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Radio frequency-Plasma-enhanced chemical vapor deposition
Vacuum-based thin film deposition techniques can be divided in two main categories: i) depositions based on a physical process such as evaporation and sputtering; and ii) depositions based on chemical processes, known as chemical vapor deposition (CVD). Plasma-enhanced chemical vapor deposition (PECVD) is a sort of CVD. In a CVD process, gas precursors are decomposed and react on a hot substrate via chemical reactions which require quite high temperatures (500-800 ℃) to take place. In the PECVD process, an electrical discharge is used to decompose the precursors, generating reactive species such as radicals or ions which will induce chemical reactions and result into the growth of a film at lower temperature (100-350 ℃). The PECVD technique is therefore low temperature process which enables the growth of films with good adhesion and uniformity properties. The plasma in the PECVD deposition is generally created by a voltage at RF (AC) frequency or DC, creating a discharge between two electrodes. A simple DC discharge can be created at a few torr between two conductive electrodes, and may be suitable for deposition of conductive materials. However, insulating films will quickly extinguish this discharge as they are deposited. The RF-PECVD has an advantage over the DC-PECVD in its capacity of depositing insulating films and to perform depositions on insulating substrates. Plasma deposition of thin films covers a wide range of materials and applications [95][96]. More particularly, PECVD is widely used for the growth of dielectric films like Si3N4 and SiO2 [97] a-Si:H [98]. It can also be used for more exotic depositions like μc-Si:H [99], and c-Si epitaxy [100]. For a-Si:H deposition, the dissociation of silane was first reported by Ogier in 1879 [101]. However, the interest for this material started after hundred years later after discovering the possibility of doping it, by Chittick and Spear and Le Comber [102], [103]. Using RF-PECVD at a 13.56 MHz frequency is today the most common way to deposit a-Si:H thin films and have allowed for uniform a-Si:H deposition on a substrate up to 5.7 m2. A schematic diagram of a PECVD deposition reactor is shown in Fig.2.2. The deposition reactor consists of a gas injection system, a vacuum chamber and of a parallel plate, capacitively coupled plasma (CCP) system; one electrode is RF powered and the other is a grounded and heated substrate holder.
This method is advantageous compared to other CVD deposition techniques because of the following benefits:
-Deposition at low temperature can be done, because the dissociation of gas precursors occurs due to collisions with high-energy electrons.
-Dense and smooth films can be deposited due to the energy brought to the growth zone by accelerated ions present in the plasma.
-Stacks can be easily deposited by varying the gas content and plasma parameters without having to remove the sample outside the reactor between two different film depositions.
Even though a wide choice of gas precursors exists for the a-Si:H deposition (Si2H6, SiF4, SiCl3H, etc…), SiH4 has been and is still the most widely studied.
The plasma formed between the two electrodes is an ionized gas which is macroscopically neutral. A transition region arises between the plasma and the electrodes and it is called sheath. As the energy of the power supply is mostly coupled to electrons, they are the most energetic species in the discharge and would just move out of the plasma if there wasn’t a potential barrier preventing them from leaving the plasma. On the other hand, ions are much less mobile because of their higher mass. They only experience an average potential, whose distribution is shown in Fig.2.3. This potential distribution implies that negatively charged species in the plasma (electrons, negative ions and negatively charged powders) experience a potential barrier which prevents them from leaving the plasma. On the other hand, positively charged ions and particles will be accelerated to the walls, thus leading to the so-called ion bombardment [104].
Matrix Distributed Electron Cyclotron Resonance-PECVD
In this section, we describe the working principle of the Matrix Distributed Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition (MDECR-PECVD). We first review the working principle of the (ECR-PECVD) before introducing the MDECR-PECVD reactor “Venus” that we have used in our experiments.
Electron Cyclotron Resonance-PECVD
In this section, the basic principal of the Electron cyclotron resonance (ECR) is first reviewed. The trajectory of an electron in a uniform static magnetic field of strength B0 without the presence of an electric field is a helicoidal trajectory with a constant radius around the magnetic lines. This cyclotronic motion of the charged particle in the magnetic field is characterized by the radius and the frequency 𝑓𝐿𝑎𝑟𝑚𝑜𝑟 and 𝑟𝐿𝑎𝑟𝑚𝑜𝑟 respectively, given by:
𝒇𝑳𝒂𝒓𝒎𝒐𝒓= 𝒒𝒆𝑩𝟎𝟐𝝅𝒎𝒆 (𝟐.𝟏)
𝒓𝑳𝒂𝒓𝒎𝒐𝒓=𝒎𝒆𝒗𝒆𝒒𝒆𝑩𝟎 (𝟐.𝟐)
Where 𝑞𝑒 is the electron charge, 𝑚𝑒 is the electron mass and 𝑣𝑒 is the electron velocity in the plane perpendicular to the magnetic field.
By supplying microwave power to the plasma, a linearly polarized electric field is superimposed on the magnetic field and will contribute to the electron motion. With its frequency in the microwave range 𝑓𝑀𝑊= 2.45 GHz, a resonance will exist when the electric field frequency is equal to the Larmor frequency: 𝒇𝑳𝒂𝒓𝒎𝒐𝒓=𝒇𝑴𝑾 (𝟐.𝟑)
This means that the motion of the electron in the field created by the magnets is resonant with the electric field generated by the microwaves and that the resonant absorption will take place at the specific value of the magnetic field of 875 Gauss. The electric field will constantly increase the electron velocity, and so its energy. When the electron energy exceeds the excitation, dissociation or ionization threshold of gas molecules, inelastic collision will happen and will result in creation of the reactive species (excited molecules, radicals or ions, respectively). To effectively couple electric field energy from the field to the electrons in resonant regime, the pressure should be low, typically below 10 mTorr to decrease the collision frequency to allow electrons to gain enough energy. On the other hand, at very low pressures, below 1 mTorr, production rate of active species drops as well, so optimal pressure range for ECR-PECVD is considered to be between 1 and 10 mTorr. Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition (ECR-PECVD) is a high plasma density, low-pressure deposition technology used for the deposition of dielectric, functional and optical thin films for different applications. It offers fast growth rates due to high ionization and gas dissociation levels at low pressures. It can also perform film depositions at low temperature (below 100℃) with good quality, due to considerable ion flux of low energy ions, which differentiates it from the RF-CCP. The most common geometry, called divergent ECR (or NTT type) consists in injecting the microwaves by a waveguide through a dielectric window in a cylindrical chamber surrounded by electromagnets. A magnetic confinement is provided by the axial magnetic field and the microwave power is absorbed in the resonant region where the electron cyclotron frequency matches the microwave frequency. The ion energy can be as well independently controlled by biasing the substrate holder. To apply ECR technology for thin film fabrication, several different designs of ECR reactors have been developed [107], including divergent ECR, microwave plasma disk reactors, distributed ECR, integrated ECR and multi-dipolar (or matrix distributed) ECR reactors.
Matrix distributed-ECR PECVD, Venus
The idea of multipolar magnetic field confinement was first proposed by Sadowski in 1967 in a spherical configuration for controlled thermonuclear reactions [108]. Its first realization for the production of large uniform low temperature plasmas is attributed to Limpaecher and MacKenzie in 1972 [109].
MDECR reactors were developed as a combination of multipolar magnetic confinement and microwave plasma in order to overcome the difficulties of DECR [110]. They use microwave antenna applicators equipped with SmCo magnets integrated into the antennas’ ends, which are installed in a matrix configuration, providing multipolar confinement. This reduces the losses of energetic electrons creating zone of uniform intense plasma in front of the wafer. It allows deposition on large surfaces, and can be expanded to larger areas by increasing the number of antennas. [In this thesis] An MDECR reactor, Venus, was used in this PhD. The microwave discharge, at 2.45 GHz, is sustained by a set of sixteen water-cooled linear antennas in a four by four matrix, each carrying on its end a permanent rare-earth magnet, with S and N polarities alternating in rows and columns. To power the microwave antennas, two magnetron generators generating up to 2000 watts each were used, each one supplying eight antennas via waveguide-to-coaxial 8-way splitters. This reactor is shown schematically in Fig.2.4. More information can be found in ref [111].
Table of contents :
Chapter 1: Introduction
1.1 Nanoparticle use in different applications
1.2 NPs for Phtovoltaics
1.2.1 Light Management
1.2.2 NPs for Lithography
1.3 Forming point contacts using different techniques in PV devices
1.3.1 Forming point contacts using lithography in PV devices
1.3.2 Forming point contacts using laser in PV devices
1.3.3 Forming point contacts using NPs in PV devices
1.4 Motivation for nanocontacts in c-Si devices
1.4.1 Rear surface modelling
1.4.2 Impact on solar cell efficiency
1.4.3 Summary and device architectures
1.5 Summary and Thesis Outline
Chapter 2: Experimental techniques
2.1 Deposition techniques
2.1.1 Spin coating
2.1.2 Radio frequency-Plasma-enhanced chemical vapor deposition
2.1.3 Matrix Distributed Electron Cyclotron Resonance-PECVD
2.1.3.1 Electron Cyclotron Resonance-PECVD
2.1.3.2 Matrix distributed-ECR PECVD, Venus
2.1.4 Atomic Layer Deposition (ALD)
2.1.5 Thermal oxidation
2.1.6 Thermal evaporation
2.1.7 RF magnetron sputtering
2.1.8 Reactive Ion Etching (RIE)
2.1.9 Doping
2.1.9.1 Doping by diffusion
2.1.9.2 Doping by ion implantation
2.2 Characterization techniques
2.2.1 Spectroscopic ellipsometry
2.2.2 Secondary electron microscopy (SEM)
2.2.2.1 Challenges using different detectors
2.2.3 Atomic force microscope (AFM)
2.2.3.1 AFM
2.2.3.2 Conductive probe atomic force microscope (CP-AFM)
2.2.4 J-V measurements
2.2.5 Electro-chemical capacitance voltage (ECV) profiling
2.2.6 Dynamic light scattering (DLS)
Chapter 3: Nanoparticle Deposition, Etching and Removal
3.1 Nanoparticle deposition
3.1.1 Method 1: NP deposition by spraying
3.1.2 Method 2: NP deposition by spin coating
3.1.2.1 50 nm PS particles
3.1.2.2 100 nm PS particles
3.1.2.3 Role of substrate surface
3.1.2.4 Summary-NP deposition by spin coating
3.1.3 Method 3: NP deposition by floating transfer technique
3.2 Nanoparticle etching by O2 plasma using MDECR
3.2.1 Effect of plasma power
3.2.2 Role of ions
3.2.3 Evolution of particle shapes
3.2.4 Summary-NP etching
3.3 Nanoparticle Removal
3.3.1 NP removal via solution
3.3.2 NP removal by tape stripping
3.3.3 Summary-NP removal
3.4 Chapter Summary
Chapter 4: Characterization of nanoholes
4.1 Forming nanoholes in thin dielectric layers
4.1.1 Approach A: Covering the NPs with a thin dielectric layer
4.1.1.1 Electrical characterization using CP-AFM
4.1.1.2 CP-AFM – Holes in films on c-Si substrates
4.1.1.3 CP-AFM – Holes in films on coated glass substrates
4.2 Forming nanoholes in thick dielectric layers
4.2.1 Approach B: Using aluminum as a mask
4.2.1.1 ZnO-glass substrates
4.2.1.2 Silicon substrates
4.2.1.3 Summary-Approach B
4.2.2 Approach C: Using a-SiCx:H as a mask
4.2.2.1 HF as wet etchant
4.2.2.2 RCA « Soft » as wet etchant
4.3 a-Si:H test PV devices
4.3.1 PIN Structure
4.3.2 NIP structure
4.3.3 Solar cell Characterization
4.3.4 Summary of a-Si:H Solar Cells
4.4 Chapter Summary
Chapter 5: Localized doped nanocontacts in cSi
5.1 Doping through nanoholes
5.1.1 Doping by diffusion through nanoholes
5.1.1.1 Description of masking layer
5.1.1.2 Experimental description
5.1.1.3 Characterization by SEM and CP-AFM
5.1.1.4 Summary – Diffusion results
5.1.2 Doping by ion implantation through nanoholes
5.1.2.1 Overview and Description of Ion Implantation Process
5.1.2.2 Experimental description
5.1.2.3 Characterization by SEM and CP-AFM
5.1.2.4 Discussion and ECV Measurements
5.1.2.5 Summary – Ion implantation results
5.2 Chapter Summary
Chapter 6: Conclusions and Perspectives
6.1 Conclusions
6.2 Perspectives
Bibliography .
