Fabrication and characterization of metal-oxide-semiconductor (MOS) capacitors based on Al2O3-ZrO2 nanolaminatesFabrication and characterization of metal-oxide-semiconductor (MOS) capacitors based on Al2O3-ZrO2 nanolaminates

Superior de Ensenada, Baja California

Maestría en Ciencias

en Nanociencias

Fabrication and characterization of

metal-oxide-semiconductor (MOS) capacitors based on Al2O3-ZrO2

nanolaminates

Thesis

submitted in partial fulfillment of the requirements for the degree of

Master of Science

Presents:

Jorge Adolfo Jurado González

Thesis defended by

Jorge Adolfo Jurado González

and approved by the committee

____________________________

Dr. Hugo Jesús Tiznado Vázquez

Advisor

Dr. Heriberto Márquez Becerra

Dr. Eduardo Antonio Murillo Bracamontes

Dr. Miguel Enrique Martínez Rosas

Dr. Sergio Fuentes Moyado

Postgraduate Coordinator in Nanosciences

Dra. Rufina Hernández Martínez

Director of Postgraduate Studies

Jorge Adolfo Jurado González©2018

Abstract of the thesis presented by Jorge Adolfo Jurado González, as partial requirement to obtain the Master of Nanoscience.

Fabrication and characterization of metal-oxide-semiconductor (MOS) capacitors based on Al2O3-ZrO2

nanolaminates

Abstract approved by:

___________________________________

Dr. Hugo Jesús Tiznado Vázquez Advisor

Today, the research and development of electronic devices involves miniaturization and better performance of these. MOS (Metal - Oxide – Semiconductor) capacitors have used Silicon Oxide (SiO2) for many years as dielectric material, but the need for miniaturization and development of these has taken this material to its limits. Many researchers have proposed as solution to change this material for a material of high k. One of the promising materials to replace SiO2 is Zirconium Oxide (ZrO2), due to its high dielectric constant, thermal stability with Silicon (Si), etc. However, one of the problems of using ZrO2 as a dielectric material is that it allows high leakage currents attributed to crystallization of material. One way to solve this problem is by adding a laminate of Aluminum Oxide (Al2O3, amorphous material) between the film to prevent crystallization. In this project, MOS capacitors were fabricated using nanolaminates of Al2O3 and ZrO2 (AZrA) as dielectric material, synthesized by the plasma enhanced atomic layer deposition technique (PEALD). The gold electrodes were deposited by thermal evaporation. The thickness and bandgap of the nanolaminates were measured by ellipsometry and UV-Vis spectroscopy, obtaining a control of thickness and modulation of the bandgap. The X-ray photoelectron (XPS) spectrum shows the characteristic peaks of Al2O3 and ZrO2, after the erosion of Argon (Ar) decreases the concentration of impurities of carbon (C) attributed to the synthesis technique (PEALD). The thin film of ZrO2 shows a roughness of root mean square (RMS) 1.686 and 0.625 nm for Al2O3 measured by Atomico Force Microscopy (AFM). The X-Ray Diffraction (XRD) spectrum shows cubic crystalline phase for the ZrO2 film and amorphous for the Al2O3 film. For the electrical characterization curves capacitance - voltage (C-V) and current - voltage (I-V) were acquired, obtaining a control of the capacitance, dielectric constant, Equivalent Oxide Thickness (EOT), leakage current and breakdown voltage.

Resumen de la tesis que presenta Jorge Adolfo Jurado González como requisito parcial para la obtención del grado de Maestro en Ciencias en Nanociencias.

Fabrication and characterization of metal-oxide-semiconductor (MOS) capacitors based on Al2O3-ZrO2

nanolaminates

Resumen aprobado por:

___________________________________

Dr. Hugo Jesús Tiznado Vazquez Director de tesis

En la actualidad, la investigación y el desarrollo de dispositivos electrónicos implican la miniaturización y un mejor rendimiento de estos. Los capacitores MOS (Metal - Oxido - Semiconductor) han utilizado óxido de silicio (SiO2) durante muchos años como material dieléctrico, pero la necesidad de miniaturización y desarrollo ha llevado este material a sus límites. Muchos investigadores han propuesto como solución cambiar este material por un material de alta constante dieléctrica (k), uno de los materiales prometedores para reemplazar el SiO2 es el óxido de zirconio (ZrO2), debido a su alta constante dieléctrica, estabilidad térmica con Silicio (Si), etc. Sin embargo, uno de los problemas del uso de ZrO2 como material dieléctrico es que permite altas corrientes de fuga, atribuidas a la cristalización del material. Una forma de resolver este problema es mediante la adición de un laminado de Oxido de Aluminio (Al2O3, material amorfo) entre la película para evitar la cristalización. En este proyecto se fabricaron capacitores MOS utilizando nanolaminados de Al2O3 y ZrO2 (AZrA) como material dieléctrico, sintetizados mediante la técnica de depósito por capa atómica asistida por plasma (PEALD, por sus siglas en ingles). Los electrodos de oro se depositaron por evaporación térmica. El grosor y el ancho de energía prohibida de los nanolaminados se midieron mediante elipsometría y espectroscopía UV-Vis, obteniendo un control del espesor y la modulación de la banda prohibida. El espectro de espectroscopia de fotoelectrones emitidos por rayos X (XPS) muestra los picos característicos de Al2O3 y ZrO2, después de la erosión de argón (Ar) disminuye la concentración de impurezas de carbono (C) atribuida a la técnica de síntesis (PEALD). La película delgada de ZrO2 muestra una rugosidad con valor cuadrático medio (RMS, por sus siglas en ingles) de 1.686 y 0.625 nm para Al2O3, medida por microscopia de fuerza atómica (AFM, por sus siglas en ingles). El espectro de difracción de rayos X (XRD, por sus siglas en ingles) muestra una fase cristalina cúbica para la película de ZrO2 y amorfa para la película de Al2O3. Para la caracterización eléctrica se hicieron curvas de capacitancia - voltaje (C-V) y corriente - voltaje (I-V), obteniendo un control de la capacitancia, constante dieléctrica, espesor de óxido equivalente (EOT), corriente de fuga y voltaje de ruptura.

Dedication

La lista es muy larga, muchas personas me han apoyado durante toda la vida.

A mis padres Jorge Adolfo Jurado Solís, María Irene González Leonardo y mi hermano Javier Saturnino

Jurado González, les debo todo a ellos, los amo.

A mi tata y mi nana Saturnino González Navarro y Cristina Leonardo Vázquez, siempre me han apoyado,

mi ejemplo a seguir, los amo.

A mis tías Patricia González Leonardo y Victoria Maribel González Leonardo, por apoyarme toda la vida y

ser como una mama para mí.

A mis tíos Rene González Leonardo, Paz Monrial Elizalde y Manual Silva por apoyarme, siempre estarán en

mi corazón.

A mis primas Cinthya Biridiana, Cris Estefany y Laura, por ser como mis hermanas, las quiero mucho.

A mi primo Alejandro Silva González, otro hermano, te quiero mucho.

A mis tías Rosa Gonzalez Navarro, Eva Gonzalez Navarro y mi tío Pimenio por ser un gran apoyo familiar,

los quiero mucho

A mi tía Artemisa Iñiguez, por apoyarme por mucho tiempo, la quiero mucho.

A los “Fantasiosos Team”, Alan Calderón, Billy Vega, Daniel Muñoz, Eddie Ojeda, Osualdo Gómez, Rodolfo

Ibarra, por ser mis mejores amigos y creer en mí.

A Dayana Badilla, Nayeli Montoya, Yahaira Delgado y Paulina Peralta, ¡por ser las mejores amigas!

A Rosaura Díaz, siempre serás parte importante de mi vida.

Acknowledgements

A Dios por todo.

Al Dr. Hugo Tiznado porque aparte de ser un excelente director de tesis, es un gran ser humano, un gran

maestro y amigo para toda la vida.

A mi comité de tesis, Dr. Eduardo Murillo, Dr. Heriberto Márquez, Dr. Miguel Martínez, por el apoyo

brindado en este proyecto.

Al Dr. Nicola Radnev Nedev por su apoyo con la caracterización eléctrica y elipsométrica.

Al M.C. David Domínguez por su apoyo con la caracterización XPS, interpretación de resultados y desarrollo

técnico del proyecto.

Al Dr. Javier López por su apoyo en la caracterización elipsométrica e interpretación de resultados

espectroscópicos.

Al Dr. Hugo Borbón por su apoyo en la capacitación del espectrofotómetro y desarrollo técnico del

proyecto.

Al Dr. Eder Lizárraga por el apoyo con la caracterización eléctrica de capacitores MOS.

A la M.C. Eloísa Aparicio por su apoyo con la caracterización XRD.

Al Ing. Israel Gradilla por el apoyo con la síntesis de películas delgadas de Au mediante evaporación

térmica.

A Alejandro Tiznado y Enrique Medina encargados del taller de máquinas y herramientas del CNyN-UNAM,

por haberme enseñado y apoyado en todo lo relacionado con máquinas y herramientas.

A la Ing. Lizeth Álvarez por el apoyo técnico en el desarrollo de este proyecto.

Al M.C. Marcelo Martínez por su apoyo técnico en el desarrollo de este proyecto.

Al Ing. Iván Ramírez por su apoyo en el desarrollo de software para mediciones eléctricas.

A Abril Botello y Gabino de Jesús por su apoyo en la síntesis y caracterización de nanolaminados.

A Ma Xiaoyi por su apoyo con la sesión fotográfica de los diferentes sistemas utilizados en este proyecto.

A los ingenieros: Jorge Vázquez, Oscar Romo, Sergio Castillo, Daniel Muñoz, Frank Romo por apoyarme en

este proyecto.

Al Centro de Nanociencias y Nanotecnología (CNyN) y al Centro de Investigación Científica y Educación

Superior de Ensenada (CICESE) por darme la oportunidad de realizar un posgrado. A mis profesores y

demás personas que laboran en los centros por su apoyo durante mi formación académica.

A la entidad DGAPA-UNAM, proyectos PAPIIT IN110018, IN112117, IA101018 y PAPIME PE100318 y

PE101317, por haber financiado este proyecto.

A la Red Temática de Energía Solar de CONACYT, proyecto 282309, por su apoyo con la beca de movilidad

Table of contents

Abstract in English……….………..………...……...……… ii

Abstract in Spanish..……….……….…….……….…….. iii

Dedication……….……….……… iv

Acknowledgements…….……….………..……….…... v

List of figures……….……….………....…... ix

List of tables……….……….……… xi

Chapter 1. Introduction ... 1

1.1 Background ... 4

1.1.1 MOS Capacitor ... 4

1.1.1.1 Energy bands model ... 4

1.2 Dielectric material ... 6

1.2.1 Zirconium oxide ZrO2 ... 6

1.2.2 Aluminum oxide - Al2O3 ... 7

1.3 Electric properties ... 7

1.3.1 Capacitance – Voltage curve ... 7

1.3.2 Current – Voltage curve ... 10

1.4 Justification ... 13

1.5 Hypothesis ... 14

1.6 Objective ... 14

1.6.1 General objective ... 14

1.6.2 Specific objective ... 14

Chapter 2 Methodology ... 15

2.1 Synthesis ... 15

2.1.1 Synthesis of nanolaminates ... 15

2.1.2 Synthesis of electrodes ... 16

2.1.3 Samples ... 17

2.2 Characterization ... 18

2.2.1 Ellipsometry ... 18

2.2.2 Spectroscopy ... 19

2.2.3 XPS ... 20

2.2.4 XRD ... 21

2.2.6 Electrical characterization ... 23

Chapter 3 Results and discussion ... 24

3.1 Thickness ... 24

3.2 Chemical composition... 26

3.2.1 ZrO2 ... 26

3.2.2 Al2O3 ... 27

3.5 XRD ... 28

3.3 Bandgap ... 29

3.3.2 Absorbance – UV-Vis ... 29

3.4 Roughness ... 32

3.6 Electrical measurement ... 36

3.6.1 I-V ... 36

3.6.2 C-V ... 39

Chapter 4 Conclusions ... 45

List of figures

Figure 1. Number of transistors over the years (Axelsson, 2016). ... 1

Figure 2. MOS capacitor structure (Hu, 2010). ... 2

Figure 3. General structure of nanolaminates, gray substrate, oxide A blue, oxide B red, where A and B are different oxides... 3

Figure 4. Energy band diagrams of the MOS capacitors under different bias conditions: (a) accumulation, (b) flatband, (c) depletion, and (d) inversion (Hehenberger, 2011). ... 4

Figure 5. Relationship between the crystalline phase of ZrO2 and its dielectric constant, (S. K. Kim &Hwang, 2007). ... 6

Figure 6. C-V curve in a MOS capacitor with a p-type substrate (Sulong, Rizman, & Kasim, 2010). ... 8

Figure 7. Design of nanolaminates for capacitors in series. ... 8

Figure 8. I-V curve characteristic of a MOS capacitor. ... 11

Figure 9. Diagram of the structure of the MOS capacitor. ... 15

Figure 10. BENEQ TFS 200 reactor. ... 16

Figure 11. Electrode design in MOS capacitor a) isometric view b) lateral view. ... 16

Figure 12. Gold evaporator JEOL model JEE-400. ... 17

Figure 13. Design of nanolaminates AZrA. ... 18

Figure 14. Vase ellipsometer M-2000, J.A. Woollam. ... 18

Figure 15. Avantes spectrometer UV-Vis. ... 19

Figure 16. XPS system SPECS, CNyN - UNAM ... 20

Figure 18. AFM, Park systems XE-70. ... 22

Figure 19. Electrical measurements station, CNyN-UNAM. ... 23

Figure 20. Thickness comparison (Expected, Ellipsometry, UV-Vis). ... 25

Figure 21. XPS spectra of ZrO2 thin film after erosion of Ar. ... 26

Figure 22. XPS spectra of Al2O3 film. ... 27

Figure 23. XRD spectra of thin films of ZrO2 180 nm, Al2O3 35 nm and ZrO2 35 nm. ... 28

Figure 24. Absorbance spectra of samples AZrA, Al2O3 and ZrO2. ... 29

Figure 25. Comparison of the band gap in the samples Al2O3, ZrO2 and AZrA. ... 30

Figure 26. Calculation of bandgap using the Kubelka-Munk method. ... 31

Figure 27. Comparison of the bandgap in the different samples. ... 31

Figure 28. Comparison of the roughness for the samples ZrO2, AZrA100, AZrA1 and Al2O3. ... 32

Figure 29. Surface roughness from a) ZrO2 and b) AZrA100 c) AZrA1 d) Al2O3 thin films (lateral view). .... 33

Figure 30. Surface roughness from a) ZrO2 and b) AZrA100 c) AZrA1 d) Al2O3 thin films (superior view). . 34

Figure 31. Measurement of electrode thickness by the step method. ... 35

Figure 32. I-V curves from MOS capacitors (Al2O3, AZrA, ZrO2). ... 36

Figure 33. Dependence of the breakdown voltage as a function of the ZrO2 cycles. ... 37

Figure 34. Comparison of breakdown voltage and bandgap in samples. ... 37

Figure 35. Comparison of breakdown voltage and roughness samples. ... 38

Figure 36. C-V curves from MOS capacitors (Al2O3, AZrA, ZrO2). ... 39

Figure 37. Capacitance slope in MOS capacitor. ... 40

Figure 39. Comparison of theoretical and expected total permittivity. ... 42

Figure 40. Comparison of total capacitance vs dielectric constant. ... 43

List of tables

Table 1. Comparison of properties of materials with high dielectric constant. Source: (Huff & Gilmer, 2005),

(Mohsenifar y Shahrokhabadi, 2015) ... 2

Table 2. Design of nanolaminates AZrA. ... 17

Table 3. XRD parameters. ... 21

Table 4. Thickness measurements (Expected, ellipsometry, UV-Vis spectroscopy). ... 24

Table 5. Absorbance peaks and bandgap from AZrA samples. ... 30

Table 6. Roughness from Al2O3 and ZrO2 samples. ... 32

Table 7. Comparison of theoretical and expected total capacitance. ... 41

Table 8. Comparison of theoretical and expected total permittivity. ... 42

Chapter 1. Introduction

In 1965 the co-founder of Intel, Gordon Moore publishes his famous article where he describes

the evolution of the density of transistors in an integrated circuit, predicts that the number of transistors

per chips could quadruple every three years, this prediction today is known as the “Moore’s Law”. Figure

1 shows the temporal evolution of the number of transistors per unit area, a clear example of Moore´s

Law (Colinge, 2007).

Figure 1. Number of transistors over the years (Axelsson, 2016).

A good example of this development is the MOSFET transistor, it´s the most important device for

the Very Large Scale Integration (VLSI) with a production greater than or equal to 107 _{transistors per chip}

(Hori, 1997). The research and development of the MOSFET transistors is based on the capacitor with

Metal-Oxide-Semiconductor (MOS) structure, that´s composed of a stacking of a semiconductor substrate

(commonly Si), a thin film of an oxide that fulfills the function of dielectric (commonly SiO2) and finally an

Figure 2. MOS capacitor structure (Hu, 2010).

Nowadays researchers are working on post-silicon technology, which will dominate the world of

computers after 2020 (Kaku, 2010), in this case in particular there´s a tendency to replace silicon oxide

(SiO2) by dielectric materials with a greater dielectric constant (k), lower leakage currents and increase

the energy of the bandgap (Xuan, Lin, & Ye, 2006). In recent years the development of MOS technology

focuses on dielectric materials such as ZrO2, Al2O3, HfO2 etc. Table 1 shows the band gap, dielectric

constant and crystal structure of some of these materials.

Table 1. Comparison of properties of materials with high dielectric constant. Source: (Huff & Gilmer, 2005),

(Mohsenifar y Shahrokhabadi, 2015)

Material Dielectric constant (k) Bandgap (eV) Crystal structure

SiO2 3.9 8.9 Amorphous

Al2O3 9 8.7 Amorphous

ZrO2 25 5.8 Monoclinic, tetragonal, cubic

HfO2 25 5.7 Monoclinic, tetragonal, cubic

SrTiO3 200 3.3 Perovskite

.

Among the materials listed in Table 1, zirconium oxide (ZrO2) has been postulated as the promising

candidate for the development of future devices, mainly due to its high dielectric constant, good thermal

stability with Si, etc. But one of the main problems when working with ZrO2 is that it allows too high leakage

currents when the material crystallizes.

METAL

OXIDE

SEMICONDUCTOR

Studies show that to solve this problem an intermediate layer of Al2O3 can be added to avoid the

crystallization of ZrO2. One way to combine these two materials is by nanolaminates, which consists of

alternating layers of different materials (Fig. 3).

Figure 3. General structure of nanolaminates, gray substrate, oxide A blue, oxide B red, where A and B are different

oxides.

In this project Metal-Oxide-Semiconductor (MOS) capacitors will be fabricated, investigating the

electrical behavior of the zirconium oxide and aluminum oxide nanolaminates used as dielectric material,

synthetized by plasma enhanced atomic layer deposition technique (PEALD). The electrical properties of

the capacitor will be characterized by capacitance-voltage (C-V) and current-voltage (IV) curves. For the

nanolaminates, it will be determining the crystal structure by X-ray diffraction (XRD), the chemical

composition by X-ray photoelectron spectroscopy (XPS), bandgap by ultraviolet-visible absorption

1.1 Background

1.1.1 MOS Capacitor

MOS capacitors (Fig. 2) are made up of a semiconductor substrate, a thin oxide film (commonly

SiO2) that fulfills the function of dielectric material which has a thickness of approximately 1.5 nanometers,

and finally a metallic material as electrode, commonly called gate when this structure is implemented in a

transitor. When the MOS structure is not polarized, there isn’t presence of charges at the metal-oxide and

oxide-semiconductor interface because the distribution of charge carriers in the semiconductor material

is homogeneous. When voltage is applied to the metal (gate voltage, Vg) and the substrate is connected

to ground, four different modes of operation are obtained: accumulation, flat band, depletion and

inversion.

1.1.1.1 Energy bands model

A good strategy to understand the different modes of operation of the MOS capacitor is to use an

energy bands model, the Fig. 4 show the band diagram of the MOS structure, for the most simple case it

is assumed:

1) No chargues in the oxide.

2) Resistivity of the oxide is infinite.

3) The work function difference between the metal and the semiconductor is zero.

Figure 4. Energy band diagrams of the MOS capacitors under different bias conditions: (a) accumulation, (b) flatband,

The operating conditions depend on the applied voltage V on the metal contact with respect to the

Fermi level of the grounded semiconductor.

 Accumulation The accumulation mode of operation occurs when a lower voltage is applied in

comparison to flat band voltage; the negative charges of the gate attract the semiconductor holes

to the semiconductor-oxide interface. The figure 4 (a) show the energy band diagram for MOS

capacitor in accumulation mode, the MOS capacitor start to store positive charge at the substrate

of surface (red charges in diagram).

 Flat Band The flat band condition refers to the diagram of the semiconductor energy band is flat,

which implies that there are no charges in the semiconductor. We need applicate voltage (VFB)

through the gate to obtain this operation mode. The voltage of the flat band (VFB) can be obtained

when it applies a gate voltage equal to the difference of the working function between the metal

and the semiconductor. The figure 4 (b) show the energy band diagram for MOS in flat band mode.

 Depletion When a higher positive voltage is applied in comparison to the flat band voltage, the

positive gate charges attract the negative charges in the semiconductor, so that the depletion

region increases more as the gate voltage increases. The figure 4 (c) show the energy band diagram

for MOS in depletion mode, the holes in the semiconductor will be repelled down in the substrate

and leave negatively charged fixed acceptor ions behind.

 Inversion When the capacitor is applied an even more positive voltage to the electrode and

negative potential to the substrate, the charges of the substrate are reversed going from type p

to n or vice versa. The figure 4 (d) show the energy band diagram for MOS capacitor in inversion

mode, in conclusion is assumed that in inversion the electrons appears at the surface of the

1.2 Dielectric material

In order to increase the capacitance per unit area, different materials have been investigated to

increase the dielectric constant (k) and the energy of the band gap (Eg) of the already exploited silicon

oxide (SiO2). This material presents a low dielectric constant, therefore a high voltage is needed to obtain

the same operational efficiency compared to structures that incorporate dielectric materials with a high k

(value above 10) (Ding et al., 2014). In addition, SiO2 shows high leakage currents in thin films (Botzakaki

et al., 2013). To solve this problem, in recent years the research and development of MOS technology

focuses on dielectric materials such as ZrO2, Al2O3, etc. (Table 1) to reduce leakage currents and obtain

high capacitance.

1.2.1 Zirconium oxide ZrO

Zirconium oxide has been postulated as the promising candidate for the development of future

devices due mainly to its high dielectric constant, good thermal stability with Si and a good bandgap. In

addition to these properties, its technological importance is greater due to its high melting point (2680 °

C), good resistance to oxidation, high refractive index (2.15-2.18) and low absorption from the near

ultraviolet to the medium infrared. It is known that the dielectric constant of ZrO2 is a function of its

crystalline structure, with average values of 20, 37 and 47 for the monoclinic, cubic and tetragonal phases

respectively (Fig. 6). The ZrO2 films with tetragonal or cubic structure produced by ALD on TiN, presented

a dielectric constant of 40 (Panda & Tseng, 2013).

1.2.2 Aluminum oxide - Al

O

One of the main problems of using ZrO2 is that it allows too high leakage currents when the

material crystallizes; one way to solve this problem is to add an intermediate layer of Al2O3 to prevent the

crystallization of ZrO2. The Al2O3 layer remains amorphous up to ~ 1000 °C (Zhang et al., 2016). Some

researchers have proposed that the Al2O3 film, which has low conductivity and high thermal stability can

be deposited before the high-k film to reduce interfacial effects. Other authors have suggested

post-deposition of the oxide film to protect the high-k film from the diffusion of residual oxygen during the

thermal process. It has been shown that the interface formed between Al2O3 and ZrO2 is

thermodynamically more stable and have a higher packing density than Al2O3 and ZrO2 layers separately,

(Oh, Shin, Park, Ham, y Jeon, 2016).

1.3 Electric properties

In this work, it is calculated the capacitance, dielectric constant, band gap, leakage current,

breakdown voltage and equivalent oxide thickness (EOT) of the MOS capacitors.

1.3.1 Capacitance – Voltage curve

The capacitance-voltage (C-V) curve is the most commonly used for the characterization of MOS

and MOSFET devices due to the ease and accuracy with which the capacitance is measured, the thickness

of the equivalent oxide (EOT), the flat band voltage (VFB), the constant dielectric (k) in other words, the

C-V curve predicts the quality of the MOS capacitor (Kar, 2013). Figure 7 shows the C-C-V characteristic curve

for a MOS capacitor with high and low frequency for a p-type substrate with their respective zones of

Figure 6. C-V curve in a MOS capacitor with a p-type substrate (Sulong, Rizman, & Kasim, 2010).

Capacitance

For the calculation of the capacitance of a nanolaminate (Fig. 8), we will consider the model of capacitors

in series, where each oxide is a capacitor connected in series with another capacitor.

Oxide B

Bilayer

C2

Oxide A C1

Oxide B

Bilayer

C2

Oxide A C1

Oxide B

Bilayer

C2

Oxide A C1

SiO2 Native oxide C3

Si Substrate Csi

Figure 7. Design of nanolaminates for capacitors in series.

For capacitors in series, total capacitance (𝐶𝑇) is given by:

1 𝐶_𝑇= 1 𝐶1+ 1 𝐶2+ 1 𝐶1+ 1

𝐶2+ ⋯ + 1 𝐶1+

1 𝐶3+

1

For N oxide layers A and B, we can use:

1 𝐶𝑇=

𝑁 𝐶1+

𝑁 𝐶2+

1 𝐶1+

1 𝐶3+

1

𝐶𝑆𝑖 (2)

After algebra: 𝐶𝑇 = 1 𝑁(𝐶1+𝐶2) 𝐶1𝐶2 + 1 𝐶1+ 1 𝐶3+ 1 𝐶𝑆𝑖 (3)

Considering only the dielectric material:

𝐶𝑇 = 1 𝑁(𝐶1+𝐶2) 𝐶1𝐶2 + 1 𝐶1+ 1 𝐶3 (4) Dielectric constant

The dielectric constant (𝑘) also called relative permittivity (𝜀𝑟) is a physical measure of the electric

polarizability of a material, (Saraswat, 2006), and it is directly related to the vacuum permittivity(𝜀0) to

obtain the absolute permittivity of a material (𝜀) by:

𝜀 = 𝜀

𝜀

In a capacitor, the dielectric constant is related to the capacitance by:

𝐶 =

𝑡

Where:

𝐶 = 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑎𝑛𝑐𝑒

𝜀0= 8.8541878176 x 10−12F/m

𝐴 = 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑜𝑟 𝑎𝑟𝑒𝑎

Clearing the dielectric constant:

𝜀

= 𝑘 =

𝜀₀𝐴

Equivalent oxide thickness

Equivalent oxide thickness (EOT) determines how much thick (nm) of silicon oxide film (SiO2)

would be required to induce the same effect as the high-k material being used and is defined by:

𝐸𝑂𝑇 = 𝑡ℎ𝑖𝑔ℎ−𝑘( 𝑘_{𝑆𝑖𝑂2}

𝑘ℎ𝑖𝑔ℎ−𝑘) (8)

Where:

𝑡ℎ𝑖𝑔ℎ−𝑘= 𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 ℎ𝑖𝑔ℎ 𝑘 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙

𝑘𝑆𝑖𝑂₂ = 𝐷𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑜𝑓 𝑆𝑖𝑂2= 3.9

𝑘ℎ𝑖𝑔ℎ−𝑘= 𝐷𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑜𝑓 ℎ𝑖𝑔ℎ 𝑘 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙

1.3.2 Current – Voltage curve

One of the major problems of current electronic devices is their electrical integrity (electrical

and/or mechanical breakdown) due mainly to high temperatures and / or high electric fields, causing the

dielectric materials to show conductivity. The MOS capacitor is not an exception to these problems. For

this reason, the characterization of this structure by the current - voltage curve (I - V) is very useful for

predicting the resistance to dielectric breakdown, in addition to obtaining important information about

the dielectric material such as trapped charge and leakage current.

This process consists of applying a voltage ramp to the structure and monitoring voltage-current

as a function of time and temperature (Blanquel, 2003). Figure 9 shows the characteristic curve I-V for a

MOS capacitor with positive voltage, where the notable increase of the current is attributed mainly to the

Figure 8. I-V curve characteristic of a MOS capacitor.

Breakdown voltage

When the MOS capacitor is subjected to extreme conditions such as high electric fields, high

current, high voltage through the gate, it causes the device to suffer a breakdown, changing its electrical

and structural properties, (Jain & Verma, 2013) measured in Volts / meter.

Bandgap

A band gap is the distance between the valence band of electrons and the conduction band,

represents the minimum energy that is required to excite an electron up to a state in the conduction band

where it can participate in conduction, (Hanania, Strenhouse, & Donev, 2015). The optical bandgap can be

𝐸 =ℎ𝑐

𝜆 (9)

Where:

𝐸 = 𝐵𝑎𝑛𝑑𝑔𝑎𝑝 𝑒𝑛𝑒𝑟𝑔𝑦

ℎ = 𝑃𝑙𝑎𝑛𝑘𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 = 6.626𝑥10−34 _𝐽/𝑠

𝑐 = 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 = 2.99𝑥108 _𝑚/𝑠

𝜆 = 𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑝𝑒𝑎𝑘 𝑖𝑛 𝑛𝑚

Another way to calculate the bandgap is from the function of Kubelka-Munk:

𝑓(𝑅∞) =

(1−𝑅_∞)2

2𝑅_∞ = 𝐾

𝑆 (10)

Where:

𝑅 = Absolute reflectance of the sample, which is the incident radiant fraction that is reflected.

𝐾 = Absorption constant.

𝑆 = Dispersion constant.

The spectrum of optical absorption, the bangap value (𝐸𝑔) and the absorption coefficient 𝛼 are related by

the following equation :

𝛼ℎ𝑣 = 𝐶(ℎ𝑣 − 𝐸𝑔)1/2 (11)

Where:

𝛼 = Coefficient of linear absorption of the material.

ℎ𝑣 = Photoelectron energy.

When a material has perfect reflectance, the absorption constant 𝐾 is equal to 2 𝛼, where 𝛼 is the

absorption coefficient of the material. In this case the coefficient 𝑆 is a constant with respect to the

wavelength, therefore the expression used to calculate the value of 𝐸𝑔 is:

(𝐾ℎ𝑣)2 _{= ℎ𝑣 − 𝐸}

𝑔 (12)

Thus, obtaining the value of K from equation 10, and graphing (𝐾ℎ𝑣)2as a function of ℎ𝑣, the value of 𝐸𝑔

is obtained from the intersection of the tangent line and the abscissa axis (ℎ𝑣), (Borbón, 2002) .

1.4 Justification

Currently, the research and development of electronic devices involves scale reduction or

miniaturization, this trend is because the new devices have more functions and at the same time need

more electronic components per unit area with a higher performance. The MOS (Metal - Oxide -

Semiconductor) structure is no exception, in addition to miniaturizing, the development involves

maintaining or increasing the capacitance, decreasing the leakage current and increasing the breakdown

voltage.

Silicon oxide (SiO2) is commonly used as a dielectric material, but once the thickness is reduced to

the nanometer range it shows high leakage currents, which directly depends on the dielectric constant. A

viable way to increase the dielectric constant (k) of the dielectric material in the MOS structure is to replace

this material (SiO2) with another with a higher dielectric constant.

In this way, MOS capacitors could be manufactured with a dielectric material with thicknesses in

the range of nanometers, maintaining the capacitance, increasing the breakdown voltage and decreasing

1.5 Hypothesis

The nanolaminated structure Al2O3/ZrO2/Al2O3 allows to control dielectric constant, bandgap as

well as the breakdown voltage in MOS capacitors.

1.6 Objective

1.6.1 General objective

To design, fabricate and electrically characterize MOS capacitors using nanolaminates

Al2O3/ZrO2/Al2O3 as a dielectric material.

1.6.2 Specific objective

 To synthesize nanolaminates type Al2O3/ZrO2/Al2O3 by PEALD.

 To characterize the nanolaminates type Al2O3/ZrO2/Al2O3.

 To fabricate MOS capacitors.

 To obtain electrical characterization of MOS capacitors by I-V and C-V curves.

 To obtain the capacitance, breakdown voltage, dielectric constant and the equivalent oxide

thickness of the MOS capacitors.

Chapter 2 Methodology

2.1 Synthesis

2.1.1 Synthesis of nanolaminates

Atomic Layer Deposition (ALD) technique is a cyclic technique that allows controlling the growth

of ultra-thin films that adopt the geometry of the substrate, one of its advantages is the possibility of

synthesizing nanolaminates (Leskela, Niinisto, & Ritala, 2014). This technique is generally divided in

thermal ALD and Plasma Enhanced ALD (PEALD), in this project PEALD was used because it allows precise

control of the thickness, homogeneity of the film, low deposition temperature, low concentration of

impurities, etc.

For the MOS structure (Fig. 10) fragments with an area between 1 and 2 cm2 _{of a silicon wafer (Si)} type P with orientation <100> were used, these fulfill the function of substrate and semiconductor material

of the MOS capacitor. For nanolaminates of aluminum oxide (Al2O3) and zirconium oxide (ZrO2) with a

sequence type Al2O3 - ZrO2 - Al2O3 (AZrA), Trimethylaluminium (TMA), 98 % STREM 93-1360, was used as

a precursor of Al and Tetrakis(ethylmethylamino)zirconium(IV) (TEMAZ), 99% STREM 40-1710 as a

precursor of Zr. An ultra-high purity (UHP) oxygen was used as the oxidizing agent and Ar UHP as the carrier

gas.

Au Superior electrode

X bilayer Nanolaminates

Oxide B

Bilayer

Oxide A

SiO

Si Substrate

Au Inferior electrode

A PEALD Beneq model TFS 200 reactor (Fig. 11) located in Unidad de Nanocaracterizacion at the

Centro de Nanociencias y Nanotecnología – UNAM was used, in which AZrA nanolaminates were

synthesized with a working pressure of 1.5 mbar and a temperature of 200 °C in the reactor, the plasma

power was 100 W. The doses of TMA, TEMAZ and Oxygen were 30 ms, 250 ms and 100 ms with a purge of

3s, 1s and 3s respectively. TEMAZ was heated to 90 °C and TMA was maintained at room temperature.

Figure 10. BENEQ TFS 200 reactor.

2.1.2 Synthesis of electrodes

Gold was deposited for the lower and upper electrode using the vacuum thermal evaporation

technique. A JEOL model JEE-400 system (Fig. 12) was used, located in Unidad de Nanocaracterización at

the Centro de Nanociencias y Nanotecnología – UNAM. For the lower electrode, a gold film with a thickness

of approximately 25 nm (see AFM results) covering the entire surface of the face was deposited on the

unpolished side of the Si fragments (Fig. 12 – Green structure). On the opposite side were placed circular

gold electrodes with a diameter of 0.75 mm and an approximate thickness of 25 nm (Fig. 12 - yellow

circular structure). Each of these electrodes is considered the upper contact of the MOS capacitor. The

number of capacitors per sample varies depending on the size of the Si fragment.

a) b)

Figure 12. Gold evaporator JEOL model JEE-400.

2.1.3 Samples

The design of the experiment consists of 13 samples with an AZrA sequence at a total thickness of

approximately 35 nm (Table 2), for which, a constant 15 cycles of Al2O3 and a varying number of ZrO2 cycles

(1 to 100 cycles), (Fig.13). For example, the sample AZrA100 has 100 cycles of ZrO2 for every 15 cycles of

Al2O3 until reaching a thickness of approximately 35 nm, obtaining 3 bilayers and ending with a layer of 15

cycles of Al2O3. A growth per cycle (GPC) of 0.09 Å/cycle for TEMAZ and 1.3 Å /cycle for TMA was

considered.

Table 2. Design of nanolaminates AZrA.

Sample #Cycles ZrO2

Thickness ZrO2

# Cycles Al2O3 Thickness

Thickness ZrO2+Al2O3

Bilayers Nanolaminate Thickness Final Al2O3 Final Thickness

ZrO2 389.0 35 0 0 35 0 35 0 35

AZrA100 100 9 15 2 11 3 33 2 35

AZrA75 75 6.8 15 2 8.8 4 35 2 37

AZrA50 50 4.5 15 2 6.5 5 32.5 2 34.5

AZrA30 30 2.7 15 2 4.7 7 32.9 2 34.9

AZrA25 25 2.3 15 2 4.3 8 34 2 36

AZrA20 20 1.8 15 2 3.8 9 34.2 2 36.2

AZrA10 10 0.9 15 2 2.9 12 34.8 2 36.8

AZrA7 7 0.6 15 2 2.6 13 34.2 2 36.2

AZrA5 5 0.5 15 2 2.5 14 34.3 2 36.3

AZrA3 3 0.3 15 2 2.3 15 34.1 2 36.1

AZrA1 1 0.1 15 2 2.1 16 33.4 2 35.4

Al2O3 15 cycles (2 nm)

ZrO2 100 cycles (9nm)

Al2O3 15 cycles (2 nm)

ZrO2 100 cycles (9nm)

Al2O3 15 cycles (2 nm)

Bilayer ZrO2 100 cycles (9nm)

Al2O3 15 cycles (2 nm)

Total thickness: 35 nm

Figure 13. Design of nanolaminates AZrA.

2.2 Characterization

2.2.1 Ellipsometry

The spectroscopic ellipsometry is based on determining the change of state of polarization of a

collimated beam of polarized monochromatic light when it falls on a material. It allows obtaining

information about the thickness of thin films and optical constants of materials (refractive index and

extinction coefficient). In this project, it was used a Vase ellipsometer M-2000, J.A. Woollam (Fig. 14)

located in Instituto de Ingenería of the Universidad Autónoma de Baja California, at Mexicali campus, to

obtain the optical information of the AZrA nanolaminates.

2.2.2 Spectroscopy

The ultraviolet-visible (UV-Vis) absorption spectrophotometry technique was used with the

spectrometer Avantes, AvaLight-DH-S-BAL, AvaSpec-2048 (Fig. 15) equipment located in the CNyN to

calculate the thickness by reflectance, obtain absorption spectra and calculate the GAP of the

nanolaminates.

2.2.3 XPS

X-ray photoelectron spectroscopy (XPS) allows the analysis of the photoelectrons emitted by the

first thin layer (1-2 nm) of a material, obtaining a characteristic spectrum with information on the atomic

composition of the sample and the oxidation state of the elements (Corzo & Nieves, 1997). In this project

the chemical composition of the thin films of ZrO2 and Al2O3 synthesized by PEALD was verified using an

XPS system of the brand SPECS with PHOIBOS 150 Wal analyzer, with Al Ka anode 1486.7 eV and base

pressure of 1E-9 Tor (Fig. 16).

2.2.4 XRD

X-ray diffraction (XRD) is a characterization technique to determine crystalline structures. It

consists of the interaction of electromagnetic radiation with crystalline solids to produce the characteristic

diffraction of each crystal, obtaining the different crystallographic planes of the material, (Billmeyer, 2004).

In this project, the crystalline structure of the thin films of ZrO2 and Al2O3 was determined using a

Panalytical brand diffractometer, X'pert PRO (Fig. 17) located in the nanocaracterization unit of the

nanoscience and nanotechnology center, the parameters used for the XRD measurement are in Table 3.

Table 3. XRD parameters.

Sample Angle (ω) Step (Δ2ω) Time (s) ZrO2 180 nm 1 0.02 1

Al2O3 35nm 1 0.2 0.5

ZrO2 35 nm 1 0.02 0.5

2.2.5 AFM

Atomic force microscopy is a technique that allows us to look at and measure the surface of a

material with great resolution and accuracy. Very small images of a size of 5 nm can be obtained, showing

only 40 to 50 atoms and very difficult as surfaces of ceramic materials, flexible polymers, human cells or

individual DNA molecules. In this project an AFM Park system-70 (Fig. 18) was used to obtain images and

surface roughness of the thin films.

2.2.6 Electrical characterization

The electrodes of the MOS capacitor were connected to a Keithley 2450 voltage source and to a

Agilent E498A LCR capacitance meter, the different measuring equipment are located in the electrical

measurements station (Fig 19). All instruments are operated using software that allows obtaining the

necessary data to generate the I-V and C-V curves.

Chapter 3 Results and discussion

3.1 Thickness

Table 4 shows the thickness obtained by ellipsometry and reflectance spectroscopy of the different

samples AZrA, Al2O3 and ZrO2. It is observed that the Al2O3 and ZrO2 samples maintain the expected

thickness (35nm) with a growth rate of 0.09 Å/cycle and 1.3 Å/cycle respectively, while the AZrA samples

show an error of ± 3 nm. This could be because the growth rate for ZrO2 is not constant at very few cycles,

more cycles are needed for the nucleation of the film. The thickness measurements obtained by UV-Vis

are similar to those obtained in ellipsometry, but with an error of ± 2 nm. This is probably because the

system used has an error range of 5 nm, in figure 20 the different thicknesses are compared.

Table 4. Thickness measurements (Expected, ellipsometry, UV-Vis spectroscopy).

Thickness (nm)

Sample Expected Ellipsometry UV-Vis

ZrO2 35 35.8 35.8

AZrA 100 35 33.7 36.5

AZrA 75 37 35.3 35.4

AZrA 50 34.5 32.0 35.2

AZrA 30 34.9 32.1 33.5

AZrA 25 36 32.6 34.5

AZrA 20 36.2 32.7 33.1

AZrA 10 36.8 32.8 31.4

AZrA 7 36.19 32.5 31.2

AZrA 5 36.3 32.7 31

AZrA 3 36.05 32.3 29.1

AZrA 1 35.44 31.8 28

Figure 20. Thickness comparison (Expected, Ellipsometry, UV-Vis).

Since the ellipsometry thicknesses values are similar to those expected, it was decided to use these

3.2 Chemical composition

3.2.1 ZrO

Chemical characterization was made by the XPS technique to ZrO2 sample with a thickness of 35

nm, the spectra obtained show the characteristic peaks of Zr (4p, 4s, 3d5/2, 3d3/2, 3p3/2 3p1/2 3s) and O (1s,

KLL) (Fig. 21). The sample surface was sputter argon cleaned (Ar+_{) and shows a low concentration of C,} this is because one of the advantages of the PEALD technique is having a lower concentration of impurities

in the thin film.

Figure 21. XPS spectra of ZrO2 thin film after erosion of Ar.

In the XPS spectrum it is shown that the surface of thin film of ZrO2 contains low concentrations

of impurities (C) (Fig. 21), this is attributed to the PEALD technique, this feature is a good option to avoid

3.2.2 Al

O

The XPS spectrum obtained from the sample Al2O3 with a thickness of 35 nm, shows the

characteristic peaks of Al (2p, 2s) and O (2s, 1s, O KLL) (Fig.22).

3.5 XRD

XRD characterization was performed on 3 samples. The first, is a special ZrO2 sample of 180 nm

that shows the characteristic peaks (220) and (200) (JCPDS Card 00-003-0640) of ZrO2 cubic phase. While

the spectra for the sample of ZrO2 35 nm and Al2O3 35 nm, only shows a characteristic peak of SiO2 (420)

(JCPDS Card 00-003-0271). The absence of peaks may be due to limited sensitivity of the equipment. The

crystallinity of the material is related to the deposition temperature and synthesis method, this can be

verified by AFM, because the crystallinity and the roughness increase proportionally. A clear example is a

thin film of ZrO2 that crystallizes easily shows a roughness of 1.5 nm, while the thin film of Al2O3 that is

amorphous shows a roughness of 0.6 nm. The XRD spectrum obtained at the sample of ZrO2 180 nm

indicates a crystallization in a cubic phase, for the sample ZrO2 35 nm does not show characteristic peaks

of ZrO2, due to the low resolution of the equipment. However, it is possible to conclude that it crystallizes

in cubic phase due to the results obtained from the sample ZrO2 180 nm. As expected, the XRD spectrum

of Al2O3 only shows characteristic peaks of Si and not of Al2O3 because this material remains amorphous

at low deposition temperatures. With the PEALD technique, a deposit of ZrO2 was made at a low

temperature (200 °C), which helped to reduce the crystallinity of the material.

3.3 Bandgap

3.3.2 Absorbance – UV-Vis

The absorption spectra of the thin films of Al2O3, ZrO2 and AZrA show a tendency to increase the

maximum absorption peak as a function of the concentration of ZrO2. The Table 5 shows the maximum

absorption peaks and the optical bandgap calculated with the equation 9 and in figure 24 shows the

comparison of the bandgap of the different samples. It´s observed that the samples ZrO2, AZrA100 and

AZrA75 maintain a similar bandgap of 3.5 eV while, from the sample AZrA50 to the sample AZrA10, it

increases proportionally, finally from the sample AZrA7 to Al2O3 a bandgap of 3.9 eV is maintained. This

shows that the bandgap can be modulated with the nanolaminates AZrA obtaining values between 3 and

4 eV.

Table 5. Absorbance peaks and bandgap from AZrA samples.

Sample Absorbance Peak (nm) Bandgap

ZrO2 350.66 3.5

AZrA100 345.39 3.5

AZrA75 347.73 3.5

AZrA50 337.20 3.6

AZrA30 328.40 3.7

AZrA25 332.51 3.7

AZrA20 323.13 3.8

AZrA10 323.13 3.8

AZrA7 317.84 3.9

AZrA5 320.78 3.8

AZrA3 314.91 3.9

AZrA1 314.32 3.9

Al2O3 321.36 3.8

Another way to calculate the bandgap is accomplished by the Kubelka-Munk method, using

equation 12, the results are shown in figure 25, in which the intersections for the sample of Al2O3 and ZrO2

are shown obtaining a bandgap value from 3.7 eV to 3.9 eV respectfully. Figure 26 shows different bandgap

values for the different samples, obtaining a value between 3.5 eV and 4.0 eV (values similar to the method

used previously), however no trend is shown between the concentration of ZrO2 and the bandgap.

Figure 26. Calculation of bandgap using the Kubelka-Munk method.

.

3.4 Roughness

The roughness was calculated by characterizing AFM of the samples ZrO2, Al2O3, AZrA1 and

AZrA100, all with a thickness of approximately 35 nm. A superficial analysis of 3 zones was made for each

sample. The XEI software was used to calculate the root mean square (RMS) value of the roughness. The

ZrO2 sample has a high roughness (1.6 nm) compared to the thin film of Al2O3 (0.5 nm), most likely because

the thin film of ZrO2 is in a crystalline phase (see XRD section) while the Al2O3 remains amorphous Table 6.

It is known that the roughness can be related to the degree of crystallinity of a given material surface.

Table 6. Roughness from Al2O3 and ZrO2 samples. Zone ZrO2 AZrA100 AZrA1 Al2O3

1 1.5 1.3 0.8 0.5

2 1.7 1.2 0.7 0.5

3 1.7 1.1 0.8 0.7

Average 1.6 1.2 0.7 0.5

a) ZrO2 – RMS 1.5 nm b) AZrA100– RMS 1.2 nm

c) AZrA1– RMS 0.8 nm d) Al2O3 – RMS 0.5 nm

a) ZrO2 – RMS 1.5 nm b) AZrA100– RMS 1.2 nm

c) AZrA1 – RMS 0.8 nm d) Al2O3 – RMS 0.5 nm

3.4.2 Electrode

The results obtained by AFM show a thickness of 23 ± 2 nm (Fig. 31) for the upper electrode of the

MOS capacitor, this is because is difficult to obtain uniform surfaces with the thermal evaporation

technique. However, this thickness is enough for electrical measurements.

3.6 Electrical measurement

3.6.1 I-V

Curves I-V of the different MOS capacitors were obtained (Fig. 37). It is observed that the samples

with higher concentration of ZrO2 show a lower rupture voltage and this increase depending on the

concentration of Al2O3.

Figure 32. I-V curves from MOS capacitors (Al2O3, AZrA, ZrO2).

The figure 38 shows the comparison of breakdown voltages for the different capacitors AZrA,

Al2O3, ZrO2. The sample ZrO2 of 35 nm (0 cycles of Al2O3) shows a breakdown voltage of 3.3 eV, a low

voltage that is attributed to the crystalline phase of ZrO2, while decreasing the number of cycles of ZrO2

and increasing those of Al2O3 is observed the tendency to a higher breakdown voltage because the

nanolaminates tend to be amorphous. Finally, the sample Al2O3 35 nm (0 cycles of ZrO2) shows a

Figure 33. Dependence of the breakdown voltage as a function of the ZrO2 cycles.

Figure 39 shows the comparison between the bandgap and the breakdown voltage of the different

MOS capacitors. The same trend is observed, the high concentration of ZrO2 causes a lower breakdown

voltage and at the same time a low bandgap value, this infers that due to the low value of bandgap it is

easier to obtain a current of electrons between the semiconductor and the conductor, causing a smaller

breakdown voltage. However, while the concentration of Al2O3 increases, the breakdown voltage

increases and at the same time the bandgap, because the Al2O3 bandgap is wider and it is more difficult to

have an electron current.

Figure 35. Comparison of breakdown voltage and roughness samples.

Figure 35 shows the comparison of the breakdown voltage and the roughness in the samples ZrO2,

AZrA100, AZrA 1 and Al2O3. At a higher concentration of ZrO2, the breakdown voltage decreases and the

roughness increases. This can be explained because the roughness is linked to the crystallinity, formation

of defects and trapped charges, causing an electron current between the semiconductor and the

conductor with greater ease, causing a lower breakdown voltage. If the concentration of Al2O3 increases,

the breakdown voltage increases and the roughness of the material decreases, attributed to the

3.6.2 C-V

Figure 36. C-V curves from MOS capacitors (Al2O3, AZrA, ZrO2).

For the theoretical values of total capacitance (TC, Table 7) , total permittivity (Tk, Table 8) and the

equivalent oxide thickness (EOT, table 9), equation 6, 7 and 8 was used respectively. The value of 453.68

Figure 37. Capacitance slope in MOS capacitor.

Figure 37 shows the slope value of the depletion zone of the C-V curves of MOS capacitors. It can

be observed that the samples with higher concentration of ZrO2 show a higher slope value, this decreases

and stabilizes from the sample AZrA25 to the sample AZrA 7, finally the value of the slope decreases again

until sample AZrA1. This indicates that it is possible to control the depletion zone of the MOS capacitors

Table 7. Comparison of theoretical and expected total capacitance.

Total Capacitance

Sample Theoretical (nF) Experimental (nF)

ZrO2 1.9 1.87

AZrA100 1.5 1.53

AZrA75 1.38 1.42

AZrA50 1.36 1.41

AZrA30 1.24 1.28

AZrA25 1.17 1.22

AZrA20 1.12 1.17

AZrA10 0.99 1.04

AZrA7 0.96 1.00

AZrA5 0.93 0.96

AZrA3 0.90 0.93

AZrA1 0.88 0.91

Al2O3 0.87 0.86

To calculate the total experimental capacitance, the thickness obtained by ellipsometry (Table 8)

and the aforementioned data were used.

Table 8. Comparison of theoretical and expected total permittivity.

Total permittivity Sample

SiO2, k= 3.9, 2.8nm Theoretical total permittivity Experimental total permittivity

ZrO2 16.57 16.69

AZrA100 13.05 12.82

AZrA75 12.74 12.45

AZrA50 11.71 11.24

AZrA30 10.75 10.24

AZrA25 10.49 9.89

AZrA20 10.10 9.50

AZrA10 9.12 8.47

AZrA7 8.72 8.12

AZrA5 8.45 7.87

AZrA3 8.14 7.55

AZrA1 7.79 7.22

Al2O3 7.61 7.61

The total dielectric constant and the total capacitance of the MOS capacitors are directly related

to the number of ZrO2 cycles contained in the sample, these increase proportionally (Fig. 37). There is also

a tendency to increase the leakage current if the concentration of ZrO2 in the sample increases and

consequently the breakdown voltage decreases.

Table 9. Comparison of theoretical and expected total EOT.

Total EOT

Sample

SiO2, k= 3.9, 2.8nm

Theoretical EOT (nm)

Experimental EOT (nm)

ZrO2 8.24 8.36

AZrA100 10.5 10.3

AZrA75 11.3 11.1

AZrA50 11.5 11.1

AZrA30 12.7 12.2

AZrA25 13.4 12.9

AZrA20 14.0 13.4

AZrA10 15.7 15.1

AZrA7 16.2 15.6

AZrA5 16.8 16.2

AZrA3 17.3 16.7

AZrA1 17.7 17.2

Al2O3 17.9 18.0

Chapter 4 Conclusions

It was possible design, fabricate and characterize electrically MOS capacitors using nanolaminates type

Al2O3/ZrO2/Al2O3 as dielectric material. At the same time, the nanolaminate allowed to control the

dielectric constant, the bandgap and the leakage current in MOS capacitors. Nanolaminates were

synthesized by PEALD technique, controlling the thickness, which was measured by ellipsometry and UV

Vis. The chemical composition of the thin films of Al2O3 and ZrO2 was checked by XPS, also absorbance

spectra were obtained by UV-Vis to measure the bandgap in the nanolaminates. The results of AFM

indicate a greater roughness as a function of the concentration of ZrO2 in the sample. In particular:

 The PEALD technique allows synthesizing thin films with lower impurities (C).

 The thickness of the thin films demonstrating different degree of error but the same thickness

tendency.

 The thin film of ZrO2 has the highest roughness with an average RMS of 1.686 nm.

 The thin film of Al2O3 has the lowest roughness with an average RMS of 0.625 nm.

 The crystallinity of ZrO2 increases in comparison with the roughness of the film.

 Gold electrodes were fabricated by thermal evaporation with an average thickness of 25 nm.

 The thin films of ZrO2 synthesized by PEALD at 200 °C show a crystallization in cubic phase.

 MOS capacitors were fabricated using AZrA nanolaminates as a dielectric material.

 The total experimental capacitance of the MOS capacitors is similar to the theoretical total

capacitance, these increases in proportion with the concentration of ZrO2 in the sample. With a

maximum total capacitance of 1.69 nF and a minimum total capacitance of 0.7 nF.

 The theoretical total permittivity is similar to the experimental total permittivity, these increases

as a function of the concentration of ZrO2 in the sample. With a maximum of 15.08 and a minimum

of 6.68.

 The EOT decreases as a function of the greater concentration of ZrO2 in the sample.

 It’s possible to control the breakdown voltage of the MOS capacitors with the AZrA nanolaminates,

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