Apparatus, device and method for wafer dicing

An apparatus, device and method for wafer dicing is disclosed. In one example, the apparatus discloses: a wafer holding device having a first temperature; a die separation bar moveably coupled to the wafer holding device; and a cooling device coupled to the apparatus and having a second temperature which enables the die separation bar to fracture an attachment material in response to movement with respect to the wafer holding device. In another example, the method discloses: receiving a wafer having an attachment material applied to one side of the wafer; placing the wafer in a holding device having a first temperature; urging a die separation bar toward the wafer; and cooling the attachment material to a second temperature, which is lower than the first temperature, until the attachment material fractures in response to the urging.

BACKGROUND

Brief Background Introduction

This specification relates generally to systems and methods for wafer dicing. There are many steps to wafer dicing and managing them efficiently and at low cost is a challenge. Further improvements are desired.

DETAILED DESCRIPTION

Semiconductor wafer pre-assembly usually occurs after full wafer fabrication. During this pre-assembly the wafer has to be separated in single chips in a dicing step. One possibility, besides normal blade dicing, is stealth laser dicing where a sawlane between two die can be reduced to 15 μm instead of 50 μm to 80 μm sawlane widths associated with blade dicing. Stealth laser often works by initiating a hair-crack within the wafer material which then enables the dies to cleave cleanly as underlying dicing tape or foil is stretched to separate the dies from one another during an expansion processing step. The dicing foil is connected to the wafer frame (FFC) and is expandable so that individually separated dies can be picked up. Expansion allows the dies to be picked up so they do not damage each other.

More details of “stealth dicing” may be found in U.S. patent application Ser. No. 13/687,110 of Sascha Moeller and Martin Lapke titled, “Wafer Separation” filed on Nov. 28, 2012 and is incorporated by reference in its entirety.

With such a reduced sawlane width, a number of Potential Good Dies Per Wafer (PGDW) can be significantly increased, especially when small dies are being fabricated. Laser dicing can also improve fracture strength, enhance fabrication speed, and reduced chipping on the front and back sides of a wafer to a minimum.

Special fabrication applications require a wafer's backside to be coated with an attachment material (e.g. Die Attach Film (DAF)). Die attach films have become an important technology to realize excellent reliability, high performance, and high speed in a packaging process, as well as to enable smaller and thinner semiconductor packages.

DAF can be thought of as a type of attaching material, adhesive, glue, etc. DAF is typically an organic material while the wafer is substantially a crystalline material. DAF backed dies at small sizes and having small lane width dimensions between dies is increasingly required by customers. However, because DAF is a soft organic, glue-like type of material, stealth laser dicing typically can not cut the DAF cleanly if at all. As a result should the underlying dicing tape or foil be stretched, the dies will stick to one another in semi-random ways, preventing the dies from being picked up individually, and reducing the yield of a wafer or requiring that the entire wafer be discarded.

FIG. 1is an example of damage which can be incurred during wafer dicing of DAF coated wafers.FIG. 1shows four dies102,104,106,108which have been laser diced and expanded, resulting in two perpendicular sawlanes110and112. Expansion has created several problems in the DAF coating, including a partially detached DAF region114, a fully detached DAF region116, and a DAF hole region118. Note that the DAF still connects the four dies102,104,106,108except in the hole118region. An attached DAF region105connected to dies104and106is also shown.

Thus when expansion occurs before the DAF can be broken, then the DAF will typically only semi-detach, as shown inFIG. 1. As discussed above, such semi-detachment occurs because of DAF flexibility, stretchiness, and/or elasticity which prevents ready separation.

In response to the concerns discussed above, additional example embodiments are now discussed. In these new embodiments, DAF coated wafers can be readily separated after laser dicing. A number of potential good dies per wafer (PGDW) with an attach material coating (e.g. DAF) can be increased significantly by reducing the saw lane up to a minimal width of 15 μm while cleanly cutting the attach material. These example embodiments extend DAF advantages to smaller dies and also allow a reduced saw lane width to increase the number of PGDW. Such embodiments enable attach material, such as DAF, to be applied to very thin dies and products that can't be diced by a blade dicing process.

Example cooling processes are used to make an attachment material more rigid and brittle, enabling easier separation after dicing. Such cooling can be applied in either a global manner, using for example a cooling cushion, or a local manner, using for example a cooled die separation bar. Global cooling in one example embodiment is a cooling cushion or in another example embodiment is a cooling liquid applied to a side of a wafer which is typically opposite to a wafer holding chuck and/or a die separation bar. Local cooling example embodiment is a cooled separation bar.

Some processes separating laser diced DAF coated wafers held in a cooled wafer holding chuck use just expansion. Since the wafer holding chuck holds a back-side of a wafer, active die separation using a die separation bar on the back-side of the wafer is prevented. This is because the die separation bar moves under a back-side of the wafer where the expansion foil is and where a cooling chuck would be placed. Processes using a cooling chuck tend not to be feasible for rectangular dies whose top surface dimensions are less than 2 mm×2 mm.

Details of the present claimed device/service are now discussed.

FIGS. 2A and 2Bare examples of a first and a second wafer processing steps. Example substrate materials can be composed in part of at least one from a group including: glass, alumina, silicon, gallium arsenide, silicon on sapphire, ceramics, plastic, a crystalline material, and other semiconductor or crystalline materials. Shown is a Silicon (Si) substrate202upon which are frontend structures204such as electronic circuits separated by a dicing street206.

A backgrinding tape208is applied to the frontend structures204surface. A first expansion material210(e.g. dicing/expansion tape/foil) is applied to the backgrinding tape208. The first expansion material210is mounted to a first wafer holding device212. In one example the first wafer holding device212is a film frame carrier (FFC) which has an 8″ diameter. In other example embodiments, the first wafer holding device212may have many other smaller or larger sizes appropriate for holding a wafer. Such other sizes include: 4″, 6″, 12″, 300 mm, as well as others. The substrate202is ground to desired thickness.

FIGS. 3A and 3Bare examples of a third and a fourth wafer processing steps. The wafer substrate202is diced by a laser from the back-side of the wafer. The laser302creates defects in the substrate202(e.g. defect/modification zone x-axis304and defect/modification zone y-axis306) which provides stress regions which can be fractured (e.g. cracked, cleaved, etc.) during subsequent steps. The normal definition of “fracture” is herein augmented to include a crack in the substrate202and/or an attachment material308(discussed below) sufficient to enable a first selected die or a first set of dies to be picked up and/or separated from a second die or second set of dies created on the wafer substrate, without sticking to or otherwise disturbing the second die or second set of dies.

An attachment material308is then applied to the backside of the substrate202. The attachment material308can be at least one from a group including: a die attach film (DAF), an adhesive, an attractive coating, a UV activated material, and other structures or materials which provide attachment properties. A second expansion material310(e.g. dicing/expansion tape/foil) is mounted to the attachment material308on the wafer substrate's202backside. A second wafer holding device312is mounted to the second expansion material310. In one example the second wafer holding device312is a 12″ film frame carrier. In other example embodiments, the second wafer holding device312may have many other smaller or larger sizes appropriate for holding a wafer. Such other sizes include: 4″, 6″, 8″, 300 mm, as well as others.

Once the wafer has been fractured by a laser, or other process, as discussed inFIG. 3, the attachment material308(e.g. the DAF tape) is now cooled so that it can become brittle and fracture (i.e. crack, break, cleave, or separate) with little or no damage to the attachment material308during expansion. In an example embodiment, cooling causes the attachment material308to be embrittled and crack under stress.FIGS. 4, 5, and 6present example embodiments of fifth and sixth processing steps for fracturing the attachment material308prior to, or during, expansion. These example embodiments are global cooling, local cooling, and a combined global and local cooling.

FIGS. 4A and 4Bare examples of a fifth and a sixth wafer processing steps using global cooling. A global cooling device402is applied to either or all of the substrate202, the frontend structures204or attachment material308. Example embodiments of the global cooling device402include: a cooling cushion, a cooling liquid, or another environmental condition which causes at least the attachment material308to cool from a first temperature to a second temperature. The first temperature is herein defined as a temperature insufficient to allow the attachment material308to fracture. An augmented definition of fracture was presented earlier. The second temperature is herein defined as a temperature sufficient to allow the attachment material308to fracture, as defined above.

In one example, shown inFIG. 4A, the attachment material308is cooled from the top side of the wafer, perhaps while in an expansion device. Depending upon a particular wafer fabrication, cooling could take place in other wafer fabrication devices as well. In theFIG. 4Aexample the global cooling device402is a cooling cushion filled with a liquid that can be cooled down to a temperature sufficient to enable the attachment material308to fracture so that the dies on the wafer can separate during an expansion step, discussed inFIG. 7. The cooling cushion would come into contact with the wafer so that the attachment material308could be cooled to the second temperature. Alternately, a cooling liquid can be coupled to cool the wafer by spraying the liquid directly on the wafer. The liquid itself can be cooled or evaporation of the liquid from the wafer can cool the attachment material308to the second temperature.

In one example embodiment the second temperature is about zero degrees Celsius. However, in other examples on production lines operating at higher speed, a −5 to −10 degrees Celsius is possible. Other wafer fabrication processes and attachment materials308may require different temperatures. The liquid in the cooling cushion in one example process can be isopropanol. Other example cooling liquids include: liquid nitrogen, Fluorinert™, propylene glycol, other organic solvents, and super-cool gas.

While in one example embodiment stretching of the second expansion material310thereby separating the wafer dies (seeFIG. 7) can be done right after or during cooling of the attachment material308,FIG. 4Bshows an example embodiment where a die separation bar404is positioned on a side of the substrate202opposite to the global cooling device402and proximate to or under the defect/modification zones304and306.

The die separation bar404is moveably coupled to the wafer holding device312such that a portion of the substrate202can then be bent over the die separation bar404to fracture the attachment material308and create a set of dies408. In one example, the fracturing occurs by positioning the die separation bar404under a selected sub-set of the substrate's202defect/modification zones304and306and urging the die separation bar404toward the substrate202such that the substrate202and attachment material308fractures.

After the die separation bar404fractures the attachment material308, the second expansion material310is stretched406thereby creating separated dies702as shown inFIG. 7. Use of the die separation bar404in conjunction with global cooling enables thin and/or small dies having attachment material308to properly break with less damage to the attachment material308.

FIGS. 5A and 5Bare examples of the fifth and the sixth wafer processing steps using local cooling. InFIG. 5A, the attachment material308is locally cooled using a cooled die separation bar502, which causes at least the attachment material308to cool from a first temperature to a second temperature, which is sufficient to fracture the attachment material308.

In theFIG. 5Aexample, the wafer assembly, including the substrate202and attachment material308, is received after theFIG. 3laser dicing step.FIG. 5Bshows an example embodiment where the cooled die separation bar502is positioned on a side of attachment material308opposite to (e.g. under) the substrate202and proximate to or under the defect/modification zones304and306.

Depending upon the particular wafer fabrication process used, the cooled die separation bar502is held under each defect/modification zone304or306such that he attachment material308is sufficiently cooled (e.g. to zero degrees Celsius as discussed above). The substrate202is then bent over the cooled die separation bar502to fracture the attachment material308under all or a selected sub-set of the substrate's202defect/modification zones304and306thereby fracturing the attachment material308and creating a set of dies506. After the cooled die separation bar502fractures the attachment material308, the second expansion material310is stretched504thereby creating separated dies702as shown inFIG. 7. Use of the cooled die separation bar502enables thin and/or small dies having attachment material308to properly break with less damage to the attachment material308.

FIGS. 6A and 6Bare examples of the fifth and the sixth wafer processing steps using both global and local cooling. InFIG. 6A, the global cooling device402is applied as discussed inFIG. 4Aand the cooled die separation bar502is applied as discussed inFIG. 5Bto create a set of dies604. Depending upon the particular wafer fabrication process used, application of both the global and local cooling can be modulated such that various production criteria are optimized, including die fabrication speed, die yield, thermal gradient minimization, reduced energy consumption, as well as others. After the cooled die separation bar502fractures the attachment material308, the second expansion material310is stretched602thereby creating separated dies702as shown inFIG. 7.

FIG. 8is an example embodiment of a local cooling device800for locally cooling the attachment material308. The local cooling device800includes a die separation bar whichFIG. 8shows in a cross-sectional, edge-on view. The die separation bar802forms a core structure upon which a first thermally conductive cutting structure804and second thermally conductive cutting structure806are attached. The die separation bar802includes a radial inlay808such that when the first and second cutting structures804and806are placed proximate to the defect/modification zones304and306, the substrate202can be bent to fracture the attachment material308.

In various example embodiments, a cooling device810is thermally coupled to at least one of either the die separation bar802, the first cutting structure804or the second cutting structure806, thereby cooling the die separation bar802for implementing local cooling as discussed inFIGS. 5 and 6. The cooling device810can take various embodiments, including a thermoelectric cooling device using thermoelectric cooling (using the Peltier effect), a liquid nitrogen based cooling device, as well as others.

Selection of the cutting structures804and806in some example embodiments impacts the thermal, production time, and energy efficiency associated with fracturing the attachment material308.

FIG. 9is one example of a flowchart for implementing a method900for wafer dicing. The method900begins in block902, by receiving a wafer having an attachment material applied to one side of the wafer. Next, in block904, placing the wafer in a holding device having a first temperature. In block906, urging a die separation bar toward the wafer. Then in block908, cooling the attachment material to a second temperature, which is lower than the first temperature, until the attachment material fractures in response to the urging.

Three example embodiments for implementing Block908are shown. In block908A, cooling a portion of the die separation bar to the second temperature until the attachment material fractures in response to the urging. In block908B, applying a cooling cushion to a side of the wafer opposite to the die separation bar; and cooling the cooling cushion to the second temperature until the attachment material fractures in response to the urging. In block908C, applying a cooling liquid to the wafer; and cooling the cooling liquid to the second temperature until the attachment material fractures in response to the urging. Blocks908A,908B, and908C may also be implemented together, in any combination.

The blocks comprising the flowcharts in the above Figures can be effected in any order, unless a specific order is explicitly stated. Also, those skilled in the art will recognize that while one example method embodiment is now discussed, the material in this specification can be combined in a variety of ways to yield other examples as well. The method just discussed is to be understood within a context provided by this and other portions of this detailed description.

Any functional and software instructions described above are typically embodied as a set of executable instructions which are effected on a computer which is programmed with and controlled by said executable instructions. Such instructions are loaded for execution on a processor (such as one or more CPUs). The processor includes microprocessors, microcontrollers, processor modules or subsystems (including one or more microprocessors or microcontrollers), or other control or computing devices. A processor can refer to a single component or to plural components.

In one example, one or more blocks or steps discussed herein are automated. In other words, apparatus, systems, and methods occur automatically. The terms automated or automatically (and like variations thereof) mean controlled operation of an apparatus, system, and/or process using computers and/or mechanical/electrical devices without the necessity of human intervention, observation, effort and/or decision.

In some examples, the methods illustrated herein and data and instructions associated therewith are stored in respective storage devices, which are implemented as one or more non-transient computer-readable or computer-usable storage media or mediums. The non-transient computer-usable media or mediums as defined herein excludes signals, but such media or mediums may be capable of receiving and processing information from signals and/or other transient media.