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497d Exam 2

What is the 2nd most abundant element in earth's crust? Raw Silicon - Silica, SiO2
What is the degree of atomic order in a solid? Crystallinity
What is the difference between Crystalline and Amorphous crystallinity? Crystalline - atoms arrange in periodic lattice; Amorphous - lack long range order
Two methods to create single crystals? Czochralski and Flat Zone Crystal Growth
Explain the Czochralski method A single crystal seed is grown into a single crystal ingot by pulling slowly upward from molten ultra-pure Si
Explain Flat zone crystal growth Seed crystal is placed at end of ingot and rod is locally melted so impurity atoms concentrate in liquid and segregate at end of crystal
For _____ resist, solubility in developer INCREASES with exposure Positive
For _____ resist, solubility in developer DECREASES with exposure Negative
What is oxidation layer? SiO2 insulating layer for ICs, MEMS; react Si wafers with oxygen atoms at high temperatures to form
General idea/two types of Physical vapor deposition Direct transfer of material by HEATING or HIGH-ENERGY ION BOMBARDMENT
Quick point on chemical vapor deposition Chemical reaction occurs in gas above surface and solid phase is absorbed onto wafer below
General ideas of wet etching Etchants SELECTIVELY remove material from thin film
How does dry etching work? Plasma and reactive ion - charged radicals interact with wafer, more directional and anisotropic than chemical etching
What are the three levels of integration of ICs and MEMS? Wafer-level, Chip-level, Board-level
Advantages/Disadvantages of wafer level? +circuitry, low noise, small devices; -inefficient space utilization and complex processing
Advantages/Disadvantages of chip level? +decouples materials and fabrication issues; -longer distance for circuitry
Disadvantages of board level? -longest wiring, large noise, large devices
How do you grow the SiO2 layer? By heating between 900-1150 C in steam or humidified oxygen stream
What is purpose of oxide layer? Serves as mask for subsequent wet etch or doping
Basic 10 steps of photolithography? Clean, photoresist, soft bake, expose, develop, hard bake, etch, inspect
Affects of pos/neg resists on polymer chains? Positive - weakens chain for more soluble resist; Negative - strengthens chain for less soluble resist
Three important aspects of resists? Tone, contrast, sensitivity
Resist - explain contrast versus sensitivity Contrast - development rate as fnc of exposure, SLOPE; Sensitivity - point where reaction starts
Three components of a resist? Polymer, sensitizer, casting solvent
When to use pos/neg resists? Positive - better when leaving most material, for isolated holes or trenches; Negative - better when taking most away, good for isolated single lines
Exposure - Substrate for near UV vs deep UV Near - flat glass; Deep - quartz (MOST)
Common wavelengths for exposure? used to be Hg (near UV = 350+), more today ArF (deep UV = 150-300)
Measurements of quality for plasma etching Defects and particles, step height, critical dimensions
Undercut for positive resists result from what? Overexposure
What are lift-off profiles desirable for? Two possibilities? Patterning to aid local deposition processes; 1.use neg resist 2.modify pos resist to undercut
What factors control resolution? Hardware, Materials, Processing
What are the control tolerances for resolution? tolerances = 1/5 of minimum feature size
What is the consistent ability to print a minimum size image? Resolution
What is the critical dimension of resolution? specific minimal feature size of an IC or microdevice to be made
Near-field resolution? Fresnel diffraction, shadow printing
Far-field resolution? Fraunhofer diffraction, projection printing
Parameter for projection printing and meaning? k1=related to wavelength and aperture
How to enhance resolution? Larger lens, short wavelength
What do we want of depth of focus and resolution? both high - tradeoff
How to enhance depth of focus? Smaller lens, long wavelength
What are RET and how do they work? Resolution enhancement technology; improve overall lithography resolution to half wavelength or better
Three properties of RET's 1.Improved resist performance by QUALITY; 2.Improved masking by PHASE SHIFTING; 3.Improved exposure by IMMERSION
What is a phase shift mask? Shifting layer on mask causes phase of light to reverse - photoresist is only sensitive to intensity, not phase
What does immersion change? Reduces NA
4 types of lithography X-ray, extreme UV, Electron and Ion-Beam, Non-radiation based
Which lithography method is most common? Not economical? extreme UV?; Electron/ion beam
What are the common chemicals for Isotropic wet etching? HNO3 (nitric acid) and HF (hydrofluoric acid)
Which combination results in best polish for iso wet etching? Low HF, high HNO3
Tips on isotropic/anisotropic wet etching ISO is faster and acids; ANISO is slower, alkalines/bases
Atoms of a material situated in a repeating or periodic three dimensional array over large distances? Crystalline
Structure of silicon? Diamond cubic, kind of FCC
Miller indices symbols: (), {}, [], <> Plane, family of planes, direction, family of directions
Etching is fastest in which direction? 110, less dense
Why use dry (plasma) etching? advanced IC with small geometries; line widths comparable to film thickness; high aspect ratio; material; less costly disposal of wastes
Explain dry etching Plasma used for stream of high energy ions to either blast with beam or chemical reaction or COMBO
4 losses of fidelity in etching? Faceting, ditching/trenching, redeposition, backscattering
What is RIE? Reactive ion etching, mostly etches down, also horizonal
What is DRIE? inhibitor coats walls to inhibit reaction there - sharp and deep
Why dry etch? No photoresist adhesion problems, anisotropic is possible, small chemical consumption BUT poor selectivity and residues leftover
Two common bases for anisotropic wet etching? KOH and EDP
Slope of resulting shape from anisotropic wet etching? 54.7, in the 111 direction
Two profiles from anisotropic wet etching? SLSP, self limiting stable profile; STP, Transitional profile
What does SLSP end in? Points or lines, NOT planes
Why do we do additive processing? Add material to substrate to make devices
What are the three main ways to deposit a conductive layer? PVD, CVD, Plasma Spray, (screen printing)
Doping - which element would you add to create an N-type? From Group V, Phosphorus
What non-uniform distribution methods exist for doping existing wafers? Thermal diffusion and ion implantation
What does Fick's law define? Diffusion
Natural oxidation thickness? Sacrificial layer thickness? 20-30 nm ; 100 nm - 1.5 um
Law defining oxide layer thickness? Healgrove?
General idea of physical vapor deposition sputtering, no chemical reaction
Analogy for sputtering? Imagine throwing large massive rocks into a lake with intent of splashing the water
General process for chemical vapor deposition? Deposition of thin film Si and Cu to react at a hot surface and deposit solid film
Two ways to help chemical vapor reaction occur? Heat the surface or dump plasma
Three common MEMS materials deposited by CVD? Polysilicon, Silicon dioxide, Phosphorus doped silica glass
What are the advantages/disadvantages of CVD? +not a line of sight method so excellent step coverage and improved uniformity; -hazardous waste by gaseous byproducts and high temperature required
Created by: aj_bianco
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