Excerpt
CONTENTS
Lists of Abbreviations and Symbols
Lists of Figures and Tables
1 Introduction
1 Theoretical Part
2 Nickel and Nickel Oxides
2.1 Introduction and Existence
2.2 Material properties of Nickel and Nickel Oxide
2.3 Application in electronic industry
3 Atomic Layer Deposition
3.1 History
3.2 Definition
3.3 Features of thermal-ALD
3.3.1 ALD growth mechanism - an ideal view
3.3.2 ALD growth behaviour
3.3.3 Growth mode
3.3.4 ALD temperature window
3.4 Benefits and limitations
3.5 Precursor properties for thermal-ALD
3.6 ALD & CVD of Nickel - A literature survey
4 Metrology
4.1 Thermal analysis of precursors
4.2 Film and growth characterization
4.2.1 Quartz Crystal Microbalance
4.2.2 Spectroscopic Ellipsometry
4.2.3 X-Ray Photoelectron Spectroscopy
4.2.4 Scanning Electron Microscopy
4.2.5 X-Ray Reflectometry and X-Ray Diffraction
4.2.6 Four Point Probe Technique
5 Rapid Thermal Processing
5.1 Introduction
5.2 Basics of RTP
5.3 Nickel Silicides-A literature survey
II Experimental Part
6 Methodologies
6.1 Experimental setup
6.2 ALD process
6.2.1 ALD process types and substrate setups
6.2.2 Process parameters
6.3 Experimental procedure
6.3.1 Tool preparation
6.3.2 Thermal analysis and ALD experiments from nickel precursors ...
6.3.3 Data acquisition and evaluation
6.3.4 Characterization of film properties
7 Results and discussion
7.1 Introduction
7.2 QCM verification with Aluminum Oxide ALD process
7.3 ALD process from the reference precursor
7.3.1 Introduction
7.3.2 TG analysis for Ni(amd) precursor
7.3.3 Thermal stability test for Ni(amd)
7.3.4 ALD process optimization
7.3.5 Film properties
7.4 Evaluating the novel Nickel precursors
7.4.1 Screening tests for precursor P1
7.4.2 Screening tests for precursor P2
7.4.3 Screening tests for precursor P3
7.4.4 Screening tests for precursor P4
7.4.5 Screening tests for precursor P5
7.5 Comparison of all nickel precursors used in this work
8 Conclusions and outlook
References
III Appendix
A Deposition temperature control & Ellipsometry model
B Gas flow plan
DEDICATION
I would like to dedicate this Master thesis to my lovely wife Maarja Sharma, my parents Varinder Sharma and Veena Sharma as well as little sister Vasudha. They have always supported me during this period. Without their never-ending support and motivation, this thesis would not have been possible.
Explore, and explore.
Be neither chided nor flattered out of your position of perpetual inquiry. Neither dogmatize or accept another's dogmatism.
Ralph Waldo Emerson
LISTS OF ABBREVIATIONS AND SYMBOLS
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LISTS OF FIGURES AND TABLES
3.3.1 Schematic illustration of an ALD cycle
3.3.2 The growth behaviour in Atomic Layer Deposition
3.3.3 Growth per cycle vs deposition temperature
4.2.1 Measuring principle of the Quartz crystal
4.2.2 Schematic illustration of Colnatec™ type RC crystal
4.2.3 The measuring principle of Spectroscopic Ellipsometry
4.2.4 OLS and material models for Al2O3 and nickel-based films
4.2.5 Schematic of photoelectric effect in XPS.
5.3.1 Binary phase diagram of Nickel-Silicon
6.1.1 Experimental setup for an ALD
6.1.2 Inside view of the tube reactor
6.1.3 Schematic of the experimental setup
6.3.1 Evolution of QCM frequency over the ALD process time
6.3.2 Change in the frquency of RC-cut quartz crystal as a function of deposition temperature
6.3.3 The approximate positions for various measurements on the silicon sample
7.2.1 The thickness of AlOx films versus number of ALD cycles
7.3.1 TGA and DTG curves for Ni(amd) precursor
7.3.2 Variation in the Ni(amd) exposure time
7.3.3 Variation in the air exposure time
7.3.4 XRR measurements for the film grown from Ni(amd)/air ALD process. . .
7.3.5 SEM picture of the film grown from Ni(amd)/air process
7.4.1 TGA and DTA curves for precursor P1
7.4.2 Thermal stability tests for precursor P1
7.4.3 Variation in the P1 exposure time
7.4.4 Variation in the H2 exposure time
7.4.5 Variation in the deposition temperature for P1/H2 process
7.4.6 SEM pictures of the films grown from P1/H2 process
7.4.7 XPS analysis of the films grown from P1/H2 process
7.4.8 TGA and DTA curves for precursor P2
7.4.9 Thermal stability tests for precursor P2
7.4.10 Deposition tests from P2/second-reactant process
7.4.11 SEM picture of the film grown from P2/second-reactant process
7.4.12 XPS analysis of the film grown from P2/second-reactant process
7.4.13 TGA and DTA curves for precursor P3
7.4.14 Thermal stability tests for precursor P3
7.4.15 Deposition tests from P3/H2 processes
7.4.16 SEM pictures of the film grown from from P3/H2 process
7.4.17 XPS analysis of the film grown from P3/H2 process
7.4.18 TGA and DTA curves for precursor P4
7.4.19 Thermal stability tests for precursor P4m
7.4.20 TGA and DTG curves for precursor P5
7.4.21 Thermal stability tests for precursor P5
A.1.1 Deposition temperature with respect to its corresponding voltage values .
A.2.1 General oscillator model parameters for NiX optical layer
A.2.2 Optical constants for the parameterized Nix layer
3.1 ALD growth of Ni-based thin-films from various precursors reported in the literature at the end of Nov, 2014. Deposition method indicates plasma or thermal-based ALD
3.2 CVD growth of Ni-based films from various precursors reported in the literature at the end of Nov, 2014
4.1 Origin of peaks in DTA curve [adapted from Wendlandt 1974]
6.1 Comparison of nickel precursors obtained from the manufacturer
6.2 ALD process parameters
7.1 Various properties of the films grown from P1/H2 process
7.2 Comparison among nickel precursors used in this work
1 INTRODUCTION
Nickel and nickel(II) oxide are widely used in advanced electronic devices [NASF 2014]. In microelectronic industry, nickel is used to form nickel silicide. The nickel mono-silicide (NiSi) has emerged as an excellent material of choice for source-drain contact applications below 45 nm node CMOS technology [Doering a. Nishi 2008; Kittl et al. 2005]. As compared to other silicides used for the contact applications, NiSi is preferred because of its low resistivity, low contact resistance, relatively low formation temperature and low silicon consumption [Chamirian et al. 2003]. Nickel is used in nickel-based rechargeable batteries and ferromagnetic random access memories (RAMs) [NASF 2014]. Nickel(II) oxide is utilized as transistor gate-oxide and oxide in resistive RAMs [Venter a. Botha 2011 ].
Atomic Layer Deposition (ALD) is a special type of Chemical Vapor Deposition (CVD) technique, that is used to deposit very smooth as well as homogeneous thin films with excellent conformality even at high aspect ratios [ Knaut et al. 2013]. It is based on selfterminating sequential gas-solid reactions that allow a precise control of film thickness down to few Angstroms. In order to fabricate todays 3D electronic devices, technologies like ALD are required.
In spite of huge number of practical applications of nickel and nickel(II) oxide, a few nickel precursors are available for thermal based ALD [ Knisley et al. 2013, p. 3226 ]. Moreover, these precursors have resulted in poor film qualities and the process properties were also limited [ Knisley et al. 2013 ]. Therefore in this master thesis, the properties of various novel nickel precursors had to be evaluated. All novel precursors are heteroleptic (different types of ligands) complexes and were specially designed by the manufacturer for thermal based ALD of pure nickel with H2 as a co-reactant.
In order to evaluate the novel precursors, a new methodology was designed to test small amounts (down to 2 g) of precursors in a very time efficient way. This methodology includes: TGA/DTA curve analyses of the precursors, thermal stability tests in which the precursors (< 0.1 g) were heated at elevated temperatures in a sealed environment for several hours, deposition experiments, and film characterizations. The depositions were monitored with the help of in situ quartz crystal microbalance, while application related film properties like chemical composition, physical phase, thickness, density, roughness and sheet resistance were investigated with the help of ex situ measurement techniques.
Prior to the evaluation of novel nickel precursors, a benchmark ALD process was developed from the reference nickel precursor (Ni(amd)) and air as a co-reactant. The main goal of developing and optimizing such benchmark ALD process was to extract standard process parameters like second-reactant exposure times, Argon purge times, total process pressure, starting deposition temperature and gas flows. These standard process parameters had to be utilized to shorten the process development task (thus saving precursor consumption) and optimize the sublimation temperature for each novel precursor. The ALD behaviour was checked in terms of growth rate by varying the nickel precursor exposure time, precursor temperature and deposition temperature.
I THEORETICAL PART
2 NICKELAND NICKEL OXIDES
2.1 INTRODUCTION AND EXISTENCE
In 1700s, nickel (Ni) was known as 'kupfernickel' and discovered in Saxony, Germany [ Stimola 2007, p.4]. Ni has atomic number (Z) 28 and holds a position within a group of transition metals in a periodic table. In nature, pure nickel is rarely found [ Sparrow 2005, p. 10 - 11 ]. Ni occurs most often in combination with Iron, Sulphur, Arsenic [Haynes et al. 2013]. Ni is widely employed in many industries like aircraft, automotive, power- electronic, semiconductor, memory, steel, rechargeable battery, glass and mint [ Davis 2000, p.7-13 ].
2.2 MATERIAL PROPERTIES OF NICKEL AND NICKEL OXIDE
Nickel is silvery-white in color, hard, malleable and ductile in nature [ Haynes et al. 2013 ]. It has bulk density of 8.9 g/cm3at R.T with a fcc crystal structure [ Haynes et al. 2013 ]. Ni has a relatively high melting point of 1,453°C [Davis 2000, p.3]. Ni is a very good conductor of heat and electricity [ Cordente et al. 2001, p.567 ] [ Davis 2000 ]. Pure nickel has a bulk resistivity value of 6.99 x 10-8(Ω-cm) at R.T. [ Haynes et al. 2013 ].
Nickel is known to form oxides like nickel monoxide (NiO), nickel dioxide (NiO2) and dinickel trioxide (Ni2O3) [ Haynes et al. 2013 ]. NiO is the most stable of all and is green in color with NaCl-type crystal structure. NiO has density of 6.7 g/cm3at R.T and melting point of 1,955°C [ Haynes et al. 2013 ]. The resistivity value for NiO has been reported in the range of 50 (Ω-cm) - 500 (Ω-cm) [ Chen et al. 2005 ] [Venter a. Botha 2011 ].
At room temperature Ni is not very reactive and does not react readily with most acids and alkalis [Alchin 2014]. Nickel occurs mostly with an oxidation state of +2, but oxidation states from -1 to +4 have also been reported [ Haynes et al. 2013 ]. On the other hand, nickel oxide (NiO) is soluble in acids but insoluble in hot and cold water because the formation of NiO prevents any further oxidation [ Haugsrud 2003 ].
Optical properties of thin nickel and Nickel(II) oxide films are different from bulk material. Ni has a high absorption coefficient in the infra-red regime, and shows less absorption in ultra-violet region because of interband transitions. Pure nickel is a ferromagnetic below its curie-temperature [ Chiaverina a. Lisensky 2014 ].
Nickel(II) oxide is p-type wide-band semiconducting material with a band-gap of 3.8 eV [Venter a. Botha 2011 ]. NiO is transparent in ultra-violet regime and visible in the near-infrared region [Venter a. Botha 2011 ]. It has some anti-ferromagnetic properties [ Ismail et al. 2013 ]. Its dielectric constant (e r) is 10.7 and refractive index is 2.3 in infra-red regime [ Chunli et al. 2010 ].
2.3 APPLICATION IN ELECTRONIC INDUSTRY
Ni is a very important material in electronics [Bradley 2011 ] [Nickel Institute 2014, p.1 ]. In microelectronic industry applications, nickel-silicide due to its low contact resistance and less silicon consumption, has been adopted and integrated to CMOS technology [ Lauwers et al. 2004 ] [ Kittl et al. 2005 ]. It has been utilized for metal gates, schottky barrier source-drain and in nano-rod contact applications [Doering a. Nishi 2008][ Léonard a. Talin 2011 ].
In electronics, many chips utilize Ni/Pd/Au coating for wire bondings and electroless nickel immersion gold (ENIG) as a diffusion barrier [Nickel Institute 2014, p.1 ]. ENIG also permits lead-free solder. ENIG process has made nickel a popular choice for mobile devices and high density interconnect applications [ Lai et al. 2012, p.80] [ Bui et al. 2010, p.305]. In magnetic hard disk drives, Ni coatings are applied to protect the disk surface from corrosion. Nickel ferrite has been used to make read-write heads [Hyie et al. 2014, p.797 ].
The other application of Ni is the development of Ni-metal hydride, nickel-cadmium, fuel cell batteries [Abu-Lebdeh a. Davidson 2013] and the anode material in Li-ion batteries [ NASF 2014] [ Lund et al. 2014].
Nickel(II) oxide is used as a gate-oxide in transistors, an oxide in dynamic random access memory (DRAM) and in making resistive as well as capacitive components [NASF 2014, p.1 ] [Venter a. Botha 2011 ]. Recently nickel(II) oxide has been utilized in resistive random access memory (RRAM), unipolar/bipolar resistance switches [Chunli et al. 2010], high performance super capacitors [Wang et al. 2012] and metal-insulator-metal (MIM) structures [Li et al. 2011 ]. Nickel(II) oxide is used in mobile touchscreens, computer displays, electrochromic display (ECD) devices [Anders Stenman 2013 ] [ Nickel Institute 2014] and p-type transparent conducting films [Venter a. Botha 2011 ].
3 ATOMIC LAYER DEPOSITION
3.1 HISTORY
Prof. V. B. Aleskovskii was first to propose the concept of ALD in his Ph.D. thesis, in 1952 [ Malygin 2013, p.61 ]. In 1964, the principle of ALD was published under the name of 'Molecular Layering' [ Puurunen 2005 ]. Later in mid-1970s, a finnish scientist Dr. Tuomo Suntola developed a real ALD technology for thin-film depositions. Since 1980s, ALD technology was adopted and modified according to industrial needs [ Parsons et al. 2013, p.050818-2 ].
3.2 DEFINITION
Atomic layer deposition is a thin film deposition technique that is based on sequential use of self-terminating surface reactions [ Miikkulainen et al. 2013]. In ALD the gas phase chemical reactions take place on any activated surface. In order to deposit a layer of material and complete the surface reaction, minimum two reactant gases are required. In contrast to a chemical vapor deposition technique, an ALD uses two different gaseous reactant pulses each separated by purging or evacuating pulse.
3.3 FEATURES OF THERMAL-ALD
3.3.1 ALD growth mechanism - an ideal view
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Figure 3.3.1 Schematic illustration of an ALD cycle: a and d denote the first step and the last step in an ALD cycle
The above figure 3.3.1 shows a four - step ALD growth mechanism and is repeated until a desired thickness of material is deposited. Most of the ALD processes are based on binary reaction mechanism where two surface reactions occur in a self - limiting sequential fashion [George 2010] and is described as follows :
1. Exposure of the reactant A: In this step the reactant A is introduced into the chamber. On exposure of reactant A an irreversible chemisorption reaction takes place on the surface of substrate and is shown in figure 3.3.1a. Three main classes of chemisorption mechanisms are ligand exchange, molecule dissociation and association [ Puurunen 2005, p. 121301 - 24 ]. The exposure of reactant A is the first step towards a self-limiting growth mechanism. A self limited growth mechanism is due to the limited availability of active sites on the surface of substrate [George 2010, p.112 ]. For instance, in figure 3.3.1a there are seven active sites available which allow the chemisoprtion of maximum seven molecules of reactant A. The active sites are the sites on the surface of substrate that are terminated by reactive groups like -OH, -H, -SH etc. These reactive groups make a surface chemical reaction possible.
2. Purging or evacuation: After the exposure of reactant A a reaction chamber is purged or evacuated with the help of non-reacting gases like argon as shown in figure 3.3.1b. Inert gas acts as a purging or transport gas. This step is necessary to remove excessive non-reacted precursor molecules as well as the reaction by-products which prevents further incorporation of any contaminations and CVD like effects. A vacuum pump or other mean is required to draw all unnecessary products out of the reaction chamber.
3. Exposure of the co-reactant B (such as O3, H2, NH3) as shown in figure 3.3.1c:During this step a co-reactant is introduced into the reaction chamber that allows to accomplish the surface reaction. This exposure further activates the surface for a chemisorption (or reaction) of the reactant A. The gaseous reaction by-products are formed during this step. A precise amount of activation energy (EA) is required to accomplish above chemical reaction [ George 2010, p.113 ]. This EA can be obtained from thermal, radiation or enhanced by plasma etc.
4. Purging or evacuation: The by-products formed during the previous step are required to be removed from the reaction chamber to prevent any unnecessary surface contaminations as well as CVD like effects. This purging step is quite similar to the step 2 and is shown in figure 3.3.1d [ Miikkulainen et al. 2013, p. 3 ].
This cycle of four steps is called an ALD cycle [ Puurunen 2005, p. 121301 - 25 ][ Sharma 2014, p.9 ]. After a saturated dose of each reactant and purge gas, a monolayer of material is deposited. Ideally, after each ALD reaction cycle a monolayer (ML) covers the activated (i.e. terminated by active sites) surface entirely and is known as the ideal growth per cycle (GPC). The ideal GPC should be constant over the number of ALD cycles. However, practically it is not the case, various phenomenons like: growth inhibition and growth enhancement can lead to different GPC over the number of ALD cycles. Therefore a cumulative GPC is calculated by dividing total thickness (Tthickness) obtained over the total number of ALD cycles (Ncyc|es) and is given by equation 3.1 [ Puurunen 2005, p. 121301 - 38 ].
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The time required to accomplish one ALD cycle (tcycie) can be denoted as 3.2.
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Where trA, tP 1, trB, tP2 are: exposure time of reactant A, purge time after exposure of reactant A, exposure time of reactant B and purge time after exposure of reactant B, respectively.
3.3.2 ALD growth behaviour
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Figure 3.3.2 ALD growth behaviour: (a) linear growth, (b) substrate-enhanced growth, (c) Type 1 substrate-inhibited growth, and (d) Type 2 substrate-inhibited growth [ Puurunen 2005]
Based on the variation of GPC with number of ALD reaction cycles, ALD growth behaviour over the number of ALD cycles can be classified into four categories. In above figure 3.3.2, actual GPC is expected to settle to a constant value after sufficient number of ALD reaction cycles. In linear growth, GPC is constant over the number of reaction cycles and is known as steady state growth regime. Linear growth can occur, e.g. if the number of reactive sites on the surface does not change with the cycle number [Puurunen 2005, p. 121301 -26 ].
In figure 3.3.2 (b) substrate-enhanced growth, the GPC is higher at the beginning (known as transient regime) than the steady state growth regime. Substrate-enhanced growth can occur, if the number of reactive sites on the substrate is higher than on the ALD-grown material [ Puurunen 2005, p. 121301 - 26 ].
Figure 3.3.2 (c) and (d) shows the growth inhibition of Type 1 and Type 2. In Type 1 the GPC is lower at the beginning of the growth than the steady state growth regime. In Type 2 the GPC reaches to a maximum value before settling. Substrate-inhibited growth is caused by a lower number of reactive sites on the substrate than on the ALD grown material. In substrate-inhibited growth of Type 2 island growth further seems to occur [ Puurunen 2005, p. 121301 - 27 ].
3.3.3 Growth mode
Growth mode is defined by the way a material is arranged on the surface during ALD growth [Puurunen 2005, p. 121301 - 27 ]. Several growth modes are possible in ALD. In 2-D growth mode (layer-by-layer, Frank-van der Merwe), new material is deposited on the lowest possible unfilled state of the material layer and one ML covers the substrate completely [ Puurunen a. Vandervorst 2004, p.7688 ] [ Lüth 2010 ].
In island growth (Volmer-Weber growth) mode, new material prefers to deposit on already grown ALD material [ Puurunen a. Vandervorst 2004, p.7688 ].
Random growth mode is based on statistical model, where the probability of new material units to grow on all surface sites is equal. Because of self-terminating reactions, random deposition causes smoother layers in ALD than in continuous deposition processes [ Puurunen 2003, p.328 ] [ Puurunen 2005, p. 121301 - 27 ].
3.3.4 ALD temperature window
For an ALD process, it is very important to define a temperature regime where deposition temperature has almost no influence on GPC [ Miikkulainen et al. 2013 ]. Figure 3.3.3 shows a plot, in which GPC is recorded as a function of deposition temperature. In case of an ideal ALD process the GPC remains constant with variation in deposition temperature, and is marked as b. Al2O3 ALD process from TMA and H2O is one example of nearly ideal ALD process [ George 2010, p.127 ]. Most of ALD processes diverge from this ideal ALD behaviour. Some ALD systems tend to follow non self-limiting behaviour because the surface species may decompose at higher temperatures (marked by c) and shows CVD like effect, while desorption of reactants can lead towards a decreased growth rate marked by e.
At lower deposition temperatures, non-ideal behaviour can still exist, e. g. condensation or physisorption of precursor molecules (marked by a) [Puurunen 2005]. In case of d, decomposition can occur, even at the minimum temperatures required for the surface reactions [George 2010, p.127]. Some ALD processes may never accomplish complete surface reaction. An incomplete surface reactions can be due to limited number of reactive surface groups, insufficient activation energy and steric-hinderance [Hurle 1994]. In case of f, the actual ALD window is not possible and the GPC is dependent on tem-
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Figure 3.3.3 Growth per cycle vs deposition temperature
perature. This can be due to decrease in number of reactive sites on the surface or incomplete reactions at higher temperatures [Puurunen 2005].
3.4 BENEFITS AND LIMITATIONS
ALD has many advantages over other thin film deposition techniques. The main advantage of ALD over CVD, PVD and sputtering is that an ALD offers a good control over film thickness down to few Angstroms. Depositions done with ALD can have better step coverage and side wall coatings. The properties of the films grown by ALD can be reproduced. ALD can allow high density, low defect density, uniformity and homogeneity over large wafers. In some cases, the physical state of films like amorphous and crystallinity can be controlled by varying the process parameters like substrate temperature [NanoTech 2011 ]. ALD requires less reactant flux as compared to CVD. ALD gives a choice to grow heterogeneous, graded index and doped layers of different materials. Due to availability of various precursors in the market, it is possible to grow almost all materials used in semiconductor industry [ Nalwa 2002, p. 116 -119 ] [ Puurunen 2005 ].
Also, in combination of PE-ALD and Flash-ALD, depositions at lower temperature can be achieved especially for substrates like polymers [Henke et al. 2014]. Films grown with the help of ALD can be of low stress due to molecular self-assembly.
The major limitation of ALD is the poor quality of the films due to incorporation of residues (like C, N, H, O etc.) as well as deviation of growth behaviour from an ideal self-limiting reaction mechanism. The residues come predominately from the precursors or other gases. Designing a good precursor for an ideal ALD behavior is very hard, this effects the ALD process the most. Many important materials like Si, Ge, Si3N4, noble metals (Ag), several multi-component oxides, cannot be deposited currently by ALD in a cost-effective way [ Emslie et al. 2013, p. 3283 ] [ Nalwa 2002 ]. Recently to overcome this difficulty, many precursors have been designed to deposit Si, Ge, Si3N4, SiO2 films but still it is required to improve the film qualities [ Miikkulainen et al. 2013 ]. Sometimes the depositions performed by ALD can lead to poor homogeneity over the complete wafer. Low temperature depositions (below 100°C) are very rare for thermal based ALD systems.
For film thickness more than 20 nm, ALD can be more time consuming as compared to other widely used deposition techniques like PVD, CVD and MBE. However, innovative roll-to-roll, spatial and batch ALD processes have been introduced to mitigate low yield and throughput problem [ Maydannik et al. 2014].
3.5 PRECURSOR PROPERTIES FOR THERMAL-ALD
The properties of precursors for thermal ALD (thALD) are different from plasma-based or flash-based ALD systems. As in case of plasma-based ALD, the low reactive precursors can also produce good results but in case of thALD, the precursors should possess the following properties [ Niinisto et al. 2014, p.222 ] [ Dussarrat 2014, p.234 ]:
1. Volatility: they must posses high volatility below 200 °C.
2. Thermal stability: they must not self decompose, neither while delivering nor at deposition temperatures.
3. Reactivity: should be highly reactive towards co-reactants and enough reactive for chemisorption to take place on initially pre-treated substrates.
4. State: preferably liquid, to prevent clogging inside the gas lines and handling problems. Also the impurity levels in precursors should be low.
Recently, a trend has been seen towards the development of heteroleptic (different ligands) precursors for ALD based systems. The heteroleptic precursor design offers higher reactivity, volatility, better thermal stability and low melting point over the ho- moleptic class of precursors [Niinisto et al. 2014, p.224]. They also improve the ALD process window, GPC, step coverage and conformality [Blasco a. Girard 2014]. Sometimes heterolepetic precursor adds an extra bulky ligand that can decrease the GPC by introducing the steric-hinderance [Dussarrat 2014].
3.6 ALD & CVD OF NICKEL - A LITERATURE SURVEY
A brief literature review is devoted to this section, that covers only a small part of the entire work done on nickel-based thin films. According to the previous research done, various precursors for both CVD as well as ALD have been adopted to deposit nickelbased thin films. As a summary, some of the main precursors for ALD and CVD of nickel-based films have been listed in Tables 3.1 and 3.2. In general, nickel metal films are obtained by two different approaches and are as follows [ Utriainen et al. 2000, p.151 ]:
1. Direct method (DM) of metal deposition: In DM, nickel metal was deposited directly from sequential surface based chemical reactions on different substrates.
2. Indirect method (IDM) of depositon for nickel-based thin films: In IDM, no pure metal was deposited but in the form of Ni-X, where X is either nitrogen, oxygen, carbon or silicon atom. In order to obtain pure metal film, a reduction step is required. A reduction of NiX based thin films into pure Ni film can be achieved by hydrogen, ammonia etc. The various deposition methods for IDM approach are, ALD, CVD, MOCVD, DLI-CVD and pulsed-CVD.
However, in the later case, reduction of denser NiO causes the structure deformation and pinholes in the resulting Ni metal-film [Utriainen et al. 2000, p.157]. Carbon and nitrogen were also left inside the film after reduction, that caused increase in the resistivity [Li et al. 2010, p.3061 ].
Recently, two patents have been granted for the development of new nickel-organic precursors forthALD, PEALD and CVD [ Lansalot-Matras et al. 2014] [Winteret al. 2014]. Unfortunately, no publication data on such precursors have been found so far. Also, two heteroleptic precursors Ni(MA)('PAMD)2 and Ni(Me—allyl)(PCAI) have been chosen for nickel-based thin film depositions by thALD and PEALD processes [ Blasco a. Girard 2014]. Recent review articles on nickel-based thin films have been studied and only the most frequently and widely used Ni-precursors have been discussed in this section [ Cloud et al. 2014 ] [ Emslie et al. 2013 ] [ Knisley et al. 2013 ].
Table 3.1 ALD growth of Ni-based thin-films from various precursors reported in the literature at the end of Nov, 2014. Deposition method indicates plasma or thermal-based ALD
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Table 3.2 CVD growth of Ni-based films from various precursors reported in the literature at the end of Nov, 2014.
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