Apparatus for monitoring ion beams with an electrically isolated aperture

An apparatus for monitoring ion beams with an electrically isolated aperture includes an ion beam source for generating an ion beam and an electrically conductive aperture plate arranged to collimate the ion beam. The aperture plate is electrically isolated from the rest of the deposition apparatus and is divided into a plurality of electrically isolated segments. A current monitoring device has an input connected to the aperture plate so as to monitor current from the aperture plate which is indicative of the ion beam performance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to ion beam sputtering and, more 
particularly, to a method for monitoring ion beams with an electrically 
isolated aperture. Still more particularly, the present invention is 
directed to a method of fabricating optical mirrors. 
2. Discussion of the Prior Art 
U.S. Pat. No. 4,992,742 to Okuda et al. discloses a device for measuring 
the distribution of charged particles. A plate having through holes is 
provided to restrict the passage of the beam. The current distribution 
induced on the plate is then measured so that a uniform ion beam can be 
provided over a target area. The present invention, on the other hand, 
measures the current on an aperture plate so that the size of the aperture 
can be adjusted to correct for beam divergence. 
U.S. Pat. No. 4,633,172 to Ekdahl, Jr. et al. shows an in-line beam current 
monitor for measuring total electron beam current. Unlike the present 
invention, Ekdahl, Jr. et al. are not concerned with measuring the current 
realized by electrically isolated aperture plates. The current measured by 
the present invention is indicative of grid condition and change in beam 
divergence. The present invention corrects the size of the aperture in 
accordance with the measured current. 
U.S. Pat. No. 4,628,209 to Wittkower shows a device for measuring the 
spatial intensity distribution of a beam. Wittkower rotates a plate having 
targets separated by apertures. As the beam passes through the aperture, 
it is received by a Faraday cup. The current observed in the Faraday cup 
measures the beam intensity passing through each of the apertures. Unlike 
the present invention, Wittkower does not measure the current at the 
aperture plate for the purpose of adjusting the aperture size. 
U.S. Pat. No. 4,135,097 to Forneris et al. and U.S. Pat. No. 4,118,630 to 
McKenna et al. each show an ion beam apparatus which controls the surface 
potential of a target surface. Forneris et al. and McKenna et al. measure 
the ion beam current of the target surface and adjacent electrically 
insulated walls in order to minimize the positive charge buildup on the 
insulated surface. The present invention is not concerned with the current 
at the target, but the current at the aperture plate. 
U.S. Re. Pat. No. 33,193 to Yamaguchi et al. discloses an ion beam 
apparatus having an adjustable aperture. However, Yamaguchi et al. do not 
adjust the aperture in response to a measured current on the aperture. 
The prior art references discussed above show conventional techniques for 
measuring beam current. However, these prior art references, do not 
measure the current from an aperture plate. The present invention, on the 
other hand, measures current from an isolated aperture plate for the 
purpose of adjusting the aperture size to correct for beam divergence. 
None of the references above seek to solve beam divergence. The present 
invention further utilizes the aperture current to signify grid condition. 
Referring now to FIG. 1, an ion beam sputtering apparatus of the prior art 
is shown. In ion beam sputtering, ions are accelerated from a region in 
which they are generated, called the plasma source 10, by means of 
suitably electrified grids 12, 14, and directed at high energies onto a 
target 33. An ion beam 16 is generally indicated by an arrow. On impact, 
material from the target is sputtered off and subsequently received by 
suitably located substrates, resulting in the deposition of target 
material. An aperture plate 20 is generally employed in such a system. The 
aperture plate 20 is typically affixed to a mounting plate 22 by means of 
mounting posts 24 or other suitable mounting apparatus. 
The aperture plate 20 serves at least two purposes. First, it intercepts 
parts of the ion beam 16 which are divergent from the main beam direction. 
These divergent parts may otherwise deleteriously impinge on materials 
other than the desired target materials inside the deposition chamber. 
Sputtering of such non-target materials in the deposition chamber is 
highly undesirable. Second, when depositing highly insulative materials, 
such as silicon dioxide, some amount of the target material may be 
deposited on the front 32, or target side, of the grid 12. Thus, 
transforming the surface of this grid from a conductor to an insulating 
coating. Such a coating may accumulate electrical charges to an extent 
that arcing occurs, with detrimental effects to the desired deposition 
process. 
By intercepting a suitable amount of the ion beam, material 30 from the 
aperture plate 20 can be back sputtered onto the grid surface to an extent 
sufficient to maintain adequate conductivity on the grid surface. Such 
back sputtering permits the accumulating charges to bleed off harmlessly, 
thus preventing arcing at the grids 12, 14. 
It is desirable to keep the size of the aperture in aperture plate 20 as 
large as possible while maintaining its functions. If the divergence of 
the ion beam changes, as, for example, from grid misalignments or from 
grid dimensional changes due to erosion during operation, the optimal 
aperture opening changes. Using the aperture of the prior art, it is 
difficult to monitor such changes either dynamically during a deposition 
process, or even statically between such processes. 
SUMMARY OF THE INVENTION 
In contrast to the prior art, the present invention provides an apparatus 
for monitoring ion beams with an electrically isolated aperture plate 
which is divided into a plurality of electrically isolated segments. An 
ion beam source, which provides an ion beam, and the electrically 
conductive aperture plate are arranged so that the aperture plate provides 
an aperture suitable for collimating the ion beam. A current monitoring 
device has an input connected to each segment of the aperture plate so as 
to monitor aperture plate current. Such an embodiment may permit 
allocating beam distribution changes among transverse spatial components. 
In operation, the present invention offers a means to monitor the 
divergence of the ion beam, and any changes in such divergence, by 
monitoring the net current drawn by the aperture plate 20. The aperture 
plate 20, is electrically isolated from all other components of the 
deposition system including the chamber, target and ion source. However, 
the aperture plate 20 is maintained at a virtual electrical ground by 
means of an operational amplifier, or equivalent. Thus the plate functions 
in a manner identical to that of a conventional aperture. The net current 
to the aperture may now be measured during the sputtering operation. 
The measured current comprises several current components such as, for 
example, incoming ions, electron emissions, sometimes called 
"secondaries", from energetic ion or atom impingement, and electrons 
available to the aperture from an electron emitter or electron emitters, 
sometimes called "neutralizers", used inside the chamber to prevent charge 
build-ups on impacted surfaces and to reduce beam spreading from 
electrical repulsive forces. The present invention permits the potential 
of the aperture plate 20 to be adjusted slightly away from ground 
potential in order to optimize the sensitivity of the device to beam 
changes. Thus the aperture plate 20 may be maintained at a slightly 
negative potential to reduce recapture of secondary electrons or 
collection of neutralizer electrons. Because of the high energy of the 
positive ions in the ion beam itself, attractive effects for these 
positive ions due to the slightly negative aperture are minimal. 
Monitoring the net current present in the aperture plate provides a measure 
of changes in beam divergence, alerting an operator to the need for 
corrective action such as, for example, grid maintenance or adjustment. 
This monitor can also serve to facilitate adjustment for optimal 
aperturing under differing parameters of ion beam operation. 
Other objects features and advantages of the invention will become apparent 
to those skilled in the art through the description of the preferred 
embodiment, claims and drawings herein, wherein like figures in the 
several views indicate like elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Now referring to FIG. 2, apparatus for monitoring ion beams with an 
electrically isolated aperture is shown. The apparatus 100 comprises a 
plasma or ion source 10, electrified grids 12, 14, a mounting plate 22, 
mounting elements 50, segmented aperture plate 20A, and an operational 
amplifier 60 having associated feedback element 62 and adjustment element 
64. The segmented aperture plate 20A is advantageously mounted so as to be 
electrically isolated. In one example, the segmented aperture plate 20A 
may be mounted to the mounting plate 22 by means of mounting elements 50 
where the mounting elements 50 comprise a material which is not 
electrically conductive. Each mounting element 50 may advantageously be 
made of any well known insulator capable of supporting the mounting plate. 
In one example, the insulators may be comprised of ceramic posts or 
similar materials. Of course, other mounting apparatus may be used. 
A conductor 70 may be electrically connected to the segmented aperture 
plate 20A. Wire 70 is routed out of the deposition chamber to the 
monitoring apparatus. In one example, the wire 70 may be routed through 
the mounting plate 22 through insulator feedthrough 54. The insulator 
feedthrough 54 may be any suitable insulator capable of isolating the wire 
70 from the mounting plate 22. The wire 70 is fed into a monitoring device 
as, for example, a conventional ammeter, or the operational amplifier 60 
as shown. In the illustrated example, the operational amplifier 60 may be 
arranged in a conventional feedback circuit having feedback element 62 and 
reference element 64, which is adjustable. 
By monitoring a voltage V at the output, which is proportional to the 
current drawn by a monitored portion of the segmented aperture plate, an 
operator may observe any changes in the current drawn by the segmented 
aperture plate 20A. This aperture plate current is indicative of ion beam 
collimation. In response to any such changes, the operator may make such 
adjustments as are required to improve beam performance. For example, the 
operator may increase the aperture opening or decrease the aperture 
opening, as the case may be, depending upon the application and the 
desired results. In another case, the grids 12, 14 may be replaced or 
cleaned. In operation, the voltage V may be connected to a visual display 
for ease in monitoring the current to the segmented aperture plate. Of 
course, other suitable current measurement devices such as conventional 
ammeters may be employed to monitor the aperture plate current. 
Referring now to FIG. 3, an embodiment of the present invention is shown 
wherein the aperture plate comprises a plurality of electrically isolated 
sections with each section monitored as discussed herein above. Further, 
each section may be connected to a readout and power supply device for 
monitoring and control. Shown in FIG. 3 is such a segmented aperture plate 
20A including segments 82A, 82B, 82C and 82D. Each segment is connected by 
means of a conductor 84, 86, 88 or 90 respectively, to monitor device 110. 
Monitor device 110 may preferably include a display 112. While the example 
herein shows a segmented aperture including four segments, it will be 
clear to those skilled in the art that the number of conductive shield 
segments 82A-82D, may be based on the resolution required for monitoring 
the aperture plate. Thus, more or fewer segments may be used depending 
upon the resolution desired. Operation of this apparatus provides an 
effective real time check of ion source performance. 
Additionally shown are processor 140 and a control means 120. The processor 
140 may advantageously be coupled to an output of the monitor 110 by line 
142 to receive monitor information. The processor 140 then processes the 
monitor information from line 142 and provides an output on line 144 to 
control apparatus 120. Control apparatus 120 uses the processed 
information from line 144 to provide control signals for each of the 
aperture segments on control lines 122, 124, 126 and 128, respectively. 
For example, in response to monitor readings which indicate that the 
aperture plate is experiencing increased current due to increased 
divergence of the ion beam, the processor may receive such information on 
line 142 in the form of digital or analog information which is then 
converted in the processor to digital information in a well known manner. 
The processor may then pass on that information to the controller which 
outputs a signal responsive to the digital information from line 144 to 
automatically reposition the voltage on the aperture segments so as to 
collimate the ion beam as necessary. Automatic mechanical adjustments may 
be made by means of a conventional stepper motor or other mechanism shown 
here as motor 200. The aperture control 120 may be designed in a 
conventional manner to operate motor 200 in response to the aperture plate 
current value, thereby eliminating the need for operator intervention. 
The invention has been described herein in considerable detail in order to 
comply with the Patent Statutes and to provide those skilled in the art 
with the information needed to apply the novel principles and to construct 
and use such specialized components as are required. However, it is to be 
understood that the invention may be carried out by specifically different 
equipment and devices, and that various modifications, both as to the 
equipment details and operating procedures, may be accomplished without 
departing from the scope of the invention itself.