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*THE THIN FILM TECHNOLOGY
Subtractive technology:
The entire substrate is coated with several layers of material. The unwanted material is removed by a series of selective photoetching processes.
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*Cross-Section of Thin Film MetallizationNiTaN
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*Thin Film Circuit
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*THIN FILM DEPOSITION TECHNOLOGIESVacuum DepositionSputtering - The material is deposited from a target by bombarding the target with charged gas ions in a plasmaEvaporation - The material is deposited by heating it until the vapor pressure is much greater than the surrounding mediumChemical DepositionElectroplating - A thin layer of material is deposited by one of the vacuum deposition processes and plated to thickness by plating in a solution of the material
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*SPUTTERING
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*SPUTTERING
PROCESS SEQUENCESystem is pumped to a high vacuum ( 800 V) between target and substrate (substrate at ground)Potential may be RF or DC
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*Basic Triode Sputtering SystemSubstrateTargetThickness Monitor
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*Planetary Sputtering System
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*Planetary Sputtering System: ChamberTargets (3)
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*Planetary Sputtering System: Substrate Holding Fixture
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*Lateral Sputtering SystemVacuumInterlockSputteringChamberControlPanel
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*EVAPORATIONOccurs when vapor pressure of material exceeds the ambient pressure
May occur from liquid state or solid state (sublimation)
Evaporation must take place in a relatively high vacuum (
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*Melting Points and PV = 10-2 torr Temperatures of Some Common Metals used in Thin Film Applications
Temperature
Melting Point
(oC)
Temperature at which
PV = 10-2 torr
(oC)
Aluminum
659
1220
Chromium
1900
1400
Copper
1084
1260
Germanium
940
1400
Gold
1063
1400
Iron
1536
1480
Molybdenum
2620
2530
Nickel
1450
1530
Platinum
1770
2100
Silver
961
1030
Tantalum
3000
3060
Tin
232
1250
Titanium
1700
1750
Tungsten
3380
3230
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*Line-of-Sight Process
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*DISADVANTAGES OF EVAPORATION
1. It is difficult to evaporate alloys such as NiCr due to the difference between the 10-2 torr temperatures. The element with the lower temperature tends to evaporate somewhat faster, causing the composition of the evaporated film to be different than the composition of the alloy. To achieve a particular film composition, the composition of the melt must contain a higher portion of the material with the higher 10-2 torr temperature and the temperature of the melt must be tightly controlled. By contrast, the composition of a sputtered film is identical to that of the target.
2. Evaporation is limited to the metals with lower melting points. Refractory metals and ceramics are virtually impossible to deposit by evaporation.
3. Reactive deposition of nitrides and oxides is very difficult to control.
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*ELECTROPLATING AND ELECTROLESS PLATINGElectroplatingThe part is connected to a negative potential (cathode) and an anode material (typically stainless steel) is connected to a positive potential.The anode and cathode are immersed in a suitable plating solution.Part must be electrically conductive (may require seed layer predeposited on the the substrate)Electroless platingPart is immersed in a plating solutionDifference in surface potentials generates EMF needed for platingRange of materials is limitedThickness is limitedVery economical for thin plating
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*
COMPARISON OF METAL DEPOSITION TECHNIQUES
PROPERTY
EVAPORATION
SPUTTERING
ELECTROLESS PLATING
PLATING
Deposition Rate
High
Medium
Medium
High
Control of Deposition
Sometimes difficult
Repeatable, easy to control
Excellent
Excellent
Coverage for complex shapes
Poor
(line of sight)
good, but nonuniform thickness
Excellent
Excellent
Step coverage
Poor
(line of sight
Good
Excellent
Excellent
Control of film stress
Limited control
High level of control
Limited control
Limited control
Control of Microstructure
Good level of control
Good level of control
Limited control
Limited control
Resistivity
Approaches bulk
Approaching bulk, but higher than evaporated
Depends on bath purity and formulation
Depends on bath purity and formulation
Adhesion to substrate
Good
Excellent
Good
Good
Chemical contamination
< 10 ppm
< 100 ppm
Depends on bath purity and formulation
Depends on bath purity and formulation
Selectivity
No
No
Possibly
Yes
Uniformity
System geometry dependent (( 3%)
Excellent
(( 3%)
Pattern sensitive
Pattern sensitive
Other advantages
Best understood of all vacuum processes
Most versatile
Simple apparatus
High aspect ratio attainable
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*Mask Alignment and Exposure System
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*Mask and Substrate
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*AlignmentSplit Beams
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*UV Exposure
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*Etching
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*THIN FILM MATERIALSTwo basic systemsNichrome/Nickel/GoldTantalum Nitride/Titanium-Tungsten/GoldResistor MaterialsNichromeTantalum NitrideBarrier MaterialsNickelTitanium/TungstenConductor MaterialsGoldCopperAluminum
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*RESISTOR MATERIALS
Also provide the adhesion to the substrateMust be a material that oxidizesForms as isolated islands, eventually joining at grain boundaries
- *NichromeAlloy of nickel and chromium (Ni80/Cr20 is typical)Maximum sheet resistance: 400 /Properties highly dependent on annealing scheduleTCR
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*Tantalum Nitride (TaN, Ta2N)Formed by reactively sputtering tantalum in the presence of nitrogenPassivated by heating in air at 400 oC for several minutesMore stable than nichromeMaximum sheet resistivity - 200 /TCR typically -70 to -120 ppmMay be improved by annealing in a vacuum for several hours or by sputtering in the presence of a trace of oxygen
Tantalum Oxynitride Films (TaO2N)Formed by adding oxygen to the nitrogen and argonHigher sheet resistivityLower TCR
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*Design of Thin Film Resistors
The resistivity of thin films can be held to a close tolerance by controlling the deposition parameters, by the precision of the etching process, and by controlling the stabilization time.
2.The designer is not limited to simple straight-line patterns. Right angle and meandering patterns are also available.
3.Line widths down to 0.001" are possible, although 0.002" are more common. This necessitates special design techniques if laser trimming is required.
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*4. Only a single sheet resistivity is available unless special deposition techniques are used. This necessitates resistors with extremely high or low aspect ratios if high or low values are required.
5. No termination effects are present, and the sheet resistivity is not a function of the resistor size.
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*Preliminary Information:
1.Nominal value of resistor2.Tolerance3.Power dissipation
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*1. Determine the minimum width from
where:PD =Power dissipated by the resistors = Sheet resistivity of the film
PR = Rated power of the resistor = 20 W/in2 for alumina at 25 oC = 40 W/in2 for beryllia at 25 oCR = Nominal value of the resistor
The width obtained from the equation should be rounded up to the nearest standard width.
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*
2.Calculate the number of squares from
The total effective length of the resistor is
Resistors deviating from a straight-line path must have allowances made for the corners since the current density tends to increase in these areas. This effect is shown in the next slide assuming L1 > 4W.
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*
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*3. Make allowances for tighter tolerances by adding trim pads. The resistance increase capability necessary for precision trimming is shown in the table below.
Tolerance
Increase Capability
1%
25%
5%
20%
10%
15%
15%
8%
20%
0%
Tolerance vs. Increase Capability
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*GEOMETRY OF TRIM PADS
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*
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*BARRIER MATERIALS
Nickel layer between nichrome and gold prevents chromium diffusion into the goldImproves wire bondability
A layer of Ti90/W10 improves adhesion of gold
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*THIN FILM CONDUCTOR MATERIALS
PROPERTY
ALUMINUM
COPPER
GOLD
Resistivity
2.66 ((-cm
1.67 ((-cm
2.35 ((-cm
Adhesion
Good
Ti or Cr
adhesion layer
used
Cr, TaN, or NiCr adhesion layer used
Deposition
Sputtering
Sputtering and plating
Sputtering and plating
Corrosion
Corrodes
(H2O and Cl)
Corrodes without barrier layers
No
Special Notes
Easiest to process
Reacts with polyamic acid -
Requires barrier layer
(Ni or Cr)
More
expensive
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*Thin Film Resistor on Silicon
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*Thin Film Microwave Circuit
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*Thin Film Active Filter
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*SUMMARYAdvantagesBetter line definitionSmaller line geometryHigher quality resistorsBetter high frequency performance
DisadvantagesHigher costMultilayer capability is limitedSingle sheet resistivity
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*HomeworkDesign the following thin film resistors:Use a film with a sheet resistivity of 200 /1. 50 , 5 %, 20 mw2. 1000 , 1 %, 100 mw3. 2 M , 1 %, 10 mwSelect the pattern, calculate the dimensions, and draw a sketch
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