Film Deposition: Basic Concept
In this technique, a thin film of polysilicon or SiO2 is deposited on a substrate by vacuum or vapor
deposition method.
* The film thickness is typically from 50A to 20,000 A.
* These films provide conducting regions within the device, electrical insulation between the metals
and protection from the environment.
Film Characteristics
1. Should be free from voids.
2. Should have high resistivity.
3. Capable of operating at high temperature.
4. Stress in the film should be controllable in order to prevent cracking.
Film Uses
• This is used for interconnection of components in IC.
• This is used for fabricating resistors and capacitors.
Physical Vapour Deposition
It is a technique which is used to deposit thin films of one atom at a time into various semiconductor
wafers.
Formation of a film on the surface of substrate consisting of following steps: -
1.Converting the condensed phase (solid) into a gaseous or vapor phase.
2.Transporting the gaseous phase for the source to the surface.
3. Condensing the gaseous source on the substrate.
Types of Physical Vapor Deposition
1) Vacuum Deposition
2) Sputtering
1) Vacuum Deposition
* In this, the source and the substrate material to be evaporated are placed in the bell jar.
* The source material is heated by an electrical unit until it vaporized.
* When the source material's vapour pressure exceeds than that existing in the bell jar, the material
vaporizes rapidly.
* The vaporized atoms radiate in all directions and condense on all lower temperature surfaces
including the substrate.
* The vaporized atoms radiate in all the directions and condense on all lower temperature surfaces
including the substrate.
2) Sputtering
In this process, metal atoms are released from a cathode constituting the source material as a result
of high energy bombardment.
These release atoms form a thin film layer on the substrate.
Chemical Vapor Deposition
Chemical Vapor Deposition uses chemicals and chemical reaction to deposit films on the wafer
surface.
The different reactors used for chemical vapor depositions are as follows: -
1. APCVD Atmospheric Pressure
2. LPCVD Low Pressure
3. PECVD Plasma Enhanced
1. APCVD Atmospheric Pressure
This is mainly used for deposition of SiO2.
In this, wafer travel from cassette to cassette on a headed chain track.
Wafer temperature is between 240 to 450 C.
The gases are injected from a shower head above the wafers and the following reaction takes place.
SiH4 + O2 -> SiO2 + 2H2
Advantages: -
• High Uniformity
• Ability to handle large diameter wafers
• High deposition rate.
Disadvantages: -
• Reactor must be cleaned frequently
• Fast gas flows are required.
2. LPCVD (Low Pressure CVD)
This reactor is used to deposit polysilicon, silicon dioxide and silicon nitride.
The reactor consists of a quartz tube heated end and pumped out form the other.
Wafer stand vertically in a quartz holder at a temperature of 900 C.
Advantages: -
Excellent Uniformity
Ability to handle large diameter wafers.
Disadvantages: -
Frequent use of toxic or inflammable gases.
3. Plasma Enhanced CVD
Plasma-enhanced chemical vapor deposition (PECVD) is a process that uses plasma to deposit thin
films of solid material on a substrate from a gas. It's a chemical vapor deposition (CVD) technique that
uses an electric field to create an energetic plasma that activates the CVD reaction.
PECVD is widely used in industry because it can operate at lower temperatures than other CVD
methods, such as low-pressure chemical vapor deposition (LPCVD). PECVD processing temperatures
typically range from 200–400°C, while LPCVD processes range from 425–900°C.
PECVD is used in a variety of applications, including: Microelectronics, Solar cells, OLEDs, and
Diamond-like carbon (DLC) coatings
* It consists of a cylindrical chamber usually glass or aluminum with aluminum plates on the top and
bottom
* Sample substrate lie on the grounded bottom electrode
* A radio frequency voltage applied to the top electrode, creates a glow discharge between the two
plates.
* Gases flow radially through the discharge which are usually introduced at the outer edge and flow
towards the center.
* Resistance heaters or high intensity lamps heat the bottom grounded electrode to a temperature
between 100 and 400 C.
Advantages: -
• Low deposition Temperature
• Excellent Uniformity.
Disadvantages: -
• Capacity is limited especially for large diameter wafers.
• Wafers must be loaded and unloaded individually.
Deposition Process of Polysilicon
* Polysilicon is used as the gate electrode in Mos devices.
* Polysilicon is also used for resistors, conductors and to ensure ohmic contact to shallow (deep)
junctions.
* The polysilicon is deposited by silane between 575 and 650c in a low-pressure reactor.
* The chemical reaction is: SiH4 -> Si + 2H2
Deposition Process of SiO2 : APCVD
Deposition Process of Silicon Nitride
* Silicon nitride is used as a mask for the selective oxidation of silicon.
*Silicon Nitride is chemically deposited by reactor silane and ammonia at atmospheric pressure at
temperature between 700 to 900 C.
3SiH4 + 4NH3 -> Si3N4 + 12H2
