Method and apparatus for fabricating mercuric iodide polycrystalline films for digital radiography

A method is provided for fabricating in a thermal evaporation system a polycrystalline film capable of directly detecting radiation. Source material is placed in a container, and the container is evacuated to create vacuum within the container. The source material is heated to evaporate the source material for depositing on a substrate. The polycrystalline film is used in as deposited form to detect the radiation.

FIELD OF THE INVENTION

This invention is related to digital radiography, and particularly to a method and apparatus for fabricating mercuric iodide (HgI2) polycrystalline films for digital radiography applications.

BACKGROUND OF THE INVENTION

Traditionally, photographic films have been used to perform X-ray imaging. Photographic film techniques have the advantages of good spatial resolution (better than 50 μm) and very large active areas. However, use of photographic films suffers from many drawbacks, including low X-ray detection efficiency, non-linearity, and slow image retrieval processes.

Thus, there currently is a growing interest in developing digital radiographic detectors for medical, scientific and industrial applications. The applications for digital radiographic detectors may include medical diagnostic applications, non-destructive evaluation of materials, X-ray diffraction of biological and other material samples, and astronomical observations. For example, some estimates indicate that, in the medical area alone, there are over 600 X-ray images produced per 1000 population per year, much of which may be performed using digital radiographic techniques.

Digital techniques in radiology typically have several benefits over traditional X-ray film analog methods. These include reduced radiation dose for an equivalent image, convenient image acquisition and retrieval (avoiding film development time and cost), digital image processing (image enhancement), computer-assisted diagnosis, and easy image storage and transmission. Furthermore, the ability to provide real time images may be advantageous in some applications.

Recently, amorphous silicon (a-Si:H) transistor-addressed arrays (amorphous silicon arrays) have become a leading technology for large area flat panel imaging. Imagers with up to 2304×3200 pixels (29.2×40.6 cm2) on a single substrate with pitch of 127 μm have been produced, and several companies have started commercial production of the amorphous silicon arrays. Smaller area but higher spatial resolution X-ray imagers are also produced using single crystal silicon CMOS readout technology. The sensitivity to X-rays is obtained by coupling a phosphor screen to either the amorphous silicon array or the CMOS readout. Typically Gd2O2S:Tb phosphor is deposited on the amorphous silicon array-based imagers, although CsI:Tl has also been used.

The detectors utilizing phosphors can be characterized as indirect detectors, which typically require a combination of processes to achieve an image. First, transfer of the X-ray energy into visible light photons by the phosphor should be accomplished, and then subsequently the light should be converted into electrical signals using light sensitive readout arrays.

Although indirect detection may be an improvement over the conventional analog technique using photographic films, this approach may suffer from deficiencies including low efficiency of the energy transfer and limited spatial resolution due to light spreading in the phosphor. The poor energy transfer is due to an inefficient process of creating and collecting visible light photons. The increased light spread is a consequence of increasing phosphor thickness to achieve better efficiency in stopping X-rays. The increased light spread can be ameliorated by use of specially grown CsI scintillators with a columnar structure when the X-rays have low energies and/or the CsI scintillators have thin layers. However, as soon as the aspect ratio (the length of the column to the diameter) increases (e.g., to account for increase in X-ray energies), the light collection within the scintillator columns decreases, further reducing the energy transfer efficiency.

Therefore, it is desirable to provide a digital X-ray detector that can provide efficient energy detection over a wide range of X-ray intensities and improved spatial resolution over phosphor-based digital X-ray detectors.

SUMMARY

In an exemplary embodiment according to the present invention, a method is provided for fabricating a polycrystalline film in a thermal evaporation system. The polycrystalline film is capable of directly detecting radiation. The method includes placing source material in a container; evacuating the container to create vacuum within the container; and heating the source material to evaporate the source material, wherein the evaporated source material is deposited on a substrate. The polycrystalline film is used in as deposited form to detect the radiation.

In another exemplary embodiment according to the present invention, a thermal evaporation system for fabricating a polycrystalline film is provided. The polycrystalline film is capable of directly detecting radiation. The system includes a container adapted for creating vacuum within and for heating source material disposed therein; a furnace enclosing at least a portion of the container, the furnace being capable of heating the container to evaporate the source material; a substrate holder for holding the substrate, on which the evaporated source material is deposited for growth of the polycrystalline film; and a temperature controlling system for maintaining the source material and the substrate at respective predetermined temperature ranges to control a growth rate of the polycrystalline film.

In yet another exemplary embodiment according to the present invention, a radiography system is provided. The radiography system includes an array detector capable of receiving radiation and generating corresponding electrical signal, the array detector comprising a polycrystalline film fabricated through sublimation on a readout substrate, wherein the polycrystalline film is used in as deposited form after being grown on said readout substrate; and an image processor coupled to the array detector to generate a displayable image from the electrical signal.

DETAILED DESCRIPTION

In an exemplary embodiment according to the present invention, a method and apparatus for fabricating polycrystalline film-based digital X-ray detectors are provided. The polycrystalline film-based digital X-ray detectors are used for efficient detection of X-ray images at high spatial resolution.

Polycrystalline films are grown through sublimation of a source material on substrates such as silicon and alumina. The source material used may be highly purified HgI2in powder form. Other metallic iodides such as lead iodide (PbI2) may also be used as the source material to produce other types of polycrystalline films. The source material in a glass ampoule is evaporated onto a substrate using a furnace of a thermal evaporation system. The substrate may comprise amorphous silicon with a TFT (thin film transistor) array or a CMOS (complementary metal-oxide semiconductor) array fabricated thereon. These arrays may be used as readout arrays when the polycrystalline film is used in a digital X-ray detector for direct detection of X-rays without using a phosphor screen.

The thickness of the grown layers, size of the grains and crystallinity can be regulated in a controlled way over a wide range without the need for heat-treating (e.g., sintering) the layers after growth by adjusting the growth parameters such as Tsource, Tsubstrate, source-substrate distance, and growth rate in specific reproducible ways. Thus, the polycrystalline films may be used in “as deposited” form to detect radiation. Detectors made from these films typically give dark current densities in the order of a few pA/cm2up to several hundred pA/cm2(pico amps/cm2) and apparent resistivities in the order of 1010to 1014Ohms-cm. X-ray sensitivity results also show these detectors have good performance. For example, low dark current, good sensitivity and linearity of the response to X-rays allow HgI2polycrystalline layers to be used in digital X-ray imaging systems.

The deficiencies associated with indirect detection may be minimized or eliminated through the use of direct conversion detectors in an embodiment according to the present invention. Thin polycrystalline films of high atomic number (high Z) and high-density semiconductor material can effectively absorb the incoming X-ray radiation and convert it directly into electrical signals, which can be read by associated readout arrays.

The efficiency of the energy transfer from X-rays to electrical signal can be an order of magnitude larger in the direct detection approach than in the case of indirect detection using phosphors due to the basic underlying physics. That is, the mean energy for creation of an electron-hole pair in a semiconductor detector is typically an order of magnitude lower than the corresponding energy necessary to generate the same signal through the scintillation approach. This results in a larger signal for the same incoming X-ray event for the direct detector approach.

The direct detector approach can significantly improve detective quantum efficiency (DQE) despite the fact that indirect detectors can achieve DQE as high as 60–80 percent. DQE values are usually quoted for very high intensities of incident X-ray photons (photon limited case). DQE is a function of the number of photons interacting with the detector and drops significantly at lower X-ray intensities. Although DQE improves with the polycrystalline semiconductor converters compared to other technologies for the whole range of the incident X-ray intensities, the DQE improves the most at lower X-ray intensities. More efficient energy conversion and better signal-to-readout noise would allow direct detection to result in better detecting method.

The improved DQE at lower intensities may be important for applications such as fluoroscopy, where the dynamic temporal aspect of the measurement favors shorter measurement times. Another important consideration for direct detectors is that the charges generated by X-rays do not spread laterally (aside from negligible spreading due to diffusion) but move instead along the applied electric field lines. Spreading of the light in the indirect scintillator approach is a well-known factor causing deteriorated spatial resolution. Thus, the direct approach offers better spatial resolution than the indirect approach. It also allows for construction of thicker, more efficient detectors without any loss in resolution due to lateral spread in the detector.

Several high Z amorphous and polycrystalline semiconductor materials may be used for this application including thallium bromide (TlBr), amorphous selenium (a-Se), lead iodide (PbI2), cadmium zinc telluride (CdZnTe), and mercuric iodide (HgI2). Of these, HgI2polycrystalline films should be used for X-ray converters due to basic characteristics of the material.

HgI2may offer the most efficient energy transfer due to high X-ray stopping power and low mean energy required for electron-hole pair generation, low dark currents, and good long-term stability with a proper surface passivation. The remaining parameters, including mobility lifetime product for electrons and holes are among the highest of all of the candidate materials. In addition, HgI2can be easily deposited by low temperature thermal evaporation without altering its stoichiometry during the sublimation process.

Before the polycrystalline film is grown on a substrate, the source material should be purified so that the growth process may use the purest possible source material. For example, in an exemplary embodiment according to the present invention, the polycrystalline film is grown using mercuric iodide (HgI2) powder with impurity concentration of major active contaminants of less than approximately 10 parts per million (PPM). In this embodiment, to purify the HgI2used for growth of the polycrystalline films, the starting compounds including mercury and iodine, such as, for example, mercuric chloride (HgCl2) and potassium iodide (KI), respectively, should be highly purified.

Then the HgI2should be synthesized using the highly purified starting compounds (e.g., HgCl2and KI). In other embodiments, source compounds other than HgCl2and KI may be purified, then used to synthesize HgI2. An exemplary process for preparation of HgI2is disclosed in N. L. Skinner et al., “Preparation and Evaluation of Mercuric Iodide for Crystal Growth,” Nucl. Instr. & Meth. A283 (1989) pp. 119–122, the contents of which are fully incorporated by reference herein.

Then HgI2may be purified through the “4XMS” process disclosed in H. A. Lamonds, “Review of Mercuric Iodide Development Program in Santa Barbara,” Nucl. Instr. & Meth. 213 (1983) pp. 5–12, the contents of which are fully incorporated by reference herein. The “4XMS” purification process includes HgI2vacuum sublimation under continuous evacuation, then thermal breakdown and coalescing of impurities in the molten HgI2when the HgI2is melted and then cooled, and finally filter sublimation under vacuum in a closed system. The filtering sublimation, for example, may be through a ceramic frit in an evacuated and sealed glass tube.

Separation between HgI2and the impurities occurs during vaporization because different materials vaporize at different temperatures. For example, HgI2 vaporizes at lower temperature than most impurities in this case. Further, some impurities coalesce and form larger particles than HgI2, and so they don't make it through the ceramic frit.

In other embodiments, other processes known to those skilled in the art may be used to purify the source compounds and the synthesized HgI2. In still other embodiments, other metallic iodides, such as, for example, lead iodide PbI2or other suitable high Z amorphous and/or polycrystalline semiconductor materials may be used to fabricate the polycrystalline film on a substrate. In each case, highly purified stoichiometric molecules are formed, and then sublimed to grow the polycrystalline film on a substrate.

FIG. 1is a schematic drawing of a thermal evaporation system100, which may be used to grow polycrystalline films, such as, for example, HgI2polycrystalline films, in an exemplary embodiment according to the present invention, using a thermal evaporation method. The thermal evaporation system100includes a furnace102, which is used to heat up source material112(e.g., HgI2powder) for evaporation through sublimation to grow polycrystalline films on a substrate114. An ampoule (bell jar)106, which may be made of Pyrex glass, is used to contain the source material112, which should be in powder form.

Vacuum118is applied to the ampoule after loading the source material112but before the growth process takes place. The furnace should be temperature controlled within a predetermined range of temperatures. Prior to and during the growth process, the ampoule106is sealed so that the vacuum is maintained within the ampoule. The sealed ampoule106should not contain undesirable impurities, such as, for example, organic based (carbon-based) and metallic based materials.

The substrate114may be fabricated from silicon, alumina, glass or other suitable materials, and may contain circuitry for electronic readout of the x-ray produced signals. When the alumina substrate is used, it may be thinly coated with palladium to provide metal contacts. In a further embodiment the contact and the HgI2may have a blocking barrier formed between them by coating the contact with a thin layer of an insulator material such as “parylene” in order to control the flow of current between the HgI2and the contact and to prevent chemical reaction between the HgI2and the contact. In one exemplary embodiment, said insulator layer is deposited over the entire substrate containing the contact.

The ampoule106should be mounted such that its opening surrounds a substrate holder and cooler104, which is used to hold the substrate114. The surface of the substrate holder and cooler104which interfaces with the substrate may, for example, comprise Teflon®. Teflon® is a registered trademark of E. I. du Pont de Nemours and Company, a Delaware corporation having a place of business at 101 West 10thSt., Wilmington, Del. 19898. The cross section116illustrates the substrate114held in place by the substrate holder and cooler104situated at the opening of the ampoule106, which abuts a top holder110. The top holder110, for example, may be made of stainless steel.

The substrate temperature controller122should be used to control the temperature of the substrate114to be at a predetermined temperature or within a range of predetermined temperatures by controlling the temperature of the substrate holder and cooler104. Therefore, the substrate holder and cooler104includes an active cooler controlled by the substrate temperature controller122. A digital thermometer120may be used to monitor the temperature of the substrate114, and may provide feedback control capability to the substrate temperature controller122. The top holder110holds the substrate holder and cooler104over the furnace102, and an insulator108keeps the stainless steel holder110substantially insulated from the furnace102.

In another exemplary embodiment according to the present invention, an additional heating element128is situated at the outside of the growth ampoule106. The heating element may have a resistive device separating elements126and thermocouples127placed at various points in the furnace and/or on the ampoule106in order to control and maintain a fixed three-dimensional temperature profile within the growth ampoule106.

A thermocouple switch may be used to switch between different thermocouples to monitor temperatures. In an embodiment where there are multiple digital thermometers each for measuring different thermocouple temperatures at various places in the furnace, the thermocouple switch124may not be needed.

It should be noted that in this and other embodiments, the process of fabricating polycrystalline films (e.g., HgI2films) are completed upon growth of the film through the evaporation process in the thermal evaporation system ofFIG. 1. In other words, in these embodiments of the present invention, no further post-deposit processing on the polycrystalline film, such as, for example, heat-treatment (e.g., sintering) to form a single, coherent, continuous coherent film, is required or used to produce the X-ray sensitive digital detector. Further insulation and ambient temperature and environmental controls may be used in other embodiments.

In preparation for film growth, the substrates may be coated with a thin layer of palladium on one side to serve as the rear electrical contact to the polycrystalline film. In embodiments wherein the polycrystalline film is grown on readout arrays, such as, for example, TFT arrays on amorphous silicon or CMOS arrays, palladium coating may not be needed since these readout arrays typically already contain metal (e.g., palladium (Pd), Indium Tin Oxide (ITO), or Titanium Tungsten (TiW)) contacts. In certain embodiments the contact and the HgI2may have a blocking barrier formed between them by coating the contact with a thin layer of an insulator material such as “parylene” in order to control the flow of current and to partially isolate the HgI2from the contact. In one exemplary embodiment, said insulator layer is deposited over the entire substrate containing the contact.

The prepared (e.g., palladium coated) substrates are mounted in the substrate holder and cooler104as seen in the cross sectional illustration116ofFIG. 1. The ampoule106, which may also be referred to as a growth ampoule, should be loaded with the source material112(e.g., high purity grade mercuric iodide (HgI2) in powder form) and evacuated to, for example, between 10−5and 10−7Torr and more particularly, between 5×10−5and 5×10−6Torr.

Prior to loading the source material, the growth ampoule should be cleaned with aqua regia or other suitable cleanser, rinsed with distilled water, and then baked for 12 hours at 300° C. The baking should remove moisture and/or other impurities remaining in the ampoule. In other embodiments, the number of hours and temperature used for baking may be different. For example, the number of baking hours may be inversely proportional to the temperature used for baking.

The ampoule106(after loading the source material112) should be placed inside the furnace102, which may also be referred to as a resistance furnace, and should be kept at Tsource, while the substrate is cooled (relative to the furnace102and the source material112) to be at a separately controlled temperature, Tsubstrate.

Several crystal growth parameters including Tsource, Tsubstrate, source-substrate distance, and vacuum may be adjusted to adjust the film growth rate and to improve conditions for film growth. For example, the temperature and distance ranges may be between 100° C. and 220° C. for the Tsource, between 10° C. and 130° C. for the Tsubstrate, and between 7 cm and 15 cm for the source-substrate distance. The growing time may range from 25 to 120 minutes, depending on Tsourceand intra-ampoule pressure after evacuation.

The ranges for source temperature and substrate temperature for growth in an exemplary embodiment according to the present invention are as follows. The range for the source temperature Tsourceis 120° C. to 160° C. Although higher temperatures may result in higher growth rates, films at high temperatures may exhibit internal stresses, which in turn may cause adhesion failures. The range for the substrate temperature Tsubstrateis 20° C. to 85° C. These conditions should result in a reasonable growth rate of approximately 2 to 5μm/min.

Substrate temperatures higher than 90° C. may produce incomplete film deposition, resulting in non-uniform growth of the film. In addition, substantially uniform temperature should be maintained throughout the substrate for uniform growth of the film. In other embodiments, precise temperatures may be achieved and maintained throughout the 3-D volume using additional heating elements, thermocouples, and controls.

It should be noted that the substrate temperature Tsubstrateof substantially higher than 100° C. may adversely affect the TFT array on the substrate. Further, it should be noted that HgI2may have an undesirable chemical reaction with some material, such as, for example, gold (Au) or aluminum (Al), if they are present during the film growth process either as impurities or in the readout array. Materials such as, for example, palladium, indium tin oxide (ITO), indium oxides and tin oxides typically do not react with HgI2, and may be used on the readout arrays.

In an exemplary embodiment according to the present invention, the physical characteristics of the produced HgI2polycrystalline film such as film thickness, grain size and texture may be controlled and verified. In other embodiments, other characteristics of the film may be controlled and verified as well. In this embodiment, films are characterized by optical microscopy for grain size and uniformity, powder X-ray diffraction for crystallinity, and radiation transmission for thickness gauging.

The optical microscopy may be performed using a high power microscope with a digital camera. For example, the high power microscope used may be Olympus® BH2-UMA microscope and the digital camera used may be Kodak® DC 120 digital camera. Olympus® is a registered trademark of Olympus Optical Co., Ltd, a Japanese Joint Stock Company having at 2-chome, Hatagaya, Shibuya-ku, Japan. Kodak® is a registered trademark of Eastman Kodak Company, a New Jersey corporation having a place of business at 343 State Street, Rochester, N.Y.

It can be seen from the optical microscopy that the polycrystalline films are made of a number of grains, each individual grain typically comprising a single crystal. The grain size (ave±σ) has been measured for a number of polycrystalline films. The measured grain size ranges from (11±5) to (160±90) μm depending on the growth parameters with smaller grain sizes resulting when the substrate is cooler. In other embodiments, grain size can be controlled by adjusting the source-substrate temperature gradient, the vacuum, and the source to substrate distance.

Through the optical microscopy characterization, it has been determined that the grain size of the HgI2layers can be regulated in a controllable and repeatable fashion from 11 to 160μm in an exemplary embodiment by selecting the substrate temperature, as illustrated, for example, inFIG. 2.FIG. 2illustrates a graph150of natural logarithm (ln) of the grain size in μm versus 1/T, where T is the substrate temperature in units of (Kelvin×10−3).

The grain size of 11 μm may be suitable for polycrystalline films deposited on TFT arrays, which may be formed on amorphous silicon substrate, and may provide adequate spatial resolution for digital radiography in many medical applications. While either very small grains (factor of one or more less than the readout pitch, and may be a factor of two or more less than the readout pitch) or a large single crystal that covers many, and possibly all the readout pixels may be the most suitable for matching to a pixelated readout, the spatial resolution of the digital X-ray detector may depend on grain sizes as well as the pitch of the readout arrays, e.g., TFT arrays. For example, typical TFT arrays may have 127μm pitch, even though TFT arrays may also have other pitches ranging from a few microns to a few hundred microns depending on the fabrication technology and process used and the application that the TFT arrays are targeted to.

FIG. 3illustrates an X-ray diffraction diagram160for a powder sample and X-ray diffraction diagrams162,164and166for polycrystalline films. The X-ray diffraction diagrams162,164and166represent films grown at the substrate temperatures of 10° C., 17° C., 85° C., respectively. The X-ray powder diffraction may be performed for grown films using a diffractometer, such as for example, a Siemens® Diffractometer. Siemens® is a registered trademark of Siemens Aktiengesellschaft, a German corporation having a place of business at Wittelsbacherplatz 2 Munich, Germany.

For each film, the texture may be estimated according to: [Σ(0 0l)/Σ(h k l)], which measures orientation of crystal, measuring peaks in different spectra. InFIG. 4, this relationship is plotted against substrate temperature (during film growth), and a correlation between the preferred orientation of the crystal with C-axis perpendicular to the substrate and substrate temperature can be deduced. An increase in preferred orientation with C-axis perpendicular to the substrate is observed with increasing temperature. See for example (002) peaks inFIG. 3. The texture value for the powder (which is random) may be used as reference as seen inFIG. 4. As can be seen fromFIG. 4, the crystallographic orientation of the film and texture may be regulated towards better values by selection of the substrate temperature.

FIG. 5illustrates a set up for measuring thickness of a polycrystalline film, such as, for example a polycrystalline HgI2film. Film thickness may be determined by using the set up ofFIG. 5via a radiation transmission method using a highly collimated (Φ=0.5 mm)241Am source (60 keV)184. The241Am source184should be collimated by a collimator182. The gamma rays passed through a HgI2polycrystalline film186should be detected using a 1″×1″ CsI(Na) scintillation crystal188coupled to a photomultiplier tube (PMT)190whose signal is then processed using a preamplifier,192that conditions the signal suitably so that it may be further connected to a multi-channel analyzer (MCA)194, on which the resulting energy spectrum may be recorded.

The attenuation of gamma rays in the layers may be obtained by subtracting the integral number of counts in the 60 keV photopeak transmitted through a substrate with a HgI2deposited layer on it from the integral number of counts in the same photopeak window transmitted through a similar but bare substrate with no HgI2deposited on it.

By use of the well-known value of the linear attenuation coefficient at 60 keV in HgI2the thickness of the films may be determined. Also, by making several collimated measurements at various locations over the surface of the grown layers, their uniformity may be measured as well. The thickness of the grown layers may vary depending on the growth conditions from 50 to 150 μm and the uniformity of thickness (±σ/mean %) in the layers may be less than +/−2% over a 4-in2area.

FIG. 6illustrates thickness of HgI2required for 99% stopping versus energy (solid line) and percentage stopping for a 500 μm HgI2film versus energy (dashed line).FIG. 8(solid line) shows the film thickness (μm) required for 99% stopping in HgI2as a function of X-ray energy. Film thickness of 150μm is sufficient for 99% stopping up to 50 keV. In fact, at X-ray mammography energies (17 keV for Mo anode X-ray tubes and 21 keV for Ag anode X-ray tubes) even 50 μm thin film may stop more than 99% of the X-rays. However, in order to obtain high efficiency for X-ray energies in the 100 keV range (150 μm gives between 70%–35% attenuation in the 100–150 keV region) thicker layers are preferred.

For example, for taking breast images in mammography, the film thickness of 20 to 50μm may be sufficient to detect adequate X-ray energy. However, when taking body images in radiography, X-ray energies in the range of 100 keV may be used, and the film may need to have a thickness of few hundred μm.

Palladium front contacts may be deposited onto the HgI2films by thermal evaporation using a vacuum coating unit, such as, for example, an Edwards Vacuum Coating Unit (Model E306A), under a vacuum of, for example, 10−4to 10−6Torr, and more particularly 10−5Torr.

Two or more kinds of contacts may be deposited onto several films. First, larger contacts covering the whole or part of the film area may be deposited for studying the X-ray ray response and uniformity of the films. After verifying an acceptable response, array contacts with sizes from a few microns2up to a few mm2each may be deposited to further study film uniformity and image capabilities. Readout may be accomplished using TFT, CMOS, or other such technology.

The HgI2polycrystalline detectors may be characterized by measuring the basic electrical properties such as dark current, resistivity, mobility, mobility-lifetime, and the linearity and sensitivity of response to X-rays. Dark current may be measured as a function of the applied voltage for all grown films by applying bias voltage between the front (entrance) and back electrodes. The measurements may be carried out using a DC voltage power supply and a Pico ammeter, which for example may be a Keithley® Model 487. Keithley® is a registered trademark of Keithley Instruments, inc., an Ohio corporation having a place of business at 12415 Euclid Ave., Cleveland, Ohio.

FIG. 7illustrates a graph of dark current density versus detector bias for 1×1 cm2detectors for several representative films. It can be seen fromFIG. 7that the film dark current is on the order of a few pA/cm2and that the apparent resistivity range of the films is 1×1014to 6×1014ohm-cm. The film dark current obtained for three films is almost three orders of magnitude better (lower) than dark current values reported previously by those skilled in the art. In addition, apparent resistivities are higher than reported data (2×1012ohm-cm). This may be due to the use of highly purified HgI2as starting (source) material and preparation of the polycrystalline films according to the present invention.

The mobility of charge carriers may be measured by irradiating the HgI2film using beta particles from a204Tl source. A pixel electrode may be connected to a fast preamplifier, and the pulses resulting from the interaction of the beta particles in the film may be displayed and recorded on a digital oscilloscope, which for example may be Tektronix® Model TDS 380, 400 MHz. Tektronix® is a registered trademark of Tektronix, Inc., an Oregon corporation having a place of business at 14200 SW KARL BRAUN DRIVE (50-LAW), Beaverton, Oreg. 97077. Charge carrier mobility may be calculated according to μ=L2/T V where L is the thickness of the layer, V the bias voltage and T the risetime.

FIG. 8illustrates an example of a captured pulse in this measurement set up, which is a voltage pulse from fast-risetime pre-amplifier collected by the digital oscilloscope (electron collection). The waveform corresponds to generation and transport of electrons in the HgI2layer, from which a mobility of 16 cm2/Vs can be obtained. This is a good charge transport value for a polycrystalline film, especially when compared with electron mobilities obtained for HgI2monocrystals (2.1 cm2/Vs≦μe≦125 cm2/Vs) known to those skilled in the art.

The mobility-lifetime product of charge carriers may be calculated using the same beta particle source measurement by means of the Hecht relation, given by
Q/Q0=(μτE/L)(1−exp(−L/μτE))
where Q is the collected charge, Q0is the charge generated initially and E is the electric field (bias voltage divided by the film thickness L). Q0may be determined experimentally by finding asymptotic value at high bias voltages where signal no longer increases. By fitting the measured data to the Hecht relation mobility-lifetime values of 6×10−5cm2/V may be calculated, which may be similar to values obtained with HgI2monocrystals as well as obtained for other HgI2layers (4×10−5cm2/V, 6.8×10−5cm2/V) as those skilled in the art would appreciate.

The response of Polycrystalline HgI2detectors to X-rays may be determined by measuring the response (detector current) to X-rays from an X-ray generator, as a function of the tube voltage (for example, in the range of 10–150 kV) and as a function of the detector's applied bias. Linearity of response may be characterized as a function of the X-ray exposure by making measurements as a function of the X-ray tube current. The exposure rate may be calibrated using a calibrated camera, such as, for example, RAD-CHECK® Plus, Model 06-526. RAD-CHECK® is a registered trademark of Victoreen, Inc. an Ohio Corporation having a place of business at 1505 Jefferson Ave., Toledo Ohio 43697

FIG. 9shows the X-ray response linearity for a representative film sample. X-rays impinging the detector are pre-filtered with a 1.7 mm Aluminum plate filter. The detector current may then be measured with a Pico ammeter, such as, for example, Keithley model 487.

FIG. 10shows the X-ray sensitivity versus exposure for several representative detectors. The X-ray response uniformity with exposure rate is very good and very repeatable for many detectors. The measured values of sensitivity compare very well with these of mercuric iodide films and with other materials (PbI2and a-Se) known to those skilled in the art.

FIG. 11is a block diagram of a digital radiography system300, in which an exemplary embodiment according to the present invention may be applied. The digital radiography system300includes a radiation detector302and an image processor308. The image processor308may be coupled to a display310for displaying processed radiographic images. The digital radiography system300, for example, may be used for X-ray imaging applications.

The radiation detector302includes an array detector304and one or more pre-amplifiers306. The array detector304may include the HgI2polycrystalline film fabricated according to an exemplary embodiment of the present invention for direct detection of radiation to generate electrical signals for the radiographic images. The electrical signals may be processed by pre-amplifiers306and applied to the image processor308for further processing to generate displayable images.

FIG. 12illustrates an array detector340, which may be fabricated using the HgI2polycrystalline film fabricated according to an exemplary embodiment of the present invention. The HgI2polycrystalline film, for example, may be capable of directly detecting X-ray. The array detector320is a blow up drawing of the array detector340. The array detector320includes column electrodes322, a guard ring324, a HgI2 polycrystalline film326, row electrodes328and a ceramic substrate330. The ceramic substrate330, for example, may be made of alumina or any other suitable ceramic substrate, and may be attached to a printed circuit board in practice.

The column and row electrodes may be fabricated by using a photolithographic technique. The electrodes may also be fabricated by evaporating, for example, Pd (palladium) contacts onto the HgI2polycrystalline films through physical masks. As illustrated inFIG. 12, the detector array320may be constructed in a cross-grid array configuration with row electrodes on the front side, and column electrodes on the back side.

FIG. 13is a block diagram of a prior art digital radiography system350, in which an exemplary embodiment according to the present invention may be applied. The digital radiography system350includes a radiation detector352and an image processor360. The digital radiography system350, for example, may be used for X-ray imaging applications.

The radiation detector352includes an array detector354, which may include HgI2polycrystalline film fabricated according to an exemplary embodiment of the present invention and have the configuration of the array detector320ofFIG. 12. The array detector354includes row and column electrodes, and are coupled to row multi-channel pre-amps356and column multi-channel pre-amps358, respectively. The pre-amps356and358may be charge sensitive pre-amps.

The outputs of the pre-amps356and358are coupled to shaping amplifiers362and digital signal generators363in the image processor360for generation of digital signals that indicate occurrence of events. The output of the shaping amplifiers362are also provided to an ADC (analog-to-digital converter)364to generate corresponding digital signals. A coincidence logic & encoder365generates displayable images by correlating the outputs from the digital signal generators363and the ADC364, and encoding the correlated output. A computer366may be used to control image processing, and the images may be displayed on a display368.

FIG. 14is a block diagram of a digital radiography system400utilizing an amorphous silicon TFT (thin film transistor) readout406in which an exemplary embodiment according to the present invention may be applied.

A radiation detector404formed of a HgI2 polycrystalline film is deposited on the TFT readout406. An X-Ray Generator402produces x-ray radiation, which is attenuated by an object403under examination. The resulting image obtained with the detector404is read out with the help of the TFT readout406and associated electronics.

The readout electronics includes the following components: amplifiers412, a multiplexer414, gate drivers408, digital sequencer410, and an A/D converter416. The image is displayed and stored in a host computer418.

In the digital radiography system400in an exemplary embodiment, a plurality of readout electrodes may be formed on the TFT readout406. Further, a single electrode may be formed on the polycrystalline film. When a bias voltage is applied between the first and second electrodes, it creates an electric field within the polycrystalline film, and the electric field facilitates signal formation in response to an x-ray radiation. In other embodiments, a plurality of electrodes may be formed on the polycrystalline film. The digital radiography system400may also includes a plurality of pre-amplifiers, each of which is capable of processing signal from one of the readout electrodes.

In the digital radiography system400in another exemplary embodiment, a plurality of readout electrodes may be formed on TFT readout406. A thin layer of insulator material may be coated on the readout substrate by depositing the insulator material on a surface of the TFT readout on which the first electrodes are formed. The thin layer of insulator material forms a blocking barrier between the first electrodes and the polycrystalline film in order to control a flow of current and to chemically isolate the polycrystalline film from the first electrodes.

It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character hereof. The present description is therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.