1-Introduction

Introduction to Advanced Micro and Nanofabrication Technologies

• Conducted by Prof. Matteo Cantoni at Politecnico di Milano, this course explores the principles and applications of micro and nanofabrication in engineering physics.

• The focus of the lecture series includes methodologies for device fabrication.

Device Fabrication Fundamentals

Methods

• Top-Down Approach: This method involves manipulating macroscopic materials to create micro-scale objects by either adding or removing material.

• Bottom-Up Approach: This involves assembling nanoscale components from smaller units, such as atoms or molecules.

  An example of bottom-up manufacturing is the synthesis of carbon nanotubes.

Case Study: Photodiodes

Key Functionality

• A photodiode must perform several critical functions:

  • Collect light

  • Convert light into an electrical signal

  • Facilitate measurement of the produced signal.

• The process relies on the generation of electron-hole pairs in a semiconductor (such as Germanium - Ge) which forms an electrical current with the help of an electric field.

Semiconductor Characteristics

• Effective light collection requires a semiconductor that can absorb light with energy greater than or equal to the bandgap of 0.66 eV, which corresponds to an infrared wavelength of 1.88 µm.

• The fundamental principles draw on semiconductor physics, optoelectronics, and material science.

Heterostructure Utilization

• Active Area Design: Upon deciding the photodiode characteristics, device active areas must be patterned using lithography, ensuring electrodes are properly insulated to prevent short circuits.

• Active Area Requirements: The thickness of the material above Ge should be minimized to reduce light attenuation, crucial for effective photodetection.

Deposition Techniques

Starting Materials

• The fabrication begins with a Ge(001) wafer, necessitating surface preparation due to contamination (from elements like C and O) and oxidation (GeO, GeO2).

• Cleaning processes can be ex-situ using chemical agents like HF or in-situ through annealing processes at ~600°C in a vacuum.

Epitaxial Growth

• The growth of layers (e.g., MgO, Fe, Au) is performed through Molecular Beam Epitaxy (MBE), aimed at establishing a highly ordered structure.

• Characterization of the structure follows layer growth, with techniques possibly employed both in-situ (XPS, XPD, etc.) and ex-situ (AFM, TEM, etc.).

Patterning Steps

• The transition from heterostructure to device involves four critical steps:

  1. Optical Lithography: Using physical masks to outline the desired device areas.

  2. Material Removal: Utilizing ion beam etching to define specific areas.

  3. Insulating Layer: Deposition of SiO2 to electrically isolate different layers.

  4. Contact Deposition: Placing Au contacts on both the top and bottom layers to connect with the external circuit.

Electrical Connections

• The device must maintain accessibility through thin wires (of Au or Al) connecting the device electrodes to external pads, accommodating conventional soldering.

• Techniques such as ultrasonic wire bonding may be used to enhance connection at the electrode area, ensuring reliability.

Final Packaging and Testing

• After fabrication, the device will transition to packaging and testing phases, ensuring proper access (both electrically and optically) to the photodiode’s active area.

• Careful design of mechanical features is necessary for integration into larger systems, facilitating real-world use.

Types of Deposition

• Various deposition methods will address critical parameters such as material choice (metals, insulators, etc.), stoichiometry, surface characteristics, and uniformity, among others.

• Exploring different techniques (PVD, CVD) will provide specific solutions tailored for diverse applications.

Conclusion

• The course emphasizes innovative techniques and real-world applications of micro and nanofabrication technologies vital for advancements within engineering physics.

• Reference literature includes M. Ohring's "Materials Science of Thin Films" to guide students through theoretical and practical aspects of thin film growth and characterization.

Introduction to Advanced Micro and Nanofabrication Technologies

Part 1: Deposition

Overview of Deposition Techniques

The deposition process is a crucial step in micro and nanofabrication technologies, where materials are applied to a substrate to create thin films or structures that are essential for device functionality.

Common Deposition Methods

1. Physical Vapor Deposition (PVD): This method involves the physical transfer of material from a source to the substrate through evaporation or sputter deposition. It's suitable for creating metal films in electronic devices.

2. Chemical Vapor Deposition (CVD): CVD relies on chemical reactions to deposit materials. It's widely used for making high-quality semiconductor films, such as silicon, which are pivotal in integrated circuits.

3. Molecular Beam Epitaxy (MBE): This technique provides highly controlled growth of thin films through the evaporation of materials in an ultra-high vacuum. MBE is particularly important in the fabrication of heterostructures for optoelectronic devices.

Application Scenarios

• Microelectronics: PVD is commonly used for fabricating interconnects and barriers in integrated circuits, ensuring reliable electrical connections.

• Optoelectronics: CVD technologies are utilized in the production of high-purity thin film solar cells, maximizing light absorption and conversion efficiency.

• Telecommunication Devices: MBE is employed for making advanced semiconductor laser diodes and detectors which are necessary for fiber optic communication systems.