Patent Application
20100081255
The field of invention relates generally to the field of microelectronic devices and, more specifically but not exclusively, relates to semiconductor integrated circuit packaging.
Electronic devices are typically combined with a workpiece and assembled in a package that is soldered or connected to a printed circuit board prior to use. Typically, a number of identical electronic devices are first formed on a single wafer. In some instances, up to several thousand identical electronic devices are formed on the wafer. Once formed, each of the electronic devices is electrically tested and sorted. Next, the wafer is sliced and diced to produce individual wafer portions known as die or chips. Each die or chip contains an individual device, such as an integrated circuit, a microprocessor, or other electronic device. Each die also includes leads, such as pins or balls, which are formed on the surface of the die.
In some instances, the cut lines for slicing and dicing a wafer into individual chips or die are scribed using a laser. Laser scribing is most often used on newer silicon devices incorporating fragile high performance dielectric materials. Laser scribing precedes singulation (slicing and dicing the wafer into individual chips or die) with a saw blade. The laser scribe process may remove some of the fragile materials without leaving mechanical defects in the device edge that can propagate into the electronic device during singulation and/or packaging. However, use of a laser scribe may also create an irregular and porous topography prior to singulation, leading to defects when exposing the topography to mechanical stress during singulation and/or packaging.
The present invention is illustrated by way of example and not as a limitation in the figures of the accompanying drawings, in which
Methods for reducing defects through selective laser scribing are described in various embodiments. In the following description, numerous specific details are set forth such as a description of an efficient method to form a normalized wafer scribe on a wafer to reduce defects during a singulation process.
It would be an advance in the art of wafer singulation and packaging to reduce defects such as front side die chipping while improving edge die monitor yield. Existing methods for singulating a wafer by first ablating a surface using multiple passes of laser pulses can create an irregular scribe surface comprising weak and porous regions. These regions may cause front side chips and micro-cracks that can propagate into the die when applying mechanical stress during the saw process and/or during packaging. Further, multiple passes of the laser scribe over some areas such as scribe street intersections combined with laser scribing of test structures with disparate material types and topographies along a scribe line can exacerbate an already irregular surface, making the die even more prone to defects formed during a saw process.
One or more methods to reshape and strengthen the laser scribe while accounting for test structures and scribe patterning of a wafer surface through controlled energy density of a series of laser pulses can enhance and normalize the remaining scribe structure prior to singulation. These methods correlate the materials and architecture of test structures along a laser scribe path or street while avoiding multiple scribe passes in common areas, such as intersections, by synchronizing the laser path with a scribe pattern and by controlling an energy density applied such as by changing ablation velocity and z-focus height to provide uniform ablation depth and width. As a result, a normalized wafer scribe may be applied over a wafer surface independent of scribe pattern and test structure layout.
One such method for scribing a wafer with a laser comprises ablating a surface of the wafer with the laser to form a heat affected zone (HAZ). Ablating the wafer surface adjacent to the trench to modify a profile of the trench and form an irregular region, the irregular region comprising a low density portion with an irregular profile. Ablating the irregular region to recast the low density portion and to reshape the irregular profile, then modulating the laser to provide a normalized wafer scribe on the wafer.
Turning now to the figures, the illustration in
The laser 110 is also movable so that multiple laser pulses can be placed in a continuous line along the surface of the wafer 120. On the surface of wafer 120 there is a line 130 which is merely representative of a scribe line. The laser 110 is connected to a controller 140. The controller 140 controls movement of the laser 110 with respect to the surface of the wafer 120. The controller 140, in one embodiment, can move the laser 110 over the surface of the wafer 120. In an alternative embodiment, the wafer 120 may be moved with respect to the laser 110. The controller 140 is capable of controlling movements of the laser 110 with respect to the wafer 120 in either embodiment. The laser scribing and cutting mechanism 100 also includes a saw wheel or grinding wheel 150 having a kerf. The saw or grinding wheel 150 is typically configured with a diamond surface so that it can cut the material of the wafer 120 to singulate the wafer 120 into a die.
The die may be a microelectronic device such as a processor that may include baseband and applications processing functions and utilize one or more processor cores and/or firmware and hardware in an Application Specific Integrated Circuit (ASIC) device. In an embodiment where the die is a processor, the die may process functions that fetch instructions, generate decodes, find operands, and perform appropriate actions, then store results. The use of multiple cores may allow one core to be dedicated to handle application specific functions such as, for example, graphics, modem functions, etc. Alternatively, the multiple processor cores may allow processing workloads to be shared across the processor cores. In another embodiment, the die is another microelectronic device such as a system on a chip, a communications chip, or a graphics chip.
Wafer 120 is made of a fragile dielectric material or similar hard material and, therefore, the saw or grinding wheel 150 must include a hard substance such as diamond. The saw or grinding wheel 150 is turned by motor 152 attached to a shaft 154. The shaft 154 is attached to the saw or grinding wheel 150. The motor 152 is also attached to the controller 140. Therefore, the controller 140 can control the speed of rotation of the grinding wheel as well as control the position of the grinding wheel or saw 150 with respect to the wafer 120.
In this embodiment, the Pass 1 (210) and Pass 2 (220) are used to ablate the wafer surface 122 to form a heat affected zone (HAZ) 230 while removing material from the wafer 120. The laser pulses directed at the wafer surface 122 irradiate the wafer 120, wherein energy from the laser pulses are absorbed by the wafer 120 at or near the wafer surface 122 to heat and remove a portion of the wafer 120 through evaporation or sublimation. A depth over which the laser energy is absorbed is a function of material type of the wafer 120 at or near the wafer surface 122, which may be a uniform material or may be a variety of disparate materials including organic material such as photoresist, one or more metals, and dielectric materials used to form alignment marks, isolation regions, and/or test structures. More particularly, the laser energy absorbed by the wafer 120 depends on the optical properties of the wafer 120 material type and on a wavelength of the laser.
The Pass 1 (210) and Pass 2 (220) impinges the wafer surface 122 in an overlapping pattern where an overlap 240 may be selected to provide between 75% to 95% overlapping coverage between the laser pulses for the dual pass laser ablation as illustrated in
A third laser pulse, Pass 3 (330) may be applied to overlap both Pass 1 (210) and Pass 2 (220) to provide a more uniform HAZ 230 with a cross-section that may resemble a trench with a substantially uniform trench bottom 310 and laterally opposing trench sidewalls 320. The trench sidewalls 320 may form an obtuse angle in reference to the trench bottom 310 or the trench sidewalls 320 may be substantially vertical in reference to the trench bottom 310. Application of a third pass may be applied to create a preferred HAZ 230 profile, wherein regions of a HAZ 230 from a dual pass process with insufficient overlap gap 240 may comprise an irregular region comprising a porous or low density portion with an irregular profile. The irregular profile may be characterized by a sharp and/or jagged topography that is prone to forming cracks when exposed to mechanical stress. Ablating the irregular region to recast and harden the low density portion, for example as a result of a porous material formed during Pass 1 (210) and/or Pass 2 (220) and to reshape the irregular profile with the Pass 3 (330) may be used to avoid defects that would otherwise be formed in the wafer 120 during a singulation process. In another embodiment (not shown), a Pass 4 and a Pass 5 may be employed in an overlapping manner to further recast and harden low density portions formed during Pass 1 (210) and Pass 2 (220).
In one embodiment, the streets 410 are formed of a consistent material such as silicon or a dielectric layer such as silicon dioxide. In another embodiment, materials in the streets 410 of the wafer 120 comprise a standard or high performance dielectric material, one or more metals, an organic layer, and/or some combination thereof. Laser ablation removes some of the fragile materials at or near the top surface of the wafer 120 without leaving mechanical defects along an edge or boundary of the wafer die 420.
Without laser scribing, mechanical defects could propagate into the wafer die 420 under normal mechanical loads and cause failures of the wafer die 420. These mechanical defects may not propagate into an active device area of the wafer die 420 until after the chip or die is packaged and leaves the factory. Laser scribing also prevents long, thin, fragile dielectric and embedded copper (Cu) metal traces from peeling up and becoming entangled in the leads that are attached to the wafer die 420. In many instances, the leads are interconnect bumps arranged in an array and are also called a ball grid array. Other types of leads can also be used in the wafer die 420. In a further embodiment (not shown), the street 410 comprises a region of a first material such as or silicon dioxide or silicon nitride in a line positioned substantially in the middle of the street 410 and bordered on each side of the line by moat regions parallel to guard rings that border the edge of the wafer dies 420. Selective use of the moats and/or guard rings offer further protection against defects that may propagate into the wafer die 420 under normal mechanical loads and cause failures of the wafer die 420.
To provide a normalized wafer scribe on the wafer, or a consistent HAZ across the streets 410 of the wafer 120, energy transferred to the wafer is modulated to compensate for elements such as alignment marks 610, test structures 620, photoresist pads or regions comprising photoresist, and isolation regions. An energy density applied by the laser 110 compensates for various elements with varied materials and configurations and is optimized by modulating, for example, an ablation velocity and/or z-focus height (in and out of the page of
The operation discussed herein may be generally facilitated via execution of appropriate firmware or software embodied as code instructions on the host processor and microcontroller, as applicable. Thus, embodiments of the invention may include sets of instructions executed on some form of processing core or otherwise implemented or realized upon or within a machine-readable medium. A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium can include an article of manufacture such as a read only memory (ROM); a random access memory (RAM); a magnetic disk storage media; an optical storage media; and a flash memory device, etc. In addition, a machine-readable medium may include propagated signals such as electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.).
A plurality of embodiments of an apparatus and methods for reducing defects through selective laser scribing have been described. The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. This description and the claims following include terms, such as left, right, top, bottom, over, under, upper, lower, first, second, etc. that are used for descriptive purposes only and are not to be construed as limiting. For example, terms designating relative vertical position refer to a situation where a device side (or active surface) of a substrate or integrated circuit is the “top” surface of that substrate; the substrate may actually be in any orientation so that a “top” side of a substrate may be lower than the “bottom” side in a standard terrestrial frame of reference and still fall within the meaning of the term “top.” The term “on” as used herein (including in the claims) does not indicate that a first layer “on” a second layer is directly on and in immediate contact with the second layer unless such is specifically stated; there may be a third layer or other structure between the first layer and the second layer on the first layer. The embodiments of a device or article described herein can be manufactured, used, or shipped in a number of positions and orientations.
However, one skilled in the relevant art will recognize that the various embodiments may be practiced without one or more of the specific details, or with other replacement and/or additional methods, materials, or components. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention. Similarly, for purposes of explanation, specific numbers, materials, and configurations are set forth in order to provide a thorough understanding of the invention. Nevertheless, the invention may be practiced without specific details. Furthermore, it is understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention, but do not denote that they are present in every embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. Various additional layers and/or structures may be included and/or described features may be omitted in other embodiments.
Various operations will be described as multiple discrete operations in turn, in a manner that is most helpful in understanding the invention. However, the order of description should not be construed as to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation. Operations described may be performed in a different order than the described embodiment. Various additional operations may be performed and/or described operations may be omitted in additional embodiments.
Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above teaching. Persons skilled in the art will recognize various equivalent combinations and substitutions for various components shown in the Figures. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
