SIMS Procedure
The goal of this experiment is to find out whether these
craters originate from natural (micrometeorite impacts)
sources or man-made (e.g. paint chips,etc.)
1D. SIMS analysis of IDE Sensors and other LDEF surfaces
The SIMS analysis may be broken down into four main portions
as follows:
1. Preparation and loading of the sample and
qualitative standard.
2. Depth profiling of contaminant layer.
3. Mass spectral analysis of remaining residue.
4. Secondary ion image acquisition
Sample and Standard Preparation
A qualitative standard has been prepared consisting of
the following materials pressed into a ~5mm square piece of
In foil: C, Al, TiO2, forsterite olivine (~20% MgO, ~75%
SiO2, ~5% FeOx plus <1% Na. K, and Ca) stainless steel (~2%
Mn, ~8% Ni, ~20% Cr, ~70% Fe), brass (Cu, Zn), Ag, and Au.
This standard should be attached to the sample holder, or to
the sample itself, using a piece of carbon tape. Try to
keep the surface flat and as close as possible to the same
level as the sample surface. You may use a piece of clean
metal or glass to flatten the foil after it has been
mounted. Check electrical continuity between the sample
surface and the sample holder. Use the standard to identify
element lines where indicated in the instructions below.
Note the Hall probe setting when viewing the standard and
again when viewing the sample. The setting has been
observed to drift within the period of one analysis (<1
hour), therefore it is desirable to identify the element
lines on the standard just prior to identifying the lines in
the sample.
In some cases, noted below, unresolvable interferences
will require the use of several isotopes and/or comparing 2-
dimensional distributions (which must be non-coincident) for
positive element identification. There will be some
situations where positive identification is not possible,
and these must be noted. each entry in the log book should
have the image number, the mass, and detailed notes about
the methods used to identify this mass as a particular
species.
Before loading samples in the SIMS sample chamber, blow
them off thoroughly with nitrogen or argon using the filter
gun and look at them carefully in the optical microscope.
Locate all of the features to be analyzed and then blow off
the sample again just prior to loading it in the chamber.
For IDE sensors, after the sample is loaded, check the
electrical resistance between the sample surface and the
Cameca sample holder to make sure it is <100 W. Reconnect
the sample bias line (steel spring) from the sample stage to
the high voltage feedthrough. After closing the sample
chamber bulkhead, and before pumping down, make sure that
the resistance between the center conductor of the sample
bias feedthrough and the instrument housing is infinite.
Locate the impact features to be analyzed through the Cameca
optical microscope and record their micrometer values before
evacuating the instrument . Also locate the In foil
standard and assure that it is in the instrument's field of
view. This is required in order to assure that a reasonable
number of impact features (at least 2, preferably more) are
within the field of view of the system. Fiducial marks will
be made on the IDE sensors by C. Simon to assist in feature
location.
Data Acquisition
Prior to any data acquisition, the conditions required
for the analysis must be chosen based on crater size and the
need for 3000 m/Dm mass resolution. Once these conditions
have been chosen, they must not be altered during the
analysis, i.e. the contrast diaphragm (CD) , the field
aperture (FA), imaged field and raster size must remain
constant throughout the analysis. CD 3 will always be used.
Settings may not be changed during analysis and other
conditions may NOT be used except after consultation with C.
Simon or D. Griffis.
1. For this analysis, It is required that the instrument
be operated at a mass resolution of 3000 m/Dm (except for
very large craters, discussed below). This resolution
requires the use of field aperture FA 2 for both the 150µm
and 400µm image fields giving a 60µm and 170µm diameter
imaged areas. For either of these imaging conditions, a
250µm raster will be used. If FA 1, 400µm imaged field is
required based on crater size, a resolution of 3000 m/Dm is
not practically acheivable. In this case, the entrance slit
setting providing 3000 m/Dm resolution determined using FA2,
400µm imaged field will be used. When using FA 1,
resolution is dramatically affected by the raster size
requiring the use of as small a raster as possible. Raster
size to be used must be determined using the formula:
Raster sizeFA1/400Field = crater size + 50µm + 1 beam
diameter.
As discussed above, three possible spectrometer
settings are chosen based on impact feature size:
Feature size £35 µm
FA2, image field=150µm dia., 250µm raster, imaged
area=60µ m dia.
35 µm 140 µm (rare in our study)
FA1, image field=400µm dia., choose a raster size based
on the formula given above. Use of this formula will result
in the smallest useful raster size (in 50µm increments
starting at 300µm raster) and the maximum mass resolution.
2. Before beginning any data acquisition, use the MAIN
program to locally calibrate to 27Al on IDE samples or a low
Z, high intensity matrix element for other samples. Note
the mass used for local calibration in the log. For local
calibration to remain valid, the ESA voltages must be
adjusted to their correct values (550.8, 509.1). Adjust
these values if necessary before proceeding with the local
calibration.
The CD, FA, image field and raster size determined
based on the criteria above must be used throughout the
analysis.
Depth Profiling
There is a ubiquitous layer of silicaceous/carbonaceous
contamination on all LDEF surfaces. The thickness of this
layer varies dramatically, even on a mm scale in specific
instances, such as proximity to the dark rings around the
electrodes on many IDE sensor surfaces. Auger analysis has
shown that the thickness of the dark layer is about twice
that of the lighter area just outside the ring. Frequently
the differences in contamination layer thicknesses are
visible, due to their C content. However, on surfaces that
have been exposed to high levels of atomic oxygen, the C has
been removed from the contamination layer and the
differences in thicknesses are not visible.
The purpose of the depth profile (DP) is to determine
the maximum thickness of this layer over an impact feature,
and then to sputter through this layer before proceeding
with the analysis. We are assuming that the contamination
layer deposited on top of an impact site cannot be thicker
than the contamination layer immediately adjacent (within ~1-
2 mm) to the site. Operator discretion must be used in
selecting the mass signals that will be used to determine
when the contamination layer has been sputtered through.
Note that the irregular surface topography will not permit
complete removal of the contamination layer using these
techniques. Our objective is to remove as little impactor
residue as possible while removing as much of the
contamination layer as possible. Note that DP's are no
longer run on laboratory produced "blank" and "standard"
impacts since we have shown that the contamination layer is
blown away by the event.
1. Depth profiles are to be done first on an area adjacent
to but not encroaching on the crater of interest. Elements
listed below are to be profiled until C, Mg and Na reach a
constant level. After the time necessary to reach these
constant levels under the profile conditions described below
has been determined, a DP run under identical conditions
including identical time must be acquired on the crater
site. Information specific to sample types is provided
below.
>>>>>> Q.: Can we assume that contaminant layer uniformly
covers sample surface? If yes -- we can use a single out-of-
crater DP for a bunch of craters. If no -- we must run an
out-of-crater DP for each and every crater.
IDE sensors: (Surface is composed of 0.1 µm of PVD Al on
top of 1 µm of SiO2 on a Si wafer) Depth profile for
12C(O), 16O(-60), 23Na(-20), 24Mg(-60), 30Si(?), and 40Ca(O)
looking for positive secondary ions: first run depth profile
out of crater (but not very far away), then run DP on crater
itself without Si. Note that Ca is a contaminant throughout
the Al layer. We assume that the bulk of the contamination
layer is removed as soon as the signals, especially C and Si
away from the crater, level off. The DP over the impact
site should be run under the same conditions for no longer
than this amount of sputter time, or until the selected mass
signals level off.
Other LDEF surfaces: Use the same procedure, but include Si
in the DP over the crater on surfaces that are not composed
of Si (Ge, Al and Au are the surfaces we have looked at to
date in addition to the IDE sensors). Be aware that most
micrometeorite residues will contain Si, so this signal
cannot be used to determine when the contamination layer has
been sputtered through over an impact site, but we do want
to have a record of the Si signal during the DP.
2. DP's must be acquired using the FA, CD, image field and
raster conditions selected for this analysis. Use a 500nA
primary ion current, a fully open energy slit translated
2.5 turns, a -60 volt sample offset, and entrance and exit
slits fully open.
3. Continue profile until C, Mg and Na intensities level
off. This assures that the contaminant layer present on all
LDEF surfaces is removed so that all data is taken on a
surface with as little remaining contamination as possible.
4. Save both the file for the "off crater" and the "on
crater" DP's and record the file names in the notebook along
with relevant conditions (i.e. beam current and times
required for contamination masses to become constant).
Mass Spectral Analysis (Bar Graph)
After completing the DP's described above, acquire bar
graph spectra using the following instructions. Before
acquisition of bargraphs, ESA and sample bias voltages must
be checked and adjusted if necessary.
REM> Practice shows that 5 (five) nA is enough to get a
good signal.
Choose Ip which gives 40Ca or 27Al reading of ~ 1..2e5
cts/s.
1. Conditions to be used for the bargraphs are the same
CD, FA, image field and raster size determined above, a -60V
offset and the energy slit fully open and translated 2.5
turns from the zero position.
2. Set the beam current to 5 na.
3. Check local calibration with MAIN program before
proceeding. On IDE samples, using the MAIN program locally
calibrate to 27Al. On other substrates, use Si or another
low mass ion for local calibration. Then, if necessary
reduce the beam current to adjust the intensity of 27Al on
IDE samples or the brightest matrix ion on other samples
(determined using MAIN) to <100,000 cts/sec for FA 2, 150µm
field, 250µm raster or to <600,00 cts/sec for all 400µm
image field/FA combinations.
4. Acquire bargraph from 1-200 AMU. Print out the bargraph
along with conditions, save file and record name of bargraph
file in notebook along with relevant conditions.
5. Translate the energy slit so that maximum matrix
intensity is obtained and set the sample energy offset to
zero. Use the instructions in 3 above to set matrix ion
intensity. Record a second bargraph and then repeat step 4
above.
Mass Resolution Adjustment
Mass resolution of 3000 m/Dm must be verified at mass
56 using the olivine standard and the raster size selected
for the analysis (250µm2 for FA 2, raster size determined by
impact crater size for FA 1 based on formula given above) to
measure the 56Si2/56Fe resolution ( m/Dm=2956). The slit
settings required must always be noted in the log.
Instrument must be warmed up for at least one hour prior to
beginning high resolution tuning. Before determination of
slit settings, ESA and sample bias voltages must be checked
and adjusted if necessary.
1. Locate the olivine standard on the standard sample.
Adjust the beam intensity to obtain an ion intensity of
greater than 50,000 cts/sec at mass 56.
2. Fully close the energy slit and then open to 0.5 on the
micrometer. Translate the energy slit to obtain the maximum
counts per second.
3. Determine the entrance and exit slit settings required
for a mass resolution (10% valley definition) of 3000 m/Dm.
4. Print out a copy of the mass scan obtained to verify
resolution. On both the mass scan and in the log, record
the entrance and exit slit settings required to obtain 3000
m/Dm.
5. Before proceeding and prior to exiting QB, all relevant
instrument parameters must be checked, especially ESA and
sample bias voltages. If ESA voltages require change, local
calibration must be checked. Refocus image (check sample
bias to make sure no residual energy aberrations are present
by making sure image is uniform on channel plate).
Positive Secondary Ion Images
All images must be acquired using magnification setting
6 in order to provide maximum utilization of channel plate
space. Make sure that the video camera is properly
positioned and focused before starting analysis. Before
image acquisition, ESA and sample bias voltages must be
checked and adjusted if necessary.
1. Exit QUICKBASIC
2. Copy C:\CAMECA\COMMON.DAT to C:\RTI\MYDIR. COMMON.DAT:
This file transfers local calibration data, its transfer to
this directory is always required. Local calibration must
be repeated if the polarity is changed or if ESA voltages
require adjustment.
3. Increase primary beam current to 500 nA. Check the
vacuum as indicated by the detector pump. At this point,
check the vacuum as indicated by the detector and
spectrometer pumps. If these vacuums are <1e-7 then turn
off detector pump to reduce noise present on the channel
plate. The detector ion pump must be turned on after each
crater to insure that the channel plate is protected.
If the vacuum does not quickly return to <1e-7, then the
analysis must be terminated.
4. Check to see that the proper field aperature and image
field are selected.
5. Run program SPACEACQ.EXE input filename ?????.img (need
to add image extension).
6. For each mass, adjust the secondary magnet to the
chosen mass. Enter the number of frames to be acquired.
Adjust the mass before you give number of frames since
adjustment is no longer possible once the frames have been
entered.
Acquire images as follows: A comparison between the
energy filtered and unfiltered bar graphs should be used to
provide qualitative information on the degree to which
molecular mass interferences are a problem. Be sure to
record all image numbers next to the mass number and notes
on species identification in the notebook. This is the only
record that correlates the stored data with the analyses.
Use the qualitative standard to identify element lines.
Interferences are listed below along with comments on
requirements for species identification. In general, a mass
resolution (m/_m) requirement <2000 results in easily
separable species. Required mass resolutions between 2000
and 3000 are achievable. Above 3000, special care must be
used to resolve species and image quality is compromised.
Species that require a mass resolution >3500 are generally
not resolvable with our SIMS system and methodology. Many
of the interference listed below are included for
completeness even though they are very easily resolved.
Some species have been left off this list, such as
fractional ion species, B and Li compounds, since they are
highly unlikely to be of concern in the LDEF samples. The
energy-filtered bargraph mass spectra should be inspected
carefully for indications of the expected (approximate)
isotopic distributions for:
Mg 79%[24], 10%[25], 11%[26]
Si 92%[28], 5%[29}, 3%[30]
K 93%[39], 7%[41]
Ca 97%[40], 0.6%[42], 0.1%[43], 2%[44], 0.2%[48]
Ti 8%[46], 7%[47], 74%[48], 5%[49], 5%[50]
Cr 4%[50], 84%[52], 10%[53], 2%[54]
Fe 6%[54], 92%[56}, 2%[57], 0.3%[58]
Ni 68%[58], 26%[60], 1%[61], 4%[62], 1%[64]
Cu 69%[63], 31%[65]
Zn 49%[64], 28%[66], 4%[67], 19%[68], 0.6%[70]
Ag 52%[107],48%[109]
The presence or absence of the (more or less) appropriate
isotope ratios for the elements above should be used
whenever possible to determine the presence or absence of an
element. While molecular interferences may skew the
abundance ratios, the presence of all isotopes in the
bargraph will indicate the possibility that the element
species are present. The absence of all isotopes in the
bargraph cannot be taken as conclusive evidence that the
elemental species are not present deeper in the sample. In
some cases it will be useful to record the images of
interfering species. Operator discretion must be used here.
The IDE samples, and other impactor residues, have a limited
lifetime under the sputter beam.
In each case, an element line must be positively identified
using the qualitative standard, non-coincident distributions
with interfering species, isotopic abundances, or
combinations of these techniques as required. The criteria
utilized to make the identifications discussed above must be
clearly described in the log book next to each image number
n sufficient detail to allow the experiment to be
reproduced. These details are critical to after-acquisition
analysis.
Note that many interfering species, while resolvable
from the element mass of interest, are not resolvable from
each other. Acquire the masses in the order given.
Depending on the thickness of the Al layer you can
acquire up to 1000 frames maximum for low intensity images.
The image will go away due to charging if you have sputtered
through the Al layer, if this occurs then note where this
occurred and take no further images. Make sure you note in
the log how many slit images you see for each mass and which
slit image you recorded. ALWAYS use the standard to
identify element lines and make note of this. This is the
only way we will know if the images are the mass as noted
above or if they are actually a molecular interference.
Consult appropriate Hall probe readings in the Log
Book. Use 19-bit readings provided by the SPACEACQ program
On bright peaks reduce beam intensity by 1 order of
magnitude. Keep the Channel Plate gain the same for all
images in a set.
(We need to set a criteria as to how many frames to
take based on intensity and need to preserve sample)
(The following sequence was suggested by
yourself during the talk between three of us (you, me
and Dieter)
Here's the sequence of acquiring the images:
1. 24Mg
2. 28Si2
3. 56Fe. If there is 56Fe -- proceed to 52Cr, 58Ni
etc. (Cr and Ni are important if there is Fe.)
If NO 56Fe is present -- proceed to 47Ti.
4. All the rest.
5. 27Al
6. 28Si.
(New sequence -- 11/01/93):
12C
23Na
24Mg
27Al
30 Si for IDE
28Si- other sensors (non-Si based)
39K
40Ca
48Ti
52Cr
56Fe - both peaks
58Ni - both peaks - not resolved, cut with slit
61Ni - maybe: need to decide
63Zn
64Cu
107Ag
197Au
24Mg -- again
56Fe/Si2 -- both peaks
7. CLOSE THE FILE If the file is not closed prior to
exiting the program, all data will be lost.
8. Exit program and reload MAIN.
9. Recheck to insure that all relevant parameters and
information are carefully noted in the log.
11. Reduce beam current to 50nA and find next area for
analysis (be sure to note the position of each impact
feature so that SEM or optical micrographs may be recorded
after SIMS analysis. Repeat the analysis beginning with the
DP analysis described above.
EDS analyses of LDEF impact craters in metal.
BEFORE PROCEEDING, Review the LDEF A0187-1 SEM/EDS results
for impact craters by Ron Bernhard and the results from
impacts in tray clamps (aluminum) NASA TM 104759 by Bernhard
and Zolensky. I have copies of these documents and they are
available through the LDEF office, Jim Jones (804)864-3795.
Most impactor residues will be present as a thin melt liner
on the bottom, walls, and/or lips of a crater. Some
residues may exist as small intact grains. Frequently the
impactor has melted and mixed with the target material. It
is impossible to determine the presence, or absence, of
impactor residues by visual inspection alone. Frequently,
very smooth looking craters have only a thin melt liner (not
detectable with EDS) or no residue at all. The best
procedure is to look for residues at 5KV in order to
maximize surface return, however this will not excite the
predominate high energy lines of Fe and other elements. A
second analysis at 15-20 KV is then performed. Typical
meteorite compositions are Si, Mg and Fe. Typical debris
compositions are aluminum oxide, Ti, Zn, stainless steel,
Cu, Ag (rare) and Au( rare). Use the thin window (or
windowless) mode in order to identify C and O. The presence
of O, even on an Al surface, can be used to identify Al2O3
residues.
Use a 10KV beam as a compromise for NASA/Lewis (Mike
Mirtich) samples. Determine the composition of the
substrate (Cr anodized Al in this case). Note the presence
of surface contaminants and and inclusions. Look carefully
at the crater lips, walls and bottom. Rotate the sample as
necessary. If residue is found, record one or more spectra
that represent the range of the residue composition. Record
micrographs to go with each spectrum. Use spot and area
mode at operator discretion..
* * *
Klaus G. Paul, 7-9-1994