Vacuum Applications in Nanomanufacturing

• Objectives– To demonstrate the use of vacuum in

manufacturing processes– To quantify the need for vacuum conditions in

each process– To define the levels of vacuum present in the

process– To identify how these levels of vacuum are

attained and measured

Vacuum in Lithography Process

• Objective of Process – Create temporary features on silicon wafer to

guide etch and deposition processes– Role of Vacuum in Process

• Electron Beam Lithography– Exclude atmospheric gases from lithography chamber to

avoid particle collisions with beam and loss of energy– Eliminate secondary emission from particles that were

inadvertently “struck” through increased MFP– Extremely high vacuum level required (10-7 to 10-9 T)

Vacuum in Deposition Processes

• Objective of Process– Add new layers or substances to a defined

region of a silicon wafer– Physical Vapor Deposition

• Sputtering• Evaporation

– Chemical Vapor Deposition• LPCVD – Low Pressure Chemical Vapor Deposition• PECVD – Plasma Enhanced CVD

Vacuum in Physical Deposition

• Sputtering– A target anode material is hit by a high energy

electron beam, dislodging atoms• Pressures of 5 – 10 mTorr• De-gas step at much higher vacuum (10-9 T) may be

used to remove oxide from target and drive off contaminants (vacuum provides a clean environment)

– Vacuum is also key to process as it minimizing gas phase collisions of particles being dislodged from target to wafer (Mean Free Path is larger)

Vacuum in Physical Deposition

• Thermal Evaporation– A material is heated to its melting point in a

vacuum environment• Pressures of 10-2 Torr to begin• Minimizes oxide formation in metals• Inert gas at higher pressure (Ar) used to transport

vapor phase element to location

– Vacuum role is to remove contaminants and, with lower atmospheric pressure, to sustain evaporation.

Vacuum in Chemical Deposition

• Chemical vapor deposition– LPCVD

• Low pressure CVD (0.1 – 1 Torr)• Deposits oxides nitrides, or polysilicon• Relatively high temperature process (>650 Deg C)

– UHVCVD – Ultra high vacuum (10 -9T)• Extremely High vacuum eliminates contaminants from reaching

surface

– PECVD - Plasma Enhanced CVD• Gas plasma used to control deposition rate

– Not possible to create a plasma at higher pressures due to mean free path being too short

– Electrons cannot gain enough energy without collision

Vacuum in The Etch Processes

• Objective– Remove material from a defined region of a

silicon wafer– Physical Etching

• Sputtering – Similar to deposition, but the “target” is the wafer! Less common today, but a low pressure method (<50mTorr)

• A purely physical process where ions from introduced gas in RF powered chamber bombard the surface

Vacuum in Etch Process (2)

• Plasma Etching– Vacuum is used to remove atmospheric gases

– Low pressure etchant gas such as CF4 is introduced into chamber where RF stream is flowing

– Gas breaks down into ions, electrons, and radicals

– CF4 dissasociates into CF3 + and F radical, which attacks silicon causing etching(2)

Vacuum in Ion Implantation

• Ion Implantation– Used to create conductive species in silicon– Creates the source and drain areas for

transistors and many other features– Ion beam of defined impurity is used– High Vacuum conditions are required to

• Ensure that no contaminant species exists• Increase mean free path so no collisions in ion beam

result

Vacuum Environments

• Creation of different vacuum levels requires different components– Pumping systems– Piping

• Measuring vacuum levels accurately requires different techniques– Gauge types– Physical Processes

Low or Rough Vacuum

760 Torr to a few Torr

Medium Vacuum A few Torr to 10-3 Torr

High Vacuum 10-3 to 10-7 Torr

Ultra-high Vacuum (UHV)

Below 10-7 Torr

Ranges of Vacuum

Vacuum Levels and Pumps

Pump Categories

Pumps

Gas Transfer Entrapment

MomentumTransfer

DragFluid

EntrainmentTurbo

Molecular

PositiveDisplacement

Rotary Pumps

VaneLobe

Piston

Dry pumps DiffusionWater JetVapor Jet

CryogenicCryosorptionSputter-IonSublimation

WorkChamber

Cryo

Blower

Rotary Vane

Ion Gauge(Hi Vac)

ThermocoupleGauge (TC)(Rough Vac)

TC Gauge

Rough Valve

Soft StartValve

ForelineValve

Heater

Purge GasValves

TemperatureTransducer

Hi VacValve

Exhaust

ConvectronGauge(Rough line)

N2 roughline backfill

Oil Trap

N2 purge(vent)

MATEC MODULE 101

Complete High Vacuum Work Chamber

Pumpdown Sequence• All valves are initially CLOSED• Soft start valve OPENS• Chamber pumps down for 60 seconds• Soft start valve CLOSES• Rough Valve OPENS• Chamber pumps down to 100 mT• Rough Valve CLOSES• Hi Vac Valve OPENS• Ion Gauge turns ON• Chamber pumps down to base pressure • Process begins at operating pressure

Pumpdown Sequence• Two different pump types are used

– Rotary Vane type for rough vacuum• Rotary vane pump is a positive displacement pump• Prior to the rotary vane pump reaching its “ultimate pressure”

(pressure at which its pumping speed goes to 0), the sequence shuts it off and the valve is closed to avoid backstreaming oil from the input.

• Crossover pressure is where this takes place

– Cryo pump for high vacuum• Cryo Pump is an entrapment type pump

– Contaminant particles are captured on its inside walls through use of very low temperatures

– Periodically Cryo pumps must be regenerated

How Can We Measure Vacuum?

• To ascertain the pressure level, gauges of different types are used– Direct gauges use pressure from the gas to

deflect a needle or move a column of mercury or other liquid

– Indirect gauges use principles of heat transfer or electrical changes that take place based on the number of gas molecules present

– Both processes are gas type dependent

Vacuum Gauges –Direct Type

• Mechanical gauges such as the diaphragm gauge shown here are usable for rough vacuum. Pressure from the gas deflects the diaphragm

Vacuum Gauges - Indirect

• Indirect gauges such as the thermocouple gauge are usable for rough to medium vacuum levels where direct pressure is too low to mechanically deflect a gauge.

Vacuum Gauges - Indirect

• Ionization gauges are useful for high vacuum measurement. where direct pressure is too low to mechanically deflect a gauge.

Typical Ranges of Gauges

Source: MATEC Module 101

references

(1)http://www.pfonline.com/articles/069901.html

(2) SS 11.26 Introduction to Semiconductor Manufacturing, Hong Xaio, Prentice Hall, Upper Saddle River, NJ C 2001

(3) MATEC Module 74 Narrative – Etch

(4) MATEC Module 26 PowerPoint