Photolithography Steps

 

Photolithography Steps

Photolithography is a critical process in the manufacturing of semiconductors, integrated circuits, and micro/nano-structures. Here's a step-by-step guide to the photolithography process:


1. Wafer Preparation


Cleaning: The silicon wafer is meticulously cleaned to remove any organic, ionic, or metallic contaminants. Common methods include:

RCA Clean: Utilizing solutions of ammonium hydroxide (NH₄OH), hydrogen peroxide (H₂O₂), and hydrochloric acid (HCl) to remove various types of contaminants.

Solvent Cleaning: Using solvents like acetone or methanol for organic residues.

Dehydration Bake: The wafer is baked to remove any moisture, which could interfere with photoresist adhesion.

Adhesion Promotion: Often, an adhesion promoter like hexamethyldisilazane (HMDS) is applied to enhance the bonding of photoresist to the substrate.


2. Photoresist Application


Spin Coating: Photoresist, a light-sensitive material, is dispensed onto the wafer. The wafer is then spun at high speeds (typically 1200 to 4800 rpm) to spread the photoresist uniformly across the surface, forming a thin layer (0.5 to 2.5 μm thick).

Soft Bake: The wafer with photoresist is heated (usually at around 90-110°C) to evaporate solvents, improving adhesion and reducing light scattering during exposure. This step also enhances the photosensitivity of the resist.


3. Exposure


Mask Alignment: A photomask with the desired pattern is precisely aligned over the photoresist layer. 

Exposure: The photoresist is exposed to light, typically ultraviolet (UV) or deep ultraviolet (DUV), through the mask. The light changes the chemical properties of the photoresist where it strikes:

Positive Photoresist: Exposed areas become more soluble in developer.

Negative Photoresist: Exposed areas become less soluble in developer.

Exposure Methods: 

Contact Printing: Mask is in direct contact with resist.

Proximity Printing: Mask is slightly above resist.

Projection Printing: Image is projected through a lens system (used in modern steppers and scanners).


4. Post-Exposure Bake (PEB)


This optional step involves heating the resist to enhance the chemical reactions started during exposure, particularly for chemically amplified resists, increasing contrast and resolution.


5. Development


Developer Application: A developer solution selectively removes either the exposed (for positive resists) or unexposed (for negative resists) photoresist. Common developers include tetramethylammonium hydroxide (TMAH) for positive resists.

Rinsing: The wafer is rinsed to stop the development process and remove any residual developer.


6. Hard Bake


The remaining photoresist pattern is baked at higher temperatures (around 120-180°C) to polymerize the resist, making it more durable for subsequent processing steps.


7. Etching or Deposition


Etching: The pattern in the photoresist acts as a mask for etching processes where unprotected areas of the substrate are removed using either wet chemicals or plasma (dry etching).

Deposition: Alternatively, the pattern can be used to selectively deposit materials onto the substrate.


8. Photoresist Removal (Stripping)


After the desired pattern has been transferred to the substrate, the photoresist is removed. This can be done with solvents like acetone for positive resists or with plasma ashing for both positive and negative resists.


9. Inspection and Metrology


The patterned wafer is inspected for defects and measured to ensure the features meet design specifications. This might involve tools like SEM (Scanning Electron Microscope) or optical inspection systems.


Key Considerations:


Cleanroom Environment: Photolithography requires extremely clean conditions to prevent particulate contamination.

Resolution: Determined by the wavelength of light, resist chemistry, and the precision of the alignment system.

Alignment: Critical for multi-layer processes to ensure layers align correctly.


Conclusion


Photolithography is a meticulous process that demands precision at every step to achieve the intricate patterns needed in modern microelectronics and microfabrication. Each step builds upon the last, ensuring that the final product matches the design specifications with high accuracy.



Photoresist Types

Photoresists are crucial materials in photolithography used for creating patterns on substrates in semiconductor manufacturing, MEMS (Micro-Electro-Mechanical Systems), and other fields where microfabrication is necessary. They are classified based on various characteristics, including their response to light, chemical composition, and application. Here are the main types of photoresists:


1. Based on Response to Light:


Positive Photoresist:

Mechanism: When exposed to light, the chemical structure of positive photoresist becomes more soluble in the developer solution. The exposed areas are removed during development, leaving behind the unexposed pattern.

Applications: Widely used in semiconductor manufacturing due to their ability to produce high-resolution patterns. They are particularly useful for creating fine details with good contrast.

Examples: Novolac-resin based photoresists like AZ series, diazonaphthoquinone (DNQ) based resists.

Negative Photoresist:

Mechanism: Light exposure causes cross-linking or polymerization, making the exposed areas less soluble or insoluble in the developer. The unexposed areas are washed away during development, leaving the exposed pattern.

Applications: Often used where high-aspect-ratio structures are needed, or in applications with less stringent resolution requirements. They are also used in PCB manufacturing and for certain MEMS applications due to their faster exposure times and lower cost.

Examples: Epoxy-based resists like SU-8, which are popular for their stability and high aspect ratios.


2. Based on Chemical Structure:


Photopolymeric Photoresist:

Mechanism: Exposure to light initiates polymerization, forming polymers from monomers. Typically used for negative resists.

Examples: Methyl methacrylate, poly(phthalaldehyde)/PAG blends.

Photodecomposing Photoresist:

Mechanism: Light exposure leads to the decomposition of the resist, usually increasing its solubility (used for positive resists).

Examples: Azide quinones, like diazonaphthoquinone (DQ).

Photocrosslinking Photoresist:

Mechanism: Light causes cross-linking of polymer chains, making them insoluble (negative resist).

Examples: Similar to photopolymeric in function but specifically refers to the cross-linking process.


3. Based on Physical State:


Liquid Photoresist:

Application: Applied via spin coating, spray coating, or dip coating. It's the most common form in semiconductor manufacturing.

Advantages: Allows for very thin, uniform layers; suitable for high-resolution patterning.

Dry Film Photoresist:

Application: Comes in pre-made sheets that are laminated onto the substrate. 

Advantages: Easier to handle for certain applications like PCB manufacturing, provides thicker resist layers, and is useful for rapid prototyping or when large areas need to be patterned.


4. Special Types:


Chemically Amplified Resists (CAR):

Mechanism: Utilize acid-catalyzed reactions to enhance sensitivity to light, allowing for lower exposure doses. This is particularly useful in deep-UV lithography where light sources have less energy.

Applications: Used in advanced semiconductor processes due to their high sensitivity and resolution capabilities.

E-beam Resists:

Mechanism: Sensitive to electron beam exposure rather than UV light, used in electron beam lithography for very high resolution.

Examples: PMMA (Polymethyl methacrylate) for positive resists, HSQ (Hydrogen Silsesquioxane) for negative resists.

Multi-Layer Resists:

Application: Used to achieve high aspect ratios or for lift-off processes where one resist layer undercuts another to create a reverse profile for metal deposition.


5. Other Classifications:


UV vs. Deep UV Photoresists: Depending on the wavelength of light used for exposure, resists are tailored to have maximum sensitivity in UV (around 365 nm) or deep UV (193 nm or 248 nm) ranges.

Thermal Stability: Some resists are formulated to withstand high temperatures, important for processes like plasma etching or high-temperature baking steps.


Conclusion


Choosing the right type of photoresist depends on the specific requirements of the application, including resolution, aspect ratio, sensitivity to light, thermal stability, and the method of processing. Each type offers unique advantages, tailored to meet the diverse needs of microfabrication technologies.

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