Selective Heat Input for Low Temperature Metallic Wafer Level Bonding
Authors: Wiemer Maik , Hofmann Christian , Vogel Klaus 
Published: December 2020

Introduction

Wafer level packaging (WLP) is a key technology for material integration in Micro-Electro-Mechanical-Systems (MEMS). To guarantee their reliable functionality, these MEMS often require a hermetically sealed and mechanically stable package. On industrial scale, MEMS packaging is done with wafer bonding technologies such as anodic, glass-frit, eutectic or solid-liquid inter diffusion bonding. These bonding technologies require process temperatures of 200 °C to more than 400 °C to form a stable bond between two substrates. To integrate heterogeneous substrates and fully processed ASICs in one package, the thermal stress has to be reduced. These requirements can be met by new innovative technologies such as reactive and inductive wafer bonding, enabling a selective heating of the bond interface in a very short time only.

Reactive wafer bonding

Reactive bonding utilizes a self-propagating exothermic reaction for selective (SER) heating of the bond interface only. The SER occurs in sputtered nanometer-scaled multilayer stack, consisting of Pd/Al, Ti/Si or CuO/Al with a total thickness between 1 µm and 4 µm. Upon ignition, the SER propagates through the multilayer stack (Fig. 1) with a reaction velocity of 1 m/s to 80 m/s, while melting the bond metallization adjacent to the reactive multilayers in the reaction zone. During solidification of the metallization, a mechanical strong, hermetic bond is formed between both substrates. To ensure close contact between the multilayers and the bond metallization, a bond pressure of 3 MPa to 5 MPa is applied to the substrates.
The reactive properties such as reaction temperature and energy level strongly depend on the reactive material system. In comparison to medium energetic Ti/Si, high energetic systems such as Pd/Al and CuO/Al generate more energy at a higher reaction temperature. The energy level of the integrated reactive material system (iRMS) can be tuned by total multilayer thickness. By varying the multilayer design, the iRMS can be adapted to the application, bond metallization and substrate material.

Inductive wafer bonding

During inductive bonding, the necessary heat input into the interface is generated by an external electromagnetic field. This field induces a voltage in the electrically conductive bond material. The resulting eddy current causes a heat flow mainly by Joule heating. To ensure an encapsulation of all MEMS devices at wafer level, the most important requirements are homogeneous heat distribution and rapid heat generation in each bond layer or structure. Considering these demands, the here presented method includes four innovative claims. The most important point includes the inductor coil as well as the conception of the inductor geometry, which has to be adapted to the bond layout for homogeneous heat generation in a single process step (I). The heating system, consisting of an induction generator and an L-C resonant circuit, is adjusted to the bonding application (II). Especially, an appropriate tuning of frequency and input power is necessary to achieve fast heating rates with a high efficiency factor and low energy losses. To ensure wafer level bonding, the inductor is integrated in a bond module, which is compatible with an industrial wafer bonder (III). With an implementation of all these components in one process, an induction-assisted wafer level bonding is accomplished.

Scope

In this paper, two innovative techniques for bonding with selective heating are presented. The focus for reactive bonding lies on the new cost efficient CuO/Al material system. Starting with the multilayer design, deposition and patterning as well as the bonding process will be explained, including the influence of the layer design on the bond strength (Fig. 2). The detailed analysis of the bond interfaces enables a comparison of the reactive bonded interface with standard wafer level packaging technologies such as thermal compression or glass-frit bonding (Tab. 1).
The focus for inductive bonding in this paper is the support and enhancement of the solid-liquid inter diffusion (SLID) bonding process at wafer level by using selective and energy-efficient induction heating of Cu-Sn layers (Fig. 3). To predict the heat distribution within the bond structures in dependence of the inductor geometry and operating frequency, Finite Element Analysis (FEA) with the electromagnetic AC/DC module of COMSOL Multiphysics is performed (Fig. 4 left). Infrared thermography imaging was used to investigate the heating distribution as well as the achieved heating rate of more than ∆T = 150 K/s (Fig. 4 right). Finally, the inductive SLID bonding is carried out in 120 s by applying a bond pressure of approx. 2 MPa and characterized by determination the bond strength.

Figure 1