Fabrication of Planar Laser Targets with Sub-Micrometer Thickness Uniformity Page: 4 of 6
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targets represented a level of precision an order of magnitude beyond that required for most laser targets, so they
could not be manufactured using standard target fabrication practices.
WORKPIECE FIXTURING
Meeting the thickness uniformity specifications required a carefully planned manufacturing process. Many
different fabrication methods were considered before selecting one that used a combination of diamond turning,
deposition, and precision assembly. The targets were built upon an aluminum disk of diameter 100 mm and
thickness 6 mm, which eventually became the 62 m baseplate of the targets. To obtain the required accuracy, all
machining was performed on a diamond turning machine outfitted with a specially designed vacuum chuck to hold
the 100 mm aluminum workpiece, as shown in Figure 3. The vacuum chuck had lands of width 50 gm, because
lands this small were known to be able to hold flat workpieces with a repeatability of 25 nm when used properly.
The workpiece diameter of 100 mm allowed the disk to be handled easily, so it could be inserted and removed from
the vacuum chuck with the required accuracy. The large diameter also had the advantage that any axial runout of
the workpiece due to a seating error on the vacuum chuck would correspond to a small angular error. The large
workpiece also contained sufficient area to measure its seating on the vacuum chuck using a capacitance probe,
which was required to metrologize the targets correctly.
100 mm workpiece And finally, a large number of 4 mm diameter targets
*- could be fabricated from a single 100 mm disk.
/ , < Obtaining the required thickness uniformity and
Coolant meeting the metrology requirements for each of the
nozzle different layers of the target required that the base upon
noz so which the targets were built be very precise. Therefore,
the two faces of the 100 mm aluminum disk had to be
machined parallel to each other. To obtain parallel faces,
the disk was diamond turned, flipped over on the vacuum
K4 !chuck, and then diamond turned again. This process was
- repeated several times until adequate parallelism was
achieved. Each time the disk was removed and placed
back onto the vacuum chuck, the axial runout of the disk
Diamond was measured using a capacitance probe to quantify the
tool seating of the part on the vacuum chuck and the
parallelism of the two faces of the disk. Using this
Figure 3. Diamond turning of a 100 mm diameter method, the two faces of the disk were machined parallel
aluminum workpiece on a custom vacuum chuck to within 0.1 gm.
DEPOSITION OF THE COPPER SAMPLE
Once a precise 100 mm aluminum disk had been obtained, the copper sample was fabricated. As shown in
Figure 1, the copper sample had a thickness of 43 gm and a width of 1 mm. The physics of the laser experiment
required that any adhesive between the copper sample and the aluminum baseplate have a thickness of less than 0.5
gm, which had to be measured to within 0.1 gm. To avoid the difficulties associated with obtaining such a thin
adhesive bond and measuring it with the required accuracy, the copper was deposited directly onto the aluminum.
Several different deposition processes were investigated, including e-beam evaporation, electroplating, and
various sputtering techniques. Each of these techniques had advantages and disadvantages that needed to be
considered. The high temperatures associated with e-beam evaporation made it difficult to maintain the form of the
aluminum substrate. Electroplating cannot be adequately performed onto the non-conductive oxide layer on
aluminum, so the aluminum substrate must first be coated with an intermediate material, such as zinc. However,
these targets could not tolerate the surface roughness created by zincation. Sputtering produces an excellent
interface between copper and aluminum, and excellent adhesion can be obtained by ion milling the aluminum prior
to sputtering. Unfortunately, sputtering processes are normally used to create sub- m coatings, and large residual
stresses are often created if the coating exceeds a thickness of a few gm.
After performing several tests with each of these methods, a technique was developed that used a
combination of sputtering and electroplating. In the sputter-seeded electroplating approach, several gm of copper
were first sputtered onto the aluminum, and additional copper was then electroplated onto the sputtered copper. The
electroplated copper was then diamond turned to the proper size and form. There were many details of this process
that had to be worked out in order to meet the requirements for the targets. The first issue that had to be addressed
was obtaining adequate adhesion between the sputtered copper and the aluminum. Testing revealed the importance
of removing the residual traces of cutting fluid from the surface of the aluminum, so the disk was carefully cleaned2
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Bono, M J; Castro, C & Hibbard, R L. Fabrication of Planar Laser Targets with Sub-Micrometer Thickness Uniformity, article, July 21, 2005; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc886253/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.