Wafer bow of substrate transfer process for GaNLED on Si 8 inch

This paper investigates the wafer bow induced during the substrate transfer process for GaN LED on Si (111) 8 inch wafers. The substrate transfer process is to transfer the thin GaN LED device layer to a carrier wafer using a CuSnCu permanent bonding layer. The process generates tensile bow on a wafer due to the high tensile stress of the Cu or the Cu_xSn_y intermetallic layer after bonding. Understanding the wafer bow evolution during the substrate transfer is very important to get a good control of the process. The high wafer bow value may cause problems for some automatic handling tools in the production line or affect process quality such as in lithography. The influence of substrate thickness and Cu metallization thickness on the wafer bow has been studied. A bow compensation layer can be used to compensate the tensile bow, thus minimizing the wafer bow after the substrate transfer process.

Introduction

Since the first appearance of GaN based blue light-emitting diodes (LEDs) in 1993 [1], the subject has attracted a lot of attention from researchers and the lighting industry. Beside many other applications, high efficiency white or blue LEDs can be a future candidate to replace incandescent light bulbs [2]. The commercially available GaN based LED products on the market at present are made on SiC or Sapphire substrates. Both substrates are rather costly and only available for small wafer sizes [3]. Other disadvantages come from either the high hardness of the substrate or bad thermal conductivity. Today GaN LED technology has not yet gained a wide acceptance from industry for general lighting applications due to its high manufacturing cost and is hindered by patents and monopolization on SiC and Sapphire substrates. This makes GaN LED on Si substrates a very promising technology as Si offers low cost production, good thermal conductivity, large wafer size and can be integrated with other devices on the same chip. Beside many advantages, however, GaN on Si also faces challenges such as wafer bow due to the lattice and CTE mismatch with the substrate [3], [4]. In this paper, wafer bow induced during the substrate transfer process for GaN LED on Si (111) 8 inch wafers have been investigated. The substrate transfer process is applied after the fabrication of LED devices on a Si substrate and is essential for the light extraction efficiency of LEDs. Due to the light absorbing property of Si, the light extraction of LEDs is hindered by the Si device substrate. To maximize light extraction, the Si device substrate should be removed from the GaN LED [5]. After the substrate transfer process, the GaN LED device layer is totally transferred to a Si carrier wafer. Besides the original bow of the wafer after the deposition of GaN, the substrate transfer process adds tensile bow to the wafer. High tensile bow is generated after the bonding step. The Cu or Cu_xSn_y in the bonding layer creates tensile stress after wafer is cooled down from the bonding temperature of 250°C to room temperature. Understanding the wafer bow evolution during the substrate transfer is very important to get a good control of the process. The high wafer bow value may cause problems for some automatic handling tools in the production line. It also affects some process quality such as in lithography.