Apparatus for intensifying radiation images

[57] ABSTRACT

A radiation-responsive layer preferably of cesium fluoride is bonded to an electrically-conductive radiationtransparent layer of material which is also utilized as an electrode. A piezoelectric polarized ferroelectric ceramic layer preferably of lead zirconate titanate is bonded to the radiation-responsive layer. An output device such as a visible-light emitting phosphor is bonded along with a radiation-opaque electrical conductive layer, also used as an electrode, to the piezoelectric layer. A source of electrical potential is connected between the two electrodes to thereby apply an electrical potential across the piezoelectric layer and the radiation-sensitive layer.

In the manufacture of the apparatus, the cesium fluoride layer is formed by vacuum deposition from a metallic cesium source in a boron trifluoride atmosphere while bleeding in small amounts of boron trifluoride to maintain a substantially constant atmospheric level thereof during evaporation. The piezoelectric layer is formed by vacuum deposition of lead zirconate titantate in an atmosphere containing trimethylamine, dimethylamine, argon and oxygen upon a bonding coat deposited on the previously-deposited cesium fluoride layer while retaining the substrate heated. During the deposition process, the trimethylamine and dimethylamine are thermally cracked thereby to provide carbon as a dopant for the lead zirconate titanate layer.

Claims, 3 Drawing Figures

BONDING COAT 20

PNOSPNOR.

PIEZOELECTRIC, LAYER 22

ANTI-FEEDBACK.

LAYER 24

<img file="US3828186A_D0001.tif" />

RADIATION- TRANSPARENT ELECTRICALLY- CONDUCTIVE

LAYER /4

SUBSTRATE /2.

PROTECTIVE LAYER /Q

RADIATION-RESPONSIVE LAYER.

PATENTED^ 61874

3,828,186

PIEZOELECTRIC

LAYER 22

BONDING, COAT 20

P//OSPVOR.

ANT!-FEEDBACK. LAYER 24

<img file="US3828186A_D0002.tif" />

Ko

SUBSTRATE /2

PROTECT/VE LAYER /Q

RADIATION-RESPONS/VP LAYER

RADIATION- TRANSPARENT

ELECTRICALLY- CONDUCTIVE LAYER /4-

<img file="US3828186A_D0003.tif" />

<img file="US3828186A_D0004.tif" />

3,828,186

2

APPARATUS FOR INTENSIFYING RADIATION IMAGES BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a radiation imageintensifying apparatus constructed in accordance with the present invention;

FIG. 2 is a schematic illustration of apparatus which may be used in manufacture of the present invention; and,

FIG. 3 is a schematic illustration, in cross-section, of an alternative embodiment of an article constructed in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is illustrated in greatly enlarged schematic form a radiation image-intensifying apparatus 10 constructed in accordance with the present invention. For the sake of a concrete illustration of its use, the apparatus of FIG. 1 shall in general be described for the case where the radiation of interest is visible light radiation. It is to be understood however that the apparatus is equally useful with other types of electro-magnetic radiation such as X-rays, ultraviolet radiation, or infrared radiation. Although the term image intensification is used herein, such is not intended to imply that the apparatus is only useful for reproducing intelligible images. For example, the apparatus might be used as a radiation detector or amplifier where no particular image, as that term is normally used, is involved.

As shown in FIG. 1, the apparatus 10 includes a transparent substrate 12 which, when visible light is the radiation of concern, may be glass. Although various glasses are useful, the preferred glass is that identified as Pyrex which has a constituency of 80 percent silicon dioxide, 4 percent sodium oxide, 13 percent boron oxide, 2 percent aluminum oxide and 1 percent other oxides. The substrate 12 is cleaned and etched as is well known to the art to thereby expose a fresh surface upon which further layers may be deposited and to assure tenacious adhesion thereof.

A layer 14 of material which is electrically conductive but also transparent to the radiation is deposited upon the cleaned surface of the substrate 12. Various such materials are known to the art such for example as aluminum, titanium, chromium, and stannous oxide. In accordance with the preferred embodiment of the present invention and for visible light, a layer of stannous oxide approximately 1,000 angstroms in thickness is preferred. The stannous oxide layer may be deposited by any means known to the art, such for example as by spraying a 5 percent stannous chloride - water solution onto the substrate 12, which has been heated to approximately 500° C. If one desires to have a greater selectivity as to bandwidth of the apparatus, then the layer 14 may be chosen for that purpose; for example, chromium may be utilized for the layer 14 and thereby effect a transparency for radiation falling within the infrared bandwidth.

A radiation-responsive layer 16 is then deposited upon the transparent layer 14. The radiationresponsive layer 16 must have the characteristic of doning electrons in response to incident radiation. By the term “doning” it is intended to mean a release of kinetic energy and may take the form of free electrons which are emitted from the layer 16 but may also include a variation in surface charge density or a meBACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to solid-state devices and more particularly to such devices adapted for intensification of incident radiation images.

2. Description of the Prior Art io

It has long been desirable to enhance images which are relatively weak and difficult to detect by means of intensification thereof. Such would be helpful in the visible light portion of the radiation spectrum particularly during low ambient light conditions. There have 15 been various prior art attempts to construct such apparatus which, although operationally successful, have met with various problems. Some of these problems are extremely limited bandwidth of transmitted radiation, large size of the resulting apparatus, the extremely high 20 voltages (kilovolts) required to obtain satisfactory operation of the apparatus, sensitivity of the apparatus to vibrations, to temperature and to external magnetic fields.

For the most part, prior art apparatus providing re- 25 suits similar to those achieved by applicant are constructed using high voltage vacuum tube circuits and technology. Some prior art devices have utilized electroluminescent apparatus of the kind comprising a layer of electroluminescent material and a layer of pho- ³⁰toconductive material. The photoconductive material is arranged to control the value of voltage applied across the electroluminescent material proportional to the intensity of incident radiation lying within the particular bandwidth of radiation to which the device is ³⁵sensitive. Such devices have been utilized singly or in tandem depending upon the particular application.

The most pertinent prior art references known to applicant are U.S. Pat. Nos. 2,816,236; 2,951,168;

2,972,694; 3,058,002; 3,072,821; 3,121,824; ⁴⁰3,132,276; 3,154,720; 3,187,184; 3,268,755;

3,286,027; 3,356,850 and 3,501,638.

SUMMARY OF THE INVENTION

A radiation-responsive layer excitable by incident ra- ⁴⁵diation within a predetermined bandwidth is bonded to a ferroelectric ceramic layer which is properly polarized to provide piezoelectric characteristics. An output-signal-producing means is connected with the piezoelectric layer and provides an output signal re- ⁵⁰sponsive to incident radiation.

In a more specific aspect of the present invention, the apparatus includes cesium fluoride as a raditionresponsive layer and carbon-doped lead zirconate titanate as the piezoelectric layer with a phosphor bonded ⁵⁵thereto as the output-signal-producing means. An electrical potential is applied across the lead zirconate titanate and cesium fluoride layers.

In accordance with the method of the present invention, the cesium fluoride layer is provided by vacuum evaporating metallic cesium through a boron trifluoride atmosphere onto a heated substrate. The lead zirconate titanate layer is provided by vacuum deposition and reactive sputtering of lead zirconate titanate ₆₅through a thermally decomposable organic compound atmosphere including oxygen to thereby epitaxially deposit carbon-doped lead zirconate titanate.

3,828,186 chanical change of form. The layer 16 may include various of the photosensitive rare earth halides but in accordance with the preferred embodiment of the present invention, the layer 16 is cesium fluoride. The cesium fluoride layer is deposited by vacuum evaporation onto the heated combination of the substrate 12 and the layer 14.

To accomplish the foregoing deposition, the substrate 12 with the layer 14 properly adhered thereto is placed in a vacuum chamber such as a bell jar 30 (FIG. 2). A source of metallic cesium is placed in a boat 32 or similar holder. The vacuum pump 38 is utilized to provide a vacuum internally of the bell jar 30 which as is well known in the art should be at least 2 X 10^-6 millimeters of mercury. Thereafter a heater 34 is energized to heat the substrate 12 to a temperature between 150° - 200° C. Thereafter, the atmosphere source and control means 40 is energized to provide an atmosphere internally of the bell jar 30 of a minumum of .07 mole percent to a maximum of 1 mole percent with a preferred approximately 1/10 mole percent of boron trifluoride from the source 42 thereof in gaseous form. Heat is then applied as indicated by the arrows 36 to the boat 32 containing the metallic cesium thereby causing the same to evaporate. During evaporation of the metallic cesium the atoms thereof travel through the boron trifluoride atmosphere and a substantial percentage of the metallic cesium reacts with the boron trifluoride thereby forming cesium fluoride which is deposited upon and adheres to the stannous oxide layer 14 previously deposited. The evaporation is continued for a time sufficient to form a layer of cesium fluoride approximately 800 angstroms in thickness upon the layer 14. During the evaporation of the metallic cesium from the source 32, the atmosphere source and control means 40 is utilized to continuously bleed boron trifluoride into the interior of the bell jar 30 thereby maintaining the desired content thereof as the atmosphere. At the percentages of boron trifluoride provided in the bell jar 30, the atmosphere is not saturated and thus it is believed that some metallic cesium in elemental form (that is, not combined with fluoride) may be deposited as an integral part of the layer 16. By providing thus a layer of radiation-responsive material 16 consisting of a majority of a rare earth halide but with a rare earth metal in its elemental form, the response of the layer 16 to incident radiation is surprisingly increased.

Immediately after deposition of the cesium fluoride layer 16, a protective coating or layer 18 must be provided thereover to preclude external contamination of the cesium fluoride. Such protective layer may include various known elements but preferably in accordance with the present invention is a layer of silicon monoxide which may be deposited by vacuum evaporation as is well known in the art. The layer of silicon monoxide should be maintained relatively thin and in accordance with the present invention is approximately 500 angstroms in thickness.

A bonding coat 20 is then deposited upon the protective layer 18 and may include any material which will provide adequate adhesion of the additional layers to be further deposited upon the cesium flouride layer 16. The bonding coat 20 in accordance with the present invention preferably includes three separate layers of metal. The first of these is titanium which is deposited directly upon the silicon monoxide layer 18 and which is then followed by a layer of aluminum and a layer of gold. These layers may be deposited by vacuum evaporation as is well known to the art and it is believed they intermix during the evaporation steps to form an intermetallic compound.

After deposition of the bonding coat 20, a layer 22 of piezoelectric material is epitaxially deposited. The epitaxially-deposited piezoelectric layer 22 is a monocrystalline material such as a ferroelectric ceramic which has been polarized. Ferroelectric ceramics of this type are well known to the art and include for example lead zirconate, barium titanate, lead metaniobate and lead zirconate titanate. In accordance with the present invention, a layer 22 is formed of lead zirconate titanate which has been carbon-doped. Once the ferroelectric ceramic has been deposited, it must be heated beyond its ferroelectric Curie temperature, and then a large unidirectional electric field is applied to the material and the material is allowed to cool to room temperature while the field remains applied. This process of polarizing a ferroelectric ceramic is well known to the art.

The carbon-doped lead zirconate titanate layer 22 is applied to the bonding coat 20 by vacuum evaporation and reactive sputtering. Such occurs by placing in the container or boat 32 lead zirconate titanate in powder form along with the substrate 12 with the various layers previously deposited thereon. The bell jar is evacuated as above referred to after which the atmosphere source and control means 40 is activated to provide an atmosphere within the bell jar of a thermally decomposable organic compound 44 in an oxidizing atmosphere. Although any thermally decomposable organic compound which upon cracking provides carbon but will not contaminate the lead zirconate titanate may be used, in accordance with the present invention, the following atmosphere is preferred: 50 percent molar composition trimethylamine, 50 percent molar composition dimethylamine, 10 percent molar composition of argon as a diluent, and 2-5 percent molar composition oxygen. Thereafter, the substrate 12 is heated to between 200° and 300° C after which heat is applied as indicated by the arrows 36 to evaporate the lead zirconate titanate. A source of potential 31 is connected through an appropriate resistor 33 in such manner as to utilize the container 32 of lead zirconate titanate as the cathode and the substrate 12 as the anode. The magnitude of the source is controlled so as to maintain a substantially constant current flow between 10 milliamps and 200 milliamps depending upon the parameters of the system such as vacuum, cooling, time and the like. By maintaining the conditions above set forth, there is obtained a combination of evaporation and reactive sputtering of the lead zirconate titanate. By providing the appropriate atmosphere and temperatures above referred to, the layer 22 is epitaxially formed upon the bonding coat 20.

After deposition of the layer 22, a layer 24 of electrically-conductive and radiation-opaque material is deposited. The purpose of this material is to provide an electrode and also to preclude feedback of light which will be emitted by the phosphor layer 26 when deposited and as will be discussed more fully hereinafter. Various materials may be utilized but in accordance with the present invention, a layer consisting of titanium and platinum black co-deposited has been found preferable. After deposition of the layer 24,

3,828,186 leads are attached to the layer 14 and the layer 24 and the polarizing field applied as above referred to.

After polarization of the ferroelectric ceramic layer 22, an output-signal-producing layer such as a lightemitting phosphor 26 is deposited. Any of the known phosphors which emit radiant energy when excited such as by application of energy thereto through friction, electron bombardment, application of an electric field or the like may be used. Such phosphors include for example zinc orthosilicate and zinc sulfide which may be activated with copper or silver. In accordance with the preferred embodiment of the present invention, the phosphor layer 26 is silver-activated zinc sulfide. The layer 26 may be applied by any of the known techniques such as evaporating, sputtering or settling out of a dilute potassium silicate solution.

Thereafter, a source 28 of driving potential is connected through the leads, as illustrated, to the electrically conductive layers 14 and 24, which operate as electrodes to apply an electrical potential across the piezoelectric layer 22 and the radiation-responsive layer 16. As is well known to the art, the application of an electrical potential to the piezoelectric material causes it to expand along one axis and contract along another axis. In addition thereto, the piezoelectric material also produces opposite charges on opposed surfaces of the material when the material is stressed by application thereto of external forces whether created by mechanical or electrical means.

It has been found that the driving source provides excellent output at a magnitude of approximately 15 volts and may vary in frequency from a few hertz to approximately 15 kilohertz. The apparatus constructed as above described has been found to provide an image intensifier with a gain of approximately 6,000.

Devices employing a plurality of elements as shown in FIG. 1 but cascaded may also be utilized in accordance with the present invention. Such a structure is illustrated in FIG. 3, and includes substrates 50,52 and 54 upon which there has been deposited layers of material as above described and as illustrated generally as a single layer 56, 58, and 60, respectively, upon each of the substrates. Thus, radiation would enter from the left as viewed in FIG. 3 and as shown by the arrows 80. A layer of optical coupling compound 62 and 64 is utilized to secure each of the devices together and provide good transmission of radiation therethrough. Sources of driving potential 66,68 and 70 are connected in the manner above described, one respectively to each of the devices. Thus, by utilizing an apparatus as constructed in FIG. 3, the radiation 80 entering from the left as shown in FIG. 3 provides an output from the phosphor layer 56 which output is transmitted to the substrate 52 with like result and also the output thereof through substrate 54 with like result. Thereby, an increased gain is provided by cascading the gains of each of the devices in the manner shown in FIG. 3.

Apparatus constructed as above described with reference to either FIG. 1 or FIG. 3 is very rugged and small in size. In view of the low voltages and non-critical frequencies for the power source, well known solid-state circuits may be used to provide the driving voltage and no particular protection is required as compared to the prior art where high voltages were used. Thus, the apparatus constructed in accordance with the invention may be used to provide image intensification for binoculars, underwater use, medical use for fluoroscopy as well as many additional uses as will be recognized by those skilled in the art.

The precise phenomena resulting in the excellent image intensification realized with apparatus when constructed in accordance with the present invention is not yet understood. It is thought that energy released by the radiation-responsive layers in response to incident radiation is coupled to the piezoelectric layers and enhances the vibration thereof. Such enhanced vibration imparts energy to the output-signal-producing layer such as the phosphors causing light to be emitted therefrom. The precise mechanism for the energy transfer is not known but is thought to be either electron emission or a physical deformation. In any event, highly surprising and unexpected results have been obtained when utilizing apparatus constructed as above described and illustrated.