EJECTION APPARATUS FOR LIFTING CHIPS OFF OF CARRIER MATERIALS

An ejection device and a method for lifting a chip (2) from a carrier material (1) having at least one needle (4), wherein the needle (4) includes at least one cutting edge (7) for cutting through the carrier material (1). A mounting device for inserting and fixing at least one needle (4) in an ejection device for lifting a chip from a carrier material, having a plate for aligning the plane of the needle tips and a removable attachment that has a recess, wherein the removable attachment can be mounted on the ejection device in such a way that the recess serves to introduce the fixing material onto the needle ends. A method for fixing the needles in an ejection device is used, wherein the at least one needle to be fixed in the ejection device is melted in.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1shows how in the prior art a chip is lifted from a carrier material. A carrier material1, for example, an adhesive carrier film, e.g. a self-adhesive carrier film, 0.9 mm in thickness, for example, is provided with a number of chips2. During operation the carrier material with the wafer or chip composite travels over the ejection device, whereby the chip to be ejected is brought into position over the ejection device. From the back side of the carrier film the ejection device lifts the chips with the needles. The prior art needles are 0.5 to 0.7 mm in diameter. By lifting the chips from the back side, that is the side facing toward the carrier film, the adhesive carrier film is detached at the edge of the chips, whereby in the prior art, needles are used with strongly rounded, conically-shaped tips so that during the lifting process the needles do not pierce or sever the carrier material. In this way, under the pressure of the needles, the carrier material is also lifted and is deformed into a generally parabolic shape. The carrier material is detached only from the edge of the chip where the carrier material is held down, since it is fastened in a frame and moreover is vacuum suctioned on. Where the film is nevertheless pierced, a small hole is formed. When the needles are raised, the carrier film is raised along with them as a result of the conical shape of the needles. At this point the chip is ready to be removed by a vacuum gripper, i.e. from the side of the chip opposite the carrier material. According to the prior art, a residual adhesion from the adhesive layer of the carrier material remaining on the chip acts on the chip such that during removal with the vacuum gripper the chip is at risk of breaking. A further drawback in this regard, as can be seen inFIG. 1, is that when the carrier film is lifted, the adjacent chips are also lifted and tipped in the process, resulting in the risk of their edges in turn sliding against the adjacent chips and becoming damaged.

This method according to the prior art is especially problematic when it comes to semiconductor wafers that are severely thinned during the fabrication process and have a wafer thickness of, for example, just 20 to 100 μm. Such thin wafers are in particular the product of MMIC processing (monolithic microwave or millimeter wave integrated circuits). These thinned wafers are mounted on a carrier material in order for the chips to be diced. The carrier material may be an adhesive carrier film. The film is stretched across a frame and the chips to be diced are sawed on the carrier film without the carrier film itself being sawed through. Sawing is done mechanically or with a laser beam. The sawed chips are separated from one another by a narrow dicing channel 5 μm to 30 μm in width and lie separately from one another on the carrier film. The prior art method is associated with high damage rates, especially for MMICS made of chips fabricated from III/V semiconductor material, for example, from GaAs or InP and thinned from 20 on down to 100 μm. In this case, the method according to the invention shown schematically inFIG. 2is advantageous in increasing the yield of functioning chips.

InFIG. 2the chips2to be lifted rest on a carrier material1, for example, an adhesive carrier film, and in this example the chip2is raised by needles8(4 needles are concealed) which lift the chip2from the carrier film and the chip composite. According to the method of the invention the ejection device is produced so that as the chip is lifted by means of the ejection device, at least one needle cuts through the carrier material during lifting. In the example inFIG. 2there are at least 8 needles, each of which are prepared with a cutting edge7. The needle tip slightly pierces the carrier film and the cutting edge, under pressure generated by the needles on the carrier film during lifting or raising, cuts through the carrier material along the lifting path.

A needle is seen as a rod-shaped device. The front portion of the needle is the portion involved in the lifting process. The needle tip is the foremost end of the needle by means of which the pressure is transferred to the chip and on which the chip rests when it is lifted. This is followed by the front portion of the needle.

The needles according to the invention may be produced from a needle blank, for example, a wire pin, the cross-section of which may be of any desired shape, for example, shapes that are round, angular or flat in cross-section. In order for the needle to pierce the carrier film as effortlessly as possible, it was found according to the invention, to prepare the front portion of the needle so that the part of the needle projecting above the original plane of the carrier film during lifting is prepared with at least one cutting edge. For this purpose, the front portion of the needle is provided with at least one edge that functions as a cutting edge for the carrier material and is sharpened and includes a surface polished as smoothly as possible. This results in minimal penetration resistance of the needle in relation to the carrier material due to the wedge-shaped cutting edge, and during the lifting process the carrier material is cut through by the at least one cutting edge of the needle. Preferably, the needle can be prepared so that it tapers or becomes thinner toward the needle tip.

An especially preferred embodiment of a needle according to the invention is shown inFIG. 3. This needle4has a triangular cut thereby giving rise to three cutting edges7which cut through the carrier material1during the lifting process.

In particular the front portion of the needle can be thinned toward the needle tip, whereby at least three lateral edges are prepared along a section of the front portion of the needle. The cut with the three cutting edges results in a triangular-shaped cross-section at the front portion of the needle.

Thus, a needle is used for lifting a chip from a carrier material that has three cutting edges.

According to the invention, it was found that the penetration resistance of the needle in the carrier material is also significantly reduced if the needle is fabricated from a wire pin with a diameter that is smaller or equal to 0.25 mm. Conventional needles used in the prior art have a diameter of from 0.5 to 0.7 mm. Such a needle thickness offers the carrier film too great a surface area such that needles of this type generally fail to pierce the carrier material. Even if a tip based on this needle diameter has a cut that sharply tapers toward the tip of the needle, such a needle offers too much resistance, with the result that when the needle is raised further the carrier film is pulled and raised along with it. In that vein, according to the invention the ejection device is operated with a needle with an appropriately small diameter such that the needle is able to penetrate the carrier material. Together with a grinding on the front portion of the needle toward the needle tip, which includes in particular at least one cutting edge it was found that as a result the carrier film is barely raised as the chip is lifted off. With a needle diameter of 0.25 mm the needle tip can be finely worked accordingly.

An ejection device with multiple needles is called for depending on the shape of the chip to be lifted or depending on the size of the chip area. Thus, the ejection device can be fitted with needles depending on the area of the chip to be lifted. Once raised, the chip rests as if on a cushion of needles, as is also shown inFIG. 2. As a result the chip no longer breaks. According to the invention four needles are inserted in the ejection device, given a chip area of 1 mm2. By selecting a smaller diameter for the needles used, it is possible to increase the needle density. For example, a chip with an area of 1×1.5 mm2was lifted simultaneously with five needles. Using multiple needles to lift a chip reduces the flexural moment acting on the chip as a result of the pressure during lifting. This reduces the risk of the chip breaking. Even very large chips can be lifted using the ejection device according to the invention. As a further example, a chip 4×1 mm2in size was lifted with 12 needles. This indicates that according to the invention needle densities are used wherein one needle is positioned on a chip area of 0.33 mm2.

According to the invention it is preferable to operate with a needle density amounting to at least 1 needle per 0.25 mm2.

FIGS. 4aand4bshow a specific embodiment of the ejection device. The ejection device is composed of a needle holder8in which the needles4can be fixed. In this example, the needle holder8of the ejection device is prepared with 12 needles. The needles4are accommodated in holes15of the needle holder.FIGS. 4aand4bshow the ejection device disassembled. In the assembled ejection device the needle holder8is inserted via a compression spring10into the needle head9and thereby attached. The allen screw16serves to connect the screw hole18in the needle holder to the slotted hole17of the needle head9. The compression spring10enables the needle head9and the needle holder8to be pressed together. That means, needle head9and needle holder8can be moved relative to one another when joined together. The advantage of this is that the needle tips5when non-operational are situated inside the needle head9. The needle tips are recess mounted approximately 0.5 mm below the surface of the ejection tool, in this case of the needle head9. During operation the carrier film1with the wafer travels over the needle head9and is thus cannot damage the needle tips7. When the corresponding chip2is correctly positioned, the needle tips7are forced upward out of the needle head9and impact the chip2located above.

Also visible inFIGS. 4aand4bis a vacuum suction connection19over which a hose is slipped for applying a vacuum at the vacuum holes20during operation of the ejection device. During operation of the ejection device, the vacuum holds the carrier material in place over the vacuum holes by means of a vacuum and thereby facilitates the lifting of the chip from the carrier material. The carrier material is suctioned and held in place by the vacuum.

The subject matter of the invention in one embodiment is to fix the needles in the needle holder in such a way that they are not warped during the fixing process. It is important, especially when the ejection device is fitted with more than one needle, that the needles tips are aligned in one plane. In this way, the mechanical forces acting on the chip as a result of the pressure of the needles is optimally distributed. For this reason, the ejection device is operated preferably with needles that are fixed in a rigid medium. By contrast, in the prior art mechanical force is expended on one side when fixing the needles in the ejection tool. The needles of the prior art are secured in their mounting holes by alien screws. By tightening the alien screws it is possible for the needles to shift slightly or become stressed in the holes, thereby no longer ensuring a uniform alignment of the needle tips.

The present invention has found that it is advantageous to fix the needles using a medium that, when solidified, permanently fixes the needles in the ejection device. Particularly suited for this is a hotmelt adhesive that changes from the liquid to the solid state when the temperature changes. The needles are guided into the holes of the needle holder, whereby the medium is applied to the needle ends in the needle holder, is applied either in the liquid state or is liquefied subsequent to application. Subsequent solidifying bonds the needles via the medium to the needle holder. In this way, the needles are fixedly mounted in the needle holder. After the ejection device is used, the needles can be removed again by means of a suitable solvent. For this reason a hotmelt adhesive is particularly suited since it can be removed completely by being bathed in solvent. For this purpose, the needle holder together with the needles is immersed in a solvent bath. A suitable solvent is acetone, for example. The hotmelt adhesive is dissolved in this way and the needles can be removed from the holes, and the ejection device can be prepared once again. A correspondingly suitable hotmelt adhesive is Crystalbond 509™.

FIG. 5shows a schematic view of a mounting device11used to insert and fix the needles4in place in the ejection device. The mounting device11consists of a flat plate12on which the needle tips5rest during mounting, and of a removable attachment13which is attached for mounting on the ejection device in such a way that a recess disposed in the removable attachment13is mounted over the needle holder8. The removable attachment13is connected to the flat plate12by the connecting piece14. In this configuration the needle holder is forced down approximately 0.5 mm by the removable attachment onto the compression spring.

In this case, the needle holder8can either be forced by the removable attachment13onto the compression spring10when the ejection device is first introduced into the mounting device,that is, before the needles4are inserted into the holes of the needle holder8and thus retained for further assembly, or the mounting device is employed so that the ejection device is first mounted in the mounting device in such a way that the needle holder is not yet forced down onto the compression spring. Only once the needles4have been inserted into the holes15and only once the adhesive is applied to the needle ends in the needle holder8is the removable attachment13moved downwardly by means of a screw or eccentric, in so doing, forces the needle holder8onto the compression spring10of the ejection device, and by means of the adhesive draws downward again any needles4that were pulled upward. This better ensures that all of the needle tips5are resting on the flat plate12when the adhesive solidifies.

To install the needles, the needles4are inserted into the holes of the ejection device. In so doing, it is preferable to insert one needle each into one hole each of the ejection device. The needles rest loosely in the holes of the ejection device and, as a result of their own weight, rest with their needle tips on the flat plate12. The flat plate12serves to align the plane of the needle tips. Therefore, the plate12includes a polished, very flat surface. The error differential of the plane of the needle tips is reduced by the mounting device according to the invention to a few μm. At this point the needles are fixed in the ejection device. In this process a hotmelt adhesive is preferably applied to the needle ends in the needle holder, the hotmelt adhesive then melted and subsequently solidifies. To liquefy the hotmelt adhesive, the mounting device11together with the ejection device is placed on a hot plate and heated by the hot plate to approximately 120° C. For this purpose, the mounting device is composed of a good thermally conductive material, thus for example, a metal plate, metal connecting piece and metal removable attachment. Tool steel is especially suitable in this regard and is also very easily ground to form a flat plate12, in particular, tempered tool steel. The hotmelt adhesive is applied via the recess which forms an opening in the removable attachment13to the needle holder8and the needle ends6disposed therein, is heated and binds the needle ends6to the needle holder8and securely fixes said needles after solidifying. This ensures that the needles are securely fixed to the tool and yet are not misaligned as a result of the fixing process. Here, a hotmelt adhesive is especially suitable that is advantageously applied as a granulate and subsequently melted. Moreover, the mounting device is preferably designed so that the removable attachment is mounted and screwed tight, thereby pressing the needle holder down approximately 0.5 mm onto the spring. The needles are then fixed. Once the ejection device is removed from the mounting device following the fixing process, the needles are recessed by approximately 0.5 mm in the ejection device.

As an option, the needles can be inserted into the ejection device under a microscope. For this purpose the ejection device is in the assembled state, consisting of needle holder, compression spring and needle head. The needle head rests on a preassembly aid made of plastic so that the tips of the needles come to rest against a soft plastic as the needles are inserted into the holes. This ensures that the tips of the needles are not damaged upon impact. With the help of this plastic pre-assembly aid the ejection device can be placed upside down on the mounting device. The pre-assembly aid is then removed. At this point the removable attachment is mounted on the ejection device and screwed tight, as described.

FIG. 6shows an ejection device prepared with needles4, wherein in this example the needle head9is visible from above and the needles4are pushed out. The holes15for the needles in the needle head9are visible in the drawing. When using needles with a diameter of 0.25 mm, the holes15are approximately 0.27 mm in diameter.

As the chip is lifted off the carrier material a vacuum flows through the vacuum holes20to the carrier material, thus pinning the carrier material to the needle head.

FIG. 7shows the preparation of a needle according to the invention which has three cutting edges (one is concealed), formed as sharp, wedge-shaped lateral edges, at the front portion of the needle and which taper toward the tip of the needle so that the tips are formed correspondingly sharp. Thus, as the needles are lifted, when the needles press against the carrier material and the chip resting thereon, the carrier material is initially pierced, and then as a result of further pressure from the needles and with that the high pressure forces of the cutting edges against the carrier material, said carrier material is cut through by the cutting edge as the needle is raised to the point at which the chip is presented for take up by the take-up tool. The pressure against the carrier material or against the carrier film as a result of the needles pressing against the carrier material and against the chip is thus removed from the carrier film as a result of it being cut through, such that the latter is not warped or raised further, and thus also with barely any pressure on the chip itself and no slipping of the adjacent chips, no damage is done to the chips. In addition, the carrier film is completely detached from the chip so that when the chip is taken up by the take-up tool, there are no residual adhesion forces holding the chip down.

In the drawing, the lateral edges7are drawn as straight lateral edges. However, they can also be ground to arch inwardly or outwardly. The lateral faces21between two cutting edges can be worked to form a flat surface between the two cutting edges. However, the lateral faces21can also be ground to form an arc between the lateral edges. To increase the sharpness of the lateral edges the lateral faces can in particular be ground to arch inwardly.

The length of the needles used here is 29 mm. However, this can be adapted depending on the ejection device and the fabrication machine, and is typically between 10 mm and 30 mm. The prior art operates with conically shaped tips. Now, according to the invention sharpened lateral edges are ground along the front portion of the needle.

The three cutting edges are ground from a wire blank at an angle β relative to one another. This angle can be approximately 120°, but can also differ from this so that the three cutting edges do not lie at the same angular distance from one another. The preferred diameter on the basis of which the needle is ground is smaller or equal to 0.25 mm. The respective angle α between height h and lateral edge or cutting edge7can be selected as required. The height h is the perpendicular that is dropped from the needle tip to the base plane, wherein the base plane is the cross-sectional area of the needle end6. In this case, a preferably selected angle α is 4° to 8°.

LIST OF REFERENCE NUMERALS