High-Vacuum Technology Course Week 2 Paul Nash HE Subject Leader (Engineering)

Vacuum Technology Outstanding enrolments Recap on last week Vapour Pressure & Mean Free Path Measuring Vacuum 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 consider the vapour pressure of a variety of materials and the effect this has on vacuum and pump down To be able to describe a variety of vacuum measurement techniques 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

Common Vacuum Units There are many varied units that are used to specify pressures The Torr, the Bar and the Pascal are in common use... .. but the Pascal is the SI recommended unit for pressure and so is the best choice for documentation 1 Atmospheric pressure is 760 mm Hg = 1 Bar = 105 Pa 1 Torr = 1 mm Hg 1 Torr = 1/760 of an atmosphere = 132 Pa 1 milliTorr = 0.13Pa = 1 μmHg 1mbar = 1/1000 Atm = 0.76 Torr = 100Pa 1 Pa = 7.6 milliTorr = 7.6 μmHg

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

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

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

Vapour 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

Vapour Pressure The vapour pressure of a substance in a chamber is important for a number of reasons. Possibility of vapourisation of the substance under low pressure May add to gas load of system Use of vaporisation 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

Typical Vapour Pressures 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

Typical Vapour Pressures

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 The average distance travelled by a molecule between collisions is termed as the Mean Free Path: For air at room temperature… Where P = Pressure in mBar This means that the Mean Free Path is about 6x10-6 cm at atmosphere and 64 metres at 10-6 mBar

Where Do We Use Vacuum in Manufacturing? 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

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

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

Measuring Vacuum 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

Vacuum Gauges There are 3 phenomena used to measure vacuum: Mechanical Vacuum Gauges There are 3 phenomena used to measure vacuum: Mechanical Displacement of materials Transport Movement of gases Ionisation Ion currents

Vacuum Gauges Vacuum systems must be monitored constantly to ensure satisfactory performance, but manufacturers seem to be reluctant to provide gauges which allow this to be done Many different types of gauges are available because each only covers a limited range of pressures Never trust a gauge unless you can check it independently Range of gauge utility

Schematic Circuit for a Pirani (hot wire) gauge Pirani gauge The Pirani is a dedicated low vacuum gauge device The resistance of the hot wire changes with the rate of heat loss (conduction) to the gas The Wheatstone bridge then measures the change in resistance of the hot wire Pirani’s are rugged and generally reliable and rarely need attention Schematic Circuit for a Pirani (hot wire) gauge

Correction Curve for Pirani Gauges Pirani calibration The calibration of a Pirani depends on thermal conductivity and so on the actual gas in the system Beware when using a crystal spectrometer as gases leaking from the counter tubes will degrade the accuracy of the Pirani gauge Correction Curve for Pirani Gauges

Penning (Cold cathode) Gauge A Penning gauge measures the ion current flowing from the cathode to the anode The magnetic field increases sensitivity by making the ions spiral as they travel to cause secondary ionization Beware - a Penning gauge reads zero current when the pressure is both very low and very high. The gauge must ‘strike’ to be operational Check with a Pirani gauge if in doubt Penning gauges require routine cleaning and testing

Capacitance Manometer Gauge head on chamber Controller and digital read-out

Capacitance Manometer A = Annular electrode D = Disk electrode S = Substrate G = Getter (in vacuum space) Differential capacitance between annulus and disk depends on pressure difference between Test Chamber and “Getter”.

Ion gauges Pressures lower than 10-5 Torr can be measured with ion gauges Mass spectrometer gauges (residual gas analyzers) are a desirable extra. These can measure partial pressures of e.g helium (for leak testing) or of water vapor.