WO2008087566A1

WO2008087566A1 - Electrical switching element

Info

Publication number: WO2008087566A1
Application number: PCT/IB2008/050067
Authority: WO
Inventors: Dagobert M. De Leeuw; Thomas C. T. Geuns; Bianca C. De Brito; Paulus A. Van Hal
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-01-15
Filing date: 2008-01-10
Publication date: 2008-07-24
Also published as: JP2010516053A

Abstract

Switching element (10) including a first layer (14) and a second layer (22), the switching element having a first state and a second state, wherein in the first state the first layer (14) and the second layer (22) are laminated at a contact area (22), and wherein in the second state the first layer and the second layer are delaminated at the contact area (22) said 5 switching element irreversibly switching from the first state to the second state upon applicat ion of a switching stimulus to the switching element.

Description

Electrical switching element

FIELD OF THE INVENTION

The invention relates to switching element for irreversibly switching between a first state and a second state.

The invention further relates to a device incorporating the electrical element.

BACKGROUND OF THE INVENTION

There is an ongoing effort for the development of cheap and easy to use write- once-read-many (WORM) memory devices. Such a device can be programmed once while it can be read many times. A well known example is a regular compact disc or a programmable read only memory (PROM) in electrical devices. All such devices require switching elements that can be switched once, but read many times.

An example of such a switching element is disclosed in US 2004/0149552. The disclosed device comprises an electronic switch used as an electronic memory element which comprises a conductive organic polymer layer sandwiched between and in contact with two metallic conductive elements. In an initial post-fabrication state the organic polymer layer is relatively highly conductive, the post fabrication state constituting a first state of the memory element that can serve to represent, for example, a binary bit 0 or 1 , depending on the encoding convention employed. A relatively high voltage pulse can be applied to between the two metal conductive elements, resulting in a marked decrease in the current carrying capacity of the organic polymer. The change in current carrying capacity of the organic polymer layer is generally irreversible, and constitutes a second stable state of the memory element that may be used to encode the other binary bit 1 or 0. In the example disclosed, the organic conductive layer constitutes commercially available polyethylenedioxythiophene (PEDOT) :pylostyrenesulphonic acid (PSS). Using PEDOT:PSS, decrease in conductivity following a high voltage pulse lies between two and three orders of magnitude.

It is a disadvantage of the device that with the choice of the PEDOT:PSS material a number of device parameters is interrelated such as the conductivity in both states and the switching voltage to be applied, since all of these rely on the same material. This hampers proper design of the switching element with respect to design specifications relation to the device requiring the switching element.

It is therefore a first object of the invention to provide a switching element and device incorporating the element that results in improved design freedom.

SUMMARY OF THE INVETNION

The invention is defined by the independent claims. The dependent claims define advantageous embodiments.

The invention is based on the insight that, during switching of a switching element, changing of the state of the switch, i.e. from the first state to the second state, is advantageously done by invoking a change of the physical integrity of the element in the form of delaminating the first layer from the second layer. The delamination is effectuated by providing either or both of the first and second layer with a compound that provides a gaseous species upon providing the switching signal such that the gas evolved presses the first layer and the second layer apart at the contact area. Here lamination is meant to be construed as meaning that there is a certain interaction between the first layer and the second layer at the contact area in the first state before switching. This could be for example adhesive molecular forces. Furthermore, delamination also discriminates the present switching mechanism and device from those wherein first and/or second layer are destroyed such as in a fuse or partly removed from the switching element as in an explosive device.

A compound providing a gaseous species upon switching is to be construed as meaning a compound that for example evaporates or chemically reacts to release a gaseous species, but not in such a from as to cause explosion of any kind. Thus, the invention is advantageous with respect to situations where for example arrays comprising closely spaced switching elements are used as in memories and/or where device integrity is crucial.

Chemical reactions include photochemical reactions that result in the compound to release a gas as well as heat induced reactions.

The invention has advantageous over the prior art device that, since switching is based on the change of the physical integrity of the device instead of having to change properties of the first layer and/or the second layer.

The switching process according to the invention involves a self limiting process. Such a process advantageously results in a reliable and controllable switching.

The switching stimulus can be an electrical signal, a radiation signal, or a heating signal. In a preferred embodiment the switching stimulus comprises an electrical signal.

In a first variant of this embodiment the electrical signal may drive a heater element that provides heat to evaporate the compound. The heater element may be provided by either of the first or second layer. This is advantageous since in that case evaporation will at least take place at the contact surface within the contact area and will therewith be effective at the area where the gas is needed. In another alternative the electrical signal may result in direct chemical reaction of the compound such as in electrolysis or decomposition. This is advantageous since in this case heating is not required. Electrolysis is advantageous since it occurs at relatively low voltage. A preferred example of a compound that can be electrolysed conveniently at low voltage is water.

In an embodiment the switching stimulus comprises radiation. This allows for example contact less switching of the element using for example visible or UV light. In that case the switching element is optically switched. In one variant the radiation heats the compound which therewith evaporates to cause the delamination. If however, heating is not preferred an advantageous variant is used in which the radiation causes the compound to chemically react to release a gas as in a photochemical reaction.

In an embodiment at least part of the contact surface is located in a cavity within a support layer. If the contact area is encapsulated, the gaseous species that are needed for the delamination are substantially prevented from diffusing into the surroundings of the contact area without exerting their delaminating effect. The effect of the support layer is dependent on its gas permeability; the lower it is the better the layer provides its function. The support layer may be any material having a suitable gas permeability. It may be of organic nature such as a photoresist or of inorganic nature as in silicondioxide. In an embodiment the switching element is an electrical switch of which the first state represents a first electrical state and the second state represents a second electrical state. With electrical state is meant electrically resistive, electrically capacitive and/or electrically inductive state or a mixture. Thus, delamination will induce an increase of the electrical resistance measured across the contact area form the first layer to the second layer. Similarly, the delamination induces a space in between the first layer and the second layer and hence the capacitance of the contact area as measured across the contact are will change upon switching. This embodiment is advantageous where the switching element must function as an electrical element such as for example to disconnect or connect different circuit parts of a device. It may also function as a fuse. Furthermore it may be advantageously used as a write once read many electrical memory element, where reading of its content can be done electrically by sensing the change in the electrical state. Note that switching may still be done according to need as described here before.

In a preferred embodiment the first layer and the second layer are electrically conducting. A resistive switching element is preferred since resistivity can be easily sensed within a memory. One of the layers may be an electrode layer. If so it is preferably made of metal since metals are relatively non permeable to gases. This is advantageous for reasons as elucidated with respect to the support layer here above.

In an embodiment at least one of the first layer and the second layer comprises a further compound, the further compound being an electrically conductive compound.

Preferably, the conductivity of the layers is determined by the further compound that differs from the compound capable of providing the gaseous species. In that case the compounds can be independently chosen providing design freedom and the opportunity to choose the conductivity of the first state according to the properties of the further compound independent from the switching conditions that predominantly depend on the compound capable of providing the gaseous species.

In an embodiment the switching element comprises a mirror layer, which mirror layer has a first shape in the first state of the switching element and a second shape, different from the first shape, in the second state of the switch. Here a mirror layer means a layer reflecting radiation. This is advantage since it allows detection of the state of the switch by using radiation, which is contactless and distinct (thus without interference) from for example an electrical switching mechanism. The mirror layer may be one of the first and second layers, but this need not be so provided that the deformation of the first and/or second layer upon the delamination translates far enough through the switching device to result in a deformation of the mirror layer. The mirror layer is preferably made of metal and hence a possible electrode layer may advantageously function as a mirror layer.

In an embodiment the device comprises a switching element according to claim 5 for electrically disconnecting two parts of the device.

In an embodiment a device comprises a switching element according to claim 8 wherein the switching element provides a switchable mirror.

In an embodiment a device comprises a switching device according to claim 1 wherein the switching element is a memory element for storing information.

In Nature 441 2006 or the non-prepublished international application IB2006/052510 is disclosed a capacitive device with a PEDOT/PSS electrode that is used to contact a gold electrode in a cavity. In between the gold and the PEDOT/PSS layer is a self assembled system or monolayer such that the device constitutes a capacitor or a molecular junction.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be further elucidated with reference to the Figures, in which:

Fig. 1 shows in cross-sectional view a first embodiment of the switching device of the invention; Figs. 2A and 2B show current- voltage and current-time plots, respectively of a switching device.

Figs. 3 A and 3B shows a plan view of a switching device before and after switching at ambient conditions.

Fig. 4 is a Scanning Electron Microscopy picture of a cross section of a switching device after application of the switching signal.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows in cross-sectional and diagrammatical view a first embodiment of the switching element of the invention. The Figure is not drawn to scale. The switching element 10 is made on a substrate 12, in this example a silicon wafer with a diameter of 4 inch (10 cm). The substrate 12 in this example is passivated with a thermally grown layer of siliconoxide (not shown). This layer is however not essential to the invention. A gold electrode 14 is made by thermal evaporation of 1 nm thick chromium or titanium followed by 40 nm thick gold and standard photolithographic patterning using a postive photoresist (HPR504™ ), mask defined exposure of the photoresist and etching of redundant gold. Afterwards the substrate is cleaned using fuming nitric acid followed by rinsing with de- ionized water.

Subsequently, an electrically insulating layer 16 is applied on top of the substrate covering also the gold electrode. A cavity 18 is defined in the insulating layer 16 above the gold electrode 14 such that the gold electrode is exposed over a contact area 20. In the present example a 500 nm thick negative photoresist (ma_N 1407 or L6000.5 ), is used as the insulating layer 16, which was patterned in a standard treatment according to specifications. In this way the silicon substrate was provided with multiple cavities each having a circular contact area with a diameter within the range of 1 to 50 micron. Before applying an intermediate layer 22 to the substrate which fills the cavity, the contact area is cleaned by subjecting them first to an UV ozone or O₂ plasma and then to a reducing step such as an ethanol bath, or an N2/O2 plasma. In this example the intermediate layer is applied by spincoating a 100 to 200 nm thick layer of a conductive polymer composition comprising poly(3,4-ethylenedioxythiophene) and polystyrenesulphonic (PEDOT/PSS) in water as supplied by H. C. Starck A.G, mixed with a few drops of a surfactant (FSO 100™, DuPont) and drying this film at room temperature in vacuum for 1 hour. The film has an electrical conductivity of approximately 1 S/cm.

Then, a second electrode 24 is provided on top of the PEDOT/PSS film. Thereto, in this example a 100 nm thick gold layer is evaporated or sputtered and structured using a photolithographic process using a 500 nm thick layer of positive photoresist (HPR504™) that is patterned such that after patterning it is removed from the substrate at all places except where the gold electrode must remain after its etching. The gold second electrode 24 is etched using a sputter etch and subsequently the PEDOT/PSS layer 22 is etched using STS RIE etching with an O₂ plasma using the gold second electrodes as a mask. After stripping of the left over positive resist, the element of Fig. 1 results.

In test switching elements, the electrodes 14 and 24 are patterned such that they are connected to bond pads located next to the cavities when viewing the substrate from the top such that contacting of the switching elements using, for example a probe station, does not destroy the cavities. A SEM image of a fabricated switching element is shown in Fig. 3A. The circular area 25 reveals the contour of the cavity of the switching element.

Current voltage characteristics (IV) of the switching elements having different contact areas are measured while keeping them under ambient conditions by connecting their first and second electrodes to a power supply and ramping the bias from 0 to 5 V. As an example a part of a current voltage characteristic 26 of a switching element having a contact area with a diameter of 50 microns is shown in Fig. 2A in order to demonstrate the switching. While up to a bias of 1 V the element behaves like a linear resistor, at a bias exceeding 1 V the resistance starts to increase slowly at first to finally increase with 5 orders of magnitude quite abruptly. The resistance increase is irreversible. Furthermore, a second recording of the switched element shows that the element still behaves as a linear resistor, but with a different resistance. Thus, while the first low resistance defines the first state of the switching element, the second high resistance defines the second state of the switching element.

The switching is also evident from the current transient (It) 28 of Fig. 2B recorded of a similar element as used for the recording of IV 26, by subjecting the switching element to a 2 V block pulse. In this case 4 orders of magnitude resistance change is induced separating the first and second state of the switching element. Thus, using a switching voltage of only 2 V, the element is conveniently switched.

Figs. 2A and 2B also show IV 30 and It 32, respectively recorded of similar switching elements as those used for recording IV 26 and It 28, while keeping the elements in vacuum conditions. No switching is observed. In addition It 34 recorded of a element at 140 °C also does not show switching. The current remains constant for long periods of time, at least longer than 10 seconds.

The switching voltage is independent of the size of the contact are within the range of contact areas given here above.

While in Fig 3 A an image of a switching element before switching is shown, the result after switching is shown in Fig. 3B. The result is typical for all switching elements switched. The electrode 24 within the region on top of the cavity bound by 25 has been pushed upward to become deformed with a spherical or globular shape. Figs. 4 A and 4B show images of a cross section of a switching element after switching, wherein it is discernible that the PEDOT/PSS layer 22 has delaminated from the gold electrode 14 to form a void or space 36 between them.

The mechanism of the switching is as follows. At voltages higher than approximately 2 V water is electrolysed to provide the gaseous species H₂ and/or O₂ that causes the delamination. It has been observed that spincoating of the PEDOT/PSS by using the mixture as described here before but with added dimethylsulphoxide, increases the conductivity of the PEDOT/PSS layer by a factor of approximately 100 while the switching voltage decreases to approximately 1 V. This is advantageous with respect to application of the switching element in devices that only have low voltage available. Thus, the device according to the embodiment provides the advantages over the prior art document of significantly lower switching voltage, increased resistive change upon switching which is advantageous as indicated within US2004/0149552 Al. Independent optimisation of switching parameters of the element and the switching state parameters.

The described embodiment may be differentiated with respect to materials as well as geometry without departing from the invention.

Thus, the geometric device construction can be altered. The electrode 24 may cover the layer 22 such that the entire layer 22 is comprised within the cavity and in between the electrodes 14 and 24. The layer 22 may be entirely within the cavity, such that its surface contacting the electrode 24 is also within the cavity. It will be appreciated that the shape and depth of the cavity may be suitably altered.

The substrate may be any substrate envisaged.

The layer 16 may be made of a different material. Advantageous are materials that provide low permeability of gases in order to keep the gas concentrated at the contact surface. Suitable other organic materials may be used. Also inorganic materials can be used such as silicondioxide or silicon nitride. This facilitates integration in semiconductor devices. If the device is an electrical switch the layer 16 is preferably electrically insulating. However, when the switching element is not an electrical switching element this does not need to be the case.

The PEDOT/PSS layer need not be patterned. In the above described example the current that runs through the device is forced through the contact area 20 since the layer 16 is insulating. Hence, gas forming region is confined to within the diameter of the cavity.

The conductivity of the intermediate layer 22 may be varied by choosing different materials for the layer. Thus, in alternative embodiments the layer 22 does not comprise an electrical conductor such as PEDOT/PSS, but a semiconductor or even a material that has lower conductivity than that of the semiconductor. The material may be of organic nature or inorganic nature. In this way, by choosing a material with different conductivity, the resistance of the first state of the switching device can be altered without having to alter the dimension (cavity diameter) of the switching device. This is advantageous with respect to circuit design where feature size is a prescribed parameter.

The material of the layer 22 is most preferably an electrically conducting polymer, e.g. a polymer material in which the conductivity arises as a result of the interaction of the dopants with the polymer material, and particularly with electrically conducting groups therein. Examples of these materials are polyanilines, polythiophenes, polyacetylenes, polypyrrols which may be substituted with side groups such as alkoxy, alkyl, aryl and the like. Alternatively, the material of the contact layer may be a material in which electrically conductive elements are incorporated, such as epoxies or other polymers filled with silver, graphite or the like. These latter materials are however distinctly less preferred in that the uniformity of the layer is substantially smaller, and hence the uniformity of the element over its surface area is reduced.

Most preferably, use is made of a poly-(3,4-substituted-thiophene) as the conducting polymer. The best known example of this class of polymers is the one with a 3,4- alkylenedioxy-substitution, that is usually referred to as PEDOT. The alkylenegroup is suitably an optionally substituted Ci-C4-alkylene group and preferably chosen herein from the group consisting of an optionally Ci to C₁₂- alkyl- or phenylsubstituted methylene group, an optionally Ci to Ci2-alkyl- or phenylsubstituted 1,2-ethylene group, a 1,3-propylene group and a 1,2-cyclohexylene group. Additives may be added to increase the conductivity and processing behavior, such as surfactant.

If the gas providing mechanism relies on electrolyses of a compound, such as water, present in a current carrying medium such as PEDOT/PSS, then by changing the conductivity of the layer 22 through replacing the PEDOT/PSS for another compound such as for example polyaniline, the resistance of the first state of the switching device voltage required to cause a certain critical current to pass through the device in order to switch the device can be controlled. Higher conductivity means lower switching voltage. There will be a region where the voltage drop over the layer is insignificantly low so that the switching voltage is substantially independent of the conductivity and within this region the resistance of the first state of the device can be adjusted according to need by choosing the appropriate material for layer 22 without altering the switching voltage.

In one embodiment a device according to that described here before is prepared wherein the intermediate layer 22 is polyvinylalcohol (PVA) having low conductivity. The, PVA layer is spincoated from a 5 weight percent solution of 87-89% hydro lysed PVA (Aldrich) in water to a layer thickness of 100 to 200 nm. The layer is dried at room temperature for 1 hour. The conductivity of the PVA layer is much lower than that of PEDOT/PSS. Hence the first state of the switching device is much higher. In an IV curve the current increases from nanoampere level at low voltage to microamperes at approximately 80V where the element switches.

Other compounds that provide the gaseous species for switching may be used in a device according to the invention. Thus for example, it could be a volatile compound. Alternatively, the compound is sensitive to heat as a consequence of a chemical reaction that proceeds through which the gaseous species is provided. In these cases the electrode 14 as advantageously used as a resistive heater, i.e. is given a resistance so high as to generate enough heat for the evaporation. Advantageously, the evaporation takes place substantially at the contact surface where delamination is desired. This embodiment provides a switching circuit that does not use the second electrode 24. Hence it is at least partly separated from an electrical circuit for measuring the resistance which would use the electrodes 14 and 24 to sample the state of the switch. Heating can also be provided through irradiation as is for example done using a laser. Also possible is the use of compounds that release the gas upon photochemical reaction.

Several methods may be used for the deposition of the layer 22. The deposition method will often depend on the material employed for the layer. Some deposition methods advantageously allow patterning in situ such as does inkjetprinting or the like.

Hence separate patterning of the layer 22 is not necessary. In another variation the layer 22 is not patterned at all.

Suitable materials for the first and or second electrode include gold, copper, conductive oxides, aluminum, doped silicon GaAs, other III-V semiconductors, mercury, nickel, platinum, palladium and the like. The bond of the layer 22 to the electrode is advantageously used to tailor the adhesion of the electrode layer to the intermediate layer. Stronger adhesion will result in more difficult delamination and thus more difficult switching.

In order to tailor the adhesion between the first layer and the second layer bonding may be tailored, using monolayers having appropriate chemical compositions.

A self-assembled system may be applied in the cavity between the first layer 14 and the second layer 22 to tailor the adhesion between these layers. The self-assembled layer may comprise one or more monolayers. In case of a gold electrode monolayers such as alkanethiols or alkanedithiols can be used . But it will be understood that the type of system used is determined by the layers that require adhesion and the extent of adhesion required.

The monolayers can be applied according to procedures as described in the non-prepublished international application IB2006/052510

The bonding may be a chemical bonding or physical bonding as elucidated in the aforementioned non-prepublished application. In the embodiment described, the first electrode 14 may function as a mirror.

In that case the switching device provides a switchable mirror, since in the first state of the switch, before switching, the mirror surface is substantially flat, while in the second state after switching the mirror has a globular shape as shown in Figs 3A and 3B. An alternative variant comprises the second electrode 24 and/or the intermediate layer 22 as the mirror. If the mirror layer is buried layer as is the layer 22, surrounding or covering layers such as 16 and or 14 and/or 24 preferably provide a transparent window such that the mirror can be detected using the radiation to be reflected by the mirror.

The switchable mirror can be used to cause changes in reflected irradiation beams, i.e. the state of the switch may be detected using a radiation based principle and system. Therewith, the switching device may for example provide an optical memory function. Those skilled in the art will know how to provide such detection systems.

Note that while detection of the state of the switch may comprise detection based on radiation such as optical detection as described here above, the switching of the device does not necessarily have to be stimulated by a radiation signal.

Thus, the switch may also be electrically switched according to any principle of the invention, for example. Alternatively, the switch may be switched using irradiation. In order to separate switching from detection conveniently.

Suitably the manufacture of the electrical element constitutes one step in the manufacture of an electronic device. Such electronic device may comprise a plurality of the electrical elements as made according to the invention and suitably also other passive and active elements. The elements of the invention may also be integrated into an array as shown, which allows the manufacture of memories. Each switching element with its electrodes is then electrically connected in series with a selection device such as a diode or a transistor between a word line and a bitline according to known art. In the case that the electronic device is an integrated circuit, it appears suitable to integrate the element of the invention within the interconnect structure, or even more suitable on top of the passivation layer. It will be understood that the first and second electrode are suitably provided as part of layers in which further patterns are defined, such as interconnects, electrodes, bond pads and the like. The manufacture hereof is carried out suitably on plate-level after which individual devices are separated from each other. Those skilled in the art will know how to integrate the element into an electronic device in order to use the element as a switch in a memory or otherwise and exploit the advantageous properties of the switching element.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element or product does not exclude the presence of a plurality of such elements or products. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that the combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. Switching element including a first layer and a second layer, the switching element having a first state and a second state, wherein in the first state the first layer and the second layer are laminated at a contact area, and wherein in the second state the first layer and the second layer are delaminated at the contact area said switching element irreversibly switching from the first state to the second state upon application of a switching stimulus to the switching element.

2. Switching element according to claim 1 wherein the switching stimulus comprises an electrical signal.

3. Switching element according to claim 1 wherein the switching stimulus comprises radiation.

4. Switching element according to claim 1 wherein at least part of the contact surface is located in a cavity within a support layer.

5. Switching element according to claim 1 wherein the switching element is an electrical switch of which the first state represents a first electrical state and the second state represents a second electrical state.

6. Switch element according to claim 5 wherein at least one of the first layer and the second layer are electrically conducting.

7 Switching element according to claim 6 wherein at least one of the first layer and the second layer comprises a further compound, the further compound being an electrically conductive compound.

8. Switching element according to claim 1 wherein the switching element comprises a mirror layer, which mirror layer has a first shape in the first state of the switching element and a second shape, different from the first shape, in the second state of the switch.

9. Device comprising a switching element according to claim 5 for electrically disconnecting two parts of the device.

10. Device comprising a switching element according to claim 8 wherein the switching element provides a switchable mirror.

11. Device comprising a switching device according to claim 1 wherein the switching element is a memory element for storing information.