Wire loops, methods of forming wire loops, and related processes

A method of forming a wire loop is provided. The method includes the steps of: (1) forming a conductive bump on a bonding location using a wire bonding tool; (2) bonding a portion of wire to another bonding location using the wire bonding tool; (3) extending a length of wire from the bonded portion of wire toward the bonding location; (4) lowering the bonding tool toward the bonding location while detecting a height of a tip of the wire bonding tool; and (5) interrupting the lowering of the wire bonding tool during step (4) if the wire bonding tool reaches a predetermined height.

FIELD OF THE INVENTION

The present invention relates to conductive bumps and wire loops utilizing conductive bumps, and to improved methods of forming conductive bumps and wire loops.

BACKGROUND OF THE INVENTION

In the processing and packaging of semiconductor devices, conductive bumps are formed for use in providing electrical interconnections. For example, such bumps may be provided for: (1) use in flip-chip applications, (2) use as stand-off conductors, (3) wire looping applications, (4) test points for testing applications, amongst others. Such conductive bumps may be formed using various techniques. One such technique is to form the conductive bumps using wire, such as on a wire bonding machine or a stud bumping machine.

Numerous techniques for forming conductive bumps on a wire bonding machine or bumping machine are disclosed in U.S. Pat. No. 7,229,906 (entitled “METHOD AND APPARATUS FOR FORMING BUMPS FOR SEMICONDUCTOR INTERCONNECTIONS USING A WIRE BONDING MACHINE”) and U.S. Pat. No. 7,188,759 (entitled “METHOD FOR FORMING CONDUCTIVE BUMPS AND WIRE LOOPS”), both of which are incorporated by reference in their entirety.

FIG. 1illustrates an exemplary sequence of forming a conductive bump on a wire bonding machine or bumping machine. At Step1, free air ball100ais seated at the tip of bonding tool102. As will be understood by those skilled in the art, prior to Step1, free air ball100ahad been formed on an end of wire100that hangs below the tip of bonding tool102using an electronic flame-off device or the like. Wire clamp104is also shown at Step1in the open position. As will be understood by those skilled in the art, wire100is provided by a wire spool on the machine (not shown). Wire100extends from the wire spool through wire clamp104(and through other structures not shown) and through bonding tool102.

After free air ball100ais formed (prior to Step1), wire100is drawn upwards (e.g., using a vacuum control tensioner or the like) such that free air ball100ais seated at the tip of bonding tool102as shown at Step1ofFIG. 1. At Step2, bonding tool102(along with other elements of a bond head assembly including wire clamp104) is lowered and free air ball100ais bonded to bonding location106(e.g., a die pad of semiconductor die106). As will be understood by those skilled in the art, the bonding of free air ball100ato bonding location106may utilize ultrasonic energy, thermosonic energy, thermocompressive energy, XY table scrub, combinations thereof, amongst other techniques.

After free air ball100ais bonded to bonding location106at Step2(where the bonded free air ball may now be termed bonded ball100b), with wire clamp104still open, bonding tool102is raised to a desired height as shown by the upward arrow in Step3. This height may be referred to as a separation height (as shown in Step3ofFIG. 1, bonding tool102has been raised such that bonded ball100bis no longer seated in the tip of bonding tool102). At Step4, with wire clamp104still open, bonding tool102is moved in at least one horizontal direction (e.g., along the X axis or Y axis of the machine) to smooth the top surface of bonded ball100b. Such smoothing provides a desirable top surface for a conductive bump, and also weakens the connection between bonded ball100band the rest of the wire to assist in the separation therebetween. At Step5, bonding tool102is raised to another height (which may be referred to as the wire tail height), and then wire clamp104is closed. Then at Step6, bonding tool102is raised to break the connection between bonded ball100b(which may now be termed conductive bump100c) and the remainder of wire100. For example, bonding tool102may be raised to an EFO height which is a position at which an electronic flame-off device forms a free air ball on wire tail100dof wire100.

Forming conductive bumps using such conventional techniques may result in certain deficiencies. Such deficiencies may include, for example: premature separation between bonded ball100band the rest of wire100due to the smoothing at step4; potential short wire tail conditions because of such premature separation; potential long wire tail conditions; and undesirably reduced smoothing of bonded ball100bin an attempt to avoid such premature separation.

Conductive bump100cshown at Step6may be used as a stand-off in a wire looping process known as stand-off stitch bonding (“SSB”). Improved wire looping techniques utilizing conductive bumps, including improved SSB techniques would be desirable.

SUMMARY OF THE INVENTION

According to an exemplary embodiment of the present invention, a method of forming a wire loop is provided. The method includes the steps of: (1) forming a conductive bump on a bonding location using a wire bonding tool; (2) bonding a portion of wire to another bonding location using the wire bonding tool; (3) extending a length of wire from the bonded portion of wire toward the bonding location; (4) lowering the wire bonding tool toward the bonding location while detecting a height of a tip of the wire bonding tool; and (5) interrupting the lowering of the wire bonding tool during step (4) if the wire bonding tool reaches a predetermined height.

DETAILED DESCRIPTION OF THE INVENTION

In certain exemplary embodiments of the present invention, during formation of stand-off stitch wire loops, methods of detecting lift-off of conductive bumps (e.g., using software of the wire bonding machine) are provided. For example, the z-axis position is measured (e.g., using a z-axis encoder or the like) to determine a height at which a second bond of an SSB wire loop is to be bonded (e.g., stitch bonded) to a previously formed conductive bump. During formation of the SSB wire loops, when the wire bonding tool (e.g., capillary) is seeking the upper surface of the conductive bump (e.g., for the “stitch on bump” bonding process) a determination is made if the bump is missing (e.g., using a servo motor control signal or the like). Such a determination may be made, for example, if the wire bonding tool is lowered during the “stitch on bump” bonding process to a height (e.g., a predetermined height) below the typical height (e.g., plus a predetermined threshold). If it is determined that the bump is missing the wire bonding process may be stopped, and an alert may be provided (e.g., to an operator, to a control room, etc.).

In connection with the formation of SSB wire loops, conductive bumps are formed. In accordance with the present invention, such conductive bumps may be formed using a process as described above with respect toFIG. 1. However, the present invention may have particular applicability to SSB processes using a bump forming process such as that shown inFIG. 2. In such a process, after deposition and bonding of a free air ball to form the conductive bump, but prior to the smoothing of the upper surface, the bonding tool is raised to a desired height (e.g., a tail height) with the wire clamp open. Then the wire clamp is closed and the wire bonding tool is lowered (to a smoothing height) to perform the upper surface smoothing of the conductive bump. This process results in a slack length of wire between the top of the bonding tool and the bottom of the wire clamp. After completion of the smoothing of the upper surface of the bump, the wire bonding tool is raised to separate the remainder of the wire from the conductive bump. In this process, the slack length of wire now facilitates a desirable wire tail length, thereby substantially reducing the potential for shorts tails and associated problems. More specifically, during a “smoothing” process (see Step6ofFIG. 2), the wire clamp remains closed which substantially reduces the potential for (or even prevents) the wire from passing through the wire bonding tool creating a stoppage in the process (which may result in a short tail error). In contrast to conventional techniques (where the wire tail is formed after the smoothing is complete), the length of the wire tail is established before the smoothing occurs (e.g., see Steps4-7ofFIG. 2). Further, the wire tail will tend to be stronger than in conventional bump forming techniques, because no smoothing motion has occurred to weaken the wire tail, thereby reducing the potential for additional errors.

FIG. 2illustrates an exemplary sequence of forming a conductive bump on a wire bonding machine or bumping machine, where such a bump may be used in connection with the methods of the present invention. At Step1, free air ball200ais seated at the tip of bonding tool202. As will be understood by those skilled in the art, prior to Step1, free air ball200ahad been formed on an end of wire200that hangs below the tip of bonding tool using an electronic flame-off device or the like. Wire clamp204is also shown at Step1in the open position.

After free air ball200ais formed (prior to Step1), wire200is drawn upwards (e.g., using a vacuum control tensioner or the like) such that free air ball200ais seated at the tip of bonding tool202as shown at Step1ofFIG. 2. At Step2, bonding tool202(along with other elements of a bond head assembly including wire clamp204) is lowered (i.e., where a lower tip of tool202is at bonding height h1) and free air ball200ais bonded to bonding location206(e.g., a die pad of semiconductor die206). As will be understood by those skilled in the art, the bonding of free air ball200ato bonding location206(to form bonded ball200b) may utilize ultrasonic energy, thermosonic energy, thermocompressive energy, XY table scrub, combinations thereof, amongst other techniques.

After bonding of free air ball200aat Step2(but before Step3) other motions may be completed as desired. For example, a fold of wire may be formed on bonded ball200bbefore Step3such as is described in U.S. Pat. No. 7,229,906. Of course, other motions and configurations of bonded ball200bare contemplated.

After free air ball200ais bonded to bonding location206at Step2(where the bonded free air ball may now be termed bonded ball200b), with wire clamp204still open, bonding tool202is raised to a desired height at Step3. This height may be referred to as a tail height (at this height in Step3ofFIG. 2the tip of wire bonding tool202is separated from bonded ball200b); however, it is understood that different heights may be selected. While the invention is not limited thereto, exemplary ranges for this height (taken with respect to the tip end of the bonding tool) are between 5-20 mils and between 10-20 mils above the top of bonded free air ball200b. At Step4, wire clamp204is closed. At Step5, bonding tool202is lowered to a desired height. This height may be referred to as a smoothing height h2(at Step5ofFIG. 2, the tool has been lowered such that the tip of bonding tool202is just barely in contact with an upper surface of bonded ball200b). While the invention is not limited thereto, exemplary ranges for this height are between 0.1-2 mils and between 1-2 mils above the height of bonding tool202at Step2(i.e., above bonding height h1). At Step5, by lowering bonding tool202with the wire clamp closed, a slack length of wire200ehas been provided below the bottom of wire clamp204and above bonding tool202. At Step6, with wire clamp104still closed, bonding tool202is moved in at least one horizontal direction (e.g., along the X axis, the Y axis, both the X and Y axes, another horizontal direction, etc.) to smooth the top surface of bonded ball200b. Such smoothing provides a desirable top surface for a conductive bump, and also weakens the connection between the bonded ball and the rest of the wire to assist in the separation therebetween. At Step7, bonding tool202is raised to break the connection between bonded ball200b(which may now be termed conductive bump200c) and the remainder of wire200. For example, bonding tool202may be raised to an EFO height which is a position at which an electronic flame-off device forms a free air ball on wire tail200dof wire200. In connection with raising bonding tool202to break the connection at Step7, ultrasonic energy or the like may also be applied to facilitate threading of slack length of wire200ethrough the tip of bonding tool202to provide wire tail200d.

InFIG. 2, the height of bonding tool202at Steps5and6is the same; however, it is understood that the height may be changed as desired from one step to the next to achieve the desired smoothing. That is, inFIG. 5the tip of the bonding tool may be lowered to a separation height (similar to the separation height of Step3ofFIG. 1) that is different from the smoothing height h2.

By providing the slack length of wire200ein this process prior to tearing wire200to separate conductive bump200bfrom the remainder of wire200, a desirable wire tail length200dis provided. Thus, the potential for short tails (where there is not enough wire to form the next free air ball hanging below the tip of bonding tool202) is substantially reduced. Thus, a desirable level of horizontal smoothing can be accomplished in Step6ofFIG. 2without substantial risk of complications related to premature separation.

The smoothing at Step6ofFIG. 2may vary considerably. For example, the smoothing operation may consist of a single horizontal motion of bonding tool202across the top surface of bonded ball200bas shown in Step6ofFIG. 2. However, multiple motions (e.g., back and forth, in different directions, etc.) may be provided in the smoothing operation. Further, the smoothing motion may be completely horizontal as shown in Step6, or may have both horizontal and vertical (e.g., upward or downward) components. Thus, the smoothed surface may be sloped in a given direction as is desired. Further still, the smoothing step of Step6may be combined with the lowering of bonding tool202of Step5into a single (e.g., simultaneous) motion. That is, the motion of bonding tool202may follow an angled path (e.g., downward and to the right as shown inFIG. 2) whereby the lowering of Step5and the smoothing of Step6are completed in a single motion.

FIGS. 3A-3Cillustrate a method of forming an SSB wire loop in accordance with an exemplary embodiment of the present invention.FIG. 3Aillustrates conductive bump400cwhich has been formed on bonding location406a(e.g., die pad406aof semiconductor die406). Semiconductor die406is supported by substrate408(e.g., leadframe408). For example, conductive bump400chas been formed according to the method described inFIG. 2or another method (e.g., such as the method shown inFIG. 1). During formation of conductive bump400c(e.g., by bonding a free air ball to bonding location406a), a height of the bonding tool is detected (and/or measured and stored into memory of the wire bonding machine). For example, the height may be height h1shown at Step2ofFIG. 2(i.e., a bonding height). In another example, the height may be height h2shown at Step5ofFIG. 2(i.e., a smoothing height). Other heights (e.g., other heights below the smoothing height h2may also be selected). For purposes of the present example, let us assume that the detected height is a bonding height h1, which is detected, for example, using a z-axis encoder of the wire bonding machine (e.g., using velocity based detection, forced based detection, etc.).

After bump400chas been formed (as shown inFIG. 3A), it is desired to now electrically connect bonding location408a(e.g., lead finger408aof leadframe408) to conductive bump400c.FIG. 3Billustrates continuous wire loop410providing electrical interconnection between lead finger408aand conductive bump400c. As is known to those skilled in the art, bonded portion410a(e.g., first bond410a, formed by bonding a free air ball to lead408a) is formed on lead finger408a. Then, length of wire410b(continuous with first bond410a) is extended toward conductive bump400c. AtFIG. 3Bwire bonding tool450is bonding (e.g., ultrasonically) a second bond portion of the wire to bump400c. Prior to this bonding operation, according to an exemplary embodiment of the present invention, a z-axis height of the wire bonding tool tip has been detected to ensure that conductive bump400chas not been previously lifted off bonding location406a. That is, when forming conductive bump400c(earlier in the process than the image shown inFIG. 3A) a height of the wire bonding tool is detected. For example, the detected height may be the bonding height h1. In such a case, in order to ensure that bump400chas not been lifted off, the height of the tip of wire bonding tool450is monitored as it descends to bond the wire to bump400cto complete the SSB wire loop. If bump400cis present (e.g., it has not been lifted off bonding location406a), then bonding tool450will make contact with bump400cat (or about) height h1. Such contact may be detected, for example, by detecting the velocity of wire bonding tool450as it descends (e.g., a change in velocity measured using the z-axis position from the encoder). Another exemplary method of detecting such contact is, for example, by detecting the force against wire bonding tool450as it descends (e.g., using a force sensor engaged with a transducer of a bond head assembly carrying wire bonding tool450).

If bump400cis not present (e.g., it has been previously lifted off bonding location406a), then wire bonding tool450will not make contact with bump400cat (or about) height h1. In such as case, it may be desired to stop movement of wire bonding tool450(e.g., to avoid damage to the device being bonded, to avoid damage to the wire bonding tool, to avoid machine downtime, etc.). However, in order to ensure that bump400cis not present, the tip of wire bonding tool450may pass a height threshold hthbelow height h1. Exemplary ranges for height hthare less than 10 micrometers below height h1(or whatever height is used as the reference height such as h2, hn, etc.), less than 7 micrometers below height h1, less than 5 micrometers below height h1, and between 3-5 micrometers.

As shown inFIG. 3B, conductive bump400cis present during the bonding of a portion of wire400(from a wire supply such as a wire spool, not shown) to conductive bump400c. As such, wire bonding tool450has not descended below height h1by at least threshold hth. Thus, conductive bump400cis not declared as missing and the bonding operation atFIG. 3Bis completed. AtFIG. 3C, the bonding operation is complete (i.e., wire portion410chas been bonded to conductive bump400c). Thus, conductive bump400cacts as a stand-off for SSB wire loop410.

The bonding of wire portion410c(e.g., second bond410c) to conductive bump400cmay be a closed loop controlled process. For example, a z-position of the bonding tool may be monitored, wherein the ultrasonic energy applied during the bonding of wire portion410cto conductive bump400cis turned off upon wire bonding tool450reaching a predetermined z-position (i.e., a position along the vertical z-axis of the wire bonding machine). More specifically, prior to the formation of second bond410con conductive bump400c, wire bonding tool450descends toward conductive bump400c. After impact between wire bonding tool450(including wire portion410ccarried by wire bonding tool450) and conductive bump400cat a certain z-position (i.e., the z-impact position), a reference position may be established (where the reference position may be, for example: the impact z-position; a z-position slightly above the impact position; the smoothing z-position (e.g., smoothing height h2at Step5ofFIG. 2); a z-position where ultrasonic energy is applied during second bond formation; a z-position at a predetermined time after impact between wire bonding tool450and the bump, amongst others). Then, the ultrasonic energy is applied to form second bond410c, that is, to bond wire portion410cto conductive bump400c(where the ultrasonic energy may be turned on before impact, upon impact, upon the bonding tool reaching the reference position, etc.).

Then, the ultrasonic energy is turned off (or reduced, for example, by at least 50% of the energy level) (e.g., with or without a predetermined time delay) upon the bonding tool reaching the predetermined z-position such that wire bonding tool450does not drive too deep into conductive bump400c. For example, the predetermined z-position may be selected relative to a reference z-position. That is, after wire bonding tool450reaches the selected reference position, the z-position is monitored (e.g., using a z-axis encoder or other technique) to determine when wire bonding tool450reaches the predetermined position. Of course, other techniques of determining the predetermined z-position (and/or the reference z-position) are contemplated within the scope of the present invention.

In the example shown inFIGS. 3A-3C, the wire bonding machine determined that the previously formed conductive bump400cwas present prior to bonding of wire portion410cto conductive bump400c. In contrast, in the example described below in connection withFIGS. 4A-4D, the conductive bump400cis absent.

Referring specifically toFIG. 4A, conductive bump400cis being formed on bonding location406a(e.g., die pad406aof semiconductor die406). That is, a previously formed free air ball (on wire400) is being bonded to bonding location406ato form bump400c. Semiconductor die406is supported by substrate408(e.g., leadframe408). During the formation of bump400c, a height is detected. As provided above, the height may be the bonding height h1(e.g., bonding height h1from Step2ofFIG. 2), the smoothing height h2(e.g., smoothing height h2from Step5ofFIG. 2), or another height hn. This height is detected (e.g., and saved into memory of the wire bonding machine). AtFIG. 4Bwire bonding tool450is raised (e.g., to form another free air ball for bonding to bonding location408a). However, while raising wire bonding tool450, conductive bump400c(now labeled as400c1as it has been raised off of its bonding location406a) has been lifted off bonding location406awith bonding tool450. For example, the lift off of conductive bump400cmay be caused by the smoothing motion (e.g., the smoothing motion of Step6ofFIG. 2).

AtFIG. 4Cwire bonding tool450has raised to a flame-off height. At this height, an electronic flame-off (EFO) assembly is used to form a new free air ball on a wire tail. However, in this case, there is no exposed wire tail because of the lift-off of conductive bump400c. Thus, during the flame-off operation, spark460afrom EFO wand460(of an EFO assembly, not shown) melts conductive bump400c1into free air ball400c2. AtFIG. 4D, free air ball400c2has been bonded to bonding location408ato form first bond410a. Then, length of wire410b(continuous with first bond410a) is extended toward bonding location406a(where conductive bump400cshould be, but it is not). During the descent of wire bonding tool450towards bonding location406a, a z-axis height of the tip of wire bonding tool450is detected to ensure that conductive bump400chas not been previously lifted off bonding location406a. In this example, conductive bump400chas been lifted off of bonding location406a(as illustrated inFIG. 4A). Thus, the wire bonding system (e.g., using software) determines that the tip of bonding tool450has passed height h1, and indeed has passed height h1by more than the height threshold hthshown inFIG. 4D. Thus, it can be declared that conductive bump400cis missing. The bonding process may now be stopped (including stopping descent of bonding tool450), and an alert may be provided (e.g., to an operator, to a control room, etc.).

The inventive techniques disclosed herein are particularly applicable to copper wire bonding. Copper wire has certain physical properties that tend to exacerbate the potential for short tail errors using conventional bumping techniques. Thus, the present invention may provide exceptional benefits to copper wire bumping and bonding processes. Of course, the inventive techniques are also applicable to other types of wire bonding including, for example, gold, aluminum, and Pd coated Cu wire bonding.

Although the present invention has been described primarily with respect to certain exemplary method steps in a predetermined order, it is not limited thereto. Certain of the steps may be rearranged or omitted, or additional steps may be added, within the scope of the present invention.

Although certain exemplary embodiments of the present invention have been illustrated and described in connection with detecting a height of the tip of a wire bonding tool (e.g., the lower tip) it is not limited thereto. It will be appreciated that a height of any portion of the wire bonding tool, or a structure that moves with the bonding tool, may be detected to achieve certain benefits of the invention described herein.

Although the present invention has been illustrated and described in connection with techniques of detecting bump lift-off, it is not limited thereto. Conductive bumps (such as bump400cillustrated herein) may have problems detected by the present invention other than lift-off. Such problems may include, for example, misplaced bumps, poorly shaped bumps, weakly bonded bumps, etc.