Instrumentation

TRACE ELEMENT DETERMINATION:
The Cameca ims 6f has a realistic limit of detection for most element in the range of 1 to 100 ppb. Such determinations have a typical spatial resolution of 10 to 20 micrometers. As an example, a ten element REE pattern for a silicate will require around 1 hour per analysis when concentrations are below 1 ppm. Elements which we CANNOT analyze well include the Nobel gases and nitrogen. Metals and transition metals are analyzed using a O- primary beam and positive secondary ions. Other elements (e.g., halogens, C, O, P, S, Au) are analyzed using a Cs+ and negative secondary ions. All such quantitative analyses require a homogeneous reference standard composed of the same mineral / material. The reference material must be measured during the same analytical session in order to correct for variations in ion yields which depend both on the chemistry and structure of the matrix.

ISOTOPIC MEASUREMENTS:
Such 'small' ion probes are capable of measuring the following isotopic systems: H/D, Li, B, O, S, Cl, and perhaps common Pb (at limited precision). Typical precision limits are on the order of 1 per mil, which is to say that Sr, Nd and similar systems are out of reach, though Hf isotopic determinations in zircons might be feasible. As an example, our routine for determining the B isotopic ratios in tourmaline requires around: 25 minutes per analysis, 10 micron diameter for 1 to 2 per mil precision. In contrast, a glass with 2 ppm of B would require: 2 hours per analysis, 20 micron diameter for 2 to 3 per mil precision.

DEPTH PROFILES: 
By measuring signal intensity vs. time and then converting time into depth (crater depth measurement using a stylus profilometer which we have installed in our laboratory) it is possible to measure diffusion profiles with a depth resolution of 5 nm. Sample surface polish is critical - roughness must be less than the required depth resolution. The primary beam is rastered over the sample (typically around 200 by 200 micron) and secondary ions are collected only from the center of the flat-bottomed crater (typical field area is 30 microns in diameter). A typical sputtering rate would be 1 to 2 microns of depth per hour.

ELEMENTAL MAPPING:
Using our Resistive Anode Encoder (RAE) we are able to qualitatively map the concentration distributions of most elements with a circa 2 micron spatial resolution. The maximum field of view for mapping is a circle of 250 micron diameter. For most elements a minimum average concentration of 10 to 50 ppm is necessary in order to produce a high contrast image in a reasonable period of time. If a suitable reference material is available, then it is possible to translate a qualitative map into a semi-quantitative distribution. By using artificial isotopes to enhance contrast, it is possible to produce isotopic ratio maps using the RAE. Realistically, a contrast of at least 20 percent in isotopic ratio is necessary in order to make an interpretable image.

INSTRUMENT SPECIFICATIONS:

The following list describes the basic capabilities of our Cameca ims 6f:

• high brightness duoplasmatron source for O+, O- and Ar+ ions, nominal accelerating voltages continuously adjustable from 5 to 17.5 kV
• high brightness cesium ion source (beam spot size 100 µm to 150 nm)
• Primary beam mass filter for removal of parasitic primary ion species and neutrals, including 4 aperture mass resolution selection capability
• Multiple immersion lens strip (4 positions)
• Continuously adjustable secondary ion extraction from -10 kV to +10 kV
• Triple focusing mass spectrometer with laminated electromagnet analyzer for fast peak switching, spherical electrostatic analyzer for energy filtering and adjustable energy slit with a 130 eV maximum bandpass
• Double ion detection capability (discrete dynode ETP electron multiplier and Faraday cup), channel plate, fluorescent screen with CCD-camera
• Transtec Ulta10 workstation (450MHz with 256MB RAM) with 21" monitor; depth profiling, mass spectra, mass interferences, isotopic ratio, energy distribution, line scan and stage scan are under full computer control; operating system Solaris 8 with Cameca software release 4.2
• Failsafe vacuum control and display
• High current normal incidence electron gun (up to 10 kV) for the analysis of electrically non-conducting samples
• Optical light microscope with color CCD-camera and zoom optical system, adjustable field of view from 0.56 mm - 1.7 mm
• Fast entry airlock - storage for two sample holders, in vacuum heating facility for sample degassing
• Computer controlled x-y sample stage (± 1µm) with sample position storage and recall, backlash circa 20µm
• Oxygen flooding attachment
• In house chill water supply, pressurized air supply and nitrogen gas supply, in room backup available

COMMERCIALLY PURCHASED UPGRADES:

Resistive Anode Encoder (RAE):
This device purchased from Cameca was installed in November 1999. It proves pulse counting digital imaging capability which greatly extends our ability to map trace element distributions at the micron scale. This technique is limited to an imaging area not larger than ~250 microns in diameter and provides information on the relative variations in a selected element with a <2 micron spatial resolution when the element is present at several 10's of ppm or more.

Residual Gas Analyzer (RGA):
This quadrupole mass analyzer (Balzers Prisma 1-100AMU) was installed on our secondary ion source in December 1999. This device allows us to measure directly the contaminant gases in our secondary ion source chamber down to partial pressures in the 1e-13 Torr range. We are using this capability to design better sample mounting techniques to reduce the H, H2 and H2O contamination within our instrument.

IN HOUSE INSTRUMENT UPGRADES:

We have developed a number of technical improvements which will expand our SIMS analytical capabilities. The following describes the upgrades which have either been installed or are currently under development.

Improved Light Optics:
The goal of this upgrade is to improve the sharpness of our sample image. A high intensity, fiber optic light source has been adapted to our system, replacing the incandescent light bulb thereby giving a brighter and more even sample illumination. We have incorporated a high resolution monitor into our system which provides improved image quality as compared to the PAL image as generated by the workstation. We are in the process of installing a set of three mirrors between the sample stage and the CCD camera, this will eliminate the image inversion between the light optic and ion images.

Coordinate Conversion Between SIMS and EMPA:
Fine details in complex geologic samples are often not visible in highly polished, gold coated samples. We have therefore developed the ability to transform sample coordinates obtained from our electron probes for use with the ion probe. With the electron probe we have the ability to image micron-scale structures both in backscattered electron and X-ray mapping modes. This standardization between the two types instruments decreases the time needed for locating important domains once a sample is place into the ion probe's secondary ion source.

Oil Free Pumping System:
This improvement to our vacuum system was designed in house and it eliminates the oil-based roughing pump from our system -- thereby leading to a much 'cleaner' system. This modification is of particular importance when analyzing samples for low concentrations of C or other elements which would be introduced by hydrocarbons derived from pump oil.  Current detection limit for carbon is in the sub-ppm range.

Enhanced Pumping of the Secondary Ion Source:
We have upgraded the ultra-high vacuum pumping system of our instrument's sample chamber. The main focus of this upgrade is the installation of a 500 l/s ion pump which is compatible with the installation of non-evaporative getter alloy. This improvement has increased the system's pumping speed by a roughly a factor of five for the all important H and H2O suite of vacuum contaminants. Due to the larger physical size of the new ion pump it was necessary to machine in house modifications to other parts of the vacuum system, and in particular to move the L4 Turbo pump towards the PBMF.

Improved Liquid Nitrogen Cold Trap:
The GFZ SIMS laboratory designed a replacement for the original liquid nitrogen cold trap on the Cameca 6f's secondary ion source.  The design was then sent to a local company which produced the new tank.  With the new design we are now able to maintain cryogenic temperatures for 30 hours on a single filling of LN2.

Sample Storage Airlock:
The Cameca ims 6f as delivered provided a sample airlock capable of holding only two samples.  This meant that only a single sample could be stored under vacuum while a measurement was in progress, meaning that a sample was typically under vacuum for only a few hours or less before being put into the secondary ion source.  As such, significant sample degassing was not avoidable.  In order to solve this shortcoming, we have designed and installed a 12 sample carousel which provides several days of advanced storage of samples under vacuum, greatly reducing outgassing during their actual measurement.

Sample LN2 Cooling:
In order to further suppress the H and H2O vacuum background of our instrument, we have installed an in-house designed sample stage which allows our sample to be cooled and maintained at liquid nitrogen temperatures (-196C) during the actual measurement process.  Cryogenic temperatures greatly reduce the outgassing rate of epoxy mounted samples and the large surface of the sample stage acts as a cold trap immediately adjacent to the sample.