Lecture 6 Metallization

Two Types of Thin Film 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)

Conducting Thin Film Applications Front-End-Of-Line (FEOL) Gate and electrodes Polysilicon Polycide Back-End-Of-Line (BEOL) Interconnection Silicides Barrier ARC

Interconnection Al-Cu alloy is commonly used material Deep sub-micron metallization …. Copper

Interconnection Copper Metallization

Silicides To reduce contact resistance of metal / semiconductor interface. TiSi2, WSi2 and CoSi2 are commonly used materials Self-aligned-silicide-process (SALICIDE)

Barrier Layer To prevent aluminum diffusion into silicon (junction-spiking) TiN is widely used barrier material

Barrier Layer To prevent aluminum diffusion into silicon TiN is widely used barrier material

ARC (anti reflective coating) to reduce “notching” during photolithography process. TiN is widely used material

Physical Vapor Deposition (PVD) Process PVD works by vaporizing the solid materials, either by heating or by sputtering, and re-condensing the vapor on the substrate to form the solid thin film.

Physical Vapor Deposition (PVD) Process Evaporation Thermal Evaporation Electron Beam Evaporation Sputtering Simple DC Sputtering DC Magnetron Sputtering DC Triode RF Diode RF Triode RF / DC magnetron

Thermal Evaporation 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 (6600C).

System needs to be under high vacuum (~ 10-6 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 re-condenses and forms a thin aluminum film.

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 metallization processes in VLSI and ULSI

Electron Beam Evaporation 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).

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.

Sputtering 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.

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.

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.

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 re-condense 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.

Simple DC Sputtering 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.

These accelerate and bombard the target, then sputtered-off the target material from the surface.

DC Magnetron Sputtering 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.

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.

CVD vs PVD 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

Basic Metallization Process 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 moisture. Prevent out-gassing during the deposition process

Titanium Deposition Process 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 resistivity aluminum oxide. To produce larger grain size, wafer is normally heated to 3500C. Collimated chamber is normally used in deep submicron IC fabrication to achieve better titanium step coverage.

Collimator allows metal atoms to move in mainly in vertical direction Significantly improve bottom step coverage

Titanium Nitride Deposition Process 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.

Al-Cu alloy Deposition Process 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.

Metal Thin Film Measurement Thickness Measurement Reflectivity Sheet Resistance Deposition Rate Film Stress Process Uniformity

Thickness Measurement Metal films such as aluminum, Ti, TiN and copper are opaque films; therefore, optical-based technique such as reflecto-spectrometry cannot be used. A destructive process is normally required to precisely measure the actual film thickness.

Step height measurement (profilometer) SEM / TEM Four point probe – indirect measurement

Acoustic 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 Acoustic wave echoes back and forth Film thickness can be calculated by; d = Vs ∆t / 2 Vs – speed of sound ∆t - time between reflectivity peaks

Reflectivity 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.

Sheet Resistance Measurement 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

Deposition Rate

Film Stress Measurement 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

Process Uniformity Max-min uniformity (Max value – Min value) / 2 x average