Mixed Orientation Si–Si Interfaces by Hydrophilic Bonding and High Temperature Oxide Dissolution

Abstract

In this paper, we describe a "quasi-hydrophobic" bonding method in which ultrathin ͑Ͻ1-2 nm͒ oxide, present on wafer surfaces during bonding, is removed after bonding by a high temperature oxide dissolution anneal to leave the desired direct Si-to-Si contact at the bonded interface. We show that the direct-silicon-bonded ͑DSB͒ interfaces produced by this method are clean enough to allow implementation of a recently described amorphization/templated recrystallization technique for changing the orientation of selected DSB layer regions from their original orientation to the orientation of the underlying handle wafer. We then present results from a related study on the dissolution of oxide layers disposed between a Si substrate and a polycrystalline overlayer, and discuss mechanisms most likely to be operative for our oxide dissolution observations. Semiconductor device technology is increasingly relying on bonded Si wafer substrates to achieve desired performance targets. Most Si wafer bonding is hydrophilic, between wafer surfaces that are oxidelike. Hydrophilic bonding is clearly the technique of choice when an oxide is desired at the bonded interface, for example, when fabricating silicon-on-insulator ͑SOI͒ structures. However, certain fabrication schemes for bulk hybrid orientation substrates 1,2 require a direct Si-to-Si bond between Si surfaces having different surface orientations, with no oxide layer at the bonded interface.Direct Si-to-Si bonding for direct-silicon-bonded ͑DSB͒ wafers is normally achieved with hydrophobic bonding, 3 a technique with some difficulties. Hydrophobic ͑H-terminated͒ surfaces are more easily contaminated than hydrophilic ones, often making it necessary to perform the bonding in a vacuum environment. In addition, the widely used surface plasma treatments developed to allow roomtemperature bonding, typically, though with some exceptions, 4 introduce surface oxygen, making them incompatible with an oxide-free bonded interface. Bonding at higher temperatures can also present difficulties because most cleaving processes for separating the bonded layer from the wafer to which it was originally attached are thermally activated and start occurring in the same temperature range as the bonding. Given these difficulties, it was thought that it might be preferable to fabricate DSB wafers with a "quasihydrophobic" bonding technique in which an ultrathin ͑1-2 nm͒ oxide would be present on one or both wafer surfaces during bonding ͑thereby allowing the bonding to be hydrophilic͒, but removed after the bonding ͑to leave the desired direct Si-to-Si contact at the bonded interface͒.The stability of buried oxide layers in Si is a topic with relevance to both Si-based devices 5 ͑due to the need for tightly controlled thicknesses of thin insulating SiO 2 layers between electrically active Si regions͒ and silicon-on-insulator ͑SOI͒ substrate fabrication. SOI substrates fabricated by separation by implanted oxygen ͑SIMOX͒ are typically subjected to internal thermal oxidation ͑ITOX͒ anneals to thicken ...

Mobility Enhancement Technology for Scaling of CMOS Devices: Overview and Status

“…A DSB substrate could be formed by a ''quasi-hydrophobic'' bonding method in which ultrathin (1 nm to 2 nm) oxide present on the wafer surfaces during bonding is removed by high-temperature (1320°C to 1325°C) oxide dissolution annealing, leaving the desired direct Si-to-Si contact at the bonding interface. 76 ATR is performed on these selected regions of Si (110) bonded to a Si(100) handle wafer. The Si regions selected for ATR are first separated from those that are not selected for ATR by SiO 2 trenches.…”

Mixed Orientation Si-Si Interfaces by Hydrophilic Bonding and High Temperature Oxide Dissolution: Wafer Fabrication Technique and Device Applications

“…Literature data on the diffusivity and solubility of O interstitials (O i ) in Si allows one to estimate the maximum "dissolvable thickness" (X diss ox ) of oxide that might be expected for low-O i content (FZ) Si wafers annealed at a given temperature and time. With the diffusivity of O i in Si given by D i (T) = 0.07 exp (-2.44 eV/ kT) cm 2 s -1 , [1] where k is the Boltzmann constant and T is the absolute temperature, and the solubility of O i in Si given by C eq i (T) = 1.53 x 10 21 exp (-1.03 eV/ kT) cm -3 , [2] one obtains (8)…”

“…We will then discuss the mechanisms for oxide dissolution most likely to be operative for our process conditions. Finally, we will show that the DSB interfaces produced by this bonding method are clean enough to allow implementation of an amorphization/templated recrystallization (ATR) technique (1,2) for changing the orientation of selected DSB layer regions from their original orientation to the orientation of the underlying handle wafer, and briefly review how the ATR technique may be used with hybrid orientation DSB wafers for device applications.…”

Fabrication of Direct Silicon Bonded Hybrid Orientation Substrate by Separation by Implanted Oxygen Layer Transfer and Oxide Dissolution Annealing

“…For realizing a DSB hybrid orientation substrate, Saenger et al and Kononchuk et al proposed a quasi hydrophilic bonding process, in which an additional post annealing after a hydrophilic bonding process is used to eliminate the interfacial oxide layer. 10,11) Recently, a newly developed approach, referred to as the separation by implanted oxygen layer transfer (SLT) process, [12][13][14] has been proposed to fabricate SOI wafers. As a novel layer transfer technique, the SLT process opens up an innovative way of synthesizing DSB hybrid orientation substrates for the hybrid orientation technology.…”