Patent Publication Number: US-9404827-B2

Title: Ionization gauge with operational parameters and geometry designed for high pressure operation

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 12/860,050, filed Aug. 20, 2010, which is a continuation of International Application No. PCT/US2009/034460, which designated the United States and was filed on Feb. 19, 2009, published in English, which application claims the benefit of U.S. Provisional Application No. 61/066,631, filed on Feb. 21, 2008. 
    
    
     The entire teachings of the above applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Ionization gauges, more specifically Bayard-Alpert (BA) ionization gauges, are the most common non-magnetic means of measuring very low pressures. The gauges have been widely used worldwide. These gauges were disclosed in 1952 in U.S. Pat. No. 2,605,431, which is herein incorporated by reference in its entirety. A typical ionization gauge includes an electron source, an anode, and an ion collector electrode. For the BA ionization gauge, the electron source is located outside of an ionization space or anode volume which is defined by a cylindrical anode screen. The ion collector electrode is disposed within the anode volume. Electrons travel from the electron source to and through the anode, cycle back and forth through the anode, and are consequently retained within, or nearby to, the anode. 
     In their travel, the electrons collide with molecules and atoms of gas that constitute the atmosphere whose pressure is desired to be measured. This contact between the electrons and the gas creates ions. The ions are attracted to the ion collector electrode, which is typically connected to ground. The pressure of the gas within the atmosphere can be calculated from ion and electron currents by the formula P=(1/S) (I ion /I electron ), where S is a coefficient with the units of 1/Torr and is characteristic of a particular gauge geometry, electrical parameters, and pressure range. 
     SUMMARY OF THE INVENTION 
     The operational lifetime of a typical ionization gauge is approximately ten years when the gauge is operated in benign environments. However, these same gauges and electron sources (cathodes) fail in minutes or hours when operated at too high a pressure or during operation in gas types that degrade the emission characteristics of the electron source. Cathode interactions with the gauge environment can lead to decreased operational life. The oxide coating on the cathode can degrade when exposed to water vapor. Degradation of the oxide coating dramatically reduces the number of electrons generated by the cathode. Exposure to water vapor results in the complete burnout of a tungsten cathode. 
     Sputtering is also a problem, when operating the ionization gauge at high pressures, such as above 10 −4  Torr. This is a problem at high pressure since there is more gas to ionize. This sputtering is caused by high impact energies between ions and components of the ionization gauge as has been demonstrated by the inventor. Ions with a high energy may impact a tungsten material that forms a collector post of the ionization gauge. This results in atoms being ejected from the collector post and envelope surfaces. This ejection carries a significant internal kinetic energy. Ejected material can travel freely to other surfaces within the line of sight of the material, and can cause gauge failure by coating the cathode or by coating of the feed-through insulators of the gauge, which can result in electric leakages. 
     The kinetic energy of the ions generated in a Bayard-Alpert ionization gauge is determined by a difference in the bias voltages between an anode grid and a collector post electrode. A bias voltage of a cathode is typically at 30 volts, and a bias voltage of the anode grid is traditionally operated at 180 volts. The collector voltage is usually fixed at a ground potential, or at a voltage near a ground potential. These voltage differentials are configured to provide 150 electron volts (eV) amount of energy for the electrons. This amount is capable of efficiently ionizing all gas species present in the gauge ionization volume. This potential difference also assures efficient transport of the electrons from the cathode to the anode volume. Efficient ionization is needed to assure an adequate signal to noise ratio from the collector at low gas density levels. 
     Operation of the anode grid at +180 volts results in energetic ions arriving at the grounded collector posts during operation. Those ions impact on the collector surfaces with kinetic energies ranging from between 0 to 180 eV. This large energy end of this spread is consistent with large sputtering yields. 
     For example, sputtering yields as large as 0.2 atoms/ion impact have been demonstrated for Ar +  ions impinging on a tungsten target with 200 eV of kinetic energy. Ions also created outside of the anode grid can also reach the envelope walls with kinetic energies as large as 180 eV. Such large kinetic energy also increases sputtering yields, and these impacts remove materials from the envelope walls and adjacent structures. 
     The present disclosure decreases the anode grid voltage at high pressure levels in order to decrease the yield of sputtering impacts. The present ionization gauge provides for a reduction in an anode grid voltage down to about 80 volts to provide for about a five fold decrease in the sputtering yields for Ar +  ions impinging on a tungsten collector surface. Reducing the cathode potential allows the anode to cathode voltage difference to still provide electrons capable of causing adequate ionization of atoms and molecules. 
     The effects of both ion energy and electron emission current on collector sputtering rates were experimentally tested in our laboratory through a long term study which tracked the operation of a large group of Micro-Ion® gauges in 35 mTorr of argon gas for several months. All tested gauges contained dual tungsten collectors of small initial diameter. As expected, the rate of collector diameter erosion (i.e. due to sputtering impacts between energetic argon ions and tungsten walls) was proportional to the electron emission current and highly dependent on ion energy. A change in grid voltage from 180V to 80V, representing an ion energy reduction from 180 to 80 eV, resulted in approximately 15-fold reduction in sputtering yields exceeding the predictions of the theoretical calculations based on current sputtering models. Gauges operated at reduced emission currents and reduced ion energies exhibited almost imperceptible collector erosion, no detectable signs of metallization of adjacent electrode structures and minimal change in filament operation parameters over time. The advantages of operation at low electron emission currents and low ion energies were fully demonstrated by this carefully monitored test. 
     There is provided an ionization gauge to measure pressure while reducing sputtering when operating at high pressure. The ionization gauge includes at least one electron source that generates electrons, and a collector electrode that collects ions formed by the collision between the electrons and gas molecules. The ionization gauge also includes an anode. The anode is configured to switch a bias voltage relative to a bias voltage of the collector electrode at a predetermined pressure to decrease a yield of sputtering impacts. 
     In one embodiment, the ionization gauge is configured so the anode operates at an initial bias voltage at one pressure range, such as, for example, below about 10 −4  Torr. Then at high pressure, the anode operates at a reduced bias voltage, such as, for example, pressures above about 10 −4  Torr. 
     The ionization gauge may also have a controller. The controller changes the bias voltage of the anode based on a pressure range of the pressure in the ionization gauge. The anode can switch the bias voltage so a potential difference between the anode and the collector is less than 90 volts. In another embodiment, the bias voltage of the anode may be switched so a potential difference between the anode and the collector is about 80 volts. In yet another embodiment, the gauge may have an electron source that operates at less than 20 volts, or that operates at about 10 volts. 
     In a further embodiment of the present disclosure, the ionization gauge has an anode grid, and the anode bias voltage can be switched from about 180 volts to 80 volts. Alternatively, the bias voltage is switched from about 180 volts to another anode bias voltage. The anode may operate at a reduced bias voltage at pressure of above about 10 −4  Torr. 
     The ionization gauge may further include that the collector electrode surrounds the anode as a triode ionization gauge. Alternatively, the collector electrode can be positioned outside of the anode. The ionization gauge may further include a second collector electrode. The second collector electrode can be positioned outside the anode to collect ions formed at high pressure. The ionization gauge can be of the Bayard-Alpert type. The gauge may also include a cold cathode electron source. 
     In yet another aspect of the present invention, there is provided a method of measuring a gas pressure from gas molecules and atoms. The method includes producing electrons from at least one electron source and transmitting the electrons to an anode to form ions. Ions formed by the collisions between the electrons and the gas molecules and atoms are collected on the collector electrode. A bias voltage of an anode relative to a bias voltage of the collector electrode is switched to reduce an impact energy of the ions on the ion collector. 
     The ion collector potential preferably is selected to be at a low near ground potential to avoid leakage currents to ground, especially for low pressures when the ion collector current is relatively small. The cathode filament potential is typically selected to about 30 volts potential relative to ground, and also relative to the ion collector potential to avoid electrons arriving at the ion collector electrode with a predetermined energy, and this is relevant for a combination of high emission currents and low ion collector currents. The anode potential is typically selected to be at about 180 volts relative to ground. 
     A potential difference between the anode and the cathode determines the energy of the electrons as they arrive at the anode. The potential difference is typically selected to be about 150 volts. The anode is relatively higher than the cathode so that the energy of the electrons available for ionization of the gas is at about 150 electron volts. 150 eV is at a fairly low slope of an ionization probability versus electron energy curve for most gases. Therefore, at 150 eV an ionization gauge sensitivity variation with electron energy is minimized. It should be appreciated that this may depend on the specific gas species. Electrons are accelerated from the cathode to the anode at an energy of about 150 eV. It should be appreciated that generally lower values of this potential difference begins to allow the onset of space charge limiting of the electron emission from the cathode. Space charge limiting imposes an electron emission limit from the cathode and can bring on high temperature operation and failure of the cathode since a typical control circuit attempts to supply power to the cathode until a desired specified electron emission current is achieved. Lowering the cathode potential allows a lower anode potential to still achieve an acceptable anode to cathode potential difference without space charge limiting. 
     Secondly, a potential difference between the anode to the ion collector determines the maximum energy of the ions as they arrive at the ion collector. Ions formed near the anode will have the maximum energy, and ions formed relatively closer to the ion collector will have relatively less energy. The potential difference between the anode to the cathode or the gauge envelope is typically about 180 volts, and this dictates the energy and, thus, the impact of the ionized atoms and molecules when they arrive at the surface of the ion collector. The potential distribution in the anode has a shape such that a majority of the anode volume is near the potential of the anode, and a majority of the ions arriving at the ion collector have the maximum energy. The potential difference of the anode to cathode or gauge envelope also dictates the energy of the ions formed outside of the anode volume when they finally arrive at any relatively lower potential surfaces, such as, for example, the cathode, the collector shield, or an envelope of the gauge. 
     Altering the above mentioned potentials allows more electrons to arrive at the ion collector and reduces the measured ion collector current. However, more electrons will not change the positive ion current arriving at the ion collector. It should be appreciated that the measured collector current equals ions arriving at the collector minus electrons arriving at the collector. Any change in the potentials that will reduce the number of ions created, such as ionization probability, or reduce the number of created ions that are collected will reduce the actual ion current to the ion collector. 
     The number of created ions collected is dependent on the ion energy and the ion collector diameter, or ion collector geometry. At relatively low potential surfaces, sputtering is directly related to the number of ions created, the number of ions arriving at the surface of interest, and the energy of those ions. The sputtering rate is relevant to the number of atoms sputtered per unit time and relative to the number of incident ions per unit time. The sputtering yield is relevant to the number of atoms sputtered per incident ion and is related to the energy of incident ions. High pressure causes large ion currents, and consequently high pressure also causes large sputtering rates. By lowering the ion energy diminishes the sputtering yield, which may decrease the sputtering rate even at a relatively high pressure. 
     According to yet another aspect of the present disclosure, there is provided a process that includes providing a substrate, evacuating a tool to perform processes on the substrate in the tool, and measuring pressure. The method for measuring pressure includes an electron source that generates electrons, and an ionization volume in which the electrons impact a gaseous substance that includes gas molecules and atoms. The anode grid voltage is decreased at high pressure levels in order to decrease the yield of sputtering impacts. A collector electrode collects ions formed by the impact between the electrons and the gaseous substance. 
     The gauge can be used in a process. The process includes conducting operations on the substrate in the vacuum environment to form a processed substrate. In another embodiment, the process can include operations that are selected from the group consisting of operations associated with manufacture of a flat panel display, magnetic media operations, solar cell manufacturing operations, optical coating operations, semiconductor manufacturing operations, and any combination thereof. Operations may also include one or more processes selected from the group consisting of: physical vapor deposition, plasma vapor deposition, chemical vapor deposition, atomic layer deposition, plasma etch operations, implantation operations, oxidation, diffusion, a vacuum lithography process, a dry strip operation, an epitaxy process, a thermal processing operation, an ultraviolet lithography operation, and any combination thereof. 
     Preferably, current is converted from the collector electrode to a pressure signal to measure pressure. The process may also include measuring a parameter of a process using an analytical tool. In one embodiment, the analytical tool may measure a parameter of the wafer. The analytical tool can be selected from the group consisting of: a scanning electron microscope, an energy dispersive X-ray spectroscopy instrument, a scanning auger microanalysis instrument, a glow discharge mass spectroscopy instrument, an electron spectroscopy chemical analysis instrument, an atomic force microscopy instrument, a scanning probe microscopy instrument, a Fourier transform infrared spectroscopy instrument, a wavelength dispersive X-ray spectroscopy instrument, an inductively coupled plasma mass spectroscopy instrument, an x-ray fluorescence instrument, a neutron activation analysis instrument, a metrology instrument, and any combination thereof. A parameter of the process can be also measured using a mass spectrometer. The mass spectrometer can be one of a gas chromatograph instrument, a liquid chromatograph instrument, an ion trap instrument, a magnetic sector spectrometer instrument, a double-focusing instrument, a time-of-flight instrument, a rotating field instrument, an ion mobility instrument, a linear quadrupole instrument, or any combination thereof. The ionization gauge in the process preferably converts the current from the collector electrode to a pressure signal. 
     In yet another embodiment, a process may include a manufacturing process step and then measuring a parameter of the process using an analytical tool and measuring pressure. The pressure measurement is performed while decreasing the yield of sputtering impacts at high pressure. The analytical tool can be any mass spectrometer, or any previously mentioned instrument. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a schematic view of a generalized ionization gauge of the present disclosure. 
         FIG. 2  is a detailed schematic view of a non-nude type ionization gauge of  FIG. 1 . 
         FIG. 3  is a schematic view of an ionization gauge including an anode coupled to an anode voltage supply for reducing an electron impact energy at high pressures according to the present disclosure. 
         FIGS. 4 through 6  show several schematic views of embodiments of ionization gauges for collecting ions using a second collector at high pressure, and for extending the measuring range of the ionization gauge. 
         FIG. 7  shows a Schultz-Phelps ionization gauge of the present disclosure. 
         FIG. 8  shows the ionization gauge of  FIG. 3  used with a cluster tool and an analytical tool for processing operations. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of preferred embodiments of the invention follows. 
     Generally, as shown in  FIG. 1 , an ionization gauge  100  of the present disclosure has at least one electron source  105  and at least one collector electrode  110 . The electron source  105  may be separated from the at least one collector electrode  110  by an optional isolation material  115  which prevents molecules and atoms of gas within a measurement chamber  117  from degrading the electron source(s)  105 . The ionization gauge  100  also includes an ionization volume and specifically an anode  120 . Anode  120  and the collector electrode  110  components may have various different configurations, and the gauge  100  is not limited to  FIG. 1 . In one embodiment, the ionization gauge  100  is a Bayard-Alpert type gauge, or an ionization gauge  100  where a heated cathode  105  is used to emit electrons toward an anode grid volume  120 . However, it should be appreciated that the gauge  100  is not limited to any specific ionization gauge configuration and the present invention encompasses several different types of gauges. 
     The Bayard-Alpert type gauge  100  is based on the ionization of gas molecules by a constant flow of electrons. The negatively charged electrons shown as reference numeral  125  are emitted at a well-controlled selectable rate from a heated cathode  105 , and may be released, or accelerated toward a positively charged anode  120 . The electrons  125  pass into and through the anode  120  and then cycle back and forth through the anode  120 . The electrons  125  are then retained within the ionization volume of the anode  120 . In this space, the electrons  125  collide with the gas molecules to produce positively charged ions. These ions are collected by the one or more ion collector(s)  110 . Collector  110  is nearly at ground potential, which is negative with regard to the positively charged anode  120 . However, this arrangement is not limiting and collector  110  may have various potential differences with respect to the anode  120 . At a constant cathode to anode voltage and electron emission current, the rate that positive ions are formed is directly correlated to the density of the gas in the gauge  100 . This signal from the collector electrode  110  is detected by an ammeter  135 , which is calibrated in units of pressure, for all pressure readings. 
     The embodiment of  FIG. 1  is shown as a nude configuration of the Bayard-Alpert type gauge  100 . It is also envisioned that non-nude type ionization gauges are also possible.  FIG. 2  shows a specific non-nude type ionization gauge  200  embodying the present disclosure. The ionization gauge  200  has similar components to the ionization gauge  100  ( FIG. 1 ) described above with the following additions. The ionization gauge  200  is housed in a tube  205 . Tube  205  is opened at one end  225  to allow molecules and atoms of gas to enter the measurement chamber  117  through a shield  220 . The shield  220  and tube  205  form a shield volume. An optional second ion collector  210  is added for high pressure measurements of very short mean free paths. 
     In operation, molecules and atoms of gas enter the measurement chamber  117  through the partially open shield  220 . The shield  220  prevents potentials external to the shield  220  from disturbing the electric charge distribution within the measurement chamber  117 . The shield  220  is maintained at a reference potential. In one embodiment, the reference potential is ground potential. 
     Turning now to  FIG. 3  the electron source (for example, a cathode filament)  105  generates electrons (represented by an electron beam  125 ) within the chamber  117  defined by the envelope  113 . The electrons  125  are used in ionizing the gas molecules in the measurement chamber  117 . The geometrical shape of the filament  105  can be a linear ribbon, a linear wire, a straight ribbon, a curved ribbon, a hairpin wire, or any other shape known in the art. In one embodiment, the cathode  105  is resistively heated to incandescence with an electrical current from cathode heating power supply  113 . The thermionically emitted electrons  125  may be released, or accelerated or directed into the measurement chamber  117  towards anode  130 . Electrons have a sufficient energy which allows the electrons to be transmitted to the ionization volume of anode  130  and have sufficient energy to enter the anode  130 . 
     A controller  105   a  is connected to the cathode bias supply  105   b , and the cathode  105  receives a cathode bias voltage of about 30 volts, and a heating voltage from power supply  113  during normal operation. Once the cathode  105  is sufficiently heated, the controller  105   a  controls the cathode  105  to maintain the appropriate electron current. The cathode bias voltage provides sufficient voltage difference from the cathode  105  to the anode  130  to transmit electrons  125  toward the anode grid  130 . Ionization occurs over an energy spread both higher and lower than the nominal design energy; see Section 5.7 on ionization gauges in  Scientific Foundations of Vacuum Technique  by Saul Dushman, 1962, which is herein incorporated by reference in its entirety. 
     The controller  105   a  also is coupled to the anode voltage supply  130   a , which delivers a bias voltage to the anode wire grid  130 . The anode  130  includes an anode bias voltage of about 180 volts when measuring pressure at high vacuum conditions. This difference (180 volts minus 30 volts) provides for 150 eV of energy for the electrons. This is the amount of kinetic energy that is gained by a single free electron  125 , when the electron  125  passes through a potential difference that is created between the cathode  105  and the anode grid  130 . This 150 eV is sufficient to ionize all gas species present in the ionization gauge volume at high vacuum conditions. 
     These ionized atoms and molecules can have a maximum energy of about 180 eV when arriving at the ion collector  110 . These ions formed in the ionization volume of the anode  130  will impact on the collector surface  110 , and when operating at high pressure, this large quantity of ions may be excessive and can increase the sputtering rate for unit time. As mentioned, sputtering yields can be as large as 0.2 atoms/ion for Ar +  ions impacting on tungsten targets with 200 eV kinetic energies, and this kinetic energy can damage the components of the gauge  100  and degrade the ionization gauge  100 . To counteract high sputtering rates, an ionization gauge design may also decrease sputtering yields. 
     The present ionization gauge  100  decreases anode grid bias voltage levels relative to the bias voltage of the collector  110  at high pressure levels to decrease the yield of sputtering collisions. A reduction of the anode grid bias voltage to about 80 volts can provide for about a five fold decrease in the sputtering yield for Ar +  ions impacting on a tungsten surface of a collector electrode  110 . Although reducing the bias voltage of the anode grid  130 , the energy of the electrons remains sufficiently high for their collisions with gas atoms and molecules to ionize all gas species present in the ionization volume in the anode grid  130 . At the same time, the kinetic energy of the ions is decreased to lessen the energy of ions arriving at the collector  110 , envelope walls, and adjacent grounded electrode structures (not shown). This occurs while providing sufficient potential difference between cathode  105  and anode  130  so electrons  125  can enter the anode grid volume  130  while reducing sputtering yields. 
     Notably, a byproduct of the bias voltage reduction can be a decrease in a sensitivity of the ionization gauge  100 . Since this reduction occurs at high pressure levels, or above 10 −4  Torr, the ion current signal received by the collector electrode  110  is relatively large. This received signal/noise level by the collector  110  is adequate for operation of the ionization gauge  100 . 
     In a first embodiment, the ionization gauge  100  includes that the controller  105   a , reduces a bias voltage of the anode grid  130  in a high pressure mode. In the embodiment shown, preferably the controller  105   a  sufficiently reduces the bias voltage supplied to the anode grid  130  by controlling anode voltage supply  130   a . The anode grid  130  operates at an anode bias voltage of less than 180 volts. This reduced bias voltage results in reduced electron kinetic energy from 150 eV, or the potential difference between the cathode  105  and the anode grid  130 . 
     Notably, this reduced electron kinetic energy of less than 150 eV is still sufficient to ionize all gas species present in the ionization gauge  100  volume. The values of threshold ionization energies required for different monatomic species can range from 3.88 eV (Cs) to 24.58 eV (He), and 15 eV (for oxygen, nitrogen, and hydrogen). Various ionization energies are possible and are within the scope of the present disclosure, depending on the gaseous material that is desired to be measured. 
     In another embodiment, the controller  105   a  outputs a control signal to the anode voltage supply  130   a  to reduce the anode bias voltage supply from 180 volts to 80 volts, or less. In this embodiment, the electron energy, or difference between the bias voltage of the cathode  105  and the anode  130  is 50 eV. This amount is sufficient to ionize the gas at high pressures without causing the degradation of the ionization gauge  100  that is attributed to high sputtering yields. This results in a decrease in the yield of sputtering impacts for Ar +  ions impinging on tungsten collector electrode  110  surface, and this 50 eV value may provide for a five-fold decrease in the sputtering yield. It should be appreciated that other anode bias voltages are also envisioned, and the present ionization gauge  100  is not limited to any specific anode bias voltage reduction. 
     In one embodiment, the ionization gauge  100  is configured so the anode  130  operates at an initial bias voltage at one pressure and then automatically operates at a reduced bias voltage at high pressures, such as, for example, above about 10 −4  Torr. The controller  105   a  can switch automatically the bias voltage so a potential difference between the anode  130  and the collector electrode  110  is less than 90 volts. 
     In another embodiment, the bias voltage of the anode  130  may be switched from about 180 volts to about 80 volts so a potential difference between the anode  130  and the collector electrode  110  is about 80 volts. The cathode filament  105 , can be supplied less than 20 volts, or can be supplied at about 10 volts so that an adequate voltage difference from the cathode  105  to the anode  130  is maintained. The anode  130  can operate at a reduced bias voltage at high pressures or above about 10 −4  Torr. 
     At low pressure, where high sputtering yields are not as much of a concern relative to the high pressure conditions discussed above, the controller  105   a  may control the anode bias voltage supply  130   a  to increase the anode bias voltage supply. The potential difference can be increased for sufficient ionization energies. This ensures that the electrons have sufficient energy to efficiently ionize a very low density of gas. 
     It should be appreciated that the bias voltage of the cathode  105  cannot be too low, and has to be above the collector electrode  110 , and the envelope wall  113 . Preferably, the cathode  105  is about 10 volts above the collector electrode  110 . 
     The present ionization gauge  100  is not limited to controlling the bias voltage of the anode grid  130 . It is envisioned that the bias voltage of the filament  105  and the collector electrode  110  can also be modified by controller  105   a . The bias voltage of the filament  105  and the collector electrode  110  can also be modified by controller  105   a  to minimize sputtering yields, and to extend the life of the ionization gauge  100 . It should also be appreciated that the ionization gauge  100  may be configured as a triode ionization gauge (not shown) or another specific type of gauge  100 . It should be appreciated that the present ionization gauge  100  is not limited to a BA ionization gauges, and may include a cold cathode electron emitter  105 . 
     The energy of the ions can be determined by the difference between the ion collector  110  and the anode  130 . Here, the anode  130  can be kept constant and the collector  110  can be raised above ground potential to reduce the energy of the ions. 
     Turning now to  FIG. 4 , there is shown another embodiment of the present disclosure. Here, the ionization gauge is a Bayard-Alpert ionization gauge  400 ; however, gauge  400  can alternatively be manufactured as a cold electron emitter ionization gauge  400  with a cold electron emitter source. Gauge  400  includes an anode grid  430 , a cathode filament  405 , and a first ion collector electrode  410   a . The anode  430  surrounds the first ion collector electrode  410   a.    
     Typically, Bayard-Alpert ionization gauges are used to measure pressure in the high vacuum and ultrahigh vacuum environment. Pressure measurement capabilities become compromised at high vacuum levels or at about 10 −4  Torr, and become even more limited at about 10 −3  Torr. One observed problem is that at higher pressures electrons scatter on their way to the anode grid  430 . Long electron trajectories are compromised by scattering collisions with neutral species. Additionally, the ability to effectively collect ions inside the anode  130  is compromised as the ion density builds up around the collector post  410   a.    
     The present ionization gauge  400  preferably collects ions at high pressure that are located outside of the anode grid  430 . This collection extends the effective pressure operating range above 10 −4  Torr. 
     In this embodiment, a second ion collector  410   b  can be positioned in a location near or closer to the cathode filament  405 . Collector electrode  410   b  assures efficient collection of ions, which are located near the electron source  405  at high pressures. Collector electrode  410   b  is configured for use as an alternative second ion collector electrode  410   b  to collect ions formed outside of the anode  430  at high pressures. Second collector electrode  410   b  extends the operating range of the gauge  400  to levels above 100 mTorr with a minimum detectable pressure limit as low as 10 −5  Torr. This provides for better overlap with a heat loss or a capacitance diaphragm pressure sensor in a combination gauge. This also provides for no lost time attributed to sensor switching during process cycles in PVD, semiconductor, and hard disk manufacturing processes. This also preserves the base measurement capabilities expected from the gauge  400  and which are needed for a vacuum qualification of the instrument. 
     The ionization gauge  400  may also include that the second ion collector electrode  410   b  is positioned near the filament  405 , and is supported in an extra filament support post (not shown). This is advantageous in a retrofit installation of the second ion collector electrode  410   b  to an existing gauge. In yet another alternative embodiment, the second collector electrode  410   b  is supported in another support structure, such as, for example, a support post which is located near the anode grid  430 . Various support configurations are possible. 
     The second ion collector electrode  410   b  can be a post  410   b  as shown in  FIG. 4 , or an electrode plate  410   c  ( FIG. 5 ), or can be formed as an electrode grid or wall  410   d  ( FIG. 6 ). The second ion collector electrode  410   c  of  FIG. 5  can be connected to an ammeter  435   a , or may alternatively be operatively coupled to the collector electrode  410   a , which is connected to the ammeter  435 . The second collector  410   b  can be connected to a different ammeter  435   a , or the same ammeter  435  as collector  410   a  to measure ions, and gauge  400  is not limited to any specific configuration. 
     Turning to  FIG. 6 , the second ion collector may be formed as an intermediate wall  410   d . Collector wall  410   d  surrounds the anode  430  and the first ion collector electrode  410   a  and the cathode  405 . 
     The intermediate wall  410   d  may be connected to a switch  440  ( FIG. 6 ). Switch  440  provides for two modes of collecting ions, namely a high pressure operation mode and a normal (high vacuum) operation mode. In the high pressure operation mode, the ionization gauge  400  collects ions using the second ion collector grid or wall  410   d . In normal (high vacuum) operation mode, the first ion collector electrode  410   a  collects ions. Gauge  400  can be switched between collecting ions using collector  410   a  or collector  410   d  using switch  440 , which can be manually controlled or electronically controlled. Pressure is measured using detected signals received by the ammeter  435  which is connected to the controller  450 . One collector would be used at high pressures, while at a different pressure, the other collector electrode would be used to measure pressure. 
     The intermediate wall  410   d  can advantageously be installed in a retrofit manner to existing BA ionization gauges  400 . In this aspect, a switch  440  can be installed in a retrofit manner and connected between the ammeter  435  and the first ion collector electrode  410   a  in an existing BA ionization gauge  400 . 
     It should be appreciated that the cathode emission level may remain constant, as well as, the voltage bias on the anode  430 . In one embodiment, the ammeter  435  would detect a pressure of 1×10 −4  Torr, and then output a signal to a controller  450 . The controller  450 , in response, would then switch from collecting ions using the first ion collector electrode  410   a  to collecting ions using the second ion collector  410   d  to collect ions formed closer to the filament  405 . 
     In another embodiment, the ion current measurement may effectively self-switch from one collector to another collector whereby the ion current diminishes significantly from the inside collector as the pressure increases from above about 10 −3  Torr, and then diminishes significantly from the outside collector as the pressure decreases below 10 −3  Torr. It should be appreciated that ion current from the outside collector is generally not a concern below 10 −4  Torr. 
     It should be appreciated that the ionization gauge  400  may be formed with more than two ion collector electrodes  410   a ,  410   d . Gauge  400  may include a second ion collector  410   d , and a third ion collector electrode (not shown), or more collector electrodes being placed near the filament  405  to collect ions at high pressures. Various configurations are possible and within the scope of the present disclosure. 
     In another embodiment shown in  FIG. 7 , an ionization gauge  100  may be configured in a Schultz-Phelps geometry with the anode  115  being arranged as a flat plate, the ion collector  110  as a parallel flat plate, and the electron source  105  positioned between those two plates  110 ,  115 . 
     Turning to  FIG. 8 , the ionization gauge  100  preferably can be used with a cluster tool  800  or another multi-chamber tool for processing operations. In one embodiment, the cluster tool  800  may include a load lock chamber  805  connected to a transfer chamber  810  by a valve  810   a . The load lock chamber  805  is sealed from ambient conditions by a valve  810   b . Both a single chamber and multi-chamber cluster tools  800  are envisioned, and the ionization gauge  100  can be used in either a single chamber or a multi-chamber tool configuration. It should be appreciated that the ionization gauge  100  is not limited for use with a vacuum chamber, and can be used in any manufacturing chamber known in the art. 
     The cluster tool  800  may also include a process module  815 . Process module  815  is also connected to the transfer chamber  810  by a valve  810   c . The tool  800  may includes multiple process modules  815  and multiple load lock chambers  805 , and the configuration shown is not limiting. The load lock chamber  805  may include a rough pump RP 1 , which also is connected to the load lock chamber  805  by a valve V 1 . Each of the load lock chamber  805 , the transfer chamber  810 , and the process module  815  may include at least one vacuum pump Vp 1 , Vp 2 , and Vp 3 . The vacuum pump may be a cryogenic vacuum pump, or another pump, such as a turbo pump or water vapor pump. Various pumping configurations are possible and within the scope of the present disclosure. 
     Preferably, a wafer (not shown) may be introduced into the load lock chamber  805 , and pumped to vacuum conditions using the rough pump Rp 1  and the vacuum pump Vp 1 . Using a wafer manipulating robot (not shown), the wafer can be manipulated to the transfer chamber  810  through the valve  810   a , and then the wafer can be placed in the process module  815  through the valve  810   c  for various deposition operations. In one embodiment, the ionization gauge  100  may be placed in one of the chambers  805 ,  810 , or  815  of the cluster tool  800 . For illustration purposes, the ionization gauge  100  is shown in the process module  815 , but is not limited to any specific chamber or location, and can be placed outside of the chamber or tool  800 . 
     The ionization gauge  100  preferably can measure pressure both base pressure (high vacuum) and higher processing pressures (mostly in the mTorr range), however this is not limiting and various operational parameters for measurement are possible and within the scope of the present disclosure. The ionization gauge  100  can be used to measure pressure in the manufacture of flat panel displays, magnetic media operations, solar cells, optical coating operations, semiconductor manufacturing operations, and other manufacturing process operations. Such processes may include physical vapor deposition, plasma vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), plasma etch operations, implantation operations, oxidation/diffusion, forming of nitrides, vacuum lithography, dry strip operations, epitaxy operations (EPI), rapid thermal processing (RTP) operations, extreme ultraviolet lithography operations, and others. Preferably, the ionization gauge  100  may also be operable with one or more analytical tools, such as, for example, a microscope or a mass spectrometer. Mass spectrometers may include gas chromatograph instruments (GC), liquid chromatograph instruments (LC), ion trap instruments, magnetic sector spectrometers instruments, double-focusing instruments, time-of-flight instruments (TOF), rotating field instruments, ion mobility instruments, linear quadrupole instruments, and others. 
     Surface analytical instruments  820  that can be used in connection with the ionization gauge  100 , and with the cluster tool  800  (or without the cluster tool  800 ) may also include scanning electron microscopes, energy dispersive X-ray spectroscopy instruments (EOS/XPS), scanning auger microanalysis instruments (Auger/SAM), glow discharge mass spectroscopy instruments (GDMS), electron spectroscopy for chemical analysis instruments (ESCA), atomic force microscopy/scanning probe microscopy instruments (AFM/SPM), Fourier transform infrared spectroscopy instruments (FTIR), wavelength dispersive X-ray spectroscopy instruments (WDS), inductively coupled plasma mass spectroscopy instruments (ICPMS), x-ray fluorescence instruments (XRF), neutron activation analysis instruments (NAA), metrology instruments, and others. It should be appreciated that this listing is not exhaustive and the gauge  100  may be used with other instruments not enumerated above. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.