Bowling forContaminants:
The New Scienceof Gas Purification
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INTRODUCTION—

Purity of gases and chemicals has always playeda critical role in the performance and reliability ofadvanced semiconductors and memory devices. Overthe past few years, fab-cleanroom purity requirements
themselves have changed exponentially in key processareas and are now approaching parts per quadrillion.

THE ROLE OF PROCESS GASES INSEMICONDUCTOR MANUFACTURING—

With hundreds of gas and specialty gas mixtures
used in an ever-increasing number of manufacturing
process steps, there are many opportunities for wafer
contamination caused by the gas supply.

To remain competitive, many semiconductor manufacturers have increased manufacturing volumes which
subsequently increases the overall consumption ofgases. In addition to increased volumes, both logic
and advanced memory devices require significantly
higher gas consumption per processed wafer to support
shrinking geometries and multi-layer device architectures such as finFETS and 3D NAND. For example, themove from 20 nm logic to 7 nm logic doubled thenumber of process steps. As a result, process gas
consumption is expected to increase over the next
five years.

Of all the bulk gases, nitrogen is currently used in
the highest volume. This ubiquitous gas is used forcountless purging applications, including pumps andvacuum chambers. Wafer fabrication facilities making
chips with 28 nm or 20 nm features can go through
20,000 to 30,000 cubic meters of nitrogen per hour.3
Extreme clean dry air (XCDA®) is also employed forpurging and sweeping cleanrooms. This has become
increasingly important as newer generations of tools
are specified to guarantee that processes are performedusing XCDA. With the forthcoming adoption of extreme
ultraviolet lithography (EUV), the use of hydrogen is
expected to increase.

Concurrently, the industry is looking to achieve higher
density at lower power which also increases thecomplexity of the processes.1

WHERE DO CONTAMINANTS ORIGINATE?

These additional process steps impact yields as there
are more opportunities to expose wafers to process
excursions. Even trace contaminants in the gas supply
can cause measurable shifts that affect chip performance by interacting with a process, potentially costing
the manufacturer thousands or even millions of dollars.2
As a result of this heightened sensitivity to molecular
contamination and increased gas consumption, semiconductor manufacturers are placing new requirements
on suppliers of both bulk and specialty gases to deliver
process gases customized to meet purity requirements.
This article explains the importance of applying purification science to managing the gas supply purity fromthe source throughout all the wafer process steps toensure the highest device yield.

—

Intrinsic contaminants originate at the gas supplier;additionally, for corrosive gases, contamination canbe picked up in the gas stream as a result of corrosionof the delivery system and tool. As such, the wafer’s
relationship with the gas supply and their journey
together begins well before they even come incontact with each other.

WHAT DO CONTAMINANTS LOOK LIKE?

—Contaminants in both bulk and specialty gas supplies
come in several forms. Categorized as particulates
or molecular contaminants, even historically benign
contaminants have become problematic and cancause defects in today’s 7 nm node devices or
3D NAND structures.
In general, contaminant classes include moisture, acids,bases, refractory compounds, and organic molecules(Table 1), but contaminants can also develop in unexpected ways. For example, if oxygen (O2) is inadvertently
introduced during the silicon (Si) ingot growth process,the resulting silicon dioxide (SiO2) deposition will cause
defects before wafer processing even begins.

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CONTAMINANT TYPESTANDARD SURROGATETYPICAL IMPURITY LEVELS(OUTLET PURITY)

Acids
NOx, SOx, Organic acids

SO2

0.1 – 1 ppmV (<1 pptV)

Atmospherics
CO2, CO, O2, N2

CO2, CO, O2, N2

0.1 – 1.0 ppmV (<100 pptV)

Bases
Amines, organoamines, silazanes

Ammonia (NH3)1 – 100 ppbV (<1 ppbV)

MetalsFe, Mn, Mo, Ni, Cr, others

E34 list, Fe, Cr

1 – 100 ppbV (<1 ppbV)

Moisture

H2O

0.1 – 25 ppmV (< 1 ppbV)

Organics
Condensable (45 – 100 amu; butane, IPA, toluene)
Non-Condensable (>100 amu; decane)

Toluene Decane

0.1 – 2 ppmV (<1 pptv)

Refractory compounds
Halogen-containing HCs, silicon compounds

HMDSO<100 ppbV (<1 pptV)

Table 1: Common gas-phase contaminant classes.

Moisture (H2O) is always a contaminant and an
unwanted source of oxygen atoms. Molecular hydrocarbon and refractory contaminants introduced during
the lithography process can wreak havoc on the stepper lens, impeding the patterning process. Carbon
dioxide (CO2) can be either a process gas or aprocess challenge.
Metal contaminants are particulates that can be picked
up in the gas stream. For example, hydrogen bromide(HBr) gas, used in dry etch processes, is very aggressive
towards stainless steel. Extremely corrosive whencontaminated with moisture, HBr leaches constituent
metals out of the stainless steel and can carry thesethrough the gas stream onto the wafer, creating
shorting defects. However, this source of metal contamination can be mitigated by properly desiccating
the gas at the delivery source.
The bottom line is that gas purification is a complicatedscience. Contamination profiles vary dramatically fromgas to gas. In many cases, it is no longer sufficient for agas supplier to provide a customer with a specification
sheet that addresses common contaminants. Customization of gas purification requires an end-to-end
solution that begins at the bulk supplier and requiresthe right purification approaches that can vary fromsystem to system, gas to gas, and process to process
as the wafer travels along its manufacturing journey.

CAPTURING THE FLAG VS. BOWLING FOR CONTAMINANTS—In simpler times, when processes were more forgiving and feature sizes were larger and planar, process
excursions and root causes were easier to identify andmitigate from the gas stream. Like the childhood game,
capture the flag, there was often only one cause toidentify and remove.
Today, as the device designs become more complex,
the sensitivities to gas contaminants rise. Processes
have become so numerous and integrated, that it
is often difficult to know how the many molecular
contaminants are contributing to process problems.
Likewise, the solutions have become much morecomplex. Contamination control has become more
like bowling, with multiple pins to knock down,
and where hitting one can cause a chain reaction.
Depending on the spin of the ball, any number ofcombinations impact the end result (Table 2).

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Process

Contaminaton effects

Process

Contaminaton effects

CMP

Corrosion

Metrology

Haze

Wafer clean

Corrosion

Deposition

Film defects, metal contamination

Dry etch

Metallic contamination

Track

Pattern defects

Epi

Lattice defects, metal contamination

ScannerHaze, pattern defects

P = Particles

W = Water/Moisture

M = Metals

O = Oxygen

Process

HCl

HBr

CxFx

NH3CO

BCl3

SixHx

Inerts

Metallic
precursors

45 nm

P,W

P,W

P,W

P,W

P,W,M

P,W

P,W,O

P,W

P

32 nm

P,W

P,W

P,W

P,W

P,W,M

P,W

P,W,O

P,W

P

28 nm

P,W,M

P,W

P,W

P,W

P,W,M

P,W

P,W,O

P,W

P

22 nm

P,W,M

P,W,M

P,W

P,W

P,W,M

P,W

P,W,O

P,W

P14 nm

P,W,M

P,W,MP,WP,WP,W,MP,WP,W,OP,W

P

10 nm

P,W,M

P,W,M

P,W

P,W

P,W,M

P,W

P,W,OP,W

P

Table 2: These charts demonstrate how different processes have different sensitivities to contaminants depending on node.

What was once a case of capturing one contaminant“flag” has become a multitude of contaminants tocontrol. This requires a broad contamination control
strategy and becomes more complex with each newdesign. This strategy requires close collaboration
between the gas manufacturer, delivery provider,and the end customer. The goal: Achieve purity bycontrolling contamination at the molecular level
to ensure the gas supply contains everything thecustomer wants, and nothing more.

THE SCIENCE BEHIND GAS PURIFICATION—Gas-phase contamination control calls for a combination of filters and purifiers installed in the rightlocations throughout the delivery system.
A filter is able to remove suspended particles fromgas streams. Purifiers are able to remove molecular
contaminants, such as H2O, O2, CO2, hydrocarbons,and others. Due to the large number of contaminant
types, gas purification is often more complex thanfiltration.
There are two dominant mechanisms in gas filtration.
The first is inertial impaction (Figure 1), which is used
when the particle is large and dense enough to passout of the fluid streamline due to its inertia. The second
mechanism is diffusion interception (Figure 2), which
captures particles that are small enough to pass outof the fluid streamline due to Brownian motion. Whenused in concert, diffusion and interception can capture
even the smallest particles, (about 0.001 µm or less).
These are the most concerning to device manufacturers,
as the current metrology tools are unable to detect
these sizes.
4PhysisorptionO
H O

Media Surface

OO

Media Surface

HOO
H O

HOO
H OOOOH O

HOO

Inertial impaction

HChemisorption
OO
OO

Diffusion interception

O
O

OO

O
O

O

O

Media Surface

O

Media Surface

O

O

O

Figure 3: Comparison of how physisorption and chemisorption areused to remove common contaminants from the gas stream toprevent wafer defects.

USING GAS PURIFICATION SYSTEMS—Media fiber

AirflowParticle

Particle pathFigures 1 and 2: Diffusion Interception.

Gas purification also employs two primary mechanisms. Physical adsorption (or physisorption) involves
the physical interaction between the impurity being
removed and the adsorbent. For example, physisorption is used to remove H2O and hydrocarbons.
Chemical adsorption (or chemisorption) involves
forming a chemical bond between surface atoms of
the adsorbent and the impurity. This is how reactiveoxides like CO and O2 are removed. (Figure 3) (Seesidebar for list of adsorbent classes and types.*)

Most gases entering the fab start out as 99.99% pure,but the actual contaminant profile can vary dramaticallyfrom lot to lot. Most modern processes require purity
levels of 99.9999% (often called “six nines”, or 6N) or
greater. Gas purification provides process stability by
maintaining consistent purity levels, which improves
yields rather than increases defects. This requires
managing facility-wide control points that represent
process risk. Table 3 illustrates these control points,
the potential contaminant risks, and the effects thesecontaminants can have on the wafer as it makes itsjourney through the process tools.

*List of adsorbent classes and types
ADSORBENT: POROUS MATERIAL CLASSESADSORBENT TYPES

Crystalline materials

Activated charcoal (carbon)

Exhibit reproducible x-ray diffraction patterns

Physical adsorption

Class includes zeolites, metal-organic frameworks (MOFs), ZIFs,ALPOs, SAPOs, some forms of carbon (i.e., CNTs, graphene)

Removal of organic and strong acid AMC

Vast array of structure types provides range of separation
selectivities

Coated carbons

Regular materials
Do not exhibit reproducible diffraction (amorphous)
Class includes activated carbons, amorphous silicas, and aluminas
Can be modified to enable/enhance separations
Catalytic and gettering materials
Typically, a formulation that contains transition metals andalumina or silica binder
Primarily used for removal of oxygen, oxidation of VOCs

Combination physical/chemical adsorption
Removal of weak acids, special chemistry also some organic AMC
Ion exchange media
Chemical adsorption
Base and acid AMC
Catalysts
Destructive adsorption/oxidation
Special materials and formulations
5ProcessContaminantEffect

TrackAcids, bases, moisture, hydrocarbons

Pattern defects

ScannerAcids, bases, refractory compounds,hydrocarbons

Haze, pattern defects

Metrology, reticle inspection, andwafer inspection

Acids, bases, refractory compounds,hydrocarbons

Haze, pattern defects

Piping welds

Moisture, oxygen

Discoloration, reduced
weld integrity

Electrochemical deposition (ECD)

Moisture, oxygen, hydrocarbons,volatile metals

Film defects, metal
contamination

Chemical vapor deposition (CVD) andPhysical vapor deposition (PVD)

Moisture, oxygen, hydrocarbons,volatile metals

Film defects, metal
contamination

Dry etch

Volatile metals

Metals contamination

EPI

Moisture, oxygen, volatile metals

Lattice defects, metal
contamination

Time

Inlet gas +molecularcontaminants
and particles

Purified gas

Purification
media

Integrated 0.003 µm
particle filter

Contaminant Level

Contaminant Level

Table 3: Processes create different contaminant risks and process excursions on the wafer.

TimeGas flow

Figure 4: Better purity = improved yield. Consistent purity = process stability.

The intent of a gas purification system or cartridge is
to create ultrapure gas as measured in “outlet purity”.
The risks to mitigate or eliminate are the process
excursions, which occur when the contaminant level
spikes outside the baseline considered to be a safe
operating range. (Figure 4).
Because a leading edge EUV scanner can cost as muchas $300 million, with 10% of the cost in the lens stackalone, protecting, for example, the optics of a lithography scanner or other costly equipment that is highly
sensitive to hydrocarbons, moisture, and othercontaminants is important for cost control.

CONCLUSION—The journey of a semiconductor device is long,complex, and varied. The role gas purity plays begins
long before the gas supply first meets the wafer in aprocess tool, and even before the first silicon crystal
begins forming the ingot from which the wafers are
sliced. To ensure the highest yield, gas purity must
begin at the gas manufacturer, and follow the siliconthrough hundreds of processes before it becomesa finished device wafer.
Amplified sensitivities due to the increased numberof processes, combined with the variety of process
flow methods, is creating very customer- and processspecific problems, requiring more than off-the-shelfsolutions. What’s needed is end-to-end collaboration
between the gas supplier, semiconductor device
manufacturer, and the end-customer.
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1——	A Tolla, PhD, “Larger Fabs + Smaller Devices =More Gases,” August 6, 2015 http://semimd.com/materials-matters/2015/08/06/larger-fabs-smallerdevices-more-gases/	A. Tolla, PhD, “Fabs Seeking Higher Quality Electronic
Materials to Meet Technology Demands”, August 27,
2015 SemiMD, http://semimd.com/materials-matters/
2015/08/27/fabs-seeking-higher-quality-electronicmaterials-to-meet-technology-demands/

2	J. Dorsch, “Gases: Completely Necessary to Semiconductor Manufacturing,” SemiMD, July 7, 2015.
http://semimd.com/blog/2015/07/15/gasescompletely-necessary-to-semiconductormanufacturing/

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