Improved direct bonding of Si and SiO2 surfaces by cleaning
`in H2SO4:H2O2 :HF
`Karin Ljungberga) and Anders So¨ derba¨ rg
`ElectronicsDivision, Department of Technology, University of Uppsala, P.O. Box 534,
`S-751 21 Uppsala, Sweden
`Thin Film and Surface Chemistry Group, Department of Inorganic Chemistry, University of Uppsala,
`P.O. Box 531,S-751 21 Uppsala, Sweden
`
`Ulf Jansson
`
`共Received 29 November 1994; accepted for publication 16 May 1995兲
`
`A method for silicon surface preparation prior to wafer bonding is presented. By cleaning the wafers
`in a H2SO4:H2O2 mixture in which a small amount of HF is added, and then rinsing in H2O, the
`bonding behavior of the surfaces is improved, compared to other pretreatments used for bonding.
`The modified SPM cleaning results in a highly fluorinated chemical oxide on the Si surface. A
`subsequent water rinse causes substitution of F by OH groups, which increase the initial attraction
`of the mating surfaces. Higher contact wave velocities and bond strengths than reported for other
`surface pretreatments have been measured, both for bare and thermally oxidized silicon
`
`surfaces. © 1995 American Institute of Physics.
`
`A variety of ways to prepare silicon surfaces prior to
`wafer bonding have been presented during the last few years,
`both for hydrophilic and hydrophobic bonding. Bare silicon
`surfaces have been reported to be well bonded, both with
`respect to bond strength and initial contact wave velocity, if
`treated in an aqueous HF solution immediately before
`contacting.1 The initial attraction is for such hydrophobic
`surfaces attributed to van der Waal’s attraction between the
`outermost atoms of the surfaces.2,3 This type of bonding
`should be used when a completely oxide-free bonded inter-
`face is desired. Wafers covered with a native or a thermal
`oxide, on the other hand, are most often treated in some
`oxidizing solution that makes the surface hydrophilic, such
`as boiling in NH4OH:H2O:H2O2, HCl:H2O:H2O2, or conc.
`HNO3. When bonding treated surfaces in this manner, the
`initial attraction is attributed to hydrogen bonding between
`adjacent hydroxyl 共OH兲 groups on the mating surfaces.4,5
`This attraction force is larger than for hydrophobic surfaces,
`resulting in higher contact wave velocity and room-
`temperature bond strength.1
`Different mixtures of oxidizing and etching agents are
`widely used for cleaning of silicon substrates. One of the
`best cleaning solutions, by means of the quality of the chemi-
`cal oxide, is the mixture of H2SO4:H2O2, called SPM 共sul-
`phuric peroxide mixture兲.6 A serious problem with this solu-
`tion is that sulphur is very difficult
`to remove in the
`following rinse, due to the viscous nature of the sulphuric
`
`acid. Verhaverbeke etal.have recently presented a way to
`
`overcome this, by addition of a small amount of hydrofluoric
`acid 共HF兲.7 HF etches the oxide during cleaning, and the
`surface turns out to be hydrophobic when removed from the
`solution. The authors’ explanation is, that the surface is com-
`pletely F passivated, i.e., all dangling bonds are occupied by
`fluorine. After a water rinse, the F passivation is removed,
`and the surface becomes hydrophilic. This is probably due to
`
`a兲Electronic mail: karin.ljungberg@teknikum.uu.se
`
`exchange of fluorine to OH groups, which takes place very
`quickly in water.8 Such an OH-terminated surface should,
`according to the above, be highly bondable. In this work,
`surfaces treated in HF-modified SPM solutions are studied
`by means of bonding properties. The surfaces, both bare sili-
`con and thermally oxidized, have been bonded and the con-
`tact wave velocity, as well as the bond strength, have been
`measured. The samples were analyzed with x-ray photoelec-
`
`diation. All spectra were corrected for charging using an ar-
`
`tron spectroscopy 共XPS兲, using monochromated AlK␣ ra-
`bitrary value of 284.8 eV for the C 1s peak of adsorbed
`The silicon wafers 关Wacker, 3 in. p(100), FZ, 1–10
`
`hydrocarbons.
`
`⍀ cm兴 were either used as-received or thermally oxidized
`共approximate thickness 1 ␮m兲. The wafers were soaked for 1
`min in a mixture of H2SO4共97%兲:H2O2共30%兲 共4:1兲 with 10
`ppm of HF added. The temperature in the solution was
`⬃100 °C, due to the heat produced in the reaction. Hence-
`forward, this mixture will be referred to as SPFM. This treat-
`ment renders the surface—either bare silicon or thermally
`oxidized—hydrophobic, i.e., the solution does not wet the
`surface when the wafer is removed from the bath. A subse-
`quent 2 min rinse in DI water is then performed, turning the
`surfaces hydrophilic. The wafers were blow-dried in N2, af-
`ter which the wafers were immediately brought together. The
`bonded area will proceed like a wave across the wafer, and
`can be observed in transmitted IR light. The velocity of this
`contact wave could thus be measured,3,9 and the amount of
`voids estimated.
`The resulting room-temperature bond strengths were
`measured using the ‘‘razor blade method’’, first introduced
`by Maszara.10 The surface energy ␥, of the bond can be
`calculated, with knowledge of Young’s modulus 共1.66⫻1011
`Pa兲, the wafer thickness 共381 ␮m兲 and the thickness of the
`
`blade 共50 ␮m兲. The crack length L, introduced by insertion
`
`of the blade, was determined by optical inspection in trans-
`mitted IR light, with an accuracy of ⫾0.25 mm.
`The measured contact wave velocities and room-
`
`650
`
`Appl. Phys. Lett. 67 (5), 31 July 1995
`
`0003-6951/95/67(5)/650/3/$6.00
`
`© 1995 American Institute of Physics
`
`TSMC1006
`IPR of U.S. Pat. No. 7,335,996
`
`

`

`TABLE I. Mean values of the measured contact wave velocities and bond
`strengths for silicon surfaces differently treated prior to bonding. Values in
`parentheses give the standard deviation.
`
`Wave velocity
`共cm/s兲
`
`Surface energy
`共J/m2兲
`
`3.5 共0.7兲
`1.5a
`2.0c
`
`0.09 共0.02兲
`0.02b
`0.05b
`
`Surface treatment
`
`SPFM and H2O rinse
`Hydrophobic, 10% aqHF
`Hydrophilic, RCA cleaned
`
`aReference 1.
`bReference 2.
`cReference 8.
`
`temperature bond strengths are given in Table I, where also,
`as comparison, corresponding values reported for hydropho-
`bic 共HF etched in 10% HF兲 and hydrophilic surfaces 共stan-
`dard RCA cleaned兲 are included.1,8 The HF content in the
`SPFM was 10 ppm for all the samples listed in Table I. As
`can be seen, the measured wave velocities are higher for the
`SPFM-treated surfaces than for both hydrophilic and hydro-
`phobic surfaces. The resulting room-temperature surface en-
`ergies are comparable to those measured for hydrophilic sur-
`faces. No significant difference in wave velocity or bond
`strength could be seen between bare and thermally oxidized
`surfaces. The quality of the bonds, in terms of bonding spon-
`taneity, bonded part of the whole wafer area, and number of
`voids, was found to be very high. The surfaces bond sponta-
`neously, i.e., once started the contact wave proceeds across
`the wafer area without any help, and there are very few vis-
`ible voids.
`XPS analyses of the SPFM treated samples prior to and
`after H2O rinse, showed peaks from Si, O, F, C, and S. The
`carbon peak can be assigned to contaminants adsorbed on the
`surface during transport in air from the cleaning solution to
`the analysis chamber. The sulphur peak, however, is due to
`adsorption of residual species on the surface during the
`cleaning step. A rough quantitative analysis, based on el-
`emental standards, shows that the amount of S is about
`1 at.%–2 at.%. No reduction in sulphur content on the sur-
`face was observed after rinsing in H2O. Figure 1 shows the
`
`SPFM treatment and 共b兲 after a subsequent water rinse. The
`
`Si 2p and F 1s peaks for 共a兲 a surface immediately after the
`Si 2p peaks show a feature at 103.7 eV, due to the presence
`
`of a thin oxide on the surface. Comparing this with native
`oxides used in Ref. 11, indicates that the thickness of the
`surface oxide is about 10 Å. The surface oxide of the SPFM
`treatment is quite different compared to the oxide obtained
`after
`a
`standard RCA 1
`cleaning
`procedure,
`i.e.,
`NH4OH:H2O:H2O2, 1:5:1 关see Fig. 1共c兲兴. Typically, RCA1
`
`yields a slightly thinner surface oxide. Moreover, the Si 2p
`
`peak from the oxide is clearly shifted towards a lower bind-
`ing energy 共103.2 eV兲. It is well known that the binding
`energy of the oxide peak is influenced by the thickness of the
`oxide film. However, the thickness difference between the
`oxides in Figs. 1共b兲 and 1共c兲 is very small, for which reason
`it is likely that the chemical shift is due to a structural dif-
`ference 共i.e., a structure induced shift兲.12 Further studies are
`required for a complete characterization of the two oxide
`surfaces.
`
`FIG. 1. F 1s and Si 2p spectra from silicon 共100兲 surfaces 共a兲 immediately
`
`after SPFM treatment 关H2O2 :H2SO4 共1:4兲, with 10 ppm HF added兴, 共b兲 after
`a subsequent rinse in deionized 共18 M⍀兲 water, and 共c兲 after cleaning in
`NH4OH:H2O:H2O2 共1:5:1兲.
`
`the surface after
`The XPS analysis also shows that
`SPFM treatment contains significant amount of fluorine. The
`
`F 1s peak was shifted to about 686.6 eV, which is typical for
`
`F atoms bonded to SiO2.11 A rough quantitative analysis
`based on elemental standards suggests that the F/Si ratio in
`the oxide is about 0.25–0.35 共the ratio has been calculated
`
`using only the Si 2p signal from the oxide兲. The high F
`
`content in the oxide explains the hydrophobic nature of the
`SPFM surface. From a chemical point of view, it is likely
`that most of the F atoms are bonded to Si atoms. Assuming
`that about 25% of all Si atoms in a 10 Å thick oxide are
`located at the surface, this should correspond to a situation
`where the F/Si ratio on the surface is about 1–1.4 共i.e., every
`surface Si atom is bonded to at least one F atom兲. Such a
`fluorinated oxide surface should certainly exhibit a highly
`hydrophobic behavior. It should be noted, however,
`that
`some fluorine can be embedded in the oxide, but this needs
`to be further investigated. After the water rinse, the fluorine
`peak is drastically reduced, giving a F/Si ratio of about 0.05
`关see Fig. 1共b兲兴. The decrease is probably due to a substitution
`of F atoms by hydroxyl 共OH兲 groups, yielding a highly hy-
`
`drophilic surface. The O 1s peak can also be seen to be
`
`slightly larger after the rinse.
`Bonded wafers were also annealed in N2 ambient at tem-
`peratures ranging from 400° to 800 °C, for 1 h, whereafter
`the bond strength was again measured.
`In Fig. 2 the surface energies versus anneal temper-
`ature are given. The corresponding values for RCA1
`共NH4OH:H2O:H2O2兲-cleaned and HF共10%兲-etched surfaces,
`from Ref. 2, are given in the same diagram. As can be seen,
`the bond strengths for SPFM-treated surfaces are slightly
`higher than for the RCA1-treated samples, but do not reach
`the values achieved for the HF-etched surfaces at anneal
`temperatures between 500 and 800 °C. Above 800 °C, the
`bond strengths were impossible to measure, since the wafers
`broke by the insertion of the blade.
`
`Appl. Phys. Lett., Vol. 67, No. 5, 31 July 1995
`
`Ljungberg, So¨ derba¨ rg, and Jansson
`
`651
`
`TSMC1006
`IPR of U.S. Pat. No. 7,335,996
`
`

`

`lier reported for other kinds of hydrophilic bonding were
`measured. The resulting bonds have very few visible voids,
`and the bond strengths were found to be slightly higher than
`reported for other hydrophilic 共RCA cleaned兲 surfaces for
`annealing temperatures up to 800 °C. The method is believed
`to have the possibility of being a useful tool for bonding
`steps both in micromechanical and microelectronic applica-
`tions, especially for bonding of oxidized surfaces.
`This work was financed by a grant from The Swedish
`Research Council for Engineering Science 共TFR兲.
`
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`FIG. 2. Measured surface energies of bonded wafer pairs, vs annealing
`temperature, for differently treated Si共100兲 surfaces. ‘‘SPFM’’ refers to
`H2O2 :H2SO4 共1:4兲, with 10 ppm of HF added, with a subsequent water
`rinse. The data for RCA1 共NH4OH:H2O:H2O2兲 and HF-treated surfaces were
`received from Ref. 2. Annealing was performed in N2 ambient for 1 h. The
`error bars for the SPFM data represent standard deviation.
`
`To summarize, a new recipe for direct bonding of Si and
`SiO2 surfaces is presented. The surfaces to be bonded are
`cleaned in a modified SPM solution 共SPFM兲, i.e., HF is
`added to a mixture of H2SO4:H2O2. This results in a surface
`covered with fluorine, which is substituted by hydroxyl
`groups by a water rinse, resulting in a highly bondable sur-
`face. Compared to the chemical oxide formed by standard
`RCA1 cleaning 共also having a high OH density兲, the oxide
`formed by SPFM is probably both structurally and chemi-
`cally different, which might explain a different bonding be-
`havior. Contact wave velocities considerably higher than ear-
`
`652
`
`Appl. Phys. Lett., Vol. 67, No. 5, 31 July 1995
`
`Ljungberg, So¨ derba¨ rg, and Jansson
`
`TSMC1006
`IPR of U.S. Pat. No. 7,335,996
`
`