Electron-beam recording medium

An electron beam recording medium with a coating, which is applied to a substrate, is sensitive to electron beam radiation and has an organic rare earth metal type phosphor, is suitable for the irreversible writing of information with low-intensity electron beams.

BACKGROUND OF THE INVENTION 
The invention relates to an electron-beam recording medium with a coating, 
applied to a substrate, which contains a phosphor and is sensitive to 
electron-beam radiation. 
The electron beam-sensitive coating is also designated hereinafter as the 
"phosphor coating" or "storage coating" for the sake of simplicity. 
A recording medium of the above-mentioned type is known from DE-OS No. 19 
63 374. The phosphors mentioned herein are thallium-doped potassium iodide 
and manganese-doped potassium magnesium fluoride. When data is written 
with an electron beam, the phosphors luminesce; at the same time, defects 
are formed which extinguish the luminescence. These defects can be 
eliminated or healed by heating. This medium, therefore, is a recording 
medium for the reversible recording of data. 
In the known recording medium the storage of data is based on the formation 
of areas of different degrees of radiation defects on the surface of the 
electron-luminescent phosphor, i.e. the thickness of the phosphor coating 
is to be selected in such a way that it is almost penetrated, but not 
quite by the electron beam which bombards the coating. Storage on the 
surface has the disadvantage that the stored information is sensitive to 
contact, weathering the other surface-damaging influences. 
The use of phosphors based on halogenide crystals in electron beam 
recording media has the further disadvantage that these crystals are 
hygroscopic. This makes the production of phosphor-based recording media 
which is simple in principle somewhat more difficult, and care must be 
taken to ensure that the phosphors are protected against air humidity 
during subsequent use of the recording medium. 
In addition, local defect concentration differences of the aforementioned 
type have a tendency to even out. This effect is presumably due to 
diffusion. The diffusion rate of the defects depends both on the 
concentration gradients of the defects in the crystals and on the 
temperature. It increases with both. The result of this is that there is a 
permanent tendency towards information loss. In the patent mentioned 
earlier, this effect which is particularly prominent at elevated 
temperature is intended after all as an erasure mechanism for the written 
information. 
The intrinsic tendency of the phosphor type contained in the known 
recording medium to restore the chemical equilibrium by diffusion of 
accumulations of localized defects imposes a natural limit on the highest 
number of 0.fwdarw.1 items of information which can be written per unit 
surface. According to the above-mentioned patent, only 10.sup.7 bits of 
digital information are stored on a disc of 4.times.6 mm.sup.2. 
DE-OS No. 30 32 611 describes organic rare earth metal salt type phosphors 
which are capable of emitting radiation when subjected to excitation by, 
among other things, electron bombardment and which are suitable, among 
other things, as photographic materials, picture-resolving materials or 
high-resolution materials, and as basic materials for the field of 
optoelectronics. 
SUMMARY OF THE INVENTION 
An object of the present invention is to create a recording medium for the 
irreversible writing of information and therefore for the recording of 
documents. 
A further object of the invention is to create a recording medium in which 
the recording process involves not only the surface but the entire volume 
of the beam-sensitive coating. 
Another object of the invention is to select phosphors for electron-beam 
recording medium which are not hygroscopic and do not have a tendency for 
the local defect concentration differences to even out. 
Yet another object is to create a recording medium with increased 
information density. 
According to the invention these objects are achieved by including in the 
electron beam-sensitive coating of the recording medium an organic rare 
earth metal type phosphor or employing an electron beam-sensitive coating 
which consists of such a phosphor. 
During the investigations which led to the invention it was shown, in fact, 
that phosphors of this type, which are described in part in DE-OS No. 30 
32 611, are already irreversibly destroyed with electron beam of low 
intensity. 
In order to ensure that the organic rare earth metal type phosphor is 
completely destroyed by the electron radiation, the thickness of the 
radiation-sensitive coating is smaller than or equal to the depth of 
penetration of the electrons.

DETAILED DESCRIPTION OF THE INVENTION 
The preferred phosphor used in the coating is a rare-earth metal salt of a 
carboxylic acid. 
Examples of suitable rare earth metal salts of carboxylic acids are those 
known from DE-OS No. 30 32 611, such as the rare earth metals salts of 
aliphatic carboxylic acids, e.g. 2,4,6-octatrienic acid and 
2,4,6,8-decatetraenic acid and their substitution products, aromatic 
carboxylic acids, e.g. m-methoxybenzoic acid, p-methoxybenzoic acid, 
m-chlorobenzoic acid, p-chlorobenzoic acid, p-bromobenzoic acid, 
dichlorobenzoic acid, p-ethoxybenzoic acid, m-nitrobenzoic acid, cuminic 
acid, (p-isopropylbenzoic acid), p-t-butylbenzoic acid, 4-benzoylbenzoic 
acid, 4-biphenyl carboxylic acid, phenylproprionic acid, 
2-chloro-6-fluorobenzoic acid, .alpha.-bromo-p-toluic acid, isophthalic 
acid, terephthalic acid, trimellitic acid and their substitution products, 
polycryclic aromatic carboxylic acids, e.g. anthracene-9-carboxylic acid 
and anthraquinone carboxylic acid and their substitution products, 
heterocyclic carboxylic acid, e.g. 2-thiophene carboxylic acid, 
3-thiophene carboxylic acid, nicotinic acid, picolinic acid, 
indole-5-carboxylic acid, pyridine-2,5-dicarboxylic acid, 
pyridine-3,4-dicarboxylic acid, 2-phenyl-4-quinolinic acid, quinaldinic 
acid and 5-methyl-2-thiopene carboxylic acid and substitution products of 
these acids, and .alpha., .beta.-unsaturated carboxylic acids, e.g. 
3-indole acrylic acid, 4-imidazole acrylic acid, 3-(2-thienyl) acrylic 
acid, .beta.-(3 -pyridyl) acrylic acid, 2,3-bis (p-methoxyphenyl) acrylic 
acid, cinnamic acid, p-methyl cinnamic acid, .alpha.-methyl cinnamic acid, 
m-chlorocinnamic acid, m-bromocinnamic acid, p-chlorocinnamic acid, 
3,5-dimethyloxycinnamic acid and 3,4 dihydroxycinnamic acid, and 
carboxylic acids such as Crocetine, Bixine and Azafrine and their 
substitution products. 
In a further preferred embodiment the applied phosphor comprise the rare 
earth metal europium, terbium, cerium or thulium. The rare earth metals 
are preferably trivalent. 
Especially preferred is europium cinnamate. 
Other phosphors which can be used in accordance with the invention are 
phthalocyanines and porphyrins in which the central metal ion is a 
trivalent rare earth metal ion, preferably europium or terbium. 
Rare earth metal chelates which, for example, contain .beta.-diketones or 
carboxyl groups as the ligand are described in DE-OS No. 30 32 611 as 
being unsatable phosphors. Consequently, these phosphors, too, are 
applicable within the framework of the invention. 
Europium-.beta.-(3-pyridyl)-acrylate, europium sorbate and europium 
diphenylacetate are also eligible as phosphors for the recording medium of 
the invention, although they have a lower luminosity factor than, for 
example, europium cinnamate. In this case it is advisable to use light 
detectors with increased sensitivity when reading out the stored 
information. 
The electron beam recording medium of the invention is produced by coating 
a substrate with a thin layer of an organic rare earth metal type 
phosphor. Metal plates, glass plates, fused silica plates, sapphire 
plates, transparent plastic foils or plates, for example, are suitable 
substrates. 
In a preferred embodiment the substrate is made of an insulating material 
and is provided on the side facing the electron-beam sensitive coating, 
with a diverging electrode such as coating of a transparent, electrically 
conductive layer or an aluminum film. If substrates with sufficient 
electrical conductivity are chosen such as, for example, glasses made 
conductive by doping or metal plates, then the conducting electrode is not 
needed. 
The phosphor coating is produced, for example, by sedimentation from a 
suspension of phosphor and organic solvent, e.g. ethanol or toluene. A 
practical thickness for the coatings thus produced is 1 to 5 .mu.m. 
To store information, the phosphor coating is irreversibly destroyed 
locally in respect to its original luminescence properties and is left at 
other places in its initial state. In this way the binary coding 
luminescent/non-luminescent as a function of location is obtained in the 
phosphor coating. Within certain limits a similar coding of the phosphor 
coating is also eligible. For this purpose the local destruction of the 
luminescence properties of the phosphor coating is made in gradations. 
Thus, for example, codings of the form luminescent/less 
luminescent/non-luminescent are obtained. 
As already mentioned, the storage of information in the case of the 
recording medium described in DE-OS No. 19 63 374 is based on the 
formation of areas of different degrees of radiation defects on the 
surface of the phosphor. In contrast to this, the surface of the recording 
medium which is the subject of the present invention is of secondary 
importance in so far as the recording process in this case involves the 
entire volume of the phosphor coating. The surface is also inscribed in 
fact, but it is not the primary carrier of the information. The benefit 
which this brings is a reduced sensitivity to contact, weathering and 
other surface-damaging influences. Subsequent treatment of the surface can 
take place. 
Furthermore, with the recording medium of the invention the radiation 
damage is localized down to molecular dimensions. Accordingly, there are 
no expansive crystal defects. The defect therefore is molecule-dependent. 
This applies to all the organic storage materials which can be used 
according to the invention. The outcome of this is that the 
above-mentioned limitations which apply to alkali halides and inorganic 
materials behaving similarly in this respect do not arise with the 
materials proposed in the present invention. 
In the case of the recording medium as claimed by the present invention, 
the local resolving power and with it the information density is 
determined by the depth of penetration of the electron beam writing the 
information, which depends on the electron energy, and the diameter of the 
electron beam. Thus, for example, commercial scanning electron microscopes 
permit focusing of the electron beam down to 5 nm. Even if the electron 
beam were defocussed to double the diameter (10 nm), 25.times.10.sup.10 
bits could thus be stored on a storage area of 4.times.6 mm.sup.2. 
The limitation due to the depth of penetration can, as already mentioned, 
be avoided by appropriate specification of the coating thickness which for 
an electron energy of 30 kev, for example, is approx. 4 .mu.m. 
For the subsequent reading of the information stored in this way the 
phosphor coating is excited into luminescence and the light emitted is 
registered as a function of the location in the phosphor coating. 
Preferred embodiments of the invention will be explained in greater detail 
below with the aid of the drawing and the following examples. 
The one FIGURE presents a schematic diagram of a read-write device with a 
recording medium arranged inside it. 
The function and the construction of the device and the recording medium 
are described below with reference to the drawing: An electron gun system 
1 is used to generate and shape a writing and/or reading electron beam 2. 
Such electron gun systems are generally known and are much used in 
electron microscopes, cathode-ray tubes etc. The device itself consists of 
an evacuated container 3. For the purpose of scanning the storage layer 4 
the electron beam is deflected line by line in the form of a raster over 
the storage layer either electrostatically or, as shown in the figure, 
magnetically by deflection coils 5. This is indicated by an arrow 6. The 
focusing plane for the electron beam is the storage layer 4. When the 
information is being written (with high electron beam intensity) the 
electron beam is blanked by a control grid, which in the case in question 
is present in the form of a Wehnelt cylinder in the electron gun system 1 
itself, at the places where destruction of the phosphor coating 4 is not 
intended. The regions in which destruction by the electron beam takes 
place are indicated by 7 in the diagram. 
The phosphor coating 4 is fully coded with information when one raster has 
been passed through. Subsequently, reading of the stored information takes 
place in a very similar manner, but with lower electron beam intensity, by 
line-by-line scanning of the phosphor coating 4. The luminescence signal 8 
thus produced is recorded synchronously with the deflection of the 
electron beam by means of a photodetector 9 (photomultiplier). In order 
that the phosphor coating 4 does not become charged during the read or 
write process due to the electrons being applied, a conductive 
under-electrode 10 is provided between phosphor coating 4 and substrate 11 
which serves to divert the charges. As already mentioned, it is also 
possible to dispense with such a diverting electrode when the substrate 11 
itself is conductive and, by virtue of this property, has assumed the 
function of the diverting electrode. As a whole, such an arrangement is 
ideally embodied in a scanning electron microscope. 
EXAMPLE 1 
60 mg europium cinnamate phosphor is dispersed in 100 cm.sup.3 toluene. 
Then, from the suspension thus produced a thin layer is precipitated by 
sedimentation on a thin glass substrate (5.times.5 cm.sup.2) which on its 
sedimentation surface has been given a transparent, electrically 
conductive coating. Such coated glasses are available commercially under 
the name "Anellglas". The transparent layer serves as an under-electrode 
and, in the earthed state, prevents the phosphor coating from charging up 
during cathode ray excitation. After the sedimentation of the layer the 
solvent which is left is carefully removed by suction so that the layer is 
not damaged. The ultimate dry adhesion of the layer is achieved by 
subsequent drying at 50.degree. C. to 70.degree. C. The weight of the 
layer amounts to 4 mg per cm.sup.2. The layer thickness is about 4 .mu.m. 
The preparation of the storage coating has thus been completed and it is 
inserted in the sample chamber of a scanning electron microscope (Philips 
PSEM 500) for the purpose of storing information. 
The method whereby the information is written is as follows: 
With the aid of the electron beam of the scanning electron microscope, an 
area of 1.times.2 mm.sup.2 of the storage layer is scanned line by line. 
By modulation of the electron beam intensity (on-off) the phosphor coating 
is successively, serially bombarded. In this process the electron beam 
intensity during the bombardment of the phosphor coating is dimensioned in 
such a way that the luminescence properties of the phosphor are 
irreversibly destroyed during the period of bombardment. The intensity of 
the electron beam amounts to 10.sup.-6 Joule/cm.sup.2 whereby a complete 
(100%) destroyment of the luminescent properties was obtained. An 
intensity of 10.sup.-7 Joule/cm.sup.2 the luminescent properties were 
destroyed for 50%. 
In this way, a flat arrangement of contiguous regions is obtained in which 
the phosphor has been destroyed or has been retained with its original 
luminescent properties, as the case may be. The coating therefore receives 
the information as to the places at which the electron beam had a high or 
a low intensity. Subsequent reading of the information stored in this way 
takes place therefore as follows: 
The phosphor coating modified by the writing operation is scanned line by 
line in the same way as during the burning-in of the information. This 
time, however, in contrast to the writing operation the intensity of the 
electron beam is not modulated. A low intensity of 10.sup.-8 
Joule/cm.sup.2 constant with time, is selected to that the phosphor is 
excited to luminescence, but it is made certain that no further 
destruction can take place. When the coating is scanned, therefor, 
luminescent light is emitted at the places of the coating where there has 
been no prior destruction. During the scanning the luminescent light is 
recorded as a function of time with a photo-multiplier (RCA type C30134A). 
This form of light detection is known to be extremely sensitive and can 
take place at high speed so that the speed of the scanning electron beam 
is determined largely by the decay time of the luminescence. 
Another advantage is the use of the transparent under-electrode. It makes 
it possible to detect the luminescent light both from the underside of the 
substrate and from the coating side. It is also possible to dispense with 
the transparent under-electrode and, instead, to have the diverting 
electrode in the form of a reflecting aluminum film such as is 
conventionally used in cathode-ray tubes at the top of the coating. In 
this case, the luminescent light is detected only from the underside of 
the coating. 
Because the speed of the scanning electron beam is constant with time, this 
method can be used to reproduce the information as to the places at which 
the phosphor coating was previously destroyed. 
With the scanning electron microscope used as the example, ion spots of the 
size of 10 (.mu.m).sup.2 were able to be obtained in this way. Therefore 
on a scanned surface area of 1.times.1 mm.sup.2 it is possible to code 
approximately 300.times.300 points with luminescent/non-luminescent 
information. The storage density depends, on the one hand, on the diameter 
of the burning-in/reading electron beam and, on the other, on the 
homogeneity and thickness of the phosphor coating and on the depth of 
penetration of the electron beam which depends on the acceleration 
voltage. 
With modern scanning electron microscopes, electron spot diameters of 5 nm 
can be achieved without difficulty. In this case the storage density is 
limited to 4.times.10.sup.12 pixels/cm.sup.2 by the electron beam 
diameter. Similar results were obtained with layers of other organic 
rare-earth metal type phosphors such as those mentioned earlier in the 
specification. 
EXAMPLE 2 
A phosphor coating is produced as described in Example 1 and the 
information is also written as in Example 1. 
The information is read out in parallel, by total-area irradiation of the 
phosphor coating with a UV lamp and imaging of the luminescence bit 
pattern on a detector array. 
EXAMPLE 3 
A 2 mm thick polymethacrylate plate containing a 4 .mu.m thick layer of 
europium cinnamate is scanned, as described in embodiment example 51 of 
DE-OS No. 30 32 611, with a focussed Co.sub.2 laser with an intensity of 
10.sup.-7 Joule/cm.sup.2 and the information is written serially by 
"on-off" modulation of the laser intensity. Reading takes place as 
describe in Example 2. 
EXAMPLE 4 
Information is written into a luminescent polymethacrylate plate as in 
Example 3. Reading of the plate is achieved serially by scanning the plate 
with a UV laser with simultaneous detection of the luminescent light.