Overview

   4 processes  Physical Vapor Deposition (PVD)     Chemical Vapor Deposition (CVD) Electrochemical Deposition (ECD-used for copper) Spin Coating (new) (Ion implantation is dealt with in FEOL) Purely physical processes: PVD, Spin coating Chemical: CVD, ECD  Note: ECD is also used as an acronym to indicate “electrical critical dimension” or “electrical CD”.

25-Apr-20 2

Deposition (general)

       Uniform step coverage, control Adhesion and Step Coverage  Conformal deposition (may not be what you want!) Void free fill  narrow space vs wide space Grain Structure, thin film properties Sticking co-efficient Contamination free; For alloys, composition Extra: Self planarizing    Metals deposited by PVD: Ti/TiN, Ta/TaN, Cu-seed, Al by CVD: W, Ti, Cu-seed by Electrochemical: Cu

3 25-Apr-20

Evaporation

 About 2 decades back  No Reaction Involved   For Aluminum, or gold in vacuum (~ micro torr)  To avoid oxidation  To get uniform coating   Chamber is also heated Evaporation source may be crucible heated with coil, electron beam melting, or just tungsten filament coated with other materials.

 Depositing alloys  E-Beam, multiple e-beams for alloy co-evaporation  Hot Plate (with alloy wire flash) Evaporation

25-Apr-20 4

Physical Vapor Deposition

   No reaction involved a.k.a. Sputtering Not the same as evaporation  Operating temperature lower than melting point  Argon ions, low pressure    dep

25-Apr-20

Control of composition Dielectric deposition Step coverage improvement (planar source vs point source)  Rotating wafer holder and heating wafer improves step coverage  Improve adhesion by sputtering ‘wafer surface’ prior to

5

Physical Vapor Deposition

 Dry argon or neon  moisture == oxidation     Pressure nano torr Argon increases pressure ==> Control amount of Ar introduced milli torr pressure during operation Ar ionized (using electrodes)   Low pressure ==> longer l (60 m vs few microns) Line of Sight vs uniform   DC/Triode/RF/magnetron sputtering Ion Metal Plasma (IMP) and Collimated

25-Apr-20 6

Controlled supply of Ar

PVD: DC sputtering

-ve Al Shield Plasma Region Shield   Virtual leak Small -ve bias on wafer  Strong -ve bias ==> Wafer sputtering Vacuum pump +ve

7 25-Apr-20

PVD: sputtering

   Target sputtering ==> Heat Aluminum oxidation ==> Sputtering stops ==>Triode sputtering  Electrons not supplied by target, rather by a separate filament  Resulting films are denser (better)  Random direction of electron from coils (or target)  ==> low efficiency of Ar ionization  Confine the field using magnets  ==> better Ar ionization  ==> better (faster) deposition rate  ==> low Ar flow rate required

25-Apr-20 8

PVD: RF sputtering

    Works mainly if the target is metal For insulator, AC voltage (at radio frequency) is used Can be used to slightly roughen (etch) wafer surface  ==> better adhesion  a.k.a. sputter etch, reverse sputter, ion milling  Also removes any contaminant ==> better electrical contact Target much smaller than wafer+chamber  Dep Rate monitored with quartz crystal  Resonant frequency changes with film thickness  1A/sec accuracy

25-Apr-20 9

PVD: Basic Mechanisms

    Direct Sputtering Emitted (desorbed) sputtering (sticking coefficient)  ions stick well, neutral atoms stick < 1 Resputtering Surface Diffusion (in, out)  more with more curvature  filling in of corners is more  For good uniform deposition  Increase target width (angle of incidence distribution?)   Pull target away (long throw) (angle of incidence distribution?) Rotate wafers around the target (similar to increasing target width)  Desorption (Sticking coeff < 1) more important than surface diffusion for step coverage

25-Apr-20 10

PVD: Basic Mechanisms

 Sputtering Yield  No of Aluminum / No of Argon ions   Depends on angle of incidence and energy of ions can be > 1  Minimum energy needed for sputtering  RF Sputtering - Reduction of necking  For good uniform deposition  Ionized Metal Plasma (IMP)    Collimated beam Better step coverage Side wall (pre-dep, post etch) < 90 degress 0 90

11 25-Apr-20

PVD: Collimated Beam

-ve Al  Equivalent to long throw Shield Shield Plasma Region Controlled supply of Ar Vacuum pump +ve

12 25-Apr-20

Ionizing Controlled supply of Ar Shield

PVD: IMP

Ti RF Shield  Ti ions also present  Similar to collimated Plasma Region Coils  For liners, IMP Ti is very common Vacuum pump RF

13 25-Apr-20

25-Apr-20

PVD: Comparison

 Comparison of effectiveness of various methods  ©Hynix (Huyndai) Ltd Sticking Coefficient (=1? <1? )

14

PVD: Comparison

Evaporation Sputter Optimal location of target

25-Apr-20 15

CVD

    Reaction occurs on surface  nucleation occurs only on surface  similar to catalysis Relatively higher pressure compared to PVD lower mean free path Anisotropic dep  better step coverage  conformal dep   APCVD, LPCVD, PECVD, HDPCVD, MOCVD Epitaxy: VPE, LPE, MBE, ALD

25-Apr-20 16

CVD

 Growing silicon on silicon (for example) Inlet Induction Heating Laminar Flow Outlet    

25-Apr-20

Atmospheric Pressure (APCVD) Cold wall. Only the wafers and wafer holders are heated  Induction heating, radiation heating Reaction (Deposition) will occur only on the wafer Hot wall not preferred

17

APCVD

  Mechanism: Boundary Layer Diffusion in gas phase is important  decrease press, increase diffusivity, increase BL thickness  Mass transfer in gas phase, adsorption, surface reaction, surface diffusion to ‘nucleating sites’ and desorption of other products  Wafer sets are not horizontal Film Dep Rate 1/T

25-Apr-20 18

Vertical Reactor

APCVD

Barrel Reactor  Horizontal  Only few wafers per chamber Heating by irradiation; better control than induction  Vertical (pancake)  few wafers per chamber, but no depletion 

25-Apr-20

Barrel Reactors  more capacity

19

APCVD

    Tilted base (susceptor) vs horizontal base Gas velocity, boundary layer thickness Diffusion - BL thickness, Kinetics-Temp High temp operation (1000 C) for single crystal growth   APCVD: Tilted base Low pressure CVD (LPCVD)  Kinetics controlled  Keep wafers close by, change temp  ==>Reactor design based on kinetics/ mass transfer

25-Apr-20 20

APCVD

 Reactions

SiCl

4  2

H

2 

Si

 4

HCl

Etch + Dep

SiH

4 

Si

 2

H

2 2 

Si

 2

HCl

 Single Crystal growth

Growth Rate (

m

/min)

Single gas Low temp Growth

25-Apr-20 0

Etch

Mole fraction of SiCl 4 0.4

21

LPCVD

 In APCVD, sometimes, reaction

may

occur in gas phase, forming particles that fall on the wafer  In LPCVD, gas phase reaction is less likely to occur  Lower temperature (i.e. Compatible with Al)  Pressure = bit less than a torr

25-Apr-20 22

LPCVD

  Hot wall (vs cold wall for APCVD) Reaction Limited (vs Mass Transf Limited for APCVD)  Used for poly, oxide, nitride, W  Poly (600 C)

SiCl

4  2

H

2 

Si

 4

HCl SiH

4 2 

Si

 

Si

2

H

2  2

HCl

25-Apr-20 23

LPCVD

 Silicon Nitride (300 or 700 C). Used for LOCOS 3

SiH

4  3

Si N

3 4  12

H

2 3

SiH Cl

2 2  4

NH

3 

Si N

3 4  6

HCl

 6

H

2  Oxide (<500 C)

SiCl

2 

SiH

 4 2

NO

2 

C H O

2 5

O

2  4 

Si



SiO

SiO

2 

O

3 2   2  2

H

2

N

SiO

2 2 

HCl

900 C 450 C;Quality issue 

Others

400+ C  TEOS, also used for PECVD, HDPCVD

24 25-Apr-20

LPCVD

 W Dep 2

WF

6  3

Si

 2

W

 3

SiF

4   Substrate reduction

WF

6  3

H

2 

W



HF

W-Silicide/ Ti Silicide

WF

6  2

SiH

4 

WSi

2  6

HF



H

2

TiCl

4  2

SiH

4 

TiSi

2  4

HCl

 2

H

2 ~300 C

25 25-Apr-20

LPCVD: Reactors

Controlled temperature (and not necessarily uniform) Gas IN Vacuum  Horizontal tube reactor  Deposition of oxide on Al  Temp of 600 C causes alloying  Need to go for PECVD

25-Apr-20 26

PECVD

    

C H O

2 5  4

Si



O

3 

SiO

2 

Others

Plasma can also be used for etching RF used for creating and sustaining plasma 400- C Used for scrubbing (cleaning) the wafer, before dep dep rate can be high, but too high a dep rate is not good (film stress, grain structure etc)  BPSG, PSG by introducing phosphine / diborane ( B2H6)  and ‘little’ bit of sputtering  Introduction of Ar in plasma causes simultaneous dep Deposited film is dense (high density plasma chemical vapor deposition= HDPCVD = HDP)

25-Apr-20 27

CVD: Other

  MOCVD (eg tri methyl gallium / Arsine) MBE (like scanning)  Easy to form hetero-junction   slow, good control growth (along with doping)  VPE  Introduce doping gas in the CVD; obtain epitaxial dep with doping  ALD  Brief Mechanism

25-Apr-20 28

Epitaxy Issues

 Epitaxy: Issues  Haze (oxidation)  spike (accelerated local growth)   stacking fault, dislocation, slip, out diffusion/ auto doping (reverse of out diffusion)

25-Apr-20 29

Electrochemical Dep

 Copper properties vs Deposition techniques

CVD Resistivity(

m 

cm) Impurities >= 2 C,O Deposition rate(nm/min) ~100 Process temp. (



C) ~250 Step coverage Good Via-filling capability Environmental impact (waste) Good Good PVD Laser reflow 1.75—2 Ar >=100 RT Fair — — Melt — Electroless 2.6

~2 Seed layer <=100 50—60 Good Poor Good Good Good Fair-Poor Poor

 Grain size, electromigration

25-Apr-20 Electrolytic ~2 — ~200 RT Good Fair-Poor Poor 30

Electrochemical Deposition

  For copper Related technique:      Electroless Catalyzed on the metal surface may occur on other surfaces (bad) may occur in solution (very bad!) may need activation deposition temperature, fluid flow patterns  Electrochemical Dep  dep in pattern: proprietary solution  need liner/ Cu seed layer  field in corners

25-Apr-20 31

Electrochemical Deposition

     Fluid flow pattern/ mass transfer limitations kinetics controlled by potential, temperature Film quality, temperature , anneal Role of additives Fill, void, Super fill (next few pages)  suppressors in solution  depletion in small trenches  Grain Size, growth in room temperature, at higher temperatures

25-Apr-20 32

Electrochemical Deposition

 Usually < 50% of max current  max current at mass transfer limited region  operation at tafel region (reaction limited region)  uniform voltage distribution is necessary (seed layer)  convection is key  DC or wave form  pulsed  can etch ‘sharp’ regions (more planarity)  need sophisticated control   acidic pH (sulfate, pyrophosphate etc) basic pH (cyanide and other solutions)

33 25-Apr-20

 Lab unit

Dep Unit

25-Apr-20 34

Sample image of Dep unit

 Controller for all parameters  © semiconductor international

25-Apr-20 35

25-Apr-20

Types of Fill: SEM

© casewestern univ

36

25-Apr-20

Types of Fill: Schematic

37

25-Apr-20

Effect of Liner/Seed

38

Example of SuperFill

 SEM Image ©semiconductor international

25-Apr-20 39

Example of superfill

 SEM Image  ©www.future-fab.com

25-Apr-20 40

Electrochemical Deposition

 Organic additives used  mechanism not very clear  could be steric effect  some of them consumed in reaction  sometime incorporated in the film  Suppressors (levelers, carriers)  carriers : polymeric surfactants (poly ethylene glycol PEG)  in presence of Cl-, adsorb on surface  levelers: multiple charged structures   adsorb on corners/edges etc, level large size, cannot go inside trench

25-Apr-20 41

Electrochemical Deposition

 Change additive concentration: plating voltage changes  Boundary layer >> feature size  Diffusion is important  additive must be consumed (otherwise, surface will be saturated)  Accelerators (brighteners)  unsaturated compounds with polar group  sulfur, oxygen, nitrogen based groups  may help nucleation  many small grains formed (brighter surface)  more growth at the trench/via bottom

25-Apr-20 42

Electrochemical Deposition

 For good dep  monitor concentration and replenish  remove products (oxidize in presence of uv)  photochemical catalysis to enhance oxidation

Summary

 Main dep technique for copper  superfill  need to control composition well  Environmental issues (waste disposal)

25-Apr-20 43

Spin Coating

    Similar to photo resist coating used for organic / mix of organic+inorganic ILD DVD, CD...

mainly used for low-k materials  low temperature  better flow characteristic / self planarizing  adhesion

25-Apr-20 44

Spin Coating

   Form solvent puddle at the center spread it by low rpm spin (500 rpm) thinning by high rpm spin (2000 rpm). Dep rate, uniformity affected by  viscosity  solid content   speed time 

© GATech

remove the ‘bead’ formation at the edge (due to surface tension)  using solvent back wash  evaporation of solvent (drying)  also depends on airflow over the wafer

25-Apr-20 45

Spin Coating: Issues

 Presence of large solid particles  comets  variable solvent evaporation rate in multi solvent solutions  Striations  due to surface tension effects and instabilities Silica-titania comet Silica-titania striation ©univ ariz

25-Apr-20 46

Spin Coating: Issues

 Striation/instability: Hypothesis Silica-titania striation ©univ ariz

47 25-Apr-20

Spin Coating: Issues

 Vacuum Chuck effects Vacuum chuck Coating on the glass

25-Apr-20

©univ ariz

48

Spin Coating: Issues

 Note: outside the O-ring variations  Near the ‘grooves’ clear variation  Hypothesis:  heat from from chuck compensates for cooling by evaporation  when there is no contact, cooling is not ‘prevented’  ==> lower evaporation when there is no contact  more pronounced in glass vs silicon (better heat conductor)

25-Apr-20 49

Spin Coating: Summary

 In R&D  Similar to photo resist coating  Goal is to use it for low-k dielectrics (insulators)  Issues yet to be resolved  Most of the issues in low-k are not with spin coating. They are with subsequent processes / integration

25-Apr-20 50