MSc Experimental Research on FinFETs


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I have experimentally studied an unconventional field effect transistor (FinFET) with a gate length of 40 nm in the subthreshold regime. Prepared devices, build a cryogenics setup together with 3 Researchers, analysed and extracted experimental data and interpreted these using the Richardson model, Yuan Taur’s model and simulations of A.Burenkov et al.
I am proud to be the first with the following experimental evidence: (1) artificial corner effect (2) interrelated physical dimensions (punch through effect) (3) a good electrostatic gate to channel coupling for devices smaller than 40 nm which could be used for

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MSc Experimental Research on FinFETs

  1. 1. Subthreshold study of undoped symmetric trigate nFinFETs Parvesh MSc project 1
  2. 2. Outline • Motivation • MOSFET (analogy) • shortening • Bands • doping • materials joined • undoped FinFET • Application of an electric field • Yuan Taur’s model • Where does current flow? • What is known in general • Setup • Results • Punchthrough (VT fluctuations), • Large/small conducting area • Barrierheight in these regions • Conclusions 2
  3. 3. Motivation GL • MOSFET- switch electronic signals • Where ? Look at your mobile phones, computers,…….. • And ask yourselves: Did it have the same size 50 years ago? The same speed maybe? • Gordon Moore: shrink at certain pace.. A human hair is 90 000 nm thick • Smaller → Faster and Cheaper and GL ~ 2000 smaller thus better • But for how long? • Nowadays: decreasing control of the current flowing in this structure • Nowadays: new geometries for better control of the current flowing in this structure • We explore this control of current flow in a new geometry switch: FinFET 3
  4. 4. Water analogy (1) Water ↔ Current When a wall present between source/drain reservoirs is pulled up : water flows (on) Good control over leakage through or over the wall and a well set open/closed state. 4
  5. 5. MOSFET – a simple on/off switch GL Current flow – high/low gate voltage much as a light switch functions Materials are physically joined: • Source – part where current flows from • Drain – part where current flows to Dimensions are 1000 000 • Gate – tune the on/off state of the device smaller than the light switch • Channel – region where current flows Current • Gate dielectric – isolation of the gate 0 VT gate voltage 5
  6. 6. Water analogy (2) Analogy with care Shortening of the canal will cause problems: • water can flow under the closed wall • building a wall might even be a problem 6
  7. 7. From MOSFET to FinFET via the scaling path GL > 50 nm solution < 50 nm Undoped channel trigate FinFETs Moore’s law Good : the smaller the faster With better control over the (1/GL), more switches in the entire Si body same area (GL2) Bad : (G and S/D) Leakage currents, VT variations 7
  8. 8. Electric current ? Physicists think of bands first! Conduction band edge E2 gap EF E1 Valence band edge distance EF is Fermi energy atomic levels Overlapping for example orbitals V ~ 1/r 2 Schrödinger's equation: − ∇2Ψ + V Ψ = EΨ 2m * Pauli exclusion principle 8
  9. 9. From semiconducting to conducting E0 Conduction band edge Conduction band edge EF EF Valence band edge Valence band edge Undoped n-doped Si shifts EF from midgap to EC Silicon highly doped Si - metal 9
  10. 10. Electronic structure of both joined E0 eγ Conduction band edge Conduction band edge EF EF Valence band edge Valence band edge Undoped n-doped Si Silicon Difference in valence and conduction band results in built-in potential 10
  11. 11. Band structure of undoped channel trigate nFinFET: flatband situation HfSiO HfSiO Si-body Gate Gate Fermi level Ec n+ S/D EF EF y Ei x Ev z Finwidth • Band alignment • Si is undoped and electrically isolated from A potential barrier results the rest from source-to-drain By lowering this barrier/wall by the gates close to S/D Fermi level current will flow through the structure 11
  12. 12. Theoretical result (1): Yuan Taur Screening of the gate field bend the bands y x z qψ ∂ 2ψ q = ni e kT ∂z 2 ε Si Current flows in the region where the source-to-drain barrier is lowest No experimental foundation of these predictions 12
  13. 13. Theoretical results (2): A.Burenkov et al. and F.J. Garcia et al. y y Y y z Z z z Sharp corners rounded corners n 13 Z Z
  14. 14. What is known of this trigate FinFET ? 4nm2 < 50 nm Proposal: Use undoped Si-body Corner effect 1. Flexible VT set by metal gate 2. good capacitive coupling over the body 3. No Corner effect: round corners 4. S/D depletion regions could overlap punchthrough (FW > GL) Aim: experimental support for undoped 14 channel trigate nFinFETs
  15. 15. Study of temperature dependent switching behavior of nFinFET Richardson-Dushman equation: * e ⎛ Eb ⎞ G = SA * T exp ⎜ − ⎟ kB ⎝ kBT ⎠ Y y y Z z z Large area Small area From which the area where current flows, barrierheight and capacitive coupling can be extracted 15
  16. 16. Experimental setup Undoped trigate Finfet S D 80 μm G 16
  17. 17. Dataset = 40 nm e ⎛ Eb ⎞ G = SA* T exp ⎜ − ⎟ kB ⎝ k BT ⎠ 4 Different Finwidths: 25 nm, 55 nm, 125 nm and 875 nm 17
  18. 18. Conductance vs. gate voltage at 100 K Linearity: thermionic transport Threshold voltage deviation once FW > GL: Punchthrough Leakage currents, typical for Punchthrough 18
  19. 19. Large FinFET GL yj ydmax LI Decrease of GL wrt. S/D FW = 3 μm < GL = 10 μm depletion region produces a leakage path No leakage current at 300 K 19
  20. 20. Active areas Y y y Z z z Below VT Above VT At threshold voltage around 14 nm2 for FW = 25 nm 20
  21. 21. Active areas Y y y Z z z Below VT Above VT • Small area at the interface • Corners n Z Z 21
  22. 22. Barrierheight e Vg = -α Eb 22
  23. 23. α , the coupling α from stability Device nr. FW (nm) α from fit measurements 1 25 1 0,7 4 55 0,7 0,8 8 125 0,14 0,7 2 875 0,03 0,8 •Punchthrough → couplings do not coincide •Small devices → good correspondence Robustness e Vg = -α Eb 23
  24. 24. Conclusions We determined the control of current flow in an unconventional nano- transistor with variable temperature measurements. It was predicted to have better control. •Wide devices show Punchthrough •These undoped channel devices show an artificial Corner effect, which could degrade device performance •Two independent measurements, thermionic measurements and stability diagram measurements, revealed same gate coupling This work will be published 24