Journal of ELECTRONIC MATERIALS, Vol. 43, No. 2, 2014
DOI: 10.1007/s11664-013-2861-z Ó 2013 TMS
Dynamics of SiO2 Buried Layer Removal from Si-SiO2-Si and Si-SiO2-SiC Bonded Substrates by Annealing in Ar ¨ . VALLIN,1 and J. OLSSON1 L.-G. LI,1,3 S. RUBINO,2 O ˚ ngstro¨m Laboratory, Solid State Electronics, Uppsala University, P.O. Box 534, 751 21 1.—The A ˚ ngstro¨m Laboratory, Applied Material Science, Uppsala University, Uppsala, Sweden. 2.—The A P.O. Box 534, 751 21 Uppsala, Sweden. 3.—e-mail:
[email protected]
Silicon-on-silicon-carbide substrates could be ideal for high-power and radiofrequency silicon devices. Such hybrid wafers, when made by wafer bonding, contain an intermediate silicon dioxide layer with poor thermal characteristics, which can be removed by high-temperature annealing in an inert atmosphere. To understand the dynamics of this process, removal of 2.4-nm-thick SiO2 layers from Si-SiO2-Si and Si-SiO2-SiC substrates has been studied at temperatures ranging from 1100°C to 1200°C. The substrates were analyzed by transmission electron microscopy, electron energy-loss spectroscopy, secondary-ion mass spectroscopy, and ellipsometry, before and after annealing. For oxide thickness less than 2.4 nm, the activation energy for oxide removal was estimated to be 6.4 eV, being larger than the activation energy reported for removal of thicker oxides (4.1 eV). Under the same conditions, the SiO2 layer became discontinuous. In the time domain, three steps could be distinguished: bulk diffusion, bulk diffusion with void formation, and bulk diffusion with disintegration. The void formation, predominant here, has an energetic cost that could explain the larger activation energy. The oxide remaining after prolonged annealing corresponds to one layer of oxygen atoms. Key words: Dynamics of oxygen outdiffusion, ultrathin SOI, hybrid substrates, Si-SiC hybrid substrates, oxide-free hybrid substrates
INTRODUCTION Silicon carbide (SiC) is a wide-bandgap semiconductor material and is utilized for high-breakdownvoltage devices as well as radiofrequency devices. It is also a very good thermally conductive material. The thermal conductivity of single-crystalline SiC is about three to four times higher than that of silicon (Si). Our earlier study showed that replacing the handle wafer and the buried oxide of siliconon-insulator (SOI) with crystalline SiC using wafer bonding gave a factor of 4 better thermal dissipation compared with commercial SOI.1 Such Si-SiC bonded hybrid substrates could be ideal for high-power and radiofrequency silicon devices.2 (Received April 2, 2013; accepted October 11, 2013; published online November 19, 2013)
Hydrophilic wafer bonding always introduces an intermediate oxide layer at the bonding interface. This layer behaves as a thermal barrier layer. In our earlier study, this layer was completely removed by oxygen outdiffusion (Ox-away).3,4 We would like to further understand the following aspects of the dynamics of the Ox-away process: (1) For partial oxide removal, do the experimental results stand up well against theory? (2) Does the layer become thinner uniformly? (3) Is the activation energy around 4.1 eV as predicted by Eq. 1 if Ox-away is the only mechanism behind the process? These are the questions we try to answer in this paper. For the Ox-away theory, the driving force is the oxygen concentration difference between the intermediate oxide layer and the top Si layer surface. On the surface of the top Si, the outdiffused oxygen is diluted by the surrounding inert gas molecules to an 541
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extremely low level that will be treated as zero. The process is temperature and time dependent, as expressed in Eq. 1. Fox ðTÞ ¼ ¼
DðTÞ Cs ðTÞ 0 dSi 1:183 1022 e4:1 eV=kT dSi
ðcm2 s1 Þ; (1)
where Fox(T) is the oxygen flux, i.e., the number of oxygen atoms that pass through the top Si layer per unit area and time. D(T) is the diffusion coefficient of oxygen in Si, which was derived from secondaryion mass spectroscopy (SIMS) measurements of indiffusion of oxygen during oxidation of float-zone Si in a wide temperature range up to 1240°C.5 dSi is the thickness of the top Si layer. Cs(T) is the solubility of oxygen in the Si layer, which is derived from SIMS measurements of indiffusion of oxygen in a bulk float-zone (FZ) Si wafer in inert or oxidizing atmosphere at 900°C to 1250°C.6 k is the Boltzmann constant. To control the oxygen consumption and avoid complex calculations for the Ox-away process, it is necessary to have both a substrate and top Si layer with low oxygen concentration. High-resolution cross-sectional transmission electron microscopy (HRXTEM) together with electron energy-loss spectroscopy (EELS) were used to investigate the oxide thickness, the overview of the bonding stack, the crystalline quality, and whether oxygen is present. To address the limitation of EELS in detecting oxygen, the oxygen profiles of the bonding layers were investigated by SIMS before and after Ox-away. To address the limitation of transmission electron microscopy (TEM) for accurate measurement of oxide thickness, we also used ellipsometry. EXPERIMENTAL PROCEDURES Bonding experiments were carried out in a class 100 cleanroom.3,4 The Si (1 0 0) layers to be transferred to the FZ wafer and the SiC wafers from the as-received SOI wafers had oxygen content less than 2.5 9 1016 cm3. In this work, it is essential to use an FZ wafer with low oxygen content (
