In the chip manufacturing process, “Start Oxide” (commonly referred to as the initial oxide layer or starting oxide layer) is a fundamental and critical processing step. Its core function is to provide physical protection, electrical insulation, and process support for subsequent fabrication steps. The following is a detailed analysis of its necessity from both technical and practical application perspectives:
I. Definition and Preparation of Start Oxide
Definition:
The initial oxide layer is a thin layer of silicon dioxide (SiO₂) formed on the surface of a silicon wafer (substrate) after cleaning, using thermal oxidation processes such as dry oxidation, wet oxidation, or steam oxidation. Its thickness typically ranges from a few nanometers to several tens of nanometers.
Purpose of Preparation:
Leveraging the chemical stability and insulating properties of silicon dioxide to build a “foundational protection layer” for subsequent processes.
II. Core Reasons for the Need for Start Oxide in Chip Manufacturing
1. As an Insulating Layer to Isolate Electrical Structures
Principle:
Silicon dioxide is an excellent insulator (with a resistivity of 10¹⁴~10¹⁶ Ω·cm) that prevents current from flowing in unintended regions.
Application Scenarios:
In MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) manufacturing, although the initial oxide layer is not the final gate oxide, it acts as a “pre-insulation layer” on the substrate surface to prevent direct contact between subsequently deposited metal or polysilicon and the silicon substrate, which could cause leakage.
In device isolation within chips (e.g., Shallow Trench Isolation, STI), the initial oxide layer can serve as a buffer layer at the trench bottom to avoid stress damage from direct contact between the silicon wafer and filling materials like silicon dioxide or silicon nitride.
2. As a Masking Layer to Control Ion Implantation Regions
Principle:
The oxide layer blocks ions (such as boron or phosphorus dopants), and windows can be precisely defined on it through photolithography and etching processes to control the ion implantation area.
Application Scenarios:
During source/drain region doping, silicon wafer areas not covered by the oxide layer receive ion implantation, while oxide-covered regions are protected, enabling precise tuning of the device’s electrical properties.
Without oxide layer masking, ion implantation would distribute uniformly across the wafer surface, failing to form the required PN junction structure for the device.
3. Protecting the Silicon Surface from Contamination and Damage
Principle:
The silicon surface is easily contaminated by impurities in the air (such as metal ions or organics) or damaged during subsequent processes by plasma or chemical solutions. The oxide layer acts as a “barrier.”
Application Scenarios:
When depositing high-stress materials like silicon nitride (SiN), the initial oxide layer (e.g., “pad oxide”) can buffer the stress on the wafer, preventing dislocations or cracks.
During processes like photoresist coating or etching, the oxide layer protects the silicon wafer from direct chemical erosion, ensuring surface flatness.
4. Providing a Smooth Base for Subsequent Thin-Film Deposition
Principle:
Even after polishing, the silicon surface still has atomic-level roughness or defects. Oxide layer growth can fill in tiny defects and form a smoother surface.
Application Scenarios:
When depositing polysilicon (for gates) or metal layers, a smooth oxide base improves film uniformity and adhesion, reducing risks of short circuits or open circuits caused by surface roughness.
In advanced nodes (e.g., FinFET, GAAFET), where device structures are at nanometer-scale 3D dimensions, the flatness of the initial oxide layer directly impacts the accuracy of subsequent nano-fabrication steps.
5. Surface Passivation to Improve Device Performance
Principle:
Dangling bonds on the silicon surface (unsaturated silicon bonds) can introduce surface states, causing device leakage or instability. The oxide layer “passivates” these dangling bonds.
Application Scenarios:
In CMOS devices, the passivation effect of the initial oxide layer reduces surface leakage current and improves the device’s on/off ratio and reliability.
III. Typical Case: Role of Start Oxide in MOSFET Fabrication
Taking traditional MOSFET as an example, the process flow and role of the initial oxide layer are as follows:
- Wafer Cleaning: Remove surface contaminants;
- Growth of Start Oxide: Form a thin oxide layer of about 5~20nm;
- Silicon Nitride Deposition: Cover the oxide with SiN as a masking layer for subsequent etching;
- Lithography and Etching: Define device isolation areas, etch the SiN and oxide to expose the silicon;
- Trench Fill and Planarization: Fill the trench with SiO₂ to form an isolation structure;
- SiN Removal: Retain the oxide as a buffer layer at the isolation interface.
Without Start Oxide, direct contact between silicon nitride and the wafer would lead to excessive stress and wafer damage, severely degrading isolation performance.
IV. Conclusion: The Irreplaceability of Start Oxide
Although the initial oxide layer seems “simple,” it is a key transitional step in chip fabrication:
From a process perspective: It is the “foundation platform” for all subsequent film deposition, ion implantation, and etching. Its absence would cause process instability.
From a device perspective: Through insulation, isolation, and protection, it directly affects the chip’s electrical performance, yield, and reliability.
Therefore, the Start Oxide process is an “indispensable first step” in chip manufacturing. Its quality (such as thickness uniformity, purity, and density) directly determines the process precision and performance ceiling of high-end chips.
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