U.S. patent number 3,840,721 [Application Number 05/269,024] was granted by the patent office on 1974-10-08 for automatic control for ion milling machine.
This patent grant is currently assigned to Commonwealth Scientific Corporation. Invention is credited to Gaines W. Monk.
United States Patent |
3,840,721 |
Monk |
October 8, 1974 |
AUTOMATIC CONTROL FOR ION MILLING MACHINE
Abstract
An ion milling machine having a pair of ion guns for bombarding
from opposite sides a sample mounted in a holder in an evacuated
chamber. In one embodiment, an ion detector mounted on an apertured
blade is periodically shifted into position for receiving the ion
beam passing through the sample from one gun while the other gun is
turned off. The signal produced by the detector is compared with a
reference signal indicating the desired thickness and milling
terminated if the detector signal indicates the desired sample
thickness has been reached.
Inventors: |
Monk; Gaines W. (Alexandria,
VA) |
Assignee: |
Commonwealth Scientific
Corporation (Alexandria, VA)
|
Family
ID: |
23025506 |
Appl.
No.: |
05/269,024 |
Filed: |
July 5, 1972 |
Current U.S.
Class: |
219/121.19;
219/121.34; 219/121.23; 250/492.1 |
Current CPC
Class: |
H01J
37/304 (20130101); H01J 37/3005 (20130101) |
Current International
Class: |
H01J
37/30 (20060101); H01J 37/304 (20060101); B23k
015/00 () |
Field of
Search: |
;219/121R,121EB,121EM,121LA,121LM
;250/49.5R,49.5A,49.5TE,49.5P,49.5T,219TH ;29/57Z |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Thin-Film Gauging Device" by Grow III, Surty and Snah; IBM
Technical Disclosure, Vol. 8, No. 11, April 1966, pp. 1584-1585.
.
"Evaporation Thickness and Rate Monitor" by Kruppa and Nuccio, IBM
Technical Disclosure, Vol. 13, No. 5, October 1970, pp.
1056-1057..
|
Primary Examiner: Goldberg; E. A.
Assistant Examiner: Peterson; G. R.
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
What is claimed is:
1. In an ion milling machine of the type having a housing, means in
said housing for holding a sample to be milled, and means for
directing at least one ion beam onto a sample in said holding means
the improvement comprising:
means for receiving radiation from said sample resulting from
milling by said ion beam and producing a signal which varies as a
function of the quantity of radiation received,
means of mounting said receiving and producing means in said
housing so that in a detecting position the radiation is received
from said sample and accordingly said signal varies as a function
of the thickness of said sample,
means for periodically shifting said mounting means from a rest to
said detecting position including timing means for shifting as a
function of time a controlled switch means from a first to second
electrical condition, and means connected to said controlled switch
means for causing said mounting means to move to align said
receiving and producing means with the ion beam received from said
sample when said controlled switch means shifts from said first to
said second electrical condition, and
means connected to said receiving and producing means for
preventing said directing means from directing said ion beam onto
said sample after said signal indicates a desired thickness has
been reached.
2. In a machine as in claim 1, the further improvement wherein said
causing means includes a solenoid.
3. In a machine as in claim 2 having at least two ion directing
means each directing a beam on an opposite side of a sample the
further improvement wherein said shifting means further includes
means connected to said controlled switch for causing one of said
guns to be rendered inoperative after said controlled switch shifts
from said first to second condition.
4. In a machine as in claim 3 the further improvement wherein said
means for causing one of said guns to be rendered inoperative
includes a first relay operative to shift a controlled switch from
a first to second position upon shifting of said controlled switch
of said timing means from said first to second condition and said
means for causing said mounting means to shift includes solenoid
means and a time delay relay connected to said first relay and
having a controlled switch connected to said solenoid for causing
said solenoid to shift said mounting means into said detecting
position a pre-determined time after said first relay shifts its
controlled switch from its first to second position.
5. In a machine as in claim 4, the further improvement wherein said
means for causing said one of said guns to be rendered inoperative
further includes valve means for supplying ion source gas to that
gun and connected to said controlled switch of said first relay for
closing off said supply when the controlled switch of said first
relay shifts from said first to said second position.
6. In a machine as in claim 4, the further improvement wherein said
preventing means includes means for producing a variable reference
signal indicating desired thickness, means for receiving and
comparing said reference signal and said signal which varies as a
function of the quantity of radiation received, and for producing a
further signal when the comparison indicates the desired thickness
has been reached, and means for preventing said two ion directing
means from thereafter operating if said further signal persists for
a pre-determined time interval.
7. In a machine as in claim 6, the further improvement wherein said
reference signal producing means includes a variable resistor
receiving and comparing means includes a D.C. amplifier with an
input connected to said variable resistor and an input to said
radiation receiving and signal producing means for providing an
output signal which varies as a function of the difference between
the inputs, and meter relay means connected to the output of said
D.C. amplifier for receiving said output signal and for shifting a
controlled switch, when said output signal reaches a pre-determined
value, from a first to second position and wherein said ion
directing means preventing means includes a time delay relay
connected to said controlled switch of said meter relay and having
a controlled switch which is shifted from a closed to an open
position if said switch in said meter relay means remains in said
second position for more than a given time interval, means
electrically connecting said controlled switch of said time delay
relay to said ion directing means so that said ion directing means
are rendered inoperative when that controlled switch is opened.
8. In a machine as in claim 7, the further improvement includes
mode switch means having at least an automatic position
electrically connecting a source of voltage to said time delay
relay so that said time delay relay maintains its controlled switch
in its open position after being shifted and at least a further
position for permitting said controlled switch of said time delay
relay to reclose and milling to resume.
9. In a machine as in claim 1 further including mode switch means
having a first position connecting said shifting and preventing
means to a source of voltage so as to be rendered operative, and at
least a second position disconnecting said shifting and preventing
means from said voltage source so as to render both
inoperative.
10. In a machine as in claim 1 wherein said machine includes first
and second ion guns for directing ion beams onto opposite sides of
said sample, the further improvement wherein said mounting means
includes a paddle shaped blade having an aperture, means pivotably
mounting said blade in said housing between one of said guns and
said sample, spring means normally urging said blade to a rest
position in which an ion beam passes through said aperture, a
solenoid, lever means connecting said solenoid to said blade so
that, when activated, said solenoid pivots said blade into a detect
position and means for mounting said receiving and signal producing
means on said blade so that ions passing through said sample
impinge on said receiving and signal producing means when said
blade is in said detect position.
11. In a machine as in claim 10, the further improvement including
a second paddle shaped blade having an aperture, means pivotably
mounting said second blade in said housing between the other of
said guns and said sample, spring means normally urging said second
blade to a rest position in which an ion beam passes through said
aperture, a second solenoid, lever means connecting said second
solenoid to said second blade so that, when activated, said second
solenoid pivots said second blade into a detect position.
12. In a machine as in claim 1, further including means for varying
a D.C. bias on said receiving and producing means so as to suppress
noise and background current.
13. In a machine as in claim 1, further including meterr means for
measuring ion current arriving at said sample.
Description
BRIEF DESCRIPTION OF THE PRIOR ART AND SUMMARY OF THE INVENTION
The invention relates to an ion milling machine and more
particularly to an improvement whereby the thickness of a sample is
periodically or continuously detected by detecting radiation
passing through the sample.
Ion milling machines which employ one or more ion guns to focus an
ion beam on a sample mounted in an evacuated chamber have been in
use for several years. Such devices remove material slowly and
uniformally so that the sample can be cleanly milled to any given
thickness, and particularly to thicknesses which are thin enough to
view with an electron microscope.
One problem in the past has been in determining exactly when the
sample has been milled to a given desired thickness. Normally, this
has been done by periodically viewing the sample and judging from
past experience when milling is completed. Alternately, the sample
can be periodically removed and checked to see whether it is at the
desired thickness. This is a time-consuming activity involving
skilled and expensive laboratory personnel. Further, this approach
necessitates nearly constant attention and precludes, for example,
letting the machine run unattended at night or the like.
Frequently, the technician tending the machine, since he is
involved in other chores, forgets to check the thickness and an
expensive sample is ruined. In addition, it is frequently desirable
to mill a sample to the thickness at which a hole has almost but
not yet appeared. This may be difficult to visually determine.
There have been suggestions in the past with respect to
automatically detecting the amount of light, for example, which is
passing through the sample as an indication of sample thickness. A
patent application, Ser. No. 89,430, entitled ION MILLING MACHINE,
and filed on Nov. 13, 1970, sets forth an approach of this type,
and the disclosure of this co-pending application is explicitly
incorporated herein by reference.
The present invention relates to a particularly advantageous
technique for automatically determining the thickness of a sample,
continuously or preferably from time to time, by detecting the
amount of radiation, such as the milling ions, passing through the
sample and terminating the milling operation when the sample
thickness has reached a desired limit. Thus, the machine can
operate unattended and no periodic supervision by a skilled
laboratory technician is required, while at the same time great
accuracy in choosing a desired thickness can be achieved.
According to one embodiment of the invention, described in detail
below, this is accomplished by a structure which includes a sensor
blade pivotably mounted within the evacuated milling housing with a
detector thereon. The sensor blade can be pivoted by a solenoid or
the like from a rest into a detecting position in which the ion or
other similar detector receives radiation passing through the
sample so that a signal is produced which varies as a function of
the received radiation and hence the thickness of the sample. A
timer is provided for periodically closing a controlled switch to
complete a current path through a first relay which when activated
cuts off the supply of gas, for example, to one of two ion guns. A
short time thereafter, a second, time delay, relay, actuated by
closing of the controlled timer switch, shifts its switch to
operate a solenoid to cause the blade and detector to be shifted
into the second position so as to receive ions passing through the
sample from the remaining operating ion gun.
Thereafter, the signal from the sensor is applied to a D.C.
amplifier and meter relay and compared with a reference voltage or
related to desired thickness. Should the sensor voltage exceed the
reference voltage by a pre-determined amount indicating that tue
desired limit for thickness has been reached, the meter relay
operates its controlled switch to complete a current path to a
conventional time delay relay. After a pre-determined interval, the
time delay relay is then actuated to operate switches to prevent
further operation of the timer and lock up the circuit.
A mode switch for operating these circuits in any of a plurality of
modes is provided and to thereafter operate the device after it has
been locked up, the mode switch is simply shifted from an automatic
to a manual position. The above operation can thereafter be
repeated. In the event that the time delay relay has not operated
within a pre-determined time, for example, 10 seconds, the
controlled switch of the timer is shifted back to its initial
position, causing the turned off ion gun to resume ion production
and direction of the ion beam onto the sample, and the sensor
solenoid to operate to shift the blade and detector back to its
initial position. Requiring that the signal indicating the
thickness persist for a given period, for example, 10 seconds,
increases the reliability of operation and prevents the turn off as
a result of transient or abnormal signals.
According to a further embodiment of the invention, as described
below, the ion guns themselves can be used as the detector with one
of the ion guns being turned off so that the ion beam from the
other gun passes into the gun and is detected. Alternately, both
guns can be continuously operated, and continuous detection
effected by putting an A.C. component on one of the guns for
continuous measuring, and permitting the ion beam to enter one or
both of the guns.
Many other objects and purposes of the invention will be clear from
the following detailed description of the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a perspective view of an ion milling machine with a
pair of ion guns.
FIG. 2 shows a top cut-away view of a milling machine, for example,
as shown in FIG. 1.
FIG. 3 shows a side cut-away view of a milling machine such as
shown in FIGS. 2 and 3.
FIG. 4 shows a front view of the sensor blade and arrangement for
shifting it from a rest into a detecting position.
FIG. 5 illustrates a schematic of a circuit for periodically
checking the thickness of a sample and preventing further milling
after the thickness reaches a pre-determined level.
FIG. 6 shows a schematic of a further embodiment wherein an ion
beam penetrating into an ion gun is employed to detect sample
thickness and terminate milling when the thickness reaches a
pre-determined level.
FIG. 7 shows a further embodiment using an ion gun to detect sample
thickness.
DETAILED DESCRIPTION OF THE DRAWINGS
Reference is now made to FIG. 1 which shows an ion milling machine
of the type discussed briefly above, and of the type shown further
in the above mentioned application, Ser. No. 89,430. Machine 20
includes a pair of conventional ion guns 22 and 24 which are
connected to a suitable gas source, for example, argon, for
providing an ion source. Gas flow to the individual guns is
individually controlled, for example, by means of dual cascade,
micro adjusted valves or other valves. These valves are used for
shutting off gas flow to one of the guns for thickness detection as
described above with regard to the discussion of the schematic of
FIG. 5. Gases other than argon, of course, may be used as desired
for different applications.
Dual ion guns 22 and 24 bombard a sample which has been loaded into
the evacuated chamber within the housing of machine 20 with ions
from opposite sides so that the bombarded portions of the sample
are gradually thinned, for example, at a rate of roughly 1-5
microns per hour. At least one viewing port 25 and a microscope 26
permit the sample to be viewed during milling.
Reference is now made to FIGS. 2 and 3 which show a cut-away view
of an ion milling machine such as shown in perspective in FIG. 1.
As indicated above, a sample is mounted on a sample holder 30
within an evacuated chamber. The sample is illuminated by a light
32 for viewing from the top, for example, through microscope
26.
Sample blades 34 and 36 are disposed between the holder and the
respective ion guns 22 and 24 for each blocking one ion beam in a
first position and permitting that ion beam to impinge on the
sample through an aperture in the blade in a second position.
Referring to FIG. 4, blade 36 is shaped like a paddle with an
aperture 44 through it, so that in the rest position the ion beam
passes through aperture 44 onto the sample in holder 30. An ion
sensing device 46 of a type which is well known in the art is
mounted on blade 36 and produces an electrical signal which varies
as a function of the number of ions impinging upon it. Blade 34 is
preferably identical to blade 36 except for the absence of detector
46 and a similar arrangement is preferably used for shifting it
from a rest to a detecting position.
Both blade 36 and blade 34 are preferably pivotably mounted in the
housing about a pin 48 with a lever 49 connecting blade 36 to a
conventional solenoid assembly 50. A spring 52 holds lever 49 in
the illustrated rest position. When solenoid 50 is actuated, lever
49 is pivoted about pin 48 in a counter-clockwise direction to
bring the detector 46 into alignment with the ion beam to detect
the beam passing through the sample and hence the sample
thickness.
As another feature, a meter 51 can be provided as shown in FIG. 3
to measure continuously the ion current arriving at the sample. A
change in detected ion current indicates a change in thickness as
more current flows through the hole. Thus, the meter provides a
continuous rough monitoring of incident current so that a change in
transmitted current indicates a change in hole size. Meter 51 is
connected between the sample and ground via a negative bias which
minimizes secondary emission effects.
Reference is now made to FIG. 5 which illustrates one schematic for
periodically carrying out a check of the thickness of a sample and
terminating the milling operation when that thickness reaches a
pre-determined level. The circuit illustrated in FIG. 5 includes a
ganged mode switch 99 with individual switch arms 100, 102, 104 and
106. As shown in FIG. 5, these switches can be manually shifted to
any of four positions numbered 1 through 4. Position 1 is the off
position for the unit in which the ion guns are kept turned off and
no detection of thickness takes place. In position 2, switch arm
106 connects lines 108 and 110, which in turn connect to a
conventional A.C. 120 volt outlet, to a time delay relay 112 via
conventional fuse 114, line 116, line 118, and line 120. When
current flows through relay coil 122, relay 112, after a short time
delay, shifts its controlled switch 124 from the illustrated
position into connection with line 126 thus completing a current
path to the conventional high voltage supply which causes the two
ion guns to operate.
Switch 124 in this position also completes a current path through
valves 130 and 132 which supply a gas, such as argon to the ion
guns so that the valves are opened to supply the gas. In this
second position, no detection of the sample thickness takes place
and the device will not automatically terminate upon detection of
some given thickness.
In a third "sample" position, blade solenoid 133 is connected to
lines 108 and 110 via switch arm 100 so that blade 34 which it
controls is shifted from its rest position into a position blocking
the ion beam from the gun 38. Switch 102 similarly completes a
current path through solenoid 50 which shifts the sensor blade 36
from its normal position permitting the ion beam to flow through
aperture 44 into its position blocking the ion beam from gun 24.
Switch arm 106 still connects lines 112 to line 116 so that the ion
guns remain operative producing an ion beam. The beam current can
then be read from a displayed meter reading.
In the fourth "automatic" position, the device operates
automatically to periodically check the thickness of a sample and
to shut down the machine, preventing further milling, when a
pre-determined sample thickness is reached. In this automatic
position, switch arm 104 connects line 116 to line 140 which is in
turn electrically connected to line 142 which is in turn connected
to switch 146 within conventional time delay relay 148. Since
switch 146 is normally in the illustraded position, line 142 is
normally electrically connected to line 150 via switch 146. Line
150 is in turn attached to one side of conventional timer motor 152
within timer 154. The other side of timer motor 152 is connected
via lines 157 and 158 to line 110 so that a complete current path
is completed through timer motor 152 via switch 146 when mode
switch arms 100, 102, 104 and 106 are in the fourth "automatic"
position.
Timer 154 includes a controlled switch 156 which is normally in the
illustrated position, and which is periodically closed, for
example, by a cam associated with motor 152 of timer 154. The
closing of switch 156 electrically connects via switch 146 line
150, which as discussed above is coupled to line 108, to one side
of relay coil 160 of relay 162. The other side of relay 162 is
connected to line 157 which is in turn connected to line 110 as
described above. The shifting of switch 156 from its illustrated
position into connection with line 159 thus completes a current
path through relay coil 160 which shifts its controlled switch 164
from a closed to open position. Opening switch 164 interrupts the
current path to valve 130 via lines 170 and 172. Valve 130 controls
the left ion gun 38 so that the left gun is thus effectively shut
off, although the high voltage source remains operative. Valve 132,
however, remains open in this automatic mode so that argon or other
gas still goes to the right gun which continues to produce an ion
beam which passes through the sample and impinges on detector 46
after it has been shifted as described below in the detecting
position.
The flow of current through coil 160 also causes its other
controlled switch 173 to shift from its illustrated position,
connecting line 159 to line 174 and completing a current path
through coil 178 of time delay relay 180. After a short time delay,
for example, ten seconds as determined by relay 180, that relay
shifts its controlled switches 184 and 186 from their illustrated
positions. The shifting of switch 184 completes a current path
between line 159 and line 190 to in turn complete a current path
through sensor solenoid 50 via switch 104. Sensor solenoid 50 then,
as described above, pivots blade 36 about pin 48 bringing detector
46 into alignment to receive the ions passing through the sample
and to produce a signal which varies as a function of the quantity
of radiation received which in turn varies as a function of the
sample thickness.
Sensor blade 36 is connected to line 201 which in turn is connected
as one input to a conventional D.C. amplifier 202. D.C. amplifier
202 includes an internal schematically illustrated amplifier 204
which receives the signal from sensor 200 as one input and
receives, as its other input, for example, a suitable reference
signal indicative of desired thickness. The output of amplifier 204
on line 206 is applied as one input to a conventional D.C. meter
relay 210 at line 212.
The other input to D.C. meter relay 210 on line 214 is ground,
connected to line 214 by switch 186, shifted from its illustrated
position by coil 178.
Meter relay 210 is the type which compares the two input signals
and closes an internal switch 216 when the one input signal exceeds
the other input signal by a pre-determined amount. Thus, switch 216
is closed whenever the signal produced by sensor 46 reaches a
pre-determined desired level indicating a pre-determined desired
thickness of the sample.
The closing of switch 216 connects line 190 to lines 220 and 222,
thus connecting line 108 to lines 220 and 222 via fuse 114, line
116, switch 104, line 140, line 142, switch 146 which still remains
in its illustrated position, line 150, timer switch 155, line 159,
switch 184 and switch 216. Line 220 is connected to one side of
conventional clutch coil 230 and line 220 is connected to one side
of the motor coil 232 via switch 234. The other side of clutch coil
230 and motor coil 232 are connected to line 110 via line 156 and
line 240.
Thus if switch 216 closes, indicating that the desired thickness
had been reached, a current is completed through coils 230 and 232.
If that current persists for a pre-determined time as determined by
the setting of time delay relay 148, motor coil 232 operates to
shift its controlled switches 146 and 234 away from their
illustrated position. This time delay insures that transient noise,
false signals from arcs, plasma background and the like will not
operate to terminate milling, and that the desired thickness has in
fact been reached.
The shifting of switch 146 completes a current path between line
142 and line 220 so that current remains flowing through clutch
coil 230. The shifting of switch 234 interrupts the current path
through motor coil 232. Clutch 230, however, maintains switches 234
and 146 in their shifted position until the current through coil
230 is interrupted. The switch controlled by clutch coil 230 is not
used.
The shifting of switch 234 interrupts the current path through
motor 152 so that the timer does not continue operating.
Accordingly, the circuit locks in this condition with a current
flow through coil 230 unitl the mode switch arms 100, 102, 104 and
106 are shifted to the off or the position number 2 in which
further milling can take place. Shifting mode switch 99 interrupts
the current flow through coil 230 so that the controlled switches
of relay 148 retrun to their illustrated positions, and the above
steps can be repeated.
If by the end of the time pre-set by timer 154, time delay relay
148 has not shifted its controlled switches indicating that the
desired thickness has not been reached, switch 155 is re-opened by
motor 152 to interrupt the current flow through relay coils 160 and
178 which return their switches to their illustrated positions,
re-completing their current flow through valve 130 so that gas
again resumes flowing to the left gun 38 and interrupting the
current path through solenoid 50 so that the blade resumes its rest
position.
A light bulb 241 labelled "Power" is connected between lines 116
and 110 for indicating that lines 108 and 110 are connected to a
source of voltage. Bulb 242 is connected in parallel with the
sensor solenoid 50 as shown for being illuminated whenever a sample
is being taken. Bulb 244 is connected so as to be illuminated
whenever the mode switches 100, 102, 104 and 106 are in the
"automatic" position 4. Bulb 244 is connected so as to be
illuminated whenever current is flowing through coil 122 to the
high voltage supply.
Bias resistor 260 can be manually varied in order to vary the D.C.
bias of the input collector from sensor 46 and suppresses noise and
background current. Power for D.C. amplifier 202 and meter relay
210 are supplied by a conventional D.C. power source 264 which is
connected between lines 156 and 150 as shown via switch 146 so that
source 264 is turned off when relay 148 locks up the circuit.
To summarize the operation of the schematic shown in FIG. 5 roughly
every fifteen minutes or so while the unit is in the automatic
mode, the argon to the left gun is shut off and a short time later,
for example, 10 seconds, one of the sensor blades is moved about
3/8 inch so that the 1/8 inch diameter collector is directly behind
the sample. If the ions passing through the sample from the right
gun exceed the setting on the relay meter, the timer will start and
if the signal remains higher than the set point for more than a
pre-set time, for example, 10 seconds, then high voltage argon flow
and other operators are turned off. If the transmitted beam does
not exceed the set point for ten seconds, the timers will return to
the system to the normal milling condition after about 30 to 45
seconds as provided by timer 154.
Reference is now made to FIGS. 6 and 7 which illustrate
schematically two other arrangements for detecting an ion beam and
shutting off the system when a pre-determined thickness of sample
has been determined. In the embodiment of FIG. 3, two ion guns 300
and 302 are shown schematically bombarding a sample 304 with ion
beams from opposite sides. Perodically, a timer 306 causes ion gun
302 to be shut off but to remain operative to receive the ions from
gun 300 which are passing through the sample. As in the embodiment
of FIG. 5, if the signal produced by the impingement of ions from
gun 300 into gun 302 indicates that the thickness has reached a
pre-determined level as determined by logic 308, a signal is
produced which shuts off ion gun 300 and terminates the milling
operation.
FIG. 7 illustrates a further embodiment in which continuous
detection of the thickness can be effected by modulating an A.C.
signal onto the transmitting ion gun 310 and detecting the A.C.
component by an A.C. detector 312 so that logic 314 shuts off ion
gun 310 whenever the desired thickness has been reached. Thus, it
is possible to eliminate the blades and use the opposite ion guns
themselves as a detector.
It should be noted that other particles can be used for detecting
purposes. Instead of the ion beam, an electron beam could be
produced and detected. Another possibility is using radio-active
particles on one blade with a radioactive detector on the other
blade. Soft particles such as beta or alpha particles are believed
best for such purposes. Alternately, a light source by
photodetector could be used in this fashion.
Many other changes and modifications in the above described
embodiments of the invention can, of course, be made without
departing from the scope of the invention. Accordingly, that scope
is intended to be limited only by the scope of the appended
claims.
* * * * *