1. Vacuum Technology

2. 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
The physical properties of partial vacuum environments were discussed in the previous presentation. In this lesson, examples of the use of vacuum to attain the desired results in nanomanufacturing are presented.

3. 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)
In e-beam lithography, an extremely focused beam of electrons is aimed at a silicon wafer that has been prepared with a chemical resist that is sensitive to this energy. A temporary pattern is created on the wafer in this way. If the MFP is increased due to high vacuum, the likelihood of a collision is reduced greatly. In the event of a collision, secondary emission from the gas particle struck can also hit the wafer.

4. 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
Deposition processes add materials to a substrate or change the properties of the substrate. Metal layers for interconnection, polysilicon for gates, and insulating or protective layers are all examples of deposition processes. These depositions can be carried out using physical or chemical methods, and in both cases, vacuum serves a role in ensuring that the process takes place in a consistent manner.

5. 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)
In sputtering, an energetic beam of ions strikes a target material of the species to be deposited. Actual mechanical dislodging of the atoms of the material occurs, and these atoms fall to the surface of a wafer. Vacuum serves the purpose of degassing (removing any stray moisture or other contaminants) from the chamber. Vacuum at somewhat higher pressures later can be used to avoid gas phase collisions with atoms falling through to the surface below.

6. Vacuum in Physical Deposition
Thermal Evaporation
A material is heated to its melting point in a vacuum environment
Pressures of 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.
In this process, a vacuum extracts the oxygen partcles. Argon, a noble gas, can be used as the transport mechanism for conducting vapor to the substrate. A direct condensation of the vapor onto the substrate is also possible under higher vacuum conditions, wherein a higher vacuum increases the mean free path in the chamber. Vacuum serves to eliminate oxygen from the process, eliminating possible contamination.

7. 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
Chemical vapor deposition processes utilize precise amounts of gases that react with the surface of a silicon wafer to form the desired species on the substrate. The chamber is pumped down and the gas is introduced. In the case of plasma enhanced CVD, the vapor entering the chamber passes by an RF field, which ionizes some atoms and creates ions, electrons and radicals. Ultrahigh vacuum use results in extremely pure thin film coatings. In plasma enhanced CVD, the energy from the plasma, created in a low pressure environment, causes the electrons to “heat up” considerably, while not affecting neutral atoms. In this case, if the pressure was higher, the mean free path would be shorter, and electrons would suffer collisions, so it would be difficult to achieve a stable plasma.

8. 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
Etching processes can be physical or chemical in nature. Sputtering was discussed before, and is similar here, except that instead of removing atoms from a target to deposit on the wafer, the wafer IS the target, and atoms are removed from it by ions of an introduced gas that have been created by putting the gas into an RF powered chamber.

9. 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)
Plasma etching processes introduce a reactant gas into a previously evacuated RF powered chamber. The vacuum level determines if a plasma will exist and of what quality it will be. At higher pressures, the plasma becomes less distributed, and the beneficial effects are reduced. Once the arc is struck, the disassociation that occurs serves to perform the etching. This is something of a combination of a physical and a chemical process.

10. Vacuum in 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
Ion beam implantation is used to put impurities into a silicon wafer for the fabrication of the source and drain areas of transistors and for many other functions. The ion implanter uses an ion beam of defined species content and targets the defined areas. The presence of unwanted gases in this environment can result in contamination of the species to be implanted. If there are molecules present in the path of the beam, uneven distribution of the species can result from collisions. The higher mean free path that exists under high vacuum conditions reduces the risk of either of these events occurring.

11. Vacuum Environments
Creation of different vacuum levels requires different components
Pumping systems
Piping
Measuring vacuum levels accurately requires different techniques
Gauge types
Physical Processes
Now that we have some ideas about where vacuum is used and the level of vacuum required, we need to see what is required to create and measure these levels. At low to medium vacuum, air particles behave with a viscous flow similar to liquids. Pump types that would be useful for pumping liquids can also be adapted for this duty. Oil is often used as a sealant in these pumps. Piping for rough vacuum systems is not critical. As vacuum levels required increase, there are fewer particles to capture and their flow becomes more laminar is nature. Molecules are more likely to adhere to small diameter tubing or sharp bends at lower vacuum. Piping tends to change to be larger in diameter and smooth finished, hence the appearance of many medium vacuum systems.

12. Ranges of Vacuum Low or Rough Vacuum 760 Torr to a few Torr
Medium Vacuum
A few Torr to Torr
High Vacuum
10-3 to Torr
Ultra-high Vacuum (UHV)
Below 10-7 Torr
Several of the processes described earlier take place in either a medium vacuum or a high vacuum environment. Although this is the case, it may have been necessary to evacuate the chamber to a high vacuum level first. The laws of gases and the properties of gases define not only the type of pump required, but also the method by which we measure pressure.
Vacuum technology applications in the 21st century cover a range of pressure that extends over more than fifteen orders of magnitude. [A Chambers] This total pressure range is divided into these four regions.
An actual vacuum is a space with a pressure less than atmosphere.

13. Vacuum Levels and Pumps
The chart in this slide shows some of the different types of pumps and the relevant vacuum ranges that they work in. If the need for vacuum levels of as low as 1 millitorr exists, a conventional pump of the rotary vane type may be acceptable. At the top of the chart, note that the term “viscous flow” is shown for the range of atmospheric pressure down to about 1 millitor. Gases behave more like liquids in this range, and the term “viscous flow” describes their behavior. “Paddle”, rotary vane, and scroll pumps are displacement pumps that move volumes of air. As the number of molecules in the space decreases, molecular flow dominates, and alternative pumping technologies are required. Popular pumps for semiconductor processing include the turbomolecular pump and the cryogenic pump.

14. Pump Categories Pumps Gas Transfer Entrapment Momentum Transfer Fluid
Drag
Fluid
Entrainment
Turbo
Molecular
Positive
Displacement
Rotary
Vane
Lobe
Piston
Dry pumps
Diffusion
Water Jet
Vapor Jet
Cryogenic
Cryosorption
Sputter-Ion
Sublimation
The positive displacement pumps, such as the rotary pump provide rough vacuum levels. In environments where a higher level of vacuum is required, a positive displacement pump serves as a roughing or backing pump to begin drawing the vacuum from the chamber. Once the pressure has dropped down to the level at which the roughing pump is no longer capable of removing more air, the momentum transfer or entrapment pump is turned on to remove the remaining molecules. This overall activity is known as pumping down to the base level.

15. Work Chamber Complete High Vacuum Work Chamber Cryo Blower Ion Gauge
Rotary Vane
Ion Gauge
(Hi Vac)
Thermocouple
Gauge (TC)
(Rough Vac)
TC Gauge
Rough Valve
Soft Start
Valve
Foreline
Heater
Purge Gas
Valves
Temperature
Transducer
Hi Vac
Exhaust
Convectron
Gauge
(Rough line)
N2 rough
line backfill
Oil Trap
N2 purge
(vent)
MATEC MODULE 101
A complete vacuum chamber system such as that shown above includes a rotary vane pump for rough vacuum and, in this example, a Cryo pump to create the high vacuum required. The sequence of operation required to create the necessary vacuum conditions is described on the next slide.

16. 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
The soft start valve opens to begin the pumpdown. A rotary pump would quickly pull most of the air out of the chamber, but in doing so, might stir up any residual particles. The soft start begins more slowly. Once the rotary pump has taken the chamber down to its capability, the rough valve closes so that air and pump oil will not escape into the chamber when the high vacuum pump begins operating, and the chamber pumps down to its base pressure (the lowest pressure that the chamber can reach). The operating pressure is usually higher than the base pressure. Pumping down to base pressure assures that residual water vapor and other contaminants have been removed before the process begins.

17. 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
The rotary pump takes the chamber down to its base pressure capability. A the crossover pressure, the high vacuum pump, a cryo begins. operating, and the chamber pumps down to its base pressure (the lowest pressure that the chamber can reach). The cryo pump is a high vacuum entrapment type pump. It traps the residual gas molecules through a high conductance inlet through a cooling means that brings the internal temperature of the pump below 10 degrees K.

18. 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
In order to determine the vacuum level present, gauges are required. As was the case for vacuum pumps, different levels of vacuum will require different types of gauges. So-called direct gauges actually utilize the pressure from the gas to move a needle or column of liquid. As the pressures drop, this becomes impractical. Indirect gauges measure some known property, such as thermal conductance of the gas present. Other gauges attempt to ionize the atoms present and in so doing, detect their presence. As the vacuum level increases, in both cases, there are fewer particles to act at all.

19. 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
The familiar diaphragm gauge has its needle connected to a metal or plastic disk that is deflected by the presence of pressure at its surface. Gauges such as this are called direct gauges as the pressure is directly responsible for their operation. This type of gauge may be usable down to 1 torr from atmospheric levels, and in lower concentrations down to the millitorr range, a diaphragm gauge may be used. All gauges are subject to the properties of the gases that they measure. As such, gauges must be calibrated for the type of gas present.

20. 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.
Indirect gauges work by measuring some property of the gas that varies with pressure rather than measuring the pressure itself. Changes in viscosity, conductivity, or ionization can be used to measure pressure indirectly. The thermocouple gauge works on the principle of heat transfer, using conductivity as its measurement method. In an environment where there are many gas atoms, as would be present in a rough vacuum, heat from an electrically heated filament would be conducted away by the gas. As the vacuum level increases, fewer gas atoms are present to carry away the heat. A thermocouple junction, capable of providing a voltage proportional to temperature, reads the temperature of the filament. A higher temperature indicates a higher level of vacuum. The gauge is known as indirect since the property being measured is not pressure. Thermocouple gauges are useful in rough to vacuum levels.

21. Vacuum Gauges - Indirect
Ionization gauges are useful for high vacuum measurement. where direct pressure is too low to mechanically deflect a gauge.
Ionization gauges, as their name implies, function by bombarding gas molecules with high-speed electrons. A high-speed electron knocks a gas molecule electron out of the valance orbit producing a charge imbalance on that gas molecule. The gas molecule becomes a positive ion. This process is called electron impact ionization. Inside the gauge, a cathode (hot or cold) provides a steady supply of electrons. The electrons are accelerated toward the positively charged electron collector or anode by a voltage difference between the two electrodes.
The number of molecules that become ionized is dependent on the pressure (number of molecules available for ionization), energy of the electron, and the type of gas. Once a positive ion is created, it is attracted to a negatively charged ion collector. In a cold cathode ionization gauge, the cathode and ion collector are the same electrode. At the ion collector surface, the gas ion picks up an electron to become a neutral gas molecule again. Then it drifts back into the vacuum. As ions impinge on the ion collector, there is a flow of electrons to neutralize the ions. This flow of electrons is called the ion or gauge current and is dependent on the pressure (From MATEC Module 101 Slide Show #4 – C 2006 MATEC

22. Typical Ranges of Gauges
The diagram indicates some of the types of direct and indirect gauges used for vacuum measurement and their approximate ranges of operation. Not all types are shown here. The reader is referred to MATEC Module 101 for furrther information.
Source: MATEC Module 101

23. references (1)http://www.pfonline.com/articles/069901.html
(2) SS 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