Patent Publication Number: US-2022238752-A1

Title: Method of Laser Treatment of a Semiconductor Wafer Comprising AlGaInP-LEDs to Increase their Light Generating Efficiency

Description:
This patent application is a national phase filing under section 371 of PCT/EP2020/063906, filed May 19, 2020, which claims the priority of European patent application 19177581.6, filed May 31, 2019, each of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to the field of manufacturing of light-emitting diode chips. 
     More particularly, the present invention relates to increasing the light generating efficiency of aluminum gallium indium phosphide LEDs. 
     BACKGROUND 
     It is well known that the light generating efficiency of aluminum gallium indium phosphide (AlGaInP) LEDs is limited due to the non-radiant recombination of electron-hole pairs at the LEDs&#39; mesa edge. This problem is particularly salient for very small AlGaInP-LEDs such as those used for high resolution monitors and screens. Indeed, as the size of the AlGaInP-LEDs decreases, their circumference to surface ratio increases, which in turn increases the relative proportion of non-radiant recombination at the mesa edge. 
     One known approach to solving this problem is to diffuse Zinc into the LED&#39;s mesa edge. This Zink diffusion leads to so-called quantum well intermixing, meaning that the bandgap of the optically active material in the LED&#39;s mesa edge is increased. This in turn means that less electron-hole pairs can reach the mesa edge. Accordingly, the electron-hole pairs are confined to the LED&#39;s center and can recombine optically to generate light. 
     However, the drawback of diffusing Zinc is an increase in the non-radiant effects in the LED&#39;s center. 
     There is thus a need for a different method of increasing the light generating efficiency of small AlGaInP-LEDs. 
     SUMMARY 
     Embodiments provide a method for treating a semiconductor wafer comprising a set of Aluminum Gallium Indium Phosphide light emitting diodes or AlGaInP-LEDs to increase the light generating efficiency of the AlGaInP-LEDs, 
     wherein each AlGaInP-LED includes a core active layer for light generation sandwiched between two outer layers, the core active layer having a central light generating area and a peripheral edge surrounding the central light generating area, 
     the method comprising the step of treating the peripheral edge of the core active layer of each AlGaInP-LED with a laser beam, thus increasing the minimum band gap in each peripheral edge to such an extent that, during later operation of the AlGaInP-LED, the electron-hole recombination is essentially confined to the central light generating area. 
     By treating the peripheral edges of the core active layers of the AlGaInP-LEDs with a laser beam, non-radiant electron-hole recombination at the LEDs&#39; edges is effectively suppressed. This increases the LEDs&#39; light generating efficiency. 
     According to one embodiment, the laser beam treatment may involve scanning the wafer with the laser beam according to a predefined pattern. 
     The photon energy of the laser beam may be higher than the minimum band gap of the core active layer and lower than the band gaps of the two outer layers such that, during the laser beam treatment, the laser beam energy is primarily transferred to the core active layer&#39;s peripheral edge. 
     Each AlGaInP-LED may be a red light LED. 
     The wavelength of the laser beam may in particular be in the range of 550 to 640 nm. 
     The laser beam may have a Gaussian shape/profile. 
     The laser beam may be generated by a pulsed laser. 
     Prior to the laser beam treatment, the wafer may be heated to a background temperature to reduce the power requirements of the laser beam treatment. 
     The laser beam power density may be between 0.1 and 100 mJ/mm2, and preferably between 1 and 10 mJ/mm2. 
     After the laser beam treatment, the wafer may be etched, thus obtaining, for each AlGaInP-LED, a chip preform, wherein, preferably, after the etching, the wafer is diced into individual AlGaInP-LED chips, e.g. by laser cutting. 
     The duration of the laser beam treatment may be between is and 10 min and preferably between 10 s and 2 min. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will now be described in detail with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic representation of a semiconductor wafer prior to being subjected to the method of the present disclosure; 
         FIG. 2  is a cross-sectional view according to the arrows II of  FIG. 1  of one AlGaInP-LED, which is part of the wafer of  FIG. 1 ; 
         FIG. 3  is a flow diagram showing the steps of obtaining individual AlGaInP-LED chips, starting from the wafer shown in  FIG. 1 , and involving the process of the present disclosure; and 
         FIG. 4  shows the effects of the method of the present disclosure on the bandgap shape of the peripheral edge of the core active layer of an AlGaInP-LED. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     With reference to  FIG. 1 , there is shown a semiconductor wafer  10  comprising a set of aluminum gallium indium phosphide light-emitting diodes (AlGaInP-LEDs)  12 . In the example of  FIG. 1 , there are twenty LEDs  12  arranged in the wafer  10 . Each LED is a PN junction, which has been formed in the wafer  10  according to well-known methods. 
       FIG. 2  is a cross section of one of the LEDs  12 . Each LED  12  includes a core active layer  14  for light generation sandwiched between two outer layers  16  and  18 . The core active layer  14  has a central light generating area  20 . This area  20  is identified by the dotted region. The central light generating area  20  is surrounded by a peripheral edge  22 . 
     With reference to the left part of  FIG. 4 , we will now describe the bandgap structure of the LED  12  shown in  FIG. 2 . The bandgap diagram in  Figure 4  shows the bandgap as a function of the depth T into the LED  12 , see  FIG. 2 . The upper outer layer  16  of the LED  12  is a P-doped layer. The bandgap shape of this P-type doped layer  16  is a P-ramp  24 , followed by a P-setback  26 . The bandgap of the P-setback  26  is denoted by BG 1 . The bandgap shape of the active layer  14  is, along the depth T, a series of quantum wells Q separated by barriers B. In the example shown in  Figure 4 , active layer  14  has two quantum wells Q separated by one barrier B. The quantum wells Q define a bandgap BG 2 , which is smaller than the bandgap BG 1 . The bandgap of the quantum wells Q is the minimum bandgap of the core active layer  14 . The lower outer layer  18  has a bandgap shape comparable to the one of the upper outer layer  16 , with a setback  28 , and ramp  30 . However, the lower outer layer  18  is N-type doped. 
     Turning now to  FIG. 3 , individual LED chips  38  with increased light generating efficiency are obtained from the wafer  10  as follows: 
     In a first step a), a hatched zone Z of the wafer  10  is treated with a laser beam L, as depicted in the small process drawing located right to the flow diagram&#39;s first arrow  100 . The laser beam treatment involves scanning wafer  10  according to a predefined pattern. More precisely, the laser beam L scans the surface of the wafer  10 , which is not taken up by the LEDs  12 , and, on top of that, the peripheral edges  22  of the core active layers  14  of the LEDs  12 . Different scanning patterns are possible, as long as the pattern involves the scanning of the peripheral edges  22  of the core active layers  14  of the LEDs  12 . 
     The photon energy of the laser beam L is higher than the minimum bandgap BG 2  of the core active layer  14  and lower than the bandgap BG 1  of the two outer layers  16  and  18 . Hence, during the laser beam treatment, the laser beam energy is primarily transferred to the core active layer&#39;s peripheral edge  22 . 
     The laser beam&#39;s wavelength is chosen in particular such that only the quantum wells Q, the barriers Band the setbacks  26 ,  28  are optically stimulated, as illustrated by the arrows E in  FIG. 4 . For example, the wavelength of the laser beam may be chosen anywhere within the range of 550 to 640 nm. 
     Furthermore, the shape or profile of the laser beam may in particular correspond to a Gaussian profile. 
     The laser beam may be a pulsed laser, such as a nano- pico- or femtosecond laser. The power density of the laser beam may be between 0.1 and 100 mJ per mm2, and preferably between 1 and 10 mJ per mm2. The overall duration of the wafer&#39;s laser beam treatment may be between 1 second and 10 minutes, and preferably between 10 seconds and 2 minutes. 
     The effect of the laser beam treatment on the bandgap structure is shown on the right hand side of  FIG. 4 . It is apparent that the laser beam L, by locally heating the treated area, effectively destroys the quantum wells Q. Hence, after the laser beam treatment, the peripheral edges  22  no longer have any core active layer  14 . The laser beam treatment results in a mixing of the quantum well material with the barrier material (so-called quantum well intermixing), which increases the bandgap. 
     After the laser beam treatment  100 , the semiconductor wafer  10  may be etched (so-called “Mesa etching”), thus obtaining for each LED  12 , a chip preform  32 . The etching step  102  is identified by the letter b) in  FIG. 3 . The etched parts of the wafer  10  are highlighted by a crosshatch pattern Y. Each chip preform  32  has a central zone  34  and a peripheral boundary  36 . The peripheral boundary  36  has been laser treated and thus lacks any core active layer  14 . In contrast thereto, the central zone  34  still has a core active layer  14 . 
     In order to obtain individual LED chips  38 , wafer  10  is diced, e.g. by laser cutting, as shown in step  104  of  FIG. 3 . 
     The laser treatment step wo may optionally be preceded by a step of heating the semiconductor wafer  10  to a background temperature to reduce the power requirements of the laser beam treatment. 
     Thanks to the laser beam treatment of the present disclosure, during operation of the LED chips  38 , the electron-hole recombination is essentially confined to the central light generating area  20 . This is because of the increased bandgap in the peripheral edges  22 , which prevents electron-hole pairs from entering the same. 
     The laser treatment method of the present disclosure is especially useful for very small red LEDs, which are e.g. used as part of high resolution monitors and displays. 
     Although the invention has been illustrated and described in detail by means of the preferred embodiment examples, the present invention is not restricted by the disclosed examples and other variations may be derived by the skilled person without exceeding the scope of protection of the invention.