Doping and Oxidation

Transcription

1 Technische Universität Graz Institute of Solid State Physics Doping and Oxidation Franssila: Chapters 13,14, 15 Peter Hadley

2 Technische Universität Graz Institute of Solid State Physics Doping Add donors (n-type) or acceptors (p-type) cm -3 impurity limit cm -3 solubility limit Dopants added during crystal growth (whole wafer) neutron transmutation (whole wafer) during epitaxy (layers) diffusion (local) ion implantation (local)

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4 Doping determines the carrier concentration

5 Crystal growth Czochralski Process add dopants to the melt images from wikipedia

6 Crystal growth Float zone Process Neutron transmutation 30 Si + n 31 Si + 31 Si 31 P + image from wikipedia

7 Chemical vapor deposition Epitaxial silicon CVD SiH 4 (silane) or SiH 2 Cl 2 (dichlorosilane) PH 3 (phosphine) for n-doping or B 2 H 6 (diborane) for p-doping. The doping can be adjusted in layers. image from wikipedia

8 Gas phase diffusion AsH 3 (Arsine) or PH 3 (phosphine) for n-doping B 2 H 6 (diborane) for p-doping wafers in a batch.

9 Gas phase diffusion Fransila C 1000 C for 1 hour ~ 1 m

10 Constant Source Diffusion dc dt For a constant source concentration C 0 at the surface: 2 D C Diffusion equation z Cz ( ) Cerfc 2 Dt t erfc x 1 e dt x 0 The concentration decreases about linearly z erfc 1 2z

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12 Limited Source Diffusion dc dt 2 D C Diffusion equation For a limited source concentration C 0 at the surface: Cz Cw 0 ( ) exp 4 Dt 2 z 4Dt w C 0 2 Cw z 4 Dt Dt 0 ( ) 1 4 Cz

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14 Diffusion is thermally activated D exp E A D0 kt B E A L 4D0 exp t kt B Diffusion length For P diffusion, 1 h, at 1200 T = 1473 K L = 1.3 m 0

15 Diffusion Interstitials and vacancies diffuse quickly and assist the dopants. BH 3 diffuses faster than B. Diffusion depends on doping concentration.

16 Solid solubility limits www-eng.lbl.gov/~shuman/next/.../diffusionin%20siliconpdf.pdf

17 Predeposition/Drive-in Oxide diffusion mask ~ 500 nm SiO 2 SiO 2 Predeposition process spin-on glass ion implantation constant source diffusion Drive-in process limited source diffusion

18 Patterned dopant regions

19 www-eng.lbl.gov/~shuman/next/.../diffusionin%20siliconpdf.pdf

20 Ion implantation X-rays are generated

21 Ion implantation More accurate control of concentration Better lateral confinement Low temperature Complex profiles through multiple implantations Less sensitive to surface preparation Requires an anneal to eliminate damage and activate dopants Dopants diffuse during the anneal Possible to implant above the solubility limit

22 Ion implantation Photoresist, oxide, or nitride. Should be thicker than projected range. High doses burn the photoresist on and make it hard to remove. Most dopants are at the projected range R p.

23 Ion implantation

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25 Rapid thermal anneal (RTA)

26 Channeling Ions travel deep into the crystal when the beam is aligned with a crystal o axis. Implantation is often done at 7 off-axis to avoid channeling.

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28 Plasma immersion ion implantation Flat panel displays and solar panels.

29 Technische Universität Graz Institute of Solid State Physics Oxidation The properties of SiO 2 were more important than Si in making Si the dominant semiconductor material. Thin gate oxide ~ 1 nm in MOSFETs Tunnel barriers in flash memory Dielectrics in capacitors Electrical isolation of components Passivation layer to protect the circuits Oxide masks for diffusion and implantation Native oxide ~ 1 nm grows in air in hours but is not good quality Thermal oxide - grows slowly, best quality Deposited oxide - grows quicker, less quality

30 Dry oxidation Si (s) + O 2 (g) SiO 2 (s) 1 hour at 900 C produces 30 nm of thermal oxide. Used for gate oxide in MOSFETs and tunnel oxide flash memories. Good layer control. Low defect density at the Si/SiO 2 interface.

31 Wet oxidation Si (s) + 2H 2 O (g) SiO 2 (s) + 2H 2 (g) The water is steam. 1 hour at 900 C produces 130 nm of thermal oxide. Faster growth than dry oxidation.

32 CVD oxides Can be deposited at lower temperature Rougher surfaces, lower breakdown field, higher etch rate in HF

33 Deal-Grove oxidation model Oxide thickness z is first linear then grows like a square root 2 A A 4Bt zt () 2 The constants are thermally activated B B 0 E A B B E A exp exp kt A A kt B 0 B

34 Deal-Grove oxidation model Wet Dry Linear dependence: Limited by the reaction rate of Si and O 2 to SiO 2 Square root behavior: Limited by oxygen diffusion through the oxide.

35 SiO 2 is amorphous gate oxide Post oxidation anneal (N 2 then H 2 ) to passivate dangling bonds D it /cm 2 defects at the interface atoms/cm 2 Fransila

36 Oxides are under compressive stress Typical stress is 300 MPa Stress will bow the wafer density quartz 2.65 g/cm 3 oxide 2.2 g/cm 3 Young's modulus quartz 107 GPa oxide 87 GPa

37 Stoney's formula f Eh s 2 s 6 h (1 ) f s The stress f in the film depends on E s Young's modulus in the substrate v s Poisson's ratio of the substrate h s thickness of the substrate h f thickness of the film curvature Only holds for uniform curvature

38 Local Oxidation of Silicon (LOCOS) thin pad oxide for stress relief

39 Controlled removal of Si 1. Thermal oxidation (good control of thickness) 2. Etch Oxide with HF Period doubling of pattern

40 Controlled removal of Si Fransila

41 Technische Universität Graz Institute of Solid State Physics Complementary Metal Oxide Semiconductor (CMOS) The dominant technology for microprocessors Low power dissipation through the use of n-type and p-type MOSFETs