DE19508617C1 - Ultrasonic key-wire bonding for microwires of aluminium@, palladium@ and/or copper@ - Google Patents

Abstract

Ultrasonic key-wire bonding process for microwires of aluminium, palladium and copper and alloys of these materials to bondable substrates at ambient temperature, using a gold wire bonding tool having at least one groove transverse to the direction of oscillation in its bonding foot and producing microwire contacts with two completely separate weld zones (9,10) in the contact area in one bonding operation. A bonding tool is also claimed.

Description

The invention relates to a method and a tool for Ultrasonic wedge wire bonding of aluminum micro wires, Palladium and copper, as well as alloys of these materials all substrates that are bondable.

Ultrasonic wire bonding at room temperature is also carried out so-called bond wedges, in contrast to capillaries in Thermocompression and thermosonic bonding. The bond wedges are offered in different versions, adapted to Wire materials, wire diameter and use in different machines and use on special Substrates (e.g. microwave components). Common to them the shaft with standardized diameter and three different ones Lengths, a phase for a defined clamping direction and a rectangular foot surface over the bonding force and Ultrasonic vibrations on the wire underneath act in the process of bonding. The wire is going through a Hole in the foot area of the wedge from one side under the Foot surface directed.

Tools with a groove are especially used for gold wire bonding provided that transverse to the direction of vibration in the foot surface is eroded. With this wire it serves to support the Form-fit between bond wedge and wire, which is essential is necessary because gold is used in comparison to others Wire materials such as B. aluminum is much more ductile. The ultrasonic wedge wire bonding with gold wire is at Room temperature is usually not used. For one reliable gold wire contact will contact the substrate Temperatures that are greater than 80 ° C warmed.

Increasing demands on process quality and  Reliability of the contacts always lead to Further developments in the process, including of its components. An example is represented by EP 0632493 describes a semiconductor device arrangement in which the contact on the carrier strip is bonded twice and so two contacts with a bond wire are created. But that requires the stitch bondability of the machine and extends it Process flow for a further positioning and Bond step.

Ultrasonic wedge wire bonding is due to the process not under the entire bond area for bond formation, she includes an annular area where the shear stress in the Contact level is greatest. Furthermore, it causes plastic wire deformation a constricting area where the Wire tears preferentially under mechanical loads. These Constriction is also gaining importance when building elements are to be contacted with high power requirements and the electrical Resistance of the wire plays a role.

The object of the invention is the area of Bond formation in relation to the total bond area increase and by better coupling the wire to the Tool the proportion of energy input into the Contact level arrives to increase to that with the acting Energy related wire deformation to reduce what the Reduced wire constriction.

According to the invention the task with the in the claims mentioned means and explained in more detail in the embodiment solved.

Neglecting the metal combination between wire and  Bond area (pad) metallization is the intensity of the Connection of the energy introduced and the adhesive force of the Contact of the product from the resulting strength and the Size of the binding area depends. With increasing Effect of energy from the tool on the wire to excite the Bond formation also increases its deformation and there is a constriction of the wire, connected with the sinking of the Wire breaking force. It is therefore aimed at only so much energy to induce that the wire breaking force is not less than that Bond breaking force is.

It has surprisingly been shown that with one in itself Known tool for gold wire bonding and use of Micro-wires with lower ductility than gold wire, two annular bond areas are generated in the bond area. The arising when using the method according to the invention Total area is larger than the individual area of a normal one Wise emerging bond area. That leads to one greater tear-off force with less necessary deformation.

It also causes the groove over the one connected to it Form fit between tool and wire for better transmission of the induced energy in the binding area.

The advantage of the solution according to the invention is that at the connection quality is increased.

The invention is described below using an exemplary embodiment explained in more detail. Show in the drawings

Fig. 1 shows a bonding tool with a detailed view of the foot surface in a sectional and rear view

Fig. 2 shows a detailed view of the foot surface with cross groove in section

Fig. 3 is a review of a bond with a welding zone

Fig. 4 is a review of a bond with two completely separate welding zones

Fig. 5 is a diagram showing the theoretical bond areas when using the conventional method compared to the invention depending on the groove width in microns

Fig. 6 is a diagram showing the tearing force of wire bridges in cN, bonded from the conventional and from the tool with a groove transverse to the direction of vibration

Fig. 7 is a review of a bond on a very narrow pad with two separate welding zones

Fig. 8 is a diagram showing the tearing force of wire bridges in cN, bonded from the conventional and from the tool with a groove transversely to the direction of vibration on a circuit board, the copper conductor tracks have an organic protective layer

Fig. 1 shows a standard tool for the ultrasonic ground of micro-wires, the diameter 100 is less than microns, with the wire guide 1, the Bondfußbreite 2, the Bondfußlänge 3 and 4 the front and back radii. 5

In Fig. 2 the foot surface is divided by the groove 6 . The groove runs transversely to the direction of vibration of the tool. The arrangement of several grooves is advantageous for longer bond foot lengths.

The standard bonding tool with a coherent foot surface only causes an annular binding area according to FIG. 3. In the case of a tool as shown in FIG. 2, there are practically two foot surfaces with a sufficiently deep and wide groove 6 . Two ring-shaped regions 9 and 10 according to FIG. 4 are formed, the total area of which, under certain conditions, is larger than the area of the ring region which arises with the standard tool.

The design of the foot surface takes into account boundary conditions described below in two or more separate areas, in the form that the associated number can form in complete welding zones (binding areas). A shape of the ultrasonic wire bond wedge with transverse groove is previously used for contacting gold wire. The Groove serves primarily to improve the transmission of the Ultrasonic power, i.e. the vibrations of the tool through that and not in the wire. This is done using the Groove-supported form fit between wire and tool. Gold wire is opposite to that in the preamble of the main claim mentioned materials (Al (x), Pd (x) and Cu (x)) ductile, the The yield point is lower. The one necessary with this wire Energy input for bond formation is at room temperature comparatively higher (often enough for a reliable Contact does not stop) and causes a stronger plastic Deformation. This leads to the formation of bonds, but in the Usually no longer to separate two areas, they go instead its one into the other. This makes the theoretically possible Surface growth inhibited. The dimensions of the groove separate into  In this case, the individual effects of the Partial areas. Energy transmission is improved, though but there is no additional binding surface. The pursuit after a wide groove, the associated reduction takes effect the size of the partial areas, which directly affects the Area of the bond areas.

The area comparison of theoretically achievable bond areas, the shape of which depends on the bond foot length, and thus the length of the bond (corresponding to the semiaxis a in FIG. 3) and wire deformation, and thus the width of the bond (corresponding to the semiaxis b in FIG. 3), clearly illustrates that Influence of the groove width on the size of the bond area.

The comparison according to FIG. 5 is based on:

• - wire diameter = 30 µm
• - degree of deformation = 1.5; b is around 22 µm
• - Bond foot length = 64 µm; a is around 32 µm
• - Bond ring width = 5 µm

Two tools with the same bond foot lengths, the same vibration amplitude (which can be neglected here), the same wire deformation and the same wide binding area are compared (bond rings 11 , FIGS . 3 and 4). When the size of the allocable in Fig. 4 semiaxes a respective groove width is 12 4 of FIG. Considered. Depending on the bond width, expressed by wire diameter and degree of deformation, the maximum permissible groove width changes directly proportionally. It is the same with the bond foot length.

The minimum groove width is limited on the one hand by manufacturing technology (currently around 15... 20 µm, EDM process) and on the other hand by the desired separation of the two foot surfaces of the transverse groove tool. The vibration amplitude counteracts this. It creates a transfer of tensions between the sub-areas below the bond foot via the material connection of the wire, which should ideally be interaction-free. The groove must be deeper as the necessary wire deformation increases. For tools for gold wire bonding, it is about 5 µm. With 30 µm AlSi1 wires on well bondable AlSi1 pads, this dimension can already lead to improvements in tear-off force of about 20% compared to comparable conventional tools (see FIG. 6). Even better results are achieved with a wire of greater hardness, in which the deformation is less. Minimum dimensions for the depth are based on the degree of wire deformation, which can vary from application to application (substrate material, pad metallization, contamination layers, etc.) regardless of the tool.

In addition to the basically achievable gain in bonding area under the conditions described, wire contacts on extremely narrow pads, which u. U. can be narrower than the bond, the coverage ratio between bond and pad also has an influence. An example of this is illustrated in FIG. 7, in which the connection pad 13 is only 13 μm wide, but the wire diameter is already 17.5 μm. Due to the larger coverage ratio of the effective binding area 14 created by two rings, as shown in FIG. 7, the tear-off force of the bonds increased from 2.39 cN to 2.97 cN (100 tests, of which the mean value in each case, an increase of around 25% ) in comparison of two tools.

The contact quality on so-called "poorly bondable" layers can also be noticeably improved, which is demonstrated in Figure 8.

Here was for the purpose of comparing two tools with a 30 µm thick AlMg1 wire on copper interconnects PCB bonded with a thin organic Passivation was coated. To establish contact between This layer must have wire and interconnect during the bonding process still to be penetrated.

This explains the small ones for this wire diameter Mean values of the tear-off forces. The difference of about 50% However, the profit of 35% with this wire still exceeds was achieved on silicon chip bond pads. For each There is an optimum application of the described method in the design of the groove (s). That affects the measure of yours Width and depth.

An assignment of these dimensions to wire diameter, Wire hardness, type and texture of the bond pad and the result in the substrate immediately below extensive tables. A size, however, in the reflecting these many influences is the wire deformation. The groove depth is based on it. It should be bigger than the depth of penetration of the foot surface of the bond wedge.

A minimum value for the minimum necessary width is derived from the vibration amplitude of the foot surface during the bonding process. The partial areas must remain separate, so the minimum width is at least twice the vibration amplitude in the area of contact formation. However, it should also be noted here that the total area of both partial foot areas decreases with increasing width. In any case, there is only a compromise for the optimal groove width. To check that the maximum permissible dimension is not exceeded, use the geometric comparison of the expected bonding area between the conventional and the tool with groove, as shown in Figure 5.

Claims (4)

1. A method for ultrasonic wedge wire bonding of micro wires made of aluminum, palladium and copper, as well as alloys of these materials on bondable substrates at room temperature, characterized in that with a known tool for gold wire bonding, which has at least one groove ( 6 ) transversely to the direction of vibration, contacts the micro-wires in a bonding process and two completely separate welding zones are generated in the contact area. 2. Tool for ultrasonic wedge wire bonding of micro wires made of aluminum, palladium and copper, as well as alloys of these materials on bondable substrates at room temperature, characterized in that at least one groove ( 6 ) running transversely to the direction of vibration is arranged in the foot surface. 3. Tool according to claim 2, characterized in that the The depth of the groove is greater than the depth of penetration of the tool in the wire during the bonding process, and that the width of the groove at least twice the vibration amplitude of the tool is. 4. Tool according to claim 2 or 3, characterized in that in the foot surface ( 2 , 3 ) a plurality of grooves ( 6 ) are arranged.