*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.

*Cross-Section of Thin Film MetallizationNiTaN

*Thin Film Circuit

*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

*SPUTTERING

*SPUTTERING

PROCESS SEQUENCESystem is pumped to a high vacuum ( 800 V) between target and substrate (substrate at ground)Potential may be RF or DC

*Basic Triode Sputtering SystemSubstrateTargetThickness Monitor

*Planetary Sputtering System

*Planetary Sputtering System: ChamberTargets (3)

*Planetary Sputtering System: Substrate Holding Fixture

*Lateral Sputtering SystemVacuumInterlockSputteringChamberControlPanel

*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 (

*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

*Line-of-Sight Process

*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.

*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

*

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

*Mask Alignment and Exposure System

*Mask and Substrate

*AlignmentSplit Beams

*UV Exposure

*Etching

*THIN FILM MATERIALSTwo basic systemsNichrome/Nickel/GoldTantalum Nitride/Titanium-Tungsten/GoldResistor MaterialsNichromeTantalum NitrideBarrier MaterialsNickelTitanium/TungstenConductor MaterialsGoldCopperAluminum

*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

*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

*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.

*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.

*Preliminary Information:

1.Nominal value of resistor2.Tolerance3.Power dissipation

*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.

*

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.

*

*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

*GEOMETRY OF TRIM PADS

*

*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

*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

*Thin Film Resistor on Silicon

*Thin Film Microwave Circuit

*Thin Film Active Filter

*SUMMARYAdvantagesBetter line definitionSmaller line geometryHigher quality resistorsBetter high frequency performance

DisadvantagesHigher costMultilayer capability is limitedSingle sheet resistivity

*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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