Mechanisms for room temperature direct wafer bonding

Plach, Thomas; Hingerl, Kurt; Tollabimazraehno, Sajjad; Hesser, Günter; Dragoi, Viorel; Wimplinger, Markus

doi:10.1063/1.4794319

Cited by 157 publications

(112 citation statements)

References 19 publications

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“…6 These experiments demonstrate that a robust bonding using O 2 plasma can be achieved even at RT but requires a long storage time. In contrast, the bonding of the wafer pairs prepared with (O 2 + CF 4 ) plasma was fairly weak at the initial step before storage.…”

Section: Resultsmentioning

confidence: 86%

“…O 2 plasma treatments have been found to increase the density of silanol group (Si-OH) on the surface and create subsurface oxide region, which is a sponge-like reservoir for water molecules. 6,20,21 To get a greater bonding strength than hydrogenbonded Si/Si pairs (0.179 J/m 2 in a calculated value), storing the bonded wafers at RT or elevated temperatures are required to convert hydrogen bonds into Si-O-Si covalent bonds. During storage at RT, the interfacial water enters into the surrounding Si-enriched oxide layers to help the surface asperities to deform (so-called "water stress corrosion"), allowing the contact area to increase.…”

Section: Treatment (<5mentioning

confidence: 99%

“…[1][2][3] To achieve a robust bonding at RT, surface treatments are performed prior to bonding, e.g., plasma activation, [4][5][6] wet chemistry, 7 or combination of these processes. 8 Among them, by introducing fluorine (F) into oxide layers before bonding, a hydrofluoric (HF) acid treatment has been found to enhance the bonding strengths of silicon or/and silicon oxide wafers at RT.…”

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confidence: 99%

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Mechanisms for Room-Temperature Fluorine Containing Plasma Activated Bonding

Wang

Liu

et al. 2017

ECS J. Solid State Sci. Technol.

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Room-temperature direct bonding is an attractive approach to combine dissimilar wafers avoiding any thermal stress by a difference of thermal expansion coefficients. In this paper, we investigate a room-temperature bonding process using fluorine containing plasma activation and propose a mechanism model to address the roles of fluorine. Details of the plasma treated surface and the bonding interface are characterized. Experimental results show that fluorinated oxide formed on the silicon surface results in a lower bonding strength at the initial bonding step before the storage. On the other hand, during storage at room temperature, fluorinated oxide asperity might be more easily softened by the interfacial water enabling a significant bonding strength enhancement. As a result, strong bonding strength of Si/Si wafer pairs, very close to the fracture energy of bulk silicon, is achieved after storage in air for 24 h, even at room temperature. Room-temperature wafer direct bonding is a state-of-the-art technique that allows wafers to be bonded at room temperature (RT, ∼25• C) without employing heat, and therefore is highly desirable to avoid temperature-related problems for many applications.1-3 To achieve a robust bonding at RT, surface treatments are performed prior to bonding, e.g., plasma activation, 4-6 wet chemistry, 7 or combination of these processes. 8 Among them, by introducing fluorine (F) into oxide layers before bonding, a hydrofluoric (HF) acid treatment has been found to enhance the bonding strengths of silicon or/and silicon oxide wafers at RT.9,10 Very strong bonding strength (also represented by "surface energy") of SiO 2 /SiO 2 , equivalent to the silicon fracture energy (∼2.5 J/m 2 ), has been realized after storage at RT for 60 h.11 Because oxide layers can be easily deposited on non-silicon materials, e.g., using thermal oxidation or chemical vapor deposition (CVD), the fluorine-enhanced oxide-to-oxide bonding process offers bonding of multiple wafers with same or different coefficients of thermal expansion (CTE) for plenty of uses. 12In contrast to produce fluorinated layers by wet chemistry, our group has developed a fluorine containing plasma activated bonding process.13 By introducing a small amount of carbon tetrafluoride (CF 4 ) into oxygen plasma, strong bonding between Si or/and SiO 2 wafers has been achieved even at RT. Moreover, void formation at the bonding interfaces could be mitigated effectively during the subsequent annealing process.14 The fluorination process by plasma surface treatments has been further simplified without requiring a fluorinated gas in recent years.15 However, the physical and chemical mechanisms enabling RT wafer bonding are still poorly understood. In this paper, we focus on characterizations of the fluorine containing plasma treated surfaces as well as the bonding interfaces. The bonding parameters are optimized, such as CF 4 gas ratio, RT storage time and storage environments. Moreover, the roles of fluorine are addressed. Based on the experimental finding...

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Section: Resultsmentioning

confidence: 86%

Section: Treatment (<5mentioning

confidence: 99%

mentioning

confidence: 99%

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Mechanisms for Room-Temperature Fluorine Containing Plasma Activated Bonding

Wang

Liu

et al. 2017

ECS J. Solid State Sci. Technol.

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“…The O 2 plasma treatment has been well known to increase the density of OH groups on surfaces as well as create subsurface oxide region, which is a sponge-like reservoir for water. [23][24][25] In contrast to the smooth surfaces after the plasma treatment, a few asperity apexes were formed on the surfaces after the VUV irradiation in air. In spite of this, the excellent bonding efficiency was achieved in our experiments and the bonding strength increased dramatically above the annealing temperature of 150…”

Section: Resultsmentioning

confidence: 86%

Direct Homo/Heterogeneous Bonding of Silicon and Glass Using Vacuum Ultraviolet Irradiation in Air

Wang

et al. 2018

J. Electrochem. Soc.

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We develop a cost-effective vacuum ultraviolet (VUV) irradiation in air combined with an in situ bonding process. The whole bonding process does not require high vacuum environments. Strong bonding strengths for Si/Si, Si/glass, and glass/glass pairs were achieved with the assistance of annealing at 200 • C. There was no crack or defect at the bonding interfaces. The excellent optical transparency of the bonded glass/glass pairs was demonstrated in the UV-visible range. On the basis of the surface and bonding interface characterizations, the low-temperature bonding mechanism was investigated and discussed. Similar to the plasma activated bonding, the internal water stress corrosion plays a crucial role in the mechanical evolution of bonding interface between the VUV irradiated surfaces during annealing. This facile bonding method offers great potential for silicon-and glass-based homo/heterogeneous integrations in microelectronics, optoelectronics and microfluidics.

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“…Many reports suggest the need for a thick oxide film to achieve a void-free bonding. [13][14][15] When a pre-bonded wafer is annealed, water molecules from the bond interface diffuse to the Si surface where they react with silicon to form silicon dioxide and hydrogen, the resulting hydrogen escapes to the interface to form voids. The hydrogen may also be dissolved in the thick oxide.…”

Section: Discussionmentioning

confidence: 99%

Low temperature bonding of heterogeneous materials using Al2O3 as an intermediate layer

Sahoo

Ottaviano

Zheng

et al. 2017

Journal of Vacuum Science &Amp; Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phen

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Integration of heterogeneous materials is crucial for many nanophotonic devices. The integration is often achieved by bonding using polymer adhesives or metals. A much better and cleaner option is direct wafer bonding, but the high annealing temperatures required make it a much less attractive option. Direct wafer bonding relies on a high density of hydroxyl groups on the surfaces, which may be difficult to achieve depending on the materials. Thus, it is a challenge to design a universal wafer bonding process. However, using an intermediate layer between the bonding surfaces reduces the dependence on the bonding materials, and thus, the bonding mechanism essentially remains the same. The authors present a systematic study on the use of Al2O3 as an intermediate layer for bonding of heterogeneous materials. The ability to achieve high hydroxyl group density and well-controlled films makes atomic layer deposited Al2O3 an excellent choice for the intermediate layer. The authors have optimized the bonding process to achieve a high interface energy of 1.7 J/m2 for a low temperature annealing of 300 °C. The authors also demonstrate wafer bonding of InP to SiO2 on Si and GaAs to sapphire using the Al2O3 interlayer.

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Mechanisms for room temperature direct wafer bonding

Cited by 157 publications

References 19 publications

Mechanisms for Room-Temperature Fluorine Containing Plasma Activated Bonding

Mechanisms for Room-Temperature Fluorine Containing Plasma Activated Bonding

Direct Homo/Heterogeneous Bonding of Silicon and Glass Using Vacuum Ultraviolet Irradiation in Air

Low temperature bonding of heterogeneous materials using Al2O3 as an intermediate layer

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