ME 585

Microfabricated devices need to have competitive advantages such as ___, microelectronics integration, batch fabrication, and price

miniaturization

Linear elastic, brittle material

silicon

Electrical characteristics of silicon can be precisely controlled by addition of

dopants

P type dopant (group 3)

boron

N type dopants (group 5)

arsenic phosphorous

Transforms signals or power from one form of energy to another

transducer

Transform stimulus signals to a domain that is detectable

sensors

Transform energy from a non-mechanical domain to the mechanical domain

actuators

Cantilever's reaction to external stimulus

mechanical stress

Observations about how physics work at different sizes

scaling laws

As a shape grows in size, its volume grows faster than its surface area

square cube law

Stiction: liquid bridge formed between suspended member and substrate during liquid etching, when liquid is removed large ____ force is generated

capillary

Volume is related to

mass, weight, mechanical and thermal inertia

Surface is related to

pressure, buoyant forces

Remaining physical dimensions are assumed to scale linearly with the ___ dimension

characteristic

The ___ is a single crystal cut in slices from a larger piece called a wafer

substrate

Devices with multiple functional elements mounted on one chip

integrated circuit

Why silicon - mechanically stable, can be integrated with electronics on the same substrate, high ___, lightweight, high melting point at ___ C, virtually no mechanical hysteresis, extremely flat

young's modulus, 1400

Silicon structure: ___ with ___ additional body atoms

FCC 4

Single crystal silicon growth, technique for large crystals

CZ

Single crystal silicon growth, technique for smaller wafers with low oxygen impurity

float zone

Review CZ vs float zone

y

Principle uses of SiO2: thermal and electric ___, as a mask in ___, and as a ___ layer

insulator etching sacrificial

SiO2 is etched with

HF

Wet oxidation grows faster but leaves more ___ and has a lower

dangling bonds, density

SiC is stable, resistant to ___, can protect MEMS components from high temperatures

oxidation

Si3N4 is a good diffusion barrier and has high resistance to oxidation and many ___

etchants

Si3N4 dep technique

LPCVD PECVD

Poly si dep technique

LPCVD

Photoresists are typically ___ and require the same type for substrate surface

hydrophobic

High hydrophilic means small contact angle from ___ surface energy

high

SiO2 surfaces are covered by a layer of water physiosorbed form air that must be removed to avoid loss of ___

adhesion

Adhesion promotion steps: ___ at 100-200 C under vacuum, then exposed to adhesion promotion agent ___

dehydration, HDMS

___ priming typically has better results

vapor

Exposure to UV light makes the area more soluble in a developer and it is washed away

positive photoresist

Photoresist application by spin coating: good homogeneity, short process times, poor ___

material utilization

Softbake: removal of residual ___ and annealing of potential film ___

solvents stress

Smaller quartz plate with a few inset die used in stepper or scanner system over multiple exposures

reticle

Related to alignment for reticle

fudicials

___ exposure: glass mask in contact with substrate and the exposure is by shadow projection using parallel light

contact

___ exposure: small gap between glass mask and substrate, gap minimizes mask damage

proximity

___ exposure: image of glass mask is projected on a resist coated substate from a distance, no risk of mask damage

projection

___ exposure achieves the highest resolution if mask (reticle) images only a single device

projection

Most common light source for mems exposure

mercury vapor lamp

I line wavelength ___ nm

365

Post exposure bake used for the mechanical relaxation of thick photoresists like

SU8

Latent image formed by exposure is converted into final 3D relief image

development

Photoresist related issues: lack of adhesion and edge bead, uncoated areas, ___ defects

pinhole

Remove residual development solvents and anneal the film to promote adhesion

hardbake

___ lithography: electron beam can be used to write patterns of very high resolution in radiation-sensitive resist

ebeam

___ can be used in nanoscratch lithography to create grooves and trenches by using the cantilever to carve

AFM

___ uses a solid or liquid source material for creating thin films

PVD

The ___ of the system directly affects the quality of the film

vacuum degree

High vacuum allows vapor molecules to travel ___ distances before collision and reduces ___ of growing film

longer contamination

In vacuum chamber, evaporation material is only evaporated when its vapor pressure is above

1 pa

___ heating: High melting point conductor is fashioned into a basket or boat. Evaporation pellets are placed on conductors and high current is passed through them

direct

___ heating: heat resistant and stable materials are used to make a crucible, and heater heats and evaporates the material through the crucible

indirect

Thermal evaporation is a ___ process

PVD

Thermal evaporation is a ___ deposition process under ___ vacuum conditions

line of sight, high

Induction heated source uses RF to create ___ currents that heat directly at the evaporant, making the process more efficient than resistance heating. Needs ___ cooling

eddy water

Electron beam heating allows high ___ density, low contamination, and high control of evaporation rate

power

In e beam heating only the ___ melts due to water cooled crucible

surface

E beam heating allows much higher temperatures for materials such as

Pt, SiO2

Plasma based deposition technique in which energetic ions are accelerated towards a target, ions strike target and atoms are ejected from the surface and incorporate into growing film surface

sputtering

For sputtering there is ___ vacuum level, high voltage is applied to initiate plasma generation, electrons in sputtering gas are accelerated away from the ___, causing ionization

high cathode

Sputtering gas chosen to have high ___

molecular weight

Reactive gas sputtering: higher ___ rates and more precisely controlled stoichiometry

deposition

In ___ sputtering magnetic fields are used to control ion velocity and behavior

magnetron

Magnetron sputtering traps electrons by ___, allowing faster deposition

target

Removing atoms by sputtering with inert gas, low surface damage etching technique

ion milling

DC sputtering is limited for ___ materials

dielectric

RF sputtering alternates electric potential of the current to avoid ___ buildup, increases ___

charge growth

Very dense thin film coatings from high current voltage spike that increases ionization

high power impulse magnetron sputtering

Nearly all materials can be deposited by ___ sputtering

magnetron

In sputtering alloys and compounds can be deposited with maintained

composition

Used to measure film thickness during sputtering

quartz crystal oscillator

Evaporation of alloys from a single source is possible if there is no ___ of the components into fragments

dissociation

Co sputtering most often used with reactive ___ sputtering

magnetron

Co sputtering uses multiple cathodes with ___ power controls

independent

Co sputtering can result in ___ damage to the substrate

heat

Better step coverage?

Sputtering

Which is slower? Sputtering or evaporation

sputtering

Easier control of film properties with

sputtering

Growth mechanism for single crystal films that receive their crystalline orientation from a substrate (also with a single crystalline structure)

epitaxial

If single crystal film and substrate are the same material

homoepitaxial

Single crystal film and substrate are different materials, typically showing a lattice mismatch

heteroepitaxial

Molecular beam epitaxy: ___ cells surrounded by liquid nitrogen

effusion

Effusion cell: Elongated crucible with radiation shields, stable up to temp of ___ C

1400

MBE: ___ vacuum conditions-> very long mean free path -> no ___

ultrahigh, collisions

Particularly suited for deposition of ceramic materials

PLD (pulsed laser deposition)

High power laser pulse ablates material from a target due to ___ interaction

photonic

In PLD highly directional ___ plume recondenses to form a film

plasma

Single target is typically sufficient for complex composition, multi target for layered structures

PLD

Most widely used deposition method

CVD

In CVD films deposited are result of chemical reaction between the reactive gas(es) and ___, and/or between the reactive gases and the ___ of the substrate surface

reactants atoms

CVD ___ reactions occur before the gas molecules reach the wafer surface, slower reaction rate, low ___ and lower ___ film

homogeneous density quality

CVD ___ reactions occur on or near the substrate surface, better ___

heterogeneous quality

Both gas phase and surface phase deposition are greatly affected by

temperature

Uses vacuum pump to reduce pressure in reaction chamber to less than 1 atm

LPCVD

Low pressure, but also uses plasma to provide higher deposition rates at lower temperatures than LPCVD

PECVD