Abstract:
A method of forming a photoresist mask on a light emitting device is disclosed. A portion of the light emitting device is coated with photoresist. A portion of the photoresist is exposed by light impinging on the interface of the light emitting device and the photoresist from inside the light emitting device. The photoresist is developed, removing either the exposed photoresist or the unexposed photoresist. In one embodiment, the photoresist mask may be used to form a phosphor coating. After the photoresist is developed to remove the exposed photoresist, a phosphor layer is deposited overlying the light emitting device. The unexposed portion of photoresist is stripped. In some embodiments, the light exposing the photoresist is produced by electrically biasing the light emitting device, or by shining light into the light emitting device through an aperture or by a focussed laser.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to light emitting devices, and more particularly, to producing a self-aligned, self-exposed photoresist pattern on a light emitting diode (LED).  
           [0003]    2. Description of Related Art  
           [0004]    Semiconductor light-emitting devices such as light emitting diodes are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness LEDs capable of operation across the visible spectrum include Group III-V semiconductors, including binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Light emitting devices based on the III-nitride materials system provide for high brightness, solid-state light sources in the UV-to-yellow spectral regions. Typically, III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. Some of these substrates are insulating or poorly conducting. Devices fabricated from semiconductor crystals grown on such substrates must have both the positive and the negative polarity electrical contacts to the epitaxially-grown semiconductor on the same side of the device. In contrast, semiconductor devices grown on conducting substrates can be fabricated such that one electrical contact is formed on the epitaxially grown material and the other electrical contact is formed on the substrate. However, devices fabricated on conducting substrates may also be designed to have both contacts on the same side of the device on which the epitaxial material is grown in a flip-chip geometry so as to improve light extraction from LED chip, to improve the current-carrying capacity of the chip, or to improve the heat-sinking of the LED die. Two types of light emitting devices have the contacts formed on the same side of the device. In the first, called a flip chip, light is extracted through the substrate. In the second, light is extracted through transparent or semi-transparent contacts formed on the epitaxial layers.  
           [0005]    Fabrication of an LED requires the growth of an n-type layer or layers overlying a substrate, the growth of an active region overlying the n-type layers, and the growth of a p-type layer or layers overlying the active region. Light is generated by the recombination of electrons and holes within the active region. After fabrication, the LED is typically mounted on a submount. In order to create an LED-based light source that emits white light or some color other than the color of light produced in the active region of the LED, a phosphor is disposed in the path of all or a portion of the light generated in the active region. As used herein, “phosphor” refers to any luminescent material which absorbs light of one wavelength and emits light of a different wavelength. For example, in order to produce white light, a blue LED may be coated with a phosphor that produces yellow light. Blue light from the LED mixes with yellow light from the phosphor to produce white light.  
           [0006]    One way to produce a phosphor-converted LED is to apply a conformal coating of phosphor over the LED after mounting on the submount. A conformally-coated phosphor-converted LED is described in more detail in application Ser. No. 09/879,547, titled “Phosphor-Converted Light Emitting Device,” and incorporated herein by reference. If the conformal coating of phosphor is not uniform, undesirable inconsistencies in the light generated by the phosphor-converted LED can result. Conventionally, an LED was conformally coated by using photo-masking techniques developed for planar semiconductors, where masks are used to define the size and shape of patterns to be printed in photoresist deposited on the LED and submount. The printed photoresist layer defines which areas are covered with phosphor.  
           [0007]    The application of conventional masking techniques to three-dimensional structures such as an LED mounted on a submount is fraught with problems including stray reflected light and depth of field artifacts in the resulting image; and imperfect alignment, both of which can result in nonuniform coating of the LED. For example, light reflected from the surfaces of the three dimensional LED structure, including the surface of the photoresist layer used for masking, may introduce exposure artifacts. Also, depth-of-field problems may lead to distortions and loss of dimensional accuracy in the image produced by the mask. Additionally, not all LEDs will have a perfect shape or be perfectly aligned with other LEDs in an array of LEDs. Shape and alignment imperfections can result in nonuniform coating. Masks cannot fully compensate for the process and object variations normally seen in a manufacturing environment, leading to imperfections and yield losses.  
         SUMMARY  
         [0008]    In accordance with an embodiment of the invention, a method of forming a photoresist mask on a light emitting device is disclosed. A portion of the light emitting device is coated with photoresist. A portion of the photoresist is exposed by light impinging on the interface of the light emitting device and the photoresist from inside the light emitting device. The photoresist is developed, removing either the exposed photoresist or the unexposed photoresist. In one embodiment, the photoresist mask may be used to form a phosphor coating on the light emitting device. The light emitting device is attached to a submount, and the light emitting device and submount are coated with photoresist. A portion of the photoresist is exposed by light impinging on the interface of the light emitting device and the photoresist from inside the light emitting device. The photoresist is developed to remove the exposed photoresist. A phosphor layer is deposited overlying the light emitting device, then the unexposed portion of photoresist is stripped. In some embodiments, the light exposing the photoresist is produced by electrically biasing the light emitting device, by shining light into the light emitting device through an aperture, or by shining light into the light emitting device by a steered, focussed laser. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    FIGS.  1 A- 1 F illustrate an LED connected to a submount at various stages during phosphor coating.  
         [0010]    [0010]FIG. 2 illustrates an embodiment of self-exposing photoresist.  
         [0011]    [0011]FIG. 3 illustrates an alternative embodiment of self-exposing photoresist.  
         [0012]    FIGS.  4 A- 4 C illustrate an alternative embodiment of phosphor coating an LED. 
     
    
     DETAILED DESCRIPTION  
       [0013]    In accordance with embodiments of the invention, light from an LED is used to expose photoresist, resulting in a photoresist pattern that is self-aligned with the LED. The process may eliminate depth-of-field, scattering, and mask alignment problems associated with the use of conventional masks, as well as problems resulting from non-uniformly sized LEDs.  
         [0014]    FIGS.  1 A- 1 F illustrate an embodiment of conformally coating an LED with phosphor using a self-aligned photoresist mask. FIG. 1A illustrates an LED  18  mounted on submount  10 . LED  18  includes a substrate  16 , an n-type region  15 , an active region  14 , and a p-type region  13 . A p-contact  12  is attached to p-type region  13 . An n-contact  11  is attached to n-type region  15 . LED  18  may be attached to submount  10  by, for example, solder (not shown) between contacts  11  and  12  and submount  10 . Other methods of attaching LED  18  to submount  10  are described in more detail in application Ser. No. 09/469,657, titled “III-Nitride Light-Emitting Device With Increased Light Generating Capability,” and incorporated herein by reference. Usually, substrate  16  is transparent, and submount  10  is opaque.  
         [0015]    In FIG. 1B, LED  18  and submount  10  are coated with a layer of photoresist  20 . Photoresist layer  20  may be, for example, a positive photoresist, meaning that when photoresist  20  is exposed to electromagnetic radiation, the radiation breaks the chemical bonds in photoresist layer  20 , making it soluble in a developer solution. The portions of photoresist  20  that are not irradiated are not soluble in a developer solution, and are therefore left behind when photoresist layer  20  is developed. Photoresist  20  may be, for example, a dry film photoresist applied by a heated vacuum coater, a liquid film photoresist, an electrophoretically deposited photoresist, a screen printed photoresist, or any other suitable photoresist. Generally, photoresist  20  is a positive acting photoresist.  
         [0016]    In FIG. 1C, photoresist layer  20   a  is exposed to light from LED  18 . Photoresist layer  20   b  is not exposed to light from LED  18 . FIGS. 2 and 3 illustrate two embodiments of exposing photoresist layer  20   a.  In an embodiment illustrated in FIG. 2, LED  18  is electrically biased in order to generate light  24 . Light  24  may be internally reflected off the photoresist covered surfaces of LED  18 , exposing the photoresist covering those surfaces. Usually, contacts  11  and  12  (shown in FIG. 1A) are highly reflective, which aids the scattering of light  24  within LED  18 . The internally reflected light  24  produces a self-aligned exposed layer of photoresist, including an annulus of controlled thickness  20   c  surrounding LED  18 .  
         [0017]    LED  18  of FIG. 2 may be electrically biased in two ways. First, a voltage may be applied to contacts (not shown) on the underside  26  of submount  10 . The contacts on underside  26  of submount  10  are electrically connected to solder bumps  28 , which are connected to contacts  11  and  12  (shown in FIG. 1A) of LED  18 . The voltage causes LED  18  to emit light  24  from the active region of LED  18 . In one embodiment, submount  10  is part of an undiced wafer of submounts with an LED attached to each submount on the wafer. A series of probes are connected to each row of submounts on the wafer. Each probe then provides a series of short voltage bias pulses, until a minimum required level of light exposure flux necessary to expose photoresist  20  has been produced in LED  18 . Second, LED  18  may be electrically biased by RF excitation. LED  18  may produce light by rectified coupling to RF fields, when submount  10  and LED  18  are placed in proximity to an RF radiator or antenna.  
         [0018]    In an embodiment illustrated in FIG. 3, LED  18  is optically pumped in order to generate light  24 . As shown in FIG. 3, a mask  30 , such as, for example, a dark field dot mask, is aligned over LED  18 . Mask  30  includes an aperture  35 . Aperture  35  is much smaller than LED  18 , in order to simplify alignment of aperture  35  over LED  18 . Aperture  35  need not be located in the center of LED  18 . Aperture  35  may be of any shape. A collimated beam of light  24  is applied to mask  30 . The light source used may be, for example, a flood light producing collimated light with a divergence less than 30°, a fiber optic cable connected to a remote light source, or a laser light source. A focussed laser light source may be used, and the laser may be steered to expose the photoresist coating multiple LEDs mounted on an undiced wafer of submounts. The light source first exposes the portion of photoresist under aperture  35 . Light  24  transmitted through aperture  35  and the photoresist layer enters LED  18 , where light  24  is reflected off the photoresist covered surfaces of LED  18 , exposing the photoresist covering those surfaces. In one embodiment, the photoresist is developed to remove the photoresist layer exposed by aperture  35 . Light is then shown through aperture  35  and the gap in the photoresist layer, and reflected off the walls of LED  18  to expose the remaining photoresist coating LED  18 . Thus, if LED is optically pumped, two cycles of photoresist exposure and developing may be required. Alternatively, LED may be a III-nitride device with an InGaN active region, and the collimated light beam may be UV light, which excites shallow UV emissions from the active region or any other layer of LED  18 . In one embodiment, the diameter of aperture  35  may be about 100 μm. LED  18  may have a top area of (1000 μm) 2 . Photoresist  20  (FIG. 1B) may have a high absorption to prevent light  24  from being transmitted through photoresist  20   a  and  20   b.    
         [0019]    In the embodiments illustrated in both FIGS. 2 and 3, the amount of light exposure (i.e. the exposure time and exposure intensity) necessary to develop photoresist  20  depends on the photoresist used. If a highly absorbing photoresist is used, the exposure time may be increased. The wavelength of light required to expose the photoresist also depends on the photoresist used.  
         [0020]    After exposure to light from LED  18 , exposed photoresist  20   a  is removed by application of a photoresist developer solution, such as a standard liquid developer. Exposed photoresist  20   a  is soluble in the developer solution, while unexposed photoresist  20   b  is not soluble in the developer solution. The developer used depends on the composition of photoresist  20 . After developing, the structure shown in FIG. 1D remains.  
         [0021]    A layer of phosphor  22  is then deposited over portions of the structure shown in FIG. 1D, as shown in FIG. 1E. Phosphor  22  may be selectively deposited by, for example, screen printing or electrophoretic deposition, both of which are described in more detail in “Phosphor-Converted Light Emitting Device,” previously incorporated by reference. After phosphor deposition and fixation, unexposed photoresist  20   b  is stripped away. The structure shown in FIG. 1F results. In one embodiment, photoresist  20  is selected such that unexposed photoresist  20   b  has a conductivity that is low enough to be an effective mask for electrophoretic deposition without a “hard-bake” which would further fix photoresist  20   b,  making photoresist  20   b  difficult to strip once phosphor  22  is deposited. In one embodiment, photoresist  20  is selected such that the hard-bake temperature is less than the maximum temperature allowed by LED  18  and submount  10  during phosphor coating and any curing steps required to set the phosphor coating.  
         [0022]    Once each LED  18  on the wafer of submounts is coated with phosphor, the submounts may be tested by probing. The wafer is then diced into individual submounts, each attached to an LED. The submounts are sorted, die-attached to a package, and encapsulated with an encapsulant. Probing, dicing, sorting, die attaching, and encapsulating steps are well known in the art of packaging light emitting diodes.  
         [0023]    In accordance with embodiments of the invention, the use of a self-exposed and self-aligned method of exposing photoresist may offer several advantages. First, since the photoresist is self-exposed by light from within LED  18 , no mask, other than possibly dot mask  30  shown in FIG. 3, is required. Dot mask  30  may be a simple inexpensive alignment jig, which will work for any size or shape of LED mounted on the submount centers of the submount wafer. Thus, costly high precision alignment of a mask with the submount wafer is avoided. The elimination of patterning by a precision mask reduces variation in the phosphor thickness caused by variations in the size, shape, placement, and mounting height of LEDs  18  relative to the mask pattern. Second, depth of field and light scattering errors in the photoresist pattern are eliminated. Third, the width of annulus  20   c  can be controlled by light exposure, reducing variations in the light output of the final packaged conformally coated LED caused by variations in the annular thickness. In one embodiment, annulus  20   c  has a width that is no greater than the thickness of the photoresist coating  20 . In one embodiment, the width of annulus  20   c  is less than 100 microns wide.  
         [0024]    FIGS.  4 A- 4 C illustrate an alternative method for creating a self aligned photoresist layer on an LED. In FIG. 4A, LED  18  is mounted on submount  10 , resulting in the same structure as shown in FIG. 1A. The structure is then coated with a layer of photoresist  40 , as shown in FIG. 4B. Photoresist  40  may be a negative photoresist filled with phosphor, fluorescent dyes, or other photoluminescent materials. In FIG. 4C, light is introduced into LED  18  by one of the method described in the text accompanying FIGS. 2 and 3. The light exposes portion  40   a  of photoresist layer  40 . Portions  40   b  are unexposed. Since photoresist  40  is a negative photoresist, when photoresist  40   a  and  40   b  is developed, portions  40   b  of the photoresist are removed, leaving portion  40   a.  The structure shown in FIG. 1F results.  
         [0025]    The above-described embodiments of the present invention are merely meant to be illustrative and not limiting. For example, the invention is not limited to III-nitride devices, and may be applied to devices made from III-phosphide or other materials systems. It will thus be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects. Therefore, the appended claims encompass all such changes and modifications as falling within the true spirit and scope of this invention.