This is a short guideline to bonding of aluminium to copper backing plate. Thermal stress can cause hugh problems with loosing targets. After several tries and failures   a very tough and good solution evolved.  This could withstand stress and is never any problems with. Several other methods  have been  tested. Several glues and no glue could withstand the stress. All had failures, were difficult to use and very expensive. The only disadvantage with the Indium procedure could be if a process is run witch is sensitive to indium contamination, it should be very careful keeping indium away from exposed areas.  Otherwise at 20 deg C indium has a quite low steam pressure so contamination should be little problem when done properly. Also this bonding is done under air pressure.  A more professional way could be to do this under vacuum, but this has never bin any problems with trapped air inside the bonded area. Vacuum bonding complicates it to much and  it is usually not necessary for highvacuum applications. For standard noncritical aluminum mirrors,  the target could be 99.95% aluminium from Curt J Lesker or other target  supplier. However  99.5% "toolshop" aluminium could also be used. This could be purchased from a metal construction company (tibnor). It is standard aluminium metal. For standard mirror coatings it is good enough. The indium could be  wire 2 mm diameter 99.99%. It is easy to use.

purchased from ebay. (Arconium)

Tools that are useful

A steel brush, somewhat smaller, size of a toothbrush is better
A steel scrape
A heat plate
A scalpel

Procedure:

Separation of old piece.

The indium melts at appr 150deg. If an old target is to be separated, there is only needed to place the backing plate to a heat plate, at slightly higher temperature then 150 deg. After everything is heated then just slide them apart. No force should be required. Just slide them apart. It is more difficult to lift them because of the suction. If a new plate, without In on, are put together or some glued or dirty, they must be carefully cleaned first, then a thin layer of indium is applied.

 Applying Indium on copper.

If indium is already on a backing plate from previously runs, then only removal of oxide is needed. The backing plate is melted on at 160deg C.  Until the thin layer of indium melts over the whole Copper surface.  A thin scrape could be used to remove the oxide. There should be a shining indium surface. Clean and blank. No oxide should be visible.

"brushing" indium on the copper surface

If the copper doesn’t have any indium, just pure cupper, then indium must be applied. Due to the thin soft copper oxide layer, Indium must be “scrubbed” into the surface. This is quite easily done. At 160 deg. Just melt indium on the cleaned and fresh cupper surface.  Just an easy mechanical scrubbing with a brush or a scalpel will remove the copper oxide and alloy the In-Cu. It is important that there is a covering In surface to avoid oxidizing.  No soldering oxide removal should be used due to the contamination. Just use pure In and scrub it into the Cu surface.  After the whole surface is covered, use a scraping metal tool or some flat plate to remove residue of oxides. A blank fine surface of pure indium, blank and silvery should be the only visible.

Aluminum surface.

Place the Sputter target on the heat plate at apr 160deg.  The indium should melt easily on the aluminium surface.  Cover the whole surface with indium.  Here it is very important to remove the Al-oxide layer.  A very careful “scrubbing” under the liquid indium surface is done. Use a scalpel or a fine stainless steel brush.(small size). It is very important to all time cover the Al surface. A careful scrubbing over the whole surface will get a good alloy in the surface In-Al.  Some In could be added under the process witch take a couple of minutes. By experience when a person is doing it, it is quite easy to see the good Alloying of In-Al. the behaviour of the surface is changed. Never use any Oxide-removal chemicals due to the contamination.

 When the whole surface is prepared by “scrubbing”, remove the excess of Al-oxide and In-oxide by scraping the surface with a tool. A clear fine silvery surface, blank and fine should appear.

Place both target and Copper backing plate on heat plate. Make sure both surfaces are blank and fine. With a thin liquid In layer on both surfaces. Both surfaces should now have a good In-alloy in the metal junction. Place some extra 1-5cm3 in the middle of the copper backing plate. Carefully place the aluminium target on the Copper plate and wove it back and forth a couple of time. Squeeze out the extra of In it should be pressed out between the plates. The two plates should be easily being moved on the sides. It should have a very low friction.  Make sure In is squished out all around. If you fell it went wrong just remove the two plates and start over. Place some extra In metal on the copper plate and start over. When you feel there is In contact over the whole plate, align the plates exactly on the right position.  Put some extra weights on top, 2-5 kg.  And shut off the heating plate. When the backing plates has cooled, remove excess of In from the target. Use knifes or some other tool to remove In until a clean surface is available.