Copper-alloy capping layers for metallization in touch-panel displays

Abstract

In various embodiments, electronic devices such as touch-panel displays incorporate interconnects featuring a conductor layer and, disposed above the conductor layer, a capping layer comprising an alloy of Cu and one or more refractory metal elements selected from the group consisting of Ta, Nb, Mo, W, Zr, Hf, Re, Os, Ru, Rh, Ti, V, Cr, and Ni.

Claims

1. A method of forming an electronic device, the method comprising: providing a substrate; depositing over the substrate a barrier layer (i) comprising an alloy of Cu and one or more refractory metal elements selected from the group consisting of Ta, Nb, Mo, W, Zr, Hf, Re, Os, Ru, Rh, Ti, V, Cr, and Ni, and (ii) comprising a plurality of crystalline grains separated by grain boundaries; depositing over the barrier layer a conductor layer comprising at least one of Cu, Ag, Al, or Au; forming a mask layer over the barrier layer and the conductor layer; patterning the mask layer to reveal a portion of the conductor layer; thereafter, applying an etchant to remove portions of the conductor layer and the barrier layer not masked by the patterned mask layer, thereby forming a sidewall comprising (i) an exposed portion of the barrier layer, (ii) an exposed portion of the conductor layer, and (iii) an interface between the exposed portion of the barrier layer and the exposed portion of the conductor layer; and annealing the substrate at a temperature sufficient to form a particulate within at least one of the grain boundaries, the particulate comprising at least one of (i) an agglomeration of at least one of the refractory metal elements or (ii) a reaction product of silicon and at least one of the refractory metal elements.

2. The method of claim 1, wherein, after the etchant is applied, the sidewall is substantially free of discontinuities notwithstanding the interface.

3. The method of claim 1, wherein the etchant comprises a mixture of phosphoric acid, acetic acid, nitric acid, and water.

4. The method of claim 1, wherein the barrier layer comprises an alloy of Ta and Cu.

5. The method of claim 1, wherein the barrier layer comprises an alloy of Nb and Cu.

6. The method of claim 1, wherein the barrier layer comprises an alloy of Ta, Zr, and Cu.

7. The method of claim 1, wherein the barrier layer comprises an alloy of Cu, Ta, and Ti.

8. The method of claim 1, wherein the substrate comprises glass.

9. The method of claim 1, wherein the substrate comprises silicon.

10. The method of claim 9, wherein the substrate comprises amorphous silicon.

11. The method of claim 1, further comprising removing the remaining portion of the patterned mask layer.

12. An electronic device comprising: a substrate; and a conductive feature disposed over the substrate, the conductive feature comprising: (i) disposed on the substrate, a barrier layer (i) comprising an alloy of Cu and one or more refractory metal elements selected from the list consisting of Ta, Nb, Mo, W, Zr, Hf, Re, Os, Ru, Rh, Ti, V, Cr, and Ni, and (ii) comprising a plurality of crystalline grains separated by grain boundaries, and (ii) disposed on the barrier layer, a conductor layer comprising at least one of Cu, Ag, Al, or Au, wherein at least one of the grain boundaries comprises a particulate therein, the particulate comprising at least one of (i) an agglomeration of at least one of the refractory metal elements or (ii) a reaction product of silicon and at least one of the refractory metal elements.

13. The electronic device of claim 12, wherein the barrier layer comprises an alloy of Ta and Cu.

14. The electronic device of claim 12, wherein the barrier layer comprises an alloy of Nb and Cu.

15. The electronic device of claim 12, wherein the barrier layer comprises an alloy of Ta, Zr, and Cu.

16. The electronic device of claim 12, wherein the barrier layer comprises an alloy of Cu, Ta, and Ti.

17. The electronic device of claim 12, wherein (i) the conductor layer comprises Cu, and (ii) the substrate is substantially free of Cu diffusion from the conductor layer.

18. The electronic device of claim 12, wherein the substrate comprises glass.

19. The electronic device of claim 12, wherein the substrate comprises silicon.

20. The electronic device of claim 12, wherein the substrate comprises amorphous silicon.

21. The electronic device of claim 12, wherein the conductive feature comprises an electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:

(2) FIG. 1A is a schematic cross-section of a thin-film transistor for a liquid crystal display;

(3) FIG. 1B is a schematic cross-section of an etched conventional TFT electrode;

(4) FIG. 2A is a plan-view schematic of the sensor array of a touch-panel display;

(5) FIG. 2B is a magnified perspective view of a portion of the sensor array of FIG. 2A;

(6) FIG. 2C is a schematic cross-section of the sensor-array portion of FIG. 2B;

(7) FIG. 2D is a schematic cross-section, along a plane perpendicular to that of FIG. 2C, of the sensor-array portion of FIG. 2B depicting an etched conventional interconnect;

(8) FIGS. 3 and 4 are schematic cross-sections of a TFT electrode during fabrication in accordance with various embodiments of the invention;

(9) FIGS. 5 and 6 are schematic cross-section of an interconnect for a touch-panel display in accordance with various embodiments of the invention;

(10) FIGS. 7A and 7B are Auger spectra graphs of mutual diffusion of Cu and Si without a diffusion barrier therebetween;

(11) FIGS. 8A and 8B are Auger spectra graphs of mutual diffusion of Cu and Si between a Si layer and a Cu-alloy capping or barrier layer in accordance with various embodiments of the invention;

(12) FIGS. 9A-9C are plan-view micrographs of the surface of a Cu-alloy capping or barrier layer as deposited (FIG. 9A), after annealing at 300° C. (FIG. 9B), and after annealing at 500° C. (FIG. 9C), in accordance with various embodiments of the invention;

(13) FIGS. 10A and 10B are plan-view micrographs taken via scanning electron microscopy (FIG. 10A) and transmission electron microscopy (FIG. 10B) of an annealed Cu-alloy capping or barrier layer on Si, in accordance with various embodiments of the invention; and

(14) FIG. 11 depicts corrosion levels after environmental corrosion testing of samples of pure Mo, pure Cu, CuTaCr alloy, and CuNbCr alloy in accordance with various embodiments of the invention.

DETAILED DESCRIPTION

(15) FIG. 3 depicts an initial step in the fabrication of a TFT gate electrode in accordance with embodiments of the present invention. As shown, a barrier layer 300 is deposited on a substrate 310 (e.g., a glass or silicon substrate) by, e.g., sputtering or other physical deposition process. A conductor layer 320 is subsequently deposited on the barrier layer 300 by, e.g., sputtering or other physical deposition process. Typically the thickness of the barrier layer 300 will be between approximately 5% and approximately 25% (e.g., approximately 10%) of the thickness of conductor layer 320. For example, the thickness of the barrier layer 300 may be approximately 50 nm, and the thickness of the conductor layer 320 may be approximately 500 nm. A mask layer 330 (e.g., photoresist) is formed over the conductor layer 320 and patterned by conventional photolithography.

(16) As shown in FIG. 4, a gate electrode 400 is then fabricated by etching the portions of the conductor layer 320 and barrier layer 300 not covered by the mask layer 330, preferably in a single-step wet etch. A wet etchant (e.g., a PAN etch) is utilized to etch away the metal layers at substantially the same rates, resulting in sidewalls 410 that are substantially smooth and/or linear and that are substantially free of any discontinuity (e.g., a stepped or nonlinear profile) at an interface 420 between the conductor layer 320 and barrier layer 300. The wet etchant may include or consist essentially of, for example, a PAN etch including or consisting essentially of 50-60 weight % phosphoric acid, 15-25 weight % acetic acid, 3-5 weight % nitric acid, and the balance DI water. Some specific examples are provided in the table below. In one preferred embodiment, the wet etchant includes or consists essentially of 50 weight % phosphoric acid, 25 weight % acetic acid, 3 weight % nitric acid, and the balance (22 weight %) DI water.

(17) TABLE-US-00001 phosphoric nitric acid, acetic acid, DI water, acid, H3PO4 HNO3 CH3COOH balance (wt %) (wt %) (wt %) (wt %) etchant 1 50 5 15 30 etchant 2 60 5 20 15 etchant 3 50 3 25 22

(18) After etching, the substrate 310 (as well as the electrode 400) is preferably substantially free of etch residue of one or both of the conductor layer 320 and the barrier layer 300 in regions proximate the gate electrode 400. In accordance with various embodiments of the invention, the wet-etching process is performed at room temperature. The wet etchant may be sprayed on the substrate 310, or the substrate 310 may be partially or completely immersed in the wet etchant. The wet-etching process may be performed as a batch (i.e., multiple-substrate) process or as a single-substrate process. In preferred embodiments, after etching the sidewalls 410 form an angle 430 with the surface of the underling substrate 310 of between approximately 50° and approximately 70°, e.g., approximately 60°. After etching, the mask layer 330 may be removed by conventional means, e.g., acetone, a commercial photoresist stripping agent, and/or exposure to an oxygen plasma.

(19) FIG. 5 depicts an initial step in the fabrication of a touch-panel sensor interconnect in accordance with embodiments of the present invention. As shown, a conductive layer 500 is deposited over a sensor 510 (e.g., a row or column sensor that may be composed of a transparent conductor such as ITO) on a substrate 520 (e.g., a glass or silicon substrate) by, e.g., sputtering or other physical deposition process. A capping layer 530 is subsequently deposited on the conductive layer 500 by, e.g., sputtering or other physical deposition process. Typically the thickness of the capping layer 530 will be between approximately 5% and approximately 25% (e.g., approximately 10%) of the thickness of conductive layer 500. For example, the thickness of the capping layer 530 may be approximately 50 nm, and the thickness of the conductive layer 500 may be approximately 500 nm. A mask layer 540 (e.g., photoresist) is formed over the capping layer 530 and patterned by conventional photolithography.

(20) As shown in FIG. 6, an interconnect 600 is then fabricated by etching the portions of the capping layer 530 and conductive layer 500 not covered by the mask layer 540, preferably in a single-step wet etch. A wet etchant (e.g., a PAN etch) is utilized to etch away the metal layers at substantially the same rates, resulting in sidewalls 610 that are substantially smooth and/or linear and that are substantially free of any discontinuity (e.g., a stepped or nonlinear profile) at an interface 620 between the capping layer 530 and conductive layer 500. The wet etchant may include or consist essentially of, for example, a PAN etch including or consisting essentially of 50-60 weight % phosphoric acid, 15-25 weight % acetic acid, 3-5 weight % nitric acid, and the balance DI water. In one preferred embodiment, the wet etchant includes or consists essentially of 50 weight % phosphoric acid, 25 weight % acetic acid, 3 weight % nitric acid, and the balance (22 weight %) DI water.

(21) After etching, the substrate 520 and electrode 510 (as well as the interconnect 600) are preferably substantially free of etch residue of one or both of the capping layer 530 and the conductive layer 500 in regions proximate the interconnect 600. In accordance with various embodiments of the invention, the wet-etching process is performed at room temperature. The wet etchant may be sprayed on the substrate 520, or the substrate 520 may be partially or completely immersed in the wet etchant. The wet-etching process may be performed as a batch (i.e., multiple-substrate) process or as a single-substrate process. In preferred embodiments, after etching the sidewalls 610 form an angle 630 with the surface of the underling substrate 520 of between approximately 50° and approximately 70°, e.g., approximately 60°. After etching, the mask layer 330 may be removed by conventional means, e.g., acetone, a commercial photoresist stripping agent, and/or exposure to an oxygen plasma.

(22) Barrier layers 300 and capping layers 530 in accordance with various embodiments of the invention also serve as effective diffusion barriers for metallic layers that include or consist essentially of, e.g., Cu, Ag, Al, or Au. Specifically, the alloying element(s) within the barrier layer 300 and/or capping layer 530 substantially prevent diffusion of a conductor layer material (e.g., Cu) into an underlying silicon substrate or an adjoining layer even after exposure to elevated temperatures (e.g., up to approximately 200° C., up to approximately 350° C., up to approximately 500° C., or even higher) for times of, e.g., up to 2 hours. FIGS. 7A and 7B show the concentrations of Cu and silicon across a Cu/silicon interface (i.e., one without a barrier layer between the Cu and silicon) as measured with Auger electron spectroscopy (AES) as fabricated (no anneal) and after anneals of 200° C.-500° C. As shown, mutual diffusion of the Cu and silicon occurs at temperatures as low as (or even lower than) 200° C., and the interface is severely diffused after an anneal at 500° C. Additionally, the Cu layer exhibits poor adhesion to silicon in the absence of a barrier layer between the Cu and silicon.

(23) FIGS. 8A and 8B show the concentrations of Cu and silicon across an interface between silicon and a barrier layer 300 or capping layer 530 that includes or consists essentially of CuTaCr as measured with AES after no anneal and anneals of 200° C.-500° C. In the illustrated embodiment, the barrier layer 300 or capping layer 530 is composed of 2 weight % Ta, 1 weight % Cr, and the balance Cu. (In another embodiment exhibiting similar behavior, the barrier layer 300 or capping layer 530 includes or consists essentially of 5 weight % Ta, 2 weight % Cr, and the balance Cu.) In contrast to the results shown in FIGS. 7A and 7B, there is negligible diffusion of Cu or silicon across the interface, even after an anneal at 500° C. for two hours. FIGS. 9A-9C are a series of scanning electron microscopy (SEM) micrographs of the surface of the barrier layer 300 or capping layer 530 as deposited (FIG. 9A), after an anneal of 300° C. for one hour (FIG. 9B), and after an anneal of 500° C. for one hour (FIG. 9C). As shown, the grain structure and size of the barrier layer 300 or capping layer 530 show no appreciable change, and there is no evidence of the formation of different phases (e.g., copper silicide phases) even after a heat treatment of 500° C. These results were confirmed by x-ray diffraction (XRD) scans of annealed structures, in which no silicide phases were detected even after anneals of 500° C. for two hours. In contrast, copper silicide phases are clearly evident in SEM and XRD performed on samples of pure Cu layers on Si that have been annealed at 500° C. for two hours.

(24) FIGS. 10A and 10B show, respectively, SEM and transmission electron microscopy (TEM) images of a barrier layer 300 or capping layer 530 that is disposed in contact with silicon (e.g., a silicon substrate and/or a silicon overlayer) and that has been annealed at 350° C. for 30 minutes. Precipitates 1000 are evident within the Cu grain boundaries 1010 of the barrier layer 300 or capping layer 530. In various embodiments the precipitates include or consist essentially of a silicide of one or more of the refractory metal alloying elements of the barrier layer 300 or capping layer 530, and such precipitates reduce or substantially eliminate Cu diffusion along the grain boundaries into the adjoining silicon.

(25) Similarly, in various embodiments of the invention, the refractory-metal dopants of barrier layers 300 and/or capping layers 530 tend to segregate to the Cu grain boundaries and provide beneficial effects even in the absence of reaction with silicon to form silicides. For example, the Cu grain boundaries may be occupied, and partially or substantially completely “blocked” with the refractory-metal dopants and thereby retard or substantially prevent oxygen diffusion along the Cu grain boundaries. In this manner, corrosion of the barrier layer 300, capping layer 530, and/or the conductive layer in contact therewith is decreased or substantially prevented. Thus, in various embodiments of the present invention, a barrier layer 300 or capping layer 530 may include or consist essentially of a polycrystalline Cu matrix doped with one or more refractory metal elements, where the grain boundaries of the layer between the doped Cu grains contain a higher concentration of the refractory metal dopant(s) that the concentration within the grains themselves. For example, the refractory metal concentration within the grain boundaries may be larger than that within the grains by a factor of 5, a factor of 10, or even a factor of 100.

(26) FIG. 11 depicts images of four different metallic samples after an environmental corrosion test conducted at 60° C. and 80% humidity for a period of 260 hours. As shown, the samples of pure Cu and pure Mo experienced much more severe corrosion that did the Cu-alloy samples in accordance with embodiments of the present invention. The two Cu-alloy samples were (1) Cu with 10 weight % Ta and 2 weight % Cr (labeled in FIG. 11 as CuTaCr), and (2) Cu with 5 weight % Nb and 2 weight % Cr (labeled in FIG. 11 as CuNbCr). The table below provides data regarding the amount of exposed surface area corroded during the environmental corrosion test for each of the samples. As indicated, the Cu-alloy samples in accordance with embodiments of the present invention experienced much less corrosion than the pure Cu and Mo samples, demonstrating the benefits of such alloys over conventional Mo diffusion barriers and capping layers, as well as over pure Cu.

(27) TABLE-US-00002 Sample Corroded Surface Area (%) Mo 6.78 Cu 3.84 CuTaCr 1.75 CuNbCr 0.83

(28) In preferred embodiments of the invention, the barrier layers 300 or capping layers 530 have low resistivity, e.g., below 10 microOhm-cm, or even below 5 microOhm-cm, even after anneals of up to 500° C., up to 600° C., or even higher temperatures. Moreover, in preferred embodiments the barrier layers 300 or capping layers 530 exhibit good adhesion to glass as measured by, e.g., an ASTM standard tape test. Embodiments of the invention also include electronic devices (or portions thereof) in which a highly conductive material (e.g., Cu, Ag, Al, and/or Au) is utilized to form all or a portion of a conductor or electrode and has both a barrier layer 300 below it and a capping layer 530 above it.

(29) The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.