EMBEDDED THIN FILMS

Abstract

A method for forming a film on a conductive substrate, comprising immersing a substrate having a conductive portion in a solution comprising a metal ion ceramic precursor for the film and a peroxide; applying a voltage potential to the conductive portion with respect to a counter electrode in the solution, sufficient to protect the conductive portion from corrosion by the solution, and drive formation of a film on the substrate, controlling a pH of the solution while limiting a production of hydrogen by electrolysis of the solution proximate to the conductive portion; and maintaining the voltage potential for a sufficient duration to produce a film on the conductive portion. An electrode may be formed over the film to produce an electrical device. The film may be, for example, insulating, dielectric, resistive, semiconductive, magnetic, or ferromagnetic.

Claims

1. A method of forming a ceramic film on a substrate having a metallic surface, comprising: immersing the substrate having the metallic surface in a solution of at least one ceramic precursor solute, the solution being corrosive to the metallic surface within a corrosive range of pH and electrical potentials with respect to a counter electrode in the solution, and is not corrosive to the metallic surface within a protective range of pH and electrical potentials with respect to a counter electrode in the solution; applying a cathodic voltage potential to the metallic surface with respect to the counter electrode in the solution, and controlling a composition of the solution, within a range adapted to: cause a conversion of the at least one ceramic precursor to a ceramic film on the metallic surface, wherein the consumption of the at least one ceramic precursor alters at least a pH of the solution proximate to the conductive surface, protect the metallic surface from corrosion, and substantially avoid generation of hydrogen gas bubbles proximate to the metallic surface.

2. The method according to claim 1, wherein the conductive surface comprises copper.

3. The method according to claim 1, wherein the at least one ceramic precursor comprises titanium ions and halide ions, and the ceramic film comprises titanium dioxide.

4. The method according to claim 1, wherein the solution comprises an alcohol, water and peroxide mixture.

5. The method according to claim 1, wherein the ceramic film comprises titanium oxide and the at least one ceramic precursor comprises titanium chloride.

6. The method according to claim 1, substrate is patterned to selectively define a dielectric layer of a capacitor in a first region and conductive electrodes substantially absent formation of the ceramic film in a second region.

7. The method according to claim 1, further comprising electrochemically converting the ceramic film formed on the metallic surface having a first composition to a ceramic film having a second composition, the second composition comprising at least one additional metal ion, substantially without corroding the metallic surface.

8. The method according to claim 7, wherein said electrochemically converting comprises immersing the ceramic film formed on the metallic surface in a second solution having a second ceramic precursor, the second solution being corrosive to the metallic surface within a second corrosive range of pH and electrical potentials with respect to a counter electrode in the solution, and is not corrosive to the metallic surface within a second protective range of pH and electrical potentials with respect to a counter electrode in the solution; applying a second cathodic voltage potential to the metallic surface with respect to the counter electrode in the solution, and controlling a composition of the second solution, within a second range adapted to: cause a conversion of the ceramic film having the first composition to the ceramic film having the second composition, wherein the consumption of the second ceramic precursor alters at least a pH of the second solution proximate to the conductive surface, protect the metallic surface from corrosion, and substantially avoid generation of hydrogen gas bubbles proximate to the metallic surface.

9. The method according to claim 8, wherein the second ceramic precursor comprises barium ions.

10. The method according to claim 1, further comprising forming a conductive layer over the ceramic film, to thereby form a capacitor having a ceramic film dielectric.

11. The method according to claim 1, wherein the ceramic film is partially conductive, further comprising forming a conductive layer over the ceramic film, to thereby form a resistor.

12. The method according to claim 1, further comprising patterning a self-assembling monolayer on the metallic surface before immersing the substrate having the metallic surface in the solution, wherein the patterning of the self-assembling monolayer controls a deposition pattern of the ceramic film.

13. The method according to claim 1, further comprising the step of altering the applied cathodic voltage potential over time.

14. The method according to claim 1, further comprising monitoring a change of composition of the solution over time, and altering at least one of the applied cathodic voltage, pH and amount of the at least one ceramic precursor added to the solution in dependence on the monitoring.

15. The method according to claim 1, wherein the ceramic film has a resistivity of between about 25 and 100 Ohms per square.

16. The method according to claim 1, wherein the ceramic film has a dielectric constant of at least 17.

17. A substrate having metallic surface with a ceramic film thereon, formed by the process according to claim 1.

18. A substrate having a substantially adherent and non-chemically corroded patterned layer of metal film, and on selected portions of the patterned layer of metal film, an electrochemically-formed ceramic film providing at least one of a resistive, capacitive, semiconductive, and environmentally-responsive electrical properties.

19. The substrate according to claim 19, wherein the ceramic film is formed by a process comprising: immersing the substrate having the metallic surface in a solution of at least one ceramic precursor solute, the solution being corrosive to the metallic surface within a corrosive range of pH and electrical potentials with respect to a counter electrode in the solution, and is not corrosive to the metallic surface within a protective range of pH and electrical potentials with respect to a counter electrode in the solution; applying a cathodic voltage potential to the metallic surface with respect to the counter electrode in the solution, and controlling a composition of the solution, within a range adapted to: cause a conversion of the at least one ceramic precursor to a ceramic film on the metallic surface, wherein the consumption of the at least one ceramic precursor alters at least a pH of the solution proximate to the conductive surface, protect the metallic surface from corrosion, and substantially avoid generation of hydrogen gas bubbles proximate to the metallic surface.

20. The substrate according to claim 18, wherein the metal film comprises copper, and the ceramic film comprises titanium oxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] FIG. 1 shows a process flow for fabrication of an embedded capacitor for a system-in-package in accordance with the present invention;

[0064] FIG. 2 shows an E.sub.H-pH diagram of the CuClH.sub.2O.sub.2H.sub.2O system ([Cl.sup.]=0.005, [H.sub.2O.sub.2]=0.05M) showing a range available for electrochemical protection of a copper substrate;

[0065] FIGS. 3A, 3B and 3C show electron micrographs of a 550 nm thick titania film under various magnifications;

[0066] FIGS. 4A and 4B show electron micrographs of a 2,000 nm thick titania film under various magnifications;

[0067] FIGS. 5A, 5B and 6A, 6B and 6C, respectively show electron micrographs comparing titania films formed with and without hydrogen peroxide on the solution;

[0068] FIGS. 7A and 7B, and 8A and 8B show, respectively comparative electron micrographs and X-ray spectrographs of titania films formed in methanol-water and water solutions, respectively;

[0069] FIG. 9 shows a stability diagram for Barium Titanate, comparing log [Ba.sup.2+] and pH with respect to the various products in solution; and

[0070] FIG. 10 shows an E.sub.H-pH diagram of the CuO.sub.2H.sub.2O system ([Cu.sup.2+]10.sup.6 M) at room temperature, showing a range available for electrochemical protection of a copper substrate.

EXPERIMENTAL RESULTS

[0071] FIG. 1 shows a process flow diagram for forming an embedded capacitor on a circuit board using an electrochemical deposition technique for the dielectric layer. In a first step, an optional self assembling monolayer (SAM) is deposited on a copper substrate using a vapor phase deposition technique. This layer is, for example, less than 4 nm thick. The optional SAM layer is then patterned using, for example, ion beam, e-beam or scanning probe microscopy (SPM) scratching of the SAM. The SAM is modified using a mask or direct write technique to provide macroscopic domains with active and inert end groups.

[0072] A ceramic film, for example having a thickness of 0.2-2.0 m is selectively deposited over the active SAM end, to form a ceramic film on the active portion of the SAM on the copper substrate from a solution of TiCl.sub.4-MeOHH.sub.2OH.sub.2O.sub.2 under an electrochemical potential of about 5V.

[0073] The inactive portion of the SAM is then masked, for example using a UV curable mask, and an electroless plating method employed to deposit a copper electrode over the ceramic dielectric exposed through the mask. The mask is then stripped, and the structure can then be heated, for example to a temperature <200 C. to remove the optional SAM layer.

[0074] FIG. 2 shows an E.sub.H-pH diagram of the CuClH.sub.2O.sub.2H.sub.2O system with [Cl.sup.]=0.005 M and [H.sub.2O.sub.2]=0.05M. As shown in the drawing, a stable reduced region for the copper, providing electrochemical protection of the copper against corrosion, lies at an electrochemical potential more negative than 0.8-1.1V at pH between 0-5, with the required voltage increasing with increasing pH. Without electrochemical protection, the TiCl.sub.4 is corrosive to the copper, and a potential of about 5V is suitable to drive the reaction while providing corrosion protection. At this potential, most of the energy is consumed in electrolysis of water. Further, hydrogen produced by the hydrolysis is evolved at the copper surface cathode, which hinders thin film deposition and can degrade the copper film.

Example 1

[0075] A ceramic film was deposited under the following conditions:

[0076] Solution: Methanol:Water=1:1 v/v, TiCl.sub.4=0.002 M, H.sub.2O.sub.2=0.01

[0077] Voltage: 5V, Electrode distance 2 cm, CD=20 mA/cm.sup.2, Time 140 s

The process produced a 550-nm thick film, which is shown in the photomicrographs of FIGS. 3A, 3B, and 3C at varying magnifications.

Example 2

[0078] A ceramic film was deposited under the following conditions:

[0079] Solution: Methanol:Water=1:1 v/v, TiCl.sub.4=0.02 M, H.sub.2O.sub.2=0.2

[0080] Voltage: 10V, Electrode distance 2 cm, CD=100 mA/cm.sup.2, Time 120 s

[0081] The process produced a 2000-nm thick film, which is shown in the photomicrographs of FIGS. 4A, 4B, and 4C at varying increasing magnifications.

Example 3

[0082] A deposition of titania films on commercial copper/FR4/copper laminates was conducted under the following conditions:

[0083] Solution: Methanol:Water=95:5 v/v, TiCl.sub.4=0.02 M, H.sub.2O.sub.2=0.1 M

[0084] Voltage: 10V, Electrode distance 2 cm, CD=100 mA/cm.sup.2, Time 20/20/20 s (three 20 second intervals). Multiple deposition periods were adopted to reduce through-the-layer cracks. Electron micrographs of the resultant films are shown in FIGS. 5A and 5B, at varying magnification.

Example 4

[0085] A deposition was conducted of titania films on 500 nm thick copper sputtered on a polyimide (PI) surface under the following conditions:

[0086] Solution: Methanol:Water=95:5 v/v, TiCl.sub.4=0.02 M. (without H.sub.2O.sub.2 in the electrolyte.)

[0087] Voltage: 10V, Electrode distance 2 cm, CD=15 mA/cm.sup.2, Time 120 s

[0088] The process, which did not use hydrogen peroxide, produces a very porous structure, with evident cracking at lower magnifications, as shown in FIGS. 6A, 6B and 6C.

Example 5

[0089] The effect of methanol on titania film deposition was investigated. FIG. 7A shows a micrograph of a titania film formed in methanol+water, while FIG. 8A shows a corresponding film formed in water without methanol. FIGS. 7B and 8B show X-ray spectrograms of samples formed in a solution of [TiCl.sub.4]=0.004M, [H.sub.2O.sub.2]=0.02M, wherein chloride peaks are evident in the water solution process that are not evident in the water-methanol process.

Example 6

[0090] Titania films formed using the conditions specified had the following properties:

TABLE-US-00001 TABLE 1 Sample 1 2 3 Solution conditions CH.sub.3OH:H.sub.2O = 95:5 CH.sub.3OH:H.sub.2O = 95:5 CH.sub.3OH:H.sub.2O = 95:5 TiClO.sub.4 0.02M H.sub.2O.sub.2 0.1M TiClO.sub.4 0.01M H.sub.2O.sub.2 0.05M TiClO.sub.4 0.02M H.sub.2O.sub.2 0.1M Deposition 10 V, 20/20/20/20/20 s 10 V, 20/20 s 7 V, 20/20/20/20 s conditions Thickness (m) 2 1.5 1 Area 10.sup.6 (m.sup.2) 0.8945 1.445 0.99 1.314 1.22 0.8959 Capacitance (nF) 0.166 0.327 0.192 0.249 0.671 0.313 Capacitance/Area 120 146 126 123 355 225 (nF/sq. in) Dielectric constant 40 53 33 33 62 39
Using an HP Impedance analyzer and Agilent 34405A DMM for capacitance measurement, the following results were obtained:

TABLE-US-00002 TABLE 2 Sample 1 Sample 2 Thickness (m) 2 1.5 Area 10.sup.6 (m.sup.2) 0.8945 1.445 Capacitance (nF) 100 kHz 0.124 0.249 0.142 0.217 Capacitance (nF) 1 MHz 0.082 0.176 0.104 0.156 Dielectric loss 100 kHz 0.0043 0.004 0.005 0.0069 Dielectric loss 1 MHz 0.51 0.52 0.64 0.67 Capacitance/Area 100 kHz 89.50 111.21 92.50 106.52 (nF/sq. in) Capacitance/Area 1 MHz 59.16 78.60 67.75 76.58 (nF/sq. in) Dielectric constant 100 kHz 31.31 38.92 24.29 27.97 Dielectric constant 1 MHz 20.70 27.51 17.80 20.11

Example 7

[0091] A barium salt (BaCl.sub.2), is added to the solution to provide Ba.sup.2+ ions. Titanium precursor solution (TiCl.sub.4) as well as TiO.sub.2 particles are converted to barium titanate (BaTiO.sub.3) in the barium-salt containing solution of higher pH. It is noted that the high pH solution is normally corrosive to the copper substrate.

[0092] The general reaction is:

Ba.sup.2++TiO.sub.2(s)+H.sub.2O.fwdarw.BaTiO.sub.3(s)+2H.sup.+

[0093] The phase diagram of this solution is shown in FIG. 9, which shows that higher [Ba.sup.2+] and higher pH favor formation of BaTiO.sub.3. Mild heating of between 55-80 C. speeds the reaction. This high pH is, however, corrosive to the copper layer.

[0094] The copper layer, however, may be protected by application of an electrochemical bias. FIG. 10 shows an E.sub.H-pH graph for the CuO.sub.2H.sub.2O system, relevant during the conversion of titania to barium titante. At a pH of 12, a voltage bias of about 0.5V is required to maintain copper in the reduced state, with no considerable oxidation. Greater negative bias voltages will cause hydrolysis of water and generation of hydrogen, which will cause the titania film to peel off of the copper.

Example 8

[0095] Original titania films are made from multiple deposition cycles of 20-40-20-10 s. The resulting film thickness is 1.5 micron. The resulting film sample was treated with 0.01M BaCl.sub.2 aqueous solution, with the pH maintained at 12.3 with 5 M KOH. The process was conducted at a temperature of 75 C. The resultant film had the following properties:

TABLE-US-00003 TABLE 3 Original Titania Converted Converted Sample Sample 1 Sample 2 Thickness (m) 1.5 1.5 1.5 Area * 10.sup.6 (m.sup.2) 0.8459 1.2347 0.6570 1.2045 0.7173 1.1845 Capacitance (nF) 0.167 0.242 0.211 0.401 0.231 0.385 C/A (nF/sq. in 127.38 126.43 207.27 214.78 207.73 209.69 Dielectric 33.5 33.2 54.1 56.34 54.67 55.13 constant

[0096] TiO.sub.2 thin films were thus fabricated on copper substrates during a low-temperature (room temperature) process for producing embedded capacitors.

[0097] The process parameters, e.g., temperature, voltage, solution composition, pH, etc., are derived from thermodynamic calculations for protection of the metallic cathode, which is in a preferred embodiment copper, but may of course be another conductive material which is, for example, subject to corrosion in aqueous TiCl.sub.4 or highly basic solutions.

[0098] Multiple deposition cycles are employed to avoid the through-the-layer cracks. Indeed, the process is generally controlled to produce a homogeneous film. On the other hand, it may be desirable to intentionally produce discrete layers, for example by selectively controlling conditions for each cycle, to produce a laminated structure of relatively homogeneous or heterogeneous composition. Further, in addition to changes in electric conditions over time, the solution may also be altered, to provide a heterogeneous layered structure and/or to permit post deposition transformation of a layer.

[0099] An optional SAM or other patterning layer may be provided on the cathode, to assist in organizing the deposited layers, though this is not required.

[0100] The process limits hydrogen evolution near the substrate, and results in good adhesion and dense microstructures.

[0101] The electrical performance of TiO.sub.2 thin film capacitors produced using the process was as follows:

[0102] Capacitance density: >120 nF/in.sup.2 (>59 nF/in.sup.2 at 1 MHz) (cf. 10 nF/in.sup.2 for commercially available ceramic particle polymer capacitors at 1 kHz)

[0103] Dielectric constant: >30 (>18 at 1 MHz).

[0104] Dielectric loss: <0.007 (at 100 kHz), <0.6 (at 1 MHz).

[0105] Breakdown voltage: >15 volts

[0106] The layer may be post-processed to transform the material. For example, a TiO.sub.2 layer may be converted to BaTiO.sub.3 by a conversion process. In thin Films, the obtained dielectric constant of BaTiO.sub.3 is >50, which represents, for example, a 67% increase over the starting TiO.sub.2 film material. The TiO.sub.2 film is converted to BaTiO.sub.3 at 55 C., pH=11.5-12.5 by 0.01 M BaCl.sub.2 in aqueous solution. This process generates acid (H.sup.+) near the cathode, and therefore the solution is titrated with KOH to maintain pH within the desired range during the process.

[0107] It is this seen that the present invention provides a method for forming films on a substrate susceptible to corrosion during film-formation or processing, which comprises providing electrodic protection of the substrate from corrosion, and limiting electrolysis of water near the substrate to improve film adhesion to the substrate. The film is preferably a titanium based dielectric, and the forming solution is preferably a ceramic precursor salt and a peroxide in a methanol-water media. The dielectric may be patterned on the substrate using a self-assembling monolayer mask.