FR2659780A1 - Optically read memory based on a ferroelectric material - Google Patents

Abstract

The invention relates to an electrically written and optically read memory based on a ferroelectric material, comprising at least two superposed layers (3, 5) of ferroelectric material, each memory element being formed by superposed areas of the said layers, the areas of one memory element being arranged in such a way that a light beam for reading may cross them, the memory also comprising means for applying electric fields (2, 4, 6) enabling the areas of each memory element to be placed in specified states of polarisation, the light beam for reading generating, on crossing a memory element, a wave of second harmonic, the amplitude of which depends on the states of polarisation of said areas.

Description

MOIRE BASED ON MATERIAL
FERRO # EL # CTRIQUE AND OPTICAL READING
The present invention relates to memories based on ferroelectric material and optical reading. It relates more particularly to programmable memories.

There are various ways of making memories from ferroelectric materials, for example from US 2,922,986, US 3,710,353, US 3,264,618, US Pat.
EP 166,938, US 3,872,318, GB 890,780, FR 2,604,283 and "A
Ferroelectric Nonvolatile Memory "(S. EATON, D. BUTLER, M.

PARRIS, D. WILSON, H. McNEILLIE, 1988, IEEE, International Solid-State Circuits Conference). All these documents concern memories based on ferroelectric materials such as BaTiO3 barium titanate, PZT (lead oxide, zirconium and titanium oxide) ceramics and PLZT (lead, lanthanum, zirconium and titanium oxide). PVDF polymer (vinyldene polyfluoride) or copolymer P (VDF-TrFE) (vinylidene fluoride / trifluoroethylene). These documents also relate to methods for reading these memories, by pyroelectric, optical or capacitive effect.

The optical reading by second harmonic generation encounters a major difficulty: the ferroelectric states -P and + PO (representative of the states of
o remanent polarization) can not be dissociated from each other on the same ferroelectric layer because they generate the same intensity of second harmonic. There is therefore only a reading contrast between states of polarization P = O and P = + P or -P0, which requires a system
o erase or reset (P = 0) for use in memory type WRM (Write Read Many).

 To overcome these drawbacks, the present invention relates to a memory based on ferroelectric material and formed of superimposed layers of this material, a memory element being formed of superimposed areas of these layers. The information will be read from the modulation of the second harmonic generation intensity resulting from these layers as a function of the ratio of their thickness to the coherence length, and the orientation of the polarization induced with respect to the incident optical wave.

 An important advantage of the invention lies in the fact that it increases the information density that can be stored in this type of memory due to the use of a multilayer system.

 Each memory element is programmable, erasable and has a number of levels adjustable according to the number of layers (reasonably from 2 to 6), the number of polarization values used (reasonably from 2 to 12) and the arrangement of directions polarization between them (parallel or antiparallel).

 In addition, the principle of optical reading makes it possible to increase the reading speed of the memory by using the parallelism of the optics (all the memory can be read simultaneously).

 Each pixel zone (a few, umZ of area) can be reprogrammed by resetting (parallel polarization) or leveling 1 by applying a reverse voltage to obtain antiparallel polarization. This operation can be done in 1, us at room temperature for maximum electric fields of 106 V / cm or 100 volts for 1 um.

 The invention therefore relates to a memory based on ferroelectric material with electrical inscription and optical reading, characterized in that it comprises at least two superposed layers of ferroelectric material, each memory element being formed of superimposed zones of said layers, the zones of the same memory element being arranged in such a way that a reading light beam can pass through them, the memory also comprising means for applying electric fields making it possible to put the zones of each element of memory in states of determined polarization, the reading light beam generating, through a memory element, a second harmonic wave whose amplitude is a function of the polarization states of said zones.

The invention will be better understood and other advantages will become apparent from the following description, given in a non-limiting manner, accompanied by the appended drawings among which:
FIG. 1 is a diagram giving the intensity of a second harmonic signal generated in an electro-optical material as a function of the interaction length,
FIGS. 2 and 3 schematically represent electro-optical material elements,
FIG. 4 represents in section a portion of memory according to the invention,
FIG. 5 is a diagram representing a hysteresis cycle,
FIG. 6 represents a device for reading a memory according to the invention,
FIG. 7 represents a variant of memory according to the invention,
FIG. 8 is a diagram representing a hysteresis cycle,
FIG. 9 represents a memory according to the invention, in the form of a disk.

1 étant appelée longueur de cohérence. La longueur de
c cohérence correspond à la relation

avec ## : longueur d'onde de l'onde électromagnétique
a w w incidente, n indice du matériau à la pulsation 2 # et n# indice du matériau à la pulsation w (pulsation de l'onde incidente).Cette relation est due au désaccord de phase entre l'onde de pulsation w et l'onde de pulsation 2z qui se propagent dans le matériau, l'indice à la pulsation Lt > ) n'étant pas égal à l'indice à la pulsation 2 w (n ).Materials with a non-centrosymmetric structure can generate a second harmonic wave in response to
aw an incident electromagnetic wave. The intensity I of this second harmonic wave is a function of the square of the interaction length 1, and therefore of the thickness of the electro-optical material traversed by the wave, according to the relation

1 being called coherence length. The length of
c consistency is the relationship

with ##: wavelength of the electromagnetic wave
aww incident, n index of the material at the pulsation 2 # and n # index of the material at the pulsation w (pulsation of the incident wave) .This relation is due to the phase mismatch between the pulse wave w and the wave of pulsation 2z propagating in the material, the index at the pulsation Lt>) not being equal to the index at the pulsation 2 w (n).

In the case of the copolymer P (VDF-TrPE) of 70/30 molar proportions (70% of VDF for 30% of TrFE) and for a wavelength λ = 1.06 m, the difference
w of index n - n is 0.007 and the coherence length 1
c is about 30 μm.

Figure 1 is a diagram showing the intensity
2 tex
I of the second harmonic signal generated in an electro-optical material as a function of the interaction length 1. The curve C1 has been plotted for # k = 0 and the curve C2 for # k # 0 (~ k = k kw, ku; k # and k2 # being the wave vectors respectively for the pulsations W and 2 #). The curve C1 has a parabolic appearance while the curve C2 has a sinusoidal shape. The curve C2 has maximum values for interaction lengths equal to the coherence length Ic or odd multiples of 1c. It has null values for multiple pairs of Ics.

c
A simple explanation of the C2 curve is that the element of ferroelectric and electro-optical material in which the second harmonic wave propagates can be subdivided into slices of thickness 1.

c
Figures 2 and 3 schematically represent electrooptical material elements of thickness 2 lc. A wave
The electromagnetic c intensity I @ incident to these elements generates a wave of second harmonic and intensity I2 #. If the two slices are polarized in the same direction (SPO and + PO) as shown in FIG. 2, the second harmonic signal produced by the second slice is in phase opposition with that produced by the first slice and the second slice. second harmonic at the output of the second slice has a zero amplitude.If the two slices are polarized in the opposite direction (+ PO and - PO) as shown in Figure 3, the signals produced by these slices are in phase and there is emission of a second harmonic signal I2 w
K (in ') K being a coefficient of proportionality.

 In the case of a ferroelectric material such as P (VDF-TrEE) of 70/30 molar proportions, the measured coherence length is substantially equal to 30 Cun. This length can be reduced to a few Cun by doping the polymer with 1 to 10% of dyes known to those skilled in the art.

This doping modifies the absorption band of the polymer in the visible range which can be adjusted in wavelength by the choice of the dye.

It is thus possible to obtain a memory system according to the polarization states of the different slices. The operating principle of such a memory is identical regardless of the thickness of the polymer layers provided that all the layers have the same thickness and that this thickness is less than or equal to the coherence length 1
c
In practice, if the coherence lengths are of the order of a few tens of pin, it is quite possible to use layers of 1 Cun thick.

 FIG. 4 represents in section a portion of memory according to the invention, formed of two layers of ferroelectric material. We will consider the case where the memory is read in transmission and in this case all its components are transparent to the reading beam. The memory comprises a static substrate 1 of glass for example on which column electrodes 2 have been deposited in mixed indium tin oxide (ITO). To obtain a first layer 3 of ferroelectric material, a ferroelectric polymer film was then deposited.

It can be P (VDF-TrFE) 70/30 spin-on. This layer 3 is then covered with an array of lines 4 ITO electrodes and a second layer 5 of the same ferroelectric polymer. The layer 5 is then covered with a uniform electrode 6 in ITO. Intersections of rows and columns define the memory elements according to a matrix system.

The two ferroelectric layers 3 and 5 are of thickness equal to the coherence length lc. Layer 5
c is first polarized uniformly and once and for all between the electrodes 4 and 6 by applying a coercive electric field of about 80 V / um. This state of polarization, for example Po, is represented in FIG. 4 by arrows. All areas of layer 5 are polarized in the same direction.

 For each image element, the states O and 1 are obtained by applying + or - the coercive voltage Vcoerc between the row electrodes 4 and the column electrodes 2.

For the inscription of the layer 3, the following type of addressing can be used in connection with FIG. 5 which represents a hysteresis cycle for a ferroelectric material. First, the entire layer 3 can be put in the state of polarization -P by applying a voltage
o Vl on the electrodes lines 4 and a voltage + Vc on the column electrodes 2. The layer 3 is then subjected to a voltage V1 - Vc, which corresponds to the point A on the hysteresis cycle of FIG. the state of polarization -P. In this way the whole memory is
o put in the memory state 0.

To put some memory elements in state 1, the registration is done line by line. Apply + V1 on the line to be written, -V c on the columns corresponding to the memory elements to put in the memory state 1, + V on
c the columns corresponding to the memory elements to be kept in the memory state 0. Thus, for the line to be written, the zones of the layer 3 subjected to a voltage V + V are
c in the state of polarization + P (point B of the cycle
hysteresis) and the corresponding memory element will be in the memory state 1.Always for this line to be written, the zones of the layer 3 subjected to a voltage V1-V are
c in the state of polarization -P (point A of the cycle
hysteresis) and the corresponding memory element will be in the memory state 0. During this time, the other row electrodes are set to potential 0 so that the corresponding areas of the layer 3 are subjected to + or - V c (points C and D of the hysteresis cycle) which does not change their state of polarization (about -PO)
FIG. 4 shows three memory elements in the memory state 1 and four memory elements in the memory state 0. In response to an optical reading signal I #, the elements in the memory state 1 generate a signal I2 # and the elements in the memory state 0 generate no signal 12w
FIG. 6 represents a device for reading a memory according to the invention with a single row of memory elements.

This device comprises a bar 10 of lasers capable of emitting pulsed light beams w and a strip li of pulsed light signal detectors 2 w. The memory, which has been designated as reference 12, is placed between the bars 10 and 11. Memory 12 only shows the substrate 1 and the ferroelectric layers 3 and 5. The memory is inclined. an angle 0 relative to the normal reading beams. This angle 9 is advantageously set between 500 and 600 to obtain a maximum detected signal I u).

An optical fiber network 13 distributes the optical read signals to the memory elements. An optical fiber network 14 collects, via microlenses 15, the optical signals generated by the memory elements towards the detector array 11.

 A more sophisticated reading system is to use a single-row array of lasers and move line by line (or column by column) so as to read a 2-dimensional memory. The bar with a single row of corresponding detectors moves synchronously with the movement of the laser array.

 Another system consists in using an N x M laser matrix and an N x M detector array in order to read a two-dimensional memory without displacement.

 By associating n ferroelectric polymer layers with a suitable number of crossed and isolated electrode arrays, a ferroelectric memory can be made wherein each memory element has a number of programmable levels depending on the number of layers and the polarization values used. The polymer layers used may be from 1 to 2 μm thick.

 The memory of Figure 4 included ferroelectric layers of thickness equal to the coherence length.

The choice of this thickness was justified by the reason that one can thus simply distinguish between a signal of maximum amplitude for 1 = the (see Figure 1) and a zero amplitude signal. On the other hand, if the ferroelectric layers have thicknesses less than the coherence length, a plurality of values can be obtained by superposition of layers for the amplitude I2 of the generated beam going from the value 0 to the value corresponding to l = 1.
c
FIG. 7 represents a memory according to the invention seen in section and comprising 4 ferroelectric films. It comprises a substrate 20 transparent to the reading optical beam on which are deposited four ferroelectric polymer layers referenced 21, 22, 23 and 24. Each layer ferroelectric is between rows and columns of electrodes transparent to the reading optical beam.

Thus the layer 21 is framed by arrays of line electrodes 25 and column electrodes 26; the layer 22 is framed by rows of electrode electrodes 27 and column electrodes 28; the layer 23 is framed by arrays of line electrodes 29 and column electrodes 30; the layer 24 is framed by arrays of row electrodes 31 and column electrodes 32. Dielectric layers 33, 34 and 35 transparent to the reading beam are interposed between the column electrodes of a ferroelectric polymer layer and the row electrodes of the next ferroelectric layer to avoid short circuits between these electrodes.

A laser 40 delivers a reading light beam 41 made parallel through the lens 42. The beam 41 is inclined at an angle θ with respect to the normal to the layers, this angle S being chosen so as to obtain a signal generated by maximum amplitude. Figure 7 shows the light beam 42 passing through a memory element. The row electrodes corresponding to the same memory element are therefore offset from one another in relation to the angle e
The thickness of the ferroelectric polymer layers may be between 1 and 5 μm, which is therefore sufficiently small relative to the coherence length. These thicknesses correspond to the parabolic progression of the beginning of the diagram of FIG. 1. The total thickness of the ferroelectric layers is at most equal to the length of coherence.

The thickness of the dielectric layers 33, 34 and 35 may be between 0.1 and 0.2 μm.

 The invention also makes it possible to take account of intermediate polarization states, for example + Po / 2 and -Po / 2.

It is thus possible to obtain four polarization states from two ferroelectric polymer layers, namely the polarization states + P0, -POJ + P / 2 and -P0 / 2. Yes
o
I is the amplitude of the light beam that can be generated in a layer polarized at Po, it will be possible to collect beams generated amplitudes proportional to 0, 1/4 12w, 9/4 I2> 7 and 4 î2W. This is summarized in Table 1 at the end of the description. This gives four levels of coding. The inscription can be carried out in the following manner in relation to FIG. 8 which represents a hysteresis cycle. One of the two layers can be placed in the polarization state + P and all the zones of this
o layer will remain in this state. It is the polarization of the second layer that will determine the final memory states.

The inscription of the second layer can be performed as follows. First the whole of the second layer can be put in the state of polarization -P by applying a voltage +
o
V on the electrodes lines and a voltage + V on the
1 c column electrodes corresponding to the second layer. This is then subjected to a voltage V1-Vc, which corresponds to point A on the hysteresis cycle of FIG. 8, that is practically the state of polarization -P. Like this
o the entire memory is put in the memory state 0. From a macroscopic point of view, the polarization of the memory is zero. Then proceed line by line. Suppose we want to write on line i the following polarization states: SPOJ -POJ + Po / 2 and -Po / 2. We must then apply + V on the line i, -V on the columns
1 c corresponding to the zones of the second layer to put in the state of polarization + P (point B of the diagram), + V on
oc the columns corresponding to the areas of the second layer to leave in the state of polarization Po, -V 'on the columns corresponding to the zones of the second layer to put in the state of polarization + Po / 2 (point E of the diagram ), + V 'on the columns corresponding to the zones of the second layer to put in the state of polarization -P0 / 2 (point F of the diagram). During this time, the other line electrodes can be subjected to the potential 0 so that the corresponding zones of the second layer are subjected to + or Ve (points C and D of the diagram) or to + or - V '(points G and
H of the diagram) which does not change their state of polarization (about -PO).

 Five polarization states can also be obtained from two ferroelectric polymer layers, ie the polarization states + Po, -P0, + Po / 2, - Po / 2 and 0. If I2 rg is the amplitude of the light beam It can be generated in a polarized layer at P, it will be possible to collect beams generated amplitudes proportional to 0 1/4 I2n>, 12w, 9/4 12w and 4 I2 #. This is summarized in Table 2 at the end of the description. We obtain five levels of codings. The inscription can be done as for the previous case including four levels of coding, except to envisage to apply a tension null on the column corresponding to the zones of the second layer to put in the state polarization 0.

 Five polarization states can still be obtained from three layers of ferroelectric polymer, ie the polarization states + Po, + Po / 2, -Po / 2, Po and 0. If I2 # is the amplitude of the light beam that is susceptible to to be generated in a polarized layer at Po, it will be possible to collect beams generated of amplitudes proportional to 0, 1/4 I2 #, I2 #, 9/4 I2 #, 4 I20, 25/4 I2 # and 9 I2W. This is summarized in Table 3 at the end of the description. Seven levels of coding are thus obtained. The preceding examples give all the indications to obtain the five necessary states of polarization.

 It is within the scope of the present invention to provide a larger number of ferroelectric layers.

It is thus possible to obtain a 4-bit coding (16 levels) with an 8-layer system.

 Figure 9 illustrates another variant of the invention. This is a ferroelectric memory on a mobile disk medium. The memory comprises a disk 50 made of insulating material and transparent to the reading beam. It supports a transparent electrode 51 on which has been formed the layer 52 of ferroelectric polymer. The layer 52 is itself covered with a uniform and transparent electrode 53 on which the ferroelectric polymer layer 54 has been formed. The layer 52 is polarized uniformly and once and for all in the manufacture of the disk by application of an electric field greater than the coercive electric field. During manufacture, it has been ensured that the electrode 51 can not be brought into contact with the metal centering axis 55 of the disk on its marking device. On the other hand, the electrode 53 is made in such a way that it is in electrical contact with the axis 55. Thus the electrode 53 can be put at the potential of the mass, at the time of the inscription, thanks to the axis 55. This inscription can be achieved by polarization of zones of the layer 54 with a Corona tip 56 device.

The layers 52 and 54 are of thickness equal to the coherence length 1c If the polarization of an area of the layer 54 takes place in the same direction as that of the polarization of the layer 52, the corresponding memory element is in the memory state 0 (case of Figure 2). In the opposite case, the memory element is in the memory state 1 (case of FIG. 3). The memory is read by the laser 57 emitting a reading light beam at an angle 8 with the normal disk. The second harmonic beam generated by the memory is detected by the photodetector 58 after passage through the filter 59. The state of the memory is revealed by the intensity at 2w generated at the point in question.

The invention can also be applied to a ferroelectric polymer neural network that can be coded on several levels by electrical addressing and can be read by a second harmonic generation optical method.

<tb><SEP> 2W
<tb> Layer <SEP> 1 <SEP> 2 <SEP> I
<tb> Status <SEP> of <SEP> + <SEP> Po <SEP> + <SEP> Po <SEP> 4 <SEP> I2 # <SEP>
<tb> polarization
<tb> Status <SEP> of <SEP> + <SEP> P <SEP> - <SEP> P <SEP> o
<tb><SEP> o <SEP> o
<tb> polarization
<tb> Status <SEP> of <SEP> + <SEP> Po <SEP> + <SEP> Po / 2 <SEP> 9/4 <SEP> I2 # <SEP>
<tb><SEP>
<tb> polarization
<tb> Status <SEP> of <SEP> + <SEP> PO <SEP><SEP> -Po / 2 <SEP> 1/4 <SEP><SEP> I2 # <SEP>
<tb><SEP>
<tb> polarization
<Tb>

TABLE I

<tb> Layer <SEP> 1 <SEP> 2 <SEP> I
<tb> Status <SEP> of <SEP> + <SEP> P <SEP> ≈ <SEP> P <SEP> 4 <SEP> 12w
<tb><SEP> o <SEP> o
<tb> polarization
<tb> Status <SEP> of <SEP> + <SEP> p <SEP> - <SEP> PO <SEP> O
<tb><SEP> o <SEP> o
<tb> polarization
<tb> Status <SEP> of <SEP> + <SEP> Po <SEP> + <SEP> Po / 2 <SEP><SEP> 9/4 <SEP> I2 # <SEP>
<tb> polarization
<tb> Status <SEP> of <SEP> + <SEP> Po <SEP> Po / 2 <SEP> 1/4 <SEP> I2 # <SEP>
<tb> polarization
<tb> Status <SEP> of <SEP> + <SEP> Po <SEP> 0 <SEP><SEP> I2 # <SEP>
<tb> polarization
<Tb>
TABLE II

<tb> Layer <SEP> 1 <SEP> 2 <SEP> 3 <SEP> I2 # <SEP>
<tb> Status <SEP> of <SEP> + <SEP> PO <SEP> Po <SEP> + <SEP> P <SEP> 9 <SEP> I2w
<tb><SEP> o <SEP> o <SEP> o
<tb> polarization
<tb> Status <SEP> of <SEP> + <SEP> Po <SEP> + <SEP> Po <SEP> 0 <SEP> 4 <SEP> I2 # <SEP>
<tb> polarization
<tb> Status <SEP> of <SEP> + <SEP> Po <SEP> + <SEP> Po <SEP> - <SEP> Po <SEP> I2Q <SEP>
<tb> polarization
<tb> Status <SEP> of <SEP> + <SEP> Po <SEP> P <SEP> + <SEP> Po / 2 <SEP> 25/4 <SEP> I2 # <SEP>
<tb> polarization
<tb> Status <SEP> of <SEP> + <SEP> Po <SEP> + <SEP> Po <SEP> -Po / 2 <SEP> 9/4 <SEP> I2 # <SEP>
<tb> polarization
<tb> Status <SEP> of <SEP> + <SEP> P <SEP> / 2 <SEP><SEP> 0 <SEP> 0 <SEP> 1/4 <SEP> I2 @ <SEP>
<tb> polarization
<tb> Status <SEP> of <SEP> o <SEP><SEP> 0 <SEP> 0 <SEP> 0
<tb> polarization
<Tb>
TABLE III

Claims (7)

 1 - memory based on ferroelectric material with electrical inscription and optical reading, characterized in that it comprises at least two superposed layers of ferroelectric material, each memory element being formed of superimposed zones of said layers, the zones of the same memory element being arranged so that a reading light beam can pass through them, the memory also comprising means for applying electric fields making it possible to put the zones of each memory element in determined states of polarization, the light beam reading reader generating, through a memory element, a second harmonic wave whose amplitude is a function of the polarization states of said zones.  2 - memory according to claim 1, characterized in that said ferroelectric material is a polymeric material.  3 - memory according to one of claims 1 or 2, characterized in that the reading light beam is incident at an angle determined with respect to the normal to said layers of ferroelectric material, this angle (0) corresponding to a second wave harmonic of maximum amplitude.  4 - memory according to any one of claims 1 to 3, characterized in that the thickness of said ferroelectric layers is equal to their coherence length for the wavelength of said reading light beam.  5 - memory according to claim 4, characterized in that it comprises two ferroelectric layers (3, 5), one of these layers (5) being uniformly polarized in a first state of polarization, the zones of the other layer (3) which can be polarized by means of a matrix system of row electrodes (4) and columns (2) at states of polarization identical to said first state or to states of polarization of equal value but of opposite sign to said first state.  6 - memory according to claim 4, characterized in that it comprises two ferroelectric layers (52, 54) placed on a support (50) in the form of a disc, one of these layers (52) being uniformly polarized in a first polarization state, the zones of the other layer (54) being polarizable by means of a Corona tip device (56) with states of polarization identical to said first state or states of polarization of equal value but of sign opposite to said first state.  7 - memory according to any one of claims 1 to 3, characterized in that the total thickness of the ferroelectric layers (21, 22, 23, 24) is less than or equal to the coherence length for the wavelength of said reading light beam, the memory comprising at least one matrix system of line electrodes (25, 27, 29, 31) and columns (26, 28, 30, 32) making it possible to put the zones of each memory element in states determined polarization.