Alloyed 2N copper wires for bonding in microelectronics devices

Abstract

An alloyed 2N copper wire for bonding in microelectronics contains 2N copper and one or more corrosion resistance alloying materials selected from Ag, Ni, Pd, Au, Pt, and Cr. A total concentration of the corrosion resistance alloying materials is between about 0.009 wt % and about 0.99 wt %.

Claims

1. An alloyed 2N copper wire for bonding in microelectronics, wherein the wire has a diameter of 10 to 250 m and consists of 2N copper and two corrosion resistance alloying materials selected from the group consisting of Ag, Ni, Pd, Au, Pt, and Cr, and wherein a concentration of the corrosion resistance alloying materials is between about 0.009 wt % and about 0.99 wt %.

2. The alloyed 2N copper wire according to claim 1, wherein the wire consists of about 0.005 wt % to about 0.07 wt % Ag, about 0.009 wt % to about 0.89 wt % Ni, and balance 2N copper.

3. The alloyed 2N copper wire according to claim 1, wherein the wire consists of about 0.005 wt % to about 0.07 wt % Ag, about 0.009 wt % to about 0.89 wt % Pd, and balance 2N copper.

4. The alloyed 2N copper wire according to claim 1, wherein the wire consists of about 0.005 wt % to about 0.07 wt % Ag, about 0.009 wt % to about 0.89 wt % Au, and balance 2N copper.

5. The alloyed 2N copper wire according to claim 1, wherein the wire consists of about 0.005 wt % to about 0.07 wt % Ag, about 0.009 wt % to about 0.89 wt % Pt, and balance 2N copper.

6. The alloyed 2N copper wire according to claim 1, wherein the wire consists of about 0.005 wt % to about 0.07 wt % Ag, about 0.009 wt % to about 0.89 wt % Cr, and balance 2N copper.

7. A system for bonding an electronic device, comprising a first bonding pad, a second bonding pad, and an alloyed 2N copper wire according to claim 1, wherein the wire is connected to the first and the second bonding pads by wedge-bonding.

8. An alloyed 2N copper wire for bonding in microelectronics, wherein the wire has a diameter of 10 to 250 m and consists of 2N copper and two corrosion resistance alloying materials selected from the group consisting of: about 0.005 wt % to about 0.07 wt % Ag and about 0.009 wt % to about 0.89 wt % Ni; about 0.005 wt % to about 0.07 wt % Ag and about 0.009 wt % to about 0.89 wt % Pd; about 0.005 wt % to about 0.07 wt % Ag and about 0.009 wt % to about 0.89 wt % Au; about 0.005 wt % to about 0.07 wt % Ag and about 0.009 wt % to about 0.89 wt % Pt; and about 0.005 wt % to about 0.07 wt % Ag and about 0.009 wt % to about 0.89 wt % Cr.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:

(2) FIG. 1 shows comparative tensile stress-strain data for 2N alloyed Cu wires according to an example embodiment;

(3) FIG. 2 shows comparative polarization scan data for 2N alloyed Cu wires according to an example embodiment;

(4) FIGS. 3(a)-(c) are SEM images illustrating loop, ball, and stitch bonds for 2N alloyed Cu wires according to an example embodiment;

(5) FIGS. 4(a)-(b) show comparative ball bond and stitch bond process window data for 2N alloyed Cu wires according to an example embodiment;

(6) FIGS. 5(a)-(b) show comparative thermal aging (high temperature storage) data for 2N alloyed Cu wires according to an example embodiment; and

(7) FIGS. 6(a)-(c) show comparative loop height data and SEM images of low loop bands for 2N alloyed Cu wires according to an example embodiment.

(8) FIG. 7 is a schematic of an electronic device according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(9) The example embodiments described herein provide alloyed 2N Cu wires for bonding in microelectronics packaging industries. The wires are prepared using high purity Cu (>99.99%) and as major alloying elements Ag, Ni, Pd, Au, Pt, Cr, Ca, Ce, Mg, La, Al, P, Fe, B, Zr and Ti. Fine wires are drawn from the alloyed Cu. The wires in example embodiments are bondable to Al bond pads, as well as Ag, Cu, Au, Pd plated surfaces. The results of HTS (high temperature storage) of the wire bonds are comparable to a commercially available 4N soft Cu reference wire, when bonded to an Al bond pad and stored at about 175 C. for about 1000 hours. Corrosion resistance of the alloyed wires is advantageously better than the 4N soft Cu reference wire. As will be appreciated by a person skilled in the art, HAST or THB (temperature humidity bias) tests are typically conducted for Cu wire bonded and epoxy molded devices using biased or unbiased conditions. During the test, the Cu wire bond interface (i.e., Cu wire welded to Al bond pad) undergoes electro-chemical based galvanic corrosion. Moisture absorption by the epoxy is the source for diffusion of hydroxyl ions (OH.sup.). Parts per million levels of halogen (Cl, Br, etc.) contamination in the epoxy are the source for Cl.sup. ions. Polarization scans recorded for wires according to example embodiments of the present invention under an electrochemical reaction of the wire in dilute HCl revealed a positive rest potential exhibiting corrosion resistance. Hence, 2N alloyed Cu wires according to example embodiments are expected to perform better on reliability studies such as HAST and THB.

(10) The 2N alloyed Cu is continuously cast into rods. Elements are added individually or combined to a maximum of about 0.99 wt. %, maintaining the purity of the wire to be 2N in the example embodiments. The cast rods are wire drawn to a fine diameter of about 10 m to 250 m. The fine wires in example embodiments advantageously exhibit good free air ball (FAB) formation, bondability, loop formation, and reliability (HTS). Hardness, tensile strength, surface oxidation, electrical resistivity, and fusing current of the wires with trace additions in example embodiments are slightly higher than for the 4N soft Cu reference wire for bonding in microelectronics packaging sectors, while advantageously revealing better corrosion resistance without drastically compromising softness.

(11) In the example embodiments, copper of 4N to 5N purity was used to prepare the alloys and was melted in a vacuum induction furnace. At least one of Ag, Ni, Pd, Au, Pt, Cr, Ca, Ce, Mg, La, Al, P, Fe, B, Zr and Ti was added into the melt and held for about 2 to 15 minutes to allow a thorough dissolution. The elements were added individually or combined. The alloy was continuously cast into about 2 mm to 25 mm rods at a slow speed. No significant loss in dopant additions was observed. These rods were cold wire drawn at room temperature (about 23-25 C.).

(12) A tungsten carbide die was used to initially draw heavy wire, and a diamond die was used for further reduction to fine wire. The wire was drawn in three stages at a drawing speed of about 15 m/s or less. The die reduction ratios were about 14-18% for heavy wires and about 4 to 12% for fine wires. During cold drawing, the wires were lubricated and intermediate annealed between stages to reduce the residual stresses. Finally, the drawn wires were strand annealed, spooled on clean anodized (plated) aluminum spools, vacuum packed and stored.

(13) Hardness was measured using a Fischer scope H100C tester with a Vickers indenter applying 15 mN force for 10 s dwell time. Tensile properties of the wires were tested using Instron-5300. The wires were bonded using a Kulicke & Soffa (K&S)-iConn bonder. The bonded wires were observed in a LEO-1450VP scanning electron microscope.

(14) The alloyed elements and ranges of additions in the example embodiments are shown in Table 1 Noble metals Ag, Au, Pd, and Pt, and metals Ni and Cr were alloyed to improve the corrosion resistance of the Cu wire. In some embodiments, Ca, Ce, Mg, La, Al, P were alloyed as deoxidizers, softening the FAB. In some embodiments, Fe, B, Zr, Ti were alloyed as grain refiners to influence FAB grains. Boron was added in some embodiments to influence the strain hardening of the wire along with Ag and Ni.

(15) TABLE-US-00001 TABLE 1 Composition (wt %) of 2N alloyed Cu wire Alloy/ Ca + Mg + Element Ag Ni Pd Au Pt Cr Ce La Al 4N soft Cu <0.012 each <0.0002 1 0.009-0.99 2 0.009-0.99 3 0.009-0.129 4 0.009-0.99 5 0.009-0.99 6 0.009-0.99 7 0.005-0.07 0.009-0.89 8 0.005-0.07 0.009-0.89 9 0.005-0.07 0.009-0.89 10 0.005-0.07 0.009-0.89 11 0.005-0.07 0.009-0.89 12 0.005-0.07 0.009-0.89 13 0.005-0.07 0.009-0.89 14 0.005-0.07 0.009-0.89 0.005 15 0.005-0.07 0.009-0.89 0.005 16 0.005-0.07 0.009-0.89 0.005 17 0.005-0.07 0.009-0.89 0.005 18 0.005-0.07 0.009-0.89 0.005 19 0.005-0.07 0.009-0.89 0.005 Alloy/ Element P S Fe B Zr Ti Total 4N soft Cu each <0.0003 each <0.0002 <100 wt. ppm 1 0.0003 0.997 2 0.997 3 0.997 4 0.997 5 0.997 6 0.997 7 0.997 8 0.997 9 0.997 10 0.997 11 0.997 12 0.008 0.997 13 0.008 0.997 14 0.997 15 0.997 16 0.008 0.997 17 0.008 0.997 18 0.008 0.005-0.02 0.005 0.002 0.997 19 0.008 0.005-0.02 0.005 0.002 0.997

(16) The mechanical and electrical properties of the alloyed wires of the example embodiments are shown in Table 2. Advantageously, the properties are close to the 4N soft Cu reference wire. A representative tensile plot of 2N alloyed Cu wire according to example embodiments is shown in FIG. 1. As can be seen from a comparison of curve 100 (2N alloyed Cu wire according to example embodiments) and curve 102 (the 4N soft Cu reference wire), the deformation behavior is advantageously similar on tensile loading, but may require higher load to plastically deform. The hardness and modulus of 2N alloyed Cu wire according to example embodiments are also slightly higher. The electrical resistivity of the 2N alloyed Cu wire according to example embodiments is advantageously equivalent to that of 4N Au wires, about 2.34 .Math.cm. This demonstrates that a maximum of about 0.99 wt % alloying advantageously does not drastically alter the deformation characteristics and electrical resistivity of the alloyed wire in example embodiments.

(17) TABLE-US-00002 TABLE 2 Corrosion, mechanical and electrical properties of 2N alloyed Cu wires Corrosion resistant Wire FAB Fusing current (for (++++ Excellent, Hardness Hardness 10 mm length, +++ very good, Alloy/ (15 mN/10 s), (15 mN/10 s), Modulus, Resistivity, 300 ms input pulse ++ Good, Element HV HV GPa .Math. cm time), mA + Satisfactory) 4N soft ~85 ~85 ~90 ~1.7 ~340 Cu 1 ~90 ~90 ~94 ~2.4 ~340 + 2 ~90 ~90 ~94 ~2.4 ~340 + 3 ~90 ~90 ~94 ~2.4 ~340 ++ 4 ~90 ~90 ~94 ~2.4 ~340 ++ 5 ~90 ~90 ~94 ~2.4 ~340 +++ 6 ~90 ~90 ~94 ~2.4 ~340 ++ 7 ~90 ~90 ~94 ~2.4 ~340 ++ 8 ~90 ~90 ~94 ~2.4 ~340 +++ 9 ~90 ~90 ~94 ~2.4 ~340 ++ 10 ~90 ~90 ~94 ~2.4 ~340 +++ 11 ~90 ~90 ~94 ~2.4 ~340 ++ 12 ~90 ~90 ~94 ~2.4 ~340 ++ 13 ~90 ~90 ~94 ~2.4 ~340 +++ 14 ~90 ~90 ~94 ~2.4 ~340 ++ 15 ~90 ~90 ~94 ~2.4 ~340 +++ 16 ~90 ~90 ~94 ~2.4 ~340 + 17 ~90 ~90 ~94 ~2.4 ~340 +++ 18 ~90 ~90 ~94 ~2.4 ~340 + 19 ~90 ~90 ~94 ~2.4 ~340 +++

(18) The corrosion resistance of 2N alloyed Cu wires according to example embodiments is significantly better than that of the 4N soft Cu reference wire (Table 2). FIG. 2 shows a representative scan of the 2N alloyed Cu wire according to example embodiments (curve 200), revealing a higher positive rest potential of 139 mV, compared to 255 mV for the 4N soft Cu reference wire (curve 202). As will be appreciated by a person skilled in the art, in a polarization scan, if the rest potential (corrosion potential) of the test element is toward positive, the element is noble. On the other hand, if the rest potential is negative, the element is active (corrosive). Therefore, the 2N alloyed Cu wire according to example embodiments is nobler than the 4N soft Cu reference wire. The scan was obtained using dilute HCl electrolyte and stirring the solution maintained at room temperature.

(19) The 2N alloyed Cu wire of example embodiments can be bonded to pads metallized (plated) with Au, Ag, Pd and Cu. On bonding to Al bond pads, the wire bonds are anticipated to have a longer reliability life, especially under HAST and THB tests. FIGS. 3(a), (b) and (c) show representative scanning electron microscope images of loop, ball and stitch bonds of a 2N alloyed 0.8 mil Cu wire according to example embodiments. With reference to FIGS. 4 and 5, it can be seen that the ball and stitch bond process window and reliability performance of the 2N alloyed Cu wire according to example embodiments and of the reference soft Cu 4N wires are nearly the same. More particularly, in FIG. 4(a), the representative ball bond process window 402 for the 2N alloyed Cu wire according to example embodiments is similar to the ball bond process window 400 of the 4N soft Cu reference wire. Similarly, in FIG. 4(b), the representative stitch bond process window 404 for the 2N alloyed Cu wire according to example embodiments is similar to the stitch bond process window 406 for the 4N soft 0.8 mil Cu reference wire. A comparison of curve 500 (FIG. 5(a)) and representative curve 502 (FIG. 5(b)) illustrates that the thermal aging of the 4N soft 0.8 mil Cu reference wire and the 2N alloyed 0.8 mil Cu wire according to example embodiments are also similar.

(20) Ultra low loop bonding of 2N alloyed Cu wires according to example embodiments for 2.4 mil height also revealed good capability similar to the 4N soft Cu reference wire. More particularly, the plot in FIG. 6(a) shows that the representative loop height measured for the bonded 2N alloyed 0.8 mil Cu wire according to example embodiments (labeled 600) is substantially the same as for the 4N soft 0.8 mil Cu reference wire (labeled 602). This indicates that 2N alloyed Cu wires according to example embodiments are soft and perform as well as the 4N soft Cu reference wire. Scanning electron microscope (SEM) images of 2N alloyed 0.8 mil Cu wires (FIGS. 6(b) and (c)) according to example embodiments showed no obvious cracks in the neck region.

(21) As an example, FIG. 7 depicts an electronic device 10 comprising two elements 11 and a wire 1. The wire 1 electrically connects the two elements 11, such as bond pads, by a ball bond or wedge bond.

(22) It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.