1. IC AND NEMS/MEMS PROCESSES

3. MATERIALS FOR NEMS/MEMS
Engineering Materials :
1) Metals.
2) Semiconductors.
3) Ceramics.
4) Polymers.
Electronic Materials :
- Comprise materials from all of the four engineering materials categories.
- They have no common physical or chemical properties : their electrical properties span the range from nearly-ideal insulators to excellent conductors.
- They are important in IC and NEMS/MEMS fabrication.

4. The bulk materials are predominantly semiconductors.
ELECTRONIC MATERIALS
To fabricate ICs and NEMS/MEMS many different kinds of bulk materials and thin films are used.
The bulk materials are predominantly semiconductors.
The most important semiconductor for ICs and NEMS/MEMS is Si.
Thin films in ICs and NEMS/MEMS are classified into four groups :
Thin films
thermal SiO2
dielectrics
Poly-Si
metals
Deposited SiO2
Deposited Si3N4

5. METALS
- Inorganic substances comprising one or more metallic elements (e. g., Fe, Al, Cu etc.).
- Metallic materials composed of two or more metallic elements are called alloys.
- Non-metallic elements added intentionally to metals may improve material quality (e. g., carbon added to iron to produce steel).
-Metals and alloys are divided into :
Ferrous metals and alloys : containing large concentrations of Fe.
Non-ferrous metals and alloys : containing no or very little Fe.
-Single crystal metals are mostly found in the three simple types of cells ; BCC, FCC, and HCP.

6. - Under different conditions of temperature and pressure, different crystal structures ( i. e., different unit cells) of the same metal are found. For example a bar of Fe at room temperature has BCC structure (a phase), however above ~ 900 °C the structure changes to FCC (g phase). Therefore at ~ 900 °C Fe undergoes a phase change.
- Metals are good thermal and electrical conductors.
- Metals are strong and ductile at room temperature and they maintain good strength both at room temperature and elevated temperatures.

7. Properties of Selected Metals used in NEMS/MEMS

8. Metallization
- Metallization is a process whereby metal films are formed on the surface of a substrate.
- The most common and important metallization method is physical vapor deposition (PVD).
- The main PVD processes are evaporation and sputtering.

9. Evaporation Chamber evacuated to ~ 10-6 - 10-7 Torr.
Crucible is heated using a tungsten or an electron beam.
The film thickness is determined by the time the shutter is opened.
The evaporation rate is a function of the vapor pressure of the metal.
In general evaporated films are highly disordered and have large residual stresses.

10. Model of Evaporation
Evaporation process comprises three steps :
Solid metal changing to a gaseous vapor
Chamber is pumped down to a pressure P given by
The concentration of gaseous metal atoms n is

11. (2) Metal gas transport to substrate and impingement
The impingement rate F is given by

12. (3) The gaseous metal must condense onto the substrate
The time tm taken by one mono-layer of metal to form is
The final thickness of the metal layer d is given by

13. Sputtering
Sputtering is a physical phenomenon involving the acceleration of ions through a potential gradient and the bombardment of these ions of a "target" or cathode.
Through momentum transfer, atoms near the surface of the target metal become volatile and are transported as vapor to the substrate. At the substrate the film grows via deposition.

14. - After evacuating the chamber to ~ 10-6 to 10-7 Torr an inert gas such as He is introduced in the chamber at a few mTorr of pressure.
- A plasma of the inert gas is then ignited.
- The energetic ions of the plasma bombard the surface of the target.
- The energy of the bombarding ions is sufficient to make some of the target atoms escape from the surface.
- The atoms land on the sample and form a thin film.

15. - Sputtered films have better uniformity than evaporated ones, and the high energy plasma overcomes the temperature limitations of evaporation.
- Most materials from the periodic table can be sputtered, as well as inorganic and organic compounds.
- The structure of sputtered films is mostly amorphous, and its stress and mechanical properties are sensitive to specific sputtering conditions.
- Some of the inert gas can be trapped in the film causing anomalies in its mechanical and structural characteristics.