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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.<\/p>

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.<\/p>

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.<\/p>

Normal static desiccators are not inert and should not be used for storage of foil<\/strong>.<\/strong><\/span> The storage atmosphere must be dynamic (flowing or pumped). If not pumped continuously oxygen, corrosive gas and the water vapor can build up.<\/p>

New plastics and even fresh paint can give off organic\u2019s 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.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t

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How fragile are foils?<\/b><\/p>
It is not possible to tell how fragile a foil will be in your environment or application.\u00a0 This depends not just on the material, but its thickness and the size.\u00a0 Most foils are stronger as they are made thicker, others become brittle.\u00a0 In addition, some materials are very temperature sensitive.\u00a0 In general foils that are not very flat, having a nice uniform wave to their surface are significantly stronger.\u00a0<\/p>
The relative fragility of foil materials is as follows:<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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Parylene, Pd, Co, Ti, Ni, Al, Au, Ag, Cu, Pt, C, Zr, Nb, Mo, Cr, In, Sn, Be, V, Si, Ge, B, Oxides<\/p>
Even the strongest of these materials is VERY fragile when very thin.\u00a0 None are strong when referred to normal every day things.\u00a0 Always exercise extreme care handling foil.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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How many pinholes will my foil have?<\/b><\/p>
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 1\u00b5 thick have few or no pinholes under even the most rigorous inspection. Foil less than 1\u00b5 thick will be more likely to have several pinholes.\u00a0 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 0.2\u00b5 thick may have more pinholes and will leak light through the bulk metal (the metal is slightly transparent).<\/p>
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 10^{5<\/sup> measured by visible light transmission\u00a0 for foil over 0.2\u00b5 thick. The maximum single pinhole allowed is 1\u00b5 (0.001mm) diameter. Rather than one large pinhole, most foil will have several, 3-6, very small pinholes. Pinholes in foil less than 0.2\u00b5 thick cannot be measured by light transmission because of bulk light leakage. The maximum acceptable total pinhole area in foil less than 0.2\u00b5 thick is 1 part in 10^{4<\/sup>. The bulk light leakage may be as much as 1 part in 10^{4<\/sup> for a 0.1\u00b5 thick foil. Foil of semiconducting materials, B, Ge and Si will have very significant visible light transmission at all thicknesses less than 1\u00b5.<\/p>}}}
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<\/sup> 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.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t}
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How long can filters be stored?<\/span><\/strong><\/p>
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.<\/p>
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\u2019s own deterioration. As a result, one should plan to use Parylene N (Light Blocks) supported filters within a year or so of their receipt.<\/p>
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.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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Vacuum pumping foil<\/b><\/p>
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, SiO_{2<\/sub>, Ta, TiO_{2<\/sub>, V and Zr). All foils less than 0.2 microns need exceptional care.<\/p>}}
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 m^{3<\/sup>. 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.\u00a0<\/p>}
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.\u00a0 If there is a bypass area equal to twice the foil area, then pumping at 1\/3 the above speeds should be safe.\u00a0 If the bypass area is as small as the foil, pumping must begin very slowly and may take overnight.\u00a0 Totally open chambers can be pumped more rapidly but the same care must be exercised.\u00a0 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.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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How accurate is the foil thickness?<\/b><\/p>
Foil thickness accuracy is more complex than it seems.\u00a0 First, unless a foil is being used as a spacer, the mechanical “thickness” is not relevant.\u00a0 Most users require a foil of a specific unit mass, micrograms\/cm^{2<\/sup><\/p>}
The Lebow Company standard “thickness” specification is +\/-10% error from exact mechanical thickness from all sources.\u00a0 The variation within any one foil is very small, often less than 1%, rarely as large as 2%.\u00a0 The variation within a single lot is usually within much less than 5%.\u00a0 The largest error is the lot error in absolute thickness.\u00a0 A nominal one micron foil lot may easily have a thickness error of 0.05 micron sometimes more.\u00a0 The principal limit in controlling the thickness of foil is metrology .\u00a0 We calibrate to the mechanical thickness of foil and can measure it within +\/-2% at one micron.\u00a0\u00a0<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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How are foil thicknesses measured?<\/b><\/p>
Lebow Company measures foil thickness during deposition using a quartz crystal microbalance.\u00a0 This is a weight measurement and is converted to thickness at standard density.\u00a0 We use this thickness to label foil.\u00a0<\/p>
Our crystal measurements are calibrated by mechanical measurement of the foil thickness.\u00a0\u00a0 We do this using a surface profiling instrument.\u00a0\u00a0 The mechanical thickness of rolled foil and C foil cannot be measured with a profilometer because the surface is too rough.\u00a0 They are measured using a high precision micrometer or by weight.\u00a0 All foil can be made and labeled directly in mass density units.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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How thick must a foil be before its opaque?<\/b><\/p>
Most metals are opaque at 0.25-0.3\u00b5 thick.\u00a0 Metals 0.2\u00b5\u00a0 thick have an attenuation of about 8-10 orders of magnitude.\u00a0 These foils will appear opaque, but can be easily seen to transmit light if inspected with a very bright light in a dark room.\u00a0 Au and Cu are less opaque than most metals.\u00a0 We call this light transmission through the metal “bulk leakage”.\u00a0 This bulk leakage is why our specification for pinholes is set for foils thicker than 0.2\u00b5.\u00a0 The bulk leakage in 0.1\u00b5\u00a0 metal foil can be large enough to exceed our maximum pinhole leakage of 1 in 10^{5<\/sup>.\u00a0\u00a0 The semiconductors,\u00a0 B,\u00a0 Ge, Si,\u00a0 as well as C have very low opacity, 3-6 orders of magnitude attenuation at 0.2\u00b5 thick.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t}
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Can vacuum tight foil be made?<\/b><\/p>
A broad selection of our materials can be made vacuum tight.\u00a0 Of particular importance are Be and Al.\u00a0 Though these foils are vacuum tight most are too thin to support 1 atmosphere of pressure difference.\u00a0 These foils find application as windows separating areas of low pressure, for example 10^{-3<\/sup> and 10^{-9<\/sup> torr.\u00a0 Where an exceptionally thin window is needed between areas of low pressure a layer of 0.1\u00b5 Parylene N can be combined with 0.1 to 0.2\u00b5 of a suitable metal.<\/p>}}
In general, it is very difficult to predict the ability of an ultrathin foil to support pressure.\u00a0 It depends not only on the strength of the foil but also on the radiation passing through the foil and the foil permeability.\u00a0 Some radiation will very quickly destroy organic (Parylene) foils.\u00a0 Some foil, though of sufficient strength, permeates too much gas.<\/p>
Vacuum tight windows supporting 1 atmosphere must be 7.5-12\u00b5 or more thick, depending on the material.\u00a0 The thinnest of these windows require great care to design and have limited lives due to the high stresses.\u00a0 We welcome the opportunity to quote atmospheric vacuum tight windows.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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Why does\u00a0 foil have wrinkles?<\/b><\/p>
Our foil intentionally has shallow wrinkles.\u00a0 These shallow wrinkles sharply increase the effective strength of the foil.\u00a0 This is because tensioning foil requires it to be put under stress.\u00a0 We try to avoid the production of drumhead tight (tensioned) foil.\u00a0 This foil is exceptionally fragile because of the imbedded tension.\u00a0 Even the very slightest bending of the mounting ring or frame may instantly break tensioned foil with imbedded stress.\u00a0 Shrinkage due to oxidation, or shrinkage at the beam impact point\u00a0 may also prematurely break the foil.\u00a0 Our ideal foil will have uniform shallow waves in it, with no tensioned areas.\u00a0 This foil will accept slight deformation of the mounting ring or shrinkage of the foil where a beam passes through without damage.\u00a0 Flat tensioned foil of most materials over 0.5\u00b5 thick can be made if needed.\u00a0 This foil is significantly more costly and requires epoxy mounting on special rings.\u00a0\u00a0<\/b><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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How flat should foils be?<\/b><\/p>
The needed flatness of a foil is determined by its application.\u00a0 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%.\u00a0 The strength gain from wrinkles far exceeds the small error they introduce.\u00a0 If foil is used as a fluorescer or a mirror,\u00a0 where instruments must focus on the foil surface,\u00a0 then the typical slight wrinkles and waves may be unacceptable.\u00a0 They\u00a0 may, however, only\u00a0 require refocusing on each foil individually, a tolerable cost for the strength and life added by slight wrinkles.\u00a0\u00a0 Occasional experiments must focus on the entire foil area, reflect a beam off the foil, or involve ultrafast events over the entire foil area.\u00a0 These experiments will benefit from tensioned flat foil.\u00a0 Tensioned foil\u00a0 of most materials over 0.5\u00b5 thick can be made.\u00a0 This foil is significantly more costly and requires epoxy mounting on special rings.\u00a0\u00a0<\/b><\/p>
Mesh supported foils will usually have\u00a0<\/b>very small, near microscopic waves in the surface.\u00a0 These form around the edges of the mesh squares because the foil is a bit larger than the mesh.\u00a0 The wrinkles have little or no effect in most applications, only introducing a slight geometrically caused apparent thickness variation.\u00a0 A pattern of wrinkles from one edge is\u00a0 very common on very thin, 0.1\u00b5 foil of soft metals (Al, In and Sn.)\u00a0 These are a common artifact of foil mounting.\u00a0\u00a0 Wrinkles are much less common on thicker or hard metals.\u00a0 We do all we can to avoid these mesh and mounting induced\u00a0 wrinkles.\u00a0\u00a0\u00a0<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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How do you clean foils?<\/b><\/p>
Typical x-ray filter foils 0.1-1.0\u00b5 thick are extremely fragile and exceptional care must be used in cleaning.\u00a0 This cleaning should not be attempted by anyone but an expert in foil handling.\u00a0 Bits of dust can be removed with a “one hair brush”, under a microscope.\u00a0 Always keep the brush hair at a grazing angle\u00a0 to the foil and lift the dust with a gentle sweeping motion.\u00a0 Be careful, even a one hair brush will pass through a very thin foil.\u00a0 Foils over 0.5\u00b5 can be dip cleaned in DI water.\u00a0 The foil must be dipped “on edge” into\u00a0 and withdrawn from the water very slowly.\u00a0 This is best done using a movable stage and mechanical foil holder.\u00a0\u00a0<\/p>
Foil over 1-2\u00b5 can be cleaned using a wider range of techniques.\u00a0 Foil strong enough to be handled can be cleaned by very gently wiping with several solvents.\u00a0 Do not ever use an ultrasonic cleaner.\u00a0 Unmounted foil may also be dipped in solvent or cleaned in a vapor degreaser.\u00a0 Suggestions for cleaning are:\u00a0<\/p>
Fingerprints, Light Oil:\u00a0 Acetone
Grease, Tar, Polymerized oils:\u00a0 Citrus solvent, 111 Trichloroethane
Si Grease, Polymerized Oils:\u00a0 Ether<\/p>
Citrus solvent leaves a residue that must be removed with acetone.<\/p>
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.\u00a0 Mounted foil may be very carefully cleaned by dragging a solvent saturated piece of soft lab wipe (Kleenex) over the surface.\u00a0 Extreme care must be taken when cleaning thin mounted foils.\u00a0 Foil under 1.0\u00b5 thick can only be cleaned by an expert.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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What are the size limits on foil Lebow Company makes?<\/b><\/p>
Most foils can be made in sizes large enough to meet all experimental needs.\u00a0 For example, some class 1 foils have been made well over 200mm.\u00a0 Foil is normally made in 25 x 75mm pieces.\u00a0 The price class of a foil is a good indicator of the maximum available size of the finished filters.<\/p>
Class 1 :\u00a0 200mm, Class 2: 100mm,\u00a0 Class 3: 50mm,\u00a0 Class 4:\u00a0 15-25mm,\u00a0 X Class 4: Special limits apply.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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Is mesh or Parylene support required?<\/b><\/p>
Mesh support is optional for all foil and is never required.\u00a0 Mesh support is often added to very thin foil to increase its strength.\u00a0 The available mesh pitches (33\u00b5-2.5\u00b5) and materials (Ni, Cu, stainless steel) are detailed under Specifications.\u00a0 Parylene support 0.1\u00b5 thick is required by our manufacturing process for thin foils of Mg, Pb, LiF, MgF_{2<\/sub>, and Zn.\u00a0 Parylene support is also widely used on other foil to add strength or gas tightness.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t}
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Foil mounting epoxy and adhesive<\/b><\/p>
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^{–<\/sup>^{9<\/sup> torr.\u00a0 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.\u00a0 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^{–<\/sup><\/b>^{10<\/sup> torr or lower.\u00a0 This problem can be minimized by pre-baking the foil in a clean, high vacuum at 10^{o<\/sup>C above its intended operating temperature.\u00a0 If the ultimate cleanliness is required, many foils can be mounted in two piece rings without adhesive or epoxy of any kind.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t}}}}}
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Instructions for Baking Ultrathin Foil<\/b><\/p>
Ultrathin metal foil may be baked but extreme care must be exercised.\u00a0 The foil is very fragile making it sensitive to damage from thermal expansion induced stress.\u00a0 Foil mounted on aluminum rings (except Al foil) can never<\/u> be baked.\u00a0 In addition, the foil has a very large surface area to volume ratio making them very sensitive to oxidation.\u00a0\u00a0 The following is a suggested procedure.\u00a0 Most foil will tolerate hard vacuum baking to 100^{o<\/sup>C.<\/p>}
Lebow Company does not guarantee any foil to be unbakable.\u00a0 Baking is done entirely at the risk of the customer.\u00a0 Mesh supported foils of brittle materials should never be heated.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t
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