LEDmanufacturing industry
that are made from the AlInGaP material family. At EV Group, which is based in St. Florian, Austria, we are supporting the manufacturing of LEDs produced with a metal bonding process. Our involvement includes the recent launch of the first tool dedicated to this fabrication step – the EVG 560HBL. This piece of equipment is designed to deliver very high yields thanks to optimisation of pressure and temperature distributions, and it sets a new benchmark for throughput of up to 176 bonds- per-hour for 2-inch wafer equivalents.
A little history… Wafer-to-wafer bonding is not new. It was developed 20-30 years’ ago to address the need for wafer-level capping of MEMS devices. Pioneers of wafer- bonding used anodic bonding and glass frit bonding to attach one wafer to another. However, these two approaches are being superseded by metal bonding technologies, which offer a lower form factor.
The metal bonding approach is the only one that is applicable to high-brightness LEDs, due to the requirement for low thermal resistance. This is not the only benefit of this type of bond, however – it can also increase the luminous efficiency of the device. It was first used in AlInGaP-based LEDs that are grown on GaAs substrates. Spontaneous emission from these devices is assumed to be isotropic, with half of all the light generated propagating towards the substrate, where most of it is absorbed, leading to lowering of overall device efficiency. Inserting a distributed Bragg reflector beneath the light-generating region of the LED could prevent this light loss to the substrate, but in practice this only works effectively on one optimised direction of light emission, and it is better to turn to wafer bonding, where a reflective layer is included in the metal stack (see Figure 1).
Manufacturing nitride LEDs with a metal bonding process presents some different challenges. Sapphire, the most widely used platform for making blue and white LEDs, has the desirable attribute of high transparency, but it is a poor heat conductor. Consequently, high-power LEDs employing a lateral design are poor at dissipating their heat and run hot, which degrades device performance. To combat this, some LED manufacturers have developed vertical LED designs, which involve substituting sapphire for another carrier with higher thermal conductivity (see Figure 1).
Switching to this design also simplifies the manufacturing process by eliminating an etching step required to form the n-contact in a lateral LED. In addition, the vertical architecture produces a vertical current path, leading to a lower forward bias and eliminating current crowding issues that are frequently seen for other LED designs. And there are
July 2011
www.compoundsemiconductor.net 29
Figure 2: A scanning electron
microscopy cross-sectional image
highlights the high quality of the join
between an InP and GaAs wafer after Au:Sn eutectic wafer bonding
other benefits too: the addition of the metal bonding layer ensures that all of the light exits from the top of the LED; and manufacturing may be simplified, because the vertical LED design uses the same process flow for different die sizes.
Every vertical LED process flow begins by depositing a stack of epitaxial layers on a substrate by MOCVD. Some engineers will then turn to the patterning of the LED dies, while others will begin with layer transfer by wafer bonding. The decision of what order to perform the various processing steps is primarily governed by the nature of downstream processing
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104