Characterization of atomic layer deposition HfO2, Al2O3

Transcription

Characterization of atomic layer deposition HfO2, Al2O3, and plasmaenhanced chemical vapor deposition Si3N4 as metal–insulator–metal
capacitor dielectric for GaAs HBT technology
Jiro Yota,a) Hong Shen, and Ravi Ramanathan
Skyworks Solutions, Inc., 2427 W. Hillcrest Drive, Newbury Park, California 91320
(Received 5 September 2012; accepted 14 November 2012; published 6 December 2012)
Characterization was performed on the application of atomic layer deposition (ALD) of hafnium
dioxide (HfO2) and aluminum oxide (Al2O3), and plasma-enhanced chemical vapor deposition
(PECVD) of silicon nitride (Si3N4) as metal–insulator–metal (MIM) capacitor dielectric for GaAs
heterojunction bipolar transistor (HBT) technology. The results show that the MIM capacitor with
62 nm of ALD HfO2 resulted in the highest capacitance density (2.67 fF/lm2), followed by
capacitor with 59 nm of ALD Al2O3 (1.55 fF/lm2) and 63 nm of PECVD Si3N4 (0.92 fF/lm2). The
breakdown voltage of the PECVD Si3N4 was measured to be 73 V, as compared to 34 V for ALD
HfO2 and 41 V for Al2O3. The capacitor with Si3N4 dielectric was observed to have lower leakage
current than both with Al2O3 and HfO2. As the temperature was increased from 25 to 150 C, the
breakdown voltage decreased and the leakage current increased for all three films, while the
capacitance increased for the Al2O3 and HfO2. Additionally, the capacitance of the ALD Al2O3 and
HfO2 films was observed to change, when the applied voltage was varied from 5 to þ5 V, while
no significant change was observed on the capacitance of the PECVD Si3N4. Furhermore, no
significant change in capacitance was seen for these silicon nitride, aluminum oxide, and hafnium
dioxide films, as the frequency was increased from 1 kHz to 1 MHz. These results show that the
ALD films of Al2O3 and HfO2 have good electrical characteristics and can be used to fabricate high
density capacitor. As a result, these ALD Al2O3 and HfO2 films, in addition to PECVD Si3N4, are
suitable as MIM capacitor dielectric for GaAs HBT technology, depending on the specific
C 2013 American
electrical characteristics requirements and application of the GaAs devices. V
Vacuum Society. [http://dx.doi.org/10.1116/1.4769207]
I. INTRODUCTION
Due to the increasing functionality and the demand for
capacity, the die size in semiconductor wafer manufacturing
must be reduced. Excluding the bond pad and scribe street
areas, metal–insulator–metal (MIM) capacitor device, which is
a key passive component in GaAs circuit designs, could consume up to 35% of the total die area.1,2 Therefore, it is critical
to increase the capacitance density of the MIM capacitor in
these designs, fabricated using GaAs process technologies,
including GaAs heterojunction bipolar transistor (HBT) technology. Increasing the capacitance density of the MIM capacitors will allow the reduction of capacitor area in these designs,
resulting in die size reduction. Furthermore, the higher capacitance density of these capacitors will allow the integration of
additional off-chip capacitors on to the GaAs die, and thereby
reducing the bill-of-materials in a multichip module.
Aside from the high capacitance density requirement, the
MIM capacitor has electrical requirements that are dependent
on its application and the design, and may be different in one
GaAs design, compared to other GaAs and most silicon
CMOS digital and analog/mixed signal designs and applications. For instance, in the majority of GaAs power amplifier
designs fabricated using HBT technology, the operating voltage is high, and the output voltage swings can be more than
20 V.1–3 Therefore, in addition to high capacitance density, it
a)
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01A134-1 J. Vac. Sci. Technol. A 31(1), Jan/Feb 2013
is required that the breakdown voltage of the capacitor to be
higher than 20 V. Additionally, low capacitor leakage current
is typically required in most GaAs applications, especially
when there are large area capacitors present in the designs and
when the devices operate at high voltage and high temperature. The leakage current of these MIM capacitors at these
conditions can be significantly higher than at normal conditions, which may lead to long term degradation and/or reliability failures. Furthermore, in most application, the
capacitance is required to be constant and not change with
applied voltage. However, there are other applications, such
as tunable capacitor, where it is required that the capacitance
be tunable and vary as a function of applied voltage.4–6
The processing thermal budget is an important consideration when fabricating many GaAs devices. It is limited due
to the degradation of the materials typically used as contact
metal to the devices, such as emitter, base, and collector contacts, and to the various epitaxial GaAs layers at high
temperatures.1,2,7–11 For instance, while the maximum temperature for many silicon interconnect process technologies
is 400 C or higher, the highest allowable temperature for
most GaAs process technologies is 300 C.1,2,12–14 Furthermore, the GaAs designs typically require thick metal interconnections, so that they can be used for heat dissipation and
high current carrier applications, and to fabricate inductors
with high quality factor (or Q). This thick metal in GaAs
technology is typically deposited by physical vapor deposition in general, and by evaporation method, in particular,
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C 2013 American Vacuum Society
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01A134-1
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01A134-2 Yota, Shen, and Ramanathan: Characterization of ALD HfO2, Al2O3, and PECVD Si3N4
which usually results in rough metal surface.1,2,12 It is known
that the underlying metal electrode surface condition and
material affect the electrical characteristics of MIM capacitors.3,15 Therefore, a uniform and conformal capacitor
dielectric film is preferred, when the underlying bottom
metal electrode is obtained using evaporation method, in
order to achieve uniform electrical characteristics. Additionally, it has been shown that capacitance of MIM capacitors
can be affected by frequency of operation, due to the capacitor dielectric itself, and due to the type of underlying metals
and films used and grown to fabricate the capacitor electrode.16,17 All these significantly limit the options available
for materials and processes to be used for the fabrication of
MIM capacitors in GaAs technologies, including GaAs HBT
technology.
The capacitor dielectric insulator films can be deposited
using various methods, including physical vapor deposition
(PVD), metal–organic chemical vapor deposition (MOCVD),
molecular beam deposition, low pressure chemical vapor
deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD), and atomic layer deposition (ALD).16–24
One of the most common methods to deposit MIM capacitor
dielectric in the semiconductor industry is the PECVD
method. The films deposited using this method typically
result in relatively good electrical, physical, chemical, and
thermal characteristics, have good film conformality, and can
be deposited at relatively low temperatures.1,2,10,13,14,25
Recently, atomic layer deposited films have also been investigated as MIM capacitor dielectric. This method results in
films that can be deposited at a low temperature and that
have good electrical characteristics, with excellent conformality and uniformity.16,18,26 However, these ALD films
have not reportedly been used as MIM capacitor dielectric in
GaAs technology.
The most widely used MIM capacitor dielectric insulator
material in the semiconductor industry is Si3N4.10,13,14,19
This silicon nitride is known to have high dielectric breakdown, which ranges from 9 to 11 MV/cm, and have relatively
high dielectric constant, which ranges from 6 to 8.9–11,13,14,19
The quality and characteristics of the Si3N4, including these
electrical characteristics, are dependent on factors, such as
the method, condition, and gases used to deposit the film.9–14
Other dielectric materials have also been considered and used
as capacitor dielectric, such as SiO2, SiOxNy, ZrO2, TiO2,
Al2O3, CaTiO3, Ta2O5, SrTiO3, and HfO2, with dielectric
constant ranging from 3.8 for SiO2 to 100 or higher for
various high dielectric constant materials.20–31 These materials, however, have mostly higher leakage current and lower
dielectric breakdown field than silicon nitride, with breakdown field ranging from 10 MV/cm for a thermal SiO2 to
much lower than 1 MV/cm for some of the higher dielectric
constant films.20–31 These high dielectric constant materials
are primarily used for MIM capacitors applications in silicon
technology, including both digital and analog/mixed signal
applications. For these silicon applications, in general, thinner films can be used in order to obtain high capacitance density and high capacitor breakdown voltage is not necessarily
required. Furthermore, in most Si-based technologies, the
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highest allowable processing temperature can be 400 C or
higher, which typically results in higher quality deposited capacitor dielectric films.
In GaAs technology, only few dielectrics are available and
satisfy the electrical requirements to be used as MIM capacitor dielectric, including Si3N4, SiOxNy, and SiO2.26,32–34
Among these films, the PECVD Si3N4 is the most widely
used, as it can be deposited at 300 C and is compatibility
with GaAs processing, with a 60 nm film having good physical, chemical, and electrical characteristics, including a high
dielectric breakdown voltage of 65 V and a breakdown field
of 10 MV/cm or higher.1,2 This PECVD Si3N4 film has been
used as capacitor dielectric in GaAs HBT,1,2,35 high electron
mobility transistor (HEMT),36,37 and other monolithic microwave integrated circuits (MMIC)18,38 technologies. Other than
Si3N4, SiOxNy, and SiO2, there have not been many studies, if
any, on other dielectric materials that have been used as MIM
capacitor dielectric for GaAs technology.
In previous studies, we have developed high capacitance
density MIM capacitors using a thin PECVD Si3N4 as the
dielectric material for GaAs HBT technology.1,2,10 In this
study, we have further characterized the PECVD Si3N4 film
and have investigated the use of ALD Al2O3 and HfO2 as
new option for MIM capacitor dielectric insulator materials
in GaAs HBT technology. This characterization includes
evaluating both the capacitance–voltage (C-V) and current–
voltage (I-V) characteristics of the MIM capacitor. Additionally, the electrical characteristics as a function of
temperature, frequency, and capacitor area have been studied, as the GaAs devices may be operating at high temperatures, high voltages, at different frequencies, and using
capacitors that vary significantly in area within a design and
from design to design. Comparison of these MIM capacitor
electrical characteristics using these three different films
will be discussed and presented. Furthermore, this investigation will discuss the factors and considerations that will
determine which material is best suitable as MIM capacitor
dielectric material for devices and designs manufactured
using GaAs HBT technology.
II. EXPERIMENT
The silicon nitride film was deposited at 300 C in a multistation sequential PECVD system (Novellus Concept-1).
The gases used for the Si3N4 deposition are SiH4, NH3, and
N2. The aluminum oxide and hafnium dioxide films were deposited using a thermal ALD reactor system (Picosun
Advanced SUNALE P-300) at 300 and 230 C, respectively.
The precursors used for the deposition of the ALD Al2O3 are
trimethyl aluminum (TMA), water, and O3, while those used
for deposition of the HfO2 are tetrakisethylmethylamino hafnium (TEMAH), water, and O3. The deposition thickness
target of all three films was 60 nm þ/ 3 nm. The measured
PECVD silicon nitride film thickness was 63 þ/ 2 nm,
while the measured film thickness of ALD aluminum oxide
and hafnium dioxide was 59 þ/ 2 nm and 62 þ/ 2 nm,
respectively. The measured refractive index of the PECVD
J. Vac. Sci. Technol. A, Vol. 31, No. 1, Jan/Feb 2013
silicon nitride was 1.875, while that of ALD aluminum oxide
and hafnium dioxide was 1.654 and 1.973, respectively.
All three films were deposited on 6 in. GaAs wafers. Both
GaAs device and bare GaAs test wafers were used in this
study. The device wafers were fabricated using GaAs heterojunction bipolar transistor technology, which include the
multiepitaxial layers and the backend metal interconnect
layers. Furthermore, these device wafers have metal–insulator–metal capacitors, in addition to the heterojunction bipolar transistor devices, which include the emitter, base, and
collector. The MIM capacitor device on the GaAs HBT wafers
includes the bottom metal electrode, the capacitor dielectric insulator, and the top metal electrode. Both these metal electrodes were deposited using evaporation method. The bottom
metal electrode in this study was fabricated on top of a
PECVD silicon nitride layer, which functions as a surface passivation and isolation layer on the GaAs wafers. This bottom
metal electrode consists of 1 lm thick Au with a thin Ti adhesion layer on top, while the top metal electrode consists of
2 lm thick Au with a thin Ti layer at the bottom. The intermetal dielectric used in this study is polybenzoxazole. The capacitor dielectric films were etched in BHF 10:1 solution to pattern
the MIM capacitor devices. In this study, a FilmTek 2000 reflectometer was used to measure the thickness and refractive
index of the ALD HfO2 and Al2O3 films, while a Rudolph FEVII ellipsometer was used to measure the PECVD Si3N4 film.
Furthermore, focus-ion beam/scanning electron microscopy
(FIB/SEM) analysis using a FEI Nova 600i instrument was
used to evaluate the fabricated MIM capacitor structures,
including the thickness and morphology of the dielectric films.
Electrical characterization was performed by collecting
both current–voltage (I-V) and capacitance–voltage (C-V)
measurements using an Agilent B1500A semiconductor device
analyzer, and using both a manual and automated probe station. The applied voltage for I-V characterization ranges from
0 to 100 V. Both the I-V and C-V measurements were performed on MIM capacitors with different areas, ranging from
100 lm2 to 10 000 lm2, and at different temperatures, ranging
from 25 to 150 C. Capacitance characterization was also performed at different frequencies, ranging from 1 kHz to 1 MHz,
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FIG. 1. Diagram and details of the metal–insulator–metal (MIM) capacitor
on GaAs.
and at different applied voltages, ranging from 5 V to þ5 V.
The accuracy of the capacitance measurements is 0.11%. For
all I-V measurements, the ground voltage is applied to the bottom metal electrode, while the bias is applied to the top metal
electrode. Figure 1 shows the diagram with the details and
configuration of the MIM capacitor used in this study.
III. RESULTS AND DISCUSSION
A. Focus-ion beam/scanning electron microscopy
analysis
Figure 2 shows the FIB/SEM image of the heterojunction
bipolar transistor device with a metal–insulator–metal capacitor structure fabricated on GaAs substrate. The image shows
the emitter, base, and collector of the HBT, with the bottom
metal of the MIM capacitor device connected to the base.
Figures 3(a)–3(c) show the higher magnification FIB/SEM
images of the MIM capacitor with 62 nm of ALD HfO2,
59 nm of ALD Al2O3, and 63 nm of PECVD Si3N4, sandwiched between the top and bottom metal electrodes. It can
be seen that the underlying, evaporated thick bottom metal
electrode of the MIM capacitor has high surface roughness.
However, all three capacitor dielectric insulator films show
good conformality, when deposited on this rough underlying
FIG. 2. FIB/SEM image showing the MIM capacitor connected to the HBT, including the emitter, base, and collector, manufactured using GaAs HBT technology.
JVST A - Vacuum, Surfaces, and Films
01A134-4
FIG. 4. Capacitance density of MIM capacitor with 63 nm of PECVD Si3N4,
59 nm of ALD Al2O3, and 62 nm of ALD HfO2 as capacitor dielectric
insulator.
die size in GaAs technology. In order to evaluate the capacitance characteristics of the dielectric insulator films of the
MIM capacitor, C-V measurements were performed. Figure 4
shows the capacitance density obtained from the MIM capacitor with an area of 4055 lm2 with capacitor dielectric insulator of 63 nm PECVD Si3N4, 59 nm of ALD Al2O3, and 62 nm
ALD HfO2. As can be seen, the PECVD Si3N4 has a capacitance density of 0.92 fF/lm2, while the ALD Al2O3 has a
capacitance density of 1.55 fF/lm2, which is 68% higher
than that of the PECVD Si3N4. The ALD HfO2 has the highest capacitance density (2.67 fF/lm2), which is higher by
190%, compared to PECVD Si3N4.
Figure 5 shows the capacitance of MIM capacitors with the
three capacitor dielectric insulator films, with capacitor areas
ranging from 100 lm2 to 10 000 lm2. As shown, the capacitance of all three films increases linearly with capacitor area,
indicating that the simple capacitor parallel-plate model can be
used to model and approximate the MIM capacitor.39 Based
on this model, the dielectric constant of the PECVD Si3N4 in
this study is calculated to be 6.5, while the dielectric constant
of the ALD Al2O3 and ALD HfO2 is calculated to be 10.3 and
18.7, respectively. Compared to the dielectric constant of
PECVD Si3N4, the dielectric constant of the ALD HfO2 is
higher by 188%, while that of ALD Al2O3 is higher by 59%.
Since most GaAs devices may be operating at high temperatures, it is important to investigate the capacitance
FIG. 3. FIB/SEM images of the MIM capacitor with (a) 59 nm of ALD
Al2O3, (b) 62 nm of ALD HfO2, and (c) 63 nm of PECVD Si3N4, as capacitor dielectric insulator, deposited between the bottom and top metal
electrodes.
metal surface. The good film conformality will lead to more
uniform insulator thickness across the MIM capacitor and
across die, and within wafer and from wafer to wafer, resulting in more uniform capacitance, leakage current, and breakdown characteristics.
B. Capacitance characterization
Metal–insulator–metal capacitors with higher capacitance
density are required in order to shrink the devices and reduce
FIG. 5. Capacitance of MIM capacitors with 63 nm of PECVD Si3N4, 59 nm
of ALD Al2O3, and 62 nm of ALD HfO2, as a function of capacitor area.
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FIG. 6. Capacitance of MIM capacitors with an area of 4055 lm2, and with
63 nm of PECVD Si3N4 capacitor dielectric, as a function of temperature,
with the applied voltage varied from 5 to þ5 V.
FIG. 8. Capacitance of MIM capacitor with an area of 4055 lm2, and with
62 nm of ALD HfO2 capacitor dielectric, as a function of temperature, with
the applied voltage varied from 5 to þ5 V.
characteristics at these high temperatures. The dependence
of the capacitance on the applied voltage and temperature of
the three capacitor dielectrics of the MIM capacitors with an
area of 4055 lm2 can be observed in Figs. 6–8. As can be
seen, the capacitance of the PECVD Si3N4 does not change
significantly when the applied voltage was varied from 5
to þ5 V. Furthermore, there is minimal change in capacitance, if any, when the temperature was increased from 25 to
150 C. Similarly, the ALD Al2O3 does not vary significantly
when the applied voltage was varied in the same voltage
range. However, an increase in temperature from 25 to
150 C results in significant increase in the capacitance from
6.21 1012 F to 6.39 1012 F (or an increase of 2.8%).
The capacitance measurements of the ALD HfO2 show that
there may be some slight dependence on the applied voltage,
where the capacitance vary by <1.0%, when the applied
voltage was varied from 5 to 0 V and from 0 to þ5 V. As
with ALD Al2O3, the MIM capacitor with ALD HfO2 significantly increases in capacitance from 1.137 1011 F to
1.141 1011 F (or an increase of 2.3%), when the temperature is increased from 25 to 150 C.
Figures 9–11 show the capacitance characteristics of the
MIM capacitors with an area of 4055 lm2, when the frequency is increased from 1 kHz to 1 MHz, and the applied
voltage is varied from 5 to þ5 V. The results show that
there is no significant dependence of capacitance on frequency and minimal dependence on applied voltage, if any,
for all three capacitor dielectric films of PECVD Si3N4,
ALD Al2O3, and ALD HfO2. This indicates that the capacitor dielectric and the type of underlying metals and films
used and grown to fabricate the MIM capacitor electrode in
this study are not dependent on frequency within the range
studied.16,17
Current–voltage characterization was performed to evaluate the leakage current and breakdown characteristics of the
MIM capacitors with the PECVD silicon nitride, ALD aluminum oxide, and ALD hafnium dioxide. The measurements
were performed on capacitors with different areas and at different temperatures. Figure 12 shows the I-V curve of the capacitor with an area of 4055 lm2 using the three films, as the
applied voltage is increased from 0 to 100 V. As can be seen,
the capacitor with 63 nm PECVD Si3N4 film results in the
lowest capacitor leakage current and the highest breakdown
voltage (73 V). The capacitor with 59 nm ALD Al2O3 results
in higher leakage current and lower breakdown voltage
(41 V), while the capacitor with 62 nm ALD HfO2 results in
the highest leakage current, and lowest breakdown voltage
(34 V). The nonlinear I-V behavior and different
59 nm of ALD Al2O3 capacitor dielectric, as a function of temperature, with
63 nm of PECVD Si3N4 as capacitor dielectric, as a function of frequency,
with the applied voltage varied from 5 to þ5 V.
C. Leakage current and breakdown characterization
59 nm of ALD Al2O3 capacitor dielectric, as a function of frequency, with
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FIG. 12. I-V characteristics, showing the leakage current and breakdown
voltage of MIM capacitor of 4055 lm2 area, with PECVD Si3N4, ALD
Al2O3, and ALD HfO2 capacitor dielectric.
characteristics in certain voltage ranges of the PECVD Si3N4
and ALD Al2O3 films in this study indicate there may be
multiple carrier conduction processes occurring, when the
voltage is biased, including Schottky, Frenkel–Poole, and
Fowler–Nordheim tunneling emissions.39,40 The ALD HfO2
show a more linear I-V characteristics, possibly suggesting
that there is only one carrier conduction process that is significant or dominant during the bias. The breakdown field of
each film in this study is shown in Fig. 13. The data show
that the PECVD Si3N4 has the highest breakdown field
among the three films, and which is 11.6 MV/cm. The breakdown field of ALD Al2O3 and HfO2 is lower by 39.7% (at
7.0 MV/cm) and 53.4% (at 5.4 MV/cm), respectively, when
compared with that of PECVD Si3N4.
The dependence of capacitor leakage current and breakdown voltage on temperature is critical for MIM applications
in GaAs technology, as in most cases, these GaAs devices
will be operating at high temperatures. Figure 14 shows the
change in leakage current and breakdown voltage of the capacitor with an area of 4055 lm2 using PECVD Si3N4 film,
when the temperature was increased from 25 to 150 C. As
expected, the leakage current increased and the breakdown
voltage decreased with increasing temperature. While at
25 C the breakdown voltage of the PECVD Si3N4 is 73 V,
at 150 C the breakdown voltage decreases to 69 V. Similar
characteristics were also observed with ALD Al2O3 and
ALD HfO2, as shown in Figs. 15 and 16. Significantly higher
leakage current and lower breakdown voltages were obtained
at higher temperatures. The breakdown voltage of ALD
Al2O3 and HfO2 decreased from 41 to 31 V and from 34 to
26 V, respectively, when the temperature was increased from
25 to 150 C.
The data shown in Fig. 14 also show that there is no significant change in the I-V characteristics of the PECVD
Si3N4 film when the temperature was varied from 25 to
150 C. Similar distinct characteristics at different voltage
ranges in these Si3N4 I-V curves were observed, indicating
that the same carrier conduction processes were present and
occurred in this temperature range. This is not the case with
the I-V curves of ALD Al2O3. While at lower temperatures,
there are multiple conduction processes present within the
film, at the high temperature of 150 C, there is only one
dominating conduction process observed, when voltage bias
is applied (Fig. 15). For ALD HfO2, the conduction mechanism is similar for all temperatures evaluated, with only one
dominant process present when bias is applied to the film
(Fig. 16). This dominant conduction mechanism in these
films may be Schottky, Frenkel–Poole, or Fowler–Nordheim
tunneling emission.39,40
62 nm of ALD HfO2 capacitor dielectric, as a function of frequency, with
FIG. 13. Breakdown field of MIM capacitor of 4055 lm2 area, with PECVD
Si3N4, ALD Al2O3, and ALD HfO2 capacitor dielectric.
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FIG. 14. I-V characteristics of MIM capacitor of 4055 lm2 area, with 63 nm
of PECVD Si3N4 capacitor dielectric, as a function of temperature.
FIG. 16. I-V characteristics of MIM capacitors of 4055 lm2 area, with 62 nm
of ALD HfO2 capacitor dielectric, as a function of temperature.
The dependence of capacitor leakage current and breakdown voltage on MIM capacitor area is also important, as
the capacitor area on GaAs circuit designs can vary significantly. Capacitor with larger area will results in higher leakage current, and which may result in earlier capacitor
breakdown. Figure 17 shows the I-V curves of MIM capacitor with PECVD Si3N4, while Figs. 18 and 19 show the I-V
curves of MIM capacitor with ALD Al2O3 and ALD HfO2,
as a function of MIM capacitor area. As can be seen, the
leakage current of all three films significantly increased,
when the capacitor area was increased from 100 lm2 to 10
000 lm2. However, no significant difference was observed in
the breakdown voltage of these films with different capacitor
areas. Furthermore, the data of both PECVD silicon nitride
and ALD aluminum oxide show that as the capacitor area
and leakage current increased, there were distinct voltage
regions observed in the I-V curves. These indicate that different current conduction mechanisms are dominating or
present in these films. These mechanisms may include
Schottky, Frenkel–Poole, and Fowler–Nordheim tunneling
emission processes.39,40 There is no change in I-V characteristics for the ALD hafnium dioxide, where there appears to
be only one or mostly one dominant conduction mechanism
present within the film, as the capacitor area is increased.
D. MIM dielectric for GaAs HBT technology
FIG. 15. I-V characteristics of MIM capacitors of 4055 lm2 area, with 59 nm
of ALD Al2O3 capacitor dielectric, as a function of temperature.
In order to reduce the die size in GaAs HBT technology,
the devices, including MIM capacitor, has to be shrunk. This
MIM capacitor area can be reduced by increasing the capacitance density of the capacitor dielectric insulator used. The
data show that by using ALD hafnium dioxide and ALD aluminum oxide, both of which were deposited at or below the
highest allowable GaAs HBT processing temperature of
300 C, the capacitance density is increased significantly,
compared to using the PECVD Si3N4. A capacitance density
that is higher by 190% and 68% can be achieved by using
62 nm of ALD HfO2 and 59 nm ALD Al2O3, respectively,
when compared to that of the 63 nm PECVD Si3N4.
The breakdown voltage of all three films is higher than
the possible output voltage swing of 20 V of the GaAs heterojunction bipolar transistors, making these films suitable for
GaAs HBT technology. However, the breakdown voltage of
these two ALD films is lower than that of the PECVD Si3N4.
Furthermore, the leakage current of capacitors with both
ALD HfO2 and Al2O3 significantly increases with higher
temperature and with larger capacitor area. While the breakdown voltage and capacitance of the PECVD Si3N4 vary
minimally, if any, when the temperature is increased, the
breakdown voltage of the two ALD films decreases and the
capacitance increases significantly. No or minimal change in
FIG. 17. I-V characteristics of MIM capacitors with 63 nm of PECVD Si3N4
as a function of capacitor area.
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capacitance, and which is inversely proportional to the thickness of the film. Therefore, all these capacitor dielectric films
can each be specifically tailored to meet some or all electrical requirements, including capacitance density, breakdown
voltage, leakage current, and other electrical characteristics
requirement of the GaAs device and circuits. Depending on
the results and specific GaAs device application, one film
may be better and more suitable that the other two films as
MIM capacitor dielectric insulator film for GaAs HBT
technology.
IV. SUMMARY AND CONCLUSIONS
FIG. 18. I-V characteristics of MIM capacitors with 59 nm of ALD Al2O3 as
a function of capacitor area.
capacitance was observed when the frequency was
increased.
Based on the deposition process and electrical characterization results, all three films can be used as MIM capacitor
dielectric in GaAs HBT technology. The selection of the
film depends on what the specific application and electrical
requirements and specifications of the GaAs devices and
designs are. These requirements and specifications include
how high the capacitance density is, how low the leakage
current is, and how high the capacitor breakdown voltage is
of the capacitor dielectric film at a specific condition, in
addition to how the electrical characteristics changes with
different temperatures, different frequencies, and for different MIM capacitor areas.
In order to achieve the required and desired electrical
characteristics of these MIM capacitors, the thickness of the
three capacitor dielectric insulator films can be adjusted and
modified. For instance, the thickness of the PECVD silicon
nitride can be reduced in order to achieve higher capacitance
density. However, this will also result in higher leakage current, in addition to lower breakdown voltage, and which is
proportional to the thickness reduction. Conversely, the
thickness of the ALD hafnium and aluminum oxide films
can be increased in order to obtain higher breakdown voltage
and lower leakage current. However, this will result in lower
ALD HfO2 and Al2O3, and PECVD Si3N4 have been
evaluated and used as MIM capacitor dielectric fabricated
using GaAs HBT technology. The results show that the capacitor with 62 nm of ALD HfO2 resulted in the highest capacitance density of 2.67 fF/lm2, while that with 59 nm of
ALD Al2O3 and 63 nm of PECVD Si3N4 resulted in a capacitance density of 1.55 fF/lm2 and 0.92 fF/lm2, respectively.
The breakdown voltage of the PECVD silicon nitride, however, was measured to be highest at 73 V, as compared to
ALD hafnium dioxide and aluminum oxide, which breakdown voltage was measured to be lower at 34 and 41 V,
respectively. The MIM capacitor with Si3N4 was observed to
have lower leakage current than both with Al2O3 and HfO2.
As the temperature was increased from 25 to 150 C, the
breakdown voltage of all three films decreased, while the
leakage current increased. The capacitance of the ALD
Al2O3 and HfO2 was shown to change, when the applied
voltage was varied from 5 to þ5 V and the temperature
was increased, while no significant change was observed on
the capacitance of the PECVD Si3N4. Furthermore, no significant change in capacitance was observed for all three
films, as the frequency was increased from 1 kHz to 1 MHz.
The results show that the two ALD HfO2 and Al2O3, like
PECVD Si3N4, are suitable as MIM capacitor dielectric in
GaAs HBT technology and can be adjusted to meet the specific application and requirements of the GaAs devices and
designs.
ACKNOWLEDGMENTS
The authors would like to acknowledge Benny Do, Mike
Sun, Mark Banbrook, Cristian Cismaru, and David Tuunanen from Skyworks Solutions, and Jay Sasserath and Frank
Lowry from LabTec for their help in this study.
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Characterization of atomic layer deposition HfO2, Al2O3

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