Initially, oxide growth rate is constant.

1. Thermal oxidation Growth Rate • Initially, oxide growth rate is constant. • As more oxide is grown, O diffusion through the oxide becomes the growth rate limiting factor. At this stage the thickness of the grown oxide becomes proportional to the square root of oxidation time. • The growth rate is also dependent on temperature, and is strongly affected by the presence of water vapor. • Possible thickness of thermal oxides is in the range of few microns.

2. The basic structural unit of thermal oxide is a Si atom surrounded tetrahedrally by four oxygen atoms. • The Si-O and O-O internuclear distances are 1.6 Å and 2.27 Å, respectively. • SiO2 or silica has several crystalline structures (the dominant one is quartz), and an amorphous structure. • Amorphous oxide has a density of ~ 2.2 gm/cm3, whereas quartz has a density of ~ 2.7 gm/cm3. • Thermally grown oxides are usually amorphous.

3. ELECTRONIC MATERIALS • To fabricate ICs and 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 MEMS is Si. • Thin films in ICs and MEMS are classified into four groups : Thin films thermal SiO2 dielectrics Poly-Si metals deposited SiO2 deposited Si3N4

4. Dielectric And Polycrystalline Silicon Film Deposition : Low Pressure Chemical Vapor Deposition (LPCVD) • The furnace is similar to the thermal oxidation furnace, with the two exceptions : (1) pressure of deposition typically between 100 and 50 mTorr, and (2) non-uniform temperature over the length of the furnace tube. • The low pressure is to allow surface catalyzed reaction of otherwise highly reactive (in some cases explosive) gases. • The temperature variation is to compensate for depletion of the reactants. • Typical LPCVD parameters are (i) pressure ; 0.2 to 2.0 Torr, (ii) gas flow ; 1 to 10 cm/s, and (iii) temperature ; 300 to 900 °C.

5. CVD of Silicon Dioxide • Low-temperature (300 to 500 °C) deposition : • SiH4 + O2 SiO2 + 2H2 • The low temperature of the deposition makes it suitable when films must be deposited over a layer of Al. • Intermediate-temperature (500 to 800 °C) deposition : • The oxide is formed by decomposing tetraethylorthosilicate (TEOS), Si(OC2H5)4. TEOS decomposes as follows : • Si(OC2H5)4 SiO2 + by-products • The high temperature of deposition makes it inappropriate for film deposition on Al, however it is used for depositing insulating films on poly-Si. • High-temperature (900 °C) deposition : • SiCl2H2 + 2N2O SiO2 + 2N2 + 2HCl 450 °C 700 °C 900 °C

6. Properties of Silicon Dioxide

7. ELECTRONIC MATERIALS • To fabricate ICs and 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 MEMS is Si. • Thin films in ICs and MEMS are classified into four groups : Thin films thermal SiO2 dielectrics Poly-Si metals deposited SiO2 deposited Si3N4

8. CVD of Silicon Nitride • In the LPCVD process, dichlorosilane and ammonia react at reduced pressure to deposit silicon nitride at 700 °C ≤ T ≤ 800 °C : • Good film uniformity, and high wafer throughput (i. e., number of wafers processed per hour) are advantages of the process. • Silicon nitride deposited by LPCVD is an amorphous dielectric containing up to ~ 8% hydrogen. • The nitride film has a very high tensile stress of approximately 1010 dynes/cm2 which is ~ 10 times that of silicon dioxide. • The resistivity of the nitride at room temperature is ~ 1016Wcm, its dielectric constant is 6, and its dielectric strength is 107 V/cm.

9. Polysilicon Deposition • It is deposited from silane in a low-pressure reactor operated between 600 and 650 °C. • The reaction :

10. LPCVD Materials, Gases, and Deposition Temperatures • Because of deposition temperature, most metals can not be placed in LPCVD furnaces.

11. Properties of Selected Electronic Materials

12. IC AND MEMS PROCESSES