Characterization of atomic layer deposition HfO ,AlO , and plasma- … · 2017-10-20 · Characterization of atomic layer deposition HfO 2,Al 2O 3, and plasma- enhanced chemical vapor

Characterization of atomic layer deposition HfO2, Al2O3, and plasma-enhanced chemical vapor deposition Si3N4 as metal–insulator–metalcapacitor dielectric for GaAs HBT technology

Jiro Yota,a) Hong Shen, and Ravi RamanathanSkyworks 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

electrical characteristics requirements and application of the GaAs devices. VC 2013 AmericanVacuum 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 con-

sume 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) tech-

nology. Increasing the capacitance density of the MIM capaci-

tors will allow the reduction of capacitor area in these designs,

resulting in die size reduction. Furthermore, the higher capaci-

tance 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 applica-

tions. For instance, in the majority of GaAs power amplifier

designs fabricated using HBT technology, the operating volt-

age is high, and the output voltage swings can be more than

20 V.1–3 Therefore, in addition to high capacitance density, it

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 tempera-

ture. The leakage current of these MIM capacitors at these

conditions can be significantly higher than at normal condi-

tions, which may lead to long term degradation and/or reli-

ability 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 considera-

tion 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 con-

tacts, and to the various epitaxial GaAs layers at high

temperatures.1,2,7–11 For instance, while the maximum tem-

perature 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 Further-

more, the GaAs designs typically require thick metal inter-

connections, 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 deposi-

tion in general, and by evaporation method, in particular,a)Electronic mail: [email protected]

01A134-1 J. Vac. Sci. Technol. A 31(1), Jan/Feb 2013 0734-2101/2013/31(1)/01A134/9/$30.00 VC 2013 American Vacuum Society 01A134-1

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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 capaci-

tors.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. Addition-

ally, it has been shown that capacitance of MIM capacitors

can be affected by frequency of operation, due to the capaci-

tor dielectric itself, and due to the type of underlying metals

and films used and grown to fabricate the capacitor elec-

trode.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 depo-

sition (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 investi-

gated 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 confor-

mality 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 break-

down, 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 materi-

als, however, have mostly higher leakage current and lower

dielectric breakdown field than silicon nitride, with break-

down 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, thin-

ner films can be used in order to obtain high capacitance den-

sity and high capacitor breakdown voltage is not necessarily

required. Furthermore, in most Si-based technologies, the

highest allowable processing temperature can be 400 �C or

higher, which typically results in higher quality deposited ca-

pacitor dielectric films.

In GaAs technology, only few dielectrics are available and

satisfy the electrical requirements to be used as MIM capaci-

tor 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 physi-

cal, 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 micro-

wave 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. Addi-

tionally, the electrical characteristics as a function of

temperature, frequency, and capacitor area have been stud-

ied, as the GaAs devices may be operating at high tempera-

tures, 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 investiga-

tion 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 mul-

tistation 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 de-

posited 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 haf-

nium (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

01A134-2 Yota, Shen, and Ramanathan: Characterization of ALD HfO2, Al2O3, and PECVD Si3N4 01A134-2

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 hetero-

junction bipolar transistor technology, which include the

multiepitaxial layers and the backend metal interconnect

layers. Furthermore, these device wafers have metal–insula-

tor–metal capacitors, in addition to the heterojunction bipo-

lar 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 in-

sulator, and the top metal electrode. Both these metal electro-

des 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 pas-

sivation and isolation layer on the GaAs wafers. This bottom

metal electrode consists of 1 lm thick Au with a thin Ti adhe-

sion layer on top, while the top metal electrode consists of

2 lm thick Au with a thin Ti layer at the bottom. The interme-

tal dielectric used in this study is polybenzoxazole. The capaci-

tor dielectric films were etched in BHF 10:1 solution to pattern

the MIM capacitor devices. In this study, a FilmTek 2000 re-

flectometer was used to measure the thickness and refractive

index of the ALD HfO2 and Al2O3 films, while a Rudolph FE-

VII 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 sta-

tion. The applied voltage for I-V characterization ranges from

0 to 100 V. Both the I-V and C-V measurements were per-

formed 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 per-

formed at different frequencies, ranging from 1 kHz to 1 MHz,

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 bot-

tom 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 microscopyanalysis

Figure 2 shows the FIB/SEM image of the heterojunction

bipolar transistor device with a metal–insulator–metal capac-

itor 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, sand-

wiched 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. 1. Diagram and details of the metal–insulator–metal (MIM) capacitor

on GaAs.

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


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, result-

ing in more uniform capacitance, leakage current, and break-

down characteristics.

B. Capacitance characterization

Metal–insulator–metal capacitors with higher capacitance

density are required in order to shrink the devices and reduce

die size in GaAs technology. In order to evaluate the capaci-

tance 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 capac-

itor with an area of 4055 lm2 with capacitor dielectric insula-

tor of 63 nm PECVD Si3N4, 59 nm of ALD Al2O3, and 62 nm

ALD HfO2. As can be seen, the PECVD Si3N4 has a capaci-

tance 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 high-

est 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 capaci-

tance 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 tem-

peratures, 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 capaci-

tor dielectric insulator, deposited between the bottom and top metal

electrodes.

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.

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.




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 capaci-

tance, 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� 10�12 F to 6.39� 10�12 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 signif-

icantly increases in capacitance from 1.137� 10�11 F to

1.141� 10�11 F (or an increase of 2.3%), when the tempera-

ture 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 fre-

quency 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 fre-

quency 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 capaci-

tor 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

C. Leakage current and breakdown characterization

Current–voltage characterization was performed to evalu-

ate the leakage current and breakdown characteristics of the

MIM capacitors with the PECVD silicon nitride, ALD alu-

minum oxide, and ALD hafnium dioxide. The measurements

were performed on capacitors with different areas and at dif-

ferent temperatures. Figure 12 shows the I-V curve of the ca-

pacitor 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

FIG. 7. Capacitance of MIM capacitor with an area of 4055 lm2, and with

59 nm of ALD Al2O3 capacitor dielectric, as a function of temperature, with

the applied voltage varied from �5 to þ5 V.


62 nm of ALD HfO2 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.

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.




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 sig-

nificant 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 break-

down 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 break-

down 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 ca-

pacitor 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 sig-

nificant 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 mecha-

nism 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


59 nm of ALD Al2O3 capacitor dielectric, as a function of frequency, with



62 nm of ALD HfO2 capacitor dielectric, as a function of frequency, with


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.

FIG. 13. Breakdown field of MIM capacitor of 4055 lm2 area, with PECVD

Si3N4, ALD Al2O3, and ALD HfO2 capacitor dielectric.




The dependence of capacitor leakage current and break-

down voltage on MIM capacitor area is also important, as

the capacitor area on GaAs circuit designs can vary signifi-

cantly. Capacitor with larger area will results in higher leak-

age current, and which may result in earlier capacitor

breakdown. Figure 17 shows the I-V curves of MIM capaci-

tor 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 dif-

ferent 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 character-

istics 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

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 capaci-

tance density of the capacitor dielectric insulator used. The

data show that by using ALD hafnium dioxide and ALD alu-

minum 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 heter-

ojunction 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 break-

down 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. 15. I-V characteristics of MIM capacitors of 4055 lm2 area, with 59 nm

of ALD Al2O3 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.

FIG. 17. I-V characteristics of MIM capacitors with 63 nm of PECVD Si3N4

as a function of capacitor area.

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.




capacitance was observed when the frequency was

increased.

Based on the deposition process and electrical characteri-

zation 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 differ-

ent 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 cur-

rent, 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

capacitance, and which is inversely proportional to the thick-

ness of the film. Therefore, all these capacitor dielectric films

can each be specifically tailored to meet some or all electri-

cal 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

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 ca-

pacitor with 62 nm of ALD HfO2 resulted in the highest ca-

pacitance density of 2.67 fF/lm2, while that with 59 nm of

ALD Al2O3 and 63 nm of PECVD Si3N4 resulted in a capac-

itance density of 1.55 fF/lm2 and 0.92 fF/lm2, respectively.

The breakdown voltage of the PECVD silicon nitride, how-

ever, was measured to be highest at 73 V, as compared to

ALD hafnium dioxide and aluminum oxide, which break-

down 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 sig-

nificant 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 spe-

cific 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 Tuuna-

nen from Skyworks Solutions, and Jay Sasserath and Frank

Lowry from LabTec for their help in this study.

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FIG. 18. I-V characteristics of MIM capacitors with 59 nm of ALD Al2O3 as

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