Invited Paper
Characterization of vapor-deposited Lu2O3:Eu3+ scintillator for x-ray imaging applications Vivek V. Nagarkar*a, S. G. Toppingb, S. R. Millera, B. Singha, C. Brecherc, V. K. Sarinb a Radiation Monitoring Devices, Inc, 44 Hunt Street, Watertown, MA, USA 02472; b College of Engineering, Boston University, 15 St. Mary’s Street, Brookline, MA, USA 02446; c ALEM Associates, 44 Hunt Street, Watertown, MA, USA 02472 ABSTRACT The europium-doped lutetium oxide (Lu2O3:Eu) transparent optical ceramic has excellent scintillation properties, namely very high density (9.5 g/cm3), high effective atomic number (67.3), light output comparable to thallium-doped cesium iodide (CsI:Tl), and emission wavelength (610 nm) for which silicon-based detectors have a very high quantum efficiency. If microcolumnar films of this material could be fabricated, it would find widespread use in a multitude of highspeed imaging applications. However, the high melting point of over 2400°C makes it extremely challenging to make microcolumnar films of this material. We have recently fabricated and characterized microcolumnar films of Lu2O3:Eu. These results are presented in this paper. Keywords: Lutetium oxide; transparent optical ceramic; TOC; microcolumnar films; physical vapor deposition; chemical vapor deposition.
1. INTRODUCTION The Lu2O3:Eu3+ transparent optical ceramic (TOC) scintillator has by far the highest density (9.5 g/cm3) of any known scintillator, high effective atomic number (67.3), excellent light output (comparable to CsI:Tl), and an emission wavelength (610 nm) for which silicon (Si) sensors have a very high quantum efficiency. Furthermore, the scintillator is completely transparent to its own light, which is essential for a high signal-to-noise ratio and reduced glare in the screen. Lu2O3:Eu3+ ceramic is easy to fabricate in a variety of shapes, and exhibits superior chemical stability, particularly with regard to moisture. It also has high optical transmissivity, and shows virtually no scattering in the scintillator volume, allowing superior imaging. This material was invented and developed by ALEM Associates, who have studied the mechanism of scintillation quite extensively, and have summarized the current understanding in a seminal publication [1]. The absorption efficiency of 5µm thick film of several 100 commonly used scintillators is shown in Figure 1. Absorption Efficiency (%)
Although Lu2O3:Eu’s unique combination of properties is Lu2O3:Eu ideally suited for digital X-ray imaging, it has not heretoLSO:Tb fore been possible to make this scintillator in the micro10 columnar form needed to achieve high spatial resolution. LuAG:Ce The primary reason for this is that Lu2O3:Eu is a highly refractory material, with a melting point in excess of 2400°C, which would ordinarily prohibit the use of conventional thermal vapor deposition typically used to produce micro1 CsI:Tl columnar structures. Additionally, dramatic variations in YAG:Ce the material properties of its constituents under vacuum conditions make it impractical to achieve an Lu2O3:Eu film GGG:Eu composition adequate to produce efficient scintillation. 0.1 However, there has been a recent breakthrough: The col0 10 20 30 40 50 60 70 80 laborative efforts between RMD, ALEM Associates, and Energy (keV) Boston University (BU) over the past few years have recently yielded microcolumnar films of Lu2O3:Eu using al- Figure 1: Absorption efficiency of commonly used scintillator materials, at a thickness of 5 µm. The very high X-ray absorption and absence of absorption edges in the 12 to 60 keV range make *
[email protected]; phone 617 668-6937; fax 617 926-9980 Lu2O3 an attractive choice for ultrahigh-resolution imaging. Penetrating Radiation Systems and Applications X, edited by F. Patrick Doty, H. Bradford Barber, Hans Roehrig, Richard C. Schirato, Proc. of SPIE Vol. 7450, 745003 © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.831031 Proc. of SPIE Vol. 7450 745003-1
ternative techniques, namely physical and chemical vapor deposition (PVD and CVD) [2,3,4]. The characterization of these films is presented in this paper.
2. VAPOR DEPOSITION OF LUTETIUM OXIDE FILMS The present state-of-the-art ceramic fabrication process involves several major manufacturing steps, such as the production of co-precipitated powders with base composition Lu2O3, containing 5 mol% Eu2O3, fabrication of TOC disks from this powder using a unique hot pressing process developed at Boston University, cutting and polishing of the disks to the needed specifications, pixelation of the disks by a laser ablation technique developed by RMD, chemical etching and/or annealing of pixelated disks, coating and bonding of the disks to fiberoptic couplings and CCD detector arrays. Both the production of the co-precipitated powder and fabrication of the TOC samples are techniques that have already been largely optimized, and we have produced circular disks of ~4 cm in diameter and several millimeters thick. However, the process, while fine for the laboratory, is not viable as currently constituted for broad-based commercial use for mammography or similar applications. For one thing, there is considerable wastage of expensive materials: Hot pressing is simply not compatible with the production of material whose planar dimensions run in the tens of centimeters but has sub-millimeter thickness. Cutting circular specimens to the required square shape typically immediately wastes at least 36% of the material, but then grinding and polishing to obtain 100 µm thick samples, discarding some 90% of the original material in total, is intolerable. Moreover, the larger the hot-pressed disk size the greater will be the time and cost of the cutting, grinding, and polishing operations. And finally, even allowing for substantial savings from large-scale commercial production, laser pixelation imposes yet another unacceptable cost burden. Thus while we have the potential for a major advance in digital radiology, this cannot be realized unless we can develop an alternative fabrication technique that is amenable to high-volume production of the material in the form of large-area thin layers at an acceptable cost. To overcome these limitations, we have explored vapor deposition methods such as physical vapor deposition (PVD) and chemical vapor deposition (CVD) as alternate fabrication methods. Both PVD and CVD are atomistic in nature, where atoms or molecules form a coating by condensing on a substrate. The primary distinction is that PVD involves strictly physical vaporization by essentially thermal means (e.g., electron beam or plasma) of material from a bulk source of the same chemical composition, while CVD generates the desired material by chemical reaction of gaseous precursors at the deposition surface. Of the two, PVD is a much simpler process wherein a gaseous stream of the desired material impinging upon a substrate held at a suitable temperature results in the formation of a film of the desired thickness and morphology. The PVD approach is already being used by RMD for the commercial production of microcolumnar films of CsI:Tl which provide superb imaging resolution without the need for pixelation. However, Lu2O3 is a highly refractory compound, with a melting point in excess of 2400°C and a correspondingly minuscule vapor pressure. It is therefore impossible to achieve the requisite vapor transport rate by conventional thermal means, but requires the focused application of high power to a very small area on the surface of the target that provides the material to be transported and deposited. We have used sputtering as a PVD technique to deposit thin films of Lu2O3 material. We have also used the CVD technique for fabricating the microcolumnar films of Lu2O3. 2.1 PVD Deposition The PVD process in this study used a radio frequency magnetron sputtering gun, with argon as sputtering gas. This generates argon plasma and accelerates the ionized particles toward the target material as depicted in Figure 2(a). There are many variables in the PVD process that can strongly affect the outcome of the coating. Some of these are independently controllable, while others are coupled so that a change in one affects the values of others. Some of the critical parameters that are under the control of the user are sputtering power, total and partial pressures of the constituents, substrate temperature, rotation speed, substrate-to-target separation, and target tilt (0 or 45 degrees). One of the greatest advantages of PVD is the flexibility of depositing at different temperatures; typically from room temperature up to 900°C. The growth mechanisms at these two extremes are drastically different from each other; at room temperature, the diffusivities are orders of magnitude lower. A material’s bond type and strength plays a significant role in room temperature deposition. For ionic compounds, room temperature deposition combined with rapid deposition rates, can produce coatings with very poorly defined crystallinity, either containing high populations of lattice defects or becoming amorphous altogether. Which of these occurs is not predictable simply from their melting points or vapor pressures; for example, deposition of lutetium silicate (Lu2SiO5) at room temperature yields an amorphous coating, whilst deposition of the more refractory lutetium oxide (Lu2O3) yields a crystalline coating. In sputtering, one of the most critical factors in determining the quality of a coating is the plasma. Argon was used as the sputtering gas, not only for its own plasma properties but also because it could serve as a carrier for small amounts of
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Exhaust
(a)
(b)
RF Power
Induction Coil
Graphite Susceptor Substrate
Graphite Susceptor Lu + Eu metal
Figure 2: Schematics of deposition equipment for fabricating Lu2O3 films: (a) PVD sputtering setup; (b) CVD reaction chamber.
reactive gases (particularly oxygen) as well. Such a mixture can be used for reactive sputtering or if we found that the coating was becoming depleted of oxygen by the sputtering process. However, we found that introduction of as little as 1% oxygen into the sputtering gas drastically decreased the sputtering efficiency (by as much as 90%), and was therefore avoided. The total pressure of the system can be considered as one the most critical parameters in sputtering; raising the total pressure increases the number of ions available for sputtering and enhances the rate of material ejection and gas interaction. The kinetic energy of the ejected material when depositing on the substrate affects numerous properties such as stress, adhesion and morphology. By decreasing the total pressure of the system, the mean free path of the ejected material increases, meaning atoms can travel further without gas interactions. The kinetic energy of the ejected material is directly related to the gas mean free path, which is related to pressure, as shown by equation 1:
λ=
RT 2π d 2 N A P
(1)
where λ is the mean free path, R is the gas constant, T is the temperature, d is the gas atom diameter, NA is Avogadro’s number and P is the gas pressure. The RF magnetron sputtering was carried out with power values of 50, 75, and 100 watts, under argon pressure of 5 mTorr, on graphite substrates. Both room temperature (no heating) and heated substrates were used for the film growth. The sputtered film growth rate varied from 0.3 – 0.8 µm/hr (for 50 to 100 W power). 2.2 CVD Deposition Our CVD study of the co-deposition of lutetium oxide and europium oxide (to form Lu2O3:Eu3+) uses the carbon dioxide (CO2) - hydrogen (H2) - halide system, as shown in the equation (2) below:
(2 − x) LuCl3 ( g ) + ( x) EuCl3 ( g ) + 3CO2 ( g ) + 3H 2 ( g ) → Lu2− x Eu x O3 ( s) + 3CO( g ) + 6 HCl ( g )
(2)
In this equation, lutetium chloride (LuCl3) and europium chloride (EuCl3) react in the gas phase with CO2 and H2 to form a solid solution of lutetium and europium oxides with byproducts of carbon monoxide (CO) and HCl. Thermodynamic calculations using HSCTM were used to ascertain the viability of this CVD process. The Gibbs free energy of reaction (ΔGrxn) at 1000°C was determined to be -439 kJ/mol for the formation of Lu2O3, compared to -170 kJ/mol for the formation of Eu2O3. Although this difference in free energy could result in a variance in the driving forces for the formation of the respective oxides, it is most likely favorable in this study since the proportion of Eu desired in the coating deposits (x in equation (2)) is relatively low (5% or less). Furthermore, Lu2O3 and Eu2O3 are the most stable phases at all elevated temperatures. Even though LuCl3 and EuCl3 are solids at room temperature, their vapor pressures at the deposition temperatures of 950°C to 1100°C are high enough to provide an adequate reactant flow. Since the metal chlorides are extremely hygroscopic and difficult to handle, they were generated in-situ by reacting chlorine gas with lutetium and europium metal, which were cut into small particles in order to provide sufficient surface area for rapid chlorination and sublimation. The temperature and flow rates were judiciously controlled to assure that reaction would be complete, with no buildup of
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solid lutetium chloride on the surfaces of the elemental metal that might interfere with smooth and continuous generation of the gaseous chloride reactants. This is a critical consideration; since the rate constant for the metal-chlorine reaction is considerably greater than that for the sublimation of the rare earth chloride, it is the latter that ultimately defines the maximum rate of the deposition itself. A schematic of the deposition chamber is shown in Figure 2(b). For CVD deposition, the process temperature was varied from 1000°C to 1300°C, and the pressure was maintained at 75 Torr. The pressure of the LuCl3 vapor was ~1.5 Torr, pressure of CO2 was 14.5 Torr, and the pressure of hydrogen was 59 Torr. Graphite was used as the substrate; with a total gas flow rate of 2 slm, the resulting growth rate was
