The Design and Application of a Sputter Ion Gun...
1
The Design and Application of a Sputter The Design and Application of a Sputter Ion Gun Apparatus to Sputter Oxides From Ion Gun Apparatus to Sputter Oxides From a Scanning Tunneling Microscope a Scanning Tunneling Microscope ’ ’ s Tips s Tips Eric Schmidt, K. W. Hipps Washington State University, Department of Chemistry and Material Science Abstract Abstract Model 04 Model 04 - - 161 2kV Sputter Ion Gun 161 2kV Sputter Ion Gun This model of sputter ion gun works in conjunction with the Ion Gun Control model 20-045. This gun generates an inert gas ion beam for sputter-etching solid surfaces. The mounting chamber containing the gun must be backfilled to a static pressure of 5 x 10 -5 torr with Ar gas. Within the ionization chamber of the gun, positive Ar ions are created by accelerating electrons from a hot tungsten filament into the anode region. The ions are then directed through a series of electrostatic lenses and focused on the target some 5 cm away. The energy of the incident beam is equal to the positive potential applied to the anode. The focusing lens uses a voltage range of 0-2000 V to determine the beam size incident on the target. To accurately measure beam size, pattern, and current density, a Faraday cup or a thin film of Ta 2 O 5 is used in place of a target to obtain desirable reproducible operating conditions. Measuring devices for determining Measuring devices for determining ideal ion gun operating conditions ideal ion gun operating conditions Ta 2 O 5 A 1000Å anodized tantalum specimen can be used for aligning and measuring the beam current density of the ion gun. It is well known that the sputter rate of this oxide is 300 Å/min using Ar + at 500eV and 1mA/cm 2 beam current density. To chemically prepare Ta 2 O 5 , an electrochemical cell using two clean 0.125 mm thick Ta foils (acting as anode and cathode), an electrolytic solution of 94.3% DI H 2 O; 5.7% conc. HNO 3 , and a power supply of 66.6 VDC is used. As the oxide forms on the anode, a gold color begins to appear. When the current within the cell drops to zero, the foil is sufficiently anodized. The 1000Å anodized Ta2O5 is then placed within the path of the ion gun and grounded through an ammeter and sputtered while measure the current. As the tantalum is sputtered, the gold color vanishes, leaving the initial silver color. The pattern left on the tantalum is used to determine the beam shape, and the time required to remove the 100nm oxide film provides the sputtering rate. Faraday cup A Faraday cup is a devise used to measure current from a beam of charged particles. A picture and schematic of the cup I designed are shown at the right. It consists of a cup with a small inner plate completely enclosed by a fine wire mesh. The outer cup, physically welded to the mesh is given a formal negative potential, while the inner cup, electrically isolated from the outer cup and wire mesh is grounded through an ammeter. As the beam, with a formal neutral charge (equal amount of positive and negative particles), is directed towards the Faraday cup the electrons are repelled and the Ar + ions are accelerated through the wire mesh and collected in on the grounded plate, and measured as a current through a floating electrometer. Results Results The ion gun was repaired and mounted into a high vacuum bell jar system and trials were run. The ideal operating conditions were determined via Ta 2 O 5 (see Table 1 for sputtering details). Using the sputter rate of Ta 2 O 5 we obtained – approximately 0.4 Å/s beamcurrent- and the literature value, the sputter rate we expect for WO 3 under the same conditions should be within 40 % standard deviation of 0.4 Å/s. To sputter a tip with 20 Å of WO3 would take around 50 sec. (Literature value) During operation, the ion gun becomes extremely hot due to the tungsten filaments (heated to about 2300 K) thus a simple cool water extension arm with soldered copper coil was designed to keep the ion gun assembly from becoming hot and outgassing. Our Faraday cup was designed and introduced into the vacuum system showing a beam current of about 20 μA when _____. Our original six-way cross had one tube that turned out to be too small for the transfer of tips to and from the gun, so a reducing cross was designed and is currently in production. Acknowledgements Acknowledgements I would like to thank the following establishments and individuals for their assistance in my project: •Dr. KW Hipps; •Dr. Louis Scudiero; •National Science Foundation: Division of Materials Research REU site program under grant number 0139125; •Washington State University’s Instrument Shop, namely Lauren Frei; •Washington State University’s Technical Services, namely Fred Schuetze The long term goal of this project is to use this technique to sputter tips used in ultra high vacuum scanning tunneling microscopy. In order to do so, a series of devices including a Faraday cup, a cooling extension, and interface and mounting hardware were designed and constructed. The ion gun, a used commercial source, was repaired and calibrated. The entire assembly was mounted on the existing STM chamber and minor revisions are in progress. The sputter rate of Ta 2 O 5 with our ion gun was determined to be 0.4 Å/(s-cm 2 -uA). References References •http://www.wsu.edu/~hipps/pdf_files/class.pdf ; •Fromhold, Jr., A. T.; Theory of Metal Oxidation, Volume 1 – Fundamentals; American Elsevier Publishing, New York; 1976; • Physical Electronics, Technician’s Model 04-161/162 2kv Sputter Ion Gun Manual, 2000, Eden Prairie, MN; •Goschnick, J; Schuricht, J; Schweiker, A; Ache, H. J; Nuclear Instruments & Methods in Physics Research, Sec. B; 1993, v. 83(3), 339-344 Figure 1: Schematic of STM tip and specimen Figure 2: Representation of reducing cross with ion gun, view port, gas leak value, and Faraday cup Figure 3: Schematic of RHK UHV STM showing where reducing cross will be mounted. Figure 4: Anodized tantalum pentoxide (above). Pure tantalum, anodized tantalum, and sputtered anodized tantalum (below). Figure 6: Bell jar system closed and open. Stage for sputtering Ta 2 O 5 can be seen. Figure 5: Faraday cup. Design (above) and finished product (below). Almost instantaneously after STM tips are finely- etched, oxides begin to grow on these metal surfaces. These oxides act as a protective layer which separates the metal from further corrosion by gaseous oxygen, but they inhibit current flow making STM imaging more difficult and they introduce anomalies in the current-voltage curve that cause the tunneling spectra to be unusable. In the real world, the tip surface is also contaminated with carbonates, hydrocarbons, and other chemical species. To obtain high quality STM images and spectra, it is essential to remove the oxide layer and contaminants without significant erosion of the metal tip. Moreover, this must be done after the tip enters the UHV environment so no new oxide grows. There are currently two preferred methods for performing this cleaning process. The first involves electron beam bombardment, the second ion beam bombardment. In either case a high energy particle is used to disrupt the oxide layer. Using Ar + ions accelerated at energies up to 2keVs, we will be blasting tips with “atomic canon balls” to dislodge the oxide layer. Introduction Introduction 45 min 60 μA/cm 2 30 mA 1000 V II 5x10 -5 torr 6.0 cm 6/21/2005 42 min 55 μA/cm 2 30 mA 1000 V I 8x10 -5 torr 6.0 cm 6/20/2005 40 min 45 μA/cm 2 30 mA 1000 V II 5x10 -5 torr 5.0 cm 6/20/2005 50 min 40 μA/cm 2 30 mA 1000 V I 8x10 -5 torr 5.0 cm 6/20/2005 45 min 15 μA/cm 2 30 mA 2000 V I 8x10 -5 torr 4.0 cm 6/17/2005 20 min 50 μA/cm 2 30 mA 2000 V I 5x10 -5 torr 3.5 cm 6/17/2005 50 min 35 μA/cm 2 30 mA 500 V I 5x10 -5 torr 3.5cm 6/17/2005 Running time Beam Current Density Emission Current Beam Voltage Filament Pressure Working Distance Date Table 1: Ion gun trials
Transcript of The Design and Application of a Sputter Ion Gun...
The Design and Application of a Sputter The Design and Application of a Sputter Ion Gun Apparatus to Sputter Oxides From Ion Gun Apparatus to Sputter Oxides From a Scanning Tunneling Microscopea Scanning Tunneling Microscope’’s Tipss Tips
Eric Schmidt, K. W. HippsWashington State University, Department of Chemistry and
Material Science
AbstractAbstractModel 04Model 04--161 2kV Sputter Ion Gun161 2kV Sputter Ion Gun
This model of sputter ion gun works in conjunction with the Ion Gun Control model 20-045. This gun generates an inert gas ion beam for sputter-etching solid surfaces. The mounting chamber containing the gun must be backfilled to a static pressure of 5 x 10-5 torr with Ar gas. Within the ionization chamber of the gun, positive Ar ions are created by accelerating electrons from a hot tungsten filament into the anode region. The ions are then directed through a series of electrostatic lenses and focused on the target some 5 cm away. The energy of the incident beam is equal to the positive potential applied to the anode. The focusing lens uses a voltage range of 0-2000 V to determine the beam size incident on the target. To accurately measure beam size, pattern, and current density, a Faraday cup or a thin film of Ta2O5 is used in place of a target to obtain desirable reproducible operating conditions.
Measuring devices for determining Measuring devices for determining ideal ion gun operating conditionsideal ion gun operating conditionsTa2O5
A 1000Å anodized tantalum specimen can be used for aligning and measuring the beam current density of the ion gun. It is well known that the sputter rate of this oxide is 300 Å/min using Ar+ at 500eV and 1mA/cm2 beam current density. To chemically prepare Ta2O5, an electrochemical cell using two clean 0.125 mm thick Ta foils (acting as anode and cathode), an electrolytic solution of 94.3% DI H2O; 5.7% conc. HNO3, and a power supply of 66.6 VDC is used. As the oxide forms on the anode, a gold color begins to appear. When the current within the cell drops to zero, the foil is sufficiently anodized.The 1000Å anodized Ta2O5 is then placed within the path of the ion gun and grounded through an ammeter and sputtered while measure the current. As the tantalum is sputtered, the gold color vanishes, leaving the initial silver color. The pattern left on the tantalum is used to determine the beam shape, and the time required to remove the 100nm oxide film provides the sputtering rate.
Faraday cupA Faraday cup is a devise used to measure current from a beam of charged particles. A picture and schematic of the cup I designed are shown at the right. It consists of a cup with a small inner plate completely enclosed by a fine wire mesh. The outer cup, physically welded to the mesh is given a formal negative potential, while the inner cup, electrically isolated from the outer cup and wire mesh is grounded through an ammeter. As the beam, with a formal neutral charge (equal amount of positive and negative particles), is directed towards the Faraday cup the electrons are repelled and the Ar+ ions are accelerated through the wire mesh and collected in on the grounded plate, and measured as a current through a floating electrometer.
ResultsResultsThe ion gun was repaired and mounted into a high vacuum bell jar system and trials were run. The ideal operating conditions were determined via Ta2O5 (see Table 1 for sputtering details). Using the sputter rate of Ta2O5 we obtained – approximately 0.4 Å/s beamcurrent- and the literature value, the sputter rate we expect for WO3 under the same conditions should be within 40 % standard deviation of 0.4 Å/s. To sputter a tip with 20 Å of WO3 would take around 50 sec. (Literature value)During operation, the ion gun becomes extremely hot due to the tungsten filaments (heated to about 2300 K) thus a simple cool water extension arm with soldered copper coil was designed to keep the ion gun assembly from becoming hot and outgassing.Our Faraday cup was designed and introduced into the vacuum system showing a beam current of about 20 µA when _____.Our original six-way cross had one tube that turned out to be too small for the transfer of tips to and from the gun, so a reducing cross was designed and is currently in production.
AcknowledgementsAcknowledgementsI would like to thank the following establishments and individuals for their assistance in my project:•Dr. KW Hipps;•Dr. Louis Scudiero;•National Science Foundation: Division of Materials Research REU site program under grant number 0139125;•Washington State University’s Instrument Shop, namely Lauren Frei;•Washington State University’s Technical Services, namely Fred Schuetze
The long term goal of this project is to use this technique to sputter tips used in ultra high vacuum scanning tunneling microscopy. In order to do so, a series of devices including a Faraday cup, a cooling extension, and interface and mounting hardware were designed and constructed. The ion gun, a used commercial source, was repaired and calibrated. The entire assembly was mounted on the existing STM chamber and minor revisions are in progress. The sputter rate of Ta2O5 with our ion gun was determined to be 0.4 Å/(s-cm2-uA).
ReferencesReferences•http://www.wsu.edu/~hipps/pdf_files/class.pdf;•Fromhold, Jr., A. T.; Theory of Metal Oxidation, Volume 1 –Fundamentals; American Elsevier Publishing, New York; 1976;•� Physical Electronics, Technician’s Model 04-161/162 2kv Sputter Ion Gun Manual, 2000, Eden Prairie, MN;•Goschnick, J; Schuricht, J; Schweiker, A; Ache, H. J; Nuclear Instruments & Methods in Physics Research, Sec. B; 1993, v. 83(3), 339-344
Figure 1: Schematic of STM tip and specimen
Figure 2: Representation of reducing cross with ion gun, view port, gas leak value, and Faraday cup
Figure 3: Schematic of RHK UHV STM showing where reducing cross will be mounted.
Figure 4: Anodized tantalum pentoxide (above). Pure tantalum, anodized tantalum, and sputtered anodized tantalum (below).
Figure 6: Bell jar system closed and open. Stage for sputtering Ta2O5
can be seen.
Figure 5: Faraday cup. Design (above) and finished product (below).
Almost instantaneously after STM tips are finely-etched, oxides begin to grow on these metal surfaces. These oxides act as a protective layer which separates the metal from further corrosion by gaseous oxygen, but they inhibit current flow making STM imaging more difficult and they introduce anomalies in the current-voltage curve that cause the tunneling spectra to be unusable. In the real world, the tip surface is also contaminated with carbonates, hydrocarbons, and other chemical species.
To obtain high quality STM images and spectra, it is essential to remove the oxide layer and contaminants without significant erosion of the metal tip. Moreover, this must be done after the tip enters the UHV environment so no new oxide grows. There are currently two preferred methods for performing this cleaning process. The first involves electron beam bombardment, the second ion beam bombardment. In either case a high energy particle is used to disrupt the oxide layer. Using Ar+ ions accelerated at energies up to 2keVs, we will be blasting tips with “atomic canon balls” to dislodge the oxide layer.
IntroductionIntroduction
45 min60
µA/cm230 mA1000 VII5x10-5 torr6.0 cm6/21/2005
42 min55
µA/cm230 mA1000 VI8x10-5 torr6.0 cm6/20/2005
40 min45
µA/cm230 mA1000 VII5x10-5 torr5.0 cm6/20/2005
50 min40
µA/cm230 mA1000 VI8x10-5 torr5.0 cm6/20/2005
45 min15
µA/cm230 mA2000 VI8x10-5 torr4.0 cm6/17/2005
20 min50
µA/cm230 mA2000 VI5x10-5 torr3.5 cm6/17/2005
50 min35
µA/cm230 mA500 VI5x10-5 torr3.5cm6/17/2005
Running time
Beam Current Density
Emission Current
Beam VoltageFilamentPressure
Working DistanceDate
Table 1: Ion gun trials
