Microfabrication Flashcards
What is microfabrication?
The process of producing MEMS devices
Where should the process be conducted?
In a clean room. Laminar air flow forces particles (smaller than 0.1um) towards the floor where they are filtered using a HEPA filter. Rated from Class 1 (1000 particles per m3) to Class 10,000 (10,000,000 particles). All surfaces are stainless steel, which is non shedding.
What is the starting material?
Doped silicon wafer. Either N - Phosphorus or Arsenic with 5 outer shell e- or P - Boron - 3 e-. Creates a semi conductor with a free electron to carry current. Normal silicon has 4 e- and as such has no free electrons
Why/how do we clean the silicon wafer?
Particles on the surface (hair, skin, fabric, tweezers etc) can cause defects in deposited materials - clean with DI water (removes electrostatically bound particles) and sonic agitation - dump rinse with DI and dry with N gas
Films on the surface can cause defects and affect adhesion. Remove with 1:3 ratio of H202:H2SO4, DI water and N2 gas
What are the methods of deposition?
Thermal oxidation
CVD
PVD
How does thermal oxidation work?
Used to deposit SO2 on surface. Uses N2 as carrier gas. Two types;
Dry - slow (0.1um/hour) but pure - gate oxide/pad oxide
Wet - uses water - converted to OH at high temp which passes through oxide layer quicker than O2. Produces thicker layer, not as pure
Operates at high temp, around 1000 degrees (increases reaction rate) and high pressure (increases passage of O2 through oxide layer to silicon)
How does CVD work?
Used to deposit Polysilicon and Silicon Nitride
Operates in the vapour phase, deposits materials onto substrate. Increase reaction by increasing temp and pressure
How does PVD work?
Used to deposit metals (Au, Al, Pt) and PZ material (PZT) Operates in a vacuum (no particles to interfere) and heat silicon wafer (to increase adhesion)
Two types:
Thermal Evaporation - used for materials with lower melting point. Uses a filament heater to heat metal.
E-beam evaporation - use charged tungsten filament to fire electrons at target anode - converts metal to gaseous atoms - condensate on any solid surface.
What are the benefits of e-beam evaporation over thermal evaporation?
Does not use a filament heater - this can provide impurities
Greater control over evaporation of metal
What binding materials do we use for Al and Au
Pt and Cr
What is photolithography?
Patterning of material
What is a photoresist?
Material that we use to produce underlying resist or layer
How do we apply a photoresist?
Spin onto wafer
Pre-bake at 80 degrees for 5 mins
What is a photomask?
Quartz glass with specific pattern
How do we use UV beams to pattern the surface?
UV beam passes through photomask and either increases solubility (positive photoresist - DNQ/Novalac) or decreases solubility (negative photoresist - epoxy based)
What is the diffraction limit of UV light?
500nm
What is the benefit and negative of E-beam lithography
Higher resolution (less than 100nm) Slower/expensive
What is the etch rate determined by?
Rate of etchant getting to surface (pressure/stirring)
Reaction of etchant at surface (temp)
What is etch rate measured by
nm/min
um/min
What is etch selectivity?
Etch rate of desired layer/Etch rate of underlying layer
What is an excellent etch selectivity?
Greater than 100
What is an ansiotropic etchant?
Directionally dependent (R=0)
How do we calculate whether an etchant is ansio or iso tropic?
Lateral etch rate/vertical etch rate
What are the two types of etching?
Wet - chemicals - isotropic
Dry - gas beam - ansiotropic
How can we etch Silicon?
KOH
How can etch Silicon Dioxide?
HF
How can we etch resist?
O2
