使用晶圆键合机的晶圆级芯片规模封装

ABSTRACT

An in-house processing capability is developed in this research for silicon-glass bonding for microfabrication and wafer level chip scale packaging (WLCSP) usinga wafer bonder. New masking technology for wet etching of glass to a depth of more than430 um is reported in this research work along with development of an anodic bondingprocess that permits electrical feedthroughs for connections to outside worldThree novel masks were developed in this work for deep wet etching of glassThey were multilayers of metals Mo/Cr/Au (mask 1) and Cr/Au/electroplated Ni (mask2) both in combination with 20 um thick Az* P4620 photoresist and anodically bondedsilicon (mask 3). Etch depths greater than 600 um in glass has been achieved usinganodically bonded silicon mask 3 above. It may be currently the only method available toachieve etch depths of 1 mm in glass. Earlier barrier of 300 um etch depth in glass usingmultilayer metal mask has effectively been broken in this work with an etch depth of430um achieved using electroplated Ni mask 2) above. A high value of 0.88 for the aspectratio, defined as the ratio of the vertical etch depth to the lateral etch distance, wasachieved using mask 1) above.

The problem of etched surface roughness observed in glass with undiluted HFetching has been alleviated by use of a combination of 50.5: 1 by volume HF:HCI:HNOEtch depths of 355 um has been achieved in silicon using 45 % KOH solutionat 50 °C with 1 um thick oxide mask. The above etch parameters also resulted in smoothetched mirror like surfaces, sharp edges in etched pits and deep trenches in silicon. Thedecontaminated etched glass and silicon substrates were aligned in-situ and bonded usingan AML 402 wafer bonder. The corner areas of the glass wafer were diced to expose the metal lines permitting electrical communication from the anodically bonded packaged chip to the outside world. The concept of WLCSP using anodic bonding has been developed and demonstrated in this research.

1.INTRODUCTION

There are many different types of chip scale packaging (CSP) commerciallyavailable. Wafer level chip scale packaging (WLCSP), a type of CSP, has been gainingmomentum in recent years because for this case all the fabrication procedures areperformed at the wafer level. WLCSp integrates the advantages of both chip scalepackaging (CSP) and wafer level packaging (WLP). Wafer bonding is one of the viablemeans to achieve WLCSP

1.1 Wafer Bonding and ts Objective

Wafer bonding may be defined as permanent or temporary binding of twosimilar or dissimilar substrates either chemically or physically. There are three types ofwafer bonding commonly employed. These are 1) direct wafer bonding (DWB), 2)intermediate layer bonding (ILB) and 3) anodic wafer bonding (AWB). AWB is used topermanently bond glass to silicon without use of adhesives. The glass and silicon wafersare heated to a typical temperature in the range of 300 - 500 C, depending on the type ofglass used for bonding. At this temperature, the alkali metal ions in glass become mobileThe glass and silicon wafers are brought into contact and a high voltage is applied acrossthem. This results in the alkali metal ions in the interface to migrate towards the cathoderesulting in an interface layer with high electric field. This phenomenon results in anelectrostatic attraction bringing the glass and silicon into intimate contact. The presenceofhigh electric field causes the oxygen anions to flow from glass to silicon resulting in an electrostatic attraction bringing the glass and silicon into intimate contact. The presenceofhigh electric field causes the oxygen anions to flow from glass to silicon resulting in ananodic reaction at the interface and therefore a permanent chemical bond between glassand silicon occurs. ILB involves deposition of an intermediate thin film layer such aslow melting point glasses, low temperature eutectics or adhesives prior to bonding.