Patent Publication Number: US-2012044610-A1

Title: Y-Doped Barium Strontium Titanate For Stoichiometric Thin Film Growth

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/825,042, filed Apr. 14, 2004, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/463,153, filed Apr. 14, 2003. Each application is incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to improved dielectric materials, more particularly to the manufacture and use of thin film yttrium-doped barium strontium titanate (hereinafter “Y-doped BST”) as a dielectric material in voltage variable capacitors, and still more particularly to a method for incorporating dopants into an ABO 3  perovskite crystal structure. 
     2. Description of the Related Art 
     BST thin films are interesting because of their large field-dependent dielectric permitivity. This property makes BST thin films highly useful, for example as dielectric materials for voltage variable capacitors. BST has a perovskite crystal structure with generic form ABO 3 , where A and B represent the 2+ and 4+ cations, respectively. 
     BST thin films can be fabricated using a variety of methods, including sol-gel processing and physical vapor deposition.  FIG. 1  illustrates a prior art method of forming BST films using sol-gels. The process starts with a sol gel film  110  that is dried to create a green body  120 . The green body  120  is then sintered or “baked”  130 . The resulting material has a more or less solid crystalline structure  140 . A problem with this structure  140 , however, is that the composition of the crystal structure is often not uniform and that it typically includes many voids. Further, sol-gels provide poor thickness control. Moreover, the green body method is not capable of producing films with a highly uniform thickness and is limited to films having a thickness no less then about 1000 Angstrom (Å). 
     Composition and stoichiometry are primary factors that determine the properties of BST films, which are characterized by the chemical formula: 
       Ba 1-x Sr x Ti 1+y O 3 . 
     For the purpose of this description, composition is defined in BST films as the ratio of Ba to Sr to Titanate and, optionally, the amount of various other dopants and impurities. Stoichiometry is defined as the ratio of Ba and Sr atoms to Ti atoms in the BST material. It is noted that the unit of measure for the composition is referenced in atomic percentages. 
     The dielectric constant of a BST film can be tailored for a specific temperature range by altering the film&#39;s barium to strontium ratio, i.e., its composition. Dielectric leakage can be reduced by altering the stoichiometry of the material, for example by making BST films with excess titanium, e.g., (0.1&gt;y&gt;0.01). Various dopants also may be used to modify the properties of BST ceramics. For instance, substituting a larger cation, such as zirconium, for titanium, can reduce the temperature dependence of the dielectric constant. 
     A problem with BST thin film development is that the complex perovskite ceramic crystal structure often causes non-stoichiometric film growth. Film stoichiometry can drastically alter the material properties of the dielectric. Stoichiometry is important because of the numerous defect incorporation and compensation methods in complex oxide materials. For example, a defect such as an oxygen vacancy will be charge-balanced by additional charge carriers, i.e., electrons, in the material. This type of defect compensation mechanism introduces carriers that degrade the insulating behavior of the BST material. 
     Another problem with BST thin-film development is that the use of physical vapor deposition techniques, such as sputtering, complicate stoichiometric film growth because of the mass and pressure dependent scattering angles for barium, strontium, oxygen and titanium. That is, these ions travel from the sputtering target to the deposition substrate at different angles. Thus, for example, the multi-component thin film resulting from the sputtering process will not necessarily reflect the stoichiometry of the sputtering target. Such non-stoichiometric growth results in a dielectric material that may exhibit poor energy quality, poor quality factor, and a low breakdown voltage. 
     In view of all of the above issues, there is a need for an improved thin film BST material, a process for growing such a material and a resulting capacitor with a dielectric that provides an increase in energy quality, quality factor and breakdown voltage, and which may have voltage-variable properties. 
     SUMMARY OF THE INVENTION 
     The present invention includes a process for doping an ABO 3  film, for example, barium strontium titanate, with yttrium (“Y”). The inventive thin film growth process places yttrium in the perovskite crystal lattice structure at some of either the A or B lattice sites, or both. That is, the yttrium can be incorporated into the perovskite structure at either: (i) an atomic site where barium or strontium atoms are normally located (an A site), or (b) an atomic site where titanate normally would be found (a B site). 
     In one embodiment, the process for BST thin-film growth in accordance with the present invention includes reducing the pressure in a sputtering chamber containing an insulating substrate and a yttrium-oxide doped BST target, raising the temperature in the chamber, starting a flow of inert gas (e.g., Ar) and oxygen into the chamber and energizing the gas to create a plasma within the chamber between the target material and the substrate. It is noted that in one embodiment, sputtering may cause oxygen to be lost in the transport process. Hence, oxygen may be flowed in to create a reactive sputtering during the deposition. 
     Using the flow of argon gas as an example, the argon ions forming the plasma bombard the target so that yttrium oxide doped BST target material is transferred to the substrate and a resulting thin film of Y-doped BST grows on the substrate. As previously mentioned, Y atoms comprising the dopant can be incorporated into the BST lattice of the deposited thin film at either the Br or Sr sites (i.e., the A sites), or at the Ti sites (i.e., the B sites). 
     An advantage of the present invention is that the use of a yttrium oxide doped BST target beneficially allows for promotion of stoichiometric crystal growth on a substrate, thereby eliminating or suppressing defects in the crystal lattice structure. In particular, if a target material does not include a perfect stoichiometric mix of Ba, Sr and Ti, or if the physical vapor deposition process alters the stoichiometry due to the mechanisms discussed above, the yttrium dopant may be incorporated at a location in the resulting thin film material normally occupied by any of these atoms, e.g., at least one of the barium or strontium lattice sites or at a titanate lattice site. Yttrium is a useful dopant for BST because of its size and stable 3+-oxidation state. It is intermediate in size between A and B site ions and can be a donor (Y 3+ :Ba 2+ :Sr 2+ ) or acceptor dopant (Y 3+ Ti 4+ ). 
     Use of a Y dopant provides yet another benefit. Yttrium oxide doped BST target material optionally allows for yttrium oxide formation at the grain boundaries in the Y-doped BST thin film. It is believed that such oxide formation inhibits current flow along the BST grain boundaries, thus producing an increased film resistivity, lower losses, and less leakage current. 
     The features and advantages discussed in this specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention has other advantages and features that will be more readily apparent from the following detailed description and the appended claims, when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram of a prior art process for thin-film growth. 
         FIG. 2  illustrates an apparatus for thin-film growth in accordance with one embodiment of the present invention. 
         FIG. 3  illustrates a process diagram for thin-film growth in accordance with one embodiment of the present invention. 
         FIGS. 4   a  through  4   c  illustrate examples of crystal structures for ABO 3  thin-films in accordance with one embodiment of the present invention. 
         FIGS. 5   a  through  5   c  illustrate examples of crystal structures for ABO 3  thin-films grown through use of a Y 2 O 3  doped BST target in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention includes an apparatus and a process for ABO 3  thin-film growth. The present invention also includes devices constructed utilizing the inventive material disclosed herein. 
     In one embodiment of the present invention, the ABO 3  thin-film is barium strontium titanate. BST has a perovskite crystal structure that may be generically referenced as ABO 3 , where A and B represent the 2+ and 4+ cations, respectively. BST thin-films have certain advantages when used with capacitance-type applications because of their large field-dependent dielectric permitivity. Thus, the value of a capacitor utilizing a BST dielectric can be varied with the application of a DC voltage. In addition, dielectric leakage can be improved by having a BST thin film with excess titanium (1% to 10%). It is believed that precipitation of TiO 2  at the BST grain boundaries (like the precipitation of yttrium oxide) helps prevent leakage currents along such grain boundary. 
     The present invention allows for tailoring the composition and stoichiometry of the BST thin-film, which, in turn, allows for altering the properties of the BST to achieve desired characteristics.  FIGS. 4   a  through  4   c  illustrate examples of crystal structures for ABO 3  thin-films, such as BST. For example,  FIG. 4   a  illustrates a lattice structure having two or more A lattice sites with a B lattice site located within the structure formed by the A lattice sites. The A lattice sites provide locations for barium or strontium and the B lattice sites provide locations for titanate. 
     When there is more barium than strontium at the A lattice sites in a BST thin-film lattice structure, the lattice becomes more elongated, for example, as illustrated in  FIG. 4   b . When there are more equal numbers of barium and strontium atoms at the A lattice sites in a BST thin-film lattice structure, the lattice is more cubic, for example as illustrated in  FIG. 4   c . However, as previously mentioned, achieving a perfect or near perfect stoichiometric lattice structure is extremely difficult if not impossible using conventional methods such as physical vapor deposition. 
       FIG. 2  illustrates an apparatus  201  for BST thin-film growth in accordance with one embodiment of the present invention that utilizes physical vapor deposition. The apparatus  201  includes a vacuum chamber  205  with ground shields  245   a ,  245   b . Between the ground shields  245   a ,  245   b  are a cathode  210 , a bonding agent  215 , a BST target  220 , argon plasma  225 , a substrate upon which the Y-doped BST thin film is to be grown  230 , and a heater  240 . Within the chamber  205 , components are ordered with the cathode  210  on top followed by the bonding agent  215 , the BST target  220 , the argon plasma  225 , the substrate  230 , and the heater  240 . Of course, the term “top” is used herein to indicate a relative direction since the chamber may be positioned in any convenient orientation. In one embodiment, the heater  240  is approximately 1 to 2.5 inches from the BST target  220 . 
     In one embodiment, the cathode  210  is comprised of a conductive material, for example, copper, stainless steel, or like material. The bonding agent  215  is comprised of a silver epoxy, indium, or like material. The BST target  220  comprises a yttrium oxide (“Y 2 O 3 ”) dopant in a BST host material. It is noted that the Y 2  0 3 -doped BST target  220  may have 0.10% to 10% yttrium and could also contain other materials and contaminants, as convenient or necessary. It is also noted that the 0.10% to 10% yttrium may be by weight, atomic percentages, mole percentages, or other appropriate ratio. 
     The argon plasma  225  may be an activated argon gas, e.g., Ar. In one embodiment, the chamber is maintained at 20 milliTorr to 50 milliTorr of Ar and a flow rate of 1 to 200 sccm. It is noted that in one embodiment, the argon gas flows through the chamber. The argon gas may contain oxygen at a ratio of, for example, 1 to 20 or 20 to 1. It is noted that the argon plasma may be confined to an area within the ground shields  245   a ,  245   b  in magnetron mode or alternatively could be throughout the chamber  205  in an RF diode mode. 
     The substrate  230  may be comprised of an insulating material that can withstand high temperatures for processing. The substrate may be comprised of aluminum oxide (“Al 2 O 3 ”), e.g., sapphire or silicon, glass, quartz, GaAs, LaAlO 3 , or MGO, or a combination thereof. The heater  240  is a conventional resistance heater that may be configured to provide heat in excess of 400° Celsius. 
       FIG. 3  illustrates a process diagram for BST thin-film growth in the apparatus  201  of  FIG. 2  in accordance with one embodiment of the present invention. The process starts  310  by loading  315  the chamber  205  with the various elements, apparatus and materials mentioned above. The chamber pressure may be at atmospheric pressure or may be pumped down to between 10 milliTorr and 60 milliTorr. Pressure in the chamber  205  is preferably pumped down to a vacuum of 10 −5  Torr to 10 −10  Torr. The temperature is raised by turning on the heater  240  so that the temperature in the chamber  205  ultimately exceeds 400° Celsius. Further, in one embodiment an appropriate temperature may be determined in terms of having the body in the chamber achieve a red glow, for example, at approximately 600° Celsius. 
     The flow  325  of argon and oxygen gas is started and then the plasma is turned on  330  by applying RF power to it. For example, in RF diode mode, 100 to 500 Watts is applied to an 8″ diameter yttrium oxide doped BST target  220 . It is noted that the plasma power may be ramped up gradually when power is supplied or may be immediately spiked (steep ramp) when power is supplied. It is also noted that the argon and oxygen gas may start flowing prior to or at the time the temperature is being raised. With the plasma energized  330 , the growth of the thin-film on the substrate begins. In addition, the process may also be applied in magnetron mode. 
     Once the thin-film growth is completed, RF power to the plasma is turned off  335  and the flow of argon and oxygen gas is turned off. The heater  240  is turned off  340  and the chamber  205  is vented  350 . The chamber may be vented to atmospheric pressure gradually or immediately. It is noted that the cathode  210  can be cooled to remove heat from the BST target to maintain bond stability. This may be done either while the plasma is turned on or after the plasma is turned off. The flow of argon and oxygen gas may be turned off after the plasma is turned off, while the heater is being turned off, or after the heater is turned off. With the process completed, a thin-film growth remains on the substrate  230 . Using this method, thin films of, for example, 10 Å to 250 m thick are achievable. 
     The present invention beneficially promotes stoichiometric BST thin-film growth. More particularly, the addition of yttrium to the BST target  220  (as a Y 2 O 3  dopant) helps promote stoichiometric growth because the yttrium can be incorporated into an A lattice site or a B lattice site in the thin-film structure. Further, the use of a yttrium dopant to promote stoiciometric growth of the BST thin-film on the substrate  230  results in increased film resistivity and lower losses, and reduces leakage current. These are generally desirable characteristics for dielectric materials in applications such as capacitors. 
       FIGS. 5   a  through  5   c  illustrate hypothetical examples of crystal structures for ABO 3  thin-films grown using a Y 2 O 3  doped BST target material in accordance with one embodiment of the present invention. In  FIGS. 5   a  and  5   b , the yttrium (“Y”) is shown at an A lattice site location, for example, where barium (“Br”) or strontium (“Sr”) normally may be found.  FIG. 5   c  illustrates the yttrium located at the B lattice site location, for example, where the titanium (“Ti”) normally may be found. 
     The present invention includes a process for influencing the location in the lattice structure where yttrium preferentially compensates for defects in the lattice structure by using a barium and strontium to titanate ratio that can be described, as follows: when ((Ba+Sr)/Ti)&gt;1 there is a tendency for the yttrium to fill the lattice site location of titanate; when ((Ba+Sr)/Ti)&lt;1 there is a tendency for the yttrium to fill the lattice site locations of either the barium or strontium. In one embodiment, an effect of these tendencies allows for locating Y at a Ba, Sr, or Ti site as illustrated in  FIGS. 5   a  through  5   c.    
     An advantage of a process in accordance with the present invention is that the use of yttrium makes the lattice structure less susceptible to the adverse effects of the defect compensation mechanisms and non-stoichiometric ratios of Ba (and/or Sr) to Ti in the lattice structure that may result from the physical vapor deposition process described above. Moreover, it is noted that the principles of the present invention apply to BST thin films in general and processes for making them, including sputtering techniques, physical vapor deposition, and chemical vapor deposition. 
     Upon reading this disclosure, those of skill in the art will appreciate still additional alternative structural and functional designs for growth of doped and un-doped BST thin films in accordance with the disclosed principles of the present invention. Thus, while particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein. Various modifications (including the use of materials other than BST and Y), which will be apparent to those skilled in the art, may be made in the arrangement, operation and details of the method, materials, apparatus and devices of the present invention without departing from the spirit and scope of the invention as defined in the appended claims and equivalents thereof.