Lecture 8 Chemical Vapor Deposition (CVD)

Transcription

1 Lecture 8 Chemical Vapor Deposition (CVD) Chapter 5 & 6 Wolf and Tauber 1/88

2 Announcements Homework: Homework Number 2 is due on Thursday (19 th October). Homework will be returned one week later Thursday (26 th October). Solutions will be also posted online on the same day. 2/88

3 Announcements Term Paper: You are expected to produce a 4-5 page term paper on a selected topic (from a list). Term paper contributes 25% of course grade. Details are on the course website now. Topics will be listed online Today (Tuesday 17 th October). You need to list your favorite topics by Monday 23 rd October. You will be assigned a topic on Friday 27 th October. The term paper should be handed in at the start of class on Tuesday 21 st November. The term paper will be returned to you in class on Thursday 30 th November. 3/88

4 Announcements Mid-Term Exam: The exam will take place on Thursday (26 th October) at 10:00 am in Gleeson 100. You will have 90 minutes. Closed book and no notes. Will cover material covered in lectures 2-9. There will be a review lecture one week before (Thursday 19 th October). There will be no lecture on Tuesday 24 th October (exam preparation). 4/88

5 Useful Links UC Berkeley Notes: MIT Notes: /sp_2005_lecture04.pdf 5/88

6 Lecture 8 CVD Growth in CMOS Introduction to Chemical Vapor Deposition CVD Growth Mechanism Growth Rate Reaction Kinetics CVD In Practice 6/88

7 Next Lecture (After Midterm) We will look at thin-film growth dynamics. E diff = 3.12 ev E diff = 0.16 ev 0.25 ML 0.50 ML 7/88

8 CVD Growth in CMOS 8/88

9 Dry Etching Processes Start with your (doped) semiconductor wafer. n + p + p + n + n + p + substrate 9/88

10 Dry Etching Processes Oxide is grown over entire substrate (Lecture 7). oxide n + p + p + n + n + p + substrate 10/88

11 Dry Etching Processes Oxide is etched in certain regions to allow contact with doped regions (Lecture 16). oxide n + p + p + n + n + p + substrate 11/88

12 Dry Etching Processes Metal is deposited everywhere on wafer (Lecture 5). oxide n + p + p + n + n + p + substrate 12/88

13 Dry Etching Processes Metal is selectively etched (Lecture 14-16). oxide n + p + p + n + n + p + substrate 13/88

14 Dry Etching Processes Wafer is covered everywhere with SiN 3 (This Lecture). SiN 3 oxide n + p + p + n + n + p + substrate 14/88

15 Dry Etching Processes Photoresist is applied (Lecture 14-15). Photoresist SiN 3 oxide n + p + p + n + n + p + substrate 15/88

16 Dry Etching Processes Photoresist is treated (Lecture 14-15). Photoresist SiN 3 oxide n + p + p + n + n + p + substrate 16/88

17 Dry Etching Processes SiN 3 and photoresist is selectively etched (Lecture 6). Photoresist SiN 3 oxide n + p + p + n + n + p + substrate 17/88

18 Dry Etching Processes Photoresist is removed etched (Lecture 14-15). SiN 3 oxide n + p + p + n + n + p + substrate 18/88

19 Dry Etching Processes Metal is deposited everywhere on wafer (Lecture 5). SiN 3 oxide n + p + p + n + n + p + substrate 19/88

20 Dry Etching Processes Metal is selectively etched. SiN 3 oxide n + p + p + n + n + p + substrate 20/88

21 Introduction to Chemical Vapor Deposition 21/88

22 Methods for Thin Film Deposition Gallium, indium, zinc oxide Thin Film Deposition Electrochemical deposition Titanium nitride Silicon nitride Chemical Vapor Deposition (CVD) Sputter Polycrystalline silicon Epitaxial silicon Atmospheric Pressure CVD (APCV) Low Pressure CVD (LPCVD) Metal Organic CVD (MOCVD) Plasma Enhanced CVD (PECVD) Sputter or Evaporation Reactive Sputter ECD Borophosphosilicate glass Material BPSG epi-si p-si Si 3 N 4 W TiN SiO 2 Si 3 N 4 Ti Al Co TiN GIZO Cu Conductor Semiconductor Insulator 22/88

23 Chemical Vapor Deposition CVD is the most widely-employed deposition technique in VLSI. In its simplest incarnation, CVD involves flowing a precursor gas or gases into a chamber containing one or more heated objects to be coated. Chemical reactions occur on and near the hot surfaces, resulting in the deposition of a thin film on the surface. This is accompanied by the production of chemical by-products that are exhausted out of the chamber along with unreacted precursor gases. 23/88

24 CVD vs PVD Physical Vapor Deposition Source material is physically deposited. Chemical Vapor Deposition Involves a chemical reaction /88

25 Chemical Vapor Deposition Free radicals transport to the surface and grow a film Energy Transport to surface Film growth Reactive species move on surface 25/88

26 Sources of Activation Energy Temperature Plasma Wafer Reaction chamber Heated wall Inlet Reaction chamber Electrode Plasma Inlet Outlet Outlet To vacuum To vacuum 26/88

27 Types of CVD Atmospheric Pressure CVD (APCVD) Advantages: High deposition rates, simple, high throughput. Disadvantages: Poor uniformity, purity is less than LPCVD Used mainly for thick oxides. E.g.: TEOS TMPO Ammonium chloride Si OC 2 H 5 4 L + CH 3 3 PO 3 L + CH 3 O 3 3 B(L) SiO 2 : B 2 O 3, P 2 O 5 S + gas products BPSG (Borophosphosilicate glass) 27/88

28 Types of CVD Low Pressure CVD (LPCVD at ~0.2 to 20 torr) Advantages: Excellent uniformity, purity Disadvantages: Lower (but reasonable) deposition rates than APCVD Used for polysilicon deposition, dielectric layer deposition, and doped dielectric deposition. E.g.: Dichloro-silane (DCS) Ammonia Silicon nitride Ammonium chloride Hydrogen 3SiCl 2 H 2 G + 10NH 3 G Si 3 N 4 S + 6NH 4 Cl G + 6H 2 (G) 28/88

29 Types of CVD Metal Organic CVD (MOCVD) Advantages.: Highly flexible > can deposit semiconductors, metals, dielectrics Disadvantages: HIGHLY TOXIC!, Very expensive source material. Environmental disposal costs are high. Uses: Dominates low cost optical (but not electronic) III-V technology, some metallization processes (W plugs and Cu) E.g.: Trimethal Arsine gas (Toxic - Gallium Gas (extremely toxic) used as chemical weapon!) Ga CH 3 3 (G) + AsH 3 (G) 3CH 4 (G) + GaAs(S) Methane gas 29/88

30 Types of CVD Plasma Enhanced CVD Plasmas are used to force reactions that would not be possible at low temperature. Advantages: Uses low temperatures necessary for back end processing. Disadvantages: Plasma damage typically results. Used for dielectrics coatings. 30/88

31 CVD Growth Mechanism 31/88

32 CVD Mechanism Requirements for CVD Growth Good thickness uniformity. High purity and density. Controlled composition and stoichiometries. High degree of structural perfection. Good electrical properties. Excellent adhesion. Good step-coverage / conformal coverage. 32/88

33 CVD Mechanism Basic Mechanism: 5). Removal of byproducts 1). Precursors fed into reactor chamber Reactor Film Susceptor 2). Transport of precursors across dead layer to substrate 3). Pyrolysis: thermal decomposition at substrate 4). Chemical reaction: Decomposed species bond to substrate 33/88

34 CVD Mechanism More Detail: Reactant molecule 1). Bulk transport Carrier gas (to maintain high pressure & slow reaction rate) 8). Bulk transport of by product 2). Transport across boundary layer J 1 D g C 3). Adsorption 4). Surface diffusion 5). Decomposition 7). Diffusion of gaseous byproduct 6). Reaction with film J 2 k i C i 34/88

35 Mass Transport We wish to quantify the growth rate as a function of gas concentration and temperature. As with our oxidation model, we assume gas concentration drops close to the substrate. The model is called the Stagnant Film Model. Assume mass transport occurs via diffusion. C G Flow Profile C S δ x boundary layer thickness 35/88

36 Mass Transport Assume mass transport occurs across boundary layer via diffusion: Flux through boundary layer J 1 = D g C g C s δ s Gas concentration on surface h g = D g δ s Diffusion coefficient of active species Gas concentration in bulk Thickness of stagnant layer C G J 1 = h g C g C s Flow Profile Mass transport coefficient (see Lecture 7) C S δ x boundary layer thickness 36/88

37 Mass Transport We can (but won t) show the thickness of the boundary layer is: Viscosity (kg/ms) δ x = ηx Lateral position (m) Velocity of gas flow ρu 0 Velocity of gas flow (m/s) u 0 Wikipedia Mass density (kg/m 3 ) δ x x boundary layer thickness 37/88

38 Mass Transport We are interested in a representative thickness of the boundary layer, so we take the average: δ = 1 L න 0 L δ x dx = 1 L න 0 L ηx ρu 0 dx = 1 L L η න ρu 0 0 x dx δ x x boundary layer thickness 0 L 38/88

39 Mass Transport We are interested in a representative thickness of the boundary layer, so we take the average: L L δ = 1 L න δ x dx = 1 0 L න 0 ηx ρu 0 dx = 1 L L η න ρu 0 0 x dx δ = 1 L Define: Reynolds Number η 2x 3/2 ρu 0 3 L 0 = 2 3 L1/2 η ρu 0 Re = u 0ρL η 39/88

40 Mass Transport We are interested in a representative thickness of the boundary layer, so we take the average: Re = u 0ρL η δ = 2 3 L1/2 η ρu 0 = 2 3 L Re = δ s Recall: Mass transport coefficient h g = D g δ s = 3 2 D g L Re Quantifies movement of mass across the boundary layer. 40/88

41 Growth Rate 41/88

42 CVD Mechanism Consider two main processes: 1). Bulk transport Reactant molecule Carrier gas 2). Transport across boundary layer J 1 D g C 3). Adsorption 4). Surface diffusion 5). Decomposition 7). Diffusion of gaseous byproduct 6). Reaction with film J 2 k i C i 42/88

43 Consider the Two Fluxes Flux to surface: Gas concentration in chamber Reaction rate: In steady state: J 1 = h g C g C s J 2 = k s C s Gas concentration at surface Reaction rate quantified through this parameter J 1 = J 2 = h g C g C s = k s C s h g C g h g C s =k s C s h g C g = k s + h g C s C s = h gc g k s + h g J 2 = k s C s = h gk s C g k s + h g Put back into J 2 : 43/88

44 Consider the Two Fluxes J 2 = k s C s = We are interested in the growth rate. Recall flux is material flowing per unit area, per unit time: m/s h gk s k s + h g #/m 2 s C g Growth velocity v = J N #/m 3 Atomic density (3D) of film v = C g N h g k s k s + h g 44/88

45 Limits of Transport v = C g N h g k s k s + h g Growth velocity (v) is limited by the slowest component. If h g < k s growth is limited by mass transport. If h g > k s growth is limited by the reaction rate on the surface. 45/88

46 Mass-Transport Limited Growth v = C g N h g k s k s + h g This is the limit where h g < k s : If h g k s then: 1 k s + h g 1 k s So we can say: v = C g N h g k s k s = h gc g N 46/88

47 Mass-Transport Limited Growth v = h gc g N Recall, from earlier we derived: So: h g = 3 2 D g L v = 3D gc g 2LN We can approximate the diffusion coefficient by: Mean free path m D g λcҧ 2 m 2 /s Mean particle velocity m/s So: Re Re v = 3λcC ҧ g 4LN Re 47/88

48 Mass-Transport Limited Growth Recall, from our Lecture on Vacuum systems (L4): Mean free path Molecular diameter λ = 1 2πd 0 2 n Recall, from earlier: Velocity of gas flow Viscosity Re = u 0ρL η v = 3λcC ҧ g 4LN Gas number density n = C g Mean particle speed Mass density Substrate Length Re Boltzmann Constant c ҧ = 8k BT πm v T 1 2 u 0 Temperature Particle mass 48/88

49 Mass-Transport Limited Growth ln v v T 1 2 u 0 Increasing T u 0 High quality, slow, layer-by-layer deposition is carried out in the the mass-transport-limited regime. 49/88

50 Reaction-Rate Limited Growth v = C g N h g k s k s + h g This is the limit where h g > k s : If h g k s then: 1 k s + h g 1 h g So we can say: v = C g N h g k s h g = k sc g N 50/88

51 Reaction-Rate Limited Growth v = k sc g N Reactions are temperature-activated processes: k s = k 0 e E A kb T Activation energy v = C g N k 0e v e E A kb T E A kb T 51/88

52 Reaction-Rate Limited Growth ln v Increasing T v e E A kb T u 0 In this regime, the flow rate is not relevant. The temperature has an exponential affect however. 52/88

53 log film growth rate Mass Transport vs Surface Reaction Arrhenius Plot Mass transport control transition slope E A Reaction control Eversteyn High T 1/T Low T 53/88

54 Reaction Kinetics 54/88

55 Reaction Kinetics We described our rate by k s = k 0 e E A kb T Rate prefactor Activation energy But how do we determine this rate constant from gas composition? I.e. can we choose our reactant concentrations to control our film growth? The short answer is, it is quite tricky. 55/88

56 CVD Mechanism Recall: 1). Bulk transport Reactant molecule Carrier gas (to maintain high pressure & slow reaction rate) 8). Bulk transport of by product 2). Transport across boundary layer 4). Surface diffusion 7). Diffusion of gaseous byproduct 3). Adsorption 5). Decomposition J 1 D g C 6). Reaction with film J 2 k i C i 56/88

57 Reaction Kinetics Consider a simple decomposition reaction: AB G A G + B(G) E.g.: SiH 4 G SiH 2 G + H 2 (G) Silane Silylene Hydrogen We have three unknown gas pressures: P SiH4 P SiH2 P H2 57/88

58 Reaction Kinetics Use the law of mass-action (we will not derive it but there is plenty of information online). K = P AP B P AB = K 0 e E A kb T Can give information about how relative concentrations affect reaction rate. 58/88

59 Example We are told: The equilibrium constant is: K = 0.153, The chamber pressure is atm (101,325 Pa), The pressure of A the pressure of B. What is the pressure of the AB? K = P AP B P AB = K 0 e E A kb T 59/88

60 Example Since the pressure in the chamber is known (1 atm): P tot = P A +P B +P AB Since the pressure of the products is equal. P A = P B P tot = 2P A + P AB Use the law of mass action: K = P AP B P AB = P A 2 P AB 60/88

61 Example From the previous slide: P tot = 2P A + P AB P A 2 = kp AB So: P A 2 = K P tot 2P A Which is a quadratic equation: P A 2 + 2KP A KP tot = 0 P A = 2K ± 4K2 + 4KP tot 2 61/88

62 Example P A = 2K ± Take positive (physical) root: 4K2 + 4KP tot 2 P A = P B = 124 Pa Easy to now work out P AB P AB = P tot 2P A P AB = 101,076 Pa 62/88

63 Example The pressure in the chamber is known (1 atm): P tot = 101,325Pa P AB = 101,076 Pa At this low value of equilibrium constant (K = 0.153) not much decomposition is occurring. K = P AP B P AB = K 0 e E A kb T 63/88

64 CVD In Practice 64/88

65 Angled Susceptor The general equation for growth speed is: v = C g N h g k s k s + h g We derived the mass-transfer coefficient as a function of the Reynold number: h g = 3 2 D g L Re 65/88

66 Angled Susceptor But recall, we approximated the boundary layer to be an average: δ = 1 L න 0 L δ x dx δ x x boundary layer thickness 0 L And the Reynolds number depends on this layer thickness: δ s = 2 L 3 Re 66/88

67 Angled Susceptor δ x x boundary layer thickness A non-uniform boundary layer Non-uniform Reynolds Number, Re Non-uniform mass-transfer-coefficient, h g Non-uniform growth rate, v 67/88

68 Angled Susceptor So, in many cases the susceptor is angled, to reduce variations in the boundary layer. Susceptor is angled at 3 to 10. More uniform u o, C g More uniform growth rate, v. 68/88

69 Atmospheric Pressure CVD High deposition rate and throughput. APCVD is Mass Transport Limited. Process is very sensitive to: Reactant concentrations (flux). Reactant flow rates. Process is less sensitive to: Temperature. Carrier gas is used to dilute reactants. Prone to gas phase nucleation. APCVD is the only CVD process not done under vacuum. Wafers being processed Pile of wafers waiting to be processed Parallel reaction chambers Robot arms APCVD Tool 69/88

70 Atmospheric Pressure CVD Little used today, but illustrative. High P, small λ. Slow mass transport. Large reaction rates. Film growth limited by mass transfer, boundary layer. Quality of APCVD Si from silane is poor, better for dielectrics. Wafers being processed Pile of wafers waiting to be processed Parallel reaction chambers Robot arms APCVD Tool 70/88

71 Silane oxidation Atmospheric Pressure CVD (APCVD). Carried out in N2 ambient pressure carrier gas. SiH 4 G + 2O 2 (G) SiO 2 S + 2H 2 (G) Silane Oxygen Silicon oxide Hydrogen Carried out ~450 C. Can be used to form gate oxides. 71/88

72 Borophosphosilicate Glass Atmospheric Pressure CVD (APCVD). Carried out at 390 C. TEOS TMPO Ammonium chloride Si OC 2 H 5 4 L + CH 3 3 PO 3 L + CH 3 O 3 3 B(L) SiO 2 : B 2 O 3, P 2 O 5 S + gas products BPSG (Borophosphosilicate glass) Used as dielectric insulator between conductor lines. 72/88

73 Low Pressure CVD (LPCVD) High deposition rate and throughput. Low pressure, few collisions: Knudsen Number Kn = λ d < 1 Mean free path Chamber dimensions Low P high D g, h g. Improves transport, reduces boundary layer. Extends reaction-controlled regime (which is where you operate). 73/88

74 Low Pressure CVD (LPCVD) Requires no carrier gas. Fewer gas-phase reactions, fewer particulates. Good conformal growth (unlike sputtering or other PVD methods which are more directional) Strong T-dependence to reaction growth rate. 74/88

75 Low Pressure CVD (LPCVD) Hot Wall Reactor Uniform T distribution. Surface reactor gets coated. System must be dedicated to one species to avoid contamination. Easier to control growth rate with T. All poly-si deposition is carried out by hot-walled LPCVD - good for low pinhole SiO 2, conformity. Cold Wall Reactor Reduce reaction rate. Reduce deposition on surfaces. Used for epitaxial silicon. 75/88

76 Silane pyrolysis This is a heat-induced reaction to deposit silicon. SiH 4 G Si S + 2H 2 (G) Silane Silicon Hydrogen Carried out ~650 C. At 1 atm, this will produce poor quality (high defect concentration) silicon. Use Low Pressure CVD (LPCVD). 76/88

77 Si-tetrachloride reduction Can be used to form high-quality Si. LPCVD. SiCl 4 G + 2H 2 (G) Si S + 4HCl(G) Silicon tetrachloride Hydrogen Carried out ~1200 C. Relative concentrations are important: v Silicon Poly-Si Single-Crystal Si Hydrogen chloride P SiCl4 P H2 77/88

78 Doping Reactions Phosphine: 2PH 3 G 2P(S) + 3H 2 (G) Phosphine Diborane: Phosphorus Hydrogen B 2 H 6 G 2B(S) + 3H 2 (G) Diborane Boron Hydrogen 78/88

79 Si-nitride compound formation This is a heat-induced reaction to deposit silicon. Dichloro-silane (DCS) Ammonia Silicon nitride Ammonium chloride Hydrogen 3SiCl 2 H 2 G + 10NH 3 G Si 3 N 4 S + 6NH 4 Cl G + 6H 2 (G) Carried out ~750 C. Use Low Pressure CVD (LPCVD). 79/88

80 Tungsten CVD Reactor Showerhead Multistation Dep Chamber Wafer Vacuum clamp Deposition Chamber Heated Pedestal Showerhead Heated Pedestal Reactor: The showerhead is like a bath shower. Reactants come through it. Important characteristics: Good step coverage. Two step deposition of tungsten Nucleation. Growth. 80/88

81 Tungsten Deposition Step 1: Nucleation: Tungsten hexaflouride Silane Tungsten Silicon tetraflouride Hydrogen 2WF 6 G + 3SiH 4 2W S + 3SiF 4 G + 6H 2 (G) Step 2: Growth: Tungsten hexaflouride Hydrogen Tungsten Hydrofluoric acid WF 6 G + 3H 2 2W S + 6HF(G) Pressure = 40 Torr. Carried out at 415 C. 81/88

82 Metal Organic CVD (MOCVD) Metal-organic just described by its contents. Similar to LPCVD. Can involve extremely toxic precursors. 82/88

83 GaAs Growth Trimethyl Gallium (TMG) reduction: Ga CH 3 3 G + H 2 G Ga S + 3CH 4 (G) Trimethal Gallium Arsine reduction: Hydrogen Gallium 2AsH 3 G 2As S + 3H 2 (G) Methane Arsine Arsenic Hydrogen Least abundant element on surface limits growth velocity. Metal Organic CVD (MOCVD). 83/88

84 Titanium Nitride Growth Ti N C 2 H L + NH 3 G TiN S + Gas Products TDEAT Ammonia Titanium nitride Pressure = 30 Torr. Temperature = 350 C. TDEAT = tetrakis-(diethylamino)-titanium 84/88

85 Plasma-Enhanced CVD (PECVD) MHz TEOS injection Multi Wafer Chamber 400 KHz Dual Frequency Deposition Heated Base Important characteristics: Low temperature. Damage from ions can occur. Reaction gases are produced from a plasma. e + O 2 O + O + e MHz 400 KHz Wafer 85/88

86 PECVD Silicon Dioxide From Silane 3SiH 4 G + 6N 2 O G + N 2 G 3SiO 2 S + 4NH 3 G + 5N 2 (G) Silane Nitrous Oxide Nitrogen Silicon Oxide Ammonia Hydrogen Pressure = = 2 Torr. Temperature = 400 C. 86/88

87 PECVD Silicon Dioxide From TEOS (Si(OC 2 H 5 ) 4 ) Si OC 2 H 5 4 L + 8O 2 G 3SiO 2 S + 8CO G + 10H 2 O(G) TEOS Oxygen Silicon Oxide Carbon monoxide Water Vapor Pressure = = 2 Torr. Temperature = 350 C. TEOS = tetraethyl orthosilicate (Si(OC 2 H 5 ) 4 ). 87/88

88 PECVD Silicon Nitride From Silane 3SiH 4 G + 4NH 3 G Si 3 N 4 S + 12H 2 (G) Silane Ammonia Silicon Nitride Hydrogen Pressure = < 1 Torr. Temperature = 400 C. 88/88