ECR Assisted Deposition of Tin And Si3N4 Thin Films For Microelectronic Applications

Abstract

The broad theme of the present research investigation is Ion Assisted Deposition of thin films and its effect on the properties of thin films. Though this activity has been of interest to researchers for more than a decade, the development of different types of ion sources with control over the ion flux and energy, makes it a current topic of interest. Ion assisted deposition was successful in depositing thin films of many material with desired qualities, however, there are certain class of materials whose deposition has been rather difficult. This has mainly been attributed to higher energies and low ion flux of conventional ion sources. The advent of ECR ion sources for thin film deposition has given impetus to the deposition of such materials. This is due to the low energy high-density plasma generated in this type of sources. Hitherto, these sources were widely used in PECVD techniques and only recently the importance of ECR sources in PVD techniques has been realized. This thesis is on the development of ECR plasma source for ion assisted deposition of thin films using PVD techniques. This thesis is organized into six chapters.

The first chapter gives an introduction on the ion assisted growth of thin films and the importance of ECR plasma. A detailed discussion on various aspects of ECR sources has been included.

The design details on the development of ECR source have been discussed in the second chapter. The performance of ECR source as analyzed by the Langmuir probe are also discussed. Variation of plasma parameters like ion density, electron temperature, plasma potential and floating potential as a function of pressure and microwave power have been studied using Langmuir probe analysis. An ion density of the order of 1011/cm3 was measured at a distance of 8 cm from the plasma source with a microwave power of 400 watts. This was comparable to the ion density reported in downstream plasma of ECR sources. The behavior of plasma parameters with variation in microwave power and pressure was explained on the basis of microwave transmission above critical ion density and the microwave power absorption. The uniformity of the plasma parameters at the substrate position (29 cm from the ECR source) was found to be ± 2% over a diameter of 12 cm, which makes the ion source suitable for ion assisted deposition.

The third chapter deals with the simulation and experimental study of the ECR sputtering process. ECR sputter type sources are equipped with cylindrical targets. The sputtered flux distribution on the substrate depends on target geometry, sputtering pressure and target-substrate distance. The effect of cylindrical geometry on the distribution of sputtered flux has been simulated by Monte Carlo methods. It is found that the sputtered flux distribution at different pressures and target-substrate distances in ECR sputter type source differs from the conventional glow discharge sputtering system equipped with planar targets. The simulated results are compared with the experimental results. The simulated data agree very well with the experimental data.

The deposition and characterization of the TiN thin films for diffusion barrier applications in copper metallization have been discussed in the fourth chapter. Titanium nitride films are prepared by ECR sputtering. The effect of high density ion bombardment on the morphology, orientation and resistivity of the films was studied. It was observed that films with atomic smoothness could be prepared by ECR sputtering. Also the high density ion bombardment has been found to be effective for the film growth in (100) orientation. The behavior of TiN films deposited by this method as a diffusion barrier in copper metallization has been investigated. The resistivity measurements and RBS depth profile studies showed that up to 700°C there is no diffusion of copper into silicon. This shows that ECR sputtered TiN can be used as an effective diffusion barrier in copper metallization.

The fifth chapter contains investigations on the ECR assisted growth of silicon nitride films. The films are characterized for composition, morphology and chemical bonding using AES, RBS, AFM, XPS and FTIR. AFM studies revealed that ion bombardment results in the reduction of surface roughness, which indicates dense film growth. The effect of ion assistance on the optical and electrical properties is studied in detail. Films prepared with microwave power ranging from 100 to 200 watts are having bandgap and refractive index of 4.9 eV and 1.92 respectively. Interface state density of silicon nitride films prepared in the above mentioned range was found to be 5x10 10 eVcm2. These films exhibited a resistivity of 10 13 Ω, cm and critical field of 4 MV/cm. The electrical conductivity in these films has been explained on the basis of Poole and Frenkel conduction. The low value of interface state density, higher resistivity, and critical field show that good quality SiN4 films can be deposited with low energy high density ECR plasma.

A detailed summary of this research investigation has been discussed in the last chapter. The thesis is concluded with a discussion on the need of focused ECR source to establish ECR assisted deposition as a versatile technique for the growth of thin films.