What is PMOS Fabrication?

PMOS (P-channel Metal-Oxide-Semiconductor) fabrication refers to the detailed process of manufacturing P-channel MOSFETs, which are fundamental components in modern integrated circuits. These transistors operate by using holes as the primary charge carriers. The fabrication involves a series of intricate steps to create the necessary layers, regions, and structures on a semiconductor substrate, typically silicon.

The core of PMOS fabrication, as described in advanced methods, includes a sequential approach. Specifically, the PMOS transistor is fabricated by a method including forming a dummy gate structure on a semiconductor substrate, forming a source region and a drain region in the semiconductor substrate on both sides of the dummy gate structure, forming an intermediate layer to cover the dummy gate structure and the semiconductor. This initial sequence lays the groundwork for defining the transistor's critical active areas.

Understanding PMOS Transistors

Before delving into the fabrication, it's essential to understand what a PMOS transistor is. A PMOS transistor consists of a gate, source, and drain terminals. Unlike NMOS transistors, PMOS devices use a p-type source and drain regions created within an n-type substrate or n-well. When a sufficiently negative voltage is applied to the gate terminal relative to the source, it attracts holes to the region under the gate, forming a conductive p-channel between the source and drain, allowing current to flow.

Key Stages of PMOS Fabrication

The fabrication of PMOS transistors is a complex multi-step process, often performed alongside NMOS fabrication in CMOS (Complementary Metal-Oxide-Semiconductor) technology. Here are the generalized key stages, incorporating the specific steps mentioned:

1. Substrate Preparation and Well Formation

• Starting Material: Fabrication begins with a highly pure silicon wafer. For PMOS, if it's a standalone PMOS process, a lightly doped n-type substrate might be used. In CMOS, a p-type substrate is common, requiring the formation of an n-well.
• N-Well Formation (for CMOS): If using a p-type substrate, an n-well is formed by implanting n-type impurities (like phosphorus or arsenic) into designated areas. This well serves as the "body" for the PMOS transistor, isolating it from other components.

2. Isolation

• Field Oxidation (LOCOS) or Shallow Trench Isolation (STI): To electrically isolate different transistors and components on the chip, thick oxide regions are grown or trenches are etched and filled with dielectric material. This prevents unwanted current leakage.

3. Gate Dielectric and Dummy Gate Formation

• Gate Dielectric Growth: A very thin, high-quality insulating layer (typically silicon dioxide, SiO2, or high-k dielectric materials) is grown or deposited on the silicon surface. This layer acts as the gate insulator, separating the gate electrode from the channel.
• Dummy Gate Structure Formation: As per the reference, a dummy gate structure is formed on this semiconductor substrate. In advanced processes, a "dummy gate" (often polysilicon) is used as a placeholder during initial high-temperature steps, which might damage the final high-performance gate material. This dummy gate defines the channel region of the transistor.

4. Source and Drain Region Formation

• Forming Source and Drain Regions: After the dummy gate is in place, source and drain regions are formed in the semiconductor substrate on both sides of the dummy gate structure. This is typically achieved through ion implantation. For PMOS, p-type impurities (like boron) are implanted into the n-well (or n-substrate) to create heavily doped p+ regions that will serve as the source and drain terminals. The dummy gate acts as a mask, defining the channel length.
• Sidewall Spacer Formation: Dielectric spacers (e.g., silicon nitride) are often formed on the sides of the gate electrode. These spacers are crucial for subsequent self-aligned implantations, creating lightly doped drain (LDD) regions that help reduce hot carrier effects and improve device reliability.

5. Intermediate Layer and Further Processing

• Intermediate Layer Formation: An intermediate layer is formed to cover the dummy gate structure and the semiconductor (including the source/drain regions). This layer, often a dielectric, protects the underlying structures during subsequent steps and acts as a foundation for further layers.
• Gate Replacement (if dummy gate used): If a dummy gate was used, it is now removed, and the actual high-performance gate material (e.g., metal gate) is deposited and patterned in its place. This "gate-last" or "replacement metal gate" process helps integrate advanced gate materials that might not withstand early high-temperature steps.
• Silicidation: A metal (like cobalt or nickel) is deposited and reacted with the silicon in the source, drain, and gate regions to form a low-resistance silicide layer. This reduces contact resistance and improves device performance.

6. Interconnect and Packaging

• Interlayer Dielectric (ILD) Deposition: More insulating layers (ILDs) are deposited over the entire wafer, filling in spaces and planarizing the surface.
• Contact and Via Formation: Holes (contacts) are etched through the ILD to reach the source, drain, and gate terminals. Vias are etched to connect different metal layers.
• Metallization: Conductive metal layers (typically copper or aluminum) are deposited and patterned to form the interconnections that link different transistors and components on the chip. Multiple layers of metal are usually used, separated by ILDs, to create a complex wiring network.
• Passivation: A final protective layer (e.g., silicon nitride) is deposited over the entire chip to protect it from moisture, contamination, and mechanical damage.
• Wafer Dicing and Packaging: The wafer is then cut into individual dies (chips), which are then packaged and tested.

Summary of PMOS Fabrication Steps

The following table provides a simplified overview of the critical steps:

PMOS fabrication is a cornerstone of modern electronics, enabling the creation of powerful and efficient integrated circuits, especially when combined with NMOS to form CMOS circuits, which are ubiquitous in microprocessors, memory, and digital logic.