Abstract
Integrated circuits generate excessive heat during operation. Effective heat dissipation is necessary to maintain optimal performance and prevent overheating. The reduced silicon wafer thickness can increase circuit layer density, support multi-chip packaging, and make chips light and compact. Backside thinning technology is an effective solution to thin the non-functional side of a silicon chip without affecting the circuit function, and can be classified into Mechanical Grinding (MG), Chemical-Mechanical Polishing (CMP), and Electrochemical Mechanical Polishing (ECMP). MG suffers from low efficiency and significant tool wear due to the high hardness of silicon. CMP compromises backside uniformity and can not accurately control wafer thickness. ECMP requires complex equipment and precise control of the chemical environment. Laser thinning can effectively solve the above issues via providing high Material Removal Rate (MRR) while maintaining a smooth and uniform surface with effective management of thermal challenges. However, the problems of current laser thinning technology can include:Non-Uniform Material Removal: The Gaussian (GAU) distribution of energy at the laser’s focal point results in deeper material removal in the central region of each scan path, with less material removed towards the edges
Unwanted Material Generation: High laser energy rapidly heats and cools silicon, resulting in the formation of amorphous silicon and silicon dioxide (SiO₂), which severely impacts the electrical conductivity of the silicon wafer.
Regenerative Effect: Laser thinning of surfaces with pre-existing scratches or cracks can exacerbate these defects, causing them to be widened and deepened and, therefore leading to increased scrap rates and reduced processing efficiency.
To address the issues in the above thinning methods, this thesis investigates a new technique for rotational laser thinning of silicon wafers using diffractive optical element (DOE). The study begins by analysing the importance of wafer thinning technology in enhancing integrated circuit performance and explores the global thinning equipment market along with the advantages and disadvantages of the main technological methods (see Chapter 1.1). It establishes the aim of developing a novel laser thinning technology to optimize the silicon wafer manufacturing process (see Chapter 1.2). This is followed by a literature review of the methods for thinning the back side of silicon wafers and the application of DOE (see Chapter 2.1 and 2.2), and gaps in these methods are identified (see Chapter 2.3). The paper then examines specific issues in traditional laser thinning technology (see Chapter 3.1) and proposes a new rotating laser thinning technique based on DOE beams (see Chapter 3.2), including the design of the DOE (see Chapter 3.3). An experimental platform is subsequently established (see Chapter 4.1). The study uses DOE laser beams and GAU laser beams to thin a flat surface and compares the results in terms of surface morphology, surface roughness, material behaviour, surface wettability, and ablation mechanisms (see Chapter 4.2). For non-flat surfaces, the study compares the thinning results of DOE laser beams and GAU laser beams concerning regeneration effect (see Chapter 4.3). It then explores the optimal processing parameters for laser thinning with DOE beams (see Chapter 4.4). Finally, the paper summarizes the results obtained (see Chapter 5.1) and outlines plans for future work (see Chapter 5.2).
Based on this study, the key conclusions can include:
Flat Surface: The uniform energy distribution of the DOE spot effectively avoids the ridge-like projections caused by GAU spots, resulting in a smooth post-processing surface.
Low Roughness: By converting traditional Gaussian laser spots into uniform linear spots, this method achieves a surface with low surface roughness.
Reduced By-products: The use of DOE-based beams minimizes the formation of amorphous silicon and oxides, as confirmed by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) analysis.
Excellent Hydrophilicity: DOE-based thinning enhances surface hydrophilicity, with consistent hydrophilic properties in all directions.
Regeneration Effect: DOE-based beams effectively mitigate the replication phenomenon encountered during laser thinning of uneven surfaces, both in width and depth directions.
This innovative technology offers a promising alternative for high-quality, efficient and controlled precision machining of silicon wafer surfaces for semiconductor device manufacturing and other high-precision applications.
| Date of Award | Jul 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Haonan Li (Supervisor), Yixiang Xu (Supervisor) & Gongyu Liu (Supervisor) |