Effect of wafer thinning methods towards fracture strength and topography of silicon die

Abstract

This paper characterizes die damage resulting from various wafer thinning processes. Die fracture strength is measured using ball breaker test with respect to die surface finish. Further study on surface roughness and topography of each surface finish is obtained by atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques. Stress relief process with 25 μm removal is able to strengthen 100 μm wafer by 20.4% using chemical wet etch and 75 μm wafer by 36.4% with plasma etch. Relatively, plasma etching shows higher fracture strength and flexibility compared to chemical wet etch. This is due to topography of the finished surface of plasma etch is smoother and rounded, leading to a reduced stress concentration, hence improved fracture strength.

Introduction

Wafer thinning is the key technology in today’s semiconductor technology trend which is heading towards thinner and smaller packages. There are many advantages of using thin chips which include reducing space, allow more functionality per unit volume in a stack die package [1]. Having thinner wafers improves the ability to dissipate heat by lowering the die’s thermal resistance [2]. In some cases, IC packaging demand smaller packaging where silicon is as thin as 30 μm [3]. Therefore to achieve this, the die has to be smaller and thinner as well which makes it become more susceptible to process related failure. To be able to fulfill this requirement, it is essential for the silicon die to possess maximum fracture strength as it improves the ability of the thin wafer to survive the mechanical and thermal stress it is subjected to by handling and further processing. Therefore, understanding the correlation of different thinning methods towards die fracture strength is of utmost importance because it impacts the integrity and reliability of semiconductor packages.

Silicon by nature is hard and brittle material, therefore mechanical grinding is the most common technique utilizing abrasive particles to take out most of the wafer thickness. Most grinding systems use a two-step process where step 1 is coarse grinding and step 2 is fine grinding [4], [5], [6]. The coarse grinding removed most of the thickness, almost 90%. A coarse abrasive grinding wheel (typically 350–500 grit diamond) removes material rapidly, but greatly damages the backside surface of the silicon layer [5], [6]. Fine grinding removes most of the damage [7] and reduce it surface roughness with its abrasive diamond of typically 2000–3000 grit. According to Pei [8], Chen and De Wolf [6] and Hadamovsky [9], back grind silicon wafers causes sub-surface damage to the wafer.

Therefore, there is a need for stress relief processes where it is increasingly important application after mechanical grinding [4]. Each type of stress relief method is aimed at removing the damaged silicon left by conventional grinding steps. Stress induced onto the wafers from mechanical grinding need to be removed by stress relief process and etching is one of the ways to enhance the die strength [4], [10]. There are two types of etching available, which is the wet chemical etch (CE) and dry chemical etch, known as plasma (PE). Wet etching used is an acidic compound containing etchants such as HF and HNO₃ [2], [4]. In plasma etching, neutral chemical species such as chlorine or fluorine atoms generated in the plasma diffuse to the substrate, where they react to form volatile products with the layer to be removed [11]. Typical reactor used for plasma etching is downstream etchers (no ions and photons) in which, generally, the plasma is excited using microwaves, detail information could be found in [12]. The neutrals strike the surface at random angles leading to isotropic, rounded features. Typically, feed gas SF₆ is used for silicon etching [11], [12]. Plasma etching has highest throughput but highest capital investment as well [13].

From a material point of view, silicon by nature is quite stable, but being brittle makes it prone to breakage. Previous studies shows that the fracture strength of a finished die would be higher if the die surface is finer [14]. Therefore, the present work investigates the effects of stress relief process such as chemical wet etching and plasma treatment on the backside characteristics. Markings left behind by each process in the form of defects and specific backside appearances are examined at two levels: through optical inspection using SEM micrograph and, for greater detail, through atomic force microscopy (AFM). The fracture strength of the die was measured as die breaking load by using ball breaker test.