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Download Chapter 11 PVD and Metallization
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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 Thermal Processes Masks Implant PR strip Photolithography Etch PR strip Packaging 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 2006/5/23 High Current Source 8 4 Electron Beam Evaporator Wafers Aluminum Charge Aluminum Vapor Electron Beam 10-6 Torr 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 -V Target Argon Plasma Wafer Chuck Metal film 2006/5/23 Wafer 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 Target Wafer Wafer Chuck 2006/5/23 15 Sputtering vs. Evaporator Sputtering •Purer film •Better uniformity •Single wafer, better process control •Larger size wafer 2006/5/23 Evaporator •More impurities •Batch process •Cheaper tool 16 8 PVD Vacuum Requirement •Residue gases on the vacuum chamber wall –H2O formation , … •Water can react with Al to form Al2O3 •Affects conductivity of interconnections •Only way to get rid of H2O: reach ultra high vacuum, 10-9 Torr 2006/5/23 17 PVD Vacuum Requirement •Cluster tool •Staged vacuum •Loading station: 106 Torr •Transfer chamber: 107 to 108 Torr •Deposition chamber: 109 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 Endura® PVD System PVD Target 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 •N2-H2 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 RF Via Hole 2006/5/23 32 16 Titanium Nitride PVD •Reactive sputtering process •Ar and N2 •N2 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 TiSi2 n+ 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 Ti/TiN TiN, ARC TiSi2 Metal 1, Al• Cu W STI n+ BPSG P-Well n+ USG p+ p+ 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 –WSi2 –TiSi2 –CoSi2, MoSi2, TaSi2, … •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 •TiSi2, WSi2, and CoSi2 are commonly used 2006/5/23 47 Silicide •TiSi2 and CoSi2 –Argon sputtering removes the native oxide –Ti or Co deposition –Annealing process forms silicide –Ti or Co don’ t react with SiO2, 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 TiSi2 n+ n- Gate oxide Titanium deposition TiSi2 Polysilicon gate Polysilicon gate n- nn+ n+ Gate oxide TiSi2 n- nn+ TiSi2 Polysilicon gate n+ Silicide annealing Gate oxide nn+ Titanium wet striping 2006/5/23 49 Tungsten Silicide •Thermal CVD process –WF6 as the tungsten precursor –SiH4 as the silicon precursor. •Polycide stack is etched –Fluorine chemistry etches WSix –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 –Copper –Gold silver –Aluminum 1.6 cm 1.7 cm 2.2 cm 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 p+ SiO2 Al Al 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 •Thermal •Electron beam •CVD –Dimethylaluminum hydride [DMAH, Al(CH3)2H] –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 WO4 and Al2O3. •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 TiSi2 2006/5/23 Ti n+ 60 30 Titanium Nitride •Barrier layer –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 Al·Cu SiO2 Si W SiO2 Si Widely tapered contact hole, PVD metal fill Narrow contact hole, void with PVD metal fill SiO2 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 Copper Pros : –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 (CoSi2) 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 Reflection detector Echoing acoustic wave TiN d = vs·t/2 TEOS SiO2 First echo Change of reflectivity Pump laser Second echo Third echo t t 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 = Vs t/2 •Vs 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 29 30 2 3 1 5 7 3 1 4 4 2006/5/23 8 28 31 5 9 12 13 27 26 49 11 10 25 48 47 24 2 9 23 3 46 45 32 14 4 1 8 22 44 7 5 21 43 33 15 6 20 16 34 42 19 17 35 18 41 36 40 37 38 39 80 40 Uniformity •Most commonly used non-uniformity definition: 49-point, 3standard 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 Force Metal Substrate 2006/5/23 83 Tensile Stress Causes Crack Force Force Metal Substrate 2006/5/23 84 42 Al Stress •Aluminum has higher thermal expansion rate than silicon Al = 23.6106 K, Si = 2.6106 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 (Rs) is a defined parameter Rs = /t •By measuring Rs, 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 A I R= L A R = Resistance, = Resistivity L = Length, A = Area of line cross-section 2006/5/23 88 44 Sheet Resistance Concepts L w t I Apply current I and measure voltage V, Resistance: R = V/I = L/(wt) For a square sheet, L = w, so R = /t = Rs Unit of Rs: ohms per square (/) 2006/5/23 89 Sheet Resistance Are you sure their resistance is the same? I Rs=/t 2006/5/23 I Rs =/t 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 P1 S1 P2 S2 P3 P4 S3 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 P1 and P4, Rs = 4.53 V/I, V is voltage between P2 and P3 –If current I is between P1 and P3, Rs = 5.75 V/I, V is voltage between R2 and R4 •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 FSG SiN FSG FSG 2006/5/23 Cu Cu 98 49 Tantalum Barrier Layer and Copper Seed Layer Deposition Cu Ta FSG SiN FSG FSG Cu Cu 2006/5/23 99 Electrochemical Plating Copper Cu Ta SiN FSG FSG FSG 2006/5/23 Cu Cu 100 50 CMP Copper and Tantalum, CVD Nitride SiN Ta Cu FSG SiN FSG Cu Cu 2006/5/23 101 Copper Metallization SiN Ti/TiN M1 Cu CoSi2 FSG FSG PSG STI 2006/5/23 Ta or TaN Cu Cu W W n+ n+ USG P-Well P-Epi P-Wafer p+ p+ N-Well 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
