DE3420509A1 - Process for reading in and out signals based on electrically polarisable layers - Google Patents

Abstract

An electrically polarisable polymer layer provided with a system of electrodes is used as a data memory. The electric signals to be stored are recorded in the polymer layer by remanent polarisation of individual domains. The written-in remanent polarisation is preserved even after switching off the signal field. The reading-out operation is based on a pyroelectric activation of the polarised domains. The activation is performed by a multiplicity of the polarised domains simultaneously inducing electric signals in the system of electrodes, which are then arranged and sampled in parallel and/or on the basis of a multiplex process. A strip matrix is arranged as the system of electrodes on the upper side and on the underside of the polymer layer. The strip-shaped electrodes of one side are parallel to one another, but form an angle with the strips of the opposite side. In the case of a data memory of such a construction, the polarised domains of the polymer layer assigned to a single conductor track of the strip matrix can be pyroelectrically activated simultaneously by a heating current pulse flowing in this conductor track. In this way, the polarised domains assigned to the conductor track can be sampled simultaneously and then, by changing over to another conductor track, the correspondingly other regions can be activated, so that altogether a selectively parallel read-out mode is achieved ... Original abstract incomplete. <IMAGE>

Description

Procedure for reading in and reading out signals based on

electrically polarizable layers The invention relates to a method for reading in and reading out binary or analog electrical signals using a data memory based on an electrically polarizable and with an electrode system contacted polymer layer. The reading process is based on the fact that the polymer layer retentively polarized in accordance with the signals to be recorded in local domains will. The readout process is based on a pyroelectric activation of the polarized Domains.

The current situation in the field of communication technology, in particular the field of data processing, is characterized by the development and the use of information stores with ever increasing data density. Currently Stationary electrical storage devices based on Si are primarily in use commercially (ROM, RAM, CCD) with storage densities around 106 bit cm # 2 and movable magnetic memories (Tape, plate, drum) around 107 bit cm -2 Electrical ones are in development Storage of higher density (production with the help of electron beam or X-ray lithography), higher density magnetic storage (replacement of pigment / binder systems by metallic layers, introduction of the perpendicular recording technique) as well as optical ones Memory, especially non-erasable optomecha- niche storage, z. B. optical disk, and erasable opto-magnetic memory based on the Kerr or Faraday effect. None of these storage facilities will - at least in the foreseeable future - not become greater storage densities than reach approx. 108 bit cm 2; so are z. B. the optical storage by the maximum Focusing the laser beam 2.

limited by approx. 1 Rm2 in their storage density.

However, memories are essential and are wanted and urgently needed higher storage densities than 108 bit / cm 2.

In addition to the storage density, other important parameters are the access time, Data transfer rate, cost per bit, lifetime of the stored information and compatibility with existing computer systems. The following data are used as target values Desired for memory of the 90s: memory densities 1011 bit / cm2 access times 10 8 sec data transfer rates 108 bit / sec Permanence 10 years and erasable.

Major development goals are a higher storage density and a shorter access time. Two development directions are currently being pursued: a) With "multi-dimensional storage" by means of laser holography or by means of "Photochemical hole burning" is the information with a laser beam in the storage medium enrolled. In the case of holographic systems, the information becomes spatially three-dimensional stored (e.g. in LiNbO2 or in polymers). With photochemical hole burning the information is read in by means of the laser, but Additionally are converted into a broadband solid-state IR absorption band per surface element (approx. 1 wem2) (Width approx. 500 GHz) up to 103 different narrow bands ("holes") with one Burned in bandwidth of 50 MHz. In both cases, there is the disadvantage that not can be deleted bit by bit and read in again, but the deletion always in packets, e.g. B. ä 103 bit in photochemical hole burning takes place.

b) The second direction of development is concerned with use of electron beams for reading in and out. The electron beam has opposite the laser has the advantage of a considerably smaller beam diameter (order of magnitude 0.005 #m). Accordingly, there is a higher spatial resolution and thus a higher one Storage density reached. With this method, a non-erasable and non-rewritable memory by burning tiny holes in ß-aluminum manufactured.

Memories based on electrically polarizable layers have been around since Developed in 1950. In U.S. Patent 2,698,928 the basic methods for described a movable memory. This is a tape / plate consisting of an electrically remanent polarizable material that is an electrically conductive Has lower layer moved past an electrode. By applying the Electrode with a voltage are areas of the electrically polarizable layer retentively polarized and thus information is stored. The electrode can become both in direct contact with the layer and a defined distance exhibit. Other methods of creating remanently polarized ones Areas are the use of electron beams or ion bombardment. To read out the saved Information is used the piezoelectric effect, i. H. on the electric polarizable layer is a pull / push by moving past a sharp Edge or exerted by using ultrasound, so that the charges are released induce a voltage in the electrode that is proportional to that during the writing process voltage used. Inorganic media are used as electrically polarizable media Ferroelectrics but also called electrically polarizable plastics.

In addition to the piezoelectric readout, at the end of the sixties the pyroelectric effect is used for reading out band-shaped memories (H. Tanaka, R. Sato, Trans. I.E.C.J. 52-A, 436 (1969); H. Niitsuma, R. Sato, Ferroelectrics 34, 37 (1980)). Here, stresses were created by heating the electrically polarized Layer generated in the grinding electrodes.

Inorganic materials were used as the electrically polarizable layer like Pb (ZrTi) 03 (= PZT) used.

A further development to a high-density memory is not known. Such a further development should not be promising either, since ceramic Ferroelectrics such as B. PZT, because of their high dielectric constant and their at the same time low coercive force not suitable, electrically polarizable Represent materials for storage of the highest density.

The high dielectric constant has a low readout signal result and the low coercive force leads to a low storage density. In one to that analogous procedures proposed by Tanaka and Sato using sliding electrodes and pyroelectric readout is used in the U.S. Patent 4,389,445 discloses a polarizable macromolecular layer, such as. B. polyvinylidene fluoride (PVDF) are used, which for the reasons given above are generally better than inorganic materials is suitable. A data memory was also made based on PVDF in US Pat. No. 4,059,827, reading in directly with an electron beam and the readout via the expansion of the electron beam on polarized PVDF domains he follows.

Reading in by means of an electron beam, however, brings with it the danger That fluorine is split off when bombarded with electrons at higher current densities from the PVDF and thus an irreversible destruction of the storage material takes place. Furthermore, in the method described above, the information can be read by means of an electron beam only after the PVDF film has been heated to a certain level Temperature (800 C) incl.

Cooling can be accomplished after the reading process. So are - because of the thermal inertia of the polymer material - only extremely low data transfer rates possible, which are of no interest in practice. The data transfer rate represents no problem with the sliding electrode method. The decisive disadvantage, But what adheres to all movable memories is the access time, which is conditional will always amount to a few 10 3 seconds due to the positioning process.

This problem does not occur with stationary storage systems based on Si For this reason, stationary storage systems were also developed in parallel to the Si-based storage systems based electrically polarizable media developed. So became by Crawford (J.C. Crawford, Ferroelectrics 3, pp. 139-146 (1972)) a ferroelectric Storage arrangement described which is compatible with the electrical storage of that time was based on Si. In this memory, electrode strips (so-called comb electrodes) on the top and bottom of a ceramic ferroelectric layer (PZT) 900 rotated against each other vapor-deposited.

Information could be saved by applying a voltage to corresponding electrodes on the top and bottom of the area at the intersection of these electrodes was polarized. Below the ferroelectric layer was an epoxy layer is applied, on which in turn a piezoelectric material was glued. This piezoelectric layer was selectively applied by applying a Tension-set vibrations so that the resulting sound wave passes over the epoxy Exerts pressure / tension on the ferroelectric storage layer and thereby in turn means the piezoelectric effect creates a voltage in the corresponding pair of electrodes was generated, depending on the sign of the present in the polarized area remanent polarization was positive or negative.

With this proposal, however, a high-density memory is not feasible because of the necessary thickness of the epoxy layer (at 10 MHz at least 0.5 mm) narrow electrodes (<100 rom), which only enable high storage densities would not be selectively approachable and, moreover, those that inevitably occur Sound reflections also address unaddressed areas with a time delay, so that partially completely incorrect readout data were obtained due to spatial interference.

A similar comb electrode system applied to a polymer such as z. B. PVDF, is presented in U.S. Patent No. 3,772,518. In this patent, however, not thinking of data storage, but solely of a light detection system similar to a VIDICON tube. A fine beam of light falls on you uniformly pre-polarized organic film, so that taking advantage of the pyroelectric Effect detects the position of the incident light beam by means of comb electrodes can be. The registered currents -11 are approx. 10 11 A. By applying the applied comb electrodes with a voltage can be analogous to Crawford information are stored and read out by means of the fine light beam. It results but in turn the problem of positioning the light beam that leads to a undesirably long access times and the limitation of the storage density the maximum possible bundling of the laser light beam to approx.

1 tjm 2 A storage tank that also avoids this disadvantage would be ideal and thus has all the properties of silicon-based memories, however Memory densities of 10 bit / cm2. In the case of Si storage systems, this is technological conditional limit of the storage density at approx. 106 bit / cm2, the invention is the Task, in combination with a data memory based on a ferroelectric Polymer layer to develop a read-in and read-out process, the highest storage density and the shortest possible access times.

This task is, starting from one with an electrode system contacted polymer layer using the remanent electrical polarization more local Domains during reading and pyroelectric activation when reading out according to the invention in that the pyroelectric activation takes place during or immediately before the readout process in such a way that a large number of the polarized domains in the electrode system induce signals which are then parallel and / or assigned and queried according to a multiplex method.

When reading in, time-stable polarized domains are created in the polymer layer. The written remanent polarization is, however, caused by free charge carriers (Electrons and holes) shielded, so that almost no external electrical Field penetrates. For this reason, the system must be disrupted to temporarily cancel the shielding effect of the compensation charges. For this purpose, the polymer layer is briefly heated during the readout process Pyroelectrically activated. The spontaneous polarization decreases, so that due to the then existing overcompensation creates an external field against the polarization field occurs that is used as a signal field.

A strip matrix is preferred as the electrode system, which by a multitude of electrically conductive, parallel strips on the top and bottom the polymer layer is formed. With a memory constructed in this way, in advantageously those assigned to a single conductor track of the strip matrix polarized domains of the polymer layer simultaneously through one in this conductor track flowing heating current pulse can be activated pyroelectrically. That way you can the polarized areas assigned to the conductor track are queried at the same time and then by switching to one other track the corresponding other areas are activated, so that a total of a selective parallel readout mode is enabled.

Surprisingly, it has been found that for pyroelectric activation of the polymer layer, extremely low thermal powers are sufficient. That's enough z. B. a -7 heating power of 10 7 watts for a global activation of the whole Memory. The written information, i.e. H. the polarization state of the polymer layer remains also retained unchanged when activated. The remanently polarized polymer layer therefore provides a storage system with high time stability and high recording density For example, there is the problem of «radiation, which is known with Si storage systems, their influence in the form of bit errors is particularly noticeable with high storage density power and requires complex correction procedures in the memory described here not. The memory can be deleted by applying a Alternating field of decreasing amplitude take place.

For pyroelectric activation when reading out, in principle the same electrode matrix can be used that is used to read in the information. Of course, separate conductor tracks can also be used for short-term global or local heating of the polymer layer is provided by means of a heating current pulse will.

Alternatively, the method according to the invention can also be carried out in this way that a dot matrix is used as the electrode system and all polarized Domains of the polymer layer globally by electromagnetic radiation or by one in the electrode system impressed heating current pulse pyroelectrically activated and the induced signals read out in parallel. pre-condition is, however, that all contacts of the dot matrix with individual connections are provided. The evaluation and assignment of the signals read out can then in turn done with a multiplexer. Another way of signal processing at global pyroelectric activation by means of electromagnetic radiation or Joul #. '' of shear heat is that the induced signals by means of time-of-flight analysis read out in parallel and assigned to the storage elements (domains) of the polymer layer will. Has as a radiation source for brief heating of the polymer layer a commercially available flashlight has proven itself.

The polymer used is advantageously polyvinylidene fluoride (PVDF) or copolymers of PVDF or mixtures of PVDF or

Copolymers of PVDF used with other polymers.

PVDF is a product available on the market, in particular for piezoelectric applications in the form of foils is commercially available. For the method according to the invention produces foils with a thickness between 0.1 and 2 Fm preferred.

PVDF films with high isotopy and high degree of orientation of the molecular chains is obtained when PVDF is evaporated from a solution between glass plates to form a film or PVDF solidify from a melt under the action of an electric field leaves or evaporated from a solution. By choosing the process parameters accordingly the important data of the polymer film, such as

the coercive force, the squareness of the hysteresis curve and the pyroelectric constant.

The data memory required for the method according to the invention is characterized by a simple structure and a high level of operational reliability the end. The comb electrode system ensures sufficient selectivity with regard to of the storage elements defined by the intersection points of the strips. Likewise only the addressed storage elements (domains) are addressed when reading out. Interference phenomena between neighboring districts are avoided.

A further development of the data memory is characterized by that several polymer layers with their electrode systems on top of each other like a grid are arranged, optionally with insulating intermediate layers, so that a three-dimensional Memory block is formed.

When using a PVDF data memory with the inventive Method achieves the following advantages: - Since PVDF has a very high coercive field strength of about 600 MV / m, can have a very stable electrical polarization within small volumes are generated, d. H. there can be high storage densities with long Lifetime can be achieved.

- Due to the relatively large pyroelectric coefficient of PVDF Even small changes in temperature are sufficient and thus a very low heating output with pyroelectric activation in order to generate easily measurable readout potentials.

- Due to the small dielectric constant of PVDF, one obtains when reading out by means of pyroelectric activation a high output voltage and hence a good signal-to-noise ratio.

- PVDF can be produced as a thin, resistant film, the data of which is physical can be optimized with regard to the intended use (e.g. by stretching, by tempering, if necessary in an electric field, and by adjusting the layer thickness).

- The information can either be in the form of binary signals (polarization state 0 or 1 or also -1 and +1) or analogously. With analog storage a polarization is generated in each memory cell, the value of which corresponds to the respective Corresponds to the signal amplitude.

In the following the invention will be explained with reference to drawings and exemplary embodiments explained in more detail.

They show: FIG. 1 the reading-in and reading-out principle in one with one Electrode system provided PVDF storage, Fig. 2 the time dependence of the through the readout signal generated the pyroelectric effect.

The PVDF memory according to FIG. 1 consists of a 1 Am thick, largely uniaxially oriented PVDF film 1, the molecular chains of which are largely parallel to Surface are directed. The opposite surfaces of the film 1 are with a Electrode system contacted. The electrode system consists of one electrode matrix each 2 and 3 on the top and bottom of the film 1. The Electrode matrix 2 in turn consists of a large number of parallel strips 21 ... 2n, the electrode matrix 3 from an analog strip system 31 ... 3mm, however, with that the strip 2 2 encloses an angle of 900. The electrode system 2.3 corresponds an X-Y coordinate network in the film plane. All strips are with connectors Mistake. There is a memory cell through a crossing point of the strips 4 defined. When reading in, all crossing points can either be parallel on one electrode addressed or selectively only a few memory areas occupied or changed will.

The strips consist e.g. B. made of gold, aluminum or nickel and are applied directly to the film by vapor deposition, sputtering or photochemical means.

The width of the strip is approx. 1 μm and the spacing between the strips varies between 1 Wm and 1000 jim depending on the desired storage density.

The reading of a signal takes place in such a way that, for. B. to the Electrodes 24 and 33 a DC voltage U is applied. This will make the one at the intersection area 4 (dotted cuboid) lying on electrodes 24/33 is remanently polarized; d. H. the polarization is retained even after the voltage is switched off. the The electric field strength required for reading in is of the order of magnitude MV / cm, d. H. the required voltage U is for a film thickness of 1 μm of the order of magnitude of 100 V. The spatial extent of the area 4 is direct correlated with the stripe width, so that z. B. at a strip width and one Strip spacing of 1 Fm a storage density of 108 bit / cm2 in Binary code is achieved. The binary code is changed by changing the sign of the retentive Polarization, d. H. by reversing the polarity of the voltage U. However, it is too a record based on the polarization states O (no polarization) and 1 (full polarization) possible. In addition, it was found that a quasi-analog Recording by applying variable voltage to the electrode matrices is possible. The individual memory cells become different in this case strongly remanently polarized.

For reading the stored in the form of polarization states Information, the same electrode system 2, 3 is used. However, it is not possible to determine the polarization states of the intersections of the strips 21 ... 2m, 31 ... 3m defined memory cells to be queried directly, since the polarization charges be shielded by free charge carriers (electrons and holes), so that after there is no external electric field. This equilibrium becomes polarized in a Domain disturbed by brief heating, this results in the relevant electrodes a potential different from 0. This effect is used for reading out. the thermal activation of a memory cell in a ferroelectric recording medium is called pyroelectric activation.

A sufficient pyroelectric activation can be obtained with a few degrees C temperature increase. For this purpose, as indicated in Fig. 1, the strip 33 briefly applied a heating current pulse (heating circuit 5) so that by the length of the strip generated electrical power the entire zone located above this strip in the PVDF film is heated. Here, the Joule heating power is applied to the film thickness and the distance between the Electrodes matched so that only the addressed areas are significant be heated. At the crossing points of the strip 33 with the strips 21 to 2 then the signals correlated with n of the heating current pulse in the relevant Induced pairs of electrodes that can be registered in parallel. In Fig. 1 is for the sake of clarity, only for those between electrodes 24 and 33 lying memory cell 4, an evaluation circuit 6 is drawn.

After the information that is generated in parallel has been sent to the central processing system 6 has been supplied, is switched to the strip 32 by means of a multiplexer 7, so that then the n-bits stored in the intersection points of 32 with 21 to 2n can be queried.

To clarify the pyroelectric readout is in the upper part of Fig. 2 the heating current pulse (pulse duration tH) and in the lower part the one caused by it Voltage (e.g. at memory cell 4) shown as a function of time. In our In the example, a temperature increase of approx. 30 ° C. was achieved with tH = 1 10 6 sec and voltages of approx. 1 V are observed. The registered rise time is in the 10th 9 seconds range, so that the resulting from this time and the multiplexer time Access time is also in the 10 9 seconds range.

For pyroelectric activation by means of a heating current pulse the same electrode system was used here that was used to query the stored information serves. Of course, the heating can also take place by separately applied conductor tracks. In this case the PVDF contains data storage In addition to the electrode matrices 2 and 3, a separate heating wire system.

Instead of a strip system 2, 3, a dot matrix can also be used individual connections can be vapor-deposited onto the PVDF surface.

Another method of thermoelectric activation is to that the activation energy is not generated by Joule heat, but by electromagnetic radiation. In practice, one proceeds in such a way that one takes the entire Memory area illuminated with a flash and the stored signals parallel In this case, if a PVDF data memory with a dot matrix is used as a Electrode system used, the memory cells can be with the help of the multiplexer be addressed selectively.

When the data memory is designed with a strip matrix as Electrode system is a direct assignment of the read out signals to the individual Storage elements not possible. An assignment of the information content to the individual Memory cells can be achieved if the triggered signal pulses one Be subjected to runtime analysis.

In the case of excitation with light, in addition to thermal excitation, direct photoelectric excitation due to an internal photo effect can also take place. However, it has not yet been clarified to what extent such an effect plays a technically decisive role.

Claims (8)

Claims: Method for reading in and reading out binary or analog electrical signals using a data memory based on a electrically polarizable polymer layer contacted with an electrode system, the signals to be recorded in local domains during the reading process is remanently polarized and activated pyroelectrically during the readout process, characterized in that the pyroelectric activation with or immediately takes place before the readout process in such a way that simultaneously a large number of the polarized Domains in the electrode system induce signals that are then parallel and / or after assigned to a multiplex method and queried. 2. The method according to claim 1, characterized in that the one assigned to a single conductor track of the electrode system designed as a strip matrix polarized domains of the polymer layer simultaneously through one in this conductor track flowing heating current pulse are activated pyroelectrically, so that when switching through this conductor track a selectively parallel readout mode by means of a multiplex method is achieved. 3. The method according to claim 1, characterized in that the electrode system a dot matrix is used and all polarized domains of the polymer layer globally by electromagnetic radiation or by a heating current pulse in one conductive layer or strip conductor on the opposite of the dot matrix Side of the polymer layer is attached, pyroelectrically activated and the induced Signals are read out in parallel. 4. The method according to claim 1, characterized in that the electrode system a strip matrix is used and that the polymer layer is globally controlled by electromagnetic Radiation is activated pyroelectrically and the induced signals by means of time-of-flight analysis can be read out in parallel. 5. The method according to claim 1 to 4, characterized in that a Polymer layer made of polyvinylidene fluoride (PVDF) or copolymers of PVDF or from blends of PVDF or copolymers of PVDF with other suitable polymers is used. 6. The method according to claim 1 to 5, characterized in that polymer layers with a thickness of I 10 μm, preferably with a thickness between 0.1 and 2 μm will. 7. The method according to claim 1 to 6, characterized in that the electrical signals read into the polymer layer by applying the Electrode system can be deleted with an alternating voltage of decreasing amplitude. 8. The method according to claim 1 to 7, characterized in that several electrically polarizable polymer layers with their electrode systems one on top of the other arranged and assembled into a three-dimensional memory block.