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Questions and Answers
What is the purpose of resist in the lithography process?
The resist is used to locally modify the substrate by being exposed to photons or electrons.
Lithography involves the transfer from design to physical structures through the use of resist and _____ processes.
development
Physical Vapor Deposition (PVD) involves direct line-of-sight impingement deposition techniques.
True
What is the purpose of wet etching?
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What type of etching is achieved with acid baths on a silicon wafer?
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What is the purpose of dry etching?
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Which statement is true about dry etching?
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Is wet anisotropic etching of silicon achieved with alkaline baths?
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Which type of etching has characteristics of poor selectivity and is rather anisotropic?
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Inhibitor driven anisotropy results in a high etch rate.
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Deep reactive ion etching involves cycling between __________ and passivation.
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What are the three stages involved in thermal evaporation?
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Which methods are used to heat up the evaporant in thermal evaporation?
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In CVD processes, plasma enhanced CVD operates at approximately ~____ Pa and ~____°C.
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Is an anisotropic etching perfectly anisotropic?
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Match the following etching techniques with their descriptions:
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What is the sensing approach used for analysis on cancer cells discussed in the content?
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What analyte is focused on in the Optofluidic sensors for the analysis of cancer cells?
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The Optical thin film thickness measurement method described is a destructive measurement approach.
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Atomic force microscopy (AFM) allows to obtain a nanoscale 3D surface map and ______ of the surface.
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What is colorimetry regulated by?
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What is the property shown by all waveguides?
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Photonic Nanofences have low optical losses.
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Photonic nanofences have a high interaction with the ________ in their environment.
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Match the following properties with Photonic Nanofences:
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What is an alternative to DRIE for fabrication, which is more complex to control?
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What advantages do microfluidic systems offer?
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What represents high side wall roughness in nanoimprint using silicon stamps?
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What is the purpose of microfabrication in relation to fluid systems?
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Micro and nanostructures can be combined in a double etching process using different PMMA layers as a mask.
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Surface Acoustic Wave (SAW) nebulization involves large sample volumes.
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____ increase mineral deposition according to the content.
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Exosomes, key to regenerative medicine, are ___ to ___ nm in size.
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Match the following technologies with their outcomes:
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What are Acoustic Radiation Forces (ARF)?
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What is the force related to the radius to the third power for nanoparticles in a standing wave?
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Increasing frequency is the only factor affecting Bjerknes forces.
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In Sound Wave Activated Nano Sieve (SWANS), the particles are trapped and manipulated using the sound field emanating from ______________.
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Match the following applications with their traditional usage:
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What is the main advantage of microfluidic systems in biological experimentation?
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What do microfluidic systems offer in terms of scale?
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Surface tension plays a role in microfluidic systems.
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What is a key challenge related to the flow rate in Surface Acoustic Wave (SAW) nebulization? Need __________ flow rate.
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Match the following techniques with their descriptions:
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Study Notes
Lithography
- Lithography is the step in micro/nanofabrication that allows the transfer from design to physical structures and enables subsequent steps for local etching or deposition of materials.
- The process involves:
- Deposit a resist on the substrate
- Locally modify the resist using electromagnetic radiation (e.g., photons or electrons)
- Develop the resist to remove exposed or non-exposed areas
- Lithography defines:
- Minimum resolution (smallest feature size)
- Structure height/depth
- Manufacturing cost
- Final structure quality
Lithography Techniques
- Direct Write Laser: uses a laser to modify the resist
- Photolithography: uses UV light to modify the resist
- Electron Beam Lithography: uses an electron beam to modify the resist
- Alternative lithographies: other methods, e.g., ion beam, X-ray, and extreme UV
Patter Transfer
- Patter transfer involves adding or removing material from the substrate in a controlled way
- Additive processes: materials are added to the substrate surface
- Techniques: mechanical, chemical, and thermal
- Examples: Si, Al, Au, Ti, W, Cu, Cr, O, N, and Ni-Fe alloys
Physical Vapor Deposition (PVD)
- PVD: direct line-of-sight impingement deposition techniques
- Thermal evaporation:
- Vapor creation using resistive heater or electron beam
- Vapor flux towards the substrate
- Condensation on the substrate
- Sputtering:
- Other PVD methods
Chemical Vapor Deposition (CVD)
- CVD: material addition through chemical reactions on the surface
- Fundamentals:
- Low-pressure CVD (LPCVD)
- Plasma Enhanced CVD (PECVD)
- Applications:
- Oxides, silicon nitride, polysilicon, and some metals (e.g., W or WSi2)
Adhesion
- Atoms arriving at the substrate:
- Adsorption: van der Waals forces (physisorption) or chemical bond (chemisorption)
- PVD: physisorption
- CVD: chemisorption
- After arrival, atoms can:
- Desorb: when physisorbed
- Diffuse: when physisorbed
- Stay fixed: when chemisorbed
Etching Processes
- Pattern transfer by etching methods:
- Dry etching: material removal using gaseous species
- Wet etching: material removal using liquid species
- Dry etching techniques:
- Vapor etching
- Ion etching/milling
- Plasma etching
- Reactive ion etching (RIE)
- Deep reactive ion etching (DRIE)
- Wet etching processes:
- Isotropic chemical wet etching
- Anisotropic etching of Si
- Micromachining
Isotropic vs Anisotropic Etching
-
Anisotropy ratio A:
- A = 0: isotropic etching
- A = 1: perfectly anisotropic etching
-
Importance of masks in etching processes### Types of Etching
-
Isotropic etching: No direction preference in the etching process
-
Anisotropic etching: Direction preference in the etching process
Types of Masks
- Resist: Normally etched during the process
- Metal: Used as a mask
- Oxide or Nitride: Used as a mask
Selectivity
- Measures how fast the material to be etched is removed compared to the mask
- Example: Removal of SiO using HF
Dry Etching Processes
-
Vapor Chemical Etching:
- Uses a chemical in gas/vapor phase to remove material
- Typically used to remove sacrificial layers
- Avoids stiction of free-standing parts in MEMS devices
- Characteristics: Very good selectivity, isotropic
-
Plasma Enabled Etching:
- Pure physical etching: Ejection of material due to high energy ions collisions
- Characteristics: Poor selectivity, rather anisotropic
- Examples: Ion milling/ion beam milling (IBE), Sputtering/ion etching
- Energy levels: 3-5 eV for physisorption, 4-10 eV for surface damage, >10 eV for ion implantation
-
Plasma Enabled Etching (continued):
- Pure chemical etching: Material removed due to reaction of neutral chemical species
- Characteristics: Good selectivity, rather isotropic
- Examples: Fluorine, chlorine
- Image: Fundamentals of Micro and Nanotechnologies, Vol II
-
Physical/Chemical Etching:
- Combined etching process
- Characteristics: Good selectivity, isotropic or anisotropic, high etch rate
- Examples: Reactive Ion Etching (RIE)
Inhibitor Driven Anisotropy
- Simultaneous etching and passivation
- Characteristics: Very high selectivity (>1:50), very anisotropic, low etch rate
- Deposition of CF type polymer chains
Deep Reactive Ion Etching (DRIE)
- Cycling between etching and passivation
- Characteristics: Very good selectivity with metals, extremely anisotropic
- Scalloping effect, micro-loading, very deep etching (even full wafer)
Wet Etching Processes
- Use of a liquid bath to remove material
- Based on a chemical reaction
- Characteristics: Isotropic, very high selectivity to masks, difficult to control
- Used in bulk micromachining, to clean surfaces, to remove whole layers, etc.
Wet Anisotropic Etching of Silicon
- Si wafer is a single crystal material
- Acid baths render isotropic etching while alkaline baths result in anisotropic etch
- Anisotropic etching can be achieved as the bonding energy of the Si atoms depend on the plane they are
- KOH is a good example of anisotropic Si etching
Micromachining
- Surface Micromachining: Deposition of poly-Si, dry etching (RIE), etching of SiO2 using HF
- Bulk Micromachining: Deposition of SiO2, dry etching (RIE), etching of Si using KOH
Summary
-
Dry etching provides a versatile and highly controllable method to remove materials
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Wet etching is a fundamental method to clean surfaces, remove large amounts of materials and process sacrificial layers
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Micromachining methods allow the creation of movable parts for MEMS applications
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Both isotropic and anisotropic etching have important applications### Thin Films Thickness Measurement
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Thin films can exhibit different colors depending on their thickness, allowing for non-destructive measurement.
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Optical thin film thickness measurement uses reflectometers and transmittometers, which are non-contact and non-invasive methods.
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Ellipsometry is a more powerful and complicated tool for measuring thin film thickness, requiring practical experience.
Microscopy Methods
- Bright field microscopy uses attenuated light, resulting in a common mode with no contrast.
- Dark field microscopy uses scattered light, resulting in edge enhancement.
- Differential interference contrast microscopy uses interfered light, resulting in a 3D appearance.
- Phase contrast microscopy uses contrast from phase shift due to the specimen, and is commonly used for live cell observation.
- Fluorescence microscopy observes fluorescent light, and is used for applications such as auto-fluorescence and cell/tissue labeling.
Optical Profilometry
- Optical profilometry is a microscope that can scan in the vertical direction to obtain 3D images.
- It has a vertical resolution of less than 1 nm and a vertical scan range of around 1 mm.
- The method is non-contact and non-destructive.
Scanning Electron Microscopy (SEM)
- SEM uses electrons instead of photons for imaging, allowing for higher resolution and magnification.
- It has a magnification of up to 500,000X and can be used for accurate dimensional measurement and nanoscale inspection.
- The method can be used for tilted inspection, but soft samples can be damaged by the high energy electron beam.
Atomic Force Microscopy (AFM)
- AFM uses a cantilever probe to scan the surface and a laser to obtain the surface height.
- The method has a height resolution of around 0.1 nm and a lateral resolution of less than 10 nm.
- It can be used to obtain a nanoscale 3D surface map and roughness of the surface.
Inspection and Metrology
- Inspection and metrology methods are fundamental to evaluate the quality of manufactured devices.
- Optical microscopy is suitable for dimensions larger than 0.5 µm, SEM is suitable for structures down to 1 nm, and AFM is suitable for 3D mapping of surfaces.
- Reflectometer/transmittometer is a simple and effective tool for measuring dielectric films, while ellipsometry is a more powerful and complicated tool.
Micro and Nanotechnologies
- Micro and nanotechnologies have enabled the development of sensors, such as optofluidic sensors for cancer cell analysis.
- Optofluidic sensors use colorimetry to detect changes in the concentration of analytes, and can be fabricated using soft-lithography.
- Microneedles are used for painless biomedical analysis, and can be fabricated using electroplating/electrodeposition.
- Photonic nanofences have been developed for enhanced environmental sensing, using evanescent fields to interact with the environment.
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Description
This quiz covers the fundamentals of lithography, including mask and mask-less lithography, their advantages and disadvantages, and common techniques such as direct write laser.
