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497d Exam 2
| Question | Answer |
|---|---|
| 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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