Patent Publication Number: US-2012032206-A1

Title: Variable height light emitting diode and method of manufacture

Description:
CROSS-REFERENCE TO RELATED INVENTION 
     The present invention is related in some aspects to commonly-owned and co-pending application Ser. No. 12/693,632, filed Jan. 26, 2010, and entitled LIGHT EMITTING DIODE (LED) AND METHOD OF MANUFACTURE, the entire contents of which are herein incorporated by reference. The present invention is also related in some aspects to commonly-owned and co-pending application Ser. No. 12/750,823, filed Mar. 31, 2010, and entitled MULTICHIP LIGHT EMITTING DIODE (LED) AND METHOD OF MANUFACTURE, the entire contents of which are herein incorporated by reference. The present invention is also related in some aspects to commonly owned and co-pending application Ser. No. 12/750,816, entitled “LIGHT EMITTING DIODE HAVING A WAVELENGTH SHIFT LAYER AND METHOD OF MANUFACTURE”, which was filed on Mar. 31, 2010, the entire contents of which are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to Light Emitting Diodes (LEDs). Specifically, the present invention relates to the manufacture of a variable height LED using a buffer layer. 
     BACKGROUND OF THE INVENTION 
     As LEDs continue to grow in popularity as an efficient technological approach, the need for continued advancement grows as well. Along these lines, obtaining white light output from LED is not only needed, but also difficult to achieve. Many approaches in the past have attempted to find new ways to obtain white light. However, many of these approaches perform such processing at the chip level instead of at the wafer level. Such an approach can result in chip waste. Moreover, none of the existing approaches vary phosphor ratios based on an underlying device measurement (such as a wavelength of a light output). For example, U.S. Pat. No. 6,650,044 forms a pedestal on top of a pad. The pedestal is a stud bump that is used for connectivity. This approach is not ideal as it is inefficient, does not provide chip level coating, is much harder to polish, and is easily contaminated. Moreover, in previous approaches such as U.S. Pat. No. 7,446,733 and 7,190,328, pads were manufactured so that all LEDs thereon had the same height. In view of the foregoing, there exists a need for an LED and associated method of manufacture that addresses the deficiencies of the related art. 
     SUMMARY OF THE INVENTION 
     In general, embodiments of the present invention provide a variable height LED and method of manufacture. Specifically, under embodiments of the present invention, a buffer layer is applied (e.g., selectively) over a wafer, and a set of LED chips is provided over the buffer layer. One role of the buffer layer is to increase a height of at least a subset of the chips. As such, the buffer layer could be applied using any processing method now known or later developed. For example, the buffer layer could be selectively deposited, etched, etc. Regardless, in a typical embodiment, the buffer layer comprises a mesa structure having a thickness less than approximately 100 μm. In addition, the mesa structure is typically constructed from three RGB wafers. 
     A first aspect of the present invention provides a method for producing a wafer having Light Emitting Diodes (LEDs) of variable heights, comprising: providing a wafer; applying a buffer layer over the wafer; and providing a set of chips over the buffer layer, the buffer layer increasing a height of at least a subset of the set of chips. 
     A second aspect of the present invention provides a method for producing a wafer having Light Emitting Diodes (LEDs) of variable heights, comprising: providing a wafer; applying a buffer layer over the wafer, the buffer layer comprising a mesa structure having a thickness less than approximately 100 μm; and providing a set of LED chips over the buffer layer, the buffer layer increasing a height of at least a subset of the set of LED chips. 
     A third aspect of the present invention provides a Light Emitting Diode (LED) device, comprising: a wafer; a buffer layer applied over the wafer, the buffer layer comprising a mesa structure having a thickness less than approximately 100 μm; and a set of LED chips applied over the buffer layer, the buffer layer increasing a height of at least a subset of the set of LED chips. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings in which: 
         FIG. 1  depicts three RGB layers being used to construct a mesa structure that is used as a buffer layer/mask according to an embodiment of the present invention. 
         FIG. 2  depicts a method of applying a buffer layer to yield a variable height LED according to an embodiment of the present invention. 
         FIG. 3  depicts a plurality of interconnected LED chips with an underlying buffer layer/mask according to an embodiment of the present invention. 
         FIG. 4  depicts a graphic representation of a chip level conformal coating (CLCC) mode in the wafer level according to an embodiment of the present invention. 
         FIG. 5  depicts the plurality of interconnected LED chips according to an embodiment of the present invention. 
         FIG. 6  depicts a specific color scheme and interconnection of a plurality of LED chips after conversion of a light output by a target LED chip to white light according to an embodiment of the present invention. 
     
    
    
     The drawings are not necessarily to scale. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. The drawings are intended to depict only typical embodiments of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As indicated above, embodiments of the present invention provide a variable height LED and method of manufacture. Specifically, under embodiments of the present invention, a buffer layer is applied (e.g., selectively) over a wafer, and a set of LED chips is provided over the buffer layer. One role of the buffer layer is to increase a height of at least a subset of the chips. As such the buffer layer could be applied using any processing method now known or later developed. For example, the buffer layer could be selectively deposited, etched, etc. It is understood in advance that although a typical embodiment of the present invention corresponds to a varying the heights of LED chips, the teachings recited herein could be applied to any type of chip. 
     Regardless, in a typical embodiment, the buffer layer comprises a mesa structure having a thickness less than approximately 100 μm. In addition, the mesa structure is typically constructed from three RGB wafers. This embodiment is shown in  FIG. 1 . Specifically,  FIG. 1  shows a mesa structure  2  comprising RGB wafers  4 A-N. Each wafer  4 A-N is separate by a predetermined distance D. In a typical embodiment, mesa structure has a thickness less than approximately 100 μm. A further shown in  FIG. 1 , a pixel  6  has R, G, and B components, each taking a constant length apart to the Z-axis (e.g., the fixed origin point. In accordance with the present invention, since a pixel has an RGB, all colors can be generated hereunder. Moreover, the Z-axis can provide variation for height. Along these lines, it is not necessary for the height of all LED chips to be uniformly varied. For example, LED chip alpha could have a height of A, LED chip beta could have a height of B, while LED gamma could have a height of C. Still yet, the teachings recited herein could be implemented in conjunction with any additional layers (e.g., a wavelength shift layer or WSL) such as those discussed in the above-incorporated applications. 
     Referring now to  FIG. 2 , the application of buffer layer  14  is schematically shown. It is understood that dispenser  23  can be used to apply some or all of the layers provided hereunder. Specifically, dispenser  23  can be used to apply buffer layer  14  over a wafer. Thereafter, a set (at least one) of chips can be positioned over buffer layer  14 . Along these lines, buffer layer  14  can be applied at various portions (e.g., selectively deposited along wafer), etched, etc. The buffer layer serves to vary the heights of at least a subset of the LED chips. In any event, as shown, there is lead frame  16  including cup  18  and the LED chip  10  is attached in the center of the cup, and the metal pad  20  and lead frame  22  are connected by the wire bond  24 . Buffer layer  14  is applied using dispenser  23 . Along these lines, it is understood that dispenser  23  can also be used to applied any of the layers (e.g., a wavelength shift layer or WSL) of any of the above-incorporated patent applications. 
     Referring to  FIG. 3 , an illustrative array/plurality of LED chips  40  is shown. As depicted, array  40  comprises chips  42 A-D electrically interconnected via red metal lines  44 . In addition, ground contacts  46  are used to connect array  40  to a ground. It should be understood in advance when the terms such as “red LED chip”, “blue LED chip”, “green LED chip” are used herein, what is meant is that the LED chips produce light having those colors (e.g., red light, blue light, green light, etc.). Thus, “red LED chip” is an abbreviated way of saying “red light producing LED chip”. As further shown, LED chips  42 A-D will be positioned over buffer layer  14 . Mentioned above, buffer layer need not have a constant thickness. Rather, wafer  47  has area  43 A where no buffer layer  14  was applied and areas  43 B-D where three different thicknesses of buffer layer  14  were applied. Such application of buffer layer  14  will result in LED chips  42 A-D having varying heights. 
     In  FIG. 4 , an additional feature of the present invention is schematically shown. In order to implement the target white color output coordinate in which it is identical about all chips  42  in wafer  47 , a proper fluorescent substance combination ratio in which it has to be coated in each unit chip by using the result of measuring the wavelength of all chips  42  is determined. For example, the combination ratio of A in case of the wavelength a, the combination ratio of B in case of the wavelength β, and the combination ratio of C in case of the wavelength γ is applied. Dispensers  34 A-C correspond to the multiple combination ratio and three coating materials are prepared. Each dispenser  34 A-C is filled with the fluorescent substance of the respectively different combination ratio A, B, or C. 
     As seen in  FIG. 4 , dispenser  34 A-C makes the coated film (fluorescent substance plus silicon material) in which it corresponds to each unit chip while at the wafer level by the dispensing method. Therefore, it is comprised of the fluorescent substance conformal coating in the adhesion layer that was previously applied to the top surface of each LED chip. In this way, the white LED chip is ultimately implemented in the wafer level through the fluorescent substance coating. Under the present invention, three examples in which it independently coats the fluorescent substance plus silicon material in the wafer level according to each unit chip  30  are proposed. 
     Regardless, referring now to  FIG. 5 , an illustrative embodiment according to the present invention is shown. As depicted, array of chips  42 A-D is drawn from chips of wafer  47 . In a typical embodiment, array of chips  42 A-D comprise four chips  42 A-D: a red LED chip  42 A, a green LED chip  42 B, a blue LED chip  42 C, and a target LED chip  42 D. Referring to  FIG. 6 , this process will be described in greater detail. As depicted, red LED chip  42 A, green LED chip  42 B, blue LED chip  42 C, and target LED chip  42 D are electrically interconnected in a 2×2 matrix format. Under the present invention, chips  42 A-D will have varying heights due to the underlying buffer layer  14 . Prior to or after their interconnection, LED chips  42 A-D are coated with the conformal coating as previously described. Specifically, a wavelength of light output of one or more LED chips  42 A-D is measured. Based on this measurement, a conformal coating is applied to one or more of LED chips  42 A-D. As described above in conjunction with  FIGS. 1-4 , a certain wavelength will result in the application of a conformal coating having a corresponding phosphor ratio (e.g., comprised of yellow, green and/or red). Moreover, the conformal coating can be applied to any or all LED chips  42 A-D using any of the isolation techniques discussed herein. Illustrative techniques for isolating the area to which the conformal coating is applied include (among others): (1) using a paraffin wax; (2) using a silk screen; and (3) using a photo resist. Using the conformal coating, the light output by target LED chip  42 D will be converted to white light. 
     The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed and, obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of the invention as defined by the accompanying claims.