Vertical InGaN light-emitting diodes with Ag paste as bonding layer

Abstract

Vertical InGaN-based light-emitting diodes (LEDs) were fabricated with a Si substrate using Ag paste as bonding layer. Vertical LEDs with Ag paste bonding layer were bonded with Si substrate at a low temperature of 140 °C. In addition to the low-temperature bonding process, the soft property of Ag paste could better alleviate thermal stress compared with conventional eutectic metal bonding layer such as Au–Sn. Under the same test conditions, these two LEDs showed similar optical and electrical properties and reliability. However, LEDs with Ag-paste bonding layer were fabricated through a low-temperature bonding process. The characteristic of soft solder enables a relatively wider process window, such as bonding pressure and temperature, and a higher yield as compared with the vertical LEDs with Au–Sn eutectic bonding layer.

Introduction

High-power GaN-based light-emitting diodes (LEDs) have been recognized as a promising next-generation general lighting source [1]. Recently, vertical thin-GaN LED structures have been identified as the most promising structure for improving the efficiency of high-power LEDs over other traditional LED structures with sapphire substrate [2], [3], [4]. Unlike in conventional LED structures, a conductive substrate and a metal reflector are implanted into the thin-GaN LEDs to improve light-extraction efficiency and reduce thermal resistance [2], [3], [4]. To achieve this goal, laser lift-off and wafer bonding techniques have been adopted to remove the epitaxial substrates (i.e., sapphires) and support the thin-GaN films through a electrical and thermal conductive carrier substrate (such as a Si substrate), respectively. Thin-GaN LEDs have been achieved using a laser pulse directed through the transparent sapphire substrate to separate GaN thin films from sapphire substrate and adhesive layers in order to integrate the LED structures with the carrier substrate [2]. In general, the adhesive layers are metals with low melting or eutectic temperatures that allow the wafer bonding process to be performed at low temperatures [3]. To enhance the light-extraction efficiency (LEE), a mirror (Al or Ag) was positioned between the LED structures and carrier substrate for a conventional thin-GaN LED. Indium-containing metals as soft solder frequently serve as adhesive layers for the wafer bonding process of thin-GaN LEDs. However, high diffusivity of indium atoms at elevated temperatures could lead to a reduction of reflectivity of the mirror because the formation of intermediate compound phases can roughen the interface between the mirror layer and the semiconductor [2], [3], [4], [5]. As a result, the LEE of the thin-GaN LEDs is discounted. Although the diffusion of indium atoms can be suppressed during the wafer bonding process using diffusion barrier layers [6], this complicates the fabrication process of thin-GaN LEDs. Indium-free bonding layers, such as Au–Sn or AuSi, elevated at approximately their eutectic temperatures, might be able to help avoid the problem of diffusion [4]. Thin-GaN LEDs featuring adequate adhesive or bonding layers serve the purpose of heat dissipation, mechanical support, and current conduction. Therefore, the presence of voids reduces the reliability of the devices. For devices bonded using hard solder, such as In-free bonding layer, the bonding layer generally has sufficient mechanical strength to endure stress without plastic deformation and does not become fatigued under thermal cycling. Voids in the hard solder layer tend to generate localized stress on the backside of the chip, which may cause die cracking either during the bonding process or during thermal cycling. In contrast, when soft solder or epoxy is used to bond the wafers, most of the stress occurs in the bonding layer because it is much softer than the GaN-based layers and carrier substrate. In this study, Ag paste was used to serve as bonding layer to connect the GaN LED epitaxial layers with the Si substrate. The Ag paste as soft solder serves the purpose of heat dissipation, mechanical support, and current conduction in the thin GaN-LEDs. To evaluate whether the thin GaN-LEDs with Ag paste bonding layer are suitable for mass production, thin GaN-LEDs with Au–Sn eutectic alloy to serve as bonding layer were also prepared for comparison. The Ag paste bonding layer was formed at atmospheric and room temperatures using the screen-printing technique, whereas the Au–Sn bonding layer was formed through conventional evaporators operating under vacuumed ambience. Therefore, Thin-GaN LEDs with Ag paste bonding layer featured a low-cost and low-temperature process compared to the vertical-type LEDs with metal bonding layers.