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Photolithography machine structure and working principle
**Introduction to Lithography Machines**
A lithography machine, also known as a mask aligner, exposure system, or lithography system, is a critical tool used in semiconductor manufacturing. The most commonly used type is the mask alignment lithography system, which is why it is often referred to as a mask alignment system. It plays a vital role in transferring circuit patterns from a mask to a silicon wafer.
**Classification of Lithography Machines**
Lithography machines are typically categorized into three types based on their level of automation: manual, semi-automatic, and fully automatic.
- **Manual**: This type requires the operator to manually adjust the X-axis, Y-axis, and angle of the mask using physical knobs. The alignment accuracy is limited and highly dependent on the operator’s skill.
- **Semi-Automatic**: In this version, alignment is guided by a CCD camera, and adjustments are made using electrically controlled stages, improving precision compared to manual systems.
- **Fully Automatic**: These systems operate with minimal human intervention. They automatically load and unload the wafer, control exposure time, and manage the entire process through pre-programmed settings. Fully automatic systems are ideal for high-volume production environments.
**Structure of a Lithography Machine**
[Image: Photolithography machine structure and working principle]
The structure of a lithography machine includes several key components such as the mask stage, wafer stage, light source, optical system, and alignment mechanism. These parts work together to ensure precise pattern transfer from the mask to the wafer.
**Performance Indicators of Lithography Machines**
Key performance metrics of a lithography machine include:
- Substrate size range
- Resolution (the finest line width it can produce)
- Alignment accuracy (how precisely the layers are aligned during multi-step exposure)
- Exposure mode (contact, proximity, projection, or direct writing)
- Light source wavelength (UV, deep UV, or EUV)
- Light intensity uniformity
- Production efficiency
Resolution is limited by the diffraction of light and depends on factors like the light source, lens quality, photoresist, and process conditions. Alignment accuracy is crucial when multiple layers are involved, ensuring that each layer is precisely positioned relative to the previous one.
**Role of Lithography Machines**
Lithography is one of the most technically complex and expensive processes in microelectronics. It determines the integration density of chips and is essential for producing advanced semiconductor devices. Without precise lithography, modern electronics would not be possible.
**Lithography Process Flow**
[Images: Lithography process flow]
The general photolithography process involves several steps: cleaning, drying, coating with photoresist, spin coating, soft baking, alignment and exposure, post-baking, development, hard baking, and etching. Each step must be carefully controlled to ensure accurate pattern transfer. More complex chips may require over 30 layers, making the lithography process even more demanding.
**Working Principle of a Lithography Machine**
The lithography machine uses a light source to project the circuit pattern from a mask onto a silicon wafer. The light beam passes through a series of optical components, including lenses that correct for aberrations and scale the image appropriately. The imaging ratio typically ranges from 5:1 to 4:1. After exposure, the wafer is chemically developed to reveal the pattern, forming the circuit on the chip.
**Key Components of a Lithography Machine**
- **Measuring Table / Exposure Table**: The platform that holds the silicon wafer. It is often double-sided for efficient processing.
- **Beam Aligner**: Ensures the laser beam is as parallel as possible before it reaches the mask.
- **Energy Controller**: Regulates the amount of energy delivered to the wafer, preventing under or over-exposure.
- **Beam Shape Setting**: Adjusts the shape of the light beam (e.g., circular or annular) to optimize optical performance.
- **Shutter**: Blocks the beam when exposure is not needed.
- **Energy Detector**: Monitors the energy level of the incoming beam and feeds back data to the controller.
- **Mask**: A transparent glass plate with a circuit design etched onto it. Masks are extremely costly due to their precision.
- **Mask Table**: Holds the mask and moves it with nanometer-level accuracy.
- **Objective Lens**: Composed of multiple lenses that focus and reduce the circuit image from the mask onto the wafer. It also corrects optical distortions.
- **Wafer**: Made from silicon crystal, available in various sizes. Larger wafers increase yield but require more precise handling. Wafers have notches or flats to define their orientation.
- **Internal Frame and Damper**: Isolates the workbench from external vibrations, maintaining stability and environmental control.
In summary, lithography machines are at the heart of semiconductor manufacturing, enabling the creation of complex and high-performance electronic devices. Their precision, reliability, and technological advancement continue to drive progress in the field of microelectronics.
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(1) Check the original appearance of the circuit board. Pay attention to cleanliness and appearance inspection.
(2) Inspect the mask, including specified color, specified thickness, quality defects, and pitting.
(3) Is electroplating sufficient? Adequate electroplating is difficult to measure. If the requirements are strict, the current requirement is based on the plating thickness.
(4) Is the hole centered around the pad (annular ring and alignment)?
(5) Is the hole size correct?
(6) Does the appearance match the artwork?
(7) Does the steel mesh match the printing position of the circuit board?
(8) Is the overall circuit board size correct?
(9) Is the PCB warped?
(10) Is there copper on the edge of the PCB board?
(11) PCB tracking width tolerance.