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LASER SEPARATION OF ENCAPSULATED
The following relates to the lighting arts. It especially relates to high intensity light emitting diode chip packages, components, apparatuses, and so forth, and to methods for producing such packages, and will be described with particular reference thereto. However, the following will also 10 find application in conjunction with other solid state light emitting chip packages such as vertical cavity surface emitting laser packages, and in conjunction with methods for producing such other packages.
The use of sub-mounts in packaging light emitting diode 15 chips, semiconductor laser chips, and other light emitting chips is well known. The light emitting chip or chips are attached to the sub-mount by soldering, thermosonic bonding, thermocompressive bonding, or another thermally conductive attachment. The light emitting chips are electrically 20 connected to bonding pads or other electrical terminals disposed on the sub-mount by wire bonding, flip-chip bonding, or another suitable technique. In some approaches, the light emitting chip is attached to the sub-mount and in thermal contact with the sub-mount, but is electrically 25 connected by wire bonds to a circuit such that the sub-mount is not part of the electrical circuit.
In a manufacturing setting, a plurality of light emitting chips are typically attached in parallel rows, or in another 3Q layout, to a large-area sub-mount wafer. The attached light emitting chips are transfer molded or otherwise encapsulated on the sub-mount wafer. Optionally, the encapsulant includes a dispersed phosphor for performing a selected wavelength conversion. For example, a group-Ill nitride 35 based light emitting diode chip emits light in the blue to ultraviolet range, and a suitable phosphor can be incorporated into the encapsulant to convert the blue or ultraviolet emission into white light. The sub-mount wafer is then diced to separate individual light emitting packages, each includ- 4Q ing one or more of the attached and encapsulated light emitting chips along with a supporting portion of the submount wafer.
Typically, the dicing of the sub-mount wafer is performed by mechanical sawing or scribing. Such mechanical sepa- 45 ration processes are readily automated, and are advantageously relatively independent of material characteristics; hence, the mechanical sawing or scribing can simultaneously cut through the transfer-molded encapsulant and the sub-mount. However, mechanical separation processes are 50 problematic in the case of sub-mounts of harder materials, such as aluminum nitride, sapphire, and the like. For these materials, a diamond-coated saw blade or a diamond-tipped scribe is used. Diamond-coated saw blades are relatively thick and generally produce cut widths or kerfs of 150 55 microns or wider, which adversely impacts device density on the sub-mount wafer. Diamond tipped scribes may produce narrower cut widths or kerfs; however, the scribe depth is limited. Hence, thicker sub-mounts cannot be diced by scribing unless the sub-mount is substantially thinned. 60
Both sawing and scribing effectively cut through any encapsulant material disposed in the dicing lanes. However, both techniques can produce roughened, striated, or otherwise damaged sidewalls that reduce light extraction efficiency. Moreover, mechanical sawing or scribing produces 65 shear forces that tend to delaminate the encapsulant, which can adversely impact device yield.
The following contemplates improved apparatuses and methods that overcome the above-mentioned limitations and others.
According to one aspect, a method is provided. A plurality of light emitting chips are attached on a sub-mount wafer. The attached light emitting chips are encapsulated. Fractureinitiating trenches are laser cut into the sub-mount wafer between the attached light emitting chips using a laser. The sub-mount wafer is fractured along the fracture initiating trenches.
According to another aspect, a method is provided. A plurality of light emitting chips are attached on a sub-mount wafer. Fracture-initiating trenches are laser ablated into the sub-mount wafer between the attached light emitting chips using a laser. The sub-mount wafer is fractured along the fracture initiating trenches.
According to yet another aspect, an apparatus is disclosed, including a plurality of light emitting chips and a sub-mount wafer. The sub-mount wafer has a front principal surface on which the light emitting chips are attached, a back principal surface opposite the front principal surface, and one or more fracture-initiating trenches disposed between the attached light emitting chips. The fracture-initiating trenches have widths less than about 75 microns.
Numerous advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the present specification.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. Except where indicated, layer thicknesses and other dimensions are not drawn to scale.
FIGS. 1A-1D show a sub-mount wafer with attached light emitting chips at various stages of a light emitting package fabrication process. FIG. 1A shows the sub-mount wafer with the chips attached; FIG. IB shows the sub-mount wafer after transfer-molded encapsulation; FIG. 1C shows the sub-mount wafer after laser cutting of fracture-initiating trenches; and FIG. ID shows one of the light emitting packages after sub-mount fracturing.
FIG. 2 shows a flow chart of an example light emitting package fabrication process.
FIG. 3 shows a flow chart of an example laser cutting process.
FIG. 4 diagrammatically shows an approach for shaping the encapsulant sidewall geometry during laser cutting. The top of FIG. 4 plots a Gaussian laser beam intensity distribution at the sub-mount; the bottom of FIG. 4 diagrammatically shows the resulting encapsulant sidewall geometry.
FIG. 5 shows a microscope image of the sidewall of a sub-mount with encapsulant after laser cutting and submount fracturing.
FIG. 6 shows a more magnified microscope image of an encapsulant sidewall after laser cutting and sub-mount fracturing.
DETAILED DESCRIPTION OF PREFERRED
With reference to FIGS. 1A-1D and FIG. 2, a plurality of light emitting chips 10 are bonded to a frontside 12 of a 5 sub-mount wafer 14 in a process operation 100. In typical embodiments, the chips 10 are attached arranged in rows; however, substantially any layout of chip attachments can be used. The light emitting chips 10 can be light emitting diodes, semiconductor lasers, or the like. In some embodi- 10 ments, the chips are attached by flip-chip bonding the chips 10 to electrical bonding pads disposed on the frontside 12 of the sub-mount wafer 14, which also makes electrical connection of the chips 10 with the sub-mount wafer 14. In other embodiments, the chips 10 are soldered or otherwise ther- 15 mally attached to the frontside 12 of the sub-mount wafer 14, and wire bonds are used to make electrical connection of the chips 10 with electrically conductive traces disposed on or in the sub-mount wafer 14. In yet other embodiments, the chips 10 are soldered or otherwise thermally attached to the 20 frontside 12 of the sub-mount wafer 14, and wire bonds are used to make electrical connection of the chips 10 with an external circuit, such that the sub-mount is not part of the electrical path. Optionally, the sub-mount wafer 14 can have an array of electrostatic discharge (ESD) protection devices 25 or other electrical circuitry disposed on or in the sub-mount wafer 14, and each chip 10 is electrically connected with such circuitry during the chip attach process 100.
Because the sub-mount wafer 14 will subsequently be separated by laser cutting (described infra), the chips 10 can 30 be attached with a relatively high density. The heat-affected zone of laser ablation for typical cutting lasers and typical sub-mount materials can be focused to about 25 microns; hence, corresponding gaps between adjacent attached chips 10 can be as small as about 25 microns. In contrast, 35 separation by sawing using a diamond-coated blade usually dictates larger gaps between adjacent chips, for example gaps of about 150-250 microns, in order to accommodate the larger widths or kerfs of the diamond-coated blade. Thus, although for illustrative purposes only twelve relatively 40 widely spaced chips 10 are illustrated in FIGS. 1A-1D, it is to be understood that the device packing densities can be substantially higher.
With reference to FIG. IB and FIG. 2, an encapsulant 20 is disposed over the attached chips 10 in a process operation 45 102. In some embodiments, the encapsulant 20 hermetically seals the light emitting chips 10 to the frontside 12 of the sub-mount wafer 14. In some embodiments, the encapsulant 20 includes a wavelength-converting phosphor that is selected to convert light generated by the light emitting 50 chips 10 to another wavelength. For example, in some embodiments the light emitting chips 10 are group Ill-nitride based light emitting diode chips emitting in the ultraviolet, and the encapsulant 20 includes a white phosphor that converts the ultraviolet emission to visible white light. In 55 other example embodiments, the light emitting chips 10 are group Ill-nitride based light emitting diode chips emitting blue light, and the encapsulant 20 includes a yellow phosphor that converts a portion of the blue light to yellow light such that the combination of direct blue emission and 60 wavelength-converted yellow fluorescence or phosphorescence approximates visible white light. In yet other example embodiments, the light emitting chips 10 are group IIIarsenide or group Ill-phosphide based light emitting diode chips that emit red, orange, green, or blue light, and the 65 encapsulant 20 contains no phosphor. These are illustrative examples; more generally, the light emitting chips 10 can be
substantially any type of light emitting diode, laser, organic semiconductor chip, or so forth, and the encapsulant 20 may or may not contain phosphor or a blend of phosphors.
In one suitable approach, the encapsulant 20 is applied by transfer molding in which each row of light emitting chips 10 are encapsulated. In the illustrated example of FIGS. 1A-1D, there are three such rows each containing four chips 10, and so there are three transfer molded strips of encapsulant 20. The encapsulant 20 extends over and covers the area of the sub-mount wafer 14 between the chips 10 in each row. In other embodiments, the light emitting chips are each individually encapsulated. In yet other embodiments, a blanket encapsulant is applied across the entire frontside 12 of the sub-mount wafer 14 to encapsulate all the chips 10 in a single encapsulation process, and the encapsulant extends over and covers substantially the entire frontside of the sub-mount wafer. For certain applications, it is also contemplated to omit the encapsulant 20, and the corresponding encapsulation process operation 102, entirely.
The encapsulated chips 10 should generally be electrically accessible unless, for example, the chips 10 are optically pumped, capacitively energized, or so forth, in which cases conductive electrical access to the chips 10 may be omitted. In some embodiments, the sub-mount wafer 14 includes electrically conductive vias passing from the frontside 12 to a backside of the sub-mount wafer 14. Such vias provide electrical connection between backside bonding pads of the sub-mount wafer 14 and electrodes of the attached chips 10. In other embodiments, printed circuitry disposed on the frontside 12 or elsewhere on or in the sub-mount wafer 14 connects with chip electrodes and extends outside of the area covered by the encapsulant 20 to provide electrical access to the light emitting chips 10.
With reference to FIG. 1C and FIG. 2, the sub-mount wafer 14 is secured to adhesive tape 24 in a process operation 104. Fracture-initiating trenches 30, 32 are laser cut into the sub-mount wafer 14 between the light emitting chips 10 in a process operation 106. The fracture-initiating trenches 30, 32 do not pass fully through the sub-mount 14 so as to sever the sub-mount 14 into pieces; rather, each fracture-initiating trench 30, 32 passes partway through the thickness of the sub-mount 14. In some embodiments, the fracture-initiating trenches 30, 32 pass about half-way through the thickness of the sub-mount 14. In the illustrated example, the fracture-initiating trenches 30 run transverse to the encapsulated rows of chips 10 and cut completely through the strips of encapsulant 20, whereas the fractureinitiating trenches 32 run parallel with the strips of encapsulant 20 and hence do not pass through the encapsulant 20.
With reference to FIG. ID and FIG. 2, the sub-mount wafer 14 is fractured in a process operation 108 at the fracture-initiating trenches 30, 32 to produce individual packages, such as the individual light emitting package 40 shown in FIG. ID which includes one of the light emitting chips 10 and portions of the encapsulant 20 and sub-mount 14. In the illustrated embodiment, each light emitting package 40 includes a single light emitting chip 10; hence, if the yield is 100% then the sub-mount wafer 14 is diced to produce twelve light emitting packages 40. Although each light emitting package 40 in the illustrated example includes one light emitting chip 10, in other embodiments each package may include two, three, or more light emitting chips. For example, each light emitting package may include a red light emitting diode chip, a green light emitting diode chip, and a blue light emitting diode chip such that the light emitting package is a full-color light emitter.