Deposited (Thin Film) Technology
Deposited technology depends on the
deposit of a thin film by vacuum
deposition. The film properties can
exhibit conductor, resistor or dielectric
behavior
The films range in thickness from a few
nanometers to a micro in thickness
Thin film technology is the basis of IC
metallization and can exhibit very fine
lines and spaces.
Schematic Representation of Vacuum Evaporator
Electron Beam Evaporation Source
Sputtering
Sputtering occurs by
accelerating
an ionized inert ion ,
usually argon,
at the target under a high
bias. The
impact knocks off atoms
from the target that
deposit on the substrate.
Three methods are usually
employed: DC, RF and
Magnetron sputtering
DC Sputtering
Photolithography for Positive and Negative
Resists
Photolithography for Positive and Negative
Resists (Cont.)
Substrate Selection
Thin film substrates must have excellent
surface finish, mechanical strength, good
thermal conductivity, and excellent
dielectric properties (dielectric constant,
insulation resistance)
Materials include glass ( poor thermal
conductivity and strength), ceramics (
alumina, beryllia, AlN), oxidized silicon , and
substrates (including metals) coated with
polymers, such as polyimide and BCB
Properties of Thin Film Substrate
Materials
Thin film Metal Conductors
Conductors
Many good conductors exhibit poor
adhesion. Use of an adhesion layer such as
Ti, Ti-W, Cr, Cr-Cu or NiCr
Conductors include Al, Au ( as Cr-Au, Ti-PdAu, TiW-Au), Cu ( with Cr or Cr-Cu)
Patterns defines by subtractive chemical
etching or additive plating. Multilayer
metallizations must have etch compatibility
Nichrome Resistor Selective Etch Subtractive Thin
Film Process
a) starting substrate with three layer deposit b) After gold etch
c)After nickel barrier etch d) After nichrome resistor etch
Thin Film Resistors
•Sheet resistivities are fixed for a given resistor material
•Typical values are from 50 Ω/sq for TaN or nichrome
to 1000 Ω/sq for complex silicides
•TCR’s are fixed by the material but are low ( ›50 ppm /
oC)
•Since sheet resistivities are fixed, need a large number
of squares for high value resistors, Often use serpentine
patterns
Multilayer Dielectrics
The major development in thin films was the
introduction of polymer dielectric layers,
based on polyimide, BCB, or other organic
formulations which allowed multilayer
fabrication and blind via generation
These films could be patterned by wet
chemical etch, dry plasma etch, and laser
ablation , either by point ablation of area
ablation through a metal mask.
Chemistry of PMDA-ODA Polyimide
Typical Polyimide Dry etch
1) Deposit
Polyimide
2) Deposit mask
3,4) Pattern mask
5) Plasma etch,
barrel or reactive
ion etch
Plated-up Via Holes for Multilayer
Thin Film
1)Define via hole,(2) plate via hole with metal,(3) sputter adhesion
layer(Cr/Cu),(4)apply photoresist and pattern,(5) electroplate Cu,
(6)remove photoresist and etch flash, plate with Ni
Via fabricated by scanning laser ablation
through a metal mask
Before mask removal
After mask removal
Ceramic Based Technologies
Conventional Thick Film Technology, based on
screen printing a cermet paste on a ceramic
substrate and air firing (850oC)
High Temperature Co-fire Ceramic (HTCC) where
the substrate and interconnect metallization are
fired at the same time, using ceramic tape and
refractory metallization and fired above 1600oC in
hydrogen
Low Temperature Co-fire Ceramic (LTCC), based
on glass/ceramic tape and thick film
metallizations, fired in air(850oC)
Thick Film Technology
Screen print cermet paste on inert, high
temperature substrate, typically Al2O3
Fire @ 850oC in air to provide desired
electrical and mechanical properties
Capable of multilayer fabrication, but
requires successive screening of
dielectric/via/metallization pastes
Thick Film Cermet Pastes
Cermet inks have four major
components:
• An active element that establishes the desired
property of the film ( resistive, conductive, and
dielectric elements)
• An adhesion promoter ( glass or reactive oxide)
• A matrix promoter, typically glass or sintering
aides
• An organic binder and solvents
Thick film inks are thixotropic, I.e.,
their viscosity increases with
increasing shear stress
Active Elements
Conductors
• Noble metals if air fired( Au, Ag, Ag-Pd,
Pt-Ag); non-noble ( Cu) if nitrogen fired
Resistors are based on conductive
oxides ( RuO2 , IrO2 , Pb2 RuO7 )
Dielectrics are vitreous, devitrifying, and
glass-ceramic mixtures
Dielectrics
Vitreous glass remelts on firing- good for
cross-over , solder dams or other protection
Devitrifying glass changes from vitreous to
crystalline state on firing. Used for multilayer
dielectrics with multiple firings, as viscosity
will react like a crystalline solid, not a glass
Glass-ceramics are mixtures of glass and
ceramics ( usually Al2O3).). Use of high
dielectric constant ceramics (BaTiO3) allows
development of cermets with higher dielectric
constants for capacitor structures.
Thick Film Resistors
The development of thick film resistors is a
major driver for the technology.
Since thick films have uniform thickness (
˜0.7 - 1.0 mils), one can consider sheet
resistivity, Rs, ( Rb/t) and R=RsL/W
Rs is given in ohms/square, since equal L
and W would be a square, independent of
absolute dimension. R is only dependent on
the number of squares. Size determines the
power rating
Thick Film Resistors (Cont..)
Thick film resistors are a mixture of the
resistive element and glass. By varying the
ratio, cermets can be fabricated that have a
range from 1 ohm to 10 MΩ/square. This is
unique to thick film technology.
Thick films also have temperature
coefficients of resistance ( TCR) in the range
of 50-150 ppm/oC
Thick film resistors have power ratings of
50W/in2
Conductors
Typical conductors are noble metals- Au,
Ag, Ag-Pd, Ag-Pt, Pt-Pd-Au
Choice of conductor system based on
•
•
•
•
•
•
•
•
•
Resistivity
Solderability
Solder leach resistance
Wire bondability
Migration resistance
Thermal-aged adhesion
Line/spacing resolution
Cost
Compatability to other materials and assembly
processes
Thick Film Paste Components
Conductor Properties
Gold• where high reliability is required and cost
justified ( highest cost material)
• eutectic die attach is used
• gold ball bonding is used
• Au forms intermetallics (Al-Au) and
dissolves in Sn readily.
• Alloying with other noble metals (Pt, Pd)
can prevent AuSn or AuAl alloys forming,
but increases bulk resisitivity.
Conductor properties
Silver based
• Pure silver has lowest cost but leaches rapidly
into solder and undergoes silver migration under
bias/humidity conditions
• highest conductivity of all thick film materials,
used as inner layer in multilayer co-fire
• Alloying with other noble metals (Pd or Pt)
reduces both tendencies to leach and migrate. Pd
is primary alloying agent but either raises
resistivity. Example: pure solder leaches in less
than 5 sec. In solder, but a 20% Pd alloy can be
withstand over 30 sec. prior to leaching
Photo-defined Thick Films
For fine line definition required for high
frequency or high density applications,
new methods of pattern definition for
thick films are required for both
metallizations and via formation.
The use of photo-defined processing,
either prior to or post firing, have yielded
sub-50µ lines and vias
Photopatterned Thick Film
Ion bean etched
10µ lines/spaces
50µ lines/spaces
35µ lines/spaces
50µ via
Ink Fabrication
Glass phase is ball milled to required particle
size
Metal powder is added, chosen for proper
density, surface area and particle shape
morphology. Particles are between 1-5
microns, with either a spherical or flat
morphology
Organics are dispersed and components
mixed in three-roll mill
Particle size is confirmed on a fineness-ofgrind (FOG) gauge and viscosity is measured
with a cup-and-cone spindle viscometer
Ink Viscosity
Viscosity is a measure of the tendency of a fluid to flow
under load and is the ratio of the shear rate of the fluid to
the shear tress applied
Ideal fluids follow “Newtonian” behavior, where the
relationship is linear. Water is nearly a Newtonian fluid
Thick films are “ thixotropic”, where shear rate/shear stress
is non-linear. As the shear rate increases, the paste
becomes more fluid
Thick films have a “yield point” or minimum pressure to
produce flow
Thick films should have some hysteresis, I.e. viscosity is
dependent on whether the pressure is increasing or
decreasing, being higher with decreasing pressure
Viscosity Response
Fluid Response to Shear
Rheology of Pd-Ag conductors
Screen Printing Terms
Screen: Fabric, Mesh, Angle of Attachment, Tension
Emulsion: Polyvinyl Alcohol or Polyvinyl Acetate are
water soluble until exposed to UV light
Stencil: a metal foil is used instead of a screen ( for
thick deposits, like solder pastes)
Squeegee: Blade used to move ink over the screen.
Downstop: Limits downward travel of the squeegee
Snapoff: Distance from the bottom of the screen to
the substrate
Screen Printing
Patterning the thick film material onto a
substrate is done by screen printing, a
process of forcing the ink through a
patterned screen, typically stainless steel
Pressure is applied to the ink by the action of
a “squeegee” traversing the screen
•
•
•
•
dependent on pressure of squeegee
angle of inclination of the squeegee blade
speed of traverse
viscosity of the ink
Screen Printing ( Cont.)
Pressure brings the screen in contact with the
substrate, which allows the ink to wet the
substrate with the screen forming a seal to the
substrate to maintain the desired dimension
As the squeegee traverses the screen, the
mesh behind the squeegee separates from the
substrate.Ink that has wetted and adhered to
the substrate is drawn through the screen
opening and remains on the substrate. The
reduction in shear causes the viscosity of the
ink to increase, and the ink to stop flowing.
Screen Printing Process
Screens For Thick Film Printing
Effect of Squeegee Attack Angle
Visualizing
the Ink
Transfer
Process
Firing
After printing the inks are dry to remove the solvent (
~ 150oC), and fired to sinter the cermet into a
functional film
Air is required on firing to provide oxygen to assure
complete binder removal (burn-out)
Can use belt, IR or programmable box furnaces with
air purges
Volume of air for belt furnace firing follows the
PLAWS equation ( volume=P(Printed paste ratio),
L(ratio of substrate area to belt surface),A is a ink
constant,typically 0.4 liters/cm2, W is the belt width
and S is belt speed)
Firing Cycle
Sintering
Sintering is the process
which allows densification
of powered materials at
temperature below their
melting points
Diffusion occurs where
particles are in contact,
bridging a “neck” between
the particles, with
continued growth and
densification
Cofire Technology
High temperature cofire ceramic (HTCC) is
based on co-firing a ceramic( usually
Al2O3)and refractory conductor ( typically W)
above 1600oC, in a reducing atmosphere
Low temperature cofire ceramic (LTCC) are
based on air co-firing a glass/ceramic
dielectric with a noble metal conductor
(Ag,Au). Firing temperature are similar to
thick film technology
Both processes are tape based processes
Process Flow for Cofired
Technology
Tape casting
Blanking
Via formation
Metallization deposition
Lamination
Firing
Brazing/(Plating for high temp cofire)
Singulation
Crossection of a Cofired Package
Tape Casting Process
Tape casting produces uniform green tapes from less
than one mil( for capacitors) to 25 mils in
thickness.LTCC is typically 4-5 mils
Tape is made by “doctor bladeing” a liquid ceramic
slurry to the desired wet thickness on a carrier film,
then drying the film and separating the green sheet
from the carrier
Tape Blanking
After casting, the “green”
tape is blanked to the
appropriate size for
additional processing .
Tooling and registration
holes are added
at this stage
LTCC PROCESS
A multilayer circuit or component made
by laminating together green (unfired)
sheets containing printed interconnection
& components and then firing the
structure to form a rigid monolithic
ceramic multilayer circuit.
Green Sheets
Lamination
Fired Ceramic
Multilayer
Cofire Semiconductor Packages
Dual-in-line construction
Pin-grid array
construction
Via Formation
Vias are fabricated by punching holes in the green
tape (mechanically with a gang punch or
programmable punch or with a laser( single point or
gang eximer))
Vias are filled with conductor pastes using a modified
form of screen printing, usually contact print through a
stencil with vacuum pull-down.
Squeegee
Tape Processing
After via formation, conductor metallization
is screen printed on each layer
Individual layers are stacked and laminated
in the green state
Parts can be cut to final size or scored prior
to firing for ease of singulation
The green part is subjected to burn-out and
firing.These steps insure the organic binder
is removed and the ceramic particles sinter
to a dense, solid structure.
High Temperature Cofire
Alumina or aluminum nitride tape, usually 10-20 mils
thick, fired in a reducing atmosphere at the sintering
temperature of the ceramic (above 1600oC). Complex
firing cycle to burn out binders and not oxidize
metallization
Metallization is refractory tungsten (sometimes Mo),
which has the same CTE as the ceramic
Exposed metallization is plated (Ni-Au) for next level
of assembly
Technology compatible with Ag-Cu eutectic brazes.
Metal components ( seal rings, lead frame) are
fabricated from Kovar/alloy 52
Low Temperature Cofire
Glass/ ceramic tape, usually 3-5 mils in
thickness, with a firing range of 850-950oC
Punched and filled vias
Internal metallization usually Ag, with top
layers Pd-Ag or Au.
Compatible with cofired/post fired thick film
resistors
Firing ramps to 1oC/min for binder burn-out
Allows buried components ( inductors,
capacitors, resistors)
Integrated RF Module
LTCC
• Integrated Passives
• 3-D designs
• Controlled Impedance
• Hi Q
• Size reduction
• Direct Chip Attach
• Rapid Prototypes
Applied Microwave & RF July/Aug 1998 pg. 45
Murata Electronics North America
LTCC
Low Temperature Cofired
Ceramics with Buried
Components
LTCC Wireless Module
RF ICs
Surface Acoustic Wave (SAW) Filter
Digital and Analog I/O
Discrete Devices
(transistors, diodes)
RF Feedthrough
PLL IC
Multilayer Ceramic
W/ Buried Components
(i.e.., resonators, filters,
capacitors...)
Baseband Processor IC
Vertical Integration in Packaging
A Paradigm Shift
# of Passives: 200
Assumptions:
Comp. Value: 20pF/20nH
Comp Size: 3 mm Sq
Q Value:
30 - 100
# Layers:
8
Area:
25 X 25 mm
Approx. Cost:
$3.60
Cost Savings:
>$4.00*
* Doesn’t anticipate cost savings
from repartitioning of functions
between IC’s and integrated
substrates