Chapter 11 PVD and Metallization

Chapter 11 PVD and Metallization 2006/5/23 1 Metallization Processes that deposit metal thin film on wafer surface. 2006/5/23 2 1

Metallization Definition Applications PVD vs. CVD Methods Vacuum Metals Processes Future Trends 2006/5/23 3 Wafer Process Flow Materials IC Fab Metalization CMP Dielectric deposition Test Wafers Masks Thermal Processes Implant PR strip Etch PR strip Packaging Photolithography Final Test Design 2006/5/23 4 2

Physical Vapor Deposition 2006/5/23 5 PVD Vaporizing solid materials Heating or sputtering Condensing vapor on the substrate surface Very important part of metallization Methods Evaporation Sputtering 2006/5/23 6 3

PVD Methods: Evaporation Filaments Flash hot plate Electron beam 2006/5/23 7 Thermal Evaporator Wafers Aluminum Charge Aluminum Vapor 10-6 Torr To Pump High Current Source 2006/5/23 8 4

Electron Beam Evaporator Wafers Aluminum Charge 10-6 Torr Aluminum Vapor Electron Beam To Pump Power Supply 2006/5/23 9 PVD Methods: Sputtering DC Diode (simplest form) RF Diode DC Magnetron (most popular) 2006/5/23 10 5

Sputtering Ar + Momentum transfer will dislodge surface atoms off 2006/5/23 11 DC Diode Sputtering Target - V Argon Plasma Wafer Chuck Metal film Wafer 2006/5/23 12 6

Schematic of Magnetron Sputtering Magnets Target Higher plasma density Magnetic field line Erosion grove 2006/5/23 13 Magnetron Sputtering Most widely used PVD system More sputter from grove Better WTW uniformity cross wafer High deposition rate Good step coverage Good process (thickness) control 2006/5/23 14 7

PVD Chamber with Shield Shield, Liner Wafer Target Wafer Chuck 2006/5/23 15 Sputtering vs. Evaporator Sputtering Evaporator Purer film Better uniformity Single wafer, better process control Larger size wafer More impurities Batch process Cheaper tool 2006/5/23 16 8

PVD Vacuum Requirement Residue gases on the vacuum chamber wall H 2 O formation, Water can react with Al to form Al 2 O 3 Affects conductivity of interconnections Only way to get rid of H 2 O: reach ultra high vacuum, 10-9 Torr 2006/5/23 17 PVD Vacuum Requirement Cluster tool Staged vacuum Loading station: 10 6 Torr Transfer chamber: 10 7 to 10 8 Torr Deposition chamber: 10 9 Torr 2006/5/23 18 9

PVD Vacuum: Pumps Wet pump (oil diffusion pump): atm to 10-3 Torr, phasing out from fabs. Rough (mechanical) pump: atm to 10-5 Torr Turbo pump: 10-2 to 10-7 Torr Cryo pump: to 10-10 Torr Ion pump: to 10-11 Torr 2006/5/23 19 PVD Target Endura PVD System PVD Chamber CVD Chamber 2006/5/23 20 10

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 2006/5/23 21 PVD vs. CVD 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 2006/5/23 22 11

Contact/Via Process Degas Pre-clean Ti PVD TiN PVD TiN CVD N 2 -H 2 plasma treatment W CVD 2006/5/23 23 Aluminum Interconnection Process Degas Pre-clean Ti PVD Al-Cu PVD TiN PVD 2006/5/23 24 12

Degas Heat wafer to drive away gases and moisture on wafer surface Outgassing can cause contamination and high resistivity of deposited metal film 2006/5/23 25 Pre-clean Remove the native oxide Reduce the contact resistance Sputtering with argon ions RF plasma 2006/5/23 26 13

Pre-clean Process Argon Plasma Ar + Native Oxide Metal 2006/5/23 27 Titanium PVD Reduce contact resistance Larger grain size with low resistivity Wafer normally is heated to about 350 C during the deposition process Improve the surface mobility Improve step coverage 2006/5/23 28 14

Collimated Sputtering Used for Ti and TiN deposition Collimator allows metal atoms or molecules to move mainly in vertical direction Reach the bottom of narrow contact/via holes Improves bottom step coverage 2006/5/23 29 Collimated Sputtering Magnets Target Plasma Collimator Film Via holes 2006/5/23 30 15

Metal Plasma System Ti, TiN, Ta, and TaN deposition Ionize metal atoms through inductive coupling of RF power in the RF coil Positive metal ions impact with the negatively charged wafer surface vertically Improving bottom step coverage Reduce contact resistance 2006/5/23 31 Ionized Metal Plasma V Target Inductive Coils Plasma Via Hole RF 2006/5/23 32 16

TiSi 2 n + Titanium Nitride PVD Reactive sputtering process Ar and N 2 N 2 molecules dissociate in plasma Free nitrogen radicals react with Ti to form a thin layer of TiN on target surface. Argon ions sputter the TiN from the target surface and deposit it on the wafer surface 2006/5/23 33 Three Applications of TiN TiN ARC, PVD Al-Cu TiN, PVD W PSG TiN glue layer, PVD & CVD 2006/5/23 34 17

Al-Cu PVD Ultra high vacuum to remove moisture and achieve low film resistivity. Cluster tool with staged vacuum dry pumps, turbo pumps and cryopump A cryopump can help a PVD chamber to reach up to 10-10 Torr base pressure by freezing the residue gases in a frozen trap 2006/5/23 35 Al-Cu PVD Standard process and hot aluminum process Standard process: Al-Cu deposition over tungsten plug after Ti and TiN deposition Normally deposit at ~ 200 C Smaller grain size, easier to etch Metal annealing to form larger grain size lower resistivity high electromigration resistance (EMR) 2006/5/23 36 18

Al-Cu PVD Hot aluminum process To fill contact and via holes with Al instead of W plug, thus reducing contact resistance Several process steps: Ti deposition Al-Cu seed layer is deposited at low <200 C Bulk Al-Cu layer is deposited at higher temperatures (450 C to 500 C) 2006/5/23 37 Applications Interconnection Gate and electrodes Micro-mirror Fuse 2006/5/23 38 19

CMOS: Standard Metallization W Ti/TiN TiN, ARC Metal 1, Al Cu BPSG TiSi 2 STI n + n + USG p + p + P-Well N-Well P-epi P-wafer 2006/5/23 39 Applications: Interconnection 2006/5/23 40 20

Applications: Interconnection Dominate the metallization processes Al-Cu alloy is most commonly used W plug, technology of 80s and 90s Ti as welding layer TiN : barrier, adhesion and ARC layers The future is --- Cu! 2006/5/23 41 Applications: Gate and Electrode Al gate and electrode Polysilicon replace Al as gate material Silicide WSi 2 TiSi 2 CoSi 2, MoSi 2, TaSi 2, Pt, Au, as electrode for DRAM capacitors 2006/5/23 42 21

Q & A Can we reduce all dimensions of metal interconnection line at the same ratio? R= l/wh. When we shrink all dimensions (length l, width w, and height h) accordingly to the shrinking of the device feature size, resistance R increases, Slower circuit and more power consumption 2006/5/23 43 Conducting Thin Films 2006/5/23 44 22

Conducting Thin Films Polysilicon Silicides Aluminum alloy Titanium Titanium Nitride Tungsten Copper Tantalum 2006/5/23 45 Polysilicon Gates and local interconnections Replaced aluminum since mid-1970s High temperature stability Required for post implantation anneal process Al gate can not use form self-aligned source/drain Heavily doped LPCVD in furnace 2006/5/23 46 23

Silicide Much lower resistivity than polysilicon TiSi 2, WSi 2, and CoSi 2 are commonly used 2006/5/23 47 Silicide TiSi 2 and CoSi 2 Argon sputtering removes the native oxide Ti or Co deposition Annealing process forms silicide Ti or Co don t react with SiO 2, silicide is formed at where silicon contacts with Ti or Co Wet strips unreacted Ti or Co Optional second anneal to increase conductivity 2006/5/23 48 24

Self-aligned Titanium Silicide Formation Ti Ti Polysilicon gate TiSi 2 Polysilicon gate TiSi2 TiSi 2 Polysilicon gate TiSi2 n - Gate oxide n - n + n + n - n - n + Gate oxide n + n - n - n + Gate oxide n + Titanium deposition Silicide annealing Titanium wet striping 2006/5/23 49 Tungsten Silicide Thermal CVD process WF 6 as the tungsten precursor SiH 4 as the silicon precursor. Polycide stack is etched Fluorine chemistry etches WSi x Chlorine chemistry etches polysilicon Photoresist stripping RTA increases grain size and conductivity 2006/5/23 50 25

Aluminum Most commonly used metal The fourth best conducting metal Silver 1.6 cm Copper 1.7 cm Gold silver 2.2 cm Aluminum 2.65 cm It was used for gate before mid-1970 2006/5/23 51 Aluminum-Silicon Alloy Al make direct contact with Si at source/drain Si dissolves in Al and Al diffuses into Si Junction spike Aluminum spikes punctuate doped junction Short source/drain with the substrate ~1% of Si alloyed in Al can stop this! Thermal anneal at 400 C to form Si-Al alloy at the silicon-aluminum interface 2006/5/23 52 26

Al-Si Junction Spike Al SiO 2 Al Al p + p + n-type Silicon 2006/5/23 53 Electromigration Aluminum is a polycrystalline material Many mono-crystalline grains Current flows through an aluminum line Electrons constantly bombards the grains Smaller grains will start to move This effect is called electromigration 2006/5/23 54 27

Electromigration Electromigration tear the metal line apart Higher current density in the remaining line Aggravates the electron bombardment Causes further aluminum grain migration Eventually will break metal lines Affect the IC chip reliability Aluminum wires: fire hazard of old houses 2006/5/23 55 Electromigration Prevention When a small percent of copper is alloyed with aluminum, electromigration resistance of aluminum significantly improved Copper serves as glue between the aluminum grains and prevent them from migrating due to the electron bombardment Al-Si-Cu alloy was used before Currently Al-Cu (0.5%) is very commonly 2006/5/23 56 28

Aluminum Alloy Deposition PVD Sputtering Evaporation CVD Thermal Electron beam Dimethylaluminum hydride [DMAH, Al(CH 3 ) 2 H] Thermal process 2006/5/23 57 Titanium Applications Silicide formation Titanium nitridation Wetting layer Welding layer 2006/5/23 58 29

Welding Layer Reduce contact resistance. Titanium scavenges oxygen atoms Prevent forming high resistivity WO 4 and Al 2 O 3. Use with TiN as diffusion barrier layer Prevent tungsten from diffusing into substrate 2006/5/23 59 Applications of Titanium Al-Cu Ti W PSG Ti TiSi 2 n + 2006/5/23 60 30

Barrier layer Titanium Nitride prevents tungsten diffusion Adhesion layer help tungsten to stick on silicon oxide surface Anti-reflection coating (ARC) reduce reflection and improve photolithography resolution in metal patterning process prevent hillock and control electromigration Both PVD and CVD 2006/5/23 61 Tungsten Metal plug in contact and via holes contact holes become smaller and narrower PVD Al alloy: bad step coverage and void CVD W: excellent step coverage and gap fill higher resistivity: 8.0 to 12 cm compare to PVD Al alloy (2.9 to 3.3 cm) only used for local interconnections and plugs 2006/5/23 62 31

Evolution of Contact Processes Al Si Cu Al Si Cu Void SiO 2 SiO 2 Al Cu W SiO 2 Si Widely tapered contact hole, PVD metal fill Si Narrow contact hole, void with PVD metal fill Si Narrow contact hole, WCVD for tungsten plug 2006/5/23 63 W Plug and TiN/Ti Barrier/Adhesion Layer Tungsten TiN/Ti Oxide 2006/5/23 64 32

Pros : Copper Low resistivity (1.7 cm), lower power consumption and higher IC speed High electromigration resistance better reliability Cons : Poor adhesion with silicon dioxide Highly diffusive in Si, heavy metal contamination Very hard to dry etch copper-halogen have very low volatility 2006/5/23 65 Copper Deposition Process 1. PVD of seed layer 2. ECP or CVD bulk layer (later slides) 3. Thermal anneal after bulk copper deposition increase the grain size improving conductivity 2006/5/23 66 33

Tantalum Barrier layer - prevent copper diffusion into silicon substrate Better partner with copper than Ti or TiN Sputtering deposition 2006/5/23 67 Cobalt Mainly used for cobalt silicide (CoSi 2 ) for gate or local interconnection. Normally deposited with a sputtering process 2006/5/23 68 34

Metal Thin Film Characteristics 2006/5/23 69 Metal Thin Film Characteristics Thickness. Stress Reflectivity Sheet resistance 2006/5/23 70 35

Thickness Measurement of Metal Thin Film TEM and SEM Profilometer 4-point probe Reflectospectrometer Acoustic measurement 2006/5/23 71 1. Profilometer Thicker film (> 1000 Å), Patterned etch process prior to measurement Stylus probe senses and records microscopic surface profile Known as -step 2006/5/23 72 36

Schematic of Stylus Profilometer Stylus Film Substrate Stage Profile Signal Film Thickness 2006/5/23 73 2. Four-point Probe Normally measure sheet resistance Commonly used to monitor the metal film thickness by assuming the resistivity of the metal film is a constant all over the wafer surface 2006/5/23 74 37

3. Acoustic Measurement New technique Directly measure opaque thin film thickness Non-contact process, can be used for production wafer 2006/5/23 75 Acoustic Measurement Laser shots on thin film surface Photo-detector measures reflected intensity 0.1 ps laser pulse heat the spot up 5 to 10 C Thermal expansion causes a sound wave It propagates in the film and reflects at the interface of different materials The echo causes reflectivity change when it reaches the thin film surface. 2006/5/23 76 38

Acoustic Method Measurement Pump laser Reflection detector TiN TEOS SiO 2 Echoing acoustic wave d = v s t/2 Change of reflectivity First echo Second echo t t Third echo 10 20 30 40 50 60 70 80 90 Time (psec) 2006/5/23 77 Acoustic Measurement Acoustic wave echoes back and forth in film The film thickness can be calculated by d = V s t/2 V s is speed of sound and t is time between reflectivity peaks The decay rate the echo is related to the film density. Also used for multi-layer film thickness 2006/5/23 78 39

Uniformity The uniformity, in fact it is non-uniformity, of the thickness, sheet resistance, and reflectivity are routinely measured during the process development and for the process maintenance. It can be calculated by measuring at multiple locations on a wafer 2006/5/23 79 Mapping Patterns for Uniformity Measurement 2 6 3 1 4 5 7 3 2 1 4 8 5 9 3029 28 27 26 49 48 1312 11 10 47 25 24 46 31 3 2 9 23 45 32 33 34 14 4 1 15 5 6 35 36 16 17 37 18 38 8 7 19 39 22 21 20 42 41 40 44 43 2006/5/23 80 40

Uniformity Most commonly used non-uniformity definition: 49-point, 3 standard deviation Clearly define non-uniformity For the same set of data, different definitions causes different results 5-point and 9-point are commonly used in production for process monitoring 2006/5/23 81 Stress Caused by mismatching between film and substrate Compressive and tensile High compressive stress causes hillocks short metal wires between different layers High tensile stress causes cracks or peels Intrinsic stress and thermal stress are commonly discussed 2006/5/23 82 41

Compressive Stress Causes Hillock Force Metal Substrate Force 2006/5/23 83 Tensile Stress Causes Crack Force Metal Substrate Force 2006/5/23 84 42

Al Stress Aluminum has higher thermal expansion rate than silicon Al = 23.6 10 6 K, Si = 2.6 10 6 K It favors tensile stress at room temperature Stress becomes less tensile when wafer is heated up later, e.g. metal annealing (~ 450 C) dielectric deposition (~ 400 C) 2006/5/23 85 Reflectivity Reflectivity change indicates drift of process A function of film grain size and surface smoothness Larger grain size film has lower reflectivity; smoother metal surface has higher reflectivity Easy, quick and non-destructive High reflectivity causes standing wave effect Anti-reflection coating (ARC) is required, Typically the TiN film 2006/5/23 86 43

Sheet Resistance Sheet resistance (R s ) is a defined parameter R s = /t By measuring R s, one can calculate film resistivity ( ) if film thickness t is known, or film thickness if its resistivity is known 2006/5/23 87 Resistance of a Metal Line L I A R = L A R = Resistance, = Resistivity L = Length, A = Area of line cross-section 2006/5/23 88 44

Sheet Resistance Concepts L t w I Apply current I and measure voltage V, Resistance: R = V/I = L/(wt) For a square sheet, L = w, so R = /t = R s Unit of R s : ohms per square ( / ) 2006/5/23 89 Sheet Resistance Are you sure their resistance is the same? I I R s = /t R s = /t 2006/5/23 90 45

Sheet Resistance For this two conducting lines patterned from the same metal thin film with the same length-towidth ratios, are their line resistance the same? Yes. 2006/5/23 91 Four-Point Probe Measurement I V P 1 P 2 P 3 P 4 S 1 S 2 S 3 Film Substrate 2006/5/23 92 46

Four-point Probe Commonly used tool for sheet resistance A current is applied between two pins and voltage is measured between other two pins If current I is between P 1 and P 4, R s = 4.53 V/I, V is voltage between P 2 and P 3 If current I is between P 1 and P 3, R s = 5.75 V/I, V is voltage between R 2 and R 4 Both configurations are used in measurement 2006/5/23 93 Copper Metallization 2006/5/23 94 47

Copper Technology Better conductor than aluminum Higher speed and less power consumption Higher electromigration resistance Diffusing freely in silicon and silicon dioxide, causing heavy metal contamination, need diffusion barrier layer Hard to dry etch, no simple gaseous chemical compounds 2006/5/23 95 Copper Technology Dual Damascene process with CMP Ta and/or TaN as barrier layer Start using in IC fabrication 2006/5/23 96 48

Copper Process Pre-deposition clean PVD barrier layer (Ta or TaN, or both) PVD copper seed layer Electrochemical plating bulk copper layer Thermal anneal to improve conductivity 2006/5/23 97 Etch trenches and via holes SiN FSG FSG FSG Cu Cu 2006/5/23 98 49

Tantalum Barrier Layer and Copper Seed Layer Deposition Cu Ta SiN FSG FSG FSG Cu Cu 2006/5/23 99 Electrochemical Plating Copper Cu Ta SiN FSG FSG FSG Cu Cu 2006/5/23 100 50

CMP Copper and Tantalum, CVD Nitride Ta SiN SiN FSG Cu FSG Cu Cu 2006/5/23 101 Copper Metallization Ti/TiN SiN CoSi 2 Ta or TaN M1 Cu FSG Cu Cu FSG W PSG W n + n + STI USG p + p + P-Well N-Well P-Epi P-Wafer 2006/5/23 102 51

Summary Mainly application: interconnection CVD (W, TiN, Ti) and PVD (Al-Cu, Ti, TiN) Al-Cu alloy is still dominant Need UHV for Al-Cu PVD W used as plug Ti used as welding layer TiN: barrier, adhesion and ARC layers The future: Cu and Ta/TaN 2006/5/23 103 52