How do you store foil?
Ultrathin foil MUST always be stored in an inert
environment. We recommend storing foils in their shipping vials (or
shipping boxes/trays with the lid removed) under vacuum or an inert gas,
or dry nitrogen for example. If foil must be stored more than a month or
two, special care is needed. High quality storage is REQUIRED,
especially for Ag, Al, Be, Cu, In, Sn and Zr. Cu is the
least stable of the common materials and must be stored under inert gas
or vacuum at all times. Cu foil cannot be exposed to air for more than a
few hours without possible damage. Ag and Be will deteriorate in a few
weeks if not stored in an inert atmosphere.
We recommend, and we use, a nitrogen flow box for
foil storage. We use the boil off from liquid nitrogen (a LS180)
so the gas is very dry and oil free. Inert gas storage boxes have
the advantage of being easy to open so foils are readily accessible.
Vacuum storage chambers are widely used for
storing foil. These work very well if the pressure is well below 1
milliTorr and they are constantly pumped. Most large labs have
these for storing microchannel plates and special optics. The
disadvantage of vacuum storage of filters is it is not very
accessible. It is not easy to return the filters to storage so
they may be left out.
Normal static desiccators are not inert and
should not be used for storage of foil. The plastics give off organics that may attack the foil
and the water vapor partial pressure can build up. The storage
atmosphere must be dynamic (flowing or pumped.)
We cannot guarantee that foils will not oxidize or develop pinholes
during storage unless they are in flowing, inert gas or high vacuum.
fragile are foils?
It is not possible to tell how fragile a foil will
be in your environment or application. This depends not just on
the material, but its thickness and the size. Most foils are
stronger as they are made thicker, others become brittle. In
addition, some materials are very temperature sensitive. In
general foils that are not very flat, having a nice uniform wave to
their surface are significantly stronger.
The relative fragility of foil materials is as
Parylene, Pd, Co, Ti, Ni, Al, Au, Ag, Cu, Pt, C,
Zr, Nb, Mo, Cr, In, Sn, Be, V, Si, Ge, B, Oxides
Even the strongest of these materials is VERY
fragile when very thin. None are strong when referred to normal
every day things. Always exercise extreme care handling foil.
Vacuum pumping foil
Ultrathin foil is among the most fragile material
ever sold. Foil WILL be broken by any, even seemingly insignificant, gas
pressure. Foil is also very sensitive to acoustic vibration. Designing
the vacuum pumping of experimental equipment using ultrathin foil
requires care. This is particularly true if the filters are less than
0.5 microns thick, large filters or those made of brittle materials (Be,
B, Cr, Ge, Mo, Nb, Si, SiO2,
Ta, TiO2, V and Zr). All
foils less than 0.2 microns need exceptional care. The equipment and
pumping schedule must be designed so there can never be even the
smallest pressure or gas vibration across a foil. This requires long,
slow pumping and a gas bypass hole near the foil with an area at least
twice that of the foil. This is particularly important where the foil is
a window between two chambers or volumes. Pumping the chambers with one
pump through a manifold, together, without a bypass hole, is never
sufficient. Normal pumping times should be at least 1 hour to 100
microns for chambers of less than 0.1 m3. Larger chambers or
those with barriers should be pumped overnight to 100 microns. A typical
small chamber pumping schedule would15 minutes for the first 10% of gas
removal, 15 more minutes for the next 30% and 15 more minutes for the
next 50% and free pumping thereafter. If the foil is a barrier
between two volumes, even if they do not divide the chamber (ie: the
window on a refrigerator,) pumping must be much slower. If there
is a bypass area equal to twice the foil area then pumping at 1/3 the
above speeds should be safe. If the bypass area is as small as the
foil, pumping must begin very slowly and may take overnight. Totally open chambers can be
pumped more rapidly but the same care must be exercised. Because
the gas force changes occur most rapidly at one atmosphere, extreme care
must be taken to start the pumping very, very gently. A throttling valve
is absolutely required and must be opened only in very tiny amounts near
atmosphere. Care must also be taken when closing chamber doors and when
installing flanges to avoid banging them shut. Slamming the door or
banging flanges can easily break filters.
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How accurate is the foil thickness?
Foil thickness accuracy is more complex than it
seems. First, unless a foil is being used as a spacer, the
mechanical "thickness" is not relevant. Most users
require a foil of a specific unit mass, micrograms/cm2
The Lebow Company standard "thickness" specification
is +/-10% error from exact mechanical thickness from all sources.
The variation within any one foil is very small, often less than 1%,
rarely as large as 2%. The variation within a single lot is
usually within much less than 5%. The largest error is the lot
error in absolute thickness. A nominal one micron foil lot may
easily have a thickness error of 0.05 micron sometimes more. The
principal limit in controlling the thickness of foil is metrology
. We calibrate to the mechanical thickness of foil and can measure
it within +/-2% at one micron.
How are foil thicknesses measured?
Lebow Company measures foil thickness during deposition using a
quartz crystal microbalance. This is a weight measurement and is
converted to thickness at standard density. We use this thickness
to label foil.
Our crystal measurements are calibrated by mechanical measurement of
the foil thickness. We do this using a surface profiling
instrument. The mechanical thickness of rolled foil and C
foil cannot be measured with a profilometer because the surface is too
rough. They are measured using a high precision micrometer or by
weight. All foil can be made and labeled directly in mass density
How thick must a foil be before its opaque?
Most metals are opaque at 0.25-0.3µ
thick. Metals 0.2µ thick have an attenuation
of about 8-10 orders of magnitude. These foils will appear opaque,
but can be easily seen to transmit light if inspected with a very bright
light in a dark room. Au and Cu are less opaque than most
metals. We call this light transmission through the metal
"bulk leakage". This bulk leakage is why our
specification for pinholes is set for foils thicker than 0.2µ.
The bulk leakage in 0.1µ metal foil can be large enough to
exceed our maximum pinhole leakage of 1 in 105.
The semiconductors, B, Ge, Si, as well as C have very
low opacity, 3-6 orders of magnitude attenuation at 0.2µ thick.
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How many pinholes will my foil have?
Lebow Company makes every effort to minimize the pinholes in our foil
but unfortunately, it is axiomatic that all foil has pinholes. Most of
these pinholes are VERY small and infrequent so we can often select
nearly pinhole free foil. Many foils over 1µ
thick have few or no pinholes under even the most rigorous inspection.
Foil less than 1µ
thick will be more likely to have several pinholes. Foil of Co,
Fe, Ni, Pd, and Pt will have fewer pinholes, while B, Be, In and Sn are
difficult materials and will have more pinholes. Foil less than 0.2µ
thick may have more pinholes and will leak light through the bulk metal
(the metal is slightly transparent). Pinholes are caused by dust,
deposition defects and damage during manufacturing. All foil delivered
to customers is very carefully inspected with a high intensity light and
selected to minimize pinholes. The maximum allowable light leakage
through pinholes is 1 part in 105 measured by visible light
transmission for foil over 0.2µ
thick. The maximum single pinhole allowed is 1µ
(0.001mm) diameter. Rather than one large pinhole, most foil will have
several, 3-6, very small pinholes. Pinholes in foil less than 0.2µ
thick cannot be measured by light transmission because of bulk light
leakage. The maximum acceptable total pinhole area in foil less than 0.2µ
thick is 1 part in 104. The bulk light leakage may be as much
as 1 part in 104 for a 0.1µ
thick foil. Foil of semiconducting materials, B, Ge and Si will have
very significant visible light transmission at all thicknesses less than
Foil to any required degree of light tightness can usually be made.
The first step in specifying this foil is to ascertain the maximum
acceptable light leakage (1 in 10-9
for example.) Then bring this requirement to the Lebow Company
engineers. Multilayer foil that is light tight (pinhole free) can be
made of many materials. Virtually all special requirements can be met.
Can vacuum tight foil be made?
A broad selection of our materials can be
made vacuum tight. Of particular importance are Be and Al.
Though these foils are vacuum tight most are too thin to support 1
atmosphere of pressure difference. These foils find application as
windows separating areas of low pressure, for example 10-3
and 10-9 torr. Where
an exceptionally thin window is needed between areas of low pressure a
layer of 0.1µ
Parylene N can be combined with 0.1 to 0.2µ
of a suitable metal.
In general, it is very difficult to predict
the ability of an ultrathin foil to support pressure. It depends
not only on the strength of the foil but also on the radiation passing
through the foil and the foil permeability. Some radiation will
very quickly destroy organic (Parylene) foils. Some foil, though
of sufficient strength, permeates too much gas.
Vacuum tight windows supporting 1 atmosphere
must be 7.5-12µ
or more thick, depending on the material. The
thinnest of these windows require great care to design and have limited
lives due to the high stresses. We welcome the opportunity to
quote atmospheric vacuum tight windows.
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Why does foil have wrinkles?
Our foil intentionally has shallow
wrinkles. These shallow wrinkles sharply increase the effective
strength of the foil. This is because tensioning foil requires it
to be put under stress. We try to avoid the production of drumhead tight (tensioned) foil. This foil is exceptionally fragile because
of the imbedded tension. Even the very slightest bending of the
mounting ring or frame may instantly break tensioned foil with imbedded
Shrinkage due to oxidation, or shrinkage at the beam impact point may
also prematurely break the foil. Our ideal foil will
have uniform shallow waves in it, with no tensioned areas. This
foil will accept slight deformation of the mounting ring or shrinkage of
the foil where a beam passes through without damage. Flat tensioned
foil of most materials over 0.5µ thick can be made if needed. This foil is
significantly more costly and requires epoxy mounting on special rings.
How flat should foils be?
The needed flatness of a foil is determined
by its application. If a foil is used in transmission, as most
are, then the typical slight wrinkles and waves introduce only a
geometric thickness error unlikely to exceed 2%. The strength gain
from wrinkles far exceeds the small error they introduce. If foil
is used as a fluorescer or a mirror, where instruments must focus
on the foil surface, then the typical slight wrinkles and waves
may be unacceptable. They may, however, only require
refocusing on each foil individually, a tolerable cost for the strength
and life added by slight wrinkles. Occasional experiments
must focus on the entire foil area, reflect a beam off the foil, or
involve ultrafast events over the entire foil area. These
experiments will benefit from tensioned flat foil. Tensioned
foil of most materials over 0.5µ thick can be made. This foil is
significantly more costly and requires epoxy mounting on special rings.
Mesh supported foils will usually have very small, near microscopic
waves in the surface. These form around the edges of the mesh squares
because the foil is a bit larger than the mesh. The wrinkles have little or no effect in most applications, only introducing
a slight geometrically caused apparent thickness variation. A
pattern of wrinkles from one edge is very common on very thin, 0.1µ foil of soft metals (Al, In and
Sn.) These are a common artifact of foil mounting. Wrinkles are much less common on
thicker or hard metals. We do all we can to avoid these mesh and
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How do you clean foils?
Typical x-ray filter foils 0.1-1.0µ
thick are extremely fragile and exceptional care must be used in
cleaning. This cleaning should not be attempted by anyone but an
expert in foil handling. Bits of dust can be removed with a
"one hair brush", under a microscope. Always keep the
brush hair at a grazing angle to the foil and lift the dust
with a gentle sweeping motion. Be careful, even a one hair brush will pass through a very
thin foil. Foils over 0.5µ
can be dip cleaned in DI water. The foil must be dipped "on edge"
into and withdrawn from the water very slowly. This is best
done using a movable stage and mechanical foil holder.
Foil over 1-2µ
can be cleaned using a wider range of techniques. Foil
strong enough to be handled can be cleaned by very gently wiping with
several solvents. Do not ever use an ultrasonic cleaner.
Unmounted foil may also be dipped in solvent or cleaned in a vapor
degreaser. Suggestions for cleaning are:
|Fingerprints, light oil
|Grease, tar, polymerized oils
||Citrus solvent, 111 Trichloroethane
|Si grease, polymerized oils
Citrus solvent leaves a residue that must be removed with acetone.
Special care must be taken cleaning mounted foils because they are
often extremely fragile and the mounting epoxy or glue is soluble
in many cleaning solvents. Mounted foil may be very carefully cleaned
by dragging a solvent saturated piece of soft lab wipe (Kleenex) over
the surface. Extreme care must be taken when cleaning thin mounted
foils. Foil under 1.0µ thick can only be cleaned by an expert.
What are the size limits on foil Lebow Company
Most foils can be made in sizes large enough to
meet all experimental needs. For example, some class 1 foils have
been made well over 200mm. Foil is normally made in 25 x 75mm
pieces. The price class of a foil is a good indicator of the
maximum available size of the finished filters.
Class 1 : 200mm, Class 2: 100mm, Class
3: 50mm, Class 4: 15-25mm, X Class 4: Special limits
Is mesh or Parylene support required?
Mesh support is optional for all foil and is never
required. Mesh support is often added to very thin foil to
increase its strength. The available mesh pitches (33µ-2.5µ)
and materials (Ni, Cu, stainless steel) are detailed under
Specifications. Parylene support 0.1µ thick is required by
our manufacturing process for thin foils of Mg, Pb, LiF, MgF2,
and Zn. Parylene support is also widely used on other foil to add strength
or gas tightness.
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Foil mounting epoxy and adhesive
The epoxy and adhesive used for mounting foil are
kept to the absolute minimum, often just a few micrograms, and will not
contaminate vacuums to 10-9 torr.
The epoxy (Emerson & Cuming Eccobond 55 with 12% catalyst 9) is
NASA approved for vacuum and our proprietary glue is specially
formulated for vacuum use. As is the case with any organic, our
epoxy and glue may contaminate ultra clean surfaces placed very close to
the foil or vacuums of10-10
torr or lower. This problem can be minimized by pre-baking the
foil in clean, high vacuum at 10oC
above its intended operating temperature. If the ultimate
cleanliness is required, many foils can be mounted in two piece rings
without adhesive or epoxy of any kind.
Instructions for Baking Ultrathin Foil
Ultrathin metal foil may be baked but extreme care must be
exercised. The foil is very fragile making it sensitive to damage
from thermal expansion induced stress. Foil mounted on aluminum
rings (except Al foil) can never be baked. In addition, the foil has a
very large surface area to volume ratio making them very sensitive to
oxidation. The following is a suggested procedure. Most foil
will tolerate hard vacuum baking to 100oC.
Lebow Company does not guarantee any foil to be unbreakable.
Baking is done entirely at the risk of the customer. Mesh
supported foils of brittle materials should never be heated.
Foils to be baked at over 100oC
must be so designated at the time of order.
Before baking above 100oC
please confirm with Lebow Company that all materials used are high
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How do you determine the VUV or soft
x-ray transmission of a filter?
Transmission of filters is determined by the basic
physics of the material chosen and its thickness. Successful filter
design requires complete knowledge of the desired measurements, the
detector and the fluxes, depending equally on the transmission in the
area of interest and the blocking of interfering radiation. The steps in
designing a filter are:
Select a candidate filter material from the
Lebow Company filter transmission curves or the CXRO foil transmission
engine at: http://henke.lbl.gov/optical_constants/filter2.html
Determine a first estimate of the filter
thickness. The filter thickness determines the transmission of the
wavelengths or energy of interest. The needed minimum transmission is
calculated from the available flux and the detector sensitivity. Compare
the needed transmission with predicted transmission using the CXRO
filter transmission engine and confirm it is high enough. Be sure to
include the oxide on all foil surfaces in your simulations. A typical 2%
oxidized Zr foil would be analyzed as a Zr100O2 foil using the density
of metallic Zr, 6.39, as a good approximation of the actual density.
Next determine the needed blocking of undesired
photons. The energies or wavelengths where blocking is required are
determined first by the range of sensitivity the detector and second by
the anticipated flux. The range of interest is the wavelength or energy
range where the detector is sensitive and there is flux. Photons outside
this range are of no interest. For example some detectors are visible
light blind so visible light is of no importance. Next the intensity of
the radiation to be rejected and the detector signal from this radiation
are calculated. These usually vary with wavelength or energy. The needed
attenuation is evaluated to determine the foil thickness required using
the CXRO curves. This is then compared with first estimate of the
thickness of the candidate material. If this material is thick enough
all is well. If it is not, then compromises must be made or a new
candidate material selected.
Once the filter material is selected evaluate the
tolerance for bulk leakage and pinholes. All materials under 200nm leak
visible light and some are quite transparent. This is done by comparing
the bulk leakage and pinhole signal (for pinholes, typically much less
than 1 in 105 of the incident flux) with the detector noise.
In most experiments the pinhole leakage noise is on the order of, or
less than, the detector noise, so pinholes can be ignored and standard
filters can be used. Bulk leakage can, however, be a problem as it can
reduce the attenuation of some 100nm metal foils to little more than 1
With luck a material will be found that will have
sufficient transmission to detect the signal and sufficient blocking to
attenuate the background and out of band photons. If not, then multi
material filters must be evaluated.
ultrathin foil be used in high flux beams?
a high flux beam passes through a foil the absorption of even a small
portion of the beam energy may significantly heat the foil. Foils
occasionally run red hot. Dissipation of the heat deposited in
foil is by radiation only. This heating is well tolerated by ductile,
tough materials like Al, Au, C, Co, Fe, Ni and Ti. Most brittle
materials (Price Class 4), B, Be, Ge, Mo, Si, and Ta will fail almost
instantly if significantly heated. Mesh backing flattens foil,
sharply reducing its tolerance for thermal stress. Where filters
must be used in a high flux beam, a micro-pinhole array (or mesh array)
or thick foil filter must be used to reduce the flux to a tolerable