8.4.
APERTURE CLEANING AND MANUFACTURE
279
of the needle is centered on the ribbon, the proper distance from one
end, using a binocular dissecting microscope [Fig. 32(a)]. The positioning
need only be approximate, and is easy to accomplish. The hole is bored
by holding the pin vise vertically and turning it with light pressure. A
piece of aluminum makes a good backing material. The hole should be
of such a size that light is barely visible through it. The tip of the copper
strip is then etched in a bath of 40-50% nitric acid. This enlarges and
smooths the hole, removes the burr, and determines the final size. Etching
must not be excessive, however, or the hole will cease to be round. The
original bored hole should be of such a size that no more than 15 or
20 sec are required to complete etching the aperture and bring it to the
desired final size. The etching process is stopped by dipping in demineralized water, blotting, and finally drying. At first one will wish to measure
the holes produced with a conventional microscope and a stage micrometer. It is not difficult with practice to make apertures within 10 or even
5u. of the desired diameter. They can be made so quickly and easily that
if some are the wrong size or asymmetrical they can be rejected at once
and another one made.
When a good vertical, smooth, round hole of proper size is obtained,
this end of the ribbon is held in a pair of jewelers' forceps, and the
aperture area clipped off [Fig. 32(b)]. The small square is mounted in the
aperture cap with the tapered bevel facing away from the electron source
to protect the sidewalls as much as possible from beam contamination.*
In an hour's time one can make quite a number of good apertures in this
way which probably do not spoil even when kept for long periods of
* When apertures are bored with a tapered hole, it is important that they be
oriented with the taper away from the electron source. The electron beam, therefore,
passes through the narrowest diameter with a relatively sharp lip first, and there is no
direct bombardment of the tapering walls by the beam. Thus contamination is
minimized; whereas if the aperture were mounted with the taper up, the whole length
of the bore wall would receive direct bombardment and contamination would be
maximal.
FIG. 32. (a) A very sharp needle mounted in a pin vise is being used to bore an
aperture hole in a strip of 2 mil copper. An aperture cap is in the lower right to
indicate size. Notice that the strip of copper is narrower than the diameter of the
aperture holder so that when the aperture area is finally cut off as a square in (b) it
will fit in the cap without clipping its corners. A piece of aluminum is used as backing
during the drilling operation. Drilling is followed by an acid etch and thorough washing before cutting the square.
APERTURE CLEANING AND MANUFACTURE
279
of the needle is centered on the ribbon, the proper distance from one
end, using a binocular dissecting microscope [Fig. 32(a)]. The positioning
need only be approximate, and is easy to accomplish. The hole is bored
by holding the pin vise vertically and turning it with light pressure. A
piece of aluminum makes a good backing material. The hole should be
of such a size that light is barely visible through it. The tip of the copper
strip is then etched in a bath of 40-50% nitric acid. This enlarges and
smooths the hole, removes the burr, and determines the final size. Etching
must not be excessive, however, or the hole will cease to be round. The
original bored hole should be of such a size that no more than 15 or
20 sec are required to complete etching the aperture and bring it to the
desired final size. The etching process is stopped by dipping in demineralized water, blotting, and finally drying. At first one will wish to measure
the holes produced with a conventional microscope and a stage micrometer. It is not difficult with practice to make apertures within 10 or even
5u. of the desired diameter. They can be made so quickly and easily that
if some are the wrong size or asymmetrical they can be rejected at once
and another one made.
When a good vertical, smooth, round hole of proper size is obtained,
this end of the ribbon is held in a pair of jewelers' forceps, and the
aperture area clipped off [Fig. 32(b)]. The small square is mounted in the
aperture cap with the tapered bevel facing away from the electron source
to protect the sidewalls as much as possible from beam contamination.*
In an hour's time one can make quite a number of good apertures in this
way which probably do not spoil even when kept for long periods of
* When apertures are bored with a tapered hole, it is important that they be
oriented with the taper away from the electron source. The electron beam, therefore,
passes through the narrowest diameter with a relatively sharp lip first, and there is no
direct bombardment of the tapering walls by the beam. Thus contamination is
minimized; whereas if the aperture were mounted with the taper up, the whole length
of the bore wall would receive direct bombardment and contamination would be
maximal.
FIG. 32. (a) A very sharp needle mounted in a pin vise is being used to bore an
aperture hole in a strip of 2 mil copper. An aperture cap is in the lower right to
indicate size. Notice that the strip of copper is narrower than the diameter of the
aperture holder so that when the aperture area is finally cut off as a square in (b) it
will fit in the cap without clipping its corners. A piece of aluminum is used as backing
during the drilling operation. Drilling is followed by an acid etch and thorough washing before cutting the square.