Device Fabrication: CVD and Dielectric Thin Film 1
Objectives Identify at least four CVD applications Describe CVD process sequence List the two deposition regimes and describe their relation to temperature List two dielectric thin films Name the two most commonly used silicon precursors for dielectric CVD 2
CVD Oxide vs. Grown Oxide SiO 2 SiO 2 Si Si Si Grown film Bare silicon Deposited film 3
CVD Chemical Vapor Deposition Chemical gases or vapors react on the surface of solid, produce solid byproduct on the surface in the form of thin film. Other byproducts are volatile and leave the surface. 4
CVD Applications FILMS PRECURSORS LPCVD SiH 4, O 2 SiO 2 (glass) PECVD SiH 4, N 2 O Dielectrics PECVD Si(OC 2 H 5 ) 4 (TEOS), O 2 LPCVD TEOS APCVD&SACVD TM Oxynitride SiH 4, N 2 O, N 2, NH 3 TEOS, O 3 (ozone) PECVD SiH 4, N 2, NH 3 Si 3 N 4 LPCVD SiH 4, N 2, NH 3 LPCVD C 8 H 22 N 2 Si (BTBAS) W (Tungsten) WF 6 (Tungsten hexafluoride), SiH 4, H 2 WSi 2 WF 6 (Tungsten hexafluoride), SiH 4, H 2 Conductors TiN Ti[N (CH 3 ) 2] 4 (TDMAT) Ti TiCl 4 Cu 5
CVD Gas or vapor phase precursors are introduced into the reactor Precursors across the boundary layer and reach the surface Precursors adsorb on the substrate surface Adsorbed precursors migrate on the substrate surface Chemical reaction on the substrate surface Solid byproducts form nuclei on the substrate surface Nuclei grow into islands Islands merge into the continuous thin film Other gaseous byproducts desorb from the substrate surface Gaseous byproducts diffuse across the boundary layer Gaseous byproducts flow out of the reactor. 6
Precursors Showerhead Forced convection region Byproducts Wafer Reactants Boundary layer Pedestal 7
Deposition Process (a) (b) Precursor arrives surface Migrate on the surface (c) (d) React on the surface Nucleation: Island formation 8
Deposition Process (e) Islands grow Islands grow, cross-section (f) (g) Islands merge Continuous thin film (h) 9
CVD Processes (1)Atmospheric Pressure CVD (APCVD) (2)Low Pressure CVD (LPCVD) (3)Plasma-Enhanced CVD (PECVD) 10
(1) Atmospheric Pressure CVD CVD process taking place at atmospheric pressure APCVD process has been used to deposit silicon oxide and silicon nitride Conveyor belt system with in-situ belt clean 11
APCVD Reactor N 2 Process Gas N 2 Wafers Wafers Heater Belt Clean Station Exhaust Conveyor Belt 12
(2) LPCVD Longer MFP Good step coverage & uniformity Vertical loading of wafer Fewer particles and increased productivity Less dependence on gas flow Vertical and horizontal furnace 13
Horizontal Conduction-Convectionheated LPCVD Adaptation of horizontal tube furnace Low pressure: from 0.25 to 2 Torr Used mainly for polysilicon, silicon dioxide and silicon nitride films Can process 200 wafers per batch 14
LPCVD System Pressure Sensor Wafers Loading Door Heating Coils To Pump Process Gas Inlet Temperature Wafer Boat Center Zone Flat Zone Quartz Tube Distance 15
(3) PECVD Developed when silicon nitride replaced silicon dioxide for passivation layer. High deposition rate at relatively low temperature. RF induces plasma field in deposition gas Stress control by RF Chamber plasma clean. 16
Plasma Enhanced CVD System Process gases Process chamber Wafer Plasma RF power By-products to the pump Heated plate 17
Step Coverage A measurement of the deposited film reproducing the slope of a step on the substrate surface One of the most important specifications Sidewall step coverage Bottom step coverage Conformality Overhang 18
Step Coverage and Conformity CVD thin film c a Structure h d b Substrate w Sidewall step coverage = b/a Bottom step coverage = d/a Conformity = b/c Overhang = (c - b)/b Aspect ratio = h/w 19
Factors Affect Step Coverage (1) Arriving angle of precursor (2) Surface mobility of adsorbed precursor 20
Arriving Angles 180 B A C 270 90 21
Arriving Angle Corner A: 270, corner C: 90 More precursors at corner A More deposition Form the overhang Overhangcan cause voids or keyholes 22
Void Formation Process Overhang Metal Dielectric Metal Dielectric Metal Void Dielectric 23
Control of Arriving Angle Changing pressure Tapering opening 24
Step Coverage, Pressure and Surface Mobility APCVD No mobility LPCVD No mobility High mobility 25
Arriving angles for tapered and straight contact holes Larger arriving angle Smaller arriving angle Nitride 26
Deposition/Etchback/Deposition Dep. Al Cu Etch Al Cu Dep. Al Cu 27
Conformal film deposition and gap fill (I) 28
Conformal film deposition and gap fill (II) 29
Conformal film deposition and gap fill (III) 30
Gap Fill of high-density plasma CVD Metal Metal Metal Metal Metal Metal Metal Metal Metal 31
Surface Adsorption Determine precursors surface mobility Affect step coverage and gap fill Physical adsorption (physisorption) Chemical adsorption (chemisorption) 32
Chemisorption Actual chemical bonds between surface atom and the adsorbed precursor molecule Bonding energy usually exceeding 2 ev Lowsurface mobility Ion bombardment with10 to 20 ev energy in PECVD processes can cause some surface migration of chemisorbed precursors 33
Physisorption Weak bond between surface and precursor Bonding energy usually less than 0.5 ev Hydrogen bonding Van der Waals forces Ion bombardment and thermal energy at 400 C can cause migration of physisorbed precursors Highsurface mobility 34
Bonding energy Relationship of bonding energy to chemical and physical adsorption Distance from surface Physisorbed precursor Chemisorbed precursor Substrate Surface 35
CVD Precursor: Silane Dielectric CVD PECVD passivation dielectric depositions Dielectric anti reflective coating (DARC) High density plasma CVD oxide processes LPCVD poly-si and silicon nitride Metal CVD W CVD process for nucleation step Silicon source for WSi x deposition 36
Dielectric CVD Precursor: Silane Pyrophoric (ignite itself), explosive, and toxic Open silane line without thoroughly purging can cause fire or minor explosion and dust line 37
Structure of Silane Molecule H H H Si H Si H H H H 38
CVD Precursor Adsorption: Silane Silane molecule is perfectly symmetrical Neither chemisorb nor physisorb Fragments of silane, SiH 3, SiH 2, or SiH, can easily form chemical bonds with surface 39
Sticking Coefficient The probability that precursor atom forms chemical bond with surface atom in one collision Can be calculated by comparing the calculated deposition rate with 100% sticking coefficient and the measured actual deposition rate 40
Sticking Coefficient The lower the sticking coefficient, the higher the surface mobility dissociate Precursors Sticking Coefficient SiH4 SiH 3 3x10-4 to 3x10-5 0.04 to 0.08 SiH 2 0.15 SiH 0.94 Silane molecule is perfectly symmetrical Neither chemisorb nor physisorb Low surface mobility having overhangs at the step corners and having poor step coverage TEOS 10-3 WF 6 10-4 TEOS: tetra-ethyl-oxy-silane, Si(OC 2 H 5 ) 4 41
CVD Kinetics Chemical Reaction Rate equation is: C.R. = A exp (-E a /kt) A: a constant E a : activation energy K: Boltzmann constant T: substrate temperature The lower the activation energy E a, the easier the chemical reaction. Precursor E a Byproduct 42
Deposition Regimes Mass-transportlimited regime Surface-reactionlimited regime ln D.R. Slope = E a /k Gas-phase-nucleation regime 1/T At lower temperature 43
Surface-Reaction-Limited Regime Chemical reaction rate can t match precursor diffusion and adsorption rates; precursors pile up on the substrate surface and wait their turn to react. D.R. = C.R. [B] [C] [] Deposition rate is very sensitive to temperature 44
Mass-Transport-Limited Regime When the surface chemical reaction rate is high enough, the chemical precursors react immediately when they adsorb on the substrate surface. Deposition rate = D dn/dx [B] [C] [] D: diffusion rate of the precursors in the boundary layer dn/dx: the gradient of the precursor concentration in the boundary layer [B] [C], etc., are precursor concentrations on the substrate surface Deposition rate is insensitive to temperature Mainly controlled by gas flow rates 45
Relation of deposition rate and temperature Deposition rate Deposition rate sensitive to temperature Deposition rate insensitive to temperature Temperature 46
CVD Reactor Deposition Regime Most single wafer process reactors are designed in mass-transport-limited regime It is easier to control the gas flow rate Plasma or unstable chemicals such as ozone are used to achieve mass-transport-limitedregime at relatively low temperature 47
Dielectric CVD, Oxide and Nitride Oxide (SiO 2 ) Nitride (Si 3 N 4 ) Similar dielectric strength, > 1x10 7 V/cm Similar dielectric strength, > 1x107 V/cm Lower dielectric constant, = 3.9 Not a good barrier for moisture and mobile ion (Na + ) Transparent to UV Higher dielectric constant, = 7.0 Good barrier for moisture and mobile ion (Na + ) Conventional nitride opaque to UV Can be doped with P and B 48
Passivation Nitride and oxide Nitride is very good barrier layer, oxide help nitride stick with metal Silane process NH 3, N 2 and nitrogen precursors, N 2 O as oxygen precursor In-situ CVD process 49
Dielectric Thin Film Characteristics Refractive index Thickness Uniformity Stress Particles 50
Refractive Index Refractive index, n = Speed of light in vacuum Speed of light in the film 51
Refractive index and refractive angle Incident light Vacuum n 1 n 1 sin 1 = n 2 sin 2 Film n 2 Refractive light For SiO 2, n=1.46 For Si 3 N 4, n=2.01 52
Film Information from R.I. Refractive index Oxygen rich Nitrogen rich 4.0 Polysilicon Oxygen rich Nitrogen rich 2.01 Si 3 N 4 Silicon rich Oxynitride Oxygen rich 1.46 SiO 2 Nitrogen rich Silicon rich 53
For the silicon- or nitrogen-rich oxide, the refractive index will be higher than the stoichiometric value of. 1.46, but it will be lower than that value when it is oxygen rich. For nitride, silicon-rich film will have a higher R.I. than 2.01 and nitrogen- or oxygen-rich films will have a lower value. 54
Ellipsometry system Linearly Polarized Incident Light Elliptically Polarized Reflected Light s p n 1, k 1, t 1 n 2, k 2 When a beam of light is reflected from the film surface, the polarization status changes. By comparing this, one can get information about the refractive index and thickness of the dielectric thin film. 55
The change in polarization of the and s components of a light beam upon reflected is determined. The fundamental equation of ellipsometry: =R p /R s =tan e I : a complex amplitude reflection ratio R p and R s : Fresnel reflection coefficient 56
Illustration of Prism Coupler Thin film Laser light Photo detector Substrate Coupling head 57
Reflected Light Intensity vs. Incidence Angle Reflected Intensity Modes 20 10 0-10 -20 58
Metricon Model 2010 Prism Coupler 59
Comparison of the Two Methods Ellipsometry Prism coupler Need know rough film thickness before hand Can measure thickness if R.I. is know Need certain thickness of the film, > 3000 Å Can measure thickness if film thick enough to support enough modes 60
Thickness Measurement One of the most important measurements for dielectric thin film processes. Determines Film deposition rate Wet etch rate Shrinkage 61
Reflection light rays and phase difference Incident light 1 2 Human eye or photodetector t Dielectric film, n( ) Substrate =2tn( )/cos t: thin-film thickness n( ): thin-film refractive index : the angle of incidence 62
Dielectric Thin Film Thickness Measurement Different thickness has different color Tilting wafer also changes the color 63
Question If you see some beautiful color rings on a wafer with a CVD dielectric layer, what is your conclusion? 64
Answer Color change indicates the dielectric thin film thickness change, thus we know the film with the color rings must have problem with thickness uniformity, which is most likely caused by a non-uniform thin film deposition process. 65
Question Why does the thin film color change when one look at the wafer from different angle? 66
Answer When one looking at wafer from a different angle, phase shift will change, thus wavelength for constructive interference will change, which causes color change It is important to hold the wafer straight when using the color chart to measure film thickness Tilt wafer makes the film thickness thicker than it actually is 67
Deposition Rate (D. R.) Deposition Rate = Thickness of deposited film Deposition time 68
Wet Etch Rate Wet Etch Rate = Thickness change after etch Etch time Wet etch rate ratio = Thickness change of the CVD film Thickness change of the thermal oxide film 69
70 Uniformity Multi-point measurement Definition Average: Standard deviation: Standard deviation non-uniformity: /x N x x x x x N 3 2 1 1 ) ( ) ( ) ( ) ( 2 2 3 2 2 2 1 N x x x x x x x x N
Stress Mismatch between different materials Two kinds of stresses, intrinsic and extrinsic Intrinsic stress develops during the film nucleation and growth process. The extrinsic stress results from differences in the coefficients of thermal expansion Tensile stress: cracking film if too high Compressive stress: hillock if too strong 71
Definitions of compressive and tensile stress Bare Wafer After Thin Film Deposition Substrate Substrate Compressive Stress Negative curvature Substrate Tensile Stress Positive curvature 72
Tensile stress Compressive stress 73
Illustration of Thermal Stress SiO 2 Si At 400 C L SiO 2 Si At Room Temperature L = T L L L: change of the dimension T: change of the temperature : coefficient of thermal expansion 74
Coefficients of Thermal Expansion (SiO 2 ) = 0.5 10 6 C 1 (Si) = 2.5 10 6 C 1 (Si 3 N 4 ) = 2.8 10 6 C 1 (W) = 4.5 10 6 C 1 (Al) = 23.2 10 6 C 1 75
Stress Measurement 2 E h 1 1 ( 1 6t R2 R1 ) : film stress (Pa) E: Young s modulus of the substrate (Pa) v: Poisson s ratio of the substrate h: substrate thickness ( m) t: film thickness ( m) R 1 : wafer curvature radius before deposition ( m) R 2 : wafer radius curvature after deposition ( m) Wafer curvature change before and after thin film deposition Mpa=10 6 Pa 1 MPa=10 7 dynes/cm 2 Laser beam scans wafer surface, reflection light indicates the wafer curvature 76
Schematic of the laser scanning stress measurement tool Laser Mirror Detector 77
Dielectric CVD Processes Thermal Silane CVD Process PECVD Silane Processes High Density Plasma CVD 78
Thermal Silane CVD Process Silane has been commonly used for silicon dioxide deposition with both APCVD and LPCVD process heat SiH 4 + 2 O 2 SiO 2 + 2 H 2 O APCVD normally uses diluted silane (3% in nitrogen) and LPCVD uses pure silane Not commonly used in the advanced fab 79
PECVD Silane Processes Silane and nitrous oxide N 2 O (laughing gas) Dissociation in plasma form SiH 2 and O Radicals react rapidly to form silicon oxide plasma SiH 4 + N 2 O SiO x H y + H 2 O + N 2 + NH 3 + heat Overflow N 2 O, using SiH 4 flow to control deposition rate 80
Question Can we overflow silane and use nitrous oxide flow rate to controlled deposition rate? 81
Answer Theoretically we can, but practically no one should even try this It is very dangerous and not cost effective Overflowing silane will create a big safety hazard: fire and explosion Silane is more expensive than nitrous oxide 82
Passivation: Silicon Nitride Barrier layer for moisture and mobile ions The PECVD nitride Low deposition temperature (<450ºC) High deposition rate Silane, ammonia, and nitrogen plasma SiH 4 + N 2 + NH 3 SiN x H y + H 2 + N 2 + NH 3 + heat Requires good step coverage, high dep. rate, good uniformity, and stress control 83
Passivation Dielectric Deposition 1. Stabilization 1 (stabilize pressure) 2. Oxide deposition (stress buffer for nitride) 3. Pump 4. Stabilization 2 (stabilize pressure) 5. Nitride deposition (passivation layer) 6. Plasma purging (eliminate SiH 4 ) 7. Pump 84
Dielectric Anti-Reflective Coating High resolution for photolithography ARC layer is required to reduce the reflection Metallic ARC: TiN, 30% to 40% reflection No longer good enough for < 0.25 m Dielectric ARC layer is used Spin-on before photoresist coating CVD 85
Dielectric Anti-Reflective Coating UV light ( ) = 2nt = /2 1 2 Photoresist t Dielectric ARC, n, k Aluminum alloy 86
Dielectric ARC PECVD silane process N 2 O as oxygen and nitrogen source plasma SiH 4 + N 2 O + He SiO x N y + H 2 O + N 2 + NH 3 + He + heat 87
Schematic of Sputtering Etch Chamber Process gases Process chamber Plasma Magnet coils By-products to the pump Chuck RF power 88
Ozonator Lighting, corona discharge plasma O 2 O + O O + O 2 + M O 3 + M (M = O 2, N 2, Ar, He, etc.) 89
Ozone generation in an ozone cell RF O 2 + N 2 O 2 + O 3 + N 2 + N 2 O + 90
Schematic of ozone concentration monitoring system Monitored by UV absorption (Beer-Lambert law): I = I 0 exp(-xcl) Mechanical chopper L Ozone cell UV sensor UV Lamp O 3 /O 2 O 3 /O 2 Analyzer 91
High-Density Plasma CVD (HDP-CVD) Dep/etch/dep gap fill needs two chambers Narrower gaps may need more dep/etch cycles to fill A tool can deposit and sputtering etch simultaneously would be greatly helpful The solution: HDP-CVD 92
Inductively coupled plasma (ICP) Electron cyclotron resonance (ECR) 93
ICP Chamber Source RF Inductive coils Ceramic cover Chamber body Plasma Wafer E-chuck Helium Bias RF 94
ECR Chamber Microwave Magnet coils Magnetic field line Plasma Wafer E-chuck Helium Bias RF 95
HDP-CVD, IMD Application Metal Metal Metal 96
HDP-CVD, Deposition Metal Metal Metal 97
HDP-CVD, Deposition Metal Metal Metal 98
HDP-CVD, Deposition Metal Metal Metal 99