(Hyo Sang Kang)
13
(Joo Hyung Lee)
1
(Jae Hwa Park)
3
(Hee Ae Lee)
1
(Won Il Park)
1
(Seung Min Kang)
4
(Sung Chul Yi)
2*
Copyright © 2019 The Korean Institute of Metals and Materials
Key words(Korean)
aluminum nitride, chemical mechanical polishing, colloidal silica, material removal rate, surface chemistry, slurry agglomeration
1. INTRODUCTION
Group III-Nitride compound semiconductor substrates such as GaN, AlN, and InN are
next-generation semiconductor materials that exhibit excellent features such as potential
for miniaturization and light weight, low power consumption, and long lifetime of
devices; demand for such material is steadily increasing [1,2]. Aluminum nitride (AlN), which has a wide bandgap of 6.2 eV, high thermal stability
and thermal conductivity, and wide critical electric fields (12.5 MV/cm), has been
applied to AlGaN-based ultraviolet optoelectronic devices; because it can provide
superior power properties, it is used mainly as an substrate for various applications
ranging from semiconductor devices to laser diodes (LD) and light emitting diodes
(LED) [3-5].
In order to apply AlN single crystal substrates to such applications, ultra-smooth,
atomically flat and defect-free the substrate surfaces are required. However, bulk
AlN single crystals are difficult to manufacture due to their high hardness and chemical
inertness [3,6]. In particular, the poor surface morphology and micro-defects inherent in the sub-surface
of the substrate are the main sources of degradation of device performance, generating
consecutive defects of the epitaxial layer grown on native substrates [7,8]. Therefore, to increase the potential performance of the material, it is essential
to obtain high optical quality and defect-free substrate surfaces [9-11].
CMP is the most effective technique for obtaining an ultra-smooth undamaged surface
of a substrate in a semiconductor process. CMP simultaneously physically and chemically
smooths the surface of the substrate; it also removes damaged layers generated by
MP on the surface and sub-surface of the substrate [1,3,6]. The principle of CMP is to push down a slurry containing both abrasive and reactive
chemicals while simultaneously rotating the pads and the wafer in the opposite direction.
The water (H2O) molecules in the slurry penetrate the atomic bonds of the wafer surface and form
an oxide layer, and this reduces the hardness of the wafer. The lower-hardness wafer
surface is polished by the physical friction of the rotating platen [12, 13]. The abrasive used in the slurry should be harder than the mixture produced in the
process, but it should be softer than the substrate and should be selected for its
ability to easily remove the mixture during post-cleaning [1,13].
The most typical materials used as abrasives for CMP slurries are silica, ceria, and
alumina; they are used differently according to the properties of the material when
processing semiconductor substrates. Ceria and alumina have selectivity and material
removal rate (MRR) for chemical additives relatively higher than those values of silica,
but it is hard to manipulate surface scratches and perform post cleaning When using
ceria and alumina [6,14]. Silica has a relatively low MRR; however, it has high dispersibility and uniformity
of particles, and it has been applied mainly to the processing of semiconductor substrates
to obtain defect-free surfaces on substrates. The colloidal silica slurry has been
mainly used for group III-nitride single crystals; studies on the composition of this
material have been actively carried out until now [1,6,14]. Although the silica slurry has relatively high surface charge and repulsive force
between particles, it can also agglomerate of silica particles by manipulating extrinsic
variables; shear rate, process time and pH etc. during CMP [15].
The agglomeration of silica particles can increase the surface roughness and number
of defects of the polished substrate. This performance used mainly to occur when CMP
was conducted in ambient air, due to the inherent characteristics of silica slurry
[15-17]. Hence, in order to prevent additional defects from agglomerated particles, optimum
process conditions must be determined.
In this study, CMP using colloidal silica at pH 9 was performed for different times
to obtain optimum conditions for fabrication of an AlN substrate. The MRR and zeta
potential of colloidal silica with different pH were analyzed to confirm the most
stable conditions of colloidal silica. The surface characteristics were characterized
by atomic force microscopy (AFM) and zeta potential and particle size analyzer. In
order to determine the extent of damaged layer and chemical changes on the AlN substrate
surface, X-ray photoelectron spectroscopy (XPS) were performed.
2. EXPERIMENTAL PROCEDURES
Bulk AlN single crystal wafers grown by physical vapor transport (PVT) with characteristics
of 2-inch width, 870 μm thickness, and c-plane (0001) were prepared, while double
side MP was conducted with water soluble diamond slurry; the final Ra values of the
mechanically polished wafers were less than 0.5 nm. The pH of the colloidal silica
slurry was manipulated by 35% HNO3 solution. The zeta potential of colloidal silica was analyzed using a zeta potential
and particle size analyzer (Otsuka Electronics, ELSZ-1000, Japan), and the MRR for
the AlN single crystal (al-polar face) were characterized using an electronic balance
(Ohaus, E02130, USA), which has an accuracy of 0.0001 g according to the weight loss,
and Equation (1), used to calculate the MRR is as follows [11,14]:
Δm (g) is the removal mass of material, ρ (g/cm3) is the density of AlN, r (cm) is the radius of the wafer, and t (hr) is the polishing
time. The CMP was conducted on an EK-3801D machine with colloidal silica slurry at
pH 9 and a SUBA-600 pad. The morphology and particle size of the colloidal silica
at pH 9 were obtained for scanning electron microscope (SEMJeol, JSM-5900LV, Japan),
and particle size distribution at pH 9 was characterized using a zeta potential and
particle size analyzer (Otsuka Electronics, ELSZ-1000, Japan). The CMP conditioner
was used to manipulate the distribution of the slurry and the roughness of the pad;
detailed CMP conditions are shown in Table 1. After CMP has been performed several times, as-polished wafers were rinsed ultrasonically
with acetone, ethanol, and deionized water for 15 minutes each; wafers were then dried
using pure N2. The surface morphology and roughness were determined using AFM (PSIA, XE-100, Korea);
zeta potential and mean particle size with increasing process time were characterized
using a zeta potential and particle size analyzer (Otsuka Electronics, ELSZ-1000,
Japan). Chemical changes on the polished surfaces using MP to CMP were analyzed via
XPS (Thermo scientific, theta probe base system, USA).
3. RESULTS AND DISCUSSION
3.1. Performance of the colloidal silica slurry
The MRR and zeta potential of the colloidal silica slurry with different pH were characterized
as shown in Fig. 1. Zeta potential is a function of electrostatic interactions, which are generated
between the wafer and particles. The zeta potential of colloidal silica was negatively
charged overall and it gradually increased from -14.5 to -50.70 mV as the pH increased
from pH 4 to 9. In acidic conditions, the colloidal silica particles with relatively
low surface charge could be easily adsorbed to the wafer surface and hinder the performance
of oxidants during CMP. With decreasing pH, the MRR also reached the highest value
(14.2 nm/h) at pH 5 and decreased rapidly to 6.0 nm/h at pH 10, this can show that
the acid condition is suitable to obtain high MRR than base condition but it can easily
cause the agglomeration of silica particles. Thus, colloidal silica in base condition
is more suitable to obtain defect-free surface. However, when it was conducted with
colloidal silica slurry above the particular process time, physical effects were dominant
than chemical effects for colloidal silica slurry. Thus, the colloidal silica at pH
9 which has the highest zeta potential value was selected; morphology, particle size,
and particle size distribution were characterized as shown in Fig. 2. The CMP with colloidal silica at pH 9 was performed for different times to investigate
physical effects for AlN substrates in detail.
3.2. Polished surface morphology analysis
Figure 3 shows the surface roughness and surface morphology according to time during CMP.
In Fig 3 (a), several defects including micro scratches and irregular pull-outs can be observed
on the surface despite the low surface roughness of 1 nm or less after MP; residue
of the diamond slurry adsorbed on the surface can also be observed. In Fig 3 (b), defects such as micro-scratches and pull-out phenomena generated on the surface
were successfully removed using the colloidal silica slurry, and the surface roughness
decreased. However, the damaged layers were not completely removed during MP, and
an additional process of 60-150 min was performed to reduce the remaining damaged
layer. Figure 3 (c) shows the significantly lower and smooth surface properties with Ra 0.395 nm,
but some damaged layers remained on the surface. Figure 3 (d) shows an ultra-smooth surface roughness and shape with Ra of 0.172 nm that the
remaining damaged layer on the surface was completely removed. However, in Fig 3 (e) and Fig 3 (f), it is confirmed that the Ra value gradually increased from 0.537 to 0.670 nm,
and the surface morphology was also uneven. It was considered that agglomeration of
the slurry particles occurred and increased due to the continuous mechanical stress
and interaction of local particles generated during CMP. Thus, after a certain period
of CMP, the slurry particles became harder, the average particle size of the slurry
increased, and the slurry became non-uniform. In order to characterize the detailed
mechanism of the interaction between colloidal silica particles during CMP, particle
size distribution and zeta potential were analyzed; results are explained in Fig 4.
3.3. Particle size distribution and zeta potential analysis
Figure 4 shows the average particle size distribution and zeta potential value of the colloidal
slurry with process time. From the initial state of process to CMP 60 min, a particle
size of about 120.8-121.2 nm was observed. After CMP 60 min, the particle size increased
significantly from 121.2 nm to 143.4 nm. While the particle size greatly increased,
the zeta potential gradually decreased about from -56.94 mV to -35.07 mV. The lower
zeta potential indicates lower surface charges of the silica particles, with decreasing
repulsive forces of particles and increasing coarsening of the particles; the coarsened
particles formed particle agglomerates. While mechanical shearing reduced many of
the soft particle agglomerates, it broke weak inter-particle interactions by van der
Waals forces during CMP and formed more densely packed particles. These processes
occurred simultaneously but formation of the densely packed particles was predominantly
generated, and it changed the particle size distribution with increasing process time.
This result implies that agglomerated particles caused other micro-defects on the
polished surface, and increased the Ra value [15-18]. Thus, in order to inhibit the formation of additional damage on the surface, it
is essential to determine the time when slurry particles change and agglomerate with
increasing process time.
3.4. XPS analysis of polished surface
The XPS analysis was performed to assess the surface chemistry as shown in Fig 5. This presents a survey scan graph that identifies the chemical composition and elements
present on the surface. The graph shows a typical spectrum obtained from XPS analysis,
of a polished AlN single crystal after MP and CMP. The photoelectron peaks represent
Al2s, Al2p, and N1s, which are the core level peaks of AlN; O1s and C1s peaks were observed
in the other peaks. As a result of the chemical composition changes induced by MP
and CMP, the intensity of the peaks at C1s and O1s rapidly decreased after CMP, and
Al and N decreased slightly. In order to analyze in detail the changes of the core
level spectra of each element, a high resolution (HD) scan was performed and results
were analyzed for MP and CMP for each condition. In the case of C1s, however, when
the specimens were measured, the carbonated compounds present in air were also detected.
For this reason, since the reliability of the component analysis was poor, results
were excluded from the precision analysis in the HD scan [19].
Figure 6 shows the O1s spectra. O2- and OH- peaks ranging from 530.7 to 532.3 eV represent aluminum trihydroxide (Al(OH)3), monohydrated alumina (AlOOH), or aluminum oxide (Al2O3) depending on the composition ratio of each compound (Al2O3·xH2O) [8,19,20]. From MP to CMP 90 min, the intensity of the hydroxide peak rapidly decreased to
less than half its original level, and the oxide peak also was shown to have a relatively
broad graph. After CMP 90 min, both peaks were maintained to constant levels of intensity
and the binding energy decreased by about 0.3 eV. It was demonstrated that MP was
performed with a water soluble diamond slurry for a stepwise polishing process in
ambient air; continuous oxidization and hydration occurred on the surface of the single
crystal. Since external oxygen and H2O combined with elemental aluminum, the reaction produced aluminum oxidehydroxide
compounds while replacing elemental nitrogen as indicated by Equation (2), as follows [1]:
It was determined that the AlOOH (peak: 531.5 eV) that occurred on the surface was
removed during CMP and that AlOOH adsorbed on the surface was hydrated continuously,
generating a small amount of Al(OH)3 (peak: 531.3 eV).
Figure 7 shows the Al2p spectra. The Al-O bond ranging from 74.3 to 75.1 eV represents AlOOH or Al(OH)3; the Al-N bond is in the range of 73.2 to 73.8 eV [20, 21]. The intensity of the AlOOH peak decreased by one third after CMP 30 min, and the
intensity of the Al-N peak increased slightly. From CMP 60 min, the intensity of the
Al-O peak increased slightly and the binding energy decreased by about 0.5 eV. After
that, the intensity of the Al-O peak was sustained at a constant level. As shown in
Fig 6, the AlOOH generated on the surface during MP was removed through CMP and the relative
density of Al-N on the surface increased.
Figure 8 shows the N1s spectra. The N-C bond is in the range of about 397.7-398.4 eV and the
N-Al bond is in the range of about 396.1 eV-396.7 eV [20, 22]. After MP, the intensity of the N-C bond (peak: 398.4 eV) gradually decreased until
CMP 150 min, and the N-Al bond (peak: 396.4 eV) was maintained at a similar intensity.
This shows that the nitrogen-carbon compound from the diamond slurry in the MP had
been removed.
4. CONCLUSION
Bulk AlN single crystals were subjected to CMP using colloidal silica slurry at pH
9 for different times; performance of the colloidal silica slurry, surface characteristics
and chemical changes on the surface were characterized. The whole damaged layer was
successfully removed, and an ultra-smooth surface, with the lowest possible surface
roughness (Ra : 0.172 nm), was obtained at CMP 90 min; this surface was not much affected
by agglomeration of colloidal silica slurry particles. While CMP was performed with
silica slurry, mechanical shearing can decrease the number of soft agglomerates; however,
mechanical shearing can break also weak interparticle interaction by van der Waals
forces, and this seems to form more densely packed particles, which can increase a
Ra value and generate additional damages on the surface. The XPS analysis was performed
and, it was determined that an aluminum oxide-hydroxide compound was formed, while
replacing nitrogen which was bonded to elemental aluminum with external oxygen and
H2O. The AlOOH adsorbed on the surface was removed during CMP and the AlOOH was hydrated
continuously, generating a small amount of Al(OH)3. It was determined that the optimum process time to minimize the occurrence of additional
defects and damaged layers on the surface was CMP 90 min.
Acknowledgements
This work was supported by the Industrial Strategic Technology Development program
funded by the Ministry of Trade Industry & Energy, KOREA (Project No. 10043791).
REFERENCES
Slack G., Schujman S., Meyer N., Schowalter L., 20060288929A1, US Patent No, (2006)
Reddy P., Bryan I., Bryan Z., Guo W., Hussey L., Collazo R., Sitar Z., J. Appl. Phys,116,
123701 (2014)
Asghar K., Das D., J. Semicond,37, 036001 (2016)
Dalmau R., Craft H., Britt J., Paisley E., Moody B., Guo J., Ji Y., Raghothamachar
B., Dudley M., Mater. Sci. Forum, Trans. Tech. Publications,923Switzerland (2018)
Reddy P., Washiyama S., Kaess F., Breckenridge M., Balderrama L. H., Haidet B. B.,
Alden D., Franke A., Sarkar B., Kohn E., Collazo R., Sitar Z., J. Appl. Phys,119,
145702 (2016)
Chen G., Ni Z., Xu L., Li Q., Zhao Y., Appl. Surf. Sci,359, 664 (2015)
Chen X. F., Siche D., Albrecht M., Hartmann C., Wollweber J., Xu X. G., Cryst. Res.
Technol,43, 651 (2008)
Rice A., Collazo R., Tweedie J., Dalmau R., Mita S., Xie J., Sitar Z., J. Appl. Phys,108,
043510 (2010)
Shi X., Pan G., Zhou Y., Xu L., Zou C., Gong H., Surf. Coat. Technol,270, 206 (2015)
Wang J., Wang T., Pan G., Lu X., ECS J. Solid State Sci. Technol,4, 112 (2015)
Wang J., Wang T., Pan G., Lu X., Appl. Surf. Sci,361, 18 (2016)
Lee J. H., Park C. W., Park J. H., Kang H. S., Kang S. H., Lee H. A., Lee J. H., In
J. H., Kang S. M., Shim K. B., J. Korean Cryst. Growth Cryst. Technol,28, 51 (2018)
Chunrong J., Baoguo Z., 2015 Int. Conference on Planarization/CMP Technology (ICPT),
p. 1, USA (2015),
Zhou Y., Pan G., Gong H., Shi X., Zou C., Colloids Surf. A Physicochem. Eng. Asp,513,
153 (2017)
Sorooshian A., Ashwani R., Choi H.K, Moinpour A., Oehler A., Mater. Res. Soc. Symp.
Proc,816 (2011)
Basim G. B., Moudgil B. M., J. Colloid Interface Sci,256, 137 (2002)
Boning J. M., Johnson D., Kim G. S., Safier P., Knutson K., 1249, Mater. Res. Soc.
Symp. Proc, (2011)
Khanna A. J., Gupta S., Kumar P., Chang F. C., Singh R. K., ECS J. Solid State Sci.
Technol,7, 238 (2018)
Bryan I., Akouala C.-R., Tweedie J., Bryan Z., Rice A., Kirste R., Collazo R., Sitar
Z., Phys. Status Solidi. C,11, 454 (2014)
Dalmau R., Collazo R., Mita S., SITAR Z., J. Electron. Mater,36, 414 (2007)
Rosenberger L., Baird R., McCullen E., Auner G., Shreve G., Surf. Interface Anal,40,
1254 (2008)
Taborda J. A. P., Landazuri H. R., Vera L. P., IEEE Sens. J,16, 359 (2016)
Figures and Table
Fig. 1.
MRR and zeta potential of the colloidal silica slurry with different pH.
Fig. 2.
(a) Particle size distribution of colloidal silica slurry at pH 9 and (b) SEM image
of colloidal silica particles.
Fig. 3.
AFM images of AlN surface after each CMP process. (a) after MP, (b) CMP 30 min, (c)
CMP 60 min, (d) CMP 90 min, (e) CMP 120 min, (f) CMP 150 min.
Fig. 4.
Mean particle size and zeta potential of silica slurry for different times.
Fig. 5.
Survey scans after MP and CMP.
Fig. 6.
Binding energies of core level of O1s scan.
Fig. 7.
Binding energies of core level of Al2p scan.
Fig. 8.
Binding energies of core level of N1s scan.
Table 1.
AlN CMP process condition.
|
Name
|
Unit
|
Quantity
|
|
Platen rotation speed
|
RPM
|
25
|
|
Carrier rotation speed
|
RPM
|
25
|
|
Applied pressure
|
kg/cm2 |
0.03
|
|
Feed rate of the slurry
|
ml/min
|
35
|
|
Diameter of plate
|
mm
|
110
|
|
Polishing pad type
|
-
|
Suba-600
|
|
Abrasive particle
|
-
|
Silica
|
|
Particle concentration
|
%
|
30
|
|
pH
|
-
|
9.02
|
|
Polishing time
|
min
|
30-150
|