Note: Descriptions are shown in the official language in which they were submitted.
108S06Z
The invention relates to a method of manufacturing electric
resistclrs from thin sheets or films of a metal or metal alloy secured to
an appropriate insulating support.
The inVention also relates to resistors obtained by the afore-
mentioned method.
There are various known methods of manufacturing resistors of
the aforementioned kind. In the known methods, windows or grooves are
engraved through a sheet of metal or metal alloy, to obtain a set of
interconnected electrically resistive filaments having a small across-
section, thus considerably increasing the effective length travelled by
the electric current through the sheet, and obtaining resistors having
very high ohmic values per unit surface.
In a known method, a mask comprising windows or grooves correspond-
ing to the shape of the filaments to be formed in the thin metal or metal-
alloy sheet or film is attached to the sheet, and the sheet or film is
immersed in a suitable chemlcal or electrochemical bath for eliminating
the metal or alloy opposite the windows or grooves in the mask (see French
Patent Specification No. 1,324,156 in the name of The Budd Company issued -
on March 4, 1963~.
In another known method described in French Patent No. 2 344 940
in the name of the present Applicant issued August 16, 1978, the metal
sheet is covered
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with an insulating mask and engraved by electrochemical
machining.
The disadvantage of the method of engraving by chemical
corroaion is that the resulting filaments have rough edges.
Owing to the roughness, it is impossible to engrave windows or
grooves having edges very close together in the metal, without
impairing the stability of the thus-obtained resistor as a
result of the electric field gradients which may exist between
the rough portions of the adjacent edges of the filaments. On
the other hand, the method has the advantage of being relatively
simple. In the method, a metal film or sheet is attached to an
insulating support and the resulting assembly is covered by a
special kind of photosensitive film. After the film has been
eliminated through an appropriate mask and developed, the
assembly is immersed in a chemical bath. After the engraving
operation, the photosensitive film is removed and the engraved
surface is covered with a protective, insulating plastics layer.
The disadvantage of the electrochemical machining method
is that it requires a number of expensive, complicated opera-
tions, i.e. deposition of a layer of copper on the surface tobe engraved, transfer of the assembly to another surface, sub-
sequent transfer to the final insulating substrate, elimination
of the copper layer, etcO
.
~08~062
On the other hand, the method can be used to obtain very regular
engraving. The resulting filaments have very smooth edges, exactly
perpendicular to the engraved surface. The thus-obtained resistors con-
sequently have very high ohmic values per unit surface and extremely low
scatter when mass-produced.
An object of the invention is to obviate the disadvantages of the
aforementioned method by manufacturing resistors having ohmic values per
unit surface which are very high and can be exactly reproduced during large-
scale manufacture, while avoiding the series of complicated operations -
required in the method of engraving by electrochemical machining.
In accordance with the invention there is provided a method of
manufacturing electrical resistors from a thin sheet or film of metal or
metal alloy secured to an insulating support, wherein a mask is attached
to the sheet and has grooves, the edges of which correspond to the shape
of the resistive electric circuit to be engraved on the sheet, wherein the
circuit is engraved by placing the assembly comprising the mask, sheet and
support in a beam of ions having a kinetic en~rgy greater than the bonding
energy of the atoms forming the mask and the sheet of metal or alloy.
In the method according to the invention, a mask is attached to
2Q the metal or alloy sheet, the mask being formed with grooves having edges
corresponding to the shape of the electrically resistive circuit to be
engraved on the sheet.
According to the invention, the method is characterised in that the
circuit is engraved by placing the assembly comprising the mask, sheet and
support in a beam of ions having a kinetic energy greater than the bonding
enelgy of the atoms forming the mask and the sheet of metal or alloy.
When the ions collide with the metal sheet and the
1085062
m~sk, the kinetic energy of the ions is transferred to the
a~oms in the ~heet and mask. If the energy is greater
than the bonding energy of the atoms, the atoms leave
the surface of tbe metal sheet and mask. ~his phenomenon,
which is called cathode sputtering (see e.g. G.~.Wehner,
Advance in electronics and Electron Physics ed s Marton
p. 239 1955) results in erosion of the surface of the
metal sheet and mask, depending inter alia on the nature
of the material forming the sheet or mask~ the nature of
the incident ions and the duration of action of the ion
~eam.
Tests have shown that the method gives extremely
fine engraving~ i.e. resistors having a high ohmic value
per unit surface (1 to 1.5 M ~/cm2 with a very slight
~catter during large-scale manufacture.
In a preferred version of the invention, the mask
material, when subjected to the ion beam, is eroded at a
greater speed than the material forming the metal sheet
to be engraved.
~h~s choice may appear surprising, since all the
known methods of engraving use masks which are not corro-
ded by the engraving agent. However, the aforementioned
mask characteristic is particularly advantageous, as will
be explained hereinafter.
Preferably, the mask is a photosensitive film
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108S06Z
based on orthoquinonediazide and the sheet to be en-
graved is made of nickel-chromium alloy.
~his kind of photosensitive film is conventionally
used in phot~-engraving. ~ickel-chromium alloy~ have
the advantage of having a very low temperature coefficient,
so that resistor~ can be obtained having ele¢tric charac-
teristics which vary only slightl~ with temperature.
Furthermore, the ratio between the speeds of erosion of
the film and of the nickel-chromium alloy is particularly
well suited for the method according to the invention.
Other features and advantages of the invention will
be clear from the following description.
~he accompanying drawings are given by way of non-
limitative example.
In the drawings :-
Figure 1 is a diagrammatic view in cross-section
on a large scale, of a metal sheet attached to a support;
Figure 2 is a view in section showing a mask at-
tached to the metal sheet in Figure 1;
Figure ~ iæ a plan view show~ng the mask attached
to the meta' sheet in Figures 1 and 2;
~igure 4 is a diagram of a device for producing an
- ion beam for working the method according to the invention;
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108S06Z
Figure 5 i8 a cross-sectional view of the ~etal
sheet secured to the support, after working the method
according to the invention;
~igure 6 is a diagram showing the progressive
engraving of the mask and of the metal sheet during the
working of the method according to the invention, and
Figure 7 i8 a plan view of a strain gauge.
~igure 1 shows a metal or metal-alloy sheet 1
- attached to an insulating, e.g. ceramics, support 2 by
j 10 a layer of adheaive 3.
, In the method according to the invention, sheet 1
j ha~ a thickness of a few microns. Alternatively, sheet 1
~ ¢a~ be directl~ attached to support 2 in the form of a
¦ thin film obtained by chemical or electrochemical deposi-
tion or by evaporation in vacuo.
In a first step of the method according to the
invention a ma~k 4 having grooves 5 (see ~igures 2 and 3)
is attached to sheet 1,~the groove edges 6 corresponding
to the shape of the electrically resistive circuit to be
engraved on sheet 1.
According to the invention, the circuit is engraved
,1 .
by placing the assembly comprising mask 4, sheet 1 and
!~ support 2 in a beam of ions having a kinetic ensrgy
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iO8S06Z
~eater than the bonding energy of the atoms forming mask
4 and the metal or metal-alloy sheet 1.
Ry way of example~ the de~ice in Figure 4 can be
used in the method of engraving. The device comprises a
ohamber 7 connected by a duct 8 to a vacuum pump (not
shown) capable of producing a vacuum of the order of
5.10-7 mmHg.
~he device also comprises an ionization chamber 10
for accelerating an electron beam 9. Chamber 10 is of
the kind described by EAUFMAN and READER (ARS Electrostatic
Propulsion Conf. Monterey col. 1960, Report No. 1374).
A gas (argon in the example) is introduced through a pipe
10a and ionized in a uniform magnetic field of a few tens
of Gauss produced by an induction winding 11 between a
cylindrical a~ode 12 and an electron-emitting filament 12a.
In the method according to the invention the beam
of ions 9 preferably comprises positive argon ions having
a kinetic energy between 1 and 2 keV, the ion current
density being between 0.5 and 5 mA/cm2. The vacuum chamber
7 contP;n~ a holder 13 having a surface 13a which is ex-
posed perpendicular to the ion beam 9 and can receive one
or more sheets 1 to be engraved.
- - In the example shown, holder 13 i~ rotatably mounted
around the axis of beam 9, to ensure uniform engraving
of the metal sheet or sheets 1 exposed to the ion beam 9.
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o8so6~ :
In this example, also, the interior of holder 13 is flo~ed
through by a cooling liquid 14.
Interaction between the ions in beam 9 and the surfaces of
mask 4 and sheet 1 exposed thereto results in the ejection of atoms and
the erosion of mask 4 and sheet 1.
In the method according to the invention, the material forming
mask 4, when acted upon by the ion beam 9, is eroded faster than the
material forming the sheet 1 to be engraved.
Preferably, mask 4 is a photosensitive film of the "photoresist"
kind, comprising a mixture of orthoquinonediazide derivatives, e.g.
2,1-naphthoquinone, 5-diazide sulphochloride, and phenolformaldehyde
resin,
Under the action of a beam of A+ ions having an energy of
1 keV and a current density between 0.5 and 0.6 mA/cm2, mask 4 constructed
of the aforementioned material is eroded at the rate of 4.5 A/sec. This
erosion speed is greater than that of the metals or alloys which may be
used for sheet 1. For example, when the sheet is an alloy of 80% nickel
and 20% chromium, the erosion speed is 2.7 A/sec under the aforementioned
conditions.
In the method according to the invention, masX 4 and sheet 1
are acted upon by ion beam 9 at least until
~,oss06Z
tlhe elimination of mask 4 and of the metal opposite
grooves 5 in mask 4. This is possible s~ce the erosion
rate of mask 4 is greater than that of sheet 1. It is
thus unnecessary to eliminate mask 4 in a subsequent step,
~s in the prior-art chemical and electrochemical methods.
Preferably, the action of ion beam 9 is continued
- after mask 4 has been eliminated and until the engraved
sheet 1 has the required ohmic value.
~igure 5 shows the result of engraving, using the
ion beam 9. Mask 4 has been completely eliminated,
leaving metal filaments 1a having round edges, separated
by grooves 15 having a maximum width 11 greater than
the init~al width 10 f the grooves 5 in mask 4.
~igure 6 shows the progressive erosion of mask 4
and metal sheet 1 by ion beam 9. ~he following ~igures
refer to the experiment in ~igure 6.
EgAI~:E 1
~hickness e1 f mask 4 : 1.3 microns
~hickness e2 f sheet 1 (ni-Cr) : 2.5 microns
Ion beam 9 : A~, energy = 1 keV
Current density: 0.6 mA/cm2
i ~ines A, B, C, D correspond to the erosion
fronts obtained after the times indicated in the following
~able;
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lO~S06Z
TAB~E I
.. _ . .
~ines ~ime (sec)
~ .
A . 676
B 1000
C 2955 (~1)
D 5000
. 9200 (t2)
t1 = time for completely eliminating mask 4
t2 = time needed to obtain complete engraving of sheet 1
~ for completely eliminating the metal opposite
the original grooves 5 in mask 4.
At the end of time t2, the metal filaments 1a in
this example have a thickness e3 of approx;m~tely 1
micron.
EXAMPLES 2 end 3
By way of comparison~ we shall give the values of
t1~ t2 and e3 obtained under the same conditions as in
the preceding Example, but using a mask 4 having initial
thicknesses e1 f 2 ~nd 3 microns.
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~08506Z
~AB~E II
_ _ _
~hickness e1 t1 t2Thickness e3
of mask 4 of filaments 1a
2 microns 4545 92001.5 microns
3 microns 6818 92001.9 microns
. . _ . .
The results in Tables I and II show that, at the end
of identical times t2, filaments 1a of increasing thickness
are obtained by uslng masks 4 of increasing thickness e1.
~his feature is noteworthy in that the method accord-
ing to the invention can easily be applied to the manufac-
ture of resistors having different ohmic values.
~hus, ~n an advantageous version of the method
according to the invention,metal or alloy sheets of identical
thickness are used and masks 4 are attached thereto and
have grooves 5 which are identical except that the thick-
nesses e1 vary in dependence on the amou~t of metal to
I be eliminated from the sheet, i.e. the desired thickness
e3 of the filaments 1a.
~ext an entire group of sheets 1 secured to support
2 and covered by masks4 ~ Yariable thick~ess e1 can be
! 20 subjected to the action of an ion beam 9 for a gi~en period.
~he following are other examples of the method accord-
ing to the in~ention.
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~08S06Z
EUAMPLE 4.
Use is made of an 80% ~i, 2~/o Cr alloy sheet 1
having a thickness of 2.5 microns and stuck to a ceramics
plate. A "photoresist" mask 4 is attached to the ~i-Cr
aheet. The mask has a thickness e1 f 1.5 microns and
has grooves 5 having a width 10 f 6 microns, the grooves
- being spaced apart by a distance ~ of 14 microns (see
Figure 2). ~he assembly is placed under a beam of A~
(positive argon) ions having a kinetic energy of 2 keV
and an ion current density between 1 and 1.5 mA/cm2.
~he thus-obtained electric resistor is a square
with sides of 5.4 mm and comprises 204 parallel filaments
1a (see Figure 5) having a thickness e~ of 1 micron and
a wiflth ~1 f 11 micro~s separated by grooves 15 having
a width 11 f 9 microns.
At the end of the operation, the metal filaments
1a are protected and electrically insulated by a coating
(not shown) of plastics, e.g. an epoxy resin.
The ohmic value of the resulting resistor is 130
kilo/ohms~ By way of comparison, if a sheet 1 identical
with the sheet previously described is ~ubjected to cor-
rGsion by electroche~ical machining, the resulting ohmic
value is n~t above 45 kilo-ohms~
~XAMP~ 5
The procedure is the same as in ~xample 4$ starting
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~08S06Z
from a mask 4 havi~g a thickness e1 of ~ microns, the
length 10 and the numb~r of furrows 5 being a~ indicated
in ~xample 4~ ~he resulting resistor has an ohmic value
of 90 kilo-ohms~
~he ohmic values of the elactri.c resistors obtained
as in ~xam~les 4 and 5 can be still further increased by
further exposure to the ion beam 9 for more than the time
t2 required for completel~ eliminating the metal of sheet
1 opposite the initial grooves 5 in mask 4.
EXAMPLE 6
It is desired that the ohmic value of the elec-tric
resistor obtained in Example 1 should be adjusted to 135
kilo-ohms. To this end, the resistor is connected to a
meaauring bridge and the ion beam 9 is stopped when the
measuring bridge indicates that the resistance is 1~5
kilo-ohms. In the Example in question, the additional
period of action of the ion beam is of the ordar of 15
to 18 seconds.
~he previous description has shown that the method
according to the invention has the following advantages:
. It ca~ be used to manufacture electric resistors
having very high ohmic values per unit surface;
It is ver~- rapid on opera.tion, since several hundred
resistors can be simultaneously subjected to the ion beam 9,
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and the complete engraving process does not take more
than a few hours;
It is simple to work, since it com~rises only a
very small number of steps;
It can be used for engraving a much larger number
of metals or alloys than the known chemical or electro-
chemical corrosion methods; consequently the method is
applicable to the construction of a wide range of resis-
tors; and
It can be used ~or mass-producing resistors having
different ohmic values, without modifying other parameters
besides the thickness of mask 4.
I ~he invention also relates, as novel indu~trial
I products, to electric resistoxs obtained by the method
according to the invention. ~he resistors differ from
the prior-art methods in that the metal filaments 1a
(see Figure 5) have a round, convex cross-section, the
convexity facing the exterior of the resistor.
~ ~ests have shown that the ohmic values per unit
i 20 surface of the resulting resistors are considerably
I . greater tharl those of resistors obtained by kno~ ~ethods
} o~ engraving. This result may be explained (a) from the
fact that the grooves 15 obtained after ionic bombardment
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108S06Z
are wider than the original grooves 5 in the mask and
(b) the cross-section of filaments la is round and convex.
The method according to the invention can also be used
for constructing thermometer probes and strain gauges.
The accompanying Figure 7, which is given by way of
non-limitative example, is a plan view of a strain gauge con- --
structed by the method according to the invention.
The strain gauge comprises an electrically resistive
circuit 9 obtained by engraving a metal or alloy sheet attached
to an insulating support 21, i.e. a ceramics or glass plate.
Circuit 20 comprises thin parallel filaments 22, 22a having
ends 23 interconnected to form a winding path which is much
longer than the dimensions of the insulating support. In the
Example shown, filaments 22a disposed along the two opposite
edges of support 21 have widened ends 24 for connecting the
strain gauge to an outer electric circuit.
The strain gauge is manufactured as follows:
The sheet of appropriate metal or alloy is attached to
the i~sulating support 21.
A mask comprising a photosensitive film is attached to
the sheet, the sheet is exposed to radiation and the film is
developed to obtain grooves reproducing the circuit 20 to be
engraved in the metal or alloy sheet; and
~08506Z
The thus-obtained mask and metal or alloy sheet is
exposed to an ion beam until the metal opposite the grooves
in the mask has been eliminated.
The following is a non-limitative example of the method
according to the invention, used for manufacturing a strain
gauge.
EXAMPLE 7
Use is made of a sheet of 80% nickel and 20% chromium
alloy having a thickness of 2.5 microns and stuck to a ceramics
plate 21. A photosensitive mask having grooves reproducing
the shape of the circuit to be engraved is attached to the
sheet. The resulting assembly is bombarded by a beam of A
(positive argon) ions being of the order of lma/cm2.
Under these conditions, the erosion rate is between 4
and 5 A/seconds for the mask and between 2.5 and 3 A/seconds
for the Ni-Cr sheet.
The resulting strain gauge is rectangular - 14 mm long
and 7 mm wide. At the end of the operation, the engraved
sheet is electrically insulated and protected from mechanical
impacts by being coated with a plastics such as an epoxy resin.
Thermometer probes are constructed as specified herein-
before, preferably starting from a metal or alloy sheet having
a temperature coefficient which is a sub-
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~,o~06Z
stantially linear function o~ the temperature between
-200C and +600C. ~h~s condition is satisfied in -the
case of platinum, nickel arld platinum-tungsten alloys.
ln the case of platinum and platinum-tungsten alloys,
the thermometer probe can be used at temperatures up to
2000C.
The follo~Jing is a non-limitative example of the
method of manufacturing a thermometer probe.
LE 8
The method is as i~dicated in Example 7, starting
from a platinum sheet having a thickness of 2.5 or
4 microns. The thermometer probe obtained by this method
can have a resistance of approximately 60 kilo-ohms/cm2.
Experience has shown that the method according to
the invention can be used to obtain thermometer probes
and strain gauges having higher ohmic values per unit
surface than gauges or probes obtained by conventional
me~hods. Ccnsequently, thermometer probes and strain
gauges can be produced having very reduced dimensions,
which is very important in certain electronic applications.
Furthermore, since the method according to the
invention is not dependent on the metal or alloy forming
the sheet to be engraved, the method is applicable to metals
or alloys which are inert towards conventional chemical
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10~506Z
or electrochemical reagents but are particularly suitable
for ma~ufacturing thermometer probes or strain gauges.
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