Metallization is the fabrication step in which proper interconnection of circuit elements is made. Aluminum is a popular metal used to interconnect ICs, both to make ohmic contact to the devices and connect these to the bonding pads on the chip's edge. Aluminum adheres well to both silicon and silicon dioxide, can be easily vacuum deposited (since it has a low boiling point), and has high conductivity. In addition to pure aluminum, alloys of aluminum are used to form IC interconnections for different performance-related reasons. For example, small amounts of copper are added to reduce the potential for electromigration effects (in which current applied to the device induces mass transport of the metal). Small amounts of silicon also are added to aluminum metallization to reduce the formation of metal "spikes" that occur over contact holes.
Metal layers are vacuum-deposited onto wafers by one of the following methods:
In this lab, a Denton Vacuum DU-502A Evaporator is used to perform Filament Evaporation.
The Evaporation process can be devided into three steps. 1) The solid aluminum metal must be changed into a gaseous vapor. 2) The gaseous aluminum must be transported to the substrate. 3) The gaseous aluminum must condense onto the substrate.
This process requires high temperature, and low pressure. The chamber of the Denton Vacuum must be "pumped down" in order to achieve the low pressure requirement. The pressure inside of the chamber is determined by the equation below.
Where P is the chamber pressure at time t, Po is the initial pressure, S is the pumping speed, Q is the rate of "outgassing", and V is the volume of the chamber.
The concentration of gaseous aluminum molecules in the vacuum chamber can be determined using:
Knowing the chamber pressure, one can calculate the impingement rate of the gaseous aluminum hitting the substrate, the time required for one monolayer of aluminum to deposit on the wafer surface, and the approximate final thickness of the aluminum on the substrate.
The Impingement Rate is given by:
The time required for one monolayer of aluminum to deposit is given by:
The final thickness of the aluminum is given by:
The evaporation process doesn't produce a uniform layer of aluminum across the substrate. The deposition rate changes as one moves radially from the center of the substrate. This radial dependence of the deposition rate is described by the following equation:
Filament evaporation is accomplished by gradually heating a filament of the metal to be evaporated. This metal may come in one of several different forms: pellets, wire, crystal, etc. Gold, platinum and aluminum are metals typically used. The PMOS process uses aluminum wire.
The metal is placed in a basket. Electrodes are connected to either side of the basket and a high current passed through it, causing the basket to heat. As power (and therefore heat) is increased, the metallic filament partially melts and is eventually vaporized. In this way, atoms of aluminum break free from the filament and deposit onto the wafers. While filament evaporation is the simplest of all metallization approaches, problems of contamination during evaporation preclude its widespread use in IC fabrication.
Flash evaporation uses the principle of thermal-resistance heating to evaporate metals. Like filament evaporation, flash evaporation offers radiation-free coatings. This technique does offer some benefits beyond filament evaporation: contamination-free coatings, speed or good throughput of wafers, and the ability to coat materials or layers that are composite in nature. Flash evaporation is accomplished by passing a continuous supply of the material to be evaporated over a ceramic structure that provides heat. The sources are usually either powder of thin wires. The ceramic "flash" bar is heated between two positively and negatively charged posts, and the metal evaporates as the source material is fed onto the bar.
Electron-beam evaporation works by focusing an intense beam of electrons into a crucible, or "pocket," in the evaporator that contains the aluminum. As the beam is directed into the source area, the aluminum is heated to its melting point, and eventually, evaporation temperature. The benefits or this technique are speed and low contamination, since only the electron beam touches the aluminum source material.
Aluminum sputtering is used commonly in IC metallization processes and is popular because the adhesion of deposited metals is excellent. RF sputtering is done by ionizing inert gas particles in an electric field (producing a gas plasma) and then directing them toward the source or target, where the energy of these gas particles physically dislodges, or "sputters off," atoms of the source material. Sputtering is a versatile tool in that many materials can be deposited by this technique, using not only RF but also DC power sources.