Patent Application
20080010798
The detailed description will refer to the following drawings, wherein like numerals refer to like elements, and wherein:
Described herein are methods of making high yield thin-film capacitors on metallic foils and metal coated substrates with large area electrodes via cofiring the dielectric with the top electrode.
As used herein, loss factor is equivalent to dissipation factor and tan delta (δ), and high dielectric constant is equivalent to high dielectric permittivity and refers to a value of greater than 500. Also, as used herein, firing is equivalent to annealing and large area electrodes or capacitors refers to electrode diameters of equal to or greater than 2 mm and high yields refers to yields above 60%.
The thin-film dielectrics on metal foil or metal coated substrates may be prepared by a variety of deposition techniques including sputtering, and chemical solution deposition. When processed with large area electrodes, the cofired thin-film capacitors described herein have high yields. The thin-film dielectrics have fired thicknesses in the range of 0.5-1.0 micron and have acceptable capacitance densities.
The capacitance density of a dielectric is proportional to its permittivity (or dielectric constant K), divided by the thickness of the dielectric. A high capacitance density capacitor can therefore be achieved by using a thin-film, high dielectric constant (“high K”) dielectric in the capacitor.
High K ferroelectric dielectrics include perovskites of the general formula ABO3, such as crystalline barium titanate (BT), lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), lead magnesium niobate (PMN) and barium strontium titanate (BST). The thin film, high dielectric constant dielectric layer of the method(s) of the present invention may comprise one or more high K ferroelectric dielectrics.
Substituent and dopant cations may be added to the high dielectric constant material to improve the dielectric properties. The properties desired in the thin-film capacitor will dictate the particular combination of added dopants.
Small quantities of suitable dopants include rare earth cations having the preferred oxide stoichiometry of R2O3, where R is a rare earth cation (e.g., Y, Ho, Dy, La, Eu). Rare earth dopants improve insulation resistance in the resulting dielectric.
Transition metal cation dopants such as Mn and Fe may also be used to improve the resistance to reduction in the dielectric and improve the insulation resistance. Other transition metal cations with the preferred oxide stoichiometry of MO2, where M is a metal, such as Zr, Hf, Sn, Ce, may also be suitable dopant cations. These transition metal cations smooth the temperature-dependence of permittivity in the dielectric by “pinching”, i.e., shifting, the three phase transitions of BaTiO3 closer to one another in temperature space.
Metal cations having the preferred oxide stoichiometry of MO, where M is an alkaline earth metal such Ca, Sr, Mg, may also be used to shift the dielectric temperature maxima to lower temperatures, further smoothing the temperature-dependent response of the dielectric.
The above-described dopants, or mixtures thereof, may be used with the perskovite, e.g., BaTiO3 or BaSrTiO3 in various concentrations. A preferred range of concentrations is between about 0 and 5 mole percent of the final formulation.
High K thin film dielectric materials can be deposited by a broad range of deposition methods including chemical solution deposition (CSD), chemical vapor deposition (CVD), evaporation, and sputtering. To obtain high K in these dielectric films, a high temperature, post deposition annealing step is required to achieve crystallization and crystal growth in the thin-film dielectric. The annealing step may be conducted at 800° C. or higher. In one embodiment, for use with copper foil, 900° C. for 30 minutes was utilized. The annealing may be undertaken under a reduced oxygen atmosphere to avoid oxidation of the metallic electrode if a base metal is utilized.
Since crystalline barium titanate and barium strontium titanate display high dielectric constants and are lead free, these are particularly useful to fabricate large area capacitors with high yield. Consequently, described herein are methods of making capacitors that comprise high yields. The dielectric is formed over the bottom electrode. The bottom electrode may be a metallic foil or an electrode deposited on a ceramic substrate. The top electrode is deposited over the dielectric and the whole structure is cofired to form the final capacitor. These capacitors have high yields with large area electrodes.
One embodiment of the present invention provides a method of making a capacitor, comprising: providing a metal foil; forming a dielectric layer over the metal foil; forming a top electrode over the dielectric; and annealing the metal foil, dielectric and top electrode; wherein upon annealing the metal foil, dielectric and top electrode form a high capacitance density capacitor. In a further embodiment, the dielectric layer above is formed by chemical solution deposition or sputtering. In yet a further embodiment of the invention, the top electrode above is formed by sputtering or evaporation.
As noted above, sputtering and chemical solution deposition (CSD) techniques are used to form the capacitor dielectrics and sputtering and evaporation techniques are used to form the top electrodes by the methods described herein above and below.
The barium strontium titanate target composition defines the composition of the dielectric formed over the first electrode. In one embodiment a target composition is Ba0.75Sr0.25TiO3 and in another a doped version, namely Ba0.5Sr0.5Nb0.004Mg0.0036Mn0.0014Ti0.988O3 is used.
The chemical precursor solution contains the desired amount of each component of the desired high dielectric constant material as well as additives useful for achieving other goals, for example, the elimination of cracks. Thus, if the desired high dielectric constant material is barium titanate, the chemical precursor solution may comprise barium acetate and titanium isopropoxide.
Pure BaTiO3 may be prepared from the following chemicals in their respective amounts:
The acetic acid is a dissolving medium for the barium acetate and the titanium isopropoxide and the acetylacetone is a stabilizing agent for titanium isopropoxide. Diethanolamine (DEA) may be added in the range of 8-12% of the weight of barium acetate in order to prevent cracking in the dielectric film. Thus, for example, to the precursor solution of the preceding paragraph, the DEA addition may total 0.58 g.
The precursor solution is deposited over the copper foil substrate. Suitable solution deposition methods include dip, slot die, gravure, spray, or spin coating. In one embodiment, spin coating was utilized and the rotation time and speed used was 30 seconds at 3000 revolutions per minute.
After deposition, the substrate containing the precursor solution is dried to remove solvents. Additional depositions may be applied to build the thickness up to the desired value. In one embodiment, a drying temperature of 250° C. for 5-10 minutes was used and six consecutive deposition and drying steps were used to achieve the final desired thickness.
After the desired dried thickness has been achieved, the substrate is heat-treated at a higher temperature to burn out and remove all or the vast majority of the organic components left in the dried solution deposited film. The heat-treatment temperature is high enough to burn out and remove the organic material but not high enough to substantially crystallize the inorganic dielectric. When heat treated, the dried dielectric decomposes to initially form very fine particles of barium and titanium oxides, carbonates, oxycarbonates and mixtures, thereof and then subsequently the carbonates and oxycarbonates decompose and the remaining oxide mixtures react to form barium titanate. In one embodiment, a temperature of 650° C. for 30 minutes, in a suitably reducing atmosphere. that protected the underlying copper foil was utilized.
A top electrode is formed over the resulting dielectric. The foil serves as the bottom electrode of the capacitor formed by this method. The top electrode can be formed by, for example, sputtering, printing, evaporation or other suitable deposition methods. In one embodiment, sputtered platinum top electrodes are used. In another copper top sputtered electrodes are used.
After deposition of the top electrode, the coated substrate is annealed. Annealing densifies and crystallizes the deposited dielectric. Annealing may be conducted at a high temperature in a low oxygen partial pressure environment for dielectrics deposited on a metallic foil substrate. The annealing temperature should be lower than the melting point of the metallic foil substrate. A suitable total pressure environment is about 1 atmosphere. In one embodiment, copper foil is used as the substrate, the furnace temperature is about 900° C., and the oxygen partial pressure is approximately 10−12 atmospheres. The annealing may be performed by ramping the furnace up to 900° C. at a rate of about 30° C./minute. The furnace is maintained at 900° C. for 30 minutes.
The annealing temperature of 900° C. described above facilitates the use of copper foil as the substrate and allows for crystallization of the deposited dielectric.
Favorable results may also be obtained at annealing temperatures higher than 900° C. Higher temperatures, for example 1200° C., combined with the appropriate atmosphere to avoid oxidation of the metallic substrate facilitate the use of various metallic substrates, such as nickel. Additionally, if the chemistry of the substrate so permits, annealing may be conducted in air, thereby dispensing with a reducing atmosphere. Such substrates may include precious metal foils or ceramic oxide compositions with precious metal electrodes deposited upon them.
The low oxygen partial pressure may be achieved by bubbling high purity nitrogen through a controlled temperature water bath. Other gas combinations such as additions of small amounts of hydrogen containing forming gas to the gas mixture are also possible. The water bath may be at a temperature of about 25° C.
The above-described annealing process for capacitors generally avoids oxidation of the copper foil or electrode to Cu2O or CuO. Oxidation is avoided by selecting an appropriate low oxygen partial pressure for the high processing temperature used during annealing. A range of oxygen partial pressures that reliably avoids oxidation of copper and does not deleteriously reduce the dielectric is between 1×10−9 and 1×10−14 atmospheres. Consequently, high quality BaTiO3, BaSrTiO3, or other high dielectric constant layers may be formed in the absence of any oxidation of the copper foil or severe dielectric degradation during annealing. Alternative metallic foils and annealing temperatures may require different atmospheres. These atmospheres may be calculated from the standard free energy of formation of oxides as a function of temperature as described by F. D. Richardson and J. H. E. Jeffes, J. Iron Steel Inst., 160: 261 (1948).
The capacitor may be optionally subjected to a re-oxygenation process to improve insulation resistance of the dielectric. Re-oxygenation may correspond to a 30 minute anneal at 450° C., at an oxygen partial pressure of approximately 10−4 atmospheres. Re-oxygenation can be integrated into the cooling step of the annealing for example, or performed as a separate step after cooling. If appropriate acceptor dopants are used as described previously, the re-oxygenation step may be dispensed with. Such acceptor dopants include manganese, magnesium, etc.
The following examples illustrate favorable properties in dielectrics prepared according to the present invention, and the capacitors incorporating the dielectrics.
A 1 micron thick barium strontium titanate (Ba0.75Sr0.25TiO3) dielectric precursor thin film was deposited onto a 2″×1″ clean 0.5 oz copper foil by magnetron sputtering. The copper foil was industry standard PLSP copper foil obtainable from Oak Mitsui Corporation. The dielectric precursor was deposited onto the smoother side or “drum” side of the copper foil. The RMS value of the copper foil was measured over an area of 5 microns by 5 microns and 20 microns by 20 microns and found to be 12 nano-meters and 20 nano-meters respectively. Deposition was undertaken at a pressure of 10 mTorr using an Ar:O2 ratio of 5:1 and a substrate temperature of 130° C. The sputter target diameter was 4 inches and the foil to target distance was 8.5 cm. The sputter source was arranged using a 25° “off-axis” geometry. The rf sputtering power was 300 watts and the deposition time was 120 minutes. After deposition, the dielectric film was annealed at 900° C. for 30 minutes in a nitrogen-based atmosphere with an oxygen partial pressure of approximately 10−12 atmospheres. An array of platinum top electrodes was deposited on top of the annealed dielectric layer by magnetron sputtering through a shadow mask. The shadow mask contained 20, 24, 24, 28, 21, 21 samples of each of 3 mm, 2 mm, 1 mm, 0.75 mm, 0.5 mm, 0.25 mm diameter electrodes respectively. The platinum electrode thickness was approximately 0.15 μm.
After deposition of the platinum electrodes, the capacitance and loss factor of the capacitors was measured for the 0.25, 0.5, 0.75, 1, 2 and 3 mm diameter capacitors using a Hewlett Packard 4192A LF impedance analyzer. Capacitors were tested at both 0 and 1 volt bias at both 10 KHz and 1 KHz using an oscillating voltage of 0.05V. Capacitors that were shorted or exhibited no capacitance or had a loss factor greater than 15% were considered to be unacceptable capacitors and included in the yield loss data. The results are shown in graphic form in
To ascertain if smoothness of the copper foil was a key factor in the low yield, very smooth 15 micron thick copper foils were made by evaporation of copper onto a glass plate. After evaporation, the foils were removed from the glass plate resulting in 2 inch by 1 inch copper foils. The RMS value of the evaporated copper foil was measured over an area of 5 microns by 5 microns and 20 microns by 20 microns and found to be 1.4 nano-meters and 1.5 nano-meters respectively and was approximately equivalent to that of the glass plate.
A 1 micron thick barium strontium titanate (Ba0.75Sr0.25TiO3) dielectric precursor thin film was deposited onto the evaporated copper foil by magnetron sputtering using the same conditions as described in example 1. Three foils were made. After deposition, the dielectric film was annealed at 900° C. for 30 minutes in a nitrogen-based atmosphere with an oxygen partial pressure of approximately 10−12 atmospheres. An array of platinum top electrodes was deposited on top of the annealed dielectric layer by magnetron sputtering through a shadow mask. The shadow mask contained 20, 24, 24, 28, 21, 21 samples of each of 3 mm, 2 mm, 1 mm, 0.75 mm, 0.5 mm, 0.25 mm diameter electrodes respectively. The platinum electrode thickness was approximately 0.15 μm.
After deposition of the platinum electrodes, the capacitance and loss factor of the capacitors was measured for the 0.25, 0.5, 0.75, 1, 2 and 3 mm diameter capacitors using the same equipment as used in example 1. Capacitors that were shorted or exhibited no capacitance or had a loss factor greater than 15% were considered to be unacceptable capacitors and included in the yield loss data. The results are shown in graphic form in
A 1 micron thick barium strontium titanate (Ba0.75Sr0.25TiO3) dielectric precursor thin film was deposited onto the drum side of a 2″×1″ clean 0.5 oz PLSP copper foil by magnetron sputtering using the same conditions as used in example 1.
The dielectric film was annealed at 900° C. for 30 minutes in a nitrogen-based atmosphere with oxygen partial pressure of about 10−12 atmospheres. An array of platinum top electrodes with diameters of 1, 2, 3, and 5 mm was deposited on top of the annealed dielectric layer by magnetron sputtering through a shadow mask. The platinum electrode thickness was approximately 0.15 micron.
The results are shown in graphic form in
A 1 micron thick barium strontium titanate (Ba0.75Sr0.25TiO3) dielectric precursor thin film was deposited onto the drum side of a 2″×1″ clean 0.5 oz PLSP copper foil by magnetron sputtering using the same conditions as used in example 1.
An array of platinum top electrodes with diameters of 1, 2, 3, and 5 mm was prepared on top of the dielectric layer by magnetron sputtering through a shadow mask. The platinum electrode thickness was approximately 0.15 micron. The dielectric film with platinum top electrodes was annealed at 900° C. for 30 minutes in a nitrogen-based atmosphere with an oxygen partial pressure of about 10−12 atmospheres.
The results are shown in graphic form in
A 1 micron thick barium strontium titanate (Ba0.75Sr0.25TiO3) dielectric precursor thin film was deposited onto the drum side of a 4 inch×1 inch clean 0.5 oz PLSP copper foil by magnetron sputtering using the conditions used in example 1. A 20 mm×20 mm top electrode contact area was defined by the use of a simple lift-off mask. A platinum top electrode was deposited on top of the dielectric layer and the lift-off mask by magnetron sputtering. The lift-off mask was removed by dissolving it in acetone. The platinum electrode thickness was approximately 0.15 micron. The dielectric film with the 20 mm by 20 mm platinum top electrode was annealed at 900° C. for 30 minutes in a nitrogen-based atmosphere with an oxygen partial pressure of about 10−12 atmospheres.
A 5 nm thick chromium adhesion layer film was deposited on a clean polished 3″ diameter single crystal <100> lanthanum aluminate (LaAIO3) substrate using rf magnetron sputtering with an argon flow of 40 sccm at a pressure of 10 mTorr. Immediately after deposition of the chromium adhesion layer and without breaking vacuum, a 0.5 micron thick copper film was deposited on top of the chromium adhesion layer to form a bottom electrode for the capacitor.
A doped barium strontium titanate (Ba0.5Sr0.5TiO3) dielectric precursor thin film was then deposited on top of the Cu film by dual magnetron sputtering at a pressure of 20 mTorr using an Ar:O2 ratio of 9:1. The sputter target diameter was 3 inches for each of the sputter sources and the foil to target distance (center to center) was approximately 4 inches for each of the two sputter sources. The sputter sources were arranged in “off-axis” geometry with the target surface nearly perpendicular to the foil surface. The rf sputtering power was 150 watts on the one source and 10 watts on the other source. The doped barium strontium titanate target composition was Ba0.5Sr0.5Nb0.004Mg0.0036Mn0.0014Ti0.988O3 for each sputter source. The deposited film thickness was estimated to be 0.5 microns based on a calibrated deposition rate of 3 nm/min. The deposited film thickness was estimated to be 0.5 microns. The dielectric film was annealed at 900° C. for 10 minutes in a nitrogen-based atmosphere with an oxygen partial pressure of about 2×10−12 atmospheres.
A 0.5 micron thick copper top electrode was then sputter deposited onto the surface of the dielectric film through a shadow mask with a pattern of 45 2 mm diameter capacitors and 40 1 mm diameter capacitors.
The 1 and 2 mm capacitors had a yield of 0%.
A 5 nm thick chromium adhesion layer film was deposited on a clean polished 3″ diameter single crystal <100> lanthanum aluminate (LaAIO3) substrate using rf magnetron sputtering with an argon flow of 40 sccm at a pressure of 10 mTorr. Immediately after deposition of the chromium adhesion layer and without breaking vacuum, a 0.5 micron thick copper film was deposited on top of the chromium adhesion layer to form a bottom electrode for the capacitor.
A doped barium strontium titanate dielectric precursor thin film was then deposited on top of the copper film using the same targets and dielectric deposition conditions used in example 6. The deposited film thickness was estimated to be 0.5 microns. A 0.5 micron thick copper top electrode was then sputter deposited onto the surface of the dielectric film through the same shadow mask as used in example 6. The dielectric film with copper top electrode contacts was annealed at 900° C. for 10 minutes in a nitrogen-based atmosphere with an oxygen partial pressure of about 2×10−12 atmospheres.
The 1 mm capacitors had a yield of 100%. The average capacitance value for these 36 capacitors was 4.47 nF (0.57 μF/cm2). The average dissipation factor was 3.0%.
The 2 mm capacitors had a yield of 93%. The average capacitance value was 23.1 nF. The average dissipation factor was 1.6%.
A 0.5 micron thick Cu film was deposited on the drum side of a clean 2″×2″0.5 oz PLSP copper foil using rf magnetron sputtering using argon at a pressure of
10 mTorr. A doped barium strontium titanate (Ba0.5Sr0.5TiO3) dielectric precursor thin film was then deposited on top of the Cu film by using the same targets and dielectric deposition conditions used in example 6. The deposited film thickness was estimated to be 0.5 microns.
A 0.5 micron thick ruthenium top electrode was then sputter deposited onto the surface of the dielectric film through a shadow mask with a pattern of 45 2 mm diameter capacitors. The dielectric film with ruthenium top electrode contacts was annealed at 900° C. for 10 minutes in a nitrogen-based atmosphere with an oxygen partial pressure of about 2×10−12 atmospheres.
The 2 mm capacitors had a yield of 80%. The average capacitance value was 43.5 nF (1.4 μF/cm2) and the average dissipation factor was 4.4%.
A 0.5 micron thick copper film was deposited on the drum side of a clean 2″×2″0.5 oz PLSP copper foil using rf magnetron sputtering using argon at a pressure of 10 mTorr.
A doped barium strontium titanate dielectric precursor thin film was then deposited on top of the copper film using the same targets and dielectric deposition conditions used in example 6. The deposited film thickness was estimated to be 0.5 microns.
A 0.5 micron thick copper top electrode was then sputter deposited onto the surface of the dielectric film through the same shadow mask used in example 6. The dielectric film with copper top electrode contacts was annealed at 900° C. for 10 minutes in a nitrogen-based atmosphere with oxygen partial pressure of about 2×10−12 atmospheres.
The 1 mm capacitors had a yield of 100%. The average capacitance value was 6.09 nF (0.75 μF/cm2). The average dissipation factor was 1.7%.
The 2 mm capacitors had a yield of 98%. The average capacitance value was 29.2 nF (0.93 μF/cm2). The average dissipation factor was 1.5%.
A doped barium strontium titanate precursor thin film of the same composition of example 6 was deposited onto the drum side of a 2″×2″ clean 0.5 oz. PLSP copper foil by dual magnetron sputtering using the same dielectric deposition conditions as used in example 6.
The dielectric film was annealed at 900° C. for 10 minutes in a nitrogen-based atmosphere with an oxygen partial pressure of about 2×10−12 atmospheres.
A 0.5 micron thick copper top electrode was then sputter deposited onto the surface of the dielectric film through the same shadow mask as used in example 6.
The 1 mm capacitors had a yield of 11%. The average capacitance value was 6.32 nF. The average dissipation factor was 5.2%.
The 2 mm capacitors had a yield of 0%.
A 1 micron thick barium strontium titanate (Ba0.75Sr0.25TiO3) dielectric precursor thin film was deposited onto the drum side of a 2″×1″ clean 0.5 oz PLSP copper foil by magnetron sputtering using the same conditions as used in example 1.
2 mm diameter copper top electrodes were prepared on top of the dielectric layer by evaporation of copper through a shadow mask. Three samples were prepared, each with a different thickness of evaporated copper. The thicknesses of the copper were 0.4 microns, 0.6 microns and 0.8 microns. The dielectric film with the 2 mm copper top electrodes was annealed at 900° C. for 30 minutes in a nitrogen-based atmosphere with an oxygen partial pressure of about 10−12 atmospheres.
The results are shown in graphic form in
A chemical solution of barium titanate was spun coated on to the drum side of a clean PLSP copper foil. A rotation speed of 3000 rpm for a period of 30 seconds was used. The film was dried at 250° C. for 8 minutes. Five more depositions were made by alternating the spin coating and drying. The films were then heat-treated at 650° C. for 30 minutes in a nitrogen-based reducing atmosphere with an oxygen partial pressure of approximately 10−17 atmospheres. This temperature was chosen from X-ray diffraction data illustrated in
An array of sixteen 3 mm diameter platinum top electrodes was deposited on top of the heat-treated dielectric layer by magnetron sputtering through a shadow mask. The platinum electrode thickness was approximately 0.1 micron. The burnt out dielectric film with platinum top electrodes was annealed at 900° C. for 30 minutes in a nitrogen-based atmosphere with an oxygen partial pressure of about 10−12 atmospheres.
The capacitor yield was 93.75% or 15 out of the 16 measured. The capacitance and loss factor versus frequency of a 3 mm capacitor is shown in
As detailed in examples 1, 2, and 3 and shown in
As detailed in examples 6 and 10 conventional processing of sputtered thin films, wherein a top copper electrode is applied after annealing the dielectric, does not allow for acceptable yields for capacitors with top copper electrodes. Conversely, as detailed in examples 7, 9, and 11 cofiring the top copper electrode with the underlying dielectric allows for significantly improved yield. Example 8 also shows that ruthenium electrodes may achieve high yield when a cofiring process is used. Example 11 shows that evaporated copper top electrodes are also usable rather than sputtering and that an optimum thickness is approximately 0.4 microns.
Example 12 shows that the cofiring process yield improvements can be extended to chemical solution deposited films.
The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only selected preferred embodiments of the invention, but it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings, and/or within the skill or knowledge of the relevant art.
The embodiments described herein above are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments, not explicitly defined in the detailed description.
