IC AND NEMS/MEMS PROCESSES

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.

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

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.

- 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.

Properties of Selected Metals used in NEMS/MEMS

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.

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.

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

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

(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

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.

- 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.

- 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.