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We have investigated the effect on a silicon surface of both wet chemical and cluster-tool UV/ozone cleaning, prior to UHV processing to fabricate MOS test structures. The physical and chemical condition of the Si surface has been examined by Scanning Tunnelling and Atomic Force Microscopy (STM, AFM) and Medium Energy Ion Scattering (MEIS). After MOS fabrication some of the structures were examined by Cross-sectional Transmission Electron Microscopy (TEM). The electrical performance of the MOS test sets were characterized by breakdown voltage measurements.
We have found correlations between the electrical performance of the MOS devices, the structure of the Si surface prior to oxidation, and the details of the UHV fabrication technique. In particular any MOS device fabricated on a Si surface thermally cleaned in UHV prior to oxidation has a poor breakdown strength. We have found that this is the result of the formation of silicon carbide on the Si surface at high temperature and the subsequent local disruption of the oxidation step of MOS fabrication by the SiC. A UHV cleaning procedure has been developed to avoid this C contamination problem.
The characteristics of the HF-treated Si-surface are investigated as a function of dipping time in dilute HF solutions. It is found that the contact angle is a very sensitive measure for the degree of oxidation of the Si-surface. The importance of obtaining a perfectly passivated surface in order to reduce the particle deposition on the surface is shown. HF-last cleans are found to be beneficial in terms of metallic contamination and gate oxide integrity. The importance of the loading ambient in furnaces is investigated after HF-treatments and RCA-cleans.
The effect of metal contamination and silicon surface defects on the gate oxide yield is investigated. The characteristics of various cleaning procedures are studied and correlated with the integrity of thin gate oxides. The standard wet cleaning recipe is optimized and a new cleaning strategy is proposed. Selective contamination experiments in chemicals and on Siwafers are used to investigate the effect of small amounts of metal contaminants on the gate oxide integrity. It is found that the characteristics of the silicon substrate play a dominant role in this. HF-last processes are investigated and a new wet cleaning strategy is proposed.
The SiO2 etching by HF solutions is studied. A new model for this etching mechanism is developed based on the existence of the dimer of HF, namely H2F2.
Then the hydrogen passivation of the Si surface is investigated. The hydrogen passivation study is found to depend on the etching mechanism. Two extremes are investigated. The buffered HF solution and the concentrated HF solution.
The problem is occurring that gate oxide integrity (GOI) might be influenced
on the residual subsurface damages induced by mirror polishing. The
correlations between mirror polishing conditions, micro roughness, and
photoconductivity amplitude (PCA) signals are measured and discussed in a
small roughness region of micro roughness Rrms such as 0.095 nm
to 0.247 nm. The theoretical analysis shows that PCA signal is possible to
reflect a subsurface lifetime.The discussions lead to result that the GOL is
mainly dependent on subsurface damages in such small roughness region, PCA
signals measured with an UV/mm-wave noncontact technique correlate closely
with the GOI yield. It is concluded that the PCA technique makes it possible
to characterize subsurface damages and to estimate GOI yield.
The changes in X-ray excited valence band of silicon oxide during
progressive oxidation of Si(111) surface in 1 Torr dry oxygen at 600–850°C
were studied. Following results are obtained: 1) energy level of top of
valence band within 0.9 nm from the SiO2/Si interface is
different from that of bulk silicon oxide by about 0.2 eV, 2) valence band
discontinuity at the interface changes periodically with the interface
The use of sulfuric acid to strip photoresist from silicon wafers is a
widely employed technique in the semiconductor manufacturing community. In
most cases, the acid is combined with hydrogen peroxide to oxidize stripped
photoresist material, though the use of sparged ozone in sulfuric solution
is also used to remove resist residues on wafers following the ashing
process. Although they have been used for many years, sulfuric acid
processes also have proven to be costly. This is due to the need for
frequent bath change-outs and the use of high temperature which impose
safety and environmental concerns. As a result of these and other
considerations, the use of mixtures of ozone and de-ionized water for
photoresist stripping has been investigated. Results show that the technique
effectively removes hard-baked resist (ashed and un-ashed) from bare silicon
and patterned wafers and produces cleaner surfaces (i.e. particles and
metals), compared to outcomes from SPM processes.
This work is an effort to determine if we can use less water effectively in
rinse processes. We have run experiments to examine our current usage and to
determine if we can use water more effectively.
This work will describe the experiments which have been done relating the
amount of chemical remaining on the wafers to variables of the: rinse such
as flow rate, wafer spacing and the cycling of the rinse valves. We will
discuss the conclusions we have reached based on those experiments and the
directions we will be taking to improve rinsing efficiency.
Comparative studies on the effect of Ultra-dilute RCA cleans,
chemical ratios in excess of 300:1, and Dilute RCA cleans,
chemical ratios around 50:1, on the integrity of thin gate oxides have been
performed. Ultra-dilute RCA chemistries have shown particle removal
efficiency, metallic contamination removal, surface roughness, Qbd, BVox and
defect density equivalent to those obtained using dilute RCA chemistries.
Furthermore ultra-dilute chemistries use less chemical leading to shorter
rinse times and thus increased throughput as compared to the dilute RCA
The use of poly-buffered LOCOS processing is a common feature of many
sub-micron integrated circuit fabrication processes. However, the silicon
layer interposed between the nitride oxidation mask and the pad oxide is
often difficult to remove. Different strategies involve dry and/or wet
etching of the film. We have demonstrated the utility of adding metals such
as Fe or Cu to a conventional phosphoric bath used to etch silicon nitride.
Briefly, the presence of the metals is thought to result in a classic
oxidation-reduction reaction between the metal and the silicon. Additions of
60ppm of Cu+2 resulted in etch rates of 20Å /min. on undoped
polysilicon at a process temperature of 165°C, whereas, the etch rate of
SiO2 was less than 1Å/min. Similar results were obtained for
additions of Fe+3 and other metals.
In the this paper, we have examined the particulate removal efficiency of
laser from solid surfaces. The silicon wafers were contaminated with alumina
particles with sizes ranging from 0.05 μm to 0.5 μm. The silicon wafers with
uniform surface-distribution of alumina particles were subjected to pulsed
laser beams at varying conditions. The results obtained have shown that line
beam lasers can remove submicron particles more efficiently from solid
surfaces. The mechanism responsible for higher particulate
removal-efficiency of line beam laser has also been discussed.
Nucleation and growth of liquid drops from the vapor can be used to locate
efficiently surface inhomogeneities such as topological defects, oxide
patches, metallic impurities, organic contamination, and particles. In this
study, nucleation of water drops was used to investigate the surfaces of
copper-contaminated silicon substrates. Hydrogen-terminated silicon (111)
substrates were dipped into copper-contaminated ultrapure water and exposed
to supersaturated water vapor. The amount of copper deposited was varied by
changing the strength of the solution. Nucleation occurred at vapor
pressures close to saturation. Higher densities of nucleated drops appeared
on areas with greater concentrations of copper. Using this technique, it was
possible to detect copper concentrations as low as 6×1011
atom/cm2. Below this concentration, treated and untreated
substrates could not be distinguished. The extreme sensitivity of the
technique to background nucleants shows its potential for efficient
screening of surfaces for a large range of inhomogeneities.
The oxidation of Si(111) and Si(100) surfaces with the high-purity
ozone(more than 98 mole %) was investigated with X-ray photoelectron
spectroscopy (XPS). Thin oxide less than 3nm thickens was formed in an
experimental chamber and the results showed that ozone oxidizes the (111)
surface faster than (100) surface. Ozone does not show the temperature
dependence on oxidation within the temperature range of 250–500 degree C for
both (111) and (100) surfaces. Ozone proceeds the oxide formation at 700
degree C where oxygen does not proceed oxide formation rapidly.
We report the formation of deuterium-terminated Si(111) and Si(100) surfaces
in deuterated aqueous KF. The reaction of a deuterated Si(100) surface with
H2O is investigated. We find that H2O etches the
surface to give hydrogen termination and oxidized silicon hydrides.
Hydrogenterminated surfaces react with various reagents under non-UHV
conditions to form other covalent bonded species, including Cl-Si and
Co(CO)4-Si. These species are characterized by polarized ATR
and transmission FTIR. On Cl-Si(111) surfaces, Cl-Si is perpendicular to the
surface. Chlorine-terminated Si(100) reacts with H2O to produce
the hydrogen terminated surface. On Co(CO)4-Si(111) surfaces, one
carbonyl is perpendicular to the surface, and the other three are
approximately parallel to the surface.
Dilute HF/RCA and IEMC/SC2 cleans have been evaluated on two process lines
with different metallic contamination levels. VPD-DSE-TXRF and SPV
techniques were used to monitor the metallic contamination. Gate oxide
integrity(GOI) tests were performed on several structures. Both HF/RCA and
IMEC/SC2 cleans have shown good Qbd and Ebd results for the clean process
line. Lower Qbd and Ebd values were obtained for both cleans in the
relatively contaminated process line. These results suggest that poor GOI is
related to the metallic contamination in the oxide or at the
The continuously increasing integration of today's advanced semiconductors
requires increasingly tight process control in the IC manufacturing steps.
This paper demonstrates the use of conductivity sensors to monitor and
control the chemical concentrations of RCA cleaning and HF etching
solutions. Electrodeless conductivity sensors were used to monitor and
regulate the concentration of these process chemicals. A linear relationship
between the conductivity of the solution and the chemical concentration was
obtained within the range studied. A chemical monitoring and concentration
scheme (ICE-1™) was developed. Different concentrations of RCA and HF
solutions were investigated. Results show that these techniques are suitable
for monitoring and controlling the concentration of chemicals in the process
tanks for better process control. These techniques provide a lower cost of
ownership of the process due to longer bath lives and the use of dilute
To form atomically flat H-passivated Si(100) surfaces, wet chemical etching
of sacrificial SiO2 layer has been examined. Roughness and
chemical overlayer thickness, as monitored by ellipsometry shows a minima at
an optimal solution of 1:0.5:30 HF(49wt\%):H2SO4
(98wt\%):H2O. A mechanistic study offers no evidence for a
chemical smoothing from preferential non-Si(100) facet etching. Silicon
planarization can be induced by rapid thermal annealing RTA of chemical
oxides. The H-terminated Si(100) surfaces are found to be moderately
reactive to ambient conditions as monitored by in-situ ellipsometry and
Auger analysis. Atomic force microscopy (AFM) measurements show Si(100)
surfaces to have a rms ∼1.0Å and Rmax values of 1.6–0.9Å. With
measured roughness incorporate into ellipsometric model, a 5Å native oxide
overlayer is rapidly incorporated into the Si(100) surface.
The applications of wet chemical cleaning at the ‘back-end’ of Integrated
Circuit (IC) or Metal-Oxide Semiconductor (MOS) fabrication are reviewed
from chemical, environmental and cost perspectives. The various classes of
commercially available “solvents” and “cryogenic” cleans are reviewed from
the perspective of process suitability, impact on device yield and waste
management. Strategies for minimizing processing concerns, as well as
alternatives to organic solvent based wet chemical processing will also be
discussed. Bulk photoresist (PR) stripping, post metal definition-, and post
window etch cleaning are used to illustrate the discussion.
The use of radiotracing as a diagnostic tool in understanding the mechanism
for metallic contamination during solvent cleans is also discussed. Data
suggesting how the chemistry and solvent composition affects alkali metal
(for example, sodium) contamination of dielectric- and barrier films during
IC processing will also be presented.
The use of malonic acid as an additive in alumina slurries used for the
chemical mechanical polishing ( CMP ) of tungsten has been explored for the
reduction of particulate contamination. The principal objective of this work
was to delineate conditions under which alumina contamination on polished
surfaces could be reduced.
The interaction between malonic acid and alumina particles has been
investigated through electrokinetic and adsorption measurements. At suitable
malonic acid concentrations and pH values, tungsten and alumina surfaces
develop a negative zeta potential resulting in conditions conducive to
reduced particulate contamination. Small scale polishing experiments have
been carried out to relate electrokinetic results to the level of
particulate contamination after polishing.
Organic adsorbates on silicon wafer surfaces exposed to superclean room air
were measured to evaluate organic contamination level of silicon wafers
stored in a clean bench up to 180min. Such Si wafers were thermally oxidized
and the dielectric degradation behavior were systematically investigated. It
is found that a carbon contamination level of half a monolayer influences
the charge to quasi-breakdown although the degradation mechanism itself