CMOS Fabrication Steps
A typical CMOS (Complementary Metal-Oxide-Semiconductor) semiconductor manufacturing process involves several key steps, each with a specific purpose. Here is an overview of these steps:
Active Region Definition
- Purpose: Define the regions where transistors will be constructed.
- Details: All areas of the die not designated as active regions are covered with a thick layer of silicon dioxide (SiO2), known as the field oxide, to insulate neighboring devices.
- Purpose: Provide additional insulation between devices by creating reverse-biased np-diodes.
- Details: An extra p+ region, known as the channel-stop implant or field implant, is added beneath the field oxide.
Formation of Wells
- Purpose: Create lightly doped p- and n-wells to house transistors.
- Details: Ion implantation is used to form n-wells for PMOS transistors and p-wells for NMOS transistors.
Source and Drain Formation
- Purpose: Create the source and drain regions of transistors.
- Details: Heavily doped n-type regions are implanted (or diffused) into the lightly doped p-type substrate for NMOS transistors, and vice versa for PMOS transistors.
- Purpose: Define the channel region between the source and drain.
- Details: A thin layer of SiO2, known as the gate oxide, is deposited. Polycrystalline silicon (polysilicon) is then deposited and patterned to form the gate of the transistor.
- Purpose: Precisely position the source and drain regions relative to the gate.
- Details: The location of the source and drain regions is defined by the polysilicon gate, allowing for precise alignment.
- Purpose: Provide electrical connections between transistors and other components.
- Details: Multiple insulated layers of metallic wires, often made of materials like aluminum or copper, are deposited and patterned to create interconnections.
- Purpose: Ensure a flat and even semiconductor surface for layer deposition.
- Details: Chemical-mechanical planarization (CMP) is used to microscopically plane the device layer and reduce step heights.
- Purpose: Provide protection for the semiconductor.
- Details: A final passivation or over-glass layer, typically made of CVD SiO2, is deposited for protection, often with an additional nitride layer for moisture resistance.
Bonding Pad Formation
- Purpose: Create openings for bonding pads used for external connections.
- Details: Etching openings are made to the pads for bonding, allowing external connections to be made.
The entire process results in a complex structure with multiple layers of materials, including transistors, insulating layers, interconnects, and protective layers. The interconnect layers play a crucial role in connecting the transistors and other components, allowing the integrated circuit to function as intended. This manufacturing process is highly intricate and precise, ensuring the proper functionality of CMOS devices.
What is the purpose of the field oxide in the CMOS fabrication process?
The field oxide serves as an insulator between neighboring devices and covers areas of the die where transistors will not be constructed.
How are lightly doped p- and n-wells formed in the CMOS fabrication process?
Lightly doped p- and n-wells are formed through ion implantation.
What materials are used for the gate of NMOS and PMOS transistors in CMOS fabrication?
The gate is covered by a thin layer of silicon dioxide (gate oxide) and conductive polycrystalline silicon (polysilicon).
What is the significance of the self-aligned process in CMOS fabrication?
The self-aligned process allows for precise positioning of the source and drain regions relative to the gate, ensuring accurate transistor construction.
What is the final step in the CMOS fabrication process after depositing the metallic interconnect layers and passivation layer?
The final processing step involves etching openings to the pads used for bonding the integrated circuit.
What is the purpose of the channel-stop implant (field implant) in CMOS fabrication?
The channel-stop implant adds an extra p+ region beneath the field oxide to create a reverse-biased np-diode, providing additional insulation between devices.
What type of material is used for intermediate planarization steps in CMOS fabrication?
Intermediate planarization steps involve the deposition of insulating material, often silicon dioxide (SiO2).
How are the threshold voltages of PMOS and NMOS transistors adjusted in CMOS fabrication?
Threshold voltages are adjusted through ion implantation in the areas just below the gate oxide.
What is the function of the sacrificial silicon nitride layer in CMOS fabrication?
The sacrificial silicon nitride layer is used during plasma etching to create trenches for insulating the devices and is later removed.
What is the primary material used for the deposition of metallic interconnect layers in CMOS fabrication?
The primary materials used for metallic interconnect layers are often Aluminum and Copper, with Tungsten used for lower layers.
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