Secondary Ion Mass SpectrometryA look at SIMS and Surface Analysis

Secondary Ion Mass Spectrometry

What are Secondary Ions?

• Ions created from the interaction between a surface and an ion beam (the primary

ions)• Require high vacuum (≤10-6 torr) in order to minimize ion-ion interactions• Analyzed using a mass spectrometer

• Studies are used to gather information about the surface being analyzed and can

fall in to two categorieso Dynamic SIMS – used for bulk analysis using a DC ion beam and a QMS

o Static SIMS – used for atomic monolayer analysis using a pulsed laser and a TOF

Target of SIMS Studies

• Typically flat surfaces or rods – CuBe– Al

• Rapidly growing interest in studying 3D– Organometallics and films from

metal or semiconductor surfaces

• These trends demand that the Mass Spectrometer have– Better Resolution– Higher Sensitivity– Mass Range Options

MAX-CS Probes are the Proven Solution

• Powerful research grade instruments. o Best performance of flange mounted systems

o Heart of Extrel’s VeraSpec Mass Spectrometer Systems

o Widest array of options of any of our systems • Mass Ranges

– 50, 120, 500, 1000, 4000, 16000 maximum masses – Based off of quad size + RF frequency

• Two Quadrupole Sizes• MS/MS option• Multiple ionization sources• Conical aperture on the Ionizer can be located close to the substrate to ionize the desorbed

neutral compoundso Energy Filter Ionizer to differentiate between secondary ions and ionized neutrals in the chamber

o Has fine control of the ionizer to perform “soft” ionization to retain the molecular ion without fragmentation

SIMS Analysis Features of MAX Probe Systems

MAX-LT Probes Provide a Quality solution at an Affordable Cost

Research grade performance at an affordable price.

– 500 and 1000 amu mass ranges– Positive and negative ion

detection

Typical applications– Thermal Programmed Desorption

(TPD)– Plasma and Chemical Vapor

Deposition (CVD) Analysis– UHV Residual Gas Analysis.

SIMS Analysis: Energy Filter Ionizer

• High Energy ions (1 keV – 100 keV) – Not easily analyzed by QMS– Should be blocked from entering the analyzer

• Center stop of the energy filter – Keeps neutrals from entering the analyzer

• Blocks line-of-sight.

• Excellent Energy Filtering– Cylindrical lens of energy filter – Bandpass lenses

• above and below the cylinder – Center stop

• Cone aperture – Allows close placement to the target surface

• Greater sensitivity to the sputtered plume

• On-axis energy filter – Fine energy scan – < 0.6 eV FWHM (full width at half maximum)

resolution

SimION Modeling of the Energy Filter Ionizer

CuBe Experiment

• The SIMS spectra for the CuBe alloy were collected using:

• MAX-120 with a tandem ionizer/energy analyzer o 19 mm Quadrupole & 2.1 MHz QC

• Base pressure: 1.6x10-9 Torr

• Primary ions: 2 keV Ar+ ions

• Incident angle (angle between the sample surface normal and the axis of the ion gun): ~ 80°

• Collection angle (the angle between the sample surface normal and the axis of the SIMS probe): ~ 10°

• All of the SIMS spectra were acquired after the sample had been sputtered with a much higher current of the

Ar+ ions. o SIMS spectra are characteristic of the unoxidized sample surface

• Ions of different kinetic energy were selected by the energy analyzer

• Ions coming out of the energy analyzer were then mass analyzed by the quadrupole mass filter o The pass energy of the energy analyzer is set by the voltage applied to the main cylindrical lens of the energy analyzer

and the kinetic energy of the selected secondary ions is close to the pass energy

SIMS Spectra: 4.8 eV Pass EnergyCopper Beryllium surface analysis

SIMS Spectra: 13.5 eV Pass EnergyCopper Beryllium surface analysis

SIMS Spectra: 20.2 eV Pass EnergyCopper Beryllium surface analysis

SIMS Spectra: 30.0 eV Pass EnergyCopper Beryllium surface analysis

SIMS Data SummaryThe Be+ and Cu+ ion intensities measured at different pass energies allowing for precise measurement of each compound

Pass Energy (eV)

Beryllium Intensity (m/z 9;

kCPS)

Copper Intensity (m/z 63; kCPS)

4.8 360 <2

13.5 120 54

20.2 100 67

30.0 92 36

Solutions to SIMS Analysis

• Characteristics of the secondary ions:o Absolute ion intensities depend on the pass energy

o The Be-to-Cu ion intensity ratio changes with the pass energy

• The Be+ and Cu+ ions from the bombarded CuBe alloy surface have very different

ion energy distributions

• Cu+ ion intensity peaks around 20 eV

• Be+ ions achieves its maximum intensity at a much lower kinetic energy

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