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

KUKUM – SHRDC INSEP Training Program 2006

School of Microelectronic Engineering

Lecture IVMetallization

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Summary of IC Processes



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 Metalization



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


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 alloy.


Physical Vapor Deposition (PVD) Process


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.


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 (660 C).



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 recondenses 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 metalization 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 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.



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.



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.


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.



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.


Sputtering System


GENERATOR RACK

PUMP FRAME

CRYOPUMPCOMPRESSOR

HEAT EXCHANGER

TRNSFORMER/MAIN ACBOX

SYSTEMCONTROLLER/ SYSTEMAC BOX

MAINFRAME

LOAD LOCK

ORIENT / DEGAS

COOL DOWNPRE CLEAN

SPUTTERCHAMBER

Cluster tool with multiple chamber. Staged vacuum;

Loading station: 10-6 Torr Transfer chamber: 10-7 to 10-8 Torr Process chamber: 10-9 Torr



APPLIED MATERIALS, ENDURA HPPVD SYSTEM


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 moiture. Prevent out-gassing during the deposition process



Pre-Clean (Pre-clean Chamber) Sputtering etch to remove native oxide on the metal surface. Prepare contact holes and vias for metal deposition.


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 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.



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.



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.


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 reflectospectrometry 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



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


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

Documents

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