Materials and fabrication methods Flashcards
Graphene
- 2D carbon sheet
- chemically most reactive form of carbon
- very high electrical and thermal conductivity
- harder than diamond and 300 times harder than steel
- electrons travels through graphene as if they have no mass with v=10^6m/s (Fermi velocity)
Four most important properties that differ Nano- and microscale material from macroscale material
1 gravitational forces become negligible and eletromagnetic forces begin to dominate
2 greater surface-to-volume ratios - mag. and chemical properties can be tuned, increased storage area relative to volume for drug delivery
3 random molecular motion become more important
4. quantum mechanism is used to describe motion and energy instead of classical mechanics (only nanoscale).
superparamagnetism
- Unstable
- describes the effect of random flips of the magnetisation direction in small ferroma. particles under the influence of temperature
- Below a certain blocking temp T_B (Curie temp.)–> behaves like a ferromagnet
- determine the upper bound for storage density in magnetic recording (ex. hard disc drives)
- Each particle will be a magnetic domain, and the overall cluster will have no remanence, i.e. it
behaves like a paramagnetic material - an external B-field is needed to magnetize the particle = the absence of an external field the particle do not retain any magnetization
Type of solid material, form of unit particle, force btw particles, example
MOLECULAR - atom or molecules, wan der waals, hydrogen bridges, glycine, surcrose
COVALENT NETWORK - atom in covalent bond network, covalent bonds, carbon (diamond), C60, SiO2
IONIC - positive and negative ions, electrostatic attraction NaCL - Sodium choride
METALIC - atoms, metalic bonds, Ni, Fe, Co
Miller indices
- define the intersection on the x,y,z axis
- Reciption of the intersection
- multi with the least common multiple of the intercepts
- Importance of miller indices: For silicon crystal, (111) plan has more atoms per area than (100)
Paramagnetic material
have a magnetic susceptibility only in the presence of an external magnetic field, i.e. no resonance when the field is removed
ferromagnetic material
become paramagnetic when heated beyond their Curie temperatures. The heating randomizes the magnetic orientation and removes any remanence
Two approaches to manufacturing
- Top-down - starting from bigger building blocks –> reduce into smaller pieces (motivated by manufacturing history)
- Resolution is limited
- uses well-established methods to pattern a bulk wafer (processes adapted from microchip manufacturing)
- thin film deposition
- Bottom-up - smaller parts are stacked together to obtain functional structures (motivated by nature´s way of growing things)
- selected atoms or molecules are added to create structures
- nanomanipulation, self-assembly or chemical methods
- long range order difficult to achieve
substrates
- The natural environment in which an organism lives.
- more atoms per cm^2 –> oxidation faster, but etching is much slower
- oxidation: an oxide layer on top of the bare substrate is often required (as a mask, insulating layer or sacrificial layer), important in microelectronics. The layer can be grown in an oxidation furnace, it consumes silicon in order to grow
substrates
- The natural environment in which an organism lives.
- more atoms per cm^2 –> oxidation faster, but etching is much slower
- oxidation: an oxide layer on top of the bare substrate is often required (as a mask, insulating layer or sacrificial layer), important in microelectronics. The layer can be grown in an oxidation furnace, it consumes silicon in order to grow
additive process
Physical Vapor Deposition (PVD) - descibes a veriety of vacuum deposition methods that can be used to produce thin film and coatings. The material goes from a condensed phase –> vapor phase –> back to a thin film condensed phase.
Chemical Vapor Deposition (CVD) - the film materials are deposited by decomposition of chemicals on the surface of substrate
Electrodeposition
Different types of PVD (physical vapor deposition)
Thermal evaporation (avdunstning):
- a source material is heated until it evaporates –> the evaporated material travels to the substrate where it condenses and is deposited.
ex. heating sources: resistance, e-beam, RF, laser… - both metals and nonmetals
Sputtering:
- Argon is ionized by a strong potential difference –> These ions acc. to a target –> After impact, target atoms are released and travel to the substrate and form a thin film of atoms
- Shadowing effect (non-conformal coating) - due to the straight trajectory of vapor molecules.
Chemical Vapor Deposition (CVD) - Reaction mechanism
- The chemicals are vaporized by heating up or reducing the air pressure –> condensate, “the hot gaseous material natually for a coating on the colder substrate <=> water vapor coats outside of the cold drinking glass on a hot day”
Ex. sunglasses
Electrodeposition
uses the electric current to reduce dissolved metals (Cu^2+) at the chatode(-) to solid atom (Cu^2+ interact with CuCl = CuCl_2) –> form a deposit on the cathode.
- electrolytic cell: consist a working (cathode) and a counter (anode) electrode. A third eletrode can be added to estimate the deposited volume by potentiometric measurements (voltage)
Lithography process
UV light and a mask with desired pattern are used to expose the photoresist and transmit the pattern onto a substrate/wafer.
- Development: exposed resist is washed away while unexposed resist remains –>
1. Deposition: metallic, semiconducting or insulating layers are evaporated or sputtered onto the surface –> photoresist is removed, leaving behind precisely deposited features
2. Wet or Dry Etch: exposed sections are etched away while the resist protects the remaining areas –> photoresist is removed, leaving behind precisely etched features
Lithopgraphy - mask
- Mask polarity: light field (region of interest is coated), dark field (region of interest is uncoated)
- alignment is never perfect –> alignment errors should be taken into account when designing a chip
Photoresist
Positive photoresist: light degrades the polymers resulting in the photoresist begin more soluble in developers
Negative photoresist: light polymerizes the rubbers in the photoresist to strengthen its resistance to dissolving in the developer
Lithography process - exposure
- Contact printing: high resolution (≤ 0.5 micrometer), mask and wafer can be easily damaged
- Proximity printing: 2-4 micrometer resolution (diffraction effect), 10-25 micrometer gap (long mask life)
- Projection printing: High resolution (≤ 0.2 micrometer), Image of mask reduced, scanning of small field, complicated and expensive optical set up. (lite som microscope)
Subtractive process - Wet etching
Wet: removes material from a wafer using liquid phase etchant (purely chemical process)
- Isotropic and anisotropic etching
- Cheaper than the dry etching
Substrative process - Dry Etching
reactive ion etching (RIE) - substrate is placed inside a reactor with several gases. The gasses => ions, which accelerates toward and reacts at the surface. (Chemical part of reactive ion etching = isotropic).
If the ions has high enough energy –> it can knock out the material without a chemical reaction (physical part of reactive ion etching = anisotropic)
Sputter etching - RIE without reactive ions. (physical part of reactive ion etching) Positive charged molecules attack to the surface (high energy bombardment)
Plasma etching - chemical part of reactive ion etching. This method can be done with simpler eq. than what RIE requires.
Wet Etching - isotropic and anisotropic
Isotropic (same etch rate in all direction - bildar en halvcirkel)
- room temperature, slightly above (<50°)
- diffusion limited
- etching is very fast
- undercuts mask
Anisotropic (has diff etch rates in lateral and vertical –> depending on the orientation of the exposed crystal plane)
- higher temperature
- reaction rate limited
- etching is slower
- does not undercut the mask
- Not very agitation sensitive
Other machining techniques
- Ultra-high precision matching
- laser machining (lithography, subtractive, additive)…
Surface and bulk micromachining
surface- adding layers to build up structures on the wafer surface, thin film deposition
bulk - removing bulk silicon to define device structures, wt or dry etching.
Nanofabrication -
- most of the micro-fabrication techniques are used as well, but some additional techniques are required in order to achieve structures with nano-scale features
lift off
Lift-off process in microstructuring technology is a method of creating structures (patterning) of a target material on the surface of a substrate (e.g. wafer) using a sacrificial material (e.g., Photoresist). It is an additive technique as opposed to more traditional subtracting technique like etching.
