E SC 412 Nanotechnology: Materials, Infrastructure, and Safety … · • As their name implies, MICs are highly mobile –MICs can move through an electronic nanoproduct even after

© 2013 The Pennsylvania State University

E SC 412

Nanotechnology: Materials, Infrastructure, and

Safety

Wook Jun Nam


Unit 4

General Processing

Concerns, Contamination and

Damage

Lecture 1

General Processing

Concerns, Contamination and

Damage


Outline

• Introduction

• What is Contamination?

• Types of Contamination

• Sources of Contamination and Their Prevention

• Materials Preparation

• Damage


Introduction

• Processing, processing equipment,

processing facilities (e.g., cleanrooms),

and the people performing processes can

all cause contamination, damage, or both

to micro and nano scaled products.

• Since this course focuses on processing,

we must first address these general

concerns.


Outline

• Introduction

• What is contamination?

• Types of contamination



• Damage


What is Contamination?

• Contamination is a general term used to describe unwanted material that adversely affects the fabrication, or synthesis of a nanoproduct.

• A nanoproduct has at least one dimension at the nanoscale.

• The scale will define the effect of the contaminate at this level.

• Contamination affects three major areas:– Processing yield

– Performance

– Reliability


Processing Yield

• Processing in a contaminated environment can cause a multitude of problems including: a change in dimensions, and an altering of the materials properties.

• There are a number of in-process quality checks to minimize contamination problems.

• Contamination results in fewer nanoproducts completing the process and, therefore, higher production costs.


Performance

• Small contamination regions may escape

in-process quality checks

• Seemingly clean materials can contain

undetected contaminants that can change

the performance of a product.


Reliability

• The most serious of contamination related problems is product failure.

• Failure is a major concern of the medical, space, and defense industries

• Reliability can also be a major concern in the consumer market. For example, an unreliable cell phone can cause a consumer to cancel their subscription.

• There can be serious financial implications if there are reliability issues.


Contamination’s Impact on

Device Yield• The impact of contamination is a function

of the size of the nanoproduct and the

size, type, and concentration of the

contaminant(s).


# of Defective

Products = 1

# of Defective

Products = 8

# of Defective

Products = 20

Yield =64-8

64 x 100 = 87.5%

Yield =256-20

256 x 100 = 92.19%

Effect of Defect size versus

Nanoproduct size

In this example, we keep the defect size constant

and vary the size of the nanoproduct.

Public Domain: Image Generated

by CNEU Staff for free use


Outline

• Introduction





• Damage


Types of Contamination

• For analysis we divide contamination into the following categories:– Particles

– Ions

– Atoms

– Molecules

• These contaminants can arise when undertaking fabrication on the nanoscale.


Particles

• These are small objects that can adhere to materials and are the most common form of contamination.

• Airborne particles suspended in the air, such as perfume, are a special class of particles known as aerosols.

• Particles possess a wide range of sizes.


Relative Particle Sizes

40 µm – Barely Visible to the Naked Eye

Gas Molecules

Virus

Tobacco Smoke

Bacteria

Fog

Adult Red Blood Cell

Flour dust, pollens

Human Hair

Beach Sand

0.1 - 1 nm

2 – 100 nm

10 – 300 nm

0.2 – 10 µm

1 – 50 µm

7.5 µm

5 – 50 µm

50 – 120 µm

100 µm and up

Public Domain: Image Generated by CNEU Staff for free use


Relative Size of Partials

Human Hair

≈100 µm Dia.

Particle ≈10 µm Dia.

Particle ≈0.5 µm Dia.

Public Domain: Image Generated by

CNEU Staff for free use


Ions

• Mobile Ionic Contamination (MIC) can contribute to corrosion and alter electrical characteristics, changing performance and reliability factors.

• Common MICs are Group I ions.


Mobile Ionic Contaminants

• Atoms in ionic form

– Only a small concentration of MICs, 1010atoms/cm2 or less, are needed to cause problems.

– For reference, surface atoms have a density of about 1014 atoms/cm2.

• So, 1010 atoms/cm2 is only 1 contaminant per 10,000 atoms.

• As their name implies, MICs are highly mobile

– MICs can move through an electronic nanoproduct even after passing electrical testing and shipping, causing failure in the field.

• Group I materials such as sodium, potassium, and lithium are very common MICs.


Typical Metal Impurities that can

Contaminate Some Nanoproducts, but are

Essential in Biological Systems

• Heavy metals

– Iron (Fe)

– Copper (Cu)

– Aluminum (Al)

– Chromium (Cr)

– Tungsten (W)

– Titanium (Ti)

• Alkali metals (found in

Group IA, very

reactive!)

– Sodium (Na)

– Potassium (K)

– Lithium (Li)


Atomic, Molecular, Thin Film,

and Residual Contamination

• Can come from processing.

• Can come from chemicals, including DI water.

• Can come from chemical reactions.

• Can come from the environment.

• Can come from humans.


Organic Molecules

• Organic refers to any compound containing carbon.

• Organic contaminates include molecules, thin films, and residues left behind by processing or exposure.

• Typical organic contaminants include bacteria, processing residues, lubricants, vapors, detergents, and solvents.

• Bacteria, for example, grow in water systems and on surfaces not cleaned regularly.– Once present in fabrication and synthesis, bacteria

can act as particulate contamination, or may contribute to organic and metallic ion concentration.


Inorganic

• Inorganic refers to any compound not containing carbon.

• Inorganic contaminants include atoms, molecules, thin films, and residue left behind by processing.

• Plasma-based processing for example, can leave behind contaminants from the chamber walls and from plasma-substrate interactions.


Outline

• Introduction





• Damage


Sources of Contamination

• Literally, everything that comes in contact with a product during manufacture is a potential contamination source.

• Major contamination sources are:– Air

– Process water

– Production facility

– Process chemicals

– Cleanroom personnel

– Process gasses


Air

• Air, due to a high concentration of particulates and aerosols, must be treated before entering a cleanroom or used in a controlled environment like a chemical hood.

• Air cleanliness levels in cleanrooms and hoods, are identified by the particulate diameters and their density in air.

• Air quality is identified by class number, this is defined as the number of particles 0.5µm in diameter or larger in a cubic foot of air.


Particle Size and Density Limits

in the Clean Air Environment


Air

• Clean air strategies for nanofabrication

and synthesis:

– Clean Work stations

– Tunnel/Bay designs

– Total cleanrooms

– Mini-environments


Clean Work Stations

• One approach in nanofabrication and synthesis

is to create individual work stations, such as

chemical hoods with air filters and non-shedding

materials.

• A large room with the work stations (or hoods)

arranged in rows, so products under fabrication

can be moved to each station, without coming in

contact with “dirty” air.


Clean Work Stations

• Filters in the clean hoods are known as High

Efficiency Particle Attenuation (HEPA) filters.

• HEPA filters consist of large, porous fibers

folded into a filter holder in an accordion design.

• HEPA filters allow a large volume of air to pass

at a low velocity (90-100 ft/min.) and have a

filtering efficiency of 99.99%.


The HEPA Filter Design

Dirty Air

Clean Air



Clean Work Stations

• A typical clean hood has a HEPA filter mounted in the top.

• “Dirty” air is pushed through the filter and exits in a laminar pattern.

• A shield directs the exiting air over the work area in the hood.

• These type of hoods are known as vertical laminar flow (VLF) hoods.


VLF Hood Cross Section


PerfilterBlower

Air Flow

Work surface

HEPA Filter

Clean

Air


Tunnel/Bay Concept

• For more stringent particulate control clean hoods are less popular because of the potential for personnel induced contamination.

• This contamination issue can be solved by dividing the fabrication/synthesis area into separate bays or tunnels.


Tunnel/Bay Concept

• In this design, clean air enters from above by HEPA filters built into the ceiling.

• Materials are less vulnerable to personnel-generated contamination because there are fewer workers in the immediate area.

• On the downside, tunnel/bay designs are more expensive to construct than hoods and are less versatile than cleanrooms, when it comes to process changes.


Tunnel/Bay DesignHEPA Filters

Clean

Air

Public Domain: Image Generated by CNEU Staff for free

use


Total Cleanroom Strategy

• This strategy employs an open fabrication/synthesis area.

• Air filtering is accomplished by HEPA filters in the ceiling with returns in the floor, to give a continuous flow of clean air.

• The continuous flow of clean air allows for a faster recovery, which is the amount of time it takes for the filters to return the area to an acceptable condition after a disturbance.

• A class 1 facility turns over air every 6 seconds!


The Total Cleanroom

Van Aznt, Peter. Microchip Fabrication 4th

Edition. McGraw Hill. New York. 2007


Mini-environments

• Rising cleanroom costs with diminishing returns on effectiveness have resulted in another concept of isolating materials in as small an environment as possible.

• The problem of how to string together a number of mini-environments, such that a product is never exposed to room air is solved by transporting the samples in a clean environment.

• One example of the industrial application of this approach is Hewlett-Packard’s Standard Mechanical Interface (SMIF).


The SMIF System

• SMIF systems have three main parts:

– The pod, or box ,for transportation of

materials.

– The isolated mini-environment at the next

process station.

– A mechanism for extracting and unloading

materials at each successive processing

station.


The SMIF System Isolated pod w/

vacuum or inert

atmosphere

Wafers in a cassette

Standard Mechanical

Interface (SMIF)

Process Chamber

Load lock

Robotic wafer

loading arm

Internal seal to

main chamber



The SMIF System

• SMIFs have the advantage of:

– Better temperature and humidity control.

– Reduced yield losses due to contamination.

• However, pods can be too heavy and expensive.

• Robots can be used to handle SMIF boxes, which drives up cost and complexity.


The Production Facility

• A clean production facility is paramount to the production industries.

• Contamination can occur at any stage of fabrication/synthesis.

• Strict contamination-level monitoring and procedural protocols are often required to insure an ultraclean environment.


The Modern Cleanroom

• The traditional cleanroom layout is the ballroomdesign, where individual process tunnels open into a central hallway.

• Every cleanroom is a trade-off between cleanliness and cost, but all are built from a primary design.– A sealed room that is supplied with clean air.

– Building materials that are non-contaminating.

– Systems to prevent accidental contamination.

– Vibration control, for sensitive equipment.


Cleanroom Elements

Cleanrooms utilize a series of techniques to keep

contamination from adversely affecting the

fabrication/synthesis process:

– Adhesive floor mats

– Static control

– Gowning Area

– Double-door

pass-throughs

– Air pressure

– Shoe cleaners

– Air showers

– Glove cleaners

– Service bays


Cleanroom Personnel

• Humans are one of the biggest sources of

cleanroom contamination.

• A clean-room operator, even after showering,

can give off between 100,000 and 1,000,000

particles per minute – this number increases

when a person is in motion.

– At two miles per hour, a human being gives off up to

5 million particles per minute!

© 2013 The Pennsylvania State University American Air Filter Company

HUMAN CONTAMINATION

NORMAL TALKING (saliva) – 2 to 3 feet

COUGHING (saliva and lung tissue) – 4 to 6 feet

SNEEZING – 10 to 15 feet @ 200 MPH, emits

1,400,000 particles


Cleanroom Personnel

• Personnel-generated particles include:

– Flakes of dead hair and skin

– Hairsprays

– Cosmetics

– Facial hair

– Exposed clothing

– Various particles from respiration

© 2013 The Pennsylvania State University American Air filter Company

© 2013 The Pennsylvania State University American Air filter Company

Cosmetic Particle AnalysisEnergy Dispersive X-ray (EDX) Analysis

from Scanning Electron Microscopy (SEM)


Cleanroom Personnel

• Humans must be isolated, as much as possible,

from direct interface with the nano materials.

• Depending upon the situation, personnel must

be covered in special cleanroom garments

(known as a “bunny suit”), consisting of a hood,

facemask, coveralls, boots, and gloves.


Cleanroom Garments

• Modern cleanroom garments are designed

to achieve:

– Total containment of body-generated particles

and aerosols.

– Zero particle release from the garment

system.

– Zero electrical-charge buildup for electrostatic

discharge (ESD).

– No release of chemical or biological residues.


Hood

Cap (underneath hood)

Boots

Safety glasses

Face

mask

Coveralls

Shoe covers

(underneath boots)

GlovesONLY

Cleanroom

paper and

notebooks

Public Domain: Image Generated by

CNEU Staff for free use


Gowning Protocol Example


Outline

• Introduction





• Damage


Process Water

• Water used in the fabrication process must go

through a rigorous filtration process and is

constantly monitored for purity.

• Water generally, is the most used chemical in

nanofabrication, mainly used in chemical

cleaning solutions, dilution of other chemicals,

and as a post-clean rinse.


Deionized Water

• DI water is a highly purified water commonly used in nanofabrication and synthesis.– A series of specially manufactured ion-exchange

resins produce DI water by removing mobile ions found in drinking water

• The water is changed from a conductive medium to a resistive medium with a resistivity of

18 megaohm-cm at 25 °C.

• DI water is a universal solvent – many substances will dissolve in it.


Deionized Water (Cont.)

• DI water purity is maintained by insuring its resistivity is relatively constant at 18 megaohm-cm.

• DI water is often referred to as 18 megaohm water, or 18 meg water.

• It is important to remember that DI water is a process chemical and caution should be observed as with any other cleanroom chemical– If consumed, DI water will leech salts out of the body,

which could be potentially fatal!


DI Water

• DI water can contain the following contaminants:– Dissolved ions

– Organic materials

– Particles

– Bacteria

– Silica

– Dissolved oxygen

• Some of these contaminants can be eliminated by the strict monitoring of filtration steps, while others, like oxygen, are a constant source of contamination.


Process Chemicals

• Acids, bases, salts, organics and solvents

used in reactions and substrates must be

of the highest purity.

• Chemical contaminants can include:

– Metals

– Particulates

– Unwanted trace chemicals.


Process Chemicals

• Unlike water, which is usually produced on site, other process chemicals are purchased and used as is.

• Chemicals are rated by grades:– Commercial

– Reagent

– Electronic

– Semiconductor

• Electronic and semiconductor grades are the cleanest chemicals and should be the purity used in nanofabrication when possible.


Process Chemicals

• Levels of chemical purity vary from manufacturer to manufacturer, but all chemical purity levels are indicated by an assay number.

– For example, an assay of 99.9% on a bottle of sulfuric acid means that the bottle contains 99.9% of sulfuric acid and 0.1% of other substances.


Process Gases

• Like process chemicals, gasses must meet strict purity level requirements.

• Gas quality is measured in four categories:

– Percentage of purity

– Water vapor content

– Particulates

– Metallic ions


Process Gases

• Gas values are measured by the assay

number, with typical values ranging from

99.99 to 99.999999%, depending on the

gas and its use in the process.

• Purity is expressed as the number of 9’s to

the right of the decimal point

– For example; a gas that is 99.999999% pure

is referred to as “six 9’s pure.”


Substrate Cleaning

• Substrate cleaning is essential at all stages of the fabrication process.

• Up to 20% of all process steps relate to some form of cleaning.

• Substrate surfaces have four general types of contamination:– Particulates

– Unwanted oxide layers

– Organic residues

– Inorganic residues


Substrate Surface Cleaning

• Substrate cleaning is a series of process

steps.

• A substrate cleaning process must:

– Remove all surface contaminants.

– Not etch or damage the substrate’s surface.

– Be safe and economical.

– Be ecologically acceptable.


Surface Scrubbers

• Stringent substrate cleanliness requirements led to the development of mechanical surface scrubbers.

• The scrubbers hold the substrate on a rotating vacuum chuck, while a rotating brush is brought near the substrate and a stream of DI water is directed onto the surface.

• The combination of the brush and substrate rotations creates a high-energy cleaning action at the surface.


Surface Scrubber

DI Water and

detergent spryer

Wafer

Rotation

Fiber Scrubber

Mechanical Scrubber



High Pressure Water Cleaning

• This method was developed to alleviate the problem of statically attached particles on glass and chrome photomasks.

• A small stream of water pressurized from 2,000 to 4,000 psi is swept across the surface, dislodging both large and small particles.

• Often a small amount of surfactant (a wetting agent) is added to the water to act as an antistatic agent.


Sulfuric Acid

• Sulfuric acid (H2SO4), heated between 90 and 125 °C, can remove most inorganic residues and particulates from a surface.

• Oxidants, usually hydrogen peroxide (H2O2), ammonium persulfate [(NH4)2S2O8], nitric acid (HNO3), or ozone (O3), are added to remove carbon residue by converting it to gaseous carbon dioxide (CO2).


Sulfuric Acid

and Hydrogen Peroxide• This mixture is an aggressive cleaning solution

and can be used to clean surfaces at all stages of processing, it is especially effective as a photoresist stripper.

• Within the semiconductor industry, this solution is known as Carro’s acid or piranha etch.

• Due to its aggressive reactivity, piranha etch is banned from use in our cleanroom, instead we use a safer form of the solution called nanostrip.


Hydrofluoric Acid (HF)

• HF is used in the cleaning process to remove native oxide, a thin, poor quality layer of SiO2

formed on silicon substrates.

• A solution of 49% HF and water is used to remove native oxides from a bare silicon substrate, while dilutions of 1:100 HF/water are used to remove native oxides on previously grown oxide layers.

• HF may also be mixed with Ammonium Fluoride to better control etch rates, this mixture is known as a buffered oxide etch (BOE).


Standard Cleans for Silicon

• In the mid 1960’s, engineers from RCA developed a two-step process for removing organic and inorganic residues from silicon substrates.

• Today, there are many methods to perform the two-step procedure.


Standard Cleans

• The RCA cleans are so effective that today the formulas are referred to as Standard Cleans. The method is used generally, and matched to the substrate.

• Standard Clean 1 (SC-1) removes organic residues and sets up a condition for the desorption of trace metals from a substrate’s surface.

• Standard Clean 2 (SC-2) removes alkali ions, hydroxides, and complex residue metals from a substrate surface.


Standard CleansClean Type Recipes

SC-1: Organics/Particles SC-1 (APM):

NH4OH—1 part

H2O—5 part

H2O2—1 part

Piranha (SPM):

H2SO4

H2O

H2O2

Acetone

IPA

H2O

SC-2: Metallic SC-2 (HPM):

HCl—1 part

H2O—6 to 8 part

H2O2—1 to 2 part

Heated to 75°-85°C

Piranha (SPM):

H2SO4

H2O

H2O2

DHF:

HF

H2O

HF: Native Oxides DHF:

HF—1 part

H2O—1000 part

BHF:

NH4F

HF

H2O

Steps Standard Clean Time

1. HF 5 min

2. Rinse (DI Water)

3. SC-1 12 min

4. Rinse (DI Water)

5. HF 5 min

6. Rinse (DI Water)

7. SC-2 12 min

8.Rinse (DI Water)

/ Dry (N2)


Spray versus Immersion Cleaning

• Immersion of a substrate into a series of chemical baths is today’s standard method of cleaning, however, as feature sizes continue to get smaller, industry has growing concerns about immersion cleaning.– Rising chemical costs.

– Immersion tanks having a tendency to re-deposit contaminants.

– Smaller feature sizes are difficult to clean efficiently with current methods.


Dry Cleaning

• The ultimate industry goal is to abandon

wet cleaning, due to contamination and

environmental impact.

• These problems have spurred interest in

vapor or gas phase cleaning, using mainly

HF/water mixtures.


Sonic Assisted Cleaning

• The addition of sonic energy waves to a tank of cleaning chemicals, greatly increases its efficiency and allows for a lower bath temperature.

• Sonic waves are energy waves generated from transducers on the outside of the tank.

• There are two ranges used for sonic assisted cleaning:– Ultrasonic

– Megasonic


Ultrasonic/Megasonic Bath

Van Aznt, Peter. Microchip Fabrication

4th Edition. McGraw Hill. New York.

2007

Cleaning Medium

Samples

Transducer


Ultrasonic Cleaning

• The ultrasonic energy range is from 20,000 to

50,000 hertz .

• Ultrasonic cleaning assists rinsing through

cavitation.

– Waves pass through the liquid causing microscopic

bubbles to form and collapse on the substrate

surface, rapidly inducing a microscopic scrubbing

action that dislodges particles.


Megasonic Cleaning

• The static or slow moving boundary at the substrate surface, a phenomenon of fluid dynamics, can hold tiny particles and prevent their exposure to the cleaning solution.

• Megasonic energy reduces the layer, exposing the particles to the cleaning solution, and increases the velocity of the solution in a phenomenon known as acoustic streaming.

• Megasonic cleaning takes place at 850 kilohertz.


Megasonic vs. Ultrasonic

Megasonic

Ultrasonic

0.001 0.01 0.1 1 10 100 1,000 10,000

Partical Diameter in Microns (µm)



Water Rinsing

• Most wet cleaning sequences are followed by a rinse in DI water to remove any residual cleaning chemicals.

• Three techniques will be discussed:

– Overflow rinsers

– Spray rinsing

– Dump rinsers


Overflow Rinsers

• An overflow rinser thoroughly cleans substrates with a constant supply of clean water.

• A box sunk into a cleaning station deck has DI water flowing in through the bottom, moving around the substrates and exiting over a dam into a drainage system.

• N2 gas also bubbles up from the bottom and aids the mixing of the chemicals on the substrate surface with the water.

• Substrate cleanliness is measured by the overflowing water’s resistivity.


Single and Three-stage

Overflow Rinse Systems

1 2 3Water inlet

To Drain

Water

InletNitrogen

Inlet



Spray Rinsing

• Spraying removes chemicals with:

– The physical force from the water’s momentum.

– Tiny droplets constantly hitting the substrate’s

surface.

• Spraying uses less water than overflow rinsers.

• However, spray water has atmospheric CO2

trapped in it, making resistivity readings invalid.


Dump Rinsers

• A combination between overflow and spray rinsing, dump rinsers possess a high cleaning efficiency, conserve water, and save space.

• Substrates are placed in a dry rinser and sprayed with DI water, at the same time the cavity of the rinser is filled with water until it begins to overflow at which time a trap door in the bottom of the rinse cavity swings open and the water is drained.– These steps are repeated many times until the

substrates are clean.


Spry-Dump Rinser

Trap doorOut to drain

Wafers

Rinse spryer

Water outlet



Substrate Drying

• Drying is an extremely important process:

– Any water left on the substrate surface can interfere with any subsequent process.

• There are many drying techniques:

– Spin-rinse dryer

– Isopropyl alcohol vapor dry

– Surface tension/Marangoni drying


Spin-rinse Dryer (SRD)

• The SRD is a centrifuge-like piece of equipment:

– Substrate boats are put in holders around the inside

surface of a drum.

– A pipe with holes for DI water and hot nitrogen is

connected to the drum center.

– The process starts with a rinse of the substrates as they

rotate around the center pipe that sprays water.

– The SRD switches to high-speed rotation that throws the

water off the substrates.

– The nitrogen aids in removing any water droplets left

behind.


Isopropyl Alcohol (IPA) Vapor

Drying• In the bottom of a dryer is a heated reserve of

IPA with a vapor zone above it.

• A substrate with residual water on the surface is suspended in the vapor zone, the IPA replaces the water.

• Chilled coils around the vapor zone condense the water vapor out of the IPA vapors and the substrate is water free.


IPA Vapor Drying

IPA Vapor

IPA liquid reserve

Condensation

of water

Heating coils

Chilling coils Vapor

Zone

Wafers



Surface Tension/Marangoni

Drying• Substrates are drawn slowly through a

water surface.

• The surface tension draws water away from the substrate surface, leaving it dry.

• The effect is enhanced when IPA and nitrogen are directed at the substrate water interface.


Outline

• Introduction





• Damage


Types of Damage

• Dimensional changes

• Bonding changes

• Material contamination

• Electro-static discharge damage

Plasma based processing is a good

example of how all these types of damage

can occur


Plasma Processing Damage

• Plasma processing damage can come from three areas of focus:– Damage can be from the inherent energy spectrum

from the plasma. Depending upon the product, it can be susceptible to X rays, light (U.V., infrared, etc.), electric charge, electric arcing, and ion bombardment damage.

– Plasma systems can also be a source of various residues that can affect performance.


Plasma Processing Damage

J. W. King, L. L. Williunis I Cirrrmt Opinion in Solid

Stule and Mirreriuls Science 7 (2003) 413424

SEM images; A: Post Etch B: Post Etch Residue Removal

A B


ESD Damage

• ESD is the uncontrolled transfer of static charge from one object to another, that can potentially damage a product.

• ESD can create a discharge of electricity measuring tens of thousands of volts.

• Though the amount of electricity transferred during ESD is small (nanocoulombs), it is still enough energy to vaporize metal features, cause dielectric breakdown, and, due to a buildup of charge, attract particulates to surfaces.

Documents

E SC 412 Nanotechnology: Materials, Infrastructure, and Safety … · • As their name implies, MICs are highly mobile –MICs can move through an electronic nanoproduct even after