Vacuum Technology Need for Vacuum Environment Vacuum processes used in nano-manufacturing Vacuum and Gas Properties Measurement and creation of a partial vacuum environment This activity presents fundamental information on vacuum. The principles of vacuum are related to a basic set of properties that relate to gases. Some of the processes that take place in the top-down nanomanufacturing environment require vacuum conditions in order to proceed correctly or to avoid contamination. The measurement of the level of vacuum present in the process is important, as the levels required vary for each type of process. To be able to understand the various concepts involved with vacuum technology, it is important that the basic concepts of vacuum and its associated terminology be discussed. The Vacuum Fundamentals Module covers the basic concepts of pressure, gas laws, and gas characteristics . These topics are fundamental to the understanding of vacuum systems. MATEC M097SS01.ppt

Learning Objectives To develop an understanding of the applications of vacuum technology in nanomanufacturing. To be able to explain the basic behavior of gases, based on the temperature, pressure, volume and molecular density present in the environment. To be able to define the basic units of vacuum Describe activities and what to expect The learning objectives of this activity include developing a basic understanding of what a vacuum is. Since vacuum defines the absence of molecules, and gases are usually what need to be removed from an environment, the knowledge of gas behaviors is essential. The units of vacuum are presented in terms of pressure, as any vacuum can be described as the absence of pressure. MATEC M097SS01.ppt

What is a Vacuum? Ideal Vacuum Actual Vacuum (Partial Vacuum) A space totally devoid of all matter. Does not exist, even in outer space! Actual Vacuum (Partial Vacuum) A space containing gas at a pressure below the surrounding atmosphere or atmospheric pressure <760T @ sea level and 00 C with no humidity Define the two types of vacuum Ideal vacuums do not exist even in our interplanetary space. There is always some minor amount of dust or material present. A vacuum is defined in terms of pressure or the lack of pressure, so any vacuum that is not ideal can be considered a partial vacuum. MATEC M097SS01.ppt

Why Might We Need Vacuum? A vacuum provides a clean environment Devoid of possible contamination from other gases that may be present from the atmosphere Devoid of particles that may react with physical processes that are intended to take place Devoid of pressure that may limit restrict a desired physical process Tell students A vacuum environment provides a “clean slate” for chemical processes. Once the nitrogen, oxygen, and other gases from the air have been removed from a vacuum chamber, known amounts of specific gases can be injected into the chamber for chemical processing to occur. In the presence of atmospheric gases, the particles of matter present can affect certain manufacturing and imaging processes Some physical processes will not occur at acceptable temperatures or will proceed much more slowly in the presence of atmospheric pressure The nanotech industry uses various levels of vacuum in the processing, visualization, and analysis stages of manufacture. In order to understand how a vacuum system works, it is necessary that the learner understand the basics of gases and gaseous evironments Once the basics of gases are understood, an understanding of the operation of the components that make up a vacuum system such as pumps, gauges, and materials will follow. MATEC M097SS01.ppt

Common Uses of Vacuum Light Bulbs Food Processing A vacuum pump removes oxygen from a light bulb so that the filament won’t “burn out” (oxidation) Food Processing Vacuum sealing eliminates oxygen from food containers to preserve the contents Plastics Manufacturing Vacuum-forming “draws” plastic sheets into shapes such as “blister packs” In our everyday lives, we use products manufactured with vacuum technology. Light bulbs are evacuated to remove the oxygen that would oxidize and “burn out” the filament of the bulb if present. In food processing, vacuum sealing eliminates the oxygen that would cause the food to decompose. Plastic sheets that have been heated are drawn by vacuum onto plastic molds to make packaging materials. Pressure is carefully controlled so that the Fluorine radical can be formed which in turn reacts with unprotected silicon forming a gas that is pumped out of the reaction chamber. Other applications: Lamps (incandescent, fluorescent, electric tubes) Melting, sintering Packaging Encapsulation MATEC M097SS01.ppt

Where Do We Use Vacuum in Nanomanufacturing? In nanomanufacturing, several applications of vacuum technology are used to support basic operations. Vacuum alone does not perform the operation, but without it, the processing attempted would not be successful. MATEC M097SS01.ppt

To Retain a Clean Surface Objective Clean surfaces Applications: Friction Adhesion Emission studies Materials testing for space Explain how a vacuum contributes to reducing the frequency of molecules striking and interacting with a surface Surfaces remain uncontaminated longer in a vacuum because the frequency of molecules striking and interacting with the surface is reduced. Time to form a monolayer is the time required for a freshly cleaved surface to be covered by a layer of gas of one molecular thickness. This time is given by the ratio between the number of molecules required to form a compact monolayer (1015 molecules per square centimeter ) and the molecular incidence rate (rate at which molecules strike a surface). MATEC M097SS01.ppt

To Create Desired Features Objective Create Insulators SiO2 SiN2 Create Conductive Layers Evaporative Coatings Sputtered Coatings To etch or remove material Plasma Etch Reactive Ion Etching Explain why this is useful Pure silicon is highly reactive. Silicon dioxide and silicon nitride are commonly used insulators in the top-down process, but in order to guarantee the properties of the insulating layers, the gases present must be provided in precise amounts. Without vacuum, both nitrogen and oxygen would be present in the chamber, creating a surface with desirable features of neither species. Sputtering occurs when a plasma of a known inert gas, such as Ar is ignited by electromagnetic energy to blast away atoms of a target that are then deposited on the device being cated. The gas species used in sputtering must be well defined for a stable plasma. The presence of atmospheric gases is not acceptable under these conditions. Time to form a monolayer is the time required for a freshly cleaved surface to be covered by a layer of gas of one molecular thickness. This time is given by the ratio between the number of molecules required to form a compact monolayer (1015 molecules per square centimeter ) and the molecular incidence rate (rate at which molecules strike a surface). Sputtering Coating System http://www.teercoatings.co.uk MATEC M097SS01.ppt

To Visualize Nano-features Objective View extremely small Objects Scanning Electron Microscopy Electron beam strikes object being viewed Backscatter of electrons is used to “image” Atmospheric molecules present may be “hit” by the beam Explain why vacuum is needed In a scanning electron microscope, a finely focused electron beam is directed at the object to be viewed. The presence of air molecules in the test chamber creates a situation where molecules of gases rather than the object being “imaged” may be struck by the beam. This reduces the clarity of the image, and may make it impossible to “see” due to the reaction of gas molecules with the beam. This time is given by the ratio between the number of molecules required to form a compact monolayer (1015 molecules per square centimeter ) and the molecular incidence rate (rate at which molecules strike a surface). http://en.wikipedia.org/wiki/Image:SEM_chamber1.JPG#file MATEC M097SS01.ppt

Click once for each question. Practice Questions Click once for each question. 1. What is an Ideal Vacuum? A space devoid of all matter. 2. What is one application of vacuum technology in nanomanufacturing? Sputtering or evaporative coating of metals or scanning electron microscopy MATEC M097SS01.ppt

The Basics of Vacuum and Pressure Vacuum can simply be thought of as a reduced air pressure environment Atmospheric Pressure comes from molecules of oxygen, nitrogen, and other gases present in air At sea level, this pressure corresponds to 14.7 PSI or 760 torr (in honor of Torricelli) which corresponds to the number of mm height of the mercury column in the barometer shown here. On the Earth, we are surrounded by an atmosphere made up mainly of nitrogen and oxygen. Although gases represent the least dense form of matter, there is still, from the particles of the gases present, a pressure on all objects on Earth from these gases. This is atmospheric pressure. Torricelli utilized a column of mercury in a pool to demonstrate the pressure from these gases. MATEC M097SS01.ppt

The Basics of Vacuum and Pressure In vacuum systems, we remove the atmospheric gases in an enclosed area Fewer molecules of gas result in lower pressure Any pressure less than or 760 torr can be considered a partial vacuum. Through various pumping devices, the molecules of air can be removed from a confined space. Since the air pressure is due to the force exerted upon the surface by the molecules present, fewer molecules mean lower pressure. MATEC M097SS01.ppt

Ranges of Vacuum Low or Rough Vacuum 760 Torr to 1Torr Medium Vacuum High Vacuum 10-3 to 10-7 Torr Ultra-high Vacuum (UHV) Below 10-7 Torr Using the table, explain the relationship between these various units of 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. MATEC M097SS01.ppt

Typical Vacuum Levels Required for Processing Evaporative Coatings 10-2 to 1 torr Light bulb manufacturing 10-3 to 10-1 Torr Scanning electron microscope 10-4 to 10-7 Torr Electron Beam Lithography Below 10-7 Torr Some processes require a higher level of vacuum than others. Evaporative coating methods used to “silver” mirrors and apply optical coatings to lenses are carried out at levels somewhat less than 1 torr. Light bulb manufacturing requires vacuum levels down to 1 millitor (10-3) torr. Scanning electron microscopy and electron beam lithography processes require a much higher level of vacuum because the presence of particles in the path of an electron beam may cause the beam to be deflected or interact with the desired process. 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. MATEC M097SS01.ppt

Gas Properties Gases consist of tiny particles called molecules or atoms. Molecules are so far apart that any attractive forces are ignored. Use the diagram to explain the properties of gas Gases represent the least dense form of matter. Comparatively speaking, the molecules of a gas are further apart creating weak, intermolecular forces of attraction between them, allowing for rapid and independent movement. At short distances, gas molecules collide with each other which causes changes in direction and velocity. These collisions create an expansion in molecular volume until the space in which the molecules are contained is filled . MATEC M097SS01.ppt

Four Qualities of a Gas Volume (V) Pressure (P) Temperature (T) Number of molecules (N) Tell students Volume (V) is simply the volume of the container that holds the gas. Pressure (P) is the force exerted by the gas on a unit area of the container. Temperature (T) is the temperature of the gas in F, C, and Kelvin (K). The number of molecules (N) is just that - the number of molecules in the container at the indicated pressure and temperature. Exactly how gases behave within a space relates to the four characteristics of gases - volume, pressure, temperature and number of molecules. MATEC M097SS01.ppt

Pressure and Molecular Density Pressure is a function of the number of molecules present in a given volume. Explain how pressure relates to the number of molecules present in a given area Molecules of a gas are continuously in motion as gases are the least stable states of matter. As the molecules strike the surface of their containment vessel and collide with each other, energy is lost. More molecules present in a given area will result in higher pressure. Drawing a vacuum reduces the number of molecules present, and hence the pressure, as there are fewer molecules present to press against the surfaces. This force is being created, not only by the molecular density (number of molecules) by the activity of the molecules. MATEC M097SS01.ppt

Pressure and Molecular Density Molecules of gases tend to spread out, evenly applying force to the containment chamber A larger volume, with the same number of molecules present, would be at lower pressure than a smaller one Boyle’s Law - a relationship between pressure and volume If we compress the area available for a given number of molecules, the pressure in the contained area increases. A partially inflated balloon that is “squeezed” will expand, illustrating the effect of higher pressure due to reduced volume. The relationship (V1)(P1) = (V2) (P2) is known as Boyle’s Law. In fact, as the relationship shows, the instantaneous pressure and volume at any point multiplied together actually result in a constant value. MATEC M097SS01.ppt

Pressure and Temperature As an equal number of molecules in an identical volume is heated, the pressure increases (Guy-Lussac’s Law) If we increase the temperature, keeping the volume and number of molecules equal, the pressure will increase. The reason behind this is that the molecular energy is increased by the heat, causing more motion of the molecules, and more collisions with the surface. The resulting effect is an increase in pressure. Consider an actual example of a tire that is cold. If we measure the air pressure, and then drive the car some distance, the heat from friction with the road surface will increase the temperature of the air in the tire. If we measure the tire pressure, it will read higher. This relationship P1/T1 = P2/T2 is Guy-Lussac’s Law. 30 psi of tire pressure means that for every square inch of rubber surface there are 30 pounds of force exerted. This force is being created, not only by the molecular density (number of molecules) in the tire, but by the activity of the molecules. MATEC M097SS01.ppt

Kelvin Scale C = .555 * (F – 32) F = 1.8 * (C + 32) K = (C + 273) Explain the conversion for Kelvin, Celsius, and Fahrenheit temperatures The absolute or Kelvin scale is used to help explain relationships in vacuum. The symbol for temperature on the Kelvin scale is K. Zero degrees Kelvin or absolute zero is the coldest possible temperature. At 0K, hypothetically there is no heat or molecular activity. Because there are no negative numbers in the Kelvin scale – that is, no below-zero values – equations are easy to work with. The Kelvin scale has the same spacing in its units as the centigrade scale: 0C = 273 K and 0K = -273 C. MATEC M097SS01.ppt

Charles’ Law Volume and Temperature Use the illustration to explain the effect that a change in temperature has on volume The temperature of the container on the right has doubled. The pressure and the number of molecules are held constant; therefore, the volume of the gas on the right has doubled. V1 / T1 = V2 / T2 MATEC M097SS01.ppt

Combined Gas Law The relationships between pressure, temperature, and volume given in Boyle’s, Charles’, and Gay-Lussac’s Law for a constant number of gas molecules can be taken together as the Combined Gas Law. This law can be used two of the 3 properties are known to find the third. (P1 * V1) / T1 = (P2 * V2) / T2 MATEC M097SS01.ppt

Click once for each question. Practice Questions Click once for each question. 1. If the pressure in a 10L chamber is 50 torr, what will the pressure for the same amount of gas be if the chamber is 20L? 25 torr 2. The temperature of a sealed vacuum chamber at 10 torr increases 10 degrees. What will happen to the pressure in the chamber? Boyle’s law defines the pressure as a linear function of volume, so doubling the volume will halve the pressure. Since Charles Law relates temperature to pressure, increasing temperature will increase the pressure. The pressure will increase MATEC M097SS01.ppt

Avogadro’s Law A volume of any gas containing 6.02 x 1023 (Avogadro’s number) atoms or molecules is said to contain 1 mole. The special condition of a gas at one atmosphere of pressure (760 Torr) and 273 K (0 C) is called standard temperature and pressure (STP). At STP one mole of any gas occupies 22.4 liters (l), this is called molar volume. MATEC M097SS01.ppt

Avogadro’s Law (2) Pressure is proportional to the number of molecules at a constant temperature. Equal volumes of gas at the same temperature and pressure contain the same number of molecules (or moles, n). P1 / n1 = P2 / n2 Tell students In the previous gas laws, N (the number of molecules) was constant. If the pressure, temperature and volume of a gas is known, then the actual number molecules can be calculated using Avogadro's Law. Avogadro's Law states that equal volumes of gas under the same conditions (temperature and pressure), contain the same number of molecules (or moles, n) and if the number of molecules increases, the pressure increases proportionally. MATEC M097SS01.ppt

Ideal Gas Law The Ideal Gas Law can be used to calculate the amount of gas (the molecular density) in a known volume, with known pressure and temperature. PV = nRT P = pressure (Torr) V = volume (liter) n = amount of gas (moles) T = temperature (K) R = universal gas constant (62.4 Torr liter per mole/K) Explain that these characteristics are not independent, but are always in relationship to each other Pressure and volume taken together are actually constant (just an application of Boyle’s Law since P1V1 = P2V2) And since Pressure and temperature share the relationship of Guy-Lussac’s Law P1/t1 = P2/T2, then there must be a relationship between the total number of moles of gas in the given space. The Ideal gas law shows that, and uses R as a “fudge factor” constant. Again, this just points out that the pressure relates back to how many molecules are in the given environment, so if we remove molecules by “pulling” a vacuum, the pressure will drop. MATEC M097SS01.ppt

Click once for each answer. Given: P1 = 50 Torr, n1 = 0.5 mole, P2 = 2 Torr, if the volume and temperature remain constant, what is n2? Solve for: 2 P1 / n1 = P2 / n2 2 = (2 Torr)*(0.5 mole) 50 Torr  2 = 0.02 mole Decreasing the pressure in a constant volume environment at the same temperature comes about by there being a smaller number of moles of gas in the chamber. MATEC M097SS01.ppt

Vapor Pressure Evaporation is the process where a liquid changes to a gaseous phase In an open environment, liquids continuously evaporate In a closed environment, eventually an equilibrium condition occurs where evaporation and condensation rates become the same. This occurs when the air becomes saturated. We are familiar with water evaporating from an open glass. If we were to put a cover on the glass, after a time, the evaporation rate would equal the condensation rate. This occurs when the air above the glass becomes saturated and cannot accept any further water, and we refer to this as a vapor. The absolute or Kelvin scale is used to help explain relationships in vacuum. The symbol for temperature on the Kelvin scale is K. Zero degrees Kelvin or absolute zero is the coldest possible temperature. At 0K,, hypothetically there is no heat or molecular activity. Because there are no negative numbers in the Kelvin scale – that is, no below-zero values – equations are easy to work with. The Kelvin scale has the same spacing in its units as the centigrade scale: 0C = 273 K and 0K = -273 C. MATEC M097SS01.ppt

Vapor Pressure We know that water changes from a liquid to a vapor state when we boil it (temp above 100 deg C) under normal atmospheric conditions. This occurs because at this temperature, the vapor pressure of the water overcomes the atmospheric pressure. If we lower the pressure, water boils at a lower temperature. Liquid water would evaporate in a closed container and the equilibrium would be met when the vapor was formed. Atmospheric pressure forces the rate to be limited. When the vapor pressure of the water equals the atmospheric pressure, boiling occurs. David.E. Goldberg (1988). 3,000 Solved Problems in Chemistry, First Edition, McGraw-Hill. ISBN 0-07-023684-4.  Section 17.43, page 321 Suppose we, on the other hand, lower the atmospheric pressure in the chamber? Then, the water should boil at a much lower temperature. In fact, the graph, and practice, show this to be true. This is useful if we are trying to remove water from a chamber. In fact, the presence of any moisture in a chamber generally forces the time it takes to evacuate it to increase dramatically. The absolute or Kelvin scale is used to help explain relationships in vacuum. The symbol for temperature on the Kelvin scale is K. Zero degrees Kelvin or absolute zero is the coldest possible temperature. At 0K,, hypothetically there is no heat or molecular activity. Because there are no negative numbers in the Kelvin scale – that is, no below-zero values – equations are easy to work with. The Kelvin scale has the same spacing in its units as the centigrade scale: 0C = 273 K and 0K = -273 C. Source:http://upload.wikimedia.org/wikipedia/commons/2/25/Water_vapor_pressure_graph.jpg MATEC M097SS01.ppt

Vapor Pressure The vapor pressure of a substance in a chamber is important for a number of reasons. Possibility of vaporization of the substance under low pressure May add to gas load of system Use of vaporization for processing Physical evaporative coatings Since the boiling point is lowered when pressure drops, if there is a substance whose vapor pressure is less than the chamber pressure, this will change the equilibrium conditions and increase the evaporation rate. Substances that were solid or liquid, such as water, will become vapors and add to the “gas load” in a chamber. As a process step, removal of water vapor to avoid affecting the process is a common requirement. This can greatly increase the time necessary to get the chamber to drop to the necessary vacuum level. Outgassing of materials due to this phenomenon can also occur. At the same time, a low pressure environment can also result in favorable conditions for a process that counts on evaporation, such as the physical evaporation method that is used to deposit metals such as aluminum on a substrate in the semiconductor process. Suppose we, on the other hand, lower the atmospheric pressure in the chamber? Then, the water should boil at a much lower temperature. In fact, the graph, and practice, show this to be true. This is useful if we are trying to remove water from a chamber. In fact, the presence of any moisture in a chamber generally forces the time it takes to evacuate it to increase dramatically. The absolute or Kelvin scale is used to help explain relationships in vacuum. The symbol for temperature on the Kelvin scale is K. Zero degrees Kelvin or absolute zero is the coldest possible temperature. At 0K,, hypothetically there is no heat or molecular activity. Because there are no negative numbers in the Kelvin scale – that is, no below-zero values – equations are easy to work with. The Kelvin scale has the same spacing in its units as the centigrade scale: 0C = 273 K and 0K = -273 C. MATEC M097SS01.ppt

Vapor Pressure of Substances Different materials have different vapor pressures, and hence, different boiling points. This chart indicates several of these, and shows why we don’t see liquid oxygen, nitrogen, or many other gases at atmospheric pressure. MATEC M097SS01.ppt

Vapor Pressure Vacuum evaporation can be used to deposit metallic coatings on surfaces. The material is melted in a heated crucible and goes from solid to vapor state. Vapor particles are deposited on the surface in straight line trajectories. Vacuum conditions allow for metal vapors to be deposited on a target. In this case, a wire crucible made of tungsten heats, through electrical current supplied, pure aluminum metal. The low pressure environment supports the evaporation, depositing the metallic vapor on the target. CHA-600 Thermal Evaporator MATEC M097SS01.ppt

Click once for each question. Practice Questions Click once for each question. 1. If a material in a vacuum environment has a vapor pressure of 50mTorr, what will happen if the chamber pressure is reduced to 5 m Torr? The material will evaporate and become part of the gas load. 2. The Vapor Pressure of Lubricant A is 10-4 Torr and for Lubricant B it is 10-8 Torr. Which lubricant would be best to use in a vacuum system and why? Lubricant B, since it is less likely to evaporate due to its lower vapor pressure when the chamber is pumped down MATEC M097SS01.ppt

Molecular Density and Mean Free Path Gas molecules collide with one another Lower pressure results in fewer molecules per unit volume. Explain how pressure relates to the number of molecules present in a given area Molecules of a gas are continuously in motion as gases are the least stable states of matter. Under normal atmospheric conditions, there are literally billions of atoms in a given space, and collisions between molecules are frequent. The “mean free path” defines how far on average one molecule may travel before colliding with another molecule. Under normal atmospheric conditions, the mfp is about 68 nM. When the vacuum level is increased to 1 millitor, the mean free path increases to 100 mm. The longer distance results from there being fewer molecules in the space. Since the Ideal Gas law shows the relationship between the number of moles of a gas and the pressure for a given volume, as we decrease the pressure, the number of molecules must drop, and the likelihood of collisions drops, simply because there aren’t as many of them there! In scanning electron microscopy, an electron beam hits the object being analyzed. If, along the way, molecules of gas in the chamber are struck by the beam or by electrons being freed from the object by the impact from the beam, the “picture” of the item being imaged will become “fuzzier”. A high vacuum removes the atmospheric particles to a great extent, so the emission “seen” is from the object being imaged alone. More molecules present in a given area will result in higher pressure. Drawing a vacuum reduces the number of molecules present, and hence the pressure, as there are fewer molecules present to press against the surfaces. This force is being created, not only by the molecular density (number of molecules) by the activity of the molecules. MATEC M097SS01.ppt

Molecular Density and Mean Free Path In the evaporative deposition system earlier described, a larger mean free path means that there are fewer molecules of air present to deflect the evaporated metal. In the thermal evaporator . a pressure of 10-8 torr from a turbo pumping system removes the atmospheric particles to a great extent, so that it is much less likely that an evaporated metal molecule will strike an air molecule as it proceeds to the target. More molecules present in a given area will result in higher pressure. Drawing a vacuum reduces the number of molecules present, and hence the pressure, as there are fewer molecules present to press against the surfaces. This force is being created, not only by the molecular density (number of molecules) by the activity of the molecules. MATEC M097SS01.ppt

Click once for each question. Practice Questions Click once for each question. 1. If the pressure in a chamber drops from 1 millitorr to 0.001 millitor, what happens to the mean free path? It increases. 2. What the condition where the rate of evaporation and condensation in a closed container are equal known as? Saturation MATEC M097SS01.ppt