Direct Bonding Mechanism of ALD-Al2O3 Thin Films

Direct bonding is used to join two mirror-polished wafers without any additional material. This technique demonstrates to be more and more attractive for microelectronics, MEMS, or optoelectronic applications as example. In many of them, Silicon-On-Insulator (SOI) substrate is used including a SiO₂ buried layer (BOX). Face to circuit performance request, SOI self-heating in these devices has to be improved, replacing BOX by adapted layers, for instance. The conciliation between the thermal and electrical properties has already pointed out different materials such as diamond, Al₂O₃, HfO₂ or Si₃N₄ [1]. Atomic Layer Deposited (ALD) amorphous Al₂O₃ thin film seems to be very interesting but different problems as for instance debonding phenomena [2][3], or defect emergence [4][5][6] could appear during their elaboration which motivated our direct bonding mechanism study. In this work, comparative results between free surfaces and bonding configurations will be done in order to establish bonding behaviour of ALD Al₂O_3.A bonding mechanism will be discussed and proposed.

The study is carried out on 300mm, lightly p-doped Si wafers. Substrates have been oxidized by plasma or thermal treatment in order to create 1.5, 20 or 145nm SiO₂ sub-layer. 20 nm-thick amorphous alumina (a-Al₂O₃) films were then deposited at low temperature (300°C) by ALD using TMA and H₂O precursors. Some samples were annealed at various temperatures under nitrogen or oxygen atmosphere in order to crystallize the amorphous alumina. Due to low chemical resistance of the amorphous ALD Al₂O₃layer, the wafers were just rinsed with de-ionized water under megasonic power, dried and contacted to each other or directly to silicon wafers or oxidised silicon wafers at room temperature (RT) and atmospheric pressure. Bonding quality is analysed at room temperature and after various annealing temperatures.

After 400°C post bonding annealing, using scanning acoustic microscopy (SAM), a high defect density is observed for several bonding type (fig 1a) which is invisible using wafer-scale IR observations (fig 1b). In the specific a-Al₂O₃/a-Al₂O₃bonding, debonding could even occur after 500°C annealing. Bonded surface analyses exhibit inhomogeneous fracture and bubble traces at the bonding interface (fig 1c).

This paper will deal with bonding mechanism and the defect origin. We will highlight the role of water/hydrogen diffusion in the bonding process and the link that could be done with the well-known [4][7] defects which could appear during O₂-atmosphere annealing of amorphous ALD Al₂O₃ free surfaces (fig2a) while no defect are seen under N₂annealing (fig2b).

Using atomic force microscopy to measure the surface roughness, the electron beam microscopy to visualize the debonding interface, the anhydrous bonding energy measurements, the FTIR-MIR [8] to characterize the chemical interface evolution and the X-ray reflectivity for monitoring the interface electron density profile, all the different bonding structures achieved in this study have been analysed (preannealing, sublayer use…).

Based on known SiO₂ hydrophilic direct bonding mechanisms [8], refined bonding mechanisms for amorphous a-Al₂O₃ bonding will be proposed. Therefore, defect free amorphous Al₂O₃bonding could be obtained overall the post bonding annealing temperature range (RT-1200°C) as shown for 300mm bonded structures (Fig 3).

One of the first applications of this work would be the elaboration of high quality SOI structure having this ALD Al₂O₃layer as an insulating layer.

ACKNOWLEDGMENT

The authors would like to thank SOITEC for its support.

REFERENCES

[1] N. Bresson, et al. Solid-State Elect. 49, 1522-1528, 2005

[2] P. Ericsson, et al., ECS PV, 576, 1997

[3] C. de Beaumont, et al., Electro Soc. 3, 231-236, 2005

[4] M. Yokoyama, et al., Semi. Sci. Tech., 28, 2013

[5] D. Landru, et al., ECS JSS and Tech, 6, 2, Q83-Q87, 2013

[6] E. Beche, et al., Micro Techn (submitted)

[7] T. Nabatame, et al., JAP, 42, 7205-7208, 2003

[8] C. Ventosa et al., JAP, 104, 2008