Wafer Bonding and Layer Splitting forMicrosystems**By Qin-Yi Tong* and Ulrich M. GöseleIn advanced microsystems various types of devices (metal-oxide semiconductor field-effect transistors, bipolar transistors,sensors, actuators, microelectromechanical systems, lasers) may be on the same chip, some of which are 3D structures innature. Therefore, not only materials combinations (integrated materials) are required for optimal device performance ofeach type but also process technologies for 3D device fabrication are essential. Wafer bonding and layer transfer are two ofthe fundamental technologies for the fabrication of advanced microsystems. In this review, the generic nature of both waferbonding and hydrogen-implantation-induced layer splitting are discussed. The basic processes underlying wafer bonding andthe layer splitting process are presented. Examples of bonding and layer splitting of bare or processed semiconductor andoxide wafers are described.1. IntroductionªWafer bondingº refers to the phenomenon that mirror-polished, flat, and clean wafers, when brought into contactat room temperature, bond to each other without using ad-hesives or external forces. The phenomenon that opticallypolished pieces can adhere or bond to each other has beenknown for a long time and was first studied by Rayleigh in1936[1]for quartz glass. But it was only in 1985 that an at-tempt was made almost simultaneously by researchers atToshiba[2]and IBM[3]to use this room-temperature adhe-sion phenomenon coupled with an appropriate heating stepfor silicon wafers in order to replace epitaxial growth ofthick silicon wafers or to fabricate silicon-on-insulator(SOI) structures, respectively. Shortly afterwards, as an ex-tension of the well-established ªanodic bondingº, the bond-ing of structured silicon wafers was applied to the fabrica-tion of micromachined pressure sensors, termed ªsiliconfusion bondingº.[4]It has been demonstrated that waferbonding is not restricted to silicon/silicon bonding but canbe applied to many kinds of materials combinations involv-ing silicon or other materials.[5]The generic nature of waferbonding was recognized in the early nineties[6]and it is oneof the two main topics of this review and will be discussedin more detail based on recent developments.In many wafer bonding applications, thinning of one wa-fer of a bonded pair is a necessary step to realize the de-sired materials combination. In SOI, a thin single crystal-line silicon layer is employed to make devices. Its thicknessis in the range of 5 nm to a few micrometers, depending onthe device structures. In the case of bonding of dissimilarmaterials, thinning of one wafer of a pair to a thickness lessthan the respective critical value for the materials combina-tion is essential. This approach prevents the generation ofmisfit dislocations in the layer and avoids cracking of thebonded pairs during subsequent thermal processing steps.[7]The most promising approach to realizing layer transfer in-volves using wafer bonding and hydrogen-implantation-in-duced layer splitting from the host wafer to a bonded de-sired substrate.[8]This approach has been applied to Si(SOI), Ge,[9]SiC,[10,11]InP,[12]and GaAs[13]layer transfer;among these, SOI wafers are now in mass production. Fullyprocessed or partially processed device layers can also betransferred from the host materials onto a desired substrateand the back side of the device layer can then be furtherprocessed for an improved device performance. The 3Ddouble-gate SOI MOSFET (metal oxide semiconductorfield effect transistor) has been considered as a promisingcandidate for ultra-small devices with a gate length ofabout 25 nm for future VLSI (very large scale integration)circuits. Several designs of the device have been reported,and wafer bonding and layer transfer appear to be essen-tial.[14]Recently, layer transfer of oxides such as sapphireand LaAlO3has also been demonstrated.[15]The genericnature of H-implantation-induced layer splitting is just be-ginning to be recognized[16]and it is therefore one of thesubjects of this review.In the following, we will first discuss our present under-standing of the wafer bonding process. The general re-Adv. Mater. 1999, 11, No. 17 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/1712-1409 $ 17.50+.50/0 1409±[*] Prof. Q.-Y. TongResearch Triangle InstituteRTP, NC 27709 (USA)Prof. U. GöseleMax Planck Institute of Microstructure PhysicsWeinberg 2, D-06120 Halle (Germany)Prof. Q.-Y. Tong, Prof. U. GöseleSchool of Engineering, Duke UniversityDurham, NC 27708 (USA)[**] We thank the members of the wafer bonding research groups at DukeUniversity and at the Max Planck Institute of Microstructure Physicsin Halle, Germany for their contributions to the wafer bonding re-search reported in this paper. The support of the Center of Semicon-ductor Research at Research Triangle Institute is greatly appreciated.We are grateful for the technical assistance in CMP by Tao Zhang, R.Rhoades, and A. Clark of Rodel, Inc. Part of the research was support-ed by grants of SEH, Japan and Intel (Duke University) and by theGerman Federal Ministry of Science, Education, Research and Tech-nology under contracts BMBF-13N6758/0 and BMBF-13N6451/1.1410 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/1712-1410 $ 17.50+.50/0 Adv. Mater. 1999, 11, No. 17quirements of wafer conditioning for successful waferbonding are described in terms of the process buildingblocks of wafer bonding: surface preparation, room-tem-perature bonding, low-temperature bond enhancement,and maintaining a bubble-free bonding interface (Sec. 2).Then we turn to the essential mechanisms involved in theH-implantation-induced layer splitting process. The condi-tions under which layer splitting can be achieved are dis-cussed in terms of layer splitting modules: platelet gen-eration, hydrogen molecule formation in microcracks, andlayer cleavage (Sec. 3). Finally, examples of wafer bond-ing and layer splitting applications are presented (Sec. 4).In this review we will not try to be exhaustive in termsof references but refer to recent conference proceed-ings,[17±20]review articles,[21±25]a special 1995 issue of thePhilips Journal of Research,[26]and a book on this sub-ject.[27]2. Process Modules of Wafer BondingThe basic requirements for good wafer bonding are:i) the materials being bonded form a covalent or chemicalbond across their interface, ii) high stresses are avoided,and
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