高功率LED技术发展近况

Recent Development in High Power LED Tchnology

 

   Tzer Perng Chen（Uxited Epitaxy Company）
Taiwan.R.O.C(联合EPITAXY公司)

 

 在过去10年里，红，黄ALGaINP与蓝绿ALGaNN LED的发光效率有了很大的提高。这种快速的发展为LED代替普通白炽灯提供了可能。传统的发光二极管的功率很低，大约为10mw至100mw。为了代替传统的白炽灯以及荧光灯，发光二极管应该提高其功率以及光强度。在本论文中，我们介绍了二种高功率LED芯片制造技术，即FLIP-CHIP技术以及薄膜转移与金属焊接技术。使用以上技术制造的高功率LED产品功率为1W至5W。
 高亮度ALGaINP红、橙、黄LED出现于二十世纪九十年代。随后，这些LED得到很多应用，例如用作交通信号、汽车照明、电话背光照明，以及室外显示等，这是LED具有高发光效率的缘故。

    这些高发光效率的LED已开始代替普通白炽灯泡以及卤灯产品。虽然发光二极管有比较好的可靠性，但直到目前它代替普通灯泡的比例仍然低于10%，其中一个主要原因是价格太高。对于白炽灯来说，每流明价格为0.001元。目前，对于LED来说，每流明价格只有0.1～0.05元，而且其发光强度仍然大于白炽灯。另一个原因是标准5mm LED在20mA的总Flux值仅为1～3流明。而大多数白炽灯或卤灯应用场合中，我们需要数百或数千流明值。因此，如LED达到数百或数千流明值时，则其组装LED尺寸会变得很大，所以LED很难在这种场合中得到使用。
 为了解决以上问题，我们有多种方法。

 

     第一，如果12mIL×12MIL单个LED的流明效率可以提高的话，则其LED价格可以降低，而且所需要的LED数量也可以降低。增加流明效率也有多种方法，其中包括提高材料质量、优化LED EPI晶片结构设计、使用非吸收衬底材料，特别芯片形状或介面结构纹理等。

    第二，可以增加注射电流密度。如果光输出及驱动电流比值为线性，同时还未饱和状态，则单个LED总FLUX值将提高好几倍（比较20m A时的数值而言）。

 

     因此，LED价格就可以下降。LED注射电流的最大值与以下二个因素有关：
 （1） LED芯片工作电流没有显著下降；
 （2） LED每W的流明效率电流没有下降太快。
 在过去，LED工作电流密度一般为20A/cm2。但目前，某些LED工作电流密度已经达到70A/cm2至100A/cm2。

   另外的方法是使用散热性能好的LED芯片或封装设计。ALGaINP与ALGaInN LED EPI-园晶片分别在GaAs以及蓝宝石衬底上生长。这些GaAs与蓝宝石衬底的导热性能不是很理想。在表1中，我们分别看到GaAS与蓝宝石的导热性分别为44W/m2K及 35W/m°K。为了更好的散热，以及降低结点温度，我们采用到下三种方法：
 （1） 让衬底变薄
 （2） 芯片-FLIP或让P-N结更接近散热片
 （3） 取走用于LED EPITAXIAL层的原有衬底，并让LED EPITAXIAL层转移到导热衬底。

    第一种方法是比较容易做到的。但是，我们很难把衬底材料的厚度降低至50μｍ以下。第二种方法已经由ＭＡＴＳＵＳＨＩＴＡ与ＬＵＭＩＬＥＤＳ公司用于提高ＧａＮ ＬＥＤ流明效率以及散热性能。ＵＥＣ以及ＳＨＯＷＡ 公司也研制出ＦＬＩＰ－ＣＨＩＰ型号的ＡＬＧａＩＮＰ ＬＥＤ以提高流明效率，并且让其具有较率的工作电流。另一方面，ＦＬＩＰ－ＣＨＩＰ工艺比较复杂而且价格昂贵。很多公司已宣称成功开发出薄膜转移式ＬＥＤ，其中是采用以上第三种方法制造的。这些公司包括ＵＥＣ，ＯＳＲＡＭ， ＮＩＣＨＩＡ， ＳＡＮＫＥＮ， ＶＰＥＣ， ＡＥＴ， ＯＰＴＯＴＥＣＨ， ＡＲＩＭＡ 以及ＥＰＩＳＴＡ，这种方法价格相对较低，可用于制造高功率ＬＥＤ芯片。在本论文中，我们讨论ＦＬＩＰ－ＣＨＩＰ以及薄膜转移技术。

 

    我们使用ＧａＮ ＬＥＤ作为一个例子来说明怎样制造ＦＬＩＰ－ＣＨＩＰ发光二极管以及它们的性能。ＧａＮ ＬＥＤ在蓝宝石衬底材料上生成。典型的ＧａＮ ＬＥＤ结构组成包括蓝宝石衬底材料，非掺透ＧａＮ缓冲层，Ｎ－型ＧａＮ层。其中，活性层含有ＩＮＧａＮ／ＧａＮ多量子阱（ＭＱＷＳ）以及Ｐ型ＧａＮ层。部分Ｐ型ＧａＮ层与积层可通过离子蚀刻方法除去以便在Ｎ－型ＧａＮ层形成欧姆触点。在ＦＬＩＰ－ＣＨＩ ＬＥＤ结构中，发光来自蓝宝石衬底。因此，有一个理想的反射器与Ｐ型ＧａＮ层接触是非常重要的。这种ＬＥＤ性能与反射器质量有很紧密的依赖性。这种反射要有高反射能力，而且Ｐ型ＧａＮ层触点电阻值要低。如果仅考虑反射能力，则铝与水银是最理想材料。但除了反射能力以外，反射器还应有很好的欧姆触点性能（与Ｐ型ＧａＮ层接触）。

 

    同时，它还要有很好的粘性。另一方面，在后期高温ＬＥＤ芯片工艺阶段，反射器的金属材料反射性能不能降低。否则，在反射器材料沉积后，ＬＥＤ芯片工艺应在低温中进行以防止出现反射的性能下降问题。其他解决方法是使用多层式反射器。例如，二层式反射器有二个成份组成。其中一个是薄金属层（作为欧姆触点层），另一个是高反射性金属层（作为反射器）。一般来说，第一个金属层仍然有一定吸收特性。为了减少光吸收，我们可以采用透明导电层代替，其中包括ＩＴＯ，ＺｎＯ，ＮＩＯ，如果它具有一个低触点电阻低的话（与Ｐ型ＧａＮ层接触）。由于具有良好的散热设计，高功率ＧａＮ ＬＥＤ可以具有ＩＡ的稳定工作电流，同时又未达到饱和状态。我们分别在３５０ｍＡ,５００ｍＡ与１Ａ情况下测试高功率ＧａＮ ＬＥＤ。在１０００小时工作以后，性能下降低于１０％。这证明这些ＡＬＩｎＧａＮ ＬＥＤ可以在大驱动电流条件下工作，如果封装设计具有良好的散热性能的话。其中，我们采用高功率ＬＥＤ薄膜转移技术。

     我们也使用新金属焊接型高亮度ＡＬＧａＩＮＰ ＬＥＤ结构。这种新结构不仅具有高流明效率，而且具有高生产能力。这是薄膜转移技术制造ＬＥＤ的一个例子。ＬＥＤ的ＥＰＩＴＡＸＩＡＬ层由蚀刻ＳＴＯＰ层，ｎ－型（ＡｌｘＧａＬ－Ｘ）０．５ＩＮ０．５Ｐ低复盖层、高复盖层、非涂掺（ＡｌｘＧａＬ－Ｘ）０．５ＩＮ０．５Ｐ活性能、高Ｃａｒｒｉｅｒ浓度及欧姆层组成的。蚀刻ＳＴＯＰ层可由Ⅲ－Ｖ化合物半导体材料组成，可以与ＧａＡｓ衬底配合。另外，它的蚀刻速度比ＧａＡｓ材料低。例如，ＩＮａＧａＰ或ＡＬＧａＡｓ材料是理想的蚀刻ＳＴＯＰ材料。对于金属焊接型ＡＬＧａＩＮＰ ＬＥＤ芯片工艺，Ｐ型栅格欧姆触点金属（例如Ａｕ－Ｂｅ）则首先沉积在高ｃａｒｒｉｅｒ浓度及欧姆触点材料上，然后是透明导电氧化层以及高反射性金属层沉积。透明导电氧化层用途是在后期欧姆触点退火工艺中防止在高反射性金属层与ＡＬＧａＩＮＰ ＬＥＤ层之间出现反应问题。插入透明导电层会恶化金属反射器的反射能力。然后ＡＬＧａＩＮＰ ＬＥＤ Ｅｐｉ－圆晶片将焊接到ＩＮ－Ｓｉ圆晶片上。在焊接以后，Ｎ－型ＧａＮｓ衬底与蚀刻ＳＴＯＰ层采用湿法化学蚀刻或干法蚀刻除去。在Ｎ型ＡＬＧａＩＮＰ低复盖层上，我们使用欧姆性Ｎｉ／ＡｕＧｅ／Ｎｉ／Ａｕ金属沉积方法制造ＬＥＤ芯片。ＬＥＤ圆晶片可以加工成１２ＭＩＬ×１２ＭＩＬ尺寸的芯片。ＡＬＧａＩＮＰ ＬＥＤ芯片可以封装到一个５ｍｍ的灯上。流明效明可以用调校型硅光探测器测定。目前６２５ｎｍ ＡＬＧａＩＮＰ ＬＥＤ的流明效率为４０ｌｍ／Ｗ以上（在２０Ａ注射电流条件下），比常用ＡＬＧａＩＮＰ ＬＥＤ?穴吸收型衬底?雪高出２至３倍，所以它与ＬＵＭＩＬＥＤＳＴＳ型ＡＬＧａＩＮＰ ＬＥＤ不相上下，参见图１。

 

     使用薄膜转移与金属焊接技术制造的ＡＬＧａＩＮＰ ＬＥＤ优点在于欧姆触点与光反射使用不用金属材料。所以，我们取得了低欧姆触点电阻值以及高反射性能的效果。反射能力可以高达９０％以上。另外，由于ＡＬＧａＩＮＰ ＬＥＤ层焊接到高导热性硅衬底上，所以ＡＬＧａＩＮＰ金属焊接型ＬＥＤ在５ｍｍ Ｔ－１３／４灯产品中具有比较高的工作电流。在１０ＭＩＬ×１０ＭＩＬ发光区域，金属焊接型ＡＬＧａＩＮＰ ＬＥＤ工作电流为７０ｍＡ，并且在工作１０００小时之后，没有出现性能下降问题。这种金属焊接技术已被应用至不同波长的ＡＬＧａＩＮＰ ＬＥＤ场合里。在５９０ｎｍ时，流明效率为７０ｌｍ／ｗ，在６０５ｎｍ时，流明效率为５９０ｎｍ。为了进一步提高金属焊接型ＡＬＧａＩＮＰ ＬＥＤ的性能，我们可以让表面变得粗糙以增加Ｅｘｔｒａｃｔｉｏｎ光效。我们使用ＡＬＧａＩＮＰ ＬＥＤ的性能，我们可以让表面变得粗糙以增加Ｅｘｔｒａｃｔｉｏｎ光效的。

    我们也使用薄膜转移与金属焊接技术来制造２７ｍｉｌ与４０ｍｉｌ芯片型高功率ＬＥＤ。图２为光输出与电流特性曲线（２７ＭＩＬ芯片高功率ＡＬＧａＩＮＰ ＬＥＤ）总流明ＦＬＵＸ值在５０ｍＡ时为５０流明左右。这种技术也已经用于４０ｍｉｌ芯片型高功率 ＡＬＧａＩＮＰ ＬＥＤ场合。在５００ｍ Ａ时，Ｆｏｒｗａｒｄ电压只有２．６Ｖ。在１０００ｍＡ时，总流明Ｆｌｕｘ值大约为９０流明。由于具有良好散热设计，所以４０ＭＩＬ高功率ＬＥＤ可以在１０００ｍＡ下工作，并且具有稳定的光输出值，参见图３。

 

 我们介绍了高功率ＬＥＤ芯片的二种制造技术，即ＦＬＩＰ－ＣＨＩＰ技术与薄膜转移以及金属焊接技术。使用以上技术制造的高功率制造的ＬＥＤ证明可以在１Ｗ－５Ｗ条件下工作。这样我们认为高功率ＬＥＤ有可能在未来的照明领域中得到应用。

 

  Ａｂｓｔｒａｃｔ
 In the past ten years，the rapid luminous efficiency improvement of AlGalnP red，yellow and AlGaInN blue and green LED has made the dream of using light emitting diodes to replace Thomas Edison’s incandescent bulb becoming feasible.Traditionally light emitting diodes only operate at very low power such as in the range from 10-100mW. To replace the conventional incandescent bulb and fluorescent lamp in lighting application，the light emitting diodes should be enabled to operate at high power and to emit high flux.A key issue for high power LED is how to extract the heat out from the light generation active region.In this paper，two important high power LED chip fabrication technologies i.e，flip chip technology，and thin film transfer and metal bonding technology will be reviewed.The high power LEDs made by using both technologies already can be operated at 1-5 watt range.

 

Ｉｎｔｒｏｄｕｃｔｉｏｎ
 The intfoductlon of high brightness AlGalnP red，orange and yellow light emitting diodes （LEDs）in the early 1990 and later the AlGaInN blue，green and white LEDs has created a lot of new applications such as traffic signals，automotive lighting，cellular phone backlighting，and full color outdoor displays for these high efficiency solid state lighting sources.These high efficiency light emitting diodes have started to replace the conventional incandescent light bulbs and halogen lamps.Although light emitting diodes have better reliability，until now the percentage of replacement is still less than 10%.One of the major reasons is that the price is too high.The dollar per lumen for incandescent bulb is about$0.001.At present the light emitting diodes can achieve only$0.l—0.05 dollar per lumen which is still about two orders of magnitude higher than the conventional incandescent bulbs.The other reason is the total flux emitted from a standard 5mm LED lamp operating at 20mA is only about 1-3 lumen.Many applications that use conventional incandescent bulbs or halogen lamps normally need at least a few hundred lumens or even a few thousand lumens.Therefore，hundreds and thousands of LEDs are needed and the total assembly size would be too big and have difficulty to fit into the space of some applications.

 To solve the above problems，there are several different approaches.First，if the luminous effciency of a 12mil x 12mil single light emitter can be improved，the dollar per lumen certainly can be reduced and the number of LEDs that are needed can be decreased.The increase of luminous efficeincy can be achieved by several ways for example，material quality improvement, better LED epi-wafer structure design，using non-absorption substrate，spedal chip shaping or surface texturing to get higher light extraction efficiency.Second，the lnjection current density can be increased.If the light output versus drive current is quite linear up to higher current without saturation, the total flux coming out from a single emitter driven by higher current with the same chip size should be several times higher compared to just operating at 20mA.Therefore，the dollar per lumen also can be reduced.The maximum current that can be injected into the LED chip depends on the following two factors.

 （l）The current which the LED chip can be operated without significant degradation.
 （2）The current which lumen per watt efficiency of LED chip doesn’t drop too much.
 In the past, normally the LEDs are operated at a current density about 20A/cm2. Now, some high flux LEDs have already operated at 70-100A/cm2.
 The other approach is to adopt better heat dissipation LED chip and package design.The AlGaInP and AlGaInN LED epi-wafers are grown on GaAa and sapphire substrates respectively.These GaAs and sapphire substrates are not good thermal conductive materials.The thermal conductivity of GaAs and sapphire are 44W/m°K and 35 W/m°K respectively as shown in table 1. To be more effectively removing the heat and lower down the junction temperature，there are three different approaches.

 （l）Thin down the substrate.
 （2）Flip the chip and let the light emitting p-n junction close to the heat sink.
 （3）Remove the original substrate that is used for the growth of the LED epitaxial layers and then transfer the LED epitaxial layers to an elecmctricaly and thermally conductive substrate.

 The first approach is the easiest one but it is difficult to thin down the Substrate to less than 50um. The second approach has already been used by Matsushita and Lumileds to improve both the luminous efficiency and heat dissipation in GaN LEDs.UEC and Showa Denko also have developed a flip chip type AlGaInP LED to enhance the lurminous efficiency and high current Operating characterstics.

However, the flip chip process is more complicated and expensive. Many companies including UEC, Usram, Nichia.Sanken, VPEC, AET，Optotech, Arima and Epistar all have announced successfully developing a thin film transfer type LED using the third approach.This approach will be the most cost effective way to make high power LED chips.In this paper, we will discuss flip chip and thin film transfer approaches in detail.

 

Ｈｉｇｈ Ｐｏｗｅｒ ＬＥＤ Ｆｌｉｐ Ｃｈｉｐ Ｔｅｃｈｎｏｌｏｇｙ
 We will use GaN LED as an example to describe how to make flip chip LEDs and their peformances.The GaN LED devices are grown on sapphire substrate.A typical GaN LED structure is composed of a saphire substrate，an un-doped GaN buffer layer, an n-type GaN layer,an active layer having InGaN/GaN multiple quantum wells（MQWs）structure，and a p-type GaN layer. Part Of the p-typ GaN layer and active layer are removed by reactive ion etching to reveal and allow the formation of an ohmlc contactic the underlying n-type GaN layer. In flip chip LED configuration，the emitted light will be extracted from the sapphire substrate side.Therefore，it is very important to have a very good reflector in contact with p-type GaN layer.The performance of high power flip chip type LED is mostly dependent on the reflector quality.This reflector is required not only high reflectivity but also lower contact resistance to the p-type GaN layer. If justconsidering the reflectivity，aluminum or silver are best candidate materials.But except reflectivity， the reflector material is also required to make good ohmic contact with p-type GaN and has better adhesion properties.Besides，the reflectivity of the reflector metal material can’t degrade during the latter high temperature LED chip process.otherwise，the LED chip process after the deposition of the reflector materials should proceed only in lower temperature to avoid the reflector degradation problem.The other approach to solve this problem is by using a multiple-layers reflector structure.For example，a two-layers reflector structure is composed of a first thin metal layer as an ohmic contact layer and a second high reflectivity metal layer as a reflector.Normally，the first thin metal layer will still have some absorption.To reduce the light absorption, a transparent conductive layer such as ITO，ZnO or NiO can be used to replace the thin metal layer if it can make good low contact resistance with the p-type GaN layer.

 With good heat dissipation design，the high power GaN LED can be stably operated up to one ampere without saturation.We tested these high power GaN LEDs at 350mA，500mA and even up to one ampere.The degradation is less than 10%after 1000hrs operation.It proves thatthesehlgh power AllnGaNLED can be oPerated athlgh driving current lf the Package design Provldes enough heat dissipation High Power LED Thin Film Tansfer Technology    we will use a new metal bonding type high brightness AIGalnP LED strcture which not only has high luminous efficiency but also is easy to manufacture with high production yield as an example to explain the thin film transfer technology.

 The epitaxial layer structure of light emitting diode consisted of an etching stop layer，an-type（AlxGal-x）0.5ln0.5P lower cladding layer，anun-doped（AlxGal-x）0.5In0.5P active layer，a p-type（AlxGal-x）0.5In0.5P upper cladding layer，and a high carrier concentration and high band gap ohmic contact layer.

 The etching stop layer can be any Ill-V compound semiconductor materials that can be lattice matched with the GaAs substrate and has an etching rate much smaller than the GaAs material. For example，InGaP or AlGaAs material can be good candidate of the etching stop layer.For the process of metal bonding type AlGaInP LED chip，a p-type grid pattern ohmic contact metal such as Au-Be was first deposited on the high carrier concentration and high band gap ohmic contact layer.A transparent conductive oxide layer and a high reflectivity metal layer were deposited subsequently.The purpose of the transparent conductive oxide layer is to avoid the interaction between the high reflectivity metal layer and the AlGaInP LED layers during the latter ohmic contact annealing process.The insertion of this transparent conductive layer can improve the reflectivity degradation of the metal reflector.The AlGaInP LED epi-wafer was then bonded to an indium solder coated silicon wafer.After bonding，the n-type GaAs substrate and the etching stop layer were removed by either wet chemical etching or dry etching.The LED chips were fabricated by depositing Ni/AuGe/Mi/Au ohmic contact metal on the n-type AlGaInP lower cladding layer.The LED epi-wafers were sawn into chips measuring 12mil×12mil.The AlGaInP LED chip was packaged Into 5mm lamp.

 The luminous emclency was measured by a calibrated Si photodetector with integrating sphere.The luminous efficiency of the present invention 625nm AlGaInP LED is more than 40lm/W（at 20mA injection current）and is 2 to 3 times higher than the luminous efficiency ofconventional absorbing substrate AlGaInP LED and comparable to Lumileds TS type AlGaInP LED as shown in Fig.l.The merits of AlGaInP LED using thin film transfer and metal bonding technologies are using different metals for ohmic contact and the light reflection.Therefore，wecan achieve both lower contact resistance and higher reflectlvlty.The reflectivity that can be reached is over 90%.Besides，because the AlGaInP LED layers are bonded to a high themal conductivity silicon substrate.This AlGaInP metal bonding type LED can be operated at higher current even in a 5mm T-13/4 lamp package.In a 10mil x 10mil emitting area metal bonding type AIGaInP LED operated at 70mA，there is nearly no degradation after 1000hrs operation.The same metal bonding technology have also been applied to AlGaInP LED with different wavelength.The luminous efficiency can reach 70lm/W at 590nm and over 80lm/W at 605nm.To further improve the performance of metal bonding type AlGaInP LED，we will roughen the top surface to increase the light extraction efficiency.

 We also used the thin film transfer and metal bonding technology to 27 mil and 40 mil chip size high power LED.Fig.2 is a light output versus current characteristic curve of a 27 mil chip siz high power AlGaInP LED.The total luminous flux is reaching close to 50 lumens at 500mA injection current.The same technology was also applied to a 40 mil chip size high power AlGaInP LED.The forward voltage is only about 2.6 volt at 500mA mectlon current.The total lumnous flux is about 90 lumens at 1000mA operating current.With good heat dissipation design，these 40 mil high power LEDs can be operated even at 1000mA and still maintain stable light output as shown in Fig.3.

 

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 We have reviewed two high power LED chips technologies i.e.，flip chip technology and thin film transfer and metal bonding technology.The high power LED made by using either one of these two technologies have already been demonstratol that it is possible to be operated at 1-5 watt range.It makes the high power LEDs to be used in the future lighting area becoming feasible.