24/10/02yacov shneider wafer cleaning processing yacov shneider processing engineer microelectronics...

24/10/02 YACOV SHNEIDER

WAFER CLEANING PROCESSING

Yacov Shneider

Processing Engineer

Microelectronics Center

Dept. of Electrical Eng.

TECHNION


Wafer 300 mm - Infineon


300 mm wafer - Infineon


IBM 300 mm FAB


IBM 300 mm FAB wet clean


Copper integration clean room


IBM 300 mm FAB wafer check


IBM 300 mm FAB mask clean


IBM 300 mm FAB engineers


Crystal silicon Ingots


VIAs before cleaning, VERTEQ


VIAs after cleaning, VERTEQ


CMP contaminations


Six layer metallization design


CLEANING CONCEPTION

• Define the nature of the contamination.

• Analytical procedures (methods) for detecting contamination.

• Cleaning procedures (techniques) to remove contaminations.

TERMINOLOGY

CLEANLINESS – The ability to control the surface atomic homogeneity. The surface is naturally covered by layers of elements (O2) or compounds (H2O).

• REQUIREMENTS for “clean” surface

• Uniform on a molecular or atomic level.

• Volatile.

• Non-interfering with diffusion or oxidation processes.

• Unable to affect the electrical properties of a finished device.


CONTAMINATION KINDS

PARTICULATES –

CHUNKS OF GRANULAR

MATTER

Diffusion locally providing unde-

sirable masking effects. Unwanted

impurities, defects. Scratches in

photomasks during contact printing.

FILMS –

ATOMIC, IONIC or

POLYMERIC

Gives uniform undesirable effects

In oxidation and evaporation processes.


ANALYTICAL METHODS FOR DETECTING CONTAMINATIONS

PARTICULATES

• OPTICAL MICROSCOPY

• Brightfield / darkfield;

• Nomarsky interference contrast;

• Phase contrast.

• SEM;

• LASER SURFSCAN;

• COLLIMATED light BEAM;

• AUGER chemical analysis;

• TEM;

• SIMS.

FILMS• ELLIPSOMETRY;• AUGER analysis;• TEM;• SIMS;• FLUORESCENCE microscopy;

TECHOLOGICAL TESTS• Water break test (hydrophobic);• Breath tests (cold chuck);• C-V technique for MOS;• Lifetime measurements.


CONTAMINATION NATURE

PARTICULATES -

CHUNKS OF GRANULAR

MATTER

DUST from abrasion grinding

and handling.

• INORGANIC “GRIT”-abrasive

particles, sand, clay (from air-

borne or chemicals).

• LINT from clothing, skin, hair -organic in nature, bacteria and etc.

FILMS -

ATOMIC, IONIC OR

POLYMERIC

ORGANIC INORGANIC

Resist residues Metal layers

left by evapora- Ions from resist

tion of solvents and reagents

Oil from water Residues from

and handling. reagents, and

handling


Introduction The device performance, reliability, and product yield of Integrated

Circuit are critically affected by the presence of chemical contaminants and particulate.

ULSI technology needs stringent and reliable means:

• To control the surface smoothness and;

• To remove metallic and organic residues.

ITRS 2003 requirements:

• Metallic impurities less 109 atoms/cm2, particles less 0.1/cm2, (for particle size greater 0.1mkm).

Process

• Standard and modified RCA Wet Cleaning.

• Dry Cleaning that meets cluster-processing system.


CLEAN wafer: free from particles, organic contamination, metal ions contamination, low surface micro roughness and native oxide films.

Basic Concepts of Cleaning• RCA Cleaning by Kern and Puotinen in 1970.

• SC-1 : NH4 OH:H2O2:H2O = 1:1:5 to 1:2:7 at 70-90oC.

• Remove organic contamination and particles by oxidation.

• SC-2 : HCL:H2O2:H2O =1:1:6 to 1:2:8 at 70-90oC.

• Remove metal contamination by forming a soluble complex.

Particles :• General Guideline (smaller than 1/10 of feature size).

• Particle adhesion occurs during the process from the equipment, ambient, gas, chemicals and DI water.


• Metallic impurity Concentration (1X1010 atoms/cm2)

• Calcium <5.0

• Cobalt <0.3

• Copper <0.4

• Chromium <0.5

• Iron 0.8

• Manganese <1.0

• Nickel <0.3

• Titanium <0.9

• Zinc <0.3

Typical concentrations of trace metallic impurities on wafer surfaces


Sulfuric Acid has the highest number of particles and HF the lowest.

• Adhesion of Particles:

1. Van der Waals Forces.

2. Forces due to the formation of an electrical double layer.

3. Forces due to capillary action around particle.

4. Chemical bond between the particle and the surface.

• Particle removal mechanisms 1. Dissolution.

2. Oxidizing degradation and dissolution.

3. Lift-off by slight etching of the wafer surface.

4. Electric repulsion between particles.

• H2O2 can oxidize the silicon surface and OH- group (from NH4OH) provide negative charge on silicon.

• The deposition of particles is a strong function of pH values of the solution. With increasing pH value above 10 results in low particle deposition (SC-1 have the highest removal efficiency).


Megasonic cleaning removes organic and inorganic particles from the surface at a temperature of less than 40oC. The high power and high frequency sonic pressure waves wet the particles first and the solvent diffuses into the interface, and then, the particle is removed from the surface.

Metal Ions Contamination• Source : Chemical solutions, Ion implantations, Plasma processes.

• Effect on Device : 1. Structural defects at the interface, after high temperature anneal.

2. Stacking faults during later Oxidation or Epi-process.

3. Increased leakage current of P-N junctions.

4. Reduced minority carrier lifetime.


TWO mechanism for precipitation of metal impurities on Si wafer.

• 1. Direct bonding to the Si by charge exchange between a metallic ion and hydrogen atoms that terminated on the Si substrate: For examples Noble metal such as Gold which has higher electronegativity.

• 2. Metals such as Al, Cr, and Fe tend to oxidize when the silicon surface is oxidized and are included in the oxide films. These oxide can be removed by HF etching.

Method for removing Metal Contamination• Currently the wet cleaning process is the most effective method for

removing metallic contamination : HF: 0.5%, H2O2: 10% clean.

• Both SC-1, SC-2 have the capability of removing metallic impurity on Si wafer due to high oxidizing mechanism of H2O2.

• Ca contamination causes rough surface and defect density in oxide: The threshold value for Ca contamination is 109 atoms/cm2.


Fe contamination: for thin oxide (<100 Ǻ) threshold concentration is as low as 1x 109 atoms/cm3.

Method for removing Organic Contamination• Source: organic vapor in the ambient, storage container, residue of

photo-resist (main source).

• Take Effect to the yield: Incomplete cleaning of the surface, leaving contaminants such as the native oxide or metal impurities which cause micro-masking effect in the Plasma process.

• Current stripping technology: Photoresist removal by Plasma and Wet cleaning (H2SO4:H2O2 = 3:1 to 4:1 at 120-130oC).

• Depletion of H2O due to high wafer temperature (120oC) cause unstable process control > Alternative: add ozone in water which can be used as strong oxidizing agent that decomposes organic impurities.

• The oxide thickness increases as the immersion time increases and with the concentration of ozone.


Based on XPS analysis, the addition of ozone can reduce residual oxide after HF dip. Ozone – unstable, toxic gas (Threshold LV 100 ppb).

Surface Microroughness problem• ULSI device need <40 Ǻ oxide: surface should be atomic flatness.

• RCA SC-1 solution cause microroughness: NH4OH acts as an etchant of the oxide while H2O2 acts as the oxidant.

• Etching and Oxidation simultaneously in SC-1 solution.

Methods to reduce surface microroughness

1. Reduce the proportion of NH4OH.

2. Reduce the temperature of the bath.

3. Reduce cleaning time.


Method for removing Native Oxide• Uncontrollable thin oxide growth, high contact resistance, and

inhibition of selective chemical vapor deposition or epitaxy.

• If this native oxide is not removed, it serves as the source of metallic impurities which diffuse into the silicon or precipitates at the interface of SiO2-Si , resulting in defects.

• Native oxide free surface is a key factor in obtaining high performance and reliability (after HF-dip: H-passivated surface).

Wet Cleaning Technology Standard Method 1. H2SO4:H2O2 (2:1 to 4:1 at 120-130oC) : Remove greasy impurities

which may be from the cassette or residues from the photoresist.

2. SC-1 : NH4OH:H2O2:H2O = 1:1:5, 70-80oC for 10 min : Remove organic films, desorption of trace metals.


• 3. 1%HF-H2O, for 10-20 sec: remove oxide and trace metals in oxide.

• 4. SC-2 : HCl:H2O2:H2:O = 1:1:6, 70-80oC for 10 min: dissolve alkali ions and hydroxides of Al+3, Fe+3, Mg+2.

• 5. DI water rinse (resistivity >17 Mohm-cm).

Equipment• 1. Immersion technique: Quartz bath to prevent leaching of Al, B, and alkalis

which can results if Pyrex glass is used.• 2. Megasonic cleaning: SC-1 solution at 35-42oC.• 3. Megasonic cleaning: SC-1, SC-2, DI-water are fed directly onto the spinning

wafer (Verteq, Semitool, SEZ).

Advanced Wet Cleaning from Ohmi (Tohoku University) 1. H2O + O3 Organic Contamination

2. NH4OH:H2O2:H2O = 0.05:1:5 Particle, Organic and Metallic impurity. Effects on micro roughness.


• HF(0.5%) + H2O2(10%) Native oxide, metallic impurity

• Ultra pure water Rinsing

• Omit RCA SC-2 if high purity HF solution is used.

Dry Cleaning Technology (25 years efforts) • Problems of wet cleaning: particle generation, drying difficulty, cost,

chemical waste, incompatibility with advanced cluster process, inflexibility.

• The dry cleaning process can solve problems: so far not completely successful.

• The dry cleaning process: require excitation energy to enhance gas-phase chemical reaction at low temperature such as plasma, particle beam, radiation, thermal heating.

• Additional energy enhance reaction but we have minimize damage on the wafer.


UV-Ozone Clean• Effective way to remove hydrocarbon, although the surface is oxide

passivated.

• Surface excitation process

• Absorbed impurity + hν (2000-3000 Ǻ UV) > Excited impurity

• Gas-phase excitation process

• O2 + hν (1849 Ǻ UV) > 2O

• O + O2 > O3

• O3 + hν (2537 Ǻ UV) > O + O2

• Excited impurity + (O + O3) > Volatile compound

• The subsequent HF/H2O vapor or Ar/H2 plasma cleaning can remove surface oxide.

HF/H2O vapor clean• HF dip promotes a hydrogen-passivated surface

• HF/H2O vapor clean induces a fluorine-terminated surface


• Hydrophobic surface (high contact angle) vs. Hydrophilic surface (low contact angle).

• With oxide layer on silicon: hydrophilic surface.

Ar/H2 Plasma Cleaning• The gas molecules are excited or ionized by remote plasma to reduce

bombardment damage on silicon wafer.

• Excited Ar ions physically sputter the surface impurity away.

• Excited Hydrogen ions chemically etch the surface.

• By proper adjustment of two process, an optimum cleaning can be obtained.

Thermal Cleaning 1. The native oxide can be removed by heating the wafer to 800oC or

above in UHV (10-10 Torr) to vaporize the oxide.


2. High temperature cleaning should be carefully examined due to etching of Si surface:

• - Si + SiO2 = 2SiO at high temperature (>800oC) and low O2 partial pressure;

• - SiO is volatile at temperatures above 750oC and oxide film is removed;

• - 2Si + O2 = 2SiO Etching of the surface (micro roughness);

• At low temperatures and high oxygen partial pressure: smooth surface with thin oxide;

• At high temperatures and low oxygen partial pressure: clean surface with roughness.

24/10/02yacov shneider wafer cleaning processing yacov shneider processing engineer microelectronics...

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