Foils "How To..."
How do you store foil?
Ultrathin foil MUST always be stored in an inert environment. It can never be left in the shipping boxes. We recommend storing foils in their shipping vials (or shipping plastic boxes/trays with the lid removed) under vacuum or an inert gas (nitrogen). 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 most reactive 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 significantly 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 micro channel plates and special optics. The disadvantage of vacuum storage of filters is they are 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 storage atmosphere must be dynamic (flowing or pumped). If not pumped continuously oxygen, corrosive gas and the water vapor can build up.
New plastics and even fresh paint can give off organic’s that may attack the foil. Extreme care needs to be taken when storing the most reactive of foil (Cu, Ag, Be). Well aged plastic flow boxes are excellent as are the stainless and glass flow boxes. Good metal boxes or vacuum chambers are also excellent.
How 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 follows:
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.
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 1µ.
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.
How long can filters be stored?
All filters less than 0.5 microns thick deteriorate with age and should not be stored more than a year or two. This deterioration takes two forms: oxidation and weakening. Unless filters are stored in exceptionally inert conditions, the metal foil filters will slowly oxidize and their transmission changes; though they may look perfect and be as strong as new. With this oxidation comes the growth of pinholes. Both the pinholes and the transmission reduction render the filter useless.
Parylene N filters (Light Blocks) lose strength after a year or so in storage. If they are exposed to UV light, they will weaken faster. In addition, of course, any metal supported by the Parylene N is undergoing it’s own deterioration. As a result, one should plan to use Parylene N (Light Blocks) supported filters within a year or so of their receipt.
In some labs there are special storage problems. We have seen filters deteriorate to uselessness in just a month or two due to storage problems. This could be caused by acids in the air produced by cleaning agents and/or new lab construction material. New construction materials emit Volatile Organic Compounds (VOC) that can expedite the oxidation process.
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 15 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.
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.
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.
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.
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: Acetone
Grease, Tar, Polymerized oils: Citrus solvent, 111 Trichloroethane
Si Grease, Polymerized Oils: Ether
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.
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 a 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 unbakable. Baking is done entirely at the risk of the customer. Mesh supported foils of brittle materials should never be heated.
- Use a vacuum furnace that permits continuous observation of the foil during baking. Be prepared to turn off the heat if the foil shifts or shows stress.
- Test bake only one foil. Do not bake an entire lot of foil without first proving the process on one foil.
- The furnace pressure must be below 5x10-5 torr during the entire baking process. Higher pressures risk oxidizing the foil.
- The foil mounting adhesive may harmlessly discolor during baking.
- Allow the foil to cool to room temperature before opening the furnace. Vent the furnace very slowly to minimize pressure shock.
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 temperature compatible.
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?
When 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 level.