Atomic layer controlled growth of SiO2 films using binary reaction sequence chemistry

Article abstract of DOI:10.1063/1.118494

SiO₂ thin films were deposited with atomic layer control using binary reaction sequence chemistry. The SiO₂ growth was accomplished by separating the binary reaction SiCl₄+ 2H₂O→SiO₂ + 4HCl into two half-reactions. Successive application of the half-reactions in an ABAB... sequence produced SiO₂ deposition at temperatures between 600 and 800 K and reactant pressures of 1-10 Torr. The SiO₂ growth was monitored using ellipsometry versus substrate temperature and reactant exposure time. The maximum SiO₂ deposition per AB cycle was 1.1 A/AB cycle at 600 K. The surface topography measured using atomic force microscopy was extremely flat with a roughness nearly identical to the initial substrate.

Full text of DOI:10.1063/1.118494

Atomic layer controlled growth of SiO 2 films using binary reaction sequence chemistry
J. W. Klaus, A. W. Ott, J. M. Johnson, and S. M. George
Citation: Applied Physics Letters 70, 1092 (1997); doi: 10.1063/1.118494
View online: http://dx.doi.org/10.1063/1.118494
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Atomic layer controlled growth of SiO2 films using binary reaction
sequence chemistry
J. W. Klaus, A. W. Ott, J. M. Johnson, and S. M. Georgea)
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309
͑Received 28 October 1996; accepted for publication 30 December 1996͒
SiO2 thin films were deposited with atomic layer control using binary reaction sequence chemistry.
The SiO2 growth was accomplished by separating the binary reaction SiCl4ϩ2H2O→SiO2
ϩ4HCl into two half-reactions. Successive application of the half-reactions in an ABAB... sequence
produced SiO2 deposition at temperatures between 600 and 800 K and reactant pressures of 1–10
Torr. The SiO2 growth was monitored using ellipsometry versus substrate temperature and reactant
exposure time. The maximum SiO2 deposition per AB cycle was 1.1 Å/AB cycle at 600 K. The
surface topography measured using atomic force microscopy was extremely flat with a roughness
nearly identical to the initial substrate. © 1997 American Institute of Physics.
͓S0003-6951͑97͒03309-3͔
The miniaturization of semiconductor microelectronic
devices to the nanoscale limit requires atomic layer con-
trolled deposition techniques. Nanoscale device fabrication
will require exquisite control over many film properties such
as thickness, morphology, crystallinity, and electrical charac-
teristics. Lower deposition temperatures must also be real-
ized because nanoscale structures are very sensitive to inter-
layer and dopant diffusion. Many of these new requirements
may be achieved by atomic layer control of growth using
binary reaction sequence chemistry.1–3
SiO2 continues to be one of the most important and
widely used materials in the microelectronics industry. Ox-
ide gate thicknesses р50 Å will be employed in future de-
vices. Higher density dynamic random access memory
͑DRAM͒ must deposit conformal SiO2 films on very high
aspect ratio trenches.4 Larger flat panel displays will require
uniform SiO2 film deposition over extremely large
substrates.5 Very thin SiO2 films may also be employed in
nanolaminate structures to tailor the mechanical, electrical,
and optical properties of materials.6
no further reaction can occur because the remaining precur-
sor has no reactivity towards the newly deposited surface
species.
If each half-reaction is self-limiting, application of the
binary reaction sequence in an ABAB... fashion should pro-
duce layer-by-layer controlled growth. The principal advan-
tage of the ABAB... binary reaction sequence approach is
that the reaction kinetics should not affect the SiO2 deposi-
tion. Provided that the surface reactions are allowed to reach
completion everywhere on the substrate, small changes in the
surface temperature, reactant pressure and exposure times
will not change the film growth per AB cycle.
In this letter, SiO2 thin films were grown using binary
reaction sequence chemistry and evaluated using in situ
spectroscopic ellipsometry and ex situ atomic force micros-
copy. The experiments were performed in an apparatus that
has been described elsewhere.8 In brief, the apparatus con-
sists of a sample load lock chamber, a central deposition
chamber and a high vacuum chamber for surface analysis.
The central deposition chamber has a base pressure of
1ϫ10Ϫ7 Torr. A recent addition to the deposition chamber is
an in situ spectroscopic ellipsometer ͑J.A. Woolam Co.
M-44͒ that simultaneously collects ellipsometric data at 44
visible wavelengths.
The ellipsometer is mounted on ports positioned at 50°
with respect to the surface normal. The ports are equipped
with gate valves to protect the birefringent-free windows
from deposition during film growth. The best sensitivity for
the spectroscopic ellipsometer is achieved at the Brewster
angle of 76° for the Si͑100͒ substrate. To increase the sensi-
tivity, these ellipsometric measurements at 50° were con-
ducted on thin Al2O3 films with ϳ300 Å thicknesses grown
on Si͑100͒. The Al2O3 films were deposited on Si͑100͒ with
excellent conformality and precise thickness control using
binary reaction sequence chemistry ͑8͒.
The Si͑100͒ substrates were rinsed with methanol and
blown free of particles before loading into the deposition
chamber. The Si͑100͒ surface was then cleaned using an an-
neal at 875 K for 1 min. This anneal was followed by a
H2O plasma exposure at room temperature to hydroxylate
the surface and remove surface carbon. Immediately follow-
ing the cleaning procedure, the substrate temperature was
Self-limiting surface reactions applied in a binary reac-
tion sequence can lead to atomic layer controlled
growth.1–3,7–9 This technique is known as atomic layer epi-
taxy ͑ALE͒ or atomic layer processing ͑ALP͒ and was first
demonstrated by Suntola and co-workers for the deposition
of ZnS films.2 Much recent research has been devoted to
developing atomic layer controlled growth techniques for the
growth of various oxides and SiO2 .7–11
A binary reaction sequence for the deposition of SiO2 is
͑7͒:
*
*
A͒ SiOH ϩSiCl →SiOSiCl ϩHCl,
͑
͑
4
3
*
*
B͒SiCl ϩH O→SiOH ϩHCl,
2
where the surface species are indicated by the asterisks. Each
half-reaction involves the reaction between a gas phase pre-
*
*
cursor and a surface functional group ͑i.e., SiOH or SiCl ͒.
The surface reaction continues until all of the initial surface
functional groups have been replaced by the new functional
group. Once the surface has achieved the new functionality,
a͒
Electronic mail: georges@spot.colorado.edu
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1092 Appl. Phys. Lett. 70 (9), 3 March 1997 0003-6951/97/70(9)/1092/3/$10.00 © 1997 American Institute of Physics
130.216.129.208 On: Mon, 08 Dec 2014 01:39:47

FIG. 1. Ellipsometric measurements of SiO2 film thickness deposited on
thin Al2O3 films on Si͑100͒ at 700 K after 5 AB cycles for various total
reactant exposures.
FIG. 2. Ellipsometric measurements of SiO2 film thickness deposited on
thin Al2O3 films on Si͑100͒ at 700 K vs number of AB reaction cycles.
ure 3 shows the surface topography after 75 AB cycles at
700 K using exposures sufficient for both half-reactions to
reach completion. Analysis of these AFM images yielded a
surface roughness of Ϯ3 Å ͑RMS͒. In comparison, the
roughness of a cleaned Si͑100͒ wafer was Ϯ2 Å͑RMS͒. The
SiO2 surface roughness also exhibits nearly the same power
spectral density versus spatial wavelength as the Si͑100͒ wa-
fer. This near equivalence confirms that the SiO2 films are
growing conformally on Si͑100͒.
ramped to the desired reaction temperature for film growth.
Earlier work on SiO2 deposition revealed that temperatures
у600 K are needed for the two half-reactions to reach
completion.7
Once a half-reaction has reached completion, additional
reactant exposure should produce no additional growth. Fig.
1 demonstrates the self-limiting nature of the half-reactions
at 700 K. The SiO2 film thickness was measured using the in
situ ellipsometer after 5 AB cycles for various total H2O and
SiCl4 reactant exposures. The SiCl4 exposures were equal to
the H2O exposures. A typical AB cycle occurred with the
following sequence: Dose SiCl4 ͑1–10 Torr͒/Evacuate
͑1ϫ10Ϫ4Torr͒/Dose H2O(1–10 Torr͒/Evacuate ͑1ϫ10Ϫ4
Torr͒.
A model for SiO2 film growth was developed that as-
sumes that the maximum number of Si atoms and SiO2 units
that can be deposited per AB cycle is equal to the number of
7
*
*
SiOH hydroxyl groups. The SiOH coverage at 700 K is
ϳ2ϫ1014 OH units/cm2.12 The SiO2 monolayer coverage
can be derived using the measured refractive index of
nϭ1.46 for the SiO2 films. This refractive index is consistent
with a density of 2.21 g/cm3 or a number density of ␳
ϭ2.2ϫ1022 SiO2 units/cm3. The thickness of a SiO2 mono-
For total H2O and SiCl4 exposures Ͻ2x109 L after 5 AB
cycles, Fig. 1 shows that the SiO2 film thickness is depen-
dent on the total reactant exposure. This behavior reveals
that the surface half-reactions have not reached completion.
In contrast, both half-reactions have saturated for reactant
exposures Ͼ2ϫ109 L after 5 AB cycles and additional re-
actant exposure results in no additional growth. The error
bars represent the 90% confidence limits and the solid line is
intended only to guide the eye.
layer can then be calculated as ␳Ϫ1/3ϭ3.5 Å. Likewise, ␳
2/3
represents an SiO2 monolayer coverage of 7.9ϫ1014 SiO2
units/cm2.
Based on the thickness and coverage of a SiO2 mono-
Figure 2 shows the in situ ellipsometric measurements of
SiO2 film thickness versus number of AB cycles at 700 K.
The growth rate at 700 K was 0.87 Å/AB cycle and the
refractive index was nϭ1.46Ϯ0.02. This refractive index
was confirmed using additional ex situ ellipsometric mea-
surements at various angles of incidence. The linear growth
rate indicates that the number of reactive surface sites re-
mains constant. The constant growth rate per AB cycle also
argues that the SiO2 film is growing conformally with no
roughening. Using the refractive index of nϭ1.46, the
growth rates at 600 K and 800 K were 1.1 Å/AB cycle and
0.75 Å/AB cycle, respectively. The decrease in the growth
rate versus temperature correlates with the thermal stability
12
*
of the SiOH surface functional groups.
The surface topography of the SiO2 films was studied
using an atomic force microscope ͑AFM͒ in tapping mode
͑Digital Instruments-Nanoscope III͒. The SiO2 films were
grown directly on Si͑100͒ for these AFM experiments. Fig-
FIG. 3. Atomic force microscope image of a SiO2 film deposited directly on
Si͑100͒ at 700 K after 75 AB cycles. The horizontal full scale is 1 ␮m. The
vertical full scale is 5 nm and the light-to-dark range is 10 Å.
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Appl. Phys. Lett., Vol. 70, No. 9, 3 March 1997 Klaus et al. 1093
130.216.129.208 On: Mon, 08 Dec 2014 01:39:47

layer, the deposition of 2ϫ1014 SiO2 units/cm2 per AB cycle
is consistent with a thickness of ϳ0.9 Å/ AB cycle. This
predicted SiO2 deposition compares favorably with the mea-
sured SiO2 growth of 0.87 Å/ AB cycle at 700 K. The
SiO2 growth per AB cycle at 600 K and 800 K were pre-
dicted to be 1.17 Å/AB cycle and 0.74 Å/AB cycle, respec-
tively. The agreement between the predicted and measured
growth rates argues that the SiO2 film density is independent
of growth temperature and consistent with a refractive index
of nϭ1.46. In addition, the agreement indicates that SiO2
film growth by binary reaction sequence chemistry is occur-
This work was supported by the Office of Naval Re-
search under Contract No. N00014-92-J-1353.
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*
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layer control using binary reaction sequence chemistry. The
SiO2 growth per AB cycle was extremely linear and AFM
measurements revealed an exceptionally flat SiO2 film with a
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*
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1094 Appl. Phys. Lett., Vol. 70, No. 9, 3 March 1997 Klaus et al.
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