MEMS制造应用的晶圆级气密封装

Abstract

In order to simplify the processing complexity andcut down the manufacturing cost, a new wafer bonding techniqueusing ultraviolet (UV) curable adhesive is introduced here formicroelectromechanical systems (MEMS) device packaging andmanufacturing applications. UV curable adhesive is cured throughUV light exposure without any heating process that is suitable forthe packaging of temperaturesensitive materials or devices. APyrex 7740 glass is chemically wet etched to form microcavitiesand utilized as the protection cap substrate. After a UV-curableadhesive is spin-coated onto the glass substrate, the substrate isthen aligned and bonded through UV light exposure with a devicesubstrate below. Electrical contact pad opening and die separationare done simultaneously by dicing. Two different testing devices,a dew point sensor and capacitive accelerometer, are built toevaluate the package strength and hermeticity After the dicingprocess, no structural damage or stiction phenomenon is found inthe packaged parallel capacitor The acceleration test results alsoindicate that the package using the Loctite 3491 UV adhesive with150 um bond width can survive more than 300 days at a 25 °Cand 100 % relative humidity working environment.

I.INTRODUCTION

HE characteristics of miniaturization, low power con-sumption, and cheap manufacturing cost have enabledmicroelectromechanical systems (MEMS) technology to be amanifest choice in the fabrication of next-generation sensingand actuating devices [1]-[3]. Nevertheless, it has still encountered great technical challenges while being commercializedMost of MEMS devices usually consist of two major components which are mechanical microstructures and solid-stateintegrated circuits (1Cs), respectively. Unlike the ICs, the mi-cromechanical structures usually contain freestanding moving parts which are formed after sacrificial layer removal of itsunderneath layer4] and need to interact with surroundingphysical environments for sensing or actuating purposes. Oneof the challenges is how to protect these delicate microme-chanical structures well with a reasonable cost and providewindows for nonelectric signal input/outputs at the same time.The packaging development has become the most crucial issuefor MEMS commercialization [4]-[10].

Meanwhile, in MEMS fabrication, the present dicing operation should be performed prior to structural release sincethe dicing is a wet process which could result in a stictionproblem to the freestanding microstructures. The cooling waterjet and particle contamination during the dicing operationcould also fail the devices. For instance, a parallel capacitoris damaged after dicing, either the overhanging electrode iswashed out (see Fig. 1(a)] or the electrode sticks to the bottomone [see Fig. 1(b)]. In order to prevent damages occurringduring the dicing,MEMS devices are, in general, fabricatedand diced on a silicon wafer first. After that, the freestandingmicromechanical structures of the devices are released die bydie for following die-level testing and packaging procedures.Although such a manufacturing flow can ensure every MEMSdevice free from the possible damages caused by the wet dicingprocess, high manufacturing cost is inevitable. On the otherhand, these damage issues could be resolved using laser ablaze[7] instead of abrasive cutting. Time-consuming, debris rede-position, and expensive dicing equipment are still inevitabledrawbacks for this solution. In this paper, a novel wafer-levelhermetic encapsulation is proposed for MEMS manufacture.hrough wafer-level processing, all devices on a wafer can besimultaneously fabricated, released, tested, and then packagedso the cost can be reduced. The approach can also effectivelyeliminate any possible damages since every device is packagedand protected prior to the final dicing operation.

Previously, several approaches have been proposed for thefabrication of a hermetic seal [11]-[14]. In these approaches, acommon scheme, called post-process packaging,is also adoptedin our new packaging process. The post-process packaging (seeFig. 2) means the packaging step is done after device fabricationprocesses, including the release of micromechanical structures.which can provide high process flexibility for various MEMSfabrications. The proposed packaging method in this paper is acombination of wafer bonding technique and microshell encap-sulation. MEMS devices and protection shells are fabricated atthe same time on two different wafers, silicon and glass substrates, respectively. Then these two substrates are assembledtogether using an appropriate “wafer bonding technique” [15]to achieve the final encapsulation of the devices. The microshell provides mechanical support, thermal path, and electrical contact for the MEMS devices.