Die Separation Strength for Deep Reactive Ion Etched Wafers

Abstract

In the electronic and microfabrication industry, die separation is one of the most critical steps in producing an undamaged, stand-alone micro-scale device. For silicon based devices, it is the predominant step governing resistance to die failure by mechanical fracture. Traditional separation methods include the use of dicing saws and/or backside grinding to dice-by-thinning. Excessive forces, vibrations, and surface contact involved with these methods can cause undesirable side-wall chipping and microcracking, which often translates to inoperable devices. Deep Reactive Ion Etching (DRIE) offers an alternative technique for die separation with less mechanical force. The DRIE process may be used to either introduce notches in one uniform step that allow for die separation via fracture in three-point bending, or to directly separate the dies by etching completely through the substrate. This work presents an analysis of the stress concentrations due to DRIE etched notches and the bending stress required to achieve die separation. The defect rate and die strength associated with DRIE-based die separated is compared with traditional saw methods for a variety of notch depths. Results indicate that the DRIE-based separation technique offers modest advantages over the traditional methods, but can also greatly reduce strength if the protective mask is over etched. It will also show that shallow trenches formed by a mechanical dicing saw resulted in stronger die than deeper trenches. vi TABLE OF CONTENTS

Die separation and rupture strength for deep reactive ion etched silicon wafers

“…The diamond composite dicing blade used for all mechanical grinding cuts had a nominal width of 70 μm. Five streets were cut aligned parallel to the 1 1 0 wafer flat, and then another five streets were cut perpendicular to the 1 1 0 wafer flat, figure 1 [20] so that square die could be obtained. Water jets helped keep the blade cool while the dicing took place.…”

“…Each street was placed 17.07 mm apart, giving a die size of 17 mm square. In all cases, the dicing saw was operated at a Reprinted from [20] with kind permission from Springer Science + Business Media B.V, Copyright 2011 (figure 3). constant rate of 1 mm s −1 .…”

“…For the dicing saw notch geometry, the maximum principal stress is located at the notch root, while the maximum principal stress for the DRIE samples is located at the corner fillet regions. For both trenching methods, the ANSYS calculated stress concentration factors for notch depths from 10% to 90% of the total wafer thickness are shown in figure 5 which is from [20].…”

“…During preliminary work on project [20], the initial specimens 100% separated via DRIE (last two columns in figure 7(b)) showed surprisingly low stress magnitude at the time of rupture. Careful inspection of these specimens revealed micro-pitting due to failure of the photoresist protection layer during the DRIE process.…”

“…This result may also have relevance to the commonly used 'dice-by-thinning' procedure for die separation, for which partial DRIE notching may be preferable to partial notches created via a dicing saw. More statistics and preliminary data can be found in [20].…”