KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering Lecture IV Metallization

` KUKUM – SHRDC INSEP Training Program 2006 Summary of IC Processes School of Microelectronic Engineering

KUKUM – SHRDC INSEP Training Program 2006 Two Types of Thin Film School of Microelectronic Engineering  Dielectric Film (CVD Process)  Oxide  Nitride  Epitaxial silicon  Conducting Film (PVD Process)  Aluminum alloy  Ti, TiN  Silicides  Copper (CVD or electroplating)  Tungsten (Metal CVD)  Polysilicon (LPCVD)

KUKUM – SHRDC INSEP Training Program 2006 Conducting Thin Film Applications School of Microelectronic Engineering  Front-End-Of-Line (FEOL)  Gate and electrodes  Polysilicon  Polycide  Back-End-Of-Line (BEOL)  Interconnection  Silicides  Barrier  ARC

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Interconnection  Al-Cu alloy is commonly used material  Deep sub-micron metallization …. Copper

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Interconnection  Copper Metalization

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Silicides  To reduce contact resistance of metal / semiconductor interface.  TiSi 2, WSi 2 and CoSi 2 are commonly used materials  Self-aligned-silicide-process (SALICIDE)

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Barrier Layer  To prevent aluminum diffusion into silicon (junction-spiking)  TiN is widely used barrier material

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Barrier Layer  To prevent aluminum diffusion into silicon  TiN is widely used barrier material

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  ARC (anti reflective coating) to reduce “notching” during photolithography process.  TiN is widely used material

KUKUM – SHRDC INSEP Training Program 2006 CVD vs PVD School of Microelectronic Engineering  CVD: Chemical reaction on the surface PVD: No chemical reaction i.e. purely physical  CVD: Better step coverage (50-100%) and gap-fill capability PVD: Poor step coverage (<15%) and gap-fill capability  CVD: Impurities in the film, lower conductivity, hard to deposit alloy. PVD: Purer deposited film, higher conductivity, easy to deposit alloy.

KUKUM – SHRDC INSEP Training Program 2006 Physical Vapor Deposition (PVD) Process School of Microelectronic Engineering  PVD works by vaporizing the solid materials, either by heating or by sputtering, and recondensing the vapor on the substrate to form the solid thin film.

KUKUM – SHRDC INSEP Training Program 2006 Physical Vapor Deposition (PVD) Process School of Microelectronic Engineering  Evaporation  Thermal Evaporation  Electron Beam Evaporation  Sputtering  Simple DC Sputtering  DC Magnetron Sputtering  DC Triode  RF Diode  RF Triode  RF / DC magnetron

KUKUM – SHRDC INSEP Training Program 2006 Thermal Evaporation School of Microelectronic Engineering  In the early years of IC manufacturing, thermal evaporation was widely used for aluminum deposition.  Aluminum is relatively easy to vaporized due to low melting point (660 C).

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  System needs to be under high vacuum (~ Torr)  Flowing large electric current through aluminum charge heats it up by resistive heating.  Aluminum starts to vaporized  When aluminum vapor reaches the wafer surface, it recondenses and forms a thin aluminum film.

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  The deposition rate is mainly related to the heating power, which controlled by the electric current.  The higher the current, the higher the deposition rate.  A significant trace amount of sodium, low deposition rate and poor step coverage.  Difficult to precisely control the proper proportions for the alloy films such as Al:Si, Al:Cu and Al:Cu:Si.  No longer used for metalization processes in VLSI and ULSI

KUKUM – SHRDC INSEP Training Program 2006 Electron Beam Evaporation School of Microelectronic Engineering  A beam of electrons, typically with the energy about 10 keV and current up to several amperes, is directed at the metal in a water-cooled crucible in vacuum chamber.  This process heats the metal to the evaporation temperature.  IR lamp is used to heat the wafer (improve step coverage)

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Better step coverage (higher surface mobility due to lamp heating)  Less sodium contamination (only part of aluminum charge is vaporized.  Cannot match the quality of sputtering deposition, therefore very rarely used in advanced semiconductor fab.

KUKUM – SHRDC INSEP Training Program 2006 Sputtering School of Microelectronic Engineering  The most commonly used PVD process for metallization  Involves energetic ion bombardment, which physically dislodge atoms or molecules from the solid metal surface, and redeposit them on the substrate as thin metal film.  Argon is normally used as sputtering atom

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  When power is applied between two electrodes under low pressure, a free electron is accelerated by the electric field.  When it collides with Ar, another free electron is generated (ionization collision). Ar becomes positively charged.  The free electron repeat this process to generate more free electrons.

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  The positively charged Ar ions are accelerated toward a negatively biased cathode, usually called target. The target plate is normally made from the same metal that to be deposited on wafer.  When these energetic argon ions hit the target surface, atoms of the target material are physically removed from the surface by the momentum transfer of the impacting ions.

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Sputtered-off atoms leave the target and travel inside the vacuum chamber in the form of metal vapor.  Eventually, some of them reach the wafer surface, adsorb and become so-called adatoms.  The adatoms migrate on the surface until they found nucleation sites and rest there.  Other adatoms recondense around the nucleation sites to form grain.  When the grains grow and meet with other grains, they form a continuous poly-crystalline metal thin film on the wafer surface.

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  The border between grains is called a grain boundary.  The grain boundary can scatter electron flows, therefore cause higher resistivity.  Grain size mainly determined by surface mobility, which is related to many other factors.

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Normally, higher temperature will result in larger grain size.  Grain size has a strong effect on film reflectivity and sheet resistance.  Film with larger grain size has less grain boundary to scatter electron flow, therefore lower resistivity.

KUKUM – SHRDC INSEP Training Program 2006 Simple DC Sputtering School of Microelectronic Engineering  The simplest sputtering system.  Wafer is placed on on the grounded electrode and the target is the negatively biased electrode, the cathode.  When a high-power DC voltage (several hundred volts) is applied, the argon atoms are ionized by electric field.

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  These accelerate and bombard the target, then sputtered-off the target material from the surface.

KUKUM – SHRDC INSEP Training Program 2006 DC Magnetron Sputtering School of Microelectronic Engineering  The most popular method for PVD metallization process, because it can achieve high deposition rate, good film uniformity, high film quality, and easy process control.  High deposition rate allow single-wafer processing, which has several advantages over batch-processing.

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  A rotating magnet is placed on top of metal target.  In a magnetic field, electrons will be constrained near magnetic field line.  This gives electrons more chances for ionization collision.  Therefore, the magnetic field serves to increase plasma density and cause more sputtering near the magnet.

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  By adjusting the location of magnets, the uniformity of the deposited film can be optimized.  Normally, a shield is installed inside the chamber to protect the chamber wall from being deposited.

KUKUM – SHRDC INSEP Training Program 2006 Sputtering System School of Microelectronic Engineering GENERATOR RACK PUMP FRAME CRYOPUMP COMPRESSOR HEAT EXCHANGER TRNSFORMER /MAIN AC BOX SYSTEM CONTROLLER / SYSTEM AC BOX MAINFRAME LOAD LOCK ORIENT / DEGAS COOL DOWN PRE CLEAN SPUTTER CHAMBER  Cluster tool with multiple chamber.  Staged vacuum;  Loading station: Torr  Transfer chamber: to Torr  Process chamber: Torr

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering APPLIED MATERIALS, ENDURA HPPVD SYSTEM

KUKUM – SHRDC INSEP Training Program 2006 Basic Metallization Process School of Microelectronic Engineering  Burn-in Step  To condition the target before processing production wafers.  Native oxide and defects on the target were removed.  De-gas (Orient/Degas Chamber)  To orient the wafer.  Heat the wafer to drive-out gases and moiture.  Prevent out-gassing during the deposition process

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Pre-Clean (Pre-clean Chamber)  Sputtering etch to remove native oxide on the metal surface.  Prepare contact holes and vias for metal deposition.

KUKUM – SHRDC INSEP Training Program 2006 Titanium Deposition Process School of Microelectronic Engineering  Normally deposited as welding layer prior to aluminum alloy deposition (reduce contact resistance)  Titanium can trap oxygen and prevent it from bonding with aluminum to form high reistivity aluminum oxide.  To produce larger grain size, wafer is normally heated to 350 C.  Collimated chamber is normally used in deep submicron IC fabrication to achieve better titanium step coverage.

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Collimator allows metal atoms to move in mainly in vertical direction  Significantly improve bottom step coverage

KUKUM – SHRDC INSEP Training Program 2006 Titanium Nitride Deposition Process School of Microelectronic Engineering  TiN is widely used as ARC, glue and barrier layers.  The deposition normally uses a reactive sputtering process.  When nitrogen flows with argon into the process chamber, some nitrogen molecules dissociate as a result of ionization collision.  Free nitrogen radicals are very reactive. They can react with sputtered Ti atoms to form TiN and deposit it on the wafer surface.  They can also react with Ti target to form a thin TiN layer on the target surface.  Argon bombardment could dislodge TiN from the target surface, redeposited on the wafer surface.

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering

KUKUM – SHRDC INSEP Training Program 2006 Al-Cu alloy Deposition Process School of Microelectronic Engineering  Needs an ultrahigh baseline vacuum to achieve low film resistivity.  Standard process  Depositing aluminum alloy over tungsten plug, after Ti and TiN wetting layer.  Normally deposited at 200 C, to achieve smaller grain size for better line patterned etch.

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Hot Aluminum Process  To allow aluminum to fill contact holes and vias, instead of W-plug  This will reduce the contact resistance between metal layers.  Aluminum: 2.9 to 3.3  Ω.cm  Tungsten: 2.9 to 3.3  Ω.cm  Aluminum is deposited at 450 to 500 C.

KUKUM – SHRDC INSEP Training Program 2006 Metal Thin Film Measurement School of Microelectronic Engineering  Thickness Measurement  Reflectivity  Sheet Resistance  Deposition Rate  Film Stress  Process Uniformity

KUKUM – SHRDC INSEP Training Program 2006 Thickness Measurement School of Microelectronic Engineering  Metal films such as aluminum, Ti, TiN and copper are opaque films; therefore, optical-based technique such as reflectospectrometry cannot be used.  A destructive process is normally required to precisely measure the actual film thickness.

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Step height measurement (profilometer)  SEM / TEM  Four point probe – indirect measurement

KUKUM – SHRDC INSEP Training Program 2006 School of Microelectronic Engineering  Accoustic Measurement  Laser shot on thin film surface  Photo-detector measures reflected intensity  Thermal expansion causes a sound wave  Propagates and reflects at interface of different materials  Accoustic wave echoes back and forth  Film thickness can be calculated by; d = Vs ∆t / 2 Vs – speed of sound ∆t - time between reflectivity peaks

KUKUM – SHRDC INSEP Training Program 2006 Reflectivity School of Microelectronic Engineering  Reflectivity change indicates a process drift.  A function of film grain size and surface smoothness  Larger grain size, lower reflectivity  Can be measured using Reflectometry (intensity of the reflected beam of light).  Reflectivity measurement results usually use the relative value to silicon.

KUKUM – SHRDC INSEP Training Program 2006 Sheet Resistance Measurement School of Microelectronic Engineering  Most important characteristics of conducting film.  Widely used to rapidly monitor the deposition process uniformity by indirectly measure the film thickness.  Four Point Probe is commonly used measurement tool

KUKUM – SHRDC INSEP Training Program 2006 Deposition Rate School of Microelectronic Engineering

KUKUM – SHRDC INSEP Training Program 2006 Film Stress Measurement School of Microelectronic Engineering  Stress is due to the mismatch between different materials  Compressive stress causes hillock, short between metal  Tensile stress causes crack, metal open, peel off  Two types of measurement  Contact – profilometer  Non-contact – capacitance measurement

KUKUM – SHRDC INSEP Training Program 2006 Process Uniformity School of Microelectronic Engineering  Max-min uniformity (Max value – Min value) / 2 x average