Deposition Lab Report

THIN FILM DEPOSITION

1. INTRODUCTION

CMOS process includes many layers above silicon substrate. These layers can be dielectric, metals or semiconductors. In this chapter different techniques for thin film deposition will be discussed. Thin films and their deposition have several issues. These issues are important when choosing deposited film and deposition methods. One issue is the quality of film. Quality includes composition, contamination level, defects, electrical and mechanical properties. Composition is important because a number of compositions are possible. There should be small level of unwanted contaminants such as halogens, oxygen, metals etc.

A second issue is that of fine and uniform thickness of film across the wafer. Another

Figure 2: Low pressure CVD used for deposition of silicon dioxide and poly silicon

There are other types of CVD techniques but all have the same idea of flushing chemical sources in gaseous form to the surface of hot wafers.

2.1.1. Atmospheric Pressure Chemical Vapor Deposition (APCVD)

This process is not commonly used nowadays but it is necessary to understand the basic concepts of CVD. The steps are:

1. Transport of gas to the wafer

2. Absorption of reactants on the wafer surface

3. Surface reactions

4. Desorption of byproducts from the surface

5. Forced exit of byproducts by diffusion

Source gas depletion occurs along the length of the susceptor. As reacting gases are consumed, their concentration becomes low with distance. The wafer susceptor is tilted as shown in figure 3. This decreases the cross sectional area of the chamber causing gas velocity to increase. Figure 3: wafer susceptor is tilted to increase the speed of the

Physical Vapor Deposition (PVD)

Physical vapor deposition is a method which mainly use physical processes to deposit film (instead of using chemical reactions). It is more flexible method allowing us to deposit almost every kind of material. There are two methods in PVD namely: evaporation and sputtering.

2.2.1. EVAPORATION

A schematic of a simple evaporator is shown in Figure. In this process the source material is heated in a vacuum chamber which has initially been pumped down to less than 10-5 torr. Evaporated atoms from the source condense on the surfaces of the wafers. The heater can be of the resistance type, using a tungsten filament which heats up when current passes through it. More popular for microelectronics use is an e-beam heater, in which a high-energy electron beam is focused onto the source material in a crucible using magnetic fields Since the evaporated atoms are transported through a high-vacuum atmosphere, very little gas-phase scattering occurs. Therefore unlike CVD systems the mean free path is very long and the atoms travel in essentially straight lines all the way from the source to the surface of the