LEARNING ACTIVITY 1: VACUUM GAUGES OVERVIEW

• This Learning Activity, Learning Activity 1, "Vacuum Gauges Overview," provides an introduction to vacuum gauge classification and range.
• Learning Activity 2, "Direct Liquid Wall Gauges," introduces the U-tube manometer and McLeod gauges.
• Learning Activity 3, "Direct Solid Wall Gauges," provides a discription of theory of operation and components for the Bourdon-tube gauge, diaphragm gauge and capacitance manometer.
• Learning Activity 4, "Vacuum Gauge Terminology," provides a game-based practice excersie for reviewing teminology used in industry.
• Learning Activity 5, "Indirect Viscosity and Thermal Gauges," introduces thermal gauges that calculate pressure from the amount of heat (energy) transferred by the gas molecules from a constant heat source.  The Thermocouple, Pirani, and convection gauges are explored in detail.
• Learning Activity 6, "Measuring Temperatures," is a lab activity (also provided as an interpretive exercise option) where learners observe the method or theory of measuring temperature with a thermocouple and relate it with that of measuring vacuum pressure with a thermocouple.
• Learning Activity 7, "Indirect Ionization Gauge," introduces ionization gauges that calculate pressure from the total charge produced when the gas is ionized. The Penning and Bayard-Alpert gauges are explored in detail.
• Learning Activity 8, "Magnetron and Extractor Gauges," the learner conducts independent research on manufacturer variations of gauges.
• Learning Activity 9, "Vacuum Gauge Questor Game," game-based practice on vacuum gauges.

• Note taking to organize your own knowledge concerning vacuum gauge classification and range
• Completion of exercises to reinforce your understanding
  

Objectives

• Classify vacuum gauges as either direct or indirect, and determine recommended type by range of operation.

Vacuum Gauge Classification

In the application of vacuum technology, vacuum gauges allow for measuring the magnitude of a vacuum.  A vacuum gauge measures the pressure in a vacuum environment by responding to the gas density (the number of gas atoms or molecules in the chamber). Remember the Ideal Gas Law (PV=nRT) which states: "As the number of gas molecules or atoms decrease in a chamber during the pump down process, the pressure decreases proportionately?" Correspondingly, as gas molecules or atoms decrease in a chamber, the pressure reading on the gauge decreases proportionately. In many industrial applications of vacuum technology, the gauge is used as an indicator of the chamber pressure during pump down and when the base pressure is reached at the end of pump down. 

There are many types of vacuum gauges to choose from depending on the application and operating pressure. In some cases, more than one type of gauge may need to be attached to the vacuum system. We classify vacuum gauges as either direct or indirect, and determine recommended type by range of operation. Type used depends on response to gas density. Several types are used in the factory environment and selection is based on physical principles underlying pressure measurements.

The most common classifications used for vacuum gauges are shown in the figure.

• Direct gauges indicate pressure by measuring the amount of displacement on a liquid or solid wall as a result of the gas force on that wall.
• Indirect gauges indicate pressure by measuring a gas property such as viscosity, heat transfer or ionization.
  

Figure 1.1. Vacuum gauge classification chart

Vacuum Gauge Ranges

Torr is the metric used in most vacuum applications.

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

Vacuum gauges have limited ranges over which they can operate.  The figure shows typical ranges for vacuum gauges used in semiconductor industry. 

• Bourdon tube gauges are used to measure rough vacuum.
• Diaphragm gauges, including capacitance manometers are used for rough vacuum to high vacuum applications such as vacuum pumps and ovens (diaphragm gauge) to plasma etchers and CVD (capacitance manometer).    
• Thermocouple, Pirani, and convection gauges are most commonly used in medium vacuum systems such as vacuum furnaces. 
• Ionization gauges are used for measurements in high and ultra high vacuum systems such as evaporators used for thin film deposition. 
• High and ultra high vacuum systems require at least two types of gauges to read the wide range of pressures from atmosphere to high and ultra high vacuum. 

Gauge labels indicate full pressure as a range. There are a few gauges that can measure pressures below 10^-10 Torr. 

Figure 1.2. Vacuum gauge ranges

Accuracy

Absolute accuracy or uncertainty (the closeness of the reading to the “true” value) is often less important than precision (the degree of repeatability from gauge to gauge and day to day).  The table shows some typical gauge accuracies under ideal conditions:

Bourdon tube	±10 Torr
Diaphragm	±5% of full scale reading
Capacitance manometer	±1 % or less of full scale reading
McLeod	±10% between 10-4 and 5X10-2 Torr of full scale reading
Spinning rotor	±2.5% between 10-7 and 10 -2  ±2.5 to 13.5% between 10 and 1 Torr
Pirani	±6% between 10-2 and 10 Torr of full scale reading
Cold cathode ionization	+100% to –50% of full scale reading
Hot cathode ionization	±10% between 10-7 and 10-4 Torr ±20% at 10-9 and 10-3 Torr ±100% at 10-10 Torr

Because of the high accuracy of a capacitance manometer, it is used when precise control of chamber process pressure is required. Also, indirect gauges respond differently to different gases.  This leads to the majority of the uncertainty in a gauge reading.

Calibration

Summary

• Vacuum gauges are classified as either direct or indirect.
• Direct gauges indicate pressure by measuring the amount of displacement on a liquid or solid wall as a result of the gas force on that wall. 
  
• Indirect gauges indicate pressure by measuring a gas property such as viscosity, heat transfer or ionization.
• Vacuum gauges have limited ranges over which they can operate.  Typical ranges for vacuum gauges used in semiconductor industry are measured in Torr. 
  
• Absolute accuracy or uncertainty is the closeness of the reading to the “true” value. 
• Under ideal conditions, the capacitance mamometer is the most accurate and can read  ±1 % or less of full scale reading making them ideal for precise control of chamber process.
• Bourdon tube gauges are used to measure rough vacuum. 
• Diaphragm gauges, which includes capacitance monometers, are used to measure rough to high vacuum applications (vacuum ovens to plasma etchers).   
• Thermocouple, Pirani, and convection gauges are most commonly used in medium vacuum systems such as vacuum furnaces. 
• Ionization gauges are used for measurements in high and ultra high vacuum systems such as evaporators used for thin film deposition.
• High and ultra high vacuum systems require at least two types of gauges. Very few gauges are capable of reading <10^-10 torr.