Thin Film Deposition. Reading Assignments: Plummer, Chap 9.1~9.4

Transcription

1 Thin Film Deposition Reading Assignments: Plummer, Chap 9.1~9.4

2 Thermally grown Deposition Thin Film Formation Thermally grown SiO 2 Deposition SiO 2 Oxygen is from gas phase Silicon from substrate Oxide grow into silicon Higher quality Both oxygen and silicon are from gas phase Deposit on substrate surface Lower temperature (no silicon bond breaking) Higher growth rate

3 Configuration of IC Nitride Oxide PD2 ARC PMD or ILD1 Metal 2, Al Cu W W USG Al Cu Metal 1, Al Cu BPSG PD1 IMD or ILD2 WCVD Sidewall spacer STI n n USG Pwell Pepi Pwafer p p Nwell STI TiN CVD

4 Primary Concerns Deposition Dielectric (oxide, nitride, SiOC, HfO 2 etc. by CVD) Semiconductor (polysi by CVD) Metal (Al, Ti, TiN, Ta, Co, Ni, Cu, Pt, Ru, W by CVD or PVD) Concerned issues Quality (composition, contamination, defect density, mechanical and electrical properties) Uniformity (thickness, conformal step coverage) Filling (via or contact hole)

5 Chemical Vapor Deposition Gases are introduced into the deposition chamber that react and form the desired films on the surface of substrate Reaction take place on the surface instead of in the gas stream (particulates) Sometimes reactants are not available in gaseous form, in which case liquid source may be used. Carrier gas : H 2, N 2, Ar

6 Chemical Vapor Deposition Atmospheric pressure chemical vapor deposition (APCVD) Low pressure chemical vapor deposition (LPCVD) Plasma enhanced chemical vapor deposition (PECVD) Metal organic chemical vapor deposition (MOCVD) Atomic layer chemical vapor deposition (ALCVD) Rapid thermal chemical vapor deposition (RTCVD)

7 Chemical Vapor Deposition 1. Reactant transfer 2. Reactant diffusion 3. Absorption 4. Surface Reaction 5. Desorption of byproducts 6. Byproduct diffusion 7. Byproduct transfer

8 Reaction Mechanisms C G Boundary Layer Gas F 1 F 2 C S Silicon h G : gas phase masstransfer coefficient k s : surfacereaction rate constant F1 = hg( CG CS) F SteadyState = kc 2 s S F1 = F2 k S CS = CG(1 ) hg Growth Rate S 1 F CG kshg ν = = N N k h Number of atoms per cm 3 G Step 2 Step 35

9 Reaction Mechanisms mole fraction in gas phase Y = C C G T Concentration of all molecules in the gas phase SiCl 2H Si 4HCl 4( g) 2( g) ( s) ( g) Y CG PG PG = = = C P P P '... T Total G G Deposition Rate C k h T S G ν = Y N k S h G

10 Refresh: In DealGrove Model DealGrove Relation (linearparabolic growth law): 2 ox T AT = B( t τ ) 2 Tox ox for t >> τ, t >> A 2 /4B = for (t τ) << A 2 /4B T ox Bt Diffusioncontrolled B = ( tτ ) A Reactioncontrolled t ox A, B : temperature, ambient composition, pressure and crystalline orientation τ is related to the initial oxide thickness t Si 0.44 t ox Top oxide surface Silicon Original silicon surface Oxidesilicon surface * DC x Ax i i A= 2 D[ ] B= τ = k h N B s I

11 Dominant Reaction Mechanisms Deposition rate is determined by k s or h G Two limiting cases Surface reaction controlled case Mass transfer controlled case If k s << h G C ν N If h G << k s C ν N T T ky S hy k S limited deposition is VERY temp sensitive. h G limited deposition is VERY geometry (boundary layer) sensitive. G

12 Arrhenius Plot of Growth Rate k S term with k S =k 0 exp(e a /kt) Growth velocity (log scale) Net Growth Velocity h G term with h G =constant Temperature sensitive Mass Transfer Controlled Flow dynamic sensitive Mixed 1/T Reaction Controlled

13 Different precursors. Change h G CVD Si by Different Precursors

14 APCVD k s decreases as the boundary layer thickness increases. Gas depletion along the length of the susceptor. The difficulties of controlling the boundary layer and gas depletion make APCVD typically not a preferable method in furnace batch progress.

15 LPCVD Standup wafers Furnace with resistance heaters Trap Exhaust scrubber Deposition Rate T S G ν = Y N k S h G If many wafers are deposited at once (furnace), operation at the surface reaction controlled region is preferable. Diffusivity is inversely proportional to the number of collisions Diffusivity increases by 100 times as pressure is reduced from 760 to 1torr C k h Vaccum Pump

16 Step Coverage Step coverage is determined by both the arriving angle and precursor surface mobility (sticking coefficient) Sidewall step coverage = b/a Conformity = b/c Bottom step coverage = d/a Overhang = (cb)/b h d w c b Aspect ratio = h/w a Arriving Angle

17 Surface Adsorption Chemisorption & physisorption Binding energy Distance from surface Chemisorbed precursor (bond formation) Low surface mobility Substrate surface Physisorbed precursor (van der Waals or dipoledipole ) High surface mobility

18 Precursor & Adsorption Pyrophoric, toxic, explosive Perfectly symmetrical tetragonal structure neither chemisorb nor physisorb Very Reactive SiH 3, SiH 2, SiH (chemisorption) Low surface mobility overhang H H Si H H

19 Precursor & Adsorption TEOS (tetraethyloxysilane) Not perfectly symmetric due to large size of ethyl group Hydrogen bond physisorb H High surface mobility H H C H H C H H C H C O O Si O H C H C H H O H H H H H C C H H H

20 Step coverage TEOS Silane

21 Void Formation Metal Dielectric Metal Dielectric Void Metal Dielectric

22 Higher current density Void Discontinuous

23 Step Coverage, Pressure and Surface Mobility APCVD (low MFP) No mobility LPCVD (high MFP) No mobility High mobility

24 What is Plasma? Plasma The plasma state is the fourth state of matter. Solid Liquid Gas PLASMA Energy (electricity) Input Gas Flow PLASMA Output Gas Flow Vacuum: to gain enough energy PLASMA: an ionized gas with equal numbers of positive and negative charges. Jyyang in NDL

25 Ionization Ionization: e A 2e A Ionization collisions generate electrons and ions. It sustains the stable plasma. Ionization rate between % 5% Nucleus Nucleus Free Electron Orbital Electron Free Electrons

26 ExcitationRelaxation ExcitationRelaxation: e A e A * A* A hν (Photos) Different atoms or molecules emit different characteristic glow colors. The change of the glow colors is used for the endpoint detection. Grounded electron Excited electron Impact electron hν h: Planck Constant ν: Frequency of Light Excited State Impact electron Nucleus Nucleus hν Ground State

27 Dissociation Dissociation: e AB e A B Electron collides with a molecule, it can break the chemical bond and generate free radicals. Free radicals have at least one unpaired electron and are chemically very reactive. e Free Radicals A B Molecule A B e

28 Sheath Potential x Electrode Bulk plasma Sheath Region V p Electron moves much faster than ions and will charge the electrode (chamber wall) negatively. Sheath Potential Dark space V f

29 Light emission E Plasma Potential Dark space sheath Potential distribution of static plasma Dark space V=0 Free electron Neutrals Ions E wall e A 2 e e A 2 (ionization) e A 2 e A A (dissociation) wall e A 2 2e A A (dissociative ionization) e A 2 e A 2 * (excitation) Plasma: electrons, ions, radicals, atoms, molecules

30 Potential Distribution DC Biased Potential distribution V(x) Cathode (target) Cathode Glow Ar Anode Wafer Cathode Sheath Negative Glow Anode sheath

31 Volt Potential Distribution RF Biased RF Source Plasma Potential DC Bias RF potential Time V V A = A m

32 PECVD Deposition of SiO 2, Si 3 N 4 as Al film has already been on the wafer Al melting point : 660 C <500 C APCVD or LPCVD can be used but will result in poor quality (porous) PECVD is employed Low temperature ( C) Excellent conformal deposition

33 Gas Inlet SiH 4,O 2 PlasmaEnhanced CVD RF power input (13.56MHz) Electrode /Target Glow Discharge Wafers Electrode Heater Vacuum Equal size of electrodes. Electron accelerated by the plasma potential to sustain the plasma. Free radicals are mainly responsible for the deposition.

34 Physical Vapor Deposition transport Gas phase Gas phase Physical Process Only Evaporation Condensation Condensed phase solid Condensed phase solid Vacuum evaporation Sputtering Molecule beam epitaxy

35 Applications: Interconnection

36 Physical Vapor Deposition PVD uses mainly physical process to deposit the films, allowing almost any material deposition Molten source(evaporation) or sputter Usually, PVD operates in a very low pressure environment Surface reaction occurs rapidly and the probability of atoms rearrangement is low Thickness uniformity, shadowing by surface topography, step coverage are challenging issues

37 Thermal Evaporator Wafers Aluminum Charge Aluminum Vapor 10 6 Torr To Pump High Current Source

38 Electron Beam Evaporator Wafers Aluminum Charge 10 6 Torr Aluminum Vapor Electron Beam To Pump Power Supply

39 evaporate Vapor Pressure Vapor pressure for reasonable deposition rate sublime Need to deposit at low pressure to reduce impurities.

40 DC sputter system V(DC) Electrode /Target Glow Discharge Wafers Electrode Heater Gas Inlet Vacuum

41 DC sputter system Potential distribution V(x) Cathode (target) Cathode Glow Ar Anode Wafer Cathode Sheath Negative Glow Anode sheath

42 DC sputter system Al _ Al target Dark space or sheath O e Secondary electron Ar Ar o Ar Al O Ar o Negative glow e Ar e Al Al Al Wafer surface Positive ions are mainly responsible for physical bombardment.

43 DC sputter deposition Target act as electrode,thus source material must be conductive Al, Ti, W Reactive Sputter Deposition Reactive gas can be introduced into system TiN, TiO 2 Controlling stoichiometry is difficult

44 RF Sputter Deposition Since the target is used as an electrode, it should be conductive Otherwise, plasma could not be initiated when the target is nonconductive RF is needed for sputtering oxide, nitride MHz is used

45 RF Sputter Deposition Matching Network RF Generator Wafers Electrode /Target Glow Discharge Electrode Gas Inlet Vacuum Heater

46 Volt Potential Distribution RF Biased RF Source Plasma Potential DC Bias RF potential Time V V A = A m

47 Problem in PVD Technology Overhangs at contact openings Thinner deposition on the sidewall Discontinuous at contact bottom

48 Effect of arrival angle distribution Low MFP: Wide range of arrival angle distribution High MFP: Narrow range of arrival angle distribution

49 Arriving Angles, Contact Holes Larger arriving angle Smaller arriving angle PSG Silicon Nitride

50 Schematic of Magnetron Sputtering Magnets Target Higher plasma density Magnetic field line Erosion grove Right angel between the direction of electric and magnetic field > spiral motion of electrons.

51 PVD vs. CVD CVD: Chemical reaction on the surface PVD: No chemical reaction on the surface CVD: Better step coverage (50% to ~100%) and gap fill capability PVD: Poor step coverage (~ 15%) and gap fill capability PVD: higher quality, purer deposited film, higher conductivity, easy to deposit alloys CVD: always has impurity in the film, lower conductivity, hard to deposit alloys