Process for reading-out information from electrically polarizable data carriers by means of electron beams

The information recorded in the data carrier in the form of a locally variable electric polarization is scanned and selected by means of an electron beam. For this purpose, the secondary electrons produced on the surface of the data carrier are used. The data carrier is simultaneously either periodically heated by radiating electromagnetic waves or charged with ultrasonics. Potential fluctuations of equal frequency thereby arise on the surface of the data carrier dependent on local polarization, which fluctuations result in a modulation of the secondary electrons. The secondary electron flow thus receives information via the polarization conditions stored in the data carrier. To recover this information, the secondary electron flow is frequency-selectively amplified and electronically evaluated according to amount and/or phase. A polyvinylidene flouride film (PVDF) is preferably used as data carrier.

BACKGROUND OF THE INVENTION 
This invention relates to a process for reading-out information from a 
layered residually or remanent electrically polarized data carrier, 
corresponding to the recorded information in local domains by scanning the 
polarized domains by means of electron beams. 
A substantial target of development in the area of data processing are 
information memories with high data capacity. Stationary electrically 
erasable memories on a silicon base with memory capacities of 10.sup.6 
bit/cm.sup.2 and movable magnetically erasable memories (such as a band, 
plate, or drum) of 10.sup.7 bit/cm.sup.2 are presently commercially in 
use. However, memories with substantially higher storage capacities than 
10.sup.8 bit/cm.sup.2 are desired and in great demand. 
In addition to the conventional, widespread semi-conductor memories and 
magnetic memories, different processes, based on other physical 
principles, for recording and selecting information from data memories 
have already been examined and described. Thus, with multi-dimensional 
storage by means of laser holography or by means of photochemical 
hole-burning, the information is entered into the storage medium by a 
laser beam. Reading-out also takes place by a laser beam. A further 
direction of development is concerned with the use of electron beams for 
reading in and reading out. The electron beam has the advantage compared 
with the laser of a substantially lower beam diameter. A higher local 
dispersion and thus a higher storage capacity is thereby achieved. With 
this method, a non-erasable and read only memory has been recently 
produced by burning minute holes in aluminum foil. 
As an analogy to the magnetic memory which is based on the magnetization of 
ferromagnetic domains, electrically polarizable media are already examined 
early on for suitability as solid-state memories. In U.S. Pat. No. 
2,698,928, several basic processes for reading in and reading out 
information in or from residually electrically polarizable data carriers 
are described. The reading-in or writing takes place on a residually 
electrically polarizable data carrier which is provided with an 
electrically conductive base layer which is passed by an electrode which 
is charged with an electric potential corresponding to the information to 
be recorded. Alternatively, the information can also be entered into the 
polarizable medium by means of electron beams. An electron beam can also 
be used according to U.S. Pat. No. 2,698,928 for reading-out. Such a high 
field strength is produced in the polarized medium by the electron beam, 
that the polarized domains are reversed in polarity. During the reversing 
of the polarity, electric potentials are produced across electrodes, which 
potentials are detected as a read-out signal. In addition, the polarized 
data carrier can be charged with ultrasonic waves. The piezoelectric 
signal stored on the data carrier is then modulated with the ultrasonic 
frequency of the waves. In this case, the signal-to-noise ratio can be 
improved by frequency selective amplifiers. 
This method of reading-out suffers from the disadvantage that the original 
polarization state and thus also the recorded information is erased by the 
relatively high field strength arising from the electron beam. 
Furthermore, the sensitivity leaves a lot to be desired; that is, it is 
difficult to achieve a sufficiently high signal-to-noise ratio. 
A read-out process based on the principle of electron beam expansion is 
described in U.S. Pat. No. 4,059,827. The data carrier here is a 
ferroelectric polyvinylidene fluoride film. The read-out signal is 
produced by the metallized back layer of the data carrier being maintained 
at such a high negative potential, that the electron beam is diverted onto 
a grid-shaped collector electrode. If the data carrier is now electrically 
polarized corresponding to the recorded information, then the deflected 
beam is moved to the side or expanded, depending on the extent of the 
polarization charges. The information recorded in the form of a residual 
electric polarization can then be recovered again on the collector 
electrode in the form of an electric potential. This process suffers from 
the disadvantage that the data carrier must remain under high vacuum after 
entering the information and until read-out, since in air, a compensation 
of the polarization charges takes place immediately and thus the read-out 
is rendered more difficult or even impossible. Moreover, the strength of 
the read-out signal is greatly dependent on the geometry of the electrode, 
such that the spacial arrangement of the electrodes and the electron beam 
guns is very critical. This demands high precision when guiding the data 
carrier and in the arrangement of the read-out head. For these reasons, a 
high susceptance to disturbance is to be expected, especially when the 
process is optimized with respect to high sensitivity and high lateral 
resolution. 
SUMMARY OF THE INVENTION 
The object of the present invention is to improve the electron beam 
read-out process of an electrically polarizable data carrier with respect 
to the lateral resolution and the read-out speed. A resolution of 
.ltoreq.1 .mu.m and a data transfer rate of .gtoreq.10.sup.6 bit/sec 
should particularly be achieved during read-out. Moreover, a high 
signal-to-noise ratio should be guaranteed in this process from the start. 
The object is achieved according to the invention by the following stages: 
(a) The areas to be selected are periodically (activation frequency W) 
heated by radiating electromagnetic waves or are changed with ultrasonic 
waves, 
(b) The secondary electrons reflected or scattered on the read-out areas 
are detected by a multiplier, 
(c) The alternating voltage occurring on the multiplier is 
frequency-selectively amplified and evaluated according to the rate and/or 
phase position in relation to the activation frequency w. 
By radiating electromagnetic waves, for example infrared light, which are 
modulated with the frequency w, the data carrier is heated on the surface 
according to the wavelength of the modulation frequency. The wavelength is 
specifically selected in the region of an absorption band of the data 
carrier material, such that the depth of penetration of the radiation is 
low and heating only takes place on the surface. The light source used for 
heating, for example a solid state laser with high frequency modulation is 
modulated with a frequency of &lt;100 MHz. Potential fluctuations of equal 
frequency are thereby produced on the surface of the read-out areas by the 
heating and depending on the local polarization, which fluctuations result 
in a modulation of the secondary electrons and the resulting secondary 
electron flow thus contains information about the polarization conditions 
stored in the data carrier. With this method of read-out, a pyroelectric 
activation of the polarization domains takes place. 
Alternatively, the polarized domains can also be piezoelectrically 
activated. For this purpose, the data carrier is specifically charged with 
ultrasonic waves via a piezoelectrically active layer applied thereon. The 
ultrasonic frequency is thereby advantageously in a range of from 0.5 to 5 
GHz, preferably from 0.5 to 2 GHz. 
The secondary electrons which are important for the read-out process are 
produced by a primary electron beam aimed at the data carrier with an 
energy of a few kV. 
It has been discovered that the secondary electron detection represents a 
highly sensitive flow which allows high frequency potential fluctuations 
of a few mV to be established on the surface of the data carrier. Owing to 
the frequency selective amplification, only the high frequency modulated 
part of the secondary electrons is amplified and indicated with a 
favorable signal-to-noise ratio. The local dispersion during read-out is 
determined by the cross section of the primary electron beam. With optimal 
adjustment, a lateral resolution of .ltoreq.1 .mu.m can be achieved. In 
principal, an assumption is thereby made that with polarizable data 
carriers, information with a storage capacity of .gtoreq.10.sup.8 
bit/cm.sup.2 can be read-out. 
Further important parameters of a data memory are then access time, the 
data transfer rate and the life of the stored information. In order to be 
able to achieve the required high data transfer rate without problems, the 
modulation frequency must be at least greater, by an order of magnitude, 
than the data transfer rate. With a data transfer rate of 10.sup.6 
bit/sec, this is safely insured with an ultrasonic frequency of 1 GHz and 
with a laser modulation frequency of 10 MHz. 
In relation to the known methods of electron beam readout, which lead to a 
reverse in the polarity of the polarized areas (see for example U.S. Pat. 
No. 2,698,928), the process according to the invention has the advantage 
that the alternating effect of the electron beam with the data carrier is 
so low that the polarization condition of the data carrier cannot be 
changed even with repeated read-out. The read-out process is always 
reversible. 
The invention is described in more detail below by means of embodiments and 
drawings.

DETAILED DESCRIPTION OF THE INVENTION 
According to FIG. 1, the layered data carrier 1 is passed by a fixed 
reading head 2, 3, 4. The layered data carrier consists of a suitable 
ferroelectric film, several .mu.m in thickness, for example, a 
polyvinylidene fluoride film (PVDF). The reading head substantially 
consists of an IR laser 2, an electron beam gun 3 and a multiplier 4 for 
detecting secondary electrons, these elements all being arranged on the 
same side. 
The PVDF layer 1 contains the stored information in the form of time-stable 
electrically polarized domains. The writing takes place in known manner by 
the signal field being impressed on the moving data carrier by means of a 
tip or by means of an electron beam (writing with a tip see U.S. Pat. No. 
2,698,928 and H. Niitsuma and R. Sato, Ferroelectric Recording using the 
pyroelectric Reproduction Technique, Ferroelectrics, 34 (1981), pages 
37-45). The entered residual polarization is, however, screened by free 
charge carriers (electrons and holes), such that if necessary, a very low 
electric field remains on the surface of the PVDF layer. For this reason, 
a rapid disturbance of the system must be carried out, which temporarily 
removes the screening action of the compensation-carrying charges and 
thereby produces a verifiable signal field. For this purpose, the PVDF 
layer is pyroelectrically activated during the read-out process. The 
pyroelectric activation here takes place by a laser beam (laser 2), which 
meets the surface of the data carrier 1 in a spacially restricted area. 
Since the spacial (lateral) resolution of the process is defined alone by 
the electron beam, the pyroelectric activation can take place on a 
relatively wide surface of the data carrier. The laser is modulated with a 
high frequency (high frequency generator 5). The surface of the data 
carrier 1 is heated by the laser beam according to the wavelength of the 
modulation frequency w. The signal field arising hereby is detected by a 
special electron probe which is described in more detail below. The 
electron probe consists of an electron gun 3 and produces a primary 
electron beam with an energy of from 1.5 to 2.5 kV which is aimed at the 
surface of the PVDF layer. If the primary electron beam meets a domain 8, 
which, as described above, is pyroelectrically activated, then the primary 
electon beam experiences an alternating effect with the surface charges 
produced by the activation in the rhythm of the modulation frequency w. 
With a collector 4 which is arranged in the vicinity of the PVDF layer 1, 
scattered or secondary electrons can be detected which are scattered or 
emitted on the surface. The collector 4, which in practice consists of a 
multiplier, is connected to a highly-sensitive narrow band amplifier 7 
which is tuned to the frequency w. In this manner, only those electrons 
are detected on the receiver side which are influenced by the signal field 
of the PVDF layer. On the other hand, the alternating action of the 
primary electron beam with the polarized domains 8 is so weak that the 
polarization condition is not changed. The stored information remains 
unchanged. 
The amplified read-out signal is passed to an evaluating circuit (not 
shown) which carries out a phase comparison with respect to the high 
frequency generator and/or an amplitude discrimination. If the primary 
electron beam meets successive opposed polarized domains 8, then the 
read-out signal changes its sign (phase jump of 180.degree.). This fact 
can be used for selecting binary coded information. If, on the other hand, 
the recorded information consists of polarized areas of the same 
direction, but different amounts of polarization, then an analogous 
read-out is possible in which only the amplitudes of the read-out signal 
are evaluated. 
The electron beam read-out must naturally take place under high vacuum. For 
this reason, the data carrier 1, the laser 2, the electron beam gun 3 and 
the multiplier 4 are situated in a high vacuum apparatus. 
According to FIG. 2, the screening action of the compensation charges on 
the surface of the data carrier 1 is temporarily removed by a 
piezoelectric activation. 
In contrast to the above-described embodiment, the data carrier 1 is here 
made up of three layers 1a, 1b, 1c. The first layer 1a is the storage 
layer consisting of a ferroelectric film, for example a PVDF film. The 
second layer 1b is a vapor-deposited aluminum layer for screening the PVDF 
film 1a. The third layer 1c is a piezoelectrically active layer and 
consists, for example, of zinc oxide or likewise of polarized 
polyvinylidene fluoride. It serves to produce the ultrasonic wave field, 
so as to activate the polarized domains 8 in the PVDF layer 1a. On the 
upper side of the piezo layer 1c, a metal electrode is arranged at a small 
spacing in the form of a tip 9. By applying a high frequency alternating 
voltage (UHF generator 5) to the tip 9 and the metallic intermediate layer 
1b, the piezo layer 1c is excited in the area of the tip 9 to ultrasonic 
oscillations which are passed on to the underlying layers 1b and 1a. The 
polarized domains 8 are thereby, as explained above, activated and the 
resulting signal fields can be detected with the electron beam probe 3, 4 
described below. 
The exposure of the data carrier 1 to ultrasonic waves can also take place 
without the piezoelectric covering layer 1c. In this case, the ultrasonic 
waves are produced by the electrostatic forces occurring between the tip 9 
and the metal layer 1b. The modulation frequency is then 2 w. 
With the arrangements according to FIG. 1 and FIG. 2, the signal 
sensitivity can be improved when electron optics are incorporated in a 
known manner between the multiplier 4 and the PVDF layer so as to draw off 
as many secondary electrons as possible. 
The electron beam probe 3, 4 can in principle be formed in a similar manner 
to known surface electron microscopes, that is, in the production of the 
process according to the invention, known technology can, by and large, be 
referred back to. 
The process according to the invention is characterized by a favorable 
signal-to-noise ratio (high signal sensitivity), by a high lateral 
resolution (conditional on the minimum cross section of the primary 
electron beam) and by a high read-out speed. With respect to a high 
read-out speed, it is favorable if the ultrasonic frequency is selected as 
high as possible. A limit is occassionally set by space-charge and 
electron transit time effects between the multiplier 4 and the data 
carrier surface. 
With further variants of the process derived from FIG. 1 or FIG. 2, a fixed 
data carrier 1 is used and the electron beam is scanned vertically with 
respect to the paper plane in FIG. 1 or FIG. 2. Moreover, the tip 9 in 
FIG. 2 can be replaced by a strip electrode which is vapor-deposited in 
the scanning direction. It is clear that with this variant, the mechanical 
expenditure is lower (since there is an elimination of the band transport 
device).