Cleaning of Silicon Wafers
Introduction
This report is dedicated to one of the most important tasks in semiconductor industrycleaning and preparation of the silicone surface for the further processing. The main goal
is to remove the contaminants from the wafer surface and to control chemically grown
oxide on the wafer surface. Modern integrated electronics would not be possible without
unless the technologies for leaning and contamination control would have been
developed, and further reduction of the contamination level of the silicone wafer is
mandatory for the further reduction of the IC element dimensions. Wafer cleaning is the
most frequently repeated step in IC manufacturing and is one of the most important
segment in the semiconductor-equipment business, and it looks as if it will remain that
way for some time. Each time device-feature sizes shrink or new tools and materials enter
the fabrication process, the task of cleaning gets more complicated.
Today, at 0.18-micron design rules, 80 out of ~400 total steps will be cleaning. While the
number of cleans increases, the requirement levels are also increasing for impurity
concentrations, particle size and quantity, water and chemical usage and the amount of
surface roughness for critical gate cleans. Not only is wafer cleaning needed now before
each new process sequence, but additional steps are often required to clean up the
fabrication process tools after a production run.
Traditionally, cleaning has been concentrated in the front end of the line (FEOL) where
active devices are exposed and more detailed cleans required. A primary challenge in
FEOL cleans is the continuous reduction in the defect levels. As a rule, a "killer defect" is
less than half the size of the device linewidth. For example, at 0.25 µm geometries, cleans
must remove particles smaller than 0.12 µm and at 0.18 µm, 0.09 µm particles.
Table 1. Particle Addition limits
Generation
Particle size
60% yield
80%yield
90% yield
95% yield
250 nm
125 nm
9.6
3.9
1.8
0.86
180 nm
90 nm
4.8
1.95
0.9
0.43
130 nm
65 nm
2.24
0.91
0.42
0.20
100 nm
50 nm
0.96
0.39
0.18
0.086
The issue is that smaller particles are physically more difficult to remove, because it is
harder to deliver the necessary force to inuscule dimensions. Thus more energy is
required to remove smaller particles. The back end of line (BEOL) has multiple metal
layers requiring more specific cleans, such as removing particles and complex organic
materials. However, the progression from 0.25 µm to .13 µm design rules has meant the
addition of new metal layers. In general, every metal added would introduce three to five
BEOL leans depending on the process. With seven metal layers at 0.18 µm, the number
of BEOL cleans is not only comparable to but is beginning to exceed the number of frontend cleans.
Figure 1. Typical IC manufacturing process flow
Types and sources of contamination
Particles- dust, pollen, clothing particles, bacteria, etc. In ordinary room there are as
much as 10 6 particles more then 0.5 micron
in diameter per cu.ft. Particles with diameter more then 20 micron will settle down
readily. Particles of diameter from 0.1 to 20
micron are the main problem.
Inorganic contaminants-salts, positive and negative ions in solution, heavy metal
atoms. Inorganics are removed by cleaning the wafer in water recirculation systems and
using special solutions.
Organic contaminants- smog, skin oil, fluxes, lubricants, solvent vapors,
monomers from plastic tubing and storage boxes that can condense on substrate. They
usually removed using strong oxidizers, gaseous or liquid.
Impurities- incorporated during the formation of substrates or overlayer films.
Generally, they cannot be removed.
Figure 2. Contamination on the silicon surface
Methods /solutions/techniques
Most cleaning methods can be loosely divided into two big groups: wet and dry methods.
Liquid chemical cleaning processes are generally referred to as wet cleaning. They rely
on combination of solvents, acids and water to spray, scrub, etch and dissolve
contaminants from wafer surface. Dry cleaning processes use gas phase chemistry, and
rely on chemical reactions required for wafer cleaning, as well as other techniques such
as laser, aerosols and ozonated chemistries. Generally , dry cleaning technologies use less
chemicals and less hazardous for environment but usually do not perform as well as wet
methods, especially for particle removal.
Wet-chemical cleaning methods.
Although no single procedure is perfect for all process steps, typical wet cleaning
sequence includes [2]:
1 . Sulfuric acid/hydrogen peroxide/ deionized water (SPM, H2SO4/H2O2/H2O@110130 degrees C ) . SPM usually used to remove organic contaminations (often called
"piranha clean").
2. Hydrofluoric acid or diluted hydrofluoric acid (HF or DHF @ 20-25 degrees C). It
removes oxides from area of interest, etches silicone oxides and dioxides, and reduces
metals contamination of the surface. Sometimes buffered oxide etch,(BOE or BHF,/
NH4/HF/H2O @60-80degrees C ) is used in place of DHF in some processes, but exposure
to it can lead to NH4F precipitation and contamination.
3. Ammonium hydroxide/ hydrogen peroxide/ DI water mixture (APM,
NH4OH/H2O2/H2O@60-80degrees C ). APM oxidizes and slightly etches to undercut and
remove particles from the surface; it also removes organic and metal contaminants .
4. Hydrochloric acid/hydrogen peroxide/DI water (HPM, HCL/H2O2/H2O@60-80degrees
C ) HPM removes metallic contaminants from silicone substrate and acts as oxidizing
agent.
5. Ultra-pure water (UPW) . Commonly called as DI water, it dilutes chemicals, and
rinses solutions after chemical cleans.
Table 2 lists the most commonly used wet cleaning methods for removal of each type of
contaminants.
Table 2.
Particles
Organic
Metals
Native/chemical oxides
APM
SPM
SPM
DHF
APM
HPM
BHF
DHF
The RCA clean [2] sequence developed by Werner Kern in the 1960s still is used widely
in semiconductor manufacturing as a critical clean for the removal of organic, metallic
and particulate contamination on wafer surfaces prior to oxide growth operations. The
typical sequence starts with SPM for heavy organic removal, followed by dip in the DHF.
So-called Standard Clean 1 (SC-1) uses APM to remove particles and Standard Clean
2(SC-2) uses HPM to remove metal contaminations. High pH SC-1 is an effective
particulate removal chemistry, aided by the high negative zeta potential of both silicon
and oxide in this pH range. SC-2 is effective at removing metallic contamination with a
pH low enough to ensure good metal oxide solubility and with the chlorideion acting as a
complexing agent. Typical composition of the traditional SC-1 is chemical ratio of (1:1:5,
NH4OH:H2O2:H2O)
at 60°C. Sometimes megasonic energy is used to increase particle removal efficiency.
Similarly,typical chemical ratio for SC-2 is (1:1:6, HCl:H2O2:H2O) at 85°C. The
composition and order of steps can vary but all wafers are rinsed in UPW after each
chemical immersion. Last few years
brought few changes, but the basic cleaning philosophy used in most fabrication
processes still based on the original RCA process.
Advances in wet cleaning
Wet cleaning account for an important part of a total chemical consumption, and
relatively small improvement in chemical usage has a large overall effect. In view of
large volume of water and chemical used, an optimization of wet cleaning steps is
urgently needed. The new environment-friendly technologies move toward the reduction
of chemicals use, recycling of water and development of improved dry processing
techniques to assist wet cleaning. Mechanical cleaning technologies such as centrifugal
spraying, water jets, brush scrubbing [10] , pressurized fluids and cryogenic ("frozen")
particles jets [12,19]. The number of cleaning and rinsing steps has also decreased by
using so-called Marangoni technique. The Marangoni principle involves the slow
withdrawal of wafers from a DI water bath to an environment of isopropyl alcohol (IPA)
and nitrogen such that only the portion of the surface that is at the interface of the liquid
and vapor phases is "drying" at any one time. In this way, uncontrolled evaporative
drying on the wafer is prevented. IPA drying provides a great advantage in hydrophobic
cleaning steps such as pre-gate, pre-silicide and pre-contact cleans. Many advances are
based on the use of the diluted chemistries and ozonated UPW as a replacement of
hydrogen peroxide or even sulphuric-based mixtures.
Megasonic
Megasonic agitation [ 1,11] is the most widely used approach to adding energy (at about
800kHz and 100,000g) to the wet cleaning process. The physics behind how particles are
removed however, is not well understood. A combination of an induced flow in the
cleaning solution (called acoustic streaming), cavitation, the level of dissolved gases and
oscillatory effects are all thought to contribute to particle removal performance. Despite
the familiarity of megasonics, there is still no common opinion of whether or not it can
make damage to the wafer . If it is true, this may prove to be a critical limitation for the
removal of particles smaller than 0.1 µm [25]. To address this potential shortcoming
associated with the megasonic process, a new technique has been developed by ProSys
(Campbell, Calif.) [9,21]. which uses acoustic wave action from a piezoelectric crystal to
cleanse wafers in fluid by producing a controlled cavitation - the formation and activity
of bubbles. By exciting ceramic piezoelectric crystals with a high-frequency AC voltage,
the resulting vibrations generate an acoustic wave that provides the cleaning mechanism.
As many as 16 piezoelectric crystals can be switched on and off in sequence, delivering
pulsed megasonic energy up to 100 Hz.
Diluted chemistries.
The development of dilute cleaning chemistries has peaked industry interest. As market
competitiveness increases, chemical consumption and associated costs become
increasingly important. Dilute (1:1:50) ammonium hydroxide (NH4OH)/hydrogen
peroxide chemistries (SC-1) have the advantage of reduced oxide loss while preserving
excellent particle removal performance. Similarly, many studies have shown that highly
dilute (1:1:60) hydrochloric acid/hydrogen peroxide (HCl/H2O2) mixtures (SC-2) are
effective at controlling metallic contamination. [3,4,5]. With implementation of
megasonic cleaning, diluted SC-1,SC-2 and HF, and opitimized timing of various
cleaning steps, this can lead to 4-50 times reduction in chemicals consumption.
Another important issue is a reduction in surface
roughness
Fig.3 AFM photo of the silicone
after standard RCA and diluted
that a
standard wet cleaning .
an rms
also can be seen when using dilute chemistries.
Atomic force microscopy (AFM) studies indicate
SC-1,1:1:5 ratio clean at temperatures >65 deg.C has
roughness of >1.5 Å (Fig. 2). With 50:1 dilute
chemistries
and temperatures <50C, rms roughness was reduced by a factor of three to <0.5 Å.
Ozonated and diluted chemistries
Ozonated chemistries can provide an environmentally friendly alternative to the use of
sulfuric acid and harmful solvent strippers. Several research groups, have investigated the
possible use of this technology for the removal of organic films. In particular, IMEC has
developed a simplified cleaning process called IMEC-clean.[6,7] It's a simple two-step
process with an optional third step before drying. The first step uses sulphuric acid/ozone
mixture (SOM) to remove the organic contaminations and grow thin chemical oxide.
Under the specific conditions, SOM can be replaced by ozonated UPW (H2O/O3) .. An
O3 diffuser is immersed in heated DI water that can be spiked with additives like acetic
acid. The wafers are then positioned directly above the liquid and exposed to a moist O3
ambient. This forms a thin condensation layer on the wafer that reduces the diffusion
limitation and allows the short-lived, reactive O3 components to reach the wafer surface.
Test results suggest that this process also may be used to completely eliminate all sulfuric
acid-based process steps in IC production.
Mechanical cleaning systems
Brush scrubbing
Fig.4 Brush scrubbing system
Brush scrubbing is considered one of the most
effective methods for removing the slurry used
in the CMP polishing process. It is used for
particle reduction
as well. Early versions of wafer scrubbers proved damaging to the wafer surface because
of high pressure
water sprays and nylon brushes. However, using brushes made of polyvinyl alcohol
(PVA), a soft, highly compressible,
sponge-like material, particle removal without damage to the surface of wafers can be
achieved. Fig.4 shows OnTrak's
wet cleaning double-sided scrubbing (DSS) system. The system is now capable of
cleaning 55 to 60 ight-inch wafers an
hour. The DSS system uses polyvinyl alcohol brushes to gently remove particles as small
as 0.12 micron without
damaging the wafer surface. Both silicon and IC manufacturers have confirmed that PVA
brush scrubbing is not only able
to remove particles on the order of a micron but is effective for the removal of submicron
particles as well., the brush
scrubber's ability to clean with room temperature dilute solutions, non-toxic chemicals
and/or DI water is well suited to
the environmental and safety requirements of today and the even more stringent
requirements of next generation
manufacturing technologies [11].
Dry cleaning methods
The introduction of dry cleaning methods into the IC manufacturing process has been
slower then expected [12], although some processes, such as CL-based chemistries[13],
Ar sputtering, and H2 pre-bakes, have been available for 30 years For the past 10 years,
the semiconductor industry has anticipated the replacement of wet chemistries with dry
cleaning technologies. Integrated, single-wafer cleaning has been contemplated and
tested since the late 1980s [24]. With the introduction of single-wafer anhydrous HF
(AHF)-vapor, UV/ozone, chemical downstream etch (CDE) and other dry processes,
integrated cleaning has become a viable approach to surface contamination control.
Table 3 lists common methods for the removal of each type of contaminant in the gas
phase.
Table 3. Commonly used dry cleaning methods.
Particles
Organics
Metals
Native/chem. oxide
Cryogenic
O2 anneal
Anneal in Cl-based
chemistries.
Ar sputter
Laser
Ozone
NO/HCl/N2 anneal
H2 anneal
CO2 snow
UV/ozone
Remote plasma HCl
Remote plasma H2
Remote plasma O2
UV/Cl2
UV/SiCl4
UV/HCl
Remote plasma
NF3/H2
AHF/H2O
AHF/alcohol solvent
UV/F2/H2
Conventional wet chemistries are very effective for most applications, though there are
growing concerns regarding environmental safety and economics. Most likely dry
methods will not directly replace wet chemistries in mainstream applications, but instead
will supplement wet techniques at various points such as single 300 mm wafer
processing. In theory, larger wafers should favor dry chemistries because of increased
demand on chemical, water and waste .disposal. Cost effectiveness may also be seen if
dry surface cleaning/conditioning modules are added to existing cluster tools. In addition
to reduction of chemicals and water, a key advantage of dry surface processing
technology is in its compatibility with process integration. [14].
Among several possible dry cleaning sequences (see Table 3), sequentialUV/O2
treatment for organics removal, AHF/alcoholic solvent process for chemical oxide
etching and UV/Cl2 exposure for volatilization of metallic contaminants and slight
etching of the silicon surface have been extensively studied. Having passed initial testing,
commercial tools are currently available. The UV/Cl2 process, initially developed at
Fujitsu, has been shown by various groups to perform well in the conditioning of silicon
surfaces, because it can etch a few monolayers of silicon without roughening the surface.
UV/Cl2 exposure at temperatures below 200°C at reduced pressure has been shown very
effective in Si surface processing prior to low-temperature epi deposition and post-RIE
surface treatments in which slight etching of silicon is needed to remove surface damage.
So while the production-worthiness of these methods has yet to be proven, the
considerable potential for performance and economic gains spurs further research and
development.
Cryogenic Cleaning
The need for very specific spot cleans is coming into focus with new emphasis on dry
processes such as vapor cleans and cryogenics. A 10-year-old technology, also referred to
as aerosol cleaning, cryogenics uses either a CO2 or an argon/nitrogen gas source.
Fig.5.Wafers cleaned under a stream of frozen argon/nitrogen in ARIES.
The use of CO2 aerosol cleaning for precision applications has been limited by problems
with recontamination. Early systems used evaporative cooling, which would freeze the
liquid droplet into a solid particle, thus distilling off the pure CO2 and concentrating
contaminants in the droplets. To address this issue, ATS Eco-Snow Systems (Livermore,
Calif.) has designed a new nozzle that prevents the deposition of residual contaminants
on the wafer surface.[7] Their approach combines purified CO2, specialized
environmental control, ultraclean automation and process control with advanced nozzle
technology. The Eco-Snow technique has successfully removed particles down to 0.15
µm with concentration levels <0.05 particles/cm2.
An argon/nitrogen aerosol mix was orginally developed at IBM because of the inherently
higher purity of gaseous sources. FSI's (Chaska, Minn.) ARIES (Fig 5) pre-cools the
gases with liquid nitrogen at high pressures and forms the solid aerosols in a vacuum
chamber. Wafers are then scanned under a linear nozzle where high velocity aerosols,
>100 m/sec, clean the surface. Greater than 99% removal of surface particles larger than
0.15 µm have been demonstrated. Applied to areas where conventional cleaning methods
are limited such as gate stacks and interconnect, yield improvements from 2 to 8% have
been indicated.
Laser cleaning
Laser cleaning can reduce particulates from a wafer surface without the use of water
chemicals and with no hazardous wastes. One approach used at Radiance Services Co.
(Bethesda, Md.) implements a KrF excimer laser to lift the contaminant from the surface
and a flowing inert gas to sweep it away. [16 ] Surface micro-roughening by the process
is less than 1%, comparable to the native roughness of a silicon waferflakes from 80 µm
to 0.09 µm has been demonstrated as well as removal of photo resist films, chemical
hazes and metallic ions. It has been hypothesized that cleaning occurs from, among
various mechanisms, a combination of light-induced surface phonons breaking particle
bonds through energy absorption, charge transfers between the surface and the particle
and photo decomposition of the particle. Because it is capable of cleaning defects less
than half the size of conventional wet cleaning, the process may become a yield enabler
for 0.18 µm design rules and below. Studies continue at Radiance and Rutherford
Appleton Laboratories (Chilton,UK) to further evaluate laser gas cleaning.
Oramir's method, called the L-Stripper, [17] uses a combination of UV excimer laser
ablation and reactive chemistry to strip the photoresist. The reactive gases are based on
ozone, nitrous oxide and very small amounts of nitrogen trifluoride. These gases become
highly reactive only during the short laser pulse (~30 nsec). This makes the process
highly selective, attacking the resist and side wall polymers but does not etch the thin
gate oxide. During the L-Stripper process, reactive gases are injected into a low vacuum
process chamber. Excimer laser pulses ( l = 248 nm), at a given repetition rate and peak
intensity, are incident on the sample using a patented optical system to prevent radiation
damage. As the laser beam scans the entire wafer, photoresist and embedded
contaminants are removed and volatilized by the photoactive process gas. The reactive
products are continuously pumped out of the process chamber through a catalytic
converter making the process safe and environmentally friendly. With this
photochemically assisted laser ablation technique, no residual residues remain even under
difficult stripping conditions such as post poly, via etch and following high dose
implants. Alpha testing was performed to evaluate particle and resist removal. No
residues were detected by SEM and Auger analysis. To verify that no metal
contamination remained after the process, VPD-TXRF tests were performed. Typical
results were 3.3 x 109 at/cm2 for Fe and 4.6 x 1010 at/cm2 for Al, which meet the SIA
Roadmap cleanliness standards and prove that, indeed, no wet chemical follow-up is
needed. Further Auger analysis indicated that no carbon or other contaminants typical of
photoresist residues were detected. The process is currently in beta-testing by a consortia
consisting of Siemens, Philips and Alcatel Microelectronics at the Frauhofer Institute in
Munich, Germany.
New materials
The technology development surrounding the replacement of SiO2-based dielectrics with
low k materials (below 3.0) and aluminum with copper is spreading out in the industry.
These new materials present the biggest challenge for cleaning as well. Though no
decision has been made, the primary dielectric for interconnects is likely to be an organic
film. This might be a big challenge for cleaning technology, because many of the
cleaning processes are designed to remove organic materials as contaminants. Therefore,
one cannot be removed without attacking the other. With oxide dielectrics, it is possible
to configure certain selectivities into the chemistry, to remove organic resist films, for
example, but leave the SiO2 untouched. However, it is not clear if the same kind of
selectivities exist with organic dielectrics. The compatibility of RCA-type chemistries is
in question. Companies that have concentrated on fine-tuning cleaning processes are now
forced to create new technologies.
Future of the cleaning systems
Commonly used wet cleaning technologies will remain dominant because of their overall
higher cleaning strength . Alternative processes that either reduce or replace chemical
usage are being investigated, because of the current challenges in submicron particle
removal, and environmental impact. Dry cleaning processes will not replace wet
cleaning, but rather complement them and will be used there wet processes are
impractical or inadequate. This will likely take place in combination with wet cleaning
steps which would effectively lead to broader use of "hybrid" sequences. In that kind of
system, gas-phase chemistries would primarily play a surface conditioning role, while
wet chemistries would remove particle and metallic contaminations. Requirements of
more stringent device specifications, environmental concerns are prompting the
development of new dry cleaning techniques such as laser, aerosols and ozonated
chemistries in parallel with the improvement of wet methods. The move to new materials,
(copper, low k dielectrics with dielectric constants in the 2.5 to 2.8 range ) bring
additional challenges and will also force the advancement of new cleaning solutions.
Wafer cleaning technology is undergoing several major transitions driven by new
materials, environmental/cost issues and continued tightening of specifications. So in
addition to more complex solutions, the end result is likely to be a more diverse set of
technology solutions that are tailored to these specific application needs.
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