Thermal oxidation Growth Rate Initially oxide growth rate

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 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.

• 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. • Si. O 2 or silica has several crystalline structures (the dominant one is quartz), and an amorphous structure. • Amorphous oxide has a density of ~ 2. 2 gm/cm 3, whereas quartz has a density of ~ 2. 7 gm/cm 3. • Thermally grown oxides are usually amorphous.

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 Si. O 2 deposited Si. O 2 dielectrics Poly-Si deposited Si 3 N 4 metals

Dielectric And Polycrystalline Silicon Film Deposition : Low Pressure Chemical Vapor Deposition (LPCVD) • The furnace is similar to thermal oxidation furnace, with the two exceptions : (1) pressure of deposition typically between 100 and 50 m. Torr, 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.

CVD of Silicon Dioxide • Low-temperature (300 to 500 °C) deposition : 450 °C Si. H 4 + O 2 Si. O 2 + 2 H 2 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(OC 2 H 5)4. TEOS decomposes as follows : 700 °C Si(OC 2 H 5)4 Si. O 2 + 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 : Si. Cl 2 H 2 + 2 N 2 O 900 °C Si. O 2 + 2 N 2 + 2 HCl

Properties of Silicon Dioxide

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 Si. O 2 deposited Si. O 2 dielectrics Poly-Si deposited Si 3 N 4 metals

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/cm 2 which is ~ 10 times that of silicon dioxide. • The resistivity of the nitride at room temperature is ~ 1016 Wcm, its dielectric constant is 6, and its dielectric strength is 107 V/cm.

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

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

Properties of Selected Electronic Materials

IC AND MEMS PROCESSES