1. Dopant Diffusion Scaling down MOSFET by 1/K calls for smaller junction depths. high deposit activation (n  N d )  Resistance  in S/D. N d (x j ) = N asubstrate W t L Concept of Sheet Resistance of doped layers.  s [  /sq.]  4 point probe or van der Pauw In MOSFETs R contact + R source + R ext < 10% R chen  s  but keep x j small to avoid DIBL (conflicting requirements ResistivitySheet resistance L/W=# squares

2. VSLS: Shallow and Heavily Doped Junctions

3. Historical Development and Basic Concepts Development (40 years) in predeposition Solid-phase diffusion from glass layer. Gas phase deposition at high temperatures (B 2 H 6, PH 3, AsH 6 )  reproducibility; good only for solid sol. (too high N s ) Replace predeposition by ion implantation; good for bigger devices but difficult for small ones (TED) Return to diffusion 1960

4. Junction Formation – Process Choice

5. Dopant Solid Solubility Concentrations above SS limits result in inactive coplexes (defects,precipitates) Metastable electrical activation Practical concenrations for active P and As As complexes

6. Diffusion from a Microscopic Viewpoint Fick’s first law; D is a diffusivity, which depends on T not direction. Fick’s second law from: D = const. Fick’s II law in 3D

7. Analytic Solutions of the Diffusion Equation In steady state Gaussian Solution in an Infinite Medium

8. Gaussian Solution near a Surface Shallow implantation or deposited layer Delta layer’s thickness << X = 2 √(Dt) (final penetration) At the surface, if oxide growth occurs  dopant segregation

9. Error Function Solution in an Infinite Medium Tabulated (or approximated) Unlimited dopant supply from the source

10. Error – Function Solution near a Surface Surface concentration set by the solubility limits of the dopants C S (T) Q erfc increases with time. Q gauss is constant in time; it is set by a predeposition (erfc) preceding the diffusion (drive-in)

11. Intrinsic Diffusion coefficients of Dopants in Silicon Arrhenius fit Fast Diffusers Slow Diffusers Intrinsic Diffusion means that N dopant < n i @ diffusion T 1000 °C, n i = 7.14 * 10 18 cm -3 (1.45 * 10 10 cm -3 @ RT). @ High dopant concentrations the diffusion is enhanced.

12. Successive Diffusion Steps T 1 followed by T 2 : Dt is a measure of thermal budget Transient Enhanced Diffusion (TED) and Concentration Enhanced Diffusion (CED) when D increases with C and/or crystallographic/point defects Computer Simulation includes diffusion enhancement Equiv. time

13. Design and Evaluation of Diffused Layer Surface Concentration determined from Rs (or  s ) and x j measurements Irvin’s curves for erfc and Gaussian profiles Example: Design a B diffusion process  s =900  /sq., xj = 3 µm for substrate C B = 10 15 cm -3 Pick 1100 °C  D=1.5 * 10 -13 cm 2 sec -1 t = 6.8 hour (ex.: the well process) Use : Q=C(0,t)(  Dt) 1/2= 4.3*10 13 cm -2 implanted or predeposition From predeposition @ 950 °C, C sol.sol =2.5*10 20 cm -3, D=4.2*10 15 cm 2 sec -1 – not valid since C spread >>n i Delta function approx. Dt predep =2.3*10 -14 << Dt drivein =3.7*10 -9 So Gaussian distribution is correct for the drive-in process

14. Manufacturing methods and Equipment Furnaces: horizontal,vertical (100 °C/min ramp) Temperatures: 800-1100 °C, N 2 (+O 2 low ) or Ar, O 2 when oxide must be grown 750-800 °C@ 5-10°C/min to 1000-1100 °C (  warpage) So  with T For TED defects induced by implantation show  D at low T than at high T. RTA goal:no diffusion but damage annealing/dopant activation Issues: 100 degC/sec ramp Single wafer processing. 1-100 sec process. Wafer T and uniformly measurement and control.

15. Measurement Methods SIMS – Secondary Ion Mass Spectroscopy – sensitivity 10 16 –10 17 cm -3. Analysis of chemical concentration of dopants (both active and non active) Mass analyzed and counted  C(X) As, P, Sb  bombarded with Cs as primary ions  produce dopant ions. B, In  O (oxygen) “Knock on” – incident beam recoils atoms into the substrate(  with ion mass Cs>O  degrades depth resolution Sputtering rate at the surface increased by oxide  use lower energies down to 200 eV- 5keV to decrease sputtering (important for shallow junctions) Multilayer structures show matrix effect (sputtering yield and ion yield) and mixing (heavy Cs) Use oxygen bleed – keep ionization yield constant. Problems:

16. Spreading Resistance R(x)   (x)  n(x) Compare with C(x) from SIMS to get dopant activation. (information on defects, clusters etc.) 8’- 34’ From Wolf, VLSi Era

17. Sheet resistance Four point probe. (Figure 3.12); for shallow junctions use the Van De Pauw structure Capacitance Voltage C-V of a MOS Capacitor  C  X D (V)  x D (N)  N(x) TEM Cross Section Preparation of samples: Very complex Delineation by etching the doped silicon in HF: HNO3:CH3COOH 1 : 40 : 20 Doped silicon etch rate depends on dopant concentrations

18. 2D Electrical Measurements Using Scanning Probe Microscopy Random distribution depends on the doping concentrations 1.3nm @ 10 20 cm -3, 6.2 nm @ 10 18 cm -3, 28.8 nm@ 10 16 cm -3 STM – Scanning Tunneling Microscopy not useful  scanning capacitance (from STM or rather AFM) and Scanning Resistance. Problems: Cross-Section=preparation hard, Image interpretation (C  N) Inverse Electrical Measurements IV, CV of devices may not match simulated characteristics  make corrections as to the doping profiles.

19. Models and Simulations Numerical Solutions of the Diffusion Equations. No predetermined boundary conditions. Planar density of atoms Atoms jump between planes Hopping to a vacancy or exchange places with frequency For stable numerical solution Max. numerical interval means that we cannot have more than available number of atoms to jump in  t Adjacent concentrations calculated  SUPREM etc

20. Modification to Fick’s Loss to Account for Electric Field Effects. Set up by the dopants @ increased concentrations Very strong effect of  at low concentrations The enhancement of diffusion by the electric field is by a factor of two @ high concentrations. F.-II Law

21. Modifications to Fick’s Laws to Account for Concentration Dependent Diffusion ISO Concentration experiments: B 11 (substrate) B 10 diffusant give D A eff Neutral and charged point defects Different Activation Energies Ex: As at 1000°C For 1*10 18 cm -3  D As = 1.43*10 -15 cm 2 sec -1 For 1*10 20 cm -3  D As = 1.65*10 -14 cm 2 sec -1 5x10 20 cm -3 10 18 cm -3 Experiment.

22. SUPREM Simulation deposition  Function doping CED Boxlike As Due to  filed effects

23. Segregation of Deposits at Interfaces Ex : Oxide Silicon Segregation h is the interface transfer coefficient. SIMS - for thick oxides can give k o (difficult measurements). - for thin oxides SIMS is inadequate (no steady state reached). So use V T or C-V areas. B segregates into the oxide P piles up at the Si Surface  Snowplow during oxidation. Flux:

24. Segregation; Interfacial Dopant Pileup. D AS << D p  steeper profile close to the interface. Dopant Loss at the surface - SIMS does not detect that. Important problem in small devices (dominant). Entrapped dopants can diffuse later. SUPREM includes a trapping flux. Very thin,  monolayer acts a sink for dopants (inactive there); Stripping of the oxide removes dopants.

25. The physical basis for diffusion on Atomic Scale Vacancy or exchange model. vacancy Typical diffusion in metals  use XRD to measure changes in lattice constant with T  extract vacancy concentration. For Si it is below the detection limit of XRD. V models used earlier for diffusion in Si especially CED; N v = f(E F ) Dopant and Si-I diffuse as bound pairs  split Dopant stays in the substitusional position, I released. Si – Interstitial (only) used in diffusion of dopants. Both are “Interstitial –” assisted diffusion. Kick - OutInterstitiency The role of interstitials-kick-out