Dicing tape and die ejection method

A tape assembly for use in wafer dicing includes a layer of adhesive dicing tape having a size at least as large as a footprint of a die, and a screening portion which is adhered to the tape. The screening portion is interposed between the layer of tape and the die when the die is adhered to the layer of tape. The screening portion covers an interior portion of the layer of tape. The screening portion is sized and shaped to leave a sufficient portion of the layer of tape underlying a perimeter of the die exposed to adhere the die to the layer of tape.

FIELD OF THE INVENTION

The present invention relates to the field of integrated circuit fabrication generally, and more specifically to separation of dies from semiconductor wafers.

BACKGROUND

During integrated circuit fabrication, a plurality of integrated circuits (dies) are formed on a single semiconductor wafer simultaneously by a series of material deposition and removal processes. The individual dies are then separated from the wafer, in a process called dicing. Typical wafer dicing involves attaching the wafer to an adhesive dicing tape, followed by separation of the dies from each other and the wafer. Separation is typically achieved by dicing with a circular saw or by scribing and breaking the wafer (if the wafer is crystalline). If the dies contain micro electromechanical systems (MEMS), the latter method is typically used, because use of a saw would produce too many particles that could interfere with the device. Even using the scribe-and-break technique, the die ejection process from the tape is non-trivial.

FIG. 1shows a die100including a MEMS structure104. The exemplary MEMS104is a mirror attached to the die100which pivots about an axis106under control of the circuitry102, and is also connected to the die by a plurality small springs, not shown. The springs (not shown) bias the mirror to the horizontal position of FIG.2. The MEMS104may be, for example, a mirror sized between about 200 microns and 300 microns, connected to the outer portion of the die100by 2 springs (not shown). Although an example of a mirror is described, this discussion applies equally to other types of MEMS with at least one moving part.

A common die-ejection procedure includes placing the adhesive tape holding the dies on top of a base that includes an array of sharp-pointed pyramid-shaped structures. A vacuum is applied between the tape and the base. The tape is thus pulled down, minimizing the contact area with the die and facilitating its removal. An example of such a machine is a Model 4800 Die Ejection Grid machine manufactured by Semiconductor Equipment Corp. of Moorpark, Calif.

FIG. 2shows a cross section of the die100of FIG.1. The die100is attached to dicing tape120, which is placed on the base110having pyramid-shaped structures.

During the ejection procedure, the tape120charges up due to charge separation at the interface and the electrostatic force on the MEMS device104can attract the moving part of the MEMS, resulting in the moving part becoming irreversibly stuck to the adhesive tape, as shown in FIG.3. When the die100is subsequently removed from the tape120, the MEMS104remains on the tape, rendering the die useless.

Although ultraviolet (UV) curable dicing tape has been developed to release items attached thereto upon curing the tape, even UV curable tapes retain a small amount of adhesion after exposure to UV radiation. This is enough to tear the MEMS104away from the outer portion100of the die.

Another attempt at protecting the MEMS involved the use of “anti-static” tapes. However, because of the small spring constant of the springs (not shown), the anti-static tapes may still develop enough charge to draw the MEMS to the tape.

In addition to the static electric charge, the sudden displacement of the tape120upon application of the vacuum can create an air flow (referred to herein as the “pneumatic effect”) that tends to move the movable part104of the MEMS into contact with the tape120.

Improved techniques are desired.

SUMMARY OF THE INVENTION

A tape assembly for use in wafer dicing includes a layer of adhesive dicing tape having a size at least as large as a footprint of a die, and a screening portion which is adhered to the tape. The screening portion is interposed between the layer of tape and the die when the die is adhered to the layer of tape. The screening portion covers an interior portion of the layer of tape. The screening portion is sized and shaped to leave a sufficient portion of the layer of tape underlying a perimeter of the die exposed to adhere the die to the layer of tape.

DETAILED DESCRIPTION

The term micro-electromechanical systems (MEMS) device as used herein is intended to mean an entire MEMS device or any portion thereof. Thus, if a portion of a MEMS device is inoperative, or if a portion of a MEMS device is occluded, such a MEMS device is nonetheless considered to be a MEMS device for purposes of the present disclosure.

FIGS. 4-6show an exemplary apparatus and method for separating a die100from a tape assembly200that is used in wafer dicing. The tape assembly200includes a layer of adhesive dicing tape202having a size at least as large as a footprint of a die, and a screening portion210which is adhered to the tape. The screening portion210is positioned on the tape202so that the screening portion210is interposed between the layer of tape and the die100when the die100is adhered to the layer of tape. The screening portion210may (but is not required to) include a conductive membrane covering an interior portion of the layer of tape202. The screening portion210is sized and shaped to leave a substantial portion of the layer of tape202underlying a perimeter of the die100exposed when the die is adhered to the layer of tape. Thus, the screening portion210does not interfere with adhering the die100to the tape202.

In the example ofFIG. 6, the screening portion210is a square conductive membrane having a length that is slightly greater than the length of the opening108surrounding the MEMS104. Preferably, the screening portion210is formed of a material that is thick enough to substantially increase the stiffness of a portion of dicing tape202to which the screening portion is adhered, but thin enough so as not to interfere with adhesion between surrounding portions of the tape202and the perimeter of the die100. Materials that are at the thicker end of this material-thickness window are also more likely to be re-usable in a manner such as described further below. An exemplary material for the screening portion210is a metal foil 10 to 25 microns thick, which may be chrome, aluminum or copper, for example.

The tape202can be any tape suitable for dicing. An exemplary tape is No. 1020 UV-curable adhesive polyvinyl chloride (PVC) tape, sold by Ultron Systems Inc. of Moorpark, Calif. A similar tape product (Adwill D-Series Tape) is available from Lintec Semiconductor of Tokyo, Japan. UV curable tapes are advantageous because, once cured, removal of the die100and/or the screening portion210from the tape202is facilitated. Where different levels of adhesion or tackiness are available, tapes having higher tackiness are advantageous, because they facilitate adhesion between the die100and the tape202, even with a portion of the tape blocked by the screening portion210. General dicing tapes that are not UV curable, and dicing tapes having lower tackiness may also be used.

The shape of the screening portion may depend on the shape of the MEMS104on the die100. For example, given a square mirror104as shown inFIGS. 4-6, a square screening portion210larger than the mirror104(and larger than the space108surrounding the mirror), but smaller than the die100is suitable. For a circular MEMS (not shown), a circular screening portion (not shown) having a diameter greater than the diameter of the MEMS (and optionally greater than the diameter of the space surrounding the MEMS) may be used.

Other shapes that are at least as large as the footprint of the MEMS104may also be used. For example, given the square mirror104, a screening portion210may be, for example, a rectangle having both sides greater than that of the space108surrounding the MEMS, or a circle having a radius at least as large as the diagonal of the space108.

The exemplary screening portion210has three effects on the tape removal process. The first effect is that the addition of the screening portion210makes the tape assembly200stiffer locally under the MEMS104, so that there is reduced movement of the portion of the tape202beneath the MEMS. This in turn reduces or eliminates the pneumatic effect, so that the MEMS104is less likely to contact the tape assembly220. Second, if screening portion210is conductive, the MEMS104, the electrical attraction between the MEMS104and the tape202is reduced or eliminated, in addition to the pneumatic effect. Without being bound to any particular theory, this may be because the MEMS is completely screened from any static charge in the tape202, or because charge beneath the MEMS is shunted towards the perimeter of the die. The third effect is that the screening portion210presents a non-adhesive surface directly beneath the MEMS104. Even if the MEMS104ever contacts the screening portion210, the MEMS104does not adhere to the screening portion.

In alterative embodiments, the screening portion210may be formed of a non-conductive material. Such embodiments would not, however, shunt the static charge in the tape away from the MEMS as the exemplary conductive membrane does.

In some embodiments, an individual screening portion210is positioned on the tape202at a respective position to underlie a respective die on a wafer when the wafer is adhered to the tape, as shown inFIGS. 4-6. A plurality of individual screening portions (not shown) may be affixed to the tape in the same manner.FIGS. 7-9show another embodiment, in which a plurality of screening portions224are joined together in a single sheet220. The plurality of screening portions224correspond to dies on a wafer150.

FIG. 7shows the tape assembly201, including dicing tape202and a patterned sheet220. The tape202may be one of the types described above, and the patterned sheet220may be formed of the same conductive membrane materials described above with reference to the screening portion210. The layer of tape202has a size at least about as large as a footprint of a die-containing portion of a wafer150from which the dies100are cut. In the example, the tape202is as large as the footprint of the entire wafer.

The patterned sheet220has a plurality of screening portions224, which are adhered to the tape202. The patterned sheet has a plurality of cutouts226, which leave a substantial portion of the perimeter (as a fraction of total perimeter length) of each of the plurality of dies100exposed when the wafer150is adhered to the layer of tape (FIG.8). The dies100and the scribe lines152between the dies are shown in phantom. The screening portions224are connected to each other and to the perimeter of the patterned sheet220by a plurality of connecting portions222.

Although the example ofFIG. 7shows four connecting portions222connected to each screening portion, any number of connecting portions may be used. Although the exemplary connecting portions222are located at the four corners of each screening portion224, the connecting portions may alternatively be located along the sides of the screening portions. Although the interior cutouts226are hexagons and the exterior cutouts are trapezoids, other cutout shapes may be used. With the cutouts226, the exemplary patterned sheet resembles a stencil. AlthoughFIG. 7shows a wafer having five dies in each row and five dies in each column, a patterned sheet can be developed to with an appropriate number of screening portions corresponding to any number of dies on a wafer (e.g., 36 screening portions per patterned sheet).

As shown inFIGS. 8 and 9, the patterned sheet220can be interposed between the layer of tape202and the wafer150when the wafer is adhered to the layer of tape. The wafer150has a die-containing portion with a plurality of integrated circuit dies100.FIG. 8shows the view from a vantage point between the tape202and the patterned sheet220(i.e., looking towards the bottom surface of the wafer. Each screening portion224is shaped and positioned to underlie an interior portion of a respective one of a plurality of dies100on the wafer150when the wafer is adhered to the layer of tape202. InFIG. 8, each of the dies100includes a respective MEMS thereon, but this is not a requirement.

The connecting portions222occupy a relatively small fraction of the perimeter of each die100, so as to not interfere with adhesion between the tape202and the perimeter of each of the plurality of dies. Further, if the four cutouts226surrounding each respective screening portion224are considered to belong to a single combined cutout228corresponding to that one screening portion, then the connecting portions222occupy a relatively small fraction of that combined cutout. As a result, good adhesion between the underlying tape202and the wafer150can be obtained, even with the patterned sheet220interposed therebetween. Each screening portion224is sized and shaped to underlie an entire micro electro-mechanical system104, while leaving a substantial portion of the layer of tape202underlying a perimeter of the corresponding die100exposed, allowing adhesion between the tape and the perimeter of each of the plurality of dies.

Although an example is provided in which every die has a MEMS, in alternative embodiments having multi-project wafer (MPW) systems, there may be MEMS on only a subset of the dies on the wafer. In a multi-project wafer, the screening portions may be placed only under the dies having MEMS.

FIG. 10is a diagram of an exemplary method of using the screening portions. In this example, the screening portions are re-used. In other embodiments, the screening portions are only used once and discarded.

At step1000, a screening portion210(or a patterned sheet220of screening portions224) is interposed between an adhesive dicing tape202and an interior portion of a die100(or an interior portion of the wafer150). For example, the tape202may be mounted (by lamination) on a dicing ring (a ring larger than the wafer). A table or a surface that supports the wafer and a roller that contacts the sheet and wafer in a uniform manner may also be used. The excess portion of the tape is cut away, and the pattern sheet220is then laminated onto the tape202.

At step1002, a perimeter of the die100(or of each die on a wafer150) is adhered to the adhesive tape202.

At step1004, the wafer150is scribed and broken to separate the dies100.

At step1006, the tape202is cured. This reduces the tackiness of the tape202to a minimum.

At step1008, a vacuum is applied between the tape202and the base110of the apparatus. This causes the tape202to pull away from the dies, towards the base110.

At step1009, the dies are removed from the tape202.

At step1010, the patterned sheet220(or individual screening portion(s) ) is removed from the tape202.

At step1012, the patterned sheet220is re-applied to another piece of dicing tape202.

At step1014, the tape assembly, with the re-used patterned sheet, is applied to another wafer.

At step1016, the dies are separated from the second wafer.