Appeal No. 95-2413
Application 07/936,865
uniformly eroded. The annular area is divided into N sectors
651 through 666 having an interior angle of "=2B/N by N/2
lines passing through the center. Concentric circles 671
through 677 are formed between circles 62 and 61 having equal
differences in radius between two adjacent circles and thus
N/2=8 circular tracks are formed. The cross points between
the straight lines and circles are labeled a through p. A
smooth curve 67 is drawn through these points to provide a
closed-loop. It is recognized that figure 8 is merely a
graphic method of construction of a spiral having a polar
equation r = b2 and having a radius extending between the
radius of inner circle 62 and the outer circle 61, which can
be mathematically represented by the equation in Sato '375.
Suzuki states (col. 5, line 59 to col. 6, line 4):
Assuming the curve 67 drawn in FIG. 8 corresponds to
the plasma region 55 in FIG. 6(a) having a very narrow
width, and the curve is rotated with an angular velocity
T around Or, then after a rotation of )T, which is equal
to " in this case, arc a-b moves to a'-b' and arc b-c to
b'-c' respectively. Each swept area by the arcs a-b and
b-c is almost proportional to radius Or-a and Or-b
respectively. On the other side, a velocity which each
arc sweeps the surface of the target is also proportional
to the radius of rotation Or-a and Or-b respectively. As
a result, any small area in the region swept by arc a-b
and b-c is exposed to a plasma for the same period of
time, and the erosion rate is almost the same.
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