Multichip light emitting diode (LED) and method of manufacture

The present invention provides a multichip LED and method of manufacture in which white light is produced. Specifically, a plurality of electrically interconnected LED chips (e.g., interconnected via red metal wire) is selected for conversion of light to white light. In a typical embodiment, the LED chips comprise: a blue LED chip, a red LED chip, a green LED chip, and a target LED chip whose light output is converted to white light. A wavelength of a light output by one or more of the plurality of chips will be measured. Based on the wavelength measurement, a conformal coating is applied to the one or more of the LED chips. The conformal coating has a phosphor ratio that is based on the wavelength. Moreover, the phosphor ratio is comprised of at least one of the following colors: yellow, green, or red. Using the conformal coating the light output of the target LED is then converted to white light. In a typical embodiment, these steps are performed at the wafer level so that uniformity and consistency in results can be better obtained. Several different approaches can be implemented for isolating the coating area. Examples include the use of a paraffin wax, a silk screen, or a photo resist. Regardless, this approach allows multiple chips to be treated simultaneously.

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.

FIELD OF THE INVENTION

The present invention generally relates to LEDs. Specifically, the present invention relates to the manufacture of multichip LEDs to produce white light.

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 LEDs 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). 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, the present invention provides a multichip LED and method of manufacture in which white light is produced. Specifically, a plurality of electrically interconnected LED chips (e.g., interconnected via red metal wire) is selected for conversion of light to white light. In a typical embodiment, the LED chips comprise a blue LED chip, a red LED chip, a green LED chip, and a target LED chip whose light output is converted to white light. A wavelength of a light output by one or more of the plurality of chips will be measured. Based on the wavelength measurement, a conformal coating is applied to the one or more of the LED chips. The conformal coating has a phosphor ratio that is based on the wavelength. Moreover, the phosphor ratio is comprised of at least one of the following colors: yellow, green, or red. Using the conformal coating, the light output of the target LED is then converted to white light. In a typical embodiment, these steps are performed at the wafer level so that uniformity and consistency in results can be better obtained. Several different approaches can be implemented for isolating the coating area. Examples include the use of a paraffin wax, a silk screen, or a photo resist. Regardless, this approach allows multiple chips to be treated simultaneously.

A first aspect of the present invention provides a method for manufacturing a multichip light emitting diode (LED chip), comprising: providing a plurality of LED chips in electrical contact with each other; applying a conformal coating to at least one of the plurality of LED chips, the conformal coating having a phosphor ratio that is based on a previous measurement; and converting a light output of a target LED chip of the plurality of LED chips to white light using the conformal coating.

A second aspect of the present invention provides a multichip light emitting diode (LED), comprising: a plurality of LED chips in electrical contact with each other, the plurality of LED chips comprising a blue LED chip, a red LED chip, a green LED chip and a target LED chip; a conformal coating applied to at least one of the plurality of LED chips, the conformal coating having a phosphor ratio that is based on a previous measurement; and wherein a light output of the target LED chip is converted to white light using the conformal coating.

A third aspect of the present invention provides a multichip light emitting diode (LED), comprising: a plurality of LED chips in electrical contact with each other, the plurality of LED chips comprising a blue LED chip, a red LED chip, a green LED chip, and a target LED chip; wherein the conformal coating applied to at least one of the plurality of LED chips, the conformal coating having a phosphor ratio that is based on a previous measurement, wherein a light output of the target LED chip is converted to white light using the conformal coating; and wherein the plurality of LED chips are interconnected with a red metal line.

DETAILED DESCRIPTION OF THE INVENTION

In general, the present invention provides a multichip LED and method of manufacture in which white light is produced. Specifically, a plurality of electrically interconnected LED chips (e.g., interconnected via red metal wire) is selected for conversion of light to white light. In a typical embodiment, the LED chips comprise a blue LED chip, a red LED chip, a green LED chip, and a target LED chip whose light output is converted to white light. A wavelength of a light output by one or more of the plurality of chips will be measured. Based on the wavelength measurement, a conformal coating is applied to the one or more of the LED chips. The conformal coating has a phosphor ratio that is based on the wavelength. Moreover, the phosphor ratio is comprised of at least one of the following colors: yellow, green, or red. Using the conformal coating the light output of the target LED is then converted to white light. In a typical embodiment, these steps are performed at the wafer level so that uniformity and consistency in results can be better obtained. Several different approaches can be implemented for isolating the coating area. Examples include the use of a paraffin wax, a silk screen, or a photo resist. Regardless, this approach allows multiple chips to be treated simultaneously.

FIGS. 1-4describe a process for treating one or more chips of the array ofFIGS. 5-8. This processing results in conversion of a light output by at least one of the array/plurality of chips to white light. To provide a proper context, the process will be described first, and then applied to the illustrative chip array shown in the drawings.

A wavelength distribution exists due to an EPI in-line MOCVD (metal organic chemical vapor deposition) process drift and facility deviation of the LED chip between each chip within a LED wafer.FIG. 1depicts the result of measuring the wavelength of blue light for chips10within an arbitrary LED wafer12. InFIG. 1, chips are unable to have the same monochromatic light output within the wafer, and it has a distribution between chips. In general, in the case of the wave length, a difference is generated between chips that are over 5 nm, and the recognition capability is that of human vision.

FIG. 2shows a RGB chromaticity diagram. Because the wavelength distribution exists similar toFIG. 1within the wafer between chips, in the case of applying a fluorescent substance of an identical combination ratio in the wafer level in order to implement white, the white color output coordinate is changed on each chip. That is, the wavelength dissemination of the white chip is enlarged.

Therefore, only when the combination ratio of the fluorescent substance applied according to the wavelength of the chip is appropriate, the same target white chip implementation is possible. In the chromaticity diagram ofFIG. 2, the wavelength of chip number1is α. This ratio of yellow, red, and green (Y, R, G) should be applied according to combination ratio A of the fluorescent substance and silicon to form the white target color output coordinate. Moreover, as to chip numbers2and3, the wavelengths of β and γ need be applied. To form the same white target color output coordinate as chip number1, ratios of B and C have to be applied to chip numbers2and3(respectively).

If the combination ratio of the coating film (fluorescent substance plus silicon) is identical for α, β, γ in which the chips are different wavelengths, then the white target color output coordinate of the three chips will be different. There is a difficulty in the BLU (back light unit), and a lamp in the LED application with the color dispersion occurrence in the product configuration if the white color output coordinate is changed. The process is implemented at the discrete chip level (not the wafer level) in the coating process of the fluorescent substance in regard to this kind of a problem (binning problem) in the LED PKG (package) process. Before implementing the dispensing process of coating the fluorescent substance, the sorting (or ranking) is made of the chips for each wavelength of the blue light output from the chips.

Referring now toFIG. 3, the process where the fluorescent substance coating14is applied is schematically shown. There is lead frame16including cup18in advance, the LED chip10is attached in the center of the cup, and the metal pad20and lead frame22are connected by the wire bond24. The coating film14(fluorescent substance plus silicon) material, which fits to the wavelength of the corresponding chip10, and in which it is designed in the LED chip surface in order to form the white target color output, is coordinated with dispensing. This described chip level fluorescent substance coating technology has several problems.

First, because of the thick coating film (over the minimum 300 um) of the fluorescent substance plus silicon, the optical mean free path is changed according to the LED chip surface location, and the color deviation is caused (in other words, binning phenomenon). When it designs optically, this color deviation occurrence brings many elements.

Second, the fluorescent substance coating process is not conducted at the wafer level, but is done at the chip level. As a result, the separate package material and process cost are additionally generated. Moreover, the chip cost of the inferior chip is generated by the white color output coordinate deviation after the package process. The sorting process is classified with chips within the wafer accompanied according to the same wavelength band because the wavelength distribution is quantified in advance according to the chip level coating. In this case, the long process time and equipment investment cost, etc., are generated. The separate sorting process is unnecessary if the fluorescent substance application process of each unit chip is conducted at the wafer level. Since it is not the package level, and the white color output coordinate is already obtained from the unit wafer level, the separate package process and material cost are not generated in addition to the inferior chip performance variation.

As indicated above, under the present invention, the white light-emitting diode manufacturing method performs the white light-emitting diode manufacturing process in the wafer level differently than the chip level packaging method. In particular, in the described lower-stage wafer level, after the wavelength data of each chip is measured in advance in order to control the fluorescent substance (yellow, blue, green) combination ratio, in which it is appropriate for each unit chip for the corresponding for each wavelength, accurately and radiate the white light in the wafer level according to each chip, the appropriately coated film is made with fluorescent substance plus silicon by using the dispensing method. In this way, the fluorescent substance conformal coating in which it has the thickness fixed, and is thin in the chip surface, is the basis of the possible method (chip level conformal coating: CLCC). In the present invention, the meaning of the wafer level points to the wafer state that discrete chips do not occur before the dicing process.

InFIG. 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 with respect to all chips30in wafer32, 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 chips30is determined. For example, the combination ratio of A in case of the wavelength α, 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. Dispensers34A-C corresponded to the multiple combination ratio and three coating materials are prepared. Each dispenser34A-C is filled with the fluorescent substance of the respectively different combination ratio A, B, or C.

As seen inFIG. 4, dispenser34A-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 chip top surface. In this way, the white LED 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 chip30are proposed.

These techniques will not be applied in the context of a plurality or an array of LED chips. Referring toFIG. 5, an illustrative array/plurality of LED chips40is shown. As depicted, array40comprises chips42A-D electrically interconnected via red metal lines44. In addition, ground contacts46are used to connect array40to 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”.

Regardless, referring now toFIG. 6, an illustrative embodiment according to the present invention is shown. As depicted, array of chips50is drawn from chips50of wafer52. In a typical embodiment, array of chips40comprise four chips42A-D: a red LED chip42A, a green LED hip42B, a blue LED chip42C, and a target LED chip42D whose light output will be converted to white light under the present invention.

Referring toFIG. 7, this process will be described in greater detail. As depicted, red LED chip42A, green LED chip42B, blue LED chip42C, and target LED chip42D are electrically interconnected in a 2×2 matrix format. Prior to or after this interconnection, LED chips42A-D are coated with the conformal coating as described above in conjunction withFIGS. 1-4. Specifically, a wavelength of light output of one or more LED chips42A-D is measured. Based on this measurement, a conformal coating is applied to one or more of LED chips42A-D. As described above in conjunction withFIGS. 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 chips42A-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 chip42D will be converted to white light.

Referring toFIG. 8, the interconnection of LED chips42A-C is shown in greater detail. As depicted, matching pairs of LED chips42A-C are interconnected using red wire44, and ground contacts46.