ELECTRO-OPTICS

Characterization of Common Electron Multipliers in Harsh

Environments

The Pittsburgh Conference 2005Poster Paper 1340-20

Bruce Laprade and Raymond CochranBURLE Electro-Optics INC

ELECTRO-OPTICSIntroduction

•Mass Spectrometers are a valuable tool foruse in demanding applications such as drug discovery,

semiconductor manufacturing and food processing. The unique capability of a mass spectrometer to identify minute amounts of unknown materials sets this instrument apart from almost all others.

•The typical Mass Spectrometer has three key components:The ionization source, the mass filter and the detector.

ELECTRO-OPTICSDiscussion

• Homeland security requirements are now driving the development of field portable instruments that can provide laboratory grade analysis.

• In order to reduce the size, weight and power consumption of a portable mass spectrometer, it is often necessary to deviate from the normal operating environment inside the vacuum system.

Objective

• The objective of this project was to characterize the performance of various electron multipliers under harsh test conditions.

ELECTRO-OPTICS

• There are 3 main types of Electron Multipliers:

- Single Channel Electron Multipliers

- Microchannel Plate based Detectors

-Discrete Dynode Multipliers

Various Types of Electron Multipliers

Single Channel Electron Multipliers

Magnum® 6 Channel Multipliers

Discrete Dynode

Microchannel Plates

Microchannel Plate Detectors

Vacuum Conditions• Typical MALDI Time-of-Flight Mass Spectrometers require an ion

flight path length of 1 to 2 meters.• Modern quadrupole based instruments typically operate at

vacuum pressures in the high 10-6 Torr range because ions need to travel 25 cm. or more before they reach the detector without colliding with residual gas molecules.

• If it were possible to shrink the flight path length requirement to less than 5cm (approx... 2”) then it should be possible to operate a system in the milli-Torr range.

• Milli-Torr vacuum levels can be achieved with simple low cost vacuum pumps.

• Multiplier performance outside of the normal 10-6 Torr operating range has not been well characterized.

The Effects of Poor Operating Pressure on Detector Gain and Noise

Single Stage MCP Analog Operation in Argon at Various

Gain Settings

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01

System Pressure (torr)

OutputCurrent(amps)

Gain 50

Gain 100

Gain 500

Gain 1000

Gain 5000

Noise vs. Operating Pressure For Various Electron Multipliers

0.1

1

10

100

1000

1.00E-06 1.00E-05 1.00E-04 1.00E-03

Operating Pressure (Torr)

Det

ecto

r N

oise

(cts

./sec

.)

5 um MCP Chevron 25 um MCP Chevron Single Channel MultiplierDiscrete Dynode Multiplier Spiraltron™

Back-filled Air

ELECTRO-OPTICS

Miniature MCP Detector Noise as a Function of Vacuum Pressure

Pressure (Torr)

DarkCurrent(amps)

1.00E-16

1.00E-15

1.00E-14

1.00E-13

1.00E-12

1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02

The Effects of Poor Operating Pressure on Multiplier Lifetime

Lifetime at High Pressure:

1

10

100

0 0.5 1 1.5 2Extracted Charge (Coulombs)

Det

ecto

r G

ain

(Mill

ions

)

1 X 10-3 Torr 5 X 10-3 Torr

MCP Chevron, 5 Micron Pore AssemblyBack-filled Air

Lifetime at High Pressure

1

10

100

0 200 400 600 800 1000 1200 1400Continuous Hours of Operation at 1uA Output Current

Det

ecto

r G

ain

(Mill

ions

)

1 X 10-3 Torr Argon

MCP Chevron, 5 Micron Pore AssemblyBack-filled Argon

Lifetime at High Pressure

1

10

100

0 200 400 600 800 1000 1200 1400Continuous Hours of Operation at 1uA Output Current

Det

ecto

r G

ain

(Mill

ions

)

1 X 10-3 Torr Argon

MCP Chevron, 5 Micron Pore AssemblyBack-filled Argon

Lifetime at High Pressure:

1

10

100

1000

0 20 40 60 80 100 120 140 160Continuous Hours of Operation at 5E-8 amps Output Current

Gain

Single MCP, 5 Micron Pore AssemblyOperated at 1.2E-2 Torr In Argon

Lifetime at High Pressure:

1

10

100

0 100 200 300 400 500 600 700Continuous Hours of Operation at 1uA Output Current

Det

ecto

r G

ain

(Mill

ions

)

1 X 10-3 Torr 5 X 10-3 Torr

MCP Chevron, 5 Micron Pore AssemblyBack-filled Air

Performance After 300 Hours:

0.1

1

10

100

1.9 2 2.1 2.2 2.3 2.4 2.5

Applied Voltage (kV)

Det

ecto

r G

ain

(Mill

ions

)

Gain Measured at 10-3 TorrMCP Chevron, 5 Micron Pore Assembly

Back-filled Air

Performance After 300 Hours:

0.000.100.200.300.400.500.600.700.800.901.00

1.9 2 2.1 2.2 2.3 2.4 2.5

Applied Voltage (kV)

Det

ecto

r N

oise

(cts

./sec

)

Noise Measured at 10-3 TorrModel 3018MA 5 Micron Pore Assembly

Back-filled Air

Experimental Apparatus

• The experimental apparatus consisted of an Inficon XPR-III Residual Gas Analyzer (0-100 AMU) utilizing a microchannel plate/faraday detector mounted to a turbo pumped vacuum chamber.

• Chamber pressure was modulated using a Granville Philips precision leak valve connected to choice backfill gases.

• Sensor pressure was monitored using a combination of calibrated Veeco ionization tubes and Granville Philips convectrons.

Miniature Mass Spectrometer and Electron Multiplier

Faraday Cup and Electron Multiplier

Ionization Sourceand Mass Filter

Argon and Hydrogen at 5 milli-Torr, Utilizing a Microchannel Plate Based Electron

Multiplier

Faraday Mode

EM Mode

Lab Air Collected at 5.5 milli-Torr

Faraday Mode

EM Mode

Chemical Durability

• The performance of an electron multiplier will degrade as a result of everyday operation. Ions from the mass filter impinge on cone of the multiplier and produce a cascading of secondary electrons, ultimately resulting in the signal used to produce the mass spectrum.

• The constant bombardment of ions on the cone can result in the development of a surface coating which will reduce the efficiency of the secondary electron emission process.

• Equivalent Electron Multipliers from three manufacturers were subjected to an extended life test.

• The multipliers were loaded in a demountable test stand and bombarded with ions created from residual gas molecules of the species typically encountered in Residual Gas Analysis (RGA) monitored systems

Gain Stability of Various Multipliers for Bench top GCMS Applications

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

0 2 4 6 8 10

Extracted Charge (Coulombs)

Gai

n at

180

0 V

BURLE MAGNUM 5900

K&M Model 7596M

Detector Technology Model 2300

Chemical Durability• Multipliers from three manufacturers were tested for chemical durability in

an Agilent 5971 GCMS Chemstation. The internal calibrant PFTBA was used to create ions ranging from mass 69 to mass 512. PFTBA is known to be an aggressive material which quickly degrades multiplier performance.

• The original instrument ion optics frame was used for this test with all multipliers.

• Each multiplier was subjected to repetitive test cycles consisting of a Maximum Sensitivity Auto tune, Gain and Linearity tests, then begin the cycle again. The instrument was not vented between cycles. The cycles were repeated until the instrument was no longer able to achieve the requirements of auto tune.

• Before each multiplier sequence began, the ion source was completely disassembled and cleaned.

Electron Multiplier Life Tests with PFTBA

1000

1200

1400

1600

1800

2000

2200

2400

2600

2800

3000

0 20 40 60 80 100 120

Autotune Cycles w ith PFTBA

Aut

otun

e M

ultip

lier V

olta

ge

Detector Technology Model2300K & M Model 7596M

BURL MAGNUM 5900

Detector TechnologyFailed Autotune after 61Cycles

K & M Failed AutotuneAfter 64 Cycles

BURLE MAGNUM®Continued to Operatebeyond 119 Cycles

ConclusionsDiscrete dynode multipliers develop ion feedback at Discrete dynode multipliers develop ion feedback at pressures above 10pressures above 10--55 TorrTorr

Single Channel Electron Multipliers Operate well at Single Channel Electron Multipliers Operate well at pressures into the mid 10pressures into the mid 10--5 5 Torr Range.Torr Range.

SPIRALTRONSPIRALTRON™™ and MAGNUM Electron and MAGNUM Electron MultipliersMultipliers®® operate well into the 10operate well into the 10--4 4 Torr RangeTorr Range

• A specially designed Microchannel Plate-based Electron multiplier has been successfully operated for over 1200 continuous hours at elevated pressures in excess of 10 milli-Torr with good performance.

• A Mass Spectrometer (Inficon XPR-III) utilizing a very short (25mm) quadrupole mass filter and this microchannel plate electron multiplier has been commercialized for process monitoring in semiconductor manufacturing applications.

Multiplier lifetime testing, with low mass ions, for Multiplier lifetime testing, with low mass ions, for models manufactured by three companies highlighted models manufactured by three companies highlighted the differences in gain performance.the differences in gain performance.

Multiplier lifetime testing in a bench top GCMS using Multiplier lifetime testing in a bench top GCMS using the internal standard PFTBA to create ions indicated the internal standard PFTBA to create ions indicated that the 6 channel MAGNUM Electron Multiplierthat the 6 channel MAGNUM Electron Multiplier®®

utilizing SPIRALTRONutilizing SPIRALTRON™™ Technology operated over Technology operated over 85% longer than the OEM multiplier.85% longer than the OEM multiplier.