How do you store foil?

Ultrathin foil MUST be stored in an inert environment.   We recommend storing foils in their shipping vials (or shipping boxes/trays with the lid removed) in vacuum or an inert gas, or dry nitrogen for example.  Under inert storage, foil will keep without changed properties for several years.  Au, C, and  Si are particularly stable.  Ag, B, Be, Fe, In, Mg, Sn, Zn and Zr  are less stable.  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. 

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 would be 50 microns of pressure reduction every 10 minutes to 400 microns, then 100 microns of reduction every 10 minutes to the minimum desired pressure. 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 units.

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?

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 stress.  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 mounting induced  wrinkles.   

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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:       

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 makes?

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 apply.

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 10^oC 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 100^oC.

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 100^oC must be so designated at the time of order.

Before baking above 100^oC please confirm with Lebow Company that all materials used are high temperature compatible.

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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 in 10³.

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.

Can ultrathin foil be used in high flux beams?