Process & Analysis Techniques

Chapter 3 : Process & Analysis Techniques

3.1 Introduction

This chapter describes in detail the fabrication processes, PECVD and ion implantation, used for the preparation of AI2O3, Er3+-doped AI2O3 and Er^+ZYb^^ co-implanted AI2O3 thin films. It also gives a detailed description of the analysis techniques used for the characterisation of the deposited thin films in terms of thickness, refractive index, surface com position, w ater content, surface structure, surface roughness and rare-earth concentration. Finally, it describes the experimental set-ups used for the absorption, photolumineseenee at 1.53pm and fluorescence lifetime measurements.

Chem ical vapour deposition (CVD) is used throughout the electronics industry for depositing semiconducting and insulating films. In common with physical vapour deposition processes, such as evaporation and sputtering, a feature of CVD processes is the chemical decomposition or reaction of a precursor gas or gases to produce reactive intermediates for film growth. Many varieties have been developed, exploiting different techniques for the production of intermediates. For example, the precursor gas can be decomposed by pyrolysis in thermally enhanced processes, such as metalorganic CVD (MOCVD) [1]. Closely related is a variety of laser induced CVD processes in which a precursor gas is heated by the absorption of infrared photons. Photo-CVD processes have also been employed to form a wide range of film materials.

In 1962 Anderson and eo-workers suggested that radio-frequency (RE) discharges could be used to create reactive species for the deposition of thin films, and thus plasm a enhanced CVD (PECVD) for optoelectronics was very soon reported [2, 3]. Recently PECVD has become increasingly important because understanding and process control have improved and because of the rapid development of requirements for new and better m aterials. PECVD is em ployed mainly in sem iconductor processing allow ing the synthesis of thin film semiconductors at moderate process temperatures (200-300°C). PECVD has also been extensively used in telecommunications for the fabrication of optical fibres and integrated optical structures. Organic films deposited by RF discharge have also been used as optical and biocompatible coatings, thin film capacitors, laser light guides in optoelectronic devices and other microelectronic applications [2].

Chapter 3: Process & Analysis Techniques

Ion implantation is another fabrication technique which together with PECVD was used for the preparation of rare-earth doped AI2O3 thin films during this research work. Ion implantation is the major technology for the introduction of impurities into solids in a uniform and reliable way. It offers accurate control of dopant composition and structural modification at any selected temperature. Its primary application is in the semiconductor industry w here it is usually the preferred choice for the electrical doping of sem iconductors [4-6]. Applications of ion im plantation have gained considerable momentum; this is mainly due to the relative ease with which one can fabricate optical w aveguides and waveguide lasers and tailor the properties of key m aterials for optoelectronics.

3.2 Plasma-Enhanced Chemical Vapour Deposition (PECVD)

3.2.1 Plasmas

The primary role of the plasma is to produce chemically active species that subsequently react via conventional pathways. Plasma is the fourth state of matter. It consists of positive ions and negative electrons in a "sea" of neutral atoms. The essential mechanisms in the plasma are ionisation and recombination; excitation and relaxation. Since these processes are always taking place in pairs, the space occupied by the plasm a remains charge neutral. The gas phase environment of a glow discharge contains electrons, various types of ions, neutral atoms, molecules and photons. Electrons and ions are characterised by their number densities ( and n- respectively) and their energies ( KT^

and KT- respectively). Since electrons are lighter than ions and neutrals, they will travel faster in the plasma environment and will be responsible for most of the energy exchange processes [7]. To maintain the discharge, the ionisation must equal recombination; this is achieved by the supply of energy. This is realised by the application of an RF voltage across the two electrodes between which the plasma is longitudinally confined. The gas must be at sufficiently low pressure for the species to be able to exchange energy via collision processes.

If a substrate is suspended in a plasma then initially it will be struck by electrons and ions with current densities given by [7]

Chapter 3: Process & Analysis Techniques

where, e is the electronic charge and and c- are the electron and ion mean speeds respectively. Because is much larger than c-, and the substrate will start immediately to build a negative charge and thus a negative potential with respect to the plasma. As a result electrons are repelled and ions are attracted. The substrate will continue to charge negatively until the electron flux is reduced by repulsion just enough to balance the ion flux. Since electrons are repelled the substrate will acquire a positive charge around it in the form of a sheath. The plasma is equipotential, except around perturbations, at a voltage known as the plasma potential. The isolated substrate can be similarly associated with a floating potential V j. Since Vj is such as to repel electrons, . In the absence of a reference only the difference is meaningful [7]. The glow intensity of the glow discharge depends on the electron number density. So, since the electron number density is lower at the sheath, it won't glow much and is noticeable as a dark space around the substrate.

If the potential in the plasma is perturbed then the plasma acts to oppose that change. The Debye length, is given by [7]

Àd = 2

n..e (3.3)

where, is the permittivity of free space, k is the Boltzmann's constant, T, is the temperature, is the electron density and e is the electronic charge. has dimensions of a length and determines how rapidly the potential perturbation is attenuated in the plasm a. So, the plasm a attenuates perturbations by forming a sheath, leaving the undisturbed region (the plasma) equipotential [7]. A more detailed analysis can be found in [7].

3.2.2 The PECVD Process And Thin Film Formation

In PECVD the plasm a is capacitatively coupled by using parallel plate electrodes separated by 4 -10cm and is used to break up inert precursor gases into reactive species. These then are accelerated by an electric field and deposited on a substrate where further reactions take place to form a thin film. Composition and optical properties can be accurately controlled and it is possible to produce novel materials which it would be difficult to create by conventional melt-glass techniques.

Chapter 3: Process & Analysis Techniques

When the plasm a proeess starts, energy from the electrie field is coupled into the gas almost entirely via the kinetic energy of a few free electrons. The electrons then acquire energy rapidly from the field and lose it slowly to elastic collisions. Soon, the electrons are capable of ionising or dissociating the precursors gas molecules and thus produce secondary electrons by electron impact reactions. The process avalanches and the discharge begins. Electrons are lost to the electrodes and walls by recombination or attachm ent reactions. To maintain a stable plasma proeess, the number of electrons generated and lost should be the same. The plasma stability is a function of the pressure. If the pressure is less than lOOmTorr then the free paths of electrons and gas molecules are too large and so the collision probability decreases, lowering the dissociation and ionisation of gas molecules. On the other hand, if the pressure is higher that 5Torr then collisions become too frequent, especially in a parallel plate reactor. As a result the uniformity of the thin films in terms of both thickness and composition decreases [2]. The properties of the deposited thin films are strongly dependent on the process parameters; RF power (10W-200W), substrate temperature (200°C-350°C), precursor gases flow rates, pressure during deposition ( 1 OOmTorr-2Torr), RF frequency, electrode separation. The deposition mechanism of the PFCVD process can be divided in the following steps: (i) The primary electrons react with the precursor gases to produce a mixture of ions and free radical reactive species, (ii) The reactive species are transported by the plasm a to the substrate surface in parallel with secondary inelastic and elastic reactions, (iii) The reactive species react, or are absorbed onto the substrate surface, (iv) The reactive species either incorporate in the growing film or re-emit from the surface back into the gas plasma [2].

The above steps which lead to the formation of a thin film are shown in Figure 3.1. At the beginning single atoms arrive at the substrate. These will either migrate across the substrate or re-evaporate back into the plasma. The atoms that migrate on the substrate surface will collide and eventually combine with other atoms creating doublets, triplets, quadruplets and so on. This stage, which is called nucléation stage, leads to the formation o f islands (Figure 3.1) each containing hundreds of atoms. During the next stage the islands grow in size, rather than in number, until they touch. This stage is called coalescence stage. Coalescence proceeds until the film reaches continuity [7].

Chapter 3: Process & Analysis Techniques