Diffusion

16 Infinite source Consider series of slices, each with thickness, x having a dose of C x. The solution for this case is simply the linear superposition of Gaussian solutions for thin slices Boundary conditions: C=0 at t=0 for x>0 C=C at t=0 for x<0 C( x, t) 2 Diffusion from an infinite source C Dt 0 2 ( x ) exp d 4Dt 16 Diffusion

17 Q Cs is the concentration at the surface and Cs=C/2 Surface conc. is constant Total Dose x Cs 2 Cs[1 erf ( )] dx Dt 0 2 Dt Time evolution of erfc profile[1] 17 Diffusion

18 Diffusivity For common impurities in silicon, D D 0 E exp( kt k is the Boltzmann constant, EA is the activation energy in ev and T is the temperature in degrees Kelvin. A ) Diffusivity for common dopants [3] 18 Diffusion

19 Solid solubility Maximum Thermodynamic concentration of dopant that can be dissolved in silicon without forming a separate phase In reality, electrical solubility is less than the solid solubility because of formation of neutral clusters with vacancies Solid solubility plots for common dopants[1] 19 Diffusion

20 Influence of Electric field Dominant when doping concentrations exceed intrinsic carrier concentrations. F h 1 C hd x 2 n i F is the flux as discussed earlier, C is the net doping concentration at x h is upper bounded by 2 C 2 C 4 20 Diffusion

21 Effect of electric field on low concentration regions[1] 21 Diffusion

22 Influence of defects DA is the effective diffusivity,da * is the normal equilibrium diffusivity under inert conditions, fi is the fraction of dopants diffusing with interstitial mechanism, fv is the fraction of dopants diffusing with vacancy-type mechanism, CI is the interstitial concentration, CV is the vacancy concentration, CI* is the interstitial concentration at equilibrium, CV* is the vacancy concentration at equilibrium 22 Diffusion

23 Oxidation enhanced diffusion P,B diffusion enhanced ; Sb retarded Oxidation of Si to SiO2 causes volume to increase induces stress which is relieved by the Si atoms moving to interstitial spaces Oxidation injects interstitials ; P,B prefer interstitial type diffusion Interstitials combine with vacancies decrease in vacancies ; Sb prefers vacancy type diffusion Plot showing effect of oxidation in diffusion of As and Sb implants[1] 23 Diffusion

24 Ion Implantation Basics Energetic and violent technique Dominant doping technique for past 20 yrs Direct bombardment of accelerated dopant ions onto the substrate Cascade of damages created in the perfect Si lattice removed by annealing Precise control on the amount and distribution of the dose Energy of ions control the distribution Ion beam current controls the dose 26 Implantation

25 Ion Implanter 27 Ion Sources: Gas : Arsine, Phosphine, Boron difluoride in a zeolite matrix ; allow rapid beam tuning Solid : elemental sources of As, P ; vaporized Ion Implantation System[5] Implantation

26 Ion Implanter Gas from the source is ionized by electrons from a filament/plasma discharge Ions are extracted by voltage and mass analyzed to select only one ion species B I K. E. qv mv r r 2 qvb 2mV q 1 I 1 2 mv B is the magnetic field, proportional to the current I, V is the external voltage applied, m is the mass of a ion, v is the velocity of an ion, q is the charge of an ion Different ions can be chosen by varying the external voltage and the current to the coils 2 Beam of B11(top) and B10 separated Courtesy: Albion Systems 28 Implantation

27 Ion Implanter The radius of curvature is proportional to square root of the mass Ions are further accelerated depending on the requirements and incident on the target The implant dose is measured by locating the sample at the end of a Faraday cup Q 1 A I q dt I is the collected beam current, A is the implant area, t is the integration time and q is the charge on the ion 29 Range of Energy and Dose needed for different applications [6] Implantation

28 Implantation profiles Implantation profiles of commonly used dopant atoms[6] Range of an ion is the actual distance travelled by it before stopping Projected Range Rp is the average distance travelled normal to the surface ΔRp is the standard deviation of the projected range also called straggle Heavy ions Smaller Rp and ΔRp Lighter ions Greater Rp & ΔRp 30 Implantation

29 Implantation profiles[7] 31 Implantation

30 32 Implantation profiles 0,6 Can be approximated to a Gaussian ( x Rp) C( x) Cp exp( 2 Rp Q 2 RpCp C(x) is the concentration distribution, Rp is the range,δrp is the straggle, Cp is the peak concentration The 2D distribution is usually assumed to be a product of vertical and lateral distribution 2 ) Range(um) Straggle(um) 0,5 0,4 0,3 0,2 0,1 Doping by Diffusion and Implantation ,1 0,09 0,08 0,07 0,06 0,05 0,04 0,03 0,02 0,01 0 Energy(KeV) Energy(KeV) Range and Straggle for As,P,B Data from BYU s Range and Straggle calculator Implantation As P B As P B

31 Pearson Model 33 Implantation

32 Channeling Crystalline Si planar and axial channels Once an ion enters a channel, it can be steered along the channel until it comes to rest either by drag or sharp collision High doses less channeling 34 Implantation

33 Impact of channeling on profiles Impact of channeling on B profile[8] 35 Implantation

34 Avoiding channeling Channeling can be reduced by Oxide screening Tilting the wafer (ideally 7degrees) Screening by amorphous Si 36 Implantation

35 Avoiding channeling 37 From: Implantation

36 Ion stopping mechanism Nuclear Stopping: Collision of ions with lattice atoms Depends on Ion energy Tends to dominate at the end of the stopping process when ions have lost much of their energy Produces damage Electronic Stopping: Nonlocal electronic Stopping Drag experienced by the ion in a dielectric medium; dissipative, does not alter the trajectory Directly proportional to the ion velocity Depends on ionization state of the ion Local electronic Stopping If the ion comes close enough to a lattice atom, momentum transfer due to e-transfer possible Subtly alters the trajectory minor compared to nuclear stopping Depends on the ion velocity 38 Implantation

37 Stopping power for common ions Total stopping power = electron stopping power+ nuclear stopping power Nuclear stopping dominates at low energies Electron stopping dominates at higher energies, for lighter atoms Stopping powers of dopants[1] 39 Implantation

38 Stopping mechanisms 40 Implantation

39 Damage During implantation Nuclear stopping ions transfer energy to lattice atoms; crystalline structure damaged Energy required to displace a Si atom to create a Frenkel pair (I +V) is 15eV Damage to the crystal is in the following ways: Creation of interstitials and vacancies Creation of local zones of amorphous material High dose implants might turn crystal to amorphous state The above two types of damage are called Primary crystalline damage ; Repaired by thermal process known as annealing But subjecting wafer to thermal process for a long time might cause diffusion of dopants - undesirable 41 Implantation

40 Annealing 43 Primary damage anneals at 400 o C Firstly I and V combine in the bulk ; this leaves only I s originating from introduction of extra atom Later vacancies and interstitials recombine at the surface Above 400 o C extra I s condense into rod shaped defects {311} planes Upon annealing after 900 o C, they start disappearing Damage less than a critical value can be repaired. For damage above critical value, {311} defects form stable dislocation loops secondary damage Steps in Annealing with time [1] {311} Ribbon Defects[1] Implantation

41 Annealing Largest interface between crystalline and amorphous Si EOR(End of Range) Defects These EOR loops are known to disappear in some instances after 60 sec anneal at 1100 o C EOR loops detrimental if present at junctions Annealing cycles are chosen to cause enough dopant diffusion so that the loops are contained in highly doped regions and are shielded from any depletion regions 44 Implantation

42 Dopant Activation Activation Dopants should occupy substitutional sites Broken bonds should be repaired to improve mobility Low primary Damage: all damage anneals out High primary Damage : Amorphization Solid Phase Epitaxy provides nearly ideal soln Partial Damage: Formation of secondary damage o C required Fraction of atoms active Fraction of atoms activated for boron implant [9] 45 Implantation

43 Annealing Annealing can be done in two ways: Furnace Annealing Rapid Thermal Annealing 46 Implantation

44 Furnace Annealing Inert ambient Nitrogen or Argon Oxide capping layer recommended to avoid evaporation of dopants Temperature range o C Time >30 mins Problem of Diffusion of implanted dopants Transient enhanced Diffusion not suited for shallow junctions Typical Furnace used for annealing 47 Implantation

45 Rapid Thermal Annealing Bank of lamps that rapidly heat a wafer Optical energy transfer Ramp rate of 100 o C/s Wafer attains uniform temperature in few ms Annealing time: s No diffusion during anneal 48 RTA furnace Schematic Implantation

46 Comparison of Diffusion and Ion implantation 50 Diffusion Advantages: No damage created Batch fabrication possible Disadvantages: Limited to solid solubility Low dose predeps difficult High temperature process Shallow junctions difficult Ion Implantation Advantages: Low temperature process Precise dose and junction depth control Implantations through thin layers of oxide/nitride possible Short process times Disadvantages: Implant damage enhances diffusion Additional cost of annealing Dislocations may cause junction leakage Channeling Comparision

47 References [1] J D Plummer, M D Deal and P B Griffin, Silicon VLSI Technology: fundamentals, practice and modelling,pearson Edu. Inc.,2001 [2] John (2010, June 1), Diffusion of impurities for IC fabrication [online].available: [3] H.Puchner, Advanced Process Modelling for VLSI Technology, Ph.D. dissertation, Dept. Elect. Eng., Technical Univ. of Vienna,Vienna,Austria, 1996 [4] National Technology Roadmap for Semiconductors (NTRS); SIA: San Jose, [5] John (2010, June 2), Ion Implantation [online].available: [6] L Rubin and J Poate(2010, Dec 2), Ion Implantation in silicon technology [online].available: [7] (2010, Dec 2), Ion Implantation Processes in Semiconductor Manufacturing [online].available: erlangen.de/lehre/mm/html/implant.htm 51 References

48 References [8] C Tian,S Gara,G Hobler and G Stingeder, Boron Implantation in Si: Channeling Effects Studied by SIMS and Simulation, Mikrochim. Acta, ser. D, vol. 107, pp , 1992 [9] B. L. Crowder and F. F. Morehead, Jr, Annealing characteristics of n-type dopants in ion-implanted silicon, Applied physics letters, ser. D, vol. 14, pp , May References

49 Thank You!!!!!