Electrografted insulator layer as copper diffusion barrier for TSV interposers
Authors: Mevellec Vincent , Suhr Dominique , Dequivre Thomas , Raynal Frédérique 
Published: October 2015

Introduction

After many years as a hypothetical possibility, 3D Integrated Circuits (3D IC) stacking has emerged as a potential key enabler for maintaining semiconductor performance trends.

Through Silicon Vias (TSVs) sit at the foundation of the 3D-IC revolution and are a key enabler for extending semiconductor integration trends into a new phase. Integrated Device Manufacturers and fabless design houses need small, high-density, fine-pitch vias for improved signal integrity and Si real-estate savings. They need them now, and cannot wait for very thin wafer processing and handling technologies to become mainstream – TSVs must cope with current mainstream wafer thickness.

Deep TSVs with Aspect Ratio (AR) greater than 10:1 elegantly fulfill both requirements. But they cannot be manufactured with acceptable yield/cost using traditional dry processes for liner, barrier and seed deposition. Chemical and Physical Vapor Deposition techniques show basic shortcomings and impose high capital investments, holding back the industry-wide adoption of 3D-IC solutions. Beyond that, physical limitations of PVD and CVD techniques prevent to reach a good step coverage of the deposited layers inside the vias, which is required to perform void-free gap filling of electroplated copper.

Electrografting polymer layer as copper diffusion barrier.

To overcomes those limitation, Alchimer develops alternative wet solutions based on electrografting (eG™) and chemical grafting (cG™) proprietary technologies[1]. Electrografting is based on surface chemistry formulations and processes [2]. It is applied to conductive and semiconductive surfaces, and enables self-oriented growth of thin coatings of various materials, especially polymer and metals, initiated by in-situ chemical reactions between specific precursor molecules and the surface.

Due to outstanding thermal, mechanical and electrical properties, electrografted polymer layers are an efficient insulation layer for TSV applications [3]. In this paper, we fill focus on the copper diffusion barrier properties of this eG polymer layer and the associated Cost of Ownership (CoO) reduction for TSV metallization.

In fact, electrografted polymer layer can be used as insulator and copper diffusion barrier, involving the removal of copper diffusion barrier and copper seed layer deposition from the conventional process flow. TSV metallization process steps can thus be reduced to 2 steps:- Step 1: insulation and diffusion barrier all together (eGpolymer)- Step 2: Copper filling (electroless copper)

We will describe in this paper the electrical, thermo-mechanical and barrier effect properties of the eG polymer film including breakdown voltage, capacitance, dielectric constant, young modulus, thermal stability and SIMS profiles after anneal to demonstrate the diffusion barrier efficiency.

Complete metallization of TSV interposers, as an example of successful integration, will be highlighted by several characterizations including SEM cross sections. In addition, CoO comparison will be presented between conventional dry processes and this new wet approach for interposer metallization on the basis of 10:1 aspect ratio TSVs.

New reliability data should also be presented, from test vehicles integrating eG layers, in extreme conditions where PECVD SiO2 cannot survive.