[0001] This invention relates to circuit protection devices, and especially to devices
for protecting electrical circuits against voltage transients that are caused by
an electromagnetic pulse, e.g. lightning, and also the transients that are caused
by electrostatic discharge.
[0002] One class of material that has been proposed for use in the manufacture of circuit
protection devices in general are the chalcogenide glasses, by which is meant glasses
formed from elements of group VIB of the periodic table (IUPAC l965 revision) together
with other elements, especially those of groups IVB and VB, for example as described
in U.S. Patent No. 3,27l,59l to Ovshinsky. Certain of these glasses can be used to
form "threshold" devices by which is meant devices that will change from a high resistance
state to a low resistance state on application of a high voltage (the lowest such
voltage being referred to as the "threshold voltage") but which will remain in their
low resistance state only for as long as a small "holding" current is maintained.
Other chalcogenide glasses can be used to form "memory" devices which will change
from a high resistance state to a low resistance state on application of a high voltage
and which will remain in the low resistance state, even when no voltage is applied,
until an appropriate, different, voltage pulse is applied. As will be appreciated,
only threshold devices are appropriate for the production of circuit protection
devices.
[0003] The chalcogenide glass materials have the advantage that they exhibit very short
switching times between their high and low resistance states where the voltage transient
that causes switching is significantly higher (e.g. about 50 V or more) than the
threshold voltage, typically less than l nanosecond, which is sufficiently fast for
protecting circuits from the transient.
[0004] In our copending British Application No. 8508304, the entire disclosure of which
is incorporated herein by reference, circuit protection devices that are formed from
a number of specific germanium/selenium/ arsenic amorphous compositions are described.
The devices described therein display surprisingly high "energies to latch", that
is to say, the devices can withstand surprisingly high electrical energies due to
the voltage transients before they latch permanently in their low resistance state.
[0005] Without being bound by any particular theory, it is believed that one factor in determining
the quality of the switch is the contact resistance between the chalcogenide glass
material and the electrodes, and that a reduction in the contact resistance can increase
the energy to latch of the switch even to the extent that the improvement in the energy
to latch caused by a substantial reduction in electrode contact resistance may even
effectively cause a composition to change from one that exhibits memory characteristics
to one that exhibits threshold characteristics. It is accordingly conjectured that
the memory characteristics previously observed by Pinto and Ovshinsky
et al could, in fact, be caused by a high electrode contact resistance.
[0006] According to the present invention, there is provided a threshold switching device
which comprises a switching element formed from an amorphous chalcogenide composition
and a pair of electrodes that are in contact with the composition, the device containing
an indium or indium containing layer in contact with the composition.
[0007] By the use of an indium containing layer it is possible in many instances to reduce
the electrical contact resistance and thereby to increase the energy to latch of the
device to allow optimisation of other properties of the compositions. Preferably the
device exhibits an energy to latch of at least 40mJ, more preferably at least 60mJ
and especially at least l00mJ for reasons given in our copending European application
No. 0l9689l.
[0008] Preferably the electrode contact resistance is sufficiently low that the overall
electrode-to-electrode resistance of the device (in its low resistance state) is
not more than l0 ohms and especially not more than l ohm, the most preferred resistance
being less than 0.l ohm.
[0009] Another advantage of the use of indium or indium containing interlayers is that the
adhesion of the electrode to the switching element can be considerably improved, to
the extent that it is possible reliably to form electrical connections to the electrodes
by wire bonding techniques. Previously proposed devices have failed on a number of
occasions by separation of the electrode from the chalcogenide glass, even after very
small electrical transients were applied. The separation, which was often associated
with sparking from the electrode, and ultimately complete destruction of the device,
can be eliminated or considerably reduced by the use of an interlayer according to
the invention.
[0010] The present invention is applicable to chalcogenide glass compositions in general,
and especially oxygen-free glasses, especially those containing sulphur and/or selenium
optionally and preferably together with group IVB and group VB elements such as germanium,
silicon, phosphorous, arsenic and antimony. Preferred glasses are those containing
germanium, selenium and arsenic, for example those disclosed in our copending British
Application described above which contain at least l5 atomic % selenium but preferably
not more than 75 atomic % selenium. The arsenic content is preferably at least l0
atomic % but preferably not more than 65 atomic %. In addition or alternatively,
the composition preferably contains at least 5 atomic % germanium but not more than
42 atomic % germanium.
[0011] The most preferred compositions contain not more than 35 atomic % germanium, more
preferably not more than 30 atomic % germanium and especially not more than 25 atomic
% germanium. Also, the compositions preferably contain at least 20 atomic % selenium
and especially at least 30 atomic % selenium but preferably not more than 70 atomic
% selenium and especially not more than 60 atomic % selenium. The compositions preferably
contain at least 20 atomic % arsenic and especially at least 25 atomic % arsenic,
but preferably not more than 60 atomic % arsenic and especially not more than 55 atomic
% arsenic.
[0012] It is possible for quantities e.g. up to l0% or sometimes more, of other materials
to be present in the compositions used in the devices according to the invention,
for example minor amounts of the elements antimony, bismuth, silicon, tin, lead, halogens
and some transition metals provided that the presence of such materials does not deleteriously
affect the properties, such as the energy to latch and/or off resistivity, to a significant
degree. It is preferred, however, for the compositions to contain substantially no
tellurium since the presence of tellurium has been found to reduce the off resistivity
of the materials severely, although, in certain circumstances, small quantities of
tellurium may be tolerated, e.g. up to l0 atomic %, but preferably less than 5 atomic
%.
[0013] The films of glass and metals are preferably deposited by vacuum techniques such
as vacuum evaporation or d.c. (in the case of metals) or r.f. (in the case of chalcogenide
glass) maagnetron sputtering. The vapour may be generated by heating an appropriate
mixture of the components (not necessarily having the same composition as the intended
glass) or the separate components may simultaneously be heated.
[0014] In one preferred method of forming the device, a substrate of suitable electrode
design and masked to the desired pattern is coated sequentially with indium (to a
thickness of about 0.l to 0.5 micrometre) and then with chalcogenide glass to about
l0 micrometres thickness in a vacuum chamber at l0⁻³ to l0⁻⁴ Pa pressure by evaporating
the coating materials from separate resistance heated boats. A second indium interlayer
and a top electrode are deposited onto the glass by a similar process, although in
order to deposit a thick metal electrode film in a reasonable time, a higher evaporation
rate may be achieved by using an electron beam heated source rather than a resistance
heated boat. In order to achieve good adhesion and a low contact resistance, the layers
should be deposited without releasing the vacuum except where necessary to change
or realign masks and then only when the substrates are cold and by admitting an inert
gas such as nitrogen or argon, in order to reduce the possibility of surface oxidation
or contamination.
[0015] In another method, an indium containing chalcogenide glass interlayer of 0.l to
0.5 micrometres in thickness is formed between the chalcogenide glass switching element
and the electrodes by a vacuum evaporation process. Preferably the indium containing
glass has the same composition (except for the presence of indium) as that of the
switching element, and preferably has an indium content of at least l%, preferably
at least 5% but preferably not more than 30%, more preferably not more than 20% and
especially not more than l5% indium, based on the total weight of the glass (including
indium).
[0016] In a modification of this method an indium layer (e.g. 0.l micrometre thick) may
be provided on each electrode and an indium containing glass interlayer (e.g. about
0.5 micrometre thick) may be provided between the glass forming the switching element
and the indium layer. As will be appreciated, this modification is one example of
a method of forming a device in which there is a concentration gradient of indium
as one penetrates the glass, which varies from about l00% indium adjacent to the electrode
to about 0% indium at a depth of 0.3 to l micrometre into the glass. This may be achieved
in the most preferred method in a variety of ways: for example a number of glass composition
having different indium contents ranging from pure indium down to low (e.g. 0.5% indium)
may be deposited sequentially onto the electrode and, after deposition of the switching
material, may be deposited in reverse order. Alternatively, indium and the switching
element glass composition may be deposited from different boats simultaneously at
the beginning and end of the deposition process. In this case the electrical power
provided to heat the two boats is continuously varied so that a high rate of deposition
of indium and a low rate of deposition of glass is achieved adjacent to the electrodes,
the deposition rate of indium falling and the glass deposition rate rising as the
distance from the electrodes increases.
[0017] It is possible for the entire switching element to contain indium. However, in this
case the device must include a layer between the switching element and at least one
of the electrodes that contains a substantially higher (e.g. at least l0%, especially
at least 20 atomic % higher) indium content. In some cases the indium may evaporate
at a faster rate than the other components and so the element may have a higher indium
content adjacent to one of the electrodes, in which case it may be appropriate to
deposit a layer of indium on top of the deposited glass immediately before depositing
the second electrode since that part of the glass layer will be depleted in indium.
[0018] The dimensions of the switching element used in the device according to the invention
will depend on the particular chalcogenide glass composition that is used to form
it, although the thickness of the switching element will usually be not more than
40 micrometres, preferably not more than 20 micrometres, but usually at least 5 and
preferably at least l0 micrometres. The cross-sectional area of the switching element,
in a plane normal to the direction of current flow therethrough, will depend on the
maximum current flow. It is preferably at least 0.5mm², the preferred size being about
lmm² for discrete devices and for the maximum pulse level, at least 2mm².
[0019] The devices may be incorporated in an electrical circuit in any suitable position,
normally being connected between a current carrying line and earth, (the term "earth"
in this context including any structure having an appropriate shape and/or capacity
so that it can absorb the charge generated by the transient, and includes for example
connection to the chassis and the like in vehicles such as aircraft), and, of course,
more than one such device may be employed in the electrical circuit. The devices are
conveniently incorporated in other electrical components for example electrical connectors,
in which case the device will usually be connected between a current carrying element
of the device and a terminal or other part of the device to be earthed e.g. a conductive
housing.
[0020] Although in most instances the device will revert to its high resistance state as
soon as the transient voltage has subsided, it is still possible for the device to
be forced into a permanent low resistance state, for example if the current associated
with the transient is unduly large or if a number of rapid transients are experienced.
As mentioned above, whether or not the device will become permanently conductive depends
on the amount of energy absorbed by the device from the transient. In some applications
it may be desirable for the protection device to fail in this way, that is to say,
so that the equipment is still protected against transients but will not function
until the protection device is replaced or reset. In other applications it may be
desirable for the device to fail in a high resistance (open circuit) state so that
the equipment will carry on functioning although unprotected from subsequent transients.
Thus in some cases the device may be connected in series to means that will exhibit
a high resistance to the intended electrical circuit current at least when the switching
element has become permanently conductive. Thus, for example, the switching element
may be connected to the current carrying line or to earth via a fuse or switch that
is capable of transmitting currents passed through it when the switching element is
in its threshold mode but will change to a high resistance state when the switching
device has become permanently conductive.
[0021] Alternatively or in addition, the device may include a capacitor to ensure that the
device exhibits a high resistance to all frequencies below the cut-off limit of the
capacitor. The use of a capacitor in series with the switching element has a number
of advantages as mentioned in our copending British Patent Application No. 8508305
entitled "Overvoltage protection Device: filed on 29th March, l985, the disclosure
of which is incorporated herein by reference. Briefly, the use of a capacitor of appropriate
size, for example from l0pF to 2 microfarads in series with the switching element
will enable the transient current to be transmitted to earth, since most of the power
of the transient current occurs at frequencies above l0 kHz, but will exhibit a high
impedance to the intended currents in the circuit which will have significantly lower
frequencies or will be a direct current. Also, the use of a capacitor will prevent
or significantly reduce the possibility of the switching element latching in its low
resistance state after a transient has occurred. Such latching may occur in the absence
of a capacitor due to direct current flow through the switching element keeping the
switching element in its low resistance state.
[0022] A device in accordance with the present invention and articles that incorporate the
device will now be described by way of example with reference to the accompanying
drawings, in which:
Figure l is a sectional view of a device in accordance with the invention with the
thickness grossly exaggerated for the sake of clarity;
Figure 2 is a side view, partly in section, of a BNC coaxial connector that incorporates
a circuit pro tection device according to the invention;
Figure 3 is a side view, partly in section, of a flat cable mass termination connector
and wafers that incorporate a circuit protection device according to the invention;
Figure 4 is an enlarged view of part of the connector shown in figure 3; and
Figure 5 is a perspective view of a modification of the wafers shown in figure 3;
[0023] Figure l of the drawings shows schematically a circuit protection device in accordance
with the present invention. The device has been formed by depositing 0.l micrometre
thick indium layer 20l onto a copper electrode 202 by a vacuum evaporation method,
followed by a l0 micrometre thick germanium/arsenic/selenium glass layer 2l0, a further
indium layer 203 and a further copper electrode 204, the further layers all being
formed by vacuum evaporation from resistance heated boats with the exception of the
copper electrode 204 which was formed by electron-beam evaporation. Adjacent to the
indium layers 20l,203 are two further layers 205,206 which comprise the germanium/arsenic/selenium
glass and a small quantity of indium. The layers 205,206 may be formed by diffusion
of part of the indium from layers 20l,203 into the glass layer 202 or may be formed
as a separate layer by any of the methods described above.
[0024] Referring to figure 2 of the accompanying drawings, a connection arrangement for
connecting two coaxial cables comprises a connector shell l and a male connector 2.
The male connector 2 comprises a pin 3, the central and rear portion of which is hollow
for receiving the central conductor of a coaxial cable to be connected (not shown).
The pin has a fluxed solder ring 4 and a number of apertures (not shown) beneath the
solder ring which communicate between the solder ring 4 and the hollow interior of
the pin 3. The rear end l0 of the pin is firmly located in a connector housing 5 by
means of an electrically insulating plastics spacer 6. The housing 5, which provides
the electrical connection between the shields of the cables to be connected, has a
termination portion 7 on which is mounted a solder impregnated braid 8 and solder
ring 9.
[0025] The rear end l0 of the pin is provided on its outer surface with an electrode, e.g.
a copper electrode followed by a l0 micrometre thick switch element having an indium/chalcogenide
glass/indium construction that has been formed thereon by a vapour deposition method,
and the outer surface of the glass element ll has been provided with a further thin
(about l0 micrometres thick) electrode, e.g. formed from copper by an electron beam
evaporation method. The electrode is electrically connected to the housing 5 by means
of a column or wire l2 of solder or other suitable conductive material.
[0026] In order to connect a coaxial cable to the connector, the outer jacket, shield and
dielectric are cut back by appropriate amounts and the cable inserted into the connector
so that the exposed end of the internal conductor is located within the hollow interior
of the pin 3, the dielectric abuts the rear end of the spacer 6 and the exposed shield
is located within the solder impregnated braid 8. The connector is then briefly heated,
for example by means of a hot-air gun, to fuse the solder rings 4 and 9 and to form
solder connections between the pin 3 and central conductor and between the braid 8
and coax cable shield.
[0027] The connector will function exactly as a standard coaxial connector until the connected
cable experiences a voltage transient whereupon the potential difference across the
thickness of the glass layer ll will cause the glass to become electrically conductive
and form a closed circuit between the central conductor and the shield.
[0028] Referring to figures 3 and 4, a mass termination connector such as that described
in British Patent No. l,522,485 (the disclosure of which is incorporated herein by
reference) is schematically shown.
[0029] The connector comprises a connector housing 2l and a pair of connector wafers 22
and 23 that can be inserted into the housing. Each wafer 22,23 has a number of (usually
20 or 40) metallic electrical conductors 24 extending therethrough which terminate
at one end either in the form of pins 25 or complementary "tuning fork" female contacts
and at the other end in the form of contacts 26 for connection to a flat cable or
to a number of small single conductor wires. The particular means used for connecting
the conductors 24 to the wires or flat cable is not shown but usually comprises one
or more solder devices for example as described in U.S. Patent Specification No. 3,852,5l7.
[0030] In each of the wafers 22 and 23 a stepped recess 27 is made that extends across the
width of the entire wafer to expose each of the conductors. in one embodiment of
this connector, a copper electrode is deposited onto the individual conductors 24
followed by a 0.l micrometre thick indium layer, a l0 micrometre thick layer 28 of
the selenium-germanium-arsenic glass described above, a further 0.l micrometre thick
indium layer and a thin, e.g. about l0 micrometres thick, electrode 29 formed e.g.
from copper, gold or aluminium on top of the indium layer. An additional conductive
layer 30 or "ground plane" of gold or aluminium is located on the wafer material in
the stepped recess 27, the ground plane being electrically earthed for example to
the metallic housing of the connector or to an earth pin. Each electrode 29 is connected
to the ground plane by means of a wire 3l formed from e.g. gold or aluminium and bonded
to the electrode 29 and ground plane 30 by conventional wire bonding techniques.
[0031] Alternatively, a single layer 28 of the glass, indium and electrode 29 may be deposited
across the entire width of the wafer (except for a single conductor which is to act
as the ground conductor) in which case only a single wire 3l is necessary for connection
to the ground plane. Alternatively the ground plane and wire can even be dispensed
with if the chalcogenide glass layer 28, and indium and electrode 29 are deposited
over all the conductors 24.
[0032] In an alternative construction, the selenium-germanium-arsenic glass layer, indium
layersand electrodes are deposited onto the common ground plane 30, and the wires
3l connect the conductors 24, after any appropriate surface preparation if necessary,
with the electrode of the glass layer.
[0033] Figure 5 shows schematically a further modification of the wafer shown in figures
3 and 4. In this form of wafer the glass layers 28, indium layers and electrodes 29
are deposited on the conductors 24 as described above and are electrically connected
to a ground plane 30 by means of wires 3l. In addition, however, a l00 nano-farad
capacitor 40 is located in the recess 27 and is connected between the ground plane
and an earth terminal or housing of the connector. In this form of device any transient
current having a frequency spectrum above about l MHz is conducted directly to earth
while any direct currents or alternating currents of frequencies significantly lower
than about l MHz are blocked by the capacitor. This modification of the device has
the advantage that it reduces or eliminates the possibility of the glass switching
layers 28 being held in their low resistance state by the direct currents in the electrical
system after the transient has been transmitted to earth.
Examples l to 3
[0034] A Ge₁₄ As₃₄ Se₅₂ glass was formed by mixing the components, which were of at least
99.99% purity, and melting the mixture in a silica ampoule under a vacuum or under
reduced argon pressure. During melting, which was carried out at temperatures of up
to l000°C and for periods of up to 48 hours, the ampoule was rocked to ensure that
a homogeneous melt was obtained.
[0035] A l0 micometre thick layer of the glass so prepared was formed by vapour deposition
at a pressure of l0⁻³ to l0⁻⁴ Pa using a resistance heated source. Deposition rates
of 0.3 to l.0 micrometres per minute were employed. The layer was provided with a
number of electrodes of varying electrode contact resistance, and the energy to latch
of the switch devices so formed was determined as described above.
[0036] In Example l a gold spring probe of l mm diameter was applied to the top surface
of the glass layer with a force of lg.wt., the opposite surface being provided with
a copper film electrode.
[0037] In Example 2 copper electrode was provided on each surface of the glass layer by
a vacuum evaporation method. However the electrodes were allowed partially to oxidize
by readmission of air into the vacuum chamber while the electrodes were hot.
[0038] In Example 3 a 0.5 micrometre thick layer of the same glass composition but containing
l5% indium was provided on each surface of the glass layer and 0.l micrometre thick
layer of indium was provided between each indium containing glass and (unoxidized)
vacuum deposited copper electrodes. The results are shown in table I.
[0039] In Example 4 a 0.2 micrometre thick layer of indium was provided between the chalcogenide
glass and each electrode by vacuum evaporation from a resistance heated boat. In Example
5 the same procedure as Example 4 was used with the exception that the indium layers
each had a thickness of 0.05 micrometres.

[0040] The energy to latch of the device, and hence of the glass material, was measured
by means of a circuit as shown in figure 6. Single shot pulses generated by means
of a pulser l were passed to the switching element 2 connected in series with a current
limiting resistor 3 having a resistance R₂ of from 40 to l00 ohms and the voltage
across the switching element was observed by means of oscilloscope 4. The voltage
generated by the pulser l was gradually increased (about 5 to l0 pulses being passed
for each voltage level from the pulser) until the switching element latched in its
low resistance state (determined by subsequently measuring its resistance.
[0041] The energy to latch the device, E
L was determined by the equation:

where J is the energy available from the pulser;
V is the peak voltage from the pulser when latching occurred;
R₁ is the internal output impedance of the pulser (5 ohms); and
R₂ is the resistance of the current limiting resistor.
[0042] This equation gives very good agreement with values obtained by integrating the current
and voltage curves of the pulses.
[0043] The resistance across the device was determined by replacing the device with standard
resistances and comparing the voltage/time curve with that of the device when subjected
to standard pulses. It was observed that the peak current flowing through the device
was approximately l ampere in all cases so that the resistance across the device (in
ohms) is numerically equal to the voltage across the electrodes (in volts).
1. A threshold switching device which comprises a switching element formed from an
amorphous chalcogenide composition and a pair of electrodes that are in contact with
the composition, the device containing an indium or indium containing layer between
the composition and at least one of the electrodes.
2. A device as claimed in claim l, which has an electrode-to-electrode resistance,
in its low resistance state, of not more than l0 ohms.
3. A device as claimed in claim 2, which has an electrode-to-electrode resistance,
in its low resistance state, of not more than l ohm.
4. A device as claimed in claim 3, which has an electrode-to-electrode resistance,
in its low resistance state, of not more than 0.l ohm.
5. A device as claimed in any one of claims l to 4, wherein the chalcogenide glass
composition comprises germanium, selenium and arsenic.
6. A device as claimed in any one of claims l to 5, wherein the concentration of indium
in the or each indium containing layer increases toward the or each electrode.
7. A device as claimed in any one of claims l to 6, wherein the or at least one indium
or indium containing layer comprises an indium containing chalcogenide layer in contact
with the switching element, and a layer substantially completely consisting of indium
between the indium containing chalcogenide layer and the electrode.
8. An electrical component which includes a threshold switching device as claimed
in any one of claims l to 7 between a current carrying line of the circuit and earth.
9. A component as claimed in claim 8, which is an electrical connector.